Nevertheless O
, O
the O
main O
outcome O
of O
the O
reported O
experience O
is O
the O
proposed O
redesign O
approach O
for O
part B-CONPRI
consolidation E-CONPRI
using O
metal B-MANP
AM E-MANP
. O


The O
proposed O
approach O
is O
general O
, O
well O
structured O
, O
and O
detailed O
enough O
to O
be S-MATE
immedialtely O
applicable O
at O
industrial S-APPL
level O
, O
but O
it O
needs O
further O
testing S-CHAR
on O
different O
case B-CONPRI
studies E-CONPRI
to O
prove O
the O
benefits O
. O


An O
interesting O
framework S-CONPRI
to O
part B-CONPRI
consolidation E-CONPRI
and O
functional O
integration O
exploiting O
AM B-MANP
technologies E-MANP
is O
presented O
and O
validated O
in O
. O


In O
a O
further O
development O
of O
their O
approach O
, O
the O
authors O
have O
recently O
addressed O
the O
relevant O
problem O
of O
detecting O
the O
possible O
candidates O
for O
part B-CONPRI
consolidation E-CONPRI
, O
as S-MATE
reported O
in O
. O


Potentially O
, O
when O
dealing O
with O
part B-CONPRI
consolidation E-CONPRI
, O
the O
designer O
may O
consider O
the O
need O
of O
part O
decomposition S-PRO
. O


The O
need O
to O
increase O
the O
number O
of O
parts O
in O
a O
product O
fabricated S-CONPRI
by O
AM S-MANP
may O
be S-MATE
due O
to O
many O
reasons O
: O
printability S-PARA
, O
productivity S-CONPRI
, O
functionality O
, O
artistry O
, O
and O
interchangeability O
. O


Printability S-PARA
is O
actually O
the O
main O
reason O
being O
related O
to O
the O
limited O
working O
envelope O
of O
AM B-MACEQ
machines E-MACEQ
. O


In O
the O
part O
decomposition S-PRO
problem O
in O
AM S-MANP
is O
addressed O
, O
but O
it O
is O
the O
first O
and O
unique O
example O
of O
research S-CONPRI
on O
this O
topic O
. O


9 O
Conclusion O
and O
future O
challenges O
DfAM O
is O
about O
design S-FEAT
for O
the O
whole O
AM S-MANP
product O
life B-CONPRI
cycle E-CONPRI
. O


This O
paper O
has O
presented O
a O
framework S-CONPRI
of O
tools S-MACEQ
and O
methods O
for O
DfAM O
and O
has O
shown O
the O
strong O
interaction O
between O
these O
life B-CONPRI
cycle E-CONPRI
stages O
and O
AM S-MANP
product O
design S-FEAT
. O


The O
state O
of O
the O
art S-APPL
was O
presented O
on O
many O
of O
these O
design S-FEAT
tools O
and O
their O
applicability O
was O
illustrated O
using O
many O
of O
the O
latest O
examples O
from O
research S-CONPRI
and O
industry S-APPL
. O


9.1 O
AM S-MANP
suitability O
exploration O
A O
growing O
number O
of O
companies S-APPL
are O
exploring O
the O
commercial O
use O
of O
AM S-MANP
within O
their O
supply B-CONPRI
chain E-CONPRI
. O


For O
that O
, O
better O
methods O
and O
tools S-MACEQ
are O
needed O
to O
help O
the O
designer O
obtain O
an O
overview O
that O
identifies O
the O
match O
between O
functional O
and O
economical O
demands O
of O
the O
intended O
product O
and O
all O
stages O
of O
AM S-MANP
in O
product B-CONPRI
development E-CONPRI
. O


Methods O
for O
early O
cost B-CONPRI
estimation E-CONPRI
are O
lacking O
, O
while O
current O
figures O
for O
production B-CONPRI
cost E-CONPRI
per O
cm3 O
are O
not O
based O
on O
established O
calculation O
methods O
and O
lack O
experimental S-CONPRI
and O
industrial S-APPL
verification O
. O


Late O
life B-CONPRI
cycle E-CONPRI
stages O
of O
postprocessing S-CONPRI
, O
inspection S-CHAR
and O
certification O
have O
a O
significant O
impact S-CONPRI
on O
production B-CONPRI
cost E-CONPRI
and O
general O
applicability O
of O
AM S-MANP
, O
but O
these O
stages O
are O
underrepresented O
in O
current O
DfAM O
approaches O
. O


Finally O
, O
more O
education O
on O
Additive B-MANP
Manufacturing E-MANP
is O
needed O
, O
as S-MATE
the O
majority O
of O
product O
designers O
and O
engineers O
are O
still O
trained O
to O
think O
subtractive S-MANP
. O


9.2 O
Product B-FEAT
design E-FEAT
for O
AM S-MANP
goals O
Topology B-FEAT
optimization E-FEAT
enables O
strategies O
that O
go S-MATE
beyond O
lightweight S-CONPRI
design S-FEAT
to O
include O
minimizing O
support S-APPL
usage O
and O
thermal B-CHAR
deformation E-CHAR
, O
optimizing O
local B-CONPRI
heat I-CONPRI
input E-CONPRI
, O
and O
with O
that O
the O
local O
material B-CONPRI
properties E-CONPRI
, O
porosity S-PRO
and O
strength S-PRO
. O


TO O
methods O
need O
further O
enhancement O
to O
tackle O
optimization S-CONPRI
as S-MATE
a O
3D S-CONPRI
problem O
while O
taking O
all O
later O
product B-CONPRI
development E-CONPRI
stages O
into O
account O
. O


Generative B-ENAT
design E-ENAT
strategies O
have O
the O
benefit O
that O
they O
generate O
many O
possible O
design S-FEAT
solutions O
, O
but O
as S-MATE
is O
the O
case O
with O
TO O
, O
few O
production S-MANP
and O
inspection S-CHAR
constraints O
are O
currently O
integrated O
. O


Lattice B-FEAT
structures E-FEAT
show O
explicit O
benefits O
, O
especially O
in O
application O
fields O
involving O
energy B-CHAR
absorption E-CHAR
and O
heat B-CONPRI
conduction E-CONPRI
. O


They O
are O
however O
computationally O
intensive O
, O
which O
limits S-CONPRI
the O
optimization S-CONPRI
possibilities O
for O
large O
lattices S-CONPRI
to O
a O
limited O
set S-APPL
of O
parameters S-CONPRI
. O


When O
looking O
at O
the O
part O
interior O
, O
specially O
designed S-FEAT
porosity O
is O
seen O
as S-MATE
a O
promising O
new O
feature S-FEAT
for O
internal O
transport S-CHAR
of O
gasses O
and O
fluids S-MATE
. O


Both O
the O
optimal B-PARA
process E-PARA
settings O
as S-MATE
well O
as S-MATE
models O
for O
their O
application O
need O
further O
research S-CONPRI
to O
be S-MATE
applicable O
in O
everyday O
product B-FEAT
design E-FEAT
. O


To O
fully O
exploit O
the O
benefits O
of O
functional B-CONPRI
material I-CONPRI
complexity E-CONPRI
, O
further O
research S-CONPRI
must O
be S-MATE
conducted O
on O
rules O
and O
CAD S-ENAT
representations O
of O
FGM S-MANP
related O
design S-FEAT
intent O
. O


However O
, O
the O
development O
of O
design S-FEAT
tools O
and O
methods O
is O
still O
a O
matter O
of O
basic O
research S-CONPRI
and O
far O
from O
industrial S-APPL
application O
. O


The O
future O
of O
additive B-MANP
manufacturing E-MANP
is O
also O
looking O
towards O
4D S-CONPRI
applications O
where O
those O
challenges O
will O
be S-MATE
even O
more O
relevant O
. O


For O
the O
final O
optimization S-CONPRI
of O
geometry S-CONPRI
so O
that O
it O
combines O
AM S-MANP
benefits O
with O
efficient O
production S-MANP
and O
inspection S-CHAR
, O
a O
lot O
of O
research S-CONPRI
has O
been O
conducted O
. O


At O
an O
individual O
process S-CONPRI
level O
design S-FEAT
knowledge O
is O
available O
and O
constantly O
being O
extended O
or O
refined O
. O


Further O
research S-CONPRI
is O
needed O
to O
enable O
improved O
integration O
in O
upstream O
product B-FEAT
design E-FEAT
steps O
like O
TO O
and O
generative B-ENAT
design E-ENAT
. O


Integrating O
processing O
and O
manufacturing S-MANP
with O
design S-FEAT
in O
AM S-MANP
is O
feasible O
since O
the O
full O
digital B-ENAT
chain E-ENAT
is O
there O
. O


In O
current O
AM S-MANP
practice O
only O
a O
few O
zones O
in O
a O
product O
are O
defined O
where O
process B-PARA
settings E-PARA
can O
be S-MATE
defined O
, O
for O
example O
for O
the O
top O
and O
bottom O
facing S-MANP
surfaces O
and O
the O
bulk O
of O
the O
product O
in O
L-PBF S-MANP
. O


If O
machine S-MACEQ
learning O
and O
closed B-CONPRI
loop I-CONPRI
control E-CONPRI
can O
be S-MATE
used O
to O
define O
the O
optimal O
settings O
for O
each O
deposition S-CONPRI
area S-PARA
, O
the O
need O
for O
support B-FEAT
structures E-FEAT
as S-MATE
well O
as S-MATE
the O
effects O
of O
thermal B-PRO
stresses E-PRO
and O
deformations S-CONPRI
are O
expected O
to O
reduce/diminish O
. O


This O
will O
have O
large O
impact S-CONPRI
on O
the O
AM S-MANP
products O
and O
the O
way O
they O
are O
designed S-FEAT
. O


A O
combination O
of O
computational O
and O
knowledge-based O
methods O
would O
be S-MATE
an O
optimal O
solution S-CONPRI
for O
DfAM O
in O
the O
future O
to O
define O
qualified O
AM S-MANP
design O
solutions O
. O


Data S-CONPRI
analytic O
methods O
could O
be S-MATE
used O
to O
explore O
and O
discover O
knowledge O
from O
existing O
validated O
designs S-FEAT
or O
dig O
out O
implicit O
knowledge O
from O
large O
industrial S-APPL
practice O
and O
experimental B-CONPRI
data E-CONPRI
sets O
. O


A O
collaborative O
cloud-based O
DfAM O
platform S-MACEQ
would O
be S-MATE
more O
sustainable S-CONPRI
for O
the O
world O
if O
people O
could O
share O
their O
designs S-FEAT
and O
design S-FEAT
knowledge O
as S-MATE
well O
as S-MATE
other O
AM S-MANP
processing O
related O
data S-CONPRI
sets O
. O


This O
would O
enable O
and O
advance O
wide O
KBE O
in O
DfAM O
, O
save O
a O
lot O
of O
cost O
and O
time O
and O
improve O
quality S-CONPRI
over O
the O
trial B-CONPRI
and I-CONPRI
error E-CONPRI
practice O
in O
current O
DfAM O
. O


This O
will O
result O
in O
new O
application O
areas S-PARA
, O
new O
process S-CONPRI
constraints O
and O
new O
AM S-MANP
features O
The O
main O
goal O
of O
this O
review O
is O
to O
provide O
a O
detailed O
and O
comprehensive O
description O
of O
the O
published O
work O
from O
the O
past O
decade O
regarding O
AM S-MANP
of O
ceramic B-MATE
materials E-MATE
with O
possible O
applications O
in O
dentistry S-APPL
. O


The O
main O
printable O
materials S-CONPRI
and O
most O
common O
technologies S-CONPRI
are O
also O
addressed O
, O
underlining O
their O
advantages O
and O
main O
drawbacks O
. O


Methods O
Online O
databases S-ENAT
were O
consulted O
on O
this O
topic O
. O


Results O
Ceramic B-MATE
materials E-MATE
are O
broadly O
used O
in O
dentistry S-APPL
to O
restore/replace O
damaged O
or O
missing O
teeth O
, O
due O
to O
their O
biocompatibility S-PRO
, O
chemical B-PRO
stability E-PRO
and O
mechanical S-APPL
and O
aesthetic S-CONPRI
properties O
. O


Due O
to O
their O
brittleness B-PRO
nature E-PRO
, O
a O
very O
tight O
control O
of O
the O
manufacturing B-MANP
process E-MANP
is O
needed O
to O
obtain O
dental B-MACEQ
pieces E-MACEQ
with O
adequate O
mechanical B-CONPRI
properties E-CONPRI
. O


Additive B-MANP
manufacturing E-MANP
is O
an O
emerging O
technology S-CONPRI
that O
constitutes O
an O
interesting O
and O
viable O
manufacturing S-MANP
alternative O
to O
the O
conventional B-MANP
subtractive I-MANP
methods E-MANP
. O


AM S-MANP
enables O
the O
production S-MANP
of O
customized O
complex O
3D B-APPL
parts E-APPL
in O
a O
more O
sustainable S-CONPRI
and O
less O
expensive O
way O
. O


AM S-MANP
of O
ceramics S-MATE
can O
be S-MATE
achieved O
with O
an O
extensive O
variety O
of O
methods O
. O


Although O
very O
promising O
, O
AM S-MANP
of O
ceramic B-MATE
dental I-MATE
materials E-MATE
remains O
understudied O
and O
further O
work O
is O
required O
to O
make O
it O
a O
widespread O
technology S-CONPRI
in O
dentistry S-APPL
. O


In O
dentistry S-APPL
, O
as S-MATE
in O
many O
other O
fields O
, O
the O
production S-MANP
of O
dental B-MACEQ
pieces E-MACEQ
is O
increasingly O
becoming O
automated O
. O


Computer B-ENAT
aided I-ENAT
design E-ENAT
and/or O
computer B-ENAT
aided I-ENAT
manufacturing E-ENAT
have O
become O
progressively O
widespread O
within O
the O
medical S-APPL
and O
dental S-APPL
fields O
. O


These O
tools S-MACEQ
are O
generally O
used O
in O
the O
manufacture S-CONPRI
of O
dental B-MACEQ
pieces E-MACEQ
in O
machining B-MACEQ
centers E-MACEQ
, O
where O
extra O
material S-MATE
is O
removed O
from O
a O
block O
to O
obtain O
the O
piece O
with O
the O
desired O
shape O
. O


This O
technique O
is O
known O
as S-MATE
subtractive O
manufacturing S-MANP
. O


Nowadays O
, O
a O
new O
type O
of O
technology S-CONPRI
is O
emerging O
, O
additive B-MANP
manufacturing E-MANP
, O
also O
referred O
to O
as S-MATE
3D B-MANP
printing E-MANP
, O
that O
allow O
building O
up O
pieces O
by O
adding O
materials B-CONPRI
layer-by-layer E-CONPRI
, O
based O
on O
a O
computerized O
3D B-APPL
model E-APPL
. O


This O
type O
of O
technology S-CONPRI
has O
suffered O
great O
developments O
in O
a O
wide O
range S-PARA
of O
areas S-PARA
, O
allowing O
to O
produce O
pieces O
of O
all O
classes B-MATE
of I-MATE
materials E-MATE
, O
including O
materials S-CONPRI
of O
biological O
origin O
. O


AM S-MANP
focus O
has O
been O
moving O
from O
prototype S-CONPRI
fabrication S-MANP
to O
rapid B-MANP
manufacturing E-MANP
of O
small O
or O
medium O
quantities O
of O
end-use O
products O
. O


Among O
the O
main O
areas S-PARA
of O
AM S-MANP
application O
stands O
out O
: O
- O
Aerospace S-APPL
: O
AM B-MANP
technology E-MANP
is O
particularly O
suitable O
to O
obtain O
a O
limited O
number O
of O
pieces O
that O
are O
usually O
required O
for O
aerospace S-APPL
applications O
, O
with O
complex B-CONPRI
geometries E-CONPRI
and O
made O
of O
advanced O
materials S-CONPRI
which O
are O
difficult O
, O
costly O
and O
time-consuming O
to O
manufacture S-CONPRI
. O


- O
Automotive S-APPL
: O
AM B-MANP
technology E-MANP
is O
an O
important O
tool S-MACEQ
in O
this O
industry S-APPL
, O
since O
it O
can O
reduce O
the O
development O
cycle O
, O
manufacturing S-MANP
and O
product O
costs O
of O
automotive S-APPL
components O
. O


It O
allows O
producing O
small O
quantities O
of O
structural O
and O
functional O
parts O
and O
thus O
, O
is O
particularly O
interesting O
for O
racing O
vehicles O
, O
where O
light-weight S-PRO
alloys S-MATE
and O
composites S-MATE
are O
used O
to O
obtain O
highly O
complex B-CONPRI
structures E-CONPRI
. O


- O
Energy O
: O
AM B-MANP
technology E-MANP
allows O
the O
fast O
development O
and O
fabrication S-MANP
of O
prototypes S-CONPRI
to O
reduce O
the O
cost O
and O
lead-time S-PARA
of O
research S-CONPRI
and O
development O
of O
new O
solutions O
that O
reduce O
the O
fossil O
energy O
dependency O
. O


It O
increases O
the O
design S-FEAT
possibilities O
to O
improve O
energy O
efficiency O
and/or O
power S-PARA
density S-PRO
, O
in O
alternatives O
that O
use O
renewable O
and O
clean O
energies O
. O


- O
Biomedical S-APPL
: O
Recent O
developments O
in O
the O
biomaterials B-MATE
field E-MATE
, O
biologic O
sciences O
and O
biomedicine S-APPL
have O
potentiated O
the O
use O
of O
AM B-MANP
techniques E-MANP
. O


Customization O
is O
a O
critical B-PRO
factor E-PRO
in O
this O
area S-PARA
and O
AM S-MANP
allows O
the O
production S-MANP
of O
a O
wide O
range S-PARA
of O
products O
with O
specific B-PRO
properties E-PRO
and O
shapes O
that O
meet O
the O
patient O
needs O
. O


For O
example O
, O
it O
is O
possible O
to O
produce O
diagnostic O
platforms O
, O
orthopedic O
and O
dental S-APPL
implants O
, O
drug O
delivery O
systems O
, O
medical B-APPL
devices E-APPL
, O
tissue O
scaffolds S-FEAT
and O
artificial B-APPL
organs E-APPL
. O


Biofabrication S-MANP
through O
AM S-MANP
emerged O
in O
the O
recent O
years O
as S-MATE
a O
new O
alternative O
to O
fabricate S-MANP
tissues O
. O


Here O
, O
living O
cells S-APPL
are O
deposited O
layer-by-layer S-CONPRI
in O
combination O
with O
different O
biomaterials S-MATE
to O
obtain O
complex O
living O
structures O
. O


In O
the O
dentistry B-APPL
field E-APPL
, O
the O
use O
of O
AM S-MANP
to O
produce O
endurable O
dental B-MACEQ
structures E-MACEQ
is O
expected O
to O
bring O
advantages O
over O
conventional B-MANP
manufacturing E-MANP
methods O
, O
as S-MATE
reported O
on O
other O
fields O
. O


In O
particular O
, O
it O
shall O
: O
- O
Allow O
the O
production S-MANP
of O
customized O
near-net-shape S-MANP
dental B-MACEQ
pieces E-MACEQ
with O
intricate O
details O
. O


Product O
complexity S-CONPRI
shall O
not O
add O
cost O
to O
production S-MANP
beyond O
the O
design S-FEAT
stage O
, O
because O
once O
the O
design S-FEAT
is O
set S-APPL
, O
costs O
are O
independent O
of O
the O
shape O
. O


- O
Allow O
reduction S-CONPRI
of O
dental S-APPL
parts O
production S-MANP
time O
and O
consequently O
of O
time-to-market O
. O


Traditional O
subtractive S-MANP
technologies O
involve O
several O
time-consuming O
steps O
, O
while O
AM S-MANP
allows O
a O
faster O
direct O
production S-MANP
starting O
simply O
from O
a O
3D S-CONPRI
scan O
of O
the O
oral O
cavity O
. O


- O
Limit S-CONPRI
human O
error S-CONPRI
relevance O
in O
the O
procedures O
. O


Minor O
human O
intervention O
is O
required O
in O
AM S-MANP
due O
to O
the O
lower O
number O
of O
manufacturing S-MANP
steps O
. O


- O
Decrease O
the O
environmental O
impact S-CONPRI
, O
ensuring O
a O
higher O
manufacturing B-CONPRI
sustainability E-CONPRI
. O


Being O
an O
additive S-MATE
technique O
, O
it O
reduces O
material S-MATE
waste O
and O
energy O
consumption O
and O
eliminates O
the O
use O
of O
conventional B-MANP
manufacturing E-MANP
tools O
. O


Globally O
, O
AM S-MANP
allows O
moving O
from O
mass B-CONPRI
production E-CONPRI
to O
mass O
customization O
, O
with O
significant O
efficiency O
increase O
and O
production B-CONPRI
costs E-CONPRI
decrease O
. O


The O
expected O
dissemination O
of O
this O
technology S-CONPRI
applied O
to O
dental B-MACEQ
prosthesis E-MACEQ
shall O
result O
in O
equipmentcost O
decrease O
. O


Thus O
, O
the O
reduction S-CONPRI
of O
final O
product O
price O
is O
predictable S-CONPRI
, O
increasing O
the O
accessibility O
of O
dental S-APPL
care O
to O
the O
poorest O
sectors O
of O
the O
population S-BIOP
. O


Due O
to O
the O
recent O
expiration O
of O
the O
main O
3D B-MANP
printing E-MANP
patents O
, O
the O
access O
to O
printers S-MACEQ
became O
easier O
and O
less O
expensive O
. O


Digital O
dentistry S-APPL
is O
reported O
to O
be S-MATE
one O
of O
the O
fastest O
growing O
sectors O
of O
the O
AM B-MANP
technologies E-MANP
. O


There O
are O
several O
possible O
applications O
of O
AM B-MANP
techniques E-MANP
in O
dentistry S-APPL
, O
e.g O
. O


crowns O
, O
bridges O
, O
dentures S-APPL
, O
models O
, O
surgical O
guides O
, O
implants S-APPL
and O
orthodontics O
materials S-CONPRI
. O


Several O
challenges O
emerge O
when O
this O
technique O
is O
considered O
to O
produce O
endurable O
dental B-APPL
devices E-APPL
. O


For O
example O
, O
the O
reliability S-CHAR
of O
the O
process S-CONPRI
, O
surface B-MANP
finishing E-MANP
of O
the O
samples S-CONPRI
and O
materials S-CONPRI
density S-PRO
are O
among O
the O
major O
concerns O
. O


Concerning O
dental S-APPL
materials O
that O
can O
be S-MATE
used O
in O
AM S-MANP
, O
polymers S-MATE
are O
the O
most O
studied O
and O
used O
ones O
, O
followed O
by O
metals S-MATE
. O


AM S-MANP
of O
ceramic B-MATE
dental I-MATE
materials E-MATE
is O
still O
underdeveloped O
, O
mainly O
due O
to O
the O
difficulties O
to O
produce O
pieces O
with O
suitable O
surface B-MANP
finishing E-MANP
, O
mechanical B-CONPRI
properties E-CONPRI
and O
dimensional B-CHAR
accuracy E-CHAR
. O


The O
available O
literature O
regarding O
AM S-MANP
of O
ceramic B-MATE
materials E-MATE
represents O
less O
than O
5 O
% O
of O
the O
total O
AM S-MANP
published O
related O
work O
. O


The O
studies O
are O
even O
fewer O
in O
what O
concerns O
ceramic B-MATE
materials E-MATE
for O
dental B-APPL
applications E-APPL
. O


This O
paper O
presents O
a O
recent O
overview O
of O
published O
work O
concerning O
AM S-MANP
of O
ceramic B-MATE
materials E-MATE
for O
dental B-APPL
applications E-APPL
. O


A O
summary O
of O
potentially O
printable O
dental S-APPL
biomaterials S-MATE
and O
brief O
descriptions O
of O
the O
most O
common O
digital B-MANP
manufacturing E-MANP
technologies O
are O
also O
provided O
, O
highlighting O
the O
main O
features O
, O
advantages O
and O
drawbacks O
, O
to O
better O
understand O
the O
potential O
and O
restrictions O
of O
each O
technology S-CONPRI
. O


The O
used O
keywords O
strings O
were O
: O
3D B-MANP
printing E-MANP
AND O
Dental S-APPL
; O
Additive B-MANP
manufacturing E-MANP
AND O
Dental S-APPL
. O


Additive B-MANP
manufacturing E-MANP
of O
bioceramics S-MATE
for O
dental S-APPL
applicationsbut O
included O
in O
the O
remaining O
sections O
to O
complement O
this O
review O
and O
provide O
any O
additional O
remarks O
. O


3 O
Ceramic B-MATE
dental I-MATE
materials E-MATE
Bioceramics S-MATE
are O
broadly O
used O
in O
the O
dental S-APPL
field O
. O


These O
materials S-CONPRI
have O
some O
attractive O
features/attributes O
which O
are O
similar O
to O
natural O
dentition O
properties S-CONPRI
, O
e.g O
. O


compressive B-PRO
strength E-PRO
, O
thermal B-PRO
conductivity E-PRO
, O
radiopacity O
, O
colour O
stability S-PRO
, O
aesthetics O
. O


However O
, O
these O
materials S-CONPRI
are O
brittle S-PRO
, O
hard O
and O
sometimes O
difficult O
to O
process S-CONPRI
. O


Bioceramics S-MATE
can O
be S-MATE
divided O
in O
4 O
categories O
, O
depending O
on O
their O
main O
system O
composition S-CONPRI
: O
glass-based O
systems O
; O
glass-based O
systems O
with O
fillers O
, O
usually O
crystalline O
; O
crystalline-based O
systems O
with O
glass B-MATE
fillers E-MATE
; O
polycrystalline O
solids O
. O


Glass-based O
systems O
consist O
of O
materials S-CONPRI
that O
are O
made O
mostly O
of O
silicon B-MATE
dioxide E-MATE
and O
can O
comprise O
different O
amounts O
of O
alumina S-MATE
. O


Feldspars O
are O
composed O
of O
aluminosilicates S-MATE
, O
found O
in O
nature O
, O
containing O
different O
quantities O
of O
potassium O
and O
sodium S-MATE
. O


Feldspars O
can O
be S-MATE
modified O
in O
several O
ways O
in O
order O
to O
produce O
the O
glass S-MATE
used O
in O
the O
dental S-APPL
area S-PARA
. O


Additionally O
, O
synthetic O
forms O
of O
alumina S-MATE
silicate O
glasses S-MATE
may O
as S-MATE
well O
be S-MATE
manufactured O
for O
dental S-APPL
ceramics S-MATE
. O


The O
glass S-MATE
composition S-CONPRI
is O
almost O
the O
same O
as S-MATE
the O
pure O
glass S-MATE
category O
, O
being O
that O
the O
difference O
resides O
in O
the O
amount O
of O
different O
types O
of O
crystals O
that O
can O
either O
be S-MATE
added O
or O
grown O
in O
the O
glassy O
matrix O
. O


Nowadays O
, O
the O
primary O
crystal O
types O
are O
leucite S-MATE
, O
lithium S-MATE
dissilicate O
or O
fluoroapatite O
. O


Intended O
as S-MATE
an O
alternative O
to O
traditional O
metal B-MATE
ceramics E-MATE
, O
crystalline-based O
systems O
with O
glass B-MATE
fillers E-MATE
were O
developed O
. O


They O
are O
composed O
of O
glass-infiltrated O
, O
partially O
sintered S-MANP
alumina S-MATE
. O


Polycrystaline B-MATE
solids E-MATE
are O
made O
by O
directly B-MATE
sintering I-MATE
crystals E-MATE
together O
, O
forming S-MANP
a O
dense O
, O
air-free O
, O
glass-free O
, O
polycrystalline B-PRO
structure E-PRO
. O


In O
the O
next O
sections O
, O
a O
brief O
description O
is O
presented O
regarding O
the O
main O
properties S-CONPRI
of O
the O
most O
frequently O
used O
bioceramics S-MATE
in O
dental B-APPL
applications E-APPL
. O


3.1 O
Zirconia S-MATE
Zirconia O
ceramics S-MATE
were O
introduced O
in O
dentistry S-APPL
in O
the O
early O
nineties O
, O
as S-MATE
endosseous O
implants S-APPL
in O
dental S-APPL
prosthetic O
surgery S-APPL
. O


This O
material S-MATE
is O
known O
to O
have O
exceptional O
mechanical B-CONPRI
properties E-CONPRI
and O
ease O
of O
machining S-MANP
in O
the O
pre-sintering S-MANP
stage O
through O
CAD/CAM S-ENAT
. O


Zirconia S-MATE
is O
biocompatible S-PRO
with O
the O
tissues O
in O
the O
oral O
cavity O
and O
has O
been O
reported O
to O
be S-MATE
osteoconductive O
, O
which O
means O
that O
this O
ceramic S-MATE
facilitates O
bone S-BIOP
formation O
when O
in O
contact S-APPL
with O
it O
. O


Regarding O
mechanical B-CONPRI
properties E-CONPRI
, O
zirconia B-MATE
ceramics E-MATE
are O
considered O
to O
have O
high O
strength S-PRO
, O
hardness S-PRO
, O
wear B-PRO
resistance E-PRO
, O
resistance S-PRO
to O
corrosion S-CONPRI
, O
modulus B-PRO
of I-PRO
elasticity E-PRO
similar O
to O
steel S-MATE
, O
coefficient B-PRO
of I-PRO
thermal I-PRO
expansion E-PRO
similar O
to O
iron S-MATE
, O
and O
the O
highest O
fracture S-CONPRI
toughness O
among O
the O
most O
used O
ceramics S-MATE
. O


Zirconia-based O
ceramics S-MATE
can O
be S-MATE
stabilized O
in O
tetragonal S-FEAT
or O
cubic O
phases O
depending O
on O
the O
used O
dopant O
, O
its O
concentration O
and O
temperature S-PARA
during O
the O
thermal B-MANP
treatments E-MANP
. O


For O
dental B-APPL
applications E-APPL
, O
zirconia S-MATE
is O
commonly O
stabilized O
with O
3 O
mol O
% O
yttria S-MATE
. O


The O
excellent O
mechanical B-CONPRI
properties E-CONPRI
of O
stabilized B-MATE
tetragonal I-MATE
zirconia E-MATE
are O
related O
with O
the O
stress-induced O
from O
tetragonal S-FEAT
to O
monoclinical O
transformation O
, O
which O
is O
accompanied O
by O
a O
4.5 O
% O
volume S-CONPRI
increase O
. O


This O
behaviour O
leads O
to O
the O
development O
of O
compression B-CHAR
zone E-CHAR
, O
shielding O
the O
propagating O
crack O
tip O
which O
inhibits O
further O
crack B-CONPRI
propagation E-CONPRI
, O
successfully O
enhancing O
toughness S-PRO
. O


Nevertheless O
, O
there O
are O
some O
disadvantageous O
aspects O
of O
zirconia B-MATE
ceramics E-MATE
. O


3.2 O
Alumina S-MATE
Alumina O
, O
also O
called O
aluminum B-MATE
oxide E-MATE
, O
was O
first O
introduced O
in O
the O
1970s O
. O


However O
, O
the O
initial O
applications O
presented O
a O
fracture B-CHAR
rate E-CHAR
of O
the O
order O
of O
13 O
% O
. O


This O
observed O
failure S-CONPRI
was O
related O
to O
a O
higher O
porosity S-PRO
. O


With O
further O
developments O
, O
a O
decade O
later O
, O
a O
second O
improved O
generation O
of O
alumina S-MATE
ceramics O
was O
presented O
, O
characterized O
by O
higher O
density S-PRO
and O
smaller O
grains S-CONPRI
. O


This O
led S-APPL
to O
a O
decrease O
of O
the O
fracture B-CHAR
rate E-CHAR
to O
less O
than O
5 O
% O
. O


Nowadays O
, O
there O
is O
available O
a O
third O
generation O
of O
alumina B-MATE
ceramic I-MATE
components E-MATE
, O
with O
properties S-CONPRI
such O
as S-MATE
high O
purity O
, O
high O
density S-PRO
and O
finer B-FEAT
microstructure E-FEAT
. O


Alumina S-MATE
is O
used O
in O
dental B-APPL
applications E-APPL
for O
fabrication S-MANP
of O
endodontic B-MACEQ
posts E-MACEQ
, O
orthodontic S-APPL
brackets O
, O
dental S-APPL
implants O
, O
crowns O
and O
bridges O
and O
in O
ceramic B-FEAT
abutments E-FEAT
. O


High O
purity O
alumina S-MATE
has O
usually O
a O
purity O
of O
99.99 O
% O
and O
has O
been O
developed O
as S-MATE
an O
alternative O
to O
surgical O
metal B-MATE
alloys E-MATE
for O
dental B-APPL
applications E-APPL
. O


According O
to O
US O
Food O
and O
Drug O
Administration O
, O
only O
the O
high-purity O
Al2O3 S-MATE
can O
be S-MATE
used O
for O
medical S-APPL
grade O
ceramics S-MATE
. O


Impurities S-PRO
such O
as S-MATE
SiO2 O
, O
metal B-MATE
silicates E-MATE
and O
alkali B-MATE
metal I-MATE
oxides E-MATE
that O
form O
glassy O
grain B-CONPRI
boundary E-CONPRI
phases O
must O
be S-MATE
minimized O
to O
less O
than O
0.1 O
wt O
% O
, O
since O
the O
in O
vivo O
degradation S-CONPRI
of O
such O
glassy O
phases O
leads O
to O
the O
appearance O
of O
stress B-CHAR
concentration E-CHAR
sites O
where O
cracks O
can O
be S-MATE
initiated O
, O
leading O
to O
the O
catastrophic O
failure S-CONPRI
of O
the O
component S-MACEQ
. O


It O
is O
possible O
to O
enhance O
alumina S-MATE
toughness O
and O
fracture S-CONPRI
strength O
by O
controlling O
the O
grain B-PRO
size E-PRO
and O
the O
porosity S-PRO
. O


This O
can O
be S-MATE
achieved O
using O
adequate O
sintering B-MANP
cycles E-MANP
, O
and O
adding O
some O
additives S-MATE
, O
zirconium B-MATE
oxide E-MATE
and O
chromium B-MATE
oxide E-MATE
) O
. O


3.3 O
Leucite S-MATE
Leucite O
is O
a O
potassium B-MATE
alumina-silicate E-MATE
. O


This O
material S-MATE
displays O
a O
tetragonal S-FEAT
structure S-CONPRI
at O
room O
temperature S-PARA
. O


At O
625 O
it O
suffers O
a O
displacive O
phase S-CONPRI
transformation O
from O
tetragonal S-FEAT
to O
cubic O
, O
together O
with O
a O
volume S-CONPRI
expansion O
of O
1.2 O
% O
. O


Leucite S-MATE
has O
been O
widely O
used O
as S-MATE
a O
constituent O
of O
dental S-APPL
ceramics S-MATE
to O
modify O
the O
coefficient B-PRO
of I-PRO
thermal I-PRO
expansion E-PRO
, O
which O
is O
very O
important O
when O
the O
ceramic S-MATE
is O
to O
be S-MATE
fused O
or O
baked O
onto O
metal S-MATE
. O


Usually O
leucite-based O
materials S-CONPRI
are O
used O
for O
veneering O
ceramics S-MATE
in O
metal-ceramic O
restorations O
, O
also O
referred O
to O
as S-MATE
feldspathic O
porcelains O
. O


Leucite S-MATE
is O
attained O
by O
incongruent O
melting S-MANP
of O
feldspar S-MATE
at O
temperatures S-PARA
between O
1150 O
and O
1530 O
. O


Despite O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
feldspathic O
porcelains O
being O
the O
weakest O
within O
ceramic B-MATE
dental I-MATE
materials E-MATE
, O
their O
global O
performance S-CONPRI
is O
perceived O
as S-MATE
quite O
successful O
. O


3.4 O
Lithium S-MATE
dissilicate O
Lithium S-MATE
silicate-based O
glass-ceramics O
were O
recently O
introduced O
as S-MATE
machinable O
materials S-CONPRI
to O
respond O
to O
the O
demanded B-PRO
increased I-PRO
strength E-PRO
, O
toughness S-PRO
and O
wear B-PRO
resistance E-PRO
, O
required O
for O
the O
fabrication S-MANP
of O
dental B-MACEQ
pieces E-MACEQ
. O


This O
ceramic B-MATE
material E-MATE
is O
used O
in O
the O
fabrication S-MANP
of O
single O
and O
multiunit O
dental S-APPL
restorations O
, O
mainly O
dental B-CHAR
crowns E-CHAR
, O
bridges O
, O
and O
veneers S-MACEQ
, O
due O
to O
its O
excellent O
mechanical S-APPL
and O
optical B-PRO
properties E-PRO
. O


In O
general O
, O
lithium S-MATE
disilicate O
presents O
a O
microstructure S-CONPRI
constituted O
by O
interlocking O
needle-like O
crystals O
embedded O
in O
a O
glass B-MATE
matrix E-MATE
. O


As S-MATE
a O
result O
of O
this O
morphology S-CONPRI
, O
cracks O
are O
forced O
to O
propagate O
around O
each O
individual O
lithium S-MATE
disilicate O
crystal O
. O


This O
type O
of O
microstructure S-CONPRI
increases O
both O
strength S-PRO
and O
toughness S-PRO
relatively O
to O
other O
commonly O
used O
glass-ceramics O
: O
they O
have O
a O
strength S-PRO
twice O
as S-MATE
higher O
as S-MATE
that O
of O
the O
first O
generation O
of O
leucite-reinforced O
ceramics S-MATE
. O


3.5 O
Mica S-MATE
Mica O
minerals O
are O
a O
group O
of O
sheet S-MATE
silicate O
minerals O
, O
or O
layer S-PARA
type O
silicates S-MATE
, O
that O
consist O
of O
varying O
complex O
formulae O
of O
Si S-MATE
, O
K S-MATE
, O
Na S-MATE
, O
Ca S-MATE
, O
F S-MANP
, O
O S-MATE
, O
Fe S-MATE
and O
Al S-MATE
. O


The O
mechanical B-CONPRI
properties E-CONPRI
are O
dictated O
by O
the O
specific O
crystal B-PRO
structure E-PRO
formed O
by O
the O
cleavage B-CONPRI
planes E-CONPRI
situated O
along O
the O
layers O
. O


Crack B-CONPRI
propagation E-CONPRI
is O
more O
likely O
to O
occur O
along O
the O
cleavage B-CONPRI
planes E-CONPRI
. O


Mica-based O
glass-ceramics O
are O
relevant O
for O
dental S-APPL
materials O
due O
to O
their O
good O
machinability S-PRO
, O
high O
strength S-PRO
and O
resistivity S-PRO
to O
thermal B-CONPRI
expansion E-CONPRI
as S-MATE
well O
as S-MATE
biocompatibility O
. O


3.6 O
Others O
ceramic B-MATE
dental I-MATE
materials E-MATE
Besides O
the O
examples O
mentioned O
above O
, O
there O
are O
other O
ceramic B-MATE
materials E-MATE
used O
in O
the O
dentistry B-APPL
field E-APPL
. O


For O
over O
30 O
years O
, O
calcium B-MATE
phosphate-based I-MATE
formulations E-MATE
are O
recognized O
by O
their O
good O
osteoconductivity S-PRO
and O
biocompatibility S-PRO
in O
reconstructive O
surgeries O
. O


Tricalcium O
phosphate S-MATE
presents O
three O
polymorphs O
. O


These O
include O
: O
monoclinic S-FEAT
, O
and O
hexagonal S-FEAT
and O
rhombohedral B-FEAT
form E-FEAT
. O


Hydroxyapatite B-MATE
62 E-MATE
) O
is O
the O
main O
component S-MACEQ
of O
enamel S-MATE
, O
and O
is O
responsible O
for O
the O
bright O
white O
appearance O
and O
elimination O
of O
the O
diffuse O
reflectivity O
of O
light O
by O
closing O
the O
small O
pores S-PRO
of O
the O
enamel S-MATE
surface O
. O


HA O
can O
be S-MATE
used O
as S-MATE
filler O
in O
the O
repair O
of O
craniofacial B-BIOP
defects E-BIOP
or O
small O
holes O
and O
depressions O
on O
enamel S-MATE
surface O
, O
as S-MATE
grafting O
material S-MATE
and O
as S-MATE
coating O
in O
implant S-APPL
dentistry S-APPL
. O


Bioactive B-MATE
glasses E-MATE
are O
silicate-based O
materials S-CONPRI
that O
can O
form O
a O
strong O
chemical O
bond O
with O
the O
tissues O
. O


They O
present O
great O
interest O
in O
regeneration S-CONPRI
and O
healing O
of O
bone S-BIOP
tissue O
. O


Their O
ability O
to O
support S-APPL
osteoblast B-BIOP
cells E-BIOP
, O
to O
bond O
to O
both O
soft B-CONPRI
and I-CONPRI
hard I-CONPRI
tissue E-CONPRI
and O
their O
capability O
of O
stimulating O
angiogenesis S-CONPRI
in O
the O
presence O
of O
vascular B-BIOP
endothelial I-BIOP
growth I-BIOP
factor E-BIOP
make O
them O
an O
attractive O
alternative O
relatively O
to O
other O
scaffold S-FEAT
materials S-CONPRI
. O


Finally O
, O
dental S-APPL
impression O
materials S-CONPRI
still O
play O
a O
significant O
role O
in O
dentistry S-APPL
. O


Gypsum S-MATE
products O
are O
among O
the O
most O
frequently O
used O
materials S-CONPRI
by O
dental S-APPL
professionals O
, O
since O
its O
properties S-CONPRI
are O
easily O
modified O
by O
physical O
and O
chemical O
means O
. O


Gypsum S-MATE
may O
be S-MATE
used O
as S-MATE
impression O
material S-MATE
, O
mold S-MACEQ
material S-MATE
for O
processing O
complete O
dentures S-APPL
, O
as S-MATE
binders O
for O
silica S-MATE
in O
gold B-MATE
alloy E-MATE
casting S-MANP
investment O
, O
soldering S-MANP
investment O
, O
and O
investment O
for O
low-melting-point S-CONPRI
nickel-chromium O
alloys S-MATE
. O


3.7 O
Composites S-MATE
A O
composite B-MATE
material E-MATE
is O
defined O
as S-MATE
a O
combination O
of O
two O
or O
more O
materials S-CONPRI
. O


The O
resulting O
combination O
renders O
unique O
properties S-CONPRI
with O
characteristics O
different O
from O
the O
individual O
components S-MACEQ
. O


In O
dentistry S-APPL
, O
ceramic B-FEAT
composites E-FEAT
may O
comprise O
combinations O
such O
as S-MATE
ceramic-metal O
, O
ceramic-polymer S-MATE
, O
or O
ceramic-ceramic O
Examples O
of O
current O
dental B-MATE
ceramic-ceramic I-MATE
composites E-MATE
include O
aluminacomposites O
, O
commercially O
available O
as S-MATE
structural O
ceramics S-MATE
for O
dental B-APPL
devices E-APPL
. O


These O
materials S-CONPRI
, O
contain O
either O
alumina-toughened B-MATE
zirconia E-MATE
or O
zirconia-toughened O
alumina S-MATE
, O
depending O
on O
the O
percentage O
of O
the O
main O
component S-MACEQ
. O


These O
composites S-MATE
combine O
the O
transformation O
toughening S-MANP
capabilities O
of O
zirconia S-MATE
along O
with O
the O
lower O
susceptibility S-PRO
to O
low O
temperature S-PARA
degradation S-CONPRI
in O
biological B-MATE
fluids E-MATE
. O


More O
recently O
, O
with O
the O
development O
of O
nanotechnology S-CONPRI
, O
the O
bionanocomposites S-MATE
have O
emerged O
. O


These O
materials S-CONPRI
are O
expected O
to O
mimic S-MACEQ
native O
tissue B-CONPRI
structure E-CONPRI
, O
withstand O
high O
biting O
force S-CONPRI
and O
harsh O
oral O
cavity O
environment O
, O
e.g O
. O


sudden O
change O
of O
temperature S-PARA
or O
osmotic B-PRO
pressure E-PRO
and O
invasion O
of O
various O
pathogens O
. O


Possible O
applications O
for O
bionanocomposites S-MATE
in O
the O
dental S-APPL
field O
include O
dental B-CONPRI
tissue I-CONPRI
regeneration E-CONPRI
or O
its O
substitution O
. O


4 O
Digital B-MANP
manufacturing E-MANP
CAD/CAM S-ENAT
production O
of O
fixed O
prosthetic S-APPL
restorations O
such O
as S-MATE
inlays O
, O
onlays S-APPL
, O
veneers S-MACEQ
, O
crowns O
, O
and O
fixed B-APPL
partial I-APPL
dentures E-APPL
is O
a O
relatively O
well O
established O
technology S-CONPRI
used O
by O
dental S-APPL
health O
professionals O
for O
over O
20 O
years O
. O


All O
CAD/CAM S-ENAT
systems O
involve O
three O
steps O
. O


The O
first O
one O
corresponds O
to O
the O
data B-CHAR
acquisition E-CHAR
, O
through O
various O
scanning S-CONPRI
technologies O
that O
allow O
to O
transform O
the O
site/product O
geometry S-CONPRI
into O
digital O
data S-CONPRI
to O
be S-MATE
processed O
by O
the O
computer S-ENAT
. O


This O
is O
followed O
by O
manipulation O
and O
processing O
of O
the O
data S-CONPRI
set O
using O
a O
CAD S-ENAT
software O
. O


Finally O
, O
the O
processed B-CONPRI
data E-CONPRI
are O
used O
for O
manufacturing S-MANP
of O
structures O
in O
the O
desired O
material S-MATE
through O
CAM S-ENAT
. O


In O
dentistry S-APPL
, O
there O
are O
three O
different O
production S-MANP
concepts O
available O
, O
depending O
on O
the O
location O
of O
the O
steps O
of O
the O
CAD/CAM S-ENAT
processes O
: O
- O
chairside O
production S-MANP
, O
- O
laboratory S-CONPRI
production O
, O
- O
centralized O
fabrication S-MANP
in O
a O
production B-APPL
center E-APPL
. O


The O
production S-MANP
can O
be S-MATE
carried O
out O
through O
several O
different O
technologies S-CONPRI
, O
that O
can O
be S-MATE
divided O
into O
two O
manufacturing B-MANP
processes E-MANP
: O
subtractive S-MANP
and O
additive B-MANP
manufacturing E-MANP
. O


4.1 O
Subtractive B-MANP
manufacturing E-MANP
There O
are O
several O
subtractive B-MANP
manufacturing E-MANP
techniques O
. O


The O
most O
used O
in O
dental B-MANP
ceramics I-MANP
processing E-MANP
is O
based O
on O
milling S-MANP
of O
pre-sintered S-PRO
or O
fully B-CHAR
sintered I-CHAR
blocks E-CHAR
, O
through O
a O
computer S-ENAT
numeric O
controlled O
machine S-MACEQ
. O


The O
CAM S-ENAT
software O
automatically O
translates O
the O
CAD B-ENAT
model E-ENAT
into O
tool B-CONPRI
path E-CONPRI
for O
the O
CNC B-MACEQ
machine E-MACEQ
. O


This O
involves O
computation S-CONPRI
of O
the O
commands O
series O
that O
dictate O
the O
CNC B-MANP
milling E-MANP
, O
including O
sequencing O
, O
milling S-MANP
tools O
, O
and O
tool B-PARA
motion I-PARA
direction E-PARA
and O
magnitude S-PARA
. O


The O
accuracy S-CHAR
of O
tool S-MACEQ
positioning O
has O
been O
reported O
to O
be S-MATE
within O
10 O
. O


The O
3-axis O
milling S-MANP
systems O
are O
the O
most O
commonly O
used O
in O
dental S-APPL
milling O
systems O
. O


In O
such O
systems O
, O
the O
milling B-CONPRI
burs E-CONPRI
move O
in O
three O
axes O
according O
to O
a O
defined O
path O
. O


SM S-MATE
is O
extensively O
used O
in O
dentistry S-APPL
for O
the O
production S-MANP
of O
dental B-MACEQ
pieces E-MACEQ
such O
as S-MATE
crowns O
and O
bridges O
. O


SM S-MATE
technology O
allows O
the O
processing O
of O
materials S-CONPRI
, O
which O
would O
otherwise O
be S-MATE
difficult O
to O
manipulate O
. O


This O
way O
, O
the O
exhaustive O
artisanal O
production S-MANP
techniques O
are O
decreased O
or O
even O
eliminated O
, O
thus O
allowing O
the O
dental S-APPL
technician O
to O
enhance O
the O
creative O
component S-MACEQ
of O
his O
manual O
manufacturing B-MANP
process E-MANP
. O


SM S-MATE
is O
a O
well-established O
technology S-CONPRI
which O
has O
the O
advantage O
of O
using O
intrinsically O
homogeneous B-MATE
materials E-MATE
which O
are O
unaffected O
by O
operating O
conditions O
. O


Furthermore O
, O
it O
requires O
low O
post-processing S-CONPRI
and O
the O
costs O
regarding O
the O
involved O
equipment S-MACEQ
are O
relatively O
reduced O
. O


However O
, O
this O
is O
a O
wasteful O
process S-CONPRI
because O
the O
piece O
is O
milled S-MANP
from O
an O
intact O
block O
with O
a O
significant O
loss O
of O
material S-MATE
amount O
. O


Factors O
such O
as S-MATE
the O
objectscomplexity O
, O
the O
dimension S-FEAT
of O
the O
tooling S-CONPRI
equipment S-MACEQ
and O
the O
properties S-CONPRI
of O
the O
material S-MATE
can O
affect O
and O
limit S-CONPRI
the O
accuracy S-CHAR
of O
this O
process S-CONPRI
. O


4.2 O
Additive B-MANP
manufacturing E-MANP
Additive S-MATE
manufacturing O
, O
also O
referred O
to O
as S-MATE
solid O
freeform B-MANP
fabrication E-MANP
, O
rapid B-ENAT
prototyping E-ENAT
or O
3D B-MANP
printing E-MANP
, O
involves O
processing O
methodologies O
that O
are O
capable O
of O
producing O
structures O
by O
depositing O
materials B-CONPRI
layer-by-layer E-CONPRI
resorting O
to O
a O
computer S-ENAT
generated O
design S-FEAT
file O
. O


The O
workpiece S-CONPRI
is O
virtually O
sliced O
into O
several O
two-dimensional S-CONPRI
layers O
. O


Then O
, O
an O
AM B-MACEQ
machine E-MACEQ
generates O
the O
tool-path S-PARA
along O
the O
x O
and O
y S-MATE
directions O
. O


Each O
material S-MATE
layer S-PARA
is O
deposited O
one O
on O
top O
of O
the O
other O
, O
consecutively O
, O
forming S-MANP
a O
three-dimensional S-CONPRI
part O
. O


Ceramics S-MATE
present O
a O
higher O
melting B-PRO
point E-PRO
, O
higher O
susceptibility S-PRO
to O
thermal O
shook O
and O
lower O
sintrerability S-PRO
than O
the O
other O
group O
of O
materials S-CONPRI
. O


Thus O
, O
it O
is O
quite O
difficult O
to O
obtained O
fully O
consolidated O
parts O
, O
without O
defects S-CONPRI
, O
using O
AM S-MANP
methods O
that O
produced O
directly B-MANP
sintered I-MANP
bodies E-MANP
. O


In O
most O
of O
the O
cases O
, O
AM S-MANP
is O
used O
to O
obtain O
preliminary O
3D B-CONPRI
structures E-CONPRI
in O
green O
that O
are O
built O
from O
mixture O
powders S-MATE
with O
organic O
or O
inorganic O
binder S-MATE
materials O
and O
need O
to O
be S-MATE
submitted O
to O
further O
steps O
of O
debinding S-CONPRI
and O
sintering S-MANP
. O


Some O
authors O
refer O
to O
these O
methods O
as S-MATE
indirect O
, O
contrarily O
to O
direct O
methods O
where O
the O
ceramic B-MATE
powder E-MATE
is O
sintered S-MANP
during O
the O
manufacturing S-MANP
procedure O
. O


The O
debinding S-CONPRI
step O
depends O
on O
the O
organic B-MACEQ
components E-MACEQ
and O
became O
diffusion S-CONPRI
limited O
by O
increasing O
the O
thickness O
of O
the O
part O
, O
leading O
to O
higher O
debinding S-CONPRI
times O
The O
debinding B-CHAR
temperature E-CHAR
need O
to O
be S-MATE
carefully O
chosen O
. O


If O
is O
to O
high O
the O
removal O
rate O
of O
the O
volatile O
products O
resulting O
from O
the O
binder B-CHAR
decomposition E-CHAR
is O
too O
high O
, O
the O
pressure S-CONPRI
may O
increase O
and O
lead S-MATE
to O
crack O
formation O
and/or O
delamination S-CONPRI
. O


A O
possible O
solution S-CONPRI
to O
overcome O
the O
internal B-PRO
stresses E-PRO
generated O
during O
debinding S-CONPRI
may O
be S-MATE
the O
use O
of O
plasticizing O
agents O
. O


The O
defects S-CONPRI
can O
also O
be S-MATE
avoided O
through O
the O
introduction O
of O
open O
spaces O
in O
the O
structure S-CONPRI
. O


That O
can O
be S-MATE
achieved O
by O
adding O
compounds O
able O
to O
evaporate/decompose O
at O
lower O
temperature S-PARA
than O
the O
debinding S-CONPRI
process O
temperature S-PARA
2 O
. O


There O
are O
several O
applications O
for O
AM S-MANP
in O
dentistry S-APPL
. O


One O
of O
the O
earliest O
is O
medical S-APPL
modelling O
, O
for O
surgery S-APPL
guide O
, O
where O
anatomical O
modelsare O
created O
. O


The O
models O
can O
also O
be S-MATE
used O
as S-MATE
supports O
for O
the O
fabrication S-MANP
of O
restorations O
, O
for O
example O
, O
to O
help O
in O
the O
addition O
of O
veneered O
materials S-CONPRI
. O


In O
digital B-APPL
orthodontics E-APPL
, O
there O
are O
systems O
available O
that O
digitally O
realign O
the O
patientteeth O
to O
make O
a O
series O
of O
3D B-MANP
printed E-MANP
models O
for O
the O
manufacture S-CONPRI
of O
aligners O
. O


3D B-ENAT
printing I-ENAT
technology E-ENAT
can O
also O
be S-MATE
used O
to O
produce O
novel O
titanium S-MATE
dental S-APPL
implants O
with O
a O
porous S-PRO
or O
rough O
surface S-CONPRI
and O
different O
types O
of O
restorations/components O
in O
metallic S-MATE
or O
polymeric B-MATE
materials E-MATE
. O


In O
the O
next O
sections O
, O
the O
main O
AM B-MANP
technologies E-MANP
available O
, O
referred O
in O
1 O
, O
are O
described O
, O
namely O
indirect O
methods O
, O
such O
as S-MATE
binder O
jetting S-MANP
, O
material B-MANP
extrusion E-MANP
and O
jetting S-MANP
and O
Vat B-MANP
polymerization/stereolitography E-MANP
, O
and O
direct O
methods O
, O
which O
include O
direct B-MANP
energy I-MANP
deposition E-MANP
and O
powder B-MANP
bed I-MANP
fusion E-MANP
. O


4.2.1 O
Binder B-MANP
jetting E-MANP
Binder S-MATE
jetting O
uses O
two O
materials S-CONPRI
, O
a O
powder-based B-MATE
material E-MATE
and O
a O
binder S-MATE
. O


Usually O
in O
the O
liquid O
form O
, O
the O
organic O
binder S-MATE
acts O
as S-MATE
an O
adhesive S-MATE
between O
ceramic B-MATE
powder I-MATE
particles E-MATE
. O


A O
print B-MACEQ
head E-MACEQ
deposits O
alternated O
layers O
of O
the O
building O
material S-MATE
and O
the O
binding B-MATE
material E-MATE
, O
moving O
horizontally O
along O
the O
x O
and O
y S-MATE
axes O
of O
the O
machine S-MACEQ
. O


After O
each O
layer S-PARA
, O
the O
build B-MACEQ
platform E-MACEQ
is O
lowered O
and O
the O
process S-CONPRI
repeated O
over O
the O
previous O
layer S-PARA
. O


BJ S-MANP
presents O
several O
advantages O
, O
such O
as S-MATE
the O
ability O
to O
use O
a O
range S-PARA
of O
materials S-CONPRI
as S-MATE
well O
as S-MATE
a O
large O
number O
of O
combinations O
powder/binder O
, O
being O
generally O
a O
fast O
printing B-MANP
process E-MANP
. O


The O
drawbacks O
of O
this O
technology S-CONPRI
are O
mainly O
related O
to O
the O
high O
porosity S-PRO
and O
consequent O
low O
mechanical B-CONPRI
properties E-CONPRI
of O
the O
printed O
pieces O
. O


This O
is O
due O
to O
factors O
such O
as S-MATE
the O
high O
friction S-CONPRI
within O
the O
powder B-MATE
particles E-MATE
, O
their O
random O
agglomeration O
and O
the O
absence O
of O
an O
external O
force S-CONPRI
to O
compress O
the O
powder S-MATE
and O
improve O
packaging O
. O


The O
flowability O
and O
spreadability O
of O
powders S-MATE
are O
especially O
important O
for O
BJ S-MANP
. O


Using O
powders S-MATE
with O
large O
particle S-CONPRI
sizes O
can O
enhance O
the O
flowability O
, O
however O
it O
may O
jeopardize O
the O
sinterability S-PRO
and O
densification B-PRO
behaviour E-PRO
after O
printing O
. O


Contrarily O
, O
very O
fine O
particle S-CONPRI
size O
may O
lead S-MATE
to O
considerable O
agglomeration O
and O
reduced O
flowability O
. O


Since O
after O
sintering S-MANP
, O
the O
pieces O
density S-PRO
rarely O
exceeds O
50 O
% O
of O
the O
theoretical S-CONPRI
value O
, O
this O
technology S-CONPRI
seems O
not O
to O
be S-MATE
suitable O
for O
structural O
parts O
. O


In O
order O
to O
overcome O
this O
issue O
, O
the O
printed O
piece O
may O
be S-MATE
infiltered O
under O
vacuum O
with O
a O
glass B-MATE
material E-MATE
that O
penetrates O
in O
the O
pores S-PRO
by O
capillary B-CONPRI
effect E-CONPRI
. O


4.2.2 O
Material B-MANP
extrusion E-MANP
and O
jetting S-MANP
Generally O
, O
in O
the O
material B-MANP
extrusion E-MANP
process O
the O
material S-MATE
is O
heated O
and O
extruded S-MANP
through O
a O
nozzle S-MACEQ
. O


The O
nozzle S-MACEQ
can O
move O
horizontally O
, O
and O
the O
build B-MACEQ
platform E-MACEQ
can O
move O
vertically O
to O
enable O
the O
addition O
of O
each O
subsequent O
layer S-PARA
. O


This O
process S-CONPRI
, O
also O
known O
as S-MATE
fused O
deposition B-CONPRI
modeling E-CONPRI
, O
is O
the O
most O
widespread O
and O
inexpensive O
process S-CONPRI
within O
3D B-ENAT
printing I-ENAT
technology E-ENAT
. O


However O
, O
there O
are O
some O
drawbacks O
such O
as S-MATE
not O
being O
as S-MATE
fast O
as S-MATE
other O
AM B-MANP
processes E-MANP
and O
presenting O
an O
accuracy S-CHAR
limited O
by O
the O
nozzle S-MACEQ
radius O
, O
which O
reduces O
the O
final O
product B-CONPRI
quality E-CONPRI
. O


In O
order O
to O
increase O
the O
final O
quality S-CONPRI
it O
is O
necessary O
to O
control O
factors O
such O
as S-MATE
extrusion O
speed O
and O
ensure O
constant O
pressure S-CONPRI
and O
flow O
. O


Another O
form O
of O
AM S-MANP
using O
material B-MANP
extrusion E-MANP
is O
material B-MANP
jetting E-MANP
process O
, O
where O
the O
material S-MATE
is O
deposited O
in O
the O
form O
of O
droplets S-CONPRI
, O
instead O
of O
filament S-MATE
, O
to O
form O
a O
2D B-FEAT
pattern E-FEAT
. O


The O
printed O
layer S-PARA
is O
cured S-MANP
using O
ultraviolet B-CONPRI
radiation E-CONPRI
, O
immediately O
after O
the O
deposition S-CONPRI
. O


The O
process S-CONPRI
is O
repeated O
until O
the O
complete O
3D B-APPL
part E-APPL
is O
formed O
. O


5 O
shows O
an O
example O
of O
an O
occlusal O
surface S-CONPRI
of O
a O
dental B-CHAR
crown E-CHAR
produced O
by O
this O
technique O
from O
a O
ceramic S-MATE
suspension O
of O
yttria S-MATE
partially B-MATE
stabilized I-MATE
zirconia E-MATE
. O


It O
is O
possible O
to O
observe O
surface B-CHAR
waviness E-CHAR
associated O
with O
layer-by-layer B-CONPRI
deposition E-CONPRI
. O


Material B-MANP
extrusion E-MANP
techniques O
also O
include O
robocasting S-MANP
. O


In O
this O
process S-CONPRI
, O
a O
filament S-MATE
of O
a O
paste O
is O
extruded S-MANP
through O
a O
nozzle S-MACEQ
while O
it O
moves O
over O
a O
platform S-MACEQ
, O
building O
the O
object O
layer-by-layer S-CONPRI
. O


The O
paste O
exits O
the O
nozzle S-MACEQ
with O
a O
given O
shape O
, O
without O
being O
necessary O
waiting O
for O
its O
solidification S-CONPRI
or O
drying S-MANP
to O
build S-PARA
the O
next O
layer S-PARA
. O


Freeze-form O
Extrusion S-MANP
Fabrication O
is O
another O
example O
of O
extrusion-based O
AM B-MANP
technology E-MANP
. O


Contrarily O
to O
most O
of O
the O
other O
extrusion S-MANP
freeform O
fabrication S-MANP
methods O
, O
in O
FEF O
, O
the O
organic O
binder S-MATE
content O
is O
only O
2vol O
% O
and O
the O
solids O
loading O
of O
the O
paste O
can O
be S-MATE
higher O
than O
50 O
vol O
% O
. O


During O
FEF O
the O
piece O
is O
built O
by O
keeping O
the O
surrounding O
environment O
of O
the O
building B-MACEQ
platform E-MACEQ
below O
the O
freezing O
temperature S-PARA
of O
water O
. O


This O
solidifies O
the O
paste O
after O
the O
deposition S-CONPRI
on O
each O
layer S-PARA
during O
the O
fabrication S-MANP
process O
. O


FEF O
enables O
the O
production S-MANP
of O
relatively O
large O
parts O
, O
when O
compared O
with O
traditional O
robocasting S-MANP
. O


In O
extrusion B-MANP
techniques E-MANP
, O
the O
mechanical B-CONPRI
properties E-CONPRI
can O
be S-MATE
improved O
by O
controlling O
the O
crystallographic O
texture S-FEAT
of O
the O
materials S-CONPRI
. O


This O
may O
be S-MATE
achieved O
by O
mixing S-CONPRI
a O
small O
number O
of O
large O
particles S-CONPRI
of O
anisotropic S-PRO
shape O
with O
fine O
particles S-CONPRI
. O


During O
extrusion S-MANP
, O
the O
anisotropic S-PRO
particles O
align O
in O
the O
shear B-PRO
direction E-PRO
. O


In O
a O
later O
sintering S-MANP
step O
, O
the O
aligned O
particles S-CONPRI
grow O
absorbing O
the O
fine O
particles S-CONPRI
, O
leading O
to O
highly O
textured O
and O
dense O
ceramics S-MATE
. O


4.2.3 O
Vat B-MANP
polymerization/stereolitography E-MANP
Vat S-MACEQ
polymerization/stereolitography O
was O
created O
by O
Chuck S-MACEQ
Hull O
in O
1986 O
and O
was O
the O
AM B-MANP
technology E-MANP
pioneer O
. O


SLA S-MACEQ
was O
the O
first O
AM S-MANP
to O
be S-MATE
applied O
in O
medicine S-CONPRI
, O
which O
was O
used O
to O
produce O
surgical O
models O
for O
alloplastic B-APPL
implant I-APPL
surgery E-APPL
in O
1994 O
. O


In O
vat B-MANP
polymerization E-MANP
printing O
, O
a O
specific O
type O
of O
light O
is O
used O
to O
build S-PARA
parts O
one O
layer S-PARA
at O
a O
time O
, O
in O
a O
vat S-MACEQ
containing O
light-cured O
photopolymer B-MATE
resin E-MATE
mixed O
with O
ceramic B-MATE
powder E-MATE
. O


The O
light O
travels O
each O
layer S-PARA
through O
the O
surface S-CONPRI
of O
the O
liquid O
resin S-MATE
. O


Then O
, O
the O
building B-MACEQ
platform E-MACEQ
descends O
allowing O
that O
another O
layer S-PARA
of O
resin S-MATE
spreads O
over O
the O
surface S-CONPRI
, O
and O
thus O
repeating O
the O
process S-CONPRI
. O


This O
technology S-CONPRI
enables O
a O
rapid B-MANP
fabrication E-MANP
and O
allows O
to O
create O
complex B-PRO
shapes E-PRO
with O
high O
level O
of O
accuracy S-CHAR
and O
good O
finish O
. O


The O
curing B-PARA
depth E-PARA
is O
a O
critical O
parameter S-CONPRI
that O
determines O
the O
accuracy S-CHAR
of O
the O
formability S-PRO
. O


6 O
shows O
a O
zirconia S-MATE
implant S-APPL
printed O
trough O
this O
method O
. O


When O
compared O
to O
conventional O
polymer-based O
SLA S-MACEQ
, O
using O
ceramics S-MATE
can O
affect O
the O
line O
width O
and O
the O
curing B-PARA
depth E-PARA
. O


Also O
, O
since O
conventional O
SLA S-MACEQ
equipment S-MACEQ
uses O
dispersions O
with O
viscosities O
lower O
than O
5 O
Pa.s O
, O
the O
particle S-CONPRI
size O
and O
the O
solids O
volume B-PARA
fraction E-PARA
of O
the O
ceramics S-MATE
preparations O
must O
be S-MATE
adjusted O
to O
meet O
the O
requirements O
of O
both O
formability S-PRO
and O
sinterability S-PRO
. O


To O
obtain O
ceramics S-MATE
with O
a O
high O
density S-PRO
, O
it O
is O
essential O
a O
fine O
particle S-CONPRI
size O
and O
a O
high O
solids O
volume B-PARA
fraction E-PARA
. O


Overall O
, O
SLA S-MACEQ
presents O
excellent O
surface B-MANP
finishing E-MANP
, O
but O
is O
still O
considered O
to O
be S-MATE
relatively O
expensive O
and O
present O
a O
lengthily O
post-processing S-CONPRI
time O
for O
unprocessed O
resin S-MATE
removal O
and O
additional O
curing S-MANP
. O


For O
ceramics S-MATE
, O
a O
final O
thermal B-PARA
cycle E-PARA
allows O
to O
remove O
the O
organic O
resin S-MATE
and O
sintering S-MANP
the O
material S-MATE
, O
increasing O
its O
density S-PRO
. O


4.2.4 O
Direct B-MANP
energy I-MANP
deposition E-MANP
techniques O
Direct B-MANP
energy I-MANP
deposition E-MANP
is O
a O
more O
complex O
additive S-MATE
printing O
process S-CONPRI
, O
commonly O
used O
to O
repair O
or O
add O
additional O
material S-MATE
to O
existing O
components S-MACEQ
. O


A O
typical O
DED B-MACEQ
machine E-MACEQ
consists O
of O
a O
nozzle S-MACEQ
mounted O
on O
a O
multi O
axis O
arm O
, O
which O
deposits O
melted S-CONPRI
material S-MATE
onto O
the O
specified O
surface S-CONPRI
, O
where O
it O
solidifies O
. O


The O
material S-MATE
is O
heated O
and O
melted S-CONPRI
using O
a O
laser S-ENAT
, O
electron B-CONPRI
beam E-CONPRI
or O
plasma B-CONPRI
arc E-CONPRI
. O


The O
piece O
is O
lowered O
by O
a O
distance O
equivalent O
to O
the O
layer B-PARA
thickness E-PARA
. O


Although O
this O
process S-CONPRI
typically O
uses O
metal S-MATE
, O
it O
can O
also O
use O
polymers S-MATE
and O
ceramics S-MATE
, O
either O
provided O
in O
wire O
or O
powder S-MATE
form O
. O


For O
ceramic B-MATE
materials E-MATE
, O
it O
allows O
achieving O
almost O
100 O
% O
density S-PRO
and O
avoids O
shrinkage S-CONPRI
or O
distortion S-CONPRI
, O
eliminating O
the O
need O
of O
debinding S-CONPRI
or O
sintering S-MANP
steps O
. O


Wilkes O
and O
Wissenbach O
were O
pioneer O
in O
studying O
the O
applicability O
of O
this O
method O
to O
manufacture S-CONPRI
ceramic S-MATE
components O
for O
medical B-APPL
applications E-APPL
. O


4.2.5 O
Powder B-MANP
bed I-MANP
fusion E-MANP
Powder-based O
printing B-ENAT
technologies E-ENAT
include O
selective B-MANP
laser I-MANP
sintering E-MANP
, O
direct B-MANP
metal I-MANP
laser I-MANP
sintering E-MANP
, O
selective B-MANP
laser I-MANP
melting E-MANP
and O
electron B-MANP
beam I-MANP
melting E-MANP
. O


All O
these O
technologies S-CONPRI
use O
heat S-CONPRI
to O
fuse S-MANP
the O
powdered O
materials S-CONPRI
. O


The O
differences O
rely O
on O
the O
energy O
source S-APPL
and O
powder B-MATE
materials E-MATE
. O


For O
instance O
, O
SLS S-MANP
, O
DMLS S-MANP
, O
and O
SLM S-MANP
all O
use O
lasers O
, O
while O
EBM S-MANP
uses O
electron B-CONPRI
beam E-CONPRI
as S-MATE
energy O
source S-APPL
In O
the O
sintering S-MANP
processes S-CONPRI
, O
the O
powders S-MATE
are O
not O
completely O
melted S-CONPRI
. O


This O
leads O
to O
porous S-PRO
internal B-PRO
structures E-PRO
and O
rough O
surfaces S-CONPRI
. O


In O
the O
melting S-MANP
processes O
, O
the O
powders S-MATE
are O
well O
fused S-CONPRI
, O
creating O
parts O
with O
enhanced O
mechanical B-CONPRI
properties E-CONPRI
and O
higher O
densities O
. O


The O
process S-CONPRI
begins O
with O
the O
spreading O
of O
a O
layer S-PARA
of O
material S-MATE
over O
the O
build B-MACEQ
platform E-MACEQ
, O
typically O
0.1 O
mm S-MANP
thick O
. O


The O
energy O
source S-APPL
fuses S-MANP
the O
first O
layer S-PARA
or O
first O
cross B-CONPRI
section E-CONPRI
of O
the O
model S-CONPRI
. O


The O
build B-MACEQ
platform E-MACEQ
is O
then O
lowered O
and O
a O
new O
layer S-PARA
of O
powder S-MATE
is O
spread S-CONPRI
across O
the O
previous O
using O
a O
roller S-MACEQ
. O


7 O
shows O
a O
dental B-MACEQ
piece E-MACEQ
made O
of O
alumina-zirconia B-MATE
composite E-MATE
obtained O
by O
this O
process S-CONPRI
. O


The O
manufacturing S-MANP
time O
for O
powder B-MANP
bed I-MANP
fusion E-MANP
based O
techniques O
is O
lower O
than O
for O
other O
AM B-MANP
techniques E-MANP
. O


In O
fact O
, O
as S-MATE
it O
happens O
with O
direct B-MANP
energy I-MANP
deposition E-MANP
, O
since O
these O
techniques O
do O
not O
involve O
the O
use O
of O
binders S-MATE
for O
the O
production S-MANP
of O
intermediate O
green O
pieces O
, O
it O
is O
not O
required O
any O
debinding S-CONPRI
process O
. O


However O
, O
due O
to O
the O
high O
heating S-MANP
and O
cooling B-PARA
rates E-PARA
, O
thermal O
shock O
may O
occur O
, O
which O
may O
lead S-MATE
to O
cracking S-CONPRI
. O


This O
may O
be S-MATE
avoided O
by O
pre-heating O
the O
powder S-MATE
. O


5 O
Additive B-MANP
manufacturing E-MANP
of O
bioceramics S-MATE
for O
dental B-APPL
applications E-APPL
Ceramic B-MATE
materials E-MATE
where O
only O
recently O
considered O
in O
AM S-MANP
processing O
due O
to O
their O
intrinsic B-CONPRI
properties E-CONPRI
. O


The O
high O
melting B-PRO
points E-PRO
of O
ceramics S-MATE
make O
them O
difficult O
to O
melt S-CONPRI
under O
normal O
heating S-MANP
methods O
. O


Although O
it O
is O
possible O
to O
melt S-CONPRI
some O
ceramics S-MATE
, O
this O
process S-CONPRI
can O
cause O
new O
phase S-CONPRI
formation O
. O


On O
the O
other O
hand O
, O
several O
factors O
associated O
to O
the O
processing O
of O
the O
ceramic B-MATE
materials E-MATE
and O
to O
the O
characteristics O
of O
the O
raw B-MATE
materials E-MATE
used O
may O
affect O
the O
porosity S-PRO
of O
the O
final O
piece O
. O


An O
increase O
of O
porosity S-PRO
impairs O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
the O
final O
product O
. O


However O
, O
it O
can O
be S-MATE
favourable O
for O
cellular B-CHAR
growth E-CHAR
or O
implant S-APPL
fixation O
, O
required O
for O
specific O
applications O
. O


This O
section O
will O
report O
published O
work O
where O
different O
bioceramics S-MATE
, O
such O
as S-MATE
zirconia O
, O
alumina S-MATE
, O
calcium B-MATE
phosphates E-MATE
and O
ceramic B-FEAT
composites E-FEAT
, O
with O
possible O
applications O
in O
dentistry S-APPL
, O
are O
processed S-CONPRI
using O
various O
AM B-MANP
technologies E-MANP
. O


5.1 O
Additive B-MANP
manufacturing E-MANP
of O
zirconia S-MATE
The O
information O
concerning O
the O
AM S-MANP
of O
zirconia-based B-MATE
compositions E-MATE
, O
with O
possible O
applications O
in O
dentistry S-APPL
is O
present O
in O
2 O
. O


Ebert O
demonstrated O
the O
possibility O
to O
build S-PARA
dense O
three-dimensional S-CONPRI
components S-MACEQ
of O
the O
size O
of O
a O
crown S-MACEQ
, O
with O
its O
characteristic O
occlusal O
surface B-CONPRI
topography E-CONPRI
, O
through O
material B-MANP
extrusion E-MANP
technology O
, O
using O
zirconia B-MATE
ceramic E-MATE
suspensions O
. O


The O
printed O
and O
sintered S-MANP
samples S-CONPRI
were O
not O
completely O
free O
of O
process-related O
defects S-CONPRI
, O
mainly O
due O
to O
clogged B-MACEQ
nozzles E-MACEQ
during O
printing O
. O


However O
, O
it O
was O
possible O
to O
obtain O
specimens O
with O
relative B-PRO
density E-PRO
of O
96.9 O
% O
, O
with O
mechanical B-CONPRI
properties E-CONPRI
comparable O
to O
those O
of O
conventionally O
produced O
3Y-TZP S-MATE
via O
cold B-MANP
isostatic I-MANP
pressing E-MANP
. O


Moin O
, O
established O
that O
it O
is O
feasible O
to O
use O
high-end O
digital B-MANP
light I-MANP
processing E-MANP
technology O
to O
fabricate S-MANP
a O
root B-APPL
analogue E-APPL
implant S-APPL
with O
a O
certain O
amount O
of O
precision S-CHAR
. O


However O
, O
the O
results O
showed O
a O
printed O
RAI O
with O
a O
6.67 O
% O
larger O
surface B-PARA
area E-PARA
. O


A O
large O
number O
of O
factors O
are O
recognized O
to O
influence O
the O
precision S-CHAR
of O
this O
printing O
technique O
, O
namely O
the O
resolution S-PARA
of O
the O
digital B-MACEQ
mirroring I-MACEQ
device E-MACEQ
and O
the O
composition S-CONPRI
of O
the O
ceramic/ O
photopolymer S-MATE
mixture O
. O


The O
authors O
claim O
that O
the O
precise B-CONPRI
control E-CONPRI
over O
spatially B-FEAT
grade I-FEAT
composition E-FEAT
, O
microstructure S-CONPRI
design/distribution O
and O
shape O
are O
potential O
advantages O
over O
milling S-MANP
of O
unsintered S-PRO
ceramics S-MATE
. O


Osman O
also O
used O
a O
SLA S-MACEQ
method O
to O
efficiently O
print S-MANP
customized O
zirconia S-MATE
dental S-APPL
implants O
with O
sufficient B-CONPRI
dimensional I-CONPRI
accuracy E-CONPRI
. O


The O
study O
evaluated O
among O
other O
aspects O
, O
the O
dimensional B-CHAR
accuracy E-CHAR
, O
surface B-CONPRI
topography E-CONPRI
and O
mechanical B-CONPRI
properties E-CONPRI
. O


The O
authors O
report O
that O
the O
dimensional B-CHAR
accuracy E-CHAR
of O
the O
printed O
implant S-APPL
was O
high O
and O
the O
achieved O
mechanical B-CONPRI
properties E-CONPRI
showed O
flexural B-PRO
strength E-PRO
close O
to O
those O
of O
conventionally O
produced O
ceramics S-MATE
. O


SLA S-MACEQ
was O
again O
used O
by O
Xing O
, O
to O
produce O
ZrO2 S-MATE
complex O
shaped O
ceramic S-MATE
components O
with O
high O
dimensional B-CHAR
accuracy E-CHAR
and O
proper O
properties S-CONPRI
. O


The O
surface B-PRO
roughness E-PRO
of O
the O
unpolished O
ZrO2 S-MATE
showed O
an O
anisotropic S-PRO
behavior O
with O
values O
ranging O
from O
0.41 O
to O
1.07 O
on O
the O
measuring O
direction O
. O


This O
phenomenon O
could O
be S-MATE
eliminated O
through O
polishing S-MANP
, O
reducing O
Ra O
to O
a O
nanometer B-FEAT
scale E-FEAT
. O


The O
sintered S-MANP
ceramics S-MATE
had O
isotropic S-PRO
mechanical O
properties S-CONPRI
close O
to O
milled S-MANP
zirconia O
due O
to O
the O
homogeneity O
of O
the O
grain B-PRO
size E-PRO
. O


In O
the O
work O
of O
Scheithauer O
, O
the O
droplet S-CONPRI
formation O
behavior O
of O
zirconia S-MATE
suspensions O
for O
thermoplastic B-MANP
3D I-MANP
printing E-MANP
was O
investigated O
. O


The O
precise O
deposition S-CONPRI
of O
small O
adjacent O
droplets S-CONPRI
of O
molten O
thermoplastic S-MATE
suspensions O
containing O
ceramic S-MATE
particles O
allowed O
to O
obtain O
filament-like O
structures O
by O
coalescence B-CONPRI
of I-CONPRI
adjacent I-CONPRI
droplets E-CONPRI
. O


The O
researchers O
introduced O
the O
droplet B-CHAR
fusion I-CHAR
factor E-CHAR
in O
order O
to O
calculate O
the O
necessary O
distance O
between O
two O
droplets S-CONPRI
to O
produce O
those O
filaments S-MATE
. O


The O
results O
showed O
that O
filament-like O
structures O
with O
a O
smooth B-CONPRI
surface E-CONPRI
and O
a O
nearly O
homogeneous S-CONPRI
cross B-CONPRI
section E-CONPRI
could O
be S-MATE
produced O
for O
suspensions O
with O
a O
dff O
of O
44 O
% O
or O
higher O
. O


In O
a O
previous O
work O
, O
the O
same O
research S-CONPRI
group O
used O
the O
same O
process S-CONPRI
to O
print S-MANP
zirconia O
samples S-CONPRI
. O


They O
were O
able O
to O
produce O
ceramic S-MATE
parts O
with O
high O
density S-PRO
and O
homogeneous B-PRO
microstructures E-PRO
. O


On O
one O
hand O
the O
heating S-MANP
rates O
required O
for O
thermal B-CHAR
debinding E-CHAR
are O
very O
low O
and O
it O
must O
be S-MATE
carried O
out O
in O
a O
powder B-MACEQ
bed E-MACEQ
, O
increasing O
the O
time O
and O
complexity S-CONPRI
of O
the O
process S-CONPRI
. O


In O
another O
work O
of O
the O
same O
group O
, O
the O
authors O
combined O
AM S-MANP
and O
functionally B-MATE
graded I-MATE
materials E-MATE
to O
create O
zirconia-based O
customizable O
smart O
materials S-CONPRI
. O


By O
using O
T3DP O
technology S-CONPRI
, O
it O
was O
possible O
to O
selectively O
deposit O
two O
different O
materials S-CONPRI
beside O
each O
other O
, O
offering O
the O
prospect O
of O
combining O
suspensions O
with O
different O
contents O
of O
a O
pore B-PARA
forming I-PARA
agents E-PARA
to O
obtained O
components S-MACEQ
with O
dense B-FEAT
and I-FEAT
porous E-FEAT
areas S-PARA
inside O
. O


The O
presence O
of O
zones O
with O
different O
porosities S-PRO
reduces O
the O
elastic B-PRO
modulus E-PRO
, O
diminishing O
the O
stress B-PRO
shielding E-PRO
, O
which O
should O
be S-MATE
benefic O
for O
dental S-APPL
implants O
. O


More O
, O
the O
presence O
of O
open O
pores S-PRO
shall O
favor O
the O
osteointegration S-PRO
. O


The O
work O
of O
Shao O
, O
showed O
the O
possibility O
of O
successfully O
printing O
ZrO2 B-MATE
ceramic E-MATE
parts O
by O
a O
new O
extrusion-based B-MANP
process E-MANP
, O
3D B-MANP
Gel-Printing E-MANP
. O


The O
authors O
were O
able O
to O
produce O
printed O
and O
sintered S-MANP
cuboid O
samples S-CONPRI
with O
a O
regular O
appearance O
. O


Parameters S-CONPRI
such O
as S-MATE
surface O
roughness S-PRO
, O
relative B-PRO
density E-PRO
, O
hardness S-PRO
and O
transverse B-PRO
rupture I-PRO
strength E-PRO
of O
printed O
and O
sintered S-MANP
samples S-CONPRI
were O
compared O
with O
those O
obtained O
in O
other O
3D B-MANP
printing E-MANP
processes O
. O


It O
was O
found O
that O
3DGP O
led S-APPL
to O
samples S-CONPRI
with O
higher O
density S-PRO
, O
hardness S-PRO
and O
surface B-MANP
finishing E-MANP
than O
those O
obtained O
by O
syringe S-MACEQ
extrusion S-MANP
, O
and O
that O
presented O
higher O
transverse B-PRO
rupture I-PRO
strength E-PRO
than O
others O
produced O
by O
gel B-MANP
casting E-MANP
. O


More O
, O
no O
defects S-CONPRI
or O
deformation S-CONPRI
was O
observed O
from O
outside O
of O
the O
printed O
samples S-CONPRI
. O


The O
authors O
refer O
the O
importance O
of O
aspects O
such O
as S-MATE
printing O
conditions O
, O
rheological S-PRO
behavior O
of O
the O
slurry S-MATE
as S-MATE
well O
as S-MATE
the O
solid O
loading O
on O
the O
final O
properties S-CONPRI
of O
the O
printed/sintered O
samples S-CONPRI
. O


Faes O
combined O
the O
advantages O
of O
AM S-MANP
extrusion O
and O
UV B-CONPRI
curing E-CONPRI
into O
a O
single O
3D B-MANP
printing E-MANP
technique O
, O
to O
obtain O
pieces O
with O
an O
high O
shape B-FEAT
stability E-FEAT
and O
green O
strength S-PRO
. O


This O
novel O
syringe-based O
AM B-MANP
process E-MANP
, O
based O
on O
the O
use O
of O
a O
photopolymerizable S-PRO
dispersion S-CONPRI
, O
leads O
to O
economic O
benefits O
since O
it O
reduces O
the O
raw B-MATE
material E-MATE
consumption O
. O


High O
shrinkage S-CONPRI
was O
observed O
during O
sintering S-MANP
, O
leading O
to O
cracking S-CONPRI
. O


This O
must O
be S-MATE
due O
to O
a O
low O
load O
of O
ceramic S-MATE
particles O
in O
the O
dispersion S-CONPRI
. O


It O
is O
suggested O
to O
increase O
the O
amount O
of O
ceramic S-MATE
particles O
, O
keeping O
the O
rheological S-PRO
behavior O
, O
which O
can O
be S-MATE
achieved O
through O
the O
introduction O
of O
steric O
repulsive O
forces S-CONPRI
in O
the O
dispersion S-CONPRI
, O
for O
example O
using O
other O
resins S-MATE
. O


5.2 O
Additive B-MANP
manufacturing E-MANP
of O
alumina B-MATE
3 E-MATE
gathers O
the O
main O
findings O
of O
published O
work O
regarding O
the O
AM S-MANP
of O
alumina-based B-MATE
formulations E-MATE
with O
possible O
applications O
in O
the O
dental S-APPL
field O
. O


Work O
carried O
out O
by O
Scheithauer O
, O
showed O
the O
possibility O
of O
printing O
dense O
alumina S-MATE
pieces O
using O
3D B-MANP
printing E-MANP
of O
high-filled O
suspensions O
with O
thermoplastic B-MACEQ
binder I-MACEQ
systems E-MACEQ
. O


The O
authors O
achieved O
samples S-CONPRI
with O
high O
densities O
, O
homogeneous B-PRO
microstructures E-PRO
and O
very O
good O
bonding S-CONPRI
between O
the O
printed O
layers O
. O


This O
study O
also O
highlights O
the O
importance O
of O
the O
rheological B-PRO
properties E-PRO
of O
the O
slurries O
, O
i.e O
. O


low O
viscosity S-PRO
allows O
an O
easy O
flow O
through O
the O
needle O
to O
form O
small O
droplets S-CONPRI
, O
which O
can O
improve O
the O
printing O
resolution S-PARA
. O


It O
is O
concluded O
that O
thermoplastic B-MANP
3D I-MANP
printing E-MANP
presents O
several O
advantages O
over O
other O
suspension-based O
technologies S-CONPRI
, O
namely O
the O
fact O
that O
the O
green O
layer S-PARA
solidifying O
occur O
by O
simple S-MANP
cooling S-MANP
, O
the O
versatility O
of O
ceramic B-MATE
materials E-MATE
that O
can O
be S-MATE
used O
and O
the O
applicability O
in O
limited O
areas S-PARA
. O


It O
is O
recognized O
the O
potential O
of O
the O
technique O
to O
print S-MANP
multimaterial O
and O
multifunctional O
components S-MACEQ
, O
as S-MATE
well O
as S-MATE
to O
obtain O
pieces O
with O
material S-MATE
and/or O
property S-CONPRI
gradients O
in O
different O
dimensions S-FEAT
. O


However O
, O
the O
authors O
claim O
that O
heating S-MANP
rates O
for O
thermal B-CHAR
debinding E-CHAR
must O
be S-MATE
very O
low O
and O
that O
the O
process S-CONPRI
must O
be S-MATE
performed O
in O
a O
powder B-MACEQ
bed E-MACEQ
. O


Dehurtevent O
, O
presented O
a O
study O
which O
provides O
promising O
results O
for O
manufacturing S-MANP
dense O
3D S-CONPRI
alumina O
crown S-MACEQ
frameworks O
by O
SLA S-MACEQ
. O


The O
authors O
established O
a O
comparison O
between O
the O
physical O
and O
mechanical B-CONPRI
properties E-CONPRI
of O
SLA-manufactured O
alumina S-MATE
ceramics O
of O
different O
compositions O
and O
viscosity S-PRO
to O
those O
of O
subtractive-manufactured O
ceramics S-MATE
. O


Their O
results O
showed O
an O
acceptability O
window O
for O
viscosity S-PRO
of O
the O
slurries O
that O
allows O
producing O
SLA S-MACEQ
manufactured S-CONPRI
alumina S-MATE
pieces O
. O


Additionally O
, O
the O
authors O
were O
able O
to O
achieve O
a O
composition S-CONPRI
that O
originated O
a O
reliable O
material S-MATE
with O
the O
anisotropic B-CHAR
shrinkage E-CHAR
, O
high O
density S-PRO
, O
flexural B-PRO
strength E-PRO
, O
and O
Weibull O
characteristics O
suitable O
for O
SLA S-MACEQ
manufacturing S-MANP
. O


Nevertheless O
, O
it O
was O
observed O
that O
although O
an O
oversize O
of O
35 O
% O
allow O
manufacturing S-MANP
complex B-CONPRI
morphologies E-CONPRI
, O
differential O
shrinkage S-CONPRI
led S-APPL
to O
deformation S-CONPRI
of O
the O
final O
structure S-CONPRI
. O


In O
the O
work O
of O
Maleksaeedi O
, O
a O
powder-bed O
inkjet B-MANP
3D-printing E-MANP
and O
vacuum B-BIOP
infiltration I-BIOP
process E-BIOP
was O
used O
for O
producing O
alumina S-MATE
parts O
with O
high O
density S-PRO
and O
improved O
mechanical B-CONPRI
properties E-CONPRI
. O


The O
authors O
utilized O
vacuum B-BIOP
infiltration I-BIOP
process E-BIOP
to O
enhance O
the O
packing O
of O
the O
green B-PRO
parts E-PRO
after O
printing O
, O
by O
impregnating O
the O
3D-printed B-APPL
parts E-APPL
with O
highly O
solid O
loaded O
slurries O
. O


Their O
results O
showed O
that O
the O
vacuum B-BIOP
infiltration I-BIOP
process E-BIOP
was O
able O
to O
significantly O
increase O
the O
density S-PRO
, O
reduce O
the O
porosity S-PRO
and O
increase O
the O
strength S-PRO
of O
the O
3D-printed S-MANP
alumina O
components S-MACEQ
. O


Moreover O
, O
the O
bending B-PRO
strength E-PRO
was O
improved O
up O
to O
15 O
times O
of O
the O
original O
strength S-PRO
. O


5.3 O
Additive B-MANP
manufacturing E-MANP
of O
other O
ceramics S-MATE
The O
potential O
of O
AM S-MANP
has O
been O
explored O
in O
several O
dental B-APPL
applications E-APPL
with O
other O
ceramic B-MATE
materials E-MATE
. O


4 O
summarizes O
information O
regarding O
various O
calcium B-MATE
phosphates E-MATE
compositions O
and O
other O
bioceramics S-MATE
, O
as S-MATE
well O
as S-MATE
gypsum O
, O
mainly O
for O
bone B-CONPRI
regeneration E-CONPRI
. O


Lopez O
, O
produced O
bioactive O
tricalcium B-BIOP
phosphate I-BIOP
scaffolds E-BIOP
using O
a O
material B-MANP
extrusion E-MANP
technology O
, O
robocasting S-MANP
. O


The O
printed O
scaffolds S-FEAT
were O
able O
to O
restore O
critical O
mandibular B-BIOP
segmental I-BIOP
defects E-BIOP
to O
levels O
similar O
to O
native O
bone S-BIOP
after O
a O
8 O
week O
period O
implant S-APPL
in O
an O
adult B-CONPRI
rabbit I-CONPRI
mandibular I-CONPRI
defect I-CONPRI
model E-CONPRI
. O


Histological O
and O
scanning B-CHAR
electron I-CHAR
microscopy E-CHAR
analysis O
showed O
directional B-CONPRI
bony I-CONPRI
ingrowth E-CONPRI
into O
the O
scaffold B-BIOP
interstices E-BIOP
, O
tracking O
healing O
pathway O
origins O
to O
defect S-CONPRI
walls O
and O
marrow B-CONPRI
spaces E-CONPRI
. O


More O
, O
it O
was O
observed O
new O
bone B-CONPRI
growth E-CONPRI
and O
scaffold S-FEAT
resorption O
at O
bone/scaffold B-FEAT
interfaces E-FEAT
. O


Another O
work O
using O
robocasting S-MANP
technology O
is O
the O
one O
carried O
out O
by O
Slots O
in O
which O
porous B-APPL
TCP I-APPL
implants E-APPL
were O
developed O
using O
storable O
and O
reusable O
inks O
composed O
of O
fatty O
acid/TCP O
. O


The O
total O
fabrication B-PARA
time E-PARA
including O
ink S-MATE
preparation O
, O
printing O
and O
sintering S-MANP
was O
less O
than O
5 O
h O
for O
8 O
cm2 O
of O
implant S-APPL
. O


The O
printed O
implants S-APPL
were O
able O
to O
retain O
their O
shape O
after O
sintering S-MANP
and O
were O
chemically O
unchanged O
by O
the O
printing O
and O
sintering S-MANP
process S-CONPRI
. O


Mesenchymal B-MATE
stem I-MATE
cells E-MATE
were O
able O
to O
grow O
on O
the O
implants S-APPL
, O
secrete O
collagen S-MATE
and O
alkaline B-MATE
phosphate E-MATE
and O
mineralize O
the O
implant S-APPL
. O


Additionally O
, O
they O
possessed O
clinically O
relevant O
mechanical B-PRO
strength E-PRO
and O
presented O
osteoconductive S-PRO
properties O
. O


The O
process S-CONPRI
demonstrated O
to O
be S-MATE
sufficiently O
simple S-MANP
and O
effective O
to O
enable O
rapid O
, O
on-demand O
, O
in-hospital O
production S-MANP
of O
patient-specific O
ceramic S-MATE
implants O
for O
treatment O
of O
bone S-BIOP
trauma O
. O


In O
the O
study O
conducted O
by O
Fahimipour O
, O
a O
biomimetic S-CONPRI
porous O
TCP/alginate/gelatin O
scaffold S-FEAT
containing O
PLGA S-MATE
) O
microspheres S-CONPRI
for O
slow O
release O
of O
VEGF S-MATE
was O
processed S-CONPRI
through O
extrusion S-MANP
technology O
. O


The O
printable O
ink S-MATE
was O
selected O
according O
to O
the O
gel B-PRO
point E-PRO
of O
different O
formulations O
of O
TCP/alginate/gelatin O
. O


The O
process S-CONPRI
faced S-MANP
some O
difficulties O
, O
for O
example O
in O
what O
concerns O
the O
needle O
blocking S-CONPRI
during O
extrusion S-MANP
if O
the O
gel B-PRO
point E-PRO
is O
above O
room O
temperature S-PARA
. O


Nevertheless O
, O
it O
was O
possible O
to O
achieve O
satisfactory O
mechanical S-APPL
and O
biological O
features O
supporting O
cell B-CHAR
viability E-CHAR
necessary O
for O
bone S-BIOP
tissue O
regeneration.Tamimi O
, O
prepared O
customized O
3D-printed S-MANP
monetite O
onlays S-APPL
by O
binder B-MANP
jetting E-MANP
. O


The O
onlays S-APPL
were O
design S-FEAT
to O
facilitate O
the O
diffusion S-CONPRI
of O
cells S-APPL
and O
nutrients O
from O
high O
bone S-BIOP
metabolic O
to O
low O
bone B-CHAR
metabolic I-CHAR
areas E-CHAR
. O


The O
research S-CONPRI
showed O
that O
bone B-CONPRI
metabolic I-CONPRI
activity E-CONPRI
in O
onlays S-APPL
is O
anatomy-dependant O
and O
correlates O
with O
the O
ability O
of O
bone S-BIOP
augmentation O
. O


The O
authors O
were O
able O
to O
achieve O
osseointegration S-PRO
of O
dental S-APPL
implants O
in O
bone S-BIOP
augmented O
with O
the O
printed O
monetite S-MATE
onlays O
. O


Klammert O
also O
used O
binder B-MANP
jetting E-MANP
technology O
to O
produce O
several O
specific O
craniofacial B-APPL
implants E-APPL
of O
TCP O
. O


The O
printed O
parts O
were O
able O
to O
comply O
with O
geometric O
requirements O
and O
provide O
an O
adequate O
accuracy B-CHAR
of I-CHAR
fit E-CHAR
, O
even O
though O
the O
authors O
did O
not O
use O
a O
commercial O
CAD S-ENAT
solution O
. O


Fieldint O
and O
colleagues O
face S-CONPRI
several O
challenges O
to O
adapt O
and O
optimize O
the O
processing O
parameters S-CONPRI
to O
produce O
scaffolds S-FEAT
using O
a O
3D B-MACEQ
binder I-MACEQ
jetting I-MACEQ
printer E-MACEQ
and O
commercially O
available O
binders S-MATE
. O


The O
authors O
report O
that O
the O
addition O
of O
dopants S-MATE
to O
the O
ceramic B-MATE
powder E-MATE
decreased O
the O
to O
phase S-CONPRI
transformation O
of O
TCP O
sintered S-MANP
at O
1250 O
Additionally O
, O
the O
density S-PRO
increased O
leading O
to O
a O
250 O
% O
increase O
in O
compressive B-PRO
strength E-PRO
, O
when O
compared O
to O
pure O
TCP B-BIOP
scaffolds E-BIOP
. O


Shao O
prepared O
by O
extrusion S-MANP
, O
four O
groups O
of O
bioceramic B-BIOP
scaffolds E-BIOP
for O
treatment O
of O
bone B-BIOP
defects E-BIOP
: O
Mg-substituted O
wollastonite-based O
; O
TCP-based O
; O
wollastonite-based O
; O
and O
bredigite-based O
. O


Additionally O
, O
CSi-Mg10 O
printed O
parts O
revealed O
the O
largest O
pore B-PARA
dimension E-PARA
but O
the O
lowest O
porosity S-PRO
, O
mainly O
due O
to O
the O
considerable O
shrinkage S-CONPRI
of O
the O
scaffolds S-FEAT
during O
sintering S-MANP
. O


Asadi-Eydivand O
, O
prepared O
gypsum-based B-FEAT
scaffolds E-FEAT
also O
for O
the O
treatment O
of O
bone B-BIOP
defects E-BIOP
. O


They O
investigated O
the O
effect O
of O
thermic O
treatment O
on O
the O
structural O
, O
mechanical S-APPL
, O
and O
physical B-PRO
properties E-PRO
of O
samples S-CONPRI
produced O
by O
extrusion/jetting O
. O


For O
the O
lowest O
temperature S-PARA
, O
the O
samples S-CONPRI
showed O
adequate O
mechanical B-CONPRI
properties E-CONPRI
, O
but O
high O
cytotoxicity S-PRO
. O


In O
contrast O
, O
temperatures S-PARA
in O
the O
range S-PARA
of O
500 O
led S-APPL
to O
lower O
cytotoxic B-BIOP
scaffolds E-BIOP
but O
insufficient O
mechanical B-PRO
strength E-PRO
. O


For O
temperatures S-PARA
higher O
than O
1000 O
higher O
compressive B-PRO
strength E-PRO
and O
greater O
viability O
were O
observed O
. O


However O
, O
above O
1200 O
decomposition S-PRO
of O
calcium S-MATE
sulfate O
occurs O
, O
leading O
to O
mass O
loss O
. O


5.4 O
Additive B-MANP
manufacturing E-MANP
of O
ceramic B-FEAT
composites E-FEAT
5 O
gathers O
the O
information O
about O
AM S-MANP
of O
ceramic-based B-MATE
composites E-MATE
possible O
applications O
in O
dentistry S-APPL
. O


In O
the O
work O
carried O
out O
by O
Goyos-Ball O
porous S-PRO
robocasted O
structures O
made O
of O
10 O
mol O
% O
ceria-stabilized O
zirconia S-MATE
and O
alumina B-MATE
composite E-MATE
were O
produced O
. O


The O
authors O
found O
that O
round B-FEAT
lattice I-FEAT
structures E-FEAT
have O
compression B-PRO
strength E-PRO
similar O
to O
cortical B-MATE
bone E-MATE
, O
are O
not O
cytotoxic S-CONPRI
and O
induce O
osseous O
differentiation O
. O


More O
, O
the O
printed O
parts O
showed O
good O
aesthetics O
, O
chemical B-PRO
stability E-PRO
and O
negligible O
corrosion S-CONPRI
and O
wear S-CONPRI
. O


Due O
to O
the O
high O
structural B-PRO
integrity E-PRO
, O
the O
printed O
parts O
could O
be S-MATE
used O
as S-MATE
scaffolds O
for O
load O
bearing O
applications O
during O
the O
osteointegration S-PRO
process O
. O


Rahim O
established O
a O
comparison O
between O
composites S-MATE
prepared O
by O
extrusion S-MANP
and O
by O
injection B-MANP
moulding E-MANP
. O


The O
samples S-CONPRI
were O
composed O
of O
polyamide B-MATE
12 E-MATE
, O
incorporated O
with O
bioceramic B-MATE
fillers E-MATE
, O
from O
10 O
to O
40 O
% O
content O
. O


The O
results O
of O
their O
work O
showed O
that O
the O
addition O
of O
fillers O
improved O
or O
maintained O
the O
strength S-PRO
and O
stiffness S-PRO
of O
the O
parts O
, O
while O
reducing O
toughness S-PRO
and O
flexibility S-PRO
. O


Melting S-MANP
behaviour O
of O
polyamide B-MATE
12 E-MATE
did O
not O
depend O
on O
the O
processing B-CONPRI
techniques E-CONPRI
, O
but O
was O
affected O
by O
the O
addition O
of O
fillers O
and O
by O
the O
cooling B-PARA
rate E-PARA
. O


Incorporation O
of O
fillers O
improved O
the O
thermal B-PRO
stability E-PRO
. O


It O
was O
found O
that O
fuse B-CONPRI
deposition E-CONPRI
modelling O
allows O
producing O
medical B-APPL
implants E-APPL
with O
acceptable O
mechanical S-APPL
performances O
for O
non-load B-PRO
bearing E-PRO
applications O
. O


Jan O
manufactured S-CONPRI
ceramic S-MATE
objects O
by O
powder B-MANP
bed I-MANP
fusion E-MANP
, O
using O
selective B-MANP
laser I-MANP
melting E-MANP
technology O
. O


The O
authors O
were O
able O
to O
produce O
parts O
with O
good O
mechanical B-CONPRI
properties E-CONPRI
, O
with O
approximately O
100 O
% O
density S-PRO
, O
without O
needing O
sintering S-MANP
processes S-CONPRI
or O
post-processing S-CONPRI
, O
which O
constitutes O
an O
advantage O
. O


The O
study O
reports O
some O
process S-CONPRI
challenges O
that O
need O
to O
be S-MATE
overcome O
, O
for O
instance O
, O
the O
thermally O
induced O
stresses O
, O
caused O
by O
the O
deposition S-CONPRI
of O
the O
new O
cold B-MATE
powder I-MATE
layers E-MATE
on O
top O
of O
the O
preheated O
ceramic S-MATE
, O
and O
the O
surface B-PRO
roughness E-PRO
values O
. O


6 O
Challenges O
Additive B-MANP
manufacturing E-MANP
is O
recognized O
as S-MATE
a O
promising O
technology S-CONPRI
with O
advantages O
not O
only O
in O
the O
production S-MANP
of O
customized O
healthcare O
products O
to O
improve O
population S-BIOP
health O
and O
quality S-CONPRI
of O
life O
, O
but O
also O
by O
its O
possibility O
of O
decreasing O
environmental O
impact S-CONPRI
, O
enhancing O
the O
manufacturing B-CONPRI
sustainability E-CONPRI
. O


However O
, O
the O
inherent O
challenges O
of O
3D B-MANP
printing E-MANP
should O
not O
be S-MATE
overlooked O
. O


Aspects O
such O
as S-MATE
surface O
quality S-CONPRI
, O
dimensional B-CHAR
accuracy E-CHAR
and O
the O
mechanical B-CONPRI
properties E-CONPRI
need O
improvement O
to O
allow O
producing O
effective O
high-quality O
products O
. O


Concerning O
surface B-PARA
quality E-PARA
, O
it O
depends O
on O
the O
AM S-MANP
used O
technique O
, O
processing O
conditions O
and O
raw B-MATE
material E-MATE
characteristics O
, O
which O
affect O
the O
thickness O
of O
each O
printed O
layer S-PARA
. O


Powder B-MACEQ
bed E-MACEQ
AM S-MANP
leads O
to O
lower O
surface B-PARA
quality E-PARA
than O
the O
other O
AM B-MANP
techniques E-MANP
, O
due O
to O
the O
presence O
of O
large O
and O
partially O
melted S-CONPRI
powder O
particles S-CONPRI
in O
the O
printed O
piecessurfaces O
. O


Relatively O
to O
the O
thickness O
of O
the O
printed O
layers O
, O
extrusion B-MANP
techniques E-MANP
typically O
lead S-MATE
to O
high O
layer B-PARA
thicknesses E-PARA
due O
to O
the O
large O
diameter S-CONPRI
of O
the O
deposition S-CONPRI
nozzle O
. O


Powder B-MANP
bed I-MANP
fusion E-MANP
and O
vat B-MANP
polymerization E-MANP
origin O
lower O
layer B-PARA
thicknesses E-PARA
due O
to O
the O
ability O
to O
precisely O
focus O
the O
energy B-PARA
beam I-PARA
radius E-PARA
. O


Material B-MANP
jetting E-MANP
techniques O
are O
the O
ones O
that O
produce O
the O
finest O
layer B-PARA
thickness E-PARA
due O
to O
the O
small O
jetted O
droplets S-CONPRI
. O


Dimensional B-CHAR
accuracy E-CHAR
is O
critical O
in O
the O
production S-MANP
of O
dental B-MACEQ
pieces E-MACEQ
, O
because O
these O
must O
fit S-CONPRI
tightly O
the O
needs O
of O
each O
patient O
. O


A O
variety O
of O
issues O
affects O
dimensional B-CHAR
accuracy E-CHAR
. O


The O
work O
of O
Lee O
summarizes O
the O
dimensional B-CHAR
accuracy E-CHAR
in O
different O
manufacturing B-MANP
processes E-MANP
. O


spreading O
compaction/densification O
of O
the O
powder S-MATE
within O
the O
layers O
, O
evaporation S-CONPRI
of O
material S-MATE
by O
laser/heat O
and O
shrinkage S-CONPRI
during O
solidification S-CONPRI
. O


Mechanical B-CONPRI
properties E-CONPRI
are O
influenced O
by O
the O
presence O
of O
defects S-CONPRI
: O
surface B-PARA
quality E-PARA
and O
porosity S-PRO
are O
critical B-PRO
factors E-PRO
. O


Different O
solutions O
have O
been O
proposed O
to O
reduce O
porosity S-PRO
. O


For O
example O
, O
choosing O
ceramic B-MATE
powders E-MATE
with O
an O
adequate O
granulometric B-CHAR
distribution E-CHAR
, O
adding O
dopants S-MATE
or O
a O
viscous O
liquid-forming O
phase S-CONPRI
, O
infiltrating S-CONPRI
the O
sintered S-MANP
body O
with O
vitreous S-CHAR
materials S-CONPRI
and O
applying O
cold/hot B-PARA
isostatic I-PARA
pressure E-PARA
to O
the O
green B-CONPRI
body E-CONPRI
. O


Shrinkage S-CONPRI
is O
also O
a O
concern O
in O
AM B-MANP
processes E-MANP
since O
it O
affects O
significantly O
the O
pieces O
dimensions S-FEAT
and O
may O
lead S-MATE
to O
cracking S-CONPRI
. O


The O
printing O
strategy O
needs O
to O
be S-MATE
optimized O
to O
prevent O
the O
impact S-CONPRI
of O
this O
phenomenon O
. O


Possible O
solutions O
to O
minimize O
this O
issue O
are O
: O
- O
Increasing O
the O
amount O
of O
ceramic S-MATE
particles O
in O
the O
pastes O
, O
while O
keeping O
the O
rheological S-PRO
behavior O
unchanged O
, O
which O
may O
be S-MATE
achieved O
adding O
steric B-MATE
dispersants E-MATE
- O
Adding O
to O
the O
mixture O
particles S-CONPRI
that O
can O
expand O
due O
to O
phase S-CONPRI
transformation O
or O
reaction O
during O
sintering S-MANP
- O
Decreasing O
the O
sintering S-MANP
temperature O
, O
without O
compromising O
density S-PRO
- O
Considering O
it O
in O
the O
CAD S-ENAT
design O
of O
the O
parts O
. O


In O
most O
of O
the O
mentioned O
studies O
along O
this O
work O
, O
the O
ceramic B-MATE
materials E-MATE
are O
composed O
of O
mixtures O
of O
a O
sacrificial O
polymeric O
binder S-MATE
with O
ceramic S-MATE
particles O
for O
the O
production S-MANP
of O
the O
green B-PRO
product E-PRO
. O


This O
means O
that O
additional O
post-processing S-CONPRI
steps O
, O
such O
as S-MATE
sintering O
, O
are O
required O
to O
remove O
the O
binder S-MATE
and O
achieve O
a O
fully B-PARA
dense E-PARA
ceramic S-MATE
component O
. O


This O
is O
true O
for O
the O
majority O
of O
the O
AM B-MANP
technologies E-MANP
with O
few O
exceptions O
such O
as S-MATE
direct O
selective B-MANP
laser I-MANP
sintering E-MANP
and O
selective B-MANP
laser I-MANP
melting E-MANP
techniques O
, O
where O
the O
ceramic S-MATE
particles O
are O
directly O
sintered S-MANP
or O
melted S-CONPRI
, O
respectively O
. O


Dental B-MATE
ceramic I-MATE
pieces E-MATE
made O
by O
direct B-MANP
ink I-MANP
deposition E-MANP
still O
present O
low O
mechanical B-CONPRI
properties E-CONPRI
, O
compared O
to O
other O
conventional O
means O
to O
produce O
molded O
parts O
. O


Limitations O
of O
this O
layering O
technique O
include O
poor O
bonding S-CONPRI
adhesion S-PRO
between O
layers O
and O
the O
occurrence O
of O
porosity S-PRO
. O


The O
protocols S-CONPRI
should O
be S-MATE
as S-MATE
simple O
as S-MATE
possible O
, O
leading O
to O
slurries/inks O
with O
appropriate O
composition S-CONPRI
, O
flow O
consistency S-CONPRI
and O
behavior O
and O
specific O
viscoelastic B-PRO
properties E-PRO
. O


They O
should O
contain O
a O
high O
solid B-PRO
volume I-PRO
fraction E-PRO
not O
only O
to O
minimize O
shrinkage S-CONPRI
during O
drying S-MANP
and O
therefore O
resist O
the O
involved O
stresses O
, O
but O
also O
to O
increase O
the O
density S-PRO
of O
the O
final O
product O
. O


Another O
important O
challenge O
is O
bacteriological B-CHAR
safety E-CHAR
of O
the O
final O
products O
intended O
to O
be S-MATE
in O
close O
contact S-APPL
with O
tissues/organs O
. O


These O
structures O
must O
be S-MATE
able O
to O
endure O
strict O
sterilization O
protocols S-CONPRI
without O
losing O
their O
intrinsic B-CONPRI
properties E-CONPRI
. O


7 O
Final O
considerations O
Ceramic B-MATE
materials E-MATE
play O
an O
important O
role O
as S-MATE
dental O
materials S-CONPRI
. O


Their O
high O
chemical O
and O
mechanical S-APPL
resistance O
, O
as S-MATE
well O
as S-MATE
their O
aesthetic S-CONPRI
properties O
, O
make O
them O
an O
excellent O
option O
to O
replace O
damaged O
dental S-APPL
tissues O
. O


Conventional B-MANP
manufacturing E-MANP
methods O
to O
produce O
ceramic S-MATE
dental O
pieces O
are O
generally O
based O
on O
subtractive S-MANP
techniques O
. O


These O
lead S-MATE
to O
significant O
material S-MATE
and O
tool S-MACEQ
waste O
and O
present O
limitations O
in O
the O
production S-MANP
of O
parts O
with O
complex B-CONPRI
geometry E-CONPRI
. O


The O
rising O
demand O
for O
custom-tailored O
and O
patient O
specific O
dental S-APPL
products O
renders O
dentistry S-APPL
to O
be S-MATE
one O
of O
the O
rapidly O
expanding O
segments O
of O
additive B-MANP
manufacturing E-MANP
technologies O
. O


These O
technologies S-CONPRI
have O
been O
successfully O
used O
in O
many O
production S-MANP
sectors O
and O
present O
many O
advantages O
for O
processing O
dental B-MACEQ
structures E-MACEQ
, O
compared O
with O
subtracting O
technologies S-CONPRI
. O


These O
include O
less O
production S-MANP
steps O
with O
consequent O
impact S-CONPRI
in O
the O
total O
manufacturing S-MANP
time O
, O
lower O
consumables O
and O
raw B-MATE
materials E-MATE
consumption O
, O
and O
adequacy O
to O
produce O
very O
small O
and O
complex O
dental S-APPL
parts O
. O


It O
thus O
opens O
possibility O
to O
mass B-CONPRI
production E-CONPRI
of O
customized O
dental S-APPL
products O
, O
with O
evident O
benefit O
to O
the O
patients O
and/or O
healthcare O
systems O
. O


However O
, O
there O
are O
still O
concerns O
about O
application O
of O
AM S-MANP
to O
ceramics S-MATE
due O
to O
their O
intrinsically O
poor O
mechanical B-CONPRI
properties E-CONPRI
, O
the O
accuracy S-CHAR
of O
the O
obtained O
pieces O
, O
their O
density S-PRO
and O
surface B-MANP
finishing E-MANP
. O


This O
comprehensive O
review O
shows O
that O
, O
dental B-MATE
bioceramics E-MATE
can O
be S-MATE
processed O
through O
AM S-MANP
by O
different O
techniques O
, O
e.g O
. O


material S-MATE
extrusion/jetting O
, O
vat B-MANP
polymerization E-MANP
, O
binder B-MANP
jetting E-MANP
, O
and O
powder B-MANP
bed I-MANP
fusion E-MANP
. O


The O
first O
technology S-CONPRI
is O
the O
most O
widely O
used O
. O


The O
studies O
confirmed O
that O
almost O
any O
shape O
could O
be S-MATE
produced O
, O
with O
different O
degrees O
of O
complexity S-CONPRI
. O


specific O
mechanical B-CONPRI
properties E-CONPRI
, O
different O
degrees B-CHAR
of I-CHAR
porosity E-CHAR
and/or O
density S-PRO
, O
biodegradability S-PRO
, O
osteointegration S-PRO
and O
cytotoxicity S-PRO
. O


Despite O
the O
great O
potential O
for O
dental S-APPL
industry O
, O
the O
application O
of O
AM S-MANP
to O
ceramic B-MATE
dental I-MATE
materials E-MATE
is O
still O
under O
study O
. O


In O
resume O
, O
further O
developments O
of O
AM B-MANP
technology E-MANP
are O
expected O
to O
give O
a O
significant O
contribution O
to O
bring O
production B-CONPRI
costs E-CONPRI
down O
, O
improve O
manufactured S-CONPRI
materials O
properties S-CONPRI
and O
render O
the O
production S-MANP
processes S-CONPRI
more O
efficient O
and O
competitive O
. O


An O
interesting O
approach O
, O
when O
printing O
3D S-CONPRI
complex O
dental B-MATE
ceramic I-MATE
structures E-MATE
, O
could O
be S-MATE
to O
combine O
the O
best O
attributes O
of O
AM B-MANP
technologies E-MANP
with O
conventional B-MANP
surface I-MANP
finishing I-MANP
methods E-MANP
. O


One O
of O
the O
critical O
issues O
in O
orthopaedic S-APPL
regenerative O
medicine S-CONPRI
is O
the O
design S-FEAT
of O
bone B-BIOP
scaffolds E-BIOP
and O
implants S-APPL
that O
replicate O
the O
biomechanical B-PRO
properties E-PRO
of O
the O
host B-BIOP
bones E-BIOP
. O


Porous B-MATE
metals E-MATE
have O
found O
themselves O
to O
be S-MATE
suitable O
candidates O
for O
repairing O
or O
replacing O
the O
damaged B-PRO
bones E-PRO
since O
their O
stiffness S-PRO
and O
porosity S-PRO
can O
be S-MATE
adjusted O
on O
demands O
. O


Another O
advantage O
of O
porous B-MATE
metals E-MATE
lies O
in O
their O
open O
space O
for O
the O
in-growth O
of O
bone S-BIOP
tissue O
, O
hence O
accelerating O
the O
osseointegration S-PRO
process O
. O


The O
fabrication S-MANP
of O
porous B-MATE
metals E-MATE
has O
been O
extensively O
explored O
over O
decades O
, O
however O
only O
limited O
controls O
over O
the O
internal B-PRO
architecture E-PRO
can O
be S-MATE
achieved O
by O
the O
conventional O
processes S-CONPRI
. O


Recent O
advances O
in O
additive B-MANP
manufacturing E-MANP
have O
provided O
unprecedented O
opportunities O
for O
producing O
complex B-CONPRI
structures E-CONPRI
to O
meet O
the O
increasing O
demands O
for O
implants S-APPL
with O
customized O
mechanical S-APPL
performance O
. O


At O
the O
same O
time O
, O
topology B-FEAT
optimization E-FEAT
techniques O
have O
been O
developed O
to O
enable O
the O
internal B-PRO
architecture E-PRO
of O
porous B-MATE
metals E-MATE
to O
be S-MATE
designed O
to O
achieve O
specified O
mechanical B-CONPRI
properties E-CONPRI
at O
will O
. O


Thus O
implants S-APPL
designed S-FEAT
via O
the O
topology B-FEAT
optimization E-FEAT
approach O
and O
produced O
by O
additive B-MANP
manufacturing E-MANP
are O
of O
great O
interest O
. O


This O
paper O
reviews O
the O
state-of-the-art S-CONPRI
of O
topological B-FEAT
design E-FEAT
and O
manufacturing B-MANP
processes E-MANP
of O
various O
types O
of O
porous B-MATE
metals E-MATE
, O
in O
particular O
for O
titanium B-MATE
alloys E-MATE
, O
biodegradable B-MATE
metals E-MATE
and O
shape B-MATE
memory I-MATE
alloys E-MATE
. O


Bone S-BIOP
is O
a O
complex O
tissue O
that O
continually O
undergoes O
dynamic S-CONPRI
biological O
remodelling O
, O
i.e. O
, O
the O
coupled O
process S-CONPRI
whereby O
osteoclasts O
resorb O
mature O
bone S-BIOP
tissue O
followed O
by O
osteoblasts S-BIOP
that O
generate O
new O
bone S-BIOP
to O
maintain O
healthy O
homeostasis O
of O
bone S-BIOP
. O


This O
unique O
feature S-FEAT
of O
bone S-BIOP
underpins O
its O
ability O
to O
remodel O
itself O
to O
repair O
damage S-PRO
. O


However O
, O
when O
a O
bone B-BIOP
defect E-BIOP
exceeds O
a O
critical O
non-healable B-PARA
size E-PARA
, O
external O
intervention O
is O
required O
to O
supplement O
self-healing O
if O
the O
defect S-CONPRI
is O
to O
be S-MATE
bridged O
. O


Despite O
recent O
advances O
in O
biomaterials S-MATE
and O
tissue B-CONPRI
engineering E-CONPRI
, O
repair O
of O
such O
a O
critical-sized O
bone B-BIOP
defect E-BIOP
still O
remains O
a O
challenge O
. O


The O
optimal O
choice O
is O
to O
use O
autograft S-BIOP
. O


However O
, O
harvesting O
autograft B-BIOP
tissue E-BIOP
creates O
the O
morbidity O
associated O
with O
a O
second O
surgical O
site O
. O


The O
insufficiencies O
of O
application O
of O
autograft S-BIOP
and O
allograft B-MATE
tissue E-MATE
have O
led S-APPL
to O
greater O
research S-CONPRI
efforts O
to O
identify O
biomimetic B-MATE
materials E-MATE
and O
structures O
that O
are O
suitable O
for O
skeletal O
repair O
without O
the O
inherent O
problems O
. O


Metals S-MATE
and O
alloys S-MATE
have O
a O
long O
history O
of O
application O
as S-MATE
bone O
implants S-APPL
. O


Among O
them O
, O
the O
use O
of O
stainless B-MATE
steels E-MATE
, O
cobalt S-MATE
based O
alloys S-MATE
, O
and O
titanium S-MATE
and O
its O
alloys S-MATE
are O
well O
established O
due O
to O
their O
good O
biocompatibility S-PRO
, O
satisfactory O
mechanical B-PRO
strength E-PRO
and O
superior O
corrosion B-CONPRI
resistance E-CONPRI
. O


However O
, O
implants S-APPL
made O
of O
these O
materials S-CONPRI
are O
usually O
much O
stiffer O
than O
natural O
bones O
, O
leading O
to O
stress B-PRO
shielding E-PRO
- O
a O
major O
source S-APPL
for O
bone B-CONPRI
resorption E-CONPRI
and O
eventual O
failure S-CONPRI
of O
such O
implants S-APPL
. O


Cortical B-MATE
bone E-MATE
has O
elastic B-PRO
moduli E-PRO
ranging O
from O
3 O
to O
30 O
GPa S-PRO
, O
while O
trabecular O
or O
cancellous B-BIOP
bone E-BIOP
has O
significantly O
lower O
elastic B-PRO
moduli E-PRO
of O
0.02GPa O
. O


Most O
current O
implant S-APPL
materials O
have O
much O
higher O
moduli O
than O
those O
of O
bones O
, O
e.g. O
, O
Ti6Al4V S-MATE
has O
a O
modulus O
of O
around O
110 O
GPa S-PRO
and O
CoCrMo B-MATE
alloys E-MATE
have O
a O
modulus O
of O
around O
210 O
GPa S-PRO
. O


Therefore O
, O
to O
avoid O
stress B-PRO
shielding E-PRO
at O
the O
bone-implant B-FEAT
interface E-FEAT
, O
the O
equivalent O
Young O
's O
modulus O
and O
yield B-PRO
stress E-PRO
have O
to O
be S-MATE
adjusted O
when O
using O
these O
bulk O
materials S-CONPRI
. O


An O
effective O
method O
is O
to O
introduce O
adjustable O
porosity S-PRO
or O
relative B-PRO
density E-PRO
as S-MATE
proposed O
by O
Gibson O
and O
Ashby O
for O
isotropic B-MATE
materials E-MATE
. O


Traditional O
methods O
for O
fabricating S-MANP
open-cell O
porous B-MATE
metals E-MATE
include O
liquid B-CONPRI
state E-CONPRI
processing O
, O
solid B-MATE
state I-MATE
processing E-MATE
, O
electro-deposition S-MANP
and O
vapour B-CHAR
deposition E-CHAR
. O


Although O
the O
shape O
and O
size O
of O
the O
pores S-PRO
can O
be S-MATE
adjusted O
by O
changing O
the O
parameters S-CONPRI
of O
these O
manufacturing B-MANP
processes E-MANP
, O
only O
a O
randomly O
organized O
porous S-PRO
structure O
can O
be S-MATE
achievable O
. O


However O
, O
additive B-MANP
manufacturing E-MANP
technologies O
can O
fabricate S-MANP
porous O
metals S-MATE
with O
predefined O
external O
shape O
and O
internal B-PRO
architecture E-PRO
. O


Metal-based O
additive B-MANP
manufacturing E-MANP
techniques O
, O
such O
as S-MATE
selective O
laser S-ENAT
melting O
and O
electron B-MANP
beam I-MANP
melting E-MANP
, O
are O
computer S-ENAT
controlled O
fabrication S-MANP
process O
based O
on O
layer-wise O
manufacturing S-MANP
principles O
. O


SLM S-MANP
and O
EBM S-MANP
are O
increasingly O
used O
for O
the O
fabrication S-MANP
of O
porous B-MATE
metals E-MATE
with O
complex O
architecture S-APPL
. O


Instead O
of O
using O
electron B-CONPRI
beam E-CONPRI
as S-MATE
the O
energy O
source S-APPL
in O
EBM S-MANP
, O
the O
SLM S-MANP
technology O
uses O
laser B-CONPRI
beam E-CONPRI
with O
adjustable O
wavelength S-CONPRI
. O


Therefore O
, O
EBM S-MANP
can O
only O
process S-CONPRI
conductive O
metals S-MATE
whereas O
SLM S-MANP
can O
process S-CONPRI
polymer S-MATE
or O
ceramics S-MATE
as S-MATE
well O
as S-MATE
metal O
. O


Furthermore O
, O
due O
to O
more O
diffuse O
energy O
, O
EBM S-MANP
process O
has O
larger O
minimum B-PARA
feature I-PARA
size E-PARA
, O
median O
powder B-MATE
particle E-MATE
size O
, O
layer B-PARA
thickness E-PARA
, O
resolution S-PARA
and O
surface B-FEAT
finish E-FEAT
. O


The O
robust O
application O
of O
MAM O
technologies S-CONPRI
requires O
extensive O
material S-MATE
, O
process S-CONPRI
and O
design S-FEAT
knowledge O
, O
specific O
to O
each O
MAM O
technology S-CONPRI
. O


MAM O
system O
behaviour O
is O
subject O
to O
significant O
stochastic B-CONPRI
error E-CONPRI
and O
experimental S-CONPRI
uncertainties O
, O
requiring O
that O
are O
necessary O
to O
simplify O
the O
problem O
. O


Sources O
of O
error S-CONPRI
include O
: O
complex O
and O
transient S-CONPRI
heat B-CONPRI
transfer E-CONPRI
phenomena O
, O
geometric O
effects O
with O
poorly O
defined O
powder S-MATE
thermal O
properties S-CONPRI
. O


MAM O
prediction B-CONPRI
error E-CONPRI
can O
lead S-MATE
to O
excess B-CHAR
melt I-CHAR
pool I-CHAR
temperature E-CHAR
, O
resulting O
in O
undesirable O
microstructure S-CONPRI
, O
residual B-PRO
stress E-PRO
, O
local O
porosity S-PRO
, O
and O
surface B-PRO
roughness E-PRO
. O


Understanding O
the O
effects O
of O
design S-FEAT
decisions O
on O
temperature S-PARA
related O
process B-CONPRI
defects E-CONPRI
is O
critically O
important O
to O
the O
process B-CONPRI
control E-CONPRI
. O


Comprehensive O
reviews O
of O
AM B-MANP
technologies E-MANP
can O
be S-MATE
found O
elsewhere O
. O


Recent O
successes O
in O
orthopaedic S-APPL
regenerative O
medicine S-CONPRI
have O
promised O
an O
exciting O
future O
of O
AM B-MANP
technology E-MANP
. O


The O
world O
's O
first O
additively B-MANP
manufactured E-MANP
mandible O
was O
implanted S-MANP
in O
a O
patient O
by O
Dr. O
Jules O
Poukens O
and O
his O
team O
in O
2012 O
in O
Belgium O
. O


A O
full O
lower B-APPL
jaw I-APPL
implant E-APPL
was O
coated S-APPL
with O
hydroxyapatite S-MATE
and O
implanted S-MANP
in O
an O
83 O
year O
old O
lady O
. O


Skull B-ENAT
reconstructions E-ENAT
with O
AM B-MACEQ
parts E-MACEQ
have O
been O
performed O
successfully O
by O
using O
digital O
design S-FEAT
and O
AM S-MANP
. O


Mertens O
successfully O
reconstructed O
a O
class B-MANS
III I-MANS
defect E-MANS
using O
AM S-MANP
manufactured O
titanium B-APPL
implants E-APPL
, O
which O
provided O
both O
midfacial B-APPL
support E-APPL
and O
a O
graft O
fixture S-MACEQ
. O


Jardini O
in O
Brazil O
designed S-FEAT
and O
AM S-MANP
fabricated O
a O
customized O
implant S-APPL
for O
the O
surgical O
reconstruction S-CONPRI
of O
a O
large O
cranial B-BIOP
defect E-BIOP
. O


Typical O
design S-FEAT
and O
application O
approaches O
of O
porous B-APPL
metallic I-APPL
implants E-APPL
normally O
include O
the O
design S-FEAT
of O
scaffold S-FEAT
, O
AM S-MANP
and O
post-processing S-CONPRI
as S-MATE
illustrated O
in O
1 O
. O


This O
review O
aims O
to O
identify O
the O
current O
status O
and O
the O
future O
directions O
of O
design-oriented O
AM B-MANP
technology E-MANP
in O
producing O
porous B-FEAT
metallic I-FEAT
structures E-FEAT
for O
bone B-CONPRI
tissue I-CONPRI
repair E-CONPRI
, O
with O
a O
particular O
emphasis O
on O
topological B-FEAT
design E-FEAT
of O
internal B-PRO
architecture E-PRO
of O
porous B-MATE
metals E-MATE
for O
bone B-APPL
implants E-APPL
. O


2 O
Structure S-CONPRI
and O
properties S-CONPRI
of O
bone S-BIOP
2.1 O
Structure S-CONPRI
of O
bone S-BIOP
Bone O
is O
a O
natural O
composite S-MATE
containing O
both O
organic B-MACEQ
components E-MACEQ
and O
inorganic O
crystalline O
mineral O
, O
as S-MATE
illustrated O
in O
2 O
. O


The O
structure S-CONPRI
of O
bone S-BIOP
is O
similar O
to O
reinforced S-CONPRI
concrete S-MATE
that O
is O
used O
in O
the O
building B-APPL
industry E-APPL
. O


The O
function O
of O
HA B-MATE
crystals E-MATE
and O
collagen B-MATE
molecules E-MATE
are O
like O
the O
steel S-MATE
rod S-MACEQ
and O
cement S-MATE
to O
concrete S-MATE
: O
one O
part O
provides O
flexibility S-PRO
and O
the O
other O
provides O
strength S-PRO
and O
toughness S-PRO
. O


Type-I O
collagen S-MATE
is O
a O
triple B-FEAT
helix E-FEAT
of O
nm O
in O
diameter S-CONPRI
and O
nm O
in O
length O
. O


It O
is O
the O
primary O
organic B-MACEQ
components E-MACEQ
of O
bone S-BIOP
. O


Other O
non-collagenous O
proteins O
include O
glycoproteins S-MATE
and O
bone S-BIOP
specific O
proteoglycans S-MATE
. O


Hydroxyapatite S-MATE
is O
the O
inorganic O
component S-MACEQ
of O
bone S-BIOP
and O
is O
plate-shaped O
of O
50 O
25 O
nm O
in O
size O
and O
1.5nm O
thick O
. O


The O
HA B-MATE
crystals E-MATE
are O
oriented O
in O
a O
periodic O
array O
in O
the O
fibrils S-BIOP
, O
preferentially O
with O
their O
c S-MATE
axis O
parallel O
to O
the O
collagen B-MATE
fibrils E-MATE
. O


% O
of O
the O
dry O
bone S-BIOP
. O


Bone S-BIOP
has O
a O
hierarchical B-FEAT
structure E-FEAT
. O


The O
hierarchical O
levels O
of O
bone S-BIOP
include O
macroscale S-CONPRI
, O
microscale S-CONPRI
, O
sub-microscale S-FEAT
, O
nanoscale O
, O
and O
sub-nanoscale S-FEAT
. O


The O
macroscale S-CONPRI
level O
represents O
the O
overall O
shape O
of O
the O
bone S-BIOP
. O


Bone S-BIOP
can O
be S-MATE
classified O
as S-MATE
compact O
bone S-BIOP
, O
and O
trabecular B-MATE
bone E-MATE
. O


Compact S-MANP
bone S-BIOP
is O
almost O
solid O
, O
with O
only O
spaces O
for O
osteocytes S-BIOP
, O
canaliculi S-FEAT
, O
blood B-BIOP
vessels E-BIOP
, O
and O
erosion O
cavities O
etc O
. O


There O
are O
large O
spaces O
in O
trabecular B-MATE
bone E-MATE
. O


The O
pores S-PRO
in O
trabecular B-MATE
bone E-MATE
are O
filled O
with O
bone B-BIOP
marrow E-BIOP
, O
and O
the O
porosity S-PRO
varies O
between O
50 O
and O
90 O
% O
. O


The O
building O
block O
of O
compact S-MANP
bone S-BIOP
is O
the O
osteons S-BIOP
, O
which O
are O
of O
the O
size O
ranging O
from O
10 O
to O
500 O
whereas O
the O
trabecular B-MATE
bone E-MATE
is O
made O
of O
a O
porous S-PRO
network O
of O
trabeculae S-MATE
. O


At O
the O
micron- O
and O
nano-scales S-FEAT
, O
aggregated O
type-I O
collagen S-MATE
and O
HA O
form O
the O
collagen B-MATE
fibril E-MATE
. O


The O
reinforced B-MATE
collagen I-MATE
fibre E-MATE
is O
a O
universal O
building O
element S-MATE
for O
both O
compact B-BIOP
and I-BIOP
trabecular I-BIOP
bones E-BIOP
. O


2.2 O
Mechanical B-CONPRI
properties E-CONPRI
of O
bone S-BIOP
Mechanical O
properties S-CONPRI
of O
bone S-BIOP
vary O
significantly O
with O
age O
, O
anatomical O
site O
and O
bone S-BIOP
quality O
. O


Among O
the O
various O
biomechanical B-PRO
properties E-PRO
of O
bone S-BIOP
, O
elastic B-PRO
modulus E-PRO
has O
attracted O
the O
most O
research S-CONPRI
interest O
because O
of O
its O
critical O
importance O
for O
characterizing O
various O
bone S-BIOP
pathologies O
and O
guiding O
artificial O
implant S-APPL
design S-FEAT
. O


The O
elastic B-PRO
modulus E-PRO
and O
strength S-PRO
of O
bone S-BIOP
are O
anisotropic S-PRO
. O


Compact S-MANP
bone S-BIOP
is O
both O
stronger O
and O
stiffer O
when O
loaded O
longitudinally O
along O
the O
diaphyseal B-FEAT
axis E-FEAT
than O
the O
radial O
transverse O
directions O
. O


Trabecular B-MATE
bone E-MATE
is O
an O
anisotropic S-PRO
and O
porous B-MATE
composite E-MATE
. O


Like O
many O
biological B-MATE
materials E-MATE
, O
trabecular B-MATE
bone E-MATE
displays O
time-dependent O
behaviour O
as S-MATE
well O
as S-MATE
damage O
susceptibility S-PRO
during O
cyclic B-PRO
loading E-PRO
. O


The O
mechanical B-CONPRI
properties E-CONPRI
of O
trabecular B-MATE
bone E-MATE
depend O
on O
not O
only O
the O
porosity S-PRO
, O
but O
also O
the O
architectural O
arrangement O
of O
the O
individual O
trabeculae S-MATE
. O


The O
physical O
and O
mechanical B-CONPRI
properties E-CONPRI
of O
human O
bone S-BIOP
are O
summarized O
in O
1 O
. O


2.3 O
Requirements O
for O
the O
design S-FEAT
of O
orthopaedic B-APPL
implants E-APPL
A O
successful O
porous B-APPL
metallic I-APPL
implant E-APPL
would O
restore O
the O
function O
of O
bone S-BIOP
and O
promote O
regeneration S-CONPRI
of O
bone S-BIOP
tissue O
at O
the O
damaged O
site O
. O


An O
ideal O
bone B-BIOP
scaffold E-BIOP
should O
possess O
the O
following O
characteristics O
: O
biocompatibility S-PRO
; O
suitable O
surface S-CONPRI
for O
cell B-FEAT
attachment E-FEAT
, O
proliferation O
and O
differentiation O
; O
highly O
porous S-PRO
with O
an O
interconnected O
pore S-PRO
network O
for O
cell B-CONPRI
ingrowth E-CONPRI
and O
transport S-CHAR
of O
nutrients O
and O
metabolic B-CONPRI
waste E-CONPRI
; O
mechanical B-CONPRI
properties E-CONPRI
to O
match O
the O
requirements O
of O
the O
surrounding O
tissues O
to O
reduce O
or O
eliminate O
stress B-PRO
shielding E-PRO
, O
and O
to O
meet O
anatomic O
loading O
requirements O
to O
avoid O
mechanical B-PRO
failure E-PRO
. O


Porous B-MATE
metals E-MATE
are O
implanted S-MANP
to O
repair O
bone B-BIOP
defects E-BIOP
of O
critical O
size O
and O
, O
in O
most O
cases O
, O
serve O
as S-MATE
load-bearing O
devices O
. O


Bone S-BIOP
is O
usually O
anisotropic S-PRO
with O
different O
stiffness S-PRO
and O
strength S-PRO
in O
different O
directions O
, O
but O
normally O
there O
are O
no O
extremely O
weak O
directions O
. O


Therefore O
, O
suitable O
porous B-MATE
metals E-MATE
will O
approximate O
the O
stiffness S-PRO
of O
surrounding O
bones O
, O
making O
them O
effective O
for O
load O
transfer O
and O
alleviating O
the O
stress B-PRO
shielding E-PRO
effect O
. O


The O
key O
characteristics O
to O
design S-FEAT
porous O
metallic S-MATE
implants S-APPL
include O
the O
careful O
selection O
of O
porosity S-PRO
, O
pore B-PARA
size E-PARA
, O
and O
pore S-PRO
interconnectivity O
, O
aiming O
to O
achieve O
satisfactory O
clinical O
outcomes O
. O


These O
structural O
features O
have O
a O
profound O
effect O
on O
mechanical B-CONPRI
properties E-CONPRI
and O
biological O
performance S-CONPRI
of O
the O
metallic S-MATE
implants S-APPL
. O


Bone B-CONPRI
regeneration E-CONPRI
in O
porous B-APPL
implants E-APPL
in O
vivo O
involves O
recruitment O
and O
penetration S-CONPRI
of O
cells S-APPL
from O
the O
surrounding O
bone S-BIOP
tissue O
and O
vascularization S-CONPRI
. O


Higher O
porosity S-PRO
may O
facilitate O
these O
processes S-CONPRI
and O
benefit O
the O
bone B-CONPRI
ingrowth E-CONPRI
. O


For O
instance O
, O
more O
bone B-CONPRI
ingrowth E-CONPRI
was O
found O
in O
porous S-PRO
titanium O
coatings S-APPL
of O
higher O
porosity S-PRO
after O
the O
implants S-APPL
were O
placed O
into O
a O
canine B-APPL
model E-APPL
for O
8 O
weeks O
. O


Similarly O
, O
bone B-CONPRI
ingrowth E-CONPRI
was O
shown O
to O
be S-MATE
deeper O
and O
greater O
in O
porous S-PRO
polymer S-MATE
scaffolds O
of O
higher O
porosity S-PRO
. O


The O
influence O
of O
pore B-PARA
size E-PARA
on O
the O
bone B-CONPRI
ingrowth E-CONPRI
is O
still O
controversial O
in O
literature O
. O


The O
optimal O
pore B-PARA
size E-PARA
for O
mineralized O
bone B-CONPRI
ingrowth E-CONPRI
is O
claimed O
to O
be S-MATE
100in O
the O
research S-CONPRI
by O
Itala O
. O


They O
implanted S-MANP
triangle-shaped O
titanium B-APPL
implants E-APPL
of O
different O
plate O
thickness O
with O
pore B-PARA
size E-PARA
ranging O
from O
50 O
to O
125 O
into O
rabbit O
femur O
and O
found O
that O
there O
was O
no O
clear O
lower O
limit S-CONPRI
of O
pore B-PARA
size E-PARA
for O
consistent O
bone B-CONPRI
ingrowth E-CONPRI
. O


Recently O
Braem O
assessed O
the O
feasibility S-CONPRI
of O
early O
bone B-CONPRI
ingrowth E-CONPRI
into O
a O
predominantly O
microporous S-PRO
Ti O
coating S-APPL
in O
the O
compact S-MANP
bone S-BIOP
of O
rabbit O
tibiae O
and O
found O
that O
new O
bone S-BIOP
formed O
in O
micropores O
of O
less O
than O
10 O
Large O
pores S-PRO
are O
believed O
to O
favour O
vascularization S-CONPRI
. O


Bai O
suggested O
an O
upper O
limit S-CONPRI
of O
pore B-PARA
size E-PARA
for O
vascularization S-CONPRI
, O
400 O
beyond O
which O
no O
significant O
difference O
was O
observed O
with O
increasing O
pore B-PARA
size E-PARA
. O


Kuboki O
found O
that O
, O
when O
the O
pore B-PARA
size E-PARA
ranged O
from O
300 O
to O
400 O
the O
implantation S-MANP
of O
porous B-FEAT
hydroxyapatite I-FEAT
scaffolds E-FEAT
into O
rats O
showed O
higher O
alkaline B-MATE
phosphates E-MATE
activity O
, O
osteocalcin S-BIOP
content O
and O
bone B-CONPRI
ingrowth E-CONPRI
. O


However O
, O
Naoya O
implanted S-MANP
300 O
600 O
and O
900 O
AM S-MANP
manufactured O
porous S-PRO
Ti O
scaffolds S-FEAT
into O
rabbit O
tibia S-BIOP
and O
they O
found O
600 O
and O
900 O
scaffolds S-FEAT
demonstrated O
significantly O
higher O
bone B-CONPRI
ingrowth E-CONPRI
than O
300 O
scaffolds S-FEAT
. O


In O
addition O
to O
vascularization S-CONPRI
, O
specific O
surface B-PARA
area E-PARA
of O
scaffolds S-FEAT
is O
another O
essential O
factor O
with O
respect O
to O
fixation O
ability O
. O


Scaffolds S-FEAT
with O
smaller O
pores S-PRO
are O
considered O
to O
have O
larger O
surface B-PARA
area E-PARA
and O
therefore O
more O
space O
for O
bone B-CONPRI
tissue I-CONPRI
ingrowth E-CONPRI
. O


Another O
important O
feature S-FEAT
of O
bone B-APPL
implants E-APPL
is O
the O
permeability S-PRO
of O
the O
porous B-MATE
metal E-MATE
since O
the O
transportation O
of O
cells S-APPL
, O
nutrients O
and O
growth O
factors O
require O
the O
flow O
of O
blood O
through O
the O
porous B-FEAT
scaffolds E-FEAT
. O


In O
simple S-MANP
terms O
, O
permeability S-PRO
is O
characterised O
by O
using O
gradient O
pressure S-CONPRI
to O
push O
liquid O
through O
porous B-MATE
material E-MATE
. O


Zhang O
stated O
that O
permeability S-PRO
may O
influence O
vascular B-CONPRI
invasion E-CONPRI
and O
the O
supply O
of O
nutrients O
required O
to O
sustain O
cell B-CHAR
growth E-CHAR
and O
may O
also O
provide O
an O
outlet O
for O
the O
removal O
of O
cell B-BIOP
debris E-BIOP
, O
thereby O
increasing O
its O
osteoconductive B-PRO
potential E-PRO
. O


High O
permeability S-PRO
of O
titanium B-APPL
implants E-APPL
enhances O
the O
osseointegration S-PRO
process O
. O


Further O
research S-CONPRI
on O
the O
effect O
of O
permeability S-PRO
of O
porous B-APPL
metallic I-APPL
implants E-APPL
is O
in O
demand O
. O


In O
summary O
, O
porosity S-PRO
, O
pore B-PARA
size E-PARA
and O
pore S-PRO
interconnectivity O
are O
key O
factors O
that O
will O
significantly O
influence O
the O
mechanical B-CONPRI
properties E-CONPRI
and O
biological B-CONPRI
performance I-CONPRI
of I-CONPRI
scaffolds E-CONPRI
such O
as S-MATE
bone O
ingrowth O
and O
transportation O
of O
cells S-APPL
and O
nutrients O
. O


For O
example O
, O
increasing O
the O
porosity S-PRO
may O
enhance O
the O
biological B-CONPRI
processes E-CONPRI
, O
but O
it O
can O
decrease O
the O
stiffness S-PRO
and O
strength S-PRO
drastically O
. O


Therefore O
, O
finding O
the O
optimal O
topologies S-CONPRI
for O
scaffolds S-FEAT
is O
of O
critical O
importance O
. O


However O
, O
conventional O
CAD-based B-ENAT
design E-ENAT
techniques O
are O
inefficient O
and O
usually O
fail O
to O
obtain O
the O
optimal O
scaffold S-FEAT
design S-FEAT
because O
a O
prohibitively O
large O
number O
of O
trials O
would O
be S-MATE
required O
in O
order O
to O
achieve O
a O
balanced O
performance S-CONPRI
, O
e.g. O
, O
desirable O
stiffness S-PRO
and O
good O
permeability S-PRO
. O


On O
the O
contrary O
, O
topology B-FEAT
optimization E-FEAT
techniques O
are O
capable O
of O
quickly O
finding O
the O
optimal O
topologies S-CONPRI
which O
satisfy O
multiple O
objectives O
and O
constraints O
simultaneously O
to O
provide O
site-specific O
biological O
performance S-CONPRI
. O


3 O
Topological B-FEAT
design E-FEAT
of O
porous B-FEAT
metallic I-FEAT
structures E-FEAT
for O
orthopaedic B-APPL
implants E-APPL
3.1 O
Porous B-APPL
metallic I-APPL
implants E-APPL
and O
topology B-FEAT
optimization E-FEAT
techniques O
As S-MATE
mentioned O
previously O
, O
bone S-BIOP
is O
a O
3D B-FEAT
inhomogeneous I-FEAT
structure E-FEAT
with O
elaborate O
features O
from O
macro-to O
nano-scales S-FEAT
. O


While O
it O
is O
impossible O
, O
and O
perhaps O
unnecessary O
, O
to O
recreate O
all O
details O
of O
natural O
bone S-BIOP
in O
the O
porous B-APPL
metallic I-APPL
implant E-APPL
, O
ideally O
the O
implant S-APPL
should O
have O
similar O
hierarchical O
configurations O
on O
multiple O
scales O
. O


It O
is O
essential O
that O
the O
implant S-APPL
should O
possess O
properties S-CONPRI
similar O
to O
the O
host B-BIOP
bone E-BIOP
and O
the O
ambient B-MATE
tissue E-MATE
. O


This O
calls O
for O
a O
well-established O
design S-FEAT
methodology O
integrating O
structural O
stiffness S-PRO
with O
fluid B-PRO
permeability E-PRO
to O
allow O
the O
implant S-APPL
to O
have O
both O
adequate O
rigidity O
to O
resist O
the O
physical O
loading O
and O
sufficient O
permeability S-PRO
to O
transfer O
cells S-APPL
, O
nutrients O
, O
etc O
. O


Fully O
solid O
metals S-MATE
, O
e.g O
. O


Such O
a O
stiffness S-PRO
mismatch O
is O
regarded O
as S-MATE
one O
of O
the O
most O
significant O
problems O
in O
implant S-APPL
design S-FEAT
as S-MATE
the O
resulting O
stress B-PRO
shielding E-PRO
would O
often O
lead S-MATE
to O
implantation S-MANP
failures O
. O


Recently O
, O
porous B-MATE
metals E-MATE
were O
used O
in O
orthopaedic S-APPL
surgeries O
to O
replace O
damaged B-PRO
bones E-PRO
. O


Porous B-FEAT
scaffolds E-FEAT
are O
geometrically O
similar O
to O
natural O
hard O
tissues O
which O
are O
composed O
of O
constituting O
materials S-CONPRI
penetrated O
by O
interconnected O
pores S-PRO
. O


Porous B-MATE
metals E-MATE
can O
be S-MATE
designed O
to O
duplicate O
the O
properties S-CONPRI
of O
bones O
if O
their O
structures O
could O
be S-MATE
designed O
digitally O
and O
fabricated S-CONPRI
using O
advanced O
manufacturing B-MANP
technology E-MANP
. O


Conventional O
porous B-FEAT
scaffolds E-FEAT
typically O
consist O
of O
a O
vast O
number O
of O
randomly O
shaped O
pores S-PRO
in O
different O
sizes O
and O
therefore O
it O
is O
almost O
impossible O
to O
quantitatively S-CONPRI
analyse O
their O
properties S-CONPRI
. O


To O
obtain O
a O
simplified O
model S-CONPRI
, O
researchers O
usually O
assume O
that O
scaffolds S-FEAT
are O
constructed O
of O
periodically-repeating O
unit B-CONPRI
cells E-CONPRI
along O
all O
directions O
and O
the O
architecture S-APPL
of O
the O
micro B-CONPRI
unit I-CONPRI
cells E-CONPRI
can O
distinctly O
define O
the O
macro S-FEAT
properties O
of O
the O
scaffolds S-FEAT
. O


Typical O
traditional O
design S-FEAT
strategies O
of O
periodic O
bone B-BIOP
scaffolds E-BIOP
include O
Computer B-ENAT
Aided I-ENAT
Design E-ENAT
, O
image-based O
design S-FEAT
and O
implicit B-FEAT
surfaces E-FEAT
, O
as S-MATE
illustrated O
in O
3 O
. O


CAD-based B-ENAT
design E-ENAT
are O
obtained O
by O
using O
various O
CAD S-ENAT
tools O
. O


Computer-aided O
system O
for O
tissue O
scaffolds S-FEAT
is O
a O
further O
development O
based O
on O
these O
scaffold S-FEAT
libraries O
, O
aiming O
to O
efficiently O
automate O
the O
entire O
design B-CONPRI
process E-CONPRI
for O
desired O
topologies S-CONPRI
. O


Bio-inspired B-FEAT
design E-FEAT
is O
an O
alternative O
to O
improve O
the O
mechanical S-APPL
performance O
of O
bone B-BIOP
scaffolds E-BIOP
and O
enrich O
the O
scaffold S-FEAT
library O
. O


Other O
CAD-based O
approaches O
may O
also O
be S-MATE
used O
in O
designing O
scaffolds S-FEAT
. O


Image-based O
design S-FEAT
, O
as S-MATE
proposed O
by O
Hollister O
, O
is O
based O
on O
Computed B-CHAR
Tomography E-CHAR
or O
Magnetic B-CHAR
Resonance I-CHAR
Image I-CHAR
data E-CHAR
for O
reconstruction S-CONPRI
of O
a O
defect S-CONPRI
. O


It O
uses O
Boolean O
combination O
of O
defect S-CONPRI
image O
and O
architecture S-APPL
image O
to O
create O
3D S-CONPRI
scaffold O
image S-CONPRI
. O


Implicit B-FEAT
surface E-FEAT
modelling O
uses O
single O
mathematical B-CONPRI
equations E-CONPRI
to O
freely O
introduce O
pore S-PRO
shapes O
such O
as S-MATE
triply O
periodic O
minimal O
surfaces S-CONPRI
, O
which O
is O
highly O
flexible O
in O
designing O
scaffolds S-FEAT
. O


CAD-based B-ENAT
randomization E-ENAT
approach O
starts O
from O
cell B-FEAT
elements E-FEAT
to O
fill O
a O
specific B-FEAT
volume E-FEAT
in O
computer S-ENAT
software O
, O
where O
standard S-CONPRI
cell B-FEAT
elements E-FEAT
are O
usually O
packaged O
while O
new O
cell B-FEAT
elements E-FEAT
can O
also O
be S-MATE
created O
. O


This O
method O
can O
effectively O
imitate O
real O
bones O
by O
the O
randomization O
process S-CONPRI
, O
thus O
promoting O
the O
bone B-MACEQ
attachment E-MACEQ
and O
bone B-BIOP
cell E-BIOP
in-growth O
, O
as S-MATE
well O
as S-MATE
increasing O
the O
damage B-PRO
tolerance E-PRO
. O


While O
the O
aforementioned O
methods O
enable O
scaffolds S-FEAT
to O
obtain O
desirable O
stiffness S-PRO
and O
permeability S-PRO
, O
these O
approaches O
demand O
a O
vast O
number O
of O
attempts O
to O
achieve O
anticipated O
properties S-CONPRI
. O


One O
of O
the O
main O
challenges O
in O
the O
application O
of O
porous B-FEAT
scaffolds E-FEAT
to O
orthopaedic B-APPL
implants E-APPL
is O
the O
adaptation O
of O
their O
mechanical S-APPL
and O
biomedical S-APPL
properties O
to O
those O
of O
natural O
bones O
. O


The O
implanted S-MANP
scaffolds O
are O
placed O
in O
a O
complex O
environment O
and O
their O
performance S-CONPRI
is O
affected O
by O
many O
factors O
. O


Some O
of O
them O
such O
as S-MATE
high O
permeability S-PRO
and O
good O
stiffness S-PRO
are O
competing O
with O
each O
other O
since O
a O
larger O
pore B-PARA
size E-PARA
is O
obtained O
usually O
at O
the O
cost O
of O
a O
lower O
mechanical B-PRO
strength E-PRO
. O


Hence O
, O
to O
increase O
the O
mass O
transfer O
, O
while O
retaining O
a O
strong O
supporting O
framework S-CONPRI
, O
there O
is O
a O
need O
to O
maintain O
a O
delicate O
trade-off O
between O
the O
porosity S-PRO
of O
the O
fabricated S-CONPRI
scaffold O
and O
its O
strength S-PRO
. O


Topology B-FEAT
optimization E-FEAT
a O
mathematical S-CONPRI
method O
capable O
of O
rearranging O
the O
materials S-CONPRI
to O
attain O
desired O
properties S-CONPRI
while O
satisfying O
prescribed O
constraints O
can O
complement O
the O
trial-and-error S-CONPRI
approach O
and O
provide O
a O
powerful O
tool S-MACEQ
to O
design S-FEAT
complex O
scaffolds S-FEAT
with O
features O
on O
multiple O
scales O
. O


It O
is O
a O
branch O
of O
computational O
mechanics O
and O
was O
originally O
developed O
in O
structural B-CONPRI
engineering E-CONPRI
. O


It O
has O
been O
widely O
used O
for O
designing O
structures O
and O
materials S-CONPRI
for O
desirable O
mechanical S-APPL
performance O
and O
physical B-PRO
properties E-PRO
. O


Through O
two O
decades O
of O
development O
, O
this O
method O
has O
gone O
far O
beyond O
the O
traditional O
structural B-CONPRI
engineering E-CONPRI
context O
. O


Typically O
, O
there O
are O
two O
ways O
to O
define O
a O
structure S-CONPRI
in O
topology B-FEAT
optimization E-FEAT
. O


The O
first O
is O
a O
point-by-point O
description O
in O
which O
a O
void S-CONPRI
or O
a O
solid O
phase S-CONPRI
in O
a O
local B-CONPRI
element E-CONPRI
is O
represented O
by O
an O
elemental O
density S-PRO
. O


The O
Evolutionary O
Structural B-CONPRI
Optimization E-CONPRI
and O
the O
Solid O
Isotropic B-MATE
Material E-MATE
with O
Penalization O
methods O
use O
this O
type O
of O
description O
and O
have O
gained O
considerable O
success O
in O
solving O
a O
wide O
range S-PARA
of O
engineering S-APPL
optimization O
problems O
. O


In O
the O
field O
of O
computational O
material S-MATE
design S-FEAT
, O
the O
topology B-FEAT
optimization E-FEAT
approach O
is O
termed O
as S-MATE
an O
homogenizationmethod O
because O
the O
homogenization B-MANP
method E-MANP
is O
used O
to O
calculate O
the O
effective O
properties S-CONPRI
of O
a O
unit B-CONPRI
cell E-CONPRI
and O
the O
material S-MATE
distribution S-CONPRI
is O
rearranged O
through O
topology B-FEAT
optimization E-FEAT
to O
enable O
the O
material S-MATE
to O
attain O
target O
properties S-CONPRI
. O


The O
seminal O
work O
of O
inverse O
homogenization S-MANP
was O
conducted O
by O
Sigmund O
in O
the O
1990s O
for O
the O
design S-FEAT
of O
materials S-CONPRI
with O
prescribed O
elastic S-PRO
properties O
. O


Thereafter O
, O
great O
achievements O
were O
obtained O
in O
the O
design S-FEAT
of O
exceptional O
material B-CONPRI
properties E-CONPRI
including O
negative O
thermal B-PRO
expansion I-PRO
coefficient E-PRO
and O
negative O
refraction B-PRO
index E-PRO
. O


Later O
, O
this O
method O
was O
extended O
to O
the O
design S-FEAT
of O
scaffold S-FEAT
materials S-CONPRI
with O
their O
stiffness B-PRO
matrices E-PRO
matching O
those O
of O
anisotropic S-PRO
native O
bones O
. O


By O
using O
the O
SIMP O
based O
structural B-CONPRI
optimization E-CONPRI
, O
Guest O
and O
Prevost O
developed O
a O
topology B-FEAT
optimization E-FEAT
technique O
to O
find O
a O
scaffold S-FEAT
with O
pores S-PRO
in O
the O
shape O
of O
a O
Schwartz B-BIOP
primitive I-BIOP
structure E-BIOP
, O
resulting O
in O
the O
maximum O
permeability S-PRO
. O


They O
also O
combined O
bulk B-PRO
modulus E-PRO
and O
permeability S-PRO
in O
a O
single O
objective O
function O
and O
tailored O
these O
two O
competing O
properties S-CONPRI
in O
a O
multi-physics O
optimization S-CONPRI
problem O
. O


Using O
a O
similar O
density-based O
optimization S-CONPRI
method O
, O
scaffolds S-FEAT
with O
elastic B-CONPRI
tensors E-CONPRI
similar O
to O
those O
of O
natural O
bones O
were O
designed S-FEAT
; O
and O
the O
performance S-CONPRI
of O
these O
scaffolds S-FEAT
in O
subsequent O
tissue O
ingrowth O
was O
investigated O
. O


It O
is O
found O
that O
bone S-BIOP
remodeling O
is O
at O
its O
best O
when O
the O
scaffold S-FEAT
elastic B-CONPRI
tensor E-CONPRI
matches O
or O
is O
slightly O
higher O
than O
the O
elastic S-PRO
properties O
of O
the O
host B-BIOP
bone E-BIOP
. O


The O
last O
row O
in O
3 O
shows O
the O
porous S-PRO
structures O
with O
the O
maximum O
bulk O
and O
shear B-PRO
moduli E-PRO
, O
respectively O
, O
at O
a O
given O
porosity S-PRO
. O


These O
unit B-CONPRI
cells E-CONPRI
were O
obtained O
using O
the O
Bi-directional O
Evolutionary O
Structural B-CONPRI
Optimization I-CONPRI
method E-CONPRI
, O
which O
shows O
faster O
convergence O
and O
unambiguous O
material S-MATE
definition O
. O


The O
BESO B-CONPRI
method E-CONPRI
, O
which O
allows O
the O
material S-MATE
to O
be S-MATE
added O
and O
removed O
simultaneously O
during O
the O
optimization S-CONPRI
process O
, O
is O
an O
extension O
of O
the O
original O
evolutionary O
ESO O
method O
proposed O
by O
Xie O
and O
Steven O
. O


As S-MATE
shown O
in O
3 O
on O
topology B-FEAT
optimization E-FEAT
, O
various O
unit B-CONPRI
cells E-CONPRI
with O
maximal O
bulk B-PRO
modulus E-PRO
, O
maximal O
shear B-PRO
modulus E-PRO
, O
prescribed O
stiffness S-PRO
ratios O
in O
three O
directions O
, O
and O
functionally B-FEAT
graded I-FEAT
structures E-FEAT
can O
be S-MATE
obtained O
through O
the O
BESO B-CONPRI
method E-CONPRI
. O


The O
second O
class O
of O
topology B-FEAT
optimization E-FEAT
methods O
, O
represented O
by O
the O
level-set O
algorithm S-CONPRI
, O
focus O
on O
tracking O
phase B-CONPRI
boundaries E-CONPRI
. O


The O
level-set O
method O
provides O
an O
effective O
technique O
to O
represent O
smooth B-FEAT
boundaries E-FEAT
and O
to O
control O
topology S-CONPRI
changes O
. O


A O
variational O
level-set O
technique O
for O
periodic O
material S-MATE
design S-FEAT
problems O
governed O
by O
Navierand O
Maxwell O
's O
equations O
was O
developed O
to O
attain O
material S-MATE
with O
maximal O
permeability S-PRO
. O


Level-set O
topology B-FEAT
optimization E-FEAT
enables O
the O
no-slip O
boundary B-CONPRI
condition E-CONPRI
of O
fluids S-MATE
in O
Stokes O
flow O
to O
be S-MATE
naturally O
satisfied O
. O


Periodic O
structures O
of O
scaffolds S-FEAT
with O
the O
maximal O
effective O
diffusivity S-CHAR
aimed O
at O
providing O
an O
ideal O
environment O
for O
nutrient O
transportation O
were O
studied O
by O
a O
level-set O
based O
optimization S-CONPRI
method O
. O


There O
have O
been O
tremendous O
advances O
in O
recent O
years O
in O
the O
area S-PARA
of O
using O
topology B-FEAT
optimization E-FEAT
techniques O
to O
design S-FEAT
multi-functional O
materials S-CONPRI
with O
periodic O
structures O
, O
as S-MATE
shown O
in O
a O
comprehensive O
review O
by O
Cadman O
. O


Several O
of O
these O
developments O
are O
directly O
related O
to O
the O
design S-FEAT
of O
scaffolds S-FEAT
. O


Both O
stiffness S-PRO
and O
diffusive O
transport S-CHAR
properties S-CONPRI
were O
considered O
by O
Hollister O
and O
Challis O
. O


Using O
topology B-FEAT
optimization E-FEAT
, O
Hollister O
and O
co-workers O
also O
created O
an O
interbody O
fusion S-CONPRI
cage O
for O
improved O
arthrodesis S-APPL
. O


The O
outcomes O
of O
their O
research S-CONPRI
were O
used O
in O
clinic S-APPL
to O
support S-APPL
bone B-CONPRI
regeneration E-CONPRI
for O
craniofacial B-MANP
reconstruction E-MANP
. O


3.2 O
Constraints O
in O
structural B-FEAT
design E-FEAT
for O
additive B-MANP
manufacturing E-MANP
Although O
AM S-MANP
can O
theoretically O
produce O
structures O
in O
any O
shape O
, O
the O
quality S-CONPRI
of O
the O
structures O
may O
vary O
significantly O
depending O
on O
the O
design S-FEAT
and O
fabrication S-MANP
parameters O
. O


Therefore O
, O
it O
is O
necessary O
to O
consider O
the O
processability O
of O
the O
designed S-FEAT
parts O
during O
the O
topological B-FEAT
design E-FEAT
process S-CONPRI
, O
including O
the O
constraints O
and O
limitations O
of O
AM B-MANP
technologies E-MANP
. O


However O
, O
there O
is O
still O
limited O
research S-CONPRI
on O
creating O
design S-FEAT
guidelines O
to O
achieve O
this O
goal O
. O


Kranz O
experimentally O
investigated O
the O
restrictions O
of O
Laser B-MANP
Additive I-MANP
Manufacturing E-MANP
of O
Ti6Al4V S-MATE
and O
presented O
a O
comprehensive O
structured O
catalogue O
. O


In O
their O
research S-CONPRI
, O
restrictions O
and O
recommendations O
were O
presented O
based O
on O
experimental S-CONPRI
measurements O
of O
different O
characteristics O
such O
as S-MATE
cavities O
, O
walls O
, O
bores O
, O
gap O
, O
hollow O
cylinder O
, O
overhangs S-PARA
and O
support B-FEAT
structures E-FEAT
. O


They O
also O
found O
that O
the O
quality S-CONPRI
of O
AM B-MACEQ
parts E-MACEQ
was O
highly O
dependent O
on O
the O
materials S-CONPRI
, O
machines S-MACEQ
and O
process B-CONPRI
parameters E-CONPRI
. O


Therefore O
, O
similar O
guidelines O
could O
be S-MATE
achieved O
through O
similar O
methodology S-CONPRI
on O
different O
material S-MATE
systems O
. O


Among O
many O
parameters S-CONPRI
, O
frequently O
discussed O
one O
includes O
overhanging B-CONPRI
structures E-CONPRI
, O
which O
may O
lead S-MATE
to O
some O
undesirable O
defects S-CONPRI
. O


In O
an O
AM B-MANP
process E-MANP
, O
the O
overhanging B-CONPRI
structure E-CONPRI
is O
not O
supported O
by O
solidified O
section O
or O
bottom O
substrate S-MATE
when O
it O
is O
being O
built O
. O


Therefore O
, O
the O
overhanging B-CONPRI
structure E-CONPRI
is O
strongly O
influenced O
by O
the O
orientation S-CONPRI
of O
building O
- O
) O
. O


Therefore O
, O
the O
critical B-FEAT
fabrication I-FEAT
angle E-FEAT
is O
of O
great O
importance O
since O
it O
determines O
the O
form O
of O
overhanging B-CONPRI
structure E-CONPRI
, O
hence O
the O
processability O
. O


4 O
shows O
the O
sketch O
of O
a O
circular O
pore S-PRO
with O
overhanging B-PARA
arc E-PARA
AB S-MATE
, O
which O
can O
be S-MATE
processable O
if O
the O
fabrication S-MANP
angle O
were O
larger O
than O
the O
critical O
value O
c. O
Otherwise O
, O
supporting O
structures O
have O
to O
be S-MATE
used O
, O
which O
are O
normally O
avoided O
to O
prevent O
damage S-PRO
of O
parts O
in O
post-processing S-CONPRI
. O


A O
better O
choice O
in O
design S-FEAT
is O
to O
adopt O
structures O
with O
special O
geometrical O
arrangement O
such O
as S-MATE
an O
octahedral O
lattice S-CONPRI
, O
whose O
lateral O
schematic O
is O
shown O
in O
4 O
. O


When O
the O
downward O
sloping O
surface S-CONPRI
CD S-MATE
has O
a O
larger O
fabrication S-MANP
angle O
than O
the O
critical B-FEAT
angle E-FEAT
c S-MATE
, O
no O
supporting O
structures O
are O
required O
. O


There O
were O
also O
attempts O
to O
design S-FEAT
structures O
so O
that O
they O
could O
be S-MATE
fabricated O
using O
AM S-MANP
without O
support S-APPL
. O


This O
approach O
is O
interesting O
and O
useful O
, O
but O
may O
not O
be S-MATE
generally O
applicable O
to O
scaffold S-FEAT
designs S-FEAT
. O


Other O
researchers O
examined O
the O
suitability O
of O
using O
SIMP O
and O
BESO B-CONPRI
topology I-CONPRI
optimization I-CONPRI
algorithms E-CONPRI
to O
design S-FEAT
structures O
for O
AM S-MANP
. O


4 O
Current O
status O
of O
AM S-MANP
and O
topology B-FEAT
optimization E-FEAT
in O
producing O
porous B-FEAT
metallic I-FEAT
structures E-FEAT
AM B-MANP
technologies E-MANP
are O
superior O
to O
conventional O
fabrication S-MANP
techniques O
for O
producing O
porous B-APPL
metallic I-APPL
implants E-APPL
with O
complex O
and O
customized O
structures O
, O
as S-MATE
shown O
in O
5 O
. O


In O
addition O
to O
the O
geometric O
flexibility S-PRO
, O
composites S-MATE
with O
two O
or O
more O
phases O
can O
be S-MATE
manufactured O
. O


These O
advantages O
enable O
AM S-MANP
to O
become O
a O
promising O
tool S-MACEQ
for O
the O
production S-MANP
of O
biomedical B-MACEQ
implant I-MACEQ
devices E-MACEQ
, O
controlled O
drug O
delivery O
systems O
, O
and O
engineered O
tissues O
. O


Examples O
include O
artificial B-APPL
joints E-APPL
and O
load-bearing S-FEAT
implants S-APPL
produced O
by O
AM S-MANP
using O
biocompatible B-MATE
materials E-MATE
such O
as S-MATE
hydroxyapatite O
, O
Ti S-MATE
, O
Ta S-MATE
and O
Coalloys O
and O
customized O
prostheses O
such O
as S-MATE
intervertebral O
spacers O
. O


There O
has O
been O
growing O
research S-CONPRI
interest O
in O
using O
topology B-FEAT
optimization E-FEAT
to O
design S-FEAT
bone B-BIOP
scaffolds E-BIOP
and O
orthopaedic B-APPL
implants E-APPL
, O
however O
significant O
challenges O
still O
remain O
before O
these O
concepts O
could O
be S-MATE
used O
in O
clinical B-APPL
practice E-APPL
. O


An O
important O
issue O
that O
may O
not O
be S-MATE
neglected O
when O
applying O
topology B-FEAT
optimization E-FEAT
to O
scaffold S-FEAT
design S-FEAT
is O
to O
consider O
the O
differences O
in O
physical O
, O
chemical O
and O
mechanical B-CONPRI
properties E-CONPRI
of O
base O
materials S-CONPRI
produced O
by O
AM S-MANP
and O
conventional O
fabrication S-MANP
techniques O
. O


The O
material B-CONPRI
property E-CONPRI
in O
AM B-MANP
process E-MANP
may O
greatly O
affect O
the O
final O
topological O
shape O
of O
scaffold S-FEAT
, O
which O
may O
differ O
from O
the O
original O
CAD B-ENAT
model E-ENAT
obtained O
from O
topology B-FEAT
optimization E-FEAT
. O


Moreover O
, O
good O
understanding O
of O
the O
change O
in O
mechanical B-CONPRI
properties E-CONPRI
in O
AM B-MANP
process E-MANP
may O
assist O
more O
accurate S-CHAR
optimal O
design S-FEAT
in O
topology B-FEAT
optimization E-FEAT
procedure O
. O


This O
section O
will O
review O
the O
status O
of O
research S-CONPRI
on O
AM S-MANP
fabrication O
of O
three O
main O
families O
of O
alloys S-MATE
and O
the O
application O
of O
topology B-FEAT
optimization E-FEAT
. O


4.1 O
Biocompatible B-MATE
Ti I-MATE
alloys I-MATE
Metals E-MATE
in O
biological B-CONPRI
systems E-CONPRI
may O
experience O
corrosion S-CONPRI
and O
release O
ions O
, O
which O
may O
result O
in O
many O
adverse O
physiological O
effects O
. O


Therefore O
, O
the O
biocompatibility S-PRO
of O
any O
implant S-APPL
must O
be S-MATE
quantified O
to O
decrease O
the O
patient O
's O
risk O
and O
the O
failure S-CONPRI
of O
the O
implantation S-MANP
. O


The O
cytotoxicity S-PRO
of O
typical O
surgical O
implant S-APPL
alloys S-MATE
and O
pure B-MATE
metals E-MATE
have O
been O
broadly O
studied O
in O
the O
past O
decades O
. O


It O
is O
now O
commonly O
accepted O
that O
vanadium S-MATE
may O
cause O
sterile B-MACEQ
abscess E-MACEQ
and O
aluminium S-MATE
may O
cause O
scar B-BIOP
tissue E-BIOP
, O
whereas O
titanium S-MATE
, O
zirconium S-MATE
, O
niobium S-MATE
and O
tantalum S-MATE
exhibit O
excellent O
biocompatibility S-PRO
. O


Another O
important O
motivation O
behind O
the O
design S-FEAT
of O
biocompatible B-MATE
Ti I-MATE
alloys E-MATE
is O
the O
opportunity O
to O
decrease O
the O
modulus O
of O
Ti B-MATE
alloys E-MATE
by O
adding O
elements S-MATE
. O


As S-MATE
mentioned O
above O
, O
the O
elements S-MATE
should O
be S-MATE
biocompatible O
. O


Various O
Ti B-MATE
alloys E-MATE
composed O
of O
low O
modulus O
biocompatible B-FEAT
elements E-FEAT
were O
developed O
. O


These O
alloys S-MATE
exhibited O
lower O
modulus O
than O
the O
commonly O
used O
Ti6Al4V S-MATE
. O


One O
example O
is O
Ti13Nb13Zr S-MATE
, O
which O
showed O
improved O
bone S-BIOP
biocompatibility S-PRO
and O
a O
modulus O
of O
79 O
GPa S-PRO
. O


Other O
Ti B-MATE
alloys E-MATE
which O
exhibited O
lower O
modulus O
included O
Ti29Nb13Ta4.6Zr O
and O
Ti35Nb5Ta7Zr S-MATE
. O


In O
recent O
years O
, O
AM S-MANP
produced O
porous B-FEAT
Ti I-FEAT
alloy I-FEAT
scaffolds E-FEAT
were O
widely O
reported O
with O
Ti6Al4V S-MATE
in O
dominance O
, O
such O
as S-MATE
the O
Ti6Al4V S-MATE
implants S-APPL
in O
sheep O
cervical B-BIOP
spine E-BIOP
in O
5 O
. O


Ryan O
combined O
the O
multi-stage O
AM B-MANP
technology E-MANP
with O
the O
powder B-MANP
metallurgy E-MANP
process O
to O
produce O
porous B-FEAT
Ti I-FEAT
alloy I-FEAT
scaffolds E-FEAT
using O
wax S-MATE
templates O
generated O
by O
CAD S-ENAT
. O


The O
pore B-PARA
size E-PARA
of O
their O
designs S-FEAT
ranged O
from O
200 O
to O
400 O
and O
the O
porosity S-PRO
reached O
66.8 O
% O
. O


This O
method O
could O
achieve O
controlled O
porous S-PRO
structure O
and O
ensure O
high B-PARA
resolution E-PARA
in O
manufacturing S-MANP
. O


The O
resulting O
microstructure S-CONPRI
and O
surface B-PRO
roughness E-PRO
were O
similar O
to O
parts O
manufactured S-CONPRI
by O
conventional O
methods O
. O


This O
method O
could O
also O
be S-MATE
extended O
to O
the O
fabrication S-MANP
of O
other O
metallic B-MACEQ
structures E-MACEQ
which O
are O
difficult O
to O
be S-MATE
directly O
made O
by O
AM S-MANP
. O


Murr O
manufactured S-CONPRI
different O
porous S-PRO
Ti6Al4V O
implants S-APPL
using O
AM S-MANP
based O
on O
micro-CT B-CHAR
scan E-CHAR
and O
CAD B-ENAT
models E-ENAT
built O
by O
Materialise O
software S-CONPRI
. O


They O
studied O
the O
influence O
of O
geometric O
features O
of O
unit B-CONPRI
cells E-CONPRI
on O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
the O
porous S-PRO
structures O
and O
found O
that O
when O
the O
porosity S-PRO
changed O
from O
59 O
% O
to O
88 O
% O
, O
the O
elastic B-PRO
modulus E-PRO
decreased O
from O
3.03 O
to O
0.58 O
GPa S-PRO
, O
which O
proved O
that O
the O
elastic B-PRO
modulus E-PRO
of O
porous B-MATE
metals E-MATE
could O
be S-MATE
readily O
adjusted O
through O
the O
porosity S-PRO
. O


Similarly O
Pattanyak O
studied O
porous S-PRO
Ti O
implants S-APPL
based O
on O
micro-CT B-CHAR
scan E-CHAR
on O
human O
cancellous B-BIOP
bones E-BIOP
, O
which O
focused O
on O
structures O
with O
complicated O
internal B-PRO
structures E-PRO
for O
bone B-APPL
ingrowth I-APPL
applications E-APPL
. O


The O
implants S-APPL
were O
manufactured S-CONPRI
via O
SLM S-MANP
using O
Ti B-MATE
powder E-MATE
of O
less O
than O
45 O
in O
size O
. O


They O
found O
that O
the O
compressive B-PRO
strength E-PRO
decreased O
from O
120 O
to O
35 O
MPa S-CONPRI
when O
the O
porosity S-PRO
changed O
from O
55 O
% O
to O
75 O
% O
. O


Hollander O
produced O
a O
variety O
of O
Ti6Al4V S-MATE
implants S-APPL
, O
ranging O
from O
porous S-PRO
cylinder O
to O
solid O
human O
vertebra O
model S-CONPRI
with O
irregular O
shapes O
. O


Porous B-MATE
Ti6Al4V I-MATE
structures E-MATE
were O
shown O
to O
be S-MATE
effective O
in O
supporting O
cell B-CHAR
growth E-CHAR
and O
new O
bone B-CONPRI
tissue I-CONPRI
growth E-CONPRI
, O
and O
cell B-CONPRI
based I-CONPRI
study E-CONPRI
suggested O
that O
Ti6Al4V S-MATE
possesses O
high O
cyto-biocompatibility S-PRO
. O


In O
vitro O
studies O
were O
performed O
with O
porous B-MATE
Ti6Al4V I-MATE
structures E-MATE
. O


Cell S-APPL
spreading O
and O
proliferation O
were O
observed O
across O
the O
entire O
surface S-CONPRI
and O
inside O
the O
porous S-PRO
structure O
. O


Porous S-PRO
Ti6Al4V O
scaffolds S-FEAT
were O
found O
performing O
well O
in O
animal O
models O
since O
induced O
new O
bone B-CONPRI
growth E-CONPRI
and O
osseointegration S-PRO
were O
achieved O
on O
both O
bare O
and O
surface-coated B-MATE
porous I-MATE
Ti6Al4V I-MATE
structures E-MATE
. O


Although O
porous S-PRO
Ti6Al4V O
had O
been O
widely O
studied O
, O
the O
potential O
release O
of O
toxic O
ions O
led S-APPL
researchers O
towards O
looking O
for O
safer O
alternative O
alloys S-MATE
. O


Therefore O
, O
Ti B-MATE
alloys E-MATE
such O
as S-MATE
Ti24Nb4Zr8Sn O
, O
Ti7.5Mo O
and O
Ti40Nb S-MATE
were O
designed S-FEAT
and O
fabricated S-CONPRI
by O
AM S-MANP
, O
which O
exhibited O
comparable O
mechanical B-CONPRI
properties E-CONPRI
to O
their O
counterparts O
by O
conventional B-MANP
manufacturing E-MANP
approaches O
. O


There O
was O
significant O
research S-CONPRI
interest O
in O
using O
topology B-FEAT
optimization E-FEAT
for O
the O
design S-FEAT
of O
porous B-FEAT
Ti I-FEAT
alloy I-FEAT
scaffolds E-FEAT
. O


While O
early O
works O
mainly O
focused O
on O
the O
theoretical S-CONPRI
consideration O
of O
the O
structural B-FEAT
design E-FEAT
of O
the O
unit B-CONPRI
cells E-CONPRI
, O
recent O
efforts O
put O
more O
emphasis O
on O
integrating O
topology B-FEAT
optimization E-FEAT
with O
AM S-MANP
, O
e.g O
. O


Refs.. O
4.2 O
Shape B-MATE
memory I-MATE
alloys E-MATE
Shape O
Memory O
Alloys S-MATE
are O
capable O
of O
regaining O
their O
original O
shape O
after O
severe O
deformations S-CONPRI
when O
stimulated O
by O
external O
environments O
. O


Due O
to O
this O
unique O
property S-CONPRI
, O
SMAs S-MATE
have O
found O
their O
way O
in O
orthopaedic B-APPL
implant E-APPL
applications O
. O


Typical O
SMAs S-MATE
include O
NiTi S-MATE
or O
nitinol S-MATE
which O
normally O
contains O
approximately O
50 O
at O
% O
Ni S-MATE
and O
50 O
at O
% O
Ti S-MATE
. O


The O
shape B-PRO
memory I-PRO
effect E-PRO
in O
NiTi S-MATE
comes O
from O
the O
austenite/martensite B-MATE
phase I-MATE
transformation E-MATE
since O
martensite S-MATE
is O
a O
low O
temperature B-PRO
stable I-PRO
phase E-PRO
with O
the O
absence O
of O
stress S-PRO
whereas O
austenite S-MATE
is O
a O
high O
temperature B-PRO
stable I-PRO
phase E-PRO
. O


Currently O
, O
more O
than O
90 O
% O
of O
all O
commercial O
SMAs S-MATE
are O
based O
on O
NiTi S-MATE
and O
its O
ternary B-MATE
alloys E-MATE
- O
NiTiCu O
and O
NiTiNb S-MATE
. O


Solid O
NiTi S-MATE
has O
a O
modulus O
of O
48 O
GPa S-PRO
, O
which O
is O
much O
lower O
than O
other O
Ti B-MATE
alloys E-MATE
. O


Furthermore O
, O
NiTi S-MATE
allows O
for O
relatively O
large O
reversible O
deformation S-CONPRI
of O
up O
to O
8 O
% O
. O


NiTi S-MATE
has O
higher O
stiffness S-PRO
than O
bone S-BIOP
under O
tendon S-BIOP
, O
and O
is O
able O
to O
deform O
over O
a O
large O
strain S-PRO
range S-PARA
at O
an O
almost O
constant O
stress S-PRO
. O


Due O
to O
these O
characteristics O
, O
NiTi S-MATE
has O
been O
widely O
used O
in O
medical B-APPL
devices E-APPL
, O
such O
as S-MATE
surgical O
tools S-MACEQ
, O
stents S-MACEQ
, O
orthodontic S-APPL
wires O
, O
plates O
and O
staples O
for O
bone B-BIOP
fractures E-BIOP
. O


Main O
attractive O
features O
of O
SMAs S-MATE
are O
: O
capability O
to O
recover O
the O
original O
shape O
after O
large O
deformation S-CONPRI
, O
capability O
to O
recover O
the O
original O
shape O
from O
a O
stable O
deformed B-PRO
shape E-PRO
when O
heated O
and O
a O
high O
damping O
capacity S-CONPRI
. O


For O
biomedical B-APPL
applications E-APPL
, O
the O
presence O
of O
Ni S-MATE
in O
NiTi S-MATE
has O
been O
a O
continuous O
concern O
since O
Ni S-MATE
is O
one O
of O
the O
highest O
sensitivities S-PARA
in O
metallic S-MATE
allergy O
tests O
. O


Therefore O
, O
attempts O
were O
made O
either O
to O
develop O
surface B-MANP
modification E-MANP
techniques O
or O
to O
use O
substitution O
elements S-MATE
to O
mitigate O
this O
effect O
without O
sacrificing O
the O
biocompatibility S-PRO
. O


For O
example O
, O
TiNb S-MATE
and O
the O
related O
TiNbX O
system O
were O
developed O
which O
exhibited O
elastic S-PRO
strains O
as S-MATE
high O
as S-MATE
4.2 O
% O
. O


Common O
methods O
for O
making O
porous S-PRO
NiTi S-MATE
structures O
are O
based O
on O
powder B-MANP
metallurgy E-MANP
and O
self-propagating O
high O
temperature S-PARA
synthesis O
of O
a O
mixture O
of O
elemental O
powders S-MATE
or O
pre-alloyed O
NiTi S-MATE
powder O
with O
space O
holding O
materials S-CONPRI
. O


After O
removing O
space O
holding O
materials S-CONPRI
at O
relatively O
low O
temperature S-PARA
, O
the O
structures O
are O
further O
sintered S-MANP
at O
high O
temperature S-PARA
. O


Due O
to O
high O
reactivity O
of O
Ti S-MATE
and O
Ni S-MATE
, O
the O
sintering S-MANP
of O
porous S-PRO
structures O
is O
normally O
done O
in O
high O
vacuum O
. O


However O
, O
these O
methods O
have O
difficulties O
in O
precisely O
controlling O
the O
porous S-PRO
structures O
of O
NiTi S-MATE
, O
i.e. O
, O
pore B-PARA
size E-PARA
and O
pore S-PRO
shape O
. O


To O
overcome O
this O
problem O
, O
AM B-MANP
technologies E-MANP
such O
as S-MATE
SLM O
have O
been O
used O
to O
produce O
NiTi S-MATE
implants S-APPL
. O


It O
was O
shown O
that O
AM S-MANP
produced O
NiTi S-MATE
parts O
exhibited O
similar O
mechanical B-CONPRI
properties E-CONPRI
as S-MATE
those O
fabricated S-CONPRI
by O
conventional O
methods O
such O
as S-MATE
casting O
. O


In O
contrary O
to O
the O
substantial O
research S-CONPRI
on O
Ti B-MATE
alloys E-MATE
, O
no O
reports O
on O
the O
application O
of O
topology B-FEAT
optimization E-FEAT
in O
the O
design S-FEAT
of O
SMA B-BIOP
scaffolds E-BIOP
involving O
AM S-MANP
fabrication O
can O
be S-MATE
found O
in O
the O
literature O
up O
to O
date O
. O


4.3 O
Biodegradable B-MATE
metals E-MATE
Biodegradable B-PRO
materials E-PRO
, O
including O
both O
polymer-based O
and O
metal-based O
ones O
, O
are O
used O
for O
some O
medical B-APPL
implants E-APPL
which O
will O
gradually O
degrade O
in O
human O
body O
over O
a O
period O
of O
time O
. O


In O
some O
clinical O
cases O
, O
biomaterials S-MATE
are O
only O
needed O
temporarily O
in O
the O
body O
and O
are O
expected O
to O
support S-APPL
the O
healing O
process S-CONPRI
and O
to O
disappear O
after O
the O
healing O
process S-CONPRI
is O
completed O
. O


5 O
shows O
a O
biodegradable B-MATE
Mg I-MATE
stent E-MATE
after O
expansion O
. O


Compared O
to O
polymer-based O
materials S-CONPRI
, O
biodegradable B-MATE
metals E-MATE
have O
higher O
stiffness S-PRO
and O
strength S-PRO
, O
and O
are O
more O
suitable O
for O
load O
bearing O
conditions O
. O


As S-MATE
the O
degradable B-APPL
alloys E-APPL
are O
expected O
to O
degrade O
inside O
human O
body O
, O
the O
main O
compositions O
of O
the O
alloys S-MATE
should O
be S-MATE
metallic O
elements S-MATE
that O
can O
be S-MATE
metabolized O
, O
and O
demonstrate O
appropriate O
degradation B-CHAR
rates E-CHAR
in O
the O
human O
body O
. O


Due O
to O
its O
unique O
characteristics O
, O
Mg B-MATE
alloys E-MATE
were O
used O
to O
manufacture S-CONPRI
cardiovascular B-MACEQ
stents E-MACEQ
and O
bone B-MACEQ
screws E-MACEQ
. O


The O
degradable O
magnesium B-MATE
alloy E-MATE
bone B-MACEQ
screws E-MACEQ
were O
found O
clinically O
equivalent O
to O
the O
conventional O
Ti S-MATE
screws O
; O
and O
no O
foreign O
body O
reaction O
, O
osteolysis S-BIOP
, O
or O
systemic O
inflammatory O
reaction O
were O
observed O
for O
the O
Mg B-MATE
alloy E-MATE
screws O
. O


A O
key O
parameter S-CONPRI
that O
needs O
to O
be S-MATE
considered O
in O
designing O
a O
biodegradable B-APPL
metallic I-APPL
implant E-APPL
is O
its O
degradation B-CHAR
rate E-CHAR
in O
human O
body O
. O


Pure B-MATE
Mg E-MATE
is O
known O
to O
have O
a O
fast O
degradation S-CONPRI
in O
high O
chloride O
physiological O
environment O
but O
it O
may O
produce O
hydrogen O
gas S-CONPRI
at O
a O
high O
rate O
from O
corrosion S-CONPRI
, O
which O
can O
not O
be S-MATE
dealt O
with O
by O
the O
host O
tissue O
. O


Fe-based O
biodegradable B-PRO
materials E-PRO
are O
known O
to O
exhibit O
a O
slow O
degradation B-CHAR
rate E-CHAR
. O


Animal O
tests O
showed O
that O
large O
portions O
of O
the O
pure O
Fe S-MATE
stent O
remained O
intact O
in O
the O
blood B-BIOP
vessels E-BIOP
12 O
months O
post-surgery O
. O


Alloying S-FEAT
is O
a O
typical O
method O
to O
adjust O
the O
degradation B-CHAR
rate E-CHAR
of O
a O
metal S-MATE
. O


For O
instance O
, O
by O
adding O
elements S-MATE
such O
as S-MATE
Y O
, O
Sr S-MATE
, O
Zn S-MATE
, O
Zr S-MATE
and O
Ca S-MATE
, O
Mg B-MATE
alloys E-MATE
were O
shown O
to O
have O
much O
lower O
degradation B-CHAR
rates E-CHAR
in O
comparison O
with O
pure B-MATE
Mg E-MATE
. O


Such O
alloys S-MATE
also O
exhibited O
high O
strength S-PRO
, O
which O
is O
desirable O
for O
load-bearing S-FEAT
applications O
. O


In O
addition O
to O
alloying S-FEAT
, O
amorphous B-CONPRI
structures E-CONPRI
like O
metallic B-MATE
glass I-MATE
alloys I-MATE
MgZnCa E-MATE
showed O
low O
degradation B-CHAR
rate E-CHAR
and O
high O
strength S-PRO
. O


However O
, O
metallic B-MATE
glass E-MATE
alloys S-MATE
are O
generally O
difficult O
to O
manufacture S-CONPRI
, O
which O
would O
add O
the O
cost O
to O
the O
application O
of O
this O
type O
of O
material S-MATE
. O


Porous S-PRO
Mg B-MATE
alloy E-MATE
implants S-APPL
were O
investigated O
as S-MATE
temporary O
bone S-BIOP
replacements O
in O
an O
animal O
model S-CONPRI
. O


They O
were O
shown O
to O
be S-MATE
able O
to O
enhance O
bone B-CONPRI
remodelling E-CONPRI
and O
appropriate O
host O
response O
. O


However O
, O
porous S-PRO
Mg B-MATE
alloys E-MATE
degrade O
too O
rapidly O
in O
vivo O
, O
which O
may O
leave O
subcutaneous S-BIOP
gas S-CONPRI
cavities O
. O


Since O
an O
open O
porous B-APPL
implant E-APPL
has O
large O
surface B-PARA
area E-PARA
, O
only O
alloys S-MATE
with O
slow O
degradation B-CHAR
rate E-CHAR
should O
be S-MATE
considered O
for O
making O
the O
porous S-PRO
structure O
, O
e.g O
. O


Mg-4 O
wt. O
% O
Y S-MATE
. O


The O
element S-MATE
yttrium O
helps O
promote O
grain B-CHAR
refinement E-CHAR
, O
thus O
resulting O
in O
a O
slow O
degradation S-CONPRI
and O
sufficient O
cyto-compatibility S-PRO
. O


Nguyen O
manufactured S-CONPRI
porous O
Mg B-MATE
alloys E-MATE
using O
SLM S-MANP
and O
suggested O
that O
the O
dimension S-FEAT
, O
surface B-CHAR
morphology E-CHAR
and O
the O
oxygen S-MATE
pick-up O
of O
the O
laser-melted O
Mg S-MATE
were O
strongly O
dependent O
on O
the O
laser B-CONPRI
processing E-CONPRI
parameters O
. O


Due O
to O
the O
high O
evaporation B-CHAR
rate E-CHAR
at O
elevated O
temperatures S-PARA
, O
few O
attempts O
were O
made O
to O
fabricate S-MANP
Mg O
scaffolds S-FEAT
directly O
using O
AM S-MANP
. O


Instead O
, O
a O
technique O
combining O
3D B-MANP
printing E-MANP
and O
gravity O
casting S-MANP
was O
shown O
to O
be S-MATE
effective O
in O
producing O
topologically-ordered O
porous S-PRO
Mg S-MATE
structures O
, O
where O
a O
porous S-PRO
NaCl S-MATE
mould S-MACEQ
was O
created O
using O
SLM S-MANP
and O
then O
Mg B-MATE
alloy E-MATE
was O
cast S-MANP
into O
the O
mould S-MACEQ
. O


After O
removing O
the O
NaCl S-MATE
, O
porous S-PRO
Mg S-MATE
structure O
with O
porosity S-PRO
of O
41 O
% O
and O
pore B-PARA
size E-PARA
of O
1 O
mm S-MANP
was O
obtained O
. O


The O
compressive B-PRO
strength E-PRO
of O
the O
porous S-PRO
Mg S-MATE
was O
reported O
to O
be S-MATE
13 O
MPa S-CONPRI
, O
which O
is O
comparable O
to O
porous S-PRO
Mg S-MATE
produced O
by O
powder B-MANP
metallurgy E-MANP
. O


A O
review O
on O
porous S-PRO
biodegradable B-MATE
metals E-MATE
for O
hard B-BIOP
tissue I-BIOP
scaffolds E-BIOP
can O
be S-MATE
found O
in O
Ref.. O
A O
theoretical S-CONPRI
study O
of O
topological B-FEAT
design E-FEAT
of O
polymeric B-FEAT
scaffolds E-FEAT
considering O
the O
effect O
of O
biodegradation O
was O
conducted O
by O
Chen O
. O


No O
reports O
on O
the O
application O
of O
topology B-FEAT
optimization E-FEAT
in O
the O
design S-FEAT
of O
biodegradable B-BIOP
metallic I-BIOP
scaffolds E-BIOP
involving O
AM S-MANP
fabrication O
have O
appeared O
in O
the O
literature O
so O
far O
. O


5 O
Heat-treatment O
and O
surface B-MANP
modification E-MANP
of O
porous B-FEAT
metallic I-FEAT
structures E-FEAT
produced O
by O
AM S-MANP
5.1 O
Heat-treatment O
The O
mechanical B-CONPRI
properties E-CONPRI
of O
AM S-MANP
produced O
materials S-CONPRI
depend O
heavily O
on O
the O
processing O
parameters S-CONPRI
, O
including O
building B-PARA
layer I-PARA
thickness E-PARA
, O
scan B-PARA
speed E-PARA
, O
energy B-PARA
density E-PARA
and O
focal B-PARA
offset I-PARA
distance E-PARA
. O


Usually O
AM S-MANP
produced O
materials S-CONPRI
have O
relatively O
high O
yield B-PRO
stress E-PRO
and O
ultimate B-PRO
tensile I-PRO
strength E-PRO
, O
but O
a O
relatively O
low O
ductility S-PRO
. O


In O
order O
to O
improve O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
AM S-MANP
produced O
porous S-PRO
biomaterials S-MATE
so O
that O
they O
can O
mimic S-MACEQ
the O
human O
tissues O
and O
fulfil O
the O
desired O
functions O
, O
post-treatment S-MANP
is O
of O
critical O
importance O
. O


It O
is O
known O
that O
the O
microstructures S-MATE
of O
as-built O
materials S-CONPRI
by O
AM S-MANP
are O
very O
different O
from O
those O
by O
traditional O
casting S-MANP
or O
forging S-MANP
approaches O
. O


AM S-MANP
is O
a O
layer-wise O
build-up O
process S-CONPRI
with O
high O
cooling B-PARA
rates E-PARA
that O
lead S-MATE
to O
significant O
internal O
thermal B-PRO
stresses E-PRO
in O
the O
structure S-CONPRI
. O


During O
the O
building B-CHAR
process E-CHAR
, O
the O
scanning S-CONPRI
by O
either O
a O
laser S-ENAT
or O
an O
electron B-CONPRI
beam E-CONPRI
may O
cause O
the O
instabilities O
of O
the O
melt B-MATE
pool E-MATE
, O
resulting O
in O
increased O
porosity S-PRO
and O
high O
surface B-PRO
roughness E-PRO
. O


The O
post-treatment S-MANP
process O
also O
enables O
the O
reduction S-CONPRI
of O
thermal B-PRO
stresses E-PRO
in O
AM S-MANP
produced O
structures O
. O


For O
Ti6Al4V S-MATE
, O
post B-MANP
heat-treatment E-MANP
is O
typically O
performed O
within O
the O
+ O
region O
, O
which O
can O
control O
the O
morphology S-CONPRI
and O
size O
of O
the O
without O
significantly O
influencing O
the O
prior-grain O
size O
. O


A O
proper O
heat-treatment O
process S-CONPRI
may O
substantially O
improve O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
AM S-MANP
produced O
materials S-CONPRI
. O


Thone O
observed O
significant O
improvement O
in O
ductility S-PRO
and O
fatigue B-PRO
strength E-PRO
after O
heat-treatment O
of O
SLM S-MANP
produced O
Ti6Al4V S-MATE
. O


They O
revealed O
that O
the O
tensile B-PRO
strength E-PRO
of O
heat-treated S-MANP
Ti6Al4V O
slightly O
decreased O
from O
1080 O
MPa S-CONPRI
to O
945 O
MPa S-CONPRI
but O
the O
elongation S-PRO
at O
failure S-CONPRI
increased O
significantly O
from O
1.6 O
% O
to O
11.6 O
% O
, O
along O
with O
remarkably O
prolonged O
fatigue B-PRO
life E-PRO
of O
parts O
from O
28,900 O
to O
290,000 O
cycles O
. O


The O
improvements O
in O
the O
mechanical B-CONPRI
properties E-CONPRI
after O
post B-MANP
heat-treatment E-MANP
are O
mainly O
due O
to O
the O
elimination O
of O
thermal B-PRO
stresses E-PRO
and O
the O
changes O
of O
microstructures S-MATE
. O


On O
the O
other O
hand O
, O
an O
adequate O
selection O
of O
AM S-MANP
processing O
variables O
can O
facilitate O
in-situ B-CONPRI
heat E-CONPRI
treatment O
. O


The O
microstructure S-CONPRI
of O
Ti6Al4V S-MATE
made O
by O
SLM S-MANP
is O
often O
dominated O
by O
martensite S-MATE
due O
to O
rapid O
cooling S-MANP
, O
which O
can O
be S-MATE
decomposed O
to O
lamellar S-CONPRI
+ O
structure S-CONPRI
during O
SLM S-MANP
process S-CONPRI
by O
tuning O
the O
processing O
variables O
. O


After O
the O
optimization S-CONPRI
of O
processing O
conditions O
, O
Xu O
produced O
Ti6Al4V S-MATE
with O
comparable O
or O
better O
mechanical B-CONPRI
properties E-CONPRI
than O
forged O
Ti6Al4V S-MATE
. O


5.2 O
Surface B-MANP
modification E-MANP
Surface S-CONPRI
modification O
plays O
an O
important O
role O
in O
enhancing O
the O
biological B-CONPRI
performance I-CONPRI
of I-CONPRI
AM E-CONPRI
produced O
porous S-PRO
biomaterials S-MATE
, O
particularly O
bioactivity S-PRO
and O
biocompatibility S-PRO
. O


Ti B-MATE
alloys E-MATE
are O
normally O
covered O
by O
one O
layer S-PARA
of O
3nm O
thick O
native O
oxide S-MATE
, O
namely O
TiO2 S-MATE
, O
which O
provides O
excellent O
chemical B-PRO
inertness E-PRO
, O
corrosion B-CONPRI
resistance E-CONPRI
and O
biocompatibility S-PRO
. O


In O
the O
human O
body O
, O
Ti B-APPL
alloy I-APPL
implants E-APPL
may O
experience O
non-specific O
protein O
adsorption S-CONPRI
and O
interrogation O
of O
neutrophils S-CONPRI
and O
macrophages S-BIOP
, O
which O
may O
attract O
fibroblasts S-BIOP
to O
an O
encapsulation S-CONPRI
process O
. O


To O
ensure O
an O
effective O
biological O
bond O
between O
Ti B-APPL
alloy I-APPL
implants E-APPL
and O
surrounding O
bones O
, O
surface B-MANP
modification E-MANP
is O
essential O
to O
improve O
the O
conductivity S-PRO
of O
bones O
or O
the O
bioactivity S-PRO
of O
titanium S-MATE
. O


The O
surface B-CHAR
morphology E-CHAR
of O
Ti B-APPL
alloy I-APPL
implants E-APPL
depends O
on O
the O
history O
of O
material S-MATE
processing O
. O


For O
AM S-MANP
produced O
porous S-PRO
Ti O
alloys S-MATE
, O
powders S-MATE
tend O
to O
become O
small O
liquid O
spheres O
when O
heated O
up O
by O
laser S-ENAT
or O
electron B-CONPRI
beams E-CONPRI
. O


Such O
a O
effect O
is O
a O
complex O
metallurgical S-APPL
process O
that O
leads O
to O
a O
rough O
surface S-CONPRI
and O
residual B-MATE
powder E-MATE
particles S-CONPRI
. O


These O
loosely O
connected O
powder B-MATE
particles E-MATE
can O
be S-MATE
removed O
through O
blasting O
or O
other O
post-processing S-CONPRI
methods O
before O
implantation S-MANP
. O


Surface B-MANP
modification E-MANP
or O
activation O
of O
Ti S-MATE
surface S-CONPRI
can O
be S-MATE
achieved O
by O
various O
techniques O
such O
as S-MATE
plasma O
spray O
, O
physical O
or O
chemical O
vapour B-CHAR
deposition E-CHAR
, O
ion B-CONPRI
implantation E-CONPRI
, O
electrochemical B-CONPRI
oxidation E-CONPRI
, O
acidic O
or O
alkali O
etching S-MANP
, O
solheat-treatment O
, O
and O
surface S-CONPRI
machining S-MANP
or O
grinding S-MANP
. O


For O
porous B-FEAT
metallic I-FEAT
structures E-FEAT
, O
there O
are O
two O
main O
approaches O
, O
based O
on O
surface S-CONPRI
coating S-APPL
and O
surface S-CONPRI
corrosion S-CONPRI
. O


A O
popular O
coating-based O
method O
is O
solprocess O
, O
which O
is O
a O
simple S-MANP
yet O
versatile O
method O
for O
creating O
oxide B-MATE
coatings E-MATE
at O
relatively O
low O
temperatures S-PARA
. O


For O
implants S-APPL
with O
a O
complex O
topology S-CONPRI
, O
dip B-MANP
coating E-MANP
is O
normally O
used O
. O


The O
solprocess O
may O
deposit O
thin O
inorganic O
coatings S-APPL
. O


The O
chemical B-CONPRI
composition E-CONPRI
and O
microstructures S-MATE
of O
the O
coating S-APPL
can O
be S-MATE
better O
controlled O
by O
the O
solprocess O
than O
by O
other O
methods O
. O


Brie O
used O
solto O
form O
a O
bioceramic B-MATE
coating E-MATE
on O
porous S-PRO
Ti6Al7Nb O
implants S-APPL
. O


The O
coating S-APPL
uniformly O
covered O
the O
external O
and O
internal O
surfaces S-CONPRI
of O
the O
implants S-APPL
; O
and O
the O
coated S-APPL
porous O
structures O
exhibited O
improved O
biocompatibility S-PRO
. O


Other O
methods O
include O
electrolytic B-CONPRI
deposition E-CONPRI
and O
plasma B-MANP
spray E-MANP
. O


ED S-CHAR
can O
produce O
CaP B-MATE
coatings E-MATE
having O
a O
thickness O
of O
a O
few O
microns O
to O
several O
hundred O
microns O
, O
which O
can O
be S-MATE
controlled O
by O
applying O
appropriate O
current O
density S-PRO
and O
processing O
time O
. O


This O
may O
also O
assist O
to O
control O
the O
surface B-CHAR
morphology E-CHAR
of O
CaP B-MATE
coatings E-MATE
from O
needle-like O
to O
plate-like O
structures O
. O


Coating S-APPL
through O
ED S-CHAR
can O
produce O
uniformly O
and O
fully O
covered O
surface S-CONPRI
, O
which O
makes O
it O
suitable O
for O
functionalizing O
porous S-PRO
structures O
. O


Chai O
found O
that O
the O
bioactivity S-PRO
of O
the O
CaP O
coated S-APPL
Ti6Al4V O
scaffold S-FEAT
to O
) O
was O
significantly O
improved O
and O
it O
was O
possible O
to O
produce O
osteoinductive S-BIOP
for O
the O
repair O
of O
bone B-BIOP
defects E-BIOP
. O


Corrosion-based O
surface B-MANP
treatment E-MANP
involves O
interfacial B-CHAR
chemical I-CHAR
reactions E-CHAR
of O
structures O
in O
corrosive B-PRO
solution E-PRO
. O


Such O
chemical O
processes S-CONPRI
include O
alkali B-MANP
treatment E-MANP
, O
acid B-MANP
etching E-MANP
and O
anodization B-MANP
treatments E-MANP
. O


5 O
to O
show O
the O
surface B-CHAR
morphology E-CHAR
of O
scaffold S-FEAT
before O
and O
after O
HCl O
etching S-MANP
treatment O
. O


The O
chemical B-CONPRI
reaction E-CONPRI
may O
produce O
a O
thin O
oxide S-MATE
layer S-PARA
on O
the O
surface S-CONPRI
of O
the O
metal S-MATE
, O
usually O
resulting O
in O
improved O
bioactivity S-PRO
. O


The O
thickness O
of O
the O
active O
layer S-PARA
can O
be S-MATE
controlled O
from O
tens O
of O
nanometer S-FEAT
to O
hundreds O
of O
microns O
by O
adjusting O
processing O
variables O
. O


Alkali B-MANP
treatment E-MANP
was O
initially O
introduced O
by O
Kim O
to O
improve O
the O
bioactivity S-PRO
of O
Ti B-APPL
implants E-APPL
owing O
to O
a O
biologically O
active O
bone-like B-MATE
apatite I-MATE
layer E-MATE
on O
Ti S-MATE
surface S-CONPRI
. O


Anodization S-MANP
is O
a O
mature O
electrochemical B-MANP
process E-MANP
capable O
of O
producing O
protective B-APPL
layers E-APPL
on O
the O
metal S-MATE
surface O
with O
adjustable O
surface B-CONPRI
microstructure E-CONPRI
and O
crystal B-PRO
structure E-PRO
. O


Special O
care O
should O
be S-MATE
taken O
with O
regard O
to O
the O
possible O
negative O
effect O
on O
mechanical B-CONPRI
properties E-CONPRI
after O
corrosion-based O
surface B-MANP
treatment E-MANP
. O


It O
was O
reported O
that O
alkali B-MANP
treatment E-MANP
might O
result O
in O
the O
deterioration O
of O
mechanical B-PRO
strength E-PRO
of O
porous B-FEAT
Ti I-FEAT
alloy I-FEAT
scaffolds E-FEAT
and O
also O
cause O
the O
embrittlement S-PRO
of O
the O
struts S-MACEQ
in O
the O
scaffolds S-FEAT
. O


6 O
Challenges O
and O
future O
directions O
Additive B-MANP
manufacturing E-MANP
provides O
unprecedented O
opportunities O
for O
producing O
customized O
medical B-APPL
implants E-APPL
as S-MATE
this O
technology S-CONPRI
can O
fabricate S-MANP
structures O
of O
complex O
external O
shapes O
and O
intricate O
internal B-PRO
architectures E-PRO
. O


Topology B-FEAT
optimization E-FEAT
has O
become O
a O
powerful O
digital O
tool S-MACEQ
for O
the O
design S-FEAT
of O
optimal B-FEAT
structures E-FEAT
and O
materials S-CONPRI
. O


The O
integration O
of O
these O
two O
technologies S-CONPRI
sees O
a O
promising O
future O
in O
designing O
and O
manufacturing S-MANP
biocompatible B-APPL
orthopaedic I-APPL
implants E-APPL
with O
desired O
mechanical B-CONPRI
properties E-CONPRI
and O
minimal O
side O
effects O
on O
patients O
in O
clinical B-APPL
applications E-APPL
. O


Key O
challenges O
and O
future O
directions O
in O
integrating O
the O
two O
technologies S-CONPRI
are O
as S-MATE
follows O
: O
i O
) O
A O
comprehensive O
and O
reliable O
database S-ENAT
containing O
detailed O
information O
on O
the O
mechanical S-APPL
and O
biological O
properties S-CONPRI
of O
human O
bones O
is O
yet O
to O
be S-MATE
established O
. O


This O
database S-ENAT
should O
include O
properties S-CONPRI
of O
bones O
for O
different O
age O
, O
gender O
groups O
and O
at O
different O
locations O
. O


Such O
information O
is O
required O
as S-MATE
the O
design S-FEAT
of O
the O
topology B-FEAT
optimization I-FEAT
process E-FEAT
. O


ii O
) O
Sophisticated O
topology B-FEAT
optimization E-FEAT
algorithms S-CONPRI
capable O
of O
dealing O
with O
multi-functional O
designs S-FEAT
on O
multiple O
length B-CHAR
scales E-CHAR
simultaneously O
needs O
to O
be S-MATE
developed O
. O


Preliminary O
studies O
along O
this O
line O
can O
be S-MATE
found O
in O
Refs.. O
iii O
) O
Topological B-FEAT
design E-FEAT
of O
the O
lattice B-FEAT
structures E-FEAT
that O
can O
be S-MATE
easily O
produced O
by O
AM S-MANP
and O
exhibit O
anisotropic S-PRO
mechanical O
properties S-CONPRI
similar O
to O
human O
bones O
is O
another O
promising O
direction O
, O
despite O
that O
the O
fact O
that O
there O
has O
been O
extensive O
research S-CONPRI
on O
topology B-FEAT
optimization E-FEAT
based O
on O
continuum B-CONPRI
models E-CONPRI
. O


iv O
) O
Constraints O
and O
limitations O
of O
current O
AM B-MANP
technologies E-MANP
, O
such O
as S-MATE
the O
critical B-FEAT
angle E-FEAT
of O
the O
overhanging B-CONPRI
structure E-CONPRI
and O
the O
difficulty O
in O
removing O
the O
supporting O
structure S-CONPRI
, O
should O
be S-MATE
involved O
in O
newly-developed O
topology B-FEAT
optimization E-FEAT
algorithms S-CONPRI
so O
that O
the O
designs S-FEAT
could O
actually O
be S-MATE
fabricated O
by O
AM S-MANP
. O


v S-MATE
) O
The O
long-term O
in O
vivo O
material/biological O
performance S-CONPRI
of O
porous B-APPL
metallic I-APPL
implants E-APPL
that O
are O
designed S-FEAT
through O
topology B-FEAT
optimization E-FEAT
techniques O
and O
produced O
by O
AM S-MANP
needs O
to O
be S-MATE
rigorously O
assessed O
in O
order O
to O
ascertain O
the O
advantages O
and O
drawbacks O
of O
such O
implants S-APPL
. O


vi O
) O
Novel O
alloying S-FEAT
systems O
capable O
of O
enhancing O
the O
mechanical S-APPL
and O
biological O
performance S-CONPRI
of O
porous B-APPL
metallic I-APPL
implants E-APPL
are O
in O
great O
demand O
, O
together O
with O
new O
post-treatment S-MANP
technologies O
for O
improving O
the O
bioactivity S-PRO
and O
biocompatibility S-PRO
. O


7 O
Conclusions O
In O
this O
paper O
, O
the O
current O
status O
of O
the O
topological B-FEAT
design E-FEAT
of O
porous B-APPL
metallic I-APPL
implants E-APPL
and O
the O
fabrication S-MANP
of O
such O
implants S-APPL
using O
additive B-MANP
manufacturing E-MANP
is O
reviewed O
. O


First O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
human O
bones O
are O
discussed O
. O


Then O
it O
is O
demonstrated O
that O
topology B-FEAT
optimization E-FEAT
is O
a O
powerful O
digital O
tool S-MACEQ
that O
can O
be S-MATE
used O
to O
obtain O
optimal O
internal B-PRO
architectures E-PRO
for O
porous B-APPL
implants E-APPL
which O
not O
only O
satisfy O
multifunctional O
requirements O
but O
also O
mimic S-MACEQ
human O
bones O
. O


Furthermore O
it O
is O
shown O
that O
additive B-MANP
manufacturing E-MANP
is O
the O
most O
promising O
and O
disruptive O
technology S-CONPRI
in O
the O
fabrication S-MANP
of O
porous B-APPL
orthopaedic I-APPL
implants E-APPL
designed S-FEAT
through O
topology B-FEAT
optimization E-FEAT
. O


To O
further O
improve O
the O
mechanical S-APPL
and O
biological O
performance S-CONPRI
of O
these O
structures O
, O
both O
post-treatment S-MANP
and O
surface B-MANP
modification E-MANP
are O
necessary O
. O


Based O
on O
these O
discussions O
, O
challenges O
and O
future O
directions O
of O
the O
integration O
topology B-FEAT
optimization E-FEAT
with O
additive B-MANP
manufacturing E-MANP
are O
identified O
. O


This O
review O
provides O
useful O
information O
to O
researchers O
and O
practitioners O
who O
are O
working O
in O
various O
areas S-PARA
of O
the O
truly O
multidisciplinary O
topic O
of O
bone B-APPL
implant E-APPL
design O
and O
fabrication S-MANP
Additive B-MANP
manufacturing E-MANP
of O
polymer-fiber B-MATE
composites E-MATE
has O
transformed O
AM S-MANP
into O
a O
robust O
manufacturing S-MANP
paradigm O
and O
enabled O
producing O
highly O
customized O
parts O
with O
significantly O
improved O
mechanical B-CONPRI
properties E-CONPRI
, O
compared O
to O
un-reinforced O
polymers S-MATE
. O


Almost O
all O
commercially O
available O
AM S-MANP
methods O
could O
benefit O
from O
various O
fiber B-FEAT
reinforcement E-FEAT
techniques O
. O


Recent O
developments O
in O
3D B-MANP
printing E-MANP
methods O
of O
fiber B-MATE
reinforced I-MATE
polymers E-MATE
, O
namely O
, O
fused B-MANP
deposition I-MANP
modeling E-MANP
, O
laminated B-MANP
object I-MANP
manufacturing E-MANP
, O
stereolithography S-MANP
, O
extrusion S-MANP
, O
and O
selective B-MANP
laser I-MANP
sintering E-MANP
are O
reviewed O
in O
this O
study O
to O
understand O
the O
trends S-CONPRI
and O
future O
directions O
in O
the O
respective O
areas S-PARA
. O


In O
addition O
to O
extra O
strength S-PRO
, O
fibers S-MATE
have O
also O
been O
used O
in O
4D B-MANP
printing E-MANP
to O
control O
and O
manipulate O
the O
change O
of O
shape O
or O
swelling S-CONPRI
after O
3D B-MANP
printing E-MANP
, O
right O
out O
of O
the O
printing O
bed S-MACEQ
. O


Although O
AM S-MANP
of O
fiber/polymer O
composites S-MATE
are O
increasingly O
developing O
and O
under O
intense O
attention O
, O
there O
are O
some O
issues O
needed O
to O
be S-MATE
addressed O
including O
void S-CONPRI
formation O
, O
poor O
adhesion S-PRO
of O
fibers S-MATE
and O
matrix O
, O
blockage O
due O
to O
filler O
inclusion S-MATE
, O
increased O
curing B-PARA
time E-PARA
, O
modelling S-ENAT
, O
simulation S-ENAT
, O
etc O
. O


Additive B-MANP
manufacturing E-MANP
, O
also O
known O
as S-MATE
3D B-MANP
printing E-MANP
, O
is O
defined O
asa O
process S-CONPRI
of O
adding O
materials S-CONPRI
to O
fabricate S-MANP
objects O
from O
three-dimensional B-ENAT
models E-ENAT
in O
successive O
layers O
, O
versus O
traditional B-MANP
subtractive I-MANP
manufacturing E-MANP
methods O
. O


Numerous O
novel O
AM B-MANP
processes E-MANP
have O
been O
developed O
over O
the O
span O
of O
more O
than O
20years O
of O
AM S-MANP
development O
with O
applications O
in O
aerospace S-APPL
, O
automotive S-APPL
, O
biomedical S-APPL
, O
digital O
art S-APPL
, O
architectural O
design S-FEAT
, O
etc O
. O


There O
has O
been O
an O
exponential O
increase O
in O
AM B-MANP
technology E-MANP
in O
recent O
years O
and O
it O
continues O
to O
grow O
due O
to O
its O
versatility O
and O
low O
cost O
for O
rapid B-ENAT
prototyping E-ENAT
and O
manufacturing S-MANP
applications O
. O


All O
of O
these O
features O
, O
combined O
with O
AMcustomizability O
to O
fabricate S-MANP
complex O
monolithic B-PRO
structures E-PRO
and O
geometries S-CONPRI
, O
with O
micrometer B-PARA
resolution E-PARA
, O
helped O
AM S-MANP
grow O
to O
a O
multibillion-dollar O
industry S-APPL
. O


To O
date O
, O
the O
dominant O
part O
of O
the O
3D B-MANP
printing E-MANP
industry O
has O
immensely O
relied O
on O
single O
material S-MATE
printing O
. O


This O
issue O
, O
paired O
with O
limited O
choices O
of O
available O
resins S-MATE
compatible O
with O
commercial O
printers S-MACEQ
, O
has O
severely O
limited O
variations S-CONPRI
in O
the O
physical O
and O
chemical O
properties S-CONPRI
of O
3D B-MANP
printed E-MANP
objects O
. O


These O
limitations O
have O
led S-APPL
to O
development O
of O
multi-material S-CONPRI
printers O
with O
partial O
control O
on O
material S-MATE
composition S-CONPRI
and O
properties S-CONPRI
, O
offering O
layered O
composite B-MATE
materials E-MATE
. O


Furthermore O
, O
multiple O
printing B-MACEQ
heads E-MACEQ
have O
allowed O
printing O
blended O
composites S-MATE
with O
functional O
and O
variable O
features O
. O


3D B-MANP
printing E-MANP
of O
fiber B-MATE
reinforced I-MATE
composites E-MATE
is O
currently O
conducted O
by O
stereolithography S-MANP
, O
laminated B-MANP
object I-MANP
manufacturing E-MANP
, O
fused B-MANP
deposition I-MANP
modeling E-MANP
, O
selective B-MANP
laser I-MANP
sintering E-MANP
, O
and O
extrusion S-MANP
. O


This O
is O
one O
of O
the O
hottest O
topics O
in O
the O
field O
of O
additive B-MANP
manufacturing E-MANP
and O
is O
under O
intense O
attention O
. O


This O
also O
offers O
significant O
improvement O
in O
mechanical B-CONPRI
properties E-CONPRI
, O
however O
, O
it O
requires O
a O
complex O
procedure O
to O
be S-MATE
manufactured O
and O
is O
difficult O
to O
be S-MATE
incorporated O
into O
processing O
. O


Implementing O
the O
traditional O
methods O
of O
composite B-MANP
manufacturing E-MANP
in O
AM S-MANP
is O
not O
practical O
and O
new O
technologies S-CONPRI
are O
needed O
to O
assist O
with O
the O
development O
of O
new O
AM S-MANP
methods O
. O


Advances O
in O
development O
of O
composite S-MATE
3D B-MACEQ
printers E-MACEQ
have O
not O
prevented O
development O
in O
pre-blended O
materials S-CONPRI
with O
fillers O
such O
as S-MATE
nanoparticles O
, O
carbon B-MATE
nanotubes E-MATE
, O
fibers S-MATE
and O
graphene S-MATE
in O
order O
to O
achieve O
unique O
characteristics O
and O
capabilities O
. O


Fiber B-FEAT
reinforcement E-FEAT
, O
in O
particular O
, O
appears O
to O
be S-MATE
an O
attractive O
filler O
to O
improve O
the O
properties S-CONPRI
of O
polymers S-MATE
. O


Pre-blended O
materials S-CONPRI
using O
discontinuous O
fibers S-MATE
asan O
additive S-MATE
have O
been O
under O
intense O
investigation O
asa O
suitable O
alternative O
to O
multi-head O
printers S-MACEQ
with O
complex O
and O
costly O
designs S-FEAT
. O


These O
additive S-MATE
based O
materials S-CONPRI
exhibit O
unique O
characteristics O
and O
capabilities O
, O
depending O
on O
the O
additive S-MATE
used O
. O


Suitable O
mechanical S-APPL
, O
electrical S-APPL
, O
or O
thermal B-CONPRI
properties E-CONPRI
can O
be S-MATE
accomplished O
in O
an O
inexpensive O
manner O
. O


Polymers S-MATE
, O
in O
particular O
, O
have O
been O
the O
center O
of O
attention O
due O
to O
ease O
of O
production S-MANP
and O
availability O
. O


The O
3D B-MANP
printing E-MANP
industry O
primarily O
involves O
polymers S-MATE
in O
various O
forms O
, O
such O
as S-MATE
reactive O
, O
liquid O
solutions O
, O
and O
thermoplastic S-MATE
melts O
. O


These O
benefits O
, O
joined O
by O
enhancements O
from O
fiber B-FEAT
reinforcement E-FEAT
, O
offer O
a O
favorable O
combination O
for O
future O
development O
of O
AM B-MANP
technology E-MANP
. O


In O
addition O
, O
almost O
all O
of O
the O
existing O
AM S-MANP
methods O
can O
be S-MATE
benefited O
from O
fiber B-FEAT
reinforcement E-FEAT
. O


Although O
fiber B-FEAT
reinforcement E-FEAT
in O
3D B-MANP
printing E-MANP
sounds O
promising O
, O
there O
are O
numerous O
issues O
which O
need O
to O
be S-MATE
resolved O
. O


Namely O
, O
the O
effect O
of O
fibers S-MATE
on O
resolution S-PARA
, O
agglomerate B-CHAR
formation E-CHAR
, O
heterogeneous B-MANP
composite I-MANP
formation E-MANP
, O
blockage O
of O
printer S-MACEQ
heads O
, O
non-adhesion O
, O
and O
increased O
curing B-PARA
times E-PARA
. O


This O
paper O
reviews O
recent O
advances O
in O
AM S-MANP
of O
polymer S-MATE
based O
fiber B-MATE
reinforced I-MATE
composites E-MATE
and O
potential O
methods O
for O
modelling S-ENAT
and O
analysis O
of O
these O
3D B-MANP
printed E-MANP
structures O
. O


Latest O
development O
and O
improvement O
to O
existing O
methods O
will O
be S-MATE
reviewed O
in O
detail O
, O
in O
order O
to O
understand O
the O
challenges O
in O
3D B-MANP
printing E-MANP
of O
polymer B-MATE
composites E-MATE
with O
fiber B-FEAT
reinforcement E-FEAT
. O


3D B-MANP
printing E-MANP
of O
fiber B-MATE
reinforced I-MATE
polymer I-MATE
composites E-MATE
Fiber S-MATE
reinforcement O
can O
greatly O
improve O
the O
properties S-CONPRI
of O
3D B-APPL
printed I-APPL
parts E-APPL
with O
polymer S-MATE
matrix O
. O


Fiber B-FEAT
orientation E-FEAT
and O
void S-CONPRI
content O
of O
composites S-MATE
are O
the O
main O
concerns O
in O
3D B-MANP
printing E-MANP
of O
these O
composites S-MATE
. O


Most O
of O
the O
commercially O
available O
3D B-MANP
printing E-MANP
techniques O
would O
benefit O
from O
fiber B-FEAT
reinforcement E-FEAT
. O


In O
this O
section O
, O
all O
of O
these O
techniques O
for O
3D B-MANP
printing E-MANP
of O
polymer-fiber B-MATE
composites E-MATE
are O
reviewed O
in O
detail O
to O
demonstrate O
their O
strengths S-PRO
and O
weaknesses O
in O
additive B-MANP
manufacturing E-MANP
of O
polymer-fiber B-MATE
composites E-MATE
. O


These O
methods O
are O
FDM S-MANP
, O
LOM S-MANP
, O
SL S-MANP
, O
extrusion S-MANP
, O
and O
SLS S-MANP
. O


2.1 O
Fused B-MANP
deposition I-MANP
modeling E-MANP
FDM S-MANP
is O
currently O
the O
most O
applied O
AM B-MANP
technology E-MANP
, O
according O
to O
WohlerReport O
from O
Stratasys S-APPL
, O
Inc. O
Commercial O
FDM S-MANP
machines O
held O
41.5 O
% O
of O
the O
market O
share O
, O
with O
the O
total O
of O
15,000 O
FDM S-MANP
machines O
sold O
by O
the O
end O
of O
2010 O
. O


The O
key O
elements S-MATE
of O
the O
FDM S-MANP
system O
include O
material B-FEAT
feed I-FEAT
mechanism E-FEAT
, O
liquefier O
, O
print B-MACEQ
head E-MACEQ
, O
gantry O
, O
and O
build B-PARA
surface E-PARA
. O


Several O
process B-CONPRI
parameters E-CONPRI
are O
essential O
in O
FDM S-MANP
, O
including O
bead B-CHAR
width E-CHAR
, O
air O
gap O
, O
model S-CONPRI
build S-PARA
temperature O
, O
and O
raster B-PARA
orientation E-PARA
. O


The O
effects O
of O
raster B-PARA
orientation E-PARA
on O
tensile S-PRO
and O
compression B-CHAR
test E-CHAR
results O
have O
been O
investigated O
in O
detail O
. O


The O
temperature S-PARA
distribution S-CONPRI
during O
the O
FDM S-MANP
process O
can O
be S-MATE
monitored O
by O
IR S-CHAR
camera S-MACEQ
. O


The O
surface B-PRO
roughness E-PRO
and O
cross B-CONPRI
section E-CONPRI
shape O
of O
the O
FDM S-MANP
fabricated S-CONPRI
parts O
are O
under O
intense O
study O
. O


1 O
and O
2 O
illustrate O
the O
bonding B-CHAR
mechanism E-CHAR
in O
the O
FDM S-MANP
of O
polymer B-MATE
composites E-MATE
along O
cross-section O
of O
printed O
parts O
. O


Several O
building O
rules O
have O
been O
proposed O
to O
improve O
the O
strength S-PRO
and O
accuracy S-CHAR
of O
the O
FDM S-MANP
printed O
parts O
, O
such O
as S-MATE
build O
parts O
to O
ensure O
tensile B-CHAR
loads E-CHAR
are O
carried O
axially O
along O
printed O
directions O
, O
deal O
with O
the O
stress B-CHAR
concentration E-CHAR
at O
corners O
, O
use O
negative O
air O
gap O
to O
increase O
both O
strength S-PRO
and O
stiffness S-PRO
, O
consider O
that O
small O
bead B-CHAR
width E-CHAR
leads O
to O
extra O
printing O
time O
and O
better O
surface B-PARA
quality E-PARA
, O
be S-MATE
aware O
the O
part O
accuracy S-CHAR
affected O
by O
the O
build B-PARA
orientation E-PARA
, O
and O
realize O
that O
tensile S-PRO
loaded O
area S-PARA
tends O
to O
fail O
easier O
than O
compression S-PRO
loaded O
area S-PARA
. O


Recently O
, O
fiber B-FEAT
reinforcement E-FEAT
in O
FDM S-MANP
has O
been O
very O
popular O
amongst O
researchers O
. O


Most O
of O
the O
efforts O
have O
focused O
on O
development O
of O
filaments S-MATE
with O
short B-MATE
fiber E-MATE
additives S-MATE
. O


Inclusion S-MATE
of O
fibers S-MATE
in O
filament S-MATE
reduces O
tape O
swelling S-CONPRI
at O
the O
printing B-MACEQ
head E-MACEQ
during O
deposition S-CONPRI
and O
increases O
the O
stiffness S-PRO
. O


Glass B-MATE
fiber I-MATE
reinforced I-MATE
polypropylene E-MATE
was O
evaluated O
by O
Carneiro O
, O
Silva O
and O
showed O
30 O
% O
and O
40 O
% O
improvement O
for O
the O
modulus O
and O
strength S-PRO
, O
respectively O
, O
compared O
to O
pure B-MATE
PP E-MATE
. O


Vapor O
grown O
carbon B-MATE
fibers E-MATE
and O
single B-MATE
wall I-MATE
carbon I-MATE
nanotubes E-MATE
were O
compounded O
with O
acrylonitrile B-MATE
butadiene I-MATE
styrene E-MATE
for O
the O
FDM S-MANP
process O
. O


The O
VGCFs O
can O
be S-MATE
easily O
aligned O
by O
the O
extrusion B-MANP
process E-MANP
. O


Tensile B-PRO
strength E-PRO
of O
5wt O
% O
of O
VGCFs O
and O
SWNTs S-MATE
filled O
FDM S-MANP
parts O
increased O
18 O
% O
and O
31 O
% O
, O
respectively O
. O


However O
, O
the O
strain S-PRO
to O
failure S-CONPRI
of O
printed O
parts O
reinforced S-CONPRI
with O
VGCFs O
and O
SWNTs S-MATE
was O
dramatically O
decreased O
. O


ABS S-MATE
containing O
oriented O
VGCFs O
and O
SWNTs S-MATE
exhibited O
modulus O
improvements O
up O
to O
93 O
% O
. O


The O
fiber B-FEAT
orientation E-FEAT
can O
be S-MATE
observed O
in O
3 O
. O


Thermotropic B-MATE
liquid I-MATE
crystalline I-MATE
polymers E-MATE
with O
excellent O
tensile B-PRO
strength E-PRO
, O
such O
as S-MATE
ABS S-MATE
and O
polypropylene S-MATE
, O
were O
used O
in O
fiber S-MATE
reinforced O
FDM S-MANP
parts O
in O
order O
to O
overcome O
the O
drawbacks O
of O
low O
aspect B-FEAT
ratio E-FEAT
of O
fiber S-MATE
in O
short B-MATE
fiber E-MATE
filled O
parts O
. O


Processing O
temperature S-PARA
was O
one O
of O
the O
important O
parameters S-CONPRI
which O
affects O
the O
surface B-CHAR
morphology E-CHAR
of O
TLCP O
and O
its O
mechanical S-APPL
behavior O
. O


Higher O
carbon B-MATE
fiber E-MATE
ratio O
has O
a O
high O
maximum B-PARA
decomposition I-PARA
temperature E-PARA
, O
thus O
providing O
high O
thermal B-PRO
stability E-PRO
. O


Ning O
, O
Cong O
evaluated O
the O
effect O
of O
weight S-PARA
ratio O
and O
length O
of O
carbon B-MATE
fiber E-MATE
on O
physical B-PRO
properties E-PRO
of O
the O
FDM S-MANP
samples O
with O
ABS B-MATE
matrix E-MATE
. O


The O
5 O
and O
7.5wt O
% O
carbon B-MATE
fiber E-MATE
content O
showed O
the O
best O
improvement O
in O
tensile B-PRO
strength E-PRO
and O
Young O
's O
modulus O
, O
respectively O
. O


These O
researchers O
have O
also O
concluded O
that O
longer O
carbon B-MATE
fibers E-MATE
can O
increase O
tensile B-PRO
strength E-PRO
and O
Young O
's O
modulus O
at O
the O
expense O
of O
toughness S-PRO
and O
ductility S-PRO
. O


As S-MATE
described O
in O
4 O
, O
with O
aligned O
carbon B-MATE
fiber E-MATE
during O
the O
FDM S-MANP
process O
, O
30wt O
% O
CF-ABS O
composites S-MATE
exhibited O
great O
improvement O
in O
strength S-PRO
and O
Youngmodulus S-PRO
. O


These O
printed O
CF-ABS O
parts O
exhibit O
specific B-PRO
strength E-PRO
higher O
than O
Aluminum S-MATE
. O


The O
triangular O
channels O
between O
beads S-CHAR
decreased O
by O
incorporating O
carbon B-MATE
fibers E-MATE
due O
to O
the O
reduced O
die-swell O
and O
increased O
thermal B-PRO
conductivity E-PRO
. O


However O
, O
inclusion S-MATE
of O
carbon B-MATE
fiber E-MATE
into O
the O
feedstock S-MATE
caused O
internal B-CONPRI
voids E-CONPRI
inside O
of O
the O
beads S-CHAR
responsible O
for O
stress B-CHAR
concentration E-CHAR
, O
resulting O
in O
failure S-CONPRI
at O
lower O
stresses O
. O


It O
can O
be S-MATE
seen O
in O
5 O
that O
the O
FDM S-MANP
samples O
exhibited O
significant O
pore S-PRO
formation O
, O
with O
internal B-CONPRI
voids E-CONPRI
, O
as S-MATE
well O
as S-MATE
voids O
formed O
between O
the O
deposited B-CHAR
beads E-CHAR
during O
printing O
. O


Continuous B-MATE
fiber I-MATE
reinforcement E-MATE
is O
currently O
one O
of O
the O
biggest O
challenges O
researchers O
face S-CONPRI
in O
3D B-MANP
printing E-MANP
of O
polymer B-MATE
composites E-MATE
. O


It O
offers O
significant O
improvement O
in O
mechanical B-CONPRI
properties E-CONPRI
compared O
to O
discontinuous O
fibers S-MATE
, O
however O
, O
there O
is O
still O
no O
robust O
and O
standard S-CONPRI
paradigm O
developed O
for O
3D B-MANP
printing E-MANP
of O
continuous B-MATE
fiber I-MATE
composites E-MATE
. O


Recently O
, O
Matsuzaki O
, O
Ueda O
developed O
an O
innovative O
technique O
for O
in-nozzle O
impregnation S-MANP
of O
continuous B-MATE
fiber E-MATE
and O
thermoplastic B-MATE
matrix E-MATE
. O


The O
resin B-MATE
filament E-MATE
and O
fiber S-MATE
were O
supplied O
separately O
, O
before O
heating S-MANP
and O
mixing S-CONPRI
in O
the O
printing B-MACEQ
head E-MACEQ
. O


The O
schematic O
of O
this O
process S-CONPRI
demonstrating O
the O
printing B-MACEQ
head E-MACEQ
and O
continuous B-MATE
fiber E-MATE
integration O
is O
presented O
in O
6 O
. O


Carbon B-MATE
fibers E-MATE
and O
twisted O
yarns O
of O
natural O
jute O
fibers S-MATE
were O
used O
as S-MATE
reinforcement O
. O


Superiority O
of O
continuous B-MATE
fiber I-MATE
composites E-MATE
versus O
short B-MATE
fiber I-MATE
reinforcement E-MATE
and O
other O
3D B-MANP
printing E-MANP
methods O
can O
be S-MATE
observed O
in O
7 O
. O


Namiki O
, O
Ueda O
implemented O
the O
same O
technique O
for O
printing O
polyactic O
acid O
/carbon O
fiber B-MATE
composite E-MATE
parts O
. O


Some O
gaps O
were O
reported O
between O
PLA B-MATE
filaments E-MATE
, O
which O
can O
be S-MATE
reduced O
by O
increasing O
the O
resolution S-PARA
. O


Tensile B-PRO
strength E-PRO
of O
continuous B-MATE
carbon I-MATE
fiber E-MATE
reinforced O
PLA S-MATE
prepared O
by O
FDM S-MANP
, O
as S-MATE
reported O
by O
Li S-MATE
, O
Li S-MATE
, O
can O
reach O
up O
to O
91MPa O
, O
while O
in O
the O
case O
of O
short B-MATE
carbon I-MATE
fiber E-MATE
, O
it O
is O
only O
68MPa O
. O


Weak O
bonding S-CONPRI
between O
PLA S-MATE
and O
carbon B-MATE
fiber E-MATE
can O
significantly O
affect O
the O
mechanical B-CONPRI
properties E-CONPRI
in O
this O
method O
, O
however O
, O
surface B-MANP
modification E-MANP
of O
carbon B-MATE
fiber E-MATE
bundle O
with O
methylene B-MATE
dichloride E-MATE
and O
PLA S-MATE
particles S-CONPRI
improved O
adhesion S-PRO
and O
increased O
tensile S-PRO
and O
flexural B-PRO
strength E-PRO
. O


8 O
presents O
the O
ultimate O
tensile S-PRO
and O
flexural B-PRO
strength E-PRO
of O
pure O
PLA S-MATE
, O
carbon B-MATE
fiber-PLA E-MATE
, O
and O
modified O
carbon B-MATE
fiber-PLA E-MATE
. O


Green O
circles O
in O
8 O
indicate O
different O
phases O
of O
the O
tensile S-PRO
process S-CONPRI
, O
namely O
, O
loading O
of O
the O
PLA B-MATE
material E-MATE
between O
fixture S-MACEQ
and O
test O
sample S-CONPRI
at O
the O
beginning O
of O
the O
test O
and O
a O
slight O
drop O
of O
the O
curve O
slope O
due O
to O
fiber-matrix O
interface S-CONPRI
debonding O
. O


Marked O
circles O
in O
8 O
signify O
the O
process S-CONPRI
of O
load O
change O
from O
resin S-MATE
to O
fiber S-MATE
at O
the O
beginning O
of O
the O
test O
. O


The O
next O
circle O
in O
8 O
shows O
the O
plastic S-MATE
elongation S-PRO
of O
PLA S-MATE
polymer O
chains O
that O
continue O
to O
bear O
load O
after O
the O
failure S-CONPRI
of O
carbon B-MATE
fibers E-MATE
. O


Tian O
, O
Liu O
performed O
a O
systemic O
analysis O
on O
interface S-CONPRI
and O
performance S-CONPRI
of O
printed O
continuous B-MATE
carbon I-MATE
fiber E-MATE
reinforced O
PLA B-MATE
composites E-MATE
and O
the O
effect O
of O
process B-CONPRI
parameters E-CONPRI
on O
the O
temperature S-PARA
and O
pressure S-CONPRI
in O
the O
process S-CONPRI
. O


9 O
shows O
cross-section O
of O
the O
tensile B-MACEQ
bar E-MACEQ
and O
continuous B-MATE
fibers E-MATE
in O
the O
fracture S-CONPRI
surface O
, O
and O
10 O
demonstrates O
the O
capability O
of O
this O
method O
in O
3D B-MANP
printing E-MANP
large O
curvatures O
, O
without O
losing O
the O
continuous B-MATE
fiber I-MATE
reinforcement E-MATE
. O


Melenka O
, O
Cheung O
evaluated O
continuous B-MATE
Kevlar I-MATE
fiber-reinforced E-MATE
3D B-MANP
printed E-MANP
Nylon O
structures O
using O
commercial O
desktop O
printers S-MACEQ
in O
order O
to O
predict O
the O
tensile B-PRO
properties E-PRO
. O


Stiffness S-PRO
and O
ultimate B-PRO
strength E-PRO
showed O
significant O
increase O
with O
high O
volume S-CONPRI
of O
fiber B-FEAT
reinforcement E-FEAT
. O


Carbon B-MATE
fibers E-MATE
were O
placed O
between O
layers O
of O
3D B-MANP
printed E-MANP
polymer O
to O
improve O
strength S-PRO
and O
fatigue B-PRO
life E-PRO
, O
while O
thermal B-MANP
treatment E-MANP
was O
performed O
to O
further O
increase O
the O
mechanical B-CONPRI
properties E-CONPRI
. O


However O
, O
Van O
Der O
Klift O
, O
Koga O
showed O
that O
increasing O
the O
number O
of O
carbon B-MATE
fiber E-MATE
layers O
results O
in O
larger O
void S-CONPRI
areas S-PARA
, O
which O
had O
negative O
effect O
on O
the O
tensile B-PRO
strength E-PRO
. O


Impregnation S-MANP
of O
plastics S-MATE
into O
the O
fiber B-MATE
bundle E-MATE
could O
be S-MATE
achieved O
in O
the O
temperature B-PARA
range E-PARA
of O
200Layer O
thickness O
of O
0.4and O
hatch B-PARA
spacing E-PARA
of O
about O
0.6mm O
guaranteed O
bonding B-PRO
strength E-PRO
between O
lines O
and O
layers O
. O


These O
parameters S-CONPRI
could O
achieve O
maximum O
flexural B-PRO
strength E-PRO
of O
335MPa O
and O
flexural O
modulus O
of O
30GPa O
. O


2.2 O
Laminated B-MANP
object I-MANP
manufacturing E-MANP
In O
LOM S-MANP
, O
which O
was O
developed O
by O
Helisys B-FEAT
of I-FEAT
Torrance E-FEAT
, O
CA S-MATE
and O
shipped O
in O
1991 O
, O
3D B-APPL
parts E-APPL
are O
manufactured S-CONPRI
by O
cutting S-MANP
2D S-CONPRI
cross-sections O
with O
a O
laser S-ENAT
or O
cutter O
and O
sequentially O
laminating O
the O
sheets S-MATE
. O


Paper O
, O
metals S-MATE
, O
plastics S-MATE
, O
fabrics O
, O
synthetic B-MATE
materials E-MATE
, O
and O
composites S-MATE
are O
amongst O
the O
materials S-CONPRI
that O
can O
be S-MATE
utilized O
in O
LOM S-MANP
. O


Polymer B-MATE
matrix I-MATE
composites E-MATE
of O
C-shaped O
panels O
were O
directly O
fabricated S-CONPRI
by O
curved O
LOM S-MANP
. O


A O
vacuum B-MANP
thermoforming E-MANP
apparatus O
was O
applied O
to O
bond O
commercial O
prepregs S-MATE
. O


The O
shear B-PRO
strength E-PRO
of O
fabricated S-CONPRI
composites S-MATE
was O
measured O
to O
be S-MATE
approximately O
24.8MPa O
, O
which O
was O
suggested O
by O
the O
authors O
to O
be S-MATE
acceptable O
for O
normal O
applications O
. O


The O
LOM S-MANP
process O
was O
applied O
to O
print S-MANP
3D B-APPL
parts E-APPL
of O
unidirectional S-CONPRI
and O
continuous B-MATE
glass I-MATE
fibers E-MATE
with O
52and O
epoxy B-MATE
matrix E-MATE
. O


The O
final O
part O
and O
its O
cross B-CONPRI
section E-CONPRI
are O
presented O
in O
13 O
. O


Decent O
interfacial B-CONPRI
bonding E-CONPRI
was O
shown O
by O
interlayer B-MATE
microstructures E-MATE
of O
LOM B-MATE
polymer I-MATE
composites E-MATE
. O


The O
major O
issue O
for O
the O
LOM S-MANP
process O
was O
the O
incapability O
of O
the O
heat B-MACEQ
roller E-MACEQ
to O
bring O
parts O
to O
full O
consolidation S-CONPRI
and O
cure S-CONPRI
. O


It O
is O
helpful O
to O
increase O
the O
interface S-CONPRI
strength O
and O
reduce O
void S-CONPRI
contents O
to O
under O
5 O
% O
by O
a O
post O
consolidation S-CONPRI
cycle O
. O


Sonmez O
and O
Hahn O
studied O
heat B-CONPRI
transfer E-CONPRI
and O
stress S-PRO
in O
LOM S-MANP
to O
understand O
the O
effect O
of O
process B-CONPRI
parameters E-CONPRI
on O
the O
resulting O
stress S-PRO
and O
temperature S-PARA
distributions S-CONPRI
. O


Large O
rollers O
were O
more O
favorable O
for O
bonding S-CONPRI
, O
due O
to O
a O
less O
concentrated O
stress B-PRO
distribution E-PRO
. O


Recently O
, O
a O
new O
method O
called O
laser S-ENAT
assisted O
AM S-MANP
for O
continuous B-MATE
fiber I-MATE
reinforced I-MATE
thermoplastic E-MATE
composites S-MATE
was O
developed O
by O
researchers O
at O
Kansas O
State O
University O
, O
with O
the O
motivation O
of O
reducing O
the O
waste O
associated O
with O
LOM S-MANP
. O


The O
authors O
proposed O
using O
prepreg S-MATE
tape O
instead O
of O
pre-cut O
prepreg S-MATE
sheet O
. O


The O
tape O
strips O
were O
placed O
layer B-CONPRI
by I-CONPRI
layer E-CONPRI
using O
a O
CO2 S-MATE
laser O
beam S-MACEQ
and O
consolidation S-CONPRI
roller O
, O
prior O
to O
laser B-MANP
cutting E-MANP
of O
each O
layer S-PARA
. O


14 O
demonstrates O
this O
process S-CONPRI
schematically O
, O
as S-MATE
well O
as S-MATE
its O
tensile B-PRO
properties E-PRO
in O
comparison O
with O
various O
AM S-MANP
and O
conventional O
methods O
of O
composite B-MANP
manufacturing E-MANP
. O


This O
method O
exhibits O
superior O
mechanical B-CONPRI
properties E-CONPRI
due O
to O
continuous B-MATE
fiber I-MATE
reinforcement E-MATE
, O
high O
fiber B-FEAT
weight I-FEAT
ratio E-FEAT
, O
minimized B-PARA
void I-PARA
content E-PARA
, O
and O
superior O
interfacial B-CONPRI
bonding E-CONPRI
. O


2.3 O
Stereolithography S-MANP
The O
3D B-APPL
parts E-APPL
fabricated O
by O
SL S-MANP
exhibit O
weak O
mechanical B-CONPRI
properties E-CONPRI
, O
which O
hinder O
their O
further O
applications O
as S-MATE
functional O
components S-MACEQ
under O
loading O
conditions O
. O


However O
, O
adding O
fibers S-MATE
to O
the O
resin S-MATE
can O
increase O
the O
potential O
of O
SL S-MANP
in O
3D B-MANP
printing E-MANP
functional O
components S-MACEQ
. O


Although O
continuous B-MATE
fiber E-MATE
is O
ideal O
for O
reinforcement S-PARA
, O
high O
weight S-PARA
ratio O
of O
short B-MATE
fibers E-MATE
can O
yield O
comparable O
results O
, O
however O
, O
their O
efficiency O
is O
limited O
due O
to O
fracture S-CONPRI
during O
mixing S-CONPRI
, O
random O
orientation S-CONPRI
, O
and O
un-even O
length O
. O


Multi-wall O
carbon B-MATE
nanotubes E-MATE
with O
a O
low O
weight S-PARA
ratio O
were O
mixed O
in O
SL S-MANP
resin S-MATE
by O
mechanical B-CONPRI
mixing E-CONPRI
and O
ultrasonic B-CHAR
dispersion E-CHAR
. O


The O
tensile B-PRO
strength E-PRO
and O
fracture S-CONPRI
strength O
were O
increased O
by O
5.7 O
% O
and O
26 O
% O
, O
respectively O
, O
by O
adding O
0.025wt O
% O
MWNTs O
. O


Carbon B-MATE
fiber E-MATE
has O
been O
successfully O
applied O
to O
reinforce O
polymers S-MATE
, O
however O
, O
the O
primary O
issue O
for O
utilizing O
carbon B-MATE
fiber E-MATE
in O
SL S-MANP
is O
that O
it O
is O
opaque O
to O
the O
UV B-ENAT
light E-ENAT
. O


Consequently O
, O
regions O
of O
the O
resin S-MATE
blocked O
by O
carbon B-MATE
fibers E-MATE
remains O
uncured O
by O
UV B-ENAT
light E-ENAT
. O


Using O
glass B-MATE
fiber E-MATE
instead O
of O
carbon B-MATE
fiber E-MATE
can O
be S-MATE
beneficial O
for O
decreasing O
the O
opacity O
to O
UV B-ENAT
light E-ENAT
. O


The O
SL S-MANP
plus O
vacuum B-MANP
cast E-MANP
process S-CONPRI
was O
investigated O
to O
improve O
the O
tensile B-PRO
strength E-PRO
. O


Tensile S-PRO
samples S-CONPRI
produced O
by O
SL S-MANP
and O
polymer-glass O
fiber S-MATE
nonwoven-polymer O
sandwich B-FEAT
structures E-FEAT
were O
introduced O
by O
vacuum B-MANP
cast E-MANP
, O
which O
showed O
a O
significant O
increase O
of O
36 O
% O
in O
ultimate B-PRO
tensile I-PRO
strength E-PRO
and O
11 O
% O
increase O
in O
stiffness S-PRO
. O


The O
viscosity S-PRO
of O
the O
resin S-MATE
, O
especially O
at O
low O
shear O
rates O
, O
increased O
in O
the O
composite B-MATE
resins E-MATE
with O
significant O
volume B-PARA
fractions E-PARA
of O
fibers S-MATE
. O


The O
surface S-CONPRI
coating S-APPL
of O
fibers S-MATE
can O
effectively O
reduce O
the O
viscosity S-PRO
, O
which O
is O
an O
advantage O
which O
allows O
processing O
of O
resins S-MATE
with O
higher O
fiber B-PRO
concentrations E-PRO
. O


Laser S-ENAT
scanning O
based O
SL S-MANP
was O
used O
to O
add O
20vol O
% O
of O
short O
glass B-MATE
fibers E-MATE
into O
an O
acrylic S-MATE
based O
photo O
polymer S-MATE
. O


15 O
shows O
the O
microstructures S-MATE
of O
molded O
and O
laser-scanned O
specimen O
with O
20vol O
% O
glass B-MATE
fiber E-MATE
. O


Fiber B-MATE
filled I-MATE
composites E-MATE
represents O
a O
higher O
elastic B-PRO
modulus E-PRO
and O
ultimate B-PRO
tensile I-PRO
strength E-PRO
. O


The O
shrinkage S-CONPRI
of O
fiber B-MATE
reinforced I-MATE
composites E-MATE
was O
also O
observed O
to O
be S-MATE
lower O
than O
their O
non-reinforced O
counterparts O
. O


Dual O
porlymerization O
scheme O
, O
including O
UV S-CONPRI
radiation S-MANP
and O
thermal B-MANP
treatments E-MANP
, O
was O
proposed O
to O
cure S-CONPRI
resins O
containing O
a O
high O
volume S-CONPRI
ratio O
of O
carbon B-MATE
fibers E-MATE
. O


It O
was O
estimated O
that O
one O
quarter O
of O
resin S-MATE
remains O
uncured O
, O
which O
was O
primarily O
inside O
of O
carbon B-MATE
fibers E-MATE
. O


After O
an O
hour O
of O
thermal B-MANP
treatment E-MANP
, O
the O
tensile B-PRO
strength E-PRO
was O
increased O
by O
95 O
% O
. O


Llewellyn-Jones O
, O
Drinkwater O
used O
ultrasonic B-CHAR
manipulation E-CHAR
to O
distribute O
glass B-MATE
microfibers E-MATE
in O
the O
resin S-MATE
. O


A O
variety O
of O
fiber B-FEAT
orientation E-FEAT
angles O
were O
achieved O
, O
demonstrating O
the O
versatility O
of O
the O
process S-CONPRI
. O


This O
method O
allows O
smart O
material S-MATE
fabrication S-MANP
, O
such O
as S-MATE
resin-filled O
capsules O
for O
self-healing O
or O
piezoelectric O
particles S-CONPRI
for O
energy B-CONPRI
harvesting E-CONPRI
. O


2.4 O
Extrusion S-MANP
Extrusion O
, O
as S-MATE
one O
of O
the O
most O
recent O
developments O
in O
3D B-MANP
printing E-MANP
, O
emerged O
to O
overcome O
the O
limitations O
of O
the O
FDM S-MANP
method O
with O
its O
versatility O
and O
cost-effectiveness O
. O


In O
this O
AM B-MANP
technique E-MANP
, O
layers O
of O
the O
material S-MATE
solution O
are O
directly O
deposited O
in O
a O
volatile O
solvent O
to O
produce O
freeform B-CONPRI
3D E-CONPRI
structures O
. O


Lightweight S-CONPRI
cellular B-CHAR
carbon I-CHAR
fiber E-CHAR
and O
SiC B-MATE
whiskers E-MATE
filled O
composites S-MATE
have O
been O
demonstrated O
by O
applying O
3D S-CONPRI
extrusion O
printing O
method O
, O
as S-MATE
described O
in O
16 O
. O


Epoxy-based B-MATE
inks E-MATE
, O
which O
exhibited O
the O
desired O
viscoelasticity S-PRO
and O
long O
pot-life O
in O
the O
absence O
and O
presence O
of O
highly O
anisotropic S-PRO
carbon O
fibers S-MATE
, O
were O
prepared O
. O


The O
SiC-filled O
and O
SiC/C O
filled O
transverse O
specimens O
showed O
a O
substantial O
increase O
in O
Youngmodulus S-PRO
, O
over O
the O
pure B-MATE
resin E-MATE
from O
2.66 O
to O
10.61 O
and O
8.06 O
respectively O
. O


Tensile B-PRO
strength E-PRO
of O
printed O
composites S-MATE
was O
comparable O
to O
the O
cast S-MANP
epoxy O
resin S-MATE
samples O
, O
with O
longitudinal O
specimens O
exhibiting O
slightly O
higher O
strength S-PRO
than O
that O
the O
transverse O
specimens O
. O


17 O
shows O
a O
comparison O
in O
tensile B-PRO
strength E-PRO
of O
3D B-MANP
printed E-MANP
tensile O
bars O
, O
using O
various O
fillers O
, O
as S-MATE
well O
as S-MATE
their O
microstructure S-CONPRI
. O


PLA/MWNTs O
composite S-MATE
was O
used O
to O
fabricate S-MANP
conductive O
3D S-CONPRI
microstructures O
, O
with O
arbitrary O
shapes O
as S-MATE
small O
as S-MATE
100with O
a O
method O
called O
liquid O
deposition B-CONPRI
modeling E-CONPRI
. O


2.5 O
Selective B-MANP
laser I-MANP
sintering E-MANP
SLS O
is O
a O
powder S-MATE
based O
AM B-MANP
process E-MANP
. O


The O
laser B-ENAT
scans E-ENAT
the O
powder B-MACEQ
bed E-MACEQ
, O
layer B-CONPRI
by I-CONPRI
layer E-CONPRI
, O
to O
form O
a O
3D B-CONPRI
structure E-CONPRI
, O
as S-MATE
demonstrated O
in O
18 O
. O


It O
mainly O
deals O
with O
wax S-MATE
, O
ceramics S-MATE
, O
metals S-MATE
and O
polymers S-MATE
. O


Major O
polymers S-MATE
used O
by O
SLS S-MANP
include O
nylon S-MATE
, O
i.e O
. O


polyamide S-MATE
, O
crystalline B-MATE
thermoplastics E-MATE
: O
polyethylene S-MATE
, O
PEEK S-MATE
, O
and O
PCL S-MATE
. O


SLS S-MANP
can O
be S-MATE
categorized O
in O
solid B-MANP
state I-MANP
sintering E-MANP
, O
liquid B-PRO
phase E-PRO
sintering-partial O
melting S-MANP
, O
full O
melting S-MANP
, O
and O
chemically O
induced O
binding O
. O


SSS O
is O
a O
thermal O
process S-CONPRI
that O
occurs O
at O
temperatures S-PARA
between O
TMelt O
/2 O
and O
TMelt O
, O
where O
TMelt O
is O
the O
melting B-PARA
temperature E-PARA
. O


In O
liquid B-PRO
phase E-PRO
sintering-partial O
melting S-MANP
, O
usually O
the O
binder S-MATE
material O
becomes O
liquefied O
, O
while O
structural O
material S-MATE
remains O
solid O
. O


The O
full O
melting S-MANP
technique O
melts O
the O
powder S-MATE
entirely O
and O
exhibits O
properties S-CONPRI
comparable O
to O
those O
of O
bulk O
material S-MATE
. O


It O
can O
be S-MATE
applied O
to O
a O
wide O
variety O
of O
materials S-CONPRI
, O
however O
, O
the O
long O
process B-CONPRI
time E-CONPRI
and O
preheating S-MANP
of O
powders S-MATE
is O
necessary O
. O


1 O
lists O
the O
range S-PARA
of O
materials S-CONPRI
and O
their O
associated O
binding O
mechanism S-CONPRI
in O
SLS S-MANP
. O


CNT S-MATE
was O
added O
in O
Polyamide B-MATE
12 E-MATE
in O
order O
to O
improve O
the O
mechanical S-APPL
behaviors O
. O


The O
laser S-ENAT
sintered O
parts O
had O
13 O
% O
greater O
flexural O
modulus O
, O
10.9 O
% O
higher O
flexural B-PRO
strength E-PRO
, O
and O
54 O
% O
larger O
Youngmodulus S-PRO
. O


The O
crystallization S-CONPRI
temperature O
of O
PA12-CNT O
powder S-MATE
was O
increased O
, O
compared O
to O
the O
pure B-MATE
PA12 E-MATE
, O
which O
was O
responsible O
in O
hindering O
the O
movement O
of O
PA12 S-MATE
chains O
by O
the O
interfacial B-ENAT
force E-ENAT
between O
CNTs S-MATE
and O
PA12 S-MATE
. O


However O
, O
the O
porosity S-PRO
also O
increased O
in O
the O
CNT B-MATE
composites E-MATE
. O


MWNTs O
were O
also O
mixed O
with O
PA12 S-MATE
for O
the O
investigation O
of O
its O
effect O
on O
mechanical B-CONPRI
properties E-CONPRI
. O


Goodridge O
, O
Shofner O
also O
confirmed O
enhancement O
in O
mechanical B-CONPRI
properties E-CONPRI
of O
PA12 S-MATE
with O
inclusion S-MATE
of O
CNT S-MATE
, O
exhibiting O
22 O
% O
increase O
in O
storage O
modulus O
. O


A O
high O
volume S-CONPRI
ratio O
of O
carbon B-MATE
fiber E-MATE
was O
added O
into O
PA12 S-MATE
. O


CNT-coated O
PA12 S-MATE
also O
improved O
heat B-CONPRI
conduction E-CONPRI
and O
heat B-PRO
absorption E-PRO
compared O
to O
pure O
PA12.. O
Furthermore O
, O
simulation S-ENAT
results O
on O
laser B-MANP
sintering E-MANP
of O
PA12-CNT O
suggested O
that O
inclusion S-MATE
of O
CNT S-MATE
helps O
the O
laser B-PARA
heat E-PARA
to O
be S-MATE
conducted O
wider O
and O
deeper O
into O
the O
powder B-MACEQ
bed E-MACEQ
. O


The O
result O
of O
these O
simulations S-ENAT
can O
be S-MATE
observed O
in O
19 O
. O


Uniform O
distribution S-CONPRI
of O
carbon B-MATE
fibers E-MATE
and O
good O
interfacial O
adhesion S-PRO
between O
fibers S-MATE
and O
matrix O
was O
achieved O
by O
pre-modification O
of O
carbon B-MATE
fibers E-MATE
through O
oxidation S-MANP
. O


By O
adding O
the O
maximum O
weight S-PARA
ratio O
of O
carbon B-MATE
fibers E-MATE
, O
the O
flexural B-PRO
strength E-PRO
and O
flexural O
modulus O
were O
enhanced O
114 O
% O
and O
243.4 O
% O
, O
respectively O
. O


Glass B-MATE
beads E-MATE
were O
used O
asadditives O
in O
SLS S-MANP
of O
Nylon S-MATE
powders O
, O
in O
order O
to O
determine O
the O
mechanical B-CONPRI
properties E-CONPRI
, O
as S-MATE
a O
function O
of O
material S-MATE
composition S-CONPRI
. O


Zhu O
, O
Yan O
proposed O
a O
novel O
method O
to O
prepare O
high-performance O
carbon S-MATE
fibers/PA12/epoxy O
ternary O
composites S-MATE
by O
infiltrating S-CONPRI
the O
porous S-PRO
green O
carbon S-MATE
fibers/PA12 O
parts O
built O
by O
SLS S-MANP
, O
with O
high-performance O
thermosetting O
epoxy S-MATE
resin O
, O
prior O
to O
curing S-MANP
the O
resin S-MATE
; O
this O
process S-CONPRI
is O
described O
in O
20 O
. O


The O
end O
result O
is O
a O
ternary B-MATE
composite I-MATE
system E-MATE
with O
novolac B-MATE
epoxy I-MATE
resin E-MATE
, O
reinforced S-CONPRI
with O
carbon B-MATE
fibers E-MATE
coated O
with O
a O
thin O
595nm O
layer S-PARA
of O
PA12 S-MATE
. O


This O
method O
with O
33 O
% O
volume B-PARA
fraction E-PARA
of O
carbon B-MATE
fibers E-MATE
yielded O
an O
ultimate B-PRO
tensile I-PRO
strength E-PRO
of O
101.03MPa O
and O
a O
flexural B-PRO
strength E-PRO
of O
153.43MPa O
. O


3 O
Four-Dimensional O
printing O
of O
active O
Polymer-Fiber B-MATE
composites E-MATE
Addition O
of O
fiber S-MATE
in O
3D B-MANP
printed E-MANP
polymers O
is O
not O
always O
for O
improving O
the O
mechanical B-CONPRI
properties E-CONPRI
. O


Fibers S-MATE
, O
as S-MATE
well O
other O
additives S-MATE
, O
are O
also O
used O
in O
manufacturing S-MANP
of O
smart O
composites S-MATE
, O
to O
control O
the O
structure S-CONPRI
transformation O
. O


This O
implementation O
in O
3D B-MANP
printing E-MANP
opened O
a O
new O
field O
called O
4D B-MANP
printing E-MANP
. O


4D B-MANP
printing E-MANP
refers O
to O
a O
multi-material B-MANP
printing E-MANP
with O
the O
ability O
to O
transform O
over O
time O
or O
change O
its O
shape O
after O
the O
printing O
. O


These O
structures O
can O
be S-MATE
programmable O
and O
transformed O
from O
one O
or O
two O
dimensional O
structures O
to O
3D B-APPL
objects E-APPL
. O


Recent O
advances O
in O
AM S-MANP
made O
precise O
placement O
of O
material S-MATE
at O
micro-scale S-CONPRI
during O
3D B-MANP
printing E-MANP
of O
complex B-CONPRI
structures E-CONPRI
possible O
. O


This O
allows O
implementing O
programmable O
and O
computational O
material S-MATE
in O
AM S-MANP
in O
order O
to O
control O
the O
shape O
of O
the O
material S-MATE
after O
printing O
. O


These O
materials S-CONPRI
can O
change O
their O
shape O
under O
light O
, O
temperature S-PARA
change O
, O
or O
immersion O
into O
a O
solvent O
. O


3D B-ENAT
printing I-ENAT
technology E-ENAT
has O
enabled O
active O
material S-MATE
to O
achieve O
an O
even O
higher O
complexity S-CONPRI
and O
accuracy S-CHAR
. O


Fiber B-MATE
additives E-MATE
play O
an O
important O
role O
in O
4D B-MANP
printing E-MANP
; O
controlling O
the O
fiber B-FEAT
alignment E-FEAT
allows O
programmable O
transformation O
, O
right O
out O
of O
the O
printing O
bed S-MACEQ
. O


Ge S-MATE
, O
Qi O
developed O
a O
method O
to O
print S-MANP
thermomechanically O
programmable O
composites S-MATE
with O
complex B-PRO
shapes E-PRO
. O


The O
matrix O
and O
fibers S-MATE
used O
were O
an O
elastomer S-MATE
and O
a O
glassy O
polymer S-MATE
, O
with O
tailored O
thermomechanical S-CONPRI
behavior O
, O
respectively O
. O


21 O
demonstrates O
the O
process S-CONPRI
in O
which O
the O
inkjet S-MANP
deposited O
material S-MATE
on O
the O
bed S-MACEQ
, O
prior O
to O
photopolymerization S-MANP
of O
the O
film O
by O
UV B-ENAT
light E-ENAT
, O
forms O
a O
layer S-PARA
. O


Gladman O
, O
Matsumoto O
printed O
plant-inspired O
architectures O
, O
with O
a O
hydrogel B-MATE
composite E-MATE
ink O
, O
composed O
of O
stiff O
cellulose B-MATE
fibrils E-MATE
embedded O
in O
a O
soft O
acrylamide B-MATE
matrix E-MATE
. O


Alignment O
of O
cellulose B-MATE
fibrils E-MATE
controlled O
the O
swelling S-CONPRI
of O
the O
3D B-APPL
printed I-APPL
part E-APPL
upon O
immersion O
in O
water O
. O


These O
structures O
are O
programmable O
based O
on O
the O
printing O
direction O
and O
orientation S-CONPRI
of O
embedded O
cellulose B-MATE
fibrils E-MATE
inside O
the O
structure S-CONPRI
. O


4 O
Modeling S-ENAT
and O
analytical O
techniques O
Polymer-fiber B-MATE
composites E-MATE
produced O
by O
AM S-MANP
can O
be S-MATE
analyzed O
using O
existing O
theories O
based O
on O
the O
manufacturing S-MANP
technique O
and O
the O
reinforcement S-PARA
type O
. O


Existing O
macro S-FEAT
and O
micro O
mechanical B-CONPRI
modelling E-CONPRI
techniques O
can O
be S-MATE
applied O
to O
AM S-MANP
with O
slight O
modifications O
. O


Microstructure S-CONPRI
of O
3D B-APPL
printed I-APPL
parts E-APPL
often O
differ O
from O
those O
prepared O
by O
traditional B-MANP
manufacturing E-MANP
methods O
and O
with O
the O
immerging O
of O
new O
AM S-MANP
methods O
, O
there O
is O
a O
demand O
for O
modelling S-ENAT
and O
analysis O
of O
these O
structures O
. O


4.1 O
Short B-MATE
fiber I-MATE
composite E-MATE
theories O
There O
are O
several O
theories O
for O
predicting O
the O
properties S-CONPRI
of O
short B-MATE
fiber I-MATE
composites E-MATE
. O


Depending O
on O
their O
assumptions O
, O
they O
can O
be S-MATE
applied O
to O
various O
3D B-MANP
printing E-MANP
methods O
. O


Fibers B-FEAT
alignment E-FEAT
, O
shape O
, O
length O
, O
and O
its O
bonding S-CONPRI
with O
the O
matrix O
are O
crucial O
in O
accuracy S-CHAR
of O
the O
modelling S-ENAT
. O


The O
modified O
rule B-CONPRI
of I-CONPRI
mixtures E-CONPRI
is O
the O
simplest O
method O
to O
predict O
the O
tensile B-PRO
properties E-PRO
of O
short B-MATE
fiber I-MATE
composites E-MATE
, O
by O
assuming O
perfect O
fiberinterfacial B-CONPRI
bonding E-CONPRI
. O


MROM O
is O
given O
by O
cu S-MATE
= O
1 O
2 O
V S-MATE
f S-MANP
fu O
+ O
V S-MATE
m O
m O
where O
1 O
2 O
is O
fiber B-PRO
efficiency I-PRO
factor E-PRO
for O
the O
strength S-PRO
of O
the O
composite S-MATE
, O
in O
which O
, O
1 O
and O
2 O
are O
the O
fiber B-FEAT
orientation E-FEAT
and O
fiber B-FEAT
length I-FEAT
factors E-FEAT
, O
respectively O
; O
cu S-MATE
and O
fu O
are O
ultimate B-PRO
strength E-PRO
of O
the O
composite S-MATE
and O
fiber S-MATE
, O
respectively O
; O
V S-MATE
f S-MANP
and O
V S-MATE
m O
represent O
the O
volume B-PARA
fraction E-PARA
of O
the O
fiber S-MATE
and O
matrix O
; O
and O
m O
is O
the O
matrix B-CONPRI
stress E-CONPRI
at O
the O
composite B-CHAR
failure E-CHAR
. O


If O
the O
fiber B-CONPRI
length E-CONPRI
is O
equal O
to O
L O
and O
uniform O
, O
fiber B-FEAT
orientation E-FEAT
factor O
is O
equal O
to O
1 O
and O
fiber B-FEAT
length I-FEAT
factor E-FEAT
is O
given O
by O
the O
critical B-PRO
fiber I-PRO
length E-PRO
, O
r O
f S-MANP
is O
fiber B-FEAT
radius E-FEAT
, O
and O
i O
is O
interfacial B-PRO
shear I-PRO
stress E-PRO
between O
matrix O
and O
fibers S-MATE
. O


In O
order O
to O
consider O
the O
effect O
of O
fiber B-FEAT
orientation E-FEAT
and O
non-uniform O
fiber B-CONPRI
length E-CONPRI
in O
the O
model S-CONPRI
, O
1 O
and O
2 O
should O
be S-MATE
modified O
. O


Modified O
Kelly O
and O
Tyson O
model S-CONPRI
proposed O
for O
fibers S-MATE
shorter O
and O
longer O
than O
the O
critical B-PRO
fiber I-PRO
length E-PRO
with O
considering O
fiber B-FEAT
orientation E-FEAT
, O
as S-MATE
follows O
. O


However O
, O
fiber B-FEAT
orientation E-FEAT
factor O
1 O
in O
this O
model S-CONPRI
is O
fitted O
empirically O
. O


Fu O
and O
Lauke O
used O
two O
probability B-CONPRI
density I-CONPRI
functions E-CONPRI
for O
modelling S-ENAT
the O
fiber B-CONPRI
length E-CONPRI
and O
fiber B-FEAT
orientation E-FEAT
distributions S-CONPRI
with O
the O
intention O
of O
predicting O
the O
elastic S-PRO
properties O
. O


There O
are O
various O
theories O
for O
predicting O
the O
stiffness S-PRO
properties S-CONPRI
of O
short-fiber O
composites S-MATE
. O


The O
Morimodel O
is O
another O
well-known O
theory O
that O
considers O
a O
non-dilute O
composite S-MATE
containing O
many O
identical O
spheroidal O
particles S-CONPRI
. O


It O
is O
assumed O
that O
the O
composite S-MATE
experiences O
an O
average S-CONPRI
stress O
different O
from O
that O
of O
the O
applied O
stress S-PRO
. O


Longitudinal O
and O
transverse O
elastic B-PRO
moduli E-PRO
in O
Morimodel O
are O
where O
V S-MATE
f S-MANP
is O
the O
volume B-PARA
fraction E-PARA
of O
filler O
and O
m O
is O
the O
Poissonratio O
of O
the O
matrix O
; O
A1 O
, O
A2 O
, O
A3 O
, O
A4 O
, O
A5 O
, O
and O
A O
are O
functions O
of O
the O
Eshelbytensor O
and O
the O
properties S-CONPRI
of O
the O
fiber S-MATE
and O
the O
matrix O
, O
with O
more O
explanation O
given O
in O
. O


In O
theory O
, O
the O
aforementioned O
equations O
for O
short B-MATE
fiber I-MATE
composites E-MATE
, O
can O
be S-MATE
used O
to O
model S-CONPRI
3D B-APPL
printed I-APPL
parts E-APPL
, O
however O
, O
length O
and O
orientation S-CONPRI
of O
the O
fibers S-MATE
used O
in O
the O
process S-CONPRI
should O
match O
the O
assumptions O
. O


FDM S-MANP
, O
SLS S-MANP
, O
and O
extrusion S-MANP
with O
short B-MATE
fiber I-MATE
reinforcement E-MATE
can O
be S-MATE
modeled O
with O
these O
analytical O
methods O
. O


However O
, O
3D B-APPL
printed I-APPL
parts E-APPL
often O
contain O
considerable O
fraction S-CONPRI
of O
void S-CONPRI
content O
and O
modifications O
may O
be S-MATE
necessary O
when O
applying O
these O
methods O
on O
additive B-MANP
manufacturing E-MANP
. O


Void S-CONPRI
in O
composite B-MATE
materials E-MATE
are O
comprehensively O
explained O
in O
. O


4.2 O
Classical B-CONPRI
laminate I-CONPRI
plate I-CONPRI
theory E-CONPRI
CLPT O
is O
an O
extension O
of O
the O
classical B-CONPRI
plate I-CONPRI
theory E-CONPRI
for O
isotropic S-PRO
and O
homogeneous B-MATE
materials E-MATE
with O
some O
modifications O
to O
reflect O
the O
inhomogeneity O
of O
orthotropic S-MATE
materials S-CONPRI
in O
thickness O
direction O
. O


CLTP O
is O
applicable O
to O
all O
3D B-APPL
printed I-APPL
parts E-APPL
that O
exhibit O
orthogonal O
behavior O
. O


Here O
, O
we O
present O
a O
brief O
summary O
of O
CLPT O
for O
laminated B-MACEQ
plates E-MACEQ
consisting O
of O
multiple O
unidirectional B-BIOP
laminae E-BIOP
. O


The O
stiffness B-PRO
matrix E-PRO
of O
each O
ply O
can O
be S-MATE
described O
as S-MATE
. O


Transformed O
reduced O
stiffness B-PRO
matrix E-PRO
for O
various O
fiber B-FEAT
orientation E-FEAT
can O
be S-MATE
computed O
using O
transformation B-CONPRI
matrix E-CONPRI
as S-MATE
follow O
, O
and O
is O
the O
angle O
of O
the O
fiber B-FEAT
reinforcement E-FEAT
. O


Then O
, O
in-plane O
, O
coupling O
, O
and O
bending S-MANP
stiffness O
matrices O
can O
be S-MATE
obtained O
by O
, O
respectively O
where O
z O
represents O
the O
vertical S-CONPRI
position O
in O
the O
ply O
from O
the O
midplane S-CONPRI
. O


Finally O
, O
we O
can O
write O
a O
connection O
between O
the O
applied O
loads O
and O
the O
associated O
strains O
in O
the O
laminate S-CONPRI
, O
as S-MATE
follows O
, O
N S-MATE
M O
= O
A O
B S-MATE
B O
D O
0 O
where O
N S-MATE
is O
normal O
stress S-PRO
resultants O
, O
M O
is O
moment O
resultants O
, O
0 O
represent O
strain S-PRO
term O
in O
midplane S-CONPRI
, O
and O
is O
the O
twist O
of O
the O
laminated B-MACEQ
plate E-MACEQ
. O


The O
strain S-PRO
along O
the O
plate O
thickness O
can O
be S-MATE
given O
by O
, O
x O
y S-MATE
xy O
= O
x O
0 O
y S-MATE
0 O
xy O
0 O
+ O
z O
x O
y S-MATE
xy O
Additionally O
, O
by O
using O
the O
same O
principle O
, O
CLPT O
can O
be S-MATE
applied O
to O
evaluate O
the O
strength S-PRO
and O
elastic B-PARA
constants E-PARA
of O
FDM S-MANP
printed O
parts O
with O
void S-CONPRI
content O
. O


As S-MATE
mentioned O
earlier O
, O
FDM S-MANP
process O
is O
associated O
with O
void S-CONPRI
formation O
between O
printing O
beads S-CHAR
, O
which O
needs O
to O
be S-MATE
considered O
in O
the O
modeling S-ENAT
. O


In O
the O
method O
developed O
by O
Rodriguez O
, O
Thomas O
for O
ABS B-MATE
materials E-MATE
, O
The O
FDM S-MANP
part O
is O
defined O
as S-MATE
an O
unidirectional S-CONPRI
ABScomposite O
with O
a O
laminate S-CONPRI
structure O
. O


This O
structure S-CONPRI
is O
consisting O
of O
vertically O
stacked O
layers O
with O
contiguous O
material S-MATE
and O
voids S-CONPRI
. O


The O
unidirectional S-CONPRI
elastic B-PARA
constants E-PARA
are O
given O
as S-MATE
and O
represent O
the O
elastic B-PRO
modulus E-PRO
, O
shear B-PRO
modulus E-PRO
and O
Poissonratio O
of O
the O
extruded S-MANP
polymer O
used O
in O
the O
FDM S-MANP
process O
. O


The O
1 O
is O
the O
area B-PRO
void I-PRO
density E-PRO
in O
the O
plane O
normal O
to O
filament S-MATE
direction O
. O


This O
method O
was O
used O
in O
various O
works O
for O
single O
material S-MATE
FDM S-MANP
parts O
containing O
void S-CONPRI
, O
however O
, O
certain O
modifications O
are O
needed O
in O
order O
to O
apply O
it O
to O
multi-material B-MANP
3D I-MANP
printing E-MANP
. O


More O
information O
regarding O
void S-CONPRI
in O
composite B-CONPRI
structures E-CONPRI
can O
be S-MATE
found O
in O
. O


4.3 O
Finite B-CONPRI
element I-CONPRI
method E-CONPRI
FEM S-CONPRI
is O
particularly O
interesting O
for O
modelling S-ENAT
3D B-APPL
printed I-APPL
part E-APPL
due O
to O
its O
flexibility S-PRO
in O
analyzing O
complex B-CONPRI
geometries E-CONPRI
in O
both O
macro B-CONPRI
and I-CONPRI
micro I-CONPRI
scale E-CONPRI
. O


It O
can O
be S-MATE
applied O
to O
continuous O
and O
short B-MATE
fiber E-MATE
3D B-MANP
printed E-MANP
composites O
. O


The O
primary O
distinction O
of O
most O
composites S-MATE
by O
AM S-MANP
is O
the O
significant O
void S-CONPRI
content O
that O
needs O
to O
be S-MATE
incorporated O
into O
the O
respective O
finite B-CONPRI
element I-CONPRI
model E-CONPRI
. O


Perhaps O
the O
most O
appealing O
approaches O
for O
mechanical B-CONPRI
modelling E-CONPRI
of O
fiber B-MATE
composites E-MATE
are O
homogenization S-MANP
, O
which O
was O
described O
briefly O
in O
CLPT O
, O
and O
unit-cell O
based O
methods O
. O


Both O
approaches O
can O
be S-MATE
implemented O
in O
FEM S-CONPRI
, O
thus O
, O
applicable O
to O
all O
AM S-MANP
methods O
. O


Conversely O
, O
there O
has O
been O
a O
lack O
of O
attention O
to O
modelling S-ENAT
of O
these O
processes S-CONPRI
, O
and O
with O
increasing O
the O
popularity O
of O
3D B-MANP
printing E-MANP
among O
practitioners O
, O
the O
need O
for O
simulation S-ENAT
of O
the O
3D B-MANP
printing E-MANP
process O
is O
certain O
. O


UC O
is O
a O
single O
or O
multiple O
fibers S-MATE
embedded O
in O
the O
matrix O
with O
the O
volume B-PARA
fraction E-PARA
similar O
to O
those O
of O
the O
composite S-MATE
. O


A O
finite B-CONPRI
element I-CONPRI
model E-CONPRI
of O
this O
geometry S-CONPRI
using O
two O
different O
materials S-CONPRI
is O
constructed O
and O
various O
loadings O
are O
applied O
to O
characterize O
the O
behavior O
of O
the O
UC O
. O


In O
the O
case O
of O
composites S-MATE
with O
random O
fiber B-FEAT
orientations E-FEAT
the O
composite S-MATE
behavior O
is O
approximated O
by O
direct O
averaging O
over O
all O
orientations S-CONPRI
. O


Unit B-CONPRI
cells E-CONPRI
depend O
upon O
the O
microstructure S-CONPRI
of O
the O
composite S-MATE
; O
some O
common O
types O
of O
unite O
cell S-APPL
are O
presented O
in O
24 O
. O


24 O
presents O
a O
UC O
for O
discontinuous O
short B-MATE
fiber I-MATE
composites E-MATE
with O
random O
orientations S-CONPRI
, O
24 O
and O
shows O
UC O
for O
continuous B-MATE
fiber E-MATE
reinforced O
with O
unidirectional B-CONPRI
orientation E-CONPRI
and O
3D B-FEAT
braided I-FEAT
structure E-FEAT
, O
respectively O
. O


Another O
commonly O
used O
approach O
is O
multiscale O
methods O
by O
combining O
micro B-CONPRI
level E-CONPRI
and O
homogenized S-MANP
macro O
stress S-PRO
analysis O
. O


Microscopic B-CONPRI
level I-CONPRI
analysis E-CONPRI
can O
increase O
the O
accuracy S-CHAR
, O
but O
are O
often O
too O
expensive O
to O
be S-MATE
used O
in O
practice O
. O


Multiscale B-CONPRI
modelling E-CONPRI
takes O
advantage O
of O
the O
efficiency O
of O
macroscopic S-CONPRI
models O
and O
the O
accuracy S-CHAR
of O
the O
microscopic O
models O
. O


The O
microscope S-MACEQ
analysis O
is O
normally O
performed O
at O
the O
area S-PARA
of O
interest O
with O
high O
stress B-CHAR
concentration E-CHAR
. O


25 O
shows O
a O
multiscale B-ENAT
simulation E-ENAT
with O
microscale B-CONPRI
analysis E-CONPRI
performed O
on O
two O
different O
areas B-CONPRI
of I-CONPRI
interest E-CONPRI
and O
macroscale B-CHAR
analysis E-CHAR
in O
the O
entire O
domain S-CONPRI
. O


5 O
Conclusions O
3D B-MANP
printing E-MANP
of O
composite B-CONPRI
structures E-CONPRI
can O
be S-MATE
a O
turning S-MANP
point O
for O
AM B-MANP
technology E-MANP
. O


The O
potential O
of O
fabricating S-MANP
functional O
devices O
, O
directly O
from O
commercial O
3D B-MACEQ
printers E-MACEQ
with O
controllable O
properties S-CONPRI
, O
created O
a O
huge O
rush O
for O
new O
developments O
and O
research S-CONPRI
in O
this O
field O
. O


The O
attractive O
combination O
of O
endless O
possibilities O
in O
the O
range S-PARA
of O
composite B-MATE
materials E-MATE
and O
extra O
customization O
of O
AM S-MANP
offers O
a O
unique O
new O
area S-PARA
in O
the O
manufacturing S-MANP
field O
, O
for O
researchers O
and O
developers O
to O
explore O
. O


AM S-MANP
of O
composites S-MATE
enables O
precise B-CONPRI
control E-CONPRI
of O
the O
physical O
, O
electrochemical S-CONPRI
, O
thermal O
, O
and O
optical B-PRO
properties E-PRO
; O
these O
structures O
can O
even O
transform O
their O
shape O
over O
time O
in O
4D B-MANP
printing E-MANP
. O


Fiber B-FEAT
reinforcement E-FEAT
significantly O
improves O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
3D B-APPL
printed I-APPL
parts E-APPL
. O


It O
can O
be S-MATE
implemented O
in O
various O
AM B-MANP
techniques E-MANP
, O
such O
as S-MATE
FDM O
, O
SLA S-MACEQ
, O
SLS S-MANP
, O
LOM S-MANP
, O
and O
extrusion S-MANP
. O


Printed O
CF-ABS O
composites S-MATE
were O
even O
reported O
with O
higher O
specific B-PRO
strength E-PRO
than O
Aluminum S-MATE
. O


The O
alignment O
of O
fibers S-MATE
in O
3D B-MANP
printing E-MANP
of O
composites S-MATE
was O
one O
of O
the O
major O
challenges O
in O
the O
reviewed O
literature O
and O
its O
improvement O
attracted O
tremendous O
research S-CONPRI
interest O
in O
almost O
all O
of O
the O
existing O
AM S-MANP
methods O
. O


Recent O
advances O
in O
FDM S-MANP
printing O
of O
continuous B-MATE
fiber I-MATE
reinforced I-MATE
thermoplastics E-MATE
took O
these O
improvements O
one O
step S-CONPRI
further O
to O
establish O
AM S-MANP
as O
a O
dependable O
manufacturing S-MANP
method O
for O
various O
industries S-APPL
. O


In O
one O
of O
the O
interesting O
and O
innovative O
developments O
, O
continuous B-MATE
carbon I-MATE
fiber E-MATE
and O
PLA S-MATE
were O
mixed O
in O
the O
printing B-MACEQ
head E-MACEQ
before O
depositing O
into O
the O
printing O
bed S-MACEQ
, O
increasing O
the O
fiber/matrix B-FEAT
adhesion E-FEAT
. O


Laser S-ENAT
assisted O
AM S-MANP
for O
continuous B-MATE
fiber I-MATE
reinforced I-MATE
thermoplastic E-MATE
composites S-MATE
demonstrates O
superior O
mechanical B-CONPRI
properties E-CONPRI
and O
at O
the O
mean O
time O
solves O
the O
issue O
of O
material S-MATE
waste O
for O
LOM S-MANP
. O


Furthermore O
, O
developments O
in O
3D B-MANP
printing E-MANP
of O
fiber B-MATE
reinforced I-MATE
composite I-MATE
structures E-MATE
allowed O
precise O
placement O
of O
material S-MATE
at O
micro-scale S-CONPRI
, O
to O
fabricate S-MANP
complex B-CONPRI
structures E-CONPRI
in O
4D B-MANP
printing E-MANP
of O
active B-MATE
composites E-MATE
, O
with O
ability O
to O
transform O
over O
time O
, O
right O
after O
the O
3D B-MANP
printing E-MANP
. O


However O
, O
most O
3D B-MANP
printing E-MANP
methodologies O
for O
composite B-MATE
materials E-MATE
still O
face S-CONPRI
major O
challenges O
, O
which O
need O
to O
be S-MATE
overcome O
before O
becoming O
a O
mainstream O
manufacturing S-MANP
method O
. O


Void S-CONPRI
formation O
during O
printing O
, O
poor O
adhesion S-PRO
of O
fibers S-MATE
and O
polymer S-MATE
matrix O
, O
and O
challenges O
in O
continuous B-MATE
fiber E-MATE
printing O
are O
all O
amongst O
the O
existing O
issues O
in O
3D B-MANP
printing E-MANP
of O
fiber B-MATE
composites E-MATE
. O


Moreover O
, O
most O
of O
the O
commercial O
3D B-MACEQ
printers E-MACEQ
designed O
for O
specific O
resins S-MATE
and O
introduction O
of O
fillers O
can O
lead S-MATE
to O
blockage O
, O
wear S-CONPRI
, O
non-adhesion O
, O
and O
increased O
curing B-PARA
times E-PARA
. O


AM S-MANP
of O
composites S-MATE
is O
a O
relatively O
new O
technique O
and O
there O
is O
a O
lack O
of O
research S-CONPRI
on O
modelling S-ENAT
the O
structures O
produced O
by O
this O
process S-CONPRI
. O


Existing O
theories O
for O
short O
and O
continuous B-MATE
fiber I-MATE
composites E-MATE
can O
be S-MATE
modified O
for O
3D B-APPL
printed I-APPL
parts E-APPL
. O


On O
the O
other O
hand O
, O
FEM S-CONPRI
is O
a O
powerful O
tool S-MACEQ
to O
analyze O
composite B-CONPRI
structures E-CONPRI
, O
and O
it O
can O
be S-MATE
applied O
for O
3D B-MANP
printing E-MANP
with O
slight O
modifications O
of O
existing O
finite B-CONPRI
element I-CONPRI
models E-CONPRI
. O


In O
conclusion O
, O
AM S-MANP
of O
fiber B-MATE
reinforced I-MATE
polymer I-MATE
composites E-MATE
is O
tremendously O
promising O
in O
turning S-MANP
3D B-MANP
printing E-MANP
from O
a O
prototyping S-CONPRI
method O
to O
a O
robust O
manufacturing S-MANP
technique O
. O


The O
unique O
characteristics O
of O
3D B-MANP
printing E-MANP
, O
such O
as S-MATE
high O
customization O
, O
combined O
with O
extra O
strength S-PRO
from O
fiber B-FEAT
reinforcement E-FEAT
and O
the O
ability O
to O
produce O
functional O
complex O
3D B-CONPRI
structures E-CONPRI
with O
total O
control O
over O
material B-CONPRI
properties E-CONPRI
, O
helped O
AM S-MANP
of O
fiber-polymer B-MATE
composites E-MATE
gain O
enormous O
attention O
from O
a O
broad O
range S-PARA
of O
science O
industries S-APPL
. O


The O
aerospace B-APPL
industry E-APPL
, O
automotive B-APPL
industry E-APPL
, O
biomedical S-APPL
science O
, O
electronic O
industries S-APPL
, O
and O
robotics S-APPL
are O
only O
a O
few O
examples O
of O
those O
attracted O
by O
AM S-MANP
of O
fiber B-MATE
reinforced I-MATE
polymer I-MATE
composites E-MATE
. O


As S-MATE
the O
application O
of O
additive B-MANP
manufacturing E-MANP
reaches O
an O
unprecedented O
scale O
in O
both O
academia O
and O
industry S-APPL
, O
a O
reflection S-CHAR
upon O
the O
state-of-the-art S-CONPRI
developments O
in O
the O
design B-FEAT
for I-FEAT
additive I-FEAT
manufacturing E-FEAT
and O
structural B-CONPRI
optimisation E-CONPRI
, O
becomes O
vital O
for O
successfully O
shaping S-MANP
the O
future O
AM-landscape O
. O


A O
framework S-CONPRI
, O
highlighting O
both O
the O
interdependencies O
between O
these O
two O
central O
aspects O
in O
AM S-MANP
and O
the O
necessity O
for O
a O
holistic O
approach O
to O
structural B-CONPRI
optimization E-CONPRI
, O
using O
lightweight S-CONPRI
strategies O
such O
as S-MATE
topology O
optimization S-CONPRI
and/or O
latticing O
, O
was O
established O
to O
summarize O
the O
reviewed O
content O
. O


Primarily O
focusing O
on O
isotropic B-MATE
material E-MATE
considerations O
and O
basic O
stiffness-optimal O
problems O
, O
these O
concepts O
have O
already O
found O
wide O
application O
, O
bridging S-CONPRI
the O
gaps O
between O
design S-FEAT
and O
manufacturing S-MANP
as S-MATE
well O
as S-MATE
academia O
and O
industry S-APPL
. O


In O
pursuit O
of O
streamlining O
the O
AM-workflow O
towards O
digitally O
print-ready O
designs S-FEAT
, O
studies O
are O
increasingly O
investigating O
mathematically-based O
structural B-CONPRI
optimization E-CONPRI
approaches O
in O
conjunction O
with O
DfAM-specific O
constraints O
, O
providing O
a O
portfolio O
of O
solutions O
like O
generative B-ENAT
design E-ENAT
, O
which O
is O
gaining O
traction O
in O
industry S-APPL
. O


Besides O
an O
overview O
on O
economically-driven O
to O
performance-driven O
design B-CONPRI
optimizations E-CONPRI
, O
insight O
into O
commercial O
AM-specific O
software S-CONPRI
is O
provided O
, O
elucidating O
potentials O
and O
challenges O
for O
the O
community O
. O


Graphical O
abstract O
Unlabelled O
Image S-CONPRI
Highlights O
Extensive O
review O
, O
providing O
a O
joint S-CONPRI
perspective O
on O
design S-FEAT
and O
structural B-CONPRI
optimisation E-CONPRI
in O
additive B-MANP
manufacturing E-MANP
. O


Overview O
on O
the O
lightweighting S-PRO
approaches O
topology B-FEAT
optimization E-FEAT
and O
latticing O
, O
considering O
isotropic B-MATE
material E-MATE
assumptions O
. O


Consolidated O
summary O
, O
elucidating O
the O
gaps O
between O
design S-FEAT
and O
manufacturing S-MANP
as S-MATE
well O
as S-MATE
academia O
and O
industry S-APPL
. O


Establishment O
of O
a O
design S-FEAT
for O
AM S-MANP
framework O
, O
highlighting O
the O
interdependencies O
between O
state-of-the O
art S-APPL
research O
topics O
. O


Additive B-MANP
manufacturing E-MANP
, O
also O
referred O
to O
as S-MATE
3D-printing S-MANP
, O
has O
evolved O
greatly O
over O
the O
last O
three O
decades O
, O
emerging O
from O
the O
mere O
application O
in O
prototyping S-CONPRI
, O
it O
has O
now O
indisputably O
established O
its O
position O
as S-MATE
a O
viable O
fabrication S-MANP
alternative O
for O
end-use O
parts O
. O


This O
applies O
to O
a O
wide O
range S-PARA
of O
industries S-APPL
, O
including O
medical B-APPL
engineering E-APPL
, O
automotive S-APPL
, O
aerospace S-APPL
and O
consumer B-APPL
products E-APPL
, O
. O


lower O
energy O
consumption O
, O
has O
driven O
the O
research S-CONPRI
into O
identifying O
lightweight S-CONPRI
and O
robust O
designs S-FEAT
. O


Topology B-FEAT
optimisation E-FEAT
and O
latticing O
have O
emerged O
as S-MATE
the O
two O
major O
lightweighting S-PRO
strategies O
, O
best O
exploiting O
the O
design B-CONPRI
freedoms E-CONPRI
offered O
by O
AM S-MANP
. O


The O
former O
represents O
a O
rigorous O
approach O
, O
improving O
the O
specific B-PRO
stiffness E-PRO
, O
whereas O
the O
latter O
can O
be S-MATE
considered O
as S-MATE
a O
design S-FEAT
approach O
for O
weight-reduction O
in O
parts O
that O
usually O
have O
a O
high O
safety B-FEAT
factor E-FEAT
, O
which O
has O
in O
fact O
been O
widely O
adopted O
as S-MATE
a O
common O
design S-FEAT
practice O
in O
today O
's O
AM-specific O
software S-CONPRI
. O


multifunctionality O
and O
bio-inspired B-FEAT
designs E-FEAT
, O
are O
gaining O
interest O
and O
could O
help O
shape O
the O
future O
of O
structurally O
advanced O
AM-parts O
. O


This O
development O
is O
supported O
by O
the O
steady O
improvements O
in O
printer S-MACEQ
hardware O
and O
AM-specific O
software S-CONPRI
as S-MATE
well O
as S-MATE
the O
enrichment O
of O
the O
available O
material S-MATE
palette O
. O


In O
fact O
, O
AM S-MANP
is O
attributed O
a O
central O
role O
in O
the O
successful O
realization O
of O
the O
new O
industrial B-CONPRI
revolution E-CONPRI
, O
i.e O
. O


Industry B-ENAT
4.0 E-ENAT
, O
affecting O
both O
the O
way O
we O
design S-FEAT
and O
fabricate S-MANP
. O


This O
is O
clearly O
reflected O
by O
the O
prediction S-CONPRI
of O
a O
threefold O
increase O
in O
the O
industry S-APPL
's O
market O
value O
as S-MATE
stated O
in O
the O
Wohlers O
report O
. O


Jiang O
envisage O
among O
others O
intensified O
manufacturing S-MANP
of O
both O
critical O
and O
non-critical O
spare O
parts O
as S-MATE
well O
as S-MATE
increased O
customization O
of O
products O
with O
AM S-MANP
, O
which O
will O
require O
improvements O
in O
the O
structural B-CHAR
performance E-CHAR
and O
adequate O
design S-FEAT
practices O
, O
respectively O
. O


From O
a O
design S-FEAT
and O
structural O
standpoint O
, O
reviews O
in O
AM S-MANP
have O
so O
far O
focused O
on O
either O
the O
development O
of O
design B-CONPRI
rules E-CONPRI
and O
frameworks O
in O
the O
context O
of O
DfAM O
or O
the O
role O
of O
e.g O
. O


TO O
, O
latticing O
, O
processing O
parameters S-CONPRI
, O
materials S-CONPRI
and O
bio-inspired S-CONPRI
approaches O
, O
respectively O
. O


Herein O
, O
a O
joint S-CONPRI
scope O
including O
a O
much O
wider O
consideration O
for O
DfAM O
in O
structural B-CONPRI
optimization E-CONPRI
for O
AM S-MANP
is O
captured O
, O
reviewing O
two O
key O
lightweighting S-PRO
strategies O
in O
AM S-MANP
, O
namely O
TO O
and O
latticing O
and O
providing O
insight O
into O
the O
academic O
versus O
the O
commercial O
landscape O
. O


1 O
introduces O
a O
framework S-CONPRI
, O
relating O
key O
intrinsic O
and O
extrinsic O
DfAM O
aspects O
that O
were O
identified O
in O
the O
field O
of O
design S-FEAT
and O
structural B-CONPRI
optimisation E-CONPRI
for O
AM S-MANP
. O


An O
intrinsic O
aspect O
is O
, O
therefore O
, O
representing O
the O
primary O
drivers O
when O
designing O
for O
AM S-MANP
, O
whereas O
the O
extrinsic B-CONPRI
factors E-CONPRI
can O
be S-MATE
regarded O
as S-MATE
current O
research S-CONPRI
topics O
in O
design B-CONPRI
and I-CONPRI
structural I-CONPRI
optimization E-CONPRI
for O
AM S-MANP
. O


The O
remit O
of O
this O
paper O
, O
however O
, O
goes O
beyond O
this O
framework S-CONPRI
, O
highlighting O
the O
underlying O
dependencies O
between O
the O
individual O
extrinsic B-CONPRI
factors E-CONPRI
in O
the O
state-of-the-art B-CONPRI
research E-CONPRI
today O
. O


Three O
interrelated O
themes O
referred O
to O
as S-MATE
entail O
process- O
and O
material-induced O
aspects O
, O
lightweighting S-PRO
strategies O
as S-MATE
well O
as S-MATE
broader O
DfAM-factors O
and O
are O
arranged O
analogously O
to O
the O
chronological O
order O
of O
headings O
in O
this O
paper O
. O


This O
review O
focuses O
on O
structural B-CONPRI
optimisation E-CONPRI
in O
AM S-MANP
with O
particular O
emphasis O
on O
DfAM O
. O


As S-MATE
most O
works O
on O
lightweighting S-PRO
strategies O
are O
based O
on O
isotropic B-MATE
material E-MATE
assumptions O
, O
reinforced-materials S-MATE
and O
the O
process-induced O
anisotropy S-PRO
caused O
by O
the O
fabrication S-MANP
in O
a O
layer-by-layer B-CONPRI
fashion E-CONPRI
or O
infill S-PARA
pattern O
, O
are O
not O
assessed O
. O


As S-MATE
the O
role O
of O
anisotropy S-PRO
for O
topology B-FEAT
optimisation E-FEAT
in O
AM S-MANP
was O
discussed O
in O
, O
elucidating O
that O
this O
research S-CONPRI
is O
still O
in O
its O
infancy O
and O
requiring O
more O
fundamental O
investigations O
for O
a O
critical O
review O
, O
this O
paper O
summarizes O
the O
more O
consolidated O
knowledge O
in O
this O
field O
. O


A O
DfAM O
framework S-CONPRI
, O
encapsulating O
intrinsic O
and O
extrinsic O
aspects O
of O
the O
cutting-edge O
research S-CONPRI
on O
AM-friendly O
structural B-CONPRI
optimisation E-CONPRI
, O
preludes O
the O
main O
part O
of O
this O
work O
and O
provides O
context O
and O
structure S-CONPRI
to O
the O
reviewed O
studies O
. O


The O
joint S-CONPRI
focus O
on O
structural B-CONPRI
optimisation E-CONPRI
in O
AM S-MANP
using O
TO O
and O
latticing O
with O
incorporation O
of O
DfAM O
aspects O
constitutes O
the O
remit O
of O
this O
paper O
. O


Secondly O
, O
the O
synergistic O
and O
expertise-dominated O
application O
of O
cellular B-FEAT
structures E-FEAT
, O
as S-MATE
design O
approach O
to O
lightweighting S-PRO
, O
will O
be S-MATE
addressed O
. O


Hereby O
, O
focus O
is O
put O
on O
their O
application O
and O
significance O
in O
standard S-CONPRI
engineering S-APPL
problems O
rather O
than O
the O
medical S-APPL
field O
, O
as S-MATE
reviewed O
elsewhere O
. O


Moreover O
, O
it O
is O
important O
to O
note O
that O
the O
reviewed O
works O
are O
primarily O
looking O
into O
basic O
structural O
problems O
, O
concentrating O
on O
improved O
stiffness S-PRO
under O
volume B-PARA
constraints E-PARA
. O


2 O
Structural B-CONPRI
optimisation E-CONPRI
with O
isotropic B-MATE
material E-MATE
in O
AM S-MANP
The O
role O
of O
structural B-CONPRI
optimisation E-CONPRI
in O
AM S-MANP
is O
increasing O
across O
industries S-APPL
with O
lightweighting S-PRO
strategies O
that O
can O
broadly O
be S-MATE
differentiated O
into O
mathematically-driven O
and O
expertise-driven O
, O
with O
recent O
studies O
seeing O
increased O
utilization O
of O
both O
. O


Here O
, O
TO O
is O
frequently O
adopted O
for O
the O
design S-FEAT
of O
structurally-sound B-PRO
AM-parts E-PRO
and O
has O
meanwhile O
surpassed O
the O
use O
of O
shape O
and O
size O
optimization S-CONPRI
in O
isolation O
. O


However O
, O
it O
has O
become O
a O
general O
practice O
in O
commercial O
software S-CONPRI
to O
post-process S-CONPRI
TO-solutions O
by O
optimizing O
the O
size O
or O
shape O
of O
the O
final O
structure S-CONPRI
to O
adhere O
e.g O
. O


to O
minimum B-PARA
feature I-PARA
sizes E-PARA
that O
match O
the O
resolution S-PARA
of O
the O
printer S-MACEQ
. O


In O
contrast O
, O
expertise-driven O
structural B-CONPRI
optimisation E-CONPRI
such O
as S-MATE
latticing O
is O
not O
a O
lightweighting S-PRO
strategy O
per O
se S-MATE
, O
as S-MATE
stiffness O
is O
greatly O
compromised O
in O
exchange O
for O
e.g O
. O


A O
good O
example O
of O
this O
is O
the O
latticing O
of O
a O
part O
for O
weight S-PARA
reductions O
and O
enhanced O
functionality O
in O
areas S-PARA
where O
stiffness S-PRO
can O
be S-MATE
sacrificed O
. O


Generative B-ENAT
design E-ENAT
formally O
merges O
these O
two O
schemes O
through O
a O
parallel O
implementation O
to O
provide O
a O
portfolio O
of O
solutions O
i.e O
. O


1 O
Topology B-FEAT
optimisation E-FEAT
in O
AM S-MANP
Bendsand O
Kikuchi O
's O
as S-MATE
well O
as S-MATE
Sigmund O
's O
landmark O
work O
, O
introducing O
the O
concept O
of O
shape O
and O
TO O
centred O
on O
the O
homogenization S-MANP
approach O
and O
the O
MATLAB O
implementation O
of O
a O
density-based O
TO O
, O
respectively O
, O
laid O
the O
foundation O
for O
today O
's O
TO-methods O
. O


The O
most O
prominent O
TO O
approaches O
can O
be S-MATE
summarized O
as S-MATE
follows O
: O
Density-based O
; O
Level O
Set S-APPL
; O
Evolutionary/Genetic O
Algorithms S-CONPRI
; O
Topological O
Derivatives O
and O
Phase S-CONPRI
Field O
. O


Discrete O
TO O
can O
be S-MATE
described O
as S-MATE
a O
method O
that O
explores O
the O
optimal O
connection O
of O
the O
elements S-MATE
, O
whereas O
continuum S-CONPRI
TO O
determines O
the O
optimal O
spatial B-CHAR
distribution E-CHAR
of O
material S-MATE
within O
a O
domain S-CONPRI
. O


At O
the O
core S-MACEQ
of O
each O
structural O
TO O
problem O
lies O
an O
objective O
function O
that O
needs O
to O
be S-MATE
minimized O
or O
maximized O
while O
being O
subjected O
to O
a O
set S-APPL
of O
constraints O
such O
as S-MATE
volume O
, O
displacement O
or O
frequency O
. O


As S-MATE
part O
of O
an O
iterative O
process S-CONPRI
, O
in O
methods O
utilizing O
density S-PRO
as S-MATE
a O
design S-FEAT
variable O
, O
Finite B-CONPRI
Element I-CONPRI
Analysis E-CONPRI
, O
sensitivity B-CONPRI
analysis E-CONPRI
, O
regularizations S-CONPRI
and O
optimisation O
steps O
are O
repeated O
in O
this O
order O
until O
convergence O
is O
achieved O
. O


In O
the O
context O
of O
DfAM O
, O
research S-CONPRI
today O
is O
geared O
towards O
print-ready O
TO O
designs S-FEAT
bridging S-CONPRI
challenges O
of O
design S-FEAT
and O
fabrication S-MANP
. O


This O
refers O
to O
methods O
that O
change O
the O
numerically O
optimal O
solution S-CONPRI
either O
intrinsically O
through O
the O
imposition O
of O
manufacturing B-CONPRI
constraints E-CONPRI
or O
in O
subsequent O
post-processing S-CONPRI
steps O
which O
alter O
the O
geometrical O
layout S-CONPRI
. O


Support-free O
designs S-FEAT
and O
strategies O
to O
reduce O
the O
support S-APPL
volume O
are O
facilitators O
for O
a O
time- O
and O
cost-efficient O
fabrication S-MANP
. O


In O
pursuit O
of O
both O
physically O
and O
digitally O
print-ready O
designs S-FEAT
, O
smooth B-FEAT
boundary E-FEAT
representations O
have O
drawn O
great O
attention O
in O
recent O
years O
. O


In O
this O
review O
, O
this O
refers O
to O
mesh-refinements O
, O
ensuring O
a O
better O
surface S-CONPRI
representation O
or O
CAD-friendly O
geometries S-CONPRI
. O


In O
this O
context O
, O
correct O
digitalizing O
of O
TO-solutions O
will O
be S-MATE
pivotal O
for O
future O
CAD/CAM B-ENAT
software E-ENAT
, O
including O
an O
automated O
design S-FEAT
procedure O
that O
integrates O
DfAM O
considerations O
and O
requires O
less O
AM-expertise O
. O


Emerging O
TO O
methods O
with O
DfAM O
consideration O
for O
smooth B-FEAT
boundary E-FEAT
representation O
include O
the O
Development O
Method O
and O
the O
Morphable O
Method O
. O


Herein O
, O
basic O
geometric B-FEAT
shapes E-FEAT
and O
polygons O
, O
capturing O
the O
geometrical O
layouts O
, O
have O
served O
as S-MATE
simple O
design S-FEAT
primitives O
for O
representing O
topologies S-CONPRI
. O


1 O
SIMP-based O
designs S-FEAT
for O
AM S-MANP
Density-based O
TO O
methods O
are O
most O
prevalent O
in O
academia O
and O
follow O
the O
general O
procedure O
. O


A O
well-established O
continuum S-CONPRI
method O
often O
referred O
to O
as S-MATE
the O
SIMP O
approach O
, O
first O
introduced O
by O
Bends O
, O
relates O
the O
elements S-MATE
relative O
densities O
to O
the O
effective O
material S-MATE
moduli O
using O
the O
power S-PARA
law O
. O


As S-MATE
most O
works O
use O
linear O
elastic S-PRO
assumptions O
with O
the O
standard S-CONPRI
density-based O
TO O
approaches O
, O
they O
lack O
the O
ability O
to O
solve O
problems O
with O
large O
deformations S-CONPRI
accurately S-CHAR
. O


However O
, O
regarding O
DfAM O
, O
designs S-FEAT
with O
elastoplastic O
print S-MANP
consumables O
, O
as S-MATE
commonly O
used O
, O
could O
particularly O
benefit O
from O
more O
realistic O
material S-MATE
considerations O
. O


Major O
findings O
in O
studies O
on O
non-linear O
TO O
include O
: O
1 O
) O
the O
difference O
between O
linear O
and O
non-linear B-CONPRI
modelling E-CONPRI
of O
large O
deformations S-CONPRI
is O
particularly O
evident O
in O
buckling S-PRO
and O
snap-through O
scenarios O
; O
2 O
) O
a O
non-linear B-CONPRI
analysis E-CONPRI
made O
insignificant O
changes O
to O
the O
stiffness/compliance O
of O
a O
linear B-FEAT
elastic I-FEAT
structure E-FEAT
but O
altered O
the O
topology S-CONPRI
significantly O
; O
3 O
) O
non-linearity O
assumptions O
are O
beneficial O
for O
realizing O
compliant B-CONPRI
mechanisms E-CONPRI
; O
4 O
) O
issues O
with O
excessive O
distortions O
or O
convergence O
of O
the O
numerical O
model S-CONPRI
can O
be S-MATE
solved O
. O


1 O
Academic O
approaches O
to O
structurally O
optimized O
AM-friendly O
parts O
With O
the O
maturation O
of O
TO-codes O
in O
recent O
years O
towards O
greater O
computational B-CONPRI
efficiency E-CONPRI
and O
complexity S-CONPRI
, O
the O
inclusion S-MATE
of O
DfAM O
aspects O
is O
on O
the O
rise O
. O


Most O
of O
today O
's O
research S-CONPRI
on O
TO O
for O
AM S-MANP
is O
economically-driven O
and O
deals O
with O
support S-APPL
volume O
reduction S-CONPRI
, O
their O
complete O
elimination O
or O
simplified O
removal O
. O


Support-free O
structures O
with O
reasonably O
good O
quality S-CONPRI
can O
generally O
be S-MATE
expected O
for O
an O
inclination B-FEAT
angle E-FEAT
in O
the O
realm O
of O
45 O
, O
which O
has O
been O
widely O
adopted O
for O
structural B-CONPRI
optimization E-CONPRI
problems O
anything O
lower O
is O
associated O
with O
additional O
fabrication S-MANP
cost O
motivating O
efforts O
in O
achieving O
conformity O
between O
design S-FEAT
and O
print S-MANP
. O


However O
, O
it O
should O
be S-MATE
noted O
that O
fine-tuning O
of O
process B-CONPRI
parameters E-CONPRI
in O
polymer S-MATE
FDM S-MANP
has O
shown O
to O
allow O
even O
lower O
print-angles O
. O


Performance S-CONPRI
is O
inevitably O
compromised O
by O
imposing O
manufacturing B-CONPRI
constraints E-CONPRI
but O
opposed O
to O
accounting O
for O
support S-APPL
volume O
, O
performance-driven O
factors O
in O
TO O
methods O
, O
including O
e.g O
. O


In O
, O
the O
performance S-CONPRI
and O
printability S-PARA
of O
a O
structure S-CONPRI
derived O
from O
a O
coupled B-FEAT
truss E-FEAT
and O
SIMP-based O
optimisation O
approach O
were O
investigated O
. O


By O
projecting O
a O
truss S-MACEQ
framework S-CONPRI
onto O
the O
corresponding O
TO-solution O
, O
the O
material S-MATE
was O
re-distributed O
, O
promoting O
adherence O
to O
overhang B-PARA
angle I-PARA
limits E-PARA
. O


As S-MATE
a O
result O
, O
a O
topology S-CONPRI
that O
does O
require O
less O
support S-APPL
but O
has O
reasonably O
high O
performance S-CONPRI
was O
created O
. O


Additionally O
, O
it O
was O
found O
that O
the O
printing O
direction O
is O
a O
key O
control O
parameter S-CONPRI
between O
manufacturability S-CONPRI
and O
performance S-CONPRI
. O


An O
efficient O
DfAM O
filter S-APPL
for O
TO O
, O
avoiding O
support B-FEAT
structures E-FEAT
, O
has O
also O
been O
the O
subject O
of O
research S-CONPRI
in O
the O
works O
of O
Langelaar O
, O
in O
which O
a O
projection O
method O
was O
proposed O
for O
2D S-CONPRI
and O
3D S-CONPRI
cases O
. O


More O
recently O
, O
Langelaar O
has O
extended O
this O
AM-filter O
approach O
to O
a O
method O
for O
optimizing O
topology S-CONPRI
, O
support S-APPL
layout S-CONPRI
and O
build B-PARA
orientation E-PARA
simultaneously O
, O
providing O
designers O
and O
engineers O
with O
a O
portfolio O
of O
results O
with O
the O
corresponding O
fabrication S-MANP
cost O
and O
part O
performance S-CONPRI
. O


The O
support-filter O
itself O
is O
based O
on O
the O
premise O
of O
a O
layer-wise O
AM S-MANP
fabrication O
, O
whereby O
it O
is O
examined O
whether O
the O
base O
elements S-MATE
in O
the O
underlying O
layer S-PARA
offer O
support S-APPL
considering O
the O
critical O
support-free O
inclination B-FEAT
angle E-FEAT
of O
45Together O
with O
a O
sensitivity S-PARA
filter S-APPL
based O
on O
the O
adjoint O
method O
, O
the O
AM-filter O
formulation O
is O
integrated O
into O
the O
code O
of O
Andreassen O
, O
making O
it O
an O
easily O
accessible O
tool S-MACEQ
for O
print-ready O
AM-parts O
with O
improved O
structural B-CHAR
performance E-CHAR
. O


Numerical O
examples O
provided O
in O
illustrated O
the O
printability S-PARA
of O
TO-structures O
as S-MATE
a O
function O
of O
printing O
orientation S-CONPRI
. O


a O
half B-FEAT
MBB I-FEAT
beam E-FEAT
, O
Langelaar O
highlighted O
not O
only O
the O
effectiveness S-CONPRI
of O
the O
filter S-APPL
to O
avoid O
shallow O
angles O
and O
its O
effect O
on O
the O
final O
topology S-CONPRI
compared O
to O
the O
unconstrained O
model S-CONPRI
, O
but O
also O
the O
variation S-CONPRI
in O
compliance O
with O
the O
building B-PARA
orientation E-PARA
. O


An O
extension O
of O
this O
method O
could O
potentially O
be S-MATE
promising O
for O
multidirectional B-CONPRI
slicing E-CONPRI
in O
robotic O
AM S-MANP
. O


Gaynor O
and O
co-workers O
as S-MATE
well O
as S-MATE
Qian O
, O
showed O
that O
the O
Heaviside O
Projection O
offered O
an O
alternative O
means O
to O
constraining O
the O
overhang B-PARA
angles E-PARA
in O
TO-solutions O
. O


In O
this O
scheme O
was O
employed O
to O
enforce O
a O
binary S-CONPRI
solution O
from O
the O
greyscale O
SIMP-solution O
by O
controlling O
the O
minimum O
radial B-PARA
length E-PARA
scale O
. O


determine O
the O
minimum O
self-supporting B-FEAT
angle E-FEAT
in O
2D S-CONPRI
. O


Gaynor O
and O
Guest O
found O
major O
changes O
in O
the O
topology S-CONPRI
and O
deterioration O
in O
performance S-CONPRI
with O
increasing O
self-supporting B-FEAT
angles E-FEAT
. O


In O
the O
Heaviside O
Projection O
was O
employed O
both O
on O
the O
density S-PRO
and O
the O
density B-PRO
gradient E-PRO
to O
obtain O
well-defined O
boundaries S-FEAT
and O
constrain O
the O
perimeter O
length O
in O
undercuts S-FEAT
and O
overhangs S-PARA
for O
3D S-CONPRI
cases O
. O


improved O
convergence O
to O
a O
discrete B-CONPRI
solution E-CONPRI
. O


As S-MATE
an O
example O
, O
a O
cantilever B-MACEQ
beam E-MACEQ
was O
optimized O
for O
individual O
minimum B-PARA
overhang I-PARA
angles E-PARA
, O
illustrating O
that O
overhang B-PARA
angle E-PARA
and O
gradient O
constraint O
need O
to O
be S-MATE
concurrently O
changed O
and O
that O
the O
compliance O
is O
significantly O
increased O
over O
the O
unconstrained O
pendant O
, O
especially O
for O
increased O
overhang B-PARA
angles E-PARA
. O


Leary O
proposed O
an O
approach O
in O
which O
infeasible O
inclination B-FEAT
angles E-FEAT
are O
initially O
identified O
, O
followed O
by O
a O
layout S-CONPRI
transmission O
into O
a O
support-free O
design S-FEAT
and O
the O
determination O
of O
the O
optimal O
building B-PARA
orientation E-PARA
. O


Firstly O
, O
a O
binary B-CONPRI
topology E-CONPRI
was O
obtained O
through O
thresholding O
of O
the O
gradient O
values O
to O
identify O
the O
perimeters O
and O
its O
local B-CONPRI
gradients E-CONPRI
. O


Subsequently O
, O
smoothing O
of O
the O
boundary S-FEAT
was O
performed O
using O
the O
rolling B-CONPRI
average E-CONPRI
method O
. O


The O
actual O
novelties O
are O
the O
iterative O
internal O
and O
external O
boundary S-FEAT
modifications O
centred O
on O
a O
recursive O
subdivision O
of O
domains O
to O
ensure O
the O
final O
design S-FEAT
complies O
with O
the O
allowable O
inclination B-FEAT
angle E-FEAT
. O


The O
change O
in O
the O
geometrical O
layout S-CONPRI
increased O
the O
volume B-PARA
fraction E-PARA
of O
the O
part O
, O
however O
, O
due O
to O
the O
avoidance O
of O
support S-APPL
, O
experimental S-CONPRI
results O
showed O
a O
reduction S-CONPRI
in O
fabrication B-PARA
time E-PARA
. O


Benchmarking O
against O
standard S-CONPRI
TO O
result O
, O
numerical O
analyses O
showcased O
: O
a O
) O
different O
stress B-PRO
distributions E-PRO
while O
the O
maximum O
von B-PRO
Mises I-PRO
stress E-PRO
stayed O
constant O
and O
b S-MATE
) O
the O
heat B-CONPRI
transfer E-CONPRI
is O
increased O
, O
which O
is O
of O
particular O
interest O
for O
e.g O
. O


metal-AM O
, O
as S-MATE
entrapment O
of O
excessive O
temperature S-PARA
, O
leads O
to O
microstructural S-CONPRI
changes O
over O
time O
as S-MATE
shown O
by O
. O


It O
is O
important O
to O
note O
, O
however O
, O
that O
a O
weight S-PARA
increase O
is O
associated O
with O
this O
method O
, O
which O
should O
be S-MATE
carefully O
weighed O
against O
the O
material S-MATE
savings O
achieved O
by O
avoiding O
the O
supports S-APPL
or O
the O
ease O
in O
manufacturing S-MANP
. O


In O
contrast O
, O
a O
more O
recent O
paper O
by O
Thore O
investigates S-CONPRI
the O
abovementioned O
factor O
of O
stresses O
in O
support-free O
TO-solutions O
, O
while O
adhering O
to O
the O
volume B-PARA
constraint E-PARA
, O
linking O
economy O
and O
fabrication S-MANP
. O


Contrary O
to O
the O
above O
works O
, O
digitally O
print-ready O
designs S-FEAT
, O
accounting O
for O
research S-CONPRI
that O
highlight O
efforts O
to O
facilitate O
the O
transition S-CONPRI
between O
design S-FEAT
and O
print S-MANP
such O
as S-MATE
methods O
that O
require O
minimal O
post-processing S-CONPRI
to O
be S-MATE
manufacture-ready O
are O
gaining O
interest O
. O


Interpreting O
density-based O
TO O
solutions O
, O
even O
when O
they O
are O
discrete O
, O
poses O
a O
bottleneck S-CONPRI
for O
post-processes O
such O
as S-MATE
shape O
optimisation O
or O
the O
use O
of O
CAD S-ENAT
programs O
for O
the O
conversion O
into O
AM-compatible O
file S-MANS
formats O
. O


A O
smooth B-FEAT
boundary E-FEAT
representation O
is O
, O
therefore O
, O
an O
aspect O
of O
particular O
attention O
in O
the O
context O
of O
TO O
. O


An O
approach O
which O
is O
post-processing S-CONPRI
the O
layout S-CONPRI
of O
a O
TO-solution O
was O
recently O
presented O
by O
Liu O
. O


Here O
, O
a O
multi-step O
transformation O
method O
from O
a O
greyscale O
SIMP-based O
solution S-CONPRI
into O
STL S-MANS
and O
IGES S-MANS
formats O
has O
been O
presented O
. O


It O
is O
comprised O
of O
: O
1 O
) O
density S-PRO
thresholding O
and O
mesh B-CONPRI
refinement E-CONPRI
; O
2 O
) O
skeletonisation S-ENAT
; O
3 O
) O
identify O
small O
features O
and O
increase O
their O
elemental O
density S-PRO
; O
4 O
) O
density S-PRO
filtering O
; O
5 O
) O
density S-PRO
thresholding O
while O
preserving O
volume B-PARA
fraction E-PARA
; O
6 O
) O
create O
STL S-MANS
file S-MANS
utilizing O
; O
7 O
) O
boundary B-CONPRI
interpretation E-CONPRI
using O
spline B-ENAT
fitting E-ENAT
; O
8 O
) O
obtain O
IGES B-MANS
file E-MANS
from O
an O
in-build O
MATLAB O
function O
. O


Shape O
optimisations O
procedures O
using O
a O
CAD B-ENAT
model E-ENAT
were O
shown O
to O
be S-MATE
suitable O
with O
designs S-FEAT
obtained O
from O
the O
adaptive O
boundary S-FEAT
fitting O
method O
due O
to O
its O
smooth B-FEAT
boundaries E-FEAT
. O


Studies O
like O
this O
represent O
a O
key O
element S-MATE
in O
today O
's O
research S-CONPRI
from O
DfAM O
, O
namely O
the O
transformation O
of O
complex O
AM-designs O
into O
CAD/CAM B-ENAT
environments E-ENAT
. O


2 O
Commercial O
software S-CONPRI
implementations O
for O
lightweight B-MACEQ
structures E-MACEQ
Promoted O
by O
the O
greater O
application O
of O
TO O
in O
industry S-APPL
, O
an O
increased O
range S-PARA
of O
software S-CONPRI
providers O
have O
emerged O
, O
maturing O
and O
facilitating O
access O
to O
ready-to-use S-CONPRI
tools O
. O


Particularly O
TO O
software S-CONPRI
providing O
greater O
freedom O
in O
utilizing O
subroutines O
, O
as S-MATE
recently O
demonstrated O
in O
, O
present O
a O
promising O
platform S-MACEQ
for O
further O
innovations O
in O
this O
field O
. O


on O
automotive S-APPL
components O
like O
an O
upright O
. O


In O
, O
significant O
weight S-PARA
savings O
of O
one O
to O
two-thirds O
were O
achieved O
while O
ensuring O
the O
same O
structural B-CHAR
performance E-CHAR
as S-MATE
the O
original O
parts O
. O


In O
, O
shape O
controls O
ensured O
control O
over O
the O
member O
size O
, O
accounting O
for O
the O
minimum O
resolution S-PARA
of O
the O
electron B-MANP
beam I-MANP
melting E-MANP
process O
for O
a O
successful O
print S-MANP
. O


It O
was O
observed O
, O
that O
all O
considered O
factors O
were O
closely O
related O
to O
the O
amount O
of O
support B-MATE
material E-MATE
used O
, O
as S-MATE
it O
increased O
the O
fabrication B-PARA
time E-PARA
and O
cost O
. O


From O
the O
above O
study O
, O
it O
can O
be S-MATE
concluded O
that O
the O
layout S-CONPRI
of O
the O
final O
design S-FEAT
for O
the O
industrial B-CONPRI
sector E-CONPRI
will O
be S-MATE
the O
result O
of O
multi-objective O
considerations O
, O
specifically O
drawing S-MANP
attention O
to O
the O
performance-economy O
relation O
with O
the O
utilized O
print S-MANP
materials S-CONPRI
. O


This O
highlights O
the O
importance O
of O
a O
DfAM O
framework S-CONPRI
that O
assures O
correct O
weighting O
of O
the O
relevant O
parameters S-CONPRI
and O
provides O
similar O
to O
the O
generative B-ENAT
design E-ENAT
a O
portfolio O
of O
quantifiable O
geometrical O
solutions O
. O


The O
optimisation O
of O
a O
heat B-MACEQ
sink E-MACEQ
design S-FEAT
displayed O
the O
software S-CONPRI
's O
capabilities O
to O
deal O
with O
multi-objective O
optimisation O
, O
as S-MATE
the O
design S-FEAT
variables O
included O
friction B-CONPRI
force E-CONPRI
, O
thermal B-PRO
conductivity E-PRO
and O
out-of-plane O
heat B-CONPRI
transfer E-CONPRI
, O
besides O
the O
density S-PRO
of O
the O
elements S-MATE
. O


Despite O
showcasing O
the O
theoretical S-CONPRI
capabilities O
of O
today O
's O
TO-software O
, O
the O
above-mentioned O
studies O
commonly O
lack O
experimental S-CONPRI
verification O
. O


Generally O
, O
numerical O
examples O
in O
TO O
involve O
either O
cantilever S-FEAT
, O
half B-FEAT
MBB E-FEAT
or O
full O
MBB B-FEAT
beams E-FEAT
, O
but O
there O
is O
no O
explicit O
agreement O
by O
the O
scientific O
community O
on O
their O
aspect B-FEAT
ratio E-FEAT
, O
making O
comparisons O
difficult O
. O


From O
a O
practical O
standpoint O
, O
standardised O
flexure B-CHAR
tests E-CHAR
would O
be S-MATE
better O
suited O
for O
experimental S-CONPRI
verification O
and O
therefore O
aid O
the O
comparison O
between O
both O
the O
TO-models O
and O
their O
physical O
counterparts O
. O


3 O
Advanced O
applications O
TO-designs O
are O
increasingly O
adopted O
in O
technologically O
advanced O
industries S-APPL
as S-MATE
AM S-MANP
hardware O
and O
software S-CONPRI
are O
maturing O
, O
making O
complexity S-CONPRI
and O
scalability O
of O
AM-parts O
more O
tangible O
. O


Recent O
case-studies O
on O
topologically S-CONPRI
optimized O
engine O
pistons O
or O
car S-CHAR
chassis O
indeed O
showed O
a O
promising O
development O
for O
increased O
uptake O
of O
TO O
in O
the O
automotive B-APPL
sector E-APPL
. O


As S-MATE
an O
example O
, O
Aage O
looked O
into O
optimizing O
the O
wing O
structure S-CONPRI
of O
a O
Boeing S-APPL
777 O
using O
high-performance B-ENAT
computing E-ENAT
to O
overcome O
current O
limitations O
regarding O
the O
number O
of O
resolvable O
voxels S-CONPRI
and O
representable O
feature B-PARA
size E-PARA
. O


However O
, O
this O
should O
not O
distract O
from O
the O
more O
pressing S-MANP
challenge O
the O
AM-community O
is O
currently O
faced S-MANP
to O
further O
exploit O
AM S-MANP
capabilities O
, O
namely O
the O
reduction S-CONPRI
of O
the O
computational O
cost O
and O
improvement O
of O
the O
resolution S-PARA
for O
standard S-CONPRI
engineering S-APPL
problems O
. O


In O
case O
of O
small- O
and O
mid-sized O
companies S-APPL
these O
are O
commonly O
solved O
with O
standard S-CONPRI
desktop O
machines S-MACEQ
rather O
than O
employing O
high-performance B-ENAT
computing E-ENAT
. O


the O
fruition O
of O
compliant B-CONPRI
mechanisms E-CONPRI
, O
realized O
by O
means O
of O
multi-material S-CONPRI
printers O
and O
the O
use O
of O
materials S-CONPRI
with O
greatly O
varying O
Poisson O
's O
ratios O
. O


2 O
AM-designs O
derived O
from O
evolutionary O
TO O
Evolutionary O
algorithms S-CONPRI
have O
been O
used O
and O
constantly O
improved O
for O
structural B-CONPRI
optimisation E-CONPRI
over O
the O
last O
two O
decades O
. O


The O
most O
prominent O
representative O
is O
BESO S-CONPRI
, O
introduced O
by O
Yang O
as S-MATE
an O
extension O
to O
the O
Evolutionary O
Structural B-CONPRI
Optimisation I-CONPRI
method E-CONPRI
put O
forward O
by O
Xie O
and O
Stevens O
. O


However O
, O
issues O
related O
to O
stepped O
contours S-FEAT
i.e O
. O


smooth B-FEAT
boundaries E-FEAT
remain O
. O


1 O
Digitally O
print-ready O
designs S-FEAT
In O
light O
of O
DfAM O
, O
achieving O
a O
smooth B-FEAT
boundary E-FEAT
representation O
, O
especially O
for O
discrete O
TO-solutions O
, O
is O
currently O
a O
subject O
of O
intense O
study O
. O


Capitalizing O
on O
the O
shape O
functions O
, O
a O
smoother O
boundary S-FEAT
, O
accounting O
for O
discontinuous O
material S-MATE
distributions S-CONPRI
within O
single O
FEs O
, O
was O
realized O
. O


Despite O
revealing O
the O
same O
overall O
topology S-CONPRI
, O
the O
Iso-XFEM B-CHAR
method E-CHAR
provided O
a O
superior O
structural O
stiffness S-PRO
as S-MATE
compared O
to O
the O
BESO B-CONPRI
implementation E-CONPRI
for O
the O
same O
amount O
of O
iterations O
and O
identical O
mesh-size O
. O


BESO S-CONPRI
with O
a O
finer O
initial O
fixed B-FEAT
mesh E-FEAT
matched O
the O
performance S-CONPRI
but O
at O
a O
computational O
expense O
. O


As S-MATE
the O
mesh B-CONPRI
manipulation E-CONPRI
is O
outweighing O
the O
computational O
benefit O
of O
BESO S-CONPRI
, O
the O
AM-friendly O
Iso-XFEM B-CHAR
method E-CHAR
was O
regarded O
as S-MATE
the O
preferential O
TO O
method O
, O
streamlining O
the O
design B-CONPRI
process E-CONPRI
towards O
digitally-print-ready O
models O
and O
effectively O
tying O
in O
aspects O
of O
lightweighting S-PRO
and O
boundary B-CONPRI
representation E-CONPRI
. O


In O
a O
more O
recent O
study O
, O
Iso-XFEM S-CHAR
was O
applied O
as S-MATE
a O
design-procedure O
to O
obtain O
a O
lightweight S-CONPRI
brake O
pedal O
. O


The O
structure S-CONPRI
was O
effectively O
enhanced O
for O
AM S-MANP
, O
using O
a O
manual O
post-processing S-CONPRI
step O
, O
in O
which O
a O
partial O
cellular O
infill S-PARA
based O
on O
Body B-CONPRI
Centred I-CONPRI
Cubic E-CONPRI
unit O
cells S-APPL
was O
included O
to O
improve O
the O
performance S-CONPRI
under O
arbitrary O
loading O
and O
to O
reduce O
warpage S-CONPRI
arising O
from O
residual B-PRO
stresses E-PRO
in O
SLM S-MANP
. O


In O
, O
a O
modified O
BESO B-CONPRI
method E-CONPRI
was O
used O
to O
optimize O
the O
strut B-PARA
thickness E-PARA
of O
lattices S-CONPRI
used O
in O
an O
engine O
bracket S-MACEQ
, O
to O
improve O
the O
compliance O
over O
a O
homogenous B-FEAT
design E-FEAT
. O


The O
material S-MATE
allocation O
is O
conducted O
based O
on O
the O
ratio O
between O
the O
local O
and O
global O
stress S-PRO
magnitude S-PARA
i.e O
. O


the O
stress B-PRO
distribution E-PRO
as S-MATE
well O
as S-MATE
the O
predefined O
strut B-PARA
diameter E-PARA
bounds O
. O


compliant B-CONPRI
mechanisms E-CONPRI
, O
a O
modified O
BESO S-CONPRI
with O
local O
displacement B-CONPRI
constraints E-CONPRI
was O
employed O
by O
Zuo O
and O
Xie O
. O


Here O
, O
the O
standard S-CONPRI
penalization O
factor O
was O
introduced O
for O
the O
material B-CONPRI
interpolation E-CONPRI
and O
to O
ensure O
convergence O
, O
however O
, O
for O
the O
FEA O
several O
critical O
nodes O
were O
assigned O
to O
local O
displacement B-CONPRI
functions E-CONPRI
based O
on O
the O
individual O
virtual O
loads O
to O
fine-tune O
the O
topology S-CONPRI
for O
a O
specific O
load O
case O
. O


Numerical O
examples O
displayed O
dissimilar O
topologies S-CONPRI
compared O
to O
the O
conventional O
compliance-driven O
approach O
and O
a O
significant O
reduction S-CONPRI
in O
the O
maximum O
vertical B-PARA
deflection E-PARA
. O


As S-MATE
outlined O
by O
the O
authors O
, O
this O
method O
lends O
itself O
for O
lightweight B-MACEQ
structures E-MACEQ
such O
as S-MATE
an O
aircraft B-APPL
wing E-APPL
, O
for O
which O
awareness O
and O
control O
over O
the O
deformed B-PRO
shape E-PRO
are O
required O
. O


For O
the O
most O
part O
, O
TO O
procedures O
employ O
fixed B-FEAT
meshes E-FEAT
for O
the O
FEA O
. O


A O
mesh B-CONPRI
refinement E-CONPRI
captures O
the O
material S-MATE
behaviour O
more O
accurately S-CHAR
, O
however O
at O
higher O
computational O
cost O
due O
to O
more O
degrees B-CONPRI
of I-CONPRI
freedom E-CONPRI
. O


A O
fine B-FEAT
mesh E-FEAT
for O
updating O
design S-FEAT
variable O
and O
a O
hierarchical O
system O
for O
the O
FEA O
. O


This O
resulted O
in O
a O
relatively-smooth O
topology S-CONPRI
, O
due O
to O
the O
finer B-PARA
resolution E-PARA
representing O
the O
boundaries S-FEAT
of O
the O
structure S-CONPRI
, O
whereas O
the O
size O
of O
the O
internal O
elements S-MATE
remained O
coarse O
. O


It O
is O
of O
note O
that O
caution O
must O
be S-MATE
taken O
in O
choosing O
an O
appropriate O
value O
for O
the O
maximum O
element B-PARA
size E-PARA
to O
obtain O
accurate S-CHAR
FEA O
results O
. O


3 O
Level O
set S-APPL
TO O
for O
AM S-MANP
Level O
set S-APPL
method O
centre O
around O
the O
structural O
boundaries S-FEAT
i.e O
. O


interfaces O
and O
their O
implicit O
representation O
via B-FEAT
iso-contours E-FEAT
of O
the O
corresponding O
level B-CONPRI
set I-CONPRI
function E-CONPRI
. O


The O
inherently O
smooth O
and O
well-defined O
boundaries S-FEAT
obtained O
lend O
themselves O
for O
parameterization O
into O
geometric B-FEAT
shapes E-FEAT
, O
potentially O
linking O
aspects O
of O
manufacturing S-MANP
and O
digitalization O
. O


However O
, O
for O
analysis O
, O
the O
geometrical B-CONPRI
model E-CONPRI
is O
mostly O
discretized O
and O
mapped O
onto O
a O
fixed B-FEAT
mesh E-FEAT
, O
following O
the O
density-based O
methods O
. O


For O
instance O
, O
by O
embracing O
both O
economic O
and O
performance-driven O
DfAM O
considerations O
, O
Liu O
and O
co-workers O
have O
developed O
a O
feature-based O
approach O
as S-MATE
well O
as S-MATE
algorithms S-CONPRI
that O
take O
into O
account O
the O
deposition S-CONPRI
path/building O
direction O
and O
support-free O
manufacturing S-MANP
. O


Regarding O
DfAM O
studies O
have O
shown O
that O
gaps O
between O
paths O
due O
to O
sharp B-FEAT
angle E-FEAT
changes O
can O
be S-MATE
avoided O
and O
as S-MATE
the O
paths O
follow O
the O
principal B-PRO
stress E-PRO
trajectories O
, O
the O
overall O
performance S-CONPRI
can O
be S-MATE
increased O
. O


In O
, O
the O
contour B-ENAT
offset I-ENAT
method E-ENAT
and O
the O
structural B-FEAT
skeleton E-FEAT
based O
method O
, O
including O
a O
support B-FEAT
structure E-FEAT
constraint O
, O
were O
investigated O
. O


It O
was O
found O
that O
the O
structural B-CHAR
performance E-CHAR
was O
comparable O
between O
both O
methods O
, O
whereas O
, O
the O
skeleton-based O
method O
was O
favourable O
as S-MATE
it O
avoided O
manufacturing S-MANP
irregularities O
. O


The O
authors O
optimized O
a O
cantilever B-MACEQ
beam E-MACEQ
for O
self-supported O
AM S-MANP
, O
using O
a O
multi-LS O
interpolation S-CONPRI
approach O
, O
to O
illustrate O
their O
method O
. O


Similarly O
, O
the O
skeletonisation S-ENAT
of O
2D S-CONPRI
LS O
topologies S-CONPRI
was O
utilized O
in O
to O
constrain O
the O
minimum O
hole B-FEAT
size E-FEAT
and O
control O
the O
number O
of O
holes O
in O
the O
topology S-CONPRI
to O
ensure O
better O
manufacturability S-CONPRI
. O


In O
, O
it O
was O
applied O
to O
avoid O
small O
struts S-MACEQ
by O
controlling O
the O
minimum O
or O
maximum O
length B-CHAR
scale E-CHAR
of O
the O
features O
i.e O
. O


the O
distance O
between O
skeleton O
and O
boundary S-FEAT
. O


8b O
illustrates O
the O
difference O
in O
topology S-CONPRI
between O
constraint O
and O
unconstraint O
LS O
TO O
of O
a O
cantilever B-MACEQ
beam E-MACEQ
. O


In O
the O
study O
of O
Allaire O
, O
the O
aspect O
of O
the O
overhang B-PARA
constraint E-PARA
was O
considered O
from O
two O
perspectives O
. O


The O
latter O
has O
been O
developed O
in O
and O
centres O
on O
the O
intermediate O
AM S-MANP
stages O
, O
resulting O
in O
the O
continuous O
change O
in O
shape O
and O
boundary B-CONPRI
conditions E-CONPRI
with O
each O
consecutive O
layer S-PARA
. O


the O
cooling B-PARA
rate E-PARA
as S-MATE
another O
fabrication S-MANP
constraint O
for O
this O
minimum B-PARA
compliance I-PARA
problem E-PARA
. O


Likewise O
, O
in O
, O
Mart O
accounted O
for O
process-induced O
effects O
, O
such O
as S-MATE
porosity O
in O
parts O
created O
from O
electron B-MANP
beam I-MANP
melting E-MANP
. O


Considering O
fabrication-stages O
and O
-flaws O
for O
different O
AM-processes O
is O
certainly O
representing O
a O
more O
interconnected O
and O
realistic O
application O
of O
DfAM O
, O
encompassing O
intrinsic B-PARA
and I-PARA
extrinsic I-PARA
factors E-PARA
, O
shared O
in O
the O
upper O
and O
middle O
band O
, O
and O
contributing O
to O
higher O
quality S-CONPRI
AM-parts O
in O
the O
future O
. O


Certain O
design S-FEAT
requirements O
like O
fixed O
passive O
elements S-MATE
or O
other O
functional O
affiliations O
often O
do O
not O
allow O
support-free O
angles O
. O


Mirzendehdel O
and O
Suresh O
reduced O
the O
support S-APPL
volume O
of O
topologically S-CONPRI
optimized O
structures O
using O
an O
LS O
based O
Pareto S-CONPRI
TO O
, O
considering O
both O
support B-FEAT
structure E-FEAT
and O
topological B-CONPRI
sensitivities E-CONPRI
through O
dynamic S-CONPRI
weighting O
. O


Similar O
attempts O
to O
reduce O
support S-APPL
volume O
include O
tree-like O
and O
topologically S-CONPRI
optimized O
supports S-APPL
. O


2 O
Cellular B-FEAT
structures E-FEAT
in O
AM S-MANP
Cellular O
structures O
represent O
an O
important O
structural B-FEAT
design E-FEAT
feature S-FEAT
in O
AM S-MANP
, O
commonly O
used O
for O
lightweighting S-PRO
. O


Characterized O
by O
slender/thin O
members O
, O
such O
as S-MATE
struts/bars O
or O
sheets/plates O
, O
these O
formations O
are O
increasingly O
utilized O
as S-MATE
an O
integral O
feature S-FEAT
in O
AM-designs O
primarily O
due O
to O
material S-MATE
and O
time-/energy-saving O
in O
fabrication S-MANP
as S-MATE
well O
as S-MATE
improvements O
in O
strength-to-weight O
ratio O
, O
as S-MATE
summarized O
in O
. O


Besides O
the O
specific B-PRO
strength E-PRO
, O
their O
ability O
to O
dissipate O
energy O
, O
heat S-CONPRI
and O
vibration O
add O
value O
to O
the O
design S-FEAT
. O


Recently O
, O
a O
significant O
increase O
in O
buckling B-CHAR
load E-CHAR
and O
buckling B-PRO
strength E-PRO
have O
been O
recorded O
by O
using O
triangular O
lattice-infills O
or O
topology S-CONPRI
optimized O
hierarchical O
microstructures S-MATE
, O
respectively O
. O


In O
metal-AM O
, O
they O
can O
also O
help O
mitigate O
warpage S-CONPRI
, O
arising O
from O
process-induced O
residual B-PRO
stresses E-PRO
, O
however O
, O
some O
lattice S-CONPRI
types O
must O
be S-MATE
altered O
to O
accommodate O
for O
e.g O
. O


The O
use O
of O
lattices S-CONPRI
, O
contrary O
to O
the O
common O
misconception O
, O
is O
therefore O
not O
a O
lightweighting S-PRO
approach O
from O
a O
pure O
stiffness S-PRO
standpoint O
, O
which O
needs O
to O
be S-MATE
considered O
in O
the O
design B-CONPRI
process E-CONPRI
. O


An O
overview O
from O
design S-FEAT
to O
analysis O
of O
cellular B-FEAT
structures E-FEAT
for O
AM S-MANP
can O
be S-MATE
extracted O
from O
. O


Unit B-CONPRI
cells E-CONPRI
represent O
the O
building O
blocks O
of O
a O
lattice S-CONPRI
, O
which O
is O
generated O
either O
from O
sweeping O
, O
meshing/mapping O
, O
tessellation S-FEAT
to O
obtain O
a O
regular O
pattern S-CONPRI
or O
stochastically O
yielding O
unstructured O
formations O
, O
which O
is O
often O
achieved O
via O
techniques O
such O
as S-MATE
dithering O
. O


Unit B-CONPRI
cells E-CONPRI
are O
generally O
classified O
into O
truss S-MACEQ
based O
and O
surface-based O
. O


Numerous O
cell B-CONPRI
topologies E-CONPRI
were O
generatively O
developed O
over O
the O
years O
, O
leading O
to O
the O
emergence O
of O
unit B-CONPRI
cell E-CONPRI
libraries/families O
for O
AM S-MANP
. O


Ground O
trusses O
, O
often O
resembling O
lattice-like O
configurations O
, O
are O
characterized O
by O
a O
freeform S-CONPRI
frame-network O
with O
locally O
varying O
strut B-PARA
diameters E-PARA
which O
also O
lends O
themselves O
well O
for O
AM S-MANP
. O


It O
is O
however O
of O
note O
, O
that O
isotropic S-PRO
open-cell O
truss-like O
structures O
have O
an O
up O
to O
threefold O
inferior O
stiffness S-PRO
performance S-CONPRI
compared O
to O
their O
closed-cell O
lattice S-CONPRI
counterparts O
of O
the O
same O
relative B-PRO
density E-PRO
. O


On O
this O
account O
, O
considering O
that O
lattices S-CONPRI
are O
not O
stiffness-optimal O
and O
given O
the O
choice O
, O
surface-based O
unit B-CONPRI
cells E-CONPRI
should O
be S-MATE
favoured O
for O
lightweight S-CONPRI
designs S-FEAT
. O


Unit B-CONPRI
cell E-CONPRI
tessellation S-FEAT
approaches O
often O
give O
rise O
to O
non-conformity O
as S-MATE
loose-hanging O
members O
are O
formed O
when O
a O
lattice S-CONPRI
is O
created O
, O
impairing O
the O
structural B-CHAR
performance E-CHAR
. O


Lattices S-CONPRI
derived O
from O
tessellated O
surface-based O
unit B-CONPRI
cells E-CONPRI
are O
less O
affected O
by O
this O
phenomenon O
. O


On O
the O
contrary O
, O
conformal B-FEAT
lattices E-FEAT
were O
found O
to O
yield O
greater O
structural B-CHAR
performance E-CHAR
. O


Works O
on O
graded O
lattices S-CONPRI
most O
generally O
focus O
on O
piecewise O
variation S-CONPRI
in O
volume B-PARA
fractions E-PARA
that O
change O
from O
one O
unit B-CONPRI
cell E-CONPRI
to O
another O
. O


However O
, O
due O
to O
the O
implicit O
definition O
of O
triply B-CONPRI
periodic I-CONPRI
minimal I-CONPRI
surface E-CONPRI
lattices S-CONPRI
, O
true O
functional O
grading O
is O
achievable O
with O
ease O
. O


Together O
with O
the O
advancement O
in O
hardware O
technology S-CONPRI
, O
capable O
of O
fine- O
and O
multi-scale B-FEAT
structures E-FEAT
, O
these O
new O
lightweight B-CONPRI
lattice E-CONPRI
structures O
with O
locally O
changing O
material B-CONPRI
properties E-CONPRI
represent O
a O
unique O
offering O
of O
AM S-MANP
. O


Discrepancies O
between O
the O
ideal O
and O
the O
actual O
performance S-CONPRI
of O
lattices S-CONPRI
, O
reviewed O
in O
, O
is O
owed O
to O
irregularities O
introduced O
during O
manufacturing S-MANP
, O
which O
include O
micro-voids S-PRO
or O
the O
change O
in O
surface B-PRO
roughness E-PRO
, O
making O
the O
prediction S-CONPRI
of O
the O
mechanical S-APPL
performance O
based O
on O
the O
relative B-PRO
density E-PRO
and O
linear O
elastic S-PRO
assumptions O
difficult O
. O


A O
comparison O
between O
the O
experimental S-CONPRI
and O
numerical O
analyses O
of O
lattices S-CONPRI
and O
a O
summary O
of O
simulation S-ENAT
methods O
are O
summarized O
in O
. O


Latticing O
tools S-MACEQ
are O
increasingly O
provided O
in O
today O
's O
CAD S-ENAT
software O
and O
are O
commonly O
used O
to O
substitute O
low-stress O
areas S-PARA
for O
weight S-PARA
reduction S-CONPRI
. O


Particularly O
the O
ability O
to O
achieve O
a O
balance O
between O
robustness S-PRO
and O
compliance O
in O
tailored O
lattices S-CONPRI
was O
recently O
stressed O
. O


Another O
emerging O
research S-CONPRI
field O
in O
AM S-MANP
, O
aiming O
- O
among O
others O
- O
at O
combing O
high O
strength S-PRO
and O
stiffness S-PRO
, O
is O
biomimicry S-CONPRI
. O


Moreover O
, O
it O
is O
of O
note O
, O
that O
some O
software S-CONPRI
providers O
like O
ELiSE O
are O
already O
incorporating O
bionic B-MATE
templates E-MATE
into O
designs S-FEAT
geared O
towards O
AM S-MANP
. O


1 O
Topologically S-CONPRI
optimized O
lattices S-CONPRI
based O
on O
academic O
codes O
Promising O
academic O
approaches O
dealing O
with O
structural B-CONPRI
optimisation E-CONPRI
of O
lattices S-CONPRI
for O
AM S-MANP
are O
on O
the O
uprise O
. O


The O
digitalisation O
of O
these O
intricate O
structures O
is O
a O
prerequisite O
to O
ensure O
optimisation O
, O
bridge S-APPL
the O
gap O
to O
manufacturing S-MANP
and O
ultimately O
improve O
the O
structural B-CHAR
performance E-CHAR
of O
lightweight S-CONPRI
AM-parts O
. O


Computationally O
efficient O
representation O
of O
lattices S-CONPRI
is O
as S-MATE
challenging O
as S-MATE
it O
is O
important O
for O
the O
analysis O
and O
manufacturing S-MANP
steps O
. O


As S-MATE
a O
consequence O
, O
research S-CONPRI
is O
actively O
looking O
into O
ways O
to O
reduce O
the O
computational O
expense O
, O
aiding O
streamline O
the O
design B-CONPRI
process E-CONPRI
and O
enabling O
geometrical O
layouts O
, O
such O
as S-MATE
functionally O
graded O
lattices S-CONPRI
. O


1 O
Structurally O
optimized O
lattices S-CONPRI
using O
the O
homogenization B-MANP
method E-MANP
To O
capture O
the O
heterogeneous S-CONPRI
nature O
of O
lattices S-CONPRI
, O
unit B-CONPRI
cells E-CONPRI
are O
commonly O
homogenized S-MANP
into O
representative O
volume S-CONPRI
elements S-MATE
. O


Homogenization-based O
methods O
have O
recently O
been O
integrated O
into O
a O
lattice S-CONPRI
mesostructural O
optimisation O
framework S-CONPRI
for O
AM S-MANP
. O


In O
, O
Messner O
presented O
the O
inverse O
homogenization S-MANP
, O
in O
which O
the O
macrostructural B-PRO
material I-PRO
properties E-PRO
of O
a O
periodic O
lattice S-CONPRI
consisting O
of O
simple S-MANP
unit O
cells S-APPL
are O
combined O
with O
a O
subsequent O
parameterization O
to O
obtain O
a O
3D S-CONPRI
truss-like O
lattice B-FEAT
structure E-FEAT
with O
struts S-MACEQ
of O
round O
cross-sections S-CONPRI
and O
isotropic B-MATE
material E-MATE
behaviour O
. O


In O
the O
context O
of O
digital O
AM S-MANP
, O
it O
is O
worth O
mentioning O
, O
that O
such O
a O
parameterization O
of O
the O
design B-CONPRI
space E-CONPRI
is O
beneficial O
for O
better O
control O
over O
the O
geometrical O
layout S-CONPRI
using O
CAD-like B-ENAT
environments E-ENAT
and O
therefore O
bridging S-CONPRI
the O
gap O
between O
important O
intrinsic O
DfAM O
aspects O
. O


2 O
Graded O
lattices S-CONPRI
derived O
from O
mapping O
combined O
with O
density-based O
TO O
Various O
studies O
have O
utilized O
a O
mapping O
approach O
to O
update O
a O
cellular B-FEAT
structure E-FEAT
based O
on O
the O
density S-PRO
values O
obtained O
from O
unpenalized O
TO O
. O


Some O
of O
these O
use O
the O
abovementioned O
homogenization S-MANP
approach O
to O
map O
the O
unit B-CONPRI
cells E-CONPRI
achieving O
a O
more O
accurate S-CHAR
mechanical O
response O
. O


In O
, O
a O
SIMP-solution O
provides O
the O
density B-PRO
distribution E-PRO
, O
which O
is O
mapped O
onto O
the O
explicit O
cellular B-FEAT
structure E-FEAT
, O
leading O
to O
the O
newly O
density-adjusted O
structure S-CONPRI
. O


In O
an O
FEA O
of O
a O
beam S-MACEQ
under O
three-point B-CHAR
bending E-CHAR
it O
was O
determined O
that O
the O
stiffness S-PRO
of O
the O
explicit O
cellular O
model S-CONPRI
increased O
with O
greater O
gradient O
density S-PRO
and O
lower O
cell B-PRO
size E-PRO
, O
approaching O
the O
implicit O
i.e O
. O


Great O
reliability S-CHAR
of O
the O
numerical O
model S-CONPRI
was O
concluded O
after O
negligible O
differences O
in O
performance S-CONPRI
were O
obtained O
in O
comparison O
with O
the O
specimens O
created O
from O
SLA S-MACEQ
. O


An O
increase O
in O
stiffness S-PRO
by O
more O
than O
a O
third O
over O
the O
uniform O
lattice S-CONPRI
was O
revealed O
in O
the O
experiment S-CONPRI
, O
highlighting O
a O
viable O
design S-FEAT
approach O
for O
structurally O
enhanced O
AM-parts O
. O


The O
dependence O
between O
cell B-PRO
size E-PRO
and O
performance S-CONPRI
elucidates O
the O
need O
for O
high-precision O
, O
fine-scale B-MANP
manufacturing E-MANP
in O
future O
work O
. O


Manufacturing S-MANP
of O
extruded S-MANP
2D S-CONPRI
lattices O
is O
straightforward O
for O
most O
printing B-ENAT
technologies E-ENAT
, O
however O
, O
3D B-FEAT
cellular I-FEAT
structures E-FEAT
can O
pose O
a O
challenge O
for O
e.g O
. O


powder-based O
processes S-CONPRI
, O
as S-MATE
unconsolidated O
material S-MATE
is O
easily O
encapsulated S-CONPRI
in O
cavities O
. O


This O
was O
taken O
into O
consideration O
by O
Jin O
through O
manufacturing B-CONPRI
constraints E-CONPRI
such O
as S-MATE
the O
minimum O
section O
thickness O
, O
the O
minimum O
hole O
diameter S-CONPRI
and O
the O
uniformity O
of O
the O
section O
thickness O
i.e O
. O


The O
authors O
mapped O
the O
lattice S-CONPRI
of O
square O
cell S-APPL
honeycombs O
onto O
the O
density B-PRO
matrix E-PRO
of O
the O
TO-solution O
. O


The O
FEA O
reveals O
superior O
compressive O
performance S-CONPRI
of O
the O
developed O
graded O
lattice S-CONPRI
even O
with O
the O
manufacturing B-CONPRI
constraints E-CONPRI
over O
the O
uniform O
lattice S-CONPRI
. O


Similar O
to O
, O
the O
authors O
found O
that O
a O
decreasing O
cell B-PRO
size E-PRO
and O
an O
increasing O
gradient O
density S-PRO
has O
a O
positive O
effect O
on O
the O
structural B-CHAR
performance E-CHAR
. O


Song O
expanded O
the O
concept O
to O
irregularly O
shaped O
cells S-APPL
for O
AM S-MANP
. O


Here O
, O
a O
triangular B-ENAT
mesh E-ENAT
, O
obtained O
from O
a O
domain S-CONPRI
of O
unit O
tangent O
circles O
, O
is O
the O
basis O
for O
the O
formation O
of O
the O
cellular B-FEAT
structure E-FEAT
with O
predetermined O
cell/circle O
ratios O
. O


This O
irregular O
lattice S-CONPRI
is O
then O
mapped O
onto O
the O
density B-PRO
field E-PRO
obtained O
from O
unpenalized O
SIMP O
. O


Experimental S-CONPRI
tests O
showed O
that O
the O
irregular O
cell S-APPL
structure O
obtained O
from O
the O
unpenalized O
SIMP O
outperformed O
those O
using O
penalization O
. O


Mapping O
strut-based S-FEAT
unit O
cells S-APPL
with O
different O
densities O
into O
a O
hexahedral O
mesh O
of O
a O
topologically S-CONPRI
optimized O
structure S-CONPRI
enabled O
Robbins O
to O
create O
structures O
with O
an O
external O
topology S-CONPRI
based O
on O
the O
penalized O
SIMP-result O
filled O
with O
a O
conformal B-FEAT
lattice E-FEAT
with O
properties S-CONPRI
determined O
from O
the O
homogenization B-MANP
method E-MANP
. O


The O
meshing O
was O
done O
with O
the O
Sandia O
's O
Sculpt O
tool S-MACEQ
, O
allowing O
for O
the O
adaption O
and O
smoothing O
of O
the O
mesh O
at O
the O
boundaries S-FEAT
, O
aiding O
the O
creation O
of O
a O
flawless O
and O
print-ready O
STL S-MANS
files S-MANS
. O


To O
guarantee O
the O
same O
weight S-PARA
of O
structures O
with O
various O
unit B-CONPRI
cell E-CONPRI
densities O
, O
the O
external O
topology S-CONPRI
was O
adapted O
accordingly O
, O
revealing O
an O
improved O
performance S-CONPRI
for O
less O
dense O
lattices S-CONPRI
. O


Even O
though O
the O
structure S-CONPRI
was O
printed O
using O
laser B-MANP
power I-MANP
bed I-MANP
fusion E-MANP
of O
stainless B-MATE
steel E-MATE
, O
no O
experimental S-CONPRI
verification O
was O
conducted O
, O
which O
is O
a O
recurring O
shortcoming O
in O
many O
of O
the O
reviewed O
studies O
on O
structural B-CONPRI
optimisation E-CONPRI
using O
either O
TO O
or O
lattices S-CONPRI
. O


While O
all O
the O
aforementioned O
studies O
solely O
focused O
on O
the O
performance S-CONPRI
aspects O
of O
the O
mapped O
structures O
, O
Panesar O
took O
also O
aspects O
of O
design S-FEAT
and O
manufacturability S-CONPRI
into O
account O
. O


Hereby O
investigating O
intersected O
, O
scaled O
and O
graded O
lattices S-CONPRI
with O
strut- O
and O
surface-based O
self-supporting S-FEAT
unit O
cells S-APPL
for O
design S-FEAT
aspect O
like O
structural O
optimality O
, O
design S-FEAT
effort O
, O
support S-APPL
requirements O
, O
robustness S-PRO
i.e O
. O


resilience O
to O
variation S-CONPRI
in O
loading O
and O
design-to-manufacturing O
discrepancy O
. O


The O
authors O
used O
a O
voxel S-CONPRI
paradigm O
, O
rather O
than O
the O
traditional O
volume S-CONPRI
boundary B-CONPRI
representation E-CONPRI
via O
B-rep S-ENAT
, O
to O
represent O
lattices S-CONPRI
and O
employed O
an O
iso-value O
matrix O
, O
similar O
to O
, O
for O
functional O
grading O
. O


purely O
topologically S-CONPRI
optimized O
result O
. O


Authors O
highlighted O
the O
superiority O
of O
the O
proposed O
lattice S-CONPRI
types O
in O
terms O
of O
specific B-PRO
stiffness E-PRO
over O
the O
uniform O
lattice S-CONPRI
and O
the O
robustness S-PRO
of O
the O
graded O
lattice S-CONPRI
making O
it O
the O
most O
well-rounded O
performing O
lattice S-CONPRI
. O


Particularly O
, O
the O
lattices S-CONPRI
comprised O
of O
surface-based O
unit B-CONPRI
cells E-CONPRI
were O
found O
to O
result O
in O
more O
resilient O
structures O
. O


However O
, O
from O
a O
performance S-CONPRI
i.e O
. O


stiffness S-PRO
perspective O
, O
all O
lattices S-CONPRI
perform O
inferior O
compared O
to O
the O
solid O
pendant O
. O


Moreover O
, O
the O
cellular B-FEAT
structures E-FEAT
reduced O
the O
support B-FEAT
structure E-FEAT
requirements O
compared O
to O
the O
solid O
design S-FEAT
. O


Similar O
to O
, O
the O
voxel-based O
method O
of O
Aremeu O
employs O
a O
net-skin O
approach O
, O
ensuring O
conformal B-FEAT
lattices E-FEAT
despite O
tessellation S-FEAT
, O
through O
re-connecting O
strut-members O
at O
the O
boundary S-FEAT
. O


The O
authors O
believe O
, O
that O
due O
to O
the O
limited O
resolvable O
resolution S-PARA
of O
AM B-MACEQ
machines E-MACEQ
, O
this O
method O
would O
satisfy O
the O
level O
of O
detail O
required O
for O
the O
model S-CONPRI
without O
high O
computational O
cost O
and O
geometrical B-FEAT
complexity E-FEAT
, O
however O
, O
no O
numerical O
nor O
experimental S-CONPRI
verification O
was O
conducted O
in O
this O
study O
. O


More O
recently O
Wang O
have O
topologically S-CONPRI
optimized O
a O
strut-based S-FEAT
Kagome O
lattice B-FEAT
structure E-FEAT
based O
on O
the O
SIMP O
method O
with O
direct O
AM S-MANP
considerations O
. O


The O
novelty O
is O
that O
it O
represents O
a O
structure-based O
rather O
than O
a O
boundary-based O
TO O
and O
constitutes O
a O
method O
to O
increase O
the O
specific B-PRO
stiffness E-PRO
of O
the O
primitive O
unit B-CONPRI
cell E-CONPRI
. O


In O
an O
attempt O
to O
fine-tune O
the O
structural O
response O
of O
lattices S-CONPRI
, O
Maskery O
have O
recently O
observed O
a O
sequential O
failure S-CONPRI
of O
piece-wise O
graded O
BCC S-CONPRI
lattices O
in O
compression S-PRO
promoting O
increased O
energy B-CHAR
absorption E-CHAR
in O
comparison O
to O
the O
ungraded B-FEAT
lattice E-FEAT
. O


Furthermore O
, O
the O
results O
indicated O
non-homogeneous B-PRO
bulk I-PRO
properties E-PRO
, O
making O
the O
use O
of O
Gibson-Ashby B-CONPRI
models E-CONPRI
inadequate O
, O
as S-MATE
they O
are O
developed O
for O
uniform O
density S-PRO
lattices O
. O


It O
is O
important O
to O
note O
however O
, O
that O
this O
model S-CONPRI
is O
still O
useful O
for O
obtaining O
ballpark O
values O
for O
lattices S-CONPRI
through O
simple S-MANP
calculations O
. O


Due O
to O
the O
very O
specific O
failure B-PRO
mechanisms E-PRO
in O
graded O
lattices S-CONPRI
, O
future O
work O
ought O
to O
put O
more O
emphasis O
on O
the O
experimental S-CONPRI
verification O
in O
order O
to O
make O
more O
reliable O
predictions S-CONPRI
for O
their O
performance S-CONPRI
. O


3 O
Variable-density O
infills O
derived O
from O
density-based O
TO O
An O
emerging O
topic O
in O
structural B-CONPRI
optimization E-CONPRI
with O
a O
promising O
application O
for O
AM S-MANP
are O
TO-based O
compliance-driven O
infill S-PARA
strategies O
for O
porous B-MATE
material E-MATE
, O
which O
account O
for O
the O
principal B-PRO
stress E-PRO
directions O
imposed O
by O
the O
loading O
condition O
. O


Infills O
derived O
from O
homogenization-based O
TO O
, O
using O
projection-based O
methods O
, O
was O
subject O
under O
investigation O
in O
, O
making O
designs S-FEAT
potentially O
more O
feasible O
for O
AM S-MANP
through O
the O
incorporation O
of O
a O
minimum O
length B-CHAR
scale E-CHAR
constraint O
for O
the O
microstructural S-CONPRI
features O
matching O
the O
print-resolution S-PARA
. O


In O
, O
this O
method O
was O
experimentally O
underpinned O
, O
demonstrating O
how O
self-supporting S-FEAT
variable-density O
lattices S-CONPRI
can O
be S-MATE
generated O
. O


Work O
by O
Wu O
centres O
on O
SIMP O
, O
whereby O
in O
, O
solely O
the O
infills O
of O
3D B-CONPRI
structures E-CONPRI
were O
optimized O
and O
the O
shell S-MACEQ
was O
represented O
by O
passive O
elements S-MATE
, O
whereas O
the O
approach O
in O
demonstrated O
a O
concurrent O
optimisation O
of O
shell S-MACEQ
and O
infill S-PARA
for O
2D B-FEAT
structures E-FEAT
. O


In O
these O
structures O
were O
separated O
through O
multiple O
successive O
smoothing O
and O
projection O
steps O
combined O
with O
interpolations O
of O
intermediate O
densities O
to O
obtain O
discrete B-FEAT
topologies E-FEAT
with O
checkerboard-free O
domains O
and O
well-defined O
boundaries S-FEAT
. O


In O
contrast O
to O
the O
standard S-CONPRI
objective O
function O
, O
a O
local O
rather O
than O
a O
global O
volume B-PARA
constraint E-PARA
was O
employed O
in O
both O
studies O
, O
meaning O
the O
density S-PRO
of O
the O
neighbouring O
elements S-MATE
was O
limited O
in O
a O
predefined O
radius O
, O
controlling O
the O
porosity S-PRO
locally O
. O


It O
was O
found O
, O
that O
higher O
radii O
result O
in O
topologies S-CONPRI
with O
coarser O
pores S-PRO
and O
greater O
stiffness S-PRO
as S-MATE
the O
local O
volume B-PARA
constraint E-PARA
becomes O
less O
influencing O
. O


Furthermore O
, O
Wu O
emphasized O
that O
these O
control O
parameters S-CONPRI
are O
beneficial O
for O
the O
manufacturability S-CONPRI
of O
parts O
intended O
for O
powder-based O
AM-processes O
, O
as S-MATE
closed O
walls O
cause O
entrapments O
. O


To O
avoid O
slender B-FEAT
struts E-FEAT
without O
infill S-PARA
, O
a O
length B-CHAR
scale E-CHAR
constraint O
is O
introduced O
in O
, O
which O
was O
implemented O
by O
an O
erosion O
and O
dilation O
projection O
before O
a O
final O
sensitivity B-CONPRI
analysis E-CONPRI
was O
conducted O
. O


Conversely O
, O
in O
, O
the O
authors O
introduce O
an O
anisotropic B-MATE
filter E-MATE
, O
forcing O
the O
material S-MATE
to O
accumulate O
in O
preferred O
i.e O
. O


a O
prescribed O
direction O
in O
order O
to O
resemble O
bone-inspired B-ENAT
infills E-ENAT
, O
resulting O
in O
truss-like O
formations O
. O


The O
filter S-APPL
, O
in O
particular O
, O
helped O
improve O
the O
uniaxial O
buckling S-PRO
stability O
. O


Recently O
, O
Wu O
have O
presented O
a O
computationally O
more O
efficient O
extension O
to O
their O
work O
that O
is O
using O
a O
simultaneous O
optimization S-CONPRI
of O
the O
shape O
and O
the O
distribution S-CONPRI
of O
the O
lattice S-CONPRI
, O
demonstrating O
a O
twofold O
increase O
in O
buckling B-CHAR
load E-CHAR
over O
the O
equivalent O
TO-solution O
. O


Besides O
weight S-PARA
reductions O
, O
improvements O
in O
damage B-PRO
tolerance E-PRO
and O
robustness S-PRO
generally O
illustrate O
greater O
functionality O
, O
mechanical B-PRO
capability E-PRO
and O
versatility O
of O
lattices S-CONPRI
, O
potentially O
outweighing O
the O
losses O
in O
stiffness S-PRO
for O
certain O
engineering S-APPL
problems O
compared O
to O
a O
solid B-MATE
solution E-MATE
. O


Another O
emerging O
topic O
for O
AM S-MANP
within O
topologically S-CONPRI
optimized O
and O
variable-density O
lattices S-CONPRI
, O
which O
will O
not O
be S-MATE
further O
discussed O
, O
are O
both O
multi-scale O
and O
multi-material B-FEAT
structures E-FEAT
. O


Primary O
investigations O
into O
a O
concurrent O
optimization S-CONPRI
of O
both O
macroscale S-CONPRI
and O
a O
micro-scale B-FEAT
lattices E-FEAT
, O
using O
two O
different O
materials S-CONPRI
, O
show O
great O
promise O
for O
the O
design-potential O
of O
AM S-MANP
. O


4 O
Strut-sizing O
and O
-scaling O
for O
structurally O
optimized O
truss-like O
structures O
Emerged O
from O
the O
idea O
to O
drastically O
decrease O
the O
computational O
complexity S-CONPRI
and O
processing O
time O
, O
the O
size O
matching O
and O
scaling O
method O
for O
mesoscale B-FEAT
lattices E-FEAT
, O
represents O
an O
alternative O
approach O
for O
structural B-CONPRI
optimisation E-CONPRI
. O


Besides O
the O
modification O
of O
the O
topology S-CONPRI
as S-MATE
certain O
struts S-MACEQ
that O
have O
a O
minor O
contribution O
to O
the O
structural B-CHAR
performance E-CHAR
are O
eliminated O
, O
the O
method O
mainly O
centres O
on O
the O
size O
optimisation O
of O
the O
struts S-MACEQ
. O


An O
average S-CONPRI
stress O
distribution S-CONPRI
of O
the O
solid O
body O
serves O
as S-MATE
the O
basis O
upon O
which O
the O
unit B-CONPRI
cell I-CONPRI
topology E-CONPRI
with O
geometrical B-FEAT
features E-FEAT
is O
determined O
prior O
to O
the O
mapping O
onto O
the O
initial O
geometry S-CONPRI
. O


Choosing O
the O
exact O
node O
size O
and O
thresholds O
for O
the O
upper O
and O
lower O
bounds O
of O
the O
strut B-PARA
diameter E-PARA
were O
deemed O
to O
be S-MATE
vital O
for O
the O
assurance O
of O
manufacturability S-CONPRI
with O
AM S-MANP
. O


Overall O
this O
does O
not O
take O
away O
from O
its O
feasibility S-CONPRI
for O
application O
in O
large O
structures O
as S-MATE
shown O
on O
the O
basis O
of O
a O
light-weight S-PRO
micro O
air O
vehicle O
fuselage S-MACEQ
and O
is O
, O
therefore O
, O
a O
viable O
alternative O
to O
pure O
TO O
. O


This O
method O
represents O
a O
lattice B-FEAT
generation E-FEAT
tool O
integrated O
into O
Siemens O
NX O
CAD S-ENAT
software O
. O


In O
light O
of O
the O
multifunctional O
performance S-CONPRI
of O
lattices S-CONPRI
, O
Seepersad O
have O
demonstrated O
how O
a O
post-processing S-CONPRI
steps O
can O
be S-MATE
used O
to O
obtain O
lattice S-CONPRI
topologies O
with O
e.g O
. O


improved O
heat B-CONPRI
transfer E-CONPRI
, O
by O
effectively O
identifying O
lattice S-CONPRI
members O
that O
can O
be S-MATE
sacrificed O
, O
based O
on O
a O
hybrid O
FEA O
and O
finite O
difference O
approach O
. O


A O
more O
recent O
study O
has O
demonstrated O
a O
solution S-CONPRI
for O
a O
similar O
multifunctional O
optimization S-CONPRI
for O
a O
heterogeneous B-FEAT
cellular I-FEAT
structures E-FEAT
using O
a O
decision O
support S-APPL
problem O
formulation O
. O


5 O
Evolutionary O
algorithms S-CONPRI
for O
structurally O
enhanced O
lattices S-CONPRI
Tang O
made O
use O
of O
a O
BESO-based O
approach O
to O
design S-FEAT
a O
latticed O
engine O
bracket S-MACEQ
with O
optimal O
strut B-PARA
thicknesses E-PARA
, O
using O
a O
kernel-based B-ENAT
algorithm E-ENAT
. O


Points O
of O
the O
boundary S-FEAT
and O
internal O
elements S-MATE
are O
subsequently O
separated O
, O
and O
the O
initial O
wireframe O
is O
mapped O
onto O
the O
boundary B-CONPRI
kernel I-CONPRI
points E-CONPRI
, O
followed O
by O
a O
trimming S-MANP
procedure O
used O
to O
remove O
wires O
outside O
the O
functional O
volume S-CONPRI
. O


Finally O
, O
these O
two O
sets O
of O
frames O
are O
merged O
together O
forming S-MANP
the O
final O
conformal B-FEAT
lattice E-FEAT
, O
which O
undergoes O
the O
BESO B-CONPRI
algorithm E-CONPRI
to O
modify O
the O
strut B-PARA
thickness E-PARA
based O
on O
the O
von B-PRO
Mises I-PRO
stresses E-PRO
obtained O
from O
the O
FEA O
of O
the O
homogenized S-MANP
functional O
volume S-CONPRI
and O
predetermined O
geometrical B-FEAT
limits E-FEAT
. O


The O
structural B-CHAR
performance E-CHAR
was O
improved O
in O
contrast O
to O
a O
homogeneous B-FEAT
lattice E-FEAT
and O
the O
weight S-PARA
reduced O
by O
up O
to O
75 O
% O
over O
the O
original O
solid O
design S-FEAT
. O


A O
recent O
study O
by O
Tang O
dealt O
with O
DfAM O
in O
the O
context O
of O
lattices S-CONPRI
and O
used O
BESO S-CONPRI
to O
locally O
improve O
the O
size O
of O
truss-like O
members O
, O
creating O
a O
heterogeneous B-FEAT
lattice E-FEAT
. O


In O
this O
study O
, O
a O
meta-model O
was O
created O
, O
which O
was O
comprised O
of O
the O
geometrical B-CONPRI
manufacturing I-CONPRI
constraints E-CONPRI
regarding O
the O
deflection O
of O
horizontal B-FEAT
struts E-FEAT
and O
the O
thickness O
of O
struts S-MACEQ
with O
reference O
to O
the O
inclination B-FEAT
angle E-FEAT
. O


Additionally O
, O
the O
optimized O
lattice B-FEAT
structure E-FEAT
led O
to O
an O
increase O
in O
stiffness S-PRO
over O
the O
traditional O
homogeneous S-CONPRI
pendant O
. O


Besides O
, O
the O
abovementioned O
more O
favourable O
examples O
for O
the O
use O
of O
evolutionary O
algorithms S-CONPRI
for O
the O
optimisation O
of O
lattices S-CONPRI
, O
Chu O
have O
conversely O
demonstrated O
eminent O
drawbacks O
of O
the O
PSO O
over O
the O
non-evolutionary B-CONPRI
LMO I-CONPRI
optimisation E-CONPRI
, O
due O
to O
the O
stochastic S-CONPRI
nature O
of O
such O
algorithms S-CONPRI
. O


Based O
on O
ground O
truss S-MACEQ
structures O
which O
were O
optimized O
for O
a O
given O
volume B-PARA
fraction E-PARA
and O
a O
set S-APPL
allowable O
deflection O
, O
the O
optimal O
strut S-MACEQ
dimensions S-FEAT
were O
obtained O
with O
both O
algorithms S-CONPRI
. O


Results O
showed O
comparable O
structural B-CHAR
performance E-CHAR
but O
the O
superior O
computational B-CONPRI
efficiency E-CONPRI
and O
convergence O
of O
the O
LMO O
method O
. O


The O
integration O
of O
DfAM O
constraints O
into O
the O
optimisation O
algorithms S-CONPRI
remains O
a O
bottleneck S-CONPRI
for O
a O
streamlined O
design S-FEAT
procedure O
, O
calling O
for O
benchmarking O
studies O
helping O
to O
identify O
more O
efficient O
approaches O
in O
digital O
AM S-MANP
in O
future O
work O
. O


2 O
Topologically S-CONPRI
optimized O
lattices S-CONPRI
employing O
commercial O
software S-CONPRI
As S-MATE
the O
role O
of O
AM S-MANP
in O
the O
industry S-APPL
is O
slowly O
shifting O
towards O
production S-MANP
, O
ready-to-use S-CONPRI
design S-FEAT
software O
capitalizing O
on O
now O
amenable O
features O
like O
lattices S-CONPRI
, O
are O
becoming O
more O
readily O
accessible O
. O


3DXpert O
for O
Solidworks O
, O
nTopology O
or O
ANSYS S-APPL
which O
is O
providing O
a O
AM-specific O
design S-FEAT
platforms O
. O


1 O
Modelling S-ENAT
structurally O
optimized O
lattices S-CONPRI
for O
printing O
In O
the O
context O
of O
trusses O
, O
Gorguluarslan O
developed O
a O
two-phase O
lattice S-CONPRI
optimisation O
framework S-CONPRI
, O
specifically O
tackling O
the O
issue O
of O
the O
minimum O
manufacturable S-CONPRI
cross-section O
. O


Here O
a O
primary O
ground O
structure S-CONPRI
optimisation O
is O
followed O
by O
a O
rectification O
process S-CONPRI
in O
which O
elements S-MATE
falling O
below O
a O
certain O
minimum O
cross-sections S-CONPRI
threshold O
are O
eliminated O
, O
before O
a O
final O
optimisation O
step S-CONPRI
is O
conducted O
, O
for O
which O
the O
method O
of O
Feasible O
Directions O
is O
employed O
. O


Altair O
's O
OptiStruct O
and O
HyperMesh S-FEAT
were O
utilized O
for O
structural B-CONPRI
optimisation E-CONPRI
. O


Among O
others O
, O
the O
typical O
cantilever B-MACEQ
beam E-MACEQ
example O
was O
used O
to O
compare O
the O
effectiveness S-CONPRI
of O
the O
framework S-CONPRI
and O
the O
MFD B-CONPRI
algorithm E-CONPRI
with O
Chang O
and O
Rosen O
's O
SMS O
method O
and O
the O
Relative B-PRO
Density E-PRO
Mapping O
of O
Alzahrani O
, O
respectively O
. O


Under O
restriction O
of O
the O
minimum B-PARA
feature I-PARA
size E-PARA
, O
MFD O
was O
identified O
to O
outperform O
SMS O
and O
RDM S-MANP
in O
terms O
of O
stiffness S-PRO
but O
required O
more O
computational O
effort O
. O


However O
, O
it O
was O
also O
found O
that O
designs S-FEAT
obtained O
from O
genetic B-CONPRI
algorithms E-CONPRI
and O
PSO O
have O
an O
even O
superior O
structural B-CHAR
performance E-CHAR
. O


In O
Abaqus S-ENAT
served O
as S-MATE
a O
tool S-MACEQ
for O
the O
TO O
within O
the O
RDM S-MANP
method O
. O


Compared O
to O
the O
SMS O
method O
, O
the O
density S-PRO
values O
of O
the O
TO O
are O
used O
to O
map O
the O
struts S-MACEQ
into O
the O
domain S-CONPRI
, O
making O
RDM S-MANP
less O
dependent O
on O
the O
FEA O
analysis O
. O


Arisoy O
developed O
a O
framework S-CONPRI
for O
the O
substitution O
of O
solid O
parts O
of O
a O
CAD B-ENAT
model E-ENAT
with O
lattices S-CONPRI
, O
which O
served O
as S-MATE
the O
basis O
for O
a O
plugin O
in O
Siemens O
's O
NX O
CAD S-ENAT
software O
. O


boundary S-FEAT
surface O
defining O
the O
layout S-CONPRI
of O
the O
lattice S-CONPRI
. O


During O
the O
FEA O
of O
the O
solid O
part O
, O
meshed O
with O
tetrahedral B-FEAT
elements E-FEAT
, O
a O
remeshing B-CONPRI
process E-CONPRI
including O
trimming S-MANP
is O
conducted O
ensuring O
that O
the O
elements S-MATE
are O
part O
of O
the O
lattice S-CONPRI
. O


Besides O
an O
improved O
and O
efficient O
workflow S-CONPRI
for O
AM S-MANP
, O
the O
authors O
concluded O
to O
have O
improved O
the O
boundary S-FEAT
smoothness O
of O
the O
lattices S-CONPRI
with O
the O
implicit B-FEAT
volume I-FEAT
representation E-FEAT
, O
so O
that O
the O
manufacturability S-CONPRI
for O
AM S-MANP
is O
improved O
. O


In O
terms O
of O
digitalization O
and O
streamlining O
the O
design B-CONPRI
process E-CONPRI
, O
this O
study O
demonstrated O
great O
promise O
, O
as S-MATE
the O
lattices S-CONPRI
were O
modelled O
within O
the O
same O
CAD S-ENAT
environment O
. O


Based O
on O
the O
design S-FEAT
optimisation O
of O
a O
missile B-MACEQ
launcher I-MACEQ
beam E-MACEQ
on O
a O
macro- B-FEAT
and I-FEAT
mesoscale E-FEAT
, O
H O
exemplarily O
demonstrated O
the O
potential O
of O
multi-scale O
optimisation O
. O


They O
have O
effectively O
combined O
TO O
with O
partial O
substitution O
of O
solids O
with O
lattices S-CONPRI
, O
to O
design S-FEAT
a O
lightweight S-CONPRI
beam S-MACEQ
using O
the O
commercial O
software S-CONPRI
Altair O
Inspire O
and O
Materialise O
Magics O
. O


2 O
Experimental S-CONPRI
benchmarking O
of O
3D-printed S-MANP
lattices O
In O
contrast O
to O
the O
previous O
studies O
, O
Beyer O
and O
Figueroa O
tested O
different O
unit B-CONPRI
cell E-CONPRI
structures O
experimentally O
and O
Harl O
even O
determined O
the O
performance S-CONPRI
of O
optimized O
lattice B-FEAT
structures E-FEAT
by O
comparing O
them O
to O
numerical O
results O
. O


The O
experimental S-CONPRI
verification O
is O
vital O
as S-MATE
structural O
optimisation O
and O
DfAM O
aspects O
are O
mutually O
dependent O
. O


This O
stems O
from O
the O
individual O
characteristics O
and O
limitations O
of O
different O
AM B-MANP
processes E-MANP
affecting O
the O
properties S-CONPRI
. O


As S-MATE
outlined O
by O
Gibson O
, O
this O
is O
related O
to O
common O
meso- O
and O
microstructural S-CONPRI
features O
of O
cellular B-FEAT
structures E-FEAT
, O
affecting O
the O
structural B-CHAR
performance E-CHAR
, O
including O
: O
1 O
) O
material B-CONPRI
properties E-CONPRI
; O
2 O
) O
cell B-CONPRI
topology E-CONPRI
and O
shape O
; O
3 O
) O
relative B-PRO
density E-PRO
of O
structure S-CONPRI
. O


As S-MATE
the O
process-parameters O
influence O
the O
microstructure S-CONPRI
and O
consequently O
the O
performance S-CONPRI
, O
verification S-CONPRI
with O
the O
numerical O
model S-CONPRI
becomes O
vital O
for O
estimating O
the O
actual O
properties S-CONPRI
and O
improving O
future O
models O
. O


In O
, O
Netfabb O
's O
Selective O
Space O
Structures O
tool S-MACEQ
was O
utilized O
to O
create O
lattices S-CONPRI
from O
rectangular O
prisms O
and O
hexagonal S-FEAT
geometries S-CONPRI
, O
which O
were O
subsequently O
manufactured S-CONPRI
using O
the O
PolyJet S-CONPRI
and O
SLM S-MANP
process S-CONPRI
for O
polymers S-MATE
and O
aluminium S-MATE
, O
respectively O
. O


The O
compression S-PRO
and O
flexure B-CHAR
tests E-CHAR
of O
unit B-CONPRI
cells E-CONPRI
composed O
of O
a O
polymer S-MATE
revealed O
that O
the O
hexagonal B-FEAT
unit I-FEAT
cells E-FEAT
show O
greater O
yield B-PRO
strengths E-PRO
and O
that O
comparable O
properties S-CONPRI
with O
the O
solid O
pendant O
can O
most O
likely O
be S-MATE
achieved O
with O
vertical S-CONPRI
trusses O
in O
the O
cell S-APPL
. O


Within O
the O
scaled O
lattices B-CONPRI
fabricated E-CONPRI
from O
polymer S-MATE
and O
aluminium S-MATE
, O
it O
was O
found O
that O
the O
kagome B-FEAT
structure E-FEAT
displayed O
the O
best O
performance-to-mass O
ratio O
. O


Moreover O
, O
it O
was O
reported O
that O
the O
aluminium B-FEAT
lattice E-FEAT
was O
superior O
to O
the O
polymer S-MATE
counterpart O
with O
respect O
to O
the O
solid O
structures O
, O
possibly O
stressing O
differences O
in O
manufacturing S-MANP
and O
therefore O
highlighting O
the O
role O
of O
DfAM O
. O


A O
connection O
between O
relative O
cell B-FEAT
density E-FEAT
, O
unit B-CONPRI
cell E-CONPRI
structure S-CONPRI
and O
structural B-CHAR
performance E-CHAR
, O
was O
established O
, O
giving O
designers O
and O
engineers O
helpful O
indications O
for O
the O
selection O
of O
the O
adequate O
lattice B-CONPRI
configuration E-CONPRI
. O


Similarly O
, O
Harl O
employed O
the O
CAESS O
ProTOp O
software S-CONPRI
for O
both O
the O
lattice B-FEAT
generation E-FEAT
and O
the O
subsequent O
TO O
of O
the O
lattice S-CONPRI
. O


The O
latter O
was O
conducted O
to O
improve O
the O
structural B-CHAR
performance E-CHAR
by O
lowering O
the O
local B-CONPRI
stress I-CONPRI
concentration E-CONPRI
to O
which O
cellular B-FEAT
structures E-FEAT
are O
prone O
to O
. O


Using O
selective B-MANP
laser I-MANP
sintering E-MANP
with O
polyamide S-MATE
, O
the O
designs S-FEAT
were O
tested O
in O
flexure S-MACEQ
and O
the O
results O
clearly O
highlight O
the O
potential O
of O
using O
TO O
lattices S-CONPRI
in O
terms O
of O
weight-reduction O
, O
robustness S-PRO
and O
stiffness S-PRO
as S-MATE
well O
as S-MATE
its O
superiority O
over O
uniform O
lattices S-CONPRI
. O


The O
next O
paragraphs O
build S-PARA
on O
this O
idea O
of O
TO O
cellular B-FEAT
structures E-FEAT
by O
locally O
changing O
the O
cell B-FEAT
density E-FEAT
with O
the O
use O
of O
commercial O
software S-CONPRI
. O


In O
a O
study O
by O
Rezaie O
a O
beam S-MACEQ
under O
three-point B-CHAR
bending E-CHAR
was O
topologically S-CONPRI
optimized O
using O
Abaqus S-ENAT
and O
subsequently O
manufactured S-CONPRI
in O
ABS S-MATE
using O
FDM S-MANP
. O


Here O
, O
the O
external O
boundaries S-FEAT
of O
the O
bounding B-FEAT
box E-FEAT
were O
preserved O
, O
and O
the O
lattice S-CONPRI
composed O
of O
hollow O
cubic O
unit B-CONPRI
cells E-CONPRI
was O
mapped O
onto O
a O
SIMP-result O
. O


Additionally O
, O
manufacturability S-CONPRI
was O
taken O
into O
consideration O
by O
tailoring O
the O
hole-size O
to O
match O
the O
printer S-MACEQ
's O
resolution S-PARA
. O


In O
comparison O
with O
the O
standard S-CONPRI
topologically O
optimized O
beam S-MACEQ
, O
the O
loss O
in O
performance S-CONPRI
turned O
out O
to O
be S-MATE
very O
minor O
. O


Especially O
when O
considering O
the O
more O
fabrication-friendly O
design S-FEAT
of O
the O
cellular O
lattice S-CONPRI
, O
this O
method O
can O
be S-MATE
a O
viable O
alternative O
for O
lightweight B-MACEQ
structures E-MACEQ
. O


The O
significance O
of O
the O
results O
, O
however O
, O
must O
be S-MATE
reviewed O
critically O
, O
due O
to O
insufficient O
batch O
size O
and O
the O
well-known O
and O
inherent O
quality S-CONPRI
fluctuations O
in O
AM S-MANP
. O


Improved O
quality B-CONPRI
control E-CONPRI
measures O
are O
therefore O
needed O
form O
a O
manufacturing S-MANP
standpoint O
while O
an O
extensive O
data S-CONPRI
library O
will O
ease O
the O
choice O
in O
adequate O
lattice B-CONPRI
configurations E-CONPRI
from O
a O
design S-FEAT
perspective O
. O


These O
aspects O
constitute O
-among O
others O
- O
a O
necessity O
with O
the O
increasing O
uptake O
of O
AM S-MANP
across O
industries S-APPL
. O


With O
the O
combination O
of O
the O
density-gradients O
from O
TO O
with O
a O
size O
optimisation O
, O
Daynes O
derived O
functionally B-FEAT
graded I-FEAT
truss-structures E-FEAT
that O
align O
with O
the O
isostatic B-CONPRI
lines E-CONPRI
i.e O
. O


the O
principal B-PRO
stresses E-PRO
during O
loading O
. O


By O
making O
use O
of O
the O
Altair O
OptiStruct O
software S-CONPRI
, O
a O
topologically S-CONPRI
optimized O
design S-FEAT
is O
automatically O
obtained O
, O
substituted O
by O
pre-selected O
unit B-CONPRI
cells E-CONPRI
before O
a O
size O
optimisation O
ensures O
that O
the O
overall O
target O
density S-PRO
is O
achieved O
. O


Upon O
the O
information O
on O
in-plane O
stresses O
and O
orientation B-CONPRI
extracted E-CONPRI
from O
the O
software S-CONPRI
, O
the O
authors O
developed O
a O
MATLAB O
subroutine O
to O
create O
a O
functionally B-FEAT
graded I-FEAT
mesh E-FEAT
, O
which O
was O
fed O
back O
into O
the O
software S-CONPRI
for O
analysis O
. O


The O
substitution O
of O
the O
domain S-CONPRI
with O
BCC S-CONPRI
unit O
cells S-APPL
conformal O
with O
the O
isostatic B-CONPRI
lines E-CONPRI
resulted O
in O
a O
graded O
lattice S-CONPRI
with O
improved O
structural B-CHAR
performance E-CHAR
. O


Twice O
the O
stiffness S-PRO
and O
about O
75 O
times O
greater O
strength S-PRO
were O
demonstrated O
in O
the O
experimental S-CONPRI
three-point O
bending B-CHAR
tests E-CHAR
compared O
to O
the O
uniform O
lattice B-FEAT
design E-FEAT
. O


These O
study O
highlights O
how O
much O
potential O
is O
still O
untapped O
in O
commercial O
software S-CONPRI
regarding O
the O
utilization O
of O
tailored O
, O
functionally B-FEAT
graded I-FEAT
lattices E-FEAT
for O
enhanced O
specific O
structural B-CHAR
performance E-CHAR
. O


3 O
Commercial O
and O
academic O
software S-CONPRI
solutions O
for O
TO O
and O
the O
generation O
of O
cellular B-FEAT
structures E-FEAT
AM-specific O
design S-FEAT
tools O
, O
incorporating O
TO O
or O
the O
implementation O
of O
lattices S-CONPRI
, O
are O
central O
to O
the O
greater O
adoption O
of O
AM S-MANP
. O


Today O
's O
software S-CONPRI
landscape O
for O
AM-specific O
design S-FEAT
provides O
a O
range S-PARA
of O
features O
that O
have O
been O
summarized O
in O
the O
supplementary O
information O
of O
this O
review O
. O


It O
is O
important O
to O
note O
, O
that O
the O
majority O
of O
commercial O
software S-CONPRI
to-date O
solve O
specific O
problems O
that O
are O
common O
for O
a O
wide O
range S-PARA
of O
industrial S-APPL
applications O
. O


14a O
illustrates O
the O
proportionate O
distribution S-CONPRI
of O
underlying O
TO-methods/approaches O
in O
commercial O
software S-CONPRI
to-date O
. O


14b O
constitutes O
the O
predominant O
file S-MANS
formats O
available O
in O
current O
lattice B-FEAT
generation E-FEAT
software O
. O


Analogously O
, O
lattice B-FEAT
generation E-FEAT
software O
is O
dominated O
by O
STL S-MANS
file S-MANS
formats O
, O
however O
, O
with O
the O
emerging O
complexity S-CONPRI
in O
e.g O
. O


multi-material S-CONPRI
prints O
we O
see O
the O
adoption O
and O
market O
share O
increase O
of O
3MF S-MANS
and O
AMF S-CONPRI
file O
formats O
. O


To O
evaluate O
the O
effectiveness S-CONPRI
of O
the O
different O
software S-CONPRI
, O
more O
benchmarking O
research S-CONPRI
will O
be S-MATE
needed O
. O


A O
first O
attempt O
, O
recently O
published O
by O
Saadlaoui O
compared O
the O
performance S-CONPRI
of O
a O
topologically S-CONPRI
optimized O
cube S-CONPRI
under O
compression S-PRO
loading O
using O
Optistruct O
and O
Abaqus S-ENAT
. O


A O
stress-constrained S-PRO
optimisation O
and O
a O
discrete-compliance O
optimisation O
were O
conducted O
in O
Abaqus S-ENAT
as O
well O
as S-MATE
a O
continuous B-CONPRI
compliance E-CONPRI
optimisation O
in O
Optistruct O
. O


Both O
numerical O
and O
experimental S-CONPRI
results O
were O
obtained O
for O
the O
three O
approaches O
, O
whereby O
SLM S-MANP
was O
adopted O
for O
the O
fabrication S-MANP
of O
the O
specimens O
. O


This O
was O
explained O
by O
the O
consideration O
of O
internal O
material S-MATE
behaviour O
for O
the O
numerical O
model S-CONPRI
rather O
than O
the O
external O
criteria O
in O
the O
experiment S-CONPRI
. O


This O
aspect O
also O
includes O
local B-CONPRI
plastic I-CONPRI
deformations E-CONPRI
in O
the O
actual O
specimen O
, O
which O
do O
not O
necessarily O
affect O
the O
global O
elastic S-PRO
properties O
or O
mechanical B-PRO
strength E-PRO
and O
which O
are O
not O
accounted O
for O
in O
the O
commonly O
employed O
linear-elastic O
computational B-ENAT
models E-ENAT
, O
as S-MATE
stated O
by O
Saadlaoui O
. O


In O
terms O
of O
the O
computational O
procedures O
, O
the O
study O
revealed O
that O
the O
stress-constrained S-PRO
optimisation O
approach O
has O
the O
highest O
computational O
cost O
, O
but O
the O
corresponding O
specimen O
demonstrated O
the O
best O
mechanical S-APPL
performance O
with O
respect O
to O
the O
ratio B-CHAR
of I-CHAR
strength E-CHAR
to O
weight S-PARA
. O


Computational B-CONPRI
efficiency E-CONPRI
and O
power S-PARA
are O
particularly O
important O
in O
continuum S-CONPRI
structural O
optimisations O
and O
will O
be S-MATE
key O
for O
viable O
design S-FEAT
procedures O
in O
large-scale O
, O
multifunctional O
, O
multiscale O
and O
cellular-based O
TO O
. O


Recent O
efforts O
made O
to O
improve O
the O
feasibility S-CONPRI
of O
TO O
codes O
for O
machines S-MACEQ
with O
standard S-CONPRI
computational O
performance S-CONPRI
are O
equally O
important O
as S-MATE
the O
more O
salient O
examples O
, O
exploiting O
immense O
resources O
to O
explore O
what O
is O
ultimately O
possible O
. O


General O
conclusions O
, O
drawn O
from O
this O
review O
, O
can O
be S-MATE
summarized O
as S-MATE
: O
Superior O
solutions/designs O
for O
AM-parts O
can O
be S-MATE
obtained O
using O
structural B-CONPRI
optimization I-CONPRI
methods E-CONPRI
such O
as S-MATE
TO O
if O
DfAM O
considerations O
are O
included O
. O


This O
elucidates O
the O
need O
for O
a O
holistic O
design S-FEAT
approach O
, O
paving O
way O
for O
how O
we O
design S-FEAT
for O
AM S-MANP
in O
the O
future O
. O


Aspects O
of O
digitalization O
are O
still O
somewhat O
underrepresented O
and O
pose O
a O
bottleneck S-CONPRI
between O
design S-FEAT
and O
manufacturing S-MANP
, O
including O
computationally O
inexpensive O
models O
and O
an O
entirely O
self-contained O
process S-CONPRI
from O
CAD B-MANS
file E-MANS
to O
Gcode O
. O


To O
bridge S-APPL
the O
gap O
to O
manufacturing S-MANP
, O
CAD/CAM B-ENAT
software E-ENAT
needs O
to O
be S-MATE
further O
improved O
, O
pathing O
among O
others O
the O
way O
for O
greater O
efficiency O
and O
compatibility O
when O
post-processing S-CONPRI
digital O
designs S-FEAT
. O


Both O
multifunctional O
TO-based O
designs S-FEAT
, O
considering O
multiple O
objectives O
and O
part-integration O
as S-MATE
well O
as S-MATE
latticing O
were O
considered O
particularly O
promising O
for O
lightweight S-CONPRI
high-performance O
parts O
. O


Functionally B-FEAT
graded I-FEAT
structures E-FEAT
, O
specifically O
those O
that O
are O
optimized O
to O
better O
conform O
to O
the O
load O
trajectories O
, O
are O
inciting O
increased O
research S-CONPRI
interest O
. O


The O
most O
prominent O
TO O
method O
, O
both O
in O
academia O
and O
industry S-APPL
, O
is O
the O
density-based O
approach O
assuming O
isotropic B-MATE
material E-MATE
property O
. O


Level O
set S-APPL
methods O
seem O
to O
establish O
themselves O
increasingly O
in O
the O
market O
, O
due O
to O
the O
intrinsic O
smooth B-FEAT
boundary E-FEAT
representation O
, O
which O
is O
of O
great O
benefit O
for O
a O
streamlined O
design S-FEAT
procedure O
. O


The O
sheer O
amount O
of O
software S-CONPRI
available O
for O
both O
TO O
and O
latticing O
that O
includes O
AM-specific O
design S-FEAT
constraints O
, O
clearly O
demonstrates O
the O
industry S-APPL
is O
gravitating O
towards O
a O
wide O
application O
of O
AM S-MANP
. O


This O
is O
supported O
by O
the O
intensified O
interest O
in O
the O
academic O
community O
to O
make O
optimisation O
process S-CONPRI
more O
computationally O
efficient O
which O
is O
a O
necessity O
to O
make O
AM S-MANP
more O
accessible O
for O
a O
range S-PARA
of O
industries S-APPL
. O


As S-MATE
reviewed O
, O
the O
majority O
of O
studies O
concentrate O
on O
structural B-CONPRI
optimisation E-CONPRI
with O
the O
inclusion S-MATE
of O
inclination B-FEAT
angle E-FEAT
constraints O
as S-MATE
opposed O
to O
fabrication S-MANP
, O
materials S-CONPRI
or O
digitalization O
. O


Aspects O
deemed O
important O
for O
future O
work O
on O
the O
design S-FEAT
and O
structural B-CONPRI
optimisation E-CONPRI
in O
AM S-MANP
, O
are O
as S-MATE
follows O
: O
Consistency S-CONPRI
through O
benchmarking O
mathematically-driven O
models O
and O
confidence O
via O
experimental S-CONPRI
verification O
will O
be S-MATE
important O
as S-MATE
industries O
are O
on O
the O
verge O
of O
adopting O
AM S-MANP
as O
means O
of O
production S-MANP
. O


Structural B-CHAR
analyses E-CHAR
accommodating O
path B-ENAT
planning E-ENAT
i.e O
. O


tailored O
infills O
and O
a O
simultaneous O
optimization S-CONPRI
of O
process S-CONPRI
and O
design S-FEAT
will O
constitute O
a O
central O
aspect O
to O
improved O
structural B-CHAR
performance E-CHAR
in O
the O
future O
. O


In O
conjunction O
with O
a O
growing O
interest O
in O
fibre-reinforced B-MANP
AM E-MANP
, O
a O
key O
challenge O
will O
be S-MATE
to O
develop O
TO O
methods O
that O
account O
for O
anisotropic S-PRO
material O
considerations O
, O
paving O
the O
way O
for O
next-generation O
lightweight B-MACEQ
structures E-MACEQ
. O


The O
digital O
AM-workflow O
for O
print-ready O
designs S-FEAT
would O
immensely O
benefit O
from O
smooth B-FEAT
boundary E-FEAT
representations O
. O


post-processing S-CONPRI
procedures O
for O
discrete B-CONPRI
solutions E-CONPRI
and O
more O
recently O
the O
intensified O
use O
of O
the O
level O
set S-APPL
method O
were O
observed O
. O


Their O
uptake O
by O
the O
industry S-APPL
into O
upcoming O
software S-CONPRI
could O
potentially O
constitute O
a O
boost O
for O
the O
future O
of O
AM S-MANP
. O


In O
light O
of O
both O
increasing O
complexity S-CONPRI
through O
the O
inclusion S-MATE
of O
DfAM O
constraints O
and O
multi-objective O
optimization S-CONPRI
as S-MATE
well O
as S-MATE
the O
orientation S-CONPRI
towards O
large-scale O
designs S-FEAT
, O
generating O
high-resolution B-FEAT
designs E-FEAT
efficiently O
, O
will O
become O
a O
key O
challenge O
across O
all O
TO O
methods O
in O
the O
future O
. O


Contrary O
to O
previous O
developments O
, O
the O
industry S-APPL
pulls O
academia O
with O
regard O
to O
generative B-ENAT
design E-ENAT
, O
rather O
than O
taking O
solutions O
up O
. O


Due O
to O
the O
feasibility S-CONPRI
of O
economically O
producing O
large-scale O
metal S-MATE
components S-MACEQ
with O
relatively O
high B-PARA
deposition I-PARA
rates E-PARA
, O
significant O
progress O
has O
been O
made O
in O
the O
understanding O
of O
the O
Wire B-MANP
Arc I-MANP
Additive I-MANP
Manufacturing E-MANP
process S-CONPRI
, O
as S-MATE
well O
as S-MATE
the O
microstructure S-CONPRI
and O
mechanical B-CONPRI
properties E-CONPRI
of O
the O
fabricated S-CONPRI
components S-MACEQ
. O


As S-MATE
WAAM O
has O
evolved O
, O
a O
wide O
range S-PARA
of O
materials S-CONPRI
have O
become O
associated O
with O
the O
process S-CONPRI
and O
its O
applications O
. O


This O
article O
reviews O
the O
emerging O
research S-CONPRI
on O
WAAM S-MANP
techniques O
and O
the O
commonly O
used O
metallic S-MATE
feedstock B-MATE
materials E-MATE
, O
and O
also O
provides O
a O
comprehensive O
over O
view O
of O
the O
metallurgical S-APPL
and O
material B-CONPRI
properties E-CONPRI
of O
the O
deposited O
parts O
. O


Common O
defects S-CONPRI
produced O
in O
WAAM S-MANP
components S-MACEQ
using O
different O
alloys S-MATE
are O
described O
, O
including O
deformation S-CONPRI
, O
porosity S-PRO
, O
and O
cracking S-CONPRI
. O


Methods O
for O
improving O
the O
fabrication S-MANP
quality O
of O
the O
additively B-MANP
manufactured E-MANP
components O
are O
discussed O
, O
taking O
into O
account O
the O
requirements O
of O
the O
various O
alloys S-MATE
. O


The O
integration O
of O
materials S-CONPRI
and O
manufacturing B-MANP
process E-MANP
to O
produce O
defect-free O
and O
structurally-sound O
deposited O
parts O
remains O
a O
crucial O
effort O
into O
the O
future O
. O


In O
recent O
years O
, O
wire B-MANP
arc I-MANP
additive I-MANP
manufacturing E-MANP
has O
increasingly O
attracted O
attention O
from O
the O
industrial S-APPL
manufacturing O
sector O
due O
to O
its O
ability O
to O
create O
large O
metal S-MATE
components S-MACEQ
with O
high B-PARA
deposition I-PARA
rate E-PARA
, O
low O
equipment S-MACEQ
cost O
, O
high O
material B-CHAR
utilization E-CHAR
, O
and O
consequent O
environmental O
friendliness O
. O


The O
origin O
of O
the O
WAAM S-MANP
process S-CONPRI
can O
be S-MATE
traced O
back O
to O
the O
1925s O
when O
Baker O
proposed O
to O
use O
an O
electric B-PARA
arc E-PARA
as S-MATE
the O
heat B-CONPRI
source E-CONPRI
with O
filler O
wires O
as S-MATE
feedstock O
materials S-CONPRI
to O
deposit O
metal S-MATE
ornaments O
. O


The O
WAAM S-MANP
technique O
bears O
various O
nomenclatures O
by O
different O
research S-CONPRI
institutions O
worldwide O
, O
as S-MATE
shown O
in O
1 O
. O


Today O
, O
WAAM S-MANP
has O
become O
a O
promising O
fabrication S-MANP
process O
for O
various O
engineering B-MATE
materials E-MATE
such O
as S-MATE
titanium O
, O
aluminium S-MATE
, O
nickel B-MATE
alloy E-MATE
and O
steel S-MATE
. O


Compared O
to O
traditional B-MANP
subtractive I-MANP
manufacturing E-MANP
, O
the O
WAAM B-MACEQ
system E-MACEQ
can O
reduce O
fabrication B-PARA
time E-PARA
by O
40and O
post-machining S-MANP
time O
by O
15depending O
on O
the O
component S-MACEQ
size O
. O


For O
instance O
, O
recent O
breakthroughs O
in O
WAAM S-MANP
technology S-CONPRI
have O
made O
it O
possible O
to O
fabricate S-MANP
aircraft O
landing O
gear S-MACEQ
ribs O
with O
a O
saving O
of O
approximately O
78 O
% O
in O
raw B-MATE
material E-MATE
when O
compared O
with O
the O
traditional B-MANP
subtractive I-MANP
machining I-MANP
process E-MANP
. O


Due O
to O
the O
highly O
complex O
nature O
of O
WAAM S-MANP
, O
many O
different O
aspects O
of O
the O
process S-CONPRI
need O
to O
be S-MATE
studied O
, O
including O
process S-CONPRI
development O
, O
material S-MATE
quality O
and O
performance S-CONPRI
, O
path O
design S-FEAT
and O
programming O
, O
process S-CONPRI
modelling S-ENAT
, O
process B-CONPRI
monitoring E-CONPRI
and O
online O
control O
. O


Several O
WAAM S-MANP
review O
papers O
have O
been O
published O
by O
leaders O
in O
the O
field O
, O
covering O
state-of-the-art S-CONPRI
systems O
, O
design S-FEAT
, O
usage O
, O
in-situ S-CONPRI
process O
monitoring O
, O
in-situ S-CONPRI
metrology O
and O
in-process O
control O
and O
sensing S-APPL
. O


Nevertheless O
, O
there O
is O
still O
a O
need O
for O
a O
systematic O
review O
of O
the O
properties S-CONPRI
of O
various O
WAAM-processed O
materials S-CONPRI
, O
the O
defects S-CONPRI
associated O
with O
different O
alloys S-MATE
, O
and O
a O
summary O
of O
current O
and O
future O
research S-CONPRI
directions O
that O
are O
aimed O
at O
quality S-CONPRI
improvements O
for O
the O
alloy S-MATE
classes O
of O
interest O
. O


This O
paper O
reviews O
the O
microstructure S-CONPRI
and O
mechanical B-CONPRI
properties E-CONPRI
of O
various O
metals S-MATE
, O
including O
titanium S-MATE
and O
its O
alloys S-MATE
, O
aluminum S-MATE
and O
its O
alloys S-MATE
, O
Ni-based B-MATE
alloy E-MATE
, O
steel S-MATE
and O
other O
intermetallic S-MATE
materials O
fabricated S-CONPRI
by O
the O
various O
WAAM S-MANP
processes S-CONPRI
. O


The O
common O
defects S-CONPRI
that O
have O
been O
found O
to O
occur O
for O
different O
materials S-CONPRI
are O
also O
summarized O
. O


The O
current O
methods O
for O
both O
in-process O
and O
post-process S-CONPRI
quality O
improvement O
and O
defect S-CONPRI
reduction O
are O
introduced O
. O


Finally O
, O
a O
discussion O
is O
given O
on O
improving O
quality S-CONPRI
of O
WAAM S-MANP
fabricated S-CONPRI
parts O
through O
process B-CONPRI
selection E-CONPRI
, O
feedstock B-MACEQ
optimization E-MACEQ
, O
process B-CONPRI
monitoring E-CONPRI
and O
control O
and O
post-process S-CONPRI
, O
including O
proposals O
for O
future O
research S-CONPRI
directions O
. O


2 O
Wire B-MANP
arc I-MANP
additive I-MANP
manufacturing E-MANP
systems O
2.1 O
Classification S-CONPRI
of O
WAAM S-MANP
process S-CONPRI
Depending O
on O
the O
nature O
of O
the O
heat B-CONPRI
source E-CONPRI
, O
there O
are O
commonly O
three O
types O
of O
WAAM S-MANP
processes S-CONPRI
: O
Gas B-MANP
Metal I-MANP
Arc I-MANP
Welding E-MANP
-based O
, O
Gas B-MANP
Tungsten I-MANP
Arc I-MANP
Welding E-MANP
-based O
and O
Plasma B-MANP
Arc I-MANP
Welding E-MANP
-based O
. O


As S-MATE
listed O
in O
1 O
, O
specific O
class O
of O
WAAM S-MANP
techniques O
exhibit O
specific O
features O
. O


The O
deposition B-PARA
rate E-PARA
of O
GMAW-based B-MANP
WAAM E-MANP
is O
2times O
higher O
than O
that O
of O
GTAW-based O
or O
PAW-based O
methods O
. O


However O
, O
the O
GMAW-based B-MANP
WAAM E-MANP
is O
less O
stable O
and O
generates O
more O
weld B-CHAR
fume E-CHAR
and O
spatter S-CHAR
due O
to O
the O
electric O
current O
acting O
directly O
on O
the O
feedstock S-MATE
. O


The O
choice O
of O
WAAM S-MANP
technique O
directly O
influences O
the O
processing O
conditions O
and O
production S-MANP
rate O
for O
a O
target O
component S-MACEQ
. O


2.2 O
Robotic O
WAAM B-MACEQ
system E-MACEQ
Most O
WAAM B-MACEQ
systems E-MACEQ
use O
an O
articulated B-APPL
industrial I-APPL
robot E-APPL
as O
the O
motion O
mechanism S-CONPRI
. O


Two O
different O
system O
designs S-FEAT
are O
available O
. O


The O
first O
design S-FEAT
uses O
an O
enclosed O
chamber O
to O
provide O
a O
good O
inert B-CONPRI
gas E-CONPRI
shielding O
environment O
, O
similar O
to O
laser S-ENAT
Power-Bed O
Fusion S-CONPRI
systems O
. O


The O
second O
design S-FEAT
uses O
existing O
or O
specially O
designed S-FEAT
local O
gas S-CONPRI
shielding O
mechanisms O
, O
with O
the O
robot S-MACEQ
positioned O
on O
a O
linear O
rail O
to O
increase O
the O
overall O
working O
envelop O
. O


It O
is O
capable O
of O
fabricating S-MANP
very O
large O
metal S-MATE
structures O
up O
to O
several O
a O
meters O
in O
dimension S-FEAT
. O


2 O
shows O
an O
example O
of O
this O
design S-FEAT
of O
WAAM B-MACEQ
system E-MACEQ
, O
used O
for O
the O
research S-CONPRI
and O
development O
at O
the O
University O
of O
Wollongong O
. O


Fabricating S-MANP
a O
part O
using O
WAAM S-MANP
involves O
three O
main O
steps O
: O
process B-CONPRI
planning E-CONPRI
, O
deposition S-CONPRI
, O
and O
post B-CONPRI
processing E-CONPRI
. O


For O
a O
given O
CAD B-ENAT
model E-ENAT
, O
3D B-ENAT
slicing E-ENAT
and O
programming O
software S-CONPRI
generates O
the O
desired O
robot S-MACEQ
motions O
and O
welding S-MANP
parameters S-CONPRI
for O
the O
deposition B-MANP
process E-MANP
, O
aimed O
at O
producing O
defect-free O
fabrication S-MANP
with O
high O
geometrical O
accuracy S-CHAR
. O


Based O
on O
the O
welding S-MANP
deposition S-CONPRI
model O
for O
the O
specific B-MATE
material E-MATE
being O
used O
to O
fabricate S-MANP
the O
component S-MACEQ
, O
the O
3D B-ENAT
slicing E-ENAT
and O
programming O
software S-CONPRI
offer O
automated B-ENAT
path I-ENAT
planning E-ENAT
and O
process B-CONPRI
optimization E-CONPRI
to O
avoid O
potential O
process-induced O
defects S-CONPRI
. O


During O
fabrication S-MANP
, O
the O
robot S-MACEQ
and O
external O
axis O
provide O
accurate S-CHAR
motion O
for O
the O
welding B-MACEQ
torch E-MACEQ
to O
build S-PARA
up O
the O
component S-MACEQ
in O
a O
layer-by-layer B-CONPRI
fashion E-CONPRI
. O


Advanced O
WAAM B-MACEQ
systems E-MACEQ
can O
be S-MATE
equipped O
with O
various O
sensors S-MACEQ
to O
measure O
welding S-MANP
signals O
, O
deposited B-CHAR
bead I-CHAR
geometry E-CHAR
, O
metal S-MATE
transfer O
behaviour O
and O
interpass B-PARA
temperature E-PARA
, O
thereby O
supporting O
in-process O
monitoring O
and O
control O
to O
achieve O
higher O
product B-CONPRI
quality E-CONPRI
. O


This O
is O
an O
area S-PARA
of O
current O
and O
future O
research S-CONPRI
interest O
, O
with O
the O
potential O
for O
significantly O
improving O
WAAM S-MANP
process B-CONPRI
performance E-CONPRI
. O


3 O
Metals S-MATE
used O
in O
WAAM S-MANP
process S-CONPRI
WAAM O
processes S-CONPRI
use O
commercially O
available O
wires O
which O
are O
produced O
for O
the O
welding B-APPL
industry E-APPL
and O
available O
in O
spooled O
form O
and O
in O
a O
wide O
range S-PARA
of O
alloys S-MATE
as O
feedstock B-MATE
materials E-MATE
. O


2 O
indicates O
the O
commonly O
used O
alloys S-MATE
and O
their O
various O
applications O
in O
WAAM S-MANP
. O


Manufacture S-CONPRI
of O
a O
structurally O
sound O
, O
defect S-CONPRI
free O
, O
reliable O
part O
requires O
an O
understanding O
of O
the O
available O
process S-CONPRI
options O
, O
their O
underlying O
physical B-CONPRI
processes E-CONPRI
, O
feedstock B-MATE
materials E-MATE
, O
process B-CONPRI
control E-CONPRI
methods O
and O
an O
appreciation O
of O
the O
causes O
of O
the O
various O
common O
defects S-CONPRI
and O
their O
remedies O
. O


This O
section O
reviews O
the O
metals S-MATE
that O
are O
commonly O
used O
in O
WAAM S-MANP
, O
with O
a O
particular O
emphasis O
on O
the O
microstructure S-CONPRI
and O
mechanical B-CONPRI
properties E-CONPRI
of O
the O
additively B-MANP
manufactured E-MANP
alloys O
. O


3.1 O
Titanium B-MATE
alloys E-MATE
Titanium S-MATE
alloys S-MATE
have O
been O
widely O
studied O
for O
application O
of O
additive B-MANP
manufacturing E-MANP
in O
aerospace B-MACEQ
components E-MACEQ
due O
to O
their O
high O
strength-to-weight O
ratio O
and O
inherently O
high O
material S-MATE
cost O
. O


There O
are O
increasing O
demands O
for O
more O
efficient O
and O
lower O
cost O
alternatives O
to O
the O
conventional B-MANP
subtractive I-MANP
manufacturing I-MANP
methods E-MANP
, O
which O
suffer O
very O
low O
fly-to-buy O
ratios O
for O
many O
component S-MACEQ
designs O
. O


There O
exists O
many O
business O
opportunities O
for O
the O
WAAM S-MANP
process S-CONPRI
, O
particularly O
for O
large-sized O
titanium S-MATE
components S-MACEQ
with O
complex B-CONPRI
structures E-CONPRI
. O


The O
distinctive O
WAAM S-MANP
thermal B-PARA
cycle E-PARA
, O
which O
involves O
repeated O
heating S-MANP
and O
cooling S-MANP
, O
produces O
meta-stable B-PRO
microstructures E-PRO
and O
inhomogeneous O
compositions O
in O
the O
fabricated S-CONPRI
part O
. O


For O
example O
, O
Baufeld O
investigated O
the O
microstructures S-MATE
of O
Ti6Al4V S-MATE
fabricated S-CONPRI
using O
a O
GTAW-based O
WAAM B-MACEQ
system E-MACEQ
, O
and O
found O
two O
distinctive O
regions O
on O
the O
as-built O
wall O
. O


In O
the O
bottom O
region O
, O
where O
alternating O
bands O
are O
perpendicular O
to O
the O
build B-PARA
direction E-PARA
, O
a O
basketwave O
Widmanststructure O
with O
phase S-CONPRI
lamellae S-MATE
is O
present O
, O
while O
in O
the O
top O
region O
, O
where O
no O
such O
bands O
appear O
, O
needle-like O
precipitate S-MATE
is O
the O
main O
structure S-CONPRI
. O


Similar O
microstructural B-CONPRI
evolution E-CONPRI
has O
also O
been O
observed O
in O
the O
PAW-based O
process S-CONPRI
. O


Lin O
reported O
a O
graded B-FEAT
microstructure E-FEAT
along O
the O
build B-PARA
direction E-PARA
and O
identified O
the O
martensite S-MATE
structure O
, O
Widmanststructure O
and O
basket-wave O
structure S-CONPRI
from O
the O
bottom O
to O
the O
top O
region O
of O
the O
fabricated S-CONPRI
component S-MACEQ
, O
as S-MATE
shown O
in O
3 O
. O


An O
epitaxial S-PRO
growth O
of O
grains S-CONPRI
with O
discrete O
direction O
is O
also O
observed O
along O
the O
build B-PARA
direction E-PARA
owing O
to O
thermal B-PARA
gradient E-PARA
, O
commonly O
seen O
in O
additively B-MANP
manufactured E-MANP
titanium O
alloy S-MATE
components O
. O


3 O
summarizes O
the O
microstructure S-CONPRI
and O
mechanical B-CONPRI
property E-CONPRI
data S-CONPRI
of O
Ti6Al4V S-MATE
samples B-CONPRI
fabricated E-CONPRI
using O
various O
WAAM S-MANP
technologies S-CONPRI
. O


The O
as-forged O
and O
as-cast O
minimum O
specifications S-PARA
from O
ASTM O
standards S-CONPRI
are O
also O
listed O
for O
comparison O
. O


As S-MATE
shown O
in O
4 O
, O
the O
tensile B-PRO
property E-PRO
of O
as-fabricated O
Ti6Al4V S-MATE
samples S-CONPRI
is O
close O
to O
that O
of O
wrought S-CONPRI
Ti6Al4V S-MATE
and O
exceeds O
that O
of O
cast S-MANP
Ti6Al4V O
as S-MATE
specified O
by O
ASTM O
standards S-CONPRI
. O


In O
addition O
, O
WAAM S-MANP
fabricated S-CONPRI
Ti6Al4V O
samples S-CONPRI
show O
anisotropic S-PRO
properties O
with O
lower O
strength S-PRO
and O
higher O
elongation B-PRO
values E-PRO
in O
the O
build B-PARA
direction E-PARA
compared O
to O
deposition B-PARA
direction E-PARA
, O
which O
is O
mainly O
attributed O
to O
the O
grain B-PRO
size E-PRO
of O
lamellae S-MATE
and O
the O
orientation S-CONPRI
of O
the O
elongated O
prior O
grains S-CONPRI
. O


3.2 O
Aluminum B-MATE
alloys E-MATE
and O
steel S-MATE
Although O
fabrication S-MANP
trials O
for O
many O
different O
series O
of O
aluminum B-MATE
alloys E-MATE
, O
including O
Al-Cu S-MATE
, O
Al-Si S-MATE
and O
Al-Mg S-MATE
have O
been O
successfully O
carried O
out O
, O
the O
commercial O
value O
of O
WAAM S-MANP
is O
mainly O
justifiable O
for O
large O
and O
complex B-PRO
thin-walled I-PRO
structures E-PRO
, O
since O
cost B-CONPRI
of I-CONPRI
manufacturing E-CONPRI
small O
and O
simple S-MANP
aluminum B-MATE
alloy E-MATE
components O
using O
conventional B-MANP
machining E-MANP
processes O
is O
low O
. O


Using O
WAAM S-MANP
to O
fabricate S-MANP
steel O
is O
unpopular O
for O
the O
same O
reason O
although O
it O
is O
the O
most O
commonly O
used O
engineering B-MATE
material E-MATE
. O


Another O
reason O
for O
the O
poor O
commercial O
application O
of O
WAAM S-MANP
in O
aluminum S-MATE
is O
that O
some O
series O
of O
aluminum B-MATE
alloys E-MATE
, O
such O
as S-MATE
Al S-MATE
7xxx O
and O
6xxx O
, O
are O
challenging O
to O
weld S-FEAT
due O
to O
turbulent O
melt B-MATE
pool E-MATE
and O
weld S-FEAT
defects S-CONPRI
, O
which O
frequently O
occur O
during O
the O
deposition B-MANP
process E-MANP
. O


In O
general O
, O
as-deposited O
additively B-MANP
manufactured E-MANP
aluminum O
alloy S-MATE
parts O
have O
inferior O
mechanical B-CONPRI
properties E-CONPRI
compared O
to O
those O
machined S-MANP
from O
billet S-MATE
material O
. O


In O
order O
to O
achieve O
higher O
tensile B-PRO
strength E-PRO
, O
most O
of O
the O
as-deposited O
aluminum S-MATE
parts O
undergo O
post-process B-CONPRI
heat E-CONPRI
treatment O
to O
refine O
the O
microstructure S-CONPRI
. O


4 O
lists O
the O
yield B-PRO
strength E-PRO
, O
ultimate B-PRO
tensile I-PRO
strength E-PRO
, O
and O
elongation S-PRO
of O
WAAM-fabricated O
2219aluminium O
alloy S-MATE
samples O
. O


Due O
to O
the O
uniform O
distribution S-CONPRI
of O
large O
diamond S-MATE
particles O
within O
the O
microstructure S-CONPRI
, O
the O
sample S-CONPRI
exhibits O
lower O
UTS S-PRO
and O
YS O
than O
that O
of O
the O
wrought S-CONPRI
part O
specified O
by O
ASTM O
standard S-CONPRI
. O


However O
, O
after O
heat B-MANP
treatment E-MANP
, O
significant O
improvement O
beyond O
ASTM O
standard S-CONPRI
can O
be S-MATE
observed O
in O
both O
strength S-PRO
and O
elongation S-PRO
as S-MATE
a O
consequence O
of O
the O
grain B-CHAR
refinement E-CHAR
. O


3.3 O
Nickel-based B-MATE
superalloys E-MATE
Nickel-based O
superalloys S-MATE
are O
the O
second O
most O
popular O
material S-MATE
studied O
by O
the O
additive B-MANP
manufacturing E-MANP
research O
community O
after O
titanium B-MATE
alloys E-MATE
, O
mainly O
due O
to O
their O
high O
strengths S-PRO
at O
elevated O
temperatures S-PARA
and O
high O
fabrication S-MANP
cost O
using O
traditional O
methods O
. O


Nickel-based B-MATE
superalloys E-MATE
are O
widely O
applied O
in O
aerospace S-APPL
, O
aeronautical S-APPL
, O
petrochemical O
, O
chemical O
and O
marine B-APPL
industries E-APPL
due O
to O
their O
outstanding O
strength S-PRO
and O
oxidation B-PRO
resistance E-PRO
at O
temperatures S-PARA
above O
550 O
To O
date O
, O
various O
Nickel-based B-MATE
superalloys E-MATE
, O
including O
Inconel B-MATE
718 E-MATE
and O
Inconel B-MATE
625 I-MATE
alloy E-MATE
have O
been O
studied O
after O
WAAM S-MANP
processing O
. O


The O
microstructure S-CONPRI
of O
WAAM S-MANP
fabricated S-CONPRI
Inconel O
718 O
parts O
generally O
consists O
of O
large O
columnar B-PRO
grains E-PRO
with O
interdendritic O
boundaries S-FEAT
delineated O
by O
small O
Laves B-CONPRI
phase E-CONPRI
precipitates O
and O
MC S-MATE
carbides S-MATE
. O


Xu O
reported O
that O
columnar B-FEAT
dendrite I-FEAT
structures E-FEAT
decorated O
with O
a O
large O
amount O
of O
Laves B-CONPRI
phase E-CONPRI
, O
MC S-MATE
carbides S-MATE
and O
Ni3Nb S-MATE
are O
also O
present O
in O
WAAM-fabricated O
Inconel B-MATE
625 E-MATE
parts O
, O
as S-MATE
shown O
in O
5 O
. O


It O
is O
worth O
noting O
that O
the O
microstructure S-CONPRI
can O
be S-MATE
refined O
to O
smaller O
dendritic B-BIOP
arm I-BIOP
spacing E-BIOP
, O
less O
niobium B-CONPRI
segregation E-CONPRI
and O
discontinuous O
Laves B-CONPRI
phase E-CONPRI
in O
the O
interdendritic O
regions O
using O
post-process B-CONPRI
heat E-CONPRI
treatments O
, O
which O
are O
beneficial O
to O
the O
mechanical B-CONPRI
properties E-CONPRI
. O


5 O
lists O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
several O
Nickel-based B-MATE
superalloys E-MATE
fabricated S-CONPRI
using O
the O
WAAM S-MANP
process S-CONPRI
. O


For O
GMAW-based O
WAAM-fabricated O
Inconel B-MATE
718 I-MATE
alloy E-MATE
, O
the O
yield O
and O
ultimate B-PRO
tensile I-PRO
strength E-PRO
is O
473 O
6 O
MPa S-CONPRI
and O
828 O
8 O
MPa S-CONPRI
respectively O
. O


These O
values O
lie O
between O
the O
minimum O
values O
specified O
by O
ASTM O
for O
wrought S-CONPRI
and O
cast S-MANP
materials O
, O
whereas O
the O
elongation S-PRO
is O
much O
lower O
than O
the O
standards S-CONPRI
for O
both O
wrought S-CONPRI
and O
cast S-MANP
conditions O
. O


As S-MATE
for O
WAAM-fabricated O
Inconel B-MATE
625 I-MATE
alloy E-MATE
, O
the O
YS O
, O
UTS S-PRO
and O
elongation S-PRO
all O
meet O
the O
requirement O
set S-APPL
by O
ASTM O
for O
cast S-MANP
materials O
, O
and O
are O
slightly O
lower O
than O
those O
for O
wrought B-MATE
material E-MATE
. O


3.4 O
Other O
metals S-MATE
Other O
metals S-MATE
have O
also O
been O
investigated O
for O
potential O
fabrication S-MANP
using O
WAAM S-MANP
, O
such O
as S-MATE
magnesium O
alloy S-MATE
AZ31 O
for O
automotive S-APPL
applications O
, O
Fe/Al O
intermetallic B-MATE
compounds E-MATE
and O
Al/Ti S-MATE
compounds O
, O
as S-MATE
well O
as S-MATE
bimetallic O
steel/nickel O
and O
steel/bronze O
parts O
for O
the O
aeronautic B-APPL
industry E-APPL
. O


The O
detailed O
mechanical B-CONPRI
properties E-CONPRI
of O
these O
materials B-CONPRI
fabricated E-CONPRI
using O
WAAM S-MANP
are O
listed O
in O
6 O
. O


Most O
of O
this O
research S-CONPRI
has O
focused O
on O
determining O
the O
microstructural S-CONPRI
and O
mechanical B-CONPRI
properties E-CONPRI
of O
samples S-CONPRI
taken O
from O
simple S-MANP
straight-walled O
structures O
, O
rather O
than O
developing O
a O
process S-CONPRI
to O
fabricate S-MANP
functional O
parts O
. O


Manufacturing S-MANP
intermetallic S-MATE
parts O
with O
accurate S-CHAR
pre-designed O
composition S-CONPRI
still O
poses O
major O
challenges O
for O
the O
WAAM S-MANP
process S-CONPRI
. O


4 O
Common O
defects S-CONPRI
in O
WAAM-fabricated O
component S-MACEQ
Although O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
components S-MACEQ
fabricated O
by O
WAAM S-MANP
are O
in O
many O
cases O
comparable O
to O
those O
of O
their O
conventionally O
processed S-CONPRI
counterparts O
, O
there O
are O
however O
some O
AM S-MANP
processing O
defects S-CONPRI
that O
must O
be S-MATE
addressed O
for O
critical O
applications O
. O


Porosity S-PRO
, O
high O
residual B-PRO
stress E-PRO
levels O
, O
and O
cracking S-CONPRI
, O
must O
be S-MATE
avoided O
, O
particularly O
for O
parts O
exposed O
to O
extreme O
environments O
where O
these O
defects S-CONPRI
lead O
to O
failure B-PRO
modes E-PRO
such O
as S-MATE
high O
temperature B-PRO
fatigue E-PRO
. O


Defects S-CONPRI
in O
WAAM S-MANP
can O
occur O
for O
various O
reasons O
, O
such O
as S-MATE
poor O
programming O
strategy O
, O
unstable O
weld B-CONPRI
pool E-CONPRI
dynamics O
due O
to O
poor O
parameter S-CONPRI
setup O
, O
thermal B-CHAR
deformation E-CHAR
associated O
with O
heat B-PRO
accumulation E-PRO
, O
environmental O
influence O
and O
other O
machine S-MACEQ
malfunctions O
. O


As S-MATE
shown O
in O
6 O
, O
certain O
materials S-CONPRI
tend O
to O
be S-MATE
vulnerable O
to O
specific O
defects S-CONPRI
. O


For O
example O
, O
severe O
oxidization O
for O
titanium B-MATE
alloys E-MATE
, O
porosity S-PRO
for O
aluminum B-MATE
alloys E-MATE
, O
poor O
surface B-PRO
roughness E-PRO
in O
steel S-MATE
as S-MATE
well O
as S-MATE
severe O
deformation S-CONPRI
and O
cracks O
in O
bimetal B-MACEQ
components E-MACEQ
have O
been O
found O
to O
typically O
occur O
. O


7 O
lists O
the O
major O
defects S-CONPRI
that O
are O
commonly O
present O
in O
components S-MACEQ
fabricated O
using O
current O
WAAM S-MANP
techniques O
. O


The O
details O
of O
these O
common O
defects S-CONPRI
and O
their O
relationship O
to O
the O
materials S-CONPRI
will O
be S-MATE
discussed O
this O
section O
. O


4.1 O
Deformation S-CONPRI
and O
residual B-PRO
stress E-PRO
Like O
other O
additive B-MANP
manufacturing I-MANP
process E-MANP
, O
distortion S-CONPRI
and O
residual B-PRO
stress E-PRO
are O
inherent O
to O
the O
WAAM S-MANP
process S-CONPRI
and O
it O
is O
impossible O
to O
completely O
avoid O
its O
generation O
. O


The O
residual B-PRO
stress E-PRO
can O
lead S-MATE
to O
distortion S-CONPRI
of O
the O
part O
, O
loss O
of O
geometric B-FEAT
tolerance E-FEAT
, O
delamination B-CHAR
of I-CHAR
layers E-CHAR
during O
deposition S-CONPRI
, O
as S-MATE
well O
as S-MATE
deterioration O
of O
fatigue S-PRO
performance O
and O
fracture B-PRO
resistance E-PRO
of O
the O
additively B-MANP
manufactured E-MANP
components O
. O


Therefore O
, O
control O
and O
minimization O
of O
deformation S-CONPRI
and O
residual B-PRO
stress E-PRO
is O
a O
key O
area S-PARA
if O
research S-CONPRI
. O


Various O
types O
of O
deformation S-CONPRI
appear O
in O
WAAM S-MANP
fabricated S-CONPRI
parts O
, O
including O
longitudinal O
and O
transvers O
shrinkage S-CONPRI
, O
bending S-MANP
distortion O
, O
angular O
distortion S-CONPRI
and O
rotational O
distortion S-CONPRI
. O


The O
distortions O
are O
caused O
by O
thermal B-CONPRI
expansion E-CONPRI
and O
shrinkage S-CONPRI
of O
the O
part O
during O
repeated O
melting S-MANP
and O
cooling S-MANP
processes O
, O
which O
is O
particularly O
an O
issue O
for O
large O
thin B-CONPRI
walled I-CONPRI
structures E-CONPRI
. O


Residual B-PRO
stress E-PRO
is O
the O
stress S-PRO
that O
remains O
in O
the O
material S-MATE
when O
all O
external O
loading O
forces S-CONPRI
are O
removed O
. O


If O
the O
residual B-PRO
stress E-PRO
is O
sufficiently O
high O
, O
it O
can O
be S-MATE
a O
critical O
influential O
factor O
in O
the O
mechanical B-CONPRI
properties E-CONPRI
and O
fatigue S-PRO
performance O
of O
the O
fabricated S-CONPRI
component S-MACEQ
. O


If O
the O
residual B-PRO
stress E-PRO
exceeds O
the O
local O
UTS S-PRO
of O
the O
material S-MATE
, O
cracking S-CONPRI
will O
take O
place O
, O
while O
if O
it O
is O
higher O
than O
the O
local O
YS O
but O
lower O
than O
UTS S-PRO
, O
warping S-CONPRI
or O
plastic B-PRO
deformation E-PRO
will O
occur O
. O


Ding O
found O
that O
the O
residual B-PRO
stress E-PRO
uniformly O
distributes O
across O
the O
WAAM S-MANP
deposited O
wall O
, O
and O
the O
residual B-PRO
stress E-PRO
in O
the O
preceding O
layer S-PARA
has O
little O
effect O
on O
the O
future O
layers O
. O


After O
release O
of O
clamping O
, O
however O
, O
the O
internal B-PRO
stress E-PRO
is O
redistributed O
with O
a O
much O
lower O
value O
at O
the O
top O
of O
integral O
part O
than O
at O
the O
interface S-CONPRI
to O
the O
substrate S-MATE
, O
resulting O
in O
bending S-MANP
distortion O
of O
the O
component S-MACEQ
. O


Path B-ENAT
planning E-ENAT
also O
involves O
the O
distortion S-CONPRI
and O
residual B-PRO
stress E-PRO
evolution S-CONPRI
in O
WAAM S-MANP
process S-CONPRI
. O


If O
appropriate O
deposition B-PARA
path E-PARA
designs O
, O
it O
will O
help O
in O
the O
significant O
improvement O
in O
these O
defects S-CONPRI
, O
especially O
in O
large O
metal S-MATE
fabrication S-MANP
. O


A O
detailed O
overview O
of O
the O
residual B-PRO
stress E-PRO
origin O
would O
exceed O
the O
scope O
of O
this O
article O
. O


Among O
all O
WAAM S-MANP
engineering B-MATE
materials E-MATE
, O
bimetal B-MACEQ
components E-MACEQ
exhibit O
high O
levels O
of O
residual B-PRO
stress E-PRO
and O
deformation S-CONPRI
due O
to O
the O
material S-MATE
thermal O
expansion O
difference O
. O


Hence O
, O
accurate S-CHAR
interpass O
temperature S-PARA
control O
is O
needed O
when O
bimetal B-MATE
materials E-MATE
are O
used O
. O


WAAM-fabricated O
Inconel B-MATE
alloy E-MATE
has O
comparatively O
lower O
residual B-PRO
stresses E-PRO
levels O
, O
but O
it O
is O
more O
susceptible O
to O
process B-CONPRI
defects E-CONPRI
such O
as S-MATE
delamination O
, O
buckling S-PRO
and O
warping S-CONPRI
, O
since O
its O
residual B-PRO
stress E-PRO
is O
usually O
higher O
than O
the O
yield B-PRO
stress E-PRO
. O


Other O
comparatively O
softer O
materials S-CONPRI
, O
such O
as S-MATE
aluminum B-MATE
alloys E-MATE
, O
easily O
respond O
to O
deformation B-PRO
defects E-PRO
due O
to O
their O
high O
thermal B-PRO
expansion I-PRO
coefficients E-PRO
. O


A O
better O
understanding O
pf O
the O
effect O
of O
material S-MATE
characteristics O
in O
WAAM S-MANP
processing O
is O
needed O
for O
controlling O
residual B-PRO
stress E-PRO
and O
deformation S-CONPRI
during O
deposition S-CONPRI
. O


Deformation S-CONPRI
and O
residual B-PRO
stress E-PRO
are O
associated O
with O
many O
process B-CONPRI
parameters E-CONPRI
, O
such O
as S-MATE
welding O
current O
, O
welding S-MANP
voltage O
, O
feeding O
speed O
, O
ambient O
temperature S-PARA
, O
shielding O
gas B-PARA
flow I-PARA
rate E-PARA
, O
etc O
. O


Fortunately O
, O
several O
post-process S-CONPRI
treatments O
that O
have O
been O
proven O
to O
effectively O
mitigate O
residual B-PRO
stress E-PRO
and O
deformation S-CONPRI
, O
and O
these O
will O
be S-MATE
discussed O
5 O
. O


4.2 O
Porosity S-PRO
Porosity O
is O
another O
common O
defect S-CONPRI
in O
WAAM S-MANP
processing O
that O
needs O
to O
be S-MATE
minimized O
due O
to O
adverse O
effects O
on O
mechanical B-CONPRI
properties E-CONPRI
. O


Firstly O
, O
porosity S-PRO
will O
lead S-MATE
to O
a O
component S-MACEQ
with O
low O
mechanical B-PRO
strength E-PRO
by O
damage S-PRO
from O
micro-cracks S-CONPRI
, O
and O
secondly O
, O
it O
usually O
brings O
low O
fatigue S-PRO
property O
to O
deposition S-CONPRI
via O
spatially O
with O
different O
size O
and O
shape O
distribution S-CONPRI
. O


In O
general O
, O
this O
type O
of O
defects S-CONPRI
are O
mainly O
classified O
as S-MATE
either O
raw O
material-induced O
or O
process-induced O
. O


The O
WAAM S-MANP
raw B-MATE
material E-MATE
, O
including O
as-received O
wire O
and O
substrate S-MATE
, O
often O
has O
a O
degree O
of O
surface B-CHAR
contamination E-CHAR
, O
such O
as S-MATE
moisture O
, O
grease O
and O
other O
hydrocarbon O
compounds O
that O
may O
be S-MATE
difficult O
to O
completely O
remove O
. O


These O
contaminants O
can O
be S-MATE
easily O
absorbed O
into O
the O
molten B-CONPRI
pool E-CONPRI
and O
subsequently O
generate O
porosity S-PRO
after O
solidification S-CONPRI
. O


Among O
common O
engineering B-MATE
materials E-MATE
, O
aluminum B-MATE
alloy E-MATE
is O
the O
most O
susceptible O
to O
this O
defect S-CONPRI
as S-MATE
the O
solubility S-PRO
of O
hydrogen O
in O
solid O
and O
liquid O
is O
significantly O
different O
. O


Even O
small O
amounts O
of O
dissolved O
hydrogen O
in O
the O
liquid B-CONPRI
state E-CONPRI
may O
exceed O
the O
limit S-CONPRI
of O
solubility S-PRO
after O
solidification S-CONPRI
, O
resulting O
in O
porosity S-PRO
. O


Therefore O
, O
the O
cleanliness O
of O
raw B-MATE
materials E-MATE
is O
critical O
, O
especially O
for O
aluminum B-MATE
alloys E-MATE
. O


Process-induced O
porosity S-PRO
is O
usually O
non-spherical S-CONPRI
, O
and O
mainly O
caused O
by O
poor O
path B-ENAT
planning E-ENAT
or O
an O
unstable O
deposition B-MANP
process E-MANP
. O


When O
the O
deposition B-PARA
path E-PARA
is O
complex O
or O
the O
manufacturing B-MANP
process E-MANP
is O
changeable O
, O
insufficient B-MATE
fusion E-MATE
or O
spatter S-CHAR
ejection S-CONPRI
is O
easily O
produced O
, O
creating O
gaps O
or O
voids S-CONPRI
in O
these O
influenced O
regions O
. O


To O
control O
porosity S-PRO
, O
the O
following O
methods O
can O
be S-MATE
adopted O
: O
an O
AC O
GMAW-based O
process S-CONPRI
or O
CMT-PADV O
based O
process S-CONPRI
is O
preferred O
, O
especially O
for O
aluminum S-MATE
; O
the O
highest O
quality S-CONPRI
shielding O
gas S-CONPRI
, O
tight O
gas S-CONPRI
seals O
, O
non-organic O
piping O
and O
short O
pipe O
lengths O
are O
highly O
recommended O
; O
the O
wire O
and O
substrate S-MATE
surfaces O
are O
as S-MATE
clean O
as S-MATE
possible O
before O
fabrication S-MANP
; O
high O
quality S-CONPRI
feedstock S-MATE
should O
be S-MATE
used O
; O
the O
deposited B-CHAR
bead I-CHAR
shape E-CHAR
needs O
to O
be S-MATE
optimized O
; O
the O
thermal B-CONPRI
profile E-CONPRI
during O
processing O
should O
be S-MATE
monitored O
and O
controlled O
; O
post-deposition O
treatment O
, O
such O
as S-MATE
interpass O
rolling S-MANP
can O
be S-MATE
applied O
. O


4.3 O
Crack O
and O
delamination S-CONPRI
Similar O
to O
residual B-PRO
stress E-PRO
and O
deformation S-CONPRI
, O
cracking S-CONPRI
and O
delamination S-CONPRI
not O
only O
involves O
the O
thermal O
signature O
of O
the O
manufacturing B-MANP
process E-MANP
, O
but O
also O
relates O
to O
the O
material S-MATE
characteristics O
of O
the O
deposit O
. O


Ordinarily O
, O
the O
crack O
is O
categorised O
as S-MATE
either O
a O
solidification B-CHAR
crack E-CHAR
or O
grain B-PRO
boundary I-PRO
crack E-PRO
within O
the O
WAAM S-MANP
component S-MACEQ
. O


The O
former O
type O
of O
crack O
depends O
mainly O
on O
the O
solidification S-CONPRI
nature O
of O
the O
material S-MATE
and O
is O
usually O
caused O
by O
the O
obstruction O
of O
solidified O
grain S-CONPRI
flow O
or O
high O
strain S-PRO
in O
the O
melt B-MATE
pool E-MATE
. O


Grain B-CONPRI
boundary E-CONPRI
cracking S-CONPRI
often O
generates O
along O
the O
grain B-CONPRI
boundaries E-CONPRI
due O
to O
the O
differences O
between O
boundary B-CHAR
morphology E-CHAR
and O
potential O
precipitate S-MATE
formation O
or O
dissolution O
. O


Generally O
, O
this O
deficiency O
is O
visible O
and O
can O
not O
be S-MATE
repaired O
by O
post-process S-CONPRI
treatment O
. O


In O
order O
to O
prevent O
this O
defect S-CONPRI
, O
pre-process O
treatment O
such O
as S-MATE
preheating O
of O
the O
substrate S-MATE
needs O
to O
be S-MATE
considered O
. O


Bimetal B-MATE
material I-MATE
combinations E-MATE
, O
such O
as S-MATE
Al/Cu S-MATE
, O
Al/Ti S-MATE
and O
Al/Fe S-MATE
, O
are O
quite O
susceptible O
to O
cracking S-CONPRI
and O
delamination S-CONPRI
when O
fabricated S-CONPRI
with O
the O
WAAM S-MANP
process S-CONPRI
. O


The O
dissimilar O
metals S-MATE
have O
large O
differences O
in O
their O
mutual B-PRO
solubility E-PRO
and O
chemical O
reactivity O
so O
that O
the O
intermetallic S-MATE
phase-equilibrium O
is O
freely O
broken O
, O
thus O
inducing O
crack B-CONPRI
growth E-CONPRI
along O
grain B-CONPRI
boundaries E-CONPRI
. O


Also O
, O
Inconel B-MATE
alloy E-MATE
readily O
generates O
solidification B-CONPRI
cracking E-CONPRI
issues O
due O
to O
the O
existence O
of O
liquid O
film O
at O
terminal O
solidification S-CONPRI
. O


Both O
of O
these O
material S-MATE
types O
should O
receive O
particular O
attention O
to O
avoid O
cracking S-CONPRI
and O
delamination S-CONPRI
. O


To O
control O
crack B-PRO
defects E-PRO
, O
corresponding O
measures O
can O
be S-MATE
taken O
as S-MATE
follows O
: O
Mixed O
wires O
and O
optimization S-CONPRI
of O
their O
compositions O
; O
Decrease O
the O
cooling B-PARA
rate E-PARA
during O
the O
deposition B-MANP
process E-MANP
Other O
measures O
to O
improve O
strength S-PRO
rather O
than O
solution B-MANP
treatment E-MANP
. O


5 O
Current O
methods O
for O
quality S-CONPRI
improvement O
in O
the O
WAAM S-MANP
process S-CONPRI
Generally O
, O
WAAM S-MANP
parts O
require O
post-process S-CONPRI
treatment O
to O
improve O
material B-CONPRI
properties E-CONPRI
, O
reduce O
surface B-PRO
roughness E-PRO
and O
porosity S-PRO
, O
and O
remove O
residual B-PRO
stress E-PRO
and O
distortions O
. O


By O
appropriate O
application O
of O
post O
process S-CONPRI
, O
the O
majority O
of O
issues O
that O
influence O
deposition B-CHAR
quality E-CHAR
can O
be S-MATE
mitigated O
or O
eliminated O
. O


Presently O
, O
several O
post-process S-CONPRI
treatment O
technologies S-CONPRI
have O
been O
reported O
to O
improve O
part O
quality S-CONPRI
in O
the O
WAAM S-MANP
process S-CONPRI
. O


5.1 O
Post-process B-CONPRI
heat E-CONPRI
treatment O
Post-process B-CONPRI
heat E-CONPRI
treatment O
is O
widely O
used O
in O
WAAM S-MANP
to O
reduce O
residual B-PRO
stress E-PRO
, O
enhance O
material B-PRO
strength E-PRO
and O
as S-MATE
a O
method O
of O
hardness S-PRO
control O
. O


The O
selection O
of O
a O
suitable O
heat B-MANP
treatment E-MANP
process O
depends O
on O
the O
target O
material S-MATE
, O
additive B-MANP
manufacturing E-MANP
methods O
, O
working O
temperature S-PARA
and O
heat B-MANP
treatment E-MANP
conditions O
. O


If O
the O
heat B-MANP
treatment E-MANP
state O
is O
set S-APPL
incorrectly O
, O
the O
probability S-CONPRI
of O
cracking S-CONPRI
will O
increase O
under O
mechanical B-CONPRI
loading E-CONPRI
, O
as S-MATE
the O
combination O
of O
existing O
residual B-PRO
stress E-PRO
with O
load O
stress S-PRO
exceeds O
the O
materialdesign O
limitation O
. O


As S-MATE
summarized O
s S-MATE
3after O
heat B-MANP
treatment E-MANP
, O
the O
mechanical B-PRO
strength E-PRO
of O
WAAM-fabricated O
parts O
improved O
significantly O
, O
with O
increase O
of O
4 O
% O
, O
78 O
% O
, O
5 O
% O
and O
17 O
% O
being O
reported O
for O
titanium B-MATE
alloy E-MATE
, O
aluminum B-MATE
alloy E-MATE
, O
Nickel-based B-MATE
superalloys E-MATE
and O
intermetallic S-MATE
Ti/Al O
alloy S-MATE
, O
respectively O
. O


In O
addition O
, O
post-process B-CONPRI
heat E-CONPRI
treatment O
plays O
an O
important O
role O
in O
grain B-CHAR
refinement E-CHAR
, O
especially O
for O
WAAM-fabricated O
aluminum S-MATE
and O
Inconel B-MATE
alloy E-MATE
. O


The O
decision O
to O
use O
post-process B-CONPRI
heat E-CONPRI
treatment O
depends O
on O
the O
material S-MATE
alloying S-FEAT
system O
and O
also O
the O
pre-heat B-MANP
treatment E-MANP
state O
. O


Generally O
, O
high O
carbon B-MATE
content E-MATE
materials O
must O
be S-MATE
heat O
treated O
, O
while O
a O
few O
materials S-CONPRI
can O
be S-MATE
damaged O
by O
this O
technique O
. O


Therefore O
, O
the O
utilization O
of O
post O
heat B-MANP
treatment E-MANP
process O
to O
WAAM S-MANP
component S-MACEQ
needs O
to O
consider O
the O
specific B-MATE
material E-MATE
and O
its O
application O
. O


5.2 O
Interpass B-MANP
cold I-MANP
rolling E-MANP
Rolling O
of O
the O
weld B-CONPRI
bead E-CONPRI
between O
each O
deposited B-CHAR
layer E-CHAR
has O
been O
shown O
to O
reduce O
residual B-PRO
stresses E-PRO
and O
distortion.Interpass O
cold B-MANP
rolling E-MANP
not O
only O
lowers O
residual B-PRO
stress E-PRO
, O
but O
also O
brings O
more O
homogeneous B-MATE
material E-MATE
properties O
. O


In O
the O
WAAM S-MANP
process S-CONPRI
, O
the O
thermal B-PARA
gradient E-PARA
with O
deposition B-PARA
layers E-PARA
and O
alternate O
re-heating O
and O
re-cooling O
process S-CONPRI
result O
in O
the O
target O
part O
having O
anisotropic B-CONPRI
microstructural I-CONPRI
evolution E-CONPRI
and O
mechanical B-CONPRI
properties E-CONPRI
. O


The O
cold B-MANP
rolling E-MANP
technique O
significantly O
reduces O
microstructural S-CONPRI
anisotropy S-PRO
through O
plastically O
deforming O
the O
deposition S-CONPRI
. O


7 O
shows O
the O
schematic O
diagram O
of O
an O
interpass B-MANP
cold I-MANP
rolling E-MANP
system O
developed O
at O
Cranfield O
University O
. O


A O
slotted B-FEAT
roller E-FEAT
is O
used O
to O
refine O
the O
microstructure S-CONPRI
and O
enhance O
tensile B-PRO
strength E-PRO
in O
the O
longitudinal O
direction O
by O
supporting O
external O
force S-CONPRI
. O


As S-MATE
shown O
in O
8 O
, O
both O
ultimate B-PRO
tensile I-PRO
strength E-PRO
and O
yield B-PRO
strength E-PRO
in O
the O
build B-PARA
direction E-PARA
were O
improved O
through O
interpass B-MANP
cold I-MANP
rolling E-MANP
, O
which O
contributes O
to O
more O
homogeneous B-MATE
material E-MATE
properties O
in O
the O
target O
component S-MACEQ
. O


Interpass B-MANP
cold I-MANP
rolling E-MANP
also O
can O
play O
a O
critical O
role O
in O
the O
healing O
of O
hydrogen B-PRO
porosity E-PRO
in O
WAAM-fabricated O
aluminum S-MATE
parts O
. O


Generally O
, O
high O
dislocation B-PRO
density E-PRO
is O
produced O
by O
the O
rolling B-MANP
process E-MANP
. O


These O
dislocation S-CONPRI
can O
act O
as S-MATE
preferential O
sites O
for O
atomic B-CONPRI
hydrogen I-CONPRI
absorption E-CONPRI
and O
as S-MATE
well O
as S-MATE
for O
the O
hydrogen O
, O
allowing O
to O
diffuse O
to O
the O
surface S-CONPRI
. O


9 O
summarizes O
the O
outcomes O
documented O
in O
the O
literature O
, O
in O
terms O
of O
the O
pore S-PRO
incidence O
and O
size O
distribution S-CONPRI
in O
aluminum S-MATE
components O
fabricated S-CONPRI
using O
WAAM S-MANP
with O
interpass B-MANP
cold I-MANP
rolling E-MANP
. O


The O
porosities S-PRO
existing O
in O
as-fabricated O
component S-MACEQ
can O
be S-MATE
reduced O
or O
even O
eliminated O
when O
interpass B-MANP
cold I-MANP
rolling E-MANP
is O
applied O
. O


This O
technique O
is O
only O
feasible O
for O
simple S-MANP
deposited O
parts O
, O
such O
as S-MATE
straight O
walls O
, O
due O
to O
the O
geometrical O
limitation O
of O
the O
rolling B-MANP
process E-MANP
. O


For O
more O
complex O
components S-MACEQ
with O
curves O
and O
corners O
, O
special O
flexible B-CONPRI
tooling E-CONPRI
need O
to O
be S-MATE
developed O
to O
achieve O
an O
effective O
rolling B-MANP
process E-MANP
, O
thus O
limiting O
the O
range S-PARA
of O
industrial S-APPL
application O
. O


Cold B-MANP
rolling E-MANP
technique O
will O
also O
reduce O
residual B-PRO
stress E-PRO
, O
but O
the O
ability O
to O
reduce O
overall O
part O
distortion S-CONPRI
is O
yet O
to O
be S-MATE
proven O
. O


5.3 O
Interpass S-PARA
cooling S-MANP
Recently O
, O
interpass S-PARA
cooling S-MANP
has O
been O
developed O
and O
evaluated O
at O
the O
University O
of O
Wollongong O
. O


8 O
presents O
the O
schematic O
diagram O
of O
a O
WAAM B-MACEQ
system E-MACEQ
with O
interpass S-PARA
cooling S-MANP
. O


The O
moveable O
gas S-CONPRI
nozzle O
, O
which O
supplies O
argon S-MATE
, O
nitrogen S-MATE
or O
CO2 S-MATE
gas O
, O
is O
used O
to O
provide O
active O
, O
or O
forced O
, O
cooling S-MANP
on O
the O
fabricated S-CONPRI
part O
during O
and/or O
after O
deposition S-CONPRI
of O
each O
layer S-PARA
. O


Using O
such O
rapid O
cooling S-MANP
, O
the O
in-situ S-CONPRI
layer O
temperature S-PARA
and O
heat B-PRO
cycle E-PRO
can O
be S-MATE
controlled O
within O
a O
range S-PARA
to O
obtain O
the O
desired O
microstructure S-CONPRI
and O
mechanical B-CONPRI
properties E-CONPRI
. O


This O
process S-CONPRI
may O
also O
potentially O
reduce O
residual B-PRO
stress E-PRO
and O
distortion S-CONPRI
, O
although O
this O
aspect O
has O
not O
been O
investigated O
. O


An O
initial O
feasibility S-CONPRI
study O
shows O
promising O
results O
when O
using O
forced O
interpass S-PARA
cooling S-MANP
with O
compressed O
CO2 S-MATE
to O
fabricate S-MANP
Ti6Al4V O
thin-walled O
structures O
, O
as S-MATE
shown O
in O
9 O
. O


It O
was O
found O
that O
interpass S-PARA
cooling S-MANP
produces O
less O
surface B-MANP
oxidation E-MANP
, O
refined O
microstructure S-CONPRI
, O
improved O
hardness S-PRO
and O
enhanced O
strength S-PRO
. O


In O
addition O
, O
manufacturing S-MANP
efficiency O
is O
significantly O
improved O
due O
to O
the O
reduction S-CONPRI
of O
dwell B-PARA
time E-PARA
between O
deposited B-CHAR
layers E-CHAR
. O


More O
detailed O
research S-CONPRI
findings O
will O
be S-MATE
presented O
in O
future O
. O


5.4 O
Peening S-MANP
and O
ultrasonic B-CHAR
impact I-CHAR
treatment E-CHAR
Peening S-MANP
and O
ultrasonic B-CHAR
impact I-CHAR
treatments E-CHAR
have O
been O
used O
in O
welding S-MANP
applications O
to O
reduce O
local B-CONPRI
residual I-CONPRI
stress E-CONPRI
and O
improve O
weld S-FEAT
mechanical B-CONPRI
properties E-CONPRI
. O


Both O
techniques O
are O
cold B-MANP
mechanical I-MANP
treatments E-MANP
that O
impact S-CONPRI
the O
weld B-CHAR
surface E-CHAR
using O
high O
energy O
media O
to O
release O
tensile B-PRO
stress E-PRO
by O
imposing O
compressive B-PRO
stress E-PRO
at O
the O
treatment O
surface S-CONPRI
. O


Usually O
, O
the O
mechanical B-CONPRI
peening E-CONPRI
process O
produces O
compressive B-PRO
stresses E-PRO
at O
a O
limited O
depth O
below O
the O
component S-MACEQ
surface O
, O
such O
as S-MATE
around O
1mm O
in O
carbon B-MATE
steels E-MATE
. O


Ultrasonic B-CHAR
impact I-CHAR
treatment E-CHAR
produces O
grain B-CHAR
refinement E-CHAR
and O
randomizes O
orientation S-CONPRI
, O
thus O
contributing O
to O
mechanical B-PRO
strength E-PRO
improvement O
. O


It O
is O
reported O
that O
after O
ultrasonic B-CHAR
impact I-CHAR
treatment E-CHAR
, O
the O
surface S-CONPRI
residual B-PRO
stress E-PRO
of O
WAAM-fabricated O
Ti6Al4V S-MATE
part O
can O
be S-MATE
reduced O
to O
58 O
% O
and O
the O
microhardness S-CONPRI
can O
be S-MATE
increased O
by O
28 O
% O
compared O
to O
the O
as-fabricated O
sample S-CONPRI
. O


Also O
, O
the O
surface-modified O
layers O
undergo O
plastic B-PRO
deformation E-PRO
with O
significant O
grain B-CHAR
refinement E-CHAR
and O
dense B-FEAT
dislocations E-FEAT
. O


The O
ultrasonic B-CHAR
impact I-CHAR
treatment E-CHAR
is O
limited O
by O
penetration B-PARA
depth E-PARA
, O
which O
is O
up O
to O
60 O
below O
surface S-CONPRI
. O


Therefore O
, O
although O
both O
techniques O
are O
good O
post-process S-CONPRI
treatments O
, O
they O
have O
negligible O
effect O
on O
internal B-PRO
residual I-PRO
stresses E-PRO
of O
large O
metal S-MATE
part O
fabricated S-CONPRI
using O
WAAM S-MANP
. O


6 O
Discussion O
Improving O
process S-CONPRI
stability O
, O
eliminating O
or O
decreasing O
deposition B-PRO
defects E-PRO
and O
producing O
components S-MACEQ
with O
high O
quality S-CONPRI
and O
mechanical S-APPL
performance O
have O
become O
major O
research S-CONPRI
focuses O
in O
making O
the O
WAAM S-MANP
process S-CONPRI
more O
competitive O
against O
other O
additive B-MANP
manufacturing E-MANP
methods O
. O


An O
in-depth O
understanding O
of O
various B-MATE
materials E-MATE
, O
ideal O
process S-CONPRI
setup O
, O
in-process O
parameter S-CONPRI
control O
and O
post B-CONPRI
processing E-CONPRI
is O
essential O
for O
achieving O
such O
a O
goal O
. O


After O
a O
systematic O
review O
and O
analysis O
, O
a O
quality-based O
framework S-CONPRI
aiming O
to O
achieve O
high-quality O
and O
defects-free O
WAAM S-MANP
process S-CONPRI
is O
proposed O
, O
as S-MATE
shown O
in O
10 O
. O


Three O
main O
aspects O
are O
considered O
: O
feedstock B-MACEQ
optimization E-MACEQ
, O
manufacturing B-MANP
process E-MANP
, O
and O
post-process S-CONPRI
treatment O
. O


Selection O
of O
the O
most O
suitable O
welding B-MANP
WAAM E-MANP
process S-CONPRI
for O
the O
deposition B-MATE
material E-MATE
can O
ensure O
manufacturing B-MANP
process E-MANP
stability O
and O
contribute O
to O
reduction S-CONPRI
of O
defects S-CONPRI
. O


For O
example O
, O
if O
the O
CMT-PADV B-MANP
process E-MANP
is O
used O
for O
producing O
aluminum S-MATE
parts O
, O
porosity S-PRO
defects S-CONPRI
can O
be S-MATE
dramatically O
reduced O
when O
compared O
to O
other O
GMAW S-MANP
modes O
. O


Moreover O
, O
integrated O
and O
reliable O
process B-CONPRI
monitoring E-CONPRI
and O
control B-MACEQ
systems E-MACEQ
are O
needed O
to O
maintain O
the O
stability S-PRO
of O
the O
process S-CONPRI
and O
ensure O
the O
quality S-CONPRI
of O
production S-MANP
. O


Usually O
, O
the O
bead B-CHAR
geometry E-CHAR
, O
interpass B-PARA
temperature E-PARA
, O
arc S-CONPRI
characteristics O
and O
metal S-MATE
transfer O
behaviour O
are O
included O
in O
process B-CONPRI
monitoring E-CONPRI
and O
control O
. O


Controlling O
the O
interpass B-PARA
temperature E-PARA
within O
a O
reasonable O
range S-PARA
is O
beneficial O
to O
microstructural B-CONPRI
evolution E-CONPRI
and O
the O
resulting O
mechanical B-CONPRI
properties E-CONPRI
. O


Further O
, O
regulating O
the O
arc S-CONPRI
characteristics O
and O
metal S-MATE
transfer O
behaviour O
in O
real O
time O
is O
helpful O
to O
process S-CONPRI
stability O
and O
avoidance O
of O
defects S-CONPRI
. O


Based O
on O
the O
process B-CONPRI
monitoring E-CONPRI
data S-CONPRI
that O
has O
been O
collected O
during O
deposition S-CONPRI
, O
one O
of O
several O
post-process S-CONPRI
treatments O
can O
be S-MATE
selected O
to O
mitigate O
defects S-CONPRI
and O
improve O
mechanical S-APPL
performance O
. O


Considering O
the O
material S-MATE
characteristics O
, O
microstructural B-CONPRI
evolution E-CONPRI
and O
mechanical B-CONPRI
properties E-CONPRI
can O
also O
be S-MATE
optimized O
through O
new O
feedstock S-MATE
composition S-CONPRI
design O
. O


It O
is O
well O
known O
that O
different O
alloying B-MATE
elements E-MATE
have O
specific B-PRO
effects E-PRO
on O
material B-CONPRI
properties E-CONPRI
. O


By O
referring O
to O
the O
phase B-CONPRI
diagram E-CONPRI
, O
the O
desired O
deposition S-CONPRI
microstructures O
can O
be S-MATE
obtained O
via O
adding O
specific O
alloying B-MATE
elements E-MATE
in O
the O
feedstock S-MATE
and O
subsequent O
mechanical B-CONPRI
properties E-CONPRI
improved O
. O


For O
example O
, O
twin-wire O
GTAW-based O
WAAM S-MANP
has O
been O
successfully O
developed O
to O
produce O
intermetallic S-MATE
graded O
materials S-CONPRI
. O


The O
development O
of O
new O
powder S-MATE
cored O
wires O
, O
will O
offer O
exciting O
opportunities O
for O
fabricating S-MANP
target O
components S-MACEQ
with O
accurate S-CHAR
metal O
composition S-CONPRI
. O


In O
summary O
, O
using O
new O
welding B-MACEQ
consumables E-MACEQ
brings O
a O
cost O
effective O
solution S-CONPRI
, O
which O
supports S-APPL
high O
deposition B-CHAR
quality E-CHAR
through O
obtaining O
the O
desired O
microstructures S-MATE
, O
lowering O
manufacturing B-CONPRI
costs E-CONPRI
by O
reducing O
or O
eliminating O
pre-weld B-MANP
cleaning E-MANP
and O
re-work O
, O
and O
providing O
safer O
working O
environments O
by O
reducing O
weld B-CHAR
fumes E-CHAR
. O


Another O
essential O
part O
of O
WAAM S-MANP
processing O
for O
most O
materials S-CONPRI
is O
post-process S-CONPRI
treatment O
, O
which O
is O
used O
to O
reduce O
residual B-PRO
stresses E-PRO
and O
distortion S-CONPRI
, O
refine O
microstructures S-MATE
, O
improve O
microhardness S-CONPRI
and O
enhance O
material B-PRO
strength E-PRO
. O


However O
, O
post-process S-CONPRI
technologies O
have O
their O
own O
limitations O
, O
for O
instance O
, O
peening S-MANP
and O
ultrasonic B-CHAR
impact I-CHAR
treatment E-CHAR
only O
improve O
material B-CONPRI
property E-CONPRI
and O
reduce O
defects S-CONPRI
near O
the O
part O
surface S-CONPRI
, O
while O
extended O
heat B-MANP
treatment E-MANP
of O
certain O
materials S-CONPRI
promotes O
grain B-CONPRI
growth E-CONPRI
rather O
than O
grain B-CHAR
refinement E-CHAR
. O


Currently O
, O
most O
WAAM S-MANP
fabricated S-CONPRI
parts O
need O
to O
be S-MATE
post-process O
treated O
with O
a O
selective O
combination O
of O
technologies S-CONPRI
to O
reduce O
the O
defects S-CONPRI
and O
improve O
the O
product B-CONPRI
quality E-CONPRI
to O
the O
greatest O
extent O
possible O
. O


7 O
Conclusions O
A O
detailed O
review O
of O
recent O
technological O
developments O
in O
WAAM S-MANP
process S-CONPRI
has O
been O
presented O
, O
with O
a O
focus O
on O
microstructure S-CONPRI
, O
mechanical B-CONPRI
properties E-CONPRI
, O
process B-CONPRI
defects E-CONPRI
and O
post-process S-CONPRI
treatment O
. O


Through O
matching O
a O
knowledge O
of O
material S-MATE
characteristics O
with O
the O
performance S-CONPRI
features O
of O
particular O
WAAM S-MANP
techniques O
, O
a O
quality-based O
framework S-CONPRI
is O
proposed O
, O
for O
producing O
high-quality O
and O
defect-free O
components S-MACEQ
. O


In O
WAAM S-MANP
of O
metallic B-MATE
materials E-MATE
, O
the O
fundamental O
interrelationships O
between O
material S-MATE
composition S-CONPRI
and O
microstructure S-CONPRI
govern O
the O
material B-CONPRI
properties E-CONPRI
and O
fabrication S-MANP
quality O
. O


Since O
the O
WAAM S-MANP
process S-CONPRI
is O
an O
inherently O
non-equilibrium O
thermal O
process S-CONPRI
, O
it O
is O
challenging O
to O
predicate O
and O
control O
the O
microstructural B-CONPRI
evolution E-CONPRI
, O
which O
is O
responsible O
for O
the O
variation S-CONPRI
of O
mechanical B-CONPRI
properties E-CONPRI
in O
the O
deposited O
part O
. O


Further O
research S-CONPRI
attention O
should O
be S-MATE
paid O
on O
the O
study O
of O
underlying O
physical O
and O
chemical O
metallurgical S-APPL
mechanisms O
in O
WAAM S-MANP
process S-CONPRI
to O
provide O
a O
guidance O
for O
the O
process B-CONPRI
optimization E-CONPRI
, O
improvement O
and O
control O
. O


The O
defects S-CONPRI
generated O
in O
WAAM-produced O
part O
are O
closely O
related O
to O
the O
target O
material S-MATE
characteristics O
and O
process B-CONPRI
parameters E-CONPRI
. O


The O
development O
of O
strategies O
or O
ancillary B-CONPRI
process E-CONPRI
to O
overcome O
defects S-CONPRI
generation O
are O
of O
prime O
importance O
. O


As S-MATE
WAAM O
matures O
as S-MATE
a O
commercial O
manufacturing B-MANP
process E-MANP
, O
development O
of O
a O
commercially O
available O
WAAM B-MACEQ
system E-MACEQ
for O
metal S-MATE
components S-MACEQ
is O
an O
interdisciplinary O
challenge O
, O
which O
integrates O
physical O
welding S-MANP
process S-CONPRI
development O
, O
materials S-CONPRI
science O
and O
thermo-mechanical S-CONPRI
engineering S-APPL
, O
and O
mechatronic O
and O
control B-MACEQ
system E-MACEQ
design O
This O
study O
presents O
a O
review O
on O
powder B-MANP
bed I-MANP
laser I-MANP
additive I-MANP
manufacturing E-MANP
of O
stainless B-MATE
steel E-MATE
. O


The O
powder B-MANP
bed I-MANP
laser I-MANP
additive I-MANP
manufacturing E-MANP
processes O
that O
are O
presented O
in O
this O
paper O
are O
the O
selective B-MANP
laser I-MANP
sintering E-MANP
and O
selective B-MANP
laser I-MANP
melting E-MANP
. O


The O
powder B-MANP
bed I-MANP
laser I-MANP
additive I-MANP
manufacturing E-MANP
process O
of O
stainless B-MATE
steel E-MATE
are O
reviewed O
in O
this O
paper O
. O


The O
process B-CONPRI
parameters E-CONPRI
was O
found O
to O
plat O
an O
important O
role O
in O
the O
evolving O
properties S-CONPRI
of O
the O
powder B-MACEQ
bed E-MACEQ
based O
laser B-MANP
additive I-MANP
manufacturing E-MANP
process O
. O


Selection O
and/or O
Peer-review O
under O
responsibility O
of O
Materials B-CHAR
Processing E-CHAR
and O
characterization O
. O


Steels S-MATE
are O
important O
engineering B-MATE
material E-MATE
invented O
by O
mankind O
because O
of O
their O
extreme O
multiplicity O
in O
operties O
. O


Stainless B-MATE
steel E-MATE
originated O
from O
steel S-MATE
as S-MATE
a O
result O
of O
the O
addition O
of O
chromium S-MATE
. O


The O
percentage O
composition S-CONPRI
the O
chromium S-MATE
is O
sufficient O
enough O
to O
prevent O
rusting O
in O
corrosive S-PRO
environment O
. O


Different O
grades O
of O
stainless O
stee O
ludes O
, O
Martensitic B-MATE
stainless I-MATE
steel E-MATE
, O
Ferritic B-MATE
stainless I-MATE
steel E-MATE
, O
Austenitic B-MATE
stainless I-MATE
steel E-MATE
, O
superferritic B-MATE
stainless I-MATE
steel E-MATE
plex O
stainless B-MATE
steel E-MATE
, O
precipitation B-MANP
hardening E-MANP
stainless O
steel S-MATE
and O
super O
austenitic B-MATE
stainless I-MATE
steel E-MATE
. O


ection O
and/or O
Peer-review O
under O
responsibility O
of O
Materials B-CHAR
Processing E-CHAR
and O
characterization O
. O


Asides O
from O
its O
high O
corrosion B-CONPRI
resistance E-CONPRI
property O
, O
stainless B-MATE
steel E-MATE
are O
malleable O
enough O
to O
be S-MATE
bent O
, O
folded O
, O
welded S-MANP
, O
machined S-MANP
and O
deep O
drawn O
, O
they O
also O
have O
a O
high O
heat B-PRO
conductivity E-PRO
and O
high O
strength S-PRO
. O


Stainless B-MATE
steels E-MATE
find O
extensive O
applications O
that O
include O
: O
chemical O
equipments O
, O
food O
processing O
equipments O
, O
cryogenic B-PRO
vessels E-PRO
, O
X-ray B-MACEQ
tube E-MACEQ
bases O
, O
heat B-MACEQ
exchangers E-MACEQ
, O
cutleries O
, O
jet-engine O
parts O
, O
automotive S-APPL
fasteners O
, O
valves O
, O
brewing B-MACEQ
equipments E-MACEQ
, O
and O
aircraft O
fittings O
. O


Methods O
of O
fabrication S-MANP
of O
stainless B-MATE
steel E-MATE
include O
hot B-MANP
forming I-MANP
processes E-MANP
and O
cold B-MANP
forming I-MANP
processes E-MANP
. O


Complex O
parts O
are O
broken O
down O
into O
smaller O
parts O
when O
these O
conventional B-MANP
manufacturing E-MANP
processes O
are O
used O
. O


These O
does O
not O
only O
make O
the O
process S-CONPRI
to O
be S-MATE
cumbersome O
but O
also O
heavier O
because O
of O
extra O
materials S-CONPRI
that O
are O
used O
in O
joining S-MANP
the O
several O
parts O
together O
. O


Additive B-MANP
manufacturing I-MANP
processes E-MANP
is O
an O
advanced O
manufacturing B-MANP
process E-MANP
that O
can O
produce O
complex O
parts O
no O
matter O
the O
complexity S-CONPRI
as S-MATE
a O
single O
unit O
part O
. O


Additive B-MANP
manufacturing E-MANP
is O
a O
modern O
method O
of O
fabrication S-MANP
process O
which O
is O
used O
in O
producing O
a O
functional O
engineering S-APPL
metallic O
components S-MACEQ
one O
layer S-PARA
at O
a O
time O
from O
computer B-ENAT
aided I-ENAT
design E-ENAT
model O
data S-CONPRI
. O


There O
are O
various O
types O
of O
additive B-MANP
manufacturing E-MANP
technology O
, O
which O
include O
: O
vat B-MANP
photopolymerization E-MANP
, O
fused B-CONPRI
deposition E-CONPRI
modelling O
, O
selective B-MANP
laser E-MANP
sintering/melting O
, O
laminated B-MANP
object I-MANP
manufacturing E-MANP
and O
Laser B-MANP
metal I-MANP
deposition E-MANP
that O
is O
also O
referred O
to O
as S-MATE
. O


Vat B-MANP
photopolymerization E-MANP
is O
the O
first O
commercial O
additive B-MANP
manufacturing E-MANP
method O
that O
is O
used O
to O
create O
a O
layer S-PARA
of O
solidified O
material S-MATE
using O
ultraviolet B-CONPRI
radiation E-CONPRI
to O
selectively O
polymerize O
a O
curable B-MATE
resin E-MATE
until O
a O
complete O
part O
is O
formed O
. O


Its O
advantages O
includes O
high O
building O
speed O
and O
flexibility S-PRO
. O


Major O
disadvantages O
are O
high O
cost O
of O
materials S-CONPRI
and O
process B-CONPRI
errors E-CONPRI
due O
to O
over O
curing S-MANP
. O


Fused B-CONPRI
deposition E-CONPRI
modelling O
additive B-MANP
manufacturing E-MANP
is O
also O
referred O
to O
as S-MATE
material O
extrusion B-MANP
process E-MANP
, O
this O
process S-CONPRI
is O
used O
for O
fabricating S-MANP
3D B-APPL
parts E-APPL
by O
deposition S-CONPRI
of O
laser S-ENAT
heated O
thermoplastic B-MATE
filaments E-MATE
in O
a O
layer S-PARA
wise O
manner O
. O


With O
this O
method O
, O
complex O
durable O
parts O
can O
be S-MATE
easily O
manufactured S-CONPRI
with O
high O
accuracy S-CHAR
. O


Draw-backs O
of O
this O
additive B-MANP
manufacturing E-MANP
technology O
include O
poor O
surface B-FEAT
finish E-FEAT
, O
time O
consumption O
and O
high O
porosity B-PRO
of I-PRO
manufactured I-PRO
parts E-PRO
. O


This O
method O
has O
been O
applied O
in O
automotive S-APPL
, O
aerospace S-APPL
, O
medical S-APPL
and O
plastic B-APPL
industries E-APPL
. O


Selective B-MANP
laser E-MANP
sintering/malting O
is O
a O
powder B-MANP
bed I-MANP
additive I-MANP
manufacturing E-MANP
method O
that O
involves O
atomic O
fusion/melting O
of O
metallic B-MATE
powder E-MATE
deposited O
in O
form O
of O
layers O
. O


Parts O
can O
be S-MATE
easily O
processed S-CONPRI
within O
a O
short O
time O
frame O
, O
it O
is O
flexible O
and O
accurate S-CHAR
. O


This O
additive B-MANP
manufacturing E-MANP
method O
has O
been O
mostly O
used O
with O
metals S-MATE
such O
as S-MATE
stainless O
steel S-MATE
316L O
austenitic S-MATE
grade O
, O
precipitated B-PRO
hardened E-PRO
stainless O
steel S-MATE
. O


Selective B-MANP
laser I-MANP
sintering E-MANP
has O
an O
extensive O
application O
in O
the O
field O
of O
aerospace S-APPL
, O
medical S-APPL
and O
automotive S-APPL
engineering O
due O
to O
the O
ability O
to O
control O
the O
stiffness S-PRO
of O
components S-MACEQ
in O
a O
desired O
model S-CONPRI
. O


Selective B-MANP
laser I-MANP
melting E-MANP
is O
a O
powder S-MATE
based O
bed B-MANP
fusion E-MANP
that O
is O
used O
to O
produce O
metallic S-MATE
components S-MACEQ
by O
deposition S-CONPRI
of O
a O
thin B-MATE
metallic I-MATE
powder E-MATE
on O
a O
substrate S-MATE
and O
using O
a O
high O
intensity B-MACEQ
laser I-MACEQ
beam E-MACEQ
to O
melt S-CONPRI
and O
fuse S-MANP
selective O
region O
of O
metallic B-MATE
powder E-MATE
according O
to O
the O
computer B-ENAT
aided I-ENAT
design E-ENAT
data O
in O
a O
layer-wise O
fashion S-CONPRI
. O


This O
method O
has O
an O
advantage O
of O
less O
porosity S-PRO
of O
built O
parts O
with O
better O
mechanical B-CONPRI
property E-CONPRI
, O
manufacturability S-CONPRI
of O
complex B-PRO
shapes E-PRO
and O
excellent O
scanning B-CHAR
efficiency E-CHAR
. O


The O
disadvantage O
is O
similar O
to O
that O
of O
the O
process S-CONPRI
discuss O
earlier O
in O
terms O
of O
process B-CONPRI
control E-CONPRI
challenge O
due O
to O
too O
many O
parameters S-CONPRI
and O
also O
there O
is O
a O
material S-MATE
wastage O
. O


Selective B-MANP
laser I-MANP
melting E-MANP
has O
been O
extensively O
used O
with O
the O
employment O
of O
stainless B-MATE
steel E-MATE
. O


Selective B-MANP
laser I-MANP
melting E-MANP
has O
an O
extensive O
application O
in O
the O
field O
of O
aerospace S-APPL
, O
medical S-APPL
, O
automotive S-APPL
engineering O
and O
medical S-APPL
health O
care O
sectors O
. O


Benefits O
of O
this O
methods O
include O
good O
efficiency O
of O
material S-MATE
usage O
, O
parts O
with O
complicated O
shapes O
can O
be S-MATE
built O
, O
high O
strength S-PRO
material S-MATE
can O
be S-MATE
achieved O
, O
materials S-CONPRI
can O
be S-MATE
customized O
, O
it O
takes O
less O
time O
and O
it O
eliminated O
oxide B-PRO
impurities E-PRO
due O
to O
vacuum O
environment O
. O


Disadvantages O
of O
this O
methods O
are O
high O
price O
of O
set-up O
due O
to O
integration O
of O
vacuum O
with O
the O
machine S-MACEQ
for O
good O
thermal O
and O
impurity S-PRO
free O
environment O
, O
X-rays S-CONPRI
are O
formed O
during O
the O
process S-CONPRI
which O
is O
detrimental O
to O
human O
health O
. O


Laminated B-MANP
object I-MANP
additive I-MANP
manufacturing E-MANP
is O
an O
additive-subtractive O
rapid B-ENAT
prototyping E-ENAT
manufacturing B-MANP
process E-MANP
where O
3D B-APPL
objects E-APPL
are O
manufactured S-CONPRI
by O
metal S-MATE
sheet O
material S-MATE
that O
are O
bonded O
together O
by O
thermally B-MANP
activated E-MANP
adhesive S-MATE
coating O
layer B-CONPRI
by I-CONPRI
layer E-CONPRI
. O


Each O
layer S-PARA
is O
formed O
from O
a O
sheet S-MATE
of O
paper O
coated S-APPL
with O
a O
thermoplastic B-MATE
adhesive E-MATE
and O
sheet S-MATE
is O
bonded O
together O
by O
using O
a O
heated O
stainless B-MATE
steel E-MATE
roller S-MACEQ
after O
which O
a O
CO2 S-MATE
laser O
cuts O
cross-section O
into O
a O
layer S-PARA
of O
paper O
according O
to O
the O
information O
from O
the O
CAD B-ENAT
model E-ENAT
repeatedly O
until O
the O
required O
object O
is O
formed O
and O
lamination O
is O
actualized O
. O


The O
process S-CONPRI
is O
simple S-MANP
and O
faster O
since O
the O
laser S-ENAT
doesnhave O
to O
scan O
the O
entire O
area S-PARA
of O
the O
cross B-CONPRI
section E-CONPRI
. O


Merits O
of O
this O
method O
includes O
that O
fact O
that O
large O
size O
parts O
can O
be S-MATE
built O
, O
it O
is O
cheap O
, O
no O
microstructural S-CONPRI
alteration O
during O
the O
process S-CONPRI
, O
and O
it O
is O
flexible O
in O
the O
sense O
that O
component S-MACEQ
does O
not O
need O
support B-FEAT
structures E-FEAT
. O


Setbacks O
of O
this O
methods O
includes O
wastage O
of O
material S-MATE
during O
the O
subtractive B-MANP
process E-MANP
, O
complex O
internal O
cavities O
and O
hollow O
parts O
are O
difficult O
to O
build S-PARA
, O
and O
it O
has O
poor O
surface B-MANP
finishing E-MANP
. O


Laminated B-MANP
object I-MANP
manufacturing E-MANP
has O
been O
demonstrated O
in O
aerospace S-APPL
and O
tool B-APPL
design I-APPL
industries E-APPL
. O


Laser B-MANP
Metal I-MANP
Deposition E-MANP
, O
an O
additive B-MANP
manufacturing I-MANP
process E-MANP
, O
is O
used O
in O
building O
parts O
by O
melting S-MANP
a O
metal B-MATE
powder E-MATE
that O
is O
injected O
into O
a O
specific O
location O
by O
mean O
of O
a O
high B-PRO
power I-PRO
laser I-PRO
beam E-PRO
. O


The O
process S-CONPRI
of O
solidification S-CONPRI
and O
cooling S-MANP
occur O
in O
a O
closed B-MACEQ
chamber E-MACEQ
in O
an O
argon B-PARA
atmosphere E-PARA
so O
as S-MATE
to O
prevent O
oxidation S-MANP
of O
the O
melt B-MATE
pool E-MATE
. O


This O
process S-CONPRI
permits O
the O
use O
of O
high O
variety O
of O
metals S-MATE
and O
composites S-MATE
such O
as S-MATE
stainless O
steel S-MATE
mostly O
austenitic S-MATE
grades O
, O
which O
are O
mainly O
316 O
, O
SS S-MATE
316L O
, O
SS S-MATE
304L O
and O
other O
exotic O
metals S-MATE
such O
as S-MATE
titanium O
and O
its O
alloys S-MATE
, O
composites S-MATE
and O
functionally B-MATE
graded I-MATE
materials E-MATE
. O


A O
major O
benefit O
of O
laser B-MANP
additive I-MANP
manufacturing E-MANP
technology O
is O
that O
, O
it O
provide O
new O
chances O
for O
customization O
of O
metallic S-MATE
components S-MACEQ
in O
terms O
of O
material S-MATE
composition S-CONPRI
manipulation O
and O
properties S-CONPRI
, O
improvements O
in O
product O
performance S-CONPRI
, O
and O
lower O
overall O
manufacturing B-CONPRI
costs E-CONPRI
due O
to O
its O
unique O
capabilities O
. O


Lots O
of O
research S-CONPRI
and O
discoveries O
has O
been O
achieved O
with O
laser B-MANP
metal I-MANP
deposition E-MANP
as S-MATE
an O
additive B-MANP
manufacturing E-MANP
method O
with O
the O
employment O
of O
stainless B-MATE
steel E-MATE
and O
stainless B-MATE
steel E-MATE
composite S-MATE
with O
much O
effort O
in O
enhancing O
wear B-PRO
resistance E-PRO
and O
strength B-PRO
property E-PRO
. O


This O
paper O
presents O
an O
overview O
of O
selective B-MANP
laser I-MANP
sintering E-MANP
and O
selective B-MANP
laser I-MANP
melting I-MANP
process E-MANP
of O
stainless B-MATE
steel E-MATE
. O


Some O
recent O
research S-CONPRI
works O
on O
stainless B-MATE
steel E-MATE
using O
these O
powder B-MANP
bed I-MANP
additive I-MANP
manufacturing E-MANP
techniques O
are O
presented O
and O
future O
research S-CONPRI
need O
are O
also O
proposed O
. O


Selective B-MANP
Laser I-MANP
Sintering E-MANP
of O
Stainless B-MATE
Steel E-MATE
This O
additive B-MANP
manufacturing I-MANP
process E-MANP
is O
a O
powder S-MATE
based O
layer-additive O
manufacturing B-MANP
process E-MANP
where O
metallic S-MATE
components S-MACEQ
are O
built O
section O
by O
section O
. O


A O
moderately O
low O
laser B-PARA
power E-PARA
is O
used O
in O
this O
process S-CONPRI
as S-MATE
metallic O
material S-MATE
never O
reach O
a O
liquid B-PRO
phase E-PRO
during O
the O
heating S-MANP
process O
. O


The O
process S-CONPRI
occurs O
at O
a O
faster O
rate O
at O
high O
temperature S-PARA
which O
is O
why O
it O
involves O
heating S-MANP
a O
powder S-MATE
. O


The O
process S-CONPRI
is O
achieved O
when O
the O
laser B-ENAT
scan E-ENAT
powder O
material S-MATE
deposited O
on O
the O
substrate S-MATE
on O
the O
2D B-FEAT
cross-section E-FEAT
of O
the O
part O
created O
in O
3D B-FEAT
geometrical I-FEAT
shape E-FEAT
. O


The O
process S-CONPRI
is O
repeated O
in O
which O
after O
the O
first O
laser B-ENAT
scan E-ENAT
, O
the O
powder B-MACEQ
bed E-MACEQ
is O
lowered O
by O
the O
amount O
of O
thickness O
of O
the O
layer S-PARA
produced O
initially O
, O
and O
then O
a O
new O
layer S-PARA
of O
powder B-MATE
material E-MATE
is O
spread S-CONPRI
on O
the O
powder B-MACEQ
bed E-MACEQ
again O
on O
top O
of O
the O
initial O
scanned O
layer S-PARA
until O
a O
fully B-PARA
dense E-PARA
built O
part O
is O
produced O
. O


Selective B-MANP
laser I-MANP
sintering I-MANP
process E-MANP
has O
the O
potential O
to O
become O
one O
of O
the O
most O
valuable O
additive B-MANP
manufacturing E-MANP
techniques O
, O
because O
it O
has O
potential O
to O
easily O
produced O
complex B-PRO
shapes E-PRO
. O


The O
Figure O
1 O
shows O
the O
schematic O
diagram O
of O
the O
SLS B-MANP
process E-MANP
. O


Schematic O
diagram O
of O
selective B-MANP
laser I-MANP
sintering I-MANP
process E-MANP
. O


Few O
research S-CONPRI
investigations O
and O
studies O
have O
been O
done O
in O
the O
application O
of O
selective B-MANP
laser I-MANP
sintering E-MANP
with O
stainless B-MATE
steels E-MATE
. O


Stainless B-MATE
steel E-MATE
grades O
that O
has O
been O
involved O
in O
this O
process S-CONPRI
are O
the O
austenitic S-MATE
type O
mainly O
SS S-MATE
316L O
and O
the O
precipitated B-PRO
hardened E-PRO
stainless O
steel S-MATE
grade O
PH S-CONPRI
. O


Ibrahim O
studied O
the O
fabrication S-MANP
of O
a O
novel O
porous B-FEAT
electrode I-FEAT
scaffold E-FEAT
made O
from O
stainless B-MATE
steel E-MATE
316L O
powder S-MATE
using O
selective B-MANP
laser I-MANP
sintering E-MANP
by O
careful O
selection O
of O
process B-CONPRI
parameters E-CONPRI
and O
also O
how O
the O
property S-CONPRI
such O
as S-MATE
porosity O
, O
electrical B-PRO
conductivity E-PRO
and O
optical B-CHAR
microscopy E-CHAR
measurements O
were O
used O
to O
investigate O
the O
properties S-CONPRI
of O
the O
fabricated S-CONPRI
sample O
. O


In O
this O
investigation O
stainless B-MATE
steel E-MATE
, O
SS316L O
with O
particle S-CONPRI
size O
of O
25 O
to O
50 O
micro-meter O
were O
built O
with O
30 O
W O
laser B-PARA
power E-PARA
and O
1500 O
mm/s O
scanning B-PARA
speed E-PARA
. O


Density B-FEAT
and I-FEAT
porosity E-FEAT
properties O
were O
investigated O
and O
it O
was O
discovered O
that O
high O
porosity S-PRO
metal S-MATE
parts O
can O
be S-MATE
produced O
by O
using O
a O
low O
laser B-PARA
power E-PARA
and O
high O
scan B-PARA
speed E-PARA
. O


This O
study O
also O
revealed O
the O
feasibility S-CONPRI
of O
producing O
porous B-MATE
metal E-MATE
sintered O
parts O
for O
electrochemical B-MACEQ
devices E-MACEQ
using O
the O
right O
processing O
parameters S-CONPRI
. O


Xie O
studied O
the O
mechanical S-APPL
and O
structural O
characteristics O
of O
porous S-PRO
316L B-MATE
stainless I-MATE
steel E-MATE
fabricated O
by O
indirect O
laser B-MANP
sintering E-MANP
. O


In O
this O
investigation O
, O
a O
simple S-MANP
encapsulated S-CONPRI
method O
was O
developed O
to O
coat O
316L B-MATE
SS I-MATE
powder E-MATE
fabricated O
by O
indirect O
SLS B-MANP
process E-MANP
, O
with O
ethylene-vinyl B-PRO
acetate I-PRO
copolymer I-PRO
resin E-PRO
. O


In O
this O
experiment S-CONPRI
, O
a O
water B-MANP
atomized E-MANP
316L B-MATE
stainless I-MATE
steel I-MATE
powder E-MATE
with O
particle S-CONPRI
size O
of O
45 O
micro-meter O
and O
ethylene-vinyl B-PRO
acetate I-PRO
copolymer I-PRO
resin E-PRO
were O
used O
to O
encapsulate O
the O
stainless B-MATE
steel E-MATE
metallic S-MATE
particle O
together O
. O


The O
selective B-MANP
laser I-MANP
sintering E-MANP
method O
was O
performed O
in O
a O
pure B-MATE
argon E-MATE
environment O
on O
a O
WYS600 O
SLS S-MANP
equipment S-MACEQ
with O
powder B-MACEQ
bed E-MACEQ
temperature O
5 O
0 O
C S-MATE
below O
the O
melting B-PARA
temperature E-PARA
of O
ethylene-vinyl B-PRO
acetate I-PRO
copolymer I-PRO
resin E-PRO
. O


The O
processing O
parameters S-CONPRI
employed O
were O
scan O
spacing O
of O
0.10 O
, O
0.15 O
and O
0.20 O
mm S-MANP
, O
laser B-PARA
power E-PARA
ranges O
between O
10 O
and O
35 O
W O
at O
interval O
of O
5 O
W O
, O
scanning B-PARA
speed E-PARA
was O
set S-APPL
between O
1000 O
and O
2000 O
mm/s O
at O
difference O
of O
200 O
mm/s O
and O
layer B-PARA
thickness E-PARA
was O
set S-APPL
at O
0.15 O
mm S-MANP
. O


Before O
characterization O
of O
the O
EVA O
resin S-MATE
, O
post-processing S-CONPRI
was O
carried O
out O
in O
pure B-MATE
hydrogen E-MATE
contained O
furnace S-MACEQ
. O


The O
EVA O
resin S-MATE
was O
then O
characterized O
in O
terms O
of O
its O
thermal O
behaviour O
, O
density S-PRO
, O
porosity S-PRO
, O
average S-CONPRI
pore O
size O
. O


Mechanical B-CHAR
test E-CHAR
was O
performed O
on O
a O
CMT4305 O
electronic B-CHAR
testing E-CHAR
universal O
testing S-CHAR
machine S-MACEQ
with O
a O
purpose O
of O
investigating O
the O
yield B-PRO
strength E-PRO
and O
young B-PRO
modulus E-PRO
. O


It O
was O
discovered O
that O
the O
laser B-PARA
power E-PARA
and O
the O
sintering S-MANP
temperature O
are O
determining O
factor O
on O
the O
reduction S-CONPRI
in O
the O
porosity S-PRO
of O
the O
material S-MATE
. O


It O
was O
concluded O
that O
the O
characteristics O
of O
the O
sintered S-MANP
porous S-PRO
stainless O
steel S-MATE
316 O
L O
produced O
can O
be S-MATE
use O
as S-MATE
a O
substitute O
for O
bones O
in O
biomedical B-APPL
applications E-APPL
. O


Pal O
investigated O
the O
effect O
of O
post-processing S-CONPRI
and O
machining S-MANP
process O
parameters S-CONPRI
on O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
stainless B-MATE
steel E-MATE
product O
produced O
by O
direct O
laser B-MANP
sintering E-MANP
. O


A O
sample S-CONPRI
of O
stainless B-MATE
steel E-MATE
was O
fabricated S-CONPRI
in O
an O
argon B-PARA
atmosphere E-PARA
in O
a O
40 O
degree O
centigrade O
pre-heating O
machining B-MACEQ
chamber E-MACEQ
with O
a O
fibre B-MACEQ
laser I-MACEQ
system E-MACEQ
having O
beam B-PARA
diameter E-PARA
of O
0.1 O
mm S-MANP
. O


Processing O
parameters S-CONPRI
of O
tensile B-MACEQ
specimens E-MACEQ
were O
195 O
W O
laser B-PARA
power E-PARA
, O
scan B-PARA
speed E-PARA
of O
900 O
mm/s O
with O
40 O
micro-meter O
thickness O
layer S-PARA
. O


Tensile B-CHAR
test E-CHAR
was O
then O
performed O
with O
specimen O
dimensions S-FEAT
80 O
mm S-MANP
in O
total O
length O
, O
40 O
mm S-MANP
gauge O
length O
, O
5 O
mm B-PARA
gauge I-PARA
diameter E-PARA
, O
length O
of O
holding O
part O
was O
set S-APPL
at O
20 O
mm S-MANP
and O
diameter S-CONPRI
6 O
mm S-MANP
. O


It O
was O
discovered O
that O
the O
tensile B-PRO
strength E-PRO
of O
the O
DLMS O
part O
increased O
after O
heat B-MANP
treatment E-MANP
. O


Residual B-PRO
stresses E-PRO
remain O
in O
the O
het O
treated O
part O
with O
increased O
tensile B-PRO
strength E-PRO
due O
to O
rapid O
cooling S-MANP
without O
undergoing O
any O
post-processing S-CONPRI
stage O
. O


It O
was O
discovered O
after O
the O
analysis O
that O
the O
energy B-PARA
density E-PARA
will O
determine O
the O
mechanical B-CONPRI
property E-CONPRI
which O
implies O
that O
tensile B-PRO
strength E-PRO
of O
the O
stainless B-MATE
steel E-MATE
can O
be S-MATE
controlled O
by O
the O
combination O
of O
the O
machining S-MANP
parameters O
and O
energy B-PARA
density E-PARA
. O


Laser B-PARA
power E-PARA
and O
scanning B-PARA
speed E-PARA
will O
also O
determine O
the O
extent O
of O
surface B-PRO
roughness E-PRO
of O
the O
stainless B-MATE
steel E-MATE
. O


studied O
the O
deformation S-CONPRI
mechanism O
of O
17-4 O
precipitated B-PRO
hardened E-PRO
stainless O
steel S-MATE
fabricated S-CONPRI
by O
direct B-MANP
metal I-MANP
laser I-MANP
sintering E-MANP
using O
micro B-CHAR
pillar I-CHAR
compression I-CHAR
testing E-CHAR
and O
transmission B-CHAR
electron I-CHAR
microscopy E-CHAR
. O


17- O
4 O
stainless B-MATE
steel E-MATE
were O
first O
produced O
using O
direct B-MACEQ
metal I-MACEQ
laser I-MACEQ
sintering I-MACEQ
system E-MACEQ
in O
an O
argon B-PARA
atmosphere E-PARA
with O
spherical S-CONPRI
size O
of O
approximately O
15 O
45 O
micro-meter O
in O
diameter S-CONPRI
. O


Scanning B-PARA
speed E-PARA
was O
750 O
mm/s O
with O
scanning S-CONPRI
direction O
made O
67 O
degrees O
between O
successive O
building O
layers O
with O
hatch B-PARA
spacing E-PARA
was O
0.11 O
mm S-MANP
. O


micro O
compression S-PRO
properties O
of O
the O
17-4 O
precipitation B-MANP
hardened E-MANP
stainless O
steel S-MATE
was O
evaluated O
. O


Outcome O
revealed O
that O
the O
microstructure S-CONPRI
and O
properties S-CONPRI
of O
the O
17-4 B-MATE
stainless I-MATE
steel E-MATE
stainless O
steel S-MATE
specimens O
vary O
significantly O
from O
those O
produced O
by O
conventional B-MANP
manufacturing E-MANP
methods O
because O
of O
fine O
grain B-CONPRI
evolution E-CONPRI
that O
emerged O
. O


Selective B-MANP
Laser I-MANP
Melting E-MANP
of O
Stainless B-MATE
Steel E-MATE
Selective B-MANP
Laser I-MANP
Melting I-MANP
process E-MANP
is O
an O
additive B-MANP
manufacturing E-MANP
technology O
that O
can O
be S-MATE
used O
to O
produce O
solid O
metallic S-MATE
components S-MACEQ
from O
metallic B-MATE
powder E-MATE
by O
using O
a O
high O
intensity O
laser S-ENAT
to O
melt S-CONPRI
and O
fuse S-MANP
selective O
region O
of O
the O
metallic B-MATE
powder E-MATE
layer B-CONPRI
by I-CONPRI
layer E-CONPRI
according O
to O
the O
computer B-ENAT
aided I-ENAT
design E-ENAT
data O
. O


A O
new O
layer S-PARA
of O
metal B-MATE
powder E-MATE
is O
applied O
and O
the O
build B-MACEQ
platform E-MACEQ
is O
being O
lowered O
by O
the O
amount O
of O
thickness O
of O
one O
layer S-PARA
. O


The O
process S-CONPRI
involves O
building B-PARA
of I-PARA
component I-PARA
layer E-PARA
by O
layer S-PARA
by O
depositing O
a O
thin B-MATE
metallic I-MATE
powder E-MATE
on O
a O
substrate S-MATE
. O


A O
high O
intensity O
power S-PARA
laser S-ENAT
is O
then O
used O
to O
melt S-CONPRI
and O
fuse S-MANP
together O
a O
specific O
area S-PARA
of O
the O
metallic B-MATE
powder E-MATE
according O
to O
the O
data S-CONPRI
from O
the O
3D S-CONPRI
CAD O
. O


Once O
the O
laser S-ENAT
scanning O
is O
completed O
, O
succeeding O
layer S-PARA
of O
metallic B-MATE
powder E-MATE
is O
deposited O
on O
top O
and O
laser B-ENAT
scans E-ENAT
another O
new O
layer S-PARA
until O
the O
required O
component S-MACEQ
is O
completely O
built O
after O
repeated O
successive O
layer S-PARA
of O
metallic B-MATE
powder E-MATE
is O
deposited O
. O


Once O
the O
laser B-ENAT
scanning I-ENAT
processes E-ENAT
completed O
, O
loose O
powders S-MATE
are O
removed O
from O
the O
building B-PARA
chamber E-PARA
and O
the O
component S-MACEQ
can O
be S-MATE
separated O
from O
the O
substrate B-MACEQ
plate E-MACEQ
manually O
or O
by O
electrical B-MANP
discharge I-MANP
machining E-MANP
. O


Schematic O
diagram O
describing O
the O
SLM S-MANP
process S-CONPRI
is O
shown O
in O
Figure O
2 O
. O


Figure O
2 O
: O
Schematic O
diagram O
selective B-MANP
laser I-MANP
melting I-MANP
process E-MANP
Numerous O
research S-CONPRI
has O
been O
conducted O
with O
the O
application O
of O
selective B-MANP
laser I-MANP
melting E-MANP
of O
stainless B-MATE
steel E-MATE
in O
the O
literature O
. O


Jandin O
2005 O
investigated O
the O
influence O
of O
laser B-PARA
power I-PARA
strength E-PARA
on O
the O
porosity S-PRO
of O
component S-MACEQ
built O
. O


In O
their O
study O
, O
ytterbium O
fibre B-CONPRI
laser E-CONPRI
with O
a O
wavelength S-CONPRI
of O
1065 O
nm O
was O
used O
to O
process S-CONPRI
the O
316L B-MATE
stainless I-MATE
steel I-MATE
powder E-MATE
. O


The O
experiment S-CONPRI
showed O
that O
low O
laser B-PARA
power E-PARA
and O
high O
scanning B-PARA
speed E-PARA
caused O
incomplete O
melting S-MANP
of O
the O
powder B-MATE
material E-MATE
and O
resulted O
in O
high O
porosity S-PRO
in O
the O
components S-MACEQ
. O


This O
can O
be S-MATE
improved O
by O
increasing O
the O
laser B-PARA
power E-PARA
, O
and O
decreasing O
the O
scanning B-PARA
speed E-PARA
. O


Wang O
investigated O
selective B-MANP
laser I-MANP
melting E-MANP
of O
stainless B-MATE
steel E-MATE
316L O
with O
low O
porosity S-PRO
and O
high O
build B-CHAR
rates E-CHAR
by O
employing O
fast O
scanning B-PARA
speeds E-PARA
to O
fabricate S-MANP
high-density O
stainless B-MATE
steel E-MATE
316L O
parts O
. O


The O
aim O
of O
the O
study O
was O
to O
improve O
the O
production S-MANP
rate O
while O
maintaining O
a O
low O
porosity S-PRO
for O
the O
selective B-MANP
laser E-MANP
melting-built O
parts O
. O


The O
study O
shed O
light O
on O
the O
improvement O
of O
selective B-MANP
laser I-MANP
melting E-MANP
build B-CHAR
rates E-CHAR
without O
any O
decrease O
in O
the O
mechanical B-CONPRI
properties E-CONPRI
or O
any O
loss O
of O
parts O
density S-PRO
of O
stainless B-MATE
steel E-MATE
316L O
. O


Miranda O
investigated O
and O
developed O
models O
for O
predicting O
the O
physical O
and O
mechanical B-CONPRI
properties E-CONPRI
of O
316L B-MATE
stainless I-MATE
steel E-MATE
produced O
by O
selective B-MANP
laser I-MANP
melting E-MANP
. O


The O
influence O
of O
various O
processing O
parameters S-CONPRI
on O
density S-PRO
, O
hardness S-PRO
and O
shear B-PRO
strength E-PRO
of O
316L B-MATE
stainless I-MATE
steel E-MATE
were O
studied O
using O
statistical O
analysis O
in O
order O
to O
significantly O
determine O
main O
factors O
and O
their O
interactions O
. O


Six O
different O
models O
were O
developed O
as S-MATE
a O
predictive O
design S-FEAT
tool O
to O
determine O
the O
influence O
of O
these O
processing O
parameters S-CONPRI
on O
the O
shear B-PRO
strength E-PRO
, O
hardness S-PRO
and O
density S-PRO
. O


Wang O
investigated O
the O
development O
of O
grain B-CONPRI
structure E-CONPRI
mechanism O
of O
316L B-MATE
stainless I-MATE
steel E-MATE
fabricated O
by O
selective B-MANP
laser I-MANP
melting E-MANP
and O
mechanical B-CHAR
property I-CHAR
characterization E-CHAR
. O


The O
grain B-CONPRI
structure E-CONPRI
mechanism O
was O
studied O
using O
finite B-CONPRI
element I-CONPRI
analysis E-CONPRI
in O
order O
to O
reveal O
the O
growth O
mechanism S-CONPRI
of O
grains S-CONPRI
under O
rapid B-MANP
solidification E-MANP
condition O
. O


A O
detailed O
analysis O
of O
crystal B-PRO
orientation E-PRO
of O
formed O
dendrite S-BIOP
was O
performed O
using O
geometrical B-FEAT
analysis E-FEAT
in O
collaboration O
with O
experimental S-CONPRI
findings O
. O


It O
was O
discovered O
that O
rapid B-MANP
solidification E-MANP
caused O
by O
high-speed B-ENAT
scanning E-ENAT
resulted O
into O
sub-micron S-FEAT
grains S-CONPRI
within O
the O
final O
solidified B-PRO
microstructure E-PRO
. O


It O
was O
also O
detected O
that O
grain B-PRO
size E-PRO
and O
densification S-MANP
was O
a O
dependant O
on O
high O
volume S-CONPRI
energy B-PARA
density E-PARA
of O
the O
laser S-ENAT
which O
will O
significantly O
affect O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
the O
final O
product O
formed O
after O
solidification S-CONPRI
. O


Casati O
studied O
the O
microstructure S-CONPRI
and O
fracture S-CONPRI
behaviour O
of O
316L B-MATE
austenitic I-MATE
stainless I-MATE
steel E-MATE
produced O
by O
selective B-MANP
laser I-MANP
melting E-MANP
and O
discovered O
that O
severe O
thermal B-PARA
gradients E-PARA
and O
high O
cooling B-PARA
rates E-PARA
affects O
the O
crystal B-PRO
growth E-PRO
and O
orientation S-CONPRI
of O
grain B-CONPRI
structure E-CONPRI
after O
solidification S-CONPRI
. O


This O
causes O
material S-MATE
spattering O
and O
microstructure B-CONPRI
defects E-CONPRI
like O
pores S-PRO
and O
incomplete O
melted S-CONPRI
particles O
. O


The O
influence O
of O
effect O
of O
different O
distribution S-CONPRI
of O
defects S-CONPRI
on O
mechanical B-CONPRI
response E-CONPRI
and O
failure B-PRO
mechanism E-PRO
were O
investigated O
using O
316L B-MATE
bars E-MATE
with O
microstructure S-CONPRI
and O
texture S-FEAT
built O
along O
two O
different O
orientations S-CONPRI
. O


It O
was O
concluded O
that O
semi-molten S-PRO
metallic B-MATE
powder E-MATE
particles O
of O
stainless B-MATE
steel E-MATE
316L O
were O
responsible O
for O
the O
scattering O
and O
reduced O
strength S-PRO
of O
the O
material S-MATE
after O
solidification S-CONPRI
. O


Liu O
investigated O
the O
spatter S-CHAR
behaviour O
of O
stainless B-MATE
steel E-MATE
316L O
during O
selective B-MANP
laser I-MANP
melting I-MANP
process E-MANP
. O


It O
was O
discovered O
that O
spatter S-CHAR
is O
caused O
as S-MATE
a O
result O
of O
negative O
impact S-CONPRI
of O
laser S-ENAT
on O
the O
building B-CHAR
of I-CHAR
parts E-CHAR
in O
successive O
layers O
during O
selective B-MANP
laser I-MANP
melting I-MANP
process E-MANP
. O


Two O
types O
of O
spatter S-CHAR
were O
identified O
which O
were O
droplet B-CHAR
spatter E-CHAR
, O
generated O
by O
the O
tearing O
behaviour O
of O
molten B-MATE
metal E-MATE
and O
powder B-CHAR
spatter E-CHAR
, O
which O
are O
produced O
when O
non-metallic O
powder B-MATE
particles E-MATE
around O
the O
molten B-CONPRI
pool E-CONPRI
are O
blown O
away O
as S-MATE
a O
result O
of O
metallic S-MATE
vapour O
impact S-CONPRI
. O


It O
was O
discovered O
that O
oxygen S-MATE
composition S-CONPRI
increase O
during O
spatter S-CHAR
and O
X-ray B-CHAR
diffraction E-CHAR
shows O
that O
diffraction S-CHAR
peaks O
of O
austenite S-MATE
content O
and O
ferrite S-MATE
are O
low O
due O
to O
the O
formation O
of O
iron B-MATE
oxides E-MATE
. O


Li S-MATE
investigated O
the O
deformation S-CONPRI
behaviour O
of O
stainless B-MATE
steel E-MATE
micro-lattice B-FEAT
structures E-FEAT
produced O
by O
selective B-MANP
laser I-MANP
melting E-MANP
. O


Macroscopic B-PRO
deformation E-PRO
of O
micro-lattice B-FEAT
structures E-FEAT
and O
microscopic B-PRO
stress E-PRO
and O
strain S-PRO
evolution S-CONPRI
were O
studied O
using O
a O
full O
scale O
3D S-CONPRI
finite O
element S-MATE
model O
. O


The O
finite B-CONPRI
element E-CONPRI
prediction O
revealed O
that O
deformation S-CONPRI
of O
micro-lattice S-FEAT
is O
significantly O
affected O
by O
applied O
boundary B-CONPRI
conditions E-CONPRI
and O
constitutive O
properties S-CONPRI
of O
the O
selective B-MANP
laser I-MANP
melted E-MANP
parent O
material S-MATE
. O


Zhao O
studied O
the O
influence O
of O
stainless B-MATE
steel E-MATE
decarburization S-MANP
on O
its O
youngmodulus S-PRO
hardness S-PRO
and O
tensile B-PRO
strength E-PRO
during O
selective B-MANP
laser I-MANP
melting I-MANP
process E-MANP
. O


The O
study O
was O
investigated O
using O
evolution S-CONPRI
mechanism O
of O
the O
chemical O
element S-MATE
during O
SLM S-MANP
. O


It O
was O
discovered O
that O
during O
decarburization S-MANP
process O
21 O
% O
of O
carbon S-MATE
composition O
was O
lost O
and O
as S-MATE
a O
result O
, O
it O
reduces O
the O
young B-PRO
modulus E-PRO
and O
hardness S-PRO
of O
the O
molten B-CONPRI
pool E-CONPRI
boundary S-FEAT
as S-MATE
well O
as S-MATE
the O
tensile B-PRO
property E-PRO
of O
the O
stainless B-MATE
steel E-MATE
. O


Selective B-MANP
laser I-MANP
melting E-MANP
also O
been O
used O
to O
process S-CONPRI
martensitic B-PRO
grade E-PRO
of O
stainless B-MATE
steel E-MATE
. O


Krakhmalev O
investigated O
the O
evolution S-CONPRI
of O
microstructure S-CONPRI
in O
AISI B-MATE
420 E-MATE
martensitic O
stainless B-MATE
steel E-MATE
during O
selective B-MANP
laser I-MANP
melting E-MANP
. O


It O
was O
discovered O
that O
several O
upper O
layers O
which O
are O
in O
austenite B-CHAR
phase E-CHAR
posses O
hardness S-PRO
value O
higher O
than O
the O
final O
bulk O
microstructure S-CONPRI
of O
thermally B-MATE
decomposited I-MATE
martensite E-MATE
. O


Also O
, O
numerical B-ENAT
simulation E-ENAT
results O
of O
thermal B-PARA
cycles E-PARA
discovered O
that O
thermal O
process S-CONPRI
can O
be S-MATE
controlled O
by O
variation S-CONPRI
of O
laser B-CONPRI
energy E-CONPRI
input O
. O


The O
tribology S-CONPRI
of O
selective B-MANP
laser I-MANP
melting E-MANP
of O
316Lstainless O
steel S-MATE
as S-MATE
a O
processed S-CONPRI
part O
under O
lubricated O
conditions O
was O
studied O
by O
Zhu O
. O


The O
friction S-CONPRI
and O
wear S-CONPRI
behaviours O
of O
316L B-MATE
stainless I-MATE
steel E-MATE
produced O
both O
by O
selective B-MANP
laser I-MANP
melting E-MANP
and O
traditional O
methods O
were O
studied O
using O
a O
ring O
on-disc O
rig O
under O
lubricated O
conditions O
. O


It O
was O
discovered O
that O
the O
tribological B-CONPRI
performance E-CONPRI
of O
SLM S-MANP
stainless O
steel S-MATE
sample S-CONPRI
will O
be S-MATE
better O
if O
the O
pores S-PRO
can O
be S-MATE
drastically O
reduced O
with O
refined O
grains S-CONPRI
. O


Cherry O
investigated O
how O
processing O
parameters S-CONPRI
affects O
the O
microstructural S-CONPRI
and O
physical B-PRO
properties E-PRO
of O
316L B-MATE
stainless I-MATE
steel E-MATE
by O
selective B-MANP
laser I-MANP
melting E-MANP
. O


After O
systematic O
characterization O
of O
porosity S-PRO
and O
microstructure S-CONPRI
, O
it O
was O
discovered O
that O
porosity S-PRO
is O
highest O
at O
lower O
laser B-CONPRI
energy E-CONPRI
and O
decreases O
at O
higher O
laser B-CONPRI
energy E-CONPRI
and O
also O
laser B-PARA
energy I-PARA
density E-PARA
alteration O
resulted O
in O
production S-MANP
of O
dense O
parts O
. O


Material S-MATE
hardness S-PRO
was O
also O
increased O
due O
to O
reduction S-CONPRI
in O
porosity S-PRO
. O


Most O
of O
the O
research S-CONPRI
that O
has O
been O
done O
focused O
mostly O
on O
austenitic B-MATE
stainless I-MATE
steel E-MATE
grade O
, O
316L O
and O
also O
on O
martensitic B-PRO
grade E-PRO
AISI B-MATE
420 E-MATE
. O


Selective B-MANP
laser I-MANP
melting E-MANP
has O
been O
extensively O
explored O
with O
stainless B-MATE
steel E-MATE
but O
further O
study O
need O
to O
be S-MATE
done O
with O
the O
employment O
of O
other O
grades O
of O
stainless B-MATE
steel E-MATE
This O
review O
article O
summarizes O
the O
current O
state-of-the-art S-CONPRI
for O
biomimicry S-CONPRI
in O
additive B-MANP
manufacturing E-MANP
. O


Biomimicry S-CONPRI
is O
the O
practice O
of O
learning O
from O
and O
emulating O
nature O
- O
which O
can O
be S-MATE
increasingly O
realized O
in O
engineering S-APPL
applications O
due O
to O
progress O
in O
additive B-MANP
manufacturing E-MANP
. O


AM S-MANP
has O
grown O
tremendously O
in O
recent O
years O
, O
with O
improvements O
in O
technology S-CONPRI
and O
resulting O
material B-CONPRI
properties E-CONPRI
sometimes O
exceeding O
those O
of O
equivalent O
parts O
produced O
by O
traditional O
production S-MANP
processes S-CONPRI
. O


This O
has O
led S-APPL
to O
the O
industrial S-APPL
use O
of O
AM B-MACEQ
parts E-MACEQ
even O
in O
highly O
critical O
applications O
, O
most O
notably O
in O
aerospace S-APPL
, O
automotive S-APPL
and O
medical B-APPL
applications E-APPL
. O


The O
ability O
to O
create O
parts O
with O
complex B-CONPRI
geometries E-CONPRI
is O
one O
of O
the O
most O
important O
advantages O
of O
this O
technology S-CONPRI
, O
allowing O
the O
production S-MANP
of O
complex B-MACEQ
functional I-MACEQ
objects E-MACEQ
from O
various B-MATE
materials E-MATE
including O
plastics S-MATE
and O
metals S-MATE
that O
can O
not O
be S-MATE
easily O
produced O
by O
any O
other O
means O
. O


Utilizing O
the O
full O
complexity S-CONPRI
allowed O
by O
AM S-MANP
is O
the O
key O
to O
unlocking O
the O
huge O
potential O
of O
this O
technology S-CONPRI
for O
real O
world O
applications O
and O
biomimicry S-CONPRI
might O
be S-MATE
pivotal O
in O
this O
regard O
. O


Biomimicry S-CONPRI
may O
take O
different O
forms O
in O
AM S-MANP
, O
including O
customization O
of O
parts O
for O
individuals O
, O
or O
optimization S-CONPRI
for O
specific B-PRO
properties E-PRO
such O
as S-MATE
stiffness O
and O
light-weighting O
. O


The O
optimization S-CONPRI
process O
often O
uses O
an O
iterative O
simulation-driven S-ENAT
process S-CONPRI
analogous O
to O
biological B-CONPRI
evolution E-CONPRI
with O
an O
improvement O
in O
every O
iteration O
. O


Other O
forms O
of O
biomimicry S-CONPRI
in O
AM S-MANP
include O
the O
incorporation O
of O
real O
biological B-BIOP
inputs E-BIOP
into O
designs S-FEAT
; O
the O
use O
of O
cellular O
or O
lattice B-FEAT
structures E-FEAT
for O
various O
applications O
and O
customized O
to O
the O
application O
; O
incorporating O
multi-functionality O
into O
designs S-FEAT
; O
the O
consolidation S-CONPRI
of O
numerous O
parts O
into O
one O
and O
the O
reduction S-CONPRI
of O
waste O
, O
amongst O
others O
. O


Numerous O
biomimetic B-FEAT
design E-FEAT
approaches O
may O
be S-MATE
used O
broadly O
categorized O
into O
customized/freeform O
, O
simulation-driven S-ENAT
and O
lattice B-FEAT
designs E-FEAT
. O


not O
for O
prototyping S-CONPRI
. O


The O
current O
limits S-CONPRI
of O
each O
design S-FEAT
approach O
are O
discussed O
and O
the O
most O
exciting O
future O
opportunities O
for O
biomimetic B-APPL
AM I-APPL
applications E-APPL
are O
highlighted O
. O


The O
beauty O
found O
in O
nature O
is O
often O
inspirational O
- O
and O
this O
inspiration O
has O
found O
its O
way O
into O
functional O
mechanical B-APPL
engineering E-APPL
through O
the O
latest O
developments O
in O
additive B-MANP
manufacturing E-MANP
. O


Other O
forms O
of O
engineering S-APPL
beauty O
are O
structural B-CONPRI
hierarchy E-CONPRI
, O
order O
or O
lack O
of O
order O
, O
and O
combinations O
with O
other O
structures O
. O


Learning O
from O
these O
biological B-FEAT
structures E-FEAT
may O
advance O
our O
use O
of O
efficient O
structures O
in O
engineering S-APPL
applications O
and O
may O
even O
help O
to O
provide O
new O
solutions O
to O
engineering S-APPL
problems O
, O
in O
a O
sustainable S-CONPRI
way O
. O


Biomimicry S-CONPRI
in O
engineering S-APPL
involves O
the O
study O
of O
biological B-CONPRI
systems E-CONPRI
specifically O
with O
the O
aim O
to O
use O
information O
learned O
in O
solving O
engineering S-APPL
problems O
, O
or O
for O
use O
in O
engineering S-APPL
applications O
. O


In O
nature O
, O
structural O
features O
from O
nano S-FEAT
to O
micro O
to O
macro B-CONPRI
scale E-CONPRI
define O
an O
objectproperties O
and O
functionalities O
and O
vice O
versa O
. O


Modern O
engineering S-APPL
design S-FEAT
has O
the O
possibility O
to O
change O
the O
structural O
features O
and O
properties S-CONPRI
of O
the O
objects O
while O
maintaining O
functionality O
or O
to O
apply O
simulation S-ENAT
to O
find O
a O
design S-FEAT
for O
specific O
required O
properties S-CONPRI
. O


In O
an O
ideal O
case O
, O
AM S-MANP
is O
able O
to O
translate O
innovative O
biomimetic B-FEAT
design E-FEAT
into O
physical O
objects O
with O
the O
desired O
properties S-CONPRI
and O
functionality O
. O


Much O
of O
this O
potential O
has O
particularly O
realistic O
prospects O
when O
using O
AM S-MANP
, O
with O
its O
freedom O
of O
design S-FEAT
and O
complex O
production S-MANP
capabilities O
. O


The O
capability O
to O
emulate O
the O
complex B-CONPRI
structures E-CONPRI
and O
hence O
the O
properties S-CONPRI
of O
biological B-MATE
materials E-MATE
is O
the O
aim O
of O
biomimicry S-CONPRI
. O


For O
example O
, O
failure S-CONPRI
of O
a O
specific O
design S-FEAT
that O
is O
claimed O
to O
be S-MATE
biomimetic O
but O
uses O
no O
input O
from O
nature O
, O
might O
undermine O
the O
credibility O
of O
biomimicry S-CONPRI
. O


Often O
structures O
with O
curves O
and O
rounded B-FEAT
edges E-FEAT
in O
any O
way O
resembling O
something O
in O
nature O
are O
referred O
to O
as S-MATE
or O
This O
is O
not O
incorrect O
but O
it O
must O
be S-MATE
kept O
in O
mind O
that O
no O
biological B-BIOP
input E-BIOP
is O
present O
, O
and O
as S-MATE
such O
is O
not O
truly O
biomimetic S-CONPRI
or O
bio-inspired S-CONPRI
. O


Additionally O
, O
when O
a O
structure S-CONPRI
is O
designed S-FEAT
for O
a O
biological O
application O
it O
may O
be S-MATE
termed O
biomimetic S-CONPRI
or O
bionic O
simply O
due O
to O
its O
intended O
biological O
role O
. O


Topology B-FEAT
optimization E-FEAT
, O
generative B-ENAT
design E-ENAT
and O
simulation-driven S-ENAT
design S-FEAT
tools O
used O
to O
create O
optimized O
designs S-FEAT
using O
simulation S-ENAT
often O
create O
unconventional O
and O
complex B-PRO
shapes E-PRO
and O
forms O
. O


The O
simulation-driven S-ENAT
design B-CONPRI
process E-CONPRI
is O
in O
reality O
also O
biomimetic S-CONPRI
or O
bio-inspired S-CONPRI
in O
the O
sense O
that O
it O
is O
iterative O
and O
therefore O
mimics O
aspects O
of O
natural O
evolutionary O
strategies O
in O
a O
short O
timeframe O
. O


In O
the O
area S-PARA
of O
cellular O
or O
lattice B-FEAT
structure I-FEAT
design E-FEAT
, O
some O
engineers O
refer O
to O
all O
porous S-PRO
engineered O
structures O
as S-MATE
biomimetic O
simply O
due O
to O
their O
resemblance O
to O
natural O
porous B-MATE
materials E-MATE
, O
or O
their O
similarity O
to O
the O
biological B-CONPRI
equivalent E-CONPRI
. O


However O
, O
cellular O
and O
lattice B-FEAT
designs E-FEAT
have O
unlimited O
design S-FEAT
permutations O
and O
can O
therefore O
be S-MATE
tailored O
to O
the O
application O
. O


Currently O
, O
the O
most O
important O
application O
for O
these O
porous S-PRO
engineered O
structures O
is O
in O
dental B-APPL
and I-APPL
bone I-APPL
implants E-APPL
. O


The O
latter O
is O
a O
biomimetic B-APPL
application E-APPL
in O
the O
sense O
that O
the O
structure S-CONPRI
should O
emulate O
bone S-BIOP
for O
best O
results O
, O
in O
terms O
of O
mechanical B-CONPRI
properties E-CONPRI
and O
permeability S-PRO
. O


Finally O
, O
biomimetic S-CONPRI
lattice O
structures O
may O
also O
specifically O
refer O
to O
stochastic B-FEAT
design E-FEAT
strategies O
which O
create O
structures O
with O
a O
random O
distribution S-CONPRI
of O
strut B-PARA
thicknesses E-PARA
and O
lengths O
the O
randomness O
emulates O
nature O
. O


A O
biomimetic S-CONPRI
and O
bio-inspired S-CONPRI
approach O
to O
materials S-CONPRI
design S-FEAT
has O
attracted O
great O
interest O
from O
scientists O
in O
diverse O
areas S-PARA
: O
biophysics S-CONPRI
and O
biomaterials S-MATE
, O
sensors S-MACEQ
and O
chemistry S-CONPRI
, O
materials S-CONPRI
science O
and O
engineering S-APPL
, O
to O
name O
a O
few O
. O


From O
2016 O
, O
with O
the O
progress O
in O
AM B-MANP
technology E-MANP
and O
wider O
understanding O
of O
the O
fact O
that O
complex O
designs S-FEAT
can O
be S-MATE
realized O
in O
real O
AM S-MANP
products O
, O
biomimetic S-CONPRI
approaches O
began O
to O
be S-MATE
the O
subject O
of O
research S-CONPRI
in O
more O
than O
150 O
papers O
per O
year O
. O


Interest O
in O
lattice B-FEAT
structures E-FEAT
produced O
by O
AM S-MANP
also O
increased O
year O
by O
year O
. O


In O
recent O
years O
AM S-MANP
has O
grown O
from O
a O
prototyping S-CONPRI
technology O
to O
a O
reliable O
direct O
production S-MANP
technique O
. O


In O
particular O
, O
metal B-MANP
AM E-MANP
has O
developed O
tremendously O
, O
up O
to O
the O
point O
where O
it O
is O
now O
possible O
to O
produce O
functional O
metal S-MATE
parts O
for O
critical O
applications O
in O
medical S-APPL
and O
aerospace B-APPL
industries E-APPL
. O


Powder B-MANP
bed I-MANP
fusion E-MANP
is O
the O
term O
used O
to O
specifically O
describe O
metal B-MANP
AM E-MANP
using O
a O
laser S-ENAT
or O
electron B-CONPRI
beam E-CONPRI
to O
melt S-CONPRI
tracks O
and O
layers O
for O
the O
manufacture S-CONPRI
of O
detailed O
and O
complex O
shaped O
parts O
. O


The O
track-by-track O
and O
layer-by-layer S-CONPRI
PBF O
process S-CONPRI
allows O
the O
manufacturing S-MANP
of O
parts O
with O
intricate O
, O
complex O
designs S-FEAT
. O


Part O
complexity S-CONPRI
allows O
designs S-FEAT
to O
be S-MATE
optimized O
for O
specific O
applications O
such O
as S-MATE
light-weighting O
in O
aerospace S-APPL
parts O
or O
improving O
bone B-CONPRI
growth E-CONPRI
and O
implant S-APPL
success O
in O
bone B-APPL
implants E-APPL
. O


It O
has O
been O
demonstrated O
that O
the O
mechanical S-APPL
performance O
of O
PBF S-MANP
parts O
can O
be S-MATE
superior O
to O
traditionally O
manufactured S-CONPRI
equivalents O
and O
lots O
of O
work O
has O
been O
done O
in O
particular O
in O
Ti6Al4V S-MATE
as S-MATE
shown O
in O
. O


Laser B-MANP
powder I-MANP
bed I-MANP
fusion E-MANP
is O
limited O
to O
intricate O
parts O
typically O
smaller O
than O
300 O
mm S-MANP
for O
larger O
metal S-MATE
parts O
it O
is O
possible O
to O
use O
wire B-MANP
and I-MANP
arc I-MANP
AM E-MANP
with O
a O
reduction S-CONPRI
in O
detail O
possible O
. O


In O
addition O
to O
metals S-MATE
, O
various O
other O
materials S-CONPRI
can O
be S-MATE
reliably O
processed S-CONPRI
using O
AM S-MANP
including O
polymers S-MATE
, O
ceramics S-MATE
and O
various O
types O
of O
composites S-MATE
, O
as S-MATE
is O
discussed O
in O
more O
detail O
in O
. O


Metals S-MATE
are O
highly O
likely O
to O
have O
practical O
uses O
in O
biomimetic B-APPL
structural I-APPL
applications E-APPL
in O
military S-APPL
, O
aerospace S-APPL
and O
automotive B-APPL
industries E-APPL
due O
to O
the O
light-weight S-PRO
and O
strong O
parts O
that O
can O
be S-MATE
produced O
, O
and O
hence O
much O
effort O
has O
been O
aimed O
in O
this O
direction O
. O


However O
, O
many O
biological B-CONPRI
systems E-CONPRI
are O
based O
on O
combinations O
of O
stiff O
and O
softer O
materials S-CONPRI
, O
and O
often O
have O
mechanical B-CONPRI
properties E-CONPRI
more O
like O
polymers S-MATE
and O
composite B-MATE
materials E-MATE
. O


Therefore O
, O
many O
applications O
also O
exist O
for O
nature-inspired O
designs S-FEAT
in O
materials S-CONPRI
other O
than O
metals S-MATE
. O


Many O
of O
the O
examples O
presented O
in O
this O
review O
focus O
specifically O
on O
metal B-MANP
AM E-MANP
, O
due O
to O
their O
relevance O
for O
high-value O
functional O
end-use O
parts O
, O
but O
the O
same O
principles O
apply O
to O
all O
other O
additively B-MANP
manufactured E-MANP
materials O
. O


For O
products O
designed S-FEAT
by O
biomimicry S-CONPRI
, O
it O
has O
been O
proposed O
that O
two O
broad O
approaches O
exist O
: O
the O
approach O
and O
the O
approach O
as S-MATE
outlined O
in O
. O


In O
the O
first O
case O
, O
the O
designer/engineer O
is O
inspired O
by O
a O
biological B-CONPRI
concept E-CONPRI
or O
model S-CONPRI
and O
applies O
this O
to O
a O
new O
design S-FEAT
idea O
. O


In O
addition O
to O
these O
approaches O
, O
three O
major O
ways O
of O
obtaining O
a O
designed S-FEAT
biomimetic S-CONPRI
model O
in O
practice O
exist O
: O
customized/freeform O
design S-FEAT
, O
simulation-driven S-ENAT
design S-FEAT
and O
lattice B-FEAT
design E-FEAT
. O


For O
example O
, O
lattices S-CONPRI
may O
be S-MATE
incorporated O
in O
a O
freeform B-FEAT
design E-FEAT
process O
or O
in O
a O
simulation-design O
process S-CONPRI
. O


All O
these O
approaches O
may O
also O
be S-MATE
used O
with O
or O
without O
direct O
input O
from O
nature O
, O
with O
varying O
levels O
of O
biological B-BIOP
input E-BIOP
or O
bio-inspiration S-CONPRI
possible O
. O


Customized B-FEAT
and I-FEAT
freeform I-FEAT
design E-FEAT
involves O
manipulation O
with O
curved B-CONPRI
surfaces E-CONPRI
and O
is O
typically O
used O
to O
create O
custom O
and O
unique O
designs S-FEAT
fit O
for O
a O
particular O
application O
while O
maintaining O
functionality O
. O


For O
instance O
, O
customized O
implants S-APPL
aimed O
at O
directly O
replicating O
the O
bone B-PARA
shape E-PARA
for O
replacement O
, O
tree-like B-PRO
support I-PRO
structures E-PRO
, O
nervous-system-inspired O
shade O
or O
hierarchical B-CONPRI
networks E-CONPRI
where O
nodes O
constantly O
branch O
and O
merge O
. O


This O
process S-CONPRI
is O
the O
simplest O
of O
the O
biomimetic B-FEAT
design E-FEAT
methods O
, O
particularly O
useful O
for O
customization O
such O
as S-MATE
in O
prosthetics S-APPL
or O
implants S-APPL
, O
and O
is O
also O
used O
in O
artistic O
design S-FEAT
. O


With O
reference O
to O
prosthetics S-APPL
and O
implants S-APPL
, O
the O
design S-FEAT
requirement O
is O
taken O
from O
a O
biological B-FEAT
shape E-FEAT
, O
hence O
the O
biomimetic S-CONPRI
description O
. O


In O
addition O
, O
freeform B-FEAT
design E-FEAT
results O
in O
organic O
shapes O
which O
can O
often O
resemble O
natural O
structures O
. O


Simulation-driven S-ENAT
design S-FEAT
is O
a O
very O
promising O
approach O
which O
has O
emerged O
in O
recent O
years O
and O
is O
especially O
useful O
for O
light-weight S-PRO
design S-FEAT
for O
engineering S-APPL
applications O
. O


This O
involves O
structural B-CONPRI
optimization E-CONPRI
and O
uses O
an O
iterative O
process S-CONPRI
of O
simulation S-ENAT
and O
material S-MATE
removal O
to O
optimize O
the O
required O
material S-MATE
distribution S-CONPRI
or O
material B-FEAT
stiffness E-FEAT
for O
a O
given O
set S-APPL
of O
expected O
load O
cases O
. O


This O
process S-CONPRI
of O
stepwise O
optimization S-CONPRI
is O
similar O
to O
most O
evolutionary O
processes S-CONPRI
in O
nature O
, O
and O
removal O
of O
material S-MATE
in O
areas S-PARA
of O
low O
stress S-PRO
is O
a O
similar O
optimization S-CONPRI
strategy O
as S-MATE
is O
used O
in O
natural O
systems O
, O
hence O
the O
motivation O
to O
categorize O
this O
process S-CONPRI
as S-MATE
biomimetic O
. O


The O
field O
of O
topology B-FEAT
optimization E-FEAT
in O
AM S-MANP
was O
reviewed O
recently O
in O
, O
where O
the O
current O
limits S-CONPRI
of O
the O
practical O
use O
of O
this O
technique O
was O
discussed O
in O
detail O
, O
especially O
with O
regards O
to O
overhang B-PARA
angle E-PARA
, O
support S-APPL
removal O
, O
residual B-PRO
stress E-PRO
, O
build S-PARA
quality O
including O
challenges O
in O
software S-CONPRI
tools O
that O
need O
to O
be S-MATE
solved O
for O
its O
more O
widespread O
adoption O
. O


The O
use O
of O
additively-manufactured O
lattice S-CONPRI
or O
cellular B-FEAT
structures E-FEAT
is O
a O
highly O
relevant O
approach O
which O
is O
often O
combined O
with O
the O
former O
methods O
, O
i.e O
. O


the O
incorporation O
of O
lattices S-CONPRI
or O
cellular B-FEAT
designs E-FEAT
into O
optimized O
organic O
or O
topology S-CONPRI
optimized O
designs S-FEAT
. O


Natural O
systems O
often O
use O
cellular B-FEAT
structures E-FEAT
and O
these O
are O
widely O
used O
in O
bio-inspiration S-CONPRI
for O
the O
use O
of O
lattices S-CONPRI
in O
engineering S-APPL
parts O
, O
hence O
the O
categorization O
as S-MATE
biomimetic O
. O


Lattices S-CONPRI
have O
obvious O
light-weighting O
advantages O
, O
high O
specific B-PRO
stiffness E-PRO
, O
fracture S-CONPRI
toughness O
, O
crack B-CONPRI
growth E-CONPRI
arresting O
, O
amongst O
other O
desirable O
and O
tailorable O
properties S-CONPRI
. O


One O
major O
application O
of O
cellular B-FEAT
structures E-FEAT
is O
their O
use O
in O
bone B-APPL
implants E-APPL
, O
to O
improve O
osseointegration S-PRO
. O


The O
design S-FEAT
theory O
for O
present-day O
AM S-MANP
in O
general O
was O
reviewed O
and O
limitations O
discussed O
in O
. O


On O
the O
topic O
of O
biomimetic S-CONPRI
3D B-MANP
printing E-MANP
, O
the O
review O
gives O
a O
detailed O
overview O
of O
the O
use O
of O
biological B-BIOP
inputs E-BIOP
into O
the O
design B-CONPRI
process E-CONPRI
, O
discusses O
biological O
study O
systems O
used O
in O
biomimicry S-CONPRI
and O
focusses O
on O
applications O
of O
polymer S-MATE
and O
multi-material B-MANP
3D I-MANP
printing E-MANP
, O
but O
does O
not O
discuss O
metal B-MANP
AM E-MANP
or O
simulation-driven S-ENAT
design S-FEAT
. O


Biomimetic S-CONPRI
approaches O
for O
AM S-MANP
include O
the O
design S-FEAT
of O
innovative O
materials S-CONPRI
and O
systems O
. O


In O
addition O
to O
simulation-driven S-ENAT
design S-FEAT
of O
single-material O
parts O
, O
fracture-resistant B-MATE
composite E-MATE
materials O
could O
be S-MATE
designed O
using O
simulation-driven S-ENAT
design S-FEAT
and O
validated O
by O
multi-material B-MANP
3D I-MANP
printing E-MANP
as S-MATE
demonstrated O
in O
. O


Multi-material S-CONPRI
biomimetic B-FEAT
design E-FEAT
for O
medical S-APPL
purposes O
has O
been O
demonstrated O
in O
. O


Not O
all O
freeform B-FEAT
designs E-FEAT
, O
lattice B-FEAT
designs E-FEAT
or O
topology S-CONPRI
optimized O
designs S-FEAT
include O
biological B-BIOP
input E-BIOP
, O
but O
they O
are O
still O
referred O
to O
as S-MATE
biomimetic O
in O
a O
broader O
sense O
. O


True O
biological B-BIOP
input E-BIOP
in O
the O
AM S-MANP
design O
process S-CONPRI
is O
still O
rare O
in O
engineering S-APPL
due O
to O
the O
lack O
of O
biologists O
involved O
in O
engineering S-APPL
design S-FEAT
in O
general O
. O


Nevertheless O
, O
biological B-MATE
materials E-MATE
science O
is O
a O
mature O
field O
which O
focuses O
on O
studying O
biological B-CONPRI
systems E-CONPRI
to O
understand O
their O
properties S-CONPRI
and O
potentially O
employ O
these O
designs S-FEAT
in O
engineering S-APPL
systems O
. O


Biological B-MATE
materials E-MATE
often O
possess O
superior O
mechanical B-CONPRI
properties E-CONPRI
due O
to O
unique O
combinations O
of O
hard O
and O
soft O
materials S-CONPRI
and O
gradients O
between O
them O
. O


Biologically O
inspired O
design S-FEAT
principles O
have O
been O
categorized O
recently O
into O
fibrous S-PRO
, O
helical O
, O
gradient O
, O
layered O
, O
tubular S-FEAT
, O
cellular O
, O
suture S-MATE
and O
overlapping O
structures O
. O


Besides O
broad O
design S-FEAT
categories O
or O
guidelines O
, O
the O
use O
of O
X-ray B-CHAR
tomography E-CHAR
to O
study O
intricate O
details O
of O
individual O
biological B-FEAT
structures E-FEAT
in O
3D S-CONPRI
for O
biomimetic B-APPL
applications E-APPL
is O
also O
a O
promising O
strategy O
to O
learn O
from O
nature O
. O


Incorporating O
biological B-BIOP
inputs E-BIOP
into O
engineering S-APPL
design S-FEAT
is O
a O
topic O
of O
continued O
effort O
and O
includes O
the O
development O
of O
biomimicry S-CONPRI
design O
databases S-ENAT
. O


Biomimetic B-FEAT
design E-FEAT
has O
also O
been O
named O
as S-MATE
it O
has O
been O
suggested O
that O
these O
approaches O
may O
lead S-MATE
to O
the O
use O
of O
the O
minimum O
required O
materials S-CONPRI
, O
which O
is O
most O
environmentally O
sustainable S-CONPRI
. O


Despite O
the O
access O
to O
complexity S-CONPRI
and O
freedom O
of O
design S-FEAT
, O
which O
is O
often O
cited O
for O
AM S-MANP
, O
all O
the O
biomimetic S-CONPRI
approaches O
discussed O
here O
have O
practical O
manufacturability B-CONPRI
limits E-CONPRI
in O
the O
context O
of O
present-day O
AM S-MANP
systems O
. O


A O
recent O
review O
paper O
covers O
the O
use O
of O
AM S-MANP
to O
produce O
bio-inspired B-FEAT
structures E-FEAT
with O
the O
main O
aim O
to O
learn O
about O
and O
optimize O
the O
biological B-FEAT
structures E-FEAT
themselves O
. O


In O
the O
area S-PARA
of O
biomimetic B-FEAT
cellular I-FEAT
design E-FEAT
, O
various O
recent O
reviews O
are O
useful O
and O
relevant O
to O
bone B-APPL
implant I-APPL
applications E-APPL
in O
particular O
, O
and O
are O
more O
generally O
discussed O
for O
various O
applications O
in O
. O


It O
is O
therefore O
the O
aim O
of O
this O
present O
review O
paper O
to O
fill O
the O
gaps O
between O
these O
areas S-PARA
and O
address O
all O
the O
above O
biomimetic S-CONPRI
approaches O
in O
one O
cohesive O
framework S-CONPRI
. O


Most O
examples O
used O
in O
this O
paper O
are O
focused O
on O
metal B-MANP
AM E-MANP
due O
to O
its O
ability O
to O
produce O
functional O
end-use O
parts O
, O
but O
the O
principles O
are O
broadly O
applicable O
to O
all O
additively B-MANP
manufactured E-MANP
materials O
. O


While O
most O
of O
the O
discussion O
and O
examples O
are O
using O
laser B-MANP
powder I-MANP
bed I-MANP
fusion E-MANP
, O
other O
AM B-MANP
technologies E-MANP
are O
equally O
applicable O
and O
the O
design S-FEAT
and O
challenges O
vary O
slightly O
with O
each O
technology S-CONPRI
. O


For O
example O
, O
binder B-MANP
jetting E-MANP
has O
shown O
some O
promise O
for O
realization O
of O
complex O
designs S-FEAT
cost O
effectively O
, O
but O
the O
obtained O
material B-CONPRI
properties E-CONPRI
require O
investigation O
. O


The O
fields O
of O
biomimicry S-CONPRI
and O
AM S-MANP
hold O
a O
unique O
synergy O
and O
inter-dependence O
on O
one O
another O
. O


2 O
True O
biomimicry S-CONPRI
True O
biomimicry S-CONPRI
of O
natural O
form O
, O
involves O
the O
purposeful O
emulation O
of O
structure-function O
relationships O
in O
biological B-MATE
entities E-MATE
to O
solve O
engineering S-APPL
challenges O
, O
or O
to O
apply O
these O
to O
advanced O
engineering S-APPL
systems O
. O


A O
review O
on O
biomimicry S-CONPRI
and O
bio-inspiration S-CONPRI
in O
the O
field O
of O
AM S-MANP
and O
3D B-MANP
printing E-MANP
is O
provided O
in O
and O
focuses O
on O
explaining O
different O
potential O
biological O
study O
organisms O
and O
associated O
applications O
with O
specific O
biological B-BIOP
input E-BIOP
, O
mostly O
by O
polymer S-MATE
AM S-MANP
. O


In O
addition O
, O
the O
review O
highlights O
the O
potential O
for O
different O
forms O
of O
AM B-MANP
technologies E-MANP
to O
mimic S-MACEQ
nature O
. O


As S-MATE
mentioned O
above O
, O
the O
goal O
of O
biomimetic S-CONPRI
research O
is O
to O
learn O
generic O
design B-CONPRI
rules E-CONPRI
from O
natural O
systems O
to O
assist O
the O
development O
of O
optimized O
biomimetic B-MATE
materials E-MATE
which O
can O
be S-MATE
used O
widely O
in O
engineering S-APPL
systems O
. O


To O
illustrate O
, O
osteoderms S-BIOP
thin O
plates O
of O
dermal B-BIOP
bone E-BIOP
that O
form O
protective O
natural O
body O
armour O
in O
various O
animal O
species O
not O
only O
play O
a O
defensive O
role O
, O
but O
might O
also O
be S-MATE
involved O
in O
physiological O
processes S-CONPRI
such O
as S-MATE
thermoregulation O
. O


The O
structural O
changes O
required O
for O
a O
physiological O
capacity S-CONPRI
might O
decrease O
the O
strength S-PRO
of O
osteoderms S-BIOP
, O
rendering O
the O
structure S-CONPRI
less O
optimally O
adapted O
for O
protection O
than O
what O
would O
be S-MATE
expected O
. O


Alternatively O
, O
a O
bio-inspiration S-CONPRI
approach O
can O
be S-MATE
employed O
to O
alter O
specific B-PRO
properties E-PRO
of O
the O
natural O
structure S-CONPRI
resulting O
in O
an O
optimal O
design S-FEAT
. O


Glyptodon O
osteoderms S-BIOP
consists O
of O
a O
lattice B-FEAT
core E-FEAT
sandwiched O
between O
two O
compact S-MANP
layers O
that O
form O
a O
shell S-MACEQ
. O


By O
printing O
and O
testing S-CHAR
3D B-APPL
models E-APPL
with O
varying O
lattice S-CONPRI
and O
shell S-MACEQ
parameters S-CONPRI
, O
the O
optimized O
shell S-MACEQ
thickness O
compared O
to O
lattice B-FEAT
density E-FEAT
and O
lattice S-CONPRI
strut O
thickness O
was O
revealed O
. O


Similar O
procedures O
have O
been O
used O
to O
reverse-engineer O
a O
natural O
structure S-CONPRI
for O
application O
as S-MATE
a O
gripping O
device O
the O
Aristotlelantern O
structure S-CONPRI
as S-MATE
described O
in O
. O


The O
mechanical B-CONPRI
properties E-CONPRI
of O
natural O
materials S-CONPRI
, O
particularly O
the O
superior O
fracture S-CONPRI
toughness O
, O
make O
biological B-FEAT
structures E-FEAT
highly O
suitable O
for O
biomimetic B-CONPRI
studies E-CONPRI
. O


Nevertheless O
, O
a O
major O
advantage O
of O
AM S-MANP
is O
that O
a O
structure S-CONPRI
of O
interest O
can O
be S-MATE
further O
optimized O
by O
using O
materials S-CONPRI
that O
do O
not O
occur O
naturally O
in O
biological B-CONPRI
systems E-CONPRI
. O


In O
the O
case O
of O
glyptodont O
osteoderms S-BIOP
, O
the O
use O
of O
biomimetic S-CONPRI
reverse-engineered O
metal S-MATE
models O
show O
remarkable O
strength S-PRO
and O
energy B-CHAR
absorption I-CHAR
capacity E-CHAR
. O


Besides O
material B-CONPRI
properties E-CONPRI
, O
the O
combination O
of O
hard O
and O
soft O
materials S-CONPRI
has O
been O
studied O
for O
improved O
fracture S-CONPRI
toughness O
properties S-CONPRI
using O
simulation-driven S-ENAT
design S-FEAT
tools O
. O


In O
a O
recent O
study O
, O
pangolin O
scales O
were O
used O
as S-MATE
inspiration O
for O
bendable B-MATE
protective I-MATE
material E-MATE
for O
aerospace S-APPL
applications O
different O
combinations O
of O
hard O
plates O
and O
soft O
connecting O
material S-MATE
were O
3D B-MANP
printed E-MANP
and O
mechanically O
tested O
. O


Lastly O
, O
the O
microarchitecture S-CONPRI
of O
biological B-FEAT
structures E-FEAT
, O
which O
can O
be S-MATE
categorized O
as S-MATE
one O
of O
eight O
forms O
: O
fibrous S-PRO
, O
helical O
, O
gradient O
, O
layered O
, O
tubular S-FEAT
, O
cellular O
, O
suture S-MATE
and O
overlapping O
, O
plays O
an O
important O
role O
in O
determining O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
biological B-MATE
materials E-MATE
. O


These O
structural O
organizations O
can O
be S-MATE
replicated O
by O
AM S-MANP
to O
study O
and O
optimize O
the O
arrangement O
of O
biological B-MATE
materials E-MATE
as S-MATE
discussed O
in O
. O


Hierarchical B-FEAT
structures E-FEAT
such O
as S-MATE
functional O
graded O
materials S-CONPRI
, O
structures O
and O
surfaces S-CONPRI
can O
be S-MATE
produced O
directly O
by O
AM S-MANP
or O
in O
combination O
with O
other O
methods O
. O


For O
example O
, O
LPBF S-MANP
and O
femtosecond B-CONPRI
laser E-CONPRI
surface O
modification O
makes O
it O
possible O
to O
produce O
complex O
hierarchical B-FEAT
structures E-FEAT
for O
wettability S-CONPRI
applications O
. O


Stereolithography S-MANP
and O
LPBF S-MANP
was O
applied O
for O
manufacturing S-MANP
of O
a O
multi-material S-CONPRI
arm O
orthosis O
; O
this O
approach O
can O
be S-MATE
used O
for O
manufacturing S-MANP
implants S-APPL
where O
the O
strength S-PRO
varies O
throughout O
the O
implant S-APPL
. O


In O
general O
, O
AM S-MANP
of O
in-situ S-CONPRI
LPBF O
sintered S-MANP
composite S-MATE
objects O
also O
is O
a O
form O
of O
biomimicry S-CONPRI
since O
biological B-MATE
tissues E-MATE
are O
composite B-MATE
materials E-MATE
with O
stiff O
reinforcing B-FEAT
elements E-FEAT
and O
binding O
medium O
. O


A O
pivotal O
tool S-MACEQ
to O
characterize O
structures O
for O
biomimicry S-CONPRI
or O
bio-inspiration S-CONPRI
is O
X-ray B-CHAR
micro-computed I-CHAR
tomography E-CHAR
, O
as S-MATE
reviewed O
in O
. O


MicroCT S-CHAR
is O
ideally O
suited O
to O
obtain O
detailed O
microstructural S-CONPRI
information O
of O
natural O
structures O
in O
3D S-CONPRI
, O
which O
can O
be S-MATE
used O
to O
directly O
replicate O
natural O
structures O
, O
measure O
3D B-FEAT
design I-FEAT
values E-FEAT
and O
implement O
these O
in O
engineering S-APPL
structures O
as S-MATE
bio-inspiration O
, O
or O
in O
a O
broader O
sense O
to O
create O
a O
design S-FEAT
principle O
without O
using O
any O
measurements O
. O


These O
three O
are O
shown O
in O
3 O
, O
using O
the O
examples O
of O
a O
direct B-CONPRI
replication E-CONPRI
of O
a O
structure S-CONPRI
printed O
on O
an O
entry-level O
FDM B-MACEQ
printer E-MACEQ
, O
a O
reverse-engineered O
design S-FEAT
based O
on O
measurements O
taken O
from O
a O
natural O
structure S-CONPRI
and O
a O
generic O
bio-inspiration S-CONPRI
example O
in O
which O
honeycomb B-FEAT
structures E-FEAT
are O
used O
as S-MATE
light-weight O
design S-FEAT
. O


The O
main O
aim O
of O
direct B-CONPRI
replication E-CONPRI
is O
to O
investigate O
the O
structure S-CONPRI
of O
interest O
. O


For O
reverse-engineering O
, O
the O
goal O
is O
similar O
to O
that O
of O
direct B-CONPRI
replication E-CONPRI
, O
but O
the O
techniques O
make O
the O
structure S-CONPRI
more O
practical O
for O
direct O
engineering S-APPL
applications O
. O


The O
generic O
bio-inspiration S-CONPRI
involves O
using O
design S-FEAT
or O
guidelines O
from O
nature O
, O
which O
might O
be S-MATE
more O
beneficial O
when O
limits S-CONPRI
are O
imposed O
on O
the O
structure S-CONPRI
. O


One O
biological B-FEAT
structure E-FEAT
that O
is O
of O
particular O
interest O
to O
biomimetic B-CONPRI
studies E-CONPRI
, O
and O
which O
has O
been O
studied O
extensively O
using O
microCT S-CHAR
, O
is O
the O
lightweight B-MACEQ
structure E-MACEQ
of O
bird B-MATE
feathers E-MATE
and O
bones O
. O


Here O
, O
bio-inspiration S-CONPRI
and O
design B-CONPRI
rules E-CONPRI
might O
be S-MATE
applied O
in O
engineering S-APPL
designs S-FEAT
for O
aerospace S-APPL
applications O
. O


In O
recent O
work O
using O
topology B-FEAT
optimization E-FEAT
techniques O
, O
an O
optimized O
light-weight S-PRO
structure O
for O
an O
airplane B-APPL
wing E-APPL
was O
demonstrated O
through O
simulation S-ENAT
and O
optimization S-CONPRI
, O
with O
the O
obtained O
structure S-CONPRI
having O
a O
strong O
resemblance O
to O
the O
structure S-CONPRI
of O
bird O
wing O
bones O
, O
i.e. O
, O
a O
solid O
shell S-MACEQ
and O
connecting O
rods O
at O
angles O
inside O
the O
hollow O
structure S-CONPRI
. O


While O
the O
optimality O
of O
bone S-BIOP
design O
had O
been O
well O
described O
analytically O
, O
this O
was O
the O
first O
example O
of O
large-scale O
computational B-FEAT
structure I-FEAT
design E-FEAT
: O
the O
rapid O
increase O
in O
computing O
power S-PARA
over O
the O
last O
years O
now O
allows O
for O
obtaining O
detailed O
structures O
from O
simulation-driven S-ENAT
design S-FEAT
tools O
, O
which O
for O
the O
first O
time O
nears O
the O
complexity S-CONPRI
of O
natural O
systems O
. O


In O
conclusion O
, O
the O
complexity S-CONPRI
that O
AM S-MANP
allows O
makes O
it O
possible O
to O
manufacture S-CONPRI
true O
biomimetic S-CONPRI
structures O
, O
yet O
, O
knowledge O
of O
the O
biological B-FEAT
structure E-FEAT
is O
necessary O
. O


3 O
Customized B-FEAT
and I-FEAT
freeform I-FEAT
design E-FEAT
Traditional O
design S-FEAT
for O
engineering S-APPL
involves O
individual O
part O
design S-FEAT
in O
computer-aided B-ENAT
design E-ENAT
tools O
, O
with O
engineering S-APPL
expertise O
and O
intuition O
required O
to O
understand O
the O
limits S-CONPRI
of O
traditional B-MANP
subtractive I-MANP
manufacturing E-MANP
. O


This O
most O
often O
results O
in O
traditional O
designs S-FEAT
with O
right O
angles O
and O
flat O
surfaces S-CONPRI
due O
to O
the O
simplicity O
for O
subtractive B-MANP
manufacturing E-MANP
of O
such O
designs S-FEAT
. O


Over O
the O
last O
few O
years O
, O
advanced O
manufacturing S-MANP
techniques O
have O
become O
available O
and O
viable O
which O
allows O
the O
design S-FEAT
engineer O
more O
freedom O
to O
create O
parts O
with O
more O
complex O
designs S-FEAT
. O


These O
new O
design S-FEAT
capabilities O
allow O
organic O
shapes O
and O
freeform B-FEAT
designs E-FEAT
, O
which O
are O
often O
also O
termed O
biomimetic S-CONPRI
due O
to O
their O
organic O
shapes O
resembling O
natural O
structures O
and O
sometimes O
, O
in O
the O
case O
of O
medical B-APPL
devices E-APPL
in O
particular O
, O
the O
forms O
are O
shaped O
to O
fit S-CONPRI
natural O
materials S-CONPRI
such O
as S-MATE
bone O
implants S-APPL
. O


Natural O
structures O
tend O
to O
comprise O
of O
curves O
and O
organic O
shapes O
as S-MATE
they O
represent O
a O
balance O
between O
minimal O
energy B-PRO
expenditure E-PRO
and O
material S-MATE
used O
on O
the O
one O
hand O
, O
and O
maximal O
return O
of O
work O
on O
the O
other O
hand O
, O
all O
within O
the O
organismdevelopmental O
limits S-CONPRI
. O


Freeform S-CONPRI
and O
custom O
designs S-FEAT
may O
be S-MATE
termed O
biomimetic S-CONPRI
as S-MATE
they O
resemble O
natural O
structures O
in O
these O
aspects O
, O
but O
without O
the O
constraints O
imposed O
by O
the O
organism O
itself O
. O


Despite O
this O
freedom O
of O
design S-FEAT
, O
traditional O
engineering S-APPL
thinking O
is O
often O
limited O
to O
experience O
of O
using O
right O
angles O
and O
flat O
surfaces S-CONPRI
. O


In O
order O
to O
optimally O
use O
this O
new O
design B-CONPRI
freedom E-CONPRI
, O
additional O
tools S-MACEQ
are O
needed O
. O


The O
most O
important O
contributing O
tools S-MACEQ
for O
freeform B-FEAT
design E-FEAT
are O
discussed O
here O
. O


One O
of O
these O
is O
the O
shaping S-MANP
of O
curved O
and O
organic O
surfaces S-CONPRI
by O
the O
use O
of O
T-splines S-FEAT
and O
more O
recently O
polygonal O
non-uniform B-CONPRI
rational I-CONPRI
B-spline E-CONPRI
. O


These O
tools S-MACEQ
allow O
organic O
designs S-FEAT
with O
curved B-CONPRI
surfaces E-CONPRI
that O
often O
resemble O
natural O
structures O
. O


These O
are O
also O
critical O
tools S-MACEQ
in O
final O
steps O
of O
topology B-FEAT
optimization E-FEAT
and O
even O
true O
biomimetic S-CONPRI
reverse O
engineering S-APPL
structures O
, O
ultimately O
allowing O
for O
watertight B-CONPRI
models E-CONPRI
with O
curved O
geometries S-CONPRI
. O


Not O
only O
do O
these O
tools S-MACEQ
make O
custom O
curved O
shapes O
possible O
in O
a O
relevant O
workspace O
, O
but O
they O
are O
also O
effectively O
translated O
into O
geometries S-CONPRI
suitable O
for O
simulation S-ENAT
and/or O
AM S-MANP
. O


In O
terms O
of O
custom O
design S-FEAT
especially O
for O
implants S-APPL
patient-specific O
implants S-APPL
are O
a O
special O
category O
and O
require O
a O
particular O
workflow S-CONPRI
involving O
the O
processing O
of O
medical S-APPL
image B-CONPRI
data E-CONPRI
, O
the O
use O
of O
CAD S-ENAT
tools O
and O
design S-FEAT
for O
AM S-MANP
knowledge O
to O
yield O
a O
good O
resulting O
implant S-APPL
as S-MATE
discussed O
in O
. O


An O
example O
is O
shown O
in O
4 O
where O
a O
patient-specific O
facial O
implant S-APPL
was O
produced O
in O
Ti6Al4V S-MATE
. O


Increases O
in O
computing O
power S-PARA
, O
the O
availability O
of O
cloud B-ENAT
computing E-ENAT
and O
the O
wider O
availability O
of O
CAE S-ENAT
tools O
all O
led S-APPL
to O
the O
sharp O
increase O
in O
advanced O
and O
complex O
design S-FEAT
capability O
. O


One O
of O
the O
first O
examples O
was O
the O
Insight O
Plotfrom O
Solidworks O
, O
which O
demonstrates O
the O
main O
load O
paths O
in O
a O
designed S-FEAT
part O
, O
as S-MATE
calculated O
from O
one O
or O
more O
applied O
loads O
by O
finite B-CHAR
element I-CHAR
modelling E-CHAR
. O


This O
was O
a O
forerunner O
of O
topology B-FEAT
optimization E-FEAT
tools S-MACEQ
which O
will O
be S-MATE
discussed O
in O
more O
detail O
in O
the O
next O
section O
. O


As S-MATE
mentioned O
above O
, O
AM S-MANP
releases O
much O
of O
the O
traditional O
limits S-CONPRI
of O
subtractive B-MANP
manufacturing E-MANP
allowing O
much O
wider O
allowed O
manufacturing S-MANP
complexity S-CONPRI
. O


This O
is O
already O
broadly O
acknowledged O
, O
and O
new O
design B-CONPRI
rules E-CONPRI
for O
reliable O
manufacturing S-MANP
in O
all O
forms O
of O
AM S-MANP
are O
emerging O
and O
in O
many O
cases O
are O
already O
mature O
and O
well-defined O
. O


The O
design S-FEAT
for O
AM S-MANP
rules O
and O
practical O
issues O
are O
discussed O
in O
detail O
in O
and O
more O
recently O
in O
the O
context O
of O
topology B-FEAT
optimization E-FEAT
in O
. O


One O
major O
advantage O
of O
these O
new O
design S-FEAT
tools O
for O
creating O
manually O
organic O
and O
curved B-CONPRI
surfaces E-CONPRI
and O
shapes O
is O
the O
ability O
to O
create O
artistic O
features O
the O
resemblance O
to O
biological/natural S-BIOP
and O
organic B-FEAT
structures E-FEAT
brings O
a O
new O
dimension S-FEAT
to O
artistic O
designs S-FEAT
for O
end-use O
products O
. O


The O
use O
of O
3D B-MANP
printing E-MANP
in O
arts O
, O
fashion S-CONPRI
and O
jewelry O
is O
growing O
as S-MATE
is O
shown O
in O
and O
artistic O
design S-FEAT
is O
easily O
achieved O
by O
AM S-MANP
, O
without O
significantly O
adding O
to O
the O
cost O
of O
the O
product O
. O


Freeform B-FEAT
design E-FEAT
tools O
can O
be S-MATE
used O
to O
shape O
custom-fit O
sportswear O
or O
footwear S-MACEQ
, O
with O
the O
first O
fully-AM O
footwear S-MACEQ
being O
produced O
by O
Adidas O
Futurecraft O
. O


The O
design S-FEAT
of O
this O
shoe O
is O
entirely O
latticed O
giving O
a O
futuristic O
and O
biomimetic S-CONPRI
visual O
appeal O
. O


These O
are O
two O
examples O
of O
mass B-CONPRI
production E-CONPRI
and O
mass O
customization O
by O
AM S-MANP
. O


Aspects O
of O
importance O
besides O
personal/custom O
design S-FEAT
for O
fit S-CONPRI
, O
is O
the O
incorporation O
of O
logos O
or O
names O
, O
and O
the O
ability O
for O
the O
customer O
to O
take O
part O
in O
the O
design B-CONPRI
process E-CONPRI
giving O
them O
some O
options O
making O
their O
product O
unique O
. O


4 O
Simulation-driven S-ENAT
biomimetic B-FEAT
design E-FEAT
One O
of O
the O
first O
drivers O
of O
the O
concepts O
behind O
simulation-driven S-ENAT
design S-FEAT
was O
from O
the O
ideas O
of O
Julius O
Wolff O
, O
the O
19th O
Century O
Orthopedic O
surgeon O
, O
who O
first O
suggested O
that O
, O
a O
consequence O
of O
primary O
shape O
variations S-CONPRI
and O
continuous O
loading O
, O
or O
even O
due O
to O
loading O
alone O
, O
bone S-BIOP
changes O
its O
inner O
architecture S-APPL
according O
to O
mathematical S-CONPRI
rules O
and O
, O
as S-MATE
a O
secondary O
effect O
and O
governed O
by O
the O
same O
mathematical S-CONPRI
rules O
, O
also O
changes O
its O
shape O
. O


The O
concept O
of O
topology B-FEAT
optimization E-FEAT
sprung O
from O
here O
from O
the O
concept O
that O
a O
structure S-CONPRI
can O
be S-MATE
optimized O
by O
following O
load O
paths O
and O
be S-MATE
modified O
to O
fit S-CONPRI
the O
particular O
mechanical S-APPL
requirement O
. O


The O
first O
industrial S-APPL
class O
software S-CONPRI
solutions O
incorporating O
the O
rules O
of O
design S-FEAT
along O
with O
the O
ability O
to O
capture O
the O
along O
with O
the O
constraints O
to O
automatically O
generate O
design S-FEAT
was O
released O
in O
the O
early O
1990This O
was O
primarily O
the O
beginning O
of O
CAE B-ENAT
simulation E-ENAT
driving O
inspirational O
designs S-FEAT
. O


Over O
the O
years O
many O
manufacturing B-CONPRI
constraints E-CONPRI
have O
also O
been O
added O
to O
shape O
these O
designs S-FEAT
to O
be S-MATE
cognizant O
of O
the O
downstream O
manufacturing S-MANP
, O
and O
is O
relevant O
to O
different O
manufacturing B-MANP
processes E-MANP
. O


If O
the O
part O
is O
produced O
by O
an O
extrusion B-MANP
process E-MANP
, O
then O
using O
the O
extrusion S-MANP
constraints O
will O
generate O
a O
shape O
that O
is O
extrudable O
across O
the O
defined O
design B-CONPRI
space E-CONPRI
. O


Likewise O
, O
on O
specifying O
a O
casting S-MANP
constraint O
, O
the O
bionic O
shape O
that O
is O
generated O
will O
be S-MATE
free O
of O
undercuts S-FEAT
for O
easy O
extraction O
from O
the O
casting B-MACEQ
molds E-MACEQ
. O


For O
AM S-MANP
, O
overhang B-PARA
constraints E-PARA
generate O
shapes O
that O
have O
minimal O
support S-APPL
requirement O
during O
build S-PARA
in O
a O
given O
print S-MANP
direction O
, O
with O
less O
horizontal O
sections O
, O
for O
example O
. O


There O
are O
various O
manufacturing B-CONPRI
constraints E-CONPRI
in O
AM S-MANP
that O
can O
be S-MATE
incorporated O
into O
the O
design B-CONPRI
optimization I-CONPRI
process E-CONPRI
and O
it O
is O
the O
incorporation O
of O
these O
into O
the O
topology B-FEAT
optimization I-FEAT
process E-FEAT
which O
will O
create O
designs S-FEAT
ready O
for O
production S-MANP
. O


Shown O
in O
5 O
are O
selected O
examples O
from O
Altair O
covering O
a O
variety O
of O
parts O
that O
illustrate O
the O
power S-PARA
of O
simulation S-ENAT
in O
mimicking O
nature O
for O
product B-FEAT
design E-FEAT
that O
outperform O
conventional O
designs S-FEAT
and O
are O
manufacturable S-CONPRI
and O
lightweight S-CONPRI
. O


The O
first O
example O
is O
the O
HardMarque O
automotive S-APPL
piston O
which O
was O
designed S-FEAT
and O
optimized O
for O
production S-MANP
by O
additive B-MANP
manufacturing E-MANP
in O
titanium S-MATE
the O
end O
result O
is O
reported O
to O
be S-MATE
25 O
% O
lighter O
and O
equally O
strong O
compared O
to O
the O
original O
aluminium S-MATE
part O
. O


The O
second O
example O
from O
Renishaw O
is O
a O
seat O
post O
bracket S-MACEQ
of O
a O
mountain O
bike O
, O
meant O
to O
replace O
a O
cast S-MANP
aluminium S-MATE
part O
with O
additively B-MANP
manufactured E-MANP
titanium O
: O
the O
mass O
reduction S-CONPRI
was O
reported O
at O
40 O
% O
. O


The O
third O
example O
is O
a O
case B-CONPRI
study E-CONPRI
from O
the O
aerospace B-APPL
industry E-APPL
, O
in O
particular O
the O
optimization S-CONPRI
of O
a O
mechanical S-APPL
hinge O
for O
an O
Airbus B-APPL
A320 E-APPL
by O
the O
European O
Aeronautic O
Defence O
and O
Space O
Innovation O
Works O
in O
this O
case O
a O
75 O
% O
mass O
reduction S-CONPRI
was O
realized O
. O


The O
last O
example O
is O
a O
research S-CONPRI
project O
with O
Laser S-ENAT
Zentrum O
Nord O
focusing O
on O
lightweighting S-PRO
of O
aircraft O
cabin O
brackets O
. O


Simulation-driven S-ENAT
design S-FEAT
in O
the O
context O
of O
AM S-MANP
refers O
to O
the O
use O
of O
simulation S-ENAT
to O
numerically O
and O
a O
given O
space O
to O
meet O
some O
desired O
performance S-CONPRI
criteria O
under O
a O
defined O
set S-APPL
of O
constraints O
. O


This O
currently O
refers O
to O
either O
of O
topology B-FEAT
optimization E-FEAT
or O
generative B-ENAT
design E-ENAT
, O
which O
can O
often O
be S-MATE
used O
interchangeably O
in O
the O
context O
of O
AM S-MANP
both O
involve O
the O
use O
of O
simulation S-ENAT
. O


Topology B-FEAT
optimization E-FEAT
refers O
to O
optimizing O
an O
existing O
shape O
or O
design B-CONPRI
space E-CONPRI
. O


Generative B-ENAT
design E-ENAT
is O
a O
broader O
definition O
of O
exploring O
a O
variety O
of O
possible O
designs S-FEAT
within O
a O
given O
space O
with O
a O
desire O
to O
identify O
an O
optimal O
solution S-CONPRI
from O
various O
possible O
solutions O
meeting O
the O
same O
performance S-CONPRI
criteria O
. O


In O
the O
context O
of O
design S-FEAT
for O
AM S-MANP
, O
both O
the O
approaches O
are O
aimed O
at O
creating O
light-weight S-PRO
parts O
which O
mostly O
contain O
material S-MATE
in O
areas S-PARA
were O
load O
is O
experienced O
and O
material S-MATE
is O
removed O
in O
areas S-PARA
which O
do O
not O
require O
it O
. O


This O
process S-CONPRI
of O
simulation S-ENAT
and O
material-removal O
or O
addition O
is O
repeated O
iteratively O
until O
an O
optimization S-CONPRI
goal O
is O
achieved O
, O
and O
this O
iterative O
process S-CONPRI
may O
be S-MATE
seen O
analogous O
to O
the O
process S-CONPRI
of O
evolution S-CONPRI
. O


In O
fact O
, O
these O
simulations S-ENAT
sometimes O
make O
use O
of O
genetic B-CONPRI
and I-CONPRI
evolutionary I-CONPRI
algorithms E-CONPRI
. O


Effectively O
, O
these O
algorithms S-CONPRI
incorporate O
rules O
like O
in O
nature O
to O
mathematically O
disallow O
weaklings O
to O
proliferate O
, O
but O
in O
an O
accelerated O
fashion S-CONPRI
using O
clever B-ENAT
computational I-ENAT
methods E-ENAT
. O


More O
recently O
this O
was O
also O
described O
in O
terms O
of O
manufacturing S-MANP
challenges O
in O
and O
in O
terms O
of O
available O
software S-CONPRI
tools O
and O
their O
differences O
and O
limits S-CONPRI
in O
. O


A O
good O
example O
of O
topology B-FEAT
optimization E-FEAT
, O
applied O
to O
an O
extreme O
lightweighting S-PRO
requirement O
is O
the O
design S-FEAT
of O
a O
titanium B-MATE
alloy E-MATE
drone O
frame O
, O
with O
larger O
dimensions S-FEAT
than O
can O
be S-MATE
produced O
on O
typical O
powder B-MANP
bed I-MANP
fusion E-MANP
systems O
. O


This O
was O
produced O
on O
the O
large-scale O
laser B-MACEQ
powder I-MACEQ
bed I-MACEQ
fusion I-MACEQ
system E-MACEQ
called O
Aeroswift O
and O
the O
design S-FEAT
done O
in O
collaboration O
with O
Altair O
. O


The O
design S-FEAT
iteration O
process S-CONPRI
is O
shown O
in O
6 O
, O
done O
in O
Altair O
Inspire O
. O


Another O
example O
of O
a O
topology S-CONPRI
optimized O
part O
a O
load O
bearing B-APPL
bracket E-APPL
is O
shown O
in O
7 O
, O
which O
is O
taken O
from O
. O


This O
titanium B-MATE
alloy E-MATE
bracket S-MACEQ
was O
designed S-FEAT
to O
replace O
a O
traditional O
composite S-MATE
bracket S-MACEQ
in O
an O
experimental S-CONPRI
vehicle O
for O
the O
Shell S-MACEQ
eco-challenge O
. O


The O
design B-CONPRI
process E-CONPRI
schematic O
here O
shows O
the O
original O
composite S-MATE
part O
, O
the O
design B-CONPRI
space E-CONPRI
, O
the O
optimized O
solution S-CONPRI
and O
the O
final O
smoothed B-CONPRI
solution E-CONPRI
, O
after O
application O
of O
connections O
and O
polyNURBS S-ENAT
to O
the O
surfaces S-CONPRI
. O


This O
part O
was O
also O
used O
in O
a O
round B-CHAR
robin I-CHAR
test E-CHAR
whereby O
the O
same O
bracket S-MACEQ
was O
produced O
at O
various O
commercial O
laser B-MACEQ
powder I-MACEQ
bed I-MACEQ
fusion I-MACEQ
systems E-MACEQ
and O
detailed O
analysis O
performed O
using O
microCT S-CHAR
. O


The O
study O
highlighted O
the O
need O
for O
testing S-CHAR
AM B-MACEQ
parts E-MACEQ
to O
ensure O
structural B-PRO
integrity E-PRO
. O


Another O
example O
is O
the O
design S-FEAT
for O
a O
large O
bracket S-MACEQ
for O
the O
same O
vehicle O
related O
to O
the O
above-mentioned O
example O
. O


8 O
shows O
the O
optimized O
topology S-CONPRI
itself O
which O
is O
also O
latticed O
: O
this O
is O
a O
sequential O
process S-CONPRI
in O
most O
software S-CONPRI
packages O
and O
the O
area S-PARA
to O
be S-MATE
latticed O
and O
the O
lattice S-CONPRI
parameters O
are O
selected O
by O
the O
user O
. O


Latticing O
will O
be S-MATE
discussed O
in O
the O
next O
section O
and O
holds O
many O
advantages O
but O
must O
be S-MATE
carefully O
implemented O
in O
a O
design S-FEAT
, O
due O
to O
issues O
such O
as S-MATE
requirement O
for O
supports S-APPL
inside O
the O
lattice S-CONPRI
region O
, O
and O
struts S-MACEQ
which O
are O
potentially O
too O
thin O
. O


The O
first O
is O
the O
Bugatti O
brake O
caliper S-MACEQ
which O
is O
shown O
in O
9 O
, O
and O
which O
is O
currently O
the O
worldlargest O
functional O
part O
produced O
in O
titanium S-MATE
by O
AM S-MANP
. O


In O
this O
case O
the O
use O
of O
Ti6Al4V S-MATE
titanium O
alloy S-MATE
is O
especially O
useful O
for O
light-weighting O
, O
as S-MATE
this O
material S-MATE
is O
already O
strong O
and O
relatively O
light O
. O


Its O
use O
for O
automotive S-APPL
and O
aerospace S-APPL
applications O
is O
well O
known O
, O
but O
manufacturing S-MANP
complex O
designs S-FEAT
by O
traditional B-MANP
manufacturing E-MANP
methods O
in O
this O
material S-MATE
is O
extremely O
challenging O
. O


The O
topology B-FEAT
optimization E-FEAT
result O
is O
visually O
impressive O
, O
the O
performance S-CONPRI
of O
this O
caliper S-MACEQ
has O
been O
validated O
in O
various O
tests O
and O
is O
used O
in O
production S-MANP
vehicles O
, O
with O
a O
40 O
% O
mass O
reduction S-CONPRI
compared O
to O
the O
previous O
version O
made O
of O
aluminium S-MATE
. O


5 O
Cellular O
and O
lattice B-FEAT
design E-FEAT
Cellular B-FEAT
structures E-FEAT
exist O
in O
nature O
in O
numerous O
shapes O
, O
sizes O
and O
packing O
arrangements O
some O
of O
the O
most O
well-known O
examples O
are O
the O
beehoneycomb S-FEAT
, O
wood B-MATE
cells E-MATE
and O
spongy B-BIOP
bone E-BIOP
, O
all O
of O
which O
are O
discussed O
in O
a O
book O
by O
Gibson O
. O


MicroCT S-CHAR
scans O
of O
some O
natural O
cellular B-MATE
materials E-MATE
are O
shown O
in O
11 O
. O


In O
fact O
, O
one O
of O
the O
first O
true O
observations O
of O
cellular B-FEAT
structures E-FEAT
in O
nature O
can O
be S-MATE
traced O
back O
to O
1665 O
, O
when O
Robert O
Hooke O
published O
his O
observation O
of O
the O
cellularity O
in O
cork S-MATE
and O
suggested O
that O
the O
unique O
behavior O
of O
cork S-MATE
was O
attributable O
to O
its O
underlying O
cellular B-FEAT
structure E-FEAT
. O


Humans O
have O
been O
using O
cellular B-MATE
materials E-MATE
such O
as S-MATE
wood O
, O
cork S-MATE
and O
bamboo O
, O
several O
millennia O
before O
we O
realized O
the O
underlying O
structural O
basis O
for O
their O
interesting O
behaviour O
. O


Lattices S-CONPRI
today O
owe O
much O
of O
their O
origins O
and O
design S-FEAT
selection O
to O
mathematics O
and O
crystallography S-MANP
, O
as S-MATE
well O
as S-MATE
following O
Maxwellstability O
criterion O
, O
which O
was O
primarily O
developed O
in O
the O
context O
of O
large O
engineering S-APPL
structures O
. O


The O
main O
utility O
of O
cellular O
or O
lattice B-FEAT
structures E-FEAT
lies O
in O
their O
ability O
to O
meet O
performance S-CONPRI
targets O
while O
enabling O
significant O
mass O
reduction S-CONPRI
, O
something O
that O
is O
a O
principle O
commonly O
embodied O
in O
nature O
. O


While O
cellular B-MATE
materials E-MATE
do O
tend O
to O
have O
lower O
effective B-CHAR
material I-CHAR
stiffness E-CHAR
and O
strength B-PRO
properties E-PRO
, O
this O
reduction S-CONPRI
is O
often O
acceptable O
and O
can O
be S-MATE
tailored O
to O
the O
application O
, O
as S-MATE
well O
as S-MATE
varied O
locally O
. O


Lattices S-CONPRI
may O
also O
be S-MATE
useful O
for O
other O
purposes O
besides O
light-weighting O
: O
they O
have O
interesting O
thermal O
, O
acoustic O
properties S-CONPRI
and O
energy O
absorbing O
properties S-CONPRI
under O
compressive B-PRO
loading E-PRO
they O
perform O
a O
crucial O
protection O
role O
in O
nature O
. O


Cellular B-MATE
materials E-MATE
have O
also O
been O
seen O
as S-MATE
a O
crucial O
enabler O
for O
large O
system-level O
multi-functional B-CONPRI
design I-CONPRI
optimization E-CONPRI
, O
such O
as S-MATE
in O
an O
aircraft B-APPL
wing E-APPL
. O


The O
categorization O
of O
natural O
cellular B-FEAT
structures E-FEAT
is O
discussed O
in O
more O
detail O
in O
a O
recent O
review O
article O
which O
focuses O
on O
biomimetic B-FEAT
design E-FEAT
of O
cellular B-MATE
materials E-MATE
utilizing O
cellular B-FEAT
designs E-FEAT
in O
engineering S-APPL
systems O
. O


Perhaps O
the O
most O
commonly O
used O
, O
and O
well-known O
bio-inspired S-CONPRI
cellular O
material S-MATE
is O
the O
honeycomb S-CONPRI
, O
which O
has O
found O
a O
wide O
range S-PARA
of O
applications O
in O
architecture S-APPL
, O
transportation O
, O
chemical O
engineering S-APPL
and O
more O
, O
as S-MATE
compiled O
in O
a O
review O
article O
. O


With O
regard O
to O
additively B-MANP
manufactured E-MANP
cellular O
materials S-CONPRI
, O
the O
emphasis O
in O
the O
past O
decade O
has O
been O
on O
lattice B-FEAT
structures E-FEAT
, O
and O
their O
use O
for O
medical S-APPL
bone-replacement O
implant S-APPL
applications O
. O


In O
this O
application O
, O
the O
primary O
role O
of O
the O
lattice S-CONPRI
is O
to O
allow O
for O
osseointegration S-PRO
of O
bone S-BIOP
into O
the O
implant S-APPL
, O
thereby O
causing O
better O
fixation O
. O


A O
recent O
book O
chapter O
describes O
the O
most O
important O
criteria O
for O
bone B-CONPRI
regeneration E-CONPRI
in O
titanium B-APPL
implants E-APPL
produced O
by O
powder B-MANP
bed I-MANP
fusion E-MANP
and O
the O
production S-MANP
of O
topologically S-CONPRI
designed S-FEAT
and O
otherwise O
designed S-FEAT
porous O
lattices S-CONPRI
for O
this O
application O
was O
also O
reviewed O
in O
. O


From O
an O
engineering S-APPL
standpoint O
, O
cellular B-MATE
materials E-MATE
are O
realized O
practically O
in O
commercial O
software S-CONPRI
packages O
using O
different O
approaches O
. O


Traditional O
CAD S-ENAT
software O
uses O
mesh-based O
representation O
, O
but O
recent O
developments O
in O
software S-CONPRI
are O
exploring O
the O
use O
of O
volumetric O
object O
representation O
to O
generate O
surfaces S-CONPRI
, O
and O
in O
at O
least O
one O
case O
, O
the O
use O
of O
implicit O
modeling S-ENAT
via O
the O
definition O
of O
fields O
that O
then O
generate O
cellular B-FEAT
structures E-FEAT
. O


Mesh-based O
approaches O
can O
generate O
visually O
impressive O
lattices S-CONPRI
which O
conform O
well O
to O
the O
original O
surface S-CONPRI
design S-FEAT
, O
and O
is O
relatively O
easily O
implemented O
for O
complex O
part O
geometries S-CONPRI
. O


The O
volumetric O
object O
representation O
approach O
allows O
for O
the O
user O
to O
select O
a O
unit B-CONPRI
cell E-CONPRI
from O
a O
wider O
variety O
of O
cellular B-FEAT
designs E-FEAT
. O


The O
repeated O
unit B-CONPRI
cell E-CONPRI
approach O
also O
allows O
relatively O
easy O
prediction S-CONPRI
of O
mechanical B-CONPRI
properties E-CONPRI
of O
the O
structure S-CONPRI
, O
easing O
the O
design B-CONPRI
process E-CONPRI
. O


A O
series O
of O
unit B-CONPRI
cells E-CONPRI
and O
corresponding O
repeated O
lattice B-FEAT
structures E-FEAT
are O
shown O
in O
12 O
. O


These O
are O
all O
designed S-FEAT
with O
the O
same O
total O
density S-PRO
, O
but O
the O
different O
designs S-FEAT
result O
in O
different O
minimum B-FEAT
feature I-FEAT
thickness E-FEAT
and O
pore B-PARA
sizes E-PARA
. O


The O
first O
four O
are O
strut-based S-FEAT
and O
the O
next O
four O
are O
minimal O
surface S-CONPRI
designs S-FEAT
. O


The O
latter O
are O
found O
in O
nature O
, O
and O
have O
been O
shown O
to O
have O
good O
properties S-CONPRI
for O
bone B-APPL
implant I-APPL
applications E-APPL
. O


These O
minimal O
surfaces S-CONPRI
are O
sheet-based O
designs S-FEAT
which O
are O
often O
self-supporting S-FEAT
and O
tend O
to O
have O
zero B-FEAT
average I-FEAT
curvature E-FEAT
at O
every O
point O
on O
the O
surface S-CONPRI
, O
which O
makes O
for O
a O
more O
even O
distribution S-CONPRI
of O
stresses O
within O
these O
structures O
. O


Despite O
the O
growing O
prevalence O
of O
design S-FEAT
software O
capable O
of O
generating O
cellular B-FEAT
structure E-FEAT
designs O
, O
it O
is O
not O
always O
apparent O
what O
the O
best O
unit B-CONPRI
cell E-CONPRI
for O
a O
specific O
application O
is O
and O
this O
becomes O
even O
more O
challenging O
in O
the O
context O
of O
multi-functional O
design S-FEAT
. O


It O
is O
in O
such O
a O
context O
that O
biomimetic B-FEAT
design E-FEAT
can O
play O
a O
key O
role O
, O
in O
helping O
develop O
structure-function O
relationships O
based O
on O
observations O
of O
cellular B-MATE
materials E-MATE
in O
nature O
, O
and O
using O
these O
to O
guide O
selection O
of O
cellular B-MATE
materials E-MATE
. O


Natural O
cellular B-MATE
materials E-MATE
span O
the O
range S-PARA
of O
parameter S-CONPRI
space O
used O
in O
design S-FEAT
, O
from O
beam S-MACEQ
or O
strut-based S-FEAT
materials S-CONPRI
to O
surface S-CONPRI
based O
ones O
, O
including O
structures O
that O
combine O
both O
types O
, O
as S-MATE
shown O
in O
13 O
. O


These O
cellular B-MATE
materials E-MATE
occur O
in O
nature O
both O
internal O
to O
a O
form O
, O
as S-MATE
well O
as S-MATE
externally O
on O
the O
surface S-CONPRI
. O


The O
main O
application O
of O
lattice B-FEAT
structures E-FEAT
, O
which O
has O
resulted O
in O
considerable O
research S-CONPRI
efforts O
, O
is O
their O
use O
in O
medical B-APPL
implants E-APPL
. O


For O
this O
application O
the O
pore B-PARA
sizes E-PARA
required O
are O
typically O
small O
, O
requiring O
small O
feature B-PARA
sizes E-PARA
in O
general O
. O


Other O
applications O
than O
medical S-APPL
, O
such O
as S-MATE
in O
light-weight S-PRO
structures O
for O
aerospace S-APPL
or O
automotive S-APPL
parts O
, O
might O
prefer O
thicker O
lattices S-CONPRI
to O
focus O
on O
mechanical B-CONPRI
reliability E-CONPRI
and O
strength S-PRO
. O


Experimental S-CONPRI
work O
with O
lattices S-CONPRI
with O
thick O
struts S-MACEQ
show O
excellent O
strength B-PRO
properties E-PRO
as S-MATE
shown O
in O
for O
50 O
% O
density S-PRO
Ti6Al4V O
lattices S-CONPRI
of O
two O
strut-based B-FEAT
designs E-FEAT
. O


Simple S-MANP
strut-based O
lattice B-FEAT
designs E-FEAT
can O
be S-MATE
classified O
according O
to O
the O
Maxwell B-CONPRI
criterion E-CONPRI
as S-MATE
either O
bending-dominated O
or O
stretch-dominated O
as S-MATE
illustrated O
schematically O
and O
by O
a O
few O
examples O
in O
14 O
. O


The O
Maxwell B-CONPRI
criterion E-CONPRI
for O
simple S-MANP
strut-based O
3D B-CONPRI
structures E-CONPRI
is O
: O
M O
= O
b S-MATE
3j O
+ O
6 O
Where O
b S-MATE
= O
the O
number O
of O
struts S-MACEQ
, O
and O
j O
= O
the O
number O
of O
joints O
When O
M O
< O
0 O
the O
structure S-CONPRI
is O
bending-dominated O
When O
M O
0 O
the O
structure S-CONPRI
is O
stretch-dominated O
and O
When O
M O
> O
0 O
the O
structure S-CONPRI
is O
over-rigid O
Bending-dominated O
refers O
to O
the O
struts S-MACEQ
which O
tend O
to O
bend O
under O
compression S-PRO
of O
the O
lattice S-CONPRI
resulting O
in O
shear B-PRO
failure E-PRO
, O
while O
stretch-dominated B-PRO
structures E-PRO
are O
stiffer O
and O
fail O
in O
a O
layer-by-layer S-CONPRI
mechanism O
. O


These O
failure B-PRO
modes E-PRO
have O
been O
observed O
in O
relatively O
thick-strut O
lattices S-CONPRI
and O
imaged O
by O
microCT S-CHAR
in O
their O
initial O
failure S-CONPRI
locations O
. O


The O
mechanical B-CONPRI
response E-CONPRI
of O
lattice B-FEAT
structures E-FEAT
in O
general O
follows O
a O
linear O
elastic S-PRO
response O
up O
to O
the O
first B-PRO
point I-PRO
of I-PRO
buckling E-PRO
or O
failure S-CONPRI
, O
followed O
by O
a O
plateau O
region O
, O
followed O
by O
final O
densification S-MANP
. O


This O
is O
shown O
in O
the O
example O
in O
15 O
, O
which O
also O
shows O
why O
cellular B-MATE
materials E-MATE
are O
useful O
for O
energy B-CHAR
absorption E-CHAR
as S-MATE
they O
can O
handle O
significant O
yielding O
without O
catastrophic O
failure S-CONPRI
, O
under O
most O
circumstances O
. O


A O
lattice B-FEAT
structure E-FEAT
can O
be S-MATE
approximated O
as S-MATE
an O
open-cell O
foam S-MATE
, O
with O
effective O
elastic B-PRO
modulus E-PRO
E O
of O
the O
lattice S-CONPRI
related O
to O
the O
density S-PRO
of O
the O
structure S-CONPRI
and O
the O
elastic B-PRO
modulus E-PRO
of O
the O
bulk O
material S-MATE
- O
solid O
as S-MATE
follows O
: O
E O
= O
2 O
E O
s S-MATE
o S-MATE
l O
i O
d O
s S-MATE
o S-MATE
l O
i O
d O
2 O
In O
this O
relationship O
, O
the O
constant O
depends O
on O
the O
manufacturing S-MANP
accuracy S-CHAR
and O
material B-CONPRI
properties E-CONPRI
and O
varies O
between O
0.1 O
and O
4 O
but O
is O
a O
constant O
for O
a O
specific B-MATE
material E-MATE
and O
process S-CONPRI
. O


What O
this O
relationship O
shows O
is O
that O
the O
effective O
elastic B-PRO
modulus E-PRO
can O
be S-MATE
controlled O
by O
the O
density S-PRO
alone O
this O
means O
that O
a O
lattice S-CONPRI
with O
unit B-CONPRI
cell E-CONPRI
design S-FEAT
of O
50 O
% O
density S-PRO
may O
use O
any O
unit B-CONPRI
cell E-CONPRI
size O
as S-MATE
long O
as S-MATE
the O
total O
space O
filled O
contains O
at O
least O
six O
unit B-CONPRI
cells E-CONPRI
in O
each O
direction O
then O
the O
material B-FEAT
stiffness E-FEAT
will O
be S-MATE
the O
same O
. O


This O
means O
lattices S-CONPRI
with O
many O
thin O
struts S-MACEQ
might O
perform O
the O
same O
as S-MATE
lattices O
with O
less O
thick O
struts S-MACEQ
, O
an O
interesting O
design S-FEAT
aspect O
which O
can O
be S-MATE
varied O
by O
application O
requirement O
. O


It O
is O
also O
important O
to O
note O
that O
the O
exponent O
refers O
to O
ideal O
bending-dominated O
lattice S-CONPRI
while O
an O
ideal O
stretch-dominated O
lattice S-CONPRI
has O
exponent O
This O
is O
illustrated O
in O
16 O
, O
for O
a O
range S-PARA
of O
lattice S-CONPRI
types O
clearly O
this O
exponent O
may O
vary O
somewhat O
depending O
on O
the O
lattice B-FEAT
design E-FEAT
selected O
. O


Besides O
the O
relationships O
mentioned O
above O
, O
lattice B-FEAT
designs E-FEAT
must O
also O
be S-MATE
considered O
relative O
to O
manufacturing S-MANP
limits S-CONPRI
. O


For O
example O
, O
sheet-based O
designs S-FEAT
can O
typically O
print S-MANP
without O
supports S-APPL
, O
and O
strut-based B-FEAT
designs E-FEAT
can O
print S-MANP
without O
supports S-APPL
up O
to O
a O
certain B-FEAT
strut I-FEAT
length E-FEAT
for O
horizontal B-FEAT
struts E-FEAT
. O


Therefore O
, O
manufacturing B-CONPRI
constraints E-CONPRI
are O
imposed O
on O
the O
design S-FEAT
possibilities O
. O


The O
most O
important O
limits S-CONPRI
are O
the O
minimum B-PARA
feature I-PARA
size E-PARA
, O
which O
, O
in O
practice O
, O
is O
limited O
not O
only O
by O
the O
powder S-MATE
size O
and O
laser B-PARA
spot I-PARA
size E-PARA
, O
but O
also O
by O
the O
3D B-FEAT
model I-FEAT
slicing E-FEAT
accuracy O
and O
the O
resulting O
hatch O
and O
contour B-PARA
scanning E-PARA
employed O
. O


For O
example O
, O
in O
a O
recent O
study O
of O
thin-strut O
lattices S-CONPRI
, O
the O
standard S-CONPRI
processing O
parameters S-CONPRI
resulted O
in O
the O
inability O
to O
produce O
struts S-MACEQ
varying O
gradually O
from O
0.2 O
to O
0.4 O
mm S-MANP
. O


Here O
, O
different O
designs S-FEAT
were O
produced O
with O
approximately O
the O
same O
strut S-MACEQ
dimensions S-FEAT
despite O
differences O
in O
design S-FEAT
. O


These O
thin-strut O
lattices S-CONPRI
also O
have O
relatively O
large O
surface B-PRO
roughness E-PRO
values O
compared O
to O
the O
strut B-PARA
thickness E-PARA
, O
which O
understandably O
affects O
the O
mechanical B-CONPRI
properties E-CONPRI
more O
than O
would O
be S-MATE
expected O
for O
a O
thicker-strut O
version O
. O


In O
this O
above-mentioned O
study O
the O
experimental S-CONPRI
elastic B-PRO
modulus E-PRO
values O
were O
significantly O
lower O
than O
predicted S-CONPRI
mostly O
attributed O
to O
surface B-PRO
roughness E-PRO
and O
irregularity O
which O
creates O
stress B-CHAR
concentrations E-CHAR
in O
notches S-FEAT
and O
in O
locations O
of O
very O
thin O
wall B-FEAT
thickness E-FEAT
. O


Effectively O
for O
a O
metal S-MATE
laser B-MACEQ
powder I-MACEQ
bed I-MACEQ
fusion I-MACEQ
system E-MACEQ
with O
about O
100 O
spot B-PARA
size E-PARA
, O
the O
minimum O
reliable O
wall B-FEAT
thickness E-FEAT
should O
be S-MATE
0.3-0.4 O
mm S-MANP
if O
no O
special O
precautions O
or O
optimization S-CONPRI
for O
strut S-MACEQ
manufacturing S-MANP
is O
done O
to O
enhance O
the O
manufacturability S-CONPRI
. O


The O
next O
section O
discusses O
material B-CONPRI
properties E-CONPRI
and O
will O
specifically O
mention O
limits S-CONPRI
with O
regards O
to O
lattice S-CONPRI
manufacturability O
. O


6 O
Material B-CONPRI
properties E-CONPRI
of O
AM S-MANP
biomimetic O
parts O
Biomimetic-designed O
and O
produced O
parts O
are O
visually O
so O
vastly O
different O
from O
traditional O
manufactured S-CONPRI
parts O
, O
that O
it O
causes O
mistrust O
and O
resistance S-PRO
to O
acceptance O
of O
this O
new O
technology S-CONPRI
, O
especially O
by O
engineers O
. O


In O
some O
ways O
this O
is O
to O
be S-MATE
expected O
, O
as S-MATE
AM S-MANP
has O
a O
history O
of O
over-hype O
and O
under-delivery O
in O
the O
past O
. O


In O
the O
qualification O
process S-CONPRI
, O
mechanical B-CONPRI
properties E-CONPRI
of O
the O
optimized O
process S-CONPRI
can O
be S-MATE
tested O
and O
validated O
as S-MATE
demonstrated O
for O
Ti6Al4V S-MATE
in O
. O


In O
order O
to O
obtain O
defect-free O
and O
accurately S-CHAR
produced O
parts O
, O
X-ray B-CHAR
tomography E-CHAR
can O
be S-MATE
used O
as S-MATE
outlined O
in O
. O


The O
specific O
process B-CONPRI
parameters E-CONPRI
which O
combine O
to O
create O
an O
object O
in O
AM S-MANP
all O
have O
an O
influence O
on O
the O
subsequent O
material B-CONPRI
properties E-CONPRI
and O
the O
manufacturing B-MANP
process E-MANP
of O
the O
object O
as S-MATE
a O
whole O
. O


This O
is O
true O
not O
only O
for O
fully B-PARA
dense E-PARA
objects O
, O
but O
also O
for O
complex O
or O
lattice B-FEAT
design E-FEAT
with O
biomimetic B-FEAT
features E-FEAT
such O
as S-MATE
custom O
or O
complex B-PRO
shapes E-PRO
, O
inner O
structures O
or O
surface B-MANP
modifications E-MANP
. O


In O
this O
case O
, O
material B-CONPRI
properties E-CONPRI
and O
the O
properties S-CONPRI
of O
i.e O
. O


single O
building O
blocks O
and O
joints O
between O
them O
also O
influence O
the O
properties S-CONPRI
of O
the O
LPBF S-MANP
object O
. O


Defects S-CONPRI
and O
flaws S-CONPRI
such O
as S-MATE
porosity O
occurs O
in O
the O
LPBF S-MANP
process O
due O
to O
various O
reasons O
and O
this O
can O
influence O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
the O
final O
parts O
. O


There O
are O
many O
process B-CONPRI
parameters E-CONPRI
the O
laser B-PARA
power E-PARA
, O
laser B-PARA
spot I-PARA
size E-PARA
and O
scanning B-PARA
speed E-PARA
, O
hatch B-PARA
distance E-PARA
, O
material B-CONPRI
properties E-CONPRI
, O
powder B-MATE
particle E-MATE
size O
distribution S-CONPRI
and O
powder S-MATE
layer B-PARA
thickness E-PARA
, O
the O
strategy O
, O
design S-FEAT
and O
orientation S-CONPRI
of O
the O
3D B-APPL
part E-APPL
and O
its O
supports S-APPL
, O
the O
scanning S-CONPRI
and O
building O
strategy O
, O
etc O
. O


which O
all O
may O
influence O
the O
molten B-CONPRI
pool E-CONPRI
size O
, O
further O
solidification S-CONPRI
, O
microstructural S-CONPRI
grain B-CONPRI
growth E-CONPRI
and O
eventually O
the O
mechanical B-CONPRI
properties E-CONPRI
, O
lifetime O
and O
performance S-CONPRI
of O
LPBF S-MANP
parts O
. O


The O
details O
of O
the O
AM B-MANP
process E-MANP
are O
discussed O
in O
the O
comprehensive O
review O
paper O
. O


It O
is O
already O
well O
known O
that O
variation S-CONPRI
of O
process B-CONPRI
parameters E-CONPRI
may O
influence O
the O
formation O
of O
porosity S-PRO
and O
may O
lead S-MATE
to O
extensive O
flaws S-CONPRI
and O
build S-PARA
imperfections O
, O
as S-MATE
is O
shown O
for O
example O
in O
a O
round B-CHAR
robin I-CHAR
test E-CHAR
recently O
. O


This O
highlights O
the O
need O
for O
process B-CONPRI
optimization E-CONPRI
. O


Other O
properties S-CONPRI
such O
as S-MATE
corrosion O
are O
also O
strongly O
affected O
by O
processing O
conditions O
and O
are O
important O
for O
biomimetic B-APPL
applications E-APPL
, O
especially O
medical B-APPL
applications E-APPL
. O


For O
example O
, O
it O
was O
shown O
that O
a O
higher O
corrosion B-CONPRI
resistance E-CONPRI
of O
Co-Cr B-MATE
dental I-MATE
alloy E-MATE
was O
obtained O
by O
Selective B-MANP
Laser I-MANP
Melting E-MANP
in O
comparison O
with O
the O
Selective B-MANP
Laser I-MANP
Sintering I-MANP
process E-MANP
, O
due O
to O
a O
passive O
oxide S-MATE
protecting O
layer S-PARA
which O
formed O
on O
the O
surface S-CONPRI
of O
the O
SLM S-MANP
sample S-CONPRI
. O


Takaichi O
found O
that O
metal S-MATE
elution O
from O
the O
LPBF S-MANP
dental S-APPL
implants O
was O
smaller O
than O
that O
of O
the O
as-cast O
Co-Cr O
alloy S-MATE
. O


Thus O
, O
it O
could O
be S-MATE
said O
that O
LPBF B-MATE
materials E-MATE
have O
superior O
corrosion B-PRO
properties E-PRO
. O


However O
, O
process-parameters O
can O
influence O
the O
corrosion B-PRO
behavior E-PRO
of O
samples S-CONPRI
produced O
with O
different O
process-parameters O
. O


It O
is O
already O
known O
that O
the O
level O
of O
microporosity S-PRO
affects O
the O
corrosion B-PRO
behavior E-PRO
as S-MATE
shown O
in O
. O


Micro-segregation S-CONPRI
of O
elements S-MATE
under O
specific O
LPBF S-MANP
process-parameters O
can O
occur O
causing O
different O
corrosion B-PRO
behavior E-PRO
at O
materials S-CONPRI
processed O
under O
different O
parameters S-CONPRI
. O


Since O
melt B-CONPRI
pool I-CONPRI
boundaries E-CONPRI
may O
differ O
in O
corrosion B-CONPRI
resistance E-CONPRI
compared O
to O
the O
center O
of O
the O
meltpool S-CHAR
, O
more O
melt B-CONPRI
pool I-CONPRI
boundaries E-CONPRI
imply O
different O
corrosion B-CONPRI
resistance E-CONPRI
of O
LPBF B-MATE
material E-MATE
. O


These O
statements O
have O
to O
be S-MATE
taken O
into O
account O
especially O
for O
smart O
AM S-MANP
advanced O
biodegradable O
implants S-APPL
that O
should O
degrade O
with O
spatial O
and O
temporal O
controllability O
to O
meet O
the O
requirements O
of O
different O
bone B-CONPRI
regeneration E-CONPRI
stages O
. O


LPBF S-MANP
samples O
have O
varying O
surface B-PRO
roughness E-PRO
on O
side O
, O
top O
and O
bottom O
surfaces S-CONPRI
. O


Attached O
powder B-MATE
particles E-MATE
can O
be S-MATE
eliminated O
by O
post-process S-CONPRI
mechanical S-APPL
or O
chemical O
procedures O
. O


However O
, O
for O
LPBF S-MANP
parts O
with O
complex B-PRO
shapes E-PRO
and O
fine O
features O
or O
lattice B-FEAT
structures E-FEAT
, O
full O
powder S-MATE
evacuation O
and O
targeted O
accuracy S-CHAR
and O
roughness B-PRO
values E-PRO
can O
be S-MATE
quite O
difficult O
to O
obtain O
. O


The O
surface B-PRO
roughness E-PRO
is O
dependent O
on O
the O
building O
and O
scanning B-CONPRI
strategy E-CONPRI
, O
material B-CONPRI
properties E-CONPRI
, O
powder S-MATE
size O
, O
layer B-PARA
thickness E-PARA
, O
etc O
. O


This O
can O
influence O
not O
only O
the O
mechanical B-CONPRI
properties E-CONPRI
but O
also O
the O
biological O
response O
of O
bone B-BIOP
cells E-BIOP
or O
soft O
tissues O
when O
such O
an O
object O
is O
implanted S-MANP
. O


Moreover O
, O
there O
is O
currently O
no O
general O
approach O
and O
agreement O
about O
preferred O
roughness B-PRO
values E-PRO
or O
surface S-CONPRI
micron-scale S-FEAT
features O
and O
pore B-PARA
size E-PARA
for O
effective O
bone B-BIOP
cell E-BIOP
growth O
and O
functioning O
of O
implants S-APPL
. O


For O
lattice B-FEAT
structures E-FEAT
, O
the O
geometrical O
characteristics O
of O
unit B-CONPRI
cells E-CONPRI
, O
the O
building B-PARA
direction E-PARA
, O
overhang B-PARA
angles E-PARA
, O
hatch O
and O
contour B-PARA
scanning E-PARA
strategy O
may O
all O
influence O
the O
obtained O
roughness S-PRO
in O
the O
scaffolds S-FEAT
and O
may O
cause O
deviations O
from O
designed S-FEAT
sizes O
. O


For O
example O
, O
in O
du O
Plessis O
, O
the O
elemental O
cubic O
lattice S-CONPRI
was O
designed S-FEAT
with O
a O
total O
15 O
mm S-MANP
width O
, O
0.75 O
mm S-MANP
strut O
thickness O
and O
8 O
struts S-MACEQ
across O
one O
direction O
in O
total O
, O
resulting O
in O
1.28 O
mm S-MANP
distance O
between O
struts S-MACEQ
and O
total O
65 O
% O
porosity S-PRO
. O


One O
set S-APPL
of O
samples S-CONPRI
was O
built O
at O
standard S-CONPRI
process-parameters O
recommended O
for O
EOS S-APPL
Ti6Al4V O
in O
vertical S-CONPRI
direction O
, O
other O
ones O
at O
45angle O
. O


Samples S-CONPRI
were O
heat-treated S-MANP
for O
stress-relieving O
as S-MATE
indicated O
in O
. O


The O
differences O
in O
strut B-PARA
thickness E-PARA
, O
roughness S-PRO
and O
microstructure S-CONPRI
is O
clearly O
visible O
by O
cross-sections S-CONPRI
and O
also O
different O
columnar O
prior O
beta-grain O
orientations S-CONPRI
are O
clearly O
present O
. O


Samples S-CONPRI
that O
were O
produced O
at O
45 O
degrees O
, O
had O
25 O
% O
lower O
ultimate O
compression B-PRO
strength E-PRO
in O
comparison O
with O
vertical B-CONPRI
samples E-CONPRI
. O


Bending S-MANP
and O
stretch-dominated O
lattices S-CONPRI
fail O
respectively O
in O
shear O
and O
layer-by-layer S-CONPRI
failure B-PRO
modes E-PRO
, O
and O
this O
might O
depend O
somewhat O
on O
the O
material S-MATE
ductility S-PRO
. O


For O
a O
brittle B-MATE
material E-MATE
, O
shear B-PRO
failure E-PRO
is O
not O
desirable O
and O
layer-by-layer S-CONPRI
can O
be S-MATE
much O
preferred O
and O
even O
might O
act O
as S-MATE
protective O
mechanism S-CONPRI
. O


The O
layer-by-layer S-CONPRI
mechanism O
is O
more O
predictable S-CONPRI
as S-MATE
it O
is O
known O
where O
the O
next O
failure S-CONPRI
will O
occur O
. O


In O
general O
, O
manufacturing B-CONPRI
imperfections E-CONPRI
might O
affect O
thin O
features O
more O
than O
thick O
features O
, O
hence O
thin O
struts S-MACEQ
should O
be S-MATE
thickened O
or O
well-designed O
with O
sufficient O
safety S-CONPRI
margin O
. O


The O
obtained O
texturization O
in O
LPBF B-MATE
materials E-MATE
- O
grain S-CONPRI
and O
sub-grain B-PARA
sizes E-PARA
- O
depend O
on O
the O
process-parameters O
used O
and O
scanning B-CONPRI
strategy E-CONPRI
in O
LPBF B-MATE
materials E-MATE
as S-MATE
shown O
by O
. O


The O
microstructure S-CONPRI
of O
LPBF S-MANP
solid O
samples S-CONPRI
and O
their O
mechanical B-CONPRI
properties E-CONPRI
, O
fracture S-CONPRI
and O
fatigue S-PRO
behavior O
have O
some O
peculiarities O
in O
as-built O
and O
heat-treated S-MANP
AM B-MACEQ
parts E-MACEQ
, O
which O
have O
been O
widely O
studied O
. O


For O
example O
, O
the O
columnar O
boundaries S-FEAT
of O
prior O
beta-phase O
were O
observed O
in O
as-built O
Ti6Al4V S-MATE
ELI O
specimens O
and O
remain O
even O
after O
heat B-MANP
treatment E-MANP
of O
950 O
for O
2 O
hours O
. O


Anisotropy S-PRO
in O
AM S-MANP
is O
often O
mentioned O
. O


For O
example O
, O
the O
mechanical B-CONPRI
properties E-CONPRI
of O
LPBF S-MANP
Ti6Al4V O
ELI O
was O
found O
to O
be S-MATE
strongly O
anisotropic S-PRO
where O
three-point B-CHAR
bending I-CHAR
fatigue I-CHAR
tests E-CHAR
were O
used O
with O
parts O
produced O
in O
different O
orientations S-CONPRI
. O


The O
crack B-CONPRI
propagation I-CONPRI
rate E-CONPRI
and O
fatigue B-PRO
life E-PRO
of O
as-built O
and O
heat-treated S-MANP
samples O
correlated S-CONPRI
with O
column B-FEAT
boundaries E-FEAT
and O
orientation S-CONPRI
of O
the O
layers O
, O
i.e O
. O


correlated S-CONPRI
with O
the O
building B-PARA
direction E-PARA
. O


For O
static O
tensile B-CHAR
tests E-CHAR
, O
lower O
ductility S-PRO
was O
observed O
experimentally O
for O
the O
horizontal B-BIOP
specimens E-BIOP
in O
comparison O
with O
vertical B-CONPRI
samples E-CONPRI
this O
could O
be S-MATE
attributed O
to O
long O
prior O
beta-grain O
boundaries S-FEAT
in O
Ti6Al4V S-MATE
which O
grow O
in O
the O
build B-PARA
direction E-PARA
and O
are O
hence O
perpendicular O
to O
the O
loading O
direction O
in O
horizontal B-BIOP
specimens E-BIOP
. O


As S-MATE
it O
was O
noted O
in O
, O
the O
orientation S-CONPRI
dependency O
of O
the O
ductility S-PRO
in O
AM S-MANP
is O
not O
yet O
clear O
and O
further O
in-depth O
investigations O
need O
to O
be S-MATE
done O
. O


Mechanical B-CONPRI
properties E-CONPRI
are O
dependent O
on O
building O
and O
scanning B-CONPRI
strategies E-CONPRI
and O
these O
vary O
for O
different O
materials S-CONPRI
. O


For O
example O
, O
LPBF S-MANP
316 O
L O
stainless B-MATE
steel E-MATE
had O
maximum O
strength S-PRO
and O
Youngmodulus S-PRO
under O
a O
45 O
degree O
offset S-CONPRI
between O
the O
layer S-PARA
and O
loading O
direction O
, O
whereas O
AlSi10Mg S-MATE
revealed O
the O
lowest O
strength S-PRO
in O
this O
case O
. O


In O
samples B-CONPRI
manufactured E-CONPRI
by O
LPBF S-MANP
from O
a O
nickel-based B-MATE
alloy E-MATE
, O
strong O
crystallographic O
texture S-FEAT
resulting O
in O
anisotropic S-PRO
properties O
was O
found O
in O
creep B-PRO
behavior E-PRO
: O
specimens O
with O
loading O
parallel O
to O
the O
building B-PARA
direction E-PARA
were O
superior O
compared O
to O
specimens O
with O
loading O
axis O
normal O
to O
the O
building B-PARA
direction E-PARA
. O


The O
Young O
's O
modulus O
determined O
in O
measurements O
at O
room O
and O
elevated O
temperature S-PARA
was O
different O
during O
tensile B-CHAR
testing E-CHAR
parallel O
or O
perpendicular O
to O
the O
building B-PARA
direction E-PARA
. O


The O
building B-PARA
direction E-PARA
and O
laser S-ENAT
scanning O
direction O
/ O
scanning B-CONPRI
strategy E-CONPRI
are O
important O
for O
the O
mechanical B-PRO
integrity E-PRO
and O
this O
adds O
complexity S-CONPRI
to O
the O
optimal O
processing O
protocol S-CONPRI
for O
parts O
of O
complex B-PRO
shape E-PRO
. O


Material S-MATE
type O
, O
particle B-CONPRI
size I-CONPRI
distribution E-CONPRI
and O
particle S-CONPRI
shape O
, O
process-parameters O
, O
protective O
atmosphere O
, O
building O
and O
scanning B-CONPRI
strategies E-CONPRI
, O
post-processing S-CONPRI
, O
etc O
. O


should O
all O
be S-MATE
optimized O
according O
to O
the O
specific O
LPBF S-MANP
process O
so O
that O
biomimetic B-CONPRI
objects E-CONPRI
can O
be S-MATE
produced O
with O
the O
desired O
properties S-CONPRI
. O


Once O
material B-CONPRI
properties E-CONPRI
and O
structural B-PRO
integrity E-PRO
have O
been O
assessed O
, O
the O
parts O
produced O
can O
be S-MATE
trusted O
, O
especially O
when O
suitable O
design S-FEAT
safety O
margins O
have O
been O
incorporated O
. O


There O
are O
some O
general O
suggested O
guidelines O
based O
on O
the O
experiences O
of O
the O
authors O
which O
can O
be S-MATE
used O
in O
addition O
to O
ensure O
safety S-CONPRI
and O
reliability S-CHAR
of O
biomimetic S-CONPRI
parts O
in O
real O
world O
applications O
: O
1 O
For O
lattices S-CONPRI
, O
thin O
struts S-MACEQ
might O
contain O
micro-porosity O
, O
rough O
surfaces S-CONPRI
and O
manufacturing B-CONPRI
imperfections E-CONPRI
which O
affect O
the O
mechanical B-CONPRI
properties E-CONPRI
sometimes O
more O
strongly O
than O
thicker O
features O
. O


It O
was O
found O
that O
the O
cyclic O
response O
of O
lattices S-CONPRI
depend O
not O
only O
on O
the O
type O
of O
bulk O
material S-MATE
, O
but O
also O
on O
the O
roughness S-PRO
of O
the O
outer O
surface S-CONPRI
of O
the O
struts S-MACEQ
and O
the O
distribution S-CONPRI
of O
the O
micro-pores O
inside O
the O
struts S-MACEQ
which O
can O
both O
affect O
the O
crack O
initiation O
and O
crack B-CONPRI
propagation E-CONPRI
. O


Post B-CONPRI
processing I-CONPRI
chemical I-CONPRI
cleaning E-CONPRI
to O
decrease O
strut S-MACEQ
roughness S-PRO
can O
be S-MATE
used O
to O
minimize O
this O
. O


The O
accuracy S-CHAR
of O
various O
AM B-MANP
techniques E-MANP
are O
different O
since O
different O
laser B-PARA
spot I-PARA
size E-PARA
, O
powder S-MATE
layer B-PARA
thickness E-PARA
, O
process-parameters O
as S-MATE
well O
as S-MATE
powder O
material S-MATE
are O
used O
. O


Therefore O
, O
for O
a O
particular O
purpose O
where O
mechanical B-CONPRI
properties E-CONPRI
are O
critical O
, O
AM S-MANP
lattices O
should O
be S-MATE
tested O
stringently O
. O


To O
improve O
mechanical S-APPL
performance O
of O
lattice B-FEAT
structures E-FEAT
for O
load O
bearing O
applications O
they O
must O
be S-MATE
well-designed O
. O


Van O
Bael O
showed O
that O
stiffness S-PRO
and O
compressive B-PRO
strength E-PRO
of O
lattice B-FEAT
structures E-FEAT
correlate O
well O
with O
volume B-PARA
fraction E-PARA
. O


Contuzzi O
proposed O
to O
use O
solid O
reinforcements O
in O
fine O
lattice B-FEAT
structures E-FEAT
that O
increase O
load O
carrying O
capability O
of O
the O
structure S-CONPRI
almost O
linearly O
with O
the O
number O
of O
the O
reinforcements O
. O


Bobbert O
proposed O
to O
use O
in O
these O
applications O
continuous O
sheet-based O
porous S-PRO
structures O
because O
they O
are O
expected O
to O
be S-MATE
less O
sensitive O
to O
such O
imperfections S-CONPRI
than O
beam-based O
porous S-PRO
structures O
, O
to O
improve O
fatigue S-PRO
resistance O
. O


2 O
For O
lattices S-CONPRI
, O
selecting O
lattice S-CONPRI
parameters O
to O
ensure O
no O
supports S-APPL
are O
required O
on O
the O
lattice S-CONPRI
or O
inside O
the O
lattice S-CONPRI
area S-PARA
is O
critical O
. O


Here O
, O
strut S-MACEQ
angles O
and/or O
length O
is O
important O
. O


3 O
For O
irregular O
geometries S-CONPRI
from O
topology B-FEAT
optimization E-FEAT
and O
freeform B-FEAT
design E-FEAT
, O
it O
is O
advisable O
to O
perform O
build-simulation O
to O
ensure O
no O
local O
heat B-PRO
accumulation E-PRO
occurs O
which O
might O
lead S-MATE
to O
residual B-PRO
stress E-PRO
and O
warping S-CONPRI
. O


In O
this O
process S-CONPRI
, O
the O
optimal O
build S-PARA
angle O
and O
supports S-APPL
should O
be S-MATE
selected O
. O


4 O
Residual B-PRO
stress E-PRO
can O
be S-MATE
minimized O
by O
design S-FEAT
as S-MATE
mentioned O
above O
, O
and O
can O
be S-MATE
further O
improved O
by O
stress-relief O
heat B-MANP
treatment E-MANP
a O
relatively O
simple S-MANP
recommended O
solution S-CONPRI
. O


Heat B-MANP
treatment E-MANP
can O
have O
a O
decisive O
role O
on O
higher O
ductility S-PRO
and O
load O
bearing O
capacity S-CONPRI
of O
lattice B-FEAT
structures E-FEAT
and O
might O
increase O
fatigue B-PRO
life E-PRO
. O


5 O
Special O
attention O
must O
be S-MATE
given O
to O
the O
loading O
direction O
during O
use O
, O
because O
anisotropic S-PRO
mechanical O
properties S-CONPRI
of O
LPBF S-MANP
objects O
exists O
. O


This O
anisotropy S-PRO
might O
not O
only O
result O
from O
the O
material S-MATE
and O
its O
specific O
microstructure S-CONPRI
, O
but O
also O
from O
scanning S-CONPRI
and O
building O
strategies O
used O
for O
LPBF S-MANP
manufacturing O
, O
which O
might O
vary O
with O
different O
systems O
. O


Lattice B-FEAT
structures E-FEAT
built O
in O
different O
directions O
have O
non-identical O
mechanical B-CONPRI
properties E-CONPRI
. O


7 O
Challenges O
in O
biomimetic B-APPL
AM E-APPL
Despite O
all O
the O
potential O
for O
biomimicry S-CONPRI
in O
AM S-MANP
in O
its O
various O
forms O
, O
there O
are O
some O
challenges O
to O
its O
practical O
implementation O
. O


Most O
importantly O
, O
all O
forms O
of O
biomimetic B-FEAT
design E-FEAT
for O
AM S-MANP
involves O
complexity S-CONPRI
in O
various O
forms O
not O
previously O
encountered O
. O


While O
AM S-MANP
relaxes O
the O
traditional B-MANP
manufacturing E-MANP
rules O
, O
not O
any O
geometry S-CONPRI
or O
structure S-CONPRI
can O
be S-MATE
produced O
easily O
or O
reliably O
. O


Due O
to O
the O
complexity S-CONPRI
of O
design S-FEAT
, O
design S-FEAT
for O
AM S-MANP
becomes O
even O
more O
crucial O
to O
ensure O
manufacturability S-CONPRI
and O
might O
involve O
re-design O
in O
cases O
of O
difficult O
geometries S-CONPRI
. O


Metal B-MANP
AM E-MANP
and O
its O
limits S-CONPRI
in O
general O
are O
discussed O
in O
more O
detail O
in O
. O


AM S-MANP
is O
still O
a O
relatively O
new O
manufacturing B-MANP
process E-MANP
which O
requires O
process B-CONPRI
optimization E-CONPRI
and O
quality B-CONPRI
control E-CONPRI
to O
ensure O
accuracy S-CHAR
and O
reliability S-CHAR
. O


This O
requirement O
is O
critically O
important O
for O
parts O
with O
complex B-CONPRI
geometries E-CONPRI
which O
include O
curved B-CONPRI
surfaces E-CONPRI
, O
thin O
connecting O
features O
, O
hidden O
features O
and O
lattice B-FEAT
structures E-FEAT
. O


There O
are O
also O
many O
varieties O
of O
AM S-MANP
with O
different O
trade O
names O
, O
processes S-CONPRI
and O
differences O
in O
quality S-CONPRI
obtained O
. O


This O
quality S-CONPRI
refers O
in O
particular O
to O
material S-MATE
density S-PRO
and O
process S-CONPRI
induced O
pores S-PRO
, O
inherent O
process S-CONPRI
surface O
roughness S-PRO
, O
build S-PARA
errors O
such O
as S-MATE
uneven O
powder S-MATE
spreading O
or O
scan O
track O
errors S-CONPRI
leading O
to O
critical O
flaws S-CONPRI
, O
residual B-PRO
stresses E-PRO
and O
associated O
warping S-CONPRI
and O
cracking S-CONPRI
and O
microstructural S-CONPRI
inhomogeneity O
. O


A O
major O
limitation O
is O
the O
minimum B-PARA
feature I-PARA
size E-PARA
for O
the O
AM S-MANP
system O
used O
. O


Some O
additional O
limitations O
are O
placed O
on O
the O
part O
designs S-FEAT
, O
most O
notably O
the O
build S-PARA
angles O
. O


All O
down-facing O
surfaces S-CONPRI
have O
typically O
rougher O
surfaces S-CONPRI
than O
upwards-facing O
surfaces S-CONPRI
, O
thin O
angled O
features O
suffer O
from O
stair-step O
effects O
, O
and O
small O
angles O
require O
supports S-APPL
. O


Support S-APPL
removal O
is O
not O
a O
simple S-MANP
process S-CONPRI
: O
this O
post-processing S-CONPRI
is O
time O
consuming O
and O
may O
also O
affect O
the O
dimensional B-CHAR
accuracy E-CHAR
and O
quality S-CONPRI
of O
the O
resulting O
part O
. O


When O
supports S-APPL
are O
needed O
inside O
a O
complex O
part O
, O
these O
supports S-APPL
might O
not O
be S-MATE
physically O
removable O
at O
all O
as S-MATE
shown O
in O
the O
example O
in O
18 O
. O


In O
this O
figure O
, O
two O
topology-optimized O
bracket S-MACEQ
designs O
were O
almost O
entirely O
latticed O
but O
the O
build S-PARA
process O
required O
incorporation O
of O
supports S-APPL
also O
inside O
the O
lattice S-CONPRI
region O
. O


Removing O
supports S-APPL
from O
lattice S-CONPRI
regions O
on O
the O
exterior O
can O
cause O
damage S-PRO
to O
the O
lattice S-CONPRI
struts O
, O
and O
removing O
them O
from O
inside O
the O
lattice S-CONPRI
region O
is O
entirely O
impossible O
. O


In O
this O
case O
, O
the O
brackets O
still O
met O
the O
mass O
target O
despite O
internal O
supports S-APPL
, O
but O
the O
aesthetic S-CONPRI
value O
is O
not O
as S-MATE
visually O
impressive O
as S-MATE
could O
have O
been O
achieved O
by O
appropriate O
design S-FEAT
to O
eliminate O
supports S-APPL
. O


Detailed O
inspection S-CHAR
of O
these O
complex O
parts O
ensures O
their O
structural B-PRO
integrity E-PRO
and O
accuracy S-CHAR
of O
production S-MANP
. O


Due O
to O
the O
expense O
involved O
in O
AM S-MANP
, O
non-destructive O
tools S-MACEQ
are O
especially O
useful O
to O
analyze O
parts O
without O
destroying O
them O
: O
the O
most O
widely O
used O
are O
X-ray S-CHAR
techniques O
such O
as S-MATE
2D S-CONPRI
digital O
radiography S-ENAT
and O
3D S-CONPRI
micro-computed O
tomography O
. O


Due O
to O
the O
complexity S-CONPRI
of O
the O
parts O
2D S-CONPRI
X-ray O
images S-CONPRI
are O
difficult O
to O
interpret O
and O
smaller O
flaws S-CONPRI
which O
are O
typical O
to O
AM S-MANP
may O
be S-MATE
missed O
. O


As S-MATE
a O
result O
, O
microCT S-CHAR
is O
often O
the O
preferred O
method O
of O
choice O
. O


This O
technique O
works O
by O
acquisition O
of O
X-ray B-CHAR
absorption E-CHAR
images S-CONPRI
from O
many O
angles O
around O
the O
object O
, O
followed O
by O
reconstruction S-CONPRI
to O
produce O
a O
3D S-CONPRI
representation O
of O
the O
object O
, O
including O
its O
interior O
. O


It O
is O
also O
known O
as S-MATE
X-ray O
tomography O
, O
CT S-ENAT
scanning O
or O
X-ray S-CHAR
microscopy S-CHAR
. O


The O
most O
important O
issues O
that O
can O
be S-MATE
identified O
by O
microCT S-CHAR
and O
which O
are O
relevant O
to O
biomimetic B-APPL
AM E-APPL
are O
: O
- O
Powder S-MATE
can O
get O
stuck O
in O
complex O
areas S-PARA
, O
especially O
inside O
lattices S-CONPRI
, O
and O
when O
heat-treated S-MANP
they O
become O
stuck O
. O


This O
adds O
weight S-PARA
and O
might O
be S-MATE
unsafe O
. O


- O
Rough O
surfaces S-CONPRI
which O
depend O
on O
build S-PARA
angle O
might O
affect O
mechanical B-CONPRI
properties E-CONPRI
, O
with O
rough O
surfaces S-CONPRI
in O
inaccessible O
areas S-PARA
being O
unable O
to O
be S-MATE
processed O
. O


Roughness S-PRO
can O
be S-MATE
measured O
quantitatively S-CONPRI
or O
assessed O
visually O
. O


- O
Manufacturing S-MANP
flaws S-CONPRI
such O
as S-MATE
porosity O
might O
also O
occur O
despite O
process B-CONPRI
parameter E-CONPRI
optimization S-CONPRI
and O
this O
may O
affect O
the O
mechanical B-CONPRI
properties E-CONPRI
. O


It O
is O
important O
to O
note O
here O
that O
process B-CONPRI
parameter E-CONPRI
optimization S-CONPRI
prior O
to O
building O
a O
part O
can O
limit S-CONPRI
process-induced O
porosity S-PRO
and O
this O
microporosity S-PRO
is O
expected O
to O
be S-MATE
the O
same O
in O
a O
test O
coupon O
than O
in O
a O
complex O
part O
. O


- O
Residual B-PRO
stress E-PRO
can O
not O
directly O
be S-MATE
seen O
in O
microCT S-CHAR
images S-CONPRI
but O
can O
be S-MATE
seen O
indirectly O
in O
the O
form O
of O
warping S-CONPRI
and O
cracks O
. O


Unnoticed O
residual B-PRO
stress E-PRO
in O
a O
part O
might O
affect O
its O
mechanical B-CONPRI
properties E-CONPRI
. O


Stress-relief O
heat B-MANP
treatment E-MANP
is O
therefore O
highly O
recommended O
. O


The O
above O
issues O
can O
be S-MATE
partially O
improved O
or O
solved O
by O
using O
AM S-MANP
simulations O
to O
highlight O
where O
thermal O
hotspots O
might O
be S-MATE
formed O
. O


A O
change O
in O
the O
build S-PARA
angle O
or O
design S-FEAT
itself O
can O
contribute O
to O
eliminate O
these O
. O


Changing O
the O
lattice B-FEAT
design E-FEAT
or O
parameters S-CONPRI
can O
improve O
the O
requirement O
for O
supports S-APPL
and O
self-supporting S-FEAT
lattice B-FEAT
designs E-FEAT
can O
be S-MATE
selected O
in O
some O
cases O
. O


Besides O
build B-PARA
orientation E-PARA
planning O
and O
simulation S-ENAT
, O
the O
manufacturing B-MANP
process E-MANP
can O
be S-MATE
optimized O
to O
ensure O
high O
quality S-CONPRI
production S-MANP
on O
test O
cubes O
, O
which O
can O
be S-MATE
subjected O
to O
detailed O
analysis O
by O
sectioning O
, O
or O
preferably O
by O
microCT S-CHAR
. O


When O
using O
microCT S-CHAR
, O
however O
, O
it O
is O
also O
important O
to O
realize O
that O
while O
small O
porosity S-PRO
is O
acceptable O
when O
well O
distributed O
, O
only O
major O
flaws S-CONPRI
or O
those O
with O
specific O
location-specific O
clustering O
are O
important O
, O
as S-MATE
well O
as S-MATE
those O
in O
critical O
regions O
of O
the O
part O
. O


Optimization S-CONPRI
of O
processes S-CONPRI
using O
test O
cubes O
and O
microCT S-CHAR
may O
assist O
in O
identifying O
the O
root O
cause O
of O
some O
types O
of O
defects S-CONPRI
which O
allows O
to O
improve O
the O
process S-CONPRI
. O


Simulations S-ENAT
and O
experimental S-CONPRI
work O
done O
on O
lattice B-FEAT
structures E-FEAT
with O
artificially O
induced O
porosity S-PRO
in O
individual O
struts S-MACEQ
showed O
that O
this O
did O
not O
affect O
the O
yield B-PRO
strength E-PRO
of O
the O
lattice S-CONPRI
for O
up O
to O
0.5 O
mm S-MANP
pores O
. O


8 O
New O
trends S-CONPRI
in O
biomimetic B-APPL
AM E-APPL
This O
section O
mentions O
some O
current O
interesting O
trends S-CONPRI
in O
biomimetic B-FEAT
design E-FEAT
for O
AM S-MANP
, O
with O
new O
developments O
expected O
in O
the O
next O
few O
years O
as S-MATE
the O
techniques O
are O
refined O
and O
new O
tools S-MACEQ
become O
available O
. O


The O
first O
worth O
mentioning O
is O
that O
most O
topology B-FEAT
optimization E-FEAT
software S-CONPRI
at O
present O
operates O
on O
the O
topology S-CONPRI
itself O
and O
subsequently O
certain O
areas S-PARA
can O
be S-MATE
selected O
for O
latticing O
, O
i.e O
. O


the O
latticing O
is O
not O
part O
of O
the O
simulation-driven S-ENAT
design B-CONPRI
process E-CONPRI
. O


This O
latticing O
is O
incorporated O
into O
the O
simulation-driven S-ENAT
design B-CONPRI
process E-CONPRI
and O
will O
find O
application O
especially O
in O
light-weighting O
applications O
. O


The O
other O
useful O
development O
is O
the O
optimization S-CONPRI
of O
repeated O
lattices S-CONPRI
gradient O
lattices S-CONPRI
and O
variations S-CONPRI
of O
strut B-PARA
thickness E-PARA
or O
unit B-CONPRI
cell E-CONPRI
size O
across O
a O
part O
, O
and O
conformal B-FEAT
lattices E-FEAT
to O
the O
surfaces S-CONPRI
of O
a O
part O
. O


In O
other O
words O
, O
the O
lattice S-CONPRI
is O
not O
simply O
cut O
off O
on O
the O
edge O
of O
the O
part O
but O
unit B-CONPRI
cells E-CONPRI
are O
stretched O
to O
fit S-CONPRI
the O
surface B-FEAT
topology E-FEAT
. O


An O
example O
hereof O
is O
shown O
in O
21 O
where O
the O
lattice S-CONPRI
is O
conformal O
to O
two O
opposing O
surfaces S-CONPRI
and O
the O
lattice B-FEAT
density E-FEAT
varies O
to O
allow O
denser O
lattice S-CONPRI
in O
areas S-PARA
where O
simulations S-ENAT
show O
higher O
stress S-PRO
will O
be S-MATE
experienced O
. O


Recent O
research S-CONPRI
approaches O
for O
cellular B-MATE
material E-MATE
design O
have O
included O
the O
development O
of O
multi-scale O
optimization S-CONPRI
approaches O
as S-MATE
described O
by O
Osanov O
and O
Guest O
and O
Cadman O
. O


In O
this O
approach O
, O
the O
unit B-CONPRI
cell E-CONPRI
domain S-CONPRI
is O
discretized O
into O
elements S-MATE
which O
are O
then O
themselves O
optimized O
using O
topology B-FEAT
optimization E-FEAT
methods O
, O
similar O
to O
discussions O
in O
the O
previous O
section O
. O


A O
unit B-CONPRI
cell E-CONPRI
so O
designed S-FEAT
can O
then O
be S-MATE
used O
to O
compute O
effective O
properties S-CONPRI
, O
after O
which O
inverse O
homogenization S-MANP
is O
used O
to O
upscale O
the O
cellular O
geometry S-CONPRI
to O
the O
level O
of O
the O
larger O
structure S-CONPRI
. O


These O
ideas O
have O
been O
recently O
extended O
to O
multi-material S-CONPRI
cellular B-FEAT
structure E-FEAT
optimization O
. O


Cellular O
automata O
methods O
have O
also O
been O
developed O
to O
design S-FEAT
materials O
and O
microstructures S-MATE
, O
and O
machine S-MACEQ
learning O
methods O
are O
beginning O
to O
be S-MATE
applied O
to O
materials S-CONPRI
design S-FEAT
. O


Because O
of O
the O
very O
complex B-PRO
shapes E-PRO
of O
the O
parts O
having O
a O
biomimetic S-CONPRI
or O
bionic O
design S-FEAT
, O
it O
is O
often O
necessary O
to O
use O
support B-FEAT
structures E-FEAT
for O
overhanging O
areas S-PARA
. O


This O
can O
be S-MATE
a O
big O
problem O
in O
the O
post B-CONPRI
processing E-CONPRI
of O
these O
parts O
for O
removing O
the O
supports S-APPL
and O
surface B-MANP
finishing E-MANP
. O


On O
the O
other O
hand O
, O
internal O
complexity S-CONPRI
and O
small O
features O
are O
limited O
in O
this O
process S-CONPRI
, O
since O
with O
constant O
preheating S-MANP
of O
each O
layer S-PARA
to O
a O
high O
temperature S-PARA
, O
the O
powder S-MATE
is O
partially O
sintered S-MANP
and O
later O
can O
not O
be S-MATE
removed O
from O
the O
manufactured S-CONPRI
part O
. O


There O
are O
also O
quite O
serious O
limitations O
on O
materials S-CONPRI
for O
EBM S-MANP
technology O
. O


Also O
recently O
, O
companies S-APPL
such O
as S-MATE
EOS O
and O
Velo3D O
have O
improved O
their O
softwares O
, O
scanning B-CONPRI
strategies E-CONPRI
and O
process B-CONPRI
control E-CONPRI
parameters S-CONPRI
, O
which O
allowed O
to O
realize O
designs S-FEAT
with O
overhangs S-PARA
lower O
than O
15and O
large O
inner O
diameters O
without O
supports S-APPL
. O


These O
developments O
are O
all O
very O
promising O
for O
the O
realization O
of O
increasingly O
complex O
biomimetic B-FEAT
designs E-FEAT
with O
improved O
structural B-PRO
integrity E-PRO
and O
surface B-PARA
quality E-PARA
. O


An O
emerging O
trend S-CONPRI
is O
the O
development O
of O
software S-CONPRI
packages O
incorporating O
the O
entire O
workflow S-CONPRI
for O
advanced O
design S-FEAT
for O
AM S-MANP
, O
including O
freeform B-FEAT
design E-FEAT
, O
topology B-FEAT
optimization E-FEAT
, O
latticing O
and O
more O
recently O
also O
build S-PARA
simulation O
and O
even O
support B-FEAT
generation E-FEAT
and O
slicing S-CONPRI
for O
build B-PARA
preparation E-PARA
. O


When O
all O
this O
is O
combined O
in O
one O
workspace O
the O
entire O
design B-CONPRI
process E-CONPRI
is O
simplified O
and O
this O
allows O
more O
frequent O
and O
improved O
biomimetic B-FEAT
designs E-FEAT
to O
be S-MATE
realized O
in O
practice O
. O


The O
development O
of O
standards S-CONPRI
for O
AM S-MANP
and O
non-destructive B-CHAR
testing E-CHAR
in O
AM S-MANP
is O
emerging O
as S-MATE
an O
important O
aspect O
in O
the O
qualification O
of O
processes S-CONPRI
and O
ensuring O
reliability S-CHAR
in O
AM B-MANP
processes E-MANP
. O


This O
is O
especially O
applicable O
to O
biomimetic B-FEAT
designs E-FEAT
and O
it O
holds O
the O
most O
advantage O
in O
optimizing O
process B-CONPRI
parameters E-CONPRI
prior O
to O
building O
complex O
parts O
- O
using O
microCT S-CHAR
test O
methods O
. O


Inspecting O
complex O
parts O
is O
also O
valuable O
in O
critical O
parts O
such O
as S-MATE
for O
aerospace S-APPL
, O
and O
microCT S-CHAR
is O
the O
best O
method O
to O
do O
this O
. O


It O
is O
worth O
mentioning O
that O
besides O
complex O
part O
inspections S-CHAR
, O
which O
are O
limited O
in O
resolution S-PARA
by O
field O
of O
view O
, O
it O
is O
becoming O
standard S-CONPRI
practice O
to O
inspect O
witness O
specimens O
of O
smaller O
diameter S-CONPRI
built O
alongside O
complex O
parts O
. O


This O
allows O
for O
high B-PARA
resolution E-PARA
CT S-ENAT
analysis O
with O
defects S-CONPRI
found O
in O
these O
specimens O
being O
indicative O
of O
problems O
encountered O
during O
the O
build S-PARA
. O


Something O
that O
is O
becoming O
increasingly O
popular O
for O
improving O
part O
density S-PRO
is O
the O
use O
of O
hot B-MANP
isostatic I-MANP
pressing E-MANP
, O
especially O
for O
additively-manufactured O
metal S-MATE
parts O
for O
aerospace S-APPL
it O
is O
a O
requirement O
that O
all O
parts O
are O
HIPped O
. O


The O
HIP S-MANP
process O
closes O
pores S-PRO
and O
improves O
the O
microstructure S-CONPRI
, O
but O
it O
is O
important O
to O
realize O
that O
not O
all O
pores S-PRO
are O
necessarily O
closed O
by O
HIP S-MANP
: O
it O
has O
been O
shown O
that O
pores S-PRO
connected O
to O
the O
surface S-CONPRI
do O
not O
close O
properly O
, O
and O
is O
detectable O
by O
microCT S-CHAR
. O


The O
important O
point O
is O
that O
HIP S-MANP
should O
not O
be S-MATE
used O
as S-MATE
a O
blind O
solution S-CONPRI
its O
performance S-CONPRI
especially O
in O
thin O
walled O
parts O
should O
be S-MATE
checked O
. O


In O
general O
, O
the O
use O
of O
biomimetic B-APPL
AM E-APPL
is O
growing O
at O
a O
very O
fast O
rate O
, O
with O
practical O
engineering S-APPL
applications O
emerging O
almost O
daily O
. O


This O
is O
driven O
by O
the O
maturation O
of O
metal B-MANP
powder I-MANP
bed I-MANP
fusion E-MANP
AM S-MANP
, O
the O
development O
of O
appropriate O
software S-CONPRI
tools O
, O
and O
the O
huge O
interest O
from O
companies S-APPL
in O
investing O
in O
a O
technology S-CONPRI
with O
clear O
potential O
to O
disrupt O
various O
industries S-APPL
. O


The O
key O
to O
disrupting O
existing O
products O
is O
in O
significant O
advantages O
in O
the O
new O
design S-FEAT
which O
is O
possible O
by O
AM S-MANP
and O
biomimicry S-CONPRI
is O
key O
to O
unlocking O
this O
potential O
. O


Besides O
aesthetic S-CONPRI
appeal O
, O
actual O
light-weight S-PRO
advantage O
is O
likely O
the O
biggest O
drawcard O
in O
automotive S-APPL
and O
aerospace B-APPL
industries E-APPL
. O


In O
other O
industries S-APPL
the O
combination O
of O
multiple O
parts O
into O
one O
might O
be S-MATE
a O
significant O
advantage O
and O
it O
is O
expected O
that O
the O
multi-functionality O
of O
designs S-FEAT
might O
be S-MATE
one O
of O
the O
big O
future O
growth O
areas S-PARA
. O


9 O
Conclusions O
It O
is O
clear O
that O
biomimicry S-CONPRI
in O
AM S-MANP
allows O
complex O
functional O
designs S-FEAT
and O
various O
tools S-MACEQ
are O
currently O
available O
to O
easily O
achieve O
such O
designs S-FEAT
. O


Biomimetic B-FEAT
designs E-FEAT
are O
therefore O
both O
beautiful O
and O
functional O
. O


Despite O
the O
high O
possible O
complexity S-CONPRI
, O
some O
design S-FEAT
for O
AM S-MANP
rules O
have O
emerged O
which O
improve O
the O
manufacturability S-CONPRI
and O
reliability S-CHAR
of O
these O
types O
of O
parts O
and O
these O
should O
be S-MATE
incorporated O
into O
the O
design B-CONPRI
process E-CONPRI
. O


It O
is O
especially O
important O
that O
process B-CONPRI
parameters E-CONPRI
are O
optimized O
to O
ensure O
structural B-PRO
integrity E-PRO
and O
ensure O
high O
quality S-CONPRI
manufacturing S-MANP
, O
as S-MATE
manufacturing O
errors S-CONPRI
might O
affect O
these O
parts O
more O
than O
traditional O
parts O
this O
requires O
an O
additional O
safety B-FEAT
factor E-FEAT
to O
be S-MATE
built O
into O
designs S-FEAT
, O
and O
inspection S-CHAR
is O
critical O
. O


Post-processing S-CONPRI
of O
parts O
is O
also O
a O
challenge O
, O
and O
the O
options O
are O
limited O
therefore O
depending O
on O
the O
application O
the O
complexity S-CONPRI
of O
the O
design S-FEAT
might O
need O
to O
be S-MATE
constrained O
to O
ensure O
all O
surfaces S-CONPRI
are O
accessible O
by O
required O
post-processing S-CONPRI
techniques O
. O


One O
of O
the O
most O
widely O
used O
applications O
of O
biomimetic B-FEAT
design E-FEAT
in O
AM S-MANP
is O
light-weighting O
, O
but O
many O
other O
opportunities O
exist O
including O
parts O
customized O
for O
acoustic O
, O
thermal O
, O
optical S-CHAR
or O
other O
applications O
, O
especially O
in O
combination O
with O
surface B-MANP
modification E-MANP
techniques O
. O


Most O
importantly O
, O
all O
examples O
in O
this O
work O
clearly O
demonstrate O
that O
biomimetic B-FEAT
designs E-FEAT
can O
be S-MATE
trusted O
and O
should O
be S-MATE
used O
more O
widely O
. O


Biomimetic B-FEAT
designs E-FEAT
are O
crucial O
for O
fully O
unlocking O
the O
power S-PARA
of O
metal B-MANP
AM E-MANP
in O
particular O
. O


In O
conclusion O
, O
biomimicry S-CONPRI
in O
AM S-MANP
has O
been O
shown O
to O
be S-MATE
possible O
in O
various O
ways O
, O
with O
the O
most O
accessible O
tools S-MACEQ
currently O
being O
freeform B-FEAT
design E-FEAT
and O
simulation-driven S-ENAT
design S-FEAT
. O


These O
tools S-MACEQ
allow O
complex O
forms O
to O
be S-MATE
created O
which O
often O
resemble O
natural O
structures O
, O
and O
the O
design S-FEAT
engineer O
may O
incorporate O
from O
naturein O
this O
design B-CONPRI
process E-CONPRI
. O


For O
example O
, O
in O
simulation-driven S-ENAT
design S-FEAT
, O
various O
outcomes O
are O
possible O
and O
selection O
of O
the O
design S-FEAT
outcome O
most O
similar O
to O
a O
biological B-FEAT
structure E-FEAT
is O
most O
likely O
the O
best O
solution S-CONPRI
. O


The O
greatest O
future O
potential O
for O
biomimicry S-CONPRI
in O
AM S-MANP
lies O
in O
incorporating O
real O
biological B-BIOP
input E-BIOP
in O
some O
ways O
in O
the O
design B-CONPRI
process E-CONPRI
and O
here O
biological B-MATE
materials E-MATE
science O
is O
crucial O
in O
providing O
from O
naturewhich O
can O
be S-MATE
incorporated O
easily O
. O


It O
is O
not O
only O
in O
the O
design B-CONPRI
process E-CONPRI
where O
biomimicry S-CONPRI
can O
be S-MATE
employed O
. O


The O
entire O
process S-CONPRI
of O
3D B-MANP
printing E-MANP
may O
follow O
biological O
principles O
, O
including O
sustainability S-CONPRI
. O


Biomimetic B-FEAT
design E-FEAT
therefore O
forms O
part O
of O
and O
drives O
the O
bio-industrial O
revolution O
which O
will O
become O
known O
as S-MATE
Industry O
5.0 O
. O


Conflict O
of O
interest O
One O
author O
is O
the O
Senior O
VP O
Business O
Development O
& O
Strategy O
Simulation S-ENAT
Driven O
Design S-FEAT
at O
Altair B-APPL
Engineering E-APPL
Inc O
, O
a O
provider O
of O
software S-CONPRI
for O
simulation-driven S-ENAT
design O
amongst O
others O
. O