Ci a ion: Os olaza, M.; A izubie a,
J.I.; Lamikiz, A.; Plaza, S.; O ega, N.
La es De elopmen s o Manu ac u e
Me al Ma ix Composi es and
Func ionally G aded Ma e ials
h ough AM: A S a e-o - he-A
Re iew. Ma e ials 2023,16, 1746.
h ps://doi.o g/10.3390/
ma16041746
Academic Edi o : Nils Ellend
Recei ed: 17 Janua y 2023
Re ised: 5 Feb ua y 2023
Accep ed: 13 Feb ua y 2023
Published: 20 Feb ua y 2023
Copy igh : © 2023 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
ma e ials
Re iew
La es De elopmen s o Manu ac u e Me al Ma ix Composi es
and Func ionally G aded Ma e ials h ough AM: A
S a e-o - he-A Re iew
Ma a Os olaza * , Jon Iñaki A izubie a , Ai zol Lamikiz , So aya Plaza and Naia a O ega
Depa men o Mechanical Enginee ing, Uni e si y o he Basque Coun y (UPV/EHU),
Plaza Ingenie o To es Que edo, 48013 Bilbao, Spain
*Co espondence: [email p o ec ed]; Tel.: +34-94-601-7347
Abs ac :
Mul i-ma e ial s uc u e ab ica ion has he po en ial o add ess some c i ical challenges
in oday’s indus ial pa adigm. While con en ional manu ac u ing p ocesses canno deli e mul i-
ma e ial s uc u es in a single ope a ion, addi i e manu ac u ing (AM) has come up as an appealing
al e na i e. In pa icula , lase -di ec ed ene gy deposi ion (L-DED) is p e e ed o mul i-ma e ial
AM. The mos ele an applica ions en isioned o mul i-ma e ial L-DED a e alloy design, me al
ma ix composi es (MMC), and unc ionally g aded ma e ials (FGM). None heless, he e a e s ill
some issues ha need o be aced be o e mul i-ma e ial L-DED is eady o indus ial use. D i en by
his need, in his li e a u e e iew, he sui abili y o L-DED o mul i-ma e ial componen ab ica ion
is i s demons a ed. Then, he main de ec s associa ed wi h mul i-ma e ial L-DED and cu en
oppo uni ies and challenges in he ield a e epo ed. In iew o he indus ial ele ance o high-
pe o mance coa ings as ools o mi iga e wea , emphasis is placed on he de elopmen o MMCs and
FGMs. The iden i ied challenges include—bu a e no limi ed o— igh ly con olling he composi ion
o he mul i-ma e ial powde mix u e injec ed in o he mel pool; unde s anding he in luence o
he he mal his o y o he p ocess on mic os uc u al aspec s, including he in e ac ions be ween
cons i uen s; and s udying he in-se ice beha iou s o MMCs and FGMs wi h ega d o hei
du abili y and ailu e modes.
Keywo ds:
mul i-ma e ial; addi i e manu ac u ing; lase di ec ed ene gy deposi ion; me al ma ix
composi es; unc ionally g aded ma e ials
1. In oduc ion o Me al Addi i e Manu ac u ing
Acco ding o he In e na ional S anda d ISO/ASTM 52900:2021, addi i e manu ac u -
ing (AM) is he “p ocess o joining ma e ials o make pa s om 3D model da a, usually
laye upon laye , as opposed o sub ac i e manu ac u ing and o ma i e manu ac u ing
me hodologies” [
1
]. Mul iple ma e ials can be p ocessed h ough AM, namely polyme s
and esins [2], ce amics [3], and me als [4].
1.1. Indus ial Con ex o Me al AM
In e ms o he indus ial ele ance and ma ke da a, me al AM is expe iencing
signi ican g ow h, ha ing eached u no e o 1.51 billion EUR in 2018 [
5
]. Mo eo e ,
acco ding o Sma Tech and Gene al Elec ic, i is expec ed o each 5.4 billion USD in
2027 [
6
]. The apid de elopmen o he me al AM ma ke has been mainly d i en by he
medical, den al, and ae ospace indus ies, whe e he popula i y o addi i e echnologies
is majo , and addi i ely buil -up componen s a e al eady being employed o end-use
pu poses. E en i AM has been widely adop ed o p o o yping applica ions, unc ional
componen manu ac u ing is g adually es ablishing i sel in he indus y oo. Mo eo e ,
AM has been demons a ed o p o ide g ea bene i s in epai applica ions [
7
], on-demand
spa e pa manu ac u ing, o low- un and cus om p oduc ion.
Ma e ials 2023,16, 1746. h ps://doi.o g/10.3390/ma16041746 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2023,16, 1746 2 o 33
In sho , AM has eme ged as a s a e-o - he-a echnology and i is quickly gaining
momen um in he indus y, owing o he signi ican esea ch e o s and ad ances made
du ing he las decades. Nowadays, i is conside ed o be a eliable and e icien echnol-
ogy o manu ac u ing ully dense s uc u al componen s. Addi ionally, i has become
an indus ially- iable echnology on accoun o he lowe cos o indus ial lase s, he
a ailabili y o high-pe o mance compu ing so wa e, and he high-quali y eeds ock ech-
nology. Mos AM echnologies, and pa icula ly me al AM p ocesses, ha e eached a
c i ical accep ance le el in he indus y. Mo eo e , some o hem ha e eached a ully
ce i ied p oduc ion s age in e ms o he echnology eadiness le els (TRL) [
8
]. Nowadays,
AM echnologies a e comple ely imme sed in many indus ies such as he manu ac u e o
medical implan s o high-pe o mance componen s in he ae ospace sec o [9].
Howe e , he e is s ill a g ea need o AM, mo e speci ically me al AM, o be included
in addi ional indus ial ields o high-end pa s manu ac u ing, as i is many imes s uck
in he esea ch phase. Fo ins ance, he capabili ies o AM in he au omo i e and machine
ool sec o s ha e been widely demons a ed in many esea ch pape s [
10
–
13
]. Ne e heless,
companies ha e no included such ad ances ye . Due o he dis up i e cha ac e o me al
AM p ocesses, many manu ac u e s s ill iden i y hei in eg a ion as a po en ial isk. Indeed,
an adap a ion o he cu en supply chain is equi ed o ully bene i om AM’s po en ial
o high-end pa p oduc ion. Las ly, he s anda disa ion o p ocesses is equi ed a he
han he ce i ica ion o he indi idual componen s. Al hough AM is conside ed o be pa
o he Fou h Indus ial Re olu ion [
14
], making i an essen ial pa o indus y 4.0 [
15
],
he business s a egies and ce i ica ion policies ha e no kep pace wi h he g ow h o
he echnology [
14
]. Consequen ly, he e is s ill a lo o wo k in p og ess in he a ea o
s anda disa ion, which will con ibu e o AM being in eg a ed in o he whole indus y.
The main AM p ocess ca ego ies cu en ly employed o me al AM a e powde bed
usion (PBF) and di ec ed ene gy deposi ion (DED) p ocesses [
16
]. Many o he s ha e he
po en ial o me al manu ac u ing, e.g., shee lamina ion (SL) o binde je ing (BJT). Fo
ins ance, BJT is slowly gaining ele ance in me al AM, and se e al po en ial applica ions
a e en isioned. None heless, his echnology is s ill a om ob aining ully dense pa s.
The e o e, i canno be conside ed a alid echnology o high- esponsibili y applica ions
ye . In sho , hese echnologies ha e lowe indus ial ele ance. Consequen ly, hey a e no
discussed in his e iew and only PBF and DED p ocess ca ego ies a e u he desc ibed.
1.2. Me al AM Ma e ials and P ope ies
The main cha ac e is ics and applica ions o ypical me al AM ma e ials a e shown
in Table 1. The a ailable ma e ial ange is con inuously expanding due o he exhaus i e
esea ch in me al AM, whils mul i-ma e ial AM is also being explo ed.
The main limi a ion o me al AM is he ac ha usion is in ol ed in he building
p ocess; hence, non-weldable and non-cas able ma e ials a e di icul o p ocess using usion
AM me hods [
17
]. Howe e , much esea ch is being ca ied ou o ace his challenge, and
non-weldable ma e ials ha e been success ully deposi ed [
18
–
20
]. AM s a egies based on
he p ecise con ol o he ene gy inpu and he uning o he he mal his o y o he pa s
du ing build-up seem o be he key aspec s o ackling his issue [21].
Addi ionally, me allu gical di e ences ha e been ound be ween addi i ely and con-
en ionally manu ac u ed componen s. The high esidual s esses and lack o in eg i y
a e c i ical o high-pe o mance applica ions, pa icula ly o hose ha equi e good
esis ance o high- empe a u e a igue [
9
]. This is c ucial in ae ospace applica ions, whe e
injec o s and o he complex pa s a e now eaching he ce i ica ion s age, bu o he c i ical
componen s, e.g., u bine blades, a e s ill in he ea ly de elopmen phase [8].
Ma e ials 2023,16, 1746 3 o 33
Table 1. Mos indus ially ex ended ma e ials o AM [22–24].
Ma e ial Main Cha ac e is ics Applica ion
Tool s eels
−High oughness
−High duc ili y
−Good esis ance o de o ma ion
Tooling o cu ing,
o ming, o shaping
p ocesses
S ainless s eels
−Co osion esis ance
−High duc ili y
−High s eng h
S uc u al and
co osion- esis an
applica ions
Ti anium alloys
−High speci ic s eng h
−Excep ional co osion esis ance
−High ac u e oughness
−Excellen a igue esis ance
−Good mechanical p ope ies a high T
−Low coe icien o he mal expansion
−Good biocompa ibili y
Ae ospace, au omo i e,
na al and biomedical
applica ions
Aluminium alloys
−Low densi y
−High speci ic s eng h
−High duc ili y and oughness
−High he mal and elec ical
conduc i i ies
Ae ospace, au omo i e,
cons uc ion and
consume goods
Nickel-based alloys
−Excellen ensile and c eep s eng hs
−Good mechanical p ope ies a high T◦
−High- empe a u e oxida ion esis ance
−High ha dness and oughness
−Low coe icien o he mal expansion
−Good weldabili y and o mabili y
Ae ospace and je
engine, s eam u bine,
pe ochemical, ene gy,
and c yogenic
applica ions
Cobal -based alloys
−High ha dness
−Excep ional wea esis ance
−Good co osion esis ance
−Good high- empe a u e pe o mance
Ae ospace and je
engines, pe ochemical,
oil and gas, medical
implan s, wea -
esis an applica ions
Coppe alloys
−High elec ical conduc i i y
−High he mal conduc i i y
−Good mechanical p ope ies
Fusion eac o s, ocke
engine, mic oelec onics
His o ically, single ma e ials and well-de ined comme cial me al alloys ha e domi-
na ed he enginee ing ma ke . Howe e , he mul i-ma e ial abili y o AM p ocesses has
unlocked a new esea ch di ec ion conce ning he build-up o mul i-ma e ial s uc u es [
25
].
In e ms o me al and me al–ce amic mul i-ma e ial componen p oduc ion, he p esence
o AM in he indus ial ma ke is s ill mino . In his ega d, se e al challenges emain
unsol ed, which inhibi i s widesp ead indus ialisa ion [
26
]. The e o e, he occu ences in
he indus ial ield o 3D-p in ed me al–ce amic mul i-ma e ials emain anecdo al. Mo e-
o e , mos appea ances o me al–ce amic addi i ely manu ac u ed pa s ha e been in he
con ex o mining and oil and gas indus ies.
Ma e ials 2023,16, 1746 4 o 33
1.3. Main Me al Addi i e Manu ac u ing P ocesses
In his sec ion, he main me al AM ca ego ies, namely PBF and DED, a e discussed.
The undamen als o he mos popula me al AM p ocesses a e b ie ly explained, and a
c i ical compa ison is p o ided, wi h an emphasis on mul i-ma e ial AM.
1.3.1. Powde Bed Fusion
The powde bed usion (PBF) e m e e s o he “addi i e manu ac u ing p ocess in which
he mal ene gy selec i ely uses egions o a powde bed” [
1
]. This ca ego y comp ises se e al
echnologies, which include bu a e no limi ed o lase PBF (L-PBF) and elec on beam PBF
(EB-PBF). Howe e , hey a e all based on he same wo king p inciple.
PBF p ocesses a e cu en ly widely accep ed in he indus y and well-es ablished in
he ae ospace and medical ields. In pa icula , L-PBF (popula ly e e ed o as selec i e
lase mel ing o SLM) is p obably he mos ex ended me al AM p ocess. In Figu e 1, a
ypical L-PBF sys em is schema ically illus a ed. On he le -hand side, he main sys em
componen s a e shown. On he igh –hand side, he ypical eeds ock equi ed o his
p ocess and an example o i s applica ion a e also included.
The L-PBF p ocess can be desc ibed as ollows:
1.
The powde deli e y pis on pushes he powde ese oi up and he ecoa e sp eads
a laye o esh powde on o ei he he building pla o m ( i s laye ) o he p e iously
deposi ed laye s (nex laye s) o o m he powde bed. This powde bed should be
p ope ly dis ibu ed o ensu e he densi ica ion o he manu ac u ed pa s;
2.
The lase deli e y sys em i adia es a lase beam, which is guided by he scanning
sys em, along he pa h p ede ined by he sliced 3D model da a. As shown in Figu e 1,
he eeds ock powde is qui e ine and he ypical diame e ange o he powde is
10–60
µ
m [
27
,
28
]. Du ing his p ocess, a mel pool is gene a ed, whose dep h needs o
exceed he laye hickness o gua an ee p ope bonding o he laye s (Figu e 2);
3.
Once he laye is inished and he pa e n is solidi ied, he build pla o m goes down
and s eps 1 and 2 a e epea ed. By o e lapping subsequen laye s ( ypically 30- o
90-
µ
m- hick) and i e a i ely epea ing his cycle, he AM pa is o med, achie ing
esul s simila o he ae ospike nozzle shown in Figu e 1[
29
]. No e ha he whole
p ocess is ca ied ou in an enclosed build chambe wi h an ine gas a mosphe e o
a oid he oxida ion and c oss-con amina ion o he pa s.
Ma e ials 2023, 16, x FOR PEER REVIEW 4 o 34
se e al echnologies, which include bu a e no limi ed o lase PBF (L-PBF) and elec on
beam PBF (EB-PBF). Howe e , hey a e all based on he same wo king p inciple.
PBF p ocesses a e cu en ly widely accep ed in he indus y and well-es ablished in
he ae ospace and medical ields. In pa icula , L-PBF (popula ly e e ed o as selec i e
lase mel ing o SLM) is p obably he mos ex ended me al AM p ocess. In Figu e 1, a
ypical L-PBF sys em is schema ically illus a ed. On he le -hand side, he main sys em
componen s a e shown. On he igh –hand side, he ypical eeds ock equi ed o his
p ocess and an example o i s applica ion a e also included.
The L-PBF p ocess can be desc ibed as ollows:
1. The powde deli e y pis on pushes he powde ese oi up and he ecoa e sp eads
a laye o esh powde on o ei he he building pla o m ( i s laye ) o he
p e iously deposi ed laye s (nex laye s) o o m he powde bed. This powde bed
should be p ope ly dis ibu ed o ensu e he densi ica ion o he manu ac u ed pa s;
2. The lase deli e y sys em i adia es a lase beam, which is guided by he scanning
sys em, along he pa h p ede ined by he sliced 3D model da a. As shown in Figu e
1, he eeds ock powde is qui e ine and he ypical diame e ange o he powde
is 10–60 µm [27,28]. Du ing his p ocess, a mel pool is gene a ed, whose dep h needs
o exceed he laye hickness o gua an ee p ope bonding o he laye s (Figu e 2);
3. Once he laye is inished and he pa e n is solidi ied, he build pla o m goes down
and s eps 1 and 2 a e epea ed. By o e lapping subsequen laye s ( ypically 30- o 90-
µm- hick) and i e a i ely epea ing his cycle, he AM pa is o med, achie ing
esul s simila o he ae ospike nozzle shown in Figu e 1 [29]. No e ha he whole
p ocess is ca ied ou in an enclosed build chambe wi h an ine gas a mosphe e o
a oid he oxida ion and c oss-con amina ion o he pa s.
Figu e 1. L-PBF sys em, eeds ock, and example applica ion [28,29].
Figu e 1. L-PBF sys em, eeds ock, and example applica ion [28,29].
Ma e ials 2023,16, 1746 5 o 33
Ma e ials 2023, 16, x FOR PEER REVIEW 5 o 34
Figu e 2. Wo king p inciple o he L-PBF p ocess.
1.3.2. Di ec ed Ene gy Deposi ion
Di ec ed ene gy deposi ion (DED) p ocesses a e hose “addi i e manu ac u ing
p ocesses in which ocused he mal ene gy is used o use ma e ials by mel ing as hey
a e being deposi ed” [1]. In hese p ocesses, he eeds ock is ypically in he o m o a
powde o wi e, and he he mal sou ce is a lase o an elec ic a c, al hough an elec on
beam can be employed oo. Th ee p ocesses a e adi ionally included in his ca ego y:
lase -di ec ed ene gy deposi ion (L-DED), in which powde o wi e is used as eeds ock
and a lase as a hea sou ce; elec on beam ee- o m ab ica ion (EBF3), in which wi e is
used as he eeds ock and an elec on beam as he hea sou ce; and wi e and a c addi i e
manu ac u ing (WAAM), in which a plasma a c is used as he ene gy sou ce and wi e as
he eeds ock. Owing o he highe popula i y o L-DED and WAAM, hey a e u he
desc ibed in his sec ion.
Lase -di ec ed ene gy deposi ion (L-DED) is he mos ex ended DED p ocess in he
indus y. The ypical se -up o an L-DED sys em is illus a ed in Figu e 3.
Mos o he ime, he eeds ock ma e ial is p o ided in powde o m, and i is ully
mel ed and well-densi ied in o de o p oduce high-quali y componen s [30]. In e ms o
he mul i-ma e ial abili y, one o he main ad an ages o powde L-DED is he wide
ma e ial a ailabili y as compa ed o wi e eeds ock. Ano he ad an age o powde L-DED
is he possibili y o in si u alloy design, as a ious elemen al powde s can be ed o
p oduce he desi ed alloy. Due o he ease o dynamically modi ying he esul ing
composi ion o he powde mix u e du ing he build-up, powde L-DED is he p e e ed
solu ion o mul i-ma e ial AM esea ch. Indeed, wi e L-DED does no sha e such
lexibili ies [31]. Consequen ly, in his e iew, he ocus is placed on powde L-DED. The
powde L-DED is desc ibed as ollows:
1. A lase beam is ocused on o a subs a e whe e a mel pool is c ea ed;
2. Simul aneously, powde pa icles a e injec ed in o he mel pool and he ma e ial is
p og essi ely added o he subs a e (Figu e 4). The powde diame e ange is
ypically 40–150 µm [32,33], as shown in Figu e 3 [34];
3. The powde pa icles a e supplied by he powde eede and d agged by an ine gas
o he nozzle. Addi ionally, a shielding gas is supplied by he nozzle, ypically a gon,
o c ea e a local p o ec i e a mosphe e, whe e he usion and solidi ica ion p ocess
akes place. In his manne , oxida ion o he added ma e ial is a oided, o a leas
minimised;
4. The e is a ela i e mo emen be ween he lase head o powde nozzle and he
subs a e, he eby deposi ing a hin laye co esponding o he c oss-sec ion o he
desi ed geome y;
5. A e a laye is comple ed, he deposi ion o he ollowing laye s a s, hence building
a h ee-dimensional componen laye -by-laye . In his manne , no only can whole
componen s be buil , bu also addi ional ea u es can be added o a p e o m, as is he
case o he pa shown in Figu e 3 [35].
Figu e 2. Wo king p inciple o he L-PBF p ocess.
1.3.2. Di ec ed Ene gy Deposi ion
Di ec ed ene gy deposi ion (DED) p ocesses a e hose “addi i e manu ac u ing p o-
cesses in which ocused he mal ene gy is used o use ma e ials by mel ing as hey a e
being deposi ed” [
1
]. In hese p ocesses, he eeds ock is ypically in he o m o a powde
o wi e, and he he mal sou ce is a lase o an elec ic a c, al hough an elec on beam can be
employed oo. Th ee p ocesses a e adi ionally included in his ca ego y: lase -di ec ed en-
e gy deposi ion (L-DED), in which powde o wi e is used as eeds ock and a lase as a hea
sou ce; elec on beam ee- o m ab ica ion (EBF3), in which wi e is used as he eeds ock
and an elec on beam as he hea sou ce; and wi e and a c addi i e manu ac u ing (WAAM),
in which a plasma a c is used as he ene gy sou ce and wi e as he eeds ock. Owing o he
highe popula i y o L-DED and WAAM, hey a e u he desc ibed in his sec ion.
Lase -di ec ed ene gy deposi ion (L-DED) is he mos ex ended DED p ocess in he
indus y. The ypical se -up o an L-DED sys em is illus a ed in Figu e 3.
Mos o he ime, he eeds ock ma e ial is p o ided in powde o m, and i is ully
mel ed and well-densi ied in o de o p oduce high-quali y componen s [
30
]. In e ms
o he mul i-ma e ial abili y, one o he main ad an ages o powde L-DED is he wide
ma e ial a ailabili y as compa ed o wi e eeds ock. Ano he ad an age o powde L-DED
is he possibili y o in si u alloy design, as a ious elemen al powde s can be ed o p oduce
he desi ed alloy. Due o he ease o dynamically modi ying he esul ing composi ion
o he powde mix u e du ing he build-up, powde L-DED is he p e e ed solu ion o
mul i-ma e ial AM esea ch. Indeed, wi e L-DED does no sha e such lexibili ies [
31
].
Consequen ly, in his e iew, he ocus is placed on powde L-DED. The powde L-DED is
desc ibed as ollows:
1. A lase beam is ocused on o a subs a e whe e a mel pool is c ea ed;
2.
Simul aneously, powde pa icles a e injec ed in o he mel pool and he ma e ial
is p og essi ely added o he subs a e (Figu e 4). The powde diame e ange is
ypically 40–150 µm [32,33], as shown in Figu e 3[34];
3.
The powde pa icles a e supplied by he powde eede and d agged by an ine
gas o he nozzle. Addi ionally, a shielding gas is supplied by he nozzle, ypically
a gon, o c ea e a local p o ec i e a mosphe e, whe e he usion and solidi ica ion
p ocess akes place. In his manne , oxida ion o he added ma e ial is a oided,
o a leas minimised;
4.
The e is a ela i e mo emen be ween he lase head o powde nozzle and he
subs a e, he eby deposi ing a hin laye co esponding o he c oss-sec ion o he
desi ed geome y;
5.
A e a laye is comple ed, he deposi ion o he ollowing laye s a s, hence building
a h ee-dimensional componen laye -by-laye . In his manne , no only can whole
componen s be buil , bu also addi ional ea u es can be added o a p e o m, as is he
case o he pa shown in Figu e 3[35].
Ma e ials 2023,16, 1746 6 o 33
Ma e ials 2023, 16, x FOR PEER REVIEW 6 o 34
Figu e 3. L-DED sys em, eeds ock, and example applica ion [34,35].
Figu e 4. L-DED wo king p inciple.
Conside ing he g owing in e es o he indus y and esea che s in mul i-ma e ial
s uc u es, he g ea es ad an age o he L-DED p ocess seems o be i s pee less mul i-
ma e ial abili y and i s capabili y o in si u modi ica ion o he eeds ock composi ion.
Addi ional nozzles o hoppe s can be in oduced in o he eeding sys em o eed se e al
alloys simul aneously [36]. In his manne , a g adual swi ch om one ma e ial o ano he
is possible and he manu ac u ing o unc ionally g aded ma e ials (FGM) is unlocked. In
addi ion, conside ing ha he eeds ock is in he o m o powde , he in si u alloy design
can also be achie ed by di ec ly mixing he elemen al cons i uen s [37]. Las ly, being a
poin -by-poin manu ac u ing p ocess, he ma e ial mic os uc u e and composi ion can
be ailo ed and speci ic loca ion-dependen physical p ope ies can be a ained. Fo
ins ance, g aded deposi ion has been employed o combine dissimila ma e ials by
ma ching he coe icien s o he mal expansion (CTEs) be ween hem. Addi ionally, he
build-up o componen s aiming o no el mechanical p ope ies h ough new alloy
designs o composi e ma e ials has been demons a ed [38].
Wi e a c addi i e manu ac u ing (WAAM) is nowadays gaining popula i y in bo h
indus y and esea ch owing o i s capabili y o deli e de ec - ee and s uc u ally sound
la ge componen s, which canno be manu ac u ed h ough o he me al AM p ocesses.
Figu e 3. L-DED sys em, eeds ock, and example applica ion [34,35].
Ma e ials 2023, 16, x FOR PEER REVIEW 6 o 34
Figu e 3. L-DED sys em, eeds ock, and example applica ion [34,35].
Figu e 4. L-DED wo king p inciple.
Conside ing he g owing in e es o he indus y and esea che s in mul i-ma e ial
s uc u es, he g ea es ad an age o he L-DED p ocess seems o be i s pee less mul i-
ma e ial abili y and i s capabili y o in si u modi ica ion o he eeds ock composi ion.
Addi ional nozzles o hoppe s can be in oduced in o he eeding sys em o eed se e al
alloys simul aneously [36]. In his manne , a g adual swi ch om one ma e ial o ano he
is possible and he manu ac u ing o unc ionally g aded ma e ials (FGM) is unlocked. In
addi ion, conside ing ha he eeds ock is in he o m o powde , he in si u alloy design
can also be achie ed by di ec ly mixing he elemen al cons i uen s [37]. Las ly, being a
poin -by-poin manu ac u ing p ocess, he ma e ial mic os uc u e and composi ion can
be ailo ed and speci ic loca ion-dependen physical p ope ies can be a ained. Fo
ins ance, g aded deposi ion has been employed o combine dissimila ma e ials by
ma ching he coe icien s o he mal expansion (CTEs) be ween hem. Addi ionally, he
build-up o componen s aiming o no el mechanical p ope ies h ough new alloy
designs o composi e ma e ials has been demons a ed [38].
Wi e a c addi i e manu ac u ing (WAAM) is nowadays gaining popula i y in bo h
indus y and esea ch owing o i s capabili y o deli e de ec - ee and s uc u ally sound
la ge componen s, which canno be manu ac u ed h ough o he me al AM p ocesses.
Figu e 4. L-DED wo king p inciple.
Conside ing he g owing in e es o he indus y and esea che s in mul i-ma e ial
s uc u es, he g ea es ad an age o he L-DED p ocess seems o be i s pee less mul i-
ma e ial abili y and i s capabili y o in si u modi ica ion o he eeds ock composi ion.
Addi ional nozzles o hoppe s can be in oduced in o he eeding sys em o eed se e al
alloys simul aneously [
36
]. In his manne , a g adual swi ch om one ma e ial o ano he
is possible and he manu ac u ing o unc ionally g aded ma e ials (FGM) is unlocked. In
addi ion, conside ing ha he eeds ock is in he o m o powde , he in si u alloy design can
also be achie ed by di ec ly mixing he elemen al cons i uen s [
37
]. Las ly, being a poin -by-
poin manu ac u ing p ocess, he ma e ial mic os uc u e and composi ion can be ailo ed
and speci ic loca ion-dependen physical p ope ies can be a ained. Fo ins ance, g aded
deposi ion has been employed o combine dissimila ma e ials by ma ching he coe icien s
o he mal expansion (CTEs) be ween hem. Addi ionally, he build-up o componen s
aiming o no el mechanical p ope ies h ough new alloy designs o composi e ma e ials
has been demons a ed [38].
Wi e a c addi i e manu ac u ing (WAAM) is nowadays gaining popula i y in bo h
indus y and esea ch owing o i s capabili y o deli e de ec - ee and s uc u ally sound
la ge componen s, which canno be manu ac u ed h ough o he me al AM p ocesses.
WAAM has been in es iga ed since he 1990s [
39
]. Howe e , a ha ime, i did no ecei e
as much a en ion as compa ed o he es o he me al AM p ocesses [40].
Ma e ials 2023,16, 1746 7 o 33
WAAM is, as he name implies, a wi e-based me al AM p ocess, in which an elec ic
a c ac s as he hea sou ce [
39
]. The e o e, adi ional a c welding p ocesses a e he basis o
WAAM, and gas me al a c welding (GMAW), plasma a c welding (PAW), o gas ungs en
a c welding (GTAW) a e commonly employed. Ne e heless, GMAW is he p e e ed choice
o WAAM. In Figu e 5, a scheme o a WAAM sys em is shown. Howe e , al e na i e
con igu a ions a e also possible, as WAAM is an open a chi ec u e p ocess. Examples o a
wi e used o WAAM [
41
] and a i anium wing spa manu ac u ed using WAAM o BAE
sys ems [42] a e also shown in Figu e 5.
Ma e ials 2023, 16, x FOR PEER REVIEW 7 o 34
WAAM has been in es iga ed since he 1990s [39]. Howe e , a ha ime, i did no ecei e
as much a en ion as compa ed o he es o he me al AM p ocesses [40].
WAAM is, as he name implies, a wi e-based me al AM p ocess, in which an elec ic
a c ac s as he hea sou ce [39]. The e o e, adi ional a c welding p ocesses a e he basis
o WAAM, and gas me al a c welding (GMAW), plasma a c welding (PAW), o gas
ungs en a c welding (GTAW) a e commonly employed. Ne e heless, GMAW is he
p e e ed choice o WAAM. In Figu e 5, a scheme o a WAAM sys em is shown.
Howe e , al e na i e con igu a ions a e also possible, as WAAM is an open a chi ec u e
p ocess. Examples o a wi e used o WAAM [41] and a i anium wing spa manu ac u ed
using WAAM o BAE sys ems [42] a e also shown in Figu e 5.
Figu e 5. WAAM sys em, eeds ock, and example applica ion [41,42].
The GMAW-based WAAM p ocess is desc ibed as ollows [43]:
1. An elec ic a c is s uck be ween he subs a e and he consumable wi e, leading o
he o ma ion o a mel pool, as shown in Figu e 6. This usion p ocess occu s unde
he p o ec ion o a shielding gas, ypically a gon o helium [44];
2. Then, as he wi e is pushed in o he mel pool, he ma e ial is used and solidi ied.
The consumable wi e is con inuously supplied. Simul aneously, he obo ic a m
mo es he welding head and a clad is o med;
3. By p ope ly o e lapping clads and by o e laying subsequen laye s, he AM
componen is gene a ed.
Figu e 5. WAAM sys em, eeds ock, and example applica ion [41,42].
The GMAW-based WAAM p ocess is desc ibed as ollows [43]:
1.
An elec ic a c is s uck be ween he subs a e and he consumable wi e, leading o
he o ma ion o a mel pool, as shown in Figu e 6. This usion p ocess occu s unde
he p o ec ion o a shielding gas, ypically a gon o helium [44];
2.
Then, as he wi e is pushed in o he mel pool, he ma e ial is used and solidi ied. The
consumable wi e is con inuously supplied. Simul aneously, he obo ic a m mo es
he welding head and a clad is o med;
3.
By p ope ly o e lapping clads and by o e laying subsequen laye s, he AM compo-
nen is gene a ed.
Ma e ials 2023, 16, x FOR PEER REVIEW 8 o 34
Figu e 6. WAAM wo king p inciple.
1.3.3. C i ical Compa ison o he Main Me al AM P ocesses
To conclude, quali a i e and quan i a i e compa isons o he abo e p ocesses a e
shown in Figu e 7 and Table 2, espec i ely.
F om a quali a i e pe spec i e, nine di e en aspec s a e conside ed, namely he
economical aspec (“cos compe i i eness”), he ma e ial p ocessabili y o ma e ial ange
ha hey can be applied o (“ma e ial selec ion”), he e iciency in e ms o he ma e ial
consump ion (“ma e ial e iciency”), he abili y (o lack he eo ) o mi iga e esidual
s esses (“ esidual s ess mi iga ion”), he quali y in e ms o he mechanical p ope ies o
he AM componen s (“mechanical p ope ies”), he geome ic lexibili y o he p ocess
(“geome ical complexi y”), he amoun o pos -p ocessing needed a e he AM p ocess
(“nea -ne -shape”), he capabili y o ma e ial addi ion in di e en geome ical su aces
(“ ee- o m abili y”), and he s aigh - o wa d abili y o p oduce mul i-ma e ial s uc u es
(“mul i-ma e ial abili y”).
Figu e 7. Quali a i e compa ison be ween L-PBF, L-DED, and WAAM. Adap ed om [45].
In Table 2, a summa ised compa ison be ween he h ee main me al AM p ocesses is
p esen ed. This compa ison elies on key indica o s o he quali y and applicabili y o
me al AM p ocesses. Namely, he maximum pa dimension, he ypical su ace inish and
dimensional accu acy a ainable, he p oduc i i y in e ms o he build a e, and he
densi ica ion o each p ocess a e assessed.
Figu e 6. WAAM wo king p inciple.
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1.3.3. C i ical Compa ison o he Main Me al AM P ocesses
To conclude, quali a i e and quan i a i e compa isons o he abo e p ocesses a e
shown in Figu e 7and Table 2, espec i ely.
Ma e ials 2023, 16, x FOR PEER REVIEW 8 o 34
Figu e 6. WAAM wo king p inciple.
1.3.3. C i ical Compa ison o he Main Me al AM P ocesses
To conclude, quali a i e and quan i a i e compa isons o he abo e p ocesses a e
shown in Figu e 7 and Table 2, espec i ely.
F om a quali a i e pe spec i e, nine di e en aspec s a e conside ed, namely he
economical aspec (“cos compe i i eness”), he ma e ial p ocessabili y o ma e ial ange
ha hey can be applied o (“ma e ial selec ion”), he e iciency in e ms o he ma e ial
consump ion (“ma e ial e iciency”), he abili y (o lack he eo ) o mi iga e esidual
s esses (“ esidual s ess mi iga ion”), he quali y in e ms o he mechanical p ope ies o
he AM componen s (“mechanical p ope ies”), he geome ic lexibili y o he p ocess
(“geome ical complexi y”), he amoun o pos -p ocessing needed a e he AM p ocess
(“nea -ne -shape”), he capabili y o ma e ial addi ion in di e en geome ical su aces
(“ ee- o m abili y”), and he s aigh - o wa d abili y o p oduce mul i-ma e ial s uc u es
(“mul i-ma e ial abili y”).
Figu e 7. Quali a i e compa ison be ween L-PBF, L-DED, and WAAM. Adap ed om [45].
In Table 2, a summa ised compa ison be ween he h ee main me al AM p ocesses is
p esen ed. This compa ison elies on key indica o s o he quali y and applicabili y o
me al AM p ocesses. Namely, he maximum pa dimension, he ypical su ace inish and
dimensional accu acy a ainable, he p oduc i i y in e ms o he build a e, and he
densi ica ion o each p ocess a e assessed.
Figu e 7. Quali a i e compa ison be ween L-PBF, L-DED, and WAAM. Adap ed om [45].
Table 2. L-PBF, L-DED, and WAAM compa ison [45–57].
Fea u e L-PBF L-DED WAAM
Pa dimensions [mm] max. 600 ×600 ×600 Vi ually unlimi ed Vi ually unlimi ed
Su ace inish, Ra [µm] 9–16 5–30 200
Dimensional accu acy [mm] 0.05–0.1 0.5–1.0 1.0–2.0
Build a e [g·min−1]3–4 6–50 300–400
Densi ica ion >99% >99% >99%
F om a quali a i e pe spec i e, nine di e en aspec s a e conside ed, namely he
economical aspec (“cos compe i i eness”), he ma e ial p ocessabili y o ma e ial ange
ha hey can be applied o (“ma e ial selec ion”), he e iciency in e ms o he ma e ial
consump ion (“ma e ial e iciency”), he abili y (o lack he eo ) o mi iga e esidual s esses
(“ esidual s ess mi iga ion”), he quali y in e ms o he mechanical p ope ies o he AM
componen s (“mechanical p ope ies”), he geome ic lexibili y o he p ocess (“geome ical
complexi y”), he amoun o pos -p ocessing needed a e he AM p ocess (“nea -ne -
shape”), he capabili y o ma e ial addi ion in di e en geome ical su aces (“ ee- o m
abili y”), and he s aigh - o wa d abili y o p oduce mul i-ma e ial s uc u es (“mul i-
ma e ial abili y”).
In Table 2, a summa ised compa ison be ween he h ee main me al AM p ocesses
is p esen ed. This compa ison elies on key indica o s o he quali y and applicabili y o
me al AM p ocesses. Namely, he maximum pa dimension, he ypical su ace inish
and dimensional accu acy a ainable, he p oduc i i y in e ms o he build a e, and he
densi ica ion o each p ocess a e assessed.
Based on Figu e 7and Table 2, i is concluded ha while good mechanical p ope ies
and ully dense componen s can be achie ed using all p ocesses, each p ocess has i s speci ic
ield o applica ion. Fi s ly, he employmen o L-PBF has an inhe en pa dimension
es ic ion. Indeed, i is limi ed by he olume o he enclosed chambe whe e he building
p ocess akes place, while L-DED and WAAM ha e no such limi a ion. Addi ionally,
WAAM pe mi s much highe deposi ion a es, meaning be e e iciency can be achie ed in
Ma e ials 2023,16, 1746 9 o 33
e ms o p oduc i i y. None heless, as a as he pa complexi y is conce ned, WAAM is
only sui ed o medium-low complexi y geome ies, while bo h L-PBF and L-DED ha e
he capabili y o manu ac u ing mo e complex componen s. Howe e , he L-PBF p ocess
is be e sui ed o high-p ecision applica ions, whe e a good su ace inish, dimensional
ole ances, and educed pos -p ocessing a e equi ed.
Ano he ele an aspec ha di e en ia es me al AM p ocesses is whe he hey ha e
a s aigh - o wa d mul i-ma e ial abili y o no . While po en ially all L-PBF, L-DED, and
WAAM p ocesses can be used o mul i-ma e ial s uc u es, only powde L-DED o e s he
equi ed lexibili y o he in si u modi ica ion o he eeds ock composi ion as an addi ional
p ocess a iable [25].
2. Addi i e Manu ac u ing o Mul i-Ma e ial S uc u es
Con en ional manu ac u ing p ocesses canno yield mul i-ma e ial componen s in a
single ope a ion. Indeed, single-ma e ial pa s mus be sepa a ely p oduced and joined
in a la e ab ica ion s ep o c ea e a composi e assembly. Con e sely, mul i-ma e ial
AM unlocks he p ocessing o composi e s uc u es wi h ei he a g aded o sha p an-
si ion be ween ma e ials, in a con inuous s ep and in a single machine [
26
]. Hence, he
manu ac u ing chain can be signi ican ly sho ened and simpli ied.
Fu he mo e, mul i-ma e ial s uc u es allow o locally ailo ed ma e ial p ope ies.
In his manne , di e en unc ionali ies can be in eg a ed in o a single componen and he
pe o mance can be subsequen ly enhanced [
58
]. Mo eo e , mul i-ma e ial s uc u es build
on he s eng hs o each o hei cons i uen s [
59
]; hus, hey ou pe o m monoli hic o
single-ma e ial solu ions. In his manne , a new ma e ial-cen ic ab ica ion pa adigm was
made a ailable, cha ac e ised by poin -wise con ol o he composi ion and he ma e ial
s uc u e [60].
Since he incep ion o AM, he e has been a s ong ocus on ee- o m abili y and
design eedom in e ms o geome y [
61
]. In con as , ma e ial-wise design eedom has
gained momen um la ely. Fo ins ance, he op imisa ion o he ma e ial dis ibu ion and
he composi ion o maximise i s u ilisa ion has been in es iga ed [
62
,
63
]. The eme ging
esea ch in he mul i-ma e ial AM ield includes he p ocessing o unc ionally g aded
s uc u es, bime allic s uc u es, o me al–ce amic composi e ma e ials [64–66].
Wi hin he se en AM p ocess ca ego ies co e ed by he ISO/ASTM s anda d [
1
], i e
o hem ha e been iden i ied as ha ing po en ial iabili y o mul i-ma e ial manu ac u -
ing [
26
]. Acco ding o Bandyopadhyay and Hee , binde je ing, ma e ial je ing, ma e ial
ex usion, di ec ed ene gy deposi ion, and shee lamina ion p ocesses ha e an inhe en abil-
i y o mul i-ma e ial AM. Howe e , ecen s udies ha e also in es iga ed mul i-ma e ial
powde bed usion p ocesses, meaning ha he mul i-ma e ial abili y o AM p ocesses is
s ill being explo ed. In Figu e 8, he mechanisms o mul i-ma e ial manu ac u ing using
di e en AM p ocesses a e depic ed. In e ms o he mul i-ma e ial abili y, a decisi e
di e en ia ion mus be made. Indeed, all o he AM p ocesses men ioned can p ocess
mixed ma e ials. Howe e , jus a ew AM p ocesses can con ol he ma e ial composi ion
locally, meaning ha he eeds ock composi ion can be hough o as an addi ional p ocess
a iable, which can be adjus ed du ing he build-up. This dis inc ion is made in Table 3.
Table 3. Di e en ia ed mul i-ma e ial abili y o AM p ocesses.
Can P ocess Mixed Feeds ock Can Con ol he Composi ion o he Feeds ock Locally
Powde Bed Fusion, Binde Je ing
Di ec ed Ene gy Deposi ion, Shee Lamina ion
1
, Ma e ial
Ex usion, Ma e ial Je ing
1
The abili y o shee lamina ion p ocesses o con ol he eeds ock composi ion locally is es ic ed o a laye -wise
disc e e a ia ion.
Ma e ials 2023,16, 1746 16 o 33
Ma e ials 2023, 16, x FOR PEER REVIEW 16 o 34
mul i-ma e ial mix u es place L-DED in a highly compe i i e posi ion o he p oduc ion
o MMC pa s and coa ings [68,132].
MMC s uc u es a e ypically ab ica ed h ough powde -based p ocesses, whe e he
ce amic and he me allic powde s a e p emixed in a ios anging om 1 o 20 w . % [133].
None heless, mix u es con aining highe amoun s o he ce amic phase a e also equen ly
epo ed. The L-DED o MMCs can be app oached in wo di e en ways, depending on
he mechanism o he o ma ion o he ein o cemen phase (Figu e 13) [22].
Figu e 13. Fo ma ion mechanisms o MMCs using L-DED. Images om [134,135].
1. Ex si u p oduc ion o MMC (Figu e 13a): The i s app oach consis s o he p ojec ion
o a powde mix u e wi h a p ecise olume ic ac ion o he ce amic phase in o he
mel pool. In ei he case, he eac ion be ween he ce amic and he me allic phase is
limi ed and con olled. To his end, he p ocess pa ame e s and he eeds ock
mo phology and g anulome y should be selec ed so as o gua an ee ha no
excessi e dissolu ion o he ce amic phase occu s. In addi ion, he p ocess pa ame e s
should be selec ed so as o gua an ee p ope bonding be ween phases.
2. In si u p oduc ion o MMC (Figu e 13b): In he second app oach, a mix u e o
elemen al powde s is in oduced in o he mel pool. The high p ocessing
empe a u es used in L-DED allow chemical eac ions be ween elemen s o occu ,
esul ing in he o ma ion o dispe se ca bides o in e me allics. Con e sely, a
ce amic–me allic powde mix u e can be ed bu he comple e decomposi ion o he
ce amic phase mus be ensu ed so ha he in si u syn hesis o dispe sed ca bides
akes place. In his manne , MMCs may be in-si u-syn hesised. In bo h cases, he
p ocess pa ame e s and he powde mo phology should be ca e ully selec ed o
acili a e he in-si u syn hesis o ca bides.
Figu e 13. Fo ma ion mechanisms o MMCs using L-DED. Images om [134,135].
Acco ding o he o mal de ini ion o he composi es, he ma e ial sys ems ob ained
h ough he second app oach do no co espond o MMCs bu a he o alloys wi h dis-
pe sed ha d phases, as no mac oscopic phases could be dis inguished. Howe e , hese
complex alloys a e o en e e ed o as MMCs in he li e a u e. In addi ion, con olling
he dissolu ion o he ce amic phase in ex-si u-p oduced MMCs is no a i ial ma e . In
ce amic- ein o ced MMCs p oduced ia L-DED, he ce amic pa icles o en su e pa ial
dissolu ion, which p omo es in e acial eac ions be ween he ce amic and me allic ma e-
ials, as shown in Figu e 14 [
136
]. As a esul , complex hie a chical s uc u es a e o en
obse ed. This phenomenon has been in es iga ed in he li e a u e; hence, he ac o s
go e ning i and i s consequences a e mo e ex ensi ely discussed in Sec ion 3.4.
Ma e ials 2023, 16, x FOR PEER REVIEW 17 o 34
Acco ding o he o mal de ini ion o he composi es, he ma e ial sys ems ob ained
h ough he second app oach do no co espond o MMCs bu a he o alloys wi h
dispe sed ha d phases, as no mac oscopic phases could be dis inguished. Howe e , hese
complex alloys a e o en e e ed o as MMCs in he li e a u e. In addi ion, con olling he
dissolu ion o he ce amic phase in ex-si u-p oduced MMCs is no a i ial ma e . In
ce amic- ein o ced MMCs p oduced ia L-DED, he ce amic pa icles o en su e pa ial
dissolu ion, which p omo es in e acial eac ions be ween he ce amic and me allic
ma e ials, as shown in Figu e 14 [136]. As a esul , complex hie a chical s uc u es a e
o en obse ed. This phenomenon has been in es iga ed in he li e a u e; hence, he
ac o s go e ning i and i s consequences a e mo e ex ensi ely discussed in Sec ion 3.4.
Figu e 14. Schema ic illus a ion o he me al–ce amic in e ac ion du ing he L-DED o MMC
coa ings and o ma ion o hie a chical s uc u es.
In essence, L-DED is a highly compe i i e echnology ha has he po en ial o o m
MMC s uc u es, owing o he high lexibili y when p ocessing he mul i-ma e ial
s uc u es [137]. A he same ime, L-DED enables he deposi ion o high-quali y su ace
coa ings, which is he main applica ion o ce amic- ein o ced MMCs. D i en by his
mo i a ion, se e al aspec s o he p oduc ion o MMCs h ough L-DED ha e been
in es iga ed. Acco dingly, he mos ele an s udies in he li e a u e a e summa ised.
3.4. Mos Rele an Li e a u e on L-DED o Me al Ma ix Composi es
The esea ch ca ied ou in his ield has ocused on he analysis o di e en ma e ial
sys ems and hei pe o mance. Fo ins ance, Nu minen e al. compa ed a ious ma e ial
sys ems consis ing o a me al binde and dispe se ca bides [138]. They ocused on
add essing ma e ial compa ibili y issues and hei e ec on he pe o mance om a
ibological pe spec i e. They concluded ha he p ope ies o MMCs we e s ongly
linked o he ma e ial selec ion and he chemical a ini y be ween he cons i uen s. Indeed,
ma e ial sys ems wi h high a ini y would p omo e he decomposi ion o he ce amic
phase, which e en ually was ound o be de imen al o he pe o mance o he coa ing.
Jiang and Ko ace ic ab ica ed MMC coa ings con aining TiC and AISI H13 ool s eel and
compa ed he ibological beha iou o his ma e ial sys em o o he ma e ial sys ems
p e iously epo ed in he li e a u e [139]. Adam e al. in es iga ed he pe o mances o
di e en ma e ial sys ems o ballis ic applica ions and s udied he sui abili y o L-DED
o he p oduc ion o MMC coa ings [140]. Zhang and Ko ace ic in es iga ed he
ibological pe o mance o MMC coa ings composed o an AISI 420 s eel ma ix and
di e en ca bides wi h he aim o p o iding some insigh s on ma e ial selec ion [128]
(Figu e 15). These s udies demons a ed he adequacy o MMC coa ings manu ac u ed by
means o L-DED o he su ace modi ica ion o componen s exposed o se e e ope a ing
condi ions and ou lined some guidelines o acili a e ma e ial selec ion. Howe e , a
limi ed ocus was placed on unde s anding how he p ocessing condi ions a ec he
pe o mance o MMC coa ings wi hin a sole ma e ial sys em. Hence, gene ic conclusions
may be d awn, bu no deep unde s anding was gained conce ning he speci ic
Figu e 14.
Schema ic illus a ion o he me al–ce amic in e ac ion du ing he L-DED o MMC coa ings
and o ma ion o hie a chical s uc u es.
In essence, L-DED is a highly compe i i e echnology ha has he po en ial o o m
MMC s uc u es, owing o he high lexibili y when p ocessing he mul i-ma e ial s uc-
u es [
137
]. A he same ime, L-DED enables he deposi ion o high-quali y su ace coa ings,
Ma e ials 2023,16, 1746 17 o 33
which is he main applica ion o ce amic- ein o ced MMCs. D i en by his mo i a ion,
se e al aspec s o he p oduc ion o MMCs h ough L-DED ha e been in es iga ed. Ac-
co dingly, he mos ele an s udies in he li e a u e a e summa ised.
3.4. Mos Rele an Li e a u e on L-DED o Me al Ma ix Composi es
The esea ch ca ied ou in his ield has ocused on he analysis o di e en ma e ial
sys ems and hei pe o mance. Fo ins ance, Nu minen e al. compa ed a ious ma e ial
sys ems consis ing o a me al binde and dispe se ca bides [
138
]. They ocused on add ess-
ing ma e ial compa ibili y issues and hei e ec on he pe o mance om a ibological
pe spec i e. They concluded ha he p ope ies o MMCs we e s ongly linked o he
ma e ial selec ion and he chemical a ini y be ween he cons i uen s. Indeed, ma e ial
sys ems wi h high a ini y would p omo e he decomposi ion o he ce amic phase, which
e en ually was ound o be de imen al o he pe o mance o he coa ing. Jiang and
Ko ace ic ab ica ed MMC coa ings con aining TiC and AISI H13 ool s eel and compa ed
he ibological beha iou o his ma e ial sys em o o he ma e ial sys ems p e iously
epo ed in he li e a u e [
139
]. Adam e al. in es iga ed he pe o mances o di e en
ma e ial sys ems o ballis ic applica ions and s udied he sui abili y o L-DED o he
p oduc ion o MMC coa ings [
140
]. Zhang and Ko ace ic in es iga ed he ibological
pe o mance o MMC coa ings composed o an AISI 420 s eel ma ix and di e en ca bides
wi h he aim o p o iding some insigh s on ma e ial selec ion [
128
] (Figu e 15). These
s udies demons a ed he adequacy o MMC coa ings manu ac u ed by means o L-DED
o he su ace modi ica ion o componen s exposed o se e e ope a ing condi ions and
ou lined some guidelines o acili a e ma e ial selec ion. Howe e , a limi ed ocus was
placed on unde s anding how he p ocessing condi ions a ec he pe o mance o MMC
coa ings wi hin a sole ma e ial sys em. Hence, gene ic conclusions may be d awn, bu no
deep unde s anding was gained conce ning he speci ic mic os uc u es ha should be
a ge ed and no me hodologies we e p oposed o ma e ialise i .
Ma e ials 2023, 16, x FOR PEER REVIEW 18 o 34
mic os uc u es ha should be a ge ed and no me hodologies we e p oposed o
ma e ialise i .
Figu e 15. MMC coa ings cons i u ed by an AISI 420 ma ix and di e en ein o cemen ma e ials.
Adap ed om [128].
O he s udies ocused on he mic os uc u al e olu ion o MMC coa ings due o he
high- empe a u e p ocessing o L-DED. As men ioned abo e, he dissolu ion o he
ce amic pa icles in ex-si u-p oduced MMCs is ha dly a oidable, and his phenomenon
a ec s bo h he mic os uc u al aspec s and mechanical p ope ies. Li e al. in es iga ed
(C , W)23C6- and WC- ein o ced Fe-based composi e coa ings. They epo ed he phase
ans o ma ions occu ing as a esul o he L-DED p ocess. Dispe se eu ec ic ca bides
we e ound in he ma ix s uc u e, bu ine ca bides also p ecipi a ed in he in e dend i ic
phase [122]. Along he same line, Zhao e al. in es iga ed he dissolu ion o he
ein o cemen phase o WC-Ni ma e ial sys ems in [141] and WC-Fe in [142]. In hei
s udies, a ho ough cha ac e isa ion p ocess o he mic os uc u e gene a ed as a esul o
he in e ac ion be ween he ein o cemen phase and he ma ix phase was ca ied ou .
They obse ed ha ca bides wi h di e en mo phologies we e dispe sed in he me al
ma ix, while e ained pa icles we e also encoun e ed. In addi ion, he main mechanisms
esponsible o he decomposi ion o he ce amic pa icles we e epo ed, namely
dissolu ion, di usion, pa ial o comple e agmen a ion, and p ecipi a ion. F om a
p ocessing pe spec i e, Mu ala e al. in es iga ed he me al–ce amic in e ace in a WC-
ein o ced Ni-based alloy. They ound ha he decomposi ion o he ein o cemen
pa icles played a majo ole in he bonding ype. Mo eo e , hey measu ed he mel pool
li e ime and co ela ed i wi h he hickness o he eac ion laye su ounding he ce amic
pa icles, as shown in Figu e 16 [126].
These s udies ha e b ough o ligh he impo ance o he in e ac ions be ween he
cons i uen phases in MMC composi es, especially conside ing he high empe a u es
inhe en o L-DED p ocessing. Howe e , ew s udies ha e been ca ied ou in ol ing a
ho ough in es iga ion and discussion o he e ec s o he p ocess pa ame e s and he
he mal cycle o he manu ac u ing p ocess. In ac , none o he p e ious au ho s ound a
co ela ion be ween he p ocess pa ame e s, mic os uc u e, and esul ing p ope ies o
he coa ings. Hence, he ole o he p ocessing pa ame e s in he me al–ce amic
in e ac ions and he mic os uc u es and p ope ies o MMC coa ings has been neglec ed,
especially in he o ma ion o unexpec ed mic os uc u al phases. I is ou o he ques ion
ha he L-DED pa ame e s a e esponsible o he he mal cycle gene a ed du ing
p ocessing. Thus, he p ocess pa ame e s a ec he dissolu ion o he ein o cemen
pa icles, and p esumably he mechanical p ope ies, such as he ha dness, o MMC
coa ings.
Figu e 15.
MMC coa ings cons i u ed by an AISI 420 ma ix and di e en ein o cemen ma e ials.
Adap ed om [128].
O he s udies ocused on he mic os uc u al e olu ion o MMC coa ings due o he
high- empe a u e p ocessing o L-DED. As men ioned abo e, he dissolu ion o he ce amic
pa icles in ex-si u-p oduced MMCs is ha dly a oidable, and his phenomenon a ec s bo h
he mic os uc u al aspec s and mechanical p ope ies. Li e al. in es iga ed (C , W)
23
C
6
-
and WC- ein o ced Fe-based composi e coa ings. They epo ed he phase ans o ma ions
occu ing as a esul o he L-DED p ocess. Dispe se eu ec ic ca bides we e ound in
he ma ix s uc u e, bu ine ca bides also p ecipi a ed in he in e dend i ic phase [
122
].
Along he same line, Zhao e al. in es iga ed he dissolu ion o he ein o cemen phase
o WC-Ni ma e ial sys ems in [
141
] and WC-Fe in [
142
]. In hei s udies, a ho ough
cha ac e isa ion p ocess o he mic os uc u e gene a ed as a esul o he in e ac ion
be ween he ein o cemen phase and he ma ix phase was ca ied ou . They obse ed ha
ca bides wi h di e en mo phologies we e dispe sed in he me al ma ix, while e ained
pa icles we e also encoun e ed. In addi ion, he main mechanisms esponsible o he
decomposi ion o he ce amic pa icles we e epo ed, namely dissolu ion, di usion, pa ial
o comple e agmen a ion, and p ecipi a ion. F om a p ocessing pe spec i e, Mu ala e al.
Ma e ials 2023,16, 1746 18 o 33
in es iga ed he me al–ce amic in e ace in a WC- ein o ced Ni-based alloy. They ound
ha he decomposi ion o he ein o cemen pa icles played a majo ole in he bonding
ype. Mo eo e , hey measu ed he mel pool li e ime and co ela ed i wi h he hickness
o he eac ion laye su ounding he ce amic pa icles, as shown in Figu e 16 [126].
Ma e ials 2023, 16, x FOR PEER REVIEW 19 o 34
Figu e 16. Reac ion laye and hickness o he ein o cemen phases o MMC coa ings wi h a ying
mel pool li e imes and a ia ions wi h espec o he scan speed. Rep oduced om [143].
In his line, ano he aspec ha has a ac ed he a en ion o esea che s is he
ha dness o MMC coa ings and he co ela ion be ween he ha dness and he olume ic
ac ion o he ein o cemen phase. Mos au ho s epo ha inc easing he olume ic
ac ion o he ein o cemen phase esul s in a subs an ial enhancemen o he ha dness.
Li e al. obse ed a g adual inc ease in ha dness wi h inc easing olume ic ac ions o
WC om 0% o 20%. They asc ibed his o he o ma ion o ca bides and he solid–solu ion
s eng hening due o la ice dis o ion [122]. Xie e al. and Raahgini e al. epo ed a nea ly
linea co ela ion be ween he ha dness and he olume ic ac ion o he ein o cemen
phase o Co-WC [144] and Ni-VC [120] ma e ial sys ems, espec i ely. Deschuy enne e
al., on he o he hand, cha ac e ised he mul i-scale ha dness o a WC- ein o ced Ni-based
alloy. In sho , hey s udied he no malised ha dness o he composi e samples, bu also
he ha dness achie ed by he me al ma ix as a esul o he mic os uc u al modi ica ion
[145]. Os olaza e al. in es iga ed he e ec o he p ocessing pa ame e s and he eeds ock
composi ion on he ha dness o he ma ix and on he ha dness o he composi e in WC-
Co coa ings h ough s a is ical eg ession me hods [146]. I was concluded ha longe
in e ac ion imes p omo ed he decomposi ion o he ce amic pa icles, which esul ed in
an inc ease in he W con en in he ma ix. A he same ime, di e en mechanisms
esponsible o he ha dening o he MMC coa ings we e epo ed, namely g ain
e inemen and ca bide p ecipi a ion, depending on he eeds ock composi ion and
p ocessing condi ions. Zhao e al., on he o he hand, quan i ied he loss in ha dness when
he dilu ion o he subs a e inc eased [141]. As expec ed, he highe he amoun o
subs a e ma e ial ha was mel ed, he lowe he ha dness o he coa ing ma e ial.
Ul ima ely, ha ing highe subs a e dilu ion al e s he composi ion o he coa ing. I he
subs a e ma e ial is so e han he coa ing ma e ial, hen he o e all ha dness o he
coa ing will dec ease.
Las ly, many au ho s ha e ied o demons a e he sui abili y o MMCs o high-
pe o mance coa ing p oduc ion by s udying hei wea esis ance. Li e al. in es iga ed
he ibological beha iou o (C , W)23C6-WC- ein o ced, Fe-based coa ings. They ob ained
good esul s in e ms o he wea esis ance o he MMC samples. Mo e in e es ingly, hey
ound ha he su ace p ope ies we e maximised o an op imal coa ing composi ion
[122]. Xie e al. s udied he wea esis ance o WC-Co MMCs and epo ed a posi i e
co ela ion be ween he wea esis ance and he WC con en o he MMC [144]. Simila
conclusions we e eached by Zhao e al. o WC-Fe [142] and WC-Ni [141] MMC coa ings.
Thei expe imen al esul s sugges ed ha a signi ican gain in ibological pe o mance
could be ob ained by inc easing he WC con en . They asc ibed his e ec o wo
phenomena, namely he e ained WC pa icles ha emained unmel ed and he dispe sed
ca bides h oughou he ma ix. Ba kowski and Kinal achie ed inc eased wea esis ance
by embedding WC pa icles in o a S elli e 6 ma ix as compa ed o he monoli hic S elli e
6 coa ings [124]. They concluded ha he excess WC con en o insu icien ha dness o
he ma ix could p omo e a mo e in ensi e wea mechanism in MMC coa ings. In a la e
Figu e 16.
Reac ion laye and hickness o he ein o cemen phases o MMC coa ings wi h a ying
mel pool li e imes and a ia ions wi h espec o he scan speed. Rep oduced om [143].
These s udies ha e b ough o ligh he impo ance o he in e ac ions be ween he
cons i uen phases in MMC composi es, especially conside ing he high empe a u es
inhe en o L-DED p ocessing. Howe e , ew s udies ha e been ca ied ou in ol ing a
ho ough in es iga ion and discussion o he e ec s o he p ocess pa ame e s and he
he mal cycle o he manu ac u ing p ocess. In ac , none o he p e ious au ho s ound a
co ela ion be ween he p ocess pa ame e s, mic os uc u e, and esul ing p ope ies o he
coa ings. Hence, he ole o he p ocessing pa ame e s in he me al–ce amic in e ac ions
and he mic os uc u es and p ope ies o MMC coa ings has been neglec ed, especially
in he o ma ion o unexpec ed mic os uc u al phases. I is ou o he ques ion ha he
L-DED pa ame e s a e esponsible o he he mal cycle gene a ed du ing p ocessing. Thus,
he p ocess pa ame e s a ec he dissolu ion o he ein o cemen pa icles, and p esumably
he mechanical p ope ies, such as he ha dness, o MMC coa ings.
In his line, ano he aspec ha has a ac ed he a en ion o esea che s is he ha dness
o MMC coa ings and he co ela ion be ween he ha dness and he olume ic ac ion o
he ein o cemen phase. Mos au ho s epo ha inc easing he olume ic ac ion o he
ein o cemen phase esul s in a subs an ial enhancemen o he ha dness. Li e al. obse ed
a g adual inc ease in ha dness wi h inc easing olume ic ac ions o WC om 0% o 20%.
They asc ibed his o he o ma ion o ca bides and he solid–solu ion s eng hening due o
la ice dis o ion [
122
]. Xie e al. and Raahgini e al. epo ed a nea ly linea co ela ion be-
ween he ha dness and he olume ic ac ion o he ein o cemen phase o Co-WC [
144
]
and Ni-VC [
120
] ma e ial sys ems, espec i ely. Deschuy enne e al., on he o he hand,
cha ac e ised he mul i-scale ha dness o a WC- ein o ced Ni-based alloy. In sho , hey
s udied he no malised ha dness o he composi e samples, bu also he ha dness achie ed
by he me al ma ix as a esul o he mic os uc u al modi ica ion [
145
].
Os olaza e al.
in es iga ed he e ec o he p ocessing pa ame e s and he eeds ock composi ion on he
ha dness o he ma ix and on he ha dness o he composi e in WC-Co coa ings h ough
s a is ical eg ession me hods [
146
]. I was concluded ha longe in e ac ion imes p o-
mo ed he decomposi ion o he ce amic pa icles, which esul ed in an inc ease in he W
con en in he ma ix. A he same ime, di e en mechanisms esponsible o he ha dening
o he MMC coa ings we e epo ed, namely g ain e inemen and ca bide p ecipi a ion, de-
pending on he eeds ock composi ion and p ocessing condi ions. Zhao e al., on he o he
hand, quan i ied he loss in ha dness when he dilu ion o he subs a e inc eased [
141
].
As expec ed, he highe he amoun o subs a e ma e ial ha was mel ed, he lowe he
ha dness o he coa ing ma e ial. Ul ima ely, ha ing highe subs a e dilu ion al e s he
composi ion o he coa ing. I he subs a e ma e ial is so e han he coa ing ma e ial, hen
he o e all ha dness o he coa ing will dec ease.
Ma e ials 2023,16, 1746 19 o 33
Las ly, many au ho s ha e ied o demons a e he sui abili y o MMCs o high-
pe o mance coa ing p oduc ion by s udying hei wea esis ance. Li e al. in es iga ed
he ibological beha iou o (C , W)
23
C
6
-WC- ein o ced, Fe-based coa ings. They ob ained
good esul s in e ms o he wea esis ance o he MMC samples. Mo e in e es ingly, hey
ound ha he su ace p ope ies we e maximised o an op imal coa ing composi ion [
122
].
Xie e al. s udied he wea esis ance o WC-Co MMCs and epo ed a posi i e co ela ion
be ween he wea esis ance and he WC con en o he MMC [
144
]. Simila conclusions
we e eached by Zhao e al. o WC-Fe [
142
] and WC-Ni [
141
] MMC coa ings. Thei
expe imen al esul s sugges ed ha a signi ican gain in ibological pe o mance could be
ob ained by inc easing he WC con en . They asc ibed his e ec o wo phenomena, namely
he e ained WC pa icles ha emained unmel ed and he dispe sed ca bides h oughou
he ma ix. Ba kowski and Kinal achie ed inc eased wea esis ance by embedding WC
pa icles in o a S elli e 6 ma ix as compa ed o he monoli hic S elli e 6 coa ings [
124
]. They
concluded ha he excess WC con en o insu icien ha dness o he ma ix could p omo e
a mo e in ensi e wea mechanism in MMC coa ings. In a la e publica ion, hey ob ained
p omising esul s when es ing he beha iou o he designed coa ing in eal condi ions
when applied o an ag icul u al ool (Figu e 17). They epo ed a 25% inc ease in he li e o
he ooling [125].
Ma e ials 2023, 16, x FOR PEER REVIEW 20 o 34
publica ion, hey ob ained p omising esul s when es ing he beha iou o he designed
coa ing in eal condi ions when applied o an ag icul u al ool (Figu e 17). They epo ed
a 25% inc ease in he li e o he ooling [125].
In his ega d, Mu ala e al. ca ied ou a comp ehensi e in es iga ion on he
con ibu ion o he ce amic phase o wea esis ance [126]. They concluded ha he
uncon olled dissolu ion o he ce amic pa icles caused he emb i lemen o he ma ix
phase. As a esul , he ac u ing and spalling o he su ace du ing wea es ing we e
p omo ed; hence, hese samples su e ed a highe wea a e. I can be concluded ha
inc easing he ein o cemen con en does con ibu e o a highe wea esis ance o MMC
coa ings, p o ided ha he ce amic pa icles do no ac u e and ha he hi d body
mechanism is a oided [120]. Simila conclusions we e p e iously eached by Fe nández
e al. [147]. In MMC coa ings, he disc e e ein o cemen pa icles ule he plas ic low o
he me al ma ix. Hence, inc easing he ein o cemen phase changes he main wea
mechanism; indeed, i swi ches om se e ely adhesi e o mildly adhesi e and ab asi e
wea [148].
Figu e 17. (a) An ag icul u al ool coa ed wi h S elli e 6 and WC and (b) mic os uc u e o he
applied coa ing. Rep oduced om [125].
The high- empe a u e wea esis ance o Co- and Ni-based MMC coa ings has also
been in es iga ed. E anmanesh e al. s udied he high- empe a u e ibological beha iou
o WC- ein o ced Ni- and Co-based ma ices [149]. The ein o ced samples showed
supe io wea esis ance and hey ound ha so ab asi e and adhesi e mechanisms we e
esponsible o he wea damage. Wang e al. also es ed he high- empe a u e wea
esis ance o WC-Co MMC coa ings and con i med he p e iously epo ed good
beha iou o his ma e ial sys em [123]. In addi ion, he expe imen al esul s showed a
posi i e co ela ion be ween he wea esis ance wi h espec o he WC con en .
Howe e , hey epo ed a signi ican d op in he he mal a igue li e when inc easing he
WC con en o e 20% w . Las ly, Ka maka e al. in es iga ed he ab asi e wea o WC-
Co- and Co-coa ed AISI H13 ool s eel subs a es a high empe a u es [109]. The ce amic-
ein o ced coa ings yielded g ea e esis ance o wea up o 650 °C as compa ed o he un-
coa ed subs a es. The supe io su ace p ope ies o he ein o ced coa ings we e mo e
e iden he highe he empe a u e o he ab asi e es .
Al hough he beha iou o MMC coa ings in e ms o wea has been ho oughly
s udied, o he aspec s ha igh ly limi hei pe o mance ha e been comple ely neglec ed.
Fo ins ance, li le e o has been de o ed o he e alua ion o he in e acial s eng h
be ween he MMC coa ing and he subs a e, which will di ec ly a ec he du abili y o
he su ace- ea ed componen s. Indeed, he lexu al s eng h o coa ed pa s and he
esidual s esses induced du ing p ocessing a e key aspec s ha limi he li e and he sa e
ope a ion o coa ed componen s [150]. I is wi hou ques ion ha he su ace p ope ies
and mechanical beha iou o MMC coa ings a e uled by he s eng h o he in e acial
bonds be ween he mac o-cons i uen s, as has been s a ed by se e al esea che s [151,152].
On he one hand, he eac ion laye gua an ees a me allu gical bond; he e o e, he
cohesion o he composi e is ensu ed and one should expec p ope load ans e be ween
he mac oscopic cons i uen s [152]. Con e sely, he dispe sion o disc e e ca bides
h oughou he ma ix may cause a loss in duc ili y, which is de imen al o he s uc u al
Figu e 17.
(
a
) An ag icul u al ool coa ed wi h S elli e 6 and WC and (
b
) mic os uc u e o he applied
coa ing. Rep oduced om [125].
In his ega d, Mu ala e al. ca ied ou a comp ehensi e in es iga ion on he con i-
bu ion o he ce amic phase o wea esis ance [
126
]. They concluded ha he uncon olled
dissolu ion o he ce amic pa icles caused he emb i lemen o he ma ix phase. As
a esul , he ac u ing and spalling o he su ace du ing wea es ing we e p omo ed;
hence, hese samples su e ed a highe wea a e. I can be concluded ha inc easing
he ein o cemen con en does con ibu e o a highe wea esis ance o MMC coa ings,
p o ided ha he ce amic pa icles do no ac u e and ha he hi d body mechanism is
a oided [
120
]. Simila conclusions we e p e iously eached by Fe nández e al. [
147
]. In
MMC coa ings, he disc e e ein o cemen pa icles ule he plas ic low o he me al ma ix.
Hence, inc easing he ein o cemen phase changes he main wea mechanism; indeed, i
swi ches om se e ely adhesi e o mildly adhesi e and ab asi e wea [148].
The high- empe a u e wea esis ance o Co- and Ni-based MMC coa ings has also
been in es iga ed. E anmanesh e al. s udied he high- empe a u e ibological beha iou
o WC- ein o ced Ni- and Co-based ma ices [
149
]. The ein o ced samples showed su-
pe io wea esis ance and hey ound ha so ab asi e and adhesi e mechanisms we e
esponsible o he wea damage. Wang e al. also es ed he high- empe a u e wea esis-
ance o WC-Co MMC coa ings and con i med he p e iously epo ed good beha iou
o his ma e ial sys em [
123
]. In addi ion, he expe imen al esul s showed a posi i e
co ela ion be ween he wea esis ance wi h espec o he WC con en . Howe e , hey
epo ed a signi ican d op in he he mal a igue li e when inc easing he WC con en o e
20% w . Las ly, Ka maka e al. in es iga ed he ab asi e wea o WC-Co- and Co-coa ed
AISI H13 ool s eel subs a es a high empe a u es [
109
]. The ce amic- ein o ced coa ings
yielded g ea e esis ance o wea up o 650
◦
C as compa ed o he un-coa ed subs a es.
Ma e ials 2023,16, 1746 20 o 33
The supe io su ace p ope ies o he ein o ced coa ings we e mo e e iden he highe he
empe a u e o he ab asi e es .
Al hough he beha iou o MMC coa ings in e ms o wea has been ho oughly
s udied, o he aspec s ha igh ly limi hei pe o mance ha e been comple ely neglec ed.
Fo ins ance, li le e o has been de o ed o he e alua ion o he in e acial s eng h
be ween he MMC coa ing and he subs a e, which will di ec ly a ec he du abili y o
he su ace- ea ed componen s. Indeed, he lexu al s eng h o coa ed pa s and he
esidual s esses induced du ing p ocessing a e key aspec s ha limi he li e and he sa e
ope a ion o coa ed componen s [
150
]. I is wi hou ques ion ha he su ace p ope ies and
mechanical beha iou o MMC coa ings a e uled by he s eng h o he in e acial bonds
be ween he mac o-cons i uen s, as has been s a ed by se e al esea che s [
151
,
152
]. On he
one hand, he eac ion laye gua an ees a me allu gical bond; he e o e, he cohesion o he
composi e is ensu ed and one should expec p ope load ans e be ween he mac oscopic
cons i uen s [
152
]. Con e sely, he dispe sion o disc e e ca bides h oughou he ma ix
may cause a loss in duc ili y, which is de imen al o he s uc u al beha iou o he
composi es [
153
]. Thus, he key lies in inding a balance in he ex en o he me al–ce amic
in e ac ion. Me allu gical bonding be ween he ma ix and he ein o cemen should be
sough , bu he emb i lemen o he me al ma ix should be p e en ed o p ese e he good
mechanical p ope ies o he ma ix. This issue may be sol ed by op imising he p ocess
pa ame e s and by uning he he mal cycle du ing p ocessing [126].
In Table 5, a summa y o he mos ele an publica ions ega ding he L-DED o MMC
coa ings is p o ided.
Table 5. Mos ele an publica ions conce ning he L-DED o MMC coa ings.
Publica ions Ma e ials Applica ion Main Resul s Limi a ions/Obse a ions
Jiang and
Ko ace ic, 2007
[139]
AISI H13 and TiC Die and mould
indus y
MMCs con aining less TiC exhibi ed
highe e osion esis ance.
No comp ehensi e discussion o
he mechanisms behind his
phenomenon was p o ided.
Nu minen, Näkki
and Vuo is o,
2009 [138]
Va ious ma ix and
ein o cemen
ma e ials
Miscellaneous
The ab asion esis ance o he MMC does
no depend solely on he ein o cemen
bu also on he ma ix.
The s udy ocused only on he
ma e ial selec ion and no
impo ance was gi en o he
p ocessing condi ions.
Ba kowski and
Ba kowska, 2017
[125]
S elli e 6 and WC Oil and gas
The massi e di e ence in ha dness
be ween he ein o cemen and ma ix
p omo ed se e e wea mechanisms.
P elimina y esul s on he e ec s
o di e en p ocess pa ame e s
we e p o ided, bu no
comp ehensi e analysis o he
unde lying phenomena was gi en
Mu ala, Pa a
Ka maka , and
Na h, 2017 [126]
Inconel 718 and WC Ae ospace
indus y
The longe mel pool li e ime p omo es
he decomposi ion o he ein o cemen
phase and is de imen al o he wea
esis ance o MMC coa ings.
The ha dness o he coa ings and
subsequen ha dening
mechanisms a e no e alua ed.
Li e al., 2021 [
122
]
Fe60 sel - luxing
alloy and WC Miscellaneous
The phase e olu ion o he mul i-ma e ial
coa ing was o mula ed and suppo ed
by mic os uc u al obse a ions.
The e ec o he p ocessing
condi ions was no conside ed and
he pe o mance o he p oposed
coa ings was no e alua ed
compa a i ely.
Zhao e al., 2022
[141]
Ni-based alloy and
WC Miscellaneous
WC pa icles su e om dissolu ion,
di usion, agmen a ion, and
p ecipi a ion mechanisms when exposed
o high empe a u es.
The s udy only ocused on
mic oha dness and no on he
ha dness o he composi e. The
in luence o he he mal cycle o
he p ocess was no conside ed.
Raahgini and
Ve di, 2022 [120]Inconel 625 and VC Miscellaneous
Though showing highe ha dness, MMC
coa ings wi h high ein o cemen
con en s su e ed a loss in wea
esis ance due o he appea ance o he
hi d body wea mechanism.
The e ec o he p ocessing
condi ions was no conside ed.
Ma e ials 2023,16, 1746 21 o 33
In e ms o he p ocessabili y o MMCs h ough L-DED, he ealisa ion o good-quali y
coa ings is s ill a challenge. Fo ins ance, ma e ial incompa ibili y is an issue ha needs o
be ackled [
36
]. Mo eo e , de ec s ela ed o he me allu gical in eg i y (po es o c acking)
a e equen ly encoun e ed [131].
The c acking o MMCs deposi ed by L-DED has been widely s udied in he li e a-
u e [
118
]. C acks in hese coa ings o igina e om he high esidual s esses gene a ed
du ing he deposi ion p ocess [
131
]. As a p ocess based on he usion and apid solidi i-
ca ion o ma e ials, pa s manu ac u ed by L-DED wi hs and high empe a u e g adien s;
hus, high he mal s esses can be p oduced i speci ic ca e is no aken [
118
]. In addi ion,
he in e ac ion be ween he pa icles and he ma ix ends o p o oke ma ix emb i lemen .
The loss o duc ili y p e en s he me al ma ix om abso bing he esidual s esses o
he manu ac u ing p ocess. Addi ionally, he ein o cemen pa icles ac as s ess concen-
a o s [
109
], which u he exace ba e c acking. Consequen ly, i has been s a ed in he
li e a u e ha hese ma e ials o en su e no only om c acking phenomena bu also
o he so s o ca as ophic ailu es due o high esidual s esses and ma e ial p ope y
misma ches, such as delamina ion [154].
Mul iple me hods ha e been p oposed o elimina e c acking, namely subs a e p e-
hea ing, he addi ion o a e ea h oxides, o he elimina ion o he sha p ansi ion be ween
subs a es and coa ings h ough he use o unc ionally g aded ma e ials (FGMs) [
117
].
D i en by his equi emen , in Sec ion 4, he o igin o FGM s uc u es is desc ibed and he
implemen a ion o he FGM s a egy o he ab ica ion o high-quali y MMCs is explo ed.
4. Lase -Di ec ed Ene gy Deposi ion o Func ionally G aded Ma e ials
4.1. O igin and De ini ion o Func ionally G aded Ma e ials
In sho e ms, FGM ma e ials a e a class o ad anced ma e ials in which spa ial
a ia ion o he ma e ial p ope ies occu s wi hin a single s uc u e [
155
]. Di e en c i e ia
can be es ablished o di e en ia e FGM s uc u es. One could classi y FGMs based on he
me hod ollowed o ob ain such a iable p ope ies. Indeed, a iable ma e ial p ope ies
can be achie ed h ough composi ional o mic os uc u al modi ica ions [
62
] (Figu e 18A).
Con e sely, a classi ica ion c i e ion based on he g adien ype has also been p oposed,
i.e., disc e e and con inuous g adien s [
22
] (Figu e 18B). Ei he way, loca ion-dependen
ma e ial p ope ies wi hou sha p ansi ions can be a ained using FGM s uc u es [155].
Ma e ials 2023, 16, x FOR PEER REVIEW 22 o 34
[117]. D i en by his equi emen , in Sec ion 4, he o igin o FGM s uc u es is desc ibed
and he implemen a ion o he FGM s a egy o he ab ica ion o high-quali y MMCs is
explo ed.
4. Lase -Di ec ed Ene gy Deposi ion o Func ionally G aded Ma e ials
4.1. O igin and De ini ion o Func ionally G aded Ma e ials
In sho e ms, FGM ma e ials a e a class o ad anced ma e ials in which spa ial
a ia ion o he ma e ial p ope ies occu s wi hin a single s uc u e [155]. Di e en c i e ia
can be es ablished o di e en ia e FGM s uc u es. One could classi y FGMs based on he
me hod ollowed o ob ain such a iable p ope ies. Indeed, a iable ma e ial p ope ies
can be achie ed h ough composi ional o mic os uc u al modi ica ions [62] (Figu e 18A).
Con e sely, a classi ica ion c i e ion based on he g adien ype has also been p oposed, i.e.,
disc e e and con inuous g adien s [22] (Figu e 18B). Ei he way, loca ion-dependen
ma e ial p ope ies wi hou sha p ansi ions can be a ained using FGM s uc u es [155].
The i s eco d o unc ionally g aded ma e ials (FGM) co esponds o he
de elopmen o his concep o o e come he limi a ion o con en ional composi e
ma e ials in he mal ba ie coa ings (TBC) o ae ospace applica ions [87,156]. As a ma e
o ac , he disco e y was d i en by he equi emen o a TBC ha would wi hs and a
empe a u e g adien highe han 1000 K [157]. Be o e coming up wi h he FGM concep ,
con en ional lamina e composi es we e es ed wi h li le success. I was obse ed ha he
sha p in e ace was esponsible o he ailu e o he composi es. Owing o he di e ence
in he CTEs be ween he me al and ce amic cons i uen s, he con en ional composi es
we e unable o endu e he high he mal g adien s [36,62]. Nowadays, FGMs a e p esen
in many o he applica ions, as a ool o ailo ing mechanical, he mal, o elec ical
p ope ies in a loca ion-dependen manne wi hin one sole componen [22,26].
Figu e 18. FGM ypes acco ding o di e en classi ica ion c i e ia: (A) mechanism o he g adual
p ope ies and (B) he g adien ypes.
In Figu e 19, an FGM ocke nozzle is shown, which was de eloped by InssTek as a
case s udy o demons a e he capabili y o L-DED o build AISI 316L o Al-b onze FGM
componen s a eal indus ial scales.
Figu e 18.
FGM ypes acco ding o di e en classi ica ion c i e ia: (
A
) mechanism o he g adual
p ope ies and (B) he g adien ypes.
Ma e ials 2023,16, 1746 22 o 33
The i s eco d o unc ionally g aded ma e ials (FGM) co esponds o he de elop-
men o his concep o o e come he limi a ion o con en ional composi e ma e ials in
he mal ba ie coa ings (TBC) o ae ospace applica ions [
87
,
156
]. As a ma e o ac , he
disco e y was d i en by he equi emen o a TBC ha would wi hs and a empe a u e
g adien highe han 1000 K [
157
]. Be o e coming up wi h he FGM concep , con en-
ional lamina e composi es we e es ed wi h li le success. I was obse ed ha he sha p
in e ace was esponsible o he ailu e o he composi es. Owing o he di e ence in
he CTEs be ween he me al and ce amic cons i uen s, he con en ional composi es we e
unable o endu e he high he mal g adien s [
36
,
62
]. Nowadays, FGMs a e p esen in many
o he applica ions, as a ool o ailo ing mechanical, he mal, o elec ical p ope ies in a
loca ion-dependen manne wi hin one sole componen [22,26].
In Figu e 19, an FGM ocke nozzle is shown, which was de eloped by InssTek as a
case s udy o demons a e he capabili y o L-DED o build AISI 316L o Al-b onze FGM
componen s a eal indus ial scales.
Ma e ials 2023, 16, x FOR PEER REVIEW 23 o 34
Figu e 19. The FGM ocke nozzle, a case s udy by InssTek (adap ed om [158]).
4.2. P oduc ion o Func ionally G aded Ma e ials
The low p ocessabili y o FGM s uc u es h ough con en ional manu ac u ing
me hods has limi ed hei de elopmen o da e. FGMs can ce ainly be p oduced h ough
apou deposi ion, powde me allu gy, o cen i ugal cas ing [62]. Howe e , hese
p ocesses ha e li le lexibili y and a e esou ce- and ime-consuming. AM p ocesses, on
he o he hand, ha e demons a ed hei sui abili y o he p oduc ion o FGM s uc u es
[159]. Indeed, he PBF echnology can be used o achie e a laye -wise a ia ion o he
mic os uc u e o a single ma e ial by modi ying he g ain o ien a ion [160], ia he
con olled apo isa ion o ce ain elemen s [161], o ia he con olled modi ica ion o he
po osi y [162]. In con as , composi ional g adien s can be a ained using DED owing o
he unp eceden ed design eedom and mul i-ma e ial abili y [22]. Many au ho s ag ee
ha L-DED is he p e e ed solu ion o he ab ica ion o FGM s uc u es [22,36,62], which
is only logical, conside ing ha he p oduc ion o FGM s uc u es equi es igh con ol o
he chemical composi ion and he lexibili y o ab ica e mul i-ma e ial componen s, while
allowing he modi ica ion o he eeds ock composi ion wi hin one single build-up [163].
In addi ion o he po en ial o he p oduc ion o loca ion-dependan ma e ial
p ope ies, FGMs ha e been p o en bene icial in mi iga ing esidual s esses [164] o
subs i u ing sha p in e aces, wi h a high isk o delamina ion ailu e [22]. In e ms o he
ope a ional beha iou , sha p ma e ial ansi ions a e conside ed s ess concen a ion
zones and complex loading condi ions may cause a ca as ophic ailu e.
4.3. The Mos Rele an Li e a u e on L-DED o Func ionally G aded Ma e ials
D i en by he bene i s o FGM s uc u es, much esea ch has been ca ied ou wi h
he aim o a aining a con olled g ading be ween di e en me al alloys. In pa icula ,
g adien s be ween Ni-based alloys and di e en g ades o s ainless s eels ha e been
ex ensi ely epo ed, due o he in e es om he powe gene a ion and ae ospace
indus ies in his ma e ial sys em. Fo ins ance, Zhang e al. s udied he mic os uc u e,
ha dness, and ensile ac u e alues o AISI 316L o Inconel 625 unc ionally g aded
s uc u es. Fi s ly, hey obse ed a ha dness g adien along he g aded egion, which
ma ched he mic os uc u e. Addi ionally, he ac u e in he ensile es ing occu ed in
he AISI 316L sec ion, while he in e aces su i ed he es ing owing o he g aded
ansi ion [165]. Su e al. s udied he mic os uc u e o AISI 316L o Inconel 718 g aded
samples and ound ha aus eni ic o ma ion was p omo ed in he g aded egion, leading
o he loss o ha dness in ce ain a eas [166]. Ca ol e al. analysed AISI 304L o Inconel
625 g adien samples, and hey ound c acks o se e al mic ons due o he o ma ion o
Figu e 19. The FGM ocke nozzle, a case s udy by InssTek (adap ed om [158]).
4.2. P oduc ion o Func ionally G aded Ma e ials
The low p ocessabili y o FGM s uc u es h ough con en ional manu ac u ing me h-
ods has limi ed hei de elopmen o da e. FGMs can ce ainly be p oduced h ough apou
deposi ion, powde me allu gy, o cen i ugal cas ing [
62
]. Howe e , hese p ocesses ha e
li le lexibili y and a e esou ce- and ime-consuming. AM p ocesses, on he o he hand,
ha e demons a ed hei sui abili y o he p oduc ion o FGM s uc u es [
159
]. Indeed, he
PBF echnology can be used o achie e a laye -wise a ia ion o he mic os uc u e o a
single ma e ial by modi ying he g ain o ien a ion [
160
], ia he con olled apo isa ion o
ce ain elemen s [161], o ia he con olled modi ica ion o he po osi y [162]. In con as ,
composi ional g adien s can be a ained using DED owing o he unp eceden ed design
eedom and mul i-ma e ial abili y [
22
]. Many au ho s ag ee ha L-DED is he p e e ed
solu ion o he ab ica ion o FGM s uc u es [
22
,
36
,
62
], which is only logical, conside ing
ha he p oduc ion o FGM s uc u es equi es igh con ol o he chemical composi ion
and he lexibili y o ab ica e mul i-ma e ial componen s, while allowing he modi ica ion
o he eeds ock composi ion wi hin one single build-up [163].
In addi ion o he po en ial o he p oduc ion o loca ion-dependan ma e ial p ope -
ies, FGMs ha e been p o en bene icial in mi iga ing esidual s esses [
164
] o subs i u ing
sha p in e aces, wi h a high isk o delamina ion ailu e [
22
]. In e ms o he ope a ional be-
ha iou , sha p ma e ial ansi ions a e conside ed s ess concen a ion zones and complex
loading condi ions may cause a ca as ophic ailu e.
Ma e ials 2023,16, 1746 23 o 33
4.3. The Mos Rele an Li e a u e on L-DED o Func ionally G aded Ma e ials
D i en by he bene i s o FGM s uc u es, much esea ch has been ca ied ou wi h he
aim o a aining a con olled g ading be ween di e en me al alloys. In pa icula , g adien s
be ween Ni-based alloys and di e en g ades o s ainless s eels ha e been ex ensi ely
epo ed, due o he in e es om he powe gene a ion and ae ospace indus ies in his
ma e ial sys em. Fo ins ance, Zhang e al. s udied he mic os uc u e, ha dness, and ensile
ac u e alues o AISI 316L o Inconel 625 unc ionally g aded s uc u es. Fi s ly, hey
obse ed a ha dness g adien along he g aded egion, which ma ched he mic os uc u e.
Addi ionally, he ac u e in he ensile es ing occu ed in he AISI 316L sec ion, while
he in e aces su i ed he es ing owing o he g aded ansi ion [
165
]. Su e al. s udied
he mic os uc u e o AISI 316L o Inconel 718 g aded samples and ound ha aus eni ic
o ma ion was p omo ed in he g aded egion, leading o he loss o ha dness in ce ain
a eas [
166
]. Ca ol e al. analysed AISI 304L o Inconel 625 g adien samples, and hey
ound c acks o se e al mic ons due o he o ma ion o ca bides in he g aded egion [
167
].
In Figu e 20, he composi ional design o he sample buil by Ca oll e al. and he e olu ion
o he Fe and Ni elemen al con en s in con as wi h he a ia ion in ha dness alues a e
shown. Li e al. jus ecen ly de eloped a mul i-physics model o be e unde s and he
go e ning phenomena in he L-DED o 316L o Inconel 718 FGM s uc u es. They ound
ha he composi ion o he eeds ock subs an ially a ec ed he mel pool size as a esul o
he di e ing he mal p ope ies. Indeed, inc easing he Inconel 718 con en educed he
hea dissipa ion and esul ed in a la ge mel pool. Mo eo e , hey ound ha laye s ha
had he same nominal composi ion con ained a non-uni o m dis ibu ion o Inconel 718,
which hey a ibu ed o he a ying mel pool sizes and he mal g adien s [168].
Ma e ials 2023, 16, x FOR PEER REVIEW 24 o 34
ca bides in he g aded egion [167]. In Figu e 20, he composi ional design o he sample
buil by Ca oll e al. and he e olu ion o he Fe and Ni elemen al con en s in con as
wi h he a ia ion in ha dness alues a e shown. Li e al. jus ecen ly de eloped a mul i-
physics model o be e unde s and he go e ning phenomena in he L-DED o 316L o
Inconel 718 FGM s uc u es. They ound ha he composi ion o he eeds ock
subs an ially a ec ed he mel pool size as a esul o he di e ing he mal p ope ies.
Indeed, inc easing he Inconel 718 con en educed he hea dissipa ion and esul ed in a
la ge mel pool. Mo eo e , hey ound ha laye s ha had he same nominal composi ion
con ained a non-uni o m dis ibu ion o Inconel 718, which hey a ibu ed o he a ying
mel pool sizes and he mal g adien s [168].
Figu e 20. Addi i ely manu ac u ed FGM om 304L s ainless s eel o Inconel 625 Ni-based alloy
and e olu ion o he Fe and Ni elemen al con en s as opposed o he ha dness. Rep oduced om
[167].
In con as , o he ma e ial combina ions and sys ems ha e been s udied oo. In his
ega d, Fan e al. in es iga ed In a o MnCu FGM samples. They epo ed a non-linea
a ia ion o he ha dness alues, which was a ibu ed o he solid–solu ion s eng hening
mechanism being p omo ed in he g adien egion, whe e he highes ha dness was
measu ed. Mo eo e , hey s udied he ensile beha iou o he g aded s uc u es and
ound ha he de o ma ion beha iou was no homogeneous due o he s ess
concen a ions. They concluded ha he s ain was di ec ly co ela ed o he ha dness
e olu ion, and ha he so e egions con ibu ed he mos s ain [169]. In con as , Ji e al.
ab ica ed Ti6Al4V o Inconel 718 g aded coa ings and hey compa ed he expec ed
mic os uc u es and phase ans o ma ions based on compu a ionally de i ed phase
diag ams and expe imen al obse a ions. They concluded ha he Ti-Ni combina ion
esul ed in he gene a ion o Ti2Ni and TiNi in e me allics, based on bo h he expe imen al
and compu a ional esul s. Mo eo e , hey s udied he high- empe a u e beha iou o
hese ma e ials and hey ound ha he di usion zone be ween he g aded laye s
inc eased wi h inc easing exposu e o high empe a u es [170]. In addi ion, Nam e al.
compa ed he deposi ion o AISI 316 on o a mild s eel subs a e wi h he Fe/AISI 316
g adien deposi ion and ound ha while many po es and c acks we e obse ed in he
sha p ansi ion, no c acking was de ec ed in he g aded sample [171].
Al hough p omising, he p oduc ion o FGM s uc u es based on mul iple ce amic
phases is s ill a challenging ma e . Based on he e iewed li e a u e, some c i ical issues
a e desc ibed he ein. The high- empe a u e p ocessing and mixing beha iou o di e en
alloys highligh he need o conside me allu gical aspec s. Indeed, he co ela ion
be ween he composi ion, mic os uc u e, and mechanical p ope ies is no linea ,
especially when phase ans o ma ions and in e me allic o ma ion occu . The e o e, om
a ma e ial science pe spec i e, a compu a ional app oach appea s o be he mos e icien
me hod o an icipa e such issues and o design he FGM s uc u e acco dingly. F om he
p ocessing pe spec i e, he e ec s o he eeds ock composi ion on he he mal aspec s o
he p ocess need o be u he unde s ood. Indeed, i has been demons a ed ha he
Figu e 20.
Addi i ely manu ac u ed FGM om 304L s ainless s eel o Inconel 625 Ni-based alloy and
e olu ion o he Fe and Ni elemen al con en s as opposed o he ha dness. Rep oduced om [167].
In con as , o he ma e ial combina ions and sys ems ha e been s udied oo. In his
ega d, Fan e al. in es iga ed In a o MnCu FGM samples. They epo ed a non-linea
a ia ion o he ha dness alues, which was a ibu ed o he solid–solu ion s eng hening
mechanism being p omo ed in he g adien egion, whe e he highes ha dness was mea-
su ed. Mo eo e , hey s udied he ensile beha iou o he g aded s uc u es and ound
ha he de o ma ion beha iou was no homogeneous due o he s ess concen a ions.
They concluded ha he s ain was di ec ly co ela ed o he ha dness e olu ion, and ha
he so e egions con ibu ed he mos s ain [
169
]. In con as , Ji e al. ab ica ed Ti6Al4V
o Inconel 718 g aded coa ings and hey compa ed he expec ed mic os uc u es and phase
ans o ma ions based on compu a ionally de i ed phase diag ams and expe imen al
obse a ions. They concluded ha he Ti-Ni combina ion esul ed in he gene a ion o
Ti
2
Ni and TiNi in e me allics, based on bo h he expe imen al and compu a ional esul s.
Mo eo e , hey s udied he high- empe a u e beha iou o hese ma e ials and hey ound
ha he di usion zone be ween he g aded laye s inc eased wi h inc easing exposu e o
high empe a u es [
170
]. In addi ion, Nam e al. compa ed he deposi ion o AISI 316 on o
a mild s eel subs a e wi h he Fe/AISI 316 g adien deposi ion and ound ha while many
Ma e ials 2023,16, 1746 24 o 33
po es and c acks we e obse ed in he sha p ansi ion, no c acking was de ec ed in he
g aded sample [171].
Al hough p omising, he p oduc ion o FGM s uc u es based on mul iple ce amic
phases is s ill a challenging ma e . Based on he e iewed li e a u e, some c i ical issues
a e desc ibed he ein. The high- empe a u e p ocessing and mixing beha iou o di e en
alloys highligh he need o conside me allu gical aspec s. Indeed, he co ela ion be ween
he composi ion, mic os uc u e, and mechanical p ope ies is no linea , especially when
phase ans o ma ions and in e me allic o ma ion occu . The e o e, om a ma e ial science
pe spec i e, a compu a ional app oach appea s o be he mos e icien me hod o an icipa e
such issues and o design he FGM s uc u e acco dingly. F om he p ocessing pe spec i e,
he e ec s o he eeds ock composi ion on he he mal aspec s o he p ocess need o be
u he unde s ood. Indeed, i has been demons a ed ha he di e ences in he he mal
p ope ies o he g aded alloys signi ican ly a ec he o ma ion o he mel pool and he
he mal his o y o he p ocess. In his ega d, Zhang e al. epo ed ins abili y and shaking
o he mel pool du ing he deposi ion o 316L on o Inconel 718, which happened only in
ce ain composi ions. They p oposed ha his e ec could be go e ned by he di e en
ine ial p ope ies o he powde s employed o ab ica e he sample [
172
]. Howe e , mo e
esea ch is needed o eally unde s and which phenomena a ec he p oduc ion o FGMs
and hei oles and signi icance.
Some publica ions ela ed o FGM s uc u es ha e ackled me al–ce amic ma e ial
sys ems, and mo e speci ically he ab ica ion o unc ionally g aded MMCs. Fo ins ance,
Wang e al. ab ica ed FGM composi es ha anged om 100% Ti6Al4V alloy o 60%Ti6Al4V
and 40% Ni-coa ed g aphi e. They epo ed a nea ly linea co ela ion be ween he ha dness
and he composi ion o he powde mix u e [
173
]. On he o he hand, Ramak ishnan
and Dinda employed he FGM s a egy o deposi MMC coa ings consis ing o Haynes
282 supe alloy and SiC [
174
]. In his ega d, Wei e al. also ocused on in oducing a
composi ion g adien in MMC pa s and hey manu ac u ed a TiC/Ti6Al4V unc ionally
g aded composi e, eaching simila conclusions o hose p e iously epo ed [175].
In Table 6, he mos ele an publica ions on he ab ica ion o FGM s uc u es h ough
L-DED a e summa ised.
Wi h he aim o explo ing he no el applica ions o FGM s uc u es and going back
o he p oduc ion o MMC coa ings, one o he main issues in he L-DED o MMCs is
he lack o me allu gical in eg i y, high c ack sensi i i y, and delamina ion, which can be
mi iga ed by implemen ing an FGM s a egy. On his basis, Xu e al. in es iga ed he c ack
sensi i i y o mono-composi ional and unc ionally g aded MMC samples. They concluded
ha unc ionally g aded ansi ions could signi ican ly con ibu e o mi iga ing c acking in
MMC coa ings, as shown in Figu e 21 [176].
Ma e ials 2023, 16, x FOR PEER REVIEW 26 o 34
Figu e 21. Reduc ion in c ack sensi i i y wi h he FGM s a egy in S elli e 6 and WC MMCs.
Adap ed om [176].
5. Conclusions
The abili y o L-DED o manu ac u e mul i-ma e ial componen s is one o he mos
compelling aspec s o his echnology, and so i has been acknowledged by indus y and
esea ch bodies. The cu en challenges ela ed o joining dissimila ma e ials, o
ins ance, can be o e come h ough he use o mul i-ma e ial L-DED, as componen s
composed o di e en ma e ials can be buil up in a single ope a ion. The p e alen
applica ions o mul i-ma e ial L-DED a e alloy design, me al ma ix composi es, and
unc ionally g aded ma e ials.
Wi h espec o alloy design, L-DED pe mi s much as e and mo e cos -e icien
expe imen al ials when es ing new alloys. This is key in he de elopmen o new high-
en opy alloys, whe e he possible composi ions o be ied ou a e mani old and
simpli ying he expe imen al es ing is absolu ely necessa y. On he o he hand, su ace
enginee s can ob ain majo bene i s om me al ma ix composi e coa ings, as hey beha e
ou s andingly well in e ms o wea . Las ly, he use o unc ionally g aded ma e ials seems
o be a key s a egy o enable he joining o a p io i incompa ible ma e ial combina ions.
The s aigh o wa d abili y o L-DED o con ol he eeds ock composi ion and modi y i
as an addi ional p ocess pa ame e has p o ided e idence o he sui abili y o his p ocess
o mul i-ma e ial ab ica ion, and i is al eady conside ed he p e ailing echnology o
o ming mul i-ma e ial s uc u es. Howe e , some issues need o be add essed be o e his
echnology is eady o indus ial use. Some o hese challenges conce n p ocess
enginee ing and ma e ials science, which a e summa ised below.
In e ms o p ocess enginee ing, he composi ion o he eeds ock injec ed in o he
mel pool needs o be igh ly con olled. I he L-DED sys em in ol es se e al powde
hoppe s, hen he indi idual mass low a e o each one o he hoppe s needs o be
p ecisely con olled, and he dynamic beha iou when a ying he composi ion o he
eeds ock du ing he build-up should be ca e ully unde s ood. Downs eam, he luid
dynamic beha iou o mul i-ma e ial powde mix u es needs o be known. Indeed, he
concen a ion o he powde mix u e by he nozzle depends on he ma e ial p ope ies.
This is especially oubling when wo king wi h me al–ce amic powde mix u es, as he
ine ial p ope ies o ce amic and me allic ma e ials di e signi ican ly. Failu e o
unde s and and p edic he beha iou o he powde eeding sys em om he powde
eede o he injec ion o he powde mix u e in o he mel pool will ine i ably cause
de ia ions be ween he nominal composi ion and he eal one.
Mo eo e , in iew o he impac o he he mal cycle du ing p ocessing on he
p ope ies and in eg i y o mul i-ma e ial L-DED, he e ec o he p ocess pa ame e s on
he he mal his o y should be in es iga ed. E en ually, he p ocess pa ame e s will be
uned o a ge a speci ic he mal cycle. Mo eo e , in he case o MMCs, he he mal
Figu e 21.
Reduc ion in c ack sensi i i y wi h he FGM s a egy in S elli e 6 and WC MMCs. Adap ed
om [176].
Ma e ials 2023,16, 1746 25 o 33
Table 6. Mos ele an publica ions conce ning he L-DED o FGM s uc u es.
Publica ions Ma e ials Applica ion Main esul s Limi a ions/ Obse a ions
Ca ol e al.,
2016 [167]
100% AISI
3104L o 100%
Inconel 625
Ae ospace and
nuclea powe
gene a ion
C acking a a p ecise
composi ion was due o he
o ma ion o ca bides, and
CALPHAD simula ions we e
able o p edic i
A c ack- ee sample could be p obably
ab ica ed, a oiding he composi ion
whe e ha d ca bides a e s able and
p one o o m.
Nam e al.,
2018 [171]
100% Fe o 100%
316L Miscellaneous
Di ec ly deposi ing 316L on o
mild s eel esul ed in c acking,
while he FGM sample had no
appa en de ec s.
No analysis o he e olu ion o he
mechanical p ope ies o beha iou o
he samples was p o ided. FGM
samples s ill showed a signi ican
amoun o po es.
Zhang, Chen,
and Liou,
2019 [165]
100% AISI 316L
o 100% Inconel
625
Die and mould
De ec - ee FGM samples we e
success ully deposi ed and
g adual ha dness was obse ed.
The ensile beha iou o he
FGM samples was in-be ween
pu e AISI 316L and Inconel 625.
I would be in e es ing o compa e he
beha iou o he FGM sample o ha
o a sample ha ing a sha p ansi ion
be ween AISI 316L and Inconel 625.
Su e al.,
2020 [166]
100% AISI 316L
o 100% Inconel
718
Nuclea powe
plan s and oil
e ine ies
The composi ional s ep o o m
he g adien a ec s he ha dness
and ensile p ope ies o he
FGM sample.
The a iabili y in FGMs designed wi h
di e en disc e isa ion s eps was
asc ibed o he he mal cycle and
p ocessing condi ions. The ac ual
e ec o he FGM design was no
p ope ly es ed, as samples ha ing
di e en sizes and amoun s o laye s
a e used o compa ison.
Os olaza e al.,
2021 [177]
100% AISI 316L
o 100% AISI
H13
Die and mould
The composi ional g adien did
no gua an ee a g adual
a ia ion o he ma e ial
p ope ies, namely he ha dness
and he co osion esis ance.
The FGM sample shows se e e
c acking, which is asc ibed o he
o ma ion o he sigma phase. The
CALPHAD me hodology could be
employed o design an FGM sample
in which he o ma ion o such ha d
phases is minimised.
Wang e al.,
2021 [173]
100% Ti6Al4V
o 40% g aphi e
60% Ti6Al4V
A mou , gea ,
and cu ing
ools
Ti-Ni-C g aded samples showed
a g adual ha dness and
mic os uc u e as a esul o
in-si u TiCx ein o cemen
o ma ions.
Fu he in es iga ions should ocus on
e alua ing he mechanical p ope ies
o FGM s uc u es as compa ed o
sha p ansi ioned samples.
5. Conclusions
The abili y o L-DED o manu ac u e mul i-ma e ial componen s is one o he mos
compelling aspec s o his echnology, and so i has been acknowledged by indus y
and esea ch bodies. The cu en challenges ela ed o joining dissimila ma e ials, o
ins ance, can be o e come h ough he use o mul i-ma e ial L-DED, as componen s
composed o di e en ma e ials can be buil up in a single ope a ion. The p e alen
applica ions o mul i-ma e ial L-DED a e alloy design, me al ma ix composi es, and
unc ionally g aded ma e ials.
Wi h espec o alloy design, L-DED pe mi s much as e and mo e cos -e icien
expe imen al ials when es ing new alloys. This is key in he de elopmen o new
high-en opy alloys, whe e he possible composi ions o be ied ou a e mani old and
simpli ying he expe imen al es ing is absolu ely necessa y. On he o he hand, su ace
enginee s can ob ain majo bene i s om me al ma ix composi e coa ings, as hey beha e
ou s andingly well in e ms o wea . Las ly, he use o unc ionally g aded ma e ials seems
o be a key s a egy o enable he joining o a p io i incompa ible ma e ial combina ions.
The s aigh o wa d abili y o L-DED o con ol he eeds ock composi ion and modi y i as
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