On he Mechanism o Fo ma ion o Bimodal G ain
S uc u e in Al–4.5Mg–0.7Sc–0.3Z Alloy P ocessed by
Lase Powde Bed Fusion
Polina Che nysho a, Te esa Gu aya, Ana Ma inez-Ames i, Hegoi Andonegi,
Sa a Singamneni, and Zhan Wen Chen*
1. In oduc ion
Along wi h he lase powde bed usion (LPBF)-addi i e
manu ac u ing (known also as 3D p in ing) g adually becoming
mo e widely applied in he las ew yea s, LPBF o aluminum
alloys has been in ensi ely s udied.
[1]
Resea ch has shown ha ,
h ough alloy modifica ion o mixing addi i es in he alloy pow-
de , he high-s eng h 2xxx and 7xxx aluminum alloys can be
p ocessed by LPBF.
[1,2]
Howe e , LPBF o pa s o s uc u es
using 2xxx and 7xxx alloys is ye o be
epo ed o sa e y-c i ical loading applica-
ions. Scalmalloy is a high-s eng h alumi-
num alloy ha has been specially de eloped
o ae ospace applica ions and p o en sui -
able o p ocessing by LPBF.
[2]
As was
explained by Schmid ke e al.,
[3]
he alloy
de elopmen was based on using an Al–
4.5Mg (5xxx) alloy wi h small addi ions o
Sc (0.66 w %) and Z (0.37 w %). The addi-
ions ha e allowed o he alloy o be age
ha denable wi h yield s eng h (σ
y
) each-
ing 500 MPa in peak-ha dening condi ion
and o he alloy o be highly p in able wi h-
ou ho c acking. Howe e , he mechanism
o o ming he mic os uc u es ee o ho
c acking du ing LPBF is ye o be unde -
s ood ully.
Following he wo k by Schmid ke e al., a
se ies o s udies we e unde aken by
Spie ings e al.,
[4–8]
u he demons a ing
he equiaxed-columna bimodal s uc u es
wi hin each ack and he possibili ies o a ain high s eng hs
a e one-s ep aging. They obse ed Al
3
(Sc,Z ) nanopa icles
30–100 nm in size and Al–Mg-oxides unde he condi ion o
low scan speed (=351 mm s
1
) using lase powe (P) o 200 W
( hus P/ =0.57 J mm
1
). They assumed ha hese pa icles
ac as nuclei o o ming he equiaxed g ains nex o he ack
bounda y.
[5]
The o iginal sou ce o Al
3
(Sc,Z ) nanopa icles, how-
e e , is less clea . Beyond he mel zone ha solidifies in o equi-
axed g ains and mo e inside he mel ack, hey sugges ha
P. Che nysho a, S. Singamneni, Z. W. Chen
Depa men o Mechanical Enginee ing
Auckland Uni e si y o Technology
Auckland 1010, New Zeland
E-mail: [email p o ec ed]z
The ORCID iden ifica ion numbe (s) o he au ho (s) o his a icle
can be ound unde h ps://doi.o g/10.1002/adem.202300135.
© 2023 The Au ho s. Ad anced Enginee ing Ma e ials published by Wiley-
VCH GmbH. This is an open access a icle unde he e ms o he C ea i e
Commons A ibu ion License, which pe mi s use, dis ibu ion and
ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly ci ed.
DOI: 10.1002/adem.202300135
T. Gu aya
Depa men o Mining & Me allu gical Enginee ing & Ma e ials Science
Uni e si y o he Basque Coun y
UPV/EHU
Bilbao 48013, Spain
A. Ma inez-Ames i
SGIke
Ad anced Resea ch Facili ies
Uni e si y o he Basque Coun y
UPV/EHU
Donos ia-San Sebas ian 20018, Spain
H. Andonegi
AZTERLAN
Basque Resea ch and Technology Alliance (BRTA)
Du ango 48200, Spain
Scalmalloy is an Al–Mg alloy wi h addi ions o Sc and Z o iginally de eloped as a
high-s eng h aluminum alloy wi h
σ
0.2
≥450 MPa o ae ospace indus y. I is
now well unde s ood ha he alloy is amendable o p ocessing by lase powde
bed usion (LPBF). Howe e , he mechanism o o ma ion o he equiaxed-
columna bimodal g ain s uc u e du ing LPBF is no asce ained ye , ully.
He ein, his gap is add essed wi h special ocus on he dis ibu ions o c i ical
elemen s such as Sc and a ious pa icles ha o m du ing LBPF. I is ound ha
s ong and weak seg ega ion o Mg and Sc, espec i ely, occu s in he final
solidifica ion a eas o he fine- and equiaxed-g ain egions. The coa se and
columna g ain egions show weak seg ega ion o Mg and no Sc seg ega ion. A
p io i knowledge on he Al–Sc eu ec ic eac ion, i s dependence on cooling a es,
and he well-known he mal and solidifica ion condi ions ela ed o he ack
loca ion du ing LPBF is used o asce ain he mechanism o o ma ion o he
bimodal g ain s uc u e. The mechanism sugges ed is subs an ia ed by he
loca ion-dependen elemen al dis ibu ions and he a ious pa icles ha a e
obse ed.
RESEARCH ARTICLE
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Al
3
(Sc,Z ) pa icles mel due o highe mel empe a u es as
p edic ed by hei simula ion. As a esul , columna g ain g ow h
occu s du ing solidifica ion. Thus, in e ec , hey ha e sugges ed
ha Al
3
(Sc,Z ) pa icles ha ing su i ed in he mel egion
nex o ack bounda y ha la e solidifies in equiaxed-g ain
solidifica ion mode ha e come om he emel ing o he p e i-
ous ack/laye . This mechanism o emel ing o he p e ious
laye lea ing Al
3
(Sc,Z ) pa icles unmel ed in he egion
nex o ack bounda y was soon mo e fi mly p oposed by
Yang e al.
[9]
As Spie ings e al.
[5]
ha e explained, Al
3
(Sc,Z ) nanopa icles ac -
ing as nuclei o he o ma ion o equiaxed g ains du ing LPBF is
easonable as Al
3
(Sc,Z ) pa icles nuclea ing α-Al equiaxed g ains
du ing con en ional cas ing has been well known. Howe e ,
equiaxed-g ain o ma ion du ing cas ing does no need p eexis ing
Al
3
(Sc,Z ) pa icles. Hyde e al.
[10]
demons a ed he g ain- efining
e ec o Al
3
Sc du ing solidifica ion o he Al–0.7w %Sc alloy which
was fi s mel ed and held a 750 °C. This mel empe a u e is abo e
he liquidus empe a u e o he alloy meaning ha Al
3
Sc pa icles
a e no p esen in he mel and Al
3
Sc nuclei o m fi s om he
mel upon cooling and a he s a o solidifica ion o he subse-
quen equiaxed α-Al-g ain g ow h. Thus, i is unclea why Al
3
(Sc,
Z ) nuclei need o come om he mel ing o he p e ious laye /
ack so as o o m equiaxed g ains du ing LPBF.
Since Spie ings e al.’s s udies, he e has con inuously been a
s ong esea ch e o on a numbe o aspec s o LPBF o Al–Mg
alloys con aining a ious amoun s o Sc and/o Z .
[11–23]
Con en s
o Sc and Z di e in a ious s udies so ha he kine ics o
o ming Al
3
(Sc,Z ) may di e . In Zhang e al.’s
[11]
s udy using
P/ =0.18 J mm
1
,Al
3
(Sc,Z ) pa icles up o 90 nm a e ound in
he as-buil s a e. In Shi e al.’s
[12]
s udy using P/ om 0.07 o
0.62 J mm
1
, in con as , he e a e no Al
3
(Sc,Z ) pa icles ha
can be de ec ed in hei scanning ansmission elec on mic oscope
(STEM) analysis. Chu yumo e al.
[13]
also could no de ec
Al
3
(Sc,Z ) pa icles in ansmission elec on mic oscope (TEM)
analysis o hei (P/ =)0.81Jmm
1
samples. In Ma e al.’s
[14]
wo k using P/ =0.27 J mm
1
,noAl
3
(Sc,Z ) pa icles could be
de ec ed.
Howe e , he sugges ion o he mechanism ela ing o emel -
ing seems o be s ill p e ailing as desc ibed in a ecen e iew on
he p og ess o aluminum-alloy LPBF
[24]
and in a e iew specifically
on LPBF o Sc-con aining aluminum alloys.
[25]
Recen ly, Ekuba u
e al.
[23]
demons a ed ha con olling ha ch spacing can con ol
he amoun o equiaxed g ains and hus can con ol he s eng h
o he alloy h ough g ain-bounda y s eng hening. A ecen e o
on he alloy design o mic os uc u e con ol in LPBF o alumi-
num alloys also was cen e ed on he modifica ion o he alloys
using Sc.
[26]
A mo e ho ough unde s anding o he ole o Sc
in o ming he bimodal mic os uc u e in Scalmalloy is hus
impo an .
In his s udy, Scalmalloy samples ha e been made using a sui -
able se o LPBF p ocess condi ions. Equiaxed and columna g ain
egions ha e been accu a ely sampled in specimens and analyzed
o imp o e he unde s anding o how he elemen s, pa icula ly Sc,
edis ibu e du ing solidifica ion. Fu he , he ole o Sc in
con olling he solidifica ion modes and he consequen o ma ion
o he bimodal g ain s uc u e is explo ed. The common knowl-
edge on he p ocess he modynamics, he mal condi ions, and
he a es a which he solid–liquid on s ad ance is used o
e alua e he esul s and in e c i ical obse a ions. The p esence
o Fe- and Mg–Si- ich pa icles along he g ain bounda ies and
he in e io s o g ains in he egions nex o and away he ack
bounda ies espec i ely will be used o subs an ia e in e ences
d awn om he esul s.
2. Expe imen al Sec ion
Samples o 6 610 mm and 10 10 55 mm in dimensions
we e buil using a Renishaw AM400 Selec i e Lase Mel ing
machine. Specific pa ame e s we e pulsed lase powe P=370 W,
scan eloci y =1600 mm s
1
, laye hickness o 30 μm, ha ch
spacing o 100 μm, meande ha ching s a egy o 67° o a ion,
and he base pla e a oom empe a u e. Conside ing pulsing lase ,
P/ <0.23 J mm, which was low bu is nowadays common, and
was a sui able LPBF condi ion o lack o usion ee and o li le
keyhole po e o ma ion. The chemical composi ion in w % o he
alloy powde , as specified in he es ce ifica e o powde supplie
(LPW), is p esen ed in Table 1. Fo ha dening ea men , samples
we e hea ed o and held a 325 °C in an elec ic hea ing u nace o
up o 4 h and hen ai cooled. Tensile samples we e machined
om he buil long samples o gauge leng h sec ion 17.9 mm
and diame e 5.05 mm and ensile es ing was conduc ed using
a Tinius Olsen H50KS es e . Mic oha dness measu emen was
conduc ed using a Leco Mic oha dness Tes e (LM800AT) wi h
a 300 g loading o 10 s.
Fo mic os uc u e analysis, samples we e fi s p epa ed ol-
lowing he no mal me allog aphic p ocedu e wi h he final pol-
ishing down o silica 50 nm. Samples we e obse ed using a
JEOL JSM 7000F field-emission-gun scanning elec on mic o-
scope (FEG-SEM) wi h 5 kV ope a ing ol age. Lamellae 50–70 nm
in hickness we e p epa ed in selec ed loca ions, ia s anda d li -
ou p o ocol using a Dual Beam Helios 650 model which consis ed
o a 30 kV field-emission scanning elec on column wi h 0.9 nm
esolu ion and a 30 kV Ga ocused-ion beam. The loca ion o a
lamella could be aken in an equiaxed g ain egion, a columna
g ain egion, o an equiaxed-columna bounda y egion. Figu e 1
is an example showing a lamella being aken ou om a columna
g ain egion. The lamellae we e analyzed using Talos F200i
field-emission-gun ansmission elec on mic oscope (FEG–TEM)
equipped wi h a B uke X-Flash100 ene gy-dispe si e X- ay
spec oscopy (EDS) spec ome e . Elemen al maps we e pe -
o med by EDS in he STEM mode unde high-angle annula
da k-field (HAADF) de ec o o Z con as imaging in STEM con-
di ions wi h a came a leng h o 200 mm using a pixel size o 2 nm,
a dwell ime o 900 s, and an image size o 512 512 pixels.
Mo eo e , EDS mic oanalyses we e ca ied ou using a p obe
cu en o 800 pA and a semi-con e gence angle o 6 m ad.
Velox so wa e was used o he composi ional map acquisi ion
and p ocessing.
Table 1. Composi ion o as- ecei ed Scalmalloy powde .
Al Mg Sc Z Mn Fe Si O Zn,Cu,Ti,V
w % Bal. 4.55 0.65 0.30 0.51 0.14 0.16 0.04 Each ≤0.02
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Figu e 1. Illus a ion o a ansmission elec on mic oscope (TEM) lamellae being aken: a) field-emission-gun scanning elec on mic oscope (FEG–SEM)
mic og aph showing an equiaxed-g ain egion on op o a columna g ain egion wi h he lamella o be aken in he columna g ain egion indica ed by he
g een ec angula , and b) he ma e ial in he on pa ha ing been aken ou by ocused-ion beam wi h ma e ial behind o be u he aken ou o o m a
lamella.
Figu e 2. Bimodal g ain s uc u e in as-buil s a e: a) FEG–SEM images showing equiaxed and columna g ains in each ack wi h acks 1–5 numbe ed
and ack bounda ies ou lined, and b) scanning ansmission elec on mic oscope (STEM) images o equiaxed g ains adjacen o ack bounda y (le ) and
nex / ansi ioning o columna g ain egion ( igh ).
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3. Resul s and Discussion
3.1. Mic os uc u es and Elemen al Dis ibu ion in
As-Buil S a e
SEM mic og aphs a e p esen ed in Figu e 2 showing he ypical
mic os uc u e in each ack s a ing om fine-equiaxed g ains a
ew mic ons in hickness nex o and along he ack bounda y
and hen columna g ains inside he whole ack. The g ain size
in he equiaxed-g ain egion nex o ack bounda y is 0.5 μm
and nex o he columna g ain egion is 1.5 μm, as shown by
he STEM mic og aphs in Figu e 2b. The g ain wid h in colum-
na g ain egion is up o 3–4μm (Figu e 2a). The bimodal mic o-
s uc u es as e iden in Figu e 2 a e ypical wi h LPBF-p ocessed
Scalmalloy as al eady s a ed in he in oduc ion. A dis inc i e ea-
u e obse ed in he p esen wo k in equiaxed g ains is ha he e
appea no pa icles inside he g ains in he fine-g ain egion (le
o Figu e 2b) bu pa icles can be seen inside each g ain in he
coa se and equiaxed g ains nex o he columna g ain egion
( igh o Figu e 2b).
The STEM image aken in he fine-equiaxed-g ain (0.5 μm
size) egion nex o ack bounda y is shown again in
Figu e 3, oge he wi h he co esponding EDS elemen al (Al,
Mg, Sc, Mn, Si, Z , Fe, and O) maps. The e a e wo majo ea-
u es in he image and he maps. The fi s is ha no Sc/Z - ich
pa icles can be de ec ed. Pa icles can be obse ed along he g ain
bounda ies and hese pa icles a e ich in Mg and Si o in Fe pos-
sibly con aining Mn bu hese pa icles a e no Sc o Z ich.
Figu e 3. STEM high-angle annula da k-field (STEM–HAADF) mic og aph, op le , and EDS elemen al maps aken and analyzed in equiaxed-g ain
egion adjacen o ack bounda y in an as-buil sample. Small a eas indica ed as 2–7, wi h he whole a ea o he map being 1, indica e a eas ha we e
composi ionally de e mined.
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Howe e , hese pa icles a e no obse ed away om he g ain
bounda ies. Inside he g ains, he e a e no pa icles (o a ew
o a ew ens o nanome e s in size), as has al eady been poin ed
ou . As is shown in Table 1, Scalmalloy con ains Fe and Si. Thus,
Mg
2
Si- and Fe-con aining in e me allic pa icles o m in he la e
s ages o solidifica ion and a e p esen in he g ain-bounda y
egions, as is commonly known in cas Al-alloy solidifica ion.
The second ea u e in Figu e 3 is he seg ega ion o Mg along
he g ain bounda ies, no jus he e y ich Mg- ich pa icles, as is
clea in he Mg map. To u he unde s and, a ea composi ions
inside he g ains and in a numbe o o he a eas each including a
sec ion o a g ain bounda y ha e been e alua ed. An EDS spec-
um can include a small Cu and a small Ga peak as a Cu g id was
used and he sample can be sligh ly con amina ed by Ga du ing
lamellae p epa a ion. They ha e hus been excluded in ZAF cal-
cula ion. Fo he p esen pu pose o examining elemen al dis i-
bu ions, only Al, Mg, and Sc a e selec ed o ZAF calcula ion,
since hese a e he p ima ily impo an elemen s o he alloys.
Thus, he composi ion is iewed no malized. No e ha , om
Table 1, a om pe cen age o Z is low and a con en a 0.3 w
%(<0.1 a %) is di ficul o EDS o accu a ely de e mine. Sc
is he majo elemen o o m Al
3
(Sc,Z ). Table 2 lis s he no mal-
ized composi ions, co esponding o he a eas ma ked in he Mg
map in Figu e 3. The (g ain-bounda y) a eas selec ed o EDS
analysis do no include any pa icles.
As lis ed in Table 2, he o e all Mg con en is 4.9 w % in A ea
1, which is he whole o he a ea in he STEM mic og aph o
Figu e 3, while a eas 2 and 3 ep esen ing he in e io egions
o he g ains showed an a e age o 3.5 w % Mg, which is signifi-
can ly (30%) lesse , compa ed o he whole a ea. In con as , he
a e age Mg con en a 6.6 w % is ypical o aaa eas 4, 6, and 7, ha
a e g ain-bounda y egions. This is a ound 34% highe Mg
con en compa ed o he whole egion esponse. Thus, he
EDS analy ical da a clea ly demons a es he Mg en ichmen in
g ain-bounda y a eas, as is eadily e idenced in he Mg map in
Figu e 3, whe e EDS analysis on A ea 5 (Figu e 3) shows a
4.8 w % o Mg. The a ea is close o he g ain bounda y bu is also
a combina ion o a eas a he g ain bounda y and he in e io .
The o e all Sc con en is low due o he ini ial Sc con en being
only 0.65 w % and he peak in an EDS spec um is clea bu no
e y s ong leading o EDS de e mina ion less ce ain in his low
w %. Howe e , alues o Sc con en lis ed in Table 2 may sugges
possibly a sligh seg ega ion o he elemen o g ain bounda y,
al hough he e is no indica ion o Sc en ichmen in he Sc
map (Figu e 3). The o e all Sc con en de e mined is 0.58 w %
(a ea 1 in Table 2), al hough Sc in he o iginal powde is
sligh ly highe . A eas 2 and 3 a e g ain in e io s and hei Sc
con en s a 0.52–0.56 w % may be iewed sligh ly lowe (3–10%)
han he o e all Sc con en . A eas 4, 6, and 7 a e p ima ily g ain-
bounda y a eas and hei Sc con en s a e 0.87, 0.96, and
0.62 w %, espec i ely. Thus, on a e age, Sc con en in g ain-
bounda y a eas de ec ed can gene ally be iewed om sligh ly
(7%) highe o conside ably (65%) highe han he o e all Sc con-
en (0.58 w %), al hough he accu acy o he low-concen a ion
de ec ion may no be e y high. Thus, Sc may also ha e edis ib-
u ed and seg ega ed a leas sligh ly o g ain bounda ies du ing
LPBF-equiaxed-g ain solidifica ion o he alloy, al hough mo e
e idence is equi ed o confi m i a weak seg ega ion o Sc o
he gain bounda ies has occu ed.
Away om he equiaxed-g ain egion, he ea u es shown in
he STEM image and EDS elemen al maps in Figu e 4 a e e y
di e en o he columna g ain egion and almos opposi e o
hose obse ed in he equiaxed-g ain egion. Fi s , pa icles ha
appea o be Mg–Si ich and Fe ich a e mos ly obse ed inside
he g ains as agains being equen ly a he g ain bounda ies. As
has been poin ed ou , e e ing o Figu e 2b, igh , in he coa se
g ain side o he equiaxed-g ain egion b idging o columna
g ain egion, pa icles a e also p esen inside he g ains.
Again, as indica ed by he Sc and Z maps in Figu e 4, he e
is no de ec able p esence o Sc- o Z - ich pa icles. Second,
he Mg map in Figu e 4 has sugges ed only a weak en ichmen
o Mg in he g ain-bounda y a eas o he columna g ain egion,
e y di e en om he s ong g ain-bounda y Mg en ichmen in
equiaxed-g ain egion shown in he Mg map in Figu e 3.
Simila o p o iding he no malized composi ions o equi-
axed g ains as explained be o e, Table 3 lis s he composi ions
co esponding o he a eas ma ked in he STEM mic og aph
in Figu e 4. The o e all composi ion (A ea 1) in Table 3 is e y
close o he alloy composi ion lis ed in Table 1. Abou 10%Mg,
=(4.55–4.12)/4.55, has been deple ed inside he g ains and seg-
ega ed a he g ain bounda ies o he columna g ains. This is a
weak seg ega ion, in compa ison o 24%Mg, =(4.55–3.47)/
4.55, ha has seg ega ed in he g ain-bounda y a eas in he
equiaxed-g ain egion. The e appea s no seg ega ion o Sc ha
can be de ec ed acco ding o he da a o Sc con en alues shown
in Table 3 o he columna g ain egion. This compa es o he
weak Sc seg ega ion o g ain bounda ies in he fine-equiaxed-
g ain egion shown in Table 2, as discussed be o e.
3.2. LPBF and Solidifica ion Pa h
Re e ing o he Al–Mg-phase diag am, he slopes o he liquidus
and solidus a e bo h nega i e meaning ha , as Mg con en
inc eases, liquidus and solidus empe a u es dec ease ( ill
18 w %). Thus, o a 4.5 w %Mg–Al alloy, Mg ejec ion du ing
solidifica ion and en ichmen in he final solidifica ion loca ion
Table 2. No malized w % o Al, Mg, and Sc de e mined by STEM–EDS in
a ea shown and each numbe in subsc ip indica es he a ea numbe
s a ed in he Mg map in Figu e 3.
A eas indica ed in Figu e 3 Al Mg Sc
Whole
1
94.49 4.93 0.58
G ain inside
2
96.06 3.42 0.52
G ain inside
3
95.93 3.51 0.56
G ain inside a e age 95.99 3.47 0.54
G ain bounda y
4
93.07 6.06 0.87
G ain bounda y
5
94.61 4.78 0.60
G ain bounda y
6
92.27 6.76 0.96
G ain bounda y
7
92.34 7.04 0.62
G ain bounda y mean 93.07 6.16 0.77
G ain bounda y s anda d de ia ion (SD) 1.09 1.01 0.18
G ain bounda y s anda d e o 0.54 0.50 0.09
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meaning a seg ega ion o g ain bounda ies is expec ed i solid–
liquid on g ow h eloci y (R) is su ficien ly low o allow o su -
ficien di usion. As is explained by Ku z and T i edi,
[26]
when R
eaches 0.8 m s
1
o is highe , solu e apping occu s du ing
apid solidifica ion. Solu e apping should mean seg ega ion
ee. Du ing LPBF, solidifica ion in a mel ack s a s a ack
bounda y wi h R=0. This is because he angle (θ) be ween
he solidifica ion on mo ing di ec ion and scan di ec ion is
90°. Nex o ack bounda y, Rinc eases e y apidly as he dis-
ance om ack bounda y inc eases. Seg ega ion in he egion
Figu e 4. STEM–HAADF mic og aph, op le , and EDS elemen al maps aken and analyzed in columna g ain egion 50 μm om ack bounda y in an
as-buil sample. Small a eas indica ed as 2–10, wi h he whole a ea o he map being 1, indica e a eas ha we e composi ionally de e mined.
Table 3. No malized w % o Al, Mg, and Sc de e mined by STEM–EDS in a eas shown in he op le map in Figu e 4.
A ea 1
Whole
A ea 2
Inside
A ea 3
Inside
A ea 4 GB A ea 5
Inside
A ea 6 GB A ea 7 GB A ea 8 GB A ea 9 Inside A ea 10 GB Inside
mean &SD
GB mean &SD
Al 94.78 94.98 95.04 94.89 94.94 93.75 95.27 94.07 95.49 93.33 95.11 0.25 94.50 0.71
Mg 4.53 4.25 4.25 4.44 4.42 5.54 4.10 5.16 3.69 5.85 4.15 0.32 4.81 0.66
Sc 0.69 0.77 0.71 0.67 0.66 0.71 0.63 0.78 0.83 0.82 0.74 0.07 0.70 0.06
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nex o ack bounda y is hus expec ed due o he e y low R
alues. The p esence o Mg–Si- and Fe-con aining pa icles only
in g ain-bounda y a eas in he fine-equiaxed-g ain egion can
also be expec ed o be he esul o seg ega ion o hese elemen s
du ing solidifica ion due o he e y low R.
Du ing lase p ocessing, as in lase welding and LPBF, θ
dec eases and Rinc eases apidly away om ack bounda y.
[27]
The exac shape o he mel pool du ing LPBF is no clea . Del
Gue cio e al.,
[28]
in ea ing di usion and seg ega ion du ing
LPBF o an Al alloy, ake θ=45°. Fo his θ alue and o =1600
mm s
1
in ou case, R=1600 mm s
1
cos45° =1.1 m s
1
.A
his R alue, solu e apping du ing solidifica ion occu s.
Howe e , θ=45° means a sho mel pool du ing LPBF, as
he leng h is compa able o he dep h o he mel pool. A
leng h/dep h a io o 2 and 3 would mean θ=63.4° and
θ=71.6°, hen, R=0.7 and 0.5 m s
1
, espec i ely. These R al-
ues a e wi hin he ange o localiza ion o di usion bu close o
he c i ical R alue o solu e apping.
[27]
Thus, only e y low Mg
seg ega ion o g ain bounda y ha has been obse ed in colum-
na g ain egion is easonable. This is also consis en wi h he
Mg
2
Si- and Fe-con aining pa icles obse ed mainly in g ain in e-
io s in he coa se equiaxed-g ains and in he whole columna
g ain egion, as he high R alues also p e en Si and Fe o di -
use o g ain bounda ies o o m pa icles he e du ing
solidifica ion.
The sligh seg ega ion o Sc o g ain bounda ies in he
equiaxed-g ain egion bu no in columna g ain egion, how-
e e , may need o conside u he . Al–Sc-phase diag am sug-
ges s ha , o he alloy con aining 0.65 w %Sc, eu ec ic should
o m a e he o ma ion o p o-eu ec ic Al
3
Sc du ing cooling.
The p o-eu ec ic Al
3
Sc o Al
3
(Sc,Z ) should ac as nuclei whe he
hey can be de ec ed o no . Howe e , he STEM image in
Figu e 3 does no display he no mal coupled eu ec ic g ow h
mo phology. This is he esul o a di o ced eu ec ic solidifica-
ion. No man e al.
[28]
illus a e ha , using an Al–0.7(w %)Sc
alloy and cooling a e up o 1000 K s
1
, no e idence o couple
g ow h could be ound unde TEM in es iga ion. They show ha
only α(Al) g ows ou wa d om he Al
3
Sc nucleus, ypical o a
di o ced eu ec ic g ow h. This g ow h hen should esul in a
small amoun o Sc being ejec ed du ing he g ow h. Thus, seg-
ega ing o a sho dis ance o g ain-bounda y a eas should
esul as, in he fine-equiaxed-g ain-bounda y egion, he g ow h
a e is low and di usion is allowed.
Mo ing away om ack bounda y, Rand cooling a e (dT/d )
inc ease e y apidly.
[27]
An inc ease in dT/d may ha e a s ong
e ec on he e ec i eness o Sc o o m and hus o g ain efine.
To illus a e, we discuss using he Al–Sc bina y sys em. The equi-
lib ium Al–Al
3
Sc eu ec ic composi ion is 0.56 w %,
[2,29]
and hus
he o ma ion o p o-eu ec ic Al
3
Sc in he p esen Scalmalloy
con aining 0.65 w %Sc is e ficien o nuclea e α(Al) in nea -
equilib ium solidifica ion condi ion o he low R alue egion.
Howe e , unde he apid solidifica ion and hus a om equi-
lib ium condi ion in he mel away om ack bounda y, he
e ec i eness o Sc o o m Al
3
Sc and o g ain efine can dimin-
ish. I has been demons a ed
[29]
ha he alues o eu ec ic com-
posi ion a e 0.6, 0.8, 1.3, and 3.0 w % o dT/d equal o 5,
10
2
,10
3
, and 10
5
Ks
1
, espec i ely. These da a o nonequilib-
ium eu ec ic composi ion sugges ha Sc in an Al–0.65 w %Sc
alloy would no be e ec i e o g ain efining i dT/d >10
2
Ks
1
.
Du ing LPBF, dT/d inc eases sha ply away om ack bound-
a y o e y high alues. Hoope
[30]
di ec ly measu ed dT/d o
ack su ace du ing LPBF o Ti6Al4V o be (1–40) 10
6
Ks
1
depending on LPBF pa ame e s used. Hye e al.
[31]
es ima ed
dT/d alues o AlSi10Mg LPBF, based on he ela ionship
be ween dT/d and seconda y a m spacing and on using he
Rosen hal equa ion, o be 10
5
–10
7
Ks
1
. Thus, i is expec ed ha
dT/d could each a leas 10
5
Ks
1
a sho dis ance away om
ack bounda y. Fo he p esen alloy o 0.65 w %Sc, al hough he
alloy also con ains 0.3 w %Z (0.09a %Z ), o ming p o-eu ec ic
Al
3
Sc o g ain efining du ing solidifica ion may hus no be
expec ed in mos pa o he mel . This e ec i eness o g ain
efining depending on dT/d may explain why inc easing base
pla e empe a u e (T
B
) inc eases he hickness o he equiaxed-
g ain egion, as obse ed o example in Yang e al.’s s udy.
[9]
The inc ease in T
B
should educe he a e o hea ans e om
he mel ack, hus educes dT/d , assis ing p o-eu ec ic Al
3
Sc
o ma ion and hus widening he equiaxed-g ain egion.
3.3. Elemen al Dis ibu ion a e Aging T ea men
Figu e 5 shows ha dness alues and ensile cu es o samples in
he as-buil and in he one-s ep aged condi ions. The esul s a e
o confi ming he aging ea men used in his s udy o be in
ag eemen wi h he da a and hea - ea men condi ions p e-
sen ed in he li e a u e. As explained in In oduc ion sec ion,
one-s ep aging ea men o he alloy o achie e mid o high
s eng h has been well unde s ood. Fu he illus a ion o Sc dis-
ibu ion a e aging ea men is no o he s udy o how Sc has
played he ole on p ecipi a ion s eng hening. Ra he , how aging
ea men has a ec ed he elemen al dis ibu ions in he LPBF
samples o he alloy p esen ed he e is o he u he suppo
o he unde s anding o he dis ibu ions in he as-buil s a e.
Thus, he sugges ed mechanism o how Sc a ec s he bimodal
mic os uc u e o med can be be e unde s ood.
The elemen al dis ibu ions in he equiaxed-g ain egion o an
aged sample a e shown in Figu e 6. The Mg map in Figu e 6
shows ha Mg con en should be s ill highe in g ain-bounda y
a eas han he con en in g ain in e io s. Bu Mg appea s o be
significan ly less en iched in g an-bounda y a eas in Figu e 6, in
compa ison o he high deg ee o en ichmen in he a eas shown
in he Mg map in Figu e 3. This is because a po ion o Mg om
he Mg- ich g ain-bounda y a eas has di used o g ain in e io s
when he sample was held a he aging empe a u e. The Si map
shown in Figu e 6 displays an e en dis ibu ion o Si, meaning
ha Mg–Si (likely Mg
2
Si) pa icles ha can be de ec ed in he as-
buil s a e in Figu e 3 ha e dissol ed du ing aging ea men . The
Fe map in Figu e 6, compa ed o ha in Figu e 3, also has sug-
ges ed ha he Fe- ich pa icles in he as-buil s a e ha e la gely
dissol ed du ing aging ea men .
Howe e , he Sc map shown in Figu e 6 sugges s he p esence
o Sc- ich pa icles a e aging ea men , as opposi e o he as-
buil s a e showing no Sc- ich pa icles in Figu e 3. As has been
explained, acco ding o he li e a u e, he s eng hening Al
3
Sc
p ecipi a es a e aging ea men can be 1–2 nm o less in size.
These small-size p ecipi a es a e no dis inguishable in he Sc
map o Figu e 6. The Sc map in Figu e 6 has sugges ed ha he e
a e Sc- ich pa icles la ge han a ew nanome e s. Obse ing
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closely he STEM mic og aph and he Sc map in Figu e 6 sug-
ges s ha mo e Sc- ich pa icles a e p esen along he g ain
bounda ies. An example o such pa icles is poin ed o by he
wo ed a ows in Figu e 6 in he STEM image and in he Sc
map. Fo ming Sc- ich pa icles in g ain-bounda y a eas du ing
aging ea men is easonable, as he e is a sligh Sc seg ega ion
o he a eas du ing solidifica ion in he equiaxed-g ain egion, as
has al eady been shown and explained. No e also ha Sc- ich pa -
icles a e also obse ed in g ain in e io s and an example is indi-
ca ed by a g een a ow in bo h he STEM image and he map in
Figu e 6. Sc- ich pa icles la ge han a ew nanome e s in size in
g ain in e io s sugges ha Sc supe sa u a ion du ing solidifica-
ion may no be homogeneous.
The elemen al dis ibu ions in he columna g ain egion o
he aged sample a e shown in Figu e 7. Li le Mg en ichmen
in he g ain-bounda y a eas is shown in he Mg map. This is
because he eadily homogeniza ion o Mg du ing aging ea -
men om he low Mg seg ega ion in g ain-bounda y a eas in
he columna g ain egion in he as-buil s a e (as shown in
Figu e 4). Many Mg
2
Si- and Fe- ich pa icles inside he g ains
Figu e 5. Mechanical es ing: a) ha dness alues o samples hea - ea ed in a ious condi ions, and b) selec i e ensile cu es o one as-buil and one
aged samples.
Figu e 6. STEM–HAADF mic og aph and elemen al maps in equiaxed-g ain egion o a sample a e aging ea men . Red a ows poin o a sec ion o a
g ain bounda y in he STEM image and o Sc- ich pa icles along he same sec ion in he Sc map. G een a ows poin s o a S - ich pa icle inside he
g ains.
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in he as-buil s a e ha e howe e emained, as is indica ed by he
Mg, Si, and Fe maps in Figu e 7. This is e y di e en om he
dissolu ion o he Mg–Si- and Fe- ich pa icles in g ain bound-
a ies in he as-buil s a e du ing aging ea men and may be he
esul o apid di usion in he fine-equiaxed-g ain egion. G ain-
bounda y di usion a e may be much highe han la ice di u-
sion a e and g ain-bounda y a eas a e la ge in he fine-equiaxed-
g ain egion. Fu he mo e, in he fine-g ain egion, elemen al
di usion only needs a sho dis ance o elemen al homogeniza-
ion in g ain in e io s. In con as , dissolu ion o pa icles mos
in g ain in e io s in he coa se and columna g ain egion
equi ing la ice di usion could be a much slowe p ocess.
This may explain he insignifican amoun o dissolu ion in
he coa se and columna g ain egion du ing he ime a aging
empe a u e. As o Sc, he Sc map in Figu e 7 shows some Sc-
ich pa icles h oughou in his aged sample. This is consis en
wi h he lack o Sc seg ega ion o g ain-bounda y a eas in colum-
na g ain egion in as-buil s a e.
4. Conclusions
The LPBF-induced elemen al dis ibu ions we e ound o be di -
e en in he solidifica ion o equiaxed o columna g ain egions
sugges i ely due o he condi ions o solidifica ion in LPBF. The
e y low g ow h and cooling a es in he egion nex o he ack
bounda y allow o Al
3
Sc o o m and ac as a nucleus o he fine-
equiaxed-g ain (0.5 μm) g ow h nex o and along he ack
bounda y. The e y low g ow h a e also allows o elemen s
o di use ou wa d du ing solidifica ion, as obse ed wi h seg e-
ga ion o elemen s o g ain bounda ies in he fine-equiaxed-g ain
egion. The equiaxed-g ain size inc eases o 1.5 μm o e a ew
mic ons dis ance om ack bounda y as a esul o he s eep
inc ease in he cooling a e away om ack bounda y. This
esul s in he shi ing o he composi ion o he di o ced eu ec ic
o highe alues han he Sc con en o he alloy. Thus, g ain-
efining e ec diminishes and g ain size inc eases. Fu he (only
a ew mic ons) away, g ain- efining e ec o Sc (wi h i s con en
o he alloy) will be o ally los , esul ing in a columna g ain
g ow h. Away om ack bounda y, he high g ow h a e
du ing solidifica ion in he columna g ain egion esul s in
highly localized di usion, p e en ing significan seg ega ion
du ing solidifica ion.
Acknowledgemen s
Open access publishing acili a ed by Auckland Uni e si y o Technology,
as pa o he Wiley - Auckland Uni e si y o Technology ag eemen ia he
Council o Aus alian Uni e si y Lib a ians.
Figu e 7. STEM–HAADF mic og aph and elemen al maps in columna g ain egion o a sample a e aging ea men . Red a ows poin o a sec ion o a
g ain bounda y in he STEM image and o Sc- ich pa icles along he same sec ion in he Sc map. G een a ows poin s o a S - ich pa icle inside he g ains.
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