esx
characteristic
to a certian
: structureof die-casting
ametersto give attention
senvalidatedin
r simulationvisibility in
e higher roundnessand
>mplexand thin section.
fferent filling condition,
Lueto high flow velocity.
and small size.With the
consideredfrom design
lurgical TransactionsA,
'erty of Al alloy during
47
\,leutonian
Fluid Mech.,
crostructur e Formati on
mi-Solid Processing
of
nisolid metal slurries
', Vol.l8(2002),No.3,
continuouscasting and
; & NonferrousAlloys,
entrite during induction
-)z
-jie: Mensuration and
alloy. Transactionsof
timization of induction
nationalConferenceon
t4,p.473-483
tgical Behaviour and
tnt Routes.Proceedings
s and CompositesS2P
SolidStatePhenomenaVols. |a1-143 (2005)pp 145-150
online at http ://www. scientific.net
@2008 Trans Tech Publications, S\uitzerland
semi-solidcastingof High-reactive
wrought Ailoys by Meansof
the Alloy AlLi2.lMgS.SScZr
(AAt420.)
MatthiasBünck1'",
FabianKüthe1'b,
AndreasBührig-polaczek1,b,
AlexanderArnold2'',BerndFriedrich2'd
and Roger5auermann.,6
1GlFoundry
Institute,RWTHAachen
2lMEProcessMetallurgy
and MetalRecycling,RWTHAachen
'Otto Junker
Grouo
"[email protected]
"[email protected],
ä",f""5#1"@.i#kä:[:].de,
Keywords:Rheocasting,
Thixoforming,
Rheocontainerprocess,Aluminium-Lithium
Abstract. Semi-solidforming offers new potentialsfor processingof high reactiveand hot crack
susceptible
aluminium-lithiumwroughtalloys.With the tailoredalloy AAl420* (AlLi2.lMg5.5
+Sc+Zr) a promising material for thixoforming with achievablehigh strengthof up to SO0Mpa
tensilestrengthand over 400MPayield strengthconcomitantwith its low density of 2,46g/cm3is
achievable.Due to high solid fractionsthe effect of solidification shrinkagecould be sufficientlv
decreased
with the resultofhot-tear-freecastins.
Simulationsupporteda critical to castautomotivetie rod was exemplarilymanufacturedvia semisolid-technology
with promisingresults.
Furthermorewith an improved and advancedheat treatment enhancedmechanicalproperties,
comparable
to thoseof rolled AAl420, were achieved.
Introduction
Wroughtalloys generallyfeaturehigh strengthcomparedwith excellentplasticity, but in classical
formingprocesseslike forging or rolling the freedomof shapeis restricted.This constrictionlimits
theapplicationof thosealloys to relativesimplegeometries.Conventionalcastingof wrought alloys
is generallynot possible,becauseof the hot tearing susceptibility.Due to theselimitationi a nicire
for SSM-processing
is given, becausewith higher solid fractionsthe effect of volume shrinkageon
tearingis significantly reduced.Thus in the majority of caseswrought alloys are under cirtain
conditionssemi-solidcastablefree from defects.
, Another advantageof semi-solidforming is the reducedprocessingtemperaturecomparedto
fully liquid casting. Hence the reactivity of oxygen-affine elements is Jignificantly ieduced.
Additionallythe life-cycle of the forming tools canbe extended.
Regardingthis benefitssemisolidcastingof wrought alloys, especiallyof high reactivewrought
alloysappearexpedient.In this researchwork the aluminium-lithium-magnesium
super-light-alloy
AAl420 was chosenand tailoredfor thixocastingby addingscandiumand zirconium. By meansof
thethixocastingprocessand the rheo containerprocess(RCP) the suitability of this alloy for SSMprocessingwas investigated.
DoE-supportedsynthesisof Al-Li-Mg-based precursor material
s of the microstructure
ilid state. Scienceand
The benefits of Al-Li alloys are the excellent mechanical properties/densityand Young's
modulus/densityratios comparedto conventionalaluminium alloys (density of Al-Li: n-r.5
g/cm'). Due to this, the disadvantage
of higher costsfor manufacturingof precursormaterialsand
alsoforming, causedby the high reactivity of the alloying elementsLi and Mg, is compensated
by
the demandsof aerospaceapplications,where even small weight reductionsresult in high added
values.
Semi€olid Processingof Alloys and CompositesX
146
To guaranteethe filling of
For SSM-forming,globulargrainedfeedstockmaterialsarenecessary.
(typically
lessthan l00pm, [l]).
grain
to
be
assured
globule
have
sizes
small
mould-areas,
small
Regardingthis, up to 0.3 weight-o/oscandiumand zirconium were added to the conventional
of theirpositiveeffectson the grainsize.In order
wroughtalloy AlLi2.lMg5.5 (AA1420),because
grain
during
feedstockmaterial production, the casting
size
on
the
its
influence
investigate
to
temperaturewas also varied (750'C, 800oC,and 850"C). With the aid of the DoE software
MObDE 5.0@,billets with variationsin chemicalcompositionwere molten and cast as precursor
material for subsequentsemi-solid processing.As the result of high reactivity of lithium with
atmosphericgasesand refractory material and its high equilibrium vapour pressurea lithiumresistantSiC-cruciblewas requiredand installed in a 3-bar argon overpressureinduction melting
fumace.The pouringof the 3kg billets took alsoplaceunderprotectivegas.
The resulting effects of the Sc and Zr addition on the grain size in the as-castcondition are
shownin Fig. l. It illustratesthe statisticalsignificance,while in contrastthe variationof the casting
temperaturehasalmostno influenceon the grain size,Fig. 2. It also showsthe contentsof Sc andZr
neededto achieve a certain grain size. The mathematicalmodel predicts a grain size of 30pm
Zr. The strongesteffectson grain sizewere observed
startingfrom contentsof 0.25oÄSc and0.25%o
to
the Al-Li-Mg matrix.
(Sc
added
Zr)
were
whenboth elements and
Pouring
ffi
f
w
f
i
+
t
f
ffiry
'
i
E
'
T
ü
L
|
The use of the non-retr
Apart from this the usage
melting metals, and in p;
80mm in inner diameteri
suitability for the thixo shc
ü
i
N
R2 Adl.=0,814
RSC=2,7184
Fig. 3: The Rheocontainer
t
ü
'
O
RI=0,167
Q2.0,599
|
!
*
ö
N=22
DF='15
containerwall, seedcrysta
the container.The heat ca
this, and as the result of
containeris placeddirectl;
and admitted with argon
containeris cooledwith ct
fraction, the containeris 1
high-pressuredie-casting
completelyin the biscuit [l
,l
,t.
Conf.LevF0,95
Fig. 1: Statistical effects of Sc and Zr addition (single addition and mixture) and the casting
temperature(Gie) on the grain size"of the alloy AlLi2.lMg5.5ScZr (left); example of the
microstructureof a billet (right).
Reheatingof the billet for
All thixo-billets were rehe;
to the softwareDiadem.I
billets were transferredm,
Btihler H-630 SC) andforr
To preventthe oxidisati
aluminiumfoils.
Forming experiments:stt
co
otE
o.10 0.ß
k
ox
o5
0$
o.m
0.6
0.10 0.15 0r
*
05
0$
o,m o.o5 0-10 0.15 020
*
025
0$
Fig.2: Dependenceof the average grain size in pm on Sc and Zr contents for the alloy
(left:750'C; centre:800oC;right: 850"C).
AlLi2.lMg5.5ZrScat 3 differentcastingtemperatures
RCP - Rheocastingof high reactive alloys
(RCP, 12,31,Fig.3) is the encapsulationand
The special feature of the Rheo-Container-Process
consequentlythe suitability for processingof highly reactive alloys such as magnesiumor
aluminiumlithium alloys. To diminish the atmospherecontactof a high oxygen affine alloy the
molten metal is poureddirectly into a lmm-walled aluminium can (container),
slightly superheated
ensuringwall contactduring melt inflow for nucleation.At first contactwith the room-tempered
Preliminary forming inves
suitability of the rheo cr
manufacturedreal compon
As a demonstratorcoml
at a total lengthof 400mm
it was appropriateto invesl
The mould was adjustedrr
was modified to warranta
critical two-channelflow a
Rheocontainerprocess:(
The tailored precursorAA
containerandprocessedat
SolidStatePhenomenaVols. 141-143
arantee
the filling of
rsthanl00pm, [1]).
to the conventional
: grainsize.In order
luction, the casting
'the
DoE software
rd castas precursor
ity of lithium with
pressurea lithiume inductionmelting
147
container
wall, seedcrystalsarebuilt and subsequently,
becauseof the flowage,finely dispersedin
thecontainer.The heat capacityof the containersufficesto cool the melt below liquidus. Due to
this, and-as the result of a high grain density, a globular microstructureforms. Thereforethe
containeris placeddirectly afterpouring into a cooling deviceand coveredimmediatelyafterwards
and admitted with argon-atmosphere
to prevent burn off. To minimise the process time the
containeris cooledwith compressed
air, [4]. At targettemperature,respectivelyat the desiredsolidfraction,the containeris transferred,togetherwith the semi-solidbillet, to the shot chamberof a
high-pressuredie-casting machine and .pressedinto the die. The folded container remains
completelyin the biscuit [5].
;-castconditionare
iationof the casting
rntentsof Sc andZr
grainsize of 30pm
sizewereobserved
A'gm i.rfkry
Pre$uE-!k
, --"1..t ,.
i
L l l
!
Pouring
-,.i
) /.
J
Cooling
Forming
Fig.3: The Rheocontainerprocessschematically
t
):, I
-"
./t
The use of the non-returnablecontainer,consistingof 99.5%-aluminium,avoids recycling [6].
Apartfrom this the usageof steelcansetc. is also possibleand imaginablefor rheocastingof high
meltingmetals, and in pa*icular for steel. Conventional lmm-walled aerosol-can-blanks,
with
80mmin inner diameterand with a total length of 280mm were chosen,becauseof their exact
suitabilityfor the thixo shotsleeveof the high-pressure
die-castingmachine.
r ,.oa',.r
i ' l \ , { ' .
. { . : .q r
(
:) and the casting
; example of the
Reheatingof the billet for Thixocasting
All thixo-billetswere reheatedusing an AEG Elothermreheatingdeviceof the type ETH I coupled
to the softwareDiadem. At the targettemperature(after a l20s long homogenisationperiod), the
billetswere transferredmanually to the shot sleeveof a high-pressuredie-castingmachine(type
BühlerH-630 SC) andformed directly.
To preventthe oxidisationof the AAl420--billets during reheating,the billets werewrappedinto
aluminiumfoils.
Formingexperiments:step form and tie rod
0.6
0ä
o25
0s
*
rnts for the alloy
; right:850"C).
encapsulation
and
rs magnesiumor
,n afline alloy the
m can(container),
re room-tempered
Preliminaryforming investigationsby meansof the step die have in the first instanceshown the
suitability of the rheo container process, [7]. To evaluate the usability also for industrially
manufactured
real components,comprehensive
castingtestswere conducted.
As a demonstratorcomponenta tie rod was chosen(Fig. a) with a minimum thicknessof 2.5mm
at a total lengthof 400mm and a weight of approx.3509 (A356). Due to the challengingcastability,
it wasappropriateto investigatethe difficult to castaluminium-lithiumwropghtalloy AA1420", [8].
Themould was adjustedregardingthe rheologicalbehaviourof semi-solidsuspensions.
The ingate
wasmodifiedto warranta laminarflow of the semi-solidmetal.The bearingsof the tie rod excitea
criticaltwo-channelflow at both endsof the component,which result in cold runs or oxide layers.
Rheocontainerprocess:experimentsand results
ThetailoredprecursorAAl420'-ingot was molten underprotectivegas,poured at 630oCinto the
containerandprocessedat a temperatureof 600oC,respectivelyat a solid fractionof 0.5 (calculated
148
Semi-SolidProcessingof Alloys and CompositesX
by ThermoCalc)in the HPDC-machine.Manufacturedtie rods were microscopicallyanalysedand
rapid solidificationof the remaining
heattreatedas well as tensiletested.During HPDC-processing
eutectic,includingthe phases
of"unexpected"
formation
the
by
melt took place in non-equilibrigm
AlzLiMg, Al,rMg,, and Al3Mf5 [9]. While heat treatmentfor 24 hours at 460"c the solidus
(Al2LiMg,
phases
these
at
480"C
temperaturewas carried out by DTA-analysis and detected
to
a more or
leading
solid
solution,
the
alpha
in
dissolved
extent
a
large
were
to
AlsMgs,AlrzMgrz)
less single phase structure (cp. Fig. a). The added elementsSc and Zr formed All(Sc"Zrr-*)
dispersJids,which inhibited undesiredgrain growth during the solution treatrnent[10,11]. After
waier quenchingfrom the solution temperaturethe componentswere agedfor 17 hours at l60oC
phaseAl3Li was formed
Accordingto literature,the primary strengthening
underaigon atmosphere.
Il0].
llat bar t€n3lon 5p€clren
Fig.4: Microstructureof semi-solidcast ZrlSc-microalloyedAA1420': homogeneitysampling
po-rition of the flat bar tension specimen(left); right hand side: microstructureof heat treated
sample.The black dots are AII(Sc,Zrr-*) particles (proved with EDX). At the grain boundaries
were detectedvia EDX.
AlzliMg-dispersoid-accumulations
Out of the cast tie rods, flat bar tensionspecimenswere sectionedand tested.Fig. 4 showsthe
sampling locationsand the good homogeneityof the microstructureover the total length of the
of AAl420- were improvedsigrrificantly.
componint.Via heattreatme,itthe mechanical:properties
the yield strengthup to 250MPa,and
400MPa,
to
over
increased
In this way the tensilestrengthwas
the elongationto fractureto an averageof 6.4Yo,almost three times higher than in'the thixocast
[8].
condition,[8]. Chemicalanalysesshowedthat the tie rodsweremanufacturedhomogeneously,
To investigate the improv€ment potentials of the heat treatrnent,sectioned samples of the
(24h at 460"C), quenchedin water and subsequentlyaged
connectinglod were solution-annealed
for various times in an oil bath at different temperatwes(140"C, 160"C, 180'C) at the Swiss
Federal Institute of Technology (ETH) Zvnch. The pyramid diamond hardness(HV5) wu
measuredand plotted againstageingtime. The hardnessbeforeheattreatmentwas 84 HV andafter
solution*tt"uiing 94 IIV. While increasingthe ageingtemperature,the maximumof hardness(130
the
FfV in eachexperiment)requiresshorterageingtime. ln order to achievea maximumhardness,
This
or
140"C435h'
160'C/25h
180"C/10h,
advisable:
parameters
are
[8]'
following annealing
revisedheattreatmentwas testedduring thixo castingexperimentswith the modified AA1420.
Despite casting in the semi-solid state hot cracks appearat the bearing points. To solve this
p.obl.-, simulationsregardingthe hotspotsat the bearingswere conductedusingMAGMAsoft and
verified duringthixocastingof AA1420 .
Thixocasting:experimer
The as-castfeedstockbill
thixoforming process.B1
Becauseof the non avail
alloy for the simulations
steel core was hot, a hot
corethe hot spotis displa
comparedto a 300.C ho
cooled beforeeachshot.I
the hot tearing tendency
forming,no morehot tear
Fig.5: MAGMAsoft-simu
simulationshowsthe disol
t
Fig. 6: Hot tearing at the b
resultsin a decreasinghot t,
A total of 12 tie rodswere,
were not further treated.A
nine sampleswere solution
; ETH Zürich the sampleswe
T7-T9 were agedfor 25 ho
Fig. 4 flat bar tensionspe
samplesper treaünentco
,Fig.7.
Due to the heattreatmen
of the longer ageing '
fracture decreasedlnotr
were raised durir
to a top of 437M1
from averaged l.57ot,
RCPexperiments).
:esx
rscopicallyanalysedand
fication of the remaining
tic, includingthe phases
at 460oC- the solidus
these phases(AlzLiMg,
ron,leadingto a moreor
1r formed Al3(Sc*Zr1_*)
lreatment[0,]l]. After
I for 17 hoursat l60oC
phaseAl3Li was formed
Solid StatePhenomenaVols. 141-143
149
Thixocasting:experimentsand results
Theas-castfeedstockbillets were tumed off to a diameterof 78mm and a lengthof l60mm forthe
thixoformingprocess.By meansof MAGIJ[Asoft-simulationsthe hot-tearproblem was solved.
Becauseof the non availability of thermo-mechanical
data of AAI420r, 4356 was used as cast
alloyfor the simulations.
A removablesteelcorewas usedfor buildingup the big bearing.If the
steelcore was hot, a hot spot directly behindthe core was the result. ötnerwise with a cold steel
corethe hot spot is displacedinto the over flow. The simulationresult using a 60"C cold steelcore
comparedto a 300oChot steel core show this effect, Fig.5. Regardingihis, the steel core was
cooledbeforeeachshot.The effect ofthis is visualisedin fig. O.D".."uiing the core temperature,
the hot tearing tendencyalso decreases.
Using a cold core that was insertedonly directly before
forming,no more hot tear appears.
IFC)
[-l
",oo
I l€13
I 1607
I ls3
fl5eE
F;d 5s3
Ig 658
Tfii
t*;
t::;
E
l5
homogeneitysampling
ructure of heat treated
t the grain boundaries
rsted.Fig. 4 showsthe
the total length of the
improvedsignificantly.
gth up to 250MPa,and
r than in the thixocast
: homogeneously,
[8].
oned samples of the
md subsequentlyaged
180"C)at the Swiss
hardness(tIV5) was
t was 84 HV and after
(130
mum ofhardness
raximumhardness.the
140.c>35h,[8]. This
odified AAl420.
points. To solve this
;ing MAGMAsoft and
r'.
"-'
Fig. 5: MAGMAsoft-simulation:comparisonbetweensteelcore temperature:60.C and 300"C; the
simulationshowsthe displacementof the hot spot
Fig.6: Hot tearing at the big bearingpoint: From Ieft to right; a decreasingsteelcore temperature
resultsin a decreasinghot teartendency
A totalof 12 tie rodswerecastandconsecutively
numberedwith Tl to Tl2. The samplesTl to T3
werenot further treated.All other sampleswere heat treatedunder argon atmospheri.Rt first all
nine sampleswere solution annealedfor 24 hours.With referenceto the improvementtestsat the
ETH Zürich the sampleswere agedafterwards:the samplesT4-T6 were agedfor 10 hoursat 180.C,
T7-T9wereagedfor 25 hoursat 160'C,andT10-Tl2 wereagedat 140'C for 35 hours.As shown
in Fig. 4 flat bar tensionspecimensweretakenfrom both sidesof the double-Tbeamof the tie rods
(6 samplesper treatmentcondition)and tensile-tested.
The resultsare illustratedbv meansof a bar
diagram,Fig. 7.
Due to the heattreatmentsa high increaseof the mechanicalpropertiescould be achieved.As a
resultof the longer ageingtime, concomitantwith the reducedageingtemperaturesthe elongation
to fracture decreased;notwithstandingthe strength remain at a similar level. The mechänical
propertieswere raisedduring the heat treatmentto almost 500MPa
Jensile strengthand the yield
strengthto a top of 437MPa. The elongationto fracture could be äoubled during the ageing at
180'Cfrom averaged1.57%oto
2.980, but theseresultsfall shortofthe expectations
(cp. risults of
the RCP experiments).
150
Semi€olid Processingof Alloys and GompositesX
r Tensile
StrengthE Yieldstrengthu Elongation
ToFracture
Solid State Phenomena Vols
online at http://www.scienl i
@ 2008 Tra:ttsTech Publica
Rheocastin
E.PM as u
I
'cslR: Materials
"emasuku@csir
300
=
zoo
F
g
Keywords: 4201 alloY
100
no HT
10h/180"C
25hl15O'C
ash/140'C
Fig.7: Mechanical properties of thixo-cast AlLi2.lMg5.5ScZr;
solution annealed at 460"C for 24 hours (HT - heat treatment).
all heat treated samples were
Assessmentand outlook
Depending on the castpart, defect free SSM-casting of AAl420- is possible. Indeed, experiments
have shown that mould adjustment is of utmost importance, especially at hot tear susceptible areas.
ln the process excellent mechanical properties of cast AAl420', comparable to those of rolled
AAl420 were achieved. As a result semi-solid processedAlLi2.lMg5.5ScZr may be used near net
shape for aerospace applications. It may also be expected that the found results are usable also for
other wrought alloys. Hence SSM-forming is a suitable process route for casting of wrought alloys.
Acknowledgement. The authors would like to thank the Deutsche Forschungsgemeinschaft (DFG),
which supports this work in the frame of a collaborative research centre (SFB 289).
References
D. Apelian: 'semisolid ProcessingRoutesand MicrostructureEvolution',Proceedings
of the 7s S2PInt.
Conf.,Tsukuba,Japn, 2002,pp. 25-30
Fehlbier, M.; ,,Herstellung, Charakterisierungund Verarbeitung teilflüssiger metallischer Werkstoffe
t2l
verschiedenerAluminium- und Magnesiumlegierungen",PhD thesisRWTH Aachen,ShakerVerlag,2003
Aguilar, J.; Fehlbier, M.; Grimmig, T.; Bührig-Polaczek,A, ,,Processingof semisolid Mg alloys",
t3l
Proceedings
of the 8'S2P ht. Conf.,Limassol,Cypms,2004.
BtiLnck,
M.; Wamken,N. Bütrig-Polaczek,A.; 'Defining theboundariesof globularto dendritictransition
t4l
ofA356 in the coolingchannelprocess';Proceedings
ofthe 5* Dec. Int. Conf. SP07;2007;pp. 615-619
Aguilar, J.; Grimmig, T.; Bührig-Polaczek,
A.: "Rheo-Container-Process
(RCP):New SemisolidForming
l5l
Method for Light Metal Alloys", Proc. of the 6th Int. Conf. MagnesiumAlloys and their Applications,
DGM, 2003,767-773
Grimmig, T.; Aguilar, J.; Btihrig-Polaczek,A.: "Optimization of the rheocastingprocess under
t6l
considerationof the main influenceparameterson the microstructure",Proceedingsof the 8tnS2P lnt.
Conf.,Limassol,Cyprus,2004
Sauermann,R; Friedrich, B.; Grimming, T.; Bünck, M.; Bütrig-Polaczek, A.: ..Developmentof
t7l
Aluminium-Lithium alloys processedby the Rheo ContainerProcess",Proceedingsof the 9ü S2P Int.
Conf.,Busan,Korea,2006,pp. 513-517
Sauermann,
R.; Friedrich,B.; Bünck, M.; Btihrig-Potaczek,
A; Uggowitzer,J.: ,,Semi-SolidProcessingof
t8j
Tailored Aluminium-Lithium Alloys for Automotive Applictions", in AdvancecdEngineeringMaterials
2007,9, No. 4,pp.253-258,WILEY-VCH VerlagWeinheim
Belov, N.A.; Eskin, D.G.; Aksenov,A.A.; "Multicomponentphasediagrams:applicationsfor commercial
i9l
aluminiumalloys",ElsevierLtd., 2005,p. 261
"Mechanicalbehaviourof aluminium-lithium
[10] EswaraPrasad,N.; Gokhaie,A.A.; RamaRao, P.;
allovs".
Sädhanä
Vol.28, PartsI & 2,2003,pp.209-246
"Scandium",Chemical
& EngineeringNews Vol. 81, AmericanChemicalSociety,2003,pp.
[11] Raade,G.;
66-68
tll
Abstract. Rheocasting
different silver conten
pressuredie casting.T
propertiesof the alloy.
the grain boundarieso
dissolveall of this Pl
decreasedmechanical
properties(strengthar
than thoserePortedfor
Introduction
Semi-solidmetal(SSl
near net shapecomPo
given to applicationsi
composition Al-4.6C
aluminium casting al
strengtheningprecipit,
thesealloys,causing:
matrix planes (primal
tensilepropertiesbein
of silverto this alloY,
industries,might outu
alloysA206 andA20l
Experimentalwork
Due to diffrculties in
alloy A (no silver- a
by an extemal suppli
elementsto a pourin
Quantris Optical Em
within the specifiedt
does not have the Al
for the analysisof Ag
Stainlesssteel cu1
metal was transferrer
castingparamet€rs:rl
The castbars(d1:12
consistedof solution
ambienttemperature
assistedwith homog