An investigation into aluminumaluminum bimetal fabricationby
squeeze castingTeng Liua, Qudong Wanga,, Yudong Suia, Qigui Wangb,
Wenjiang DingaaNational Engineering Research Center of Light Alloys
Net Forming and State Key Laboratory of Metal Matrix Composite,
Shanghai Jiao Tong University, 200030 Shanghai, PR ChinabGeneral
Motors Global Powertrain Engineering, 823 Joslyn Avenue, Pontiac,
MI 48340-2920, USAarti cle i nfoArticle history:Received 29
September 2014Accepted 26 November 2014Available online 4 December
2014Keywords:Aluminum alloySqueeze
castingInterfaceMicrostructureMechanical
propertiesabstractAluminumaluminumbimetal were prepared by casting
liquid A356 aluminumalloy onto 6101aluminum extrusion bars and
solidifying under applied pressure. The effect of surface
treatment, pouringtemperature and applied pressure on
microstructure and mechanical properties of the bimetal was
inves-tigated. The results showed that sound metallic bonding could
be produced by electro-plating the solid6101 aluminum alloy with a
layer of zinc coating and carefully controlling the casting
temperature. Withthe application of pressure during solidication
process, the tensile strength exhibited more promisingresults than
that made by gravity casting, for both A356 aluminum alloy matrix
and bimetal. However,with the increase of applied pressure, A356
aluminum alloy matrix and bimetal showed different behav-iors. For
A356 aluminum alloy matrix, the tensile strength increased with the
increase of applied pres-sure, for bimetal it
appearedtobeindependent onthemagnitudeof
theappliedpressureandthevalue remained steady. The fracture
analysis indicated that during the tensile test of bimetal, the
crackinitiation began with initial fracture of eutectic Si in the
transition zone then extended in the transitionzone. The tensile
strength of the bimetal fabricated by squeeze casting method was
improved by about10%, from 145 MPa to 155 MPa, as compared with
that made by gravity casting. The process
presentedinthisstudyprovidesapromisingandeffectiveapproachtocreateametallicbondingbetweenanaluminum
insert and various aluminum melts to develop advanced functional
and structural materials. 2014 Elsevier Ltd. All rights reserved.1.
IntroductionLight metal castings have been extensively used in the
automo-tiveandaerospaceindustriesfor lightweight
applications[1,2].When one single light material alone does not
satisfy the require-mentsofhighperformanceandefciencyatlowcost,
bimetallicdesignandmanufacturingappearstobeanidealsolution.
Com-poundcastingisdenedasaproductiontechnologywheretwometals, one
insolidstate while the other inliquidstate, arebrought into contact
with each other and thus a continuous metal-lic transition occurs
from one metal to the other [3]. Because of itshigh efciency and
low cost, this method has drawn great atten-tion in a variety
systems, such as magnesium alloy and aluminumalloy [46], aluminum
alloy and cast iron [7], aluminum alloy andcopper [8,9],
grayironandcopper [10], magnesiumalloyandmagnesium alloy [11].
However, the application of this method isstill very limited in
aluminumalloy [3,12]. Because solid
aluminumalloysarealwaysnaturallycoveredwithanoxidelm,
whichisthermodynamically stable andnot easily wettable by
metallicmelts.Apromisingapproachof joiningaluminumalloys was
pre-sentedbyPapis et al. [3] byreplacingtheoxidelayer
withaelectro-depositedzinccoating. Couplesof
AlMg1substrateandvarious aluminum alloys were successfully produced
in a labora-tory-scale. Defect-freeinterfaces
couldberealizedbypreciselyoperating in an Ar 6.0 atmosphere.
However, the characterizationof the joint interface is not well
investigated and the mechanicalproperties of thejoint areunknown.
BasedonPapiss method,Rubner et al. [12] and Koerner et al. [13]
focused on the realizationof aluminumaluminum bimetals using high
pressure die casting.However, the very high and locally varying
melt velocities some-times completely washedaway the zinc coating,
furthermore,bonding strength of the aluminumaluminumbimetal is
stillunexplored.Squeeze casting is a technical process in which
metal is solidi-edunder pressure, andcanberegardedas
acombinationofdie-casting and closed die forging [14,15].
Application of
pressurehttp://dx.doi.org/10.1016/j.matdes.2014.11.0510261-3069/
2014 Elsevier Ltd. All rights reserved.Corresponding author at:
National Engineering Research Center of Light AlloysNet Forming,
Shanghai Jiao Tong University, 800, Dongchuan Road, Shanghai200240,
PR China. Tel.: +86 21 54742715; fax: +86 21 34202794.E-mail
address: [email protected] (Q. Wang).Materials and Design 68
(2015) 817ContentslistsavailableatScienceDirectMaterials and
Designj our nal homepage: www. el sevi er . com/ l ocat e/ mat
deson molten metal during solidication may cause a series of
effects,suchas change of solidicationrate, change of
meltingpoint,change of phase diagramandreductionof gas
andshrinkageporosities [16]. As aconsequence,
squeezecastingcomponentsalways exhibit superior mechanical
properties and casting sound-nesscomparedwiththeconventionalones.
Theparametersthataffect thecast
microstructureandwhichneedtobeoptimizedarepouringtemperature,
mouldtemperature, appliedpressure,time delay between pouring of the
melt in the die and applicationof pressureanddurationof
pressureapplication. Amongall theparameters, the effect of applied
pressure and pouring
temperaturehasbeenextensivelystudied[14,17,18], andit
isbelievedthatthesetwoparametersareofsignicantinuenceonmicrostruc-tureandmechanical
properties. Manyresearchershavecarriedout research work on squeeze
casting of aluminum alloy[17,19,20]. It is proved that squeeze
casting can effectively improvethemechanical
propertiesbyenhancingthea-Al solidsolutionphase, rening the
microstructure and homogenizing the eutecticphase [21].6101
aluminum alloy has high strength, excellent thermal
andelectricalconductivity, whileA356aluminum
alloyhasexcellentcastingcharacteristics,
hightensileandfatigueproperties. A356aluminum alloy6101 aluminum
alloy bimetal can combine theiradvantages. Therefore, the present
study focused on the realizationof A356 aluminumalloy6101
aluminumalloy bimetal usingsqueeze casting method. The effects of
surface treatment, pouringtemperature and applied pressure on
microstructure and mechan-ical properties of squeeze cast bimetal
were investigated. Themechanismof interface formationandfracture
behavior werediscussed.2. Materials and methods2.1. Materials and
surface treatmentA commercial 6101 aluminum alloy was used as the
solid insertmaterial, beforethesqueezecastingprocedure,
theinsertswerecut into rectangular bars with a dimension of 60 10
2.5 mm3.A commercial A356 aluminum alloy was used as the casting
mate-rial. Thechemical compositions ofthematerials
aretabulatedinTable1. Thetensilestrengthof as-gravity-cast
A356and6101inserts are about 145 MPa and 200 MPa respectively.The
6101 aluminum alloy was received in rolled condition,
inordertoremovethethicklmscontainingoxidesandlubricantremaindersonthesurface,
aprocedurewhichcombinesseveralaluminum surface pre-treatment was
developed, including degre-asing, alkali erosion, acid pickling,
rst zinc treatment, zinc retreat-ment and second zinc treatment.The
result of the zinc treatment is a zinc layer with the thick-ness of
300500 nm, because the deposition stops as soon as thesurface is
completely covered with zinc and ion-exchange
reactionhasnodrivingforceanymore[12]. Electro-platingmethodwasthen
operated onto the 6101 aluminum alloy insert in order to fur-ther
increase the thickness of the zinc layer, the desired thicknesswas
adjusted by controlling the coating time.2.2. Compound castingAn
80-ton vertical hydraulic press was used for direct squeezecasting.
The mold was preheated to 250 C. Before casting, the
elec-tro-plated 6101 aluminum alloy insert was pre-seated at the
bot-tomof themold, thenA356liquidmetal waspouredintothemold,
andsolidiedunderappliedpressure. Thebimetalsamplewas thus produced
by squeeze casting method with a dimensionof u55 mm 50 mm.
TheprocessisschematicallypresentedinFig.
1.ThecastingparametersforbimetalsstudiedinthisstudyareshowninTable2.
Intherst round, thepouringtemperatureand applied pressure were kept
at 700 C and 30 MPa respectively,while the condition of surface
treatment of 6101 aluminum alloyinsert varied. Inthesecondround,
pouringtemperaturevariedfrom660 C to 740 C, while applied pressure
was kept at30
MPaand6101aluminumalloyinsertswereallelectro-platedwith 5 lm zinc
coating. In the last round, the applied pressure var-ied from10 MPa
to 50 MPa, while pouring temperature was kept at700
Cand6101aluminumalloyinsertswereall electro-platedwith 5 lm zinc
coating.2.3. Metallographic examinationMicrostructure samples were
prepared with standardmetallographic procedure. The polished
samples were furtheranodizedat 30 Vfor 30 s ina2%solutionof
uoroboric acid.Table 1The chemical compositions of the materials
(wt.%).Alloys Chemical compositions (wt.%)Si Cu Mg Mn Zn Fe Ti B
Other AlA356 7.5 0.2 0.4 0.1 0.1 0.2 0.2 0 0.15 Bal6101 0.49 0.23
0.92 0 0 0.45 0 0 0.1 BalFig. 1. Schematic illustration of the mold
(a) and the tensile specimens in mm (b).Table 2Casting parameters
for bimetals.Round Surface treatment Pouringtemperature
(C)Appliedpressure (MPa)1 Degreased 700 30Zinc treated5 lm zinc
coating10 lm zinc coating2 5 lm zinc coating 660 307403 5 lm zinc
coating 700 1050T. Liu et al. / Materials and Design 68 (2015) 817
9Microstructurecharacterizationwascarriedout
withanopticalmicroscope (OM) and a scanning electron microscope
(SEM).
Theelementdistributionwasanalyzedbyenergydispersivespectro-scope
(EDS) attached to SEM.2.4. Mechanical testingTensile samples for
both A356 aluminum alloy matrix and
A356aluminumalloy-6101aluminumalloybimetal weretakenfromthe middle
part of the cylindrical sample, then cut into rectangulartensile
specimens according to the ISO 6892-1:2009 standard [22].The
tensile specimen of bimetal had a sandwich structure(A356,
interface region, 6101, interface region, A356) in the
gaugesection, which is shown schematically in Fig. 1. Tensile
testing wascarriedout onaZwick/Roell-20 kNmaterial test machineat
astrainrateof8.33 104s1atambienttemperature. Toinsurerepeatability,
atleastthreesamplesweretestedineachtestingcondition. Hardness of
the bimetal was also measured across theinterface region.3.
Results3.1. Effect of surface treatment on microstructure and
mechanicalproperties of bimetalSurface treatment is the
prerequisite factor that determines theinterface formation during
the casting procedure [3]. In this paper,the inuence of surface
treatment on microstructure and mechan-ical properties is
investigated.Fig. 2 shows the microstructures of the interface
region of A356aluminumalloy6101 aluminumalloy bimetals, which
weresqueezecastatpouringtemperatureof700 Candappliedpres-sure of 30
MPa, while under different surface treatment conditions.The SEM
micrographs of the interface region along with the corre-sponding
concentration maps of element Si, Zn and O for each sam-ple are
presented in Fig. 3. The microstructure of A356 aluminumalloy shows
typical casting aluminum structure consisting of den-dritic a-Al
phase and uniformly dispersed eutectic Si particles.
Themicrostructureof 6101aluminumalloyshowstypical
wroughtaluminumalloy structure consisting of ne elongated
grains.When the6101 aluminum alloyinsertwas degreased,therewasa
clear interface between the two materials because of the
oxidelayer. The same phenomenon was observed for the bimetal
fabri-catedwiththe6101aluminum insertzinctreated.
However,theinterfacefracturedundertheappliedpressureandinthesefrac-turedspots,
metallicbondingformedbetweenA356aluminumalloyand6101aluminum alloy.
Whenthe6101aluminum alloyinsert was electro-plated with 5 lm or 10
lm zinc layer,desiredmetallic bonding could be realized. It can be
observed that alongthe interface, there is no aggregation of
element O or Zn,and nodefectsordiscontinuitiesaredetected.
Thereisa100 lmthicktransition zone intheinterface
region,themicrostructureshowsneequiaxedgrainedstructurewitheutecticSi
alongthegrainboundaries.Vickers hardness and Tensile strength were
evaluated for bime-tal prepared under different surface treatment.
As shown in Fig. 4,the Vickers hardness values of 6101 aluminum
alloy and A356 alu-minum alloy are in the range of 5565 and 8595
respectively. Forbimetal
madeunderdegreasedandzinctreatedconditions, thehardness changes
abruptly. While for bimetal prepared with6101 aluminumalloy
electro-plated with 5 lm or 10 lm zinc coat-ing, there is a
transition zone between the two bonded materials,whose hardness is
6575. Thus, the Vickers hardness changesgraduallyfrom5565to8595.
ThetensilestrengthofbimetalsmadeunderdifferentsurfacetreatmentconditionsareshowninFig.
5. Thefracturedsurfacesandthecrosssectionviewsofthefractured
specimen are presented in Fig. 6. For bimetal made
underdegreasedandzinctreatedconditions, theA356aluminum alloyand
6101 aluminum alloy are partially metallic bonded, the
tensilestrengths are very low. While for bimetal made with the 6101
alu-minumalloy electro-plated with 5 lmor 10 lmzinc layer, the
ten-sile strength are about 155 MPa. The fracture occurs in
thetransitionzonealongtheinterface. Thereisnotmuchdifferencein
tensile strength when the thickness of zinc coating varied from5 lm
to 10 lm.3.2. Effect of pouring temperature on microstructure and
mechanicalproperties of bimetalIn the formation of the metallic
bonding between two differentaluminum alloys, the pouring
temperature needs to be controlledprecisely. The microstructures of
A356 aluminum alloy6101 alu-minum alloybimetalsprepared
atdifferentpouring temperatureare presented in Fig. 7. The applied
pressure was kept at 30 MPa,andthe6101aluminumalloyinserts
wereelectro-platedwith5 lmzinccoating.
TheSEMmicrographsoftheinterfaceregionalongwiththecorrespondingconcentrationmapsofelementSi,Zn
and O for each sample are shown in Fig. 8. When the
pouringtemperaturewas low, therewas aclear interfacebetweenthetwo
aluminum alloys, beside, element Zn and O tended to aggre-gate
along the interface. When the pouring temperature was high,the 6101
aluminum alloy inserts would melt into the A356 moltenmetal. Only
whenthe pouring temperature was controlledataround700 C, the
metallic bonding betweenA356aluminumalloy and 6101 aluminum alloy
could be realized.The results of Vickers hardness across the
interface and tensilestrength are presented in Figs. 9 and 10
respectively. For bimetalmade at pouring temperature of 660 C, the
A356 aluminum alloyand 6101 aluminumalloy are partially metallic
bonded, the
Vickershardnesschangesfrom5565to8595abruptlyandthetensilestrengthisabout
20 MPa.
Thedesiredmetallicbondingcanbeachievedwhenthepouringtemperatureis
700 C,
theVickershardnesschangesgraduallyacrosstheinterfaceandthetensilestrength
is about 155 MPa.3.3. Effect of applied pressure on microstructure
and mechanicalproperties of bimetalSolidication under pressure is
the most distinctive feature ofsqueeze casting. The effect of
applied pressure (ranging from10 MPato50
MPa)onmicrostructureandmechanicalpropertiesof A356
aluminumalloy6101 aluminumalloy bimetals wasinvestigated.Fig. 11
presents the microstructures of interface region for sam-ples made
under different applied pressures, which were
preparedatpouringtemperatureof700 C, andthe6101aluminumalloyinserts
were electro-plated with 5 lm zinc coating. Sound metallicbonding
between A356 aluminum alloy and 6101 aluminum
alloycanberealizedforallthreebimetals.
Thereisnoaggregationofelement O or Zn, and no defects or
discontinuities are detected.The results of Vickers hardness for
bimetal made under differ-ent appliedpressureareshowninFig. 12.
Ascanbeobserved,the hardness changes gradually across the
interface. The hardnessof 6101 aluminum alloy stays in the range of
5565, the hardnessof transition zone stays in the range of 6575,
while the hardnessof A356aluminum alloy increases gradually withthe
increaseofthe applied pressure. The results of tensile strength are
presentedinFig. 13. Withtheapplicationof
pressureduringsolidicationprocess, the tensile strength exhibits
more promising results thanthat made by gravity casting, for both
A356 aluminum alloy matrixand bimetal. However, with the increase
of applied pressure, A35610 T. Liu et al. / Materials and Design 68
(2015) 817aluminum alloy matrix and bimetal show different
behaviors. ForA356aluminumalloymatrix,
thetensilestrengthincreaseswiththe increase of applied pressure,
for bimetal it appears to be inde-pendent on the magnitude of the
applied pressure and the valueremains steady. In addition, for
bimetals made under applied pres-sureof0 MPa,
thefractureoccursinA356aluminumalloypart,while for bimetal made
under applied pressure varied from10 MPa to 50 MPa, fracture occurs
in the transition zone (Fig. 6f).4. DiscussionAfter pouringthe
moltenmetal, A356aluminumalloywasbrought into contact with the zinc
coating rst, then encountered6101 aluminum alloy inserts after the
zinc coating was dissolvedinto the molten metal [13]. The surface
region of the 6101 alumi-num alloy inserts melted partially due to
the heat. Then the
6101aluminumalloyinsertsservedastheheterogeneousnucleationsubstratefortheprimarya-Al
phase[23,24], undercoolingwasdevelopedinthemoltenmetal, a-Al
dendritesstartedtogrowfrom the 6101 aluminum alloy side towards the
A356 aluminumalloy side, which parallels to heat ow but in the
opposite direc-tion. It is believed that the local fusion
determines the formationof a sound metallic bonding [25].For
bimetals made under different surface treatment,
thedegreasedsamples exhibitedpoor performance becauseof
theexistence of the oxide layer. The same phenomenon was
observedFig. 2. Microstructures of the interface region of bimetals
made at different surface treatment conditions (a and b, degreased,
c and d, zinc treated, e and f, 5 lm zinc coating,g and h, 10 lm
zinc coating).T. Liu et al. / Materials and Design 68 (2015) 817
11for zinc treated sample, because the 300500 nm thick zinc
coatingalone was insufcient. During experiments, the thin zinc
layer willevaporate and reoxidation of the aluminum insert will
occur [12].Electro-plating method was thus introduced to increase
the thick-ness of the zinc layer. Under this condition, metallic
bonding canbe successfully produced. For pouring temperature, when
operatedFig. 3. SEM micrographs of the interface region along with
the corresponding concentration maps of element Si, Zn and O for
bimetals made at different surface treatmentconditions (a0a3,
degreased, b0b3, zinc treated, c0c3, 5 lm zinc coating, d0d3, 10 lm
zinc coating).Fig. 4. Hardness proles measured across the interface
region for bimetals made atdifferent surface treatment
conditions.Fig. 5. Tensile strength for bimetals made at different
surface treatment conditions.12 T. Liu et al. / Materials and
Design 68 (2015) 817at alowvalue, eventhemeltingof
zinccoatingcouldnot beachieved, element Zn and O tended to
aggregate along the inter-face, mechanical
bondingwouldformbetweenA356aluminumalloyand6101aluminumalloy.
Whenoperatedatahighvalue,the 6101 aluminum alloy insert would
severely melt that made itno longer able to serve as insert
material with high strength andexcellent thermal and electrical
conductivity. So in order to achievetheformationof
asoundmetallicbonding, the6101aluminumalloy inserts need to be zinc
coated, the pouring temperature needto be carefully controlled as
well.It canbe observedthat signicant improvement of tensilestrength
was achieved for A356 aluminum alloy matrix and bime-tal
whensolidiedunderappliedpressure. ForA356aluminumalloymatrix,
theimprovementisattributedtothereductionofshrinkage and gas
porosities and the renement of the microstruc-ture [26]. At
present, there are usually two theories to explain theFig. 6.
Fractured surfaces and the cross section views of the fractured
specimens made at different surface treatment conditions (a and b,
degreased, c and d, zinc treated, eand f, 5 lm zinc coating, g and
h, 10 lm zinc coating).T. Liu et al. / Materials and Design 68
(2015) 817 13grain renement mechanism in squeeze casting process.
First, theimprovement of heat transfer during solidication [27]. It
is wellknownthat anairgapwouldformduringconventional
castingprocess, when asolid shell forms and shrinks away from the
dieby thermal contraction [28]. The air gap is one of the most
impor-tant factors that control overall solidication behavior
[29,30]. Theheat ux can be justied by the simplied equation
suggested byLee et al. [31]:h KgapXgap1where Kgap is the average
thermal conductivity of the gas in the gap,Xgapisthegapsize.
Withtheapplicationof
thepressureduringsolidicationthegapbetweenthemoldandthesolidiedshelldecreasesdramatically.
Thustheheattransfercoefcientandthecoolingrateisincreased,
thenthereoccurstherenementofthemicrostructure.Second, thechangeof
phasediagramunderappliedpressure[32]. According to the
ClausiusClapeyron equation:DTfDP TfVlVsDHf2where Tf is the
equilibrium freezing temperature, Vl and Vs are thespecic volumes
of the liquid and solid respectively, and Hf is thelatent heat of
fusion. Substitutingthethermodynamicsequationforvolume,
theeffectofpressureonfreezingpointmayroughlybe estimated as follows
[33]:P P0expDHfRTf 3Fig. 7. Microstructures of the interface region
of bimetals made at different pouring temperatures (a and b, 660 C,
c and d, 700 C).Fig. 8. SEMmicrographsof
theinterfaceregionalongwiththecorrespondingconcentrationmapsof
element Si, ZnandOfor bimetalsmadeat different pouringtemperatures
(a0a3, 660 C, b0b3, 700 C).14 T. Liu et al. / Materials and Design
68 (2015) 817where P0, DHf and R are constants. With the increase
of pressure P,Tf should increase, thus inducing a high undercooling
compared toconventional castingprocess.
SowhentheA356aluminumalloyis solidied under pressure, the tensile
strength is higher than thatof made by gravity
casting.Thephasediagram alsochangeswiththeapplicationofpres-sure.
Applied pressure has an effect on the equilibrium phase byshifting
the liquidus and solidus lines, and by changing the
eutecticcomposition. In the AlSi alloy system, the eutectic point
is shiftedtowardhigherconcentrationof Si
asappliedpressureincreases[34,35]. As a result, for hypoeutectic
AlSi alloy, with the increaseFig. 9. Hardness proles measured
across the interface region for bimetals made atdifferent pouring
temperatures.Fig. 10. Tensile strength for bimetals made at
different pouring temperatures.Fig. 11. Microstructures of the
interface region of bimetals made at different applied pressures (a
and b, 10 MPa, c and d, 30 MPa, e and f, 50 MPa).T. Liu et al. /
Materials and Design 68 (2015) 817 15of appliedpressure, the amount
of eutectic Si decreases. It
issuggestedbyYehandLiu[36]thatthemechanicalpropertiesofAlSi base
alloys depends on the amount, size and shape of Si. Sothe decrease
of Si further promotes the increase of tensile strengthof
A356aluminumalloymatrix. At the same time, the inter-solubility of
constituent Si together with the solubility of impurityand trace
elements is expected to increase with the application ofpressure,
which results in the increase of hardness [33].Interestingly, it
canbeobservedthat thetensilestrengthofA356 aluminumalloy6101
aluminumalloy bimetal did notincrease with the increasing of
applied pressure, and the
hardnessvalueintransitionzonedidnotincreaseneither.
Asmentionedabove, the 6101aluminumalloy servedas the
heterogeneousnucleationsubstrateforprimarya-Al phase[23,24].
Duringthesolidicationprocess,
thenon-preheated6101aluminuminsertmaterial chilled the molten metal
around it. Besides, the partiallyfusion of the 6101 aluminum insert
also absorbed the latent heatfromthe molten metal [8,10]. Thus the
liquid A356 aluminumalloysolidied and transition zone formed in a
relatively high velocitythat the pressure may have not applied onto
the liquid metal. Sowiththeincreaseof theappliedpressure,
thetensilestrengthand the hardness value did not change, therefore,
to increase theappliedpressureinordertofurther
increasethetensilestrengthof bimetal seems to be unnecessary in
this circumstance.It is widely accepted that for aluminumaluminum
bimetal, thetensilefracturewill belocatedinthealloywithlower
tensilestrength, while the interface region remains well [2325].
Interest-ingly, in this study the results were quiet different that
the fracturewaslocatedinthetransitionzone. However,
thereisnocontro-versy between these two results. Because in the
study mentionedabove, thebimetalswerereceivedingravitycastingstate.
Theinterface region is solid solution strengthened by the
interdiffusioneffect and the microstructure is more compact in
transition zone.During tensile test, fracture occurs when the UTS
of the alloy withlower strength is reached. While in the present
study, the bimetalwere prepared with squeeze casting method. The
alloy with lowerstrength is A356 aluminum alloy, and its tensile
strength increasedsignicantly because of the applied pressure. The
transition zone,onthe other hand, was still receivedingravity
casting statebecauseof thehighsolidicationvelocity.
Soduringthetensiletest,
thecrackinitiationbeganwithinitialfractureofeutecticSiin the
transition zone then extended in the transition zone,
whiletheA356aluminumalloypart and6101aluminumalloypartremains well.
However, compared with that made by gravity cast-ing, the tensile
strength of the bimetal is improved by about 10%.5.
ConclusionsInthisstudy, theA356aluminum alloy-6101aluminum
alloybimetalweresuccessfullyfabricatedbysqueezecastingprocess.The
effect of surface treatment, pouring temperature and
appliedpressure on microstructure and mechanical properties of the
bime-tal were investigated. The following conclusions were
obtained:(1)Soundmetallicbondingcanbeproducedsuccessfullybyelectro-plating
a layer of zinc coating on the 6101
aluminumalloyinsertsandcarefullycontrol of
thesqueezecastingtemperature.(2)Withtheapplicationofpressureduringsolidicationpro-cess,
thetensilestrengthexhibitsmorepromisingresultsthan that made by
gravity casting, for both A356 aluminumalloy matrix and
bimetal.(3)With the increase of applied pressure, A356 aluminum
alloymatrix and bimetal show different behaviors. For A356
alu-minum alloy matrix, the tensile strength increases with
theincreaseof appliedpressurebecauseof
thereductionofinherentdefects, therenementofmicrostructure
andthedecrease of the amount of eutectic Si. For bimetal it
appearsto be independent on the magnitude of the applied
pressureand remains steady.(4)During the tensile test, the crack
initiation begins with initialfracture of eutectic Si in the
transition zone then extends inthe transition
zone.(5)Withtheincreaseofpressure, thehardnessvalueofA356aluminum
alloy matrix increases, while the hardness valueof transition zone
stays at about 6575.(6)For bimetal fabricated with squeeze casting
method, tensilestrength is improved by about 10%, from145 MPa to155
MPa, as compared with that made by gravity
casting.AcknowledgmentThe authors gratefully acknowledge the
nancial support of theGeneral Motors Company in Pontiac, USA.Fig.
12. Hardness proles measured across the interface region for
bimetals madeat different applied pressures.Fig. 13.
TensilestrengthforbimetalsandA356aluminumalloymatrixmadeatdifferent
applied pressures.16 T. Liu et al. / Materials and Design 68 (2015)
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