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: wangqudong@sjtu.edu.cn (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) 817References[1] Zhang J, Luo G, Wang Y, Shen Q, Zhang L. 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