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Constituents evolution of LFEC 7050 aluminum alloy during processing

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Constituents evolution of LFEC 7050 aluminum alloy during processing

    Constituents evolution of LFEC 7050

    aluminum alloy during processing Science

    PressTrans.NonferrousMet.Soc.China17(2007)s249s253

    Transactionsof

    NonferrousMetals

    SocietyofChina

    WWW.CSU.educn/ysxb/

    ConstituentsevolutionofLFEC7050aluminumalloyduringprocessing ZUOYu_bo(左玉波),WANGShuang(~),ZHAOZhi_hao(赵志浩),

    CUIJianzhong(催建忠),ZHANGHaitao(张海涛),TUGan.feng(涂赣峰).

    1?KeyLaboratoryofElectromagneticProcessingofMaterials,

    MinistryofEducation,NortheasternUniversity,

    Shenyang110004,China;

    2.CollegeofSoftware,NortheasternUniversity,

    Shenyang110004,China;

    3.SchoolofMaterialsandMetallurgy,NortheasternUniversity,Shenyang110004,China Received15July2007;accepted10September2007

    Abstract:Thed120miningotsof7050aluminumalloyweremadebylowfrequencyelectromagneticcastingfLFEC1and

    conventionalDCcastingprocess,respectively.Afterhomogenizationtreatmenttheingotswereextrudedtorodsandthesolutionand

    agingtreatmentwerecarriedouttbrtherods.ConstituentsevolutionduringprocessingandeffectsofLFEConconstituentsand

    remnantconstituentswerestudied.Theresultsshowthat7050aluminumalloymainlycontainsAI.Zn.Mg.CutypeandAI.CuFe

    peconstituents.A1.Zn-MgCu

peconstituentsdissolveduringhomogenization,whileA1.Cu.Fetvpeconstituentscouldnot

    dissolve.Atterhomogenizationtreatment,themainremnantconstituentisA17Cu2Fewhich

    crushesandorientsalongtheextrusion

    directionafterextrusion.ComparedwithDCprocess,bytheprocessofLFEC, theconstituentsorremnantconstituentsaresmallerin sizeandlessincontent.TheLFECprocessshowssignificantimprovementinelongationbyL

    FECinbothas..caststateandfinal

    state

    Keywords:7050aluminumalloy;lowfrequencyelectromagneticcasting;microstructures;

    constituents

    lIntrOductiOn

    7050aluminumalloyisthepremierchoicefor

    aerospaceapplicationsrequiringthebestcombinationof strength,stresscorrosioncrackingresistanceand

    toughness[3].7050aluminumalloycontainshigh

    contentalloyingelements,so,ithasseveresegregation tendency,which1eadstoeutecticphasesandcoarse

    constituentsformedatthegrainboundaries.Coarse

    constituentsnotonlycausehottearingduringcastingbut

    alsodecreasestrength,plasticifracturetoughnessand ~tiguestrengthafterheattreatment[48].Inorderto

    obtaingoodcombinedproperties,thecoarseconstituents havetobecontrolled.Ithasbeenreported[91thatfor

    aluminumalloyingot.severa1benefitssuchasbetter

    metalfluidity,finerdispersedporosityandsecondphases, higherresistancetohottearing,higher~tiguestrength, andbetterpressuretightnessarisefromtheformationof afineequiaxedmicrostructureratherthanacoarse

    dendriticstructure.Themasteralloysbasedonthe

    A1.Ti.Bsystemhavebeenusedextensivelyforrefining

    aluminumalloygrainstructure.But,7050aluminum alloycontainsZr.andZrpoisonsA1.TiBsystemgrain refiners[10121.Itisnecessarytofindanewwayto

    refine7050aluminumalloygrains.Usingexterna1field suchasultrasonic[13141,electromagneticfield[1520]

    torefinealuminumalloystructureattractsmuchattention BasedonCREM[1516],anewtechnique,low

    frequencyelectromagneticcasting(LFEC)processwas developedbyCUIandhisco.workers17201.Inthe

    previouswork,thestudieswerefocusedontherefining effectofLFEConas.caststructures,buteffectsonthe structuresduringprocessingsuchashomogenization, deformationattract1ittleattention.Inthiswork.LFEC wasusedtomake7050aluminumalloyingotsandthe effectsoflowfrequencyelectromagneticfieldonthe constituentsnotonlyinas.caststatebutalsoafter homogenizationanddeformationwerestudied. 2Experimental

    A16.1%Zn-2.3%Mg2.2%Cu0.13%Zrwasusedas

    theexperimentalmateria1.

    Foundationitem:Projects(2005cB6237071supposedbytheNationalKeyBasicResearchPr

    ogramofChina

    Correspondingauthor:ZUOYubo;Tel:+8624

    83687734;E-mail:zuoyubo@epm.neuedu.cn

    s250ZUOYu?bo,etaFTrans.NonferrousMet.Soc.China17(2007) Thed120mmingotsweremadebyLFEC(30Hz,

    120A)andconventionalDCcastingprocess,

    respectively.TheschematicoftheLFECprocessis showninFig.1.

C

    ForcedconvectionMeniscus

    Fig.1SchematicofLFECprocess

    ttop

    ductionCO

    Aftercastingprocess,theingotswerehomogenized at420?for12handthenat465?for36h.

    Subsequently.theingotswereextrudedtorodsofl7mm indiameter,withanextrusionratioof28.Solution treatment(470?,3h)andageingtreatment(120.C,

    12h,werecarriedoutfortherods.

    Thesamplesobtainedfromtheingotandrodswere groundandpolished,thenwereetchedwithKeller'S reagentf2mLHF,3mLHCL.5mLHNOand190mL

    H,O1.Microstructureswereobservedunderoptical microscope(LeicaDMR).Scanningelectronmicroscope (SEM)SSX550,energydispersivespectrometer(EDS) andX.raydiffractionfXRD1instrumentD/Max.Rawere usedforobservingmicrostructuresandanalyzing constituents.Themechanicaltestswereconductedat roomtemperature.Theaveragevalueofthreetestresults wasusedasthefinalvalue.

    3Resultsanddiscussion

    3.1Majorconstituentsin7050aluminumalloyingots TheresultsofX-raydiffraction,inFig.2,showthat therearemainlyMgZn2(q),A12CuMg(S)andA17Cu2Fe constituentsin7050aluminumalloyingots.No differenceinconstituenttypebetweenDCandLFEC ingotswasfound.

    Themicrostructuresandconstituentsobservedby

    SEMareshowninFig.3.Therearemassiveconstituents (Fig.3,points1,3)andeutecticstructuresatgrain boundaries(Fig.3,points2).AsshowninTable1, energydispersivespectrometry(EDS)revealsthatthe constituentslikepoint1areA1-Zn-Mg-Cutypephases, theconstituentslikepoint3areA1-Cu-Fetypephases, andtheeutecticcontainsa(A1)andA1-Zn-Mg-Cu) typephases.

    20/(.)

    Fig.2XRDpatternof7050aluminumalloyingots Fig.3Majorconstituentsof7050aluminumalloy Table1Compositionofconstituentsof7050aluminumalloy (massfraction,%)

    Ithasbeenreportedthatther/(MgZn2)phase dissolvessomealuminumandcopper,andthenMg(A1, Cu,Zn)2[21_22]isformed.Inthesameway,theSphase isfoundtobeassociatedwithzincintherangeof 25%30%.Soin7050aluminumalloyingotsboththe

ZUOYubo,etal/Trans.NonferrousMet.

    Soc.China17(20071

    phaseandSphasecontainAI,Zn,Mg,Cuelements.The XRDdistinguishesconstituentsbystructure,whilethe EDSdistinguishesconstituentsbycomposition.Thisis believedtobethereasonwhytherearedifferencein resultsbetweenXRDandEDS.

    Themicrostructuresof7050alloyingotareshown inFig.4.AsshowninFig.4,theconstituentsofLFEC ingotaresmallerthanthatofDCingot.Thequantitative

    analysisofconstituentsandeutecticphaseareafraction wascarriedoutwithsoftwareSISCIAS8.0.

    The

    statisticalvalueofareafractionofconstituentsand eutecticstructuresinLFECingotf7.2%)iSlessthanthat ofDCingot(8.1%).

    Fig.4Microstructuresof7050aluminumalloyingots:(a) LFEC;fb1DC

    3.2Constituentsevolutionduringprocessing AsshowninFig.5,fortheLFECingot,theXRD

    resultsshowthatther/(MgZn2)phasedissolvesduring homogenization,precipitatesduringextrusionand redissolvesduringsolutiontreatment.While.A11Cu,Fe phasedoesnotdissolveduringhomogenization, extrusionandsolutiontreatment.

    Afterhomogenization,theeutecticsdisappearand theconstituentsbecomemuchlessincontentthanthatof ascastingot.TheremnantconstituentsofLFECingot aresmallerinsizeandlessinareafractionthanthoseof DCingot.TheEDSresultsshowthatthemainremnant constituentsareAI-Cu??Fetypephasesandtheyare

    believedtobeAlCu,Fe.

    s25l

    20/(.)

    Fig.5XRDpatternsofLFEC7050aluminumalloy Theremnantconstituents

    werealsoobservedbySEM

    of7050aluminumalloy

    Backscatteredelectron

    imagesofthemicrostructuresinthestateof

    homogenization,extrusionandsolutiontreatmentare showninFig.6.Theremnantconstituentsmarkedin Fig.6wereanalyzedwithenergydispersivespectrometry (EDS)andtheresultsarelistedinTable2.

    Table2Compositionofremnantconstituentsof7050 aluminumalloy(massfraction,%)

    Duringextrusion.therearetwophenomenainthe constituentevolution.Ononehand,theAICUFetVpe

    remnantconstituentscrushandorientalongtheextrusion direction.Ontheotherhand,manyprecipitatephases whicharesmallerinsizethanremnantconstituentswere formed.Soafterextrusionthealloymainlycontains crushedremnantconstituentsandnewlyformed precipitatephases.

    Atiersolutiontreatment.almostallthenewly formedprecipitatephasesdissolve,whilethecrushed remnantconstituentscouldnotdissolve.So.after solutiontreatment,themainremnantconstituentsarethe

    s252ZUOYubo,etal/Trans.NonferrousMet.Soc.China17(2007) Fig.6Remnantconstituentsof7050aluminumalloy:(a),(b)Homogenization;(c),(d)Extrusi

    on;(e),(f)Solutiontreatment;(a),(c),

    (e)LFEC;(b),(d),(f)DC

    crushedA1CuFetypephases.Thestatisticalanalysis showsthattheareafractionoftheremnantconstituents inrodmadebyLFECingot(1.1%)islessthanthatof rodmadebyDCingot(1.3%),whichishelpfulfor improvingmechanicalproperties.

    3.3Mechanicalpropertiesof7050alloy

    Theascasttensilestrengthandtheelongationof LFECingots(292.5MPa.2-3%1arehigherthanthoseof DCingots(274.0MPa,1.67%).ItiSbelievedthosethe improvementofascastmechanicalpropertiesresults fromfnermicrostructuresandconstituentsofLFEC ingots.

    Afterhomogenization,extrusion,solutiontreatment andartificialageingtreatment.theLFECprocessdonot showsignificantimprovementinfinaltensilestrength (from676.5to677.5MPa).buttheimprovementin elongmioniSveryremarkable(from11.23%tol3-20%). Itisbelievedthattheimprovementoffinalelongation resultsfromfinerandlessremnantconstituents. 4Conclusions

    1,7050aluminumalloymainlycontains

    A1ZnMgCutypeandA1CuFetypeconstituents.

    A1..Zn..Mg..Cutypeconstituentsdissolveduring homogenization.whileAlCuFetVpeconstituentscould

    notdissolve.Atierhomogenizationtreatment,themain remnantconstituentsareAl1Cu,Fephaseswhichare crushandorientalongtheextrusiondirectionafter extrusion.ComparedwiththeDCprocess,bytheLFEC process,theconstituentsorremnantconstituentsare smallerandlessinthestatesofascast.homogenization,

    extrusion,andsolutiontreatment.

    2,Forthe7050aluminumalloy,theascast

    mechanicalpropertiesofLFECingotarehigherthan

    ZUOYubo,etal/Trans.NonferrousMetSoc.China17(2007)s253 thoseofDCingot.TheLFECprocessdoesnotshow

    significantimprovementinfinaltensilestrength(from 676.5to677.5MPa),buttheimprovementinelongation isveryremarkable(from11.23%to13.20%).

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