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Void damage evolution of LF6 aluminum alloy welded joints under external load and thermal cycling conditions

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Void damage evolution of LF6 aluminum alloy welded joints under external load and thermal cycling conditionsOF,of,VOID,LF6,ALLOY,UNDER,Void,alloy,under,void

    Void damage evolution of LF6 aluminum

    alloy welded joints under external load

    and thermal cycling conditions

EERPress

    Availableonlineatwww.sciencedirect.com

    .

    

    

    ScienceDirect

    Trans.NonferrousMet.Soc.China20(2010119681973

    Transactionsof

    NonferrousMetals

    SocietyofChina

    wwwtnmscc11

    VoiddamageevolutionofLF6aluminumalloyweldedjointsunder

    externalloadandthermalcyclingconditions

    MIGuo.fa(米国发),ZHAODa.wei(赵大为),DONGCuifen(董翠粉),

    NIUJi.tai(牛济泰).,J1NCheng(~)

    1.SchoolofMaterialsScienceandEngineering,HenanPolytechnicUniversity,Jiaozuo454003,China;

    2.SchoolofMaterialsScienceandEngineering,HarbinInstituteofTechnology,Harbin150001,China;

    3.SchoolofMaterialsScienceandEngineering,DalianJiaotongUniversity,Dalian116028,China

    Received16October2009;accepted22March2010

Abstract:Basedonthesimulatedaerospacethermalcyclingtests.theeffectofthermalcycleo

    nthevoiddamageevolution

    mechanismofLF6aluminumalloyweldediointwasinvestigated.Theresultsshowthatmicro

    .voidsformaroundthesecondphase

    particlesunderthethermalcyclingtests.Thetherma1stresscoupledwithextema1stressleads

    todislocationspile.uparoundthe

    particles,andwhenthedislocationdensityreachesacertaindegree,thestressconcentrationw

    illexceedthebondingstrengthatthe

    interfacebetweenparticlesandmatrix.resultingintheformationofmicro

    cracks.ThenumericalsimulationisSuCCeSSfully

    implementedwiththefiniteelementtodescribethevoiddamageevolutionoftheweldedioint

    underthermalcyclingconditions.

    Keywords:aluminumalloy;weldedjoint;therma1stress;numericalsimulation;micro

    void

    1IntrOductiOn

    Duringthespacevehiclesmovinginlowearthorbit,

    someshellconstructionsaresubjectedtothespacecyclic

    thermalloadaswellastheworkingloads.Theextrcme

    temperaturerangeofthethermalcyclingencounteredis

    93-393K1-31.Whichwillcausetheperformances

    deteriorationandmateria1microstructuredamage[4].

    Muchworkhadbeendoneonthedamageofcomposites

    inducedbythermalcycling[56],butlittleattentionhas

    beengiventoinvestigatethethermalcyclinginduced

    damageofmetalsoritsweldedjoints.

    Aluminumalloyisanimportantlightweight

    constructionmaterialinaerospaceindustries[7].

    Aluminumalloyweldedstructureshavebeenwidely

    usedtoconstructtheshellmoduleofspacevehicles.But

    theweldediointoftenbecomestheweakestpartofthe

wholeweldedconstructioninsomeconditions8111.In

    ordertoevaluatethe1ong.1ifereliabilityandguarantee thesafetyoftheweldedstructureunderservices.itis essentialtodetermineitsvoiddamagemechanisms.In thiswork,thevoidnucleationandevolutioninaLF6 aluminumalloybuttweldedjointwerestudiedunder

    simulatedspacethermalcyclingconditions. 2Experimental

    2.1Materialsandweldingcondition

    Atypica1compositionoftheparentaluminumalloy LF6sheetusedinthisstudyispresentedinTable1.The sheetswim5mminthicknesswerebuttweldedby

    variablepolarityplasmaarcwelding(VPPAW)usingthe samefillerwiremateria1.Table2liststheVPPAW conditions.Weldediointspecimenswerecutacrossthe butt-joinweldedsheets.asshowninFig.1.

    Table1Chemicalcompositionoftestmaterial(massfraction, %

    Foundationitem:Project(90205035)supposedbytheNationalNaturalScienceFoundation

    ofChina

    Correspondingauthor:ZHAODa-wei;Tel:+863913987503;E

    mail:peten@hpu.edu.cn

    DOI:10.1016/S10036326(09)604039

    MIGuofa,etal/Trans.NonfeITOUSMet.Soc.China20(2010)19681973

    Weld

    Fig.1Dimensionofweldedjointspecimens(rnllq) X--raynon?-destructivedetectionswerecarriedout ontheseweldedspecimens.Thespecimenswithany detectabledefectwereeliminatedtoensuretheresultsT

    consistencyinthefollowingtests.

2.2Thermalcyclingtest

    Thermalcyclingtestswereconductedusinga homemademachine.Severalweldediointspecimens wereputintothethermalisolationcavityofthemachine. Duringthetest.anexternaltensileloadof100MPawas appliedtothespecimens.Thespecimenswereheated usingaresistantheaterandcooledbysprayingliquid nitrogenintothetestcavity.Inordertosimulatethe lOW.earthorbitthermalcycle.thecycletemperature rangeiS173-373KandthecyclicperiodiS90min.A thermocouplewasfixedonthespecimensurfaceinorder torealizerealtimetemperaturemonitor.A11test

    parameterswerepreprogrammedandinputmtothe supervisorycontrolcomputer,andatotalofl00cycles wererecordedforeachtest.Aschematicdiagramof thermalcyclingofonecycleiSshowninFig.2. Tostudythechangeofmicrostructuresanddamage evolution,especiallythenucleationofthemicro.void, Table2Parametersofweldingprocess

    \\\

    Fig.2Schematicdiagramofthermalcycling

    1969

    differentsectionsoftheweldedjointspecimenswere sampledbeforeandafterthethermalcyclingtest.The microstructuresofthesesampleswereobservedusingan opticalmicroscopeforthemesoscale.Thefracture surfacesofthetensilespecimenswereobservedbySEM. Typicalchemicalcompositionsweredetermined quantitativelyusingEDS.Dislocationconfiguration characteristicswereappraisedusingTEM.The

    numericalsimulationwasimplementedtodescribethe voiddamageevolutionoftheweldediointunderthermal cyclingconditions.

    3Resultsanddiscussion

    Theoriginalmicrostructuresofdifferentzonesof theweldedjointareshowninFig.3.Thesecondphase DCNE:directcurrentnegativeelectrode;DCPE:directcurrentpositiveelectrode

    Fig.3MicrostructuresofweldedLF6aluminumalloyjoint:(a)Fusionzone;(b)Heat

    affectedzone

    1970MIGuofa,etal/Trans.NonferrousMet.Soc.China20(2010)19681973

    canberesolvedfromthematrixorganizationafter corrosion,andblackparticlesarethesecondphases. FromFig.3(a),itcanbeseenthatthesecondphasesare fineanddistributeevenlyintheweldzone.However.in theheataffectedzone(HAZ)(Fig.3fo)),thesecond phasesarerelativelycoarseandaggregateatthegrain boundaries.Theseimpuritiesplayacrucialrolein degradationofthemechanicalpropertiesduringthe thermaIcyclingtest.

    3.1Fracturecharacteristicsanddamage

    TherepresentativeSEMfractographsofthewelded jointspecimensbeforeandafterthethermalcyclingtest arecomparedinFig.4.whichcanproviderich informationforthefailureprocess.

    InFig.4(a),severalparticlescanbeseenonthe fracturesurface.Therearenoobviousvoidsfound aroundtheseparticles.However,thelargeranddeeper voidsareobservedinFig.4(b).Inthelargervoids, particlesarevisible.Thefractureoftheweldedioint specimensafterthermalcyclingmainlyoccursinthe

    HAZ.Itistheevidencethatvoidsforrnaroundthe particlesduringthethermalcyclingprocess.The particlesinthewelded{ointsbecomethesourceof failureunderthermalcyclingconditions?214].The

    particlesintheweldregionwereidentifiedbymeansof EDSanalysis.Theyareacomplexmixtureofphase (AIsMgs),impuritiesofMg2Si,andafewA1-Mn-Fe compounds.

    3.2Dislocationconfiguration

    Dislocationconfigurationcharacteristicsofsamples inHAZafter1O0cyclesareshowninFig.5.inwhichthe darkparticlesarebelievedtobethesecondphase particlesandimpurities.

    AsillustratedbyAandBinFig.5.thedislocation pileupgroupsaroundtheparticlesgenerateintheHAZ. DuetothelargedifferencesinthermaIandmechanical parametersbetweentheseparticlesandthebasealloy, largestressesincludingthemismatchstressandapplied stressareinducedundertherma11oadcyclingconditions. whichleadstodislocationglide.Intheglideprocess,the blockingoftheparticlesresultsindislocationpile.up groupsaroundtheseparticles.Thestressconcentrationin blockingparticlesowingtodislocationpile-upgroupsis r:

    F"r0(1)

    wherel-istheresolvingshearstressvalueofstress concentrationalongtheslipdirection,whichiscausedby dislocation.AccordingtoEq.(1),thestressconcentration generatedinfrontofdislocationpile.uDgroupsisntimes aslargeasr0Whenthedislocationdensityaroundthe

    particlesaccumulatetoacertaindegree,thestress concentrationbetweentheparticlesandthematrix exceedstheinterfacebondingstrength,andmicrocracks

    willgenerateattheinterface.Particlesgraduallyseparate fromthebasealloywiththecontinuationofthetest cycle.

    Fig.4Representativefractographsofweldedjoints:(a)Before thermalcyclingtest;(b)After100cyclesofthermalcycling Fig.5Dislocationconfigurationcharacteristicsofsamplesin HAZafter100cyclesofthermalcycling

    MIGuofa,etal/Trans.NonferrousMet.Soc.China20(2010)19681973

    4Theoreticalanalysisofvoidnucleation

    Forthepurposeofunderstandingthethermal

    cyclingassistedvoidsformation,itisconvenientto considerasphericalvolumeunitcontainingaparticleof sphericalshape,asshowninFig.6115].

    Fig.6Sphericalvolumeunitcontainingparticle Eachconstituentphaseisassumedtobe

    elasto-plasticallyperfectandisotropicsolid,buttheyield strengthofthesecondphaseparticleismuchhigherthan thatoftheAlalloymatrix.Andtheinterfaceofthe matrixandthesecondphaseparticleisassumedtobe wellbonded.FromgeneralizedHpoke'slaw,thestrain duringtemperaturevarymgcanbeexpressedas

    =

    (2)+

    =+(2)

    whereEiselasticmodulus,visthePoissonratio,Tisthe temperatureand.cisthecoefficientofthermalexpansion. r,

    o-randstr0arethestain,stressatradialdirectionand tangentialdirection,respectively.OLSSONetal[14

    calculatedtheinterfacepressurePduringtemperature varyingintheelastoplasticframeworkwithnoother

    externalload.Usingsubscript1torefertothematrix materialandsubscript2torefertosecondphaseparticle, theinterfacepressureinducedbytemperaturevarying canbeexpressedby[16]

    p=

    (

    2M

    j

    el(1_)]

    (3)

    whereistheparticlevolumefraction,andinthisunit cell,~o=(r21r1).lissocalledelasticmismatch parameterandgivenby

    c

    l97l

    (4)

    AccordingtoEqs.(3)and(4),wecanseethatthe interfacepressurePcanbepositiveornegative,which indicatesthattheinterfacecanbeintractionduring temperaturevaryingevenifthereisnoexternalstress applied.Ingeneral,al>Gt2and00<Me,<3/2,so,when ?T>0.p<O.theinterfaceisinthestatusoftraction.This meansthethermalmismatchstressPcanassistthevoid nucleation.

    Aboveanalysisisundertheconditionofnoextemal loadapplied.Ifthematerialisappliedwithanexternal

    stressduringthermalcyclingprocess,thethermal mismatchstressPcanassociatetheextema1lpadto controlthevoidnucleationprocess.

    5NumericaIsimulationofvoidnucleation

    Inthepresentstudy,finiteelement(FE)calculations ofeel1structureshavebeenwidelyusedtosimulateand studythebehaviorofporoussolids.Inthenumerical calculationscarriedouthere,acylindricalrepresentative volumeelement(RVE1insteadofasphericalRVEis selectedbecausel,thecylindricalRVEiswidelyusedto analyzevoidnucleationinGursontypeporous materials[17],2)itisconvenienttoapplyboundary conditionsandconstraints.and31theabovetheoretical thermoanalysisresultsofthesphericalRVEarevery similartotheFEcalculationsofthecylindricalRVEas longastheparticlevolumefractionisnotSOlarge. Arepresentativeeellmodelwasestablishedto analyzethermalstress.assistedvoidsformationf181.The eellmodelshowninFig.7isassumedtobeacylinderof matrixcontainingasphericalparticleatitscenter.The heightofthecylinderisequaltoitsdiameter.Theradius oftheparticleisonefourthofthatofthecylinder.DHeto periodicalsymmetry,thecylindercanberepresentedby theplaneABCD.whichcanbeconvertedintoafinite elementgridbyusingaxisymmetricelements. DeformedA

    r(b)

    Fig.7Cylindricalaxisymmetricmodel:(a)Representativeunit cellmodel;(b)Finiteelementcellmodel

1972MIGuo?fa,etal/Trans.NonferrousMet.Soc.China20(2010)19681973

    TheboundariesACandCDwerecons~ainedinthe anddirections,respectively.Asetofcohesive elementsiSmodeledattheinterfacebetweentheparticle andthematrix.Thevalueofdebondingstrainofthe cohesiveelementsiSspecifiedas0.1.Thetypical propertiesoftheconstituentphasesinthisuniteell modelarelistedinTable3.

    Twoanalysisstepsweredefined.Inthefirststep,a Table3Typicalmaterialsparametersat20.C constantmechanicalload,=l00MPa,wasapplied alongboundaryABintheverticaldirection.Inthe secondstep,asine'typetemperaturecyclingwas superposed.Thecyclicrangewasbetween173and 373Kandtheperiodwas90min.

    TheFEMsimulationvoidnucleationinextemal loadandthernla1cyclingconditioniSshowninFig.8and thestressvaluesatthevariousstagesofthevoid nucleationprocessweredisplayedbydifferentgrey scales.

    AsillustratedinFigs.8(a)and8(b),itcanbeseen thatthebiggishstresscanbefoundattheinterface,and theyallextendoutwardfromtheinterfaceandreduce gradually.Whenthematrixandparticleremainbonded, themaximumstressinthematriXdoesnotoccuratthe tensileaxis(Azone),butalongtheparticlesurfaceat (C)

    (e)(f)

    Fig.8Simulationforvoidnucleationprocess:(a)Bondinginterface;(b)Locationofmaximu

    mstress;(C)Firstfailureelement;(d)

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