The-Mechanisms-of-Resistance-Spot-Welding-of-Magne

TheMechanismsofResistanceSpotWeldingofMagnesiumtoSteelL.LIU,L.XIAO,J.C.FENG,Y.H.TIAN,S.Q.ZHOU,andY.ZHOUAnoveltechnologyhasbeendevelopedfortheresistancespotwelding(RSW)ofmagnesiumtosteelproducingjointstrengthashighas95pctofthatofMgtoaMgjoint.Themechanismsofthedissimilarjoininghavebeeninvestigatedusingmechanicaltestingandmetallurgicalexaminationemployingscanningelectronmicroscopy,energy-dispersiveX-rayspectroscopy,andX-raydiffraction.TheresultsshowthatthemechanismsofjoiningduringRSWofamagnesiumalloytoZn-coatedsteelinvolvebrazewelding,solid-statebonding,andsoldering.ThejointformationincomparisonofRSWofZn-coatedsteelwithsteel,Au-platedNi,andbareNisheetsisdiscussed.Apossibleexpansionofthistechnologyalsoissuggested.DOI:10.1007/s11661-010-0333-0TheMinerals,Metals&MaterialsSocietyandASMInternational2010I.INTRODUCTIONTHEcontinuedeffortsbyautomobilemanufacturerstoreducevehicleweighthaveledtoanincreaseduseofhighstrength-to-weight-ratiomagnesiumalloysintheautomobilearchitecture.Therefore,tasksofdevelopingmagnesium-to-magnesiumsimilarjointsandmagnesiumtootherssuchassteelmustbefaced.Theweldabilityofmagnesiumalloysbyvariousmethods,includinglaser,[1]frictionstir,[2]laser-tungstenhybrid,[3]andresistancespotwelding(RSW),[4]hasbeenexploredindetail.However,onlyafewstudieshavefocusedonthedissimilarjoiningofmagnesiumtosteels.Buttjoints[5]andoverlapjoints[6]ofMgtosteelwereperformedbyfrictionstirweldingwithajointstrengthofaround70pctofMgbasematerialand3.7kN(20mmweldseamlength),respectively.Laser-GTA(gastungstenarc)hybridweldingwasusedtomakeoverlapjointsofMgtosteel.[7]However,itwashardtocontroltheseprocessestoavoidmetallicoxideslikeMgO,ZnO,Fe2O3,orAl2O3,[5–7]whichcouldreducethestrengthofwelds.Theshearstrengthofdissimilarjointswasimprovedfrom56pctto100pctofthebasematerialbyinsertinganickelinterlayer.[8]RSWisaprimaryjoiningmethodintheautoindustry.IfdissimilarjointscouldbeperformedsuccessfullybyRSW,thenthecostscouldbeattractiveasaresultofthelackofneedfornewequipmentandtheinherentlow-costcharacteristicsofRSW.LimitedpublicationshavefocusedoncharacteristicsofMg/MgsimilarRSWjoints,suchasnuggetgrowthcharacteristics,[9]effectsofsurfaceconditionsandweldingparameters,[4,10]andsolidifica-tionmorphologies.[11]However,virtuallynoliteraturehasbeenpublishedontheRSWofMgtosteel.OnlyafewstudiesonRSWofAltosteelhavebeenpub-lished.[12–15]Mg-freeAlfoilwasinsertedbetweenAlandsteeltosuppresstheformationofanintermetalliclayer.[12]AnAllayerwascoldrolledtosteelasatransitiontoaidtheresistanceweldingprocess.[14]AlaminateofsteelandAlwasinsertedbetweenAlandsteelsothesametypesofmetalscouldfaceeachother.[15]ThemainproblemsofweldingAltosteelusingRSWaretheformationofbrittleintermetalliccompounds,wet-tingofAltosteel,andlargedifferencesofelectricconductivityandthermalphysicalpropertiesbetweenAlalloysandsteel.[12,13]SomeoftheseproblemsmaybeofconcernaswellinRSWofMgtosteel.TheobjectivesofthecurrentworkweretodevelopanRSWprocessforweldingmagnesiumalloytoDP600steelandtoinves-tigatethemechanismsofthejoiningprocessusingmetallurgicalexaminationandmechanicaltesting.II.EXPERIMENTALPROCEDURESThematerialsusedinthisworkwerecommercial-gradetwin-rollstripcastMgalloyAZ31Bandzinc-coatedDP600steelsheetswiththicknessesof1.5mmand1.2mm,respectively.Lap-weldedjointsweremadebyassemblingtestcouponsthatwerecuttoapproxi-mately25mmwidthand100mmlength.Specimenswerecutparalleltotherollingdirectionofthesheetsandultrasonicallycleanedfor5minutesinacetone.Priortowelding,thesurfacesofthemagnesium,couponsalsowerecleanedwithasolutionof2.5pct(wt/vol)chromicacidtominimizesurfaceoxidesbecauseoftheirrecognizednegativeeffectsonbothweldstrengthandRSWelectrodelife.[4]L.LIU,Ph.D.Candidate,iswiththeDepartmentofMechanicalandMechatronicsEngineering,UniversityofWaterloo,WaterlooN2L3G1,Canada,andtheStateKeyLaboratoryofAdvancedWeldingProductionTechnology,HarbinInstituteofTechnology,Harbin150001,P.R.China.L.XIAO,Ph.D.Candidate,andY.ZHOU,Professor,arewiththeDepartmentofMechanicalandMechatronicsEngineering,UniversityofWaterloo.Contacte-mail:[e-mail],Professor,andY.H.TIAN,AssistantProfessor,arewiththeStateKeyLaboratoryofAdvancedWeldingProductionTechnology,HarbinInstituteofTechnology.S.Q.ZHOU,Professor,iswiththeSchoolofMechanicalScienceandEngineering,HuazhongUniversityofScienceandTechnology,Wuhan430074,P.R.China.ManuscriptsubmittedDecember5,2009.ArticlepublishedonlineJune22,2010METALLURGICALANDMATERIALSTRANSACTIONSAVOLUME41A,OCTOBER2010—2651ThemicrostructureofAZ31BisshowninFigure1(a).TheAZ31alloyconsistedofequiaxedgrainswithanaveragegrainsizeof7.5lm,measuredaccordingtoASTME112-96.RollingtracksareshownclearlyonthesurfaceoftheAZ31Bsheets(Figure1(b)).Aninspec-tionoftheDP600steelmicrostructurewithscanningelectronmicroscopy(SEM)inFigure2(a)showedthetypicaldualphase(DP)structurecomprisingmartensiteislandswithinthepolygonalferritematrix.ThesurfaceoftheDP600steelwashot-dippedwithzinc.Thethicknessofthezinclayerwasabout9lmto12lmmeasuredbySEM(Figure2(c)).EnergydispersiveX-ray(EDX)showedthezinclayercontained3wtpct