Mechanical properties and stress corrosion cracking behaviour of AZ31 magnesium alloy laser weldment.docx

Mechanical properties and stress corrosion cracking behaviour of AZ31 magnesium alloy laser weldment.docx

《Mechanical properties and stress corrosion cracking behaviour of AZ31 magnesium alloy laser weldment.docx》由会员分享，可在线阅读，更多相关《Mechanical properties and stress corrosion cracking behaviour of AZ31 magnesium alloy laser weldment.docx（15页珍藏版）》请在冰豆网上搜索。

MechanicalpropertiesandstresscorrosioncrackingbehaviourofAZ31magnesiumalloylaserweldment

Mechanicalpropertiesandstresscorrosioncrackingbehaviourof

AZ31magnesiumalloylaserweldments

P.B.SRINIVASAN,S.RIEKEHR,C.BLAWERT,W.DIETZEL,M.KOÇAK

InstituteofMaterialsResearch,GKSS-ForschungszentrumGeesthachtGmbHGeesthachtD-21502,Germany

Received25January2010;accepted7April2010

Abstract:

AnAZ31HPmagnesiumalloywaslaserbeamweldedinautogenousmodewithAZ61fillerusingNd-YAGlasersystem.Microstructuralexaminationrevealedthatthelaserbeamweldmetalsobtainedwithorwithoutfillermaterialhadanaveragegrainsizeofabout12μm.Themicrohardnessandthetensilestrengthoftheweldmentsweresimilartothoseoftheparentalloy.However,thestresscorrosioncracking（SCC）behaviourofboththeweldmentsassessedbyslowstrainratetensile（SSRT）testsinASTMD1384solutionwasfoundtobeslightlyinferiortothatoftheparentalloy.Itwasobservedthatthestresscorrosioncracksoriginatedintheweldmetalandpropagatedthroughtheweldmetal-HAZregionsintheautogenousweldment.Ontheotherhand,intheweldmentobtainedwithAZ61fillermaterial,thecrackinitiationandpropagationwasintheHAZregion.Thelocalizeddamageofthemagnesiumhydroxide/oxidefilmformedonthesurfaceofthespecimensduetotheexposuretothecorrosiveenvironmentduringtheSSRTtestswasfoundtoberesponsiblefortheSCC.

Keywords:

AZ31magnesiumalloy;laserwelding;microstructure;mechanicalproperties;slowstrainratetensiletest;stresscorrosioncracking;fractography

1Introduction

Automobileandaircraftapplicationsdemandlightweightandhighperformancematerials,andwroughtmagnesiumalloysareslowlyreplacingsteelsandaluminiumalloysinthoseindustries[1].Magnesiumalloys,ingeneral,haveexcellentcastabilityandmoderateformability,andhencecomponentsaremadefrombothcastandwroughtMgalloys.Eventhoughthejoiningofmagnesiumalloyscanbeaccomplishedbygastungstenarcwelding[2í4].Recentlylaserbeamwelding（LBW）,electronbeamwelding（EBW）andfrictionstirwelding（FSW）processesarewidelyemployedforthejoiningofthesealloys[5í9].ThisisduetoreduceddefectlevelsandconsequenthigherjointefficienciesthatresultfromthecontrolledpowerbeamandFSWprocessing.WhiletheFSWprocessisasolidstateprocessthatdoesnotinvolveanyfillermaterials,theLBWprocesscanbeusedtoproducemagnesiumalloyweldmentswithorwithouttheemploymentofadditionalfillermaterials.Formanyapplications,inadditiontothemechanicalproperties,thecorrosionresistanceofmaterialsmustbetakenintoconsideration.

Magnesiumalloysgenerallyhavebeenregardedas

materialswithapoorcorrosionresistance[10].However,thegeneralcorrosionresistanceofmagnesiumalloyswithcontrolledlevelsofimpuritiesisclaimedtobebetterthanthatofcarbonsteelinatmosphericexposuretestsfor2yearsattheTexasGulfCoast[11].

Thecorrosionbehaviourofmagnesiumalloysissignificantlyinfluencedbytheimpurities,especiallythoseonthesurface[12].Inthecaseofweldments,inadditiontothesurfaceimpurities,thecorrosionbehaviourcanbeinfluencedbythegrainsize,weldmetalcomposition,distributionofmicro-constituentsandresidualstresses,etc.Furthermore,theproblemofstresscorrosioncracking（SCC）isprominentinmagnesiumalloys[13].Theenvironmentallyassistedcrackingofmagnesiumalloysisreportedtooccurduetoeitherthegalvanicallyinducedanodicdissolutionofthematrixpromotedbyamorenoblesecondaryphaseoratomichydrogen[13í15].Researchersreportedbothintergranularandtransgranularmodesofcrackingexistedinmagnesiumalloysandtheirweldments[16í19].ThereareafewpublicationsbyourgroupandotherresearcherswhichaddressthecorrosionandSCCbehaviourofweldmentsobtainedbyGTAW,FSWandautogenousLBW[20í22].Joiningofmagnesiumalloys

byLBWismostlydoneinautogenousmode.Inthis

work,laserbeamweldsfromanAZ31HPmagnesiumalloywereproducedinautogenousmodeandwiththeintroductionofanaluminium-rich（AZ61）fillermaterial.AsthepublishedinformationwaslimitedontheSCCbehaviourlaserbeamweldmentsofmagnesiumalloys,anattemptwasmadetocorrelatethemicrostructure-mechanicalpropertyüSCCbehaviouroftheseweldments.

2Experimental

RolledAZ31HPmagnesiumalloysheetof2.5mminthicknesswithanominalchemicalcompositionofMg-（2.5í3.5）%Al-（0.6í1.4）%Zn-（0.2í0.6）%Mn（massfraction）wasusedinthisexperiment.Weldswereproducedbyjoiningtwopiecesofsize200mm×330mm×2.5mmwithaNd-YAGlasersysteminautogenousmodeandwiththeuseofad1.2mmAZ61wire.Thefollowingoptimizedweldingparameterswereused:

laserpower2.2kW;weldingspeed90mm/s;focusingonsurface;heliumshielding16L/minattopside,40L/minatbottomside.Thewirefeedratewas1.25m/minwhileweldingwithAZ61fillerwire.

Themetallographicspecimens（cross-sections）werepolishedsuccessivelywith500,1200and2500gritemerysheetsfollowedbyfinalpolishingwithcolloidalsilicasolutionandetchedinasolutioncontaining3.5gpicricacid,6.5mLaceticacid,20mLwaterand100mLethanol.Microstructuralexaminationwasperformedusingascanningelectronmicroscope（SEM）.Microhardnessevaluationacrosstheweldmentswascarriedoutunderaloadof1Nwithaloadingdwelltimeof20s.

TheSCCsusceptibilityoftheparentalloyandtheweldmentswasassessedbyslowstrainratetensiletests（SSRT）inASTMD1384solutioncontaining148mg/LNa2SO4,165mg/LNaCland138mg/LNaHCO3in1L

distilledwateratanominalstrainrateof10í6sí1

followingtheISOstandard7359üPart7[23].ThegeometryanddimensionsofthespecimensemployedfortheSSRTtestsarepresentedinFig.1.Theweldreinforcementsinthefaceandrootregionsofboththeweldmentspecimenswereremovedbygrindingsuccessivelywith320,500and1200gritemerysheets,andthesamelevelofsurfacefinishwasgiventotheparentspecimensbeforeSSRTtests.ReferenceSSRTtestswereperformedontheparentalloyandweldmentspecimensinairatastrainrateof10í5sí1.TheelongationofthespecimensintheSSRTtests（inairandinASTMsolution）wasmeasuredbyemployingtwolinearvariabledisplacementtransducers（LVDTs）.AstheelongationvaluesweremeasuredwiththeLVDTsfittedtothespecimengripsandnotinthegaugesectionofspecimens,itisreferredasapparentstraininthediscussion.TheSSRTtestedspecimenswereexamined

byanopticalmicroscopetoidentifythefracturelocation,andthefailedsurfaceswereinvestigatedbySEMtoassessthenatureoffracture.

Fig.1DimensionsoftensilespecimenusedinSSRTtests（Unit:

mm）

3Resultsanddiscussion

3.1Microstructure

Scanningelectronmicrographsoftheparentalloy,theautogenousweldmetalandtheweldmetalobtainedwithAZ61fillerareshowninFigs.2（a）í（c）,respectively.Theparentalloyisanaluminiumleanalloy,thesecondaryphaseȕparticleswereobservedinverysmallquantitiesrandomlydistributedinthematrix.Boththeweldmetalshadafinergrainsize（averagegrainsizeofabout12μm）comparedwiththeparentalloy.Theweldmetalswereobservedtocontainarelativelylargenumberoffineparticlesofsize<1μmspreaduniformlyacrossthematrixandalongthegrainboundaries.ItshouldbepointedoutthatthevolumefractionoftheseparticleswashigherintheweldmetalwithAZ61fillerthanintheautogenousweldmetal.Theformationofsuchparticleswasreportedinlaserbeamandelectronbeamweldmetalsbyotherresearchers[22,24].WhileBOBBYetal[22]reportedtheparticlesasAlrichphaseinanautogenousAZ31laserweldmetal,COELHOandhisco-workers[25]identifiedthemasMg17（Al,Zn）12phasebyusingTEMdiffractionpatternsinyetanotherAZ31magnesiumalloylaserbeamweldmetal.CHIetal[24]alsoreportedtheparticleswereMg17Al12phaseintheEBweldsofAZ31,AZ61andAZ91alloys.TheEDSpointanalysismadeontheparticlesshowedthattheywerericherinAlandZn,whichcorroboratedthereportsofCHIetal[24]andCOELHOetal[25].

3.2Microhardness

Fig.3showsthemicrohardnessdistributionintheweldmentsobtainedwithorwithoutfillermaterial.ThehardnessvalueswereintherangeofHV0.1（70±10）intheentireweldment（parent,HAZandweldmetalregions）.NeithertheevolutionofafinegrainedstructurenorthepresenceoftheAl-Znrichparticlesdidinfluencethehardnessoftheweldmetal.Similarobservationsonthehardnessoflaserbeamweldmetalsofmagnesiumalloys

Fig.2SEMimagesofAZ31parentalloy（a）,weldmetalobtainedwithoutfiller（b）andweldmetalobtainedwithAZ61filler（c）

Fig.3Microhardnessprofileacrosslaserbeamweldments

werereportedinearlierinvestigations[22,25].

3.3Tensileproperties

TheplotsofstressvsapparentstrainpresentedinFig.4showthatallthethreespecimenshaveanultimatetensilestrengthvaluearound250MPa.Theparentalloyhadregisteredanapparentstrainvalueofabout55%againstavalueofabout46%registeredforthetwo

weldmentspecimens.Aftertensiletests,theweldmentspecimenswereexaminedintheopticalmicroscopetoidentifythefracturelocation.TheopticalmacrographoftheautogenousweldmentspecimenSSRTteste