2011-Effect of tool design and process parameters on properties of Al alloy 6016 friction stir spot

friction stir welding

JournalofMaterialsProcessingTechnology211 (2011) 972–977

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JournalofMaterialsProcessing

Technology

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EffectoftooldesignandprocessparametersonpropertiesofAlalloy6016frictionstirspotwelds

W.Yuana,R.S.Mishraa, ,S.Webba,Y.L.Chenb,B.Carlsonb,D.R.Herlingc,G.J.Grantc

a

CenterforFrictionStirProcessing,DepartmentofMaterialsScienceandEngineering,MissouriUniversityofScienceandTechnology,Rolla,MO65409,USAGeneralMotorR&DCenter,Warren,MI48090,USAc

Paci cNorthwestNationalLaboratory,Richland,WA99356,USA

b

articleinfoabstract

Frictionstirspotwelding(FSSW)ofAlalloy6016-T4sheetwasevaluatedusingaconventionalpin(CP)toolandoff-centerfeature(OC)tool.Toolrotationspeedandplungedepthwerevariedtodeterminetheeffectofindividualprocessparameteronlap-shearseparationload.Maximumseparationloadofabout3.3kNwasobtainedbyusinga0.2mmshoulderpenetrationdepthwith1500rpmtoolrotationspeedfortheCPtooland2500rpmfortheOCtool.Threedifferentweldseparationmodesunderlap-shearloadingwereobserved:interfacialseparation,nuggetfractureseparationanduppersheetfractureseparation.Microhardnesspro leforweldcrosssectionindicatednodirectrelationshipbetweenmicrohardnessdistributionandseparationlocations.

© 2010 Elsevier B.V. All rights reserved.

Articlehistory:

Received26July2010

Receivedinrevisedform7December2010Accepted17December2010

Available online 24 December 2010Keywords:

FrictionstirspotweldingTooldesignAlalloy6016

ProcessparameterSeparationmode

1.Introduction

Weightsavingintheautomotiveindustryisbecomingincreas-inglyimportantandcanbeenhancedbyusinglight-weightaluminumalloyforvehicles;particularlyclosurepanelssuchashoods,decklidsandlift-gates.Resistancespotwelding(RSW),currentlythemostcommonlyusedjoiningtechniqueinthevehi-cleindustry,hasapplicationsforlow-carbon,high-strengthandcoatedsteels.However,RSWofaluminumalloysheetsisfraughtwithmanydisadvantages,whichincludeporosityandcracks,asreportedbyThorntonetal.(1996)andGeanetal.(1999).AsevereelectrodetipwearproblemhasalsobeenencounteredduringRSW(Khanetal.,2007).Recently,FSSWforjoiningaluminumalloysheethasbeendevelopedbyMazdaMotorCorporation(Sakanoetal.,2001)andKawasakiHeavyIndustry(Iwashita,2003).

Similartofrictionstirwelding,whichwasdevelopedbyTWI,UKin1991(Thomasetal.,1991),FSSWisasolid-stateweldingtechnique.DuringplungetypeFSSW,arotatingtoolwithaprotrud-ingpinisinsertedintotheoverlappingsheetstoapredetermineddepth.Afteracertaindwelltime,itisretractedandakeyholeisleft.Thefrictionalheatgeneratedatthetool-workpieceinterfacesoftensthesurroundingmaterial,andtherotatingandmovingpincausesthematerial owinboththecircumferentialandtheaxial

Correspondingauthor.Tel.:+15733416361;fax:+15733416934.E-mailaddress:rsmishra@mst.edu(R.S.Mishra).0924-0136/$–seefrontmatter© 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2010.12.014

directions(Lathabaietal.,2006).Theinter-mixingoftheplasticizedmaterialandforgingpressureappliedbythetoolshoulderresultintheformationofasolidbondregion(Freeneyetal.,2006).

ThestrengthofweldsiscriticalwhenapplyingFSSWtoload-bearingcomponents.Thisstrengthisaffectedmainlybytoolgeometryandprocessparameters.Toolgeometry,suchasshoul-derdiameterandshape,pinshape,length,diameterandfeatureisakeyparametertoaffectheatgenerationandmaterial ow(MishraandMa,2005).Currently,aconcavetoolshoulderisthemostcommonshoulderdesigninFSSW,thoughsome attoolshoul-derdesignshavealsobeenused.Aruletal.(2005)reportedthatconcaveshouldertoolproducedhigherjointstrengththanthe atshouldertoolduringFSSWofaluminum5754.Badarinarayanetal.(2009a)alsoreportedthatspotweldsmadeusingconcavepro leshoulderexhibitedhigherweldstrengththanthoseusingconvexor atshoulderasaresultofthehighesteffectivetopsheetthick-nessachievedusingconcaveshoulder.Recently,Tozakietal.(2010)proposedanewlydesignedscroll-groovedshouldertoolwithoutaprobewhichproducedmuchhigherfailurestrengthofaluminumalloy6061weldsthantoolwithprobe.Thetoolpinisdesignedtodisruptthefayingsurface,transportandshearadjacentmaterialandgeneratefrictionalheatinthethicksheet.Themostcurrentpindesigninopenliteratureisconventionalcylindricalandconicalpinwithorwithoutthreads.Valantetal.(2005)havereportedathree-pinfeaturedesign.Recently,Mishraetal.(2007)havereportedtheconceptofOCtooltohavebettercontroloverthematerialsweepduringFSSW.Badarinarayanetal.(2009a,b)presentedtheeffectof

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Fig.1.Schematicillustrationoflap-shearspecimen.

toolgeometryonstaticstrengthandhookformationandshowedthattheprobegeometrysigni cantlyaffectedthehookformation.Processparametersarealsokeyfactorsthataffectthestrengthoftheweldjoints.Themechanicalbehaviorofspotweldedaluminumalloyshasbeenstudied.Sakanoetal.(2001)andAruletal.(2008)showedthatthelapshearload rstincreased,andthendecreasedasthetoolrotationspeedincreased.Freeneyetal.(2006)andTozakietal.(2007)reportedthathigherweldstrengthcanbeattributedtoalargerstirzonesizeattainedbyloweringtoolrotationspeed.However,Lathabaietal.(2006)reportedthatahighertoolrotationspeedof3000rpmwasoptimumforAlalloy6060-T5andpenetra-tiondepthhadsigni cantin uenceonthetensileshearstrength.Ontheotherhand,Freeneyetal.(2006)andMitlinetal.(2006)reportedthatincreasingpenetrationdepthhadnoin uenceonthefailureload.Tranetal.(2009)showedthatfailureloadofdissim-ilaraluminumalloyspotfrictionweldsincreasedwithprocessingtimeasaresultofenlargedwelddiameter.Theliteratureonhowtheseprocessparametersaffectweldstrengthisdiverse,andonlyageneralcomparisoncanbeachievedbecauseofdifferentalloyuse,variedalloythicknessandtooldesign.

Inthispaper,twodifferenttoolswiththesameshoulderdiam-eterandconcaveshapebutdifferentpinfeatureswerecompared.Onewasaconventionaltoolwithastepspiralpin,p-sheartestswereusedtoinvestigatesystematicallytheeffectofindividualprocessparameteronweldstrength.Crosssectionalmicrostruc-turesoftheweldedandseparatedspecimenswereanalyzedtooutlinethedifferentseparationmodesunderlap-sheartests.2.Experimentalprocedures

Fig.3.Opticalmacrographsofspotweldsmadeusingconventionalpintoolat(a)1500rpmand(b)2500rpm;off-centerfeaturetoolat(c)1500rpmand(d)2500rpm.

off-center0.8mmlonghemisphericalpinfeatures.BothtoolsweremachinedfromDensimettungstenalloy.Thespotweldingmachinewasoperatedunderpositioncontrolmode.FortheOCtool,astop-then-retractionmodewasused.

AnMTStestingmachinewasusedtoevaluatethreelap-shearspecimensforeachweldingconditioninordertoobtainarepre-sentativeaveragemaximumlap-shearseparationload.Specimenswerepulledatarateof0.02mm/s.Inadditiontomechanicaltest-ing,twoweldsineachconditionwerecross-sectionedandmountedformetallographicstudiesandmicrohardnesstests.Sampleswerepreparedandetchedusinga5%HFreagenttodetermineweldmor-phology.Microhardnesstestswereperformedoncrosssectionsofweldsmadebybothtools.Vickersmicrohardnessmeasurementsweretakenat0.5mmbelowtheuppersheetsurfacewith1.0mmintervalusingadiamondindenterwitha0.5kgfloadand10sdwelltime.ThesampleswerekeptinafreezerbetweenFSSWrunsandmicrohardnesstests.3.Resultsanddiscussion

1.0mmthickAlalloy6016-T4sheetswereusedinthisstudy.Alalloy6016isalowCu,Mg–Sialloythatgainedpopularityasskinmaterialforcarbodypanelsduetoitsdesirabledentresistanceandrelativelyhighformability(Hirthetal.,2001).Aluminumsheetswereshearedtoadimensionof127.0mmlongand38.1mmwide.Fig.1showsalap-shearspecimenusedtoinvestigatethestrengthofthewelds.Thespecimenhada38.1mmsquareoverlaparea.

AplungetypeFSSWmachinewithaxialloadcapacityof22.2kNandspindlerotationspeedsupto3000rpmwasused.Duringtheweld,axialforce,torqueandtimeweredatalogged.TwotoolsareshowninFig.2.TheCPtool,whichisaconventionaltoolwithacenterpin,hasaconcaveshoulderwith10.0mmdiameteranda1.5mmlongstepspiralpinwithrootdiameterof4.5mmandtipdiameterof3.0mm.TheOCtoolistheoff-centerfeaturetoolwiththesameconcaveshouldershapeanddiameter,and

three

3.1.Macrostructure

Fig.3showstypicalcrosssectionsofaluminumalloyspotwelds.Theregioninsideofthedashedlinesindicatesthedynamicrecrys-tallizedzonewhichisgenerallyreferredasnuggetzoneorstirzoneandtheregionrightadjacentisthethermomechanicallyaffectedzone(TMAZ)(MishraandMa,2005).AsFSSWofaluminumalloyisgenerallyperformedwithoutadditionalspecimencleaningoroxideremoving,ahookingorhookfeature(indicatedbyblackarrowinFig.3)originatingfromthefayingsurfaceofthetwosheetsisoftenobservedduetotheuncompletedbreak-upofaluminumoxide lm.Basedonthedistributionoftheoxide lm,thebondingconditionsbetweentwosheetscanbede nedascompletelymet-allurgicalbonded,partiallymetallurgicalbondedand

unbonded

Fig.2.Macroimagesof(a)Conventionalpintooland(b)Off-centerfeaturetool.

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p-shearseparationloadandfrictionalheatinputasafunctionoftoolrotationspeed.

(Badarinarayanetal.,2009a).Fig.3(a)and(b)showsthecrosssec-tionsofweldsmadeat1500rpmand2500rpmusingCPtool.Alargerbondedregionwasachievedatalowerrpmwitharela-tively athookingwhichappearstodisappearattheinterfaceofthenuggetregion.Athigherrpms,ratherthandisappearing,thehook-ingtendedtocurveupwardsontheouteredgeofthenuggetregion,whicheffectivelydecreasesthebondedregion.Thedistincthook-ingfeatureresultsfromthedifferenceinmaterial owatlowerandhighertoolrotationrates.DuringFSSW,materialundergoessevereplasticdeformationandthermalcycle.Softmaterialistransporteddownbythesynergyoffeaturedpinandtoolshoulder,andthenitisreleasedand owsupwardintothestirzone(Suetal.,2007).Thisprocesscontinuesuntiltoolretracts.Toolrotationratein uencesthematerial owbyvaryingthefrictionalheatinputandvolumeofmaterialtransportedandreleasedwhichdirectlyaffecttheforma-tionofstirzone.ThefrictionalheatinputundervarioustoolrotationrateswascalculatedandshowninFig.4basedonthetoolprocess-ingforce,spindletorqueandprocessingtime.Detailsofheatinputcalculationcanbefoundelsewhere(Linetal.,2011).Theheatinputincreasedwithincreaseintoolrotationrate,whichinturnreducedtheviscosityofmaterialunderthetoolshoulderandaroundthepin,i.e.highertoolrotationrateleadstobettermaterial owabil-ity,consequentlymorematerialdisplacedperunittime,andlessshearandforgingcomponentsrequiredfromthetooltodeformthematerial.Thiscanbeveri edbythepro leofstirzone.Atlowertoolrotationrate,theboundarybetweenstirzoneandTMAZchangesmildlyandhookingbecomesstirredandmoredispersed.How-ever,sharpinnercurvedstirzonepro lecanbeobservedathighertoolrotationrateandhookingappearstobejustextrudedbythereleasedmaterialandsubsequentlysqueezedbydownwardmate-rial ow.Fig.3(c)and(d)showsthecross-sectionsofweldsmadeat1500rpmand2500rpmusingtheOCtool.UnliketheresultsfortheCPtool,thehookingwasnotpronounced,especiallyat1500rpmwheretwosheetswerenotwellbondedandthesizeofthebondedregionincreasedfortheOCtoolfrom1500rpmto2500rpm.Itsug-geststhattheOCtoolgeometrygeneratesalowerheatinputforgivenprocessparametersandassuchisabletoincreasetheamountofmaterialintothestirzonewithinthisrpmrangeversustheCPtool.SimilartotheCPtool,thefrictionalheatincreases(notedfromFig.4)andtheviscositydecreasewhenthetoolrotationrateincreases.Aslightlyupwardhookingfeatureformedtoaccommo-datethematerialreleaseduringtoolplunging.Thematerial owduringFSSWusingtheOCtoolismorecomplexduetotheasym-metricarrangementofpinsandthehemisphericalpinfeature.The

hemisphericalfeatureisnotpronetointroducesigni cantverticalmaterial ow,butenhancedmaterialsweepinhorizontalplanes,thisisseenbythelimitedupwardhookingfeature;thesmallvol-umeofpinwithhemisphericalfeaturesandshallowtoolplungedepthisthesuggestedreasonforthis.

Itshouldbenotedthatforthesameprogrammedtoolplungedepthunderdisplacementcontrol,theaccuracyofplungedepthisbetterathighertoolrotationspeedsforbothtools.Thisislikelytoberelatedtothefactthatatlowerrpmsthereislowerheatinputandagreaterresistancetothetoolwhichisaffectedbythecomplianceofthemachine.Valantetal.(2005)reportedasimilarobservationandindicatedthatforagivenpositionoftheZ-axisser-vomotor,axialloadontheZ-axisspindleand nitestiffnessofthemachinedeterminedtheactualtoolpenetrationdepth.Althoughastop-then-retractionmodewasusedfortheOCtool,themotorinertiaandsoftermaterialaroundthepinswerepulledoutwiththetool,whichleftaholeintheweldcenter.

p-sheartest

Forthe rstsetofruns,toolrotationspeedwasvariedfrom1000rpmto2500rpminstepsof500rpm;whileplungedepthwaskeptconstant.Fig.4showstheeffectoftoolrotationspeedonlap-shearseparationloadandfrictionalheatinput.FortheCPtool,thelap-shearseparationload rstincreasedthendecreasedastherotationspeedincreased,withapeakloadof2.8kNat1500rpm.However,fortheOCtool,thelap-shearseparationloadincreasedasthetoolrotationspeedincreased,withamaximumvalueof3.0kNat2500rpm.Nodirectrelationshipbetweenfrictionalheatinputandseparationloadwasobserved,especiallywhentheCPtoolwasused(higherheatinputwithlowerseparationload).Thevariationinlap-shearseparationloadwithtoolrotationspeedisrelatednotonlytothefrictionalheat,butalsotothematerial owwhichaffectsthesizeofbondedregionandhookingfeaturesaswell.Suetal.(2005)haveshownapositivecorrelationbetweenthebondedareaandthelap-shearstrength.Highertoolrotationspeedisbelievedtogeneratemorefrictionalheat,whichisbene cialforformationofalargerbondedregion.Bozzietal.(2010)proposedthatthesizeandlocationofthestirzoneandtheunweldedinter-facetipslopplayedadeterminantroleontheweldstrength.TheupwardandinwardclimbinghookingobservedfortheCPtoolathighertoolrotationratessigni cantlydecreasedthebondedregionandinducedtheeasycrackpropagationduringlap-shearloading,resultinginthereducedseparationload.However,largerbondedregionandlimitedupwardandoutwardhookingmadetheweldsstrongerathighertoolrotationspeedswiththeOCtool.

Forthesecondsetofruns,thepenetrationdepthwasvariedbyincreasingthedepthby0.1mmincrements.Consideringthe nitestiffnessofthemachine,actualshoulderpenetrationdepthwasmeasuredbyweldcrosssection.Thetoolrotationspeedwas1500rpmfortheCPtooland2500rpmfortheOCtool.Fig.5showshowshoulderpenetrationdepthaffectstheseparationload.HighpenetrationdepthhasbeenreportedtogeneratehighweldstrengthbyAruletal.(2005)andLathabaietal.(2006).Cur-rentresultsindicatedtheseparationload rstincreasedandthendecreasedwithplungedepthforweldsmadeusingbothtools.Apeakvalueof3.3kNwasobservedforweldsmadeusingbothtoolswitha0.2mmshoulderpenetration.Continuallyincreasingthepenetrationdepthdecreasedthelap-shearseparationloadofweldsmadeusingbothtools.Yinetal.(2010)suggestedthathighfailureloadswereachievedforlargebondedwidths,smallratioofthedistancefromthesheetintersectiontothetipofthehookregiontotheinitialsheetthicknessandoutwardscurvedfeatureofhook.Withtheincreaseofplungedepth,thesizeofbondedregionincreased,however,hookingwentupandthethicknessoftheuppersheetundershoulderindentationdecreased,whichreduced

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Fig.6.AschematicplotofseparationmodesforspotweldsmadeusingCPtoolandtestedunderlap-shearloadingcondition.

p-shearseparationloadasafunctionofshoulderpenetrationdepth.

theratioindicatedbyYinetal.(2010).Finally,thetopsheetundertheoutercircumferenceofthetoolshoulderbecametheweakestload-bearingcrosssection.

3.3.Separationmodesofweldsunderlap-shearloadingconditionThestrengthofweldsdependsnotonlyonthesizeofthebondedregion,butalsoonthehookingfeaturesandthicknessofthetopsheetattheoutercircumferenceoftheshoulderindenta-tion.Theweakestpoint,ofcourse,isthelocationforseparation.Badarinarayanetal.(2009b)pointedthatacombinationofstressconcentration,mechanicalpropertyofthematerial,andeventheload-carryingareainfrontofthehooktipdeterminedfurthercrackpropagation.Theseparationmodeofspecimenchangeswhenweldingprocessparametersvary.Mitlinetal.(2006)haveindicatedthattoolpinpenetrationdepthhadastrongeffectonthesepara-tionmodeofwelds.Suetal.(2005)haveshownenergyinputduringFSSWin uencedthefracturemodeduringmechanicaltesting.

Inthisstudy,threedifferentseparationmodesunderlap-sheartestswereobservedforbothtools.Aschematicplotofsepara-tionmodesforweldsmadeusingCPtoolisshowninFig.6.

For

eachseparationmode,theinitialcrackstartedfromthefayingsur-facedepictedaspoint“O”.ForthemodeIF,interfacialseparation,thecrackpropagatedalongthehookingtopointA,thenalongthecircumferencetoA .Inthiscase,thecrackpropagatedparalleltotheuppersheetsurfaceandseparatedwithlimitedplasticityattheweldnuggetleadtoseparationload.ModeNF,nuggetfrac-tureseparation,thecrackpropagatedalongthehookingintothestirzoneandthenpropagatedtopointB;afterthatitcametopointB alongtheinnercircumferenceortoB1,B2thentoC alongtheoutercircumference.ModeUSF,uppersheetfracturesepara-tion;inthiscase,signi cantlymorefrictionalheatinputgeneratedalargebondedregionandthehookingmergedintothenuggetcompletely.Whenthecrackreachedthestirzone,itfollowedtheboundaryofTMAZandnuggettothethinnestpartoftopsheet,wherefracturehappenedatpointCthenpropagatedtopointC alongtheoutercircumferenceoftheshoulderindentation.Inthiscase,weldsdisplayedcertainplasticityondeformationandsepa-ratedatahigherload.TheseparationrouteofmodeUSFisshowninFig.7withtheweldcross-sectionindicatedatdifferentlap-sheartestingextensions.Brokensampleswithvariousseparationmodesafterlap-sheartestingareshowninFig.8.

ThemaximumseparationloadwasachievedundermodeUSFseparationconditionforweldsmadeusingbothtools;separa-tionmodedidnotchangeastoolpenetrationdepth

continued

Fig.7.SeparationrouteofweldsmadeusingCPtoolandseparatedundermodeUSFcondition.

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Fig.8.BrokensamplesmadeusingCPtool,separatedby:(a)modeIF,(b)and(c)modeNF,and(d)modeUSF.Thephotosshow,fromlefttoright,viewsofthetopsheet,theundersideofthetopsheetandthebottomsheet.

toincrease.However,themaximumseparationloaddecreasedgreatly,potentiallybecausethethinuppersheetundertheshoulderindentationbecameweakestanddominatedthe nalseparation.TherelationbetweenseparationloadandextensionofweldsmadeusingtheCPtoolandthemodeofseparationispresentedinFig.9.Theresultsindicatedthreeseparationmoderegionsandtwomodetransitionregions.AstheseparationmodeshiftedfromIFtoUSF,theseparationloadsandextensionswereatleastdoubled.Noobvi-ousrelationshipbetweenweldingparametersandseparationmodewasobserved.Generally,theIFmodewaspresentedatlowerheatinputconditions(lowerrpm,lowerplungedepth)whenthesizeofbondedregionwassmallandinterfacialbondingwasweak.Astheheatinputincreasedbyincreasingrpmandplungedepth,thesepa-rationmodechangedfromIFtoNForUSFdependedontheratioofthesizeofbondedregiontotheeffectivethicknessofuppersheet,andthefeatureofthehooking.Asmallerratioandinwardhook-ingpromotedtheNFmode,andalargerratioandoutwardhookingpreferredtheUSFmode.3.4.Microhardnesstest

Thetensilepropertiesoftheweldaredependentonthestrengthdistributionwithintheweldonlywhenitisfreeofdefects.Tounderstandtherelationshipbetweenstrengthdistributionandseparationlocations,microhardnesspro lesforweldcrosssectionsweretakenatthreedifferentshoulderpenetrationdepthswhichcorrespondedtothreeseparationmodes.Microhardnesstestswereperformed0.5mmbelowtheuppersheetsurface.TheresultsinFig.10showatypicalheat-affectedzone(HAZ)which

under-Fig.9.Separationloadasafunctionofextensionforweldsseparateatdifferent

modes.

Fig.10.Vickersmicrohardnessdistributionsofweldsatdifferentshoulderpene-trationdepths.

goesthermalcycleswithlowestmicrohardness.TheHAZmovedawayfromtheweldcenterresultingfromhigherthermalinputastheshoulderpenetrationdepthincreased.Nodirectrelationshipbetweenmicrohardnessandseparationlocationswasobserved,sincethemicrohardnessincreasedwhenclosetotheweldcen-terandtherewasnotmuchvariationinmicrohardnessinnuggetsaspenetrationdepthincreased;however,theseparationlocationshiftedawayfromtheweldcenterwithincreaseofpenetrationdepth.Theseparationlocationsweresigni cantin uencedbythehookingwhichnotonlyaffectedthesizeofbondedregionandsubsequentratioofsizeofbondedregiontoeffectivethicknessofuppersheet,butalsointroducedstressconcentrationattheendofunbondedregionduringshearloading,whichin uencedfurthercrackpropagationandthe nallocationforfracture.4.Conclusions

Alalloy6016sheetswerefrictionstirspotweldedbyusingbothCPandOCtools.Resultsindicatethattoolrotationspeedandplungedepthprofoundlyin uencedlap-shearseparationloads.Bothtoolsexhibitedmaximumweldseparationload:about3.3kNat0.2mmshoulderpenetrationdepth;differenttoolrotationspeeds,1500rpmfortheCPtooland2500rpmfortheOCtool.Threedifferentseparationmodeswereobservedforweldsmadeusingbothtools,interfacialseparation,nuggetfractureseparationanduppersheetfractureseparation.Thehighestseparationloadwasachievedunderuppersheetfracturemode.Microhardnesstestsindicatednodirectrelationshipbetweenmicrohardnessandseparationmodes,andtheHAZwasthesoftestregion.

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Acknowledgments

Theauthorsgratefullyacknowledgethesupportof(a)theNationalScienceFoundationthroughgrantNSF-EEC-0531019and(b)GeneralMotorsCompanyandFrictionStirLinkfortheMissouriUniversityofScienceandTechnologysite.References

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