Materials&Design,Vol.18,Nos.4r6,pp.269273,1997Q1998PublishedbyElsevierScienceLtdPrintedinGreatBritain.Allrightsreserved0261-3069r98$19.00q0.00()PII:S0261306997000629FrictionstirweldingforthetransportationindustriesW.M.ThomasU,E.D.NicholasTWI,AbingtonHall,Abington,CambridgeCB16AL,UKReceived16June1997；accepted17June1997()ThispaperwillfocusontherelativelynewjoiningtechnologyfrictionstirweldingFSW.Likeallfrictionweldingvariants,theFSWprocessiscarriedoutinthesolid-phase.Genericallysolid-phaseweldingisoneoftheoldestformsofmetallurgicaljoiningprocessesknowntoman.Frictionstirweldingisacontinuoushotshearautogenousprocessinvolvinganon-consumablerotatingprobeofhardermaterialthanthesubstrateitself.Inaddition,FSWproducessolid-phase,lowdistortion,goodappearanceweldsatrelativelylowcost.Essentially,aportionofaspeciallyshapedrotatingtoolisplungedbetweentheabuttingfacesofthejoint.Onceenteredintotheweld,relativemotionbetweentherotatingtoolandthesubstrategeneratesfrictionalheatthatcreatesaplasticisedregionaroundtheimmersedportionofthetool.Thecontactingsurfaceoftheshoulderedregionofthetoolandtheworkpiecetopcontactingsurfacealsogeneratesfrictionalheat.Theshoulderedregionprovidesadditionalfrictiontreatmenttotheweldregionaswellaspreventingplasticisedmaterialbeingexpelled.Thetoolisthentranslatedwithrespecttotheworkpiecealongthejointline,withtheplasticisedmaterialcoalescingbehindthetooltoformasolid-phasejointasthetoolmovesforward.AlthoughtheworkpiecedoesheatupduringFSW,thetemperaturedoesnotreachthemeltingpoint.Frictionstirweldingcanbeusedtojoinmostaluminiumalloys,andsurfaceoxidepresentsnodifficultytotheprocess.Trialsundertakenuptothepresenttimeshowthatanumberoflightweightmaterialssuitablefortheautomotive,rail,marine,andaerospacetransportationindustriescanbefabricatedbyFSW.Q1998PublishedbyElsevierScienceLtd.Allrightsreserved.Keywords:frictionstirwelding；transportation；solidphaseIntroductionRecently,anovelfrictionweldingprocessfornon-fer-rousmaterialshascapturedtheattentionofthefabri-cationindustry.ThisrelativelynewprocesscalledFric-.tionStirWeldingFSWisasolid-phaseprocessgivinggoodqualitybuttandlapjoints14.TheFSWprocesshasprovedtobeidealforcreatinghighqualityweldsinanumberofmaterials,includingthosewhichareex-tremelydifficulttoweldbyconventionalfusionprocesses5.ThebasicprincipleoftheprocessisillustratedinFigure1.Theprocessoperatesbygeneratingfrictionalheatbetweenarotatingtoolofhardermaterialthantheworkpiecebeingwelded,insuchamannerastothermallyconditiontheabuttingweldregioninthesoftermaterial.Thetoolisshapedwithalargerdiame-tershoulderandasmallerdiameter,speciallyprofiledprobe.Theprobefirstmakescontactasitisplungedintothejointregion.Thisinitialplungingfrictioncon-tactheatsacylindricalcolumnofmetalaroundtheprobeaswellasasmallregionofmaterialunderneaththeprobe.Thedepthofpenetrationiscontrolledbythelengthoftheprobebelowtheshoulderofthetool.ThecontactingshoulderappliesadditionalfrictionalUCorrespondencetoW.M.Thomas.Tel.:q4401223891162；fax:q4401223892588；e-mail:wmthomastwi.co.ukheattotheweldregionandpreventshighlyplasticisedmaterialfrombeingexpelledduringtheweldingopera-tion.Oncetheshouldermakescontacttheadjacentthermallysoftenedregiontakesupafrustumshapecorrespondingtothatoftheoveralltoolgeometry.Thethermallysoftenedregionappearsmuchwideratthetopsurfaceincontactwiththeshoulder,taperingdowntotheprobediameter.Thecombinedfrictionalheatfromtheprobeandtheshouldercreatesaplasticisedalmosthydrostaticconditionaroundtheimmersedprobeandthecontactingsurfaceoftheshoulderedregionoftheworkpiecetopsurface.Materialflowsaroundthetoolandcoalescesbehindthetoolasrela-tivetraversebetweensubstrateandtherotatingtooltakesplace.Frictionstirweldingcanberegardedasaautogenouskeyholejoiningtechnique.Theconsoli-datedweldsaresolid-phaseinnatureanddonotshowfusionweldingdefects.Noconsumablefillermaterial,shieldinggas,oredgepreparationisnormallyneces-sary.Thedistortionissignificantlylessthanthatcausedbyanyfusionweldingtechnique.Exploratorydevelopmentworkhasencompassedaluminiummaterialsfrom1to75mmthick.FSWweldingtrialsEarlyexploratorydevelopmenttrialswerecarriedoutMaterials&DesignVolume18Numbers4/61997269Frictionstirweldingforthetransportationindustries:W.M.ThomasandE.D.NicholasFigure1FrictionstirweldingtechniqueFigure2Transversemacrosectionof50mmthick6082T6aluminiumalloy.Platematerialweldedfrombothsides,showingweldnuggetprofileandflowcontourswith6082T6aluminiumalloymaterialinthicknessesrangingfrom1.6to12.7mm.Recenttrialshaveex-tendedthethicknessrangeupwardsto75mmintwopasses.MetallographicexaminationAseriesof50and75mmthickweldsin6082T6conditionaluminiumalloy,whichdemonstratethepos-sibilitiesfortheprocessforthickplateareshowninFigures24.Themacrosectionsfromtheseweldsarecharacterisedbywelldefinedweldnuggetsandflowcontours,almostsphericalinshape,thesecontoursaredependantonthetooldesignandtheweldingparame-tersandprocessconditionsused.Forheattreatablematerialsawell-definedheataffectedzonesurroundstheweldnuggetregionandextendstotheshoulderdiameteroftheweldattheplatesurface.Theweldnuggetitselfistheregion,wherefulldynamicre-crys-tallisationoccursthatcomprisesofafineequiaxedgrainstructure.Themeasuredgrainsizeisintheorderof24mmindiameter.Typicallytheparentmetalchemistryisretained,withoutanysegregationofalloy-ingelements.Ahardnesstraversetakenfrom50mmthicktestweldrecordedthefollowingvalues:vParentmetal100HV2.5vWeldnugget65HV2.5vHAZregion52HV2.5FractographySamplesofwelded50mmthick6082T6platewerenotchedintheparentmaterial,andtheweldnuggetregionandthenfracturedbybending.Thefracturesurfaceswereexaminedusingscanningelectronmi-croscopy.BoththeweldnuggetandtheparentmaterialfailedinaductilemannerbythemicrovoidcoalescencemechanismasshowninFigure3.However,therewasanabsenceofrelativelylargemicrovoidsintheweldnuggetsampleandthismaybeaconsequenceofthebreakupoftheprimaryconstituentparticlesduringstirwelding.MechanicalintegrityForplatethicknessupto50mmthick,transverseMaterials&DesignVolume18Numbers4/61997270Frictionstirweldingforthetransportationindustries:W.M.ThomasandE.D.Nicholas.Figure3Fractographycomparisonbetweenweldnuggetandparentmetalin50mmthick6082T6aluminiumalloyplate.aFractureinweld.nugget.Scanningmicroscopyofnickbreakbend,fractureface.bFractureinparentmetal.Scanningmicroscopyofnickbreakbend,fracturefaceFigure4Threepointbendandtensiletestin75mmthick6082T6aluminiumalloyFSWplatesectionswerehammerbendtestedto1808.Whiletransversesectionstakenfrom75mmthickplatewerethreepointbendtestedasshowninFigure4.Anum-beroftensiletestswerecarriedoutwithfailuretypi-callyoccurringintheHAZregionat175Nmm2asshowninFigure4.Thelocalisedreductioninspecimenthicknessduringthetensiletestingcorrespondswithregionsofreductioninhardness.ProcesscharacteristicsExperimentalresearchworkisbeingcarriedouratTWItoevaluatearangeofmaterialsanddevelopotherFSWtoolsdesignedtoimprovetheflowofplasti-cisedmaterialaroundtheprobeitselfandtoenablesubstantiallythickerplatestobejoinedandenablerelativelyhightraverseratestobeachieved.Figure5illustratesthenaturaldynamicorbitinher-entlyassociatedwitheverytypeofrotarymachine.Thiseccentricitymusttoagreaterorlesserextentbepartofthefrictionstirweldingprocesscharacteristics.Eccen-tricityallowshydromechanicallyincompressibleplasti-cisedmaterialtoflowmoreeasilyaroundtheprobe.Itfollowsthatanominalbiasoff-centreornon-circularprobewillalsoallowplasticisedmaterialtopassaroundtheprobe.Essentiallyitistherelationshipbetweenthegreatervolumeofthedynamicorbitoftheprobeandthevolumeofthestaticdisplacementoftheprobe,thathelpsprovideapathfortheflowofplasticisedmaterialfromtheleadingedgetothetrailingedgeoftherotatingtool.Anumberoftoolgeometriesandtoolattitudefordifferentmaterialshavebeenreportedinthelitera-ture3,6,7.Fortoolspositionedperpendiculartotheworkpiecetheleadingedgeoftherotationtoolpro-videsafrictionalpreheateffectheatandsubsequentthermalsofteningoftheworkpieceinfrontoftheprobe.Thispreheatcanbeofadvantagewhendealingwithharderordifficulttoweldmaterials.Thegreatertheareaoftheshoulderedregionoftherotatingtoolmakingcontactwiththejointsurfacethegreaterthe