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Faye, Ameth and Balcaen, Yannick and Lacroix, Loïc and Alexis, Joël Effects of welding parameters on the microstructure and mechanical properties of the AA6061 aluminium alloy joined by a Yb: YAG laser beam. (2021) Journal of Advanced Joining
Processes, 3. 1-11. ISSN 2666-3309 Official URL:
https://doi.org/10.1016/j.jajp.2021.100047
Effects of welding parameters on the microstructure and mechanical
properties of the AA6061 aluminium alloy joined by a Yb: YAG laser beam
Ameth Faye, Yannick Balcaen, Loic Lacroix, Joel Alexis
∗Laboratoire Génie de Production, Ecole Nationale d’Ingénieurs de Tarbes, Université de Toulouse, LGP, ENIT/INPT, 47 av. d’Azereix, BP1629, 65016 Tarbes cedex, France
Keywords:
Laser welding process Aluminium alloy Microstructure study Engineering properties
a b s t r a c t
Inthisstudy,theeffectsofYb:YAGlaserweldingparametersonthemicrostructureandmechanicalpropertiesof AA6061-T4jointswereanalysed.Sampleswithoutanyweldingdefectssuchasporosity,meltpoolcollapse,and hotcrackingwereproducedwithdifferentweldingparameters.Energeticprocessingparametershadasignificant influenceonthefusionzone(FZ)andheat-affectedzone(HAZ)microstructureanddimensions,inadditiontolocal andglobalmechanicalproperties.Theweldbeadwidthincreasedwithincreasingpowerandenergydensities andreducedtheweldbeadtensileproperties.Highweldingtravelspeedproducedamoreelongatedweldpool andripplesresultedina‘V’shape.Asignificantquantityofaxialheterogeneousnucleationwasobservedinthe FZcentreoftheseweldbeadsbecauseofthelow-energydensity.Incontrast,lowweldingtravelspeedproduced C-shapedripplesandafewaxialgrainnucleationsitesintheFZ.Thelatterwasevidenceofaslowersolidification rate.Dendritesecondaryarmspacingmeasurementsconfirmedthishypothesis.Itwasobservedthataxialgrains intheFZcentreimprovetheweldbeadtensileproperties.Comparedtothebasematerial(BM),thehardness intheFZwasreducedandidenticalintheHAZ.TheFZhardnessdependedontheweldingparameters.Digital imagecorrelation(DIC)strainmeasurementsindicatedhigherdeformationneartheFZ,butwhengeometrical defectswereremoved,theFZdeformedmorehomogeneously.
Introduction
Laserweldingisamodernprocessusedintheaircraftindustry,re- placingtraditionaljoiningtechniques,suchasboltingorriveting.This joiningprocesshasbeenwidelyappliedtothefabricationofaluminium alloyweldedstructures(Chuetal.,2018;Pardaletal.,2017;Fuetal., 2014).Highpowerdensityandlowheatinputaresomeoftheadvan- tagesofthelaserweldingprocess.Inaddition,itcansimplifyproduct designandreducecosts(Katayama,2013).
Lightalloysareemployedin theaircraftandautomobileindustry for weightreduction. Al–Mg–Si alloyshave good formability, corro- sionresistance,weldability,andrecyclingpotential(Eckermannetal., 2008).However,severalproblemsassociatedwithgeometricalandmet- allurgicaldefects(porosities,cracks,andmetalevaporation)havebeen observedonAl–Mg–Siweldbeads(Sánchez-Amayaetal.,2009).Dur- ingthelaser weldingof these alloys, processing parameter manage- mentis importantbecausepower density,energydensity, andinter- actiontime contribute toweld bead quality (Sánchez-Amaya et al., 2009;KuoandLin,2006).Laserweldingisdifficulttoapplytoalu- miniumalloysbecauseofthehighreflectivity,highthermalconductiv- ity,andlowviscosityofthesealloys(KuoandLin,2006;Pierronetal.,
∗Correspondingauthor.
2007).Thesephysicalproperties,combinedwithlowbeamenergy,re- sult ina lackofpenetration(Alfieria etal.,2015). Ahighfeedrate thatis toofastoranincorrect weldinganglecanalsoleadtoincor- rectbeadgeometries(OladimejiandTaban,2016).Microporesusually occurwhenlaser-weldingaluminiumalloys.Themicropores’principal sources arehydrogencontaminationinduced bypoor surfaceprepa- ration(Zhangetal.,2016; PaleocrassasandTu, 2007)andahydro- gen solubility difference between theliquid andsolid phase of alu- minium (0.036vs. 0.69 cm3/100 g at themelting pointof 660 °C) (Tiryakioğlu,2019).Macroporesmayalsooccurduringkeyholeinsta- bility(Rasmussen,2008).Hotcrackingoccursattheendofalloysolidifi- cationwhenthedendriticskeletonisnotsufficientlyformedtoresistde- formationandthepermeabilityoftheliquidmediumisverylow.Foran AA6061aluminiumalloy,thetemperaturerangeresultinginbrittleness isbetween580°Cand596°Cforrapidcoolingratesencounteredduring theweldingprocess(Giraud,2010).Hotcrackingoccursinthemelting zone(MZ),orsometimesintheheat-affectedzone(HAZ),ifaportionof thegrainsarepartiallymelted(Cicală etal.,2004;Liuetal.,2006).This partialmelting,calledliquation,isoftennotedatgrainboundariesthat havealowmeltingpointduetochemicalsegregation(Paleocrassasand Tu,2007).Mathersdefinedacriterionforcracksusceptibilitybasedon thechemicalcompositionofthealuminiumalloy(Mathers,2002).Al- E-mailaddresses:[email protected](A.Faye),[email protected](Y.Balcaen),[email protected](L.Lacroix),[email protected](J.Alexis).
Table1
ChemicalcompositionofAA6061aluminiumalloysheet(inwt%).
Alloy Mg Si Cu Fe Mn Zn Cr Ti V Al
AA6061 0.86 0.69 0.30 0.49 0.10 0.16 0.19 0.03 0.01 Bal.
Fig.1. Drawingsofweldedcouponsandsam- plessizesandlocationalongweldseam.
Fig.2. Experimental setup toweldAA6061 aluminiumalloysamples(Fayeetal.,2020).
loysinthe6000series,particularlyalloy6061, aretherefore predis- posedtohotcrackingbasedontheirchemicalcomposition.Toreduce thistendencytohotcracking,weldingofthisalloyisrecommendedwith asilicon-richfillermetalsuchasalloy4043(Rasmussen,2008).More- over,hotcrackingdependsonthemorphologyofthemicrostructureof themeltedzone.Itdecreasesinthepresenceofanequiaxedgrainzone inthecentreofthewelds,whichfavoursliquidflowandsmallgrain sizes(Kou,2003).ThelatterresultwasquestionedbyNiel,whodemon- stratedthatthepermeabilityoftheequiaxedzoneincreaseswithgrain sizeinTIGweldsofthe6061-T6alloy(Niel,2011).Finally,although weldingiscarriedoutatatmosphericpressure,vaporisationofcertain elementsisinevitable(magnesium,silicon,zinc,etc.).Thislossofchem- icalelementssignificantlyreducesthepropertiesofthemeltzone.The mechanicalpropertiesofweld beadsdependon theirmicrostructure.
Wangetal.foundthatequiaxedgrainsimprovedthetensilestrengthof theweldbead(Wangetal.,2016).
Inthisframe,theYb:YAGdisk-laserweldingoftheAA6061alu- minium alloyis discussed. Plateswith thicknessof 1mm weresuc- cessfullyweldedwithvariedparameters.Afterwelding,theseamchar- acteristics such as penetration, geometry, and microstructure were investigated. The weldability range of AA6061-T4 aluminium al- loy, welded with Yb: YAG laser, was defined by the NF L 06-395 2010standard (Fayeet al.,2020). Theinfluenceof theweldingpa- rameters on the geometry and microstructure, such as grain mor- phology and secondary dendrite arm spacing, was studied on the resultant sound weld beads. The mechanical properties of seams were investigated at different scales and related to microstructural features.
Table2
Weldingconditionsadoptedforspecificanalysis.
Welding parameters Power (W) Welding travel speed (m min −1) Focal diameter (µm) Power density (W cm −2) Energy density (J mm −2)
A 2500 2 150 1.4 ×107 637
B 2290 3 155 1.2 ×107 376
C 1200 4 134 8.5 ×106 171
D 1500 3.4 165 7 ×106 204
Fig.3. WeldabilitydomainoftheAA6061-T4alloyjoinedbytheYb:YAGlaser.
Materialandmethods
Theexperimentalmaterialwasa1mmthickAA6061sheet,naturally aged(T4).Immediatelyafterquenching,thematerialwasmetastable, and during natural aging at room temperature, the microstructure evolvedasa resultof thecontinuousformation anddevelopmentof Mg–Si-richclusters(Liuetal.,2015;Morleyetal.,2006;Werinosetal., 2016).Thecompositionofthealuminiumsheet,determinedbyanOx- fordFOUNDRY-MASTERsparkOESspectrometer,islistedinTable1. Priortowelding,theworkpieceswerepolishedwithP600SiCpaper andcleanedwithanalcoholsolutiontoremovetheoxidefilmonthe samplesurface.Theweldswereperformedwithin1h.
AsshowninFig.1,sheetswiththegivendimensionswerewelded first,andtensilespecimenswerelasercut.
Toavoidanydistortionanddiminishtheresidualstressvariationbe- tweenoneweldingconditiontoanother,theplateswereclamped.The experimentalsetupisshowninFig.2(Fayeetal.,2020).Aflatwelded plateresultinginatensilespecimenwasobtained.Theclampingsystem allowsthesamplestobe maintainededge-to-edge,withpreciseposi-
tioningofthepartingline.Theshieldinggasonthereversesidesweeps alongtheentirelengthofthebead,intheoppositedirectiontothatof thewelding.Theprimarychamberisdesignedtocontainmetalvapours fromtheweldbeadundergasprotection.Thebottomofthesecondary chamberconsistsofa copperreflector,whichreflectsthefraction of thebeamthatcanpassthroughtheweldbeadtowardsthewallofthis chamber.
ThelaserweldingmachinewasaTRUMPFTruLaserCell3000cou- pledwithaYb:YAG3.3kWsource.Thebeamwasguidedbya2-in-1 coaxialopticalfibre(100𝜇mforthecoreCFand400𝜇mfortheouter fibreOF),generatingaconventionalorannular-shapedpowerdensity distribution.
Theinfluenceofparameterssuchasbeampower,weldingspeed,and focalpointdiameteronthebeadgeometry,porosity,andcrackingwas previouslyanalysed (Fayeetal.,2020) forbothfibreconfigurations.
Tominimisethesamplenumberwhileattemptingtomaximisethein- formationregardingtherelationshipbetweenprocessparametersand beadcharacteristics,experimentaldesignsweregeneratedforeachfi- breusingtheCORICOTMsoftware.Atotalof108sampleswerewelded andcharacterisedforthisstudy(36withtheOFand72withtheCF).
Thepowerdensitywiththeouterfibrewasfoundtobeinsufficientto weldAA6061alloysheetswithathicknessof1mm(Liuetal.,2015).
Allweldbeadsobtainedwiththisfibreshowedalackofpenetrationor cracks.Therefore,inthiswork,onlythecorefibrewasused.Thewelds wereperformedinabuttjointconfigurationwithnofillermetal.The laserpowervariedbetween500and3000W.Theweldingtravelspeed variedfrom1and8mmin−1,whilethefocaldiametervariedfrom120 and370µm.Forthisfibre,thecalculatedpowerdensityrangevaried from4.6×105to2.2.×107Wcm−2,whiletheenergydensityrange variedfrom2×101to1.6×103Jmm−2.Argonwasusedasshielding gaswithflowratesof40lmin−1and20lmin−1forupperandlower weldbeadprotection,respectively.Thegeometryoftheproducedsam- pleswasinvestigatedbyopticalmicroscopy,usingaLEICAWildM420 withMICROVISIONArchimedacquisitionandanalysissoftware.There- sultswerecomparedtothealuminiumalloystandard,andaweldability rangeofAA6061forthedifferentprocessingparameterswasdefined.
Repeatabilitywasstudied.Theweldabilityrangedefinedinaprevious studywasexaminedresultinginthesameweldqualitybeingobserved
Fig.4. Weldbeadwidthvariationasafunctionof:(a)energydensityand(b)powerdensity.
Fig.5. EBSDinversepolefigure(IPFx)mapsintheRD-TDplaneofwelds.
aswaspreviouslyreported.Thecompletemethodwasdescribedinlit- erature(Liuetal.,2015).Weldbeadmicrostructureswerestudiedby meansofopticalandelectronmicroscopy.Opticalmicroscopyutilised anOLYMPUSPMG-3.TheJEOL7000F FEG-SEMwasused forEBSD mappingwithanOXFORDNordlysFastcamera.Sampleswerepolished usingBUELHERVibrometusinganOPSsuspensiononboththesheet planeandtransversesections.TheEBSDmapstepsizewassetat1and 2µm.OnlytherepresentativeA,B,C,andDsampleswerestudied.The weldingparametersaregiveninTable2.Thesolidificationratewases- timatedinthefusionzone(FZ)bymeasuringthe𝛼-anglebetweenthe weldingdirectionandthesolidificationfrontfromtheripplesorienta- tionontheweldbeadtopsurface,asshownlaterinFig.7.Theripples areconsideredasreliablemarkersoftheweldpoolposteriorgeome- try.Thesolidificationspeedisgovernedbytheweldingtravelspeed (Savage,1976):
𝑅=𝑉𝑠cos𝛼 (1)
whereRisthelineargrowthrateatanypointofthesolidificationfront, Vs theweldingtravelspeed,and𝛼 istheanglebetweenthewelding directionandthegrowthdirection ofthesolidification front.The𝛼- anglemeasurementswereperformedwithImageJanalysissoftwareon macrographsobtainedwiththeabovementionedbinocular,usingax32 magnification.Themeasurementswererecordedin200µmincrements leadingfromtheFZaxistoitsperiphery.Eachreported𝛼valuewasan averageoftenindividualmeasurements.
Inaddition,themainmechanicalpropertiessuchashardnessand tensile properties were evaluated. Transversetensile tests were per- formedwiththeassistanceofdigitalimagecorrelation(DIC)techniques.
Thus,thelongitudinalstrainfieldsoftworepresentativeweldbeads(A andDconditions)weremeasuredonbothP600polishedandunpolished samplestostudytheinfluenceofgeometricaldefects.TheDICwaspro- cessedwithARAMISV5softwareataspatialresolutionof0.42mm.
Nanoindentation tests werecarried out with an MTSnanoinden- ter,designedtoprovidelowloadswithnanoscaledepthmeasurements todeterminethematerialhardness.Theindentationincrementswere 50µm,andtheapproachspeedtothesurfacewas10nms−1.Theim- printlimitdepthwas200nm.
Resultsanddiscussion
WeldabilityrangeoftheAA6061aluminiumalloy
Variousaspectsofmetallurgicalweldingdefectswereobservedin aluminiumalloyweld beads.However,theinvestigation ofYb: YAG laserweldabilityofAA6061aluminiumalloysindicatedthatsoundweld beadsareobtainedwithcorefibre,havingafocaldiameterintherange from120to370µm(Fig.1)(Fayeetal.,2020).Thiswasdeterminedby varyingdifferentprocessingparameters.Thecorefibreallowswelding oftheAA6061aluminiumalloyasaresultofthehigh-powerdensitiesof thelaserbeam.Thiscorrespondstoanenergyandpowerdensityranging from5×101to1.2×103 Jmm−2 and5×106to2×107Wcm−2, respectively.Compliantweldbeadswereobtainedforafocaldiameter lessthanorequalto240µmandatravelspeedlessthan4mmin−1 (Liuetal., 2015).Soundweld beadswereselectedforthepresented analysis.
Weldbeadsgeometry
Forthesoundweldbeads,theheataffectedzonewidth(WHAZ)and thefaceandrootFZwidths(WfandWrrespectively)aremeasured.
Fig.3presentstheweldbeadwidthvariationwithvarying(a)energy and(b)powerdensities.Theweldbeadwidthincreaseswiththeenergy andpowerdensities.InagreementwithSánchez-Amayaetal.(2009), highlaser power, lowfocal diameter,andlow welding travelspeed favourlargeFZandHAZ(Fig.4).
Fig.6. EBSDinversepolefigure(IPFz)mapsintheND-TDplaneofwelds.
Microstructure
Grainsmorphologyoftheweldbeadtopsurfaceandcross-section wereobservedandstudiedthroughEBSDmaps(Figs.5and6).Foreach condition,theweldbeadwascomposedofanaxialstructuregrowing alongtheweldingdirectionintheweldcentreline;acolumnarstruc- turewaspresentfromthebasemetaltothecentralzone.Thiscolum- nargraingrowthoccurredinthedirectionofthelargestthermalgradi- ent.Thegrowthdependeduponthemoltenpoolrearshape,andcon- sequently,ontheweldingparameters.Withintheweldabilitydomain, highlevelsforpowerandenergydensity(here1.4×107Wcm−2and 6.37×102Jmm−2,respectively,forsampleA)displayednegligibleger- minationinthecentralzone.Thecentreoftheweldwascomposedof asetofcolumnargrainsalmostparalleltotheweldingdirectiontrans- latedoversignificantdistances.Noheterogeneousnucleationleadingto noequiaxedgrainwasobserved.Thehighenergydensitiesinduceda thermalgradient,promotingtheformationofthesecolumnargrainsin thecentreoftheweldseam.ForsampleA,thecolumnargraindirection variedfromtheweldedgetowardsitscentre.Thegrain’scurvedmor- phologymayindicateanellipticalweld poolinducedbybothahigh linearenergydensityandalowweldingtravelspeed.Germinationin thecentralzonewasgreaterforthelowerlinearenergyweldbead(B, C,andDsamples).SampleDFZobtainedwithalowpowerdensityand averymoderateenergywassignificantlynarrowerthantheotherones.
SampleDalsoexhibitedagreatergraindensityinthecentralzonebe- causeofthehighweldingspeed.Asexpected(Gäumannetal.,1999; Kurzetal.,2001),the<001>grainsdevelopedfavourablyfromthefu- sionline(IPFxmapsofFig.5andIPFzmapsofFig.6).
Fig.7presentsthemacrographsfortheA,B,C,andDsamplesfrom thetopsurfaceoftheweldbeads.TheweldbeadofsampleAhadC- shapedripples,causingasignificantvariationinthesolidificationfront growthrate.Bycontrast,samplesB,C,andDripplesweremostlyV- shaped.Theripples’shapedependedonthemoltenpoolgeometryand maybecomparedtothegrain’sorientation.
Fig.8presentsthesolidificationspeed(lineargrowthrate)variation asafunctionofthedistancefromtheweldbeadaxis.Fig.9presentsthe
axialzonewidthasafunctionoftheweldingparameters.Thecentral zonewidthisdeterminedbythetransitionofthelateralcolumnarmi- crostructuretoamicrostructureconsistinggrains,paralleltotheweld- ingdirection(bothwithandwithoutheterogeneousnucleation).These results indicatethatthesolidificationratewaslower atallpoints in thesampleA.Thissamplewasweldedwithalowertravelspeedand higherpowerandenergydensities.Asexpected(Guitterezetal.,1996), fastweldingtravelspeedproducedamoreelongatedmoltenpool,which favouredadistinctivegrainstructurecomposedofcolumnargraininthe peripheryandacentralzonenucleation.ForsampleA,theseenergetic conditionsfavouredcolumnargraingrowthandreducedthecentralnu- cleationofnewgrains.ForsampleC,thesolidificationratewasmore influencedbytheweldingtravelspeed.SampleCwasweldedwiththe highesttravelspeed;consequently,thegreatestsolidificationratewas observed.
Toestimatethesolidificationtime,thesecondaryinter-dendriticarm spacing 𝜆2 was measured in theFZ. These measurementswere per- formedonSEMmicrographsobtainedonalltheoptimisedweldbeads, asshowninFig.10a.Eachmeasurementvaluepresentedbelowcorre- spondstotheaverageof30measurementsmadeinthecentreoftheFZ.
Fig.10bpresentsthesecondarydendritearmspacingasafunctionof theenergydensity.Agreaterenergydensityfavoursdendritegrowth.
Dependingontheweldingconditions,𝜆2variesfrom2µmto6µm.Ac- cordingtoEq.(2),themeasured𝜆2valuesareonaverage,corresponding toasolidificationtimeestimatedbetween0.015and0.3s.
𝜆2=5.5( 𝑀 .𝑡𝑓)1∕3
(2) wheretf=thesolidificationtime(s),
M=aconstantdependingonthematerial(4×10-18m3s-1foralu- minium6061,KurzandFisher,1998).
Asignificantstandarddeviationispresentinthesemeasurements.
Thismayberelatedtothetechniqueprecisionlevelusedtoestimate𝜆2 butalsotothedendrite’sorientationwithrespecttothemetallographic cross-sectionplane.
InSection “Mechanicalproperties”, theeffects of microstructure, suchasgrowthmorphologiesanddendritic/cellularspacingscale,on
Fig.7.Ripplesorientationattheweldbedtopsurface:a)seamA,b)seamB,c)seamC,andd)seamD;e)Schematicshowingthedeterminationofthe𝛼-angle betweentheweldingdirectionandthesolidificationdirection.
Fig.8. Evolutionofthesolidificationfrontspeedasafunctionofthedistance dfromtheweldbeadcentre.
themechanical properties (ductility, strength,andhardness)arede- termined.Grain orientationandtheabundance ofheterogeneousnu- cleated grains have important effects on the ductility of the weld bead(RamasamyandAlbright,2000;RamasamyandAlbright,2001).
According to current literature, such microstructures increase weld strengthandreducesolidificationcrackingsusceptibilityasaresultof betterresistancetocrackformationandpropagation(Wangetal.,2016; Gäumannetal.,1999).
Mechanicalproperties Tensileproperties
Forthewelded AA6061-T4aluminiumalloy, thehigher ultimate tensile strengthwas230MPa,compared tothebasematerialtensile strengthof250MPa(Fig.11).However,despitetheoptimisingofweld- ingparameters,thetotalelongationtofractureislow(equaltoorbe- low11%)comparedtothatofthebasematerial.Similarresultswere demonstratedbyWangetal.(2016)duringlaserweldingoftheAA6061- T6aluminiumalloy.Thelaserweldingprocessinducedaductilityde- creaseofupto70%.Asexpected(RamasamyandAlbright,2000,2001), theglobalductilityreductionisduetostrainlocalisationinthelower
Fig.9. Evolutionoftheaxialzonewidthasafunctionoftheweldingparameters(a)powerdensity,(b)energydensity,and(c)travelweldingspeed.
Fig.10. (a)Secondarydendritearmspacingmeasurementsand(b)itsvariationasafunctionoftheenergydensity.
strengthFZ.InFig.11b,thetensiletestspecimenfracturezoneoccurs attheweldbeadandpreciselyintheFZ.Fig.12showsthevariationin tensilepropertiesasafunctionoftheFZwidth.Theweldbeadductility andstrengthdecreaseastheweldbeadwidthincreases.
Forthedifferentweldingparameters,Fig.13showsthattheultimate tensilestrengthandtotalelongationtofractureincreaseastheaxialzone increases,aspreviouslyreported(Wangetal.,2016;Gäumannetal., 1999).
Cross-sectionalopticalmicrographsandtensilepropertiesareshown in Figs. 14 and15, respectively. Minor strengtheningisobserved in sampleA.Despitethisgeometricalfeature,themechanicalbehaviour issimilaramongthesamples.SampleDhadgreatertensileproperties thansampleA.Afterpolishing(Fig.14(b)),theultimatetensilestressof
sampleAwassignificantlygreaterthanthatofsampleD.Alocalstrain fieldanalysiswouldhelpunderstandthebehaviouroftheseseams.
Axialstrainmapsaregivenfordifferentaverageaxialstrainvaluesin Fig.16.StrainislocalisedintheFZanditssurroundingsinbothpolished andunpolishedtensiletestspecimensbecauseofthelowerstrengthex- hibitedintheFZ.ThestrainismorehomogeneousinsampleDFZ.This differencecanbeexplainedbythegrainmorphologyintheirFZ,but geometricalfeaturescanalsocontributetothisbehaviour.Afterpolish- ing,theglobalmechanicalbehaviourofsampleAweldbeadimproves andis betterthanthatof sampleD.Itis importanttonotethatthe mechanicalpropertiesofweldjointsarenotonlycontrolledbythemi- crostructurebutalsobyweldingdefectssuchasgeometricalfeatures.
Theinitialdefect(reinforcement)observedontheunpolishedsampleA
Fig.11. (a)Stress–straincurvesofweldedsamplesandbasematerialand(b)fracturezone.
Fig.12. Variationoftheoptimizedsamplestensilepropertiesasafunctionof theFZwidth.
generatedstrainlocalisationintheHAZ.Polishingthesampleremoves thislocalisationandallowsacontinuousstraindistribution.
AsobservedinFigs.5and6,thereisaspecificgrainstructureinthe centreoftheFZwhichissurroundedbythecolumnargrains.Heteroge- neousnucleationcanfavourthehomogenouslocalelongationobserved insampleD.Otherwise,insampleAFZ,theweldcentreiscomposedof
asetofcolumnargrainsnearlyparalleltotheweldingdirection.Such anisotropiccolumnarstructuresarereportedtobeverysusceptibleto tearing(Savage,1976;Gäumannetal.,1999).ThisindicatesthattheFZ dendriticstructuresareweakercomparedtothebasematerial,equiaxed microstructure.Dependingontheweldingparameters,priortopolish- ing,theweldbeadofsampleDproducedwithalowerpowerandenergy densitydisplayedbetterlocalmechanicalbehaviourthansampleA.The improvementinthebehaviourofsampleAisaresultoftheextrathick- nessremovalduringthepolishingprocess.Inlaserweldingjoints,ithas beenreportedthattheirregulargeometryinducestheinitialpositionof thefractureandconsequentlyreducestheweldbeadtensileproperties (Wangetal.,2016).Stressconcentrationsareobserved ininhomoge- neousjointsorimperfectFZgeometry(Zhaoetal.,1999).
Hardnessproperties
Nanohardnessprofilesweremeasuredfromthreedifferentlocations alongthetransversedirection,asshowninFig.17.Theresultsshowthat regardlessoftheweldingconditions,theminimumhardnesswaslocated intheFZ(approximately1GPa).Inaddition,theHAZhardness was equaltothatofthebasematerial(about1.2GPa)(Fig.18).Asexpected (Sánchez-Amayaetal.,2009;PaleocrassasandTu,2007),thematerial hardnessdecreasesafterlaserwelding.Bycontrast,thisworkindicates thattheheat-affectedHAZzonenanohardnessisequivalenttothatof thebasemetalnanohardness,asreportedbyWangetal.(2016).This decreaseinhardnesswithintheFZisaresultofthedissolutionofthe
Fig.13. Evolutionofthetensilepropertiesasafunctionoftheaxialzonewidth:(a)ultimatetensilestrengthand(b)totalelongationtofracture.
Fig.14. Opticalmicrographs:(a)SampleAwithextrathicknessand(b)SampleD.
Fig.15. Stress–straincurvesofsamplesAandD:(a)unpolishedsamplesand(b)polishedsamples.
Fig.16. ComparisonofthestrainfieldsmeasuredonAandDspecimensatdifferentaverageglobalelongations(a)unpolishedsamplesand(b)polishedsamples.
Fig.17. Opticalmicrographshowingindentationlines.
hardeningprecipitatesandtheincreaseindendritesizewithincreasing energydensity.Consequently,thisexplainsthecauseofthereduction intheultimatetensilestrengthincreasingwithincreaseinthewidthof theweldsinrelationtotheinitiallengthofthespecimen.
Conclusion
SoundweldbeadswereobtainedwithaYb:YAGlaserbeamforthe AA6061T4alloy.Awiderangeofprocessparametersettingswereuti- lized.Theweldbeadsizeandrippleshapedependedonprocessparam- eterssuchaspowerdensity,energydensity,andweldtravelspeed.The latterdefinedtheweldmoltenpoolshapeandsolidificationspeedlevel.
Therefore,grainswithdifferentsizesandorientationswereformedin theFZ.Theellipticalmoltenpoolisevidenceofaof2mmin−1welding travelspeed,whichpromotedaxialgraingrowthonarelativelynarrow
zoneandlimitednucleation.Moreelongatedweldmoltenpoolswere observedforhigherweldingtravelspeeds.Inthiscase,V-shapedrip- pleswereobservedandthecalculatedsolidificationspeedwasgreater.
Themicrostructuresoftheseseamspresentagerminationofgrainsin thecentralzoneoftheFZ,whichbecomesstrongerastheenergydensity decreases.Thesecondarydendritearmspacingmeasurementsremained consistentwiththesestatements.Themechanicalpropertiesofthesam- plesweretestedbytensiletests.Basedonstrainfieldmeasurements,the higheststrainlevelwasobservedintheweldbeadFZ,andfracturesap- pearedinthiszone.Thesecondarydendritearmspacingdidnotappear tohaveasignificantroleinthemechanicalpropertiesoftheFZ.Indeed, nanohardnessmeasurementsintheFZindicatedasignificantreduction comparedtothatofthebasemetalandHAZ.However,theresultswere similarforallweldingconditions.Thehighermechanicalpropertiesob- servedforlowenergydensitysamplesarecreditedtobothreductionsin theFZwidthandcentralgerminationphenomenon.Afollowingstudy willinvestigatetheeffectofapost-weldT6heattreatmentontheme- chanicalbehaviourofthedifferentmicrostructurescontainedintheFZ andHAZ.
DeclarationofCompetingInterest
Theauthorsdeclarethattheyhavenoknowncompetingfinancial interestsorpersonalrelationshipsthatcouldhaveappearedtoinfluence theworkreportedinthispaper.
Acknowledgements
The authors would like to acknowledge Jade Pécune, Nathalie Aubazac,andJeanDenisBéguinfortheirtechnicalsupport.
Fig.18. Nanohardnessprofilesoftheweldjointcross-section(a)A,(b)B,(c)C,and(d)Dsamples.
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