Duncan
P.
Hand
,
Krystian
L.
Wlodarczyk
aWeldingEngineeringandLaserProcessingCentre,CranfieldUniversity,Cranfield,Bedfordshire,MK430AL,UnitedKingdom
bInstituteofPhotonicsandQuantumSciences,SchoolofEngineeringandPhysicalSciences,Heriot-WattUniversity,Edinburgh,EH114AS,UnitedKingdom cSPILasersUKLtd,CosfordLane,SwiftValleyIndustrialEstate,Rugby,CV211QN,UnitedKingdom
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Articlehistory: Received9May2016 Receivedinrevisedform 23September2016 Accepted31October2016 Availableonline2November2016 Keywords:
Laserprocessing Lasertexturing Laserspotwelding Surfacemodification Dissimilarmetals
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Laserweldingofdissimilarmetals(steelandaluminium)wasinvestigatedwiththeaimtoincreasethe maximumtensileshearloadoftheFe-Aljoints.Theincreasewasachievedbytexturingthesurfaceof steelpriortothelaserspotweldingprocesswhichwasperformedinalap-jointconfigurationwiththe steelpositionedontopofthealuminiumandwithatexturefaceddowntothealuminiumsurface.This configurationenabledanincreaseofthebondingareaofthejoints,becausethemoltenaluminiumfilled inthegapsofthetexture,withouttheneedofincreasingtheprocessenergywhichtypicallyleadsto thegrowthoftheintermetalliccompounds.Differenttextures(containinghexagonallyarrangedcraters, parallellines,gridandspiralpatterns)weretestedwithdifferentlaserweldingparameters.TheFe-Al jointsobtainedwiththetexturedsteelwerefoundtohaveupto25%highermaximumtensile-shearload thanthejointsobtainedwiththeuntexturedsteel.
©2016TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
Oneofthechallengesintheautomotiveindustryistoreducethe carbonemissionofmanufacturedvehicles(Church,2015;Havrilla, 2014;Lesemannetal.,2008).Thiscanbedonebyincorporating lightweightmetallicalloys,suchasaluminiumalloys,withmore genericautomotivemodulusoftenmadeofuncoatedmildsteelin ordertoreducethetotalmassofavehicle.Unfortunately,joining thesetwodissimilarmetalsisdifficultduetothechemical reac-tionsbetweenaluminiumandsteelthatleadtotheformationof intermetalliccompounds(IMC)atthemetals’interfaceduringthe weldingprocessandthusthemechanicalpropertiesofthewelds aredeteriorated(Martinsenetal.,2015).
Athinsheetofsteel(DC04grade)canbesuccessfullyweldedto analuminiumalloy(6111-T4)usingaconductionmodelaserspot weldingprocess(Pardaletal.,2014).Thisnovelweldingtechnique reliesontheincidenceofadefocusedlaserbeamonthetopsurface ofsteelandthegenerationofheatresultantfromthelaser-material interactionthatisconductedtothealuminiumthroughthesteel. Inthisprocesstheweldingparameters needtobecontrolledin ordertomelt onlythealuminiumat theinterface.Thebacking
∗Correspondingauthor.
E-mailaddress:g.n.rodriguespardal@cranfield.ac.uk(G.Pardal).
barunderneaththeAlalloyisusedtofacilitatetheheat extrac-tionandminimisetheIMCgrowth(Borrisutthekuletal.,2007).This techniquediffersfromregularlaserweldingbecausehereonly alu-miniumismeltedwhilststeelremainsinsolidstate.Althoughthis approachhelpstominimisethemixingofAlwithFe,duetoa max-imumsolubilityof12%AlinFe,athinIMClayerisstillproduced whichleadstoareductioninweldstrength.Asreportedby(Pardal etal.,2014),thethicknessoftheIMClayerdependsonthe temper-atureprofilegeneratedbythelaserspotandtheheatdistribution acrosstheinterface.However,thethermalfieldisalso responsi-bleforgeometryofthefusionzoneofthealuminiumandthusthe dimensionofthebondingarea.Previousresultsreportedby(Meco etal.,2015)suggestthatwhenalargerbondingareaiscreatedthe jointshavehighermaximumshearload.Onewouldexpectalinear evolutionofthetensile-shearloadofdissimilarmetaljointswith thebondingarea.However,thistrendisnotobservedindissimilar metaljoiningbecausetheIMClayerbecomesthicker simultane-ouslywiththegrowthofthebondingarea.
Resistancespotwelding(RSW)canalsobeusedforjoiningthin sheetsofsteeltoanaluminiumalloy,asdemonstratedby(Zhang et al.,2011)and (Qiu et al.,2009a,b).Thefirst ofthese papers reportedweldingofa1mmthickgalvanizedhighstrengthsteelto 1.5mmthick6008aluminiumalloyandobtainedtheFe-Aljoints withthemaximumtensile-shearloadof3.3kN.Bycorrelatingthe mechanicalstrengthofthejointswiththeprocessparameters,it http://dx.doi.org/10.1016/j.jmatprotec.2016.10.025
Table1
Chemicalcompositioninweight%ofAl5083andCR4steelaccordingtothestandardsBSEN573-3:2009andEN10130-2006,respectively.
Element Al Fe Mn Ti Mg Cu Si Zn Cr C P Other
Al5083 Bal. 0.4max 0.4–1.0 0.05–0.25 4.0–4.9 0.1max 0.0–0.4 0.0–0.1 0.05–0.25 – – 0.0–0.15
CR4 – Bal. 0.6max – – – – – – 0.12max 0.045max
wasfoundthatthetensile-shearloadofthejointsincreasedwith theincreasingweldingcurrentandtime.Detailedexaminationof thejointsprovidedevidencethattheincreaseoftheprocess param-etersleadstotheincreaseoftheweldnuggetdiameteraswellasto thegrowthofIMC.Thesecondofthesepapers(Qiuetal.,2009a,b) describeduseoftheRSWprocesswithacoverplatetojoin1mm thickSPCCsteelto1mmthick5052aluminiumalloy.This weld-ingapproachenabledthegenerationoftheFe-Aljointswiththe maximumtensile-shearloadofapproximately5kN.Althoughthe authorsconfirmedthatthetensile-shearloadincreaseswiththe increasingweldnuggetdiameter,theyalsoprovidedevidencethat thepresenceoftheIMClayerreducesthemaximumloadofthe dissimilarFe-AljointscomparedtotheAl–Aljoints.Undersimilar weldingconditions,themaximumtensile-shearloadoftheAl–Al jointswasabout6kN,andhencea1kNhigherthanthatobtained withtheFe-Aljoints.Theuseofmetallicinsertswasalso investi-gatedinRSWforjoining0.8mmthicklowcarbonsteelto1mm thick5XXXseriesaluminiumalloy(OikawaandSaitoh,1999).The insertmetalsheetswerealuminiumcladsteelsheetsmanufactured byhotrollingwithadirectresistanceheatingprocess.Theseinserts werepositionedbetweenthesteelandthealuminiumsheetswith thesteelsideoftheinsertfacingthesteelandthealuminiumside oftheinsertfacingthealuminiumsheet.Thismetallic combina-tionwasfoundtobestrongerthanthatwithoutthemetallicinsert (3.3kNcomparedto2kN).
Analternativeapproachtojointhinsheetsofgalvanizedsteel to6061aluminiumalloybyspotweldinghasbeeninvestigated usingavariantofarcweldingprocesscalledColdMetalTransfer, alsoknownasCMT(Caoetal.,2014).Inthisstudy,4043aluminium wirewasusedtocreateaspotplugbyfillinginaholepreviously machinedonthemetallicsheetwhichwaspositionedonthetopof thelap-joint.Thematerialstackingconfigurationandwelding vari-ableswereinvestigatedintermsofmicrostructureandmechanical strength.Theauthorsfoundthatwhenthesteelwaspositionedon thetoptherewasnopresenceofIMCsinthemiddleonthe sur-faceofthefracturedjoint.Ontheotherhand,whenaluminium waspositionedonthetop,themoltenaluminiumwasdeposited ontothesteelsurfaceandthereactionbetweenFeandAloccurred, formingFeAl3andFe2Al5atthejointinterface.Thefractureload
ofthejointswiththealuminiumonthetopwasabout5kNwhich washigherthanwhenthesteelwasonthetoporevenwhenthe jointwasproducedbetweenthecouponsofaluminium(3kN).The reasonfortheseresultswasthenuggetdiameterthatinthe steel-aluminiumjointwassmallerthanthatinthealuminium-steeljoint configuration.
Amodifiedfrictionstirspotwelding(FSSW),calledabrasion cir-cleFSSWwasusedby(Chenetal.,2012)tojoin6111-T4aluminium alloytoDC04lowcarbonsteel.TheauthorsshowedthatIMCswere notformedonthejointandthemaximumtensile-shearload regis-teredwasnear4kN.TheabrasioncircleFSSWenabledtheincrease ofthebondingareabytranslatingthetoolthroughanorbitalpath. Inthispaper,analternativespotweldingapproachtoincrease thebondingareabetweensteelandaluminiumalloywithout mod-ifying thelaser-generated temperature profile is proposed and investigated.Ananosecondpulsedlaserisusedfortexturingthe surface of steel prior to laser spot welding in order to locally increasethebondingareabetweenthesetwo dissimilarmetals. Theaimofthisworkistoinvestigatetheimpactofvarious laser-generatedtexturesonthemechanicalproperties(strength)ofthe
Table2
MechanicalstrengthofAl5083andCR4steel. Ultimatetensile
strength,MPa
Yieldstrength,MPa Hardness,HV
Al5083 275–350 125min 87
CR4 270–410 280max 105
Fig1. Schematicofthelasersystemusedfortexturingthesurfaceofsteel.
Fe-Alweldsaswellastounderstandthemechanismoftheir for-mation.
2. Experimentalprocedure
2.1. Material
A 1mm thick 5083 aluminium alloy and 0.85mm thick CR4 grade mild steel were used in theexperiments. The steel and aluminium coupons were 100mm long and 40mm wide. Tables1and2showthechemicalcompositionandthemechanical propertiesofthesetwometals.
2.2. Lasertexturingofsteel
2.2.1. Methodologyandexperimentalsetup
Fig.1showsaschematicofthelasermachiningsystemusedfor texturingthesteelcoupons.ThelasersourcewasapulsedSPI20W fibrelaser(G4EP-S)thatprovided220nslongpulses(fullwidth10% maximum)withapulserepetitionfrequencyof28kHz,wavelength of1064nmandmaximumpulseenergyof0.71mJ.Thelowcost, highflexibilityandacceptableabsorptionofthiswavelengthmake
Fig.2.Steelcouponswiththelaser-generatedtexturescontaining30mdeep par-alleltroughs(left)anda30mdeepspiraltrough(right).
thistypeoflaserwellsuitedfortexturingthesurfaceofsteel(Dunn etal.,2015,2014;Vilhenaetal.,2009).Thelaserbeamwasdelivered totheworkpieceviaagalvoscannerequippedwitha160mmfocal lengthF-thetalens.ThegalvoscannerwascontrolledbyaPCusing SCAPSSamlightsoftware.Thenominalspotdiameteronthework piecewas22m(1/e2ofitsmaximumintensity,M2∼1.1).
Priortolasertexturing,bothsurfacesofthesteelcouponswere cleanedwithacetonetoremoveoilanddebris.Later,theworkpiece wasplacedinametalvessel(enclosure)containingaglasswindow. Thisvesselallowedthesteelcouponstobemachinedunder low-pressureargongasinordertoavoidoxidationofthesteel,which couldhaveanimpactonthequalityofwelds.
Priorto texturinga largequantityof thesteel coupons,the laser-inducedcraterswerecharacterizedforvariouslaser process-ingsettings,inparticularpulseenergy(EP)andnumberoflaser
pulses(N). Thediameterand depthof thesecraterswere mea-suredusing an opticalmicroscope (Leica DM6000M) and a 3D surfaceprofilometer(InfiniteFocus®Alicona).Followingthe cali-brationprocess,sevendifferenttesttexturesweregeneratedon thesurfaceofthesteelcoupons.Theareaselectedfortexturing waseithera40mmby40mmsquareora40mmdiametercircle, asshowninFig.2,dependingonthetexturepattern.Eachtexture wasproducedonatleast12coupons,providingarelativelylarge numberofsamplesfortestingvarioussettingsforlaserspot weld-ing,inparticularaveragepower(PA)andinteraction(irradiation)
time.
2.2.2. Calibration
IndividualcratersweregeneratedonthesurfaceofCR4mild steelbyvaryingthepulseenergy(EP)between0.04mJand0.71mJ
andvaryingthenumberoflaserpulses(N)perspotbetween1and 10,byrepeatedlypassingthelaserbeamovertheworkpiece.The depthofcratersobtainedforvariousvaluesofEPandNisshownin Fig.3.Theerrorbarsplottedonthesegraphsarethestandard devi-ationsofthreeseparatemeasurementsforthecratersgenerated withtheconstantpulseenergyandnumberoflaserpulses.
Fig.3a)revealsthatthedepthofthecratersincreases signifi-cantlyandalmostlinearlywithincreasingnumberoflaserpulses. Thismeansthat each laser pulse(with energy above the abla-tionthreshold,0.04mJ)removesapproximatelythesamedepthof materialfromthelaser-irradiatedarea.Fig.3b,inturn,showsthat thedepthofthecratersincreasesinsignificantlywithincreasing pulseenergy,inparticularforalownumberoflaserpulses.
Thediameterofthecraterswasalsomeasuredandtheresults arepresentedinFig.4.Theyshowthecraterdiameterisalmost unchangedfortheincreasingnumberoflaserpulses,inparticular
Fig.3.Depthofcratersasafunctionof:a)numberoflaserpulsesfordifferentpulse energies,b)pulseenergyfordifferentnumberoflaserpulses.Solidlinesaretrend lines.
Fig.4.Diameterofcratersasafunctionofnumberoflaserpulsesfordifferentpulse energies.Solidlinesaretrendlines.
Table3
Listoflaserprocessingparametersusedforthegenerationofvarioustexturesonthesurfaceofthesteelcoupons.
Texture Pattern EP,mJ Numberofpulses v,mm/s Processingrate,s/cm−2 D,m ,m SD,m
A Hexagonal 0.55 10 2240 9.5 30 65 80 B Hexagonal 0.55 10 3500 4.6 30 65 125 C Hexagonal 0.17 12 3500 5.5 30 40 125 D Hexagonal 0.17 12 1400 25.7 30 40 50 E Lines 0.17 5 350 30.6 30 40 50 F Spiral 0.17 5 350 22.2 30 40 50 G Grid 0.17 5 350 61.2 30 40 50
forEP<0.21mJ.Forhigherpulseenergies,itwasobservedthatthe
craterdiameterincreasesatarateof<1.5mperpulse.
Ingeneral,theresultspresentedinFigs.3and4seemto fol-lowthesimilartrendsthatwereobservedintheothertypesof steel(100Cr6andgrade304)byDunnetal.(Dunnetal.,2014)and Vilhenaetal.(Vilhenaetal.,2009).
2.2.3. Generationoftesttexturesonsteel
Theresultspresented in theprevioussection(Section2.2.2) showclearlythatitispossibletoalmostindependentlycontrolboth thedepthandthediameterofthelaser-inducedcratersonthe sur-faceofgradeCR4mildsteel.Byusingthecorrectpulseenergyand thenumberoflaserpulses,thedimensionofthecraterscanhence betailoredandthereforevarioustexturescanbegenerated.
Sevendifferenttexturesweregeneratedonthesurfaceofthe steelcoupons.Threedimensionalsurfaceprofilesofthesetextures measuredwiththeAliconainstrumentareshowninFig.5.Four ofthemcomprisedofcratersthatwerearrangedinahexagonal pattern(seeTexturesA–D),eachtexturehavingcraterswitha nom-inaldepth(D)of30m(±5m),diameter()ofeither40mor 65m,andseparationdistance(SD)betweenthecratersofeither
50m,80mor125m.Theotherthreetexturescontainedeither paralleltroughs(seeTextureE),aspiraltrough(seeTextureF)or agridoftroughs(seeTextureG).Thetroughsinallthreetextures wereapproximately30mdeep,40mwideandwereseparated byafixeddistanceof50m.
Table3providesalistoflaserprocessingparameters(i.e.pulse energy,numberoflaserpulses,andscanspeed)thatwereusedfor thegenerationofthetexturesshowninFig.5.TexturesA-D (con-tainingcratersarrangedinahexagonalpattern)weregenerated followingthetexturingproceduredescribedin(Dunnetal.,2014). Variousseparationdistancesbetweenthecraterswereobtained bymodifyingthescanspeed(v)andhatchdistance(h).Thehatch distancewascalculatedusingthefollowingformula:
h= √ 3 2 ×
v
PRF (1)wherePRFisthepulserepetitionfrequency.
ThetexturesE-G(thosecontainingtroughs)weregeneratedby movingthelaserbeamwithaspeedof350mm/s.Sincethepulse overlapatthisspeedwasapproximately57%,whichcorresponds toslightlymorethantwopulsesperlaserspot,itwasnecessaryto apply5laserpassesinordertoobtainapproximately30mdeep troughs(basedontheresultsinFig.4).
2.3. Laserweldingoftexturedsteeltoaluminium 2.3.1. Methodologyandexperimentalsetup
Allthesurfacesofthealuminiumandtheuntexturedsurfaceof thesteelcouponswerecleanedbeforeweldingwithanabrasive padandacetonetoremovecontaminants.Thetexturedsurfaceof thesteelwasleftasprocessed.
Amulti-modeIPGCWfibrelaserwithamaximumpowerof 8kW and a wavelength of 1070nm wasused for welding the
Table4
LaserweldingparametersusedforTexturesA-D(preliminarytests).
Dbeam,mm P,kW ti,s PD,MW/m2 Esp,kJ 13.00 2.00 3.0 15.1 6.0 13.00 2.00 4.0 15.1 8.0 13.00 2.00 5.0 15.1 10.0 13.00 2.00 6.0 15.1 12.0 13.00 2.67 3.0 20.1 8.0 13.00 3.67 3.0 27.6 11.0 13.00 4.00 3.0 30.1 12.0
laser-texturedsteeltothealuminium.Thewelding processwas performed using a 13mm diameter laser beam that was defo-cusedontheuntexturedsideofthesteel,asshowninFig.6a.The laser-texturedsurfaceofthesteelwasfaceddown,beinginavery closecontactwiththealuminiumcoupon.Thesteelandaluminium couponswerepositionedontheclampingsysteminalap-joint con-figurationwithanoverlapof35mm(Fig.6b)andclamped(Fig.6c). Theringonthesteelapplieduniformloadonthesubstratearound theweldandithadthreeoutletstoshieldtheweldwithargon.
Thelaser powerandirradiationtime wereusedasvariables for welding the laser-textured steelsamples to thealuminium coupons.Theanalysisoftheweldingresults,however,wasmade usingthefundamentallasermaterialinteractionparameters,i.e. thepower density (PD),interaction time (ti) and specificpoint
energy(Esp),becausetheseparameterscanbeusedtotransferthe
weldingprocessbetweenvariouslasersystems(Assuncaoetal., 2012;WilliamsandSuder,2011).ThePD,tiandEspvalueswere
calculatedbasedonthesystemparameters,usingthefollowing equations:
Powerdensity,MW/m2 PD= Power
Abeam
(2) Interactiontime,s ti=irradiationtime (3) Specialpointenergy, kj Esp=PD×ti×Abeam (4)
Thespecificpointenergycorrespondstotheenergytransferred tothematerialthroughthelaserspotanditiscalculatedbythe productofthepowerdensitybytheinteractiontimeandtheareaof thelaserbeam(Abeam).Theinteractionofthelaserwiththematerial
inducesathermalfieldthatresultsinpartialmeltingofthesteel. Astheheatconductsthroughthesteelandistransferredtothe aluminium,thetemperatureatthejointinterfaceincreasesand meltingofthealuminiumoccurs.Theenergyoftheweldingprocess mustbecontrolledtoavoidthemeltingofthefullthicknessofthe steelsheet.Thisprocessenablesthewettingofthesteelsurfaceby themoltenaluminiumreducingthediffusionofFeinAl.
Thelaserweldingparametersthatwereusedinour prelimi-naryandfinal weldingexperimentsarelistedinTables4and5, respectively.
2.3.2. Tensilesheartest
The mechanical tensile-shear tests were performed using a 100kNINSTRON5500Rtensiletestmachine.Theexperimentswere
Fig.5.TesttexturesgeneratedonthesurfaceofgradeCR4mildsteel.TexturesA–Dcontaincratersarrangedinahexagonalpattern,textureEcontainsparalleltroughs, textureFcontainsaspiraltrough,andtextureGcontainsagridoftroughs.ThesurfaceprofileswereobtainedwiththeAliconainstrument.
Fig.6.a)Schematicofthelaserspotweldingprocess,b)steelandAlsampledimensionsandexperimentaloverlapandc)photographoftheclampingsystemusedfor weldingsteeltoaluminium.
carriedoutwithconstantcross-headspeedof1mm/minandat roomtemperature.Theloadanddisplacementwereacquiredby
aNationalInstrumentssystemconnectedtoalaserextensometer withagaugelengthof50mm.
Fig.7.TensileshearloadobtainedwithTexturesA–Dasafunctionof:a)interactiontime(ti),whilePD=15.1MW/m2andb)powerdensity(PD),whileti=3s.
Table5
LaserweldingparametersusedforTexturesD-F(finaltests).
Dbeam,mm P,kW ti,s PD,MW/m2 Esp,kJ 13.00 2.67 3.00 20.1 8.0 13.00 2.67 4.00 20.1 10.7 13.00 2.67 5.00 20.1 13.4 13.00 2.25 3.00 17.0 6.8 13.00 3.00 3.00 22.7 9.0 Table6
RelativecontactareaattheFe-AlinterfaceestimatedforTexturesA-D.
Texture A B C D
Relativecontactarea(S) 42% 73% 90% 44%
3. Resultsanddiscussion
3.1. Preliminarytestsforhexagonalgeometry
Apreliminarystudywascarriedouttodeterminethebest tex-tureforlaserspotweldingintermsofcraterdepth,diameterand spacingfordifferentweldingparameters.Thelaserwelding param-eterslistedinTable4wereusedandtheexperimentalresultsare showninFig.7.Inthisexperimentnocomparisonwasmadewith theuntexturedsamples.
Fig.7a)showsthatTexturesAandDalwaysprovidedhigher tensile-shearloadof the joints (>4kN)than TexturesB and C, independentlyontheinteractiontime(ti).Poorermechanical
per-formanceofthejoints withTexturesBandCmayberelatedto thelargespacingbetweencraters,whichinthesetwotextureswas 125mcomparedto80and50mforTexturesAandD, respec-tively.Tobetterillustratethis,therelativecontactarea(S)ofeach texturewascalculatedandtheresultsareinTable6.TheSvalues werecalculatedasfollows:
S=ContactAreaTotalArea .100% (5)
whereContactAreaisthewhiteareashowninFig.8(expressed in mm2 units), whereas Total Area is equal to 0.25mm2
(500m×500m).
Alargespacingbetweencraters(asinTexturesBandC) repre-sentsasmallersurfaceareaavailableonthesteelforthemolten aluminiumtoflowandcreatethebondingbetweenbothmetals. TheresultsinFig.8suggestthattheinteractiontime(between3 and6s)doesnotsignificantlyaffectthemaximumtensileshear loadofthejoints.Therewasonlyonepoint(obtainedwith Tex-tureBandti=6s)whichprovidedatensileshearloadof<3kN.
Ingeneral,theresultsindicatethatatexturewithsmallerspacing
Table7
RelativecontactareaattheFe-AlinterfaceestimatedforTexturesE-G. TextureE(Lines) TextureF(Spiral) TextureG(Grid)
Relativecontactarea(S) 20% 23% 4%
betweencratersandsmallercontactareashouldbeusedtoimprove maximumshearloadofthejoint.
Asimilartrendwasobservedforthejointsproducedwiththe differentpowerdensity(Fig.7b).Foranyvalueofpowerdensity, TexturesBandCseemtohavelowermaximumtensileshearload whencomparedtoTexturesAandD.Thereisalsoadifferencein themechanicalperformanceofthejointsproducedwithTextures AandD.ThejointsproducedwithTextureDweretheonlyonesin whichthetensileshearloaddidnotdeterioratewiththeincreaseof thepowerdensity.Alltheothertextureshadconsiderablyreduced thetensileshearloadofthejointsforpowerdensityvaluesabove 25MW/m2.Thisshowsthatnotonlythespacingbetweencraters
isimportantbutalsothecraterdiameterneedstobecontrolledto obtainthemaximumtensileshearloadforawiderangeoflaser weldingparameters.
3.2. Resultsforseveralgeometricpatterns
Inthepresentsection,thecharacteristicsofTextureDwereused forthegenerationofdifferentpatternsonthesteelsurface,viz.grid, lines,andspiral.Theperformanceofthenewtextureswastested fordifferentlevelsofthespecificpointenergybyvaryingthepower densityandtheinteractiontime.Theaimwastoinvestigatehow thetextureswithdifferentgeometricalpatternsaffectthe mechan-icalstrengthandthewettingareaofthejoints,andcomparethese featuresagainsttheuntexturedsamples.
Thecontactareabetweenthesteelandthealuminiumvaries withthepatternduetotheirdifferentgeometricaldesigns.Fig.9 illustrateshowthelaser-generatedpatternsaffectthecontactarea atthe metalsinterface,whereas Table7 providesthevalues of therelativecontactareathatwereestimatedforthesethree tex-tures.Thetexturewiththesmallestcontactareaisthegridpattern (S=4%).Thelineand spiralpatternswerefoundtohavesimilar contactareas(S=20%).
Thetensileshearloadofthejointsproducedwiththenew geo-metricpatternswasevaluatedagainsttheuntexturedsamples(see Fig.10).Thejointswereproducedwithconstantpowerdensityof 20.1MW/m2andvariableinteractiontime.Fig.10showsthatthe
spiraltexturedidnotgenerategoodjointswhentheinteraction time(ti)was4s.Thejointgeneratedwithti=4shadatensileshear
Fig.8.SchematicofTexturesfromAtoD.Whitearearepresentsthecontactareabetweenthealuminiumandthesteelsurface,whereasbluecirclesrepresentcraters.(For interpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.).
Fig.9.Schematicoflaser-generatedtexturescomprisingparalleltroughs(TextureE),spiralpattern(TextureF)andagridoftroughs(TextureG).Whitearearepresentsthe contactareabetweenthealuminiumandthesteelsurface,whereasbluearearepresentthelaser-texturedarea.(Forinterpretationofthereferencestocolourinthisfigure legend,thereaderisreferredtothewebversionofthisarticle.).
Fig.10.Tensileshearloadasafunctionoftheinteractiontimefordifferent laser-texturedanduntexturedsamples.TheresultswereobtainedwithPD=20.1MW/m2.
Withintheexperimentalrange,thetensileshearloadofthe jointswasnearlyconstantwithinteractiontime.Thistrendwas observedforallthetestedsamples,withtheexceptionofthespiral. Whencomparingtotheuntexturedjoints,whichhadanaverage tensileshearloadof4kN,thejointsproducedwiththetextures containinglines,gridandhexagonally-arrangedcratersshoweda highertensileshearload,withanaveragevalueoftensile-shear loadof5kN.Thejointwiththehighesttensileshearloadof>5.5kN wasobtainedwiththetextureGcontainingthegridpatternusing PD=21.1MW/m2andt
i=5s.Incontrast,thejointwiththelowest
tensileshearload(<1kN)wasobtainedwiththetexture contain-ingthespiralpatternusingtheinteractiontimeof4s.Thislow tensileshearloadisaconsequenceoftheplugfailure.Sucha fail-ureoccurstypicallyduetothestrongreactionbetweenFeandAl whichleadstothemixingofthesetwometalsintheliquidstate duringtheweldingprocess.Theplugfailureobservedinthis sam-pleischaracteristicofabrittleweldwheretheweldnuggetismade ofbrittleIMCphases.ThehighdiffusionofFeintoAlandvice-versa, whenbothmetalsaremixedinliquidstate,enhancestheformation ofseveralIMCsatthejointinterface,deterioratingthemechanical propertiesofthejoints.
Thenearly constantevolution of thetensileshear loadwith theinteractiontimeforallthetexturedanduntexturedsamples indicatesthattheinteractiontimemaynotbethemostimportant fundamentallasermaterialinteractionparameterindetermining themechanical propertiesofthejoints.Themechanical proper-tiesofthejointsaredependentonthebondingareaandtheIMCs formedduringtheweldingprocess.Thefactthatwithinthe exper-imentalrangetheinteractiontimedoesnotchangethemaximum tensileshearloadofthejointscouldbeeitherduetothe simultane-ousgrowthoftheIMCsandthebondingareawhichinthiscaseare bothinbalance,orbecausetheinteractiontimehasasmalleffect onthegrowthoftheIMCandbondingarea.
Fig. 11 shows the impact of power density on the tensile shearloadofthejointsforconstantinteractiontimeof3s.With 17MW/m2 ofpowerdensity,allsampleswithtexturedsurfaces
haveconsiderablybettermechanicalpropertiesthanthe untex-turedsamples.Onceagaintheresultsshowthatthepatternsarean effectivewayofimprovingthemechanicalpropertiesofthelaser spotweldedjointswithoutincreasingtheprocessenergy.Asthe powerdensityincreasedto20MW/m2,allofthetexturedsamples
stillshowedhighertensileshearloadthantheuntexturedsamples, albeitwitha lowerdifference.Forthemaximumpowerdensity tested,thetrendwasnotverified.Twotextures,thosecontaining thegridandlines,showedlowertensileshearloadwhencompared withtheuntexturedsamples.Thisresultmaybejustifiedbythe poorheatconductionfromFetoAl,inparticularforthegridpattern
Fig.11. Tensileshearloadasafunctionofthepowerdensityfordifferent laser-texturedanduntexturedsamples.Theresultswereobtainedwithti=3s.
wheretherelativecontactareawasverysmall(S=4%).Ingeneral, thepresenceofcraters(airgaps)inducesathermalresistanceatthe interfacebetweenthemetals,causinganincreaseoftemperature andconsequentlymeltingofthesteel.Thereforethetemperatureof thealuminiumisnotsignificantlyincreased,andthewettingarea remainsalmostunchanged.
The joints produced withthe spiral and hexagonal patterns showedhighertensileshearloadathigherpowerdensitiesthanthe otherpatterns.Thisresultmaybeexplainedbythelargercontact areapromotedbythesetwolastpatternswhichallowshigherheat conductionandmoremeltingofAlandthereforebetterwettingof thesteelbymoltenAl.
The textures processed on the steel not only changed the thermal field acrossthesamples,but alsochanged thewetting behaviourandhencethebondingareabetweentheAlandsteel. Intheexperimentthebondingarea,whichisalsoknownasthe areaoftheweldnugget,wasdeterminedbymeasuringthe frac-turesurfacesofthesteelandaluminiumaftertensile-sheartesting. Thefracturesurfacesrevealedtwodistinctregions:adarkcentral regionandabrightperipheralregion.Here,itshouldbenotedthat similarfracturesurfaceswereobservedinresistancespotwelded jointsbetweensteeland5052aluminiumalloybyQiuetal.(Qiu etal.,2009a,b).TheauthorsanalysedtheseregionsbySEMand EDSandtheyfoundthattheperipheralareaofthesteelcontained asignificantamountofAl,whereasthedarkareainthecentrewas identifiedasamixtureofAlandFe.Theresultssuggestedthatthe fracturepropagatedthroughthealuminiumlocatedonthe periph-eralregionandthroughtheIMCslocatedinthecentralareaofthe weld.ThefailurethatoccurredacrosstheIMCsresultedfromthe factthattheIMClayerwasrelativelyuniformandthick,andthus morefragile.Ontheotherhand,theIMCsontheperipheralarea werediscontinuousandthin,andthereforeit wasbelievedthat thefailureoccurredonthealuminium.
Similaranalysisofthefracturesurfaceswasperformedinthe presentedwork.Fig.12showstheopticalmicrographsofselected areasonthesteelsurfaceafterthetensile-sheartest.
The left column of Fig. 12 presentsthe regions outside the weldnuggetswhichwerenotwettedbythemoltenaluminium. Thesemicrographsrevealdifferenttexturesthatweregenerated onthesurfaceofthesteelbeforewelding.Themiddlecolumnof Fig.12presentstheperipheralregionoftheweldnuggets, show-ingtheimpactofthelaser-generatedpatternsontheflowofthe moltenaluminiumduringtheweldingprocess.Thesemicrographs showclearlythatthemoltenaluminiumpenetratesintothe micro-channels, confirming that the laser-generated patterns play an importantroleintheformationofthebondingareabetweenFe
Fig.12.Micrographsshowingthefracturesurfaceonthesteelsideaftertensile-sheartesting.ThesampleswereproducedusingPD=20.1MW/m2andt i=3s.
andAl.Inthecaseofthetexturescontainingthespiralandparallel lines,thedirectionoftheflowofthemoltenaluminiumis con-strainedbytheorientationofthemicro-channels,whilethegrid andhexagonalpatternspermitthemoltenaluminiumtobespread inmorethanonedirection.Thesteelsurfaceoftheuntextured sam-plewaswettedbythemoltenaluminium,buttheflowdirectiondid notfollowanypreferentialdirection.
Thecrosssectionsoftheweldnuggetsareshown inFig.13. ThesefractographsshowtheIMClayerthatwasformedinthe cen-tralregionoftheweldnugget,whichprovesthatthetextureswere successfullywetbythemoltenaluminium,inparticularthose com-prisingthegrid,spiralandhexagonally-arrangedcraters,andthat
thebondingareawasincreased.Thefractographsalsoindicatethat thegrowthoftheIMClayerwasinfluencedbythetexture geome-try.ThiscanbeclearlyobservedinFig.13c)–e)wheretheIMCwas formedinsidethelaser-generatedmicro-channels.Forthe untex-turedsteel,theIMClayerwasthinbutitwasspreadcontinuously acrossthemetalsurface,ascanbeseeninFig.13a).
Thebondingareaofalljointswasmeasuredtopermita quanti-tativeanalysisoftheeffectofthepatternsandweldingparameters ontheweldnugget.Thebondingareawasmeasuredfrom macro-graphstakenontheFesideatfractrographicplane.Thetotalareaof theweldnuggetsresultantfromtheweldingprocesswithvariable
Fig.13.Cross-sectionalfractographyofthejointshowingonlythesteelplatefor:a)untexturedandlaser-texturedsteelwithb)parallellines,c)grid,d)hexagonal-arranged cratersande)spiralpatterns.
Fig.14.Totalbondingarea(central+peripheral)fortheweldedsamplesandplottedfortheincreasing:a)interactiontimeandb)powerdensity.
interactiontimeandpowerdensityispresentedinFig.14a)andb), respectively.
Thetexturedjointshadsimilaroreven alargerweld nugget areathantheuntexturedjointsfortheinteractiontimesof3sand 4s.Fortheinteractiontimeof5s,however,thebondingareaof theuntexturedjointswaslargerthanthatofthetexturedjoints.A similartrendwasobservedinFig.14b.Forpowerdensitiesupto 20MW/m2,allthetexturedjointshadweldnuggetswithequalor
evenlargertotalareasincomparisonwiththeuntexturedjoints. Thisshowsthatforidenticalweldingparameterstheintroduction ofa textureonthesteelsurfacecanimprovethewettabilityof themetal,makingtheweldnuggetslarger.Forpowerdensities of22.7MW/m2,however,thesurfacescontainingthelines,grid
andspiralpatternsinducedasmallerbondingareathanthe untex-turedsurface.Thisexplainsthelowtensileshearloadofthejoints producedwiththegridandlinetextures(seeFig.11).
Fig.15showsthecorrelationbetweenthetensileshearstrength (whichwascalculatedforeachsample)and(a)theinteractiontime (ti)and(b)thepowerdensity(PD).
Withtheincreaseinpowerdensity,allofthetexturedsamples havehighermechanicalpropertiesthantheuntexturedsamples.
Theonlyexceptionswerethejointsobtainedwiththespiraland linespatternsatPD=22.7MW/cm2.
The highest tensile shear strength of approximately 60MPa was obtained with the sample containing the grid pattern at PD=22.7MW/cm2.Thissamplehadamaximumshearloadof2kN,
butthewettingareawasverysmall,duetothehighpowerdensity andtheuncontrolledmixingofbothmetalsduringwelding.Even thoughthisresultedinthehigheststrengthobtainedforallofthe testedsamples,thiswasnotthesamplewiththehighestload.This showsonceagainthenon-linearrelationshipbetweenthetensile strengthandthebondingareawhentheintermetalliccompounds arepresent.
Comparingtheresultsobtainedinthisworkwiththeresults availableintheliterature,onecanobserveanimprovementonthe tensileshearloadoftheFe-Aljoints.Forexample,themaximum tensile-shearloadof5kNwasobtainedbyresistancespotwelding (Qiuetal.,2009a,b),whilsttheresultsreportedinthepresentwork hadamaximumtensile-shearloadof5.6kN.
Basedontheresultspresentedinthis section,it canbe con-cluded that thelaser-generatedpatterns changethewetting of thesteelsurfacebythemoltenaluminium,oftenimprovingthe
mechanicalstrengthofthejoints.Theyalsodeterminethe dimen-sionandqualityofthebondingareabecausethepatternsallow themoltenaluminiumtoflow intomicro-channels onthesteel surface,interlockingthetwometalstogether.Thelaser-generated texturesalsodeterminetheamountofheatthatistransferredto thealuminium.Therefore,theresultsshowthatifonewantsto increasetheultimatetensileshearloadoftheFe-Aljointsanduse awiderrangeofweldingparameterstoproducethejointsthenthe besttexturetochooseisthehexagonalpattern.However,thejoints producedwiththehexagonalpatterndidnothavethemaximum tensile-shearloadnorthelargestbondingareabutthesefeatures wereconstantwithintherangeoftheexperimentalparameters. Ontheotherhand,inordertoproduceFe-Aljointswithatensile shearloadof>5.5kN,thebesttexturestochoosearethegridand spiralpatterns.Theloadof>5.5kNcanbeobtainedwiththegrid patternwhenPD=20.1MW/m2andt
i=5s,whereaswiththespiral
patternwhenPD=22.7MW/m2andt i=3s.
4. Conclusions
The conclusions presented in this paper are based on pre-liminarytestsperformedtoinvestigatetheinfluenceofdifferent texture patterns on the mechanical performance of dissimilar metallaserspotweldedjoints.Varioustexturescontainingcraters arrangedindifferentpatterns:hexagonal,parallellines,gridsand spiralswereinvestigatedinordertodeterminetheimpactofthe textureonthemaximumtensile-shearloadofthejoints.The tex-turesweretestedwithdifferentlaserenergylevels,bychanging eitherthe powerdensity or interaction time, duringlaser spot weldingtounderstandhowthetextureswouldaffectthethermal fieldinthejoinedmetalsaswellasthewettingofthesteelsurface bythemoltenaluminium.
Themainconclusionsofthispreliminarystudyareasfollows:
•DefectfreeFe-Aljointswereproducedbylaserspotweldingusing laser-texturedsteel;
•Comparedtotheuntexturedsamples,thelaser-texturedsurfaces enabledthejointstoincreasethemaximumtensile-shearloadby approximately25%;
•The maximum tensile-shearload obtained in this work was equivalenttothatreportedintheliteratureforjointsbetween Al–Al(Qiuetal.,2009a,b);
•Thejoints welded withthehexagonal texturehad maximum tensile-shearloadwith40mdiametercraters,30mdeep, sep-aratedbyadistanceof50m;
•Texturescomprisinglines,gridandspiralpatternshadasmaller workingenvelopethantheuntexturedsamples;
•Theresultssuggestthatthemaximumtensile-shearloadwas improvednotonlyduetoanincreaseinthebondingarea,but alsoduetothequalityanduniformityoftheweldnuggets.
Acknowledgements
Theresearchcovered inthis paperwasfunded bythe Engi-neeringandPhysicalSciencesResearchCouncil(EPSRC),theCentre forInnovativeManufacturinginLaser-basedProductionProcesses (GrantNo.:EP/K030884/1).Additionalsupportwasprovidedby AliconaandSPILasersUKLtd..Dataunderlyingthisstudycanbe accessedthroughtheCranfieldUniversityrepositoryat:http://dx. doi.org/10.17862/cranfield.rd.4205307.
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