A-Study-of-Spot-Welding-of-Advanced-High-Strength-

JournalofIronandSteelResearch,International,2011,18(Supplement1-2)724AStudyofSpotWeldingofAdvancedHighStrengthSteelsforAutomotiveApplicationsBholeS.D.,MaC.,KhanM.S.,ChenD.L.(DepartmentofMechanicalandIndustrialEngineering,RyersonUniversity,350VictoriaStreet,Toronto,ON,M5B2K3,Canada)Abstract:Theapplicationofadvancedhighstrengthsteelsinautomotiveindustryhashighlightedtheneedforresearchintospotweldabilityofthesesteels.Usingweldlobediagrams,thespotweldabilityofDP600steelwasfoundtobepoorwithconventionalweldschedules.Anenhancedweldscheduleconsistingoftwopulseswithreducedcurrentonthesecondpulsegaveasubstantialincreaseinthelobewidth;thefirstpulseremovedthezinccoatingandthesecondpulsecontrolledthenuggetgrowth.Adataacquisitionsystemwasdesignedtomonitorweldexpulsionduringtheweldoperation.Ofthethreepossiblecontrolstrategiesproposed,especiallywithACweldingequipment,thedynamicresistancesignaliseasilyobtainedandtheleastexpensive.Expulsionphenomena,microstructuralcharacterizationandmechanicalpropertiesofspot-weldedhotdippedgalvanizedDP600steelandinterstitialfreesteelwereinvestigated.FurtherworkondissimilarweldsinDP600andHSLA350wasalsoconductedwithemphasisontensileandfatiguepropertiesandfracturecharacteristics.TheperformanceofdissimilarspotweldswasdifferentfromthatofthesimilarspotweldsineachoftheHSLA350andDP600steels.TheDP600weldpropertiesplayedadominatingroleinthehardnessandtensilepropertiesofthedissimilarspotwelds.However,thefatigueperformanceofthedissimilarweldswassimilartothatoftheHSLAwelds.Detailswillbepresentedattheconference.Keywords:resistancespotwelding;dynamicresistance;nuggetdiameterandhardness;dissimilarmaterials;microstructure;mechanicalproperties;fatigue;fracturemode1IntroductionAdvancedhighstrengthsteelshavebeenusedintheautomotiveindustryforweightreduction,safetyimprovementsandcostsaving.Thehigherinitialworkhardeningratewithexcellentuniformandtotalelongationcombinetogivedualphase(DP)steelsamuchhigherultimatetensilestrength(UTS)andalowerratioofyieldstrength(YS)toUTSthanconventionallowcarbonsteelsatasimilarlevelofYS.ThesecharacteristicshavemadeDPsteelsattractiveforautomotiveapplications[1].ImprovedresistanceweldingperformanceinDPsteelscanbeachievedthroughmultipleenhancedweldschedulesandmonitoredbyweldlobediagrams.Astrategyforweldexpulsionmonitoringbasedonthedynamicresistanceisproposed.Inautomotiveapplications,adissimilarcombinationofmaterialtypeandthicknessarespecifiedinordertotailormaterialpropertiesforlocalrequirements[2].2LobeDiagramsandWeldSchedulesForaspecificmaterial,thelobediagramprovidesarangeofweldingparameterstoproduceanacceptablespotweld[3].Hardenabilityelementsinadvancedhighstrength(AHSS)steelsareusuallyconsidereddetrimentaltoresistancespotweldingbecausetheycanleadtonarrowerweldlobe[4]whichrepresentsalimitedselectionofweldingparametersforachievingacceptablewelds.Developingaweldschedulewithalargelobewidthforthesesteelsallowsgreaterflexibilityforaproductionweldingenvironment.Singlepulseweldingisthesimplestscheduleusedforspotwelding(Table1).At9cycleszincispresentatthefayinginterface.Fromcycle1to13,zincgraduallymeltsandispushedawayfromthecentretowardstheperipheryoftheelectrodes.Thezincthatispushedawayfromthecentreformsazinc‘halo’(liquidzincseal)alongtheperipheryoftheelectrodes.ThisobservationThe6thInternationalConferenceonHighStrengthLowAlloySteels725Table1WeldingparametersusedfornuggetgrowthstudiesinsinglepulseforDP600ElectrodeSheetThickness(mm)DressedfaceDia.(mm)Force(kN)Cooling(l/mm)WeldTime(cy)HoldTime(cy)WeldingRate(welds/Min)2.08.06.66.0261015isinaccordancewiththatofmanyresearchers[5,6].Variousinteractingphenomenaobservedwerezincremoval,startofsubstratemeltingandnuggetgrowth.Onceanuggetisformeditgrowsveryquickly(23to26cycles)withtimeandleadstoexpulsion.Thefastnuggetgrowthandexcessiveheatingduringalaterstageresultinearlyexpulsion,smallerexpulsionnuggetdiameter,andpushtheexpulsionlinetotheleft.Thisresultsinlowerlobewidth.Lee’s[7]workshowedthatthefirststageinnuggetgrowth(1to17cycles)iscontactresistancedominatedandthelaststage(23to26cycles)isbulkresistancedominated.ThehigherbulkresistivityofDP600[8],andthefactthatthebulkresistancedominatesthelaterstageofnuggetgrowth,leadstoextremelyfastnuggetgrowthduringthespotweldingofDP600steel.Controloverthisnuggetgrowthisnecessarytoavoidprematureexpulsion,i.e.expulsionbeforethenuggetgrowstoitsmaximumsize.Thusthenuggetgrowthcontrolcanbeachievedbyreducingtheheatinputwhenthenuggetisgrowingfast.MultiplepulseweldingscheduleshavereceivedmoreattentioninrecentyearsforweldingAHSS[4].Anenhancedpulsewasdesignedfromadoublepulsebyreducingcurrentonthesecondpulse.Holdtimeof2cycleswasmaintainedafterthefirst13cycles.Onceconductivelayerofzincwasremoved,currentonthesecondpulsewasreducedtodecreaseheatinputandslowdownnuggetgrowth.Aweldingcurrentjustpriortoexpulsionof11.4kAwasused.The100%-87.5%enhancedpulseshowedslowernuggetgrowthandresultsinbiggernuggetdiameter,althoughhigheramountsofcurrentarenecessarytoachievetheweldinshorterweldingtime.Fig.1showslobesat26cyclesforDP600withsinglepuls