StudyonthehydroformingprocessforautomobileradiatorsupportmembersMun-YongLeea,*,Sung-ManSohna,Chang-YoungKangb,Sang-YongLeecaSungWooHitechCo.Ltd.,Pusan619-731,SouthKoreabPukyungNationalUniversity,PusanSan100,SouthKoreacKoreaInstituteofMachineryandMaterials,Changwon641-830,SouthKoreaAbstractTubularhydroformingwasusedtomanufactureradiatorsupportmembersthatsupportradiatorcomponentssuchasaradiatorandacoolingfan,whichisplacedinfrontofcarbody.Newtooldesignconceptnamedmechanicalcamslidingsystemwasintroducedforthepre-formingprocessinwhichastartingtubewassimultaneouslypre-bendedandnarrowedtoapproximatelyfitthepartshape.Sinceirreversiblewrinklesappearatthebendedregionduringpre-formingwithoutinternalpressure,pre-formingwasperformedafterinternalpressure(C24150bar)wasaddedintothetube.Fromhydroformingtests,itwasknownthatradiatorsupportmemberswerehydroformedattheinternalpressurerangingfrom420to780bar.However,radius(20R)ofthecorneratrectangularsectionofthepartsshowedlargerthandesignspecification(10R).Inordertoinvestigatethestrainhardeningeffectsofthehydroformedparts,tensilepropertiesofthepartshydroformedat420barwerecomparedtothoseofstartingtubes.Throughthisinvestigation,itwasrevealedthattensilestressofhydroformedpartswasincreasedby22.1%,whereaselongationofthesepartswasreducedby49.7%.Concerningthefunctionofthispart,itisbelievedthatthesectionalshapeandthesetensilevaluesareenoughtoassembleandsupporttheradiatormembers.However,becausetoolwearanddamageoccurringinthepre-formingofastartingtubeevidentlyresultindeterioratingdimensionalreproducibility,itisnecessarytosolvethisproblemassociatedwithmechanicalcamslidingsystemwhenconsideringmassproductionofthispart.Inthispaper,thehydroformingprocessofradiatorsupportmembersincludingnewtooldesignconceptwasdescribedindetailandtestresultswereprovidedtoshowhydroformingpossibilityforradiatorsupportmembers.Keywords:Hydroforming；Automobileradiator；Pre-forming1.IntroductionRecentautomobiletechnologiesarefocusedonthedevelopmentoflowemissionandhighperformancevehicles,whichcouldbesuccessfullyrealizedbyreducingtheweightofvehicleswithoutanysacrificeofvehiclesafety.Accordingly,designcriteriaassociatedwithacarbodyandachassishavebeenvariedtoprovidemaximumstiffness,dimensionalstability,fatiguelifeandcrashworthinesswithminimummassandcost.Inordertoachievethesegoals,hydroformingtechnologiesarebeingcurrentlyusedorconsideredtomanufactureseveralauto-motivepartsincludedincarbodysystems,chassissystemsandpowertrainsystems1.Inparticular,tubularhydroforming,whichmakesone-stepformingofcomplexclosedhollowpartspossible,iseffectivelyusedforhighvolumeapplicationinautomotiveindustrybecausevariousclosedsectionsresultinginthehighestbendingandtorsionalstiffnesssectionalshapecanbeeasilyformedbyuseofthistechnology.ThatisthereasonwhyavarietyofhighvolumeautomotivepartshavebeenhydroformedinEuropeancountriesandtheUS.Inthisviewpoint,hydroformingtechnologieswillbecomemorepopulartomanufacturehighlyengineeredpartscharacter-izedbylightweightandhighquality2,3.However,consideringtheapplicationofhydroformingtechnologiesinautomotiveparts,itshouldbealsonotedthatthereareseveralproblemsrelatedtotooldesignanddevicesaswellastheestablishmentoftheoptimumprocessingparameters.Furthermore,sinceautomotivepartshavevar-iousdifferentdesignshapesaccordingtoautomobilemakersJournalofMaterialsProcessingTechnology130131(2002)115120*Correspondingauthor.andtypes,optimumtooldesignandprocessparameterscannotbeunitedasacommontechnologicalstandard.Therefore,whenapartisplannedtodevelopusinghydro-forming,thetooldesignandoptimumprocessparameteraswellasthepossibilityofhydroformingshouldbeonlydeterminedbydirecthydroformingtrialsfortheintendedrealparts.Thisstudywascarriedouttocheckthehydro-formingpossibilityofarealpart,aradiatorsupportmember,whichisusuallymanufacturedbystampingofsheetmetalsandweldingofstampedparts.2.Experimentalprocedureforhydroformingofradiatorsupportmembers2.1.ExaminationofhydroformabilityforatubematerialSinceradiatorsupportmembersareastructuralcompo-nent,whichsupportsbothofaradiatorandacoolingfanlocatedinfrontofacarbody,therequirementforthispartisonlyconsideredasstaticstrengthensuringsafesupport.Therefore,whenatubeissubstitutedforaconventionalweldedstructurehavingclosedsection,itispossibletoreducethethicknessofthepartduetohigherstrengthinducedbystrainhardeningaftertubeformingprocess.However,inthisstudy,tubethicknesskeptthesamelevelasthethickness(1mm)oftheconventionalmembertoavoidtheproblemrelatedtosealing,whichmayoccurwhenthethicknessofthememberisreduced.Thetubematerialusedforthisstudyisahotrolledplateforautomobilestructure(SAE1010).Thisplatewasrolledalonglongitudinaldirectionandweldedtomaketubeshapeandthenextrudedtofitthefinalsize.Mechanicalpropertiesaftereachprocess,whichisperformedtoobtainastartingtubeforhydroforming,aresummarizedinTable1.Yieldstressandelongationofas-receivedplateswere32.2kgf/mm2and42.8%,respectively.However,aftertubinganddrawing,yieldstresswasreducedto29.2kg/mm2andelongationwasincreasedto43.7%,whichwasduetosubsequentannealingtreatment.Accordingly,itwasconfirmedthatstartingtubeshavegoodpropertiesforhydroforming.AccordingtomaindesignfactorsforhydroformingofradiatorsupportmembersdescribedinTable2,itwasexpectedthatthispartcouldbecompletelyformedatbelow1000barwithoutfeeding.Inaddition,comparingforminglimitdiagram(FLD)tothestraindistributionofahydro-formedpartobtainedbyFEManalysis(Fig.1),itwasassumedthatthepartcouldbesafelyhydroformedwithoutTable1MechanicalpropertiesaftereachprocessforhydroformingofradiatorsupportmembersSAE1010Tensilestress(kgf/mm2)Yieldstress(kgf/mm2)Elongation(%)As-received41.732.242.8Aftertubing40.935.140.2Afterdrawing37.529.243.7Afterhydroforming45.844.822.0Table2HydroformingdesignfactorsofradiatorsupportmembersFactorsHydroforming(stamping)SPEC(mm)C3160:3C2thickness1:0C21390Expansionratio(%)7MinimumR(mm)10Internalpressure(bar)1000Feeding(mm)NoPress(t)1000Weight(kg)1.75(2.7)Thenumberofsubparts(ea)1(2)Fig.1.ResultsofFEManalysisforhydroformingprocessofaradiatorsupportmemberanddistributionofstrainintheFLD(courtesyHyundaiMotorsCo.Ltd.):(a)afrontviewofdie-setforhydroformingofradiatorsupportmembers；(b)asideviewofdie-setforhydroformingofradiatorsupportmembers.116M.-Y.Leeetal./JournalofMaterialsProcessingTechnology130131(2002)115120anyriskresultedfromseverelylocalizedstrain.However,duringrealhydroformingtrials,alotoffailuresoccurredonthebendedregionsofthepartindicatedbyFig.1.Thereasonforfrequentfailuresmaybeassociatedwiththepre-strainsinducedbythepre-bendingbeforefinalhydroforming,whichleadstoseverelocalthinningduringfinalforming.Finally,anattentionneedstobepaidregardingthehydro-formingadvantageofthispart:asdescribedinTable2,theweightandthenumberofsubpartsisreducedusinghydro-formingcomparedtotheconventionalparts.2.2.ToolsystemforhydroformingofradiatorsupportmembersUnliketheconventionalhydroforming,inwhichthepre-formingispreviouslyconductedbyuseofseparatetools,thetoolusedforthisstudywasspeciallydesignedtodoboththepre-formingandthehydroformingwithasingletool.Inparticular,itisnotedthatmechanicalcamslidingsystemwasusedfortubenarrowing,inwhichthecross-sectionalshapeinatubechangesfromcirculartypetoellipticaltype.Fig.2illustratestheschematictool-operatingconcept.Asshowninthisfigure,pre-bendingiscompletedbydieclosingwhiletheslidersassembledatthebottomdiearelaterallymovedtonarrowatube.Accordingly,thepre-formingforthispartwasautomaticallyfinishedwhendieisclosed.Afterthedieiscompletelyclosed,hydroformingisdonetoexpansethepre-formedtubebyfeedingthefluid.Duringhydroformingprocess,thecross-sectionalshapeofthepre-formedpartwasfittofinaldesignshape(rectangularshape).Finally,whenhydroformingisfinishedandupperdieislifted,bothslidersareautomaticallyreturntooriginalpositionbyspringforceandthenthehydroformedpartcanbeeasilyejectedwithoutanyinterferencebetweenapartanddiecavity.Withthisuniquetool,realpartswerehydroformedusing2500thydraulicpress,asshowninFig.3.Sincetheinternalpressureneededforhydroformingofthispartwasevaluatedasbelow1000barthroughpreviousFEMsimulation,itwasassumedthat2500tpresshasenoughcapacityforthistest.2.3.HydroformingofradiatorsupportmembersVariousinternalpressuresfrom420to780barwereusedforhydroformingtrialsofaradiatorsupportmember.Sincealotoftubeswererupturedattheinternalpressureof780barduringhydroforming,internalpressurewasgradu-allydiminisheduntil420bar.Byobservingtheexternalappearanceofhydroformedparts,itcouldbefoundthattheparthydroformedattheinternalpressureof420barshowsthealmostsameshapecomparedtotheparthydroformedatthehigherinternalpressureof780bar.Therefore,theinter-nalpressureof420baristheoptimumvaluetobuildaradiatorsupportmemberusinghydroformingwithouttheriskoftubeburst(Fig.4).Ofcourse,somecurvedregionsatcornerofthepartwerenotcompletelysatisfiedwithdesignFig.2.Schematicdiagramshowingbasictooloperatingconcept:(a)asupperdieisdescending,astartingtubeisbendedtofitintothediecavity(note:speciallydesignedclampingdevice,wasusedtoavoidsealingproblemthatmayoccurwhentheendregionofthetubeisrotatedbythebendingmoment)；(b)asupperdieisdescending,slidersarelaterallymovedtonarrowatube.Lateralmotionofslidersisachievedbythecontactbetweenthesidefacesofslidersandupperdiehavingthematedgradientprofilewitheachother(mechanicalcamslidingsystem).M.-Y.Leeetal./JournalofMaterialsProcessingTechnology130131(2002)115120117specification(Fig.5).However,sincetheseregionsdonotseriouslyinfluencedonthesubsequentassemblingofthispart,itisbelievedthatthedesignspecificationmaybeeasilychangedtopermitthedimensionofthesehydro-formedparts.Thereisonethingtobenoted；theinternalpressureofaround150barwaspreviouslyaddedjustbeforethepre-formingbecauseirreversiblewrinklesappearedatthebendedregionwithouttheinternalpressureduringthepre-formingprocess.Fig.5representsthedetailedhistoryoftheinternalpressureusedforhydroformingoftheseparts.Accordingly,thisprocessissomewhatdifferentfromtheconventionalhydroformingprocesses,inwhichmechanicalpre-formingsuchasbendingandnarrowingisperformedinseparatetoolsandformingdeviceswithoutinternalpressure4,5.3.Hydroformingtestresults3.1.SomeproblemsassociatedwithhydroformingprocessesFig.6(a)showshydroformedpartshavingincompleteexternalshapeattheupperregionofbothendsintheparts.Moreover,itwasdiscoveredthatmostofthefailuresduringhydroformingoccurredattheseregions.Therefore,itseemsthatthesepositionsweresufferedfromveryseverelocaldeform