JournalofMaterialsProcessingTechnology170(2005)181186StampingformabilityofpuretitaniumsheetsFuh-KuoChen,Kuan-HuaChiuDepartmentofMechanicalEngineering,NationalTaiwanUniversity,Taipei10764,Taiwan,ROCReceived20October2003；receivedinrevisedform12April2005；accepted4May2005AbstractBecauseofhexagonalclose-packed(HCP)crystalstructures,commerciallypuretitanium(CPTi)showslowductilityatroomtemperature,andrequiresthermalactivationtoincreaseitsductilityandformability.Inthepresentstudy,theformabilityofCPTisheetsatvarioustemperatureswasstudiedbytheexperimentalapproach.TensiletestswerefirstconductedtoinvestigatethemechanicalbehaviorofCPTisheetsatvarioustemperatures.Forminglimittests,V-bendtests,andcupdrawingtestswerealsoperformedtoexaminethestampingformabilityofCPTisheetsatvarioustemperatures.TheexperimentalresultsindicatethatCPTisheetscouldbeformedintoshallowcomponentsatroomtemperature,althoughtheformabilityislimitedincoldforming.Inaddition,theresultsobtainedfromtheV-bendtestsrevealthatspringbackcanbereducedatelevatedformingtemperatures.TheexperimentalresultsobtainedinthepresentstudycanbeofhelptothediedesignofstampingCPTisheets.2005ElsevierB.V.Allrightsreserved.Keywords:Puretitaniumsheet；Formability；Forminglimit；V-bend；Springback1.IntroductionInthepresentstudy,theformabilityofstampingCPTimerciallyrialfromfofAmongingoficsphone,at(HCP)attemperatureHo14T0924-0136/$doi:10.1016/j.jmatprotec.2005.05.004Duetoitslightweightandhighspecificstrength,com-puretitanium(CPTi)hasbeenapotentialmate-forstructuralcomponents,andattractsmuchattentiontheelectronicsindustryrecently.Theprincipalmanu-acturingprocessofCPTihasbeenpressformingbecauseitscompetitiveproductivityandsuperiorperformance.thefabricationprocessesofpressforming,stamp-ofCPTisheetsisespeciallyimportantfortheproductionthin-walledstructuralcomponentsusedintheelectron-products,suchasthecovercasesofnotebook,mobileetc.TheCPTisheetusuallyexhibitslimitedductilityroomtemperaturebecauseofitshexagonalclose-packedstructure.Althoughtheformabilitycanbeimprovedelevatedtemperatures,amanufacturingprocessatroomisalwaysdesiredforthecost-effectivereason.wever,mostresearchofCPTiisfocusedonmicrostructure,andtheliteratureregardingformabilityofstampingCPisheetsisnotprofound.Correspondingauthor.Tel.:+886233662701；fax:+88623631755.E-mailaddress:.tw(F.-K.Chen).pertiestestsroomoftestseefrontmatter2005ElsevierB.V.Allrightsreserved.wasinvestigatedusingtheexperimentalapproach.ThepropertiesofCPTisheetsatvarioustemperaturesfromroomtemperatureto300Cwereobtainedfromxperimentalresults.Inaddition,theimportantformingchar-ofCPTisheets,suchasforminglimit,springback,limitingdrawingratio,werealsoexaminedbyexperi-MechanicalpropertiestestsatvariousesThestressstrainrelationsarethefundamentalinfor-forthestudyofformabilityofasheetmetal.Asabove,theformabilityofCPTisheetsislimitedattemperatureandcanbeimprovedatelevatedformingInordertoexaminethevarietyofmechanicalofCPTisheetsatdifferenttemperatures,tensilewereperformedatvarioustemperaturesrangingfromtemperatureto300Candunderdifferentstrainrates0.1,0.01,0.001,and0.0001/s,respectively.ThetensilespecimensmadeofJISGrade1CPTisheetsof0.5mm182ProcessingFig.imensthicknessTherollingrollingalongmachine.wwerewerewwereconandneeringformensTheFig.lartions,iswdirections.byoccursofdeformationhigheringation.TrespectiofathefromthreeF.-K.Chen,K.-H.Chiu/JournalofMaterials1.Truestressstrainrelationsatroomtemperatureobtainedfromspec-inthethreedirections.werepreparedaccordingtotheASTMstandards.specimenswerecutalongplanescoincidingwiththedirection(0),andatanglesof45and90tothedirection.Thespecimenswerewirecuttoavoidburrstheedge.ThetensiletestswereconductedusinganMTS810testFortestsatelevatedtemperatures,aheatingfurnaceasmountedontheMTS810testmachine.Thespecimensheatedto100,200,and300Cbeforethetensiletestsperformed.Duringtests,thetemperatureofspecimenaskeptconstantuntilthespecimenwasstretchedtofailure.Inthepresentstudy,theengineeringstressstrainrelationsfirstobtainedfromtheexperimentaldataandthenwerevertedintothetruestressstrainrelationsaccordingto=0(1+e)and=ln(1+e),whereandweretruestresstruestrain,0andewereengineeringstress,andengi-strain,respectively.ThetruestressstrainrelationsCPTisheetsatroomtemperatureobtainedfromspeci-cutinthethreedifferentorientationsareshowninFig.1.anisotropicbehaviorisobservedinFig.1.Itisseenin1thatthe0specimenhasahigheryieldstrengthandagerelongationthanthespecimensintheothertwodirec-thedifferenceinelongationbeingmoresignificant.Italsoobservedthatthe0specimendisplaysasignificantork-hardeningpropertyamongthespecimensinthethreeTheseresultsareconsistentwiththoseobtainedIshiyamaetal.5.Theyfoundthattheslipdeformationinboththe0and90directionsinthebeginningstagethetest.Duringfurtherdeformationstage,thetwinningincreasesfasterinthe0directionandproducesresistanceagainsttheslipofdislocations,resultinglargervaluesinyieldstrength,workhardening,andelon-TheaverageyieldstressandelongationoftheCPisheetatroomtemperatureareabout352MPaand28%,vely.ThoughthevaluesofyieldstressandelongationtheCPTisheetatroomtemperaturearenotfavorableindeepdrawingprocesscomparedtothoseofcarbonsteels,yarefeasibleforstampingofrelativelyshallowproductstheformabilitypointofview.Fig.2showstheoriginalanddeformedspecimensinthedirections.ItisnoticedinFig.2thatthe0specimenunderspecimenmodemationatdiftiforinnoticedtointhatobtainedousforrelationsformedCPatures.thethatroomelongation100Fig.menTechnology170(2005)181186Fig.2.Originalanddeformedspecimensinthethreedirections.goesuniformdeformationbeforefracture,whilethe90displaysanobviousnecking,andthedeformationof45specimenliesbetweenthoseofothertwomodes.Inordertoexaminetheeffectofstrain-rateonthedefor-ofCPTisheets,thetensiletestswerealsoperformedroomtemperatureunderdifferentramspeeds,resultinginferentstrain-ratesof0.1,0.01,0.001,and0.0001,respec-vely.Thetruestressstrainrelationsatvariousstrain-ratesthe0specimenareshowninFig.3.Asignificantdropthestressstraincurvesfromstrain-rate0.1to0.001isinFig.3,andthestressstraincurvesbecomecloseeachotherafterwards.Thesametrendsarealsoobservedthetensiletestsforthe45and90specimens.ItindicatesastablestressstrainrelationsforCPTisheetscanbeunderthestrain-ratessmallerthan0.001.ThetruestressstrainrelationsofCPTisheetsatvari-temperaturesrangingfromroomtemperatureto300Cthespecimenof0directionareshowninFig.4.TheshowninFig.4areobtainedfromthetestsper-atstrain-rateof0.001.ItisseeninFig.4thattheTisheetexhibitsbetterformabilityatelevatedtemper-Thestressstraincurvesgetlowerproportionallytoincreaseoftestingtemperature.ItistobenotedinFig.4theelongationofthespecimendoesnotincreasefromtemperatureto100Casexpected,onthecontrary,thegetssmallerwhenthespecimenisheateduptoC.However,theelongationbecomeslargerattesting3.Truestressstrainrelationsatvariousstrain-rates(1/s)for0speci-atroomtemperature.ProcessingFig.Fig.temperaturesroomhappenstheatureThe0ningproducingandrtransvuniaxialobtainedFig.90fromF.-K.Chen,K.-H.Chiu/JournalofMaterials4.Truestressstrainrelationsatvarioustemperaturesfor0specimen.5.Truestressstrainrelationsatvarioustemperaturesfor45specimen.higherthan100C.Thegreaterelongationattemperatureisquiteunusual.Butthisphenomenononlytothe0specimen.Forthe45and90specimens,elongationcontinuouslyincreasesasthetestingtemper-getselevated,asshowninFigs.5and6,respectively.greaterelongationatroomtemperatureoccurredinthespecimenmightbeduetothefastincreaseofthetwin-deformationinthe0directionatroomtemperature,higherresistanceagainsttheslipofdislocations,resultinginalargerelongation.Anotherindexofanisotropyistheplasticstrainratio,i.e.-value,whichisdefinedastheratioofplasticstrainintheersedirectiontothatinthethicknessdirectioninatensiletest.Inthepresentstudy,ther-valuewasfromthetensiletestsforspecimensof0,45,and6.Truestressstrainrelationsatvarioustemperaturesfor90specimen.0vethansheetsr3.ingpresentandtemperaturesrelatinging3.1.formingacceptedmetalperformedusingelectrochemicallydeformedingellipseThestrainssamefromSimilarentationseachatsphericalmeasuredspecimenplottednate,thelimitafracturebeitstemperature.fromatTechnology170(2005)181186183directionsatroomtemperature.Ther-valuesmeasuredspecimensstretchedto20%are4.2,2.2,and2.1forthe,45,and90specimens,respectively.Sinceahigherr-alueindicatesbetterdrawability,itshowsthatCPTisheetsxhibitbetterdeepdrawingqualityintherollingdirectiontheothertwodirections.AlsotheanisotropyofCPTiwasconfirmedagainfromthesignificantdifferenceof-values.StampingformabilityofCPTisheetsInadditiontothebasicmechanicalproperties,thestamp-formabilityofCPTisheetswasalsoexamined.Inthestudy,theforminglimittestsatroomtemperature,theV-bendtestsandcircularcupdrawingtestsatvariouswereperformed.ThetestresultswerediscussedtotheformingpropertiesofCPTisheetsinastamp-process.ForminglimittestsSinceKeelerandBackofen6introducedtheconceptoflimitdiagram(FLD)in1963,ithasbeenawidelycriterionforthefracturepredictioninthesheet-forming.TodetermineanFLD,stretchingtestswereforsheet-metalspecimensofdifferentwidthsasemi-sphericalpunch.Thespecimenswerefirstetchedwithcirculargridsthatwouldbeintoellipsesafterbeingstretched.Theengineer-strainsmeasuredalongthemajor-andminor-axesofthearetermedthemajor-andminor-strain,respectively.yarealsotheprincipalstrainsontheplanewherethearemeasured.Inthepresentstudy,rectangularspecimenshavingthelengthof100mm,butwithdifferentwidthsranging10to100mminanincrementof10mm,weretested.totensiletests,theCPTisheetwascutatthreeori-totherollingdirection,i.e.,0,45,and90,forsizeofspecimen.Duringthetests,specimensclampedperipherywerestretchedtofailureovera78mmsemi-punch.Theengineeringmajor-andminor-strainsinthelocationclosesttothefractureforeachwererecorded.Themajor-andminor-strainswereagainstoneanotherwiththemajorstrainastheordi-andthecurvefittedintothestrain-pointswasdefinedasforminglimitcurve.Thediagramshowingthisformingcurveiscalledtheforminglimitdiagram.TheFLDisveryusefulcriterionforthepredictionoftheoccurrenceofinastampingprocess.Accordingtothepreviousanalysis,theCPTisheetcouldformedatroomtemperature.Inordertofurtherconfirmfeasibility,theforminglimittestswereperformedatroomFig.7showstheforminglimitcurveobtainedthetestresults.ItisseeninFig.7thatthemajorstrainthelowestpointofthecurve,whichisalsotheplanestrain184ProcessingTechnology170(2005)181186deformationorstampinginsheetsufCP3.2.ofprocess.toformingishaspunchfrompared.ofFmensusedinsignificanttest.100,Fig.foranglesspringbackandtiF.-K.Chen,K.-H.Chiu/JournalofMaterialsFig.7.Forminglimitcurveatroomtemperature.mode,is0.34.Comparedwithcold-rolledsteelsstainlesssteels,thisvalueisalittlelower.However,forofshallowproducts,theforminglimitcurveshownFig.7indicatesagreaterpossibilityofformingofCPTiatroomtemperature.Thismakesitpossibletoman-actureelectronicscomponentsatroomtemperatureusingTisheets.V-bendtestsSinceCPTihasalowervalueofelasticmodulusthanthatsteel,springbackcouldbemuchsignificantinabendingInthepresentstudy,theV-bendtestswereperformedexaminethespringbackpropertyofCPTisheetsatvarioustemperatures.ThetoolingusedintheV-bendtestsshowninFig.8.ItcanbeseeninFig.8thatthelowerdieanopeningangleof90.Inordertostudytheeffectofradiusonspringback,thetoolingsetswithpunchradii0.5to5.0mm,inanincrementof0.5mm,werepre-TheCPTisheetwithathicknessof0.5mm,alength60mm,andawidthof15mm