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JournalofMaterialsProcessingTechnology1702005181–186StampingformabilityofpuretitaniumsheetsFuhKuoChen∗,KuanHuaChiuDepartmentofMechanicalEngineering,NationalTaiwanUniversity,Taipei10764,Taiwan,ROCReceived20October2003receivedinrevisedform12April2005accepted4May2005AbstractBecauseofhexagonalclosepackedHCPcrystalstructures,commerciallypuretitaniumCPTishowslowductilityatroomtemperature,andrequiresthermalactivationtoincreaseitsductilityandformability.Inthepresentstudy,theformabilityofCPTisheetsatvarioustemperatureswasstudiedbytheexperimentalapproach.TensiletestswerefirstconductedtoinvestigatethemechanicalbehaviorofCPTisheetsatvarioustemperatures.Forminglimittests,Vbendtests,andcupdrawingtestswerealsoperformedtoexaminethestampingformabilityofCPTisheetsatvarioustemperatures.TheexperimentalresultsindicatethatCPTisheetscouldbeformedintoshallowcomponentsatroomtemperature,althoughtheformabilityislimitedincoldforming.Inaddition,theresultsobtainedfromtheVbendtestsrevealthatspringbackcanbereducedatelevatedformingtemperatures.TheexperimentalresultsobtainedinthepresentstudycanbeofhelptothediedesignofstampingCPTisheets.©2005ElsevierB.V.Allrightsreserved.KeywordsPuretitaniumsheetFormabilityForminglimitVbendSpringback1.IntroductionInthepresentstudy,theformabilityofstampingCPTimerciallyrialfromfofAmongingoficsphone,atHCPattemperatureHo1–4T09240136/doi10.1016/j.jmatprotec.2005.05.004Duetoitslightweightandhighspecificstrength,compuretitaniumCPTihasbeenapotentialmateforstructuralcomponents,andattractsmuchattentiontheelectronicsindustryrecently.TheprincipalmanuacturingprocessofCPTihasbeenpressformingbecauseitscompetitiveproductivityandsuperiorperformance.thefabricationprocessesofpressforming,stampofCPTisheetsisespeciallyimportantfortheproductionthinwalledstructuralcomponentsusedintheelectronproducts,suchasthecovercasesofnotebook,mobileetc.TheCPTisheetusuallyexhibitslimitedductilityroomtemperaturebecauseofitshexagonalclosepackedstructure.Althoughtheformabilitycanbeimprovedelevatedtemperatures,amanufacturingprocessatroomisalwaysdesiredforthecosteffectivereason.wever,mostresearchofCPTiisfocusedonmicrostructure,andtheliteratureregardingformabilityofstampingCPisheetsisnotprofound.∗Correspondingauthor.Tel.886233662701fax88623631755.Emailaddressfkchenccms.ntu.edu.twF.K.Chen.sheetsmechanicalrangingeacteristicsandments.2.temperaturmationmentionedroomtemperatures.propertiestestsroomoftest–seefrontmatter©2005ElsevierB.V.Allrightsreserved.wasinvestigatedusingtheexperimentalapproach.ThepropertiesofCPTisheetsatvarioustemperaturesfromroomtemperatureto300◦Cwereobtainedfromxperimentalresults.Inaddition,theimportantformingcharofCPTisheets,suchasforminglimit,springback,limitingdrawingratio,werealsoexaminedbyexperiMechanicalpropertiestestsatvariousesThestress–strainrelationsarethefundamentalinforforthestudyofformabilityofasheetmetal.Asabove,theformabilityofCPTisheetsislimitedattemperatureandcanbeimprovedatelevatedformingInordertoexaminethevarietyofmechanicalofCPTisheetsatdifferenttemperatures,tensilewereperformedatvarioustemperaturesrangingfromtemperatureto300◦Candunderdifferentstrainrates0.1,0.01,0.001,and0.0001/s,respectively.ThetensilespecimensmadeofJISGrade1CPTisheetsof0.5mm182ProcessingFig.imensthicknessTherollingrollingalongmachine.wwerewerewwereconσandneeringformensTheFig.lartions,iswdirections.byoccursofdeformationhigheringation.TrespectiofathefromthreeF.K.Chen,K.H.Chiu/JournalofMaterials1.Truestress–strainrelationsatroomtemperatureobtainedfromspecinthethreedirections.werepreparedaccordingtotheASTMstandards.specimenswerecutalongplanescoincidingwiththedirection0◦,andatanglesof45◦and90◦tothedirection.Thespecimenswerewirecuttoavoidburrstheedge.ThetensiletestswereconductedusinganMTS810testFortestsatelevatedtemperatures,aheatingfurnaceasmountedontheMTS810testmachine.Thespecimensheatedto100,200,and300◦Cbeforethetensiletestsperformed.Duringtests,thetemperatureofspecimenaskeptconstantuntilthespecimenwasstretchedtofailure.Inthepresentstudy,theengineeringstress–strainrelationsfirstobtainedfromtheexperimentaldataandthenwerevertedintothetruestress–strainrelationsaccordingtoσ01eandεln1e,whereσandεweretruestresstruestrain,σ0andewereengineeringstress,andengistrain,respectively.Thetruestress–strainrelationsCPTisheetsatroomtemperatureobtainedfromspecicutinthethreedifferentorientationsareshowninFig.1.anisotropicbehaviorisobservedinFig.1.Itisseenin1thatthe0◦specimenhasahigheryieldstrengthandagerelongationthanthespecimensintheothertwodirecthedifferenceinelongationbeingmoresignificant.Italsoobservedthatthe0◦specimendisplaysasignificantorkhardeningpropertyamongthespecimensinthethreeTheseresultsareconsistentwiththoseobtainedIshiyamaetal.5.Theyfoundthattheslipdeformationinboththe0◦and90◦directionsinthebeginningstagethetest.Duringfurtherdeformationstage,thetwinningincreasesfasterinthe0◦directionandproducesresistanceagainsttheslipofdislocations,resultinglargervaluesinyieldstrength,workhardening,andelonTheaverageyieldstressandelongationoftheCPisheetatroomtemperatureareabout352MPaand28,vely.ThoughthevaluesofyieldstressandelongationtheCPTisheetatroomtemperaturearenotfavorableindeepdrawingprocesscomparedtothoseofcarbonsteels,yarefeasibleforstampingofrelativelyshallowproductstheformabilitypointofview.Fig.2showstheoriginalanddeformedspecimensinthedirections.ItisnoticedinFig.2thatthe0◦specimenunderspecimenmodemationatdiftiforinnoticedtointhatobtainedousforrelationsformedCPatures.thethatroomelongation100Fig.menTechnology1702005181–186Fig.2.Originalanddeformedspecimensinthethreedirections.goesuniformdeformationbeforefracture,whilethe90◦displaysanobviousnecking,andthedeformationof45◦specimenliesbetweenthoseofothertwomodes.InordertoexaminetheeffectofstrainrateonthedeforofCPTisheets,thetensiletestswerealsoperformedroomtemperatureunderdifferentramspeeds,resultinginferentstrainratesof0.1,0.01,0.001,and0.0001,respecvely.Thetruestress–strainrelationsatvariousstrainratesthe0◦specimenareshowninFig.3.Asignificantdropthestress–straincurvesfromstrainrate0.1to0.001isinFig.3,andthestress–straincurvesbecomecloseeachotherafterwards.Thesametrendsarealsoobservedthetensiletestsforthe45◦and90◦specimens.Itindicatesastablestress–strainrelationsforCPTisheetscanbeunderthestrainratessmallerthan0.001.Thetruestress–strainrelationsofCPTisheetsatvaritemperaturesrangingfromroomtemperatureto300◦Cthespecimenof0◦directionareshowninFig.4.TheshowninFig.4areobtainedfromthetestsperatstrainrateof0.001.ItisseeninFig.4thattheTisheetexhibitsbetterformabilityatelevatedtemperThestress–straincurvesgetlowerproportionallytoincreaseoftestingtemperature.ItistobenotedinFig.4theelongationofthespecimendoesnotincreasefromtemperatureto100◦Casexpected,onthecontrary,thegetssmallerwhenthespecimenisheatedupto◦C.However,theelongationbecomeslargerattesting3.Truestress–strainrelationsatvariousstrainrates1/sfor0◦speciatroomtemperature.ProcessingFig.Fig.temperaturesroomhappenstheatureThe0ningproducingandrtransvuniaxialobtainedFig.90fromF.K.Chen,K.H.Chiu/JournalofMaterials4.Truestress–strainrelationsatvarioustemperaturesfor0◦specimen.5.Truestress–strainrelationsatvarioustemperaturesfor45◦specimen.higherthan100◦C.Thegreaterelongationattemperatureisquiteunusual.Butthisphenomenononlytothe0◦specimen.Forthe45◦and90◦specimens,elongationcontinuouslyincreasesasthetestingtempergetselevated,asshowninFigs.5and6,respectively.greaterelongationatroomtemperatureoccurredinthe◦specimenmightbeduetothefastincreaseofthetwindeformationinthe0◦directionatroomtemperature,higherresistanceagainsttheslipofdislocations,resultinginalargerelongation.Anotherindexofanisotropyistheplasticstrainratio,i.e.value,whichisdefinedastheratioofplasticstrainintheersedirectiontothatinthethicknessdirectioninatensiletest.Inthepresentstudy,thervaluewasfromthetensiletestsforspecimensof0◦,45◦,and6.Truestress–strainrelationsatvarioustemperaturesfor90◦specimen.0vethansheetsr3.ingpresentandtemperaturesrelatinging3.1.formingacceptedmetalperformedusingelectrochemicallydeformedingellipseThestrainssamefromSimilarentationseachatsphericalmeasuredspecimenplottednate,thelimitafracturebeitstemperature.fromatTechnology1702005181–186183◦directionsatroomtemperature.Thervaluesmeasuredspecimensstretchedto20are4.2,2.2,and2.1forthe◦,45◦,and90◦specimens,respectively.Sinceahigherralueindicatesbetterdrawability,itshowsthatCPTisheetsxhibitbetterdeepdrawingqualityintherollingdirectiontheothertwodirections.AlsotheanisotropyofCPTiwasconfirmedagainfromthesignificantdifferenceofvalues.StampingformabilityofCPTisheetsInadditiontothebasicmechanicalproperties,thestampformabilityofCPTisheetswasalsoexamined.Inthestudy,theforminglimittestsatroomtemperature,theVbendtestsandcircularcupdrawingtestsatvariouswereperformed.ThetestresultswerediscussedtotheformingpropertiesofCPTisheetsinastampprocess.ForminglimittestsSinceKeelerandBackofen6introducedtheconceptoflimitdiagramFLDin1963,ithasbeenawidelycriterionforthefracturepredictioninthesheetforming.TodetermineanFLD,stretchingtestswereforsheetmetalspecimensofdifferentwidthsasemisphericalpunch.Thespecimenswerefirstetchedwithcirculargridsthatwouldbeintoellipsesafterbeingstretched.Theengineerstrainsmeasuredalongthemajorandminoraxesofthearetermedthemajorandminorstrain,respectively.yarealsotheprincipalstrainsontheplanewherethearemeasured.Inthepresentstudy,rectangularspecimenshavingthelengthof100mm,butwithdifferentwidthsranging10to100mminanincrementof10mm,weretested.totensiletests,theCPTisheetwascutatthreeoritotherollingdirection,i.e.,0◦,45◦,and90◦,forsizeofspecimen.Duringthetests,specimensclampedperipherywerestretchedtofailureovera78mmsemipunch.Theengineeringmajorandminorstrainsinthelocationclosesttothefractureforeachwererecorded.Themajorandminorstrainswereagainstoneanotherwiththemajorstrainastheordiandthecurvefittedintothestrainpointswasdefinedasforminglimitcurve.Thediagramshowingthisformingcurveiscalledtheforminglimitdiagram.TheFLDisveryusefulcriterionforthepredictionoftheoccurrenceofinastampingprocess.Accordingtothepreviousanalysis,theCPTisheetcouldformedatroomtemperature.Inordertofurtherconfirmfeasibility,theforminglimittestswereperformedatroomFig.7showstheforminglimitcurveobtainedthetestresults.ItisseeninFig.7thatthemajorstrainthelowestpointofthecurve,whichisalsotheplanestrain184ProcessingTechnology1702005181–186deformationorstampinginsheetsufCP3.2.ofprocess.toformingishaspunchfrompared.ofFmensusedinsignificanttest.100,Fig.foranglesspringbackandtiF.K.Chen,K.H.Chiu/JournalofMaterialsFig.7.Forminglimitcurveatroomtemperature.mode,is0.34.Comparedwithcoldrolledsteelsstainlesssteels,thisvalueisalittlelower.However,forofshallowproducts,theforminglimitcurveshownFig.7indicatesagreaterpossibilityofformingofCPTiatroomtemperature.ThismakesitpossibletomanactureelectronicscomponentsatroomtemperatureusingTisheets.VbendtestsSinceCPTihasalowervalueofelasticmodulusthanthatsteel,springbackcouldbemuchsignificantinabendingInthepresentstudy,theVbendtestswereperformedexaminethespringbackpropertyofCPTisheetsatvarioustemperatures.ThetoolingusedintheVbendtestsshowninFig.8.ItcanbeseeninFig.8thatthelowerdieanopeningangleof90◦.Inordertostudytheeffectofradiusonspringback,thetoolingsetswithpunchradii0.5to5.0mm,inanincrementof0.5mm,werepreTheCPTisheetwithathicknessof0.5mm,alength60mm,andawidthof15mmwasusedasspecimens.ortestsatelevatedtemperatures,bothtoolingandspeciwereenclosedinaheatingfurnace.NolubricantwasintheVbendtestsincethefrictionalconditionhasaneffectonthespringbackoccurredintheVbendThebendingtestswereconductedatroomtemperature,200,and300◦C,respectively.Afterbendingtests,theFig.8.ToolingusedintheVbendtests.forsmallerbend,notedbacksheetarctocomplespringbackFig.imens9.Relationsbetweenspringbackandpunchradiusatroomtemperaturespecimensofthreedirections.ofbentspecimensweremeasuredbyaCMM,andtheangleswerecalculated.Figs.9and10showtherelationshipsbetweenspringbackpunchradiusatroomtemperatureand300◦C,respecvely.Itisseeninbothfiguresthatthespringbackdecreasessmallerpunchradiiregardlessoftemperaturechange.Thepunchradiuscauseslargerplasticdeformationattheandhencereducestheeffectofspringback.ItisalsoinbothFigs.9and10thatnegativevaluesofspringoccurforsmallerpunchradii.ThisisbecausethattheonthestraightsidesofVshapeisdeformedintoanatthebeginningofbendingprocess,andtheloadappliedflattenthearcattheendofbendingprocessresultsinaxstressdistributionthatcausesanegativevalueof7.Comparingbothfigures,itisobservedthat10.Relationsbetweenspringbackandpunchradiusat300◦Cforspecofthreedirections.Fig.11.Punchanddieusedincircularcupdrawingtests.