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首页 Warm forming of aluminium-magnesium alloy sheet

Warm forming of aluminium-magnesium alloy sheet.pdf

Warm forming of aluminium-magne…

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2010-05-24 0人阅读 0 0 0 暂无简介 举报

简介:本文档为《Warm forming of aluminium-magnesium alloy sheetpdf》,可适用于工程科技领域

JournalofMechanicalWorkingTechnvlogy,()©ElsevierScientificPublishingCompany,AmsterdamPrintedinTheNetherlandsWARMFORMINGOFALUMINIUMMAGNESIUMALLOYSHEET*FSHEHATA,MJPAINTERandRPEARCEDepartmentofMaterials,CranfieldInstituteofTechnology,Cranfield(England)(ReceivedJune,)IndustrialsummaryAtthistime,whenenergyconservationisgainingpopularity,theincreaseduseofaluminiumsheetsformotorcarbodyconstructiontoreducetheoverallweightofthecar,andsouselessfuel,isappropriatePurealuminium,asusedinthedaysofhandmadebodies,isnowquiteoutofthequestionforthemassproducedcar,duetoinherentsoft°ness,andcurrentlyanumberofmediumstrengthalloysareusedInthe'sanaluminiumalloycontainingmagnesiumwasgenerallyemployedforpressformingpanelsforthemoreexpensiverangeofcarsThiswasanonheattreatablestrainhardeningalloy,butinevitablywasofreducedcoldductility,becauseofthealloyingadditionInthe's,aninterestinwarmformingemergedanditwasdiscoveredthatanaluminiummagnesiumalloycouldgiveatotalelongationofaboutatabout°CattheappropriatestrainrateTheworkherereportedcoversthewholerangeofcommercialaluminiummagnesiumalloystoattemperaturesfromto°Coverarangeofstrainrates,andtestswerecarriedoutinbothuniaxialandbiaxialtensionInuniaxialtension,(i)thehigherthemagnesiumcontent,(ii)thelowerthestrainrate,and(iii)thehigherthetemperature,thegreateristheelongationSimilarlytosuperplasticity,thiselongationismvaluedependent,iethehigherthevalueofmthegreatertheavailableelongationAnotherinterestingfindingisthatpriorcoldrollingofthesheetbeforewarmformingisbeneficialtototalelongationThelimitednumberofbiaxialtestscarriedoutindicatethatinthisdeformationmodethematerialislesssensitivetotemperatureandstrainratethaninuniaxialtension,suggestingthatpracticalexploitationmaybepossiblewithoutsophisticatedtemperatureandpressspeedcontrolIntroductionIngeneral,sheetmetalformingiscarriedoutatambienttemperatureElevatedtemperaturesaresometimesusedthoughforexample,berylliumandmagnesiumalloysrequirehightemperaturestoactivatethenecessaryslipsystemsforbiaxialductilityMoremundanely,heavysteelitemssuchasbracketsandchassispartsareoftenpreheatedbeforebendingtoavoidcracking,especiallywheretightradiiarerequired*PaperpresentedattheInternationalConferenceonWarmWorking,Sunderland,UK,September,Itseemsverylikelythatcertainofthemediumstrengthaluminiumalloyswillsoonbeusedforthemassproductionofmotorcarbodyparts,fortheweightincreasesresultingfromtheimplementationofcurrentsafetyregulationsmustbebothalleviatedandredistributed,whilstfromanenergyconservationstandpoint,lightercarsuselessfuelAsthebodyisthesingleheaviestiteminthemotorcar,itisherethatsignificantweightsavingscanbeachieveditisrealistictosaveuptoofthebodyweight(saykginsteel)bytheuseofaluminiumalloysheet,andretainthesamestiffnessandstrengthInthe'stheA~Mgalloyswereusedextensivelyforthemanufactureofbodypanels(doors,bonnet,boot)fortheRoverPOverthelifeofthismodelabouttenyears,bodiesweremanufactured,utilizingsometofA~Mgsheet,mostlymmthickMuchexpertisewasgatheredmostlyinthethenPressedSteelCoLtdontheprocessing,pressing,weldingandfinishingofthisalloyWhenthismodelfinished,aluminiumalloyswerestillused,butinmuchmoremodestquantitiesandthismagnesiumcontainingalloywassupplantedbyanACuMgalloyofhigherstrengthandroughlyequivalentductility,plusanimportantextrapropertyitagehardenedduringpaintstoring,whilsttheA~MgsoftenedThis,verybriefly,isthepositiontodayThewholeproblemwiththeformingoftwophasealloys,asopposedtosinglephasealloysorpuremetals(brass,austeniticstainlesssteel,extradeepdrawingsteel(Fe))isthatthestrengtheningeffectofthesecondphasereducesductilitythisisparticularlyapparentinthealuminiumalloys,wheretotalelongationscaneasilybehalvedThus,thoughtshaveturnedtoformingatelevatedtemperatures,forallmetalsaremoreductilewhenhotthanwhencoldHotformingofsheetmetalimmediatelysuggestssuperplasticity,where,generallyunderratherspecialconditionsofalloycomposition,temperatureandstrainrate,largeelongations()canbeproducedItwouldbedesirable,however,ifsomemoremodestincreaseinductilitycouldbeproduced,inconventionalalloys,atratherlowtemperaturesunderuncriticaloperatingconditions,anditisfromthisspringboardthatworkonthewarmformingofaluminiumalloyswaslaunchedFigshowstheresultsofsomeunpublishedwork(anasidefromanotherresearchprogramme)carriedoutatCranfieldin,inwhichlocalelongationsofat°CwerefoundinanAMgalloyinuniaxialtensionataninitialtruestrainrate(~i)of×sec~withnoattempttooptimizetheconditionsWhatthisworkdidshowwasthatthediffuseneckformedaftermaximumloadinuniaxialtensionwasgreatlyextended,suggestingthatasthetemperaturerisesandstrainhardeningbecomeslesseffectiveasaproductofstableflow(nvaluecontrolled),strainratehardeningtakesovertherole(mvaluecontrolled)ThisiselegantlydemonstratedinFigforanAMgalloyfromAyreswhichshowsthefallineuandconcurrentriseinedInanindustrialpressing,thepointatwhichapressingfails,eitherbytheformationofalocalneckorbyfracture,followsdiffuseneckingwhichEi='xseczooiwOo',oTEMPERATURE°CFigElongationversustemperatureforanAMgalloyAfterAyres(Ref,)EL=gxseczoz~~oqwIncreasingedm~eFa~TEMPERATURE°CFigTheeffectoftemperatureupontheuniformstrain(eu)andthediffusestrain(ed)inanAMgalloyitselffollowsuniformstrainingandsoitisgenerallyunimportantwhichflowmodecontributestoasuccessfulpartthepressshopcriteriononlyrequiresneitherweaknessnoracrackinthepartThus,anoverallincreaseinductilityshouldbeadvantageousTheexistenceofstrainratehardeninghasalwaysbeenrecognisedbythoseconcernedwiththehotworkingofmetalsathighratesofstrainbutm,thestrainratehardeningexponentfromtherelationshipa=k~mneverexceedsinthehotworkingexperimentsofrefthoughm=insomesuperplasticalloysAyresshowsm=insomeofhisexperimentsOOoz<oJuu,~Motsukietal£i=xlO'sec~e~PeGrce'sResult~L="x$eCI~o~o'IuO~goTEMPERATURE°CFigElongationversustemperatureforasuperplasticAIMgalloyMatsukietalhavestudied,basically,anAMgalloy,andshown"fullsuperplasticity"elongationsofatei=Xsecandm=,butat°CResultsfromrefarereplottedasthepresentFigwherealsoisshownapointrelatingtotheupperendofthecurveofFigthelatterfitsratherwell,eventhoughthetwostrainratesusedarenotexactlythesameOneconclusionfromthispastworkisclearIncertainaluminiummagnesiumalloys,asignificantincreaseofductilitycanbeachievedbyraisingthetemperature,evento°CThisisbecausem,whichisapprox,zeroatroomtemperatureforfacecentredcubicmetals,attainsavalue>~inwarmworkingandthestrainratehardeningwhichthisachievesmorethancompensatesforn~Phenomenologically,thisbehaviouristhesameassuperplasticity,ie,thegreaterthemvalue,thehigherthetotalelongationThisisthestartingpointfortheexperimentsnowtobedescribedMaterialsArangeofcommercialsheetaluminiummagnesiumalloys(thicknessmm)containingbetweenandmagnesiumwereusedforthisworkDesignations,nominalcompositionsandhardnessaregiveninTableExperimentalThewarmformingbehaviourofallthesealloyshasbeenevaluatedbybothconventionaluniaxialtensiontestingand,oftheMgseries,bylaboratorypressformingTABLEAlloyDesignationHardness(Sv)AnnealedcommercialpurityaluminiumMg()AnnealedaluminiummagnesiumMg()ColdrolledaluminiummagnesiumMg()ColdrolledaluminiummagnesiumMg()AnnealedaluminiummagnesiumMg()ColdrolledaluminiummagnesiumMg()ColdrolledaluminiummagnesiumMg()AnnealedaluminiummagnesiumMg()ColdrolledaluminiummagnesiumMg()ColdrolledaluminiummagnesiumMg()(a)TensiontestingStandardmmXmmgaugelengthtensilespecimens,cutintherollingdirection,weredeformedtofractureinanInstronkgf(kN)tensiletestingmachine,usingarangeoftestingtemperatures(°C)andcrossheadspeeds(mmmin)Thestrainratehardeningexponentm,fromtherelationshipo=k~rn,wasobtainedbytheBackofenstepchangemethod,betweenelongation(b)PunchstretchingtestsSomehotpressingtestswereperformedontheAMgalloyusingalaboratorypressspeciallydesignedandbuiltforBirminghamUniversityforpressformingatelevatedtemperaturesStretchformingtestsusingafullyclampedblankandammhemisphericalpunchlubricatedwithPTFEfilmwerecarriedoutoverarangeoftemperatures(°C)andpunchspeeds(mmmin)andasimultaneousXYrecordingofpunchloadvspunchheightenabledthepunchheightatfracturetobedeterminedResultsanddiscussionResultsaregiveninFigsFigshowstheeffectoftemperatureontotalelongationataparticularstrainrateforallthealloysThisshowsthatthetotalelongationofMg()isnotsignificantlyaffectedbyatemperatureriseof°CbutthatthethreealloysallshowaboutfivefoldincreasesandthehigherthemagnesiumcontentthehighertheelongationFigshowstheeffectofinitialstrainrateontotalelongationAgainMg()isvirtuallyinsensitive,whilstthethreeAIMgalloysallshowincreasesinelongationwithdecreasingstrainrateFigsandshowtheeffectofinitialhardness(temper)onelongationatelevatedtemperaturesInbothcasesthesofttemperismostductileuptoofQOMg()xMg()~,'~Mg()oMg()£="xl'secJL,~~~JGl,O'i~oI~~o~~~TEMPERATURE°CFigTheeffectofMgcontentuponelongationoverthetemperaturerangecoveredcMg()xMg()t,zMg()oMg()TESTTEMPERATURE°COzxIOI,LINITIALSTRAINRATE(,~~wsecFigTheeffectofinitialstrainrateonelongationforallalloysinvestigatedoMg()xZMg()AAMg()E=x~sec~oozdo~~~iiiiiiTEMPERATURE°CFigEffectofinitialsheethardnessuponelongationforanAIMgalloyAMg()xMg()oMg()~i=x~sec~wTEMPERATURE°CFigTheeffectofinitialhardnessuponfinalelongationforAMgalloysabout°CwhereafterthehardesttemperisbestFigshowstheeffectoftemperatureonmvalueforMg()atthreedifferentstrainratesItwillbeseenthatm=fortheloweststrainrateandthehighesttemperature,afigurewellabovethatusuallyfoundinconventionalhotworkingm,o~secxxlOsecz~x~secxIiiIIIITEMPERATURE°CFigTheeffectofstrainrateuponmvalueforMg()overarangeoftemperaturesandstrainratesFigshowstherelationshipbetweenelongationandmvalueforallalloystestedThisisreminiscentofthecurvespublishedbyWoodfordandLeeandBackofenforalargenumberofdifferentsuperplasticalloys,althoughtheirmaterialsshowedhighermvaluesandconsequentlyhigherelongationsSofarthedatapresentedhavebeenconcernedwithparameterswhicheffectductility,underthesesocalledwarmworkingconditionsFigshowstheeffectofnominalmagnesiumcontentonthemaximumflowstress,definedasPmaxAowherePmaxisthemaximumloadinuniaxialtension,usuallyreachedinthesetestsatstrains~<andAoistheoriginalcrosssectionalareaofthespecimenHere,aswouldbeexpected,flowstressriseswithadditionofthealloyingelementThereductioninductilitywhichincreasingstrengthnormallyproducesis,however,notapparent,forasFigshows,otherthingsbeingequal,thetotalelongationrisesfrominMg()toinMg()InFigthefallinstresswithtemperatureforMginallconditionsIDataFrc~AllAlloysTested~C~CwC","•t•~mVALUEFigRelationshipbetweenmandelongationforallalloystestedfT~i~Ox~¢uJc~RhTEMREi=×IseiIiIlNOMINALMAGNESIUMFigEffectofcompositionuponflowstressataparticulartemperatureandstrainrate,,,O,gOfioxMg(z,)oMg()<""o!Mg()~"E~x"seC~o"MatsukietoL("~T~"o•,,'""~,TEMPERATURE"CFigEffectoftemperatureonflowstressforAIMgalloys,ItogetherwiththeresultsofMatsukietalfortheirsuperplasticAMgalloy,isshownThisdemonstratesthatthewarmformingdeformationprocessrequiresconsiderablyhigherstressesthandoesgenuinesuperplasticityeventhoughajudiciouslydottedlineconnectsMatsuki'sresultstothepresentresultsinareasonablysmoothmannerItseemsimportantthatthetemperaturerange°Cshouldnowbeinvestigatedinthesealloys,ifonlyforcompletenessInanattempttobridgethegapbetweenconventionallaboratorytestingandtheproductionofpressings,alimitednumberofhotpunchstretchingexperimentswerecarriedoutResultsforMg(),Mg()andMg()areshowninFigThefirstthingthatisapparentisthelargeincreaseinmaximumcupheighth*,betweenroomtemperatureandanyofthetesttemperatures,whichrangefromto°CSecondly,thestrainrateeffectapparentinthetensiletestisalsoapparenthereThirdly,asinthetensionteststheinitialcoldrolledconditionofthesheetisimportantForinstance,at°Candapunchspeedofmmmin(~×sec),Mg()gaveh*=mmandMg()gaveh*=mmThiseffectisapparentlynotasmarkedhereasinuniaxialtensionItwouldbesurprisingifamoremarkedeffectwasnotachievedwiththematerialsofhighermagnesiumcontentsOneusefuleffectseemstobetherathersmalleffectofraisingthetemperaturefromto°CinMg()atapunchspeedofmmminThissuggeststhatinpracticewarmformingcouldperhapsbecarriedoutwithoutsophisticatedtemperaturecontrolAgaintheeffectofpunchspeedseemslesscriticalthanmighthavebeenexpected,suggestingthatwarmformingmightevenbesuccessfulatconventionalpressspeedsB,£E•=Bwn~cJ~u~~""wIaoMg{}~'~x~~#~punchspeed•mmminxMg(),,~xII,Mg()o~ot~~~,ITEMPERATURE°CFigCupheightversustemperatureforthreeAMgalloys,testedatdifferentpunchspeedsConclusions()Enhancedductilityisfoundinthealuminiummagnesiumalloyseriesattemperatureslessthan°C(Tin)butatcomparativelylowstrainrates(~Xsecl)()Increasingthemagnesiumcontentuptoincreasestheductilityaswellasincreasingthestrengthatalltemperaturesandstrainrates()Theinitialdegreeofcoldworkisimportant,inthatitinfluencestheelongationattainableAtlowtemperaturesthesoftesttemperisbestthissituationisreversedatthehighesttemperatures()Theparametermiscentraltowarmworkingasitistosuperplasticity,thehigherthemvaluethehighertheelongationandthelowertheflowstressforagivenalloy()Thereductioninflowstress,inwarmworking,isanorderofmagnitudelessthanthatinsuperplasticalloys()Theeffectsdescribedforuniaxialtensionarebroadlyfollowedinthepunchstretchingexperiments,showingthatbiaxialbehaviourcanbepredictedfromuniaxialdata,asisthecaseinconventionalsheetmetalforming()NewmethodsneedtobedevisedfortheefficientexploitationofthisphenomenonAcknowledgementsWewouldliketothankAlcanLtd,BanburyAlusuisseLtd,ZurichandBirmetalsLtd,Birmingham,fortheprovisionofmaterialandtheDepartmentofIndustrialMetallurgy,UniversityofBirmingham,fortheuseoftheirhotpressingfacilityReferencesRAAyres,MetTrans,A,A()CMSellarsandWJMeGTegart,IntMetRev,Review,()KMatsuki,YUetani,MYamadaandYMurakami,MetSciJ,()WABackofen,IRTurnerandDHAvery,TransASM,()DJBarrett,SheetMetInd,()DAWoodford,TransASM,()DLeeandWABackofenTransMetSocAIME,()

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