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OptimizationofDieDesignforForgingaTurboChargerImpellerandaRingGearUsingProcessSimulationJayGunasekera,PhD,DSc,PE,ProfessorofMechanicalEngineering,OhioUniversity,USA,MazyadAlMoheibandFahadAlMufadi,FormerPhDstudentsatOhioUniversity,USASYNOPSISTheobjectiveofthisprojectwastooptimizethepreformandfinaldiedesignfortwocomplexautomotiveforgedproductsaturbineimpellerandaringgearfortwodifferentforgingcompaniesintheUS.Theturbineimpellerhastohaveaminimumeffectiveplasticstrainof0.5inordertoincreasethetoughnessandresistfractureduetotheveryhighcentrifugalstresses.Itisalsoimportanttodistributethestrainandthegrainsizeasuniformlyaspossiblethroughoutthefinishedforgedpart,soastoachievethebestmechanicalpropertiesfortheAl2618turbinewheel.OptimizationofgrainsizewasperformedbydeterminingoptimaltemperatureandaveragestrainratebytheuseofZenerHollomonParameter.ThesecondprojectwastooptimizethediedesignforasteelRingGear,soastoreducethenumberofforgingstagesandalsoreducethematerialwastageduetoexcessiveflash.ThesoftwareusedwasMSC.SuperForge,thepredecessorofSimufact.forming,whichiscapableofcheckingthediefilling,defectformationanddiecontactinthefinalstage.Itcanalsodetermineanddisplayavarietyofusefulparameterssuchastheeffectiveplasticstrain,effectivestrainrate,effectivestress,materialflow,temperature,forcetimerelationshipandfinalshapebyusingSuperforgeFVFiniteVolumesimulation.Itisconcludedthatthesoftwarecanbeeffectivelyusedtooptimizetheforgingprocesstomaximizethemechanicalstrength,minimizematerialscrapforgingstagesandhencereducetheoverallcostofmanufacture.1.INTRODUCTIONTheobjectiveofthisprojectwastooptimizethepreformandfinaldiedesignfortwocomplexautomotiveforgedproducts.Thefirstpartisaturbochargerimpellerorturbinewheelmadeofaluminum.Thispartrotatesatveryhighspeedupto100,000RPM,acceleratesrapidlyfromrestandhasveryhighcentrifugalstresses.Thenewpreformdieshavetobedesigned,sothattheeffectiveplasticstraininthedeadmetalzoneofthispartcanbeincreasedtoavaluegreaterthan0.5.Theyieldstrengthcanalsobeincreasedbytheoptimizationofthenewpreformdiedesignsinceitincreasestheloweffectiveplasticstraininthedeadmetalzone.Thisalsoleadstoanearuniformeffectiveplasticstrainthroughouttheformedproduct.ReferringtoFigure1,itcanbeseenthataflatpreformdiewaspreviouslyusedintheforgedrotatingpartwithalloyAA2618.ReferringtoFigure2,adiewiththefinalcontourwasusedtoobtainaneffectiveplasticstraingreaterthan0.5inthefinalproduct.However,thisdoesnotresultinanoveralluniformplasticstraingreaterthan0.5.AproblemassociatedwiththisproductisthepresenceofloweffectiveplasticstrainwhichisdisplayedinFigures1and2bytheregionsofblueknownasDeadMetalZoneDMZ1.Figure1StraincontourswithflatdieFigure2FinalstraincontourswithflatdieThegoalistoachievethebestmechanicalpropertiesthroughouttheforgedrotatingpartmadefromAA2618.Themainadvantageofthehotforgingoperationisgainedbydecreasingtheinhomogenitiesoftheworkpieceporositiesareeliminatedbecauseoffusionofcavities.AnothergoalistheoptimizationoftheZenerHollomanparameter,Z,bydetermininganoptimaltemperatureandanaveragestrainrate,inordertogetanindicationofthegrainsizeofthematerial.TheZenerHollomanparameterincreaseswithanincreaseintheaveragestrainrateandtheaveragestrainrateincreaseswithanincreaseintheeffectiveplasticstrainordecreaseinforgingtime.Itisalsoincreasedbyadecreaseintheforgingtemperature.Thecoarsecolumnargrainsarereplacedbysmallerequiaxed,recrystallizedgrainsthatgiveanincreaseinductilityandtoughness.Thiswoulddecreasethestrengthinadirectforging,but,apreformwherethemagnitudeofthestrainhasbeenincreasedwouldpreservethestrengthofthematerial.Thesecondpartisaringgear2,andtheobjectiveherewastoreducethenumberofforgingstagesandalsoreduceanymaterialwastage.FEMforwardsimulationhasplayedasignificantroleinpredictingthedeformationflowpatternsandhasimprovedthequalityoftheproduct.However,themainroleofFEMistoverifythediedesignsaccomplishedbyusingempiricalrelationshipsorbasedonengineeringpractice3.Usually,anumberofpreformsareneededinordertoachievethefinalcomplexshapefromtheinitialsimpleshapewiththeoptimalpropertiesandgeometricaltoleranceinmetalformingprocesses.Forgingpreformdesignisdeterminedviabackwarddeformationsimulationsusingaproceduresimilartodiedesignprocedurewherethedieshapesandprocessparametersaredeterminedbasedonthefinalproductshapeaswellasthematerialpropertiesrequirements.Consequently,forgingpreformdesignprocessusingbackwardsimulationhasaveryimportantfunctioninforgingdiedesignprocess.Optimizingtheentireforgingprocesstoobtainthedesiredforgingpropertiessuchasachievingproperdiefill,reducingthematerialwaste,reducingthediewear,obtainingfavorablegrainflowandtheloadrequiredcanbefulfilledbyusingadequateandappropriatepreforms3.UBETUpperBoundElementTechniquewasusedforthebackwardsimulationtoobtainanoptimalperformandthenFiniteVolumeMethodSimufact.formingsoftwarewasusedtodotheforwardsimulationandverifythedesign.UBEThasbeendevelopedandusedbymanyresearchersforexampleLeeetal.4usedUBETtoanalyzetheforgingload,diefilling,andtheeffectivestrainforforgingswithandwithoutflashgap.Theprogramwasappliedtobothaxisymmetricandnonaxisymmetriccloseddieforgingaswellasplanestraincloseddieforgingwithribwebtypecavity.Theresultsobtainedfromthisstudywerecomparedwithexperimentalresultsinwhich,agoodagreementwasachieved.ApreformdesignapproachthatincorporatesbothFEMbasedforwardsimulationsandUBETbasedbackwardsimulationswasdevelopedbyLiu,etal.5.Bramley,6,hasemployedTEUBA,whichisaUBETbasedcomputerprogramfortheprocessofforgingpreformdesignusingreversesimulations.Thisapproachisbasedonreversingtheflowbystartingfromthedesiredfinalshapewiththedievelocitiesreversedinsuchawaythatthematerialattheendofthedeepestdiecavityisconsideredtohaveafreeboundaryandmaterialflowsbackwarduptocertaintimestepwherethediesareseparatedfromthebillet,whichgivesthepreformoftheprocess.AfiniteelementbasedinversediecontacttrackingmethodtodesigntheperformdieshapesofagenericturbinediskforgingprocesswasusedbyZhao,etal.7.Finally,M.MohelibandJ.S.Gunasekera8usedUBETforbackwardsimulationinRingRollingandforforgingaRingGear.TheRingGearprojectisreportedinthispaper.ThetheoryofUBETcanbefoundinanumberofexcellentpublicationsandwillnotberepeatedhere.2.TURBINEWHEELANALYSIS1ThedevelopmentofFiniteElementAnalysisFEAtechniqueshasprovidedanimportantlinkbetweenadvancesindieandequipmentdesignandanimprovedunderstandingofmaterialsbehavior.InputstotheFEcodesincludethecharacteristicsoftheworkpiecematerialflowstressandthermalpropertiesandthetool/workpieceinterfacefrictionandheattransferproperties,aswellasworkpieceandtoolinggeometries.Typicaloutputsincludepredictionsforformingload,strain,strainrateandtemperaturecontourplots,andtoolingdeflections.Themethodofstudyforthismodelis1.ThemodelsarefirstmadeinCADsoftwaresuchasSOLIDEDGEforthebilletsandforpreformupsettingdiesaswellascloseddieforginginboththeupperandlowerdies.ThismodelisexportedforthreedimensionalFEAtechniquessuchasFVFiniteVolumeanalysissimulationofactualdieforgingoftherotatingpartwithSUPERFORGE9tofindflawsinthedesignofthepreform.2.Tofocusonoptimizingthepreformdesign.3.Todefinethebestpreformdesignandfinishedworkpiecebasedonoptimizationresultsandtoverifytheapplicabilityofthismethod.Oneofthemostimportantaspectsofthecloseddieforgingprocessisthedesignofpreformsorblockerstoachieveadequatemetaldistribution.Withtheproperpreformdesign,defectfreemetalflowandcompletediefillcanbeachievedinthefinalforgingoperationandmetallossesintoflashcanbeminimized.Thedeterminationofthepreformconfigurationisanespeciallydifficulttaskinvolvingthoroughmetalflowunderstanding.The3DmodelingsoftwareSolidEdgeisusedtomodelparts,billetsanddies.SolidEdgehasanoptionbywhichvolumeofthemodeledpartcanbefound.SolidEdgeprovidestheoptionofBooleanoperationbywhichaspecificshapecanbesubtractedoraddedtoanothershape.Forthisresearch,lowerdieandupperdiearemodeledwithoutBooleanoperations.SimufactSUPERFORGEisusedtosimulatetheforgingprocess.3.FINITEVOLUMEMETHODThetraditionalfiniteelementmeshdistortswhenaneffortismadetotrackthedeformationofthematerial.However,whenthefinitevolumemethodisused,thecomputationalmeshusesafinitevolumemeshwithanunchangingframeofreferencewhenthematerialofthebilletflowsthroughthemesh.Theenergy,themassandthemomentumofthematerialaretransportedfromonecomponenttoanother.Thegridpointsforthefinitevolumesolverarepresetinspaceandtheelementsarejustpartitionsofthespacedefinedbytheconnectedgridpoints.Thematerialofthebilletbeneathanalysismovesthroughoutthefinitevolumemesh.Thus,themovementofthematerialviatheelementsofconstantvolumeiscomputedbythefinitevolumesolver.Thediesworklikeaboundarytotheflowofmaterialinaforgingsimulationemployingthefinitevolumemesh,whereasthestressespresentinthematerialcontainedbythefinitevolumemeshapplyforcesonthesurfacesofthedies.Inthefinitevolumetechnique,themeshmustbebigenoughtocoverthematerialoftheworkpieceoncedeformationhastakenplace.Afundamentalfiniteelementmeshalsoactslikeacontainerandthematerialcannotdepartthemesh.Fromafinitevolumemesh,stresswavereflectionsandpressurebuildupdevelop,butarenotsignificantenoughtobeanalyzed.FVMcomputermodelingisfavorableforsimulatinggrossmaterialdeformationsintrinsicinforgingoperationsduetothisdistinctivefeature.Moreover,therequirementforremeshingtechniqueswhichareusuallythoughttobethemajorbottlenecksin3Dforgingsimulationsbasedonthefiniteelementmethod,istotallyeliminated9,10.Figure3Modelofupperdies,lowerdiesandbillets
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