OptimizationofDieDesignforForgingaTurbo-ChargerImpellerandaRingGearUsingProcessSimulationJayGunasekera,PhD,DSc,PE,ProfessorofMechanicalEngineering,OhioUniversity,USA,MazyadAl-MoheibandFahadAl-Mufadi,FormerPhDstudentsatOhioUniversity,USASYNOPSIS:Theobjectiveofthisprojectwastooptimizethepreformandfinaldiedesignfortwocomplexautomotiveforgedproducts(aturbineimpellerandaringgear)fortwodifferentforgingcompaniesintheUS.Theturbineimpellerhastohaveaminimumeffectiveplasticstrainof0.5inordertoincreasethetoughnessandresistfractureduetotheveryhighcentrifugalstresses.Itisalsoimportanttodistributethestrainandthegrainsizeasuniformlyaspossiblethroughoutthefinishedforgedpart,soastoachievethebestmechanicalpropertiesfortheAl2618turbinewheel.Optimizationofgrainsizewasperformedbydeterminingoptimaltemperatureandaveragestrainrate(bytheuseofZener-HollomonParameter).ThesecondprojectwastooptimizethediedesignforasteelRingGear,soastoreducethenumberofforgingstagesandalsoreducethematerialwastageduetoexcessiveflash.ThesoftwareusedwasMSC.SuperForge,thepredecessorofSimufact.forming,whichiscapableofcheckingthediefilling,defectformationanddiecontactinthefinalstage.Itcanalsodetermineanddisplayavarietyofusefulparameterssuchas；theeffectiveplasticstrain,effectivestrainrate,effectivestress,materialflow,temperature,force-timerelationshipandfinalshapebyusingSuperforge-FV(FiniteVolume)simulation.Itisconcludedthatthesoftwarecanbeeffectivelyusedtooptimizetheforgingprocesstomaximizethemechanicalstrength,minimizematerialscrap&forgingstagesandhencereducetheoverallcostofmanufacture.1.INTRODUCTION:Theobjectiveofthisprojectwastooptimizethepreformandfinaldiedesignfortwocomplexautomotiveforgedproducts.Thefirstpartisaturbo-chargerimpeller(orturbinewheel)madeofaluminum.Thispartrotatesatveryhighspeed(upto100,000RPM),acceleratesrapidlyfromrestandhasveryhighcentrifugalstresses.Thenewpreformdieshavetobedesigned,sothattheeffectiveplasticstraininthedeadmetalzoneofthispartcanbeincreasedtoavaluegreaterthan0.5.Theyieldstrengthcanalsobeincreasedbytheoptimizationofthenewpreformdiedesignsinceitincreasestheloweffectiveplasticstraininthedeadmetalzone.Thisalsoleadstoanearuniformeffectiveplasticstrainthroughouttheformedproduct.ReferringtoFigure1,itcanbeseenthataflatpreformdiewaspreviouslyusedintheforgedrotatingpartwithalloyAA2618.ReferringtoFigure2,adiewiththefinalcontourwasusedtoobtainaneffectiveplasticstraingreaterthan0.5inthefinalproduct.However,thisdoesnotresultinanoveralluniformplasticstraingreaterthan0.5.AproblemassociatedwiththisproductisthepresenceofloweffectiveplasticstrainwhichisdisplayedinFigures1and2bytheregionsofblueknownasDeadMetalZone(DMZ)1.Figure1StraincontourswithflatdieFigure2FinalstraincontourswithflatdieThegoalistoachievethebestmechanicalpropertiesthroughouttheforgedrotatingpartmadefromAA2618.Themainadvantageofthehotforgingoperationisgainedbydecreasingtheinhomogenitiesoftheworkpiece；porositiesareeliminatedbecauseoffusionofcavities.AnothergoalistheoptimizationoftheZener-Hollomanparameter,Z,bydetermininganoptimaltemperatureandanaveragestrainrate,inordertogetanindicationofthegrainsizeofthematerial.TheZenerHollomanparameterincreaseswithanincreaseintheaveragestrainrateandtheaveragestrainrateincreaseswithanincreaseintheeffectiveplasticstrainordecreaseinforgingtime.Itisalsoincreasedbyadecreaseintheforgingtemperature.Thecoarsecolumnargrainsarereplacedbysmallerequiaxed,recrystallizedgrainsthatgiveanincreaseinductilityandtoughness.Thiswoulddecreasethestrengthinadirectforging,but,apreformwherethemagnitudeofthestrainhasbeenincreasedwouldpreservethestrengthofthematerial.Thesecondpartisaringgear2,andtheobjectiveherewastoreducethenumberofforgingstagesandalsoreduceanymaterialwastage.FEMforwardsimulationhasplayedasignificantroleinpredictingthedeformationflowpatternsandhasimprovedthequalityoftheproduct.However,themainroleofFEMistoverifythediedesignsaccomplishedbyusingempiricalrelationshipsorbasedonengineeringpractice3.Usually,anumberofpreformsareneededinordertoachievethefinalcomplexshapefromtheinitialsimpleshapewiththeoptimalpropertiesandgeometricaltoleranceinmetalformingprocesses.Forgingpreformdesignisdeterminedviabackwarddeformationsimulationsusingaproceduresimilartodiedesignprocedurewherethedieshapesandprocessparametersaredeterminedbasedonthefinalproductshapeaswellasthematerialpropertiesrequirements.Consequently,forgingpre-formdesignprocessusingbackwardsimulationhasaveryimportantfunctioninforgingdiedesignprocess.Optimizingtheentireforgingprocesstoobtainthedesiredforgingpropertiessuchasachievingproperdie-fill,reducingthematerialwaste,reducingthediewear,obtainingfavorablegrainflowandtheloadrequiredcanbefulfilledbyusingadequateandappropriatepre-forms3.UBET(UpperBoundElementTechnique)wasusedforthebackwardsimulationtoobtainanoptimalperformandthenFiniteVolumeMethod(Simufact.formingsoftware)wasusedtodotheforwardsimulationandverifythedesign.UBEThasbeendevelopedandusedbymanyresearchers；forexampleLeeetal.4usedUBETtoanalyzetheforgingload,diefilling,andtheeffectivestrainforforgingswithandwithoutflashgap.Theprogramwasappliedtobothaxisymmetricandnon-axisymmetriccloseddieforgingaswellasplanestraincloseddieforgingwithrib-webtypecavity.Theresultsobtainedfromthisstudywerecomparedwithexperimentalresultsinwhich,agoodagreementwasachieved.Apre-formdesignapproachthatincorporatesbothFEM-basedforwardsimulationsandUBET-basedbackwardsimulationswasdevelopedbyLiu,etal.5.Bramley,6,hasemployedTEUBA,whichisaUBET-basedcomputerprogramfortheprocessofforgingpre-formdesignusingreversesimulations.Thisapproachisbasedonreversingtheflowbystartingfromthedesiredfinalshapewiththedievelocitiesreversedinsuchawaythatthematerialattheendofthedeepestdiecavityisconsideredtohaveafreeboundaryandmaterialflowsbackwarduptocertaintimestepwherethediesareseparatedfromthebillet,whichgivesthepre-formoftheprocess.Afiniteelement-basedinversediecontacttrackingmethodtodesigntheperformdieshapesofagenericturbine-diskforgingprocesswasusedbyZhao,etal.7.Finally,M.MohelibandJ.S.Gunasekera8usedUBETforbackwardsimulationinRingRollingandforforgingaRingGear.TheRingGearprojectisreportedinthispaper.ThetheoryofUBETcanbefoundinanumberofexcellentpublicationsandwillnotberepeatedhere.2.TURBINEWHEELANALYSIS1ThedevelopmentofFiniteElementAnalysis(FEA)techniqueshasprovidedanimportantlinkbetweenadvancesindieandequipmentdesignandanimprovedunderstandingofmaterialsbehavior.InputstotheFEcodesincludethecharacteristicsoftheworkpiecematerial(flowstressandthermalproperties)andthetool/workpieceinterface(frictionandheattransferproperties),aswellasworkpieceandtoolinggeometries.Typicaloutputsincludepredictionsforformingload,strain,strainrateandtemperaturecontourplots,andtoolingdeflections.Themethodofstudyforthismodelis:1.ThemodelsarefirstmadeinCADsoftwaresuchasSOLIDEDGEforthebilletsandforpreform(upsettingdies)aswellascloseddieforginginboththeupperandlowerdies.Thismodelisexportedforthree-dimensionalFEAtechniquessuchasFV(FiniteVolume)analysis(simulation)ofactualdieforgingoftherotatingpartwithSUPERFORGE9tofindflawsinthedesignofthepreform.2.Tofocusonoptimizingthepreformdesign.3.Todefinethebestpreformdesignandfinishedworkpiecebasedonoptimizationresultsandtoverifytheapplicabilityofthismethod.Oneofthemostimportantaspectsoftheclosed-dieforgingprocessisthedesignofpreforms(orblockers)toachieveadequatemetaldistribution.Withtheproperpreformdesign,defect-freemetalflowandcompletediefillcanbeachievedinthefinalforgingoperationandmetallossesintoflashcanbeminimized.Thedeterminationofthepreformconfigurationisanespeciallydifficulttaskinvolvingthoroughmetalflowunderstanding.The3DmodelingsoftwareSolidEdgeisusedtomodelparts,billetsanddies.SolidEdgehasanoptionbywhichvolumeofthemodeledpartcanbefound.SolidEdgeprovidestheoptionofBooleanoperationbywhichaspecificshapecanbesubtractedoraddedtoanothershape.Forthisresearch,lower-dieandupper-diearemodeledwithoutBooleanoperations.SimufactSUPERFORGEisusedtosimulatetheforgingprocess.3.FINITEVOLUMEMETHODThetraditionalfiniteelementmeshdistortswhenaneffortismadetotrackthedeformationofthematerial.However,whenthefinitevolumemethodisused,thecomputationalmeshusesafinitevolumemeshwithanunchangingframeofreferencewhenthematerialofthebilletflowsthroughthemesh.Theenergy,themassandthemomentumofthematerialaretransportedfromonecomponenttoanother.Thegridpointsforthefinitevolumesolverarepresetinspaceandtheelementsarejustpartitionsofthespacedefinedbytheconnectedgridpoints.Thematerialofthebilletbeneathanalysismovesthroughoutthefinit