Modified material constitutive models for serrated chip formation simulations an

Modi?edmaterialconstitutivemodelsforserratedchipformationsimulationsandexperimentalvalidationinmachiningoftitaniumalloyTi–6Al–4V

¨zelnMohammadSima,Tu?grulO

ManufacturingAutomationResearchLaboratory,DepartmentofIndustrialandSystemsEngineering,RutgersUniversity,Piscataway,NJ08854,USA

articleinfo

Articlehistory:

Received19June2010Receivedinrevisedform7August2010

Accepted9August2010

Availableonline19August2010Keywords:Machining

Titaniumalloys

FiniteelementsimulationsFlowsoftening

abstract

Titaniumalloyspresentsuperiorpropertiessuchashighstrength-to-weightratioandresistancetocorrosionbut,possesspoormachinability.Inthisstudy,in?uenceofmaterialconstitutivemodelsandelastic–viscoplastic?niteelementformulationonserratedchipformationformodelingofmachiningTi–6Al–4Vtitaniumalloyisinvestigated.Temperature-dependent?owsofteningbasedmodi?edmaterialmodelsareproposedwhere?owsofteningphenomenon,strainhardeningandthermalsofteningeffectsandtheirinteractionsarecoupled.Orthogonalcuttingexperimentshavebeenconductedwithuncoatedcarbide(WC/Co)andTiAlNcoatedcarbidecuttingtools.Temperature-dependent?owsofteningparametersarevalidatedonasetofexperimentaldatabyusingmeasuredcuttingforcesandchipmorphology.FiniteElementsimulationsarevalidatedwithexperimentalresultsattwodifferentrakeangles,threedifferentundeformedchipthicknessvaluesandtwodifferentcuttingspeeds.Theresultsrevealthatmaterial?owstressand?niteelementformulationgreatlyaffectsnotonlychipformationmechanismbutalsoforcesandtemperaturespredicted.ChipformationprocessforadiabaticshearinginmachiningTi–6Al–4Valloyissuccessfullysimulatedusing?niteelementmodelswithoutimplementingdamagemodels.

&2010ElsevierLtd.Allrightsreserved.

1.Introduction

TitaniumTi–6Al–4Va+balloy,themostcommonlyusedtitaniumalloyinaerospaceandbiomedicalindustries,exhibitshighlyfavorablepropertiessuchasahighstrength-to-weightratio,goodheattreatmentcapability,lowdensity,goodbio-compatibilityandresistancetocorrosion.TitaniumTi–6Al–4Valloyisconsidereddif?culttomachinebecauseofitslowthermalconductivity,highchemicalreactivitywithcuttingtoolmaterialsatelevatedtemperatures,andlowmodulusofelasticityeventhoughitsannealedhardnessisonlyabout35HRC[1].Thatisbecause,theheatgeneratedduringmachiningisconcentratedatthecuttingtool’srakefaceinadditiontothevibrationsgeneratedduringmachiningcausedbyitslowmodulusofelasticityandserratedchipformationresultinginrapidtoolwearandpoorworkpiecesurfacequality.Therefore,selectingcuttingconditions,toolmaterialanditscoating,andcuttingedgegeometryisimportantnotonlytoincreasetheproductivityofmachiningoperationbutalsotoobtainadesirablesurfaceintegrity(i.e.residualstresses,surfaceroughness,etc.)ofthe?nishedmachined

n

Correspondingauthor.Tel.:+17324451099;fax:+17324455467.

¨zel).E-mailaddress:ozel@rutgers.edu(T.O

part.Hence,comprehensivereviewsonmachinabilityoftitanium

alloysareprovidedin[1–4].

Manyexperimentalworkshavebeendedicatedtounderstandthechipformationmechanismanditsimplicationsintitaniummachining.Forthispurpose,quick-stoptestsinorthogonalcuttingareoftencarriedouttocaptureinstantaneouschipformationandchipmorphology[5,6]oraftermachining,imagesofpolishedandetchedchipcross-sectionsarecapturedwiththehelpofopticalorscanningelectronmicroscopy[4,7–11].Suchchipimagesdepicthighlyserratedshapewithregionsofco-existingadiabaticshearbandsandcracks(seeFig.1).Thesesegmented,butcontinuous,chipsareoftenformedathighcuttingspeeds.Adiabaticshearbands,aformoffailuremechanismandaprecursortoductilefractureoccurintitaniumalloys,areusuallyverynarrow(5–500mm)andtheyconsistofhighlyshearedmaterial.Adiabaticshearbandsbecomeapparentafter60m/mincuttingspeedandusuallyaboveafeedof0.05mm/rev.

Inroomtemperature,titaniumhashexagonalclose-packed(hcp)crystallinestructureknownasa-Tibutitformsbody-centeredcubic(bcc)crystallinestructureat8271C(1100K)knownasb-Ti.Thisisoftenreferredasallotropicphasetransformation[12].Itisclaimedthatduringmachiningprocess,duetolowthermalconductivityofTi–6Al–4Valloy,temperaturemayriseuprapidlytoaboveallotropicphasetransformation

0890-6955/$-seefrontmatter&2010ElsevierLtd.Allrightsreserved.doi:10.1016/j.ijmachtools.2010.08.004

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