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ofg,isprocess.ultrasonictheultrasonicKeywordsUltrasonicmachiningTurningFiniteelementmodellingMicrostructuretheexistent,conventionalturningCTtechnology.frequencyultrasonicvibration,superimposedontheconventionalmovementofthecuttingtoolFig.1,hasprovedtobeeffectiveinmachiningintractablemetalalloysaswellasbrittlematerials,suchasceramicsandcuttingforces7,8.trolsystem.Thissystemstabilisestheturningprocesswithultrasonicvibrationandmakesthisprocesshighlycontrollable.Thedetaileddescriptionofthisnovelcontrolsystemisgivenin9,10.TheexperimentalpartofthispaperstudiesUATwithautoresonantcontrolincomparisontoconventionalturning.AnotherimportantissueconcerningUATistheCorrespondingauthor.Tel.441509227504fax441509Conventionalmachiningofmodernnickelandtitaniumbasedsuperalloys,usedinaerospaceapplications,causeshightooltemperaturesandsubsequentfastwearofcuttingedgesevenatrelativelylowcuttingspeeds.Agrowingdemandformachiningtheseintractablematerialsrequiresnewadvancedturningtechnologies.Suchatechnologywasintroducedin1960shighNevertheless,uptothepresentdayUAThasnotbeenwidelyintroducedintoindustrialenvironment.ThemainreasonforitissensitivityoftheUATprocesstotheloadappliedtothecuttingtip,resultinginthelossofcuttingefficiencywhentheloadchangesoradifferenttipisused.However,thislimitationhasrecentlybeeneliminatedwiththeinventionoftheautoresonantcon1.IntroductionTurningisamachiningprocess,whereathinsurfacelayerofthetreatedmaterialisremovedfromaworkpiecebyasharpwedgeshapedcuttingtoolformingacylindricalsurface.Thistechnologyhasbeenusedforcenturiesmainlyforcuttingvarioustypesofmetallicmaterials.However,intherecentyears,arangeofnewalloysandcompositematerialshasbeendevelopedforvariousengineeringapplications.Manyofthesenewmaterialsbecomemuchmoredifficulttocutwithglass.Thistechnology,calledultrasonicallyassistedturningUAT,demonstratesarangeofbenefitsinmachininghardmetalalloysadecreaseincuttingforcesofuptoseveraltimes1–4,improvementinsurfacefinishbyupto50comparedtoCT5andnoisereduction6.Asformachiningofbrittlematerials,ceramicsandglasspresentlyrequireprolongedandexpensivepostprocessingtoobtainthesurfacequalityrequiredforopticalcomponentsUATallowsobtainingmirrorsurfacefinishinmachiningthesematerialsaswellasconsiderablereductionintoolwearandaverageUltrasonicallyassistedturningandexperimentalV.I.Babitsky,A.V.Mitrofanov,WolfsonSchoolofMechanicalandManufacturingEngineerinAbstractUltrasonicallyassistedturningofmodernaviationmaterialsamplitudeaC2515lmsuperimposedonthecuttingtoolmovement.nonlinearresonantmodeofvibrationthroughoutthecuttingworkpiecesmachinedconventionallyandwiththesuperimposedprocessandnanoindentationanalysesofthemicrostructureofmodelprovidesnumericalcomparisonbetweenconventionalandcuttingforcesandcontactconditionsattheworkpiece/toolinterface.C2112004ElsevierB.V.Allrightsreserved.Ultrasonics422004227502.Emailaddressv.silberschmidtlboro.ac.ukV.V.Silberschmidt.0041624X/seefrontmatterC2112004ElsevierB.V.Allrightsreserved.doi10.1016/j.ultras.2004.02.001aviationmaterialssimulationsstudyV.V.SilberschmidtLoughboroughUniversity,LeicestershireLE113TU,UKconductedwithultrasonicvibrationfrequencyfC2520kHz,AnautoresonantcontrolsystemisusedtomaintainthestableExperimentalcomparisonofroughnessandroundnessforvibration,resultsofhighspeedfilmingoftheturningmachinedmaterialarepresented.ThesuggestedfiniteelementturningofInconel718intermsofstress/strainstate,81–86www.elsevier.com/locate/ultrasmechanicsofthisprocess.ThereareonlyafewsourcesoftheworkpieceparalleltotheXaxis,Fig.1b,orinthefeeddirection,i.e.alongtheaxisoftheworkpieceZaxis,Fig.1b.Aselfsustainedresonantmodeofvibrationofthiscuttingsystemisimplementedviatheautoresonantcontrolsystem,whichisdescribedindetailin9,10.ArangeofturningtestshasbeenconductedtocomparetheusageofUATandCTformachiningaviationmaterials.Thedetaileddescriptionofthesetestscanbefoundin5.AmongthematerialsusedforthetestsisInconel718–ahighgradeheatresistantNibasedsuperalloywidelyusedintheaerospaceindustry.Thismaterialisveryabrasiveandcausesthetoolbluntingandhighcuttingtemperatureswhenmachinedconventionally.Thesurfacequalityobtainedbyturningisoneofthecrucialfactorsinmetalcuttingandisextremelysensitivetoanychangesinthemachiningprocess.ThesurfacefinishofspecimensiscomparedintermsofaverageroughnessmeasurementðRaÞandmeasurementofroundnessthepeaktovalleymeasure,usingtheTays42200481–86Fig.1.Experimentalsetupforultrasonicallyassistedturninga,andaschemeofrelativemotionoftheworkpieceandcuttingtool82V.I.Babitskyetal./Ultrasonicattemptingtodescribetheprocessesintheworkpiece/cuttingtoolinteractionzoneandtheirinfluenceonthestructureofthemachinedmaterial3,6,11.Theseworksstudymostlythedynamicsoftheultrasonicmachineunitandnottheresponseofthetreatedmaterialtothistechnology,whileaclearunderstandingofmechanicalprocessesinthematerialduringUATwouldcertainlyallowafurtherdevelopmentoftheUATtechnology.ThemainaimofthispaperistostudyexperimentallyandnumericallythematerialmechanicsoftheUATprocess.2.ExperimentalstudiesTheexperimentalsetupusedtostudyUATisshowninFig.1.Theworkpieceisclampedinthechuckoftheuniversallatheandrotateswithaconstantspeed.Highfrequencyelectricimpulses,fedtotheinputoftheultrasonictransducer,excitevibrationinpiezoceramicringsduetothepiezoelectriceffect.Thevibrationamplitudeisintensifiedintheconcentratorandtransmittedtothetoolholderatthethinendoftheconcentrator.Resultantvibrationofthecuttingtipfixedinthetoolholderreaches15lmi.e.30lmpeaktopeakatafrequencyofabout20kHz.ThevibrationcanbeappliedeitherinthedirectiontangentialtothesurfaceinorthogonalUATwithtangentialvibrationb.lorHobson–Talysurf4surfacemeasurementinstrument.Thefollowingcuttingparametersareusedtomachinetestedspecimensdepthofcutd¼08mm,feedrates¼005mm/rev,andcuttingspeedv¼17m/min.ThesameparametersareusedforbothUATandCT,withsuperimposedultrasonicvibrationinthefeeddirectionappliedforUAT.Fig.2ashowsrepresentativeaxialprofilesofthemachinedsurfaceoftheInconel718.ItisobviousthatmagnitudesofRaarereducedbynearly50forspecimensmachinedwithUAT.Furthermore,theregularityofthesurfaceprofileisgreatlyimproved,asthesurfaceFig.2.SurfacequalityofInconel718specimensmachinedwithUATandCTaxialsurfaceprofilesa,roundnessprofilesb.Cuttingparametersd¼08mm,s¼005mm/rev,v¼17m/min.direction.Apparently,thereasonfortheseimprovementsisthes42200481–8683changeofthenatureofthecuttingprocess,whichistransformedintotheonewithmultipleimpacthighfrequencyinteractionbetweenthecuttingtoolandchipduetoappliedultrasonicvibration.Thisleadstochangesinmaterialdeformationprocessesandfrictionforces,andincreaseinthedynamicstiffnessofthelathetoolworkpiecesystem6,11duetothevibrationfrequencylevelsconsiderablyexceedingitsnaturalfrequency.Inadditiontomeasurementsofthesurfacequality,themicrostructureofthemachinedsurfacehasbeeninvestigated.Inconel718workpiecesaremachinedunderthesamecuttingconditionsv¼36m/min,d¼01mm,s¼003mm/revwithapplicationofultrasonicvibrationintangentialdirectionandwithoutit.Then,nanoindentationanalysesofthesurfacelayersareperformedwiththeNanoTestPlatformmadebyMicroMaterialsLtd.Accordingtotheresultsofthesetests,thewidthofthehardenedsurfacelayer,whichresultsfromtheextensivedeformationandhightemperatureprocessesduringtheturningprocedures,fortheultrasonicallymachinedspecimenishalfthesizethatoftheconventionallymachinedone40and80lm,respectively.Furthermore,theaveragehardnessofthislayerforUATabout15GPaisahalfofthatforCTandconsiderablyclosertothehardnessoftheuntreatedmaterialabout7GPa.Thehardnessofthematerialnonlinearlyincreaseswithariseintheleveloftheresidualplasticstrains.Hence,nanoindentationtestsindicatelowerresidualstrainsinthesurfacelayerforworkpiecesmachinedwithUATandaconclusioncanbedrawnthattheUATprocedureisconsiderablymoredelicatetotheworkpiecematerial.3.NumericalanalysisofUATFiniteelementFEsimulationsareamajortoolformodellingofmachiningprocesses.Ithasbeenusedformodellingofturningforsome30years.Theoverviewofthestateoftheartinmetalcuttingsimulationscanbefoundin13,14.However,uptotheauthorsknowledge,nomodelsforUAThavebeendevelopeduntilnow.ThetwodimensionalFEmodelofbothCTandUATdebecomessmootherintheaxialdirection.AconsiderableimprovementisalsoobtainedforroundnessofmachinedworkpiecesFig.2bapeaktovalleyvalueofroundnessmeasures4.20lmforCT,whereasitattainsonly1.89lmforUAT.Hence,theroundnessisimprovedby40whenultrasonicvibrationissuperimposeduponthemovementofthecuttingtool.Itisworthnoticingthatsimilarresultshavebeenobtainedbyotherresearchers7,12utilisingvibrationinthetangentialV.I.Babitskyetal./UltrasonicscribedinthispaperisbasedonthecommercialFEcodeMSC.Marc15.Anorthogonalturningprocess,i.e.thecuttingprocesswherethetooledgeisnormaltobothcuttingandfeeddirections,withtangentialvibrationisconsidered.Fig.1bshowsaschemeofthemodelledrelativemotionoftheworkpieceandcuttingtooltherotationaxisofthecylindricalworkpieceisorthogonaltotheplaneofthefigure.Theworkpiecemoveswithaconstantvelocity,whereasthetoolvibratesharmonicallyarounditsequilibriumpositionwithfrequencyf¼20kHzandamplitudea¼15lm,correspondingtothevaluesusedinexperimentalstudies.Otherparametersofsimulationsareuncutchipthicknesst1¼01mmwhichcorrespondstothedepthofcut,rakeangleofthetoolc¼10C176,cuttingspeedV¼9m/min.Suchparametersofvibrationandofthecuttingprocessprovideseparationofthecutterfromthechipwithineachcycleofultrasonicvibration.ThematerialconstantsforagedInconel718aretakenfrom16.Kinematicalboundaryconditionsfortheworkpieceareappliedtoitsleft,rightandbottomsidesFig.1b,whereasitstopsurfaceisfreeVxjAH¼VVxjFG¼VVxjHG¼VVyjHG¼0Thermalboundaryconditionsincludeconvectiveheattransferfromtheworkpiece,chipandtoolfreesurfacestotheenvironmentC0koTon¼hðTC0T1Þ,wherekistheconductivity,hisaconvectiveheattransfercoefficient,T1istheambienttemperature.ThethermalfluxpassingfromthechiptothecutteralongthecontactlengthLcFig.1bisdescribedasfollowsq¼HðTchipC0TtoolÞ,whereHisacontactheattransfercoefficient,TchipandTtoolarechipandtoolsurfacetemperatures,respectively.Themodeltakesintoconsiderationthefollowingfactors,importantformetalturningsimulationsandaffectingstressandstraingeneration1contactinteractionandfrictionatthetoolchipinterface2nonlinearmaterialbehaviour,includingstrainrateeffects,namelythedependenceofthematerialsyieldstressonstrainrates3thermomechanicalcoupling,i.e.interconnectionbetweenmechanicalandthermalpartsoftheproblem.AsfollowsfromFEsimulations,theUATprocessduringonecycleofvibrationcouldbedividedintofourmainstages.DuringthefirststageFig.3a,thecutterapproachesthechipinthesecondstage,thecuttingtoolcontactsthechipandstartspenetratingintotheworkpiececausingthechipseparation.TheattainmentofthemaximumpenetrationdepthischaracterizedbythehighestlevelofgeneratedstressesintheprocesszoneandmarkstheendofthesecondstageFig.3b.Thefollowingstageisunloadingthevelocitydirectionofthetoolchangesanditmovesbackwards,butremainsincontactwiththechipevenafterthemomentwhenthespeedofthetoolexceedsthecuttingspeedduetotheelasticspringbackofthechip.Duringthisphase,framespersecondwiththeareaoftheimagecomprisingabout4mm2.Fig.4ademonstratesaframeoftheUATfilmingshowinganinteractionbetweenthecuttingtipandworkpiece.ThedifferencesbetweenCTandUATinchipseparationmanifestinsuchspecificfeaturesofthetheelasticstrainsintheprocesszonedecrease.Thelaststage,startingwiththefullseparationofthecuttingedgefromthechip,isthewithdrawalofthecutterfromthechipFig.3c.TheintermittentcharacterofthechipcuttingtoolcontactdeterminesthemaindifferencesinthestressdistributionforCTandUAT.ThestressstateduringCTisnearlyquasistatic,asshowninnumericalsimulationsFig.3d,withthehighestequivalentstressesconcentratedinprimaryandsecondaryshearzones,i.e.zonesaroundlineBEFig.1bandnexttotherakefaceEK.Conversely,thestressstateinUATisinherentlytransientstressesreachmaximumlevels,similartothoseofFig.3.DistributionofequivalentstressesduringUATatdifferentmomentsofasinglecycleofvibrationcutterapproachingthechipa,cutterinfullcontactwiththechipb,andcuttermovingawayfromthechipcandCTd.84V.I.Babitskyetal./UltrasonicCT,duringthepenetrationpartofthecycleofultrasonicvibrationFig.3b,whereasthestressmagnitudeissignificantlylowerduringtheotherstagesofthecycleFig.3aandc,whenthecuttermovesawayfromthechipornotincontactwithit.Hence,averagestressesgeneratedinthematerialand,consequently,theintegrallevelofinteractionforcesbetweenthecuttingtoolandworkpieceareconsiderablysmallerforUAT.Thisexplainsareductioninaveragecuttingforcesbyseveraltimesreportedinmanyexperimentalstudies1,3,4.4.StudyofchipformationAchipformationprocessisoneofthemostimportantcharacteristicsinmetalcutting.Hence,itisofparticularinteresttostudythedifferencesinthechipformationarisingfromsuperimposedultrasonicvibration.Intheexperimentalpartofthisstudy,ahighspeeddigitalcameraKodakEktaproHSMotionAnalyzer4540isusedforrealtimeobservationsofthechip–toolinteractionduringbothUATandCTofInconel718.Filmingspeedisintherangefrom9000upto27000Fig.4.ChipformationframesofhighspeedfilmingaandnumericalFEsimulationsb.s42200481–86processasthesizeandshapeoftheprocesszone,andthetypeoftheproducedchip.TheareaofthevisibleprocesszoneforUATanditssizeintheradialvertical,Fig.4adirectionareconsiderablysmallerthanthoseforCT.DeformationprocessesforUATarelocalizedinthedirectvicinityofthecuttingedgealongthesurfaceoftheworkpieceandarenotobservedunderneaththecutter,incontrasttotheCTprocess.ThiscorrelateswellwiththeresultsofnanoindentationtestsindicatingsmallerhardenedlayerfortheUATmachinedsurface.Finally,highspeedobservationsshowedthatsuperimposedultrasonicvibrationmakestheprocessofchipformationmoreregular,resultinginanincremental,continuouschipformationprocess.Incontrast,theobservedCTprocessproducesessentiallysegmentedchip,duetoforcedirregularvibrationofthecuttingtoolandtearinglikechipseparation.AscanningelectronmicroscopeSEMstudyofmicrostructureofchipsproducedwithUATandCTconfirmsthisobservation,revealingacontinuouschipwithsmallserrationsforUATandstronglysegmentedchipwithvividshearbandsforCT.
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