航空材料超聲輔助車削的仿真與實驗研究@中英文翻譯@外文翻譯

ofg,isprocess.ultrasonictheultrasonicKeywords: Ultrasonic machining; Turning; Finite element modelling; Microstructurethe existent, conventional turning (CT) technology.frequency ultrasonic vibration, superimposed on theconventional movement of the cutting tool (Fig. 1), hasproved to be eective in machining intractable metalalloys as well as brittle materials, suchas ceramics andcutting forces 7,8.trol system. This system stabilises the turning processwith ultrasonic vibration and makes this process highlycontrollable. The detailed description of this novelcontrol system is given in 9,10. The experimental partof this paper studies UAT with autoresonant controlin comparison to conventional turning.Another important issue concerning UAT is the*Corresponding author. Tel.: +44-1509-227504; fax: +44-1509-Conventional machining of modern nickel- and tita-nium-based superalloys, used in aerospace applications,causes high tool temperatures and subsequent fast wearof cutting edges even at relatively low cutting speeds.A growing demand for machining these intractablematerials requires new advanced turning technologies.Such a technology was introduced in 1960s: high-Nevertheless, up to the present day UAT has notbeenwidelyintroducedintoindustrialenvironment.Themain reason for it is sensitivity of the UAT process totheloadappliedtothecuttingtip,resultinginthelossofcuttingeciencywhentheloadchangesoradierenttipis used. However, this limitation has recently beeneliminated with the invention of the autoresonant con-1. IntroductionTurning is a machining process, where a thin surfacelayer of the treated material is removed from a work-piece by a sharp wedge-shaped cutting tool forming acylindrical surface. This technology has been used forcenturies mainly for cutting various types of metallicmaterials. However, in the recent years, a range of newalloys and composite materials has been developed forvarious engineering applications. Many of these newmaterials become muchmore dicult to cut withglass. This technology, called ultrasonically assistedturning (UAT), demonstrates a range of benets inmachininghardmetalalloys:adecreaseincuttingforcesof up to several times 14, improvement in surfacenishby up to 50% compared to CT 5 and noisereduction 6. As for machining of brittle materials,ceramics and glass presently require prolonged andexpensive post-processing to obtain the surface qualityrequired for optical components; UAT allows obtainingmirrorsurfacenishinmachiningthesematerialsaswellas considerable reduction in tool wear and averageUltrasonically assisted turningand experimentalV.I. Babitsky, A.V. Mitrofanov,Wolfson School of Mechanical and Manufacturing EngineerinAbstractUltrasonically assisted turning of modern aviation materialsamplitude a C25 15 lm) superimposed on the cutting tool movement.nonlinear resonant mode of vibration throughout the cuttingworkpieces machined conventionally and with the superimposedprocess and nanoindentation analyses of the microstructure ofmodel provides numerical comparison between conventional andcutting forces and contact conditions at the workpiece/tool interface.C211 2004 Elsevier B.V. All rights reserved.Ultrasonics 42 (2004)227502.E-mail address: v.silberschmidtlboro.ac.uk (V.V. Silberschmidt).0041-624X/$ - see front matter C211 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.ultras.2004.02.001aviation materials: simulationsstudyV.V. Silberschmidt*Loughborough University, Leicestershire LE11 3TU, UKconducted withultrasonic vibration (frequency f C25 20 kHz,An autoresonant control system is used to maintain the stableExperimental comparison of roughness and roundness forvibration, results of high-speed lming of the turningmachined material are presented. The suggested nite-elementturning of Inconel 718 in terms of stress/strain state,8186/locate/ultrasmechanics of this process. There are only a few sourcesof the workpiece (parallel to the X-axis, Fig. 1b), or inthe feed direction, i.e. along the axis of the workpiece(Z-axis, Fig. 1b). A self-sustained resonant mode ofvibration of this cutting system is implemented viathe autoresonant control system, which is described indetail in 9,10.A range of turning tests has been conducted tocompare the usage of UAT and CT for machining avi-ation materials. The detailed description of these testscan be found in 5. Among the materials used for thetestsisInconel718ahigh-gradeheat-resistantNi-basedsuperalloy widely used in the aerospace industry. Thismaterial is very abrasive and causes the tool bluntingand high cutting temperatures when machined conven-tionally.The surface quality obtained by turning is one of thecrucialfactorsinmetalcuttingandisextremelysensitiveto any changes in the machining process. The surfacenishof specimens is compared in terms of averageroughness measurement Ra and measurement ofroundness (the peak-to-valley measure), using the Tay-s 42 (2004) 8186Fig. 1. Experimental setup for ultrasonically assisted turning (a), anda scheme of relative motion of the workpiece and cutting tool82 V.I. Babitsky et al. / Ultrasonicattempting to describe the processes in the workpiece/cutting tool interaction zone and their inuence on thestructureofthemachinedmaterial3,6,11.Theseworksstudy mostly the dynamics of the ultrasonic machineunit and not the response of the treated material to thistechnology, while a clear understanding of mechanicalprocesses in the material during UAT would certainlyallow a further development of the UAT technology.The main aim of this paper is to study experimentallyand numerically the material mechanics of the UATprocess.2. Experimental studiesThe experimental setup used to study UAT is shownin Fig. 1. The workpiece is clamped in the chuck of theuniversal lathe and rotates with a constant speed. Highfrequency electric impulses, fed to the input of theultrasonic transducer, excite vibration in piezoceramicrings due to the piezoelectric eect. The vibrationamplitude is intensied in the concentrator and trans-mitted to the tool holder at the thin end of the con-centrator. Resultant vibration of the cutting tip xed inthe tool holder reaches 15 lm (i.e. 30 lm peak-to-peak)at a frequency of about 20 kHz. The vibration can beapplied either in the direction tangential to the surfacein orthogonal UAT with tangential vibration (b).lor HobsonTalysurf 4 surface measurement instrument.The following cutting parameters are used to machinetested specimens: depthof cut d 0:8 mm, feed rates 0:05 mm/rev, and cutting speed v 17 m/min. Thesame parameters are used for bothUAT and CT, withsuperimposed ultrasonic vibration in the feed directionapplied for UAT.Fig. 2a shows representative axial proles of themachined surface of the Inconel 718. It is obvious thatmagnitudes of Raare reduced by nearly 50% for speci-mens machined with UAT. Furthermore, the regularityof the surface prole is greatly improved, as the surfaceFig. 2. Surface quality of Inconel 718 specimens machined with UATand CT: axial surface proles (a), roundness proles (b). Cuttingparameters: d 0:8 mm, s 0:05 mm/rev, v 17 m/min.direction.Apparently, the reason for these improvements is thes 42 (2004) 8186 83change of the nature of the cutting process, which istransformed into the one with multiple-impact high-frequency interaction between the cutting tool andchip due to applied ultrasonic vibration. This leadsto changes in material deformation processes and fric-tion forces, and increase in the dynamic stiness of thelathe-tool-workpiece system 6,11 due to the vibrationfrequency levels considerably exceeding its natural fre-quency.In addition to measurements of the surface quality,the microstructure of the machined surface has beeninvestigated. Inconel 718 workpieces are machined un-der the same cutting conditions (v 3:6 m/min, d 0:1mm, s 0:03 mm/rev) withapplication of ultrasonicvibration in tangential direction and without it. Then,nanoindentation analyses of the surface layers are per-formed withthe NanoTest Platform made by MicroMaterialsLtd.Accordingtotheresultsofthesetests,thewidth of the hardened surface layer, which results fromthe extensive deformation and high temperature pro-cesses during the turning procedures, for the ultrasoni-cally machined specimen is half the size that of theconventionally machined one (40 and 80 lm, respec-tively). Furthermore, the average hardness of this layerfor UAT (about 15 GPa) is a half of that for CT andconsiderably closer to the hardness of the untreatedmaterial (about 7 GPa). The hardness of the materialnonlinearly increases with a rise in the level of theresidual plastic strains. Hence, nanoindentation testsindicate lower residual strains in the surface layer forworkpieces machined with UAT and a conclusion canbe drawn that the UAT procedure is considerably moredelicate to the workpiece material.3. Numerical analysis of UATFinite element (FE) simulations are a major tool formodelling of machining processes. It has been used formodellingofturningforsome30years. Theoverviewofthe state of the art in metal cutting simulations can befoundin13,14.However,uptotheauthorsknowledge,nomodelsforUAThavebeendevelopeduntilnow.Thetwo-dimensional FE model of bothCT and UAT de-becomes smoother in the axial direction. A considerableimprovement is also obtained for roundness of ma-chined workpieces (Fig. 2b): a peak-to-valley value ofroundness measures 4.20 lm for CT, whereas it attainsonly 1.89 lm for UAT. Hence, the roundness is im-proved by 40% when ultrasonic vibration is superim-poseduponthemovementofthecuttingtool.Itisworthnoticingthatsimilarresultshavebeenobtainedbyotherresearchers 7,12 utilising vibration in the tangentialV.I. Babitsky et al. / UltrasonicscribedinthispaperisbasedonthecommercialFEcodeMSC.Marc 15. An orthogonal turning process, i.e. thecutting process where the tool edge is normal to bothcutting and feed directions, withtangential vibration isconsidered. Fig. 1b shows a scheme of the modelledrelative motion of the workpiece and cutting tool; therotation axis of the cylindrical workpiece is orthogonalto the plane of the gure. The workpiece moves with aconstantvelocity,whereasthetoolvibratesharmonicallyaround its equilibrium position withfrequency f 20kHz and amplitude a 15 lm, corresponding to thevaluesusedinexperimentalstudies.Otherparametersofsimulationsare:uncutchipthicknesst1 0:1mm(whichcorresponds to the depth of cut), rake angle of the toolc 10C176, cutting speed V 9m/min.Suchparametersofvibration and of the cutting process provide separationofthecutterfromthechipwithineachcycleofultrasonicvibration. The material constants for aged Inconel 718are taken from 16.Kinematical boundary conditions for the workpieceare applied to its left, right and bottom sides (Fig. 1b),whereas its top surface is free:VxjAH V ; VxjFG V ; VxjHG V ; VyjHG 0:Thermal boundary conditions include convective heattransfer from the workpiece, chip and tool free surfacesto the environment: C0koT=on hT C0 T1, where k isthe conductivity, h is a convective heat transfer coe-cient, T1is the ambient temperature. The thermal uxpassing from the chip to the cutter along the contactlength Lc(Fig. 1b) is described as follows: q HTchipC0Ttool, where H is a contact heat transfer coecient, Tchipand Ttoolare chip and tool surface temperatures,respectively.The model takes into consideration the followingfactors, important for metal turning simulations andaecting stress and strain generation: (1) contact inter-action and friction at the tool-chip interface; (2) non-linear material behaviour, including strain-rate eects,namely the dependence of the materials yield stress onstrain rates; (3) thermomechanical coupling, i.e. inter-connection between mechanical and thermal parts ofthe problem.As follows from FE simulations, the UAT processduring one cycle of vibration could be divided into fourmain stages. During the rst stage (Fig. 3a), the cutterapproachesthechip;inthesecondstage,thecuttingtoolcontacts the chip and starts penetrating into the work-piece causing the chip separation. The attainment of themaximum penetration depth is characterized by thehighest level of generated stresses in the process zoneand marks the end of the second stage (Fig. 3b). Thefollowingstageisunloading:thevelocitydirectionofthetool changes and it moves backwards, but remains incontact with the chip even after the moment when thespeed of the tool exceeds the cutting speed (due tothe elastic spring-back of the chip). During this phase,framespersecondwiththeareaoftheimagecomprisingabout 4 mm2.Fig. 4a demonstrates a frame of the UAT lmingshowing an interaction between the cutting tip andworkpiece.ThedierencesbetweenCTandUATinchipseparation manifest in suchspecic features of thethe elastic strains in the process zone decrease. The laststage, starting with the full separation of the cuttingedge from the chip, is the withdrawal of the cutter fromthe chip (Fig. 3c).The intermittent character of the chip-cutting toolcontact determines the main dierences in the stressdistributionforCTandUAT.ThestressstateduringCTis nearly quasistatic, as shown in numerical simulations(Fig. 3d), with the highest equivalent stresses concen-trated in primary and secondary shear zones, i.e. zonesaround line BE (Fig. 1b) and next to the rake face EK.Conversely, the stress state in UAT is inherently tran-sient: stresses reachmaximum levels, similar to those ofFig. 3. Distribution of equivalent stresses during UAT at dierentmomentsofasinglecycleofvibration:cutterapproachingthechip(a),cutter in full contact with the chip (b), and cutter moving away fromthe chip (c) and CT (d).84 V.I. Babitsky et al. / UltrasonicCT,duringthepenetrationpartofthecycleofultrasonicvibration (Fig. 3b), whereas the stress magnitude is sig-nicantlylowerduringtheotherstagesofthecycle(Fig.3a and c), when the cutter moves away from the chip ornot in contact withit. Hence, average stresses generatedin the material and, consequently, the integral level ofinteraction forces between the cutting tool and work-piece are considerably smaller for UAT. This explains areduction in average cutting forces (by several times)reported in many experimental studies 1,3,4.4. Study of chip formationA chip formation process is one of the most impor-tant characteristics in metal cutting. Hence, it is ofparticular interest to study the dierences in the chipformation arising from superimposed ultrasonic vibra-tion.Intheexperimentalpartofthisstudy,ahigh-speeddigital camera (Kodak Ektapro HS Motion Analyzer4540) is used for real-time observations of the chiptoolinteraction during bothUAT and CT of Inconel 718.Filming speed is in the range from 9000 up to 27000Fig.4.Chipformation:framesofhigh-speedlming(a)andnumerical(FE) simulations (b).s 42 (2004) 8186process as the size and shape of the process zone, andthe type of the produced chip. The area of the visibleprocess zone for UAT and its size in the radial (vertical,Fig. 4a) direction are considerably smaller than thoseforCT.DeformationprocessesforUATarelocalizedinthedirectvicinityofthecuttingedgealongthesurfaceofthe workpiece and are not observed underneath thecutter,incontrasttotheCTprocess.Thiscorrelateswellwiththe results of nanoindentation tests indicatingsmaller hardened layer for the UAT machined surface.Finally, high-speed observations showed that super-imposed ultrasonic vibration makes the process of chipformation more regular, resulting in an incremental,continuous chip formation process. In contrast, theobserved CT process produces essentially segmentedchip,duetoforcedirregularvibrationofthecuttingtooland tearing-like chip separation. A scanning electronmicroscope (SEM) study of microstructure of chipsproduced withUAT and CT conrms this observation,revealing a continuous chip with small serrations forUAT and strongly segmented chip with vivid shearbands for CT.It should be pointed out that experimental measure-UAT and CT processes. However, due to the stressesExperimental comparison of the temperatures in thecutting tip and chip showed 50% higher temperaturesfor UAT. This was explained by additional energycominginto the cutting system withultrasonic vibrationand dierent chip formation processes when ultrasonicvibration is applied.AcknowledgementsAuthors would like to thank Dr. G.K. Hargrave,LoughboroughUniversity,fordevelopingandprovidingments of temperature during turning are limited to thesurface temperature distribution. Numerical (FE) sim-ulations are, therefore, an important tool, which allowsan in-depth look at the details of transient thermalproperties inside the workpiece and cutting tip. Fig. 5bshows a temperature plot of the cutting tipworkpieceinteraction during the UAT process, as obtained in FEsimulations. There is a good correlation betweenA shape of the numerically modelled chip (Fig. 4b) isin a good agreement withthat experimentally observed.This plot of the FE-simulation results provides an in-sight into the distribution of equivalent plastic strains inthe cutting region. Plastic strains attain maximal valuesnear the cutting edge and along the newly formed sur-face of the specimen.5. Study of tool and chip temperaturesDuring the turning process, the work of plasticdeformation and friction between the cutting tool andworkpiece can result in a signicant temperature in-crease in both the workpiece and cutter. The tempera-ture increase, in its turn, changes material properties,suchas the yield stress, temperature conductivity andspecic heat, thus inuencing deformation processes inthe workpiece. Evolution of temperatures during turn-ing is, therefore, an important characteristic feature ofthe comparison between CT and UAT.Infraredlminghasbeenemployedtoexperimentallymeasure the temperature distribution in the area ofinteraction between the workpiece and cutting tip.Agema 880 Thermovision System was used