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Surface integrity in electro-discharge machining, ultrasonicmachining, and diamond saw cutting of ceramic compositesDeng Jianxina,*, Lee TaichiubaDepartment of Mechanical Engineering, Shandong University of Technology, Jinan 250061, Shandong Province, PR ChinabDepartment of Manufacturing Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, PR ChinaReceived 24 October 1999; received in revised form 6 November 1999; accepted 3 February 2000AbstractGood surface integrity is frequently required for structural ceramic components, since the lifetime of a ceramic component is inmost cases determined by its surface features. In this study, the surface integrity of electro-discharge machined, ultrasonicmachined, and diamond saw cut ceramic composites has been investigated and compared. The surface roughness, hardness, andtopography of the machined surface were examined. Flexural strength and its distributions were used to evaluate the e?ect ofmachining process on the surface integrity of the machined specimens. Results show that the machining process had an importante?ect on the surface quality of the machined ceramics. Electro-discharge machining of ceramics created a heat-a?ected surface layerwith poor surface integrity, such as cracks and craters, and the variable surface damages on the electro-discharge machined speci-mens resulted in low Weibull modulus. Within most machining conditions, the ultrasonic machined and diamond saw cut speci-mens behaved fairly consistently, and the flexural strength and Weibull modulus were higher than that of the electro-dischargemachined specimens. # 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved.Keywords: A. Finishing; B. Composites; Machining1. IntroductionThe production of ceramic composite in the firedconditions is still unable to meet the accuracy requiredof a finished component with complex geometry, thisinevitably means that further processing has to be per-formed. Machining processes such as grinding, electro-dischargemachining(EDM),ultrasonicmachining(USM), diamond saw cutting, and laser beam machin-ing are often used and are the major contributor to theoverall cost of a ceramic component 1,2. The relativelylow fracture toughness of ceramics makes them verysensitive to the damage, which is introduced into theceramic surface during machining. The type of pro-duced damage includes surface and sub-surface cracksand unfavorable residual stresses. The combination ofsurface cracks and residual stresses can greatly a?ect thecharacteristics of ceramics and play an important role inthe final strength of a ceramic component.The strength of ceramic materials depends on both itsinherent resistance to fracture and the stochasticallydistributed defects such as grain boundaries, pores andcracks, which can lead to immense variations instrength.Fractureanalysishaveshownthatthemechanisms responsible for the failure of ceramics arein most cases surface defects, which are often originatedduring mechanical treatment in the final machining.These machining damages on the surface limit thestrength and determine the strength distribution sincethey influence crack growth under stress 35. It is quitecommon to observe that fracture strength varies sig-nificantly among di?erent specimens of the same mate-rial. This is partly due to the randomly distributedsurface damages. For this reason, a statistical measure isrequired to account for its variability. Several studies68 have shown that the Weibull modulus of flexuralstrength can be used to characterize the surface integrityof the machined ceramics. As the Weibull modulusdescribes the distribution of individual values of theflexural strength within a population. A high value ofWeibull modulus implies a narrow distribution ofstrength, and denotes a good material with a high0272-8842/00/$20.00 # 2000 Elsevier Science Ltd and Techna S.r.l. All rights reserved.PII: S0272-8842(00)00024-9Ceramics International 26 (2000) 825830* Corresponding author. Tel.: +86-531-294-0972; fax: +86-531-295-5999.E-mail address: (D. Jianxin).degree of homogeneity of properties. Whereas low Wei-bull modulus indicates that the size distribution of thestrength-controlling cracks is broader, which may beresultedfrom surface damages during the finalmachining.In this study, the surface integrity of electro-dischargemachined (EDMed), ultrasonic machined (USMed),and diamond saw cut Al2O3/TiC/Mo/Ni ceramic com-posite have been investigated. The surface roughness,hardness, and topography of the machined surfaceswere examined. Flexural strength and its distributionwere used to evaluate the e?ect of machining process onthe surface integrity of the machined specimens. Thepurpose was to compare the surface integrity, and tocharacterize the e?ect of machining process on thestrength and its distribution.2. Materials and experimental procedures2.1. Material preparationThe material used in the present investigation was analumina-based ceramic composite fabricated by theauthors. Monolithic Al2O3(average particle size 0.5 mm)was used as the baseline material. Additions of TiC par-ticles (average particle size 0.8 mm) were added to Al2O3matrix according to the combinations listed in Table 1.The final densification was accomplished by hot press-ing with a pressure of 36 MPa in argon atmosphere for8 min to produce a ceramic disk. The required sinteringtemperature was in the range of 16001800?C. Smallamount Mo and Ni metal phases were added to lowerthe electrical resistivity and to increase the interfacebonding strength between the particles. Table 1 lists themechanical properties of this ceramic composite.2.2. Machining methods and experimental proceduresEDM is a thermal process where material is removedbyasuccessionofelectricaldischargesoccurringbetween the tool electrode and the workpiece. EDMprovides a mean of machining of ceramic materials,irrespective of their hardness and strength, provided thattheir electrical conductivity is of the order of 100 ?cm.The wire-EDM machine used in this study was a Mit-subishi Wire System Model DWC 90G with a transistorpulse circuit having a maximum machining current of30 A. The wire electrode used was a brass wire of 0.25mm diameter. The average machining voltage was set at55 V and the current Ipwas set at 5, 10, and 14 A,respectively.USM is an abrasive machining process by which hardand brittle materials can be machined whether con-ducting or not, and the workpiece experiences no ther-mal damage. In the ultrasonic machining, the abrasiveparticles in slurry with water are under a tool, which isexcited of an ultrasonic frequency with small amplitude(normally less than 75 mm), and the material is removedprimarily by impact of the abrasive particles on theceramic surface. In this study, USM was conducted byusing a J93025 machine tool (made in China) with apower of 250 W and frequency of 1625 kHz. The staticload was of 10 N and the abrasives used were of 80, 120,and 240-grit B4C powders, respectively.Diamond saw cutting was carried out using a StruersAccutom-2 machine tool with a diamond blade. The feed-rate was of 0.5 mm/min and the speed of the diamondblade was of 6 m/s.A minimum of 10 specimens (3?4?16 mm) from eachmachining condition were prepared for the measure-ment of flexural strength. Three-point-bending modewas used to measure the flexural strength over a 10 mmspan at a crosshead speed of 0.5 mm/min. The strengthswere then analyzed by two-parameter Weibull distribu-tion. The surface roughness measurements for themachined specimens were carried out on a Talysurf 10system. Vickers hardness was tested on Zwick3212hardness tester. The microstructures of the machinedsurfaces were observed by scanning electron microscopy(SEM).3. Results and discussion3.1. Surface topography of the machined Al2O3/TiC/Mo/Ni ceramic compositeFig. 1(a) shows the typical SEM micrograph of theEDMed surface of the Al2O3/TiC/Mo/Ni composite,which reveals quite a number of spark-induced craters.The overall surface appears pitted with molten-lookingbumps of varying diameter and morphology, significantsurface damage with micro-cracks and small dropletscan also be seen. The probability of finding such fea-tures on the EDMed surface was significantly greater. Itcan be concluded that Al2O3/TiC/Mo/Ni ceramic com-posite are EDMed by either melt formation for lowmelting phases or thermal spalling for refractory phases,the e?ect of micro-cracking is more pronounced for thematerial removal and surface formation. The possiblemechanism for the formation of the craters in EDMedsurfaces is that sparks are formed at the conductiveTable 1Mechanical properties of Al2O3/TiC/Mo/Ni ceramic compositeComposition(vol %)Flexuralstrength(MPa)Fracturetoughness(MPa.m1/2)Hardness(GPa)Al2O3/50%TiC/5% Mo/Ni9005.0420.5826Deng Jianxin, Lee Taichiu/Ceramics International 26 (2000) 825830phase such as Mo and Ni, which melts and may evapo-rate. The high thermal conductivity of these metal pha-ses allows deeper melting, which may further increasethe machined surface roughness. Fig. 1(b) shows clearlyat higher magnification the craters and re-solidifiedlayer of the surface. It can be seen that the surfacedamage caused by EDM leads to severe Mo and Nierosion and exposure of Al2O3and TiC grains. Thepossibility and extent of subsurface damage wereexplored by preparing cross section view through theeroded crater, perpendicular to the specimen surface.Fig. 2 is a SEM micrograph, taken near the center of thecrater base, showing the subsurface damage. It revealsthat there is a distinct heat-a?ected surface layer. Thissurface layer exhibits a darker color than the inside core,and the depth is about 8 mm in thickness. The combina-tion of surface cracks and craters can greatly e?ect thecharacteristics of ceramics and play an important role inthe strength of the ceramic as can be seen later.The USMed ceramic surface was quite di?erent fromthose of the EDMed. Fig. 3 shows the typical SEMmicrograph of the USMed surface of Al2O3/TiC/Mo/Niceramic composite. It reveals that the material isremoved primarily by brittle fracture. The fracture modeis obviously mixed transgranular and intergranular. Themachined surface is characterized by numerous smallchipped region zones. In comparison with Fig. 1, itexhibited a relative smooth surface, and there is no dis-tinct crack on the USMed surface.Typical SEM micrograph of the diamond saw cut sur-face of Al2O3/TiC/Mo/Ni ceramic composite is shown inFig. 4. It can be seen that the machined surface containsFig. 1. SEM micrographs of the EDMed surface of Al2O3/TiC/Mo/Ni ceramic composite.Fig. 2. Cross-sectional SEM micrograph of the EDMed surface ofAl2O3/TiC/Mo/Ni ceramic composite.Fig. 3. SEM micrograph of the USMed surface of Al2O3/TiC/Mo/Niceramic composite.Deng Jianxin, Lee Taichiu/Ceramics International 26 (2000) 825830827grooves produced by micro cutting and pits producedby microfracture. On the flanks of the grooves smallspalling occurs. This spalling may indicate the presenceof brittle fracture during chipping. There was very littleevidence of pullout of individual grains on the diamondsaw cut surface.3.2. Surface roughness and hardness of the machinedAl2O3/TiC/Mo/Ni ceramic compositeThe results of the surface roughness and hardness ofthe EDMed, USMed, and diamond saw cut surfaces arelisted in Table 2. It can be seen that the EDMed surfacesshowed the highest surface roughness and the lowesthardness. The highest surface roughness of the EDMedspecimens attributed to the existence of surface cracks,craters and droplets as can be seen in Fig. 1. While thelow surface hardness of the EDMed ceramic may beresulted from its heat-a?ected layer as shown in Fig. 2.A decrease of surface roughness in EDM can be reachedby using smaller current, because of a decreasingamount of discharge energy density. The surface hard-ness shows no noticeable di?erences among these twoabrasive machining processes.3.3. Flexural strength and its distribution of themachined Al2O3/TiC/Mo/Ni ceramic compositeWeibull plot of strength distributions of the EDMedspecimens is shown in Fig. 5. The slopes of the lines arethe Weibull modulus for each distribution. A higherslope indicates less variability in the strength data. Table3 summarizes the Weibull modulus and average strengthmeasured under each condition. Referring to Tables 1and 3, it is evident that the average strength values of theEDMed specimens are considerably low, only 6880% ofthe original value of the material. The value of Weibullmodulus is in the range of 6.38.7. This indicates that theEDM process has a detrimental e?ect on the flexuralstrength of the machined specimens. The plot for thehigh current condition shows more scatter. The compo-site EDMed with a current of 5 A had an averagestrength of 716 MPa and a Weibull modulus of 8.7.When the current was of 10 A, the average strength wasdecreased to 665 MPa and Weibull modulus to 7.1.Current of 15 A resulted in further decrease in strengthof 619 MPa and Weibull modulus of 6.3. As higher dis-charge current leads to a higher discharge energy den-sity, thus increasing the material removal rate and thesurface roughness. Surface flaws are introduced that canlead to catastrophic failure of the specimen.One of the most serious problems in ceramic machin-ing is the initiation of cracks on the workpiece surface.Table 2Surface roughness and hardness of the machined Al2O3/TiC/Mo/Niceramic compositeSpecimenConditionsRa (mm)Hv(GPa)EDMedU=55 V, Ip=514 (A)1.642.5514.716.5USMedB4C abrasive,grit size 802400.150.8518.219.8Diamondsaw cutSaw speed V=6 m/s0.651.4218.520.2Fig. 4. SEM micrograph of the diamond saw cut surface of Al2O3/TiC/Mo/Ni ceramic composite.Fig. 5. Weibull plot of the strength distribution of the EDMed Al2O3/TiC/Mo/Ni ceramic composite.Table 3Weibull parameters for the strength distribution of the EDMed cera-mic compositeSpecimenCurrentIp(A)WeibullmodulusAverage flexuralstrength (MPa)EDMed146.3619107.166558.7716828Deng Jianxin, Lee Taichiu/Ceramics International 26 (2000) 825830This is due to the fact that the crack initiation results inthe considerable reduction of the strength of themachined components, and the strength is inverselyrelated to the size and distribution of existing flaws predominantly surface flaws. The basic factors thatcontrol the strength of brittle materials will determinethe flaw size generated during machining. The spark-induced cracks and craters on the EDMed surfaces mayserve as fracture origins and cause degradation ofstrength, and the variable surface damages on theEDMed specimens resulted in low Weibull modulus.Weibull plot of strength distributions of the USMedspecimens is shown in Fig. 6. Table 4 summarizes theWeibull modulus and average strength measured undereach condition. It is evident that the average strengthvalues of the USMed specimens are higher than that ofthe EDMed specimens. The value of Weibull modulus isincreased, in the range of 14.316.9. This indicates thatthe USM process has little strength limiting damages tothe machined ceramic surfaces compared with the EDMprocess. The smaller the abrasive grit-size, the higherthe strength and Weibull modulus. Weibull plots ofstrength distributions of the diamond saw cut specimensare shown in Fig. 7. Table 5 summarizes the Weibullmodulus and average strength. The diamond saw cutspecimens with saw speed of 6 m/s had an averagestrength of 846 MPa and a Weibull modulus of 19.2. Itcan be seen that within the most machining conditions,the USMed and diamond saw cut specimens behavedfairly consistently, and the flexural strength and Wei-bull modulus were higher than that of the EDMedspecimens.4. ConclusionsThe surface integrity of EDMed, USMed, and dia-mond saw cut Al2O3/TiC/Mo/Ni ceramic compositeshave been investigated. The following conclusions wereobtained:1. EDM of Al2O3/TiC/Mo/Ni ceramic compositecreated a heat-a?ected surface layer with poorsurface integrity such as spark-induced cracks andcraters. The surface integrity and residual strengthof the EDMed specimens decreased with increas-ing current, and the variable surface damages onthe EDMed specimens resulted in low Weibullmodulus.2. USMed and diamond saw cut surfaces are of bet-ter quality than the EDMed surfaces within themost machining conditions. The strength andWeibull modulus of these two abrasive machinedspecimens were much higher than that of theEDMed specimens. An improvement in both flex-uralstrengthandWeibullmoduluscanbeachieved with
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