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微凹坑超声加工设计及试验,微凹坑,超声,加工,设计,试验
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Material removal mechanisms in precision machining of new materialsAbstractModern-day products are characterised by high-precision components. A wide range of materials, includingmetals and their alloys, ceramics, glasses and semiconductors, are finished to a given geometry, finish,accuracy and surface integrity to meet the service requirements. For advanced technology systems, demandsfor higher fabrication precision are complicated by the use of brittle materials. For efficient and economicalmachining of these materials, an understanding of the material removal mechanism is essential. This paperfocuses on the different material removal mechanisms involved in machining of brittle materials. 2001Published by Elsevier Science Ltd.Keywords: Brittle; Defects; Ductility; Material removal; Precision machining1. IntroductionUltra-precision machining technology has been developed over recent years for the manufactureof cost-effective and quality-assured precision parts for several industrial applications such aslasers, optics, semiconductors, aerospace and automobile applications. Precision manufacturingdeals with the realisation of products with high shape accuracy and surface quality. The accuracymay be at the nanometric level. Several machining techniques can be mentioned here like diamondturning, grinding, lapping, polishing, honing, ion and electron-beam machining, laser machining,etc. Efficient overviews of the processes are given in Refs. 13.Ultra-precision machining technology has been highly developed since the 1980s mainlybecause of its high accuracy and high productivity in the manufacturing of optical, mechanicaland electronic components for industrial use. For many advanced technology systems, higherfabrication precision is complicated by the use of brittle materials. The past decade has seen atremendous resurgence in the use of ceramics in structural applications. The excellent thermal,chemical and wear resistance of these materials can be realised because of recent improvementsin the overall strength and uniformity of advanced ceramics 4.Ceramic materials have been widely adapted as functional materials as well as structuralmaterials in various industrial fields and their application to precision parts is also increasing 5.However, the high dimensional accuracy and good surface quality required for precision parts arenot necessarily obtained by the conventional forming and sintering process of ceramic powders.Thus precision finishing of the ceramics after forming and sintering is recognised as a key technologyto precision ceramic parts 6.The quantity of ceramic material to be removed by the finishing process must be very small,so that microcracks do not remain on the finished surface. Abrasive processes such as grindingor lapping with diamond abrasives have generally been adopted for precision finishing of ceramics79.However, it is expected that better surface integrity and higher production rates can berealised by cutting processes. Compared with other processes, cutting is also advantageous inmachining complex shapes.Brittle materials can be divided into three groups: amorphous glasses, hard crystals andadvanced ceramics. Advanced ceramics are a modern development. They are made from fineporous particles that are formed, consolidated and thermally treated under precisely controlledconditions. Use of these materials enables development of high-technology devices and systemsthat simply could not be produced otherwise 10.The same statement could be made about theuse of certain crystalline materials (e.g., semiconductors) and advanced high-temperature glasses.2. Ductile regime machiningImprovements in machining tolerances have enabled researchers to expose the ductile materialremoval of brittle materials. Under certain controlled conditions, it is possible to machine brittlematerials like ceramics using single- or multi-point diamond tools so that material is removed byplastic flow, leaving a crack-free surface (Fig. 4). This process is called ductile regime machining.Ductile regime machining follows from the fact that all materials will deform plastically if thescale of deformation is very small. Another way of viewing the ductile regime machining problemis that described by Miyashita 17, as shown in Fig. 5. The material removal rates for grindingand polishing are compared and there is a gap in which neither technique has been utilised. Thisregion can be termed the micro-grinding gap since the region lies in between grinding and polishing.This gap is important because it represents the threshold between ductile and brittle grindingregimes for a wide range of materials like ceramics, glasses and semiconductors.2.1. Principle of ductile regime machiningThe transition from brittle to ductile mode during machining of brittle materials is described in terms of the energy balance between strain energy and surface energy 18. Localised fracturesproduced during application of load are of interest in machining of brittle materials. Machiningis an indentation process during which indentation cracks are generated, and these cracks play animportant role in ductile regime machining 19.A critical penetration depth dc for fracture initiation is described as follows 20where Kc is the fracture toughness, H is the hardness, E is the elastic modulus and b is a constantwhich depends on tool geometry. Fig. 6 shows a projection of the tool perpendicular to the cuttingdirection. According to the energy balance concept, fracture damage will initiate at the effectivecutting depth and will propagate to an average depth yc. If the damage does not continue belowthe cut surface plane, ductile regime conditions are achieved. The cross-feed f determines theposition of dc along the tool nose. Larger values of f move dc closer to the tool centreline.Another interpretation of ductile transition phenomena is based on cleavage fracture due to thepresence of defects 21. The critical values of a cleavage and plastic deformation are affectedby the density of defects/dislocations in the work material. Since the density of defects is not solarge in brittle materials, the critical value of fracture depends on the size of the stress field. Fig 7 shows a model of chip removal with size effects. When the uncut chip thickness is small, thesize of the critical stress field is small to avoid cleavage. Consequently a transition in the chip2.2. Material removal mechanisms in ductile regime machiningMachining generates a useful surface by intimate contact of two mating surfaces, namely the workpiece and abrasive tool. However, the micromechanisms of material removal differ from material to material depending upon the microstructure of both workpiece and tool material.Generally, during high-precision machining of brittle materials, tools having large negative rake angles are used (as high as -30). The negative rake angle provides the required hydrostatic pressure for enabling plastic deformation of the work material beneath the tool radius. During conventional machining with a single-point tool, the rake angle will be positive or close to 0.With positive rake angle, the cutting force will generally be twice the thrust force. Hence the deformation ahead of the tool will be in a concentrated shear plane or in a narrow plane as shown in Fig. 8. During the grinding process, it is generally agreed that the tool will have a large negative rake angle and also that the cutting force is about half of the thrust force Fig. 8(b). In ultraprecision machining of brittle materials at depths of cut smaller than the tool edge radius, the tool presents a large negative rake angle and the radius of the tool edge acts as an indenter as shown in Fig. 8(c). This represents indentation sliding of a blunt indenter across the workpiece surface. This is similar to a situation where the tool is rigidly supported and cuts the workpiece under a stress such that no median vents are generated but the material below the tool is plastically deformed due to large hydrostatic pressure as in Fig. 8(d).3. Material removal in glass and ceramicsThe ductile grinding of optical glass is considered as the most perfect adaptation of a machining method to the material 22. Glass is an inorganic material supercooled from the molten state to the solid state without crystallising. Glasses are non-crystalline (or amorphous) and respond intermediate between a liquid and a solid; i.e., at room temperature they behave in a brittle manner1838 P.S. Sreejith, B.K.A. Ngoi / International Journal of Machine Tools & Manufacture 41 (2001) 18311843but above the glass transition temperature in a viscous manner. The high brittleness of glass is due to the irregular arrangement of atoms. In crystalline materials like metals, the atoms have a fixed arrangement and regularity described by Miller indices, whereas glass structure does not show any definite orientation 23.The unique physical and mechanical properties of ceramics such as hardness and strength,chemical inertness and high wear resistance have contributed to their increased application in mechanical and electrical components. The advanced ceramics for structural and wear applications include alumina (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC), zirconia (ZrO2) and SiAlON. The nature of atomic bonding determines the hardness of the material as well as the Youngs modulus. For ductile metallic-bonded materials the ratio E/H is about 250, while for covalentbonded brittle materials the ratio is about 20. The ratio will lie in between these values for ionicbonded materials. Low density and low mobility of dislocations are the reasons for the high hardness of some of brittle materials.4. Gentle grindingThere is an alternative hypothesis called “gentle” machining wherein it is believed that plastic deformation is not involved exclusively in the material removal 26. According to this theory, since the mode of deformation (plastic/brittle) depends on the state of the stress and not on the magnitude of the stress, it is difficult to assume that the mode of deformation will change by merely changing the depth of cut keeping all other parameters constant. Investigations have shown that, in order for brittle materials to deform in a ductile manner, considerable hydrostatic stress and/or temperature are required. Reducing the depth of cut will merely decrease the stress without changing the stress state. Therefore this theory suggests that the superior quality of the surface produced at lower depth of cut is due to the above effect and not necessarily to plastic deformation. At smaller depths of cut, microcracks may be formed but they may not propagate to form larger cracks. Hence grinding at extremely small depth of cut can be called gentle grinding rather than ductile grinding.5. Material removal with microfractureDuring conventional machining operations on brittle materials most of the material is removed by brittle fracture, enabling higher removal rates. Fig. 10 shows the various stages of indentation. The material below the indenter is initially subjected to elastic deformation 27,28. As indentation continues, the material below is subjected to high hydrostatic pressure and hence an inelastic/plastic deformation zone is produced Fig. 10(a). At some point, a deformation-induced flaw develops into a median vent and subsequently can develop into a median crack during unloading Fig. 10(b). Further increase in load produces growth of the vent as in Fig. 10(c). On unloading the vent begins to close Fig. 10(d). During indenter removal, lateral vents begin to initiate near the base of the plastic deformation zone below the contact and spread out laterally on a plane closely parallel to the specimen surface. This is due to the presence of a residual tensile stress field. Upon complete removal of the indenter, the lateral vents continue to extend towards the specimen surface and may finally lead to material removal by chipping. Crack forma-tion is generally due to the residual stress field, which results from a mismatch in the elasticplastic deformation process 29. 6. Material removal without microfractureIt is well known that the extent of plastic deformation is determined by the magnitude of the hydrostatic stress. Under high hydrostatic pressures brittle materials are capable of ductile behaviour 30 at room temperatures. Such a condition exists at light loads under the indenter in indentation testing. Immediately below the indenter, the material is assumed to behave as a radially expanding core exerting uniform hydrostatic pressure on its surroundings, encasing the core in an ideally plastic region. Beyond the plastic region lies the elastic matrix 31. Fig. 11 shows a model for elasticplastic indentation of brittle materials. A model for material removal without microfracture was developed by Lawn et al. 27. The elastic/plastic field beneath the indenter is resolved into elastic and residual components. The reversible elastic component assumes only a secondary role in the fracture process although it does enhance the
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