MODERN MATERIALS FOR HIGH SPEED MACHINING

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1 MODERN MATERIALS FOR HIGH SPEED MACHINING Prof. Eng. Stanislaw LEGUTKO, Ph.D., Assistant. Eng. Pawel TWARDOWSKI Poznan University of Technology, POLAND Abstract: The High Speed Machining, sintered tool materials, especially cemented carbides have been presented in the paper. Cemented carbides have become so popular tool materials, because they are relatively resistant to wear and high temperatures, although not as ceramic materials. In addition, they are characterized by much greater ductility (resistance to bending) compared to ceramic materials. Key words: cemented carbides, HSM 1. NATURE OF MACHINING WITH HIGH SPEED - HSM Machining with high speed (HSM - High Speed Machining or HSC - High Speed Cutting) is a variety of machining with increased cutting speed v c, compared to conventional machining. There is not determined the exact limit, when conventional machining goes into HSM. Frequent and very legitimate indicator is the cutting speed v c. However, it should be noted that there is no single, fixed speed at which begins range of HSM [8]. This depends on several other factors. One of them, and also very important is the type of work material. Application of increased cutting speeds is associated with increase of temperature in the cutting zone. As a result, in the vicinity between the points of contact of the tool and work piece, is separated a lot of heat. The temperature increases with cutting speed, until in some point stabilizes and sometimes may even decreases. Therefore, the main effects of this type of machining, e.g. HSM, are directly related to the increasing temperature. HSM machining, like any other, has advantages and disadvantages, which the most important are listed in Table 1. table 1. Advantages and disadvantages of HSM machining ADVANTAGES DISADVANTAGES high rate of removal allowance, excessive wear of the wedges, shortening the time of machining, the need of special tools using (special reduced cutting forces, materials for cutting wedges, precision good thermal dissipation, which cause performance), reduction of deformations in work piece the need for special machine tools and tool holders To the tool materials and tools for cutting are high demands when it comes to applying them in a HSM. The most important are: good wear resistance at high speeds, good dynamical balance, maximum roundness deviation ± 0,001 mm, high rigidity, large grooves for chips removing, special geometry of the wedge to avoid chipping and to assure resistance on the effect of centrifugal force. 44

2 2. TOOL MATERIALS IN MACHINING In the present era, there are tool materials, which allow optimizing practically of any machining operation. Among the entire spectrum of the proposed tool materials, one can always choose one, with which there is, possibility to fulfill the optimization criterion while machining determined work piece. Cemented carbides and high speed steels play currently an absolutely dominating role. Cemented carbides are the material for around 50% of the wedges of cutting tools used in industry. High speed steel is about 40%, and for the other tool materials, only about 10% (fig.1). Fig.1 Percentage of groups of tool materials (elaborated on the basis of [2]) The properties of particular tool materials and fields of their application are presented in many publications, for examples [1, 2, 4 and 9]. Valuable information about this can be found in the information materials of companies which are producing cutting tools, although they are usually don t give details about chemical compositions and preparation technology. From cutting ability point of view the important characteristics of tool materials are: the hardness, toughness and, in the case of ceramic tool materials, resistance on fracture toughness. 3. SINTERED TOOL MATERIALS USED IN HSM Application of increased cutting speeds and higher requirements for efficiency concerning productivity brings, as already mentioned, increasing of temperature in the cutting zone. As the result searching is still in progress of materials resistant to heat and thermal shock. The temperature of the process can sometimes exceed 1200 K. This is one of the main reasons for the elimination of cutting material, which is High Speed Steel (HSS) using to the HSM processing. Therefore, the basic groups of tool materials used in the HSM are: cemented carbides, cermets, ceramic tools, super hard materials (PKD - polycrystalline diamond, CBN). However, the practical range of cermets applications, ceramics and super hard materials tool is very limited compared to cemented carbides [3, 10]. Cemented carbides are used primarily for the manufacture of indexable inserts, milling cutters, drills with carbide inserts. Their share in tool materials market is about 50% (fig.2) [9]. The share of manufactured chips made by cemented carbide tools in machining operations comes even to 80% [9]. Cemented carbides because of their structure might be included into sintered metal matrix composites reinforced with particles (Fig. 2). Cemented carbides are sintered from hard metal carbides with high-melting properties: WC, TiC, TaC, NbC, VC and the metallic matrix, mostly cobalt and less - nickel or iron [9]. 45

3 Carbide structure (about 85% ) Phase combining (about 15% ) Fig.2 Carbide structure [own elaboration based on the materials from company FRAISA] High hardness of carbides ( HV) decides on their good resistance against abrasive wear. Physical and mechanical properties of carbides are depending mainly on the chemical and phase composition, shape and size of carbides and their volume fraction in the structure. Much the same fragility of metal carbides with high-melting properties substantially restricts their use during the machining process, as the working elements of cutting tools. These are in fact sensitive materials on the dynamic impact loads. Therefore, in order to exploit their assets and ensure that acceptable toughness were delivered as part of sintered metal matrix composites. Manufacturers of carbides can affect the type, size and share of grains in the materials structure and in such way optimize their properties. Standard grain size is 1,5 to 3 µm, in finegrained cemented - less than 1 µm and in case of ultra fine-grained carbides - less than 0,5 µm. A new generation of so-called nano-carbides has a grain size of 0,1 to 0,2 µm. Distances between the powder particles in nano-carbides are so small that they can be sintered at lower temperatures. Reducing the size of grains is one of the ways to improve the cutting ability of these cutting tool materials. Other ways are: preventing the proliferation of grains TiC during the sintering with cobalt matrix, use the platelet grains of tungsten carbide-oriented of a pre-estimated way, apply a gradient structure, in which the outer layer has a higher content of binder phase, which provides increase toughness of inserts and protects against the propagation of surface cracks; cobalt-enriched surface layer with a thickness of 13 to 25 µm contains 2 to 3 times more cobalt than the core of material. Cemented carbides with type WC+Co were divided depending on particles sizes of WC into the following groups [5, 9, 12]: coarse - with an average diameter of 3 to 30 µm (Fig. 3a), standard - with an average diameter of 1,5 to 3 µm, fine - an average diameter of 0,5 to 1,5 µm, ultrafine - with an average diameter less than 0,5 µm (Fig. 3b), nano-carbides - with an average diameter of 0,1 to 0,2 µm. Tungsten carbide with cobalt owes its widespread use as a component of cemented carbides, including high bending strength and very high coefficient of elasticity and thermal conductivity. WC grain size has a huge impact on the properties of tungsten carbide wedges, and especially on their flexural strength and hardness. If the particle diameter of WC is more than 1,5 µm, one can observe the increase of bending strength and hardness decrease with the tungsten carbide wedge with increased WC grain size. 46

4 a) b) Fig.3 Carbide structure: a) coarse b) ultrafine [7] When the WC grains have a diameter smaller than 1,5 µm, one can observe in the same time increase in bending strength and hardness of tungsten carbide wedge (Fig. 4), the greater, the smaller the grain WC is. Analysis showed that this contributed to breakthrough in their views that the increase in hardness must result in a decrease of ductility [12]. Properties of cemented carbides depend largely on the phase composition, size and shape of their grains and also from the volume fraction in structure (fig.4). Fig.4 Effect of grain size and phase content on the properties of cobalt carbide [authors' elaboration based on the materials from company FRAISA] Although cemented carbides have a high degree of bending strength (Fig. 5), however they have small hardness comparing with newer materials such as boron nitride (CBN) or ceramic tools. This limits their use for HSM machining hardened steel. In order to improve hardness and surface properties of cemented carbides they are covered by antiwear coatings such as TiN, TiAlN, TiC etc. 47

5 Micro cutter 0,5 mm under bending test a) b) Fig.5 Bending test a) carbide cutter, b) carbide fibers [own elaboration based on the materials from company FRAISA] At high cutting speeds there are used wedges coated with Al2O3, which compared with layers of TiC and TiN behaves at temperatures above 1200 K the highest hardness and lowest thermal conductivity, which makes that heat produced during the cutting is dissipated mainly by the chip and only in small part taken over by the wedge. Al2O3 layers are also characterized by huge resistance to oxidation (which provides excellent protection against chemical reactions with the work piece), low coefficient of friction and low propensity to interlock with the feet of iron. The wear resistant layers are usually applied with coatings made by chemical vapor deposition (CVD - Chemical Vapor Deposition) and physical vapor deposition (PVD Physical Vapor Deposition) [6]. Layer, which one usually receives, have thickness of about 3 to 5 µm with a very high hardness, mostly within the limits of HV, significantly increases the resistance of cutting tools against abrasive wear. Deposition of hard layers on cutting wedges provides: increasing tool life and their resistance to catastrophic failure (seizure, chipping, breaking), increase machining capacity (mainly due to increase of cutting speed), improve the tribological properties and hardness of the wedge s surface, improve the quality of machined surface object. In the works [8] it was found that cemented carbides coated with TiN / TiCN are dedicated to cutting materials with hardness not exceeding 42 HRC, while the TiAlN coatings can incorporate cutting materials with hardness of 42 HRC or more. While cutting AISI D2 tool steel with hardness of 60 HRC using carbide with TiAlN and TiCN coatings proved that the wear of WC with TiAlN were half less as compared to using WC with TiCN coating [11]. Other authors of research [3] presented that the cemented carbide with TiAlN coating showed two to five times better durability of wedge (depending on cutting speed) than the carbide with TiCN+Al2O3+TiN layers and also cermets during high speed cutting of tool steel with a hardness of 58 HRC. However, carbides with TiCN+Al2O3+TiN layers and also cermets showed close to each other durability. During last year s, increasingly often can be found diamond coatings applied on the core material, which is cemented carbide (Fig. 6). They ensure even greater durability compared to ceramic coatings. Diamond coating 48

6 Fig.7 Diamond coating on cemented carbide [own elaboration based on materials from company FRAISA] 4. CONCLUSIONS Cemented carbides have definitely the biggest application among today's tool materials. It should be emphasized, based on the authors' own experience in the implementation of high speed machining in industrial conditions that this also applies to machining of HSM. In case of tool materials, one tends to get as much toughness with huge resistance to wear. Such condition, in perfect degree, no tool material doesn t meet and existing materials are either too brittle (for example: ceramics and super hard materials), or too little resistant to wear (for example HSS steels). Cemented carbides have become so popular tool material, because they are relatively resistant to wear and high temperatures, although not as ceramic materials. In addition, they are characterized by much greater ductility (resistance to bending) compared to ceramic materials. The use of coatings has enabled the use of carbide steel machining hardened in conditions of HSM. Cemented carbides with coatings have a much greater resistance to high temperatures, which in the case of machining hardened steel under conditions of HSM is very important. In addition, increases abrasion resistance and the cutting wedge is strengthening. As a result, durability of coated cutting tool (not only cemented carbide) increases several times. Therefore, now many tools companies offering tools in which is very hard to find carbide tools (not only) without coatings? The main role of coating is reduced, in large part, to increased wedge life, and thus to reduce manufacturing costs. Currently, in relation to machining with high cutting speeds are mainly practical applications of tools with coatings, including, as it mentioned, mainly coated cemented carbides. REFERENCES 1. Cichosz, P., Darlewski, J., Oczoś, K.E., Nowoczesne trendy rozwojowe w obróbce skrawaniem, Prace Naukowe Instytutu Technologii Maszyn i Automatyzacji Politechniki Wrocławskiej, Nr 78, Oficyna Wydawnicza Politechniki Wrocławskiej, Wrocław 2000, str Destefani, J., Cutting tools 101 materials, Manufacturing Engineering, September

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