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Modern cutting technology is developing in the direction of high speed, high precision and high efficiency. At the same time, cutting technology has higher requirements for tool performance. Polycrystalline Cubic Boron Nitride (PCBN) is a new tool material that is synthesized by artificial methods and is second only to natural diamond. It is the most suitable tool material for high-speed cutting of iron-based metals.
Polycrystalline cubic boron nitride (PCBN) was first developed by GE in the last century. Due to its high hardness and wear resistance, high thermal stability and high temperature hardness, excellent chemical stability and good PCBN materials. The thermal conductivity and low coefficient of friction, so the PCBN tool has been valued by the developed countries such as the United States, Germany, Britain, and Japan since its inception. After more than 30 years of development, foreign research on PCBN tools has basically entered a mature stage, and products are also developing in a diversified and serialized direction. There are also many units and scholars in China who are engaged in the development and research of PCBN tools, but compared with foreign countries, there is a lag in the research of PCBN material synthesis and tool cutting performance.
As one of the major replacement tools in the 21st century, PCBN tools account for a growing proportion of the total use of tools in industrialized countries. US GE believes that in industrialized countries, PCBN tool materials will have a market of 620 million US dollars. The focus of the “10th Five-Year Plan†of China's machine tool industry is to develop CNC machine tools and their supporting components. In 2005, the domestic PCBN blade demand has reached 662,000 pieces (equivalent to 30mm pieces). According to market research and scientific predictions, the total value of PCBN tool consumption in China will reach 540 million yuan by 2010.
2 PCBN blade development
From 1957, the United States GE company developed cubic boron nitride (CBN) single crystal powder, the successful development of PCBN tools in the 1970s, after more than 30 years of development, PCBN tools have gradually matured. At present, PCBN tools are mostly welded by PCBN blades and tool holders or indexable inserts. In recent years, there have been more and more types of integrated PCBN blades. The PCBN insert is a composite whole formed by directly sintering a PCBN layer of about 0.5 mm on a cemented carbide substrate. This material not only has the high hardness, high thermal stability and high chemical stability of PCBN, but also has the advantages of good strength and good weldability of the cemented carbide, so that the PCBN tool can not only cut workpieces of various hardnesses, but also Easy to produce. The quality of the PCBN blade directly affects the cutting performance of the tool.
2.1 PCBN blade sintered PCBN blade is mainly produced in two ways: one is a single polymerization method, that is, a hexagonal boron nitride (HBN) is directly polymerized into a high-density sintered body; the other is a secondary polymerization method. High-density cubic boron nitride (CBN) is first synthesized, and then sintered into high-concentration cubic boron nitride by high temperature and high pressure. At present, the secondary polymerization method of adding a binder is more common:
(1) CBN sintering by adding a metal binder There are many types of metal binders, mainly Ni, Co, Ti, Ti-Al, and the like. At the sintering temperature, the metal binder becomes a liquid phase, and the appearance of the liquid phase promotes the sintering of CBN, and the sintering can be carried out at a slightly lower temperature and pressure. In addition, since CBN is insoluble in the metal binder, a dissolution precipitation process which promotes shrinkage and crystal growth does not occur. During the sintering process, the cobalt in the cemented carbide penetrates into the CBN grain boundary in a liquid state, and the cemented carbide and the CBN sintered body are closely bonded together. PCBN tools with metal binders have lower hardness than pure CBN sintered tools, but have good toughness. The AMB90 of Element Six of the United Kingdom, the BZN6000 of GE of the United States, and the MB710 of Mitsubishi of Japan are all sintered from a metal binder.
(2) CBN sintering with ceramic binder The ceramic binder mainly includes TiN, TiC, Al2O3 and the like. Since cracks are easily generated during the sintering process, iron elements (iron, cobalt, nickel) and Mo or Mo2C are added to the ceramic binder to increase the strength between the ceramic ions, thereby acting as a fluid pressure transmitting medium, which is advantageous. A sintered body having no deformation inside is formed, and molybdenum or the like can also improve the wettability between the ceramic particles and the iron element. The content of binder in ceramic binder PCBN is generally higher, and the toughness is worse than that of metal binder tools, but its high temperature resistance is better. Such products include DBA80 and DBC50 of Element Six of the United Kingdom, BZN7000S and BZN8200 of GE of the United States, NBX300 and NBX3200 of Sumitomo of Japan.
(3) Pure CBN sintering In the CBN polycrystal, polycrystalline cubic boron nitride can also be directly sintered without a binder. The CBN undergoes recrystallization and grain growth during sintering, and the crystal grains of the sintered CBN polycrystal are not the original CBN single crystal particles. This PCBN tool has good wear resistance and long life, but it is brittle and the temperature is harsh. It is difficult to make a blade with high toughness and large size.
2.2 Factors affecting the quality of PCBN blades The quality of PCBN inserts is affected by many factors, among which the changes in various process parameters during the sintering process have the greatest impact on the quality of PCBN inserts. The composition of the binder/catalyst material, the matrix material, and the surface cleanliness and graininess of the particles all determine the performance of the PCBN blade.
(1) Selection of the substrate Since the tungsten carbide cemented carbide has high hardness, good thermal conductivity and toughness, a tungsten carbide-containing carbide containing Co is often used as the PCBN blade substrate. Co as a solvent and binder can significantly improve the sintering degree of CBN and enhance the strength of PCBN [3], but the content of Co in tungsten carbide can not be too high, otherwise it will affect the wear resistance of PCBN blade and shorten Tool life.
(2) Cleanliness and granularity of CBN grains The surface cleanliness of CBN grains will directly affect the sintering quality of PCBN. Therefore, CBN grains must be treated strictly before sintering to remove moisture from the surface of the grains. And impurities such as oxides. The method used is mainly heating under vacuum or a reducing gas such as hydrogen or ammonia for 1 to 2 hours. Otherwise, excessive impurities may affect the bonding between the CBN-CBN particles and the CBN and the binder, so that the strength of the PCBN blade is reduced and the wear resistance is lowered.
The size of the CBN particles not only affects the cutting surface quality of the PCBN tool but also plays a role in the sintering ability of the binder during the sintering of the PCBN. In general, the smaller the CBN particle size, the better the cutting surface quality of the PCBN tool, the better the impact resistance and wear resistance of the tool. Therefore, the hardened steel is processed and high surface quality is required (ie, the PCBN tool is realized). When the car is worn, the CBN particles in the PCBN tool used should take a small value. On the other hand, since the sintering of the PCBN insert is achieved by the "capillary phenomenon", that is, the penetration of various binder elements into the CBN particles, if the CBN particles are too small, the gap between the CBN particles is reduced, thereby The amount of penetration of the binder element is reduced. Therefore, the CBN particles should be selected to be larger when sintered. Considering the above two factors comprehensively, the mixing particle size should be selected more when CBN is sintered, and different particle size ranges are determined according to the bonding ability of the selected binder.
(3) CBN grain content CBN content has a great influence on the hardness and thermal conductivity of PCBN tools. The higher the CBN content, the higher the hardness of the tool and the better the thermal conductivity. High-content PCBN cutters (typically 80% to 90% CBN) are based on direct bonding between CBNs and have high hardness and high thermal conductivity. These PCBN tools are suitable for processing high hardness alloys and materials with high hardness points in the structure, such as chilled cast iron, heat resistant alloys, etc. At present, such tool blades mainly include BZN6000 of GE Company, AMB90 of Element Six Company, BN100 and BN600 of Sumitomo Electric Company. Low-content PCBN inserts are mostly ceramic binders, which have good heat resistance and are easy to process hardened steel (alloy steel, bearing steel, die steel, carbon steel, etc.), and use the metal softening effect formed by heat retention in the cutting zone for cutting. GE's BZN8100, BZN8200, Element Six's DBC50, Sumitomo Electric's NB300, NB220, and Seco's CBN10, CBN100, CBN150, etc. all fall into this category.
(4) Binder required for sintering CBN: 1 The closer the physical and chemical properties are to CBN, the less the weakening of the cutting performance of the PCBN tool after sintering; 2 The temperature is easy to reach or at this temperature. It has good plasticity; 3 has sufficient chemical activity relative to CBN and has catalytic properties for converting hexagonal boron nitride (HBN) to CBN.
At present, the commonly used binders can be classified into metal binders (such as Ni, Co, Ti, Ti-Al, etc.) and ceramic binders (such as TiN, TiC, TiCN, Al2O3, etc.) according to their physical and chemical properties; According to the action, it can be divided into catalysts (such as Al, AlN, AlB2, Si, etc.) and solvent (such as Ti, Ni, Co, TiN, TiC, TiCN, etc.). Both the type and amount of binder have different effects on the performance of the PCBN insert. Carbide, nitride, carbonitride can improve the chemical wear resistance and impact resistance of PCBN inserts, but too high content will reduce tool hardness and shorten tool life; cobalt is the most commonly used binder to improve CBN sintering. Titanium sintering time; Ti ceramic binder can improve the toughness of PCBN insert; aluminum and aluminum compounds can react with CBN particles and other binders to make CBN particles bond more firmly and improve tool wear resistance; The mixture with Al, AlN and AlB2 is an effective catalyst for the conversion of HBN to CBN. Adding a small amount of Al and Si to the ceramic binder can also enhance the bond between CBN and form a continuous ceramic phase [12]; The PCBN composite sheet with nickel as a binder is highly conductive and suitable for cutting with low-cost electric spark.
Due to the different binders, PCBN inserts now tend to have two characteristics: one is high wear resistance (high content CBN, catalytic binder), and the other is good impact resistance (low content CBN) , ceramic binder).
3 PCBN blade market status
At present, the manufacturers of PCBN inserts at home and abroad are uneven, and there are great differences in the quality of PCBN inserts. However, in general, domestic PCBN inserts have a certain gap in terms of quality, specifications and varieties compared with foreign products. Table 1 compares the grades produced by major PCBN blade manufacturers at home and abroad and their main components. (slightly)
As can be seen from Table 1, with the deepening of research and manufacturing, foreign PCBN blades have now become serialized, which brings specialization of applications; on the other hand, the size of PCBN composite sheets is also large, and the available specifications are currently 57mm. , 74mm, 101.6mm, etc. Large composite sheets not only reduce the cost per unit area, but also improve the utilization rate, and are more suitable for users to manufacture blades of various specifications. In addition, the introduction of integrated inserts (eg, Seco CBN100, CBN30, CBN300, etc.) overcomes the cutting depth limitations due to the small size of the welded PCBN inserts and the risk of PCBN failure due to solder joint failure during processing; PCBN inserts Materials have also been developed in a variety of forms, coated PCBN inserts have been successfully developed and used in industrial production (such as Sumitomo BNC80, BNC150, BNC200; Seco CBN50C, CBN400C, etc.).
Although China's research and manufacture of PCBN materials began earlier, it has limited the application and development of PCBN tools because of the previous practice in the domestic manufacturing industry to use hard alloys and ordinary abrasives to process iron-based metals. At present, although the number of manufacturers capable of manufacturing PCBN inserts in China is large, the overall strength is poor, the types of products are small, and the quality is relatively poor. Therefore, in the automotive industry and other key users to cut difficult materials, China's PCBN blades are mostly dependent on imports.
4 PCBN blade development trend
Due to its unparalleled superiority in processing iron-based metals, PCBN tools are suitable for high-speed cutting technology and “car-based grinding†technology, which not only saves costs but also greatly improves production efficiency, which will make PCBN tools widely available in China. Promotion. At the same time, the tool manufacturer's position from a pure tool supplier has risen to become a key partner of the user's enterprise to improve production efficiency and product quality, reduce manufacturing costs, making the PCBN blade material develop in a diversified and serialized way to adapt to different Cutting of materials. At present, PCBN composite materials, coating technology, and PCBN nanotechnology have become the main directions for the development of PCBN blade materials in the future.
Research and manufacturing status and development of PCBN inserts
1 Introduction