[Technology +] Research on the Status Quo and Development of Polycrystalline Diamond Tools
Wu Chang, Song Pengtao, Liu Haihua*, Lai Xiqing, Deng Ping
(Zhuhai Rossini Watch Industry Co., Ltd., Zhuhai, Guangdong 519085)
Abstract: This paper introduces the development and performance characteristics of polycrystalline diamond tools, and briefly describes the manufacturing process, processing parameters and aging mechanism of polycrystalline diamond tools. Nowadays, the development of machining technology has made the cutting tool technology develop in the direction of high-speed cutting, high-precision, dry cutting and grinding, making the use of super-hard tools high in efficiency and high in stability.
Key words: polycrystalline diamond; tool; performance; manufacturing technology; processing parameters; failure mechanism
Chinese Library Classification: TG711 Documentary Code: A Article ID: 2095-2945(2019)09-0080-03
01, overview
Superhard tools play an important role in the automotive, aerospace, energy, military and mechanical fields [1]. Superhard tools are tools made of superhard materials such as diamond and cubic boron nitride (CBN) and their composite materials. Currently, they are mainly PCD (Polycrystalline Diamond, PCD) and PcBN (polycrystalline cubic boron nitride) tools. . Polycrystalline diamond is a polycrystalline material obtained by sintering diamond micropowder through a metal bond such as cobalt under high temperature and high pressure. Although the hardness is slightly lower than that of single crystal diamond, the crystal grains are disorderly arranged, isotropic, and has no solution. The polycrystalline diamond tool has a low coefficient of friction, excellent thermal conductivity and a low coefficient of expansion. Its hardness is 2 to 4 times that of cemented carbide and its tool life is more than 10 times that of cemented carbide. Moreover, the price of raw materials used in polycrystalline diamond is several times lower than that of natural diamond. Polycrystalline diamond composite tool uses cemented carbide as the base material, has weldability and good compatibility with polycrystalline diamond, and has good toughness and hardness. Therefore, polycrystalline diamond has both diamond hardness and hard alloy. Toughness and weldability [2].
02, the development history of polycrystalline diamond tools
As a superhard tool material, diamond has been used in processing for hundreds of years. In the tool development process, the tool material used in the tool was from 1890 to 1950, and the typical material used was high speed steel. In 1927, Germany first developed hard alloy materials, and then the hard alloy material was widely used. In the field of tool manufacturing. From 1950 to 1959, with the successful synthesis of synthetic diamonds in Sweden and the United States, superhard materials—diamonds began to gradually replace cemented carbide materials as the main tool materials. From 1970 to 1979, with the successful birth of polycrystalline diamond prepared by high-pressure synthesis technology, synthetic polycrystalline diamond became an effective substitute for natural diamonds. The material problem of diamond tools was effectively solved, making the use of diamond tools From stone, electronics and automobiles to the aerospace and aerospace sectors [3].
03, polycrystalline diamond tool characteristics
Diamond tools have the advantages of high hardness, high compressive strength, good thermal conductivity and good wear resistance. These characteristics (advantages) are closely related to the crystal state of diamond: the reason why the diamond hardness is extremely high is that it has a "diamond structure", and the so-called "diamond structure" means that in the diamond crystal, the four valence electrons of the carbon atom have a tetrahedral structure. Bonding, and each carbon atom forms a covalent bond with four adjacent atoms, and this structure has strong directivity and adhesion. Polycrystalline diamond is sintered from fine-grained diamonds and binders with different orientations [4]. Because of this, polycrystalline diamonds are characterized by isotropicity, although their wear resistance and hardness are not as good as single crystals. Diamond, but it is more difficult to crack along a single cleavage plane than single crystal diamond, and it also has hardness and wear resistance second only to single crystal diamond [5].
The surface Vickers hardness of polycrystalline diamond is greater than or equal to 1800 HV. The surface Vickers hardness of cemented carbide is far less than the Vickers hardness of polycrystalline diamond. Polycrystalline diamond tools do not appear to be slow due to heat dissipation during production. The problem of heat collection and workpiece burn is higher than copper (Cu), up to 700W/mK; the cutting force of polycrystalline diamond tools can be significantly reduced because the friction coefficient of polycrystalline diamond tools is generally only 0.1 to 0.3. It is only about one-third of the friction coefficient of cemented carbide; the coefficient of thermal expansion of polycrystalline diamond is only 0.0000009 to 0.00000118, which is about one-fifth of that of cemented carbide. In addition, polycrystalline diamond tools do not easily produce sticky knives during production and use, and the chips do not easily bond to the tips to form chips because the affinity between polycrystalline diamond and non-metallic and non-ferrous materials is extremely low.
Polycrystalline diamond tools are mainly used in the following two aspects: (1) processing of non-ferrous materials, processing of non-ferrous metals, and the use of common tools, it is easy to produce tool wear, low processing efficiency and other defects. Polycrystalline diamond tools can show excellent processing performance. (2) For the processing of non-metallic materials, polycrystalline diamond cutters are suitable for processing non-metallic materials that are difficult to process, such as stone, hard carbon, carbon fiber reinforced plastic (CFRP) and wood-based panels.
04, polycrystalline diamond tool manufacturing process
Figure 1 Production process of polycrystalline diamond tool
The manufacturing process of the polycrystalline diamond tool is shown in Figure 1. The manufacturing process of polycrystalline diamond tools mainly includes the manufacture and sorting of polycrystalline diamond micropowder, the synthesis, grinding and cutting of polycrystalline diamond compacts, and the welding and sharpening of polycrystalline diamond inserts.
4.1 Manufacturing composite sheets
Polycrystalline diamond composite panels are prepared by mixing synthetic diamond particles/powder with metal binders (such as Co, Ni, etc.) at high temperatures and pressures (1,000 to 2,000 degrees Celsius, 50,000 to 100,000 Prepared by sintering under atmospheric pressure. During the sintering process, the binder melts to form a bonding bridge between diamond crystals mainly composed of iron (Fe), cobalt (Co), nickel (Ni), etc., and the diamond crystal is embedded in the bond in the form of an embedded bridge. In the skeleton of the bridge. For sintered composite panels, grinding and polishing as well as other corresponding physical and chemical treatments are still required [6].
4.2 Cutting process of polycrystalline diamond composite blade
Polycrystalline diamond composite sheets have high hardness, so special processing methods must be used for subsequent cutting, such as Wire Cut Electrical Discharge Machining (WEDM), Electrical Discharge Machining (EDM), and Ultrasonic Machining. (Supersonic Machining, referred to as SM), Laser Processing (LP), High-pressure Waterjet Machining (HWM) and other processes [7].
Among the five processing methods of appeal, Electrical Discharge Machining (EDM) is more effective. Since the bonding bridge exists in the polycrystalline diamond, the working fluid in the vicinity of the electrode metal is formed into a discharge channel by using a pulse voltage under the condition of the conductive working fluid. A discharge spark is locally generated, and a transient high temperature causes a specific portion of the polycrystalline diamond to melt and fall off, thereby forming a desired blank of various shapes, such as a triangle or a rectangle. The efficiency and surface quality of EDM-polycrystalline diamond composite sheets are affected by factors such as the particle size of the diamond, the layer thickness of the polycrystalline diamond composite sheet, and the electrode quality.
4.3 Welding process of polycrystalline diamond composite blade
Most polycrystalline diamond composite plates and bodies are brazed and polycrystalline diamond composite plates are welded to the cemented carbide substrate. Welding methods mainly include laser welding, vacuum diffusion welding, vacuum brazing, high frequency induction brazing, and the like. At present, due to the low cost of high frequency induction heating brazing, it is widely used for the welding of polycrystalline diamond blades.
In the welding process of polycrystalline diamond composite and cemented carbide substrate, the factors that have great influence on the performance of the tool after welding are: welding temperature and flux selection. It is important to control the soldering temperature during the soldering process. On the other hand, if the soldering temperature is too high, the polycrystalline diamond composite sheet is easily oxidized and further graphitized, resulting in "excessive ablation", which affects the solderability of the polycrystalline diamond composite sheet and the cemented carbide substrate. Sex. The traditional manual welding method has low production efficiency and unstable product quality. Nowadays, the welding of polycrystalline diamond composite sheets and cemented carbide substrates adopts automatic high-frequency welding technology, which has high welding efficiency and good product quality consistency.
4.4 Grinding process of polycrystalline diamond tools
The hardness of the polycrystalline diamond composite blade is very high, so the removal rate is extremely low, about one thousandth or even one ten thousandth of the removal rate of the cemented carbide. Therefore, the sharpening process of the polycrystalline diamond tool is mainly performed by using a resin-bonded diamond wheel or a ceramic-bonded diamond wheel. Since the abrasives of the resin bond diamond grinding wheel and the ceramic bond diamond grinding wheel are also diamond particles/powder, which is the same as the polycrystalline diamond tool, the grinding rule is complicated.
Grinding wheels for resin or ceramic bonding options should be based on the type of grinding machine and the processing conditions. Since the EDM technology is not affected by the hardness of the grounded workpiece, grinding the polycrystalline diamond with the EDM technology has a better grinding effect. The grinding of some complex-shaped polycrystalline diamond tools (such as woodworking tools) cannot be performed by the traditional diamond grinding wheel. Only such a tool for complex shape EDM grinding can be used. With the continuous application of polycrystalline diamond tools and the development of EDM technology, Electrical Discharge Grilling (EDG) technology will become a trend in polycrystalline diamond tool grinding.
05. Cutting parameters and failure mechanism of polycrystalline diamond tools
5.1 Cutting speed
Polycrystalline diamond tools can be machined at higher speeds, and cutting speed is an important factor influencing the quality of the process. Although the cutting efficiency can be improved, the machining efficiency of the tool can be improved. However, due to the high hardness and brittleness of the polycrystalline diamond tool, the cutting temperature and the cutting force increase under the high-speed rotation state, and the instantaneous collision between the tool and the cutting sample causes the blade tip to be brittle, so the polycrystalline diamond tool needs Re-sharpening. Therefore, the cutting speed of polycrystalline diamond tools should also be different when processing different workpiece materials.
5.2 Feed
The feed of polycrystalline diamond tools directly affects the quality of the workpiece and the service life of the polycrystalline diamond tool. If the feed amount of the polycrystalline diamond tool is too large, the geometric area of ​​the polycrystalline diamond tool in contact with the sample will be multiplied, resulting in an increase in the surface roughness of the machined surface; if the feed rate is too small, the process efficiency is lowered. Therefore, for different equipment and processing materials, it is an important factor to improve the processing quality and processing efficiency by continually trying to find the appropriate feed amount.
5.3 Cutting depth
In addition, the depth of cut also has a large effect on the service life of the tool. If the cutting force of the polycrystalline diamond tool increases, the cutting heat increases, which accelerates tool wear and affects tool life. In addition, an increase in the depth of cut tends to cause the cutting edge of the polycrystalline diamond tool to break. Therefore, when different polycrystalline diamond tools of different particle sizes are used, the cutting performance is different when different workpieces are processed under different processing conditions, so the actual cutting parameters of the polycrystalline diamond tool should be determined by a large number of tests and processing conditions. .
5.4 Failure mechanism
The failure modes of conventional tools are abrasive wear, bond wear (cold weld wear), diffusion wear, oxidative wear and thermoelectric wear. The wear forms of polycrystalline diamond tools mainly include polycrystalline layer damage, bond wear and diffusion wear. Studies have shown that when polycrystalline diamond tools are used to process metal matrix composites, the failure modes are mainly bond wear and failure caused by intergranular cracks in diamond grains; when processing high hardness and high brittle materials, polycrystalline diamond tools Bond wear is not obvious; when processing low-brittle materials, tool wear increases, and bond wear plays a leading role.
06, the conclusion
At present, in the field of stainless steel cutting, traditional hard alloys and high-speed tool steel tools are still used. The application of polycrystalline diamond tools has not been reported.
According to reports, when cutting stainless steel materials, polycrystalline diamond tools are prone to "sticking" due to the softness of stainless steel. At the same time, if the feed rate is too large, the surface roughness of the stainless steel is low, the polycrystalline diamond tool is easy to wear and easy to collapse; if the feed amount is too small, the surface hardening of the stainless steel will occur, and the specific processing parameters are difficult to adjust. However, in the actual use process, how to use superhard polycrystalline diamond tools in the field of stainless steel cutting still needs to be studied in detail in specific production practices.
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