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Application of polycrystalline diamond tools

Diamond has been used as a superhard tool material in cutting processing for hundreds of years. In the development process of cutting tools, from the end of the nineteenth century to the middle of the twentieth century, high-speed steel was the main representative of the cutting tool materials; in 1927, Germany first developed carbide tool materials and gained wide application; The United States separately synthesized artificial diamonds, and cutting tools have since entered a period represented by superhard materials. In the 1970s, people used high-pressure synthesis technology to synthesize polycrystalline diamond (PCD), which solved the problem of scarce and expensive natural diamond, and expanded the application of diamond tools to aviation, aerospace, automotive, electronics, and stone And many other fields.

Performance characteristics

Diamond tools have the characteristics of high hardness, high compressive strength, good thermal conductivity and wear resistance, and can obtain high machining accuracy and machining efficiency in high-speed cutting. The above characteristics of diamond tools are determined by the state of diamond crystals. In diamond crystals, the four valence electrons of carbon atoms form a bond according to a tetrahedral structure, and each carbon atom forms a covalent bond with four adjacent atoms, thereby forming a diamond structure. So that diamond has extremely high hardness. Since the structure of polycrystalline diamond (PCD) is a fine-grained diamond sintered body with different orientations, although a bonding agent is added, its hardness and wear resistance are still lower than single crystal diamond. However, since the PCD sintered body is isotropic, it is not easy to crack along a single cleavage surface.

Main indicators

① The hardness of PCD can reach 8000HV, which is 8-12 times that of cemented carbide;

②The thermal conductivity of PCD is 700W / mK, which is 1.5 to 9 times that of cemented carbide, even higher than PCBN and copper, so the heat transfer of PCD tools is rapid;

③ The friction coefficient of PCD is generally only 0.1 to 0.3 (the friction coefficient of cemented carbide is 0.4 to 1), so PCD tools can significantly reduce the cutting force;

④The thermal expansion coefficient of PCD is only 0.9 × 10 ^ -6 ～ 1.18 × 10 ^ -6, which is only equivalent to 1/5 of the cemented carbide, so the PCD tool has small thermal deformation and high processing accuracy;

⑤The affinity between the PCD tool and non-ferrous metal and non-metallic materials is very small, and the chip is not easy to stick to the tool tip to form a built-up edge during the processing process.

Application

The research on PCD tools has been carried out earlier in industrialized countries, and its application has been relatively mature. Since the first synthetic diamond was synthesized in Sweden in 1953, the research on the cutting performance of PCD tools has obtained a lot of results, and the application scope and usage of PCD tools have expanded rapidly. The internationally renowned manufacturers of synthetic diamond composite sheets mainly include DeBeers of the United Kingdom, GE of the United States and Sumitomo Electric Co., Ltd. of Japan. According to reports, in the first quarter of 1995, Japan’s PCD tool output reached 107,000. The application range of PCD tools has expanded from initial turning to drilling and milling. A survey conducted by a Japanese organization on superhard tools shows that the main considerations for people choosing PCD tools are based on the advantages of surface accuracy, dimensional accuracy and tool life after PCD tool processing. The synthesis technology of diamond composite sheet has also been greatly developed. DeBeers has launched a polycrystalline diamond composite sheet with a diameter of 74 mm and a layer thickness of 0.3 mm.

The domestic PCD tool market is also expanding with the development of tool technology. China FAW Group has more than 100 PCD turning tool use points, and many wood-based panel companies also use PCD cutting tools for wood product processing. The application of PCD tools has also further promoted the research on its design and manufacturing technology. Tsinghua University, Dalian University of Technology, Huazhong University of Science and Technology, Jilin University of Technology, Harbin Institute of Technology, etc. are all actively conducting research in this area. There are dozens of domestic companies engaged in R & D and production of PCD tools, such as Shanghai Schubert, Foshan Bonner, Changzhou Merck Tools, Zhengzhou Xinya, Nanjing Leitz, Shenzhen Runxiang, Chengdu Tool Research Institute, and Chengdu Daimendi. The processing range of PCD tools has expanded from traditional metal cutting to stone processing, wood processing, metal matrix composites, glass, engineering ceramics and other materials. Analysis of the application of PCD tools in recent years shows that PCD tools are mainly used in the following two aspects: ① Difficult to process non-ferrous metal materials: When using ordinary tools to process non-ferrous metal materials, the tools tend to wear, and the processing efficiency is low. Defects, and PCD tools can show good processing performance. For example, PCD tools can effectively process a new type of engine piston material-hypereutectic silicon aluminum alloy (the research on the processing mechanism of this material has made a breakthrough). ②The processing of difficult-to-process non-metallic materials: PCD tools are very suitable for the processing of difficult-to-process non-metallic materials such as stone, hard carbon, carbon fiber reinforced plastic (CFRP), and artificial plates. For example, Huazhong University of Science and Technology realized glass processing with PCD tools in 1990; the application of reinforced composite floors and other wood-based plates (such as MDF) is becoming more and more extensive. Processing these materials with PCD tools can effectively avoid defects such as easy tool wear.

Manufacturing Technology

Manufacturing process

The manufacturing process of PCD tools mainly includes two stages:

The manufacture of PCD composite tablets: PCD composite tablets are made of natural or synthetic diamond powder and binders (including cobalt, nickel and other metals) at a certain ratio at high temperature (1000 ~ 2000 ℃), sintered under high pressure (50,000 ~ 100,000 atmospheres). During the sintering process, due to the addition of the bonding agent, a bonding bridge with TiC, SiC, Fe, Co, Ni, etc. as the main component is formed between the diamond crystals, and the diamond crystals are embedded in the skeleton of the bonding bridge in the form of covalent bonds. Usually the composite sheet is made into a disk with a fixed diameter and thickness, and the sintered composite sheet needs to be ground and polished and other corresponding physical and chemical treatments.

PCD blade processing: The processing of PCD blades mainly includes the steps of cutting the composite sheet, welding the blade, and sharpening the blade.

Cutting process

Because the PCD composite sheet has high hardness and wear resistance, special processing techniques must be used. The processing of PCD composite sheets mainly adopts several technological methods such as wire electric discharge cutting, laser machining, ultrasonic machining, and high-pressure water jet.

Process method-process characteristics

EDM-Highly concentrated pulse discharge energy and strong discharge explosion force melt the metal in PCD material, part of the diamond is graphitized and oxidized, part of the diamond is shed, the process is good and the efficiency is high

Ultrasonic processing-low processing efficiency, large diamond powder consumption, and large dust pollution

Laser processing-non-contact processing, high efficiency, small processing distortion, poor processability

Among the above processing methods, the effect of EDM is better. The presence of the bonding bridge in the PCD makes it possible to process the EDM composite sheet. Under the condition of working fluid, using pulse voltage to make the working fluid near the electrode metal form a discharge channel, and generate a discharge spark locally, the instantaneous high temperature can cause the polycrystalline diamond to melt and fall off, thereby forming the required triangle, rectangle or Square cutter head blank. The efficiency and surface quality of EDM machining of PCD composite sheets are affected by factors such as cutting speed, PCD particle size, layer thickness and electrode quality. Among them, the reasonable choice of cutting speed is very critical. Experiments show that increasing the cutting speed will reduce the quality of the processed surface. The cutting speed is too low will produce “arch wire” phenomenon, and reduce cutting efficiency. Increasing the thickness of the PCD blade will also reduce the cutting speed.

Welding process

In addition to the mechanical clamping and bonding method, the PCD composite sheet and the knife body are mostly combined by brazing to press the PCD composite sheet on the cemented carbide substrate. Welding methods mainly include laser welding, vacuum diffusion welding, vacuum brazing, high-frequency induction brazing, etc. High-frequency induction heating brazing with low investment and low cost is widely used in PCD blade welding. In the process of blade welding, the choice of welding temperature, flux and welding alloy will directly affect the performance of the tool after welding. During the welding process, the control of the welding temperature is very important. If the welding temperature is too low, the welding strength is not enough; if the welding temperature is too high, the PCD is easily graphitized and may cause “over-burning”, which affects the PCD composite sheet and cemented carbide The combination of substrates. In the actual processing, the welding temperature can be controlled according to the holding time and the degree of redness of the PCD (generally lower than 700 ℃). Foreign high-frequency welding mostly uses automatic welding process, which has high welding efficiency and good quality, which can achieve continuous production; domestic use mostly uses manual welding, which has low production efficiency and insufficient quality.

Sharpening process

The high hardness of PCD makes its material removal rate extremely low (even only one ten thousandth of the removal rate of cemented carbide). The PCD tool sharpening process mainly uses ceramic bond diamond grinding wheels for grinding. Because the grinding between grinding wheel abrasives and PCD is the interaction between two materials with similar hardness, the grinding law is more complicated. For high-grit, low-speed grinding wheels, the use of water-soluble coolant can improve the grinding efficiency and grinding accuracy of PCD. The choice of grinding wheel bond should depend on the type of grinding machine and the processing conditions. Because EDM technology is hardly affected by the hardness of the workpiece being ground, the use of EDG technology for grinding PCD has great advantages. The grinding of certain PCD tools with complex shapes (such as woodworking tools) also has a huge demand for this flexible grinding process. With the continuous development of EDM grinding technology, EDG technology will become a main development direction of PCD grinding.

Design Principles

PCD granularity

The choice of PCD particle size is related to the tool processing conditions. For example, when designing a tool for finishing or super finishing, a PCD with high strength, good toughness, good impact resistance, and fine grain should be selected. Coarse grain PCD tools can be used for general roughing. The particle size of the PCD material has a significant influence on the wear and breakage performance of the tool. Studies have shown that the larger the PCD particle number, the stronger the wear resistance of the tool. The tool wear test results when machining SiC-based composite materials using the tools of DeBeers SYNDITE 002 and SYNDITE025 PCD materials show that the SYNDITE002PCD material with a particle size of 2 μm is easier to wear.

The PCD particle size is submicron, 2μm, 5μm, 10μm, 25μm and 2 ~ 30μm. Among them, the CDW010 material with 10μm particle size is a universal type, with good chipping resistance and wear resistance. Generally, the larger the particle size, the more resistant the blade material Wear, and the wear resistance of CDW302 material with a mixed particle size of 2 ~ 30μm is better. As shown in the figure, it is a comparison of chipping resistance and wear resistance of different PCD particle sizes, as well as a comparison of discharge characteristics and grindability.

PCD Material	Polycrystalline diamond PCD particle size	Technical process
CDW850 CDW002 CDW005	Fine particle size, average particle size is submicron,2μm,5μm	Mainly used for finishing, ultra-precision processing technology.
CDW010	General-purpose medium particle size with an average particle size of 10μm	Both roughing and finishing
CDW025 CDW302	Coarse particle size 25μm and mixed particle size	It is mainly used for rough machining and can obtain a higher service life.

Blade thickness

Normally, the layer thickness of the PCD composite sheet is about 0.3 to 1.0 mm, and the total thickness after adding the cemented carbide layer is about 2 to 8 mm. The thinner PCD layer is beneficial to the EDM of the blade. DeBeers’ 0.3mm thick PCD composite sheet can reduce the grinding force and increase the cutting speed of EDM. When the PCD composite sheet is welded to the blade body material, the thickness of the cemented carbide layer should not be too small to avoid delamination caused by the difference in stress between the two materials’ joining surfaces.

Structural design

The geometric parameters of PCD tools depend on the specific processing conditions such as workpiece conditions, tool materials and structure. Since PCD tools are often used for the finishing of workpieces, the cutting thickness is small (sometimes even equal to the cutting edge radius of the tool), which is a small amount of cutting. Therefore, the clearance angle and the clearance of the cutting tool have a significant impact on the processing quality. The smaller clearance angle, Higher flank quality can play an important role in improving the processing quality of PCD tools.

The connection methods of PCD composite sheet and cutter bar include mechanical clamping, welding, indexable and other methods

Mechanical clamping-consists of a standard knife body and interchangeable blades that can be made into various collection angles, with the advantages of quick change and easy regrind-small and medium-sized machine tools

Integral welding-compact structure, easy production, can be made into small-sized tools-special tools or tools that are difficult to clamp, used for small machine tools

Machine clamp welding-the blade is welded to the cutter head, and a standard cutter bar can be used, which is convenient for sharpening and adjusting the cutter head position-automatic machine tool, CNC machine tool

Indexable-compact structure, reliable clamping, no need for re-grinding and welding, can save auxiliary time and improve tool life-ordinary general-purpose machine tools

Cutting parameters

Influence of cutting parameters of PCD tools on cutting performance

Cutting speed

PCD tools can be cut at extremely high spindle speeds, but the impact of changes in cutting speed on machining quality cannot be ignored. Although high-speed cutting can improve the machining efficiency, under high-speed cutting, the increase in cutting temperature and cutting force can cause damage to the tool tip and make the machine tool vibrate. When processing different workpiece materials, the reasonable cutting speed of the PCD tool is also different. For example, the reasonable cutting speed of milling Al2O3 reinforced floors is 110 ～ 120m / min; the reasonable cutting speed of turning SiC particle reinforced aluminum-based composite materials and silicon oxide-based engineering ceramics 30 ～ 40m / min.

Feed rate

If the feed amount of the PCD tool is too large, the residual geometric area on the workpiece will increase, resulting in an increase in surface roughness; if the feed amount is too small, the cutting temperature will increase and the cutting life will decrease.

Cutting depth

Increasing the cutting depth of the PCD tool will increase the cutting force and increase the cutting heat, thereby aggravating the tool wear and affecting the tool life. In addition, the increase in cutting depth is likely to cause PCD tool chipping.

PCD tools of different granularity grades show different cutting performance when processing different workpiece materials under different processing conditions, so the actual cutting parameters of PCD tools should be determined according to the specific processing conditions.

Failure mechanism

The wear forms of cutters are mainly abrasive wear, bond wear (cold welding wear), diffusion wear, oxidation wear, thermoelectric wear, etc. The failure modes of PCD tools are different from traditional tools, mainly manifested as broken polycrystalline layer, adhesive wear and diffusion wear. Studies have shown that when PCD tools are used to process metal matrix composites, the failure modes are mainly bond wear and microscopic intergranular cracks caused by diamond grain defects. When processing high-hardness, high-brittle materials, the bond wear of PCD tools is not obvious; on the contrary, when processing low-brittle materials (such as carbon fiber reinforced materials), the wear of the tools increases, and the bond wear plays a leading role

Main application areas of PCD tools

(1) Aerospace field

Compared with other materials, composite materials have the characteristics of light weight, high strength, high temperature resistance, and corrosion resistance, making them ideal materials for the aerospace industry. With the wide application of aerospace composite materials, how to properly select and reasonably use cutting tools for efficient and high-quality cutting is a very important issue.

The tool materials widely used in the aerospace manufacturing industry mainly include cemented carbide and PCD tool materials. Among them, PCD tool materials account for an increasing proportion. In today’s aerospace manufacturing, both of them become the leading tool in the development of aerospace industry tools , The scope of application is quite wide.

(2) Automotive industry

PCD cutters were used in the United States in 1973. Since the application of PCD tools in the automotive industry, the use of PCD tools in the North American automotive industry has grown rapidly. The main object of the American automobile company’s processing with PCD tools is the aluminum alloy cylinder block of the engine.

The application of PCD tools covers almost all aluminum alloy parts processing fields in automobiles, including special step compound tools, cast aluminum and aluminum alloy processing taps, various high-precision hole compound reamers, compound drill bits, large-diameter drill bits, and difficult material processing. Drill bits and so on. Domestic auto companies such as FAW-Volkswagen have widely used PCD tools to process automotive aluminum alloy parts (such as automotive aluminum alloy pistons and aluminum alloy wheels).

Vacuum brazing furnaces for brazing polycrystalline diamond tools have the advantages of high vacuum, high welding hardness, and no need for post-weld treatment. They gradually replace high-frequency induction brazing methods.

SIMUWU company specializes in the development of a vacuum brazing furnace for diamond tools. It can be used for the vacuum brazing of rod and sheet diamond tools to process polycrystalline diamond tools in batches.