The Performance and Application of PCD

Ⅰ. Introduction of PCD tools

PCD tool is made of superhard material PCD (polycrystalline diamond) and the tool matrix is produced through a special process.

1. Characteristics of PCD tools

The polycrystalline powder has the characteristics of high hardness, high compressive strength, good thermal conductivity, and good wear resistance.

2. Main indicators of PCD tools

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

(2) The thermal conductivity of PCD is 700W/mK, which is 1.5-9 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;

(4) The coefficient of thermal expansion of PCD is only 0.9×10^-6～1.18×10^-6, which is only 1/5 of that of cemented carbide, so the thermal deformation of PCD tools is small and the machining accuracy is high;

(5) The affinity between PCD tools and non-ferrous metals and non-metallic materials is very small, and the chips are not easy to adhere to the tool tip during the machining process to form a built-up edge.

Ⅱ. PCD tool manufacturing technology

1. Manufacturing process

The manufacturing process of PCD tools mainly includes two stages:

(1) The manufacture of PCD composite sheet: PCD composite sheet is made of natural or synthetic diamond powder and bonding agent (containing cobalt, nickel and other metals) sintered in a certain proportion at high temperature (1000～2000℃) and high pressure (50,000～100,000Torr). 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 components is formed between the diamond crystals, and the diamond crystals are embedded in the framework of the bonding bridge in the form of covalent bonds. Usually the composite sheet is made into a disc with a fixed diameter and thickness, and the sintered composite sheet needs to be ground and polished and other corresponding physical and chemical treatments.

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

2. Cutting process

Because PCD tools have high hardness and wear resistance, special processing techniques must be used. At present, the processing of pcd pcbn composite sheets mainly adopts several technological methods such as wire EDM, laser processing, ultrasonic processing, and high-pressure water jet. The comparison of their technological characteristics is shown in Table 1.

Table2: The characteristics of the connection between the PCD composite sheet and the cutter bar and their applications

Among the processing methods, the effect of EDM is better. The bonding bridge in PCD makes it possible for EDM composite sheets. In the presence of working fluid, the pulse voltage is used to make the working fluid near the electrode metal form a discharge channel, and a discharge spark is generated locally. The instant high temperature can melt and fall off the polycrystalline diamond, thereby forming the required triangle, rectangle or square cutter head blanks. The efficiency and surface quality of the EDM PCD composite sheet are affected by factors such as cutting speed, PCD particle size, layer thickness and electrode quality. The reasonable choice of cutting speed is very important. Experiments show that increasing the cutting speed will reduce the surface quality of the machined. However, if the cutting speed is too low, it will cause the phenomenon of "spinning" and reduce the cutting efficiency. Increasing the thickness of the PCD blade will also reduce the cutting speed.

3. Welding process

In addition to mechanical clamping and bonding methods, the combination of the PCD composite sheet and the cutter body is mostly to press the PCD composite sheet on the cemented carbide substrate by brazing. The welding methods mainly include laser welding, vacuum diffusion welding, vacuum brazing, high-frequency induction brazing and so on. At present, high-frequency induction heating brazing with low investment and low cost is widely used in PCD blade welding. In the blade welding process, the selection of welding temperature, flux and welding alloy will directly affect the performance of the tool after welding. In the welding process, the control of the welding temperature is very important. If the welding temperature is too low, the welding strength will be insufficient; if the welding temperature is too high, the PCD is easy to graphitize and may cause "overburning", which affects the PCD composite sheet and the cemented carbide combination of substrates. In the actual processing process, the welding temperature can be controlled according to the holding time and the degree of PCD turning red (generally it should be lower than 700°C). Foreign high-frequency welding mostly uses automatic welding technology, which has high welding efficiency and good quality, can realize continuous production. Manual welding is mostly used in the domestic process, which has low production efficiency and unsatisfactory quality.

4. Sharpening process

The high hardness of PCD makes the material removal rate extremely low (only one ten thousandths of the removal rate of cemented carbide). At present, the PCD tool sharpening process mainly uses ceramic bond diamond grinding wheels for grinding. Because the grinding between the grinding wheel abrasive and PCD is the interaction between two materials with similar hardness, the grinding law is more complicated. For high-granularity, low-speed grinding wheels, the use of water-soluble coolant can improve the grinding efficiency and precision of PCD. The choice of grinding wheel bond depends on the type of grinding machine and processing conditions. Since the electric discharge grinding (EDG) technology is almost unaffected by the hardness of the workpiece to be ground, the use of EDG technology to grind PCD has greater advantages. The grinding of some complex-shaped PCD tools (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 the main development direction of PCD grinding.

Ⅲ. Classification and design principles of PCD tools:

1. Classification of PCD tools

(1) PCD tools for metal cutting

PCD tools for metal cutting are mainly divided into welded PCD tools and indexable PCD tools in terms of structure. In recent years, welded PCD tools with shank have developed rapidly in the automotive and parts industries. There are mainly PCD milling cutters, PCD boring cutters, PCD reamers, or a combination of the above two or even three types of tools. The tool holder forms are mainly cylindrical shanks, BT shanks (BT40 and BT50), and SK shanks (SK40 and SKS0) , HSK handle (HSK63 and HSK100), etc.

(2) PCD tools for wood processing

PCD tools are also widely used in the wood processing industry. PCD woodworking tools can be divided into two categories: PCD saw blades and PCD shaped woodworking milling cutters.

2. Design principles of PCD tools

(1) PCD granularity

The choice of PCD particle size is related to tool processing conditions. For example, when designing tools for finishing or super finishing, PCD with high strength, good toughness, good impact resistance and fine grain should be selected. Coarse grain PCD tools can be used for general rough machining. The particle size of the PCD material has a significant effect on the wear and breakage properties of the tool. Studies have shown that the larger the PCD particle size, the stronger the wear resistance of the tool.

(2) Blade thickness

Under normal circumstances, 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 thickness is conducive to the EDM of the blade. When the PCD composite sheet is welded to the cutter 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 bonding surfaces of the two materials.

(3) Structural design

The geometric parameters of the PCD tool depend on the specific processing conditions such as the condition of the workpiece, the tool material and the structure. Because PCD tools are often used for finishing of workpieces, the cutting thickness is small (sometimes even equal to the cutting edge radius of the tool), which belongs to micro-cutting. Therefore, the clearance angle and the flank face 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 the PCD composite sheet and the cutter bar include mechanical clamping, welding, indexing, etc. The characteristics and application scope are shown in Table 2.

Table2: The characteristics of the connection between the PCD composite sheet and the cutter bar and their applications

Ⅳ. PCD tool cutting parameters

1. Cutting speed

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

2. Feed rate

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

3. Cutting depth

Increasing the cutting depth of PCD tools will increase the cutting force and cutting heat, which will aggravate tool wear and affect tool life. In addition, the increase in cutting depth is likely to cause chipping of PCD tools.

The cutting performance of PCD tools of different particle size grades is different when processing different workpiece materials under different processing conditions. Therefore, the actual cutting parameters of PCD tools should be determined according to the specific processing conditions.

Ⅴ. The development and application prospects of PCD tools

At present, the gap between our country's tool technology and foreign countries is mainly manifested in the research and development of high-performance tool materials, tool preparation technology and tool use technology. For PCD tools, since foreign materials can be purchased, the key lies in tool preparation technology and tool use technology. In the future, with the rapid development of China's automobile and aerospace abrasive powder suppliers industries, the preparation technology and use technology of PCD tools will surely receive attention and development.

In the future, aluminum, manganese and other light alloys as well as titanium alloys, sintered alloys and other high-strength alloys will be increasingly used as materials for automotive power system components. Diamond tools capable of processing these materials are expected to have a very large growth. In addition, because PCD tools have high hardness, high heat resistance, long life, chip edge sharpness and good thermal conductivity, etc., they can meet the requirements of high-speed, high-efficiency, high-precision, new materials and new shapes for automotive parts processing. Therefore, PCD tools are bound to be widely used.