Application and Prospect of superhard tool materials

1、 Introduction

In recent years, due to the great development of the automobile and aircraft manufacturing industry, a large number of light materials (A1-Si, a1-mg alloys) and composite materials have been applied. At present, these materials are usually milled with cemented carbide tools. However, when the cutting speed is increased, the wear of cemented carbide tools will be intensified. In order to ensure the durability of the tools, the milling of parts is often carried out at a lower cutting speed (rough machining 30 m / min, finish machining 100 m / min), and the machining efficiency is low (material removal rate 3-13 cm3 / min). Therefore, in the manufacturing process of light material parts, the machinability is very poor, which is shown as follows:

(1) Low processing efficiency;

(2) Serious tool wear;

(3) Machining accuracy and surface quality are unstable.

There is no doubt that seeking a high-speed, high-efficiency and low-cost machining method has become a hot research topic.

As superhard materials (polycrystalline diamond and polycrystalline cubic boron nitride) tools enter the field of machining, they gradually show their superior machining performance. Their ultra-long service life, high machining quality and high production efficiency are incomparable to those of other materials. In the past, it was mainly used for finishing. In recent years, due to the improvement of the production process of artificial superhard tool materials, the control of raw material purity and grain size, the use of composite materials and hot pressing technology, etc., the application range has been continuously expanded. In addition to being suitable for general finishing and semi finishing, it can also be used for rough machining. It is internationally recognized as one of the most promising tool materials for improving productivity.

2、 Performance characteristics of superhard tool materials

According to the 1981 International Conference on hard material science, materials with hardness greater than 1000hv can be called hard materials, and materials that can process such materials as hard alloy (hardness 1600-1800hv), corundum (2000hv), silicon carbide (2200hv) are called superhard materials. Generally speaking, superhard materials refer to artificial diamond and CBN (cubic boron nitride) materials with similar hardness and performance to natural diamond. Because the market price of natural diamond is very high, polycrystalline cubic boron nitride (PCBN), Artificial Polycrystalline Diamond (PCD) and their composites are mostly used in the production of superhard tools in China. Superhard tool materials have the following performance characteristics:

(1) High hardness. The hardness of the tool material must be higher than that of the workpiece material. The hardness of PCD can reach 8000hv, which is 8-12 times of that of cemented carbide; CBN crystal structure is similar to diamond, so it has similar hardness and strength to diamond; The microhardness of CBN fine powder is 8000-9000hv, and the hardness of its sintered PCBN is generally 3000-5000hv

(2) High wear resistance. The tool material should have good wear resistance, which depends on the mechanical properties of the material. The chemical composition and structure of the tool wear resistance is the tool wear resistance. Generally, the higher the hardness of the tool, the better the wear resistance. The more hard spots (such as carbides and nitrides) in the metallographic structure of the tool, the smaller the particles and the more uniform the distribution, the better the wear resistance of the tool. The heat resistance of tool material is the main symbol to measure the cutting performance of the tool. It is usually measured by the performance of maintaining high hardness at high temperature, also known as thermal hardness. The higher the high-temperature hardness of the tool material, the better the heat resistance, and the stronger the resistance to plastic deformation and wear at high temperature.

(3) Sufficient strength and toughness. The tool material must have sufficient strength and toughness to resist impact and vibration. For example, when turning 45 steel, under the condition of back draft AP = 4mm and feed f = 0.5mm/r, the cutting force borne by the blade reaches 4000N. It can be seen that the tool material must have high strength and strong toughness. The toughness of general tool materials is expressed by impact toughness AK, which reflects the brittleness resistance and edge breaking ability of tool materials.

(4) High heat resistance. The ability to maintain high hardness, wear resistance, strength and toughness at high temperature.

(5) Good thermal conductivity and processability. The higher the thermal conductivity is, the longer the service life of the tool will be; If the coefficient of linear expansion is small, the thermal deformation can be reduced; In order to facilitate manufacturing, it is necessary to have good forging performance, heat treatment performance, welding performance, processing performance, etc., and to pursue high performance price ratio.

(6) Having a lower coefficient of friction. Low friction coefficient can lead to small cutting force, lower cutting temperature and improved surface quality.

Because of the high hardness of superhard tool materials, it has always been a difficult problem to produce chip breaking grooves like cemented carbide blades. The emergence of cutting-edge laser engraving technology has solved this problem. The following figure shows the PCD / CBN blade with chip breaking groove manufactured by dicut diamond tool company in Germany with cutting-edge laser processing technology.

3、 Application of superhard tool materials

（1） Diamond cutter

As a kind of superhard tool material, diamond has been used in cutting for hundreds of years. In the 1970s, polycrystalline diamond (PCD) was synthesized by high-pressure synthesis technology, which solved the problem of scarce natural diamond and high price, and accelerated the development of artificial diamond. In addition to using graphite as raw material to sinter diamond at high temperature and high pressure, many people now develop a new process of producing diamond with pure methane at normal temperature and pressure.

The application of this high hardness material in metal cutting is mainly reflected in the following two aspects:

(1) Processing of difficult to process non-ferrous metal materials: when ordinary tools are used to process difficult to process non-ferrous metal materials, defects such as easy wear of tools and low processing efficiency often occur, while PCD tools can show good processing performance. For example, PCD tool can effectively process hypereutectic silicon aluminum alloy, a new type of engine piston material (a breakthrough has been made in the research on the processing mechanism of this material).

(2) Processing of hard to process non-metallic materials: PCD tools are very suitable for processing hard to process non-metallic materials such as stone, hard carbon, carbon fiber reinforced plastic (CFRP) and artificial plates. For example, Huazhong University of technology realized glass processing with PCD tools in 1990; At present, the application of laminate flooring and other wood-based panels (such as MDF) is becoming more and more extensive. Processing these materials with PCD tools can effectively avoid the defects such as easy wear of tools. For example, due to the requirements of weight reduction and multi-function, modern automobile gearboxes are more and more made of die-casting aluminum alloy. There are many stepped forming holes on them. These holes have strict requirements on surface roughness, roundness, cylindricity, concentricity, burr free and other quality. In order to meet the needs of high-efficiency and high-precision processing of formed holes, Japanese company has developed a PCD forming step reamer. The high-quality PCD insert material is selected, and the high-precision form grinding technology is used to ensure the shape of the cutting edge, to obtain excellent and consistent dimensional accuracy and surface roughness, so that all cutting edges have excellent cutting performance under different processing loads. However, it is well known that diamond reacts with W, Ta, Ti, Zr, Fe, Ni, Co, Mn, Cr, Pt, etc. at high temperature, and chemical wear occurs with ferrous metals (iron carbon alloys) during processing. Diamond is not used for processing ferrous metals.

（2） Cubic boron nitride (CBN)

Cubic boron nitride (CBN) is another superhard material after artificial diamond. It is characterized by its hardness second only to artificial diamond (up to 8000hv ~ 9000hv), good wear resistance, high thermal stability and high temperature resistance of 1300 ℃ ~ 1500 ℃. In addition, it has good thermal conductivity and small friction coefficient. At present, CBN single crystal is generally prepared by high temperature and high pressure method.

The cubic boron nitride tool can cut hardened steel, chilled cast iron, high-temperature alloy, etc. at the cutting speed of processing ordinary steel and cast iron, thus greatly improving the productivity. The machining accuracy and surface quality are enough to replace grinding when the parts are hardened by finish turning. According to the relevant articles in the United States, the surface finish of the workpiece can always be maintained at 16um by turning instead of grinding. Under normal conditions, the surface finish can reach 6-8um At present, the new technology of replacing grinding with vehicle is being used in many industrial departments. For example, automobile manufacturers use this method to semi finish and finish the transmission shaft, various shaft transmission chains, engines, brake discs and brake rotors; Aircraft manufacturers use this method to manufacture aileron gears and landing gear. From oil fields to steel plants, we can see the application of the method of replacing grinding with vehicles. Machine tools, tools, heavy trucks, agricultural machines and tools, medical equipment, can molds, and auto parts all take the replacement of grinding by vehicles as an integral part of their production process.

（3） Polycrystalline cubic boron nitride (PCBN)

At present, polycrystalline cubic boron nitride (PCBN) tools in the market can be divided into three types according to composition and manufacturing method: monolithic polycrystalline cubic boron nitride tools, polycrystalline cubic boron nitride composite sheets and electroplated cubic boron nitride tools. It has high hardness, good wear resistance, strong heat resistance, good thermal conductivity, large chemical inertia and low friction coefficient. It is commonly used as tool material in turning and surface milling of hardened steel.

At present, PCBN is usually prepared by high temperature and high pressure method.

At present, 50% of PCBN tools are used in the automobile manufacturing industry, including the processing of automobile engine housings, brake discs, transmission shafts, cylinder holes, engine inlet and outlet valve seats, etc. in addition, about 20% are used for the processing of heavy equipment (such as rollers). In recent years, with the rapid development of computer processing technology and the widespread use of numerical control machine tools, the application of PCBN tools that can achieve high efficiency, high stability and long-life machining has become increasingly popular. At the same time, many advanced cutting concepts have been introduced, such as high-speed cutting, hard machining, turning instead of grinding, dry cutting, etc. PCBN tool material has become an indispensable tool material in modern cutting.