The high hardness and excellent physical and mechanical properties of diamond make diamond tools an indispensable and effective tool for machining all kinds of hard materials. The adhesion of matrix metal matrix to diamond (embedding ability of matrix) is one of the main factors affecting the service life and performance of diamond tools.
Because of the high interfacial energy between diamond and general metals and alloys, diamond particles can not be infiltrated by general low melting point alloys, and the adhesion is very poor. In the traditional manufacturing technology, diamond particles are embedded in the metal matrix of the matrix only by the mechanical clamping force generated by the cold shrinkage of the matrix, and no firm chemical bond or metallurgical bond is formed, which leads to the easy separation of diamond particles from the metal matrix of the matrix during work, which greatly reduces the service life and performance level of diamond tools. The utilization rate of diamond in most impregnated tools is low, and a large number of expensive diamonds fall off and are lost in waste chips during work. Lin Zengdong and others took the lead in using diamond surface metallization technology to endow the diamond surface with many new characteristics, such as excellent thermal conductivity and thermal stability, improve its original physical and chemical properties and improve its wettability to metal or alloy solutions.
Since 1970s, the problem of diamond surface metallization has attracted great attention from diamond tool manufacturers at home and abroad. Many people devote themselves to the study of diamond surface metallization during sintering, and add or pre-attach strong carbide metal powder to the matrix material (this kind of diamond does not react with the coating before heating, but belongs to the diamond coating) in order to realize chemical bonding with diamond during sintering. Although the literature has demonstrated that some metals, such as tungsten (unoxidized), can form WC layer on diamond surface at low temperature (about 800℃), from the perspective of the technology used to realize pre-metallization of diamond surface, it is necessary to heat 1 hour above 600℃ in vacuum to obtain ideal bonding force. According to the sintering conditions of commonly used impregnated diamond tools at present, it is unlikely that a metallized layer will be formed on the diamond surface when heated at 900℃ for about 5 minutes under non-vacuum or low vacuum conditions. Because it is rich in active metal atoms (Ti, V, Cr, etc.). ) and metallurgical bonding between adhesive and diamond through interfacial reaction is an atomic diffusion process, which is very insufficient according to the temperature and such a short time used for hot pressing. Under the condition of solid state sintering (sometimes there is a small amount of metal or alloy liquid with low strength and melting point), the chemical bonding or metallurgical bonding force of matrix to diamond is weak or not formed at all.
Pre-metallization of diamond surface is not the ultimate goal, but one of the measures to realize chemical metallurgy combination with matrix metal. After the coated diamond is sintered into saw (drill) teeth, all the diamonds exposed on the section lose their coating, and the surface of the residual pit where the diamond falls off is very smooth. This phenomenon seems to indicate that diamond and matrix have not reached the level of chemical coating. Therefore, even if the surface pre-metallization of diamond is realized, it is impossible for the traditional solid powder metallurgy sintering method to realize the firm combination of diamond and matrix material.
At the end of 1980s, people began to explore the brazing technology of diamond tools. Some transition group elements (such as Ti, Cr, W, etc. ) is plated on the surface of diamond and reacts with it to form carbide on the surface. Through the action of this layer of carbide, diamond, binder and matrix can be combined by solid-state chemical metallurgy through brazing, thus realizing the real diamond surface metallization, which is the principle of diamond brazing. It can be seen from published patents and articles that this technology can make the maximum edge value of diamond reach 2/3 of the particle size and increase the tool life by more than 3 times, while the conventional value is less than 1/3, so the allowable edge value can be obtained when the cutting operation reaches the stable edge value. Therefore, brazing technology is expected to achieve a firm combination between the matrix metal (solder) and the matrix material-diamond and steel matrix.
2. Research status of brazed diamond tools
At present, brazing diamond (or cubic boron nitride) tools has become a hot technology, but it is limited to single-layer tools, and there is no achievement in realizing multi-layer "impregnation". The research on brazing technology abroad began in the late 1980s, but its application was limited to single-layer tools because of its complicated work. The research on high temperature brazing technology in China started late, and compared with developed countries, the breadth and depth of the research are far from enough, so the progress is very slow at present. However, with China's entry into WTO, the pace of research will be gradually accelerated.
Research Status of (1) High Temperature Brazing Diamond Tools Abroad
AKChattopadhyay et al. of Switzerland used flame spraying method (oxygen-acetylene torch) to plate solder alloy (72%Ni, 14.4%Cr, 3.5%Fe, 3.5%Si, 3.35%B, 0.5%O2) on the tool steel substrate, and arranged diamond (uncoated) cloth on the solder surface. Cr in the solder alloy, as a strong carbide element, is enriched to the diamond surface during brazing, realizing the metallization of the diamond surface.
The method introduced by Wiand et al. in American patent is: adding Ni-Cr metal powder and organic binder to make solder coating, adhering the coated diamond to the tool steel substrate, then coating the solder coating, and then heating to a moderate temperature for a certain time to eliminate volatile substances. Heating in a vacuum furnace (vacuum degree 1 .333x10-2pa) or a dry hydrogen furnace to about 1 100℃, keeping the temperature1hour, and completing diamond surface metallization at the same time of brazing.
In some patents, Ni-Cr alloy solder is also used for brazing, and the solder also includes Fe, B or Si, Mo, etc. For example, in document [14], Ni-Cr alloy solder containing Si or Si and Ti is used to realize brazing in vacuum furnace, and the brazing temperature is1126 ~1176℃; Document [15] uses copper-based brazing filler metal containing W, Fe, Cr, B and Si to braze diamond grinding wheels; Document [16] uses Ag-Mn-Zr silver-based solder to braze diamond tools instead of electroplating tools.
ATrenker et al. in Germany used nickel-based active solder and nickel-based solder to realize the combination of diamond and matrix respectively during brazing. Compared with electroplating tools, it can be seen that the performance of high temperature brazing diamond tools is much better than electroplating diamond tools. The initial grinding performance of brazing tools (using active solder and PDA989 and PDA665 diamonds) is more than 3.5 times that of electroplating tools (nickel-based solder and PDA665 diamonds), and its service life is more than 3 times that of electroplating tools. Because of the large chip holding space of brazing tool, the free cutting surface of diamond abrasive particles and the large space between abrasive particles, the chips are easy to be removed, so the grinding performance of brazing diamond tool is good.
(2) The research status of high temperature brazing diamond tools in China.
On the basis of the research on brazing diamond at home and abroad, the Fourth Military Medical University and Xi Jiaotong University adopted the vacuum furnace (vacuum degree is 0.2Pa) high-temperature brazing method, using NiCr 13P9 alloy as brazing filler metal and adding a small amount of Cr powder, and brazing at high temperature (950℃) and pressure (4.9MPa), thus realizing the firm combination of diamond and steel matrix. The brazing filler metal is evenly distributed on the surface of the grinding wheel, and the diamond has been brazed firmly, which makes the surface of the grinding wheel feel sharp and rough. Solder is evenly distributed among diamond abrasive particles, and the height of diamond cutting edge is high. Its durability is obviously improved compared with electroplating grinding wheel, and only a small amount of diamond falls off after work.
Xiao Bing of Nanjing University of Aeronautics and Astronautics adopted the method of high frequency induction brazing. Using Ag-Cu alloy and Cr powder as interlayer materials, induction brazing was carried out in air at 780℃ for 35s, which realized the firm combination of diamond and steel matrix. Yao Zhengjun et al. used the method of induction brazing in Ar gas shielded furnace, using Ni-Cr alloy powder as brazing filler metal, vacuum induction brazing for 30 seconds, and brazing temperature was 1050℃, which realized the firm connection between diamond and steel matrix. By using scanning electron microscope, X-ray energy dispersive spectrometer and X-ray diffraction structure analysis, it is found that a chromium-rich layer is formed at the diamond interface during brazing, and the chromium-rich layer reacts with C element on the diamond surface to form Cr3C2 and Cr7C3, which are the main factors to achieve high bonding strength between the alloy layer and diamond. Grinding experiments with large cutting depth, slow feed and heavy load were carried out. From the surface morphology of the grinding wheel after grinding, the diamond does not fall off as a whole, and the diamond abrasive particles wear normally, which shows that the diamond has high grip strength and is suitable for high-efficiency grinding.
Taiwan Province China Grinding Wheel Company (KNIK. Inc) introduced a single-layer uniform diamond high-temperature brazing bead, which reduced the diamond consumption by 50% and increased the cutting speed by 2 times without reducing its service life.
On the basis of research at home and abroad, the author's research group used Ni82CrBSi alloy flake filler metal to evenly distribute diamonds on the filler metal, so as to realize brazing in a low vacuum hot pressing sintering furnace, and made a preliminary study on brazing diamond tools, exploring how to apply brazing technology to impregnated tools. Starting with optimizing the static structural parameters such as diamond arrangement in matrix, diamond particle size and concentration, and dynamic parameters such as effective diamond quantity and diamond spacing, the ordered arrangement of single-layer diamonds in the horizontal plane is realized, and then the diamonds in the working layer are staggered in the longitudinal direction by stacking method, so as to realize the continuous working ability of diamonds. In order to test the diamond inlay ability of matrix, a special diamond inlay bit was made, and five cutting experiments were carried out, and its maximum average cutting value was measured. By testing the height of diamond cutting edge of brazed single-layer cutter (diamond is 45/50 mesh), it is found that the maximum cutting edge value can reach more than 70%. It can be seen that brazing technology can greatly improve the bonding strength between diamond and matrix. The simulation experiment of reinforced concrete drilling with diamond bit (φ63mm) shows that the bit can still work when the teeth are worn nearly 2 mm Theoretically, two layers of diamonds are involved in the work, which seems to indicate that "impregnation" can be realized, and the specific application technology is still under further study.
3. The problems of brazing technology in the application of diamond tools.
There are many difficulties to be solved in diamond brazing: ① it is required that the brazing filler metal has good wettability and bonding strength to diamond and matrix; (2) The choice of brazing filler metal and brazing process should ensure the stability of diamond, so as to reduce or avoid the erosion of diamond by brazing filler metal; (3) Because the thermal expansion coefficient between diamond and metal matrix is very different, the welding residual stress is also very large, which reduces the strength of the joint; (4) The melting point of the solder is higher than the working temperature of the diamond tool, so we should find a metal (alloy) material with a lower melting point and close to the expansion coefficient of diamond as the solder, and then consider adding some active elements to improve the wettability and affinity to diamond, so as to achieve the purpose of bonding diamond and meeting the mechanical properties of the matrix. In addition, the key technologies such as the realization mode of diamond surface metallization, the matching and selection of surface metal and solder, and the selection of solder and gas medium need to be further matured and optimized.
The service efficiency and service life of diamond tools depend not only on the firmness of diamond abrasive particles embedding, but also on the wear resistance of the matrix. The strength of the matrix itself, the distribution of diamond in the matrix and the concentration of diamond will all affect the wear resistance of the matrix, so how to make the matrix reach the ideal state is also a problem worthy of attention in the future work.
4. Conclusion
Brazing technology can realize the chemical metallurgical bonding of diamond, bonding agent (brazing alloy material) and metal matrix interface with high bonding strength. Because of the high bonding strength on the interface, only a very thin thickness of the bonding agent is enough to firmly hold the abrasive particles, and its exposure height can reach 70% ~ 80%, which makes the abrasive more fully utilized and greatly improves the service life and processing efficiency of the tool. Compared with the traditional process, the allowable maximum cutting edge value of diamond tools can be increased by more than 50%, and the diamond consumption per unit volume of workpiece materials can be reduced by more than half without increasing or reducing the power consumption of tools. Compared with electroplating tools, it also shows unparalleled advantages. In a word, brazing technology has a good development prospect in the diamond tool manufacturing process, and should be industrialized as soon as possible.