Brazing technology in diamond tools application
1. application of technology in diamond tools
The high hardness and excellent physical and mechanical properties of diamond make diamond tools an indispensable and effective tool for machining various hard materials. The bonding ability of matrix metal to diamond (the encapsulation 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 common metals and alloys, diamond particles can not be infiltrated by common low melting point alloys, and the bonding property is very poor. In traditional manufacturing technology, diamond particles are embedded in the matrix metal only by the mechanical clamping force produced by the cold shrinkage of the matrix, but do not form a solid one. Chemical bonding or metallurgical bonding results in the easy separation of diamond particles from matrix metal, which greatly reduces the life and performance of diamond tools. Most of the impregnated tools have low utilization rate of diamond, and a large number of expensive diamonds are lost in the scrap during work. Lin Zengdong took the lead in utilizing the Metallization Technology of diamond surface to endow diamond surface with many new characteristics, such as excellent thermal conductivity, good thermal stability, improving its original physical and chemical properties, improving its wettability to metal or alloy solution and so on.
The problem of surface metallization of diamond has attracted great attention from diamond tool manufacturers both at home and abroad in the 1970s. Many people devote themselves to the study of metallization of diamond surface in sintering process, adding or pre-bonding strong carbide metal powder to matrix material (this kind of diamond does not react with coating before heating, only belongs to diamond coating), so as to expect them to be solid in sintering process. The chemical bonding of diamond is now in progress. Although it has been demonstrated that some metals such as tungsten (not oxidized) can form a WC layer on the surface of diamond at lower temperatures (around 800 C), the ideal bonding strength can be obtained only by heating for one hour above 600 C in vacuum. According to the sintering conditions of pregnant diamond cutting tools, it is impossible to form a metallized layer on the surface of diamond when heating at 900 C for about 5 minutes in non-vacuum or low vacuum. Because both the enrichment of active metal atoms (Ti, V, Cr, etc.) on the surface of diamond and the metallurgical bonding between bond and diamond are atom diffusion processes, this process is extremely inadequate according to the temperature used in hot pressing and such a short time. Under the condition of solid phase sintering (sometimes with a small amount of low strength and low melting point metal or alloy liquid), the chemical bonding or metallurgical bonding of matrix to diamond is very weak or will not form at all.
Premetallization of diamond surface is not the ultimate goal, but only one of the measures to achieve chemical metallurgical bonding with matrix metals. After sintering the coated diamond into saw (drill) teeth, all the diamonds exposed on the fracture surface lost coating, and the surface of the residual pit of the diamond was very smooth. This phenomenon seems to indicate that the diamond and matrix can not reach the level of chemical encapsulation. Therefore, even if the surface of diamond is pre-metallized, the traditional solid phase powder metallurgy sintering method can not achieve a solid combination between diamond and matrix materials.
In the late 80s of last century, people began to explore brazing technology for diamond tools. Certain transition group elements (such as Ti, Cr, W, etc.) are coated on the surface of diamond and reacted with them to form carbides on the surface. Through the action of this layer of carbide, diamond, binder and matrix can be soldered to achieve a solid chemical metallurgical bonding, thus realizing the true surface metallization of diamond, which is the principle of diamond brazing. From the published patents and articles, it can be seen that the maximum value of diamond cutting edge can reach 2/3 of the diamond particle size, and the tool life can be increased more than three times. However, the value is less than 1/3 under the normal conditions, allowing the cutting edge value to be obtained when the cutting edge is stable. Therefore, brazing technology is expected to achieve a solid bonding between matrix metal (brazing metal) and parent material - Diamond and steel matrix.
At present, brazing diamond (or cubic boron nitride) tools have become a hot technology, but only limited to single-layer tools, for multi-layer realization of "impregnation" has not yet been published. Foreign brazing technology research began in the late 1980s, but because of the complexity of the work is still in the experimental stage, its application is limited to single-layer tools; domestic high-temperature brazing technology research started late, compared with developed countries, the breadth and depth of research is far from enough, so the progress is very slow, but With China's entry into WTO, the pace of research will gradually accelerate.
(1) research status of high temperature brazing diamond tools abroad
A K Chattopadhyay et al. of Switzerland plated the brazing alloy (72% Ni, 14.4% Cr, 3.5% Fe, 3.5% Si, 3.35% B, 0.5% O2) on the tool steel substrate by flame spraying (oxygen-acetylene gun) and laid the diamond (uncoated) on the solder layer, then brazed the diamond and steel for 30 seconds under the protection of 1080 C and argon gas. Matrix bonding. As a strong carbide element, Cr in brazing alloy enriches on the surface of diamond and metallizes the surface of cashed diamond during brazing.
The method introduced by Wiand et al. in the U. S. patent is: solder (Ni-Cr) metal powder and organic binder made of brazing paint, coated diamond bonded to tool steel substrate, then coated with brazing paint, and then heated to a moderate temperature and held for a certain time to remove volatile substances. In vacuum furnace (vacuum 1.333 x10-2Pa) or dry hydrogen furnace, heating to about 1100, holding for 1 hour, brazing at the same time complete the surface metallization of diamond.
Some patents also use Ni-Cr alloy solder to realize brazing, which includes Fe, B elements or Si, Mo and so on. For example, the Ni-Cr alloy filler metal containing Si or Si and Ti is used to braze in vacuum furnace, and the brazing temperature is between 1126 and 1176 degrees Celsius; the diamond grinding wheel is brazed with Cu-based filler metal containing W, Fe, Cr, B and Si; and the diamond tool is brazed with Ag-Mn-Zr silver-based filler metal to replace the electroplating tool.
Compared with electroplated tools, the performance of brazed diamond tools is much better than that of electroplated diamond tools. The initial grinding performance of brazed tools (using active filler metals and PDA989, PDA665 diamonds) is 3.5 times higher than that of electroplated tools (nickel-based filler metals and PDA665 diamonds), and the life of brazed tools is 3 times longer than that of electroplated tools. Above; because brazing tools have a large chip space, diamond abrasives have a larger free cutting surface and more space between the abrasives, so that the chip is easy to be removed, so brazing diamond tools grinding performance is good.
(2) research status of high temperature brazing diamond tools in China
NiCr13P9 alloy was used as filler metal and a small amount of Cr powder was used to braze in vacuum furnace (vacuum degree 0.2Pa) at high temperature (950 C) and pressure (4.9MPa), thus achieving a solid bonding between diamond and steel substrate. The brazing filler metal distributes evenly on the surface of the grinding wheel. The diamond has been soldered firmly. It feels sharp and rough when touching the surface of the grinding wheel. The distribution of filler metal between diamond grains is uniform, and diamond cutting edge height is high. Its durability is obviously improved compared with the electroplated grinding wheel, and only a small amount of diamond drops after work.
Using Ag-Cu alloy and Cr powder as the interlayer material, the bonding between diamond and steel substrate was realized by induction brazing in air for 35 seconds and brazing temperature for 780 C. Yao Zhengjun et al. used induction brazing method in Ar gas shielding furnace, using Ni-Cr alloy powder as solder, vacuum induction brazing for 30 seconds, brazing temperature 1050 C, realized the solid connection between diamond and steel matrix. By means of scanning electron microscopy, X-ray energy dispersive spectrometry and X-ray diffraction structure analysis, it is found that Cr-rich layer is formed at the interface of Cr element diamond and reacts with C element on the surface of diamond to form Cr3C2 and Cr7C3, which are the main factors to achieve high bonding strength between alloy layer and diamond. The grinding experiment is carried out with deep cutting, slow feed and heavy load. From the surface morphology of grinding wheel, there is no diamond peeling off, and diamond abrasive is normal wear. It shows that diamond has higher holding strength and is suitable for high efficiency grinding.
Ni82CrBSi alloy sheet brazing filler metal was used to braze diamond in low vacuum hot-pressing sintering furnace with diamond evenly distributed on the filler metal sheet. From the optimization of static structural parameters such as diamond distribution in matrix, diamond particle size and concentration, and dynamic parameters such as effective diamond quantity and diamond spacing, the orderly arrangement of single layer diamond in transverse plane is realized, and the diamond in working layer is continuously distributed in staggered direction by lamination method. Working ability. In order to test the ability of matrix to encapsulate diamond, a diamond bit for surface mounting was specially made. The cutting experiments were carried out five times and the maximum average cutting value was measured. By measuring the cutting edge height of brazed single-layer tool diamond (45/50 mesh diamond), 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 results of reinforced concrete drilling simulation experiment on diamond bit (_63mm) show that the bit can continue to work even when the drill tooth wears nearly 2 mm. In theory, two layers of diamond have been involved in the work, which seems to indicate that the "impregnation" can be achieved, and the specific application process is still under further study.
3. the problems of brazing technology in the application of diamond tools.
There are many difficulties to be solved urgently in diamond brazing: 1. Requiring good wettability and bonding strength of brazing filler metal to diamond and matrix; 2. Choosing brazing materials and brazing technology to ensure the stability of diamond to reduce or avoid the erosion of brazing filler metal to diamond; 3. Thermal expansion system of diamond and metal matrix. Because of the large difference in number, the welding residual stress is also large, reducing the strength of the joint; 4. The melting point of the solder is higher than the working temperature of the diamond tool, so the metal (alloy) with low melting point and close to the diamond expansion coefficient should be looked for as the solder, and some active elements should be added to improve the diamond performance. Wettability and affinity can achieve the purpose of bonding diamond and satisfying the mechanical properties of matrix. In addition, the key technologies such as the realization of surface metallization of diamond, the matching and selection of surface metals and solders, and the selection of solders and gas media need to be further matured and optimized.
The service efficiency and life of diamond tools depend not only on the firmness of diamond abrasives, but also on the wear resistance of matrix. The strength of the matrix itself, the distribution of diamond in the matrix, the concentration of diamond and so on will have an impact on the wear resistance of the matrix. Therefore, how to make the matrix achieve the ideal state is also a problem worthy of attention in future work.
Brazing technology can realize the chemical metallurgical bonding of diamond, binder (brazing alloy material) and metal matrix interface, and has higher bonding strength. Because of the high bonding strength on the interface, only a very thin bond thickness is needed to hold the abrasive grains firmly, and the exposed height can reach 70%-80%, which makes the use of abrasive more fully and greatly improves the tool life and processing efficiency. Compared with the traditional technology, the maximum allowable 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 under the condition that the power consumption of the tools is not increased or reduced. Compared with electroplating tools, it also shows unparalleled advantages. In a word, brazing technology has a good development prospect in the process of diamond tool manufacturing, and it should be industrialized as soon as possible.