Brazing diamond

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The high hardness and excellent physical and mechanical properties of diamond make diamond tools an indispensable and effective tool for processing various hard materials. The bonding of matrix metal to diamond is one of the main factors affecting the service life and performance of diamond tools.

1. Overview of Brazed Diamond
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 have poor cohesion. 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, without forming a solid chemical bond or metallurgical bond, resulting in diamond particles. It is easy to separate from metal matrix of matrix in work, which greatly reduces the 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 the waste. Diamond surface metallization technology is used to endow the diamond surface with many new characteristics, such as excellent thermal conductivity, good thermal stability, improving its original physical and chemical properties, and improving its wettability to metal or alloy solution.
The problem of diamond surface metallization has attracted great attention from the diamond tool manufacturing industry at home and abroad in the 1970s. Many people devote themselves to the research of diamond surface metallization in sintering process, adding or pre-bonding strong carbide metal powder on the matrix material (this kind of diamond does not react with the coating before heating, but belongs to diamond coating), in order to realize the chemical bonding of diamond in sintering process. Although some metals such as tungsten (not oxidized) can form WC layer on the diamond surface at low temperature (about 800 C), it is necessary to heat it for more than one hour at 600 C in vacuum to obtain an ideal bonding force from the process used to pre-metallize the diamond surface. According to the sintering conditions of commonly used impregnated diamond cutting tools at present, it is unlikely that metallized layer will be formed on the surface of diamond when heated at 900 C for about 5 minutes in non-vacuum or low vacuum. Because both enrichment of active metal atoms (Ti, V, Cr, etc.) to diamond surface and metallurgical bonding of binder to diamond by interfacial reaction are atomic diffusion processes, which are extremely inadequate according to the temperature used for hot pressing and such a short time. The chemical bonding or metallurgical bonding force of matrix to diamond is very weak or will not form at all under solid state sintering (sometimes with a small amount of low strength and low melting point liquid metal or alloy).
Premetallization of diamond surface is not the ultimate goal, but only one of the measures to achieve chemical metallurgical bonding with matrix metals. When the coated diamond is sintered into sawtooth, the exposed diamond on the fracture surface loses its coating, and the surface of the residual pit of the diamond is very smooth. This phenomenon seems to indicate that the diamond and matrix have not reached 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 the solid bonding between diamond and matrix materials.
In the late 1980s, people began to explore brazing technology for diamond tools. Some transition group elements (such as Ti, Cr, W) are coated on the surface of diamond, and carbides are formed by chemical reaction with them. Through the effect of this layer of carbide, diamond, binder and matrix can be soldered to achieve a solid chemical metallurgical bonding, thus realizing the real diamond surface metallization, which is the principle of diamond brazing. From the published patents and articles, it can be seen that this technology can make the maximum cutting edge value of diamond reach 2/3 of the particle size and the tool life increase more than three times, while under the conventional condition, the value is less than 1/3, which allows the cutting edge value to be obtained when the cutting operation reaches the stable cutting edge value. Therefore, brazing technology is expected to achieve a solid bond between matrix metal (brazing metal) and matrix material – Diamond and steel matrix.

2. Present situation of brazed diamond
At present, brazing diamond (or cubic boron nitride) tools has begun to become a hot technology, but it is limited to single-layer tools, and no results have been published for multi-layer “impregnation”. Overseas brazing technology research started in the late 1980s, but its application is limited to single-layer tools because of the complexity of the work. Domestic high-temperature brazing technology research started relatively late, compared with developed countries, the breadth and depth of research is far from enough, so the current progress is very slow, but with China’s accession to the WTO, the pace of research is bound to gradually. Speed up.
(1) Research status of high temperature brazing diamond tools abroad
AK Chattopadhyay et al. in Switzerland used flame spraying method (oxygen-acetylene welding torch) to deposit brazing alloy (72% Ni, 14.4% Cr, 3.5% Fe, 3.5% Si, 3.35% B, 0.5% O 2) on tool steel substrate, and to arrange diamond (uncoated) on the solder layer, then induction brazing at 1080 C and under argon protection for 30 seconds to achieve the bonding of diamond with steel matrix. As a strong carbide element, Cr in solder alloys enriches on the surface of diamond and metallizes the surface of real cash diamond during brazing.
The method introduced by Wiand et al. in American patent is that solder (Ni-Cr) metal powder and organic binder are added to make brazing paint, coated diamond is bonded to tool steel matrix, then brazing paint is applied, then heated to a moderate temperature and kept for a certain time to remove volatile substances. In vacuum furnace (vacuum 1.333 *10-2Pa) or dry hydrogen furnace, the diamond surface metallization is completed at the same time of brazing.
Some patents also use Ni-Cr alloy filler metal to realize brazing. The filler metal also includes Fe, B elements or Si, Mo, etc. For example, in reference [14], the Ni-Cr alloy filler metal containing Si or Si and Ti is used to braze diamond grinding wheel in vacuum furnace, the brazing temperature is 1126-1176 C; in reference [15], the copper-based filler metal containing W, Fe, Cr, B and Si is used to braze diamond grinding wheel; in reference [16], the Ag-Mn-Zr silver-based filler metal is used to braze diamond tools instead of electroplating tools.
In the brazing process, ATrenker et al. in Germany adopted nickel-based active solder and nickel-based solder respectively to realize the bonding of diamond and matrix. Compared with electroplating tools, the performance of high temperature brazed diamond tools is much better than that of electroplated diamond tools. The initial grinding performance of brazing tools (using active solders and PDA989, PDA665 diamonds) is more than 3.5 times of that of electroplating tools (nickel-based solders and PDA665 diamonds), and the service life of brazing tools is more than 3 times of that of electroplating tools. Diamond Abrasives have larger free cutting surface and more space between abrasives, which makes it easy to remove chips, so the grinding performance of brazed diamond tools is good.
(2) Research status of high temperature brazed diamond tools in China
Based on the research of brazing diamond at home and abroad, the Fourth Military Medical University and Xi’an Jiaotong University brazed diamond in vacuum furnace (vacuum 0.2Pa), using NiCr13P9 alloy as filler metal, with a small amount of Cr powder, under high temperature (950 C) and pressure (4.9MPa), so as to achieve a solid bonding between diamond and steel matrix. The filler metal is evenly distributed on the surface of the grinding wheel. The diamond has been soldered firmly. Touching the surface of the grinding wheel feels very sharp and rough. The brazing filler metal distributes evenly among diamond abrasives and the cutting edge height of diamond is high. Its durability is obviously improved compared with electroplated grinding wheel, and only a small amount of diamond falls off after working.
Xiao Bing and others of Nanjing University of Aeronautics and Astronautics used high frequency induction brazing method. Ag-Cu alloy and Cr powder were used as interlayer materials. Induction brazing was carried out in air for 35 seconds at 780 C. The solid bonding between diamond and steel matrix was realized. Yao Zhengjun et al. used induction brazing method in Ar gas protection furnace, using Ni-Cr alloy powder as filler metal, vacuum induction brazing for 30 seconds, brazing temperature 1050 C, to achieve a solid connection between diamond and steel matrix. By means of scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS), combined with X-ray diffraction (XRD), it was found that during brazing, Cr-rich layer formed at the interface of Cr-element diamond and reacted with C-element on the surface of diamond to form Cr3C2 and Cr7C3, which are the main factors for achieving high bonding strength between alloy layer and diamond. The grinding experiments were carried out with large cutting depth, slow feed and heavy load. From the surface morphology of the grinding wheel, no diamond peeled off, and the diamond abrasive was normal wear, which indicated that the diamond had higher holding strength and was suitable for high efficiency grinding.
Taiwan China Grinding Wheel Company (KNIK. Inc.) has introduced single layer uniformly distributed diamond brazed beads at high temperature. Without reducing its service life, the amount of diamond used is reduced by 50%, and the cutting speed is increased by two times.
On the basis of the research at home and abroad, the author’s research group used Ni82CrBSi alloy sheet brazing filler metal, diamond was evenly distributed on the filler metal, and brazed in low vacuum hot-pressing sintering furnace. The brazed diamond tool was preliminarily studied, and how to use brazing technology in the impregnating tool was explored. Starting from optimizing the 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 then the diamond in working layer is arranged in staggered arrangement in longitudinal direction by stacking method, so that the diamond can work continuously. In order to test the ability of matrix to encapsulate diamond, a surface-mounted diamond bit was specially made. Five cutting experiments were carried out and its maximum average cutting edge value was measured. Diamond cutting edge of single layer tool by brazing.

3. Problems in Brazing Diamond
There are many difficulties in diamond brazing which need to be solved urgently: 1) good wettability and bonding strength of filler metal to diamond and matrix are required; 2) the selection of brazing materials and brazing technology should ensure the stability of diamond to reduce or avoid the erosion of diamond by filler metal; 3) because the thermal expansion coefficients of diamond and metal matrix are quite different, the welding residual stress is also large. (4) The melting point of filler metal is higher than the working temperature of diamond tools, so metal (alloy) materials with lower melting point and similar expansion coefficient of diamond should be sought as filler metal, and some active elements should be added to improve the wettability and affinity of diamond, so as to achieve the purpose of bonding diamond and satisfying the mechanical properties of matrix. In addition, the key technologies of diamond surface metallization, 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 abrasive inlay, but also on the wear resistance of matrix. The strength of the matrix itself, the distribution of diamond in the matrix and the concentration of diamond will affect the wear resistance of the matrix. Therefore, how to make the matrix achieve an ideal state is also a problem worthy of attention in future work.

4. Summary of Brazing Diamond
Brazing technology can realize chemical metallurgical bonding of diamond, bond (brazing alloy material) and metal matrix interface, and has high bonding strength. Because of the high bonding strength on the interface, only a very thin bond thickness is needed to hold the abrasive firmly, and its exposed height can reach 70%-80%. This makes the use of abrasive more fully, and greatly improves the tool life and processing efficiency. Compared with traditional technology, the maximum allowable cutting edge value of diamond tools can be increased by more than 50%. Under the condition that the power consumption of tools does not increase or decrease, the diamond consumption per unit volume of workpiece materials can be reduced by more than half. Compared with plating tools, it also shows incomparable advantages. In a word, brazing technology has good prospects in the manufacturing process of diamond tools. It should be industrialized as soon as possible.