Diamond abrasive particles have excellent properties such as high hardness, strong wear resistance and excellent thermal conductivity, and are widely used in the processing of hard, brittle and difficult-to-process materials such as cemented carbide, ceramics, glass, and gemstones. However, there is a high interface energy between the diamond with covalent bond structure and the general metal, which is difficult to weld, and the general mechanical mosaic and physical adsorption do not have enough holding force . Since the early 1990 s, foreign countries and Taiwan region of China have successively studied the use of high-temperature brazing technology to develop a new generation of diamond tools. Active metal elements (such as Ti, Cr, Mo, W, etc.) are used to form a chemical metallurgical bond at the interface between metal brazing filler metal and superhard abrasive grains, which greatly improves the holding strength of bonding agent on diamond abrasive grains and significantly improves the service life of grinding wheels [2-3].
However, during the test, it is found that brazed diamond tools still have brazing quality problems such as diamond easy graphitization, brittle intermetallic compounds at diamond brazing interface affecting brazing strength, and residual stress concentration in brazed joints. At the same time, due to the development of high-efficiency precision manufacturing technology, the brazing technology of diamond abrasive grains has also proposed new requirements such as the ability of brazing materials to intelligently control diamond shedding according to the wear state of abrasive grains, matching the wear rate between brazing materials and diamonds, improving the heat dissipation performance of brazed joints and increasing the chip tolerance space .
Therefore, how to scientifically transform the new requirements into the diamond brazing quality evaluation standard, and how to improve the diamond brazing technology to meet these new requirements is a work of great significance. This paper summarizes the current evaluation methods of diamond brazing quality, discusses the research progress of the factors and mechanism of diamond brazing quality at home and abroad, and looks forward to the development of diamond brazing technology in the future.
1Diamond Brazing Quality Evaluation MethodLaw
Since the diamond brazing technology was put forward in the early 1990 s, scholars engaged in the research of diamond brazing technology mainly judge the quality of diamond brazing from the aspects of micro-morphology characteristics of diamond brazing, friction and wear performance of brazed diamond tools, diamond brazing strength, residual stress of diamond brazed joints, etc.
Microstructure Characteristics of 1.1 Brazed Diamond
The micro-morphology of diamond brazing mainly includes: diamond surface morphology and solder morphology, as well as the interface compound morphology between diamond, solder and matrix. Among them, the diamond surface morphology can reflect the degree of thermal damage or graphitization, the brazing material morphology can reflect the degree of wetting and spreading of the brazing material to the diamond, and the interface compound morphology between the diamond, the brazing material and the substrate can reflect the chemical metallurgical reaction state between the three.
Deng Zhaohui et al.  observed the microscopic morphology of diamond brazed joints by scanning electron microscopy. As shown in fig. 1, part of the brazing filler metal powder in fig. 1(a) is not completely melted, and the wetting and spreading of the brazing filler metal to diamond is not sufficient; Fig. 1 (B) diamond is still complete in crystal form, sharp edges and corners, and there is no obvious thermal etching trace, and the brazing filler metal and diamond are combined densely, and the brazing filler metal powder is fully spread; while fig. 1(c) diamond has a certain degree of thermal etching phenomenon. Khalid et al.  observed by transmission electron microscopy (TEM) that the interfacial compound TiC formed between diamond and brazing filler metal is divided into two layers, and the TiC in the second layer is brittle elongated columnar morphology (Figure 2).
Fig.1 Surface morphology of diamond brazed in vacuum at different temperatures for 10min
Fig.2 TEM diamond brazing interface compound TiC layered morphology
Friction and wear properties of 1.2 brazed diamond abrasive particles
During the abrasive grinding process, the abrasive particles are subjected to thermal loads from the impact action and contact of the workpiece. If the strength of the brazed joint is insufficient to hold the abrasive grains in the high-load processing state, the abrasive grains will be abnormally detached. According to this principle, the friction and wear test of diamond abrasive particles is carried out, and the results can be used to judge the brazing performance. The surface wear of new brazed diamond fiber grinding wheel and ordinary resin bond diamond grinding wheel was observed by Dino-Lite digital microscope . As shown in fig. 3, when there are more falling pits on the working surface of the ordinary grinding wheel, the brazed diamond fiber grinding wheel seldom has the whole diamond falling off. Huang Hui et al  also observed the abrasive wear state of brazed diamond grinding wheel and ordinary sintered grinding wheel through Hirox 3D video system.
Fig.3 Working surface of grinding wheel
1.3 diamond brazing strength
Brazing strength is an important direct evaluation standard of high-performance brazed joints, according to the direction of force is divided into tensile strength and shear strength, although the tensile strength can directly reflect the strength of the bond on the material interface, but the shear strength is more in line with the grinding process of the force state. Swiss SebastianBuhl et al. applied a load parallel to the height of 50μm on the contact surface, and analyzed the shear strength at different brazing temperatures and different holding times. The experimental diagram and results are shown in Figure 4 .
Fig. 4 Strength Test
Residual stress of 1.4 diamond brazed joint
Residual stress is an important factor affecting the performance of diamond brazed joints, and excessive residual stress will lead to cracks in brazed joints, resulting in abnormal shedding or wear of abrasive particles during processing. At present, the research on residual stress of diamond brazed joints mainly adopts experimental measurement and finite element simulation analysis. Khalid et al.  found strain contrast at the contact interface between TiC layer and diamond by transmission electron microscopy. As shown at point B in FIG. 2 (B), this phenomenon occurs because the thermal deformation of diamond and TiC is restricted due to the mismatch of thermal expansion coefficients during the brazing process.
Buhl, Ding Wenfeng  and others used Raman spectroscopy to test the residual stress of brazed diamond joints. With the rapid development of computer technology and numerical analysis technology, many researchers have used numerical simulation to analyze the residual stress of diamond brazed joints. For example, Sun Fenglian et al.  analyzed the influence of brazing process parameters on the distribution and size of residual stress in diamond brazed joints through finite element simulation tests, and predicted the dangerous area of post-weld cracks.
2Main influencing factors of diamond brazing performance
In the process of preparing brazed diamond tools, the brazing powder, the type of coated diamond, and the brazing process parameters (including brazing temperature, holding time, brazing atmosphere, etc.) have a great influence on the brazing performance of diamond.
2.1 solder composition
In the process of diamond brazing, the diamond is connected by the melting of the brazing filler metal, and the performance of the brazing filler metal itself greatly affects the performance of the diamond brazing joint. In the process of designing the composition and ratio of the solder, the following basic requirements should be considered:
(1) the solder has a suitable melting point. Melting point can not be higher than the diamond graphitization temperature, can not be too low, if the melting point is too low, in the grinding process, may be due to the higher grinding temperature lead to softening of the solder, resulting in premature shedding of abrasive particles. At present, Ni-Cr brazing filler metal is the most widely used, but its brazing temperature is high (above 900 ℃), diamond has the tendency of graphitization, which affects the strength and tool life of brazed diamond . Ag-Cu to the low melting point of the solder, the use temperature of the joint after welding cannot exceed 500 ℃, and the solder contains precious metals, the cost is high, and the use is also subject to certain restrictions. Compared with Ag-based solder, copper-based solder has the characteristics of low sintering temperature, low cost, good formability and sinterability, and good compatibility with other elements, but Cu is almost non-wetting to diamond .
(2) the brazing filler metal has good infiltration and diffusion effect on diamond. Better infiltration and diffusion can ensure the formation of a strong chemical metallurgical bond between the brazing filler metal and the diamond abrasive particles, and improve the brazing strength. Zhang Fenglin et al. improved the wetting properties of the solder alloy to diamond by adding Cr and Ti metal powders to the solder alloy .
(3) The brazing filler metal should have a stable and uniform composition to reduce the segregation phenomenon and the loss of volatile elements during the brazing process. Sun Fenglian et al.  prevented the production of excessive highly brittle metal compounds by optimizing the amount of active metals added. Wang Yi et al.  established the regression equation of Cu-Ni-Sn-Ti active solder composition and brazing performance by using mixed regression design, and analyzed the influence of solder element mass fraction on solder performance.
(4) the solder has a certain strength and hardness. In the grinding process, if the strength and hardness of the solder is not enough, it will lead to rapid wear and lose the ability to hold the abrasive particles. By analyzing the interface between diamond, brazing filler metal and matrix, Khalid et al.  found that the addition of Ti element also increases the strength and wear resistance of the brazing filler metal itself, but too much Ti content will lead to an increase in the melting point of the alloy, an increase in the number of metal compounds and an increase in the brittleness of the brazed joint.
(5) Avoid large residual stress at the cross section of diamond and solder due to the mismatch of physical properties.
2.2 diamond coating
Diamond coating refers to plating a layer of affinity metal on the surface of diamond, and make a strong chemical bond between the coating and diamond, reduce the surface energy of diamond, make it easy to be infiltrated by metal bonding agent, improve the weldability of diamond surface, and realize the strong metallurgical combination between diamond and metal. Ma Bojiang et al.  conducted brazing experiments by using diamond particles coated with a layer of amorphous carbon film on the surface, and found that the surface of the diamond immersed under the brazing material layer generated a chromium carbon compound with a more uniform distribution of nucleation points, and the brazing material had a good brazing effect on the diamond. Deng Zhaohui et al.  used Cu-10Sn-5Ti brazing filler metal powder to vacuum braze Ti-plated diamond on steel substrate, and found that diamond greatly reduced thermal damage and graphitization due to the protection and isolation of Ti-plated layer, and the crystal form of diamond was complete.
2.3 brazing process parameters
Although according to the diamond brazing process heating is different, the brazing process can be divided into high temperature furnace brazing, high frequency induction brazing, laser sintering brazing, etc., but regardless of the kind of brazing process, the impact on the performance of diamond brazing is mainly achieved by brazing temperature, holding time, brazing atmosphere and other brazing process parameters.
2.3.1 Brazing temperature and holding time
The process of the melting and filling of the brazing filler metal and the chemical metallurgical reaction between the melting brazing filler metal and the diamond surface is inseparable from the brazing temperature and the holding time. Xiang Sun Zu et al.  believe that the brazing temperature has an important influence on the quality of the brazed joint and the uniformity of the abrasive particle distribution, as well as the climbing height of the brazing material on the abrasive particle surface. Xu Zhengya et al.  studied the grinding performance of brazed diamond specimens with different holding times, and concluded that if there is no holding stage, the amount of interfacial reaction products is very small, only a small amount of compounds are generated in the local part of the diamond abrasive grains, and the abrasive grains are easy to fall off.
2.3.2 Brazing atmosphere
Because diamond is easy to oxidize and graphitize at high temperature, the brazing process of diamond is generally carried out in vacuum or protective atmosphere, but different brazing atmosphere has different effects on diamond brazing morphology, intermediate carbide formation, diamond graphitization and abrasive grain damage. Chen Yan et al.  conducted an experimental study on brazing diamond abrasive grains in a furnace in two different atmospheres of vacuum and Ar gas, and concluded that different brazing atmospheres lead to different diamond wear characteristics. Wu Qiao and equal people  used Cu-10Sn-5Ti solder powder to braze diamond in air, Ar gas protection and vacuum atmosphere respectively, and found that when Ar gas protection and vacuum brazing, the solder was fully wetted and spread, and the degree of diamond graphitization was very small.
From the current research status of grinding wheels, the quality of diamond brazing is mainly analyzed by the micro-morphology of diamond brazing, the friction and wear properties of brazed diamond tools, the strength of diamond brazing, and the residual stress of diamond brazed joints. However, from the point of view of high-efficiency precision machining, the diamond brazing strength is not the greater the better, excessive brazing strength will lead to dull diamond abrasive particles can not fall off, and dull abrasive particles will bring grinding burns, grinding cracks and other processing quality problems. In addition, for multi-layer diamond brazing grinding wheel, if there is a certain matching relationship between the brazing material wear rate and the diamond wear rate, it will be possible to realize the timely compensation of the next layer of diamond after the previous layer of diamond wear.
In addition, the grinding technology has higher requirements for the heat dissipation performance and chip tolerance performance of the processing tool. These requirements are rarely given in the current diamond brazing quality evaluation methods. Therefore, with the development of high-efficiency precision manufacturing technology, the requirements for diamond brazing technology are also increasing. As for how to carry out the next step of research, the author believes that the following points need to be considered:
(1) Further study and analyze the influence factors and mechanism of diamond brazing strength, and realize the control method of diamond abrasive brazing strength by optimizing the combination of related brazing process technology.
(2) Through the selection of diamond brazing filler metal and the optimization of the formula, the wear performance between the matrix material and the abrasive particles is matched, so that the abrasive particles are exposed automatically and evenly during the grinding process without dressing.
(3) By improving the relevant process, on the basis of realizing that the grinding wheel substrate material has a suitable holding force on the abrasive particles, the pore-forming agent is introduced to improve the porosity of the substrate material, increase the chip space and heat dissipation capacity, and realize the thermal damage-free processing of the workpiece.