1 Introduction
Superabrasive grinding wheels made of diamond or cubic boron nitride (CBN) abrasives have been widely used in all aspects of grinding because of their excellent grinding properties. Diamond grinding wheels are special effects tools for grinding hard and hard materials such as cemented carbide, glass, ceramics and precious stones.
In recent years, with the rapid development of high-speed grinding and ultra-precision grinding technology, higher requirements have been placed on the grinding wheel. The grinding wheel of ceramic and resin bonding has been unable to meet the production needs. The metal bond grinding wheel has its bonding strength. High performance, good formability, long service life and other significant characteristics have been widely used in production. The metal bond diamond grinding wheel mainly has two types of sintering and electroplating depending on the manufacturing method. In order to give full play to the role of super-hard abrasives, foreign countries began to develop a new type of grinding wheel by high-temperature brazing process from the early 1990s, namely single-layer high-temperature brazing super-hard abrasive grinding wheel. At present, this kind of grinding wheel is still in the research and development stage.
2. Sintered diamond
Grinding wheel sintered metal bond grinding wheel is mostly made of bronze and other metals as a bonding agent, and is manufactured by high-temperature sintering method. It has high bonding strength, good formability, high temperature resistance, good thermal conductivity and wear resistance, long service life and can withstand large The load. Since the grinding wheel inevitably has shrinkage and deformation during the sintering process, the grinding wheel must be shaped before use, but the dressing of the grinding wheel is difficult. At present, the grinding wheel rolling method commonly used in production not only takes time and labor in dressing, but also the diamond particles fall off during the dressing process, the dressing wheel itself consumes a lot, and the shaping precision is low. In recent years, scholars from various countries have carried out research work on the application of special processing methods to trim metal bond diamond grinding wheels, mainly including electrolytic dressing method, electric spark trimming method and composite trimming method. The electrolytic dressing method is fast, but the shaping precision is not high; the electric spark dressing method has high precision, can be shaped and sharpened, but the shaping speed is slow; the composite dressing method has electrolytic sparking composite trimming method and mechanical chemical compound trimming method. Etc., the dressing effect is better, but the system is more complicated, so the dressing problem of the sintered diamond wheel is still not well solved. In addition, because the manufacturing process of the grinding wheel determines that the surface topography is random, the geometry and distribution of each abrasive grain and the height of the cutting edge are inconsistent, so only a few high cutting edges are cut to the workpiece during grinding, limiting Further improvement in grinding quality and grinding efficiency.
3. Electroplated diamond
Advantages of grinding wheel electroplated diamond wheel: 1 electroplating process is simple, investment is small, easy to manufacture; 2 no need to trim, easy to use; 3 single layer structure determines that it can reach a very high working speed, currently abroad has reached 250 ~ 300m / s; 4 Although there is only a single layer of diamond, there is still enough life; 5 For the rollers and grinding wheels with high precision requirements, electroplating is the only manufacturing method. Because of these advantages, electroplated grinding wheels occupy an undisputed dominant position in high-speed, ultra-high-speed grinding. Defects in electroplated diamond grinding wheel: There is no strong chemical-metallurgical bond on the bonding surface between the plating metal and the substrate and the abrasive. The abrasive is actually embedded in the coating metal by mechanical embedding, so the holding force is small, and the diamond particles are under load. In heavy-duty high-efficiency grinding, it is easy to fall off (or the coating is peeled off), resulting in overall failure; in order to increase the holding force, the thickness of the coating must be increased. As a result, the bareness of the abrasive grains and the space of the chip are reduced, and the grinding wheel is prone to blockage. The heat dissipation effect is poor, and the surface of the workpiece is prone to burns. At present, the domestic electroplated grinding wheel manufacturing has not yet realized the optimal topography of the grinding wheel according to the processing conditions. The inherent disadvantages of the single-layer electroplated diamond grinding wheel will inevitably limit its application in efficient grinding.
4. Single layer brazed diamond
In order to give full play to the role of diamond, the grinding wheel should try to increase the holding force of the bonding agent on the diamond and improve the bonding strength of the grinding wheel. The single-layer high-temperature brazing super-abrasive grinding wheel can overcome the shortcomings of the electroplated grinding wheel, and can realize the chemical metallurgical combination between the diamond, the bonding agent and the metal matrix, and has a high bonding strength, and only needs to maintain the thickness of the bonding layer in the grinding. 20% to 30% of the height of the grain can firmly hold the abrasive grains in high-load high-speed and high-efficiency grinding, so that the abrasive height of the brazing wheel can reach 70% to 80%, thus increasing the chip space. The grinding wheel is not easy to block, and the use of abrasive is more sufficient.
Under the same processing conditions as electroplated grinding wheels, the single-layer high-temperature brazing super-abrasive grinding wheel has lower grinding force, power loss and lower grinding temperature, which means higher working speed can be achieved, which is 300-500m/s. The above ultra-high speed grinding has special significance. Single-layer high-temperature brazing, non-coated diamond grinding wheel and Cr-silver-based solder single-layer brazing wheel using high-frequency induction brazing method, using Ag-Cu alloy with Cr added as brazing filler metal, brazing in air at 780 ° C for 35 s , natural cooling, can achieve a firm connection between diamond and steel substrate. X-ray energy spectroscopy and X-ray diffraction analysis revealed that Cr3C2 was formed between Cr and diamond, and (FexCry)C was formed between the steel and the steel matrix. The comparison with the non-Cr solder showed that the alloy layer and diamond were realized. Both the steel and the matrix have the main factors of high bonding strength, and it is confirmed by grinding experiments that the diamond has a high holding strength. The advantage of this process is that the brazing temperature is low and the damage to diamond is small. The disadvantage is that the silver-based solder has a low melting point, and the wear-resistant high-temperature performance is poor, and the application in high-efficiency heavy-duty grinding is limited. Ni-Cr alloy single-layer brazing wheel The foreign diamond brazing process is: firstly, a layer of Ni-Cr alloy layer is flame sprayed on the steel substrate with an oxyacetylene torch. This layer of active metal can be used as a brazing material to directly braze the diamond. The abrasive particles were then brazed for 30 s in argon at 1080 °C. In the process of flame spraying the alloy layer, since the surface of the steel substrate is easily oxidized, the consistency of the thickness of the bonding agent layer after brazing and the uniformity of the abrasive arrangement are difficult to effectively control. Wu Zhibin et al. placed the diamond abrasive grains directly on the Ni-Cr alloy sheet or powder, pressed the diamond abrasive grains with ceramic blocks, and then brazed on a vacuum high-frequency induction machine for 30 seconds, the brazing temperature was 1080 ° C; or in argon Brazing is performed in a gas-protected radiant heating furnace, and the brazing temperature, the holding time, and the cooling rate are appropriately controlled. Scanning electron microscopy X-ray energy spectroscopy and X-ray diffraction analysis showed that during the brazing process, the Cr element in the Ni-Cr alloy separated to form a Cr-rich layer on the diamond surface and reacted with the element on the diamond surface to form Cr3C2. Cr7C3, the alloy layer reacts with the steel matrix to form carbides of (FexCry)C while being well infiltrated with diamond. Therefore, this brazing process can ensure high bonding strength between the alloy layer and the diamond and steel matrix. The heavy-duty grinding experiment proved that the diamond is normal wear and there is no whole diamond shedding. The advantage of this process is that the strength of the Ni-Cr alloy itself is high, and the bonding strength after brazing is higher than that of the silver-based alloy brazing; the Ni-Cr alloy has a high melting point and good wear resistance and high temperature performance. However, it still has certain limitations. Because of the high brazing temperature (1080 ° C), it is easy to cause thermal damage of diamond and reduce the strength of diamond. Brazing with vacuum or argon protection can minimize the thermal damage and oxidation of diamond. . Ma Chufan et al. selected NiCr13P9 alloy as the active brazing filler metal, and added a small amount of Cr powder in the vacuum furnace to heat it to 950 °C for brazing. A single-layer high-temperature brazed diamond grinding wheel for dental use was developed. Scanning electron microscopy showed that there was a silver-white alloy around the diamond. X-ray diffraction analysis confirmed the formation of Cr3C2. It is this carbide layer that achieves a high bonding strength between the diamond and the steel matrix. The grinding experiment also confirmed that the diamond has a high holding strength, and the life and grinding efficiency of the single-layer high-temperature brazed diamond grinding wheel are significantly improved compared with the electroplated grinding wheel. Single-layer high-temperature brazing diamond grinding wheel due to poor thermal stability of diamond, graphitization will occur at 800 °C, so higher brazing temperature will inevitably cause thermal damage of diamond and reduce the strength of diamond; Harmful elements can cause corrosion and graphitization of the diamond, so a layer of active metal and its alloy can be applied to the diamond surface before brazing.
The plating technology of superabrasives mainly includes chemical vapor deposition, ion plating, thermal evaporation, vacuum micro-evaporation plating and the like. Chemical vapor deposition of Cr, vacuum micro-evaporation of Ti, etc. can effectively improve the surface properties of diamond. In the brazing process, by virtue of the intermediate effect of the coating, in addition to the easier metallurgical chemical bonding between the diamond and the binder, the reaction speed of the carbon atoms on the diamond surface is greatly increased due to the barrier effect of the coating on the oxygen in the hot air. Decreasing, at the same time, the strong carbide forming elements in the coating react with the carbon atoms on the diamond surface to form carbides, which close the dangling bonds on the diamond surface and increase the resistance of the oxidation reaction, thereby suppressing Fe, Co, and Ni in the binder. The corrosion of the diamond by the elements and the graphitization process of the diamond itself enable the brazed abrasive to maintain its original strength and crystal form.
Source: Mechanical Expert Network
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