Research progress on brazing superabrasive tools

1 Overview Superabrasive tools made of diamond and cubic boron nitride (CBN) are widely used in the grinding of various metal materials, processing of hard and brittle materials such as stone, ceramics, optical glass, as well as road maintenance and geological mining. In many engineering fields such as oil and gas drilling, the market demand is very large. The super-hard abrasive tools currently used in production are generally produced by multi-layer sintering or single-layer electroplating processes. The abrasive grains are only mechanically embedded and embedded in the bonding layer, and the holding force is not large, and the processing is heavy. It is easy to cause the abrasive to fall off prematurely due to insufficient holding power, resulting in waste. On the other hand, in the sintering and electroplating tools, the abrasives are randomly distributed, the exposed height of the abrasive grains is not large, the chip space is small, and the adhesion and clogging of the abrasive chips are easily generated during the grinding process, thereby reducing the processing performance of the tool and Service life.
Due to the above drawbacks and drawbacks of the multi-layer sintered and single-layer electroplated abrasive tools, their application in high-efficiency grinding and high-speed/ultra-high-speed grinding is greatly limited. To this end, in the past decade or so, many scholars at home and abroad have begun to study the use of brazing technology to produce single-layer super-hard abrasive tools. The starting point is the melting, wetting, and the infiltration at the interface between the abrasive, the brazing material and the substrate during high-temperature brazing. Diffusion, compounding and other interactions (so-called chemical metallurgical effects) fundamentally improve the holding strength of the matrix and the brazing alloy to the abrasive. Figure 1 shows the microscopic morphology of the interface between the brazing diamond single abrasive particles and the binder. It can be seen that the solder exhibits good wettability to the diamond abrasive grains, and the combination is tight, without gaps and grooves. The diamond abrasive grains are intact, the surface is free of cracks, and the exposed height is large. Compared with traditional multi-layer sintered superabrasive tools and single-layer electroplated superabrasive tools, brazed superabrasive tools have the following characteristics: 1 metallurgical chemical bonding between abrasives, brazing fillers and matrix can be improved The bonding strength, the tool has a long service life; 2 the abrasive grain has a high exposure height (up to 70% to 80% of the abrasive grain height), the chip space is large, it is not easy to block, the use of the abrasive is more sufficient; 3 the grinding force, The power consumption and grinding temperature are lower; 4 is environmentally friendly, in line with the trend of green manufacturing advocated today.
Brazed superabrasive tools have become a hot topic in today's superabrasive tools due to their unique advantages. This paper summarizes the research status and research results of brazing superabrasive tools at home and abroad from brazing solder, process and method, and analyzes the bonding strength and interface microstructure of different brazing materials, processes and methods. influences. On this basis, the optimal arrangement of abrasive grains and the method of orderly arrangement of abrasive grains on the surface of single-layer brazed superabrasive tools are discussed.
2 Research status of brazed diamond tools There is a high interfacial energy between diamond and general metals and alloys, and the surface is not easily infiltrated by molten metal or alloy. According to theoretical analysis and literature reports, under certain conditions, certain transition elements such as Ti, V, Cr, Zr, Mo, W, etc., under certain conditions, can form carbides with carbon on the surface of diamonds. The role of this layer of carbide, diamond, solder and steel matrix can achieve a strong metallurgical bond by brazing, which is the principle of high-temperature soldering of diamond. The quality of diamond brazing depends to a large extent on the solder used, the brazing process and the method. Therefore, the key to improving the holding force of diamond abrasive grains is to find active solder, brazing method and brazing process.
2.1 Nickel-based alloy brazing process and method of brazing diamonds The brazing process adopted by AK Chattopadhyay in Switzerland is: firstly using flame spraying method (oxy-acetylene torch) to braze the alloy (the main component is 72% Ni, 14.4%) Cr, 3.5% Fe, 3.5% Si, 3.35% B, and 0.5% O2) are plated on the tool steel substrate, and the diamond is placed on the solder layer, and then high frequency induction brazing is performed at 1080 ° C under argon protection. The combination of diamond and steel matrix is ​​achieved in seconds. The Cr in the solder alloy acts as a strong carbide element to enrich the surface of the diamond during brazing to achieve surface metallization of the diamond.
Germany's A Trenker et al. used a brazing method in a vacuum furnace to use an active brazing filler metal and a nickel-based brazing filler metal in the high-temperature brazing process to achieve the combination of diamond and matrix. In the experiment of processing glass, the performance of the high-temperature brazed diamond tool is superior to that of the electroplated diamond tool. The initial grinding performance of the brazing tool is more than 3.5 times that of the electroplating tool, and the life is more than three times that of the electroplating tool. .
Xiao Bing of Nanjing University of Aeronautics and Astronautics used the high-frequency induction brazing method to vacuum braze the Ni-Cr alloy solder for 30 seconds and the brazing temperature at 1050 °C to braze the diamond firmly on the steel substrate. X-ray diffraction analysis revealed that the alloy brazing filler metal and the diamond formed Cr7C3 and Cr23C7 at the interface, so the brazing process can ensure high strength bonding between the alloy brazing filler metal layer and the diamond. In the subsequent large-load grinding experiment, no diamond detachment occurred, indicating that the brazing material has a high holding strength for diamond. Yao Zhengjun and others used the method of brazing in the Ar gas protection furnace to control the brazing temperature of 1050 ° C, the holding time of 6 min and the cooling rate of 30 ° C / min using Ni-Cr alloy powder as the brazing material, achieving a firm connection between the diamond and the steel substrate. . Scanning electron microscopy and X-ray energy spectroscopy, combined with X-ray diffraction structure analysis, found that the Cr element in the Ni-Cr alloy was separated during the brazing process to form a Cr-rich layer at the diamond interface, and reacted with the C element on the diamond surface to form Cr3C2. Cr7C3, which is the main factor for achieving a higher bond strength between the alloy layer and the diamond. Heavy-duty grinding experiments show that the diamond is normally worn and there is no whole diamond shedding, which can be applied to efficient grinding. Lu Jinbin et al. used the method of brazing in a vacuum furnace to use Ni-Cr alloy as the brazing material. The diamond was directly arranged on the Ni-Cr alloy, and the brazing process was properly controlled to achieve a firm connection between the diamond and the steel substrate. Through the analysis of the microstructure of the interface between diamond and brazing filler metal, it is found that the brazing filler metal will form a Cr-rich layer at the diamond interface and react with the C element on the diamond surface to form Cr3C2 and Cr7C3. Cr7C3 is in the form of bamboo shoots and Cr3C2 is in the form of tablets. Growth. Finally, the grinding and contrast experiments confirmed that the diamond has a higher bonding strength with the brazing filler metal. Ma Bojiang et al. used high-frequency induction brazing method to braze Ti diamond and uncoated diamond with two different compositions of Ni-Cr solder under the same brazing process conditions. Two nickel-based active brazing materials were used in the test. The difference between the two was 4% Cr (mass fraction). High-frequency induction brazing was carried out under the protection of Ar gas. The brazing temperature was less than 1100 ° C and the temperature was kept for several seconds. The results show that the composition of the brazing filler metal and the diamond plating of Ti make the carbide composition and morphology of the diamond surface different, which affects the strength of their bonding with diamond. On this basis, a single-layer brazed diamond tool was fabricated by high-frequency induction brazing in vacuum furnace and argon protection. The results show that the tools produced by these two processes can produce carbides that play a key role in improving the diamond holding force at the diamond-solder interface, but the interface structure is different. Under brazing conditions in the resistance furnace, the product has a two-layer structure, the inner layer product is Cr3C2, and the outer layer product is Cr7C3; under high frequency brazing conditions, there is only a single layer product Cr3C2.
Ma Chufan of the Fourth Military Medical University and others used the method of brazing in a vacuum furnace to use NiCr13P9 alloy as the brazing filler metal, while adding a small amount of Cr powder, and brazing at a high temperature (950 ° C) pressure and a vacuum degree of 0.2 Pa. Achieving a strong bond between the diamond and the steel matrix, a dedicated single-layer brazed diamond wheel for the dental CEREC CAD/CAM system was developed. Scanning electron microscopy showed that the silver-white alloy was wrapped around the diamond perimeter, the brazing material was evenly distributed between the diamond abrasive grains, the diamond was brazed firmly, and the diamond blade height was high. X-ray diffraction analysis can observe the Cr3C2 diffraction peak, indicating the formation of Cr3C2. It is through the carbide layer on the diamond interface that the alloy material achieves a high holding power for diamond. Grinding experiments have confirmed that diamonds have high holding strength, and the durability and grinding efficiency of brazed diamond wheels are significantly higher than those of ordinary electroplated grinding wheels.
Huang Hui of Huaqiao University used the method of high-frequency induction brazing to use Ni-Cr alloy as the brazing material, and tried to directly braze the diamond abrasive grains in the air. The firm welding of the diamond to the steel substrate is achieved by proper control of the brazing current and brazing time. Grinding experiments show that the diamond tool made by this method has a high bonding strength between the diamond abrasive grains and the matrix. The diamond abrasive grains do not fall off during the whole process, and the wear process of the diamond abrasive grains is normal wear. In addition, attempts have been made to braze diamond segments using high frequency induction in a self-made vacuum furnace. By adjusting the heating time and the heating power to control the heating temperature of the solder, a firm connection between the diamond and the substrate is achieved, and a diamond grinding disc is produced. In the process of grinding granite, the height of the diamond abrasive grains and the wear state of the abrasive grains were tracked, revealing the wear performance of the brazed diamond tools during the processing.
2.2 Process and method for brazing diamond by silver-based alloy brazing material Xiao Bing et al. of Nanjing University of Aeronautics and Astronautics used high-frequency induction brazing method to add Ag-Cu alloy with Cr powder as brazing filler metal and induction brazing in air. In seconds, the brazing temperature is 780 ° C, achieving a strong bond between the diamond and the steel matrix. X-ray energy spectrum and X-ray diffraction analysis revealed that Cr3C2 was formed at the diamond interface. A comparison test with an Ag-Cu alloy solder without Cr powder shows that the alloy brazing material strongly holds the diamond through this carbide layer on the diamond interface.
Sun Fenglian of Harbin Institute of Technology used brazing method in vacuum furnace to test the CVD diamond thick film with Ag-Cu-Ti active solder alloy foil as filling material. Experimental parameters: vacuum degree 5×10-3Pa, brazing temperature 920°C, heating and cooling rate 30°C/min, holding time 20min. X-ray diffraction analysis confirmed that the new compound at the interface between the diamond and the solder was a TiC layer. It is this carbide layer that creates a metallurgical bond between the solder and the diamond, creating a strong bond between the diamond thick film and the base metal. On this basis, the effects of different brazing temperatures (850 ° C, 880 ° C, 910 ° C, 940 ° C and 970 ° C) on the diamond bond strength were discussed. The test results show that the shear strength is the highest at 940 ° C, up to 133 MPa. It can be seen that the brazing temperature directly affects the bonding strength between the brazing material and the diamond and the matrix.
Li Dan of Harbin University of Science and Technology used the brazing technology in vacuum furnace to test the wet state of Ag-Cu-Ti solder on the diamond surface. Three Ag-Cu-Ti solders were used, and the mass fraction of Ti was 5%, 10%, and 15%, respectively. Brazing process parameters: vacuum degree 6.65×10-3Pa, heating and cooling rate 30°C/min, brazing temperature 950°C, holding time 25min. It was found that the Ag-Cu-Ti solder with 10% Ti has better wettability to diamond and higher bonding strength. Guan Yucong et al. used a vacuum induction brazing method to test the brazing process of Ag-Cu-Ti brazing diamond abrasive grains, and discussed the effect of brazing temperature and brazing state on bonding strength. Brazing process: a secondary heating process is used, the brazing temperatures are 890 ° C, 910 ° C and 940 ° C, respectively, and the vacuum is 0.2 Pa. The test results show that the bonding strength of the bonding interface is the highest when the brazing temperature is 50 °C (940 °C) higher than the melting temperature of the brazing material. X-ray diffraction analysis shows that the new compound between the interface of diamond and brazing filler metal is TiC. When the brazing temperature is 940 ° C, the uncoated diamond is vacuum brazed, and the bonding strength of the Ag-Cu-Ti solder alloy foil is higher than that of the Ag-Cu eutectic alloy foil and the Ti foil.
Wang Chengyong of Guangdong University of Technology used high-frequency induction brazing to high-frequency braze diamond in air or under local gas protection. Main brazing process: Place brazing sheets (mainly Ag-Cu-Zn, Ag-Cu-Ti or other Ag-Cu based alloys) and 102 flux on the substrate, and then wrap the diamond with metal powder (chromium powder and titanium). The powder is placed on the brazing sheet, or the metal powder is evenly sprinkled on the brazing sheet on which the diamond abrasive grains are placed, and then high-frequency brazing is performed to achieve a strong chemical-metallurgical bond between the diamond, the brazing material and the substrate.
2.3 Copper-based alloy solder brazing diamond process and method Pakistan's FA Khalid et al. used the method of brazing in a vacuum furnace to study the microstructure of the diamond interface with Cu-14.4Sn-10.2Ti-1.5Zr alloy as the brazing filler metal. . Brazing process: vacuum degree 2×10-8Pa, brazing temperature 930°C, holding time 10min, cooling rate 20°C/min. Scanning electron microscopy and X-ray energy spectrum analysis showed that the titanium carbide formed at the diamond-solder interface had a two-layer structure, the first layer was a cubic TiC, and the second layer was a slender or cylindrical TiC.
Meng Weiru of Xi'an Jiaotong University used the method of brazing in a vacuum furnace to test the adaptability of diamond brazing filler metal. Three kinds of BNi2 (NiCrSiB), BNi7 (NiCrP) and self-made CuSnNiTi brazing filler metals containing strong carbide forming elements Cr and Ti were used respectively, and the respective brazing temperatures were 1050 ° C, 950 ° C and 900 ° C, respectively, and the holding time was 10 min. And a single layer brazed diamond circular saw blade (φ125 mm) under a vacuum of 0.13 Pa. Scanning electron microscopy observation of morphology and X-ray energy spectrum analysis showed that the three solders have good wettability to diamond. The Cr and Ti elements in the solder will diffuse to the diamond surface and C element in the diamond. Combines to form carbides. The formation of carbides causes a chemical-metallurgical bond between the diamond, the brazing filler metal and the matrix, which improves the holding power of the diamond, but the bonding condition and the sawing performance vary with the brazing filler metal. The test confirmed that the self-made copper-based solder has a low brazing temperature, the thermal damage of the diamond during brazing is small, and the diamond has a good holding power, and the brazing material has good adaptability to the cut stone, and effectively improves the hardness. Utilization of diamonds.
Taiwan University used Cu-15Ti-10Sn alloy brazing filler metal to compare the effects of brazing method (brazing temperature 925 ° C, heat preservation 5 min) and laser brazing method (brazing time 10 seconds) on the microstructure of diamond interface. Under the brazing condition in the vacuum furnace, a continuous transition layer (TiC film) is formed on the surface of the diamond; and under the condition of laser brazing, a discontinuous transition layer is formed on the surface of the diamond.
The above studies show that different solder alloys (including solder composition, content, state, etc.), brazing methods and brazing process parameters directly affect the interaction between the solder and the matrix and diamond, the form of the product and The microstructure of the interface is combined to affect the bond strength and the quality and performance of the brazing tool.
3 Research Status of Brazed CBN Tools Diamond tools are only suitable for processing non-ferrous metals, while CBN tools are suitable for efficient grinding of iron group metals, which are complementary to the applicable processing objects. However, CBN is extremely chemically stable, and it is more difficult to braze it than diamond brazing. At present, reports on CBN brazing research in domestic and foreign literatures are much less than diamond brazing.
The foreign research on CBN brazing began in the early 1990s. The brazing process is usually: firstly depositing a layer of TiC film on the surface of CBN abrasive by chemical vapor deposition (CVD) at a deposition temperature of 1000 ° C and a deposition time of 90 min. Then, vacuum induction brazing is carried out at 1040 ° C using Ni-Cr alloy, and the Ni-Cr alloy brazing filler metal can effectively infiltrate the coated CBN abrasive grains, thereby successfully welding the coated CBN on the substrate. In addition, the Cr-containing nickel-based solder (such as Ni-P alloy: Ni90%, P10%) can also be used to saturate the coated CBN abrasive grains well.
In recent years, Nanjing University of Aeronautics and Astronautics has also conducted research on CBN brazing and has made some progress. The process used is: using Ag-Cu-Ti alloy as brazing filler metal (main component: 67% Ag, 20% Cu, 12% Ti), heating in a vacuum resistance furnace to brazing temperature (1000 ° C) and suitable for holding The time was then cooled to room temperature with the furnace and it was successful. Scanning electron microscopy, X-ray energy spectrometer and X-ray diffractometer were used to observe and analyze the microstructure of CBN and solder joint interface. It was found that the element Ti in the alloy solder diffused and concentrated on the surface of CBN, and acicular TiB2 and TiN were formed. A chemical metallurgical bond is formed between the CBN abrasive grains and the solder joint, which is the main reason for good wettability and high bonding strength between CBN and Ag-Cu-Ti solder. Subsequent grinding comparison tests show that the brazed CBN grinding wheel has a higher abrasive holding strength than the electroplated CBN grinding wheel. Under the same grinding conditions, the grinding temperature of the single-layer brazed CBN grinding wheel is significantly lower than that of the electroplated CBN grinding wheel, especially under the condition of large depth of cut. Therefore, the brazing wheel has obvious advantages over the electroplated grinding wheel in reducing the grinding temperature, increasing the bonding strength of the abrasive, and prolonging the service life of the grinding wheel.
4 Optimized arrangement problem of super-hard abrasive In order to fully utilize the superiority of brazing super-abrasive tool and improve the processing performance and processing effect of the tool, the arrangement of the abrasive grains on the tool surface and the three-dimensional shape design are an important factor. The preferred and orderly arrangement of the abrasives can effectively improve the processing performance of the tool. For example, in the performance comparison test of diamond saw blade processing Sichuan red granite slab (Mohs hardness 7-8, thickness 20mm), the life of high-temperature single-layer brazed diamond saw blades after abrasive arrangement is not optimized. The single-layer high-temperature brazing diamond saw blade and the multi-layer sintered diamond saw blade are 120% and 81.7%, and the processing efficiency is 1.5 times and 4.9 times, respectively. China's Taiwan's China Wheel Company's single-layer uniform diamond high-temperature brazing beads can reduce the amount of diamond by half and the cutting speed by two times without reducing the tool life. It is said that if single-layer brazing technology can be applied to stone processing tools, it will be a revolution in the design of diamond processing tools. The result is not only a substantial increase in production efficiency, but also a significant reduction in processing costs.
The optimal arrangement of abrasives includes the arrangement of abrasive particles on the surface of the tool, the particle size and concentration of the abrasive particles, the height and contour of the abrasive particles, the dynamic effective abrasive grain spacing, the number of abrasive grains, and other static and dynamic parameters. And geometric parameters. Among them, the effective abrasive particle spacing directly determines the working load of a single abrasive grain. The inappropriate abrasive grain spacing usually leads to problems such as premature abrasive failure, which reduces the processing efficiency and service life of the tool, so it is a parameter that needs to be optimized. The parameter that has the greatest influence on the machining state. In many processing situations, different parts of the tool require different particle sizes and concentrations to make more efficient use of the abrasive. Therefore, it is necessary to optimize the abrasive grain arrangement according to the specific application and processing requirements to produce a superabrasive tool with the best performance. In addition to effective control and improved processing quality, the abrasive optimization layout technology can also estimate tool life and better evaluate the performance of brazed superabrasive tools.
At present, the technologies for achieving orderly arrangement of abrasives mainly include replication technology, template method and laser rapid prototyping technology. However, the application of these technologies in industrial production needs further research. How to realize the automatic and orderly arrangement of abrasive grains is an important issue in the development of brazing tools.
(1) Copying technology Copying technology is suitable for the manufacture of diamond tools with small particle size and fine powder grade. Since the ordered diamonds formed by this technique have the same diamond specifications, the gaps are uniform, and they are duplicated, they are called replication techniques. The principle of the replication technology is to make a regular die hole in the order of the desired pattern by forming a sized die, then deposit a diamond film in the die hole by CVD, and finally remove the Si template and diamond. The film is bonded to the substrate to achieve an orderly arrangement of the diamond, and the tool base can be any curved surface.
(2) One-time use of shell mold cloth method The principle of this method refers to the shell material in investment casting and its manufacturing process, firstly coating a layer of refractory coating on the working surface of the tool (the thickness is controlled at the abrasive height) About half of the time), after it is cured, a cloth groove (strip) or a hole (dot cloth) having a pitch corresponding to the effective abrasive grain spacing is processed on the coating; and then filled in the groove or hole of the coating layer to form a paste. The solder alloy, and spread the abrasive grains, and then compact the abrasive grains to the surface of the substrate; finally, a layer of refractory paint is further coated on the coating with the abrasive and the brazing material to further fix the position of the abrasive grains. After hardening and high temperature roasting, it is sent to a vacuum furnace for brazing; after brazing, the outer layer shell is removed and cleaned to obtain a single layer brazing tool article with optimized morphology. This fabric technology meets the needs of end faces, cylindrical faces and a variety of arbitrarily complex profiled tools.
(3) Using the hole template to realize the orderly arrangement of the abrasive grains According to the optimized arrangement requirements, the holes arranged in the diameter of the diamond (or CBN) abrasive grains and the depth of the abrasive grain height of 70% are processed on the ceramic template. Then arrange the abrasive grains according to the holes. The thickness of the alloy solder after melting is about 30% of the height of the abrasive grains. Since the ceramic template does not have weldability, it is easy to remove after brazing. The brazing process not only ensures the orderly arrangement of the abrasive grains, but also has a high contour, and can ensure that the abrasive grains have a height of 70%.
(4) Ordered array method This method firstly fixes a layer of solder on the substrate by welding or gluing, and then fixes a layer of paraffin on the solder. The height of the paraffin is the same as the height of the superhard abrasive, using mechanical force. The steel column is pressed into the paraffin to form a hole slightly larger than the super-hard abrasive grain, and the position of the steel column is regularly adjusted by stepping the steel column or the base body, and holes of different densities can be obtained on the base body, and the super-hard abrasive grains are obtained. The super-hard abrasive grains that have not been dropped into the holes are formed into an "ordered array", and the paraffin wax is burned off during brazing, and the super-hard abrasive grains are formed in an orderly arrangement state after brazing. A laser beam can also be used to form an ordered array.
(5) Using laser rapid prototyping technology to achieve orderly arrangement of abrasive grains. Based on the optimized results, the SLS method of laser rapid prototyping technology is used, and the CAD data processing software is used for slicing. The lasers are pre-distributed at a certain scanning speed. The abrasive particles on the surface of the workpiece surface are scanned according to the specified topography requirements. By effectively controlling the laser intensity, pulse period and spot diameter, a certain welding temperature and time are ensured, so that the abrasive grains scanned by the laser are shaped according to the shape. The appearance is required to be brazed to the substrate in an orderly manner, and after removing the abrasive grains not scanned by the laser, a single-layer brazing tool in which the abrasive grains are arranged in an order can be obtained.
(6) Dispensing method Referring to the principle of the dispenser, the appropriate nozzle size is selected to form a suitable size droplet. The control nozzle sprays the adhesive on the substrate according to the specified topography requirements, and then spreads the abrasive on the surface of the substrate, and removes the excess abrasive to form an orderly arrangement of the abrasive grains on the substrate, and the solder is dried after the glue is dried. The powder is evenly arranged between the abrasive grains and then brazed.
5 Conclusion High-temperature brazing technology can achieve chemical-metallurgical bonding between diamond (or CBN), solder and metal matrix interface, with high bonding strength. Compared to electroplating and sintering tools, brazed superabrasive tools have many advantages and will become a new trend in the future. At present, a small number of countries such as Europe and Japan (including Taiwan in China) have begun trial production of single-layer brazing super-hard abrasive tools in small batches, but to achieve high-volume industrial production and popularization in processing, the manufacturing process - regardless of Whether it is a brazing process or an abrasive arrangement - there are still many problems that need to be further solved. In short, brazing super-abrasive tools have a good application prospect, which can bring huge social and economic benefits. It is necessary to increase research and development efforts and achieve industrialization as soon as possible.