Sintered metal bond grinding wheels are mostly made of bronze and other metals by high temperature sintering. They have high bonding strength, good formability, high temperature resistance, good thermal conductivity and wear resistance, long service life and can bear large loads. Due to the inevitable shrinkage and deformation in the sintering process of grinding wheel, it is necessary to reshape the grinding wheel before use, but it is difficult to dress the grinding wheel. At present, the commonly used grinding wheel double-roll dressing method is not only time-consuming and laborious, but also the diamond particles fall off during dressing, which consumes a lot of dressing wheel itself and has low dressing accuracy.
In recent years, scholars from all over the world have successively carried out research work on dressing metal-bonded diamond grinding wheels by special machining methods, mainly including electrolytic dressing and EDM dressing. Electrolytic dressing method is fast, but the shaping accuracy is not high; EDM dressing method has high precision and can be used for shaping and grinding, but the shaping speed is slow. Composite dressing methods include electrolytic EDM composite dressing method, mechanochemical composite dressing method and so on. The dressing effect is good, but the system is complex, and the dressing problem of sintered diamond grinding wheel has not been well solved.
In addition, because the grinding wheel manufacturing process determines that its surface morphology is random, the geometric shape, distribution and cutting edge height of each abrasive particle are inconsistent, and only a few higher cutting edges cut into the workpiece during grinding, which limits the further improvement of grinding quality and grinding efficiency.
2 electroplated diamond grinding wheel
Advantages of electroplated diamond grinding wheel:
① simple electroplating process, less investment and convenient manufacture;
(2) no trimming is needed, and the use is convenient;
③ Single-layer structure determines that it can reach a high working speed, which has reached 250,300 m/
(4) Although the diamond has only a single layer, it still has enough life;
⑤ For roller grinding wheels with high precision, electroplating is the only manufacturing method.
Because of these advantages, the high-speed and ultra-high-speed grinding of electroplated grinding wheel occupies an undisputed dominant position. There are some defects in electroplated diamond grinding wheel: there is no firm chemical metallurgical bond between the coated metal and the matrix and the abrasive, and the abrasive is actually only mechanically embedded with the coated metal, so the clamping force is small, the diamond particles are heavily loaded, and it is easy to fall off (or the coating falls off in pieces) in efficient grinding, resulting in overall failure; In order to increase the retention, it is necessary to increase the thickness of the coating. As a result, the chip space at the exposed height of abrasive particles is reduced, the grinding wheel is easy to block, the heat dissipation effect is poor, and the surface of the workpiece is easy to burn. At present, the best shape of electroplated grinding wheel in China has not been optimized according to the requirements of machining premise, and these inherent shortcomings of single-layer electroplated diamond grinding wheel will greatly limit its high-efficiency grinding application.
Single-layer brazed diamond grinding wheel
In order to give full play to the role of diamond, we should try our best to increase the holding power of bonding agent to diamond and improve the bonding strength of grinding wheel. Single-layer high-temperature brazing superhard grinding wheel can overcome the shortcomings of electroplating grinding wheel, realize the chemical metallurgical bonding between diamond bond and metal matrix, and have high bonding strength. Only by keeping the thickness of the bonding layer at 20% to 30% of the abrasive height, can the abrasive particles be firmly held by high-load high-speed and high-efficiency grinding, and the exposed height of the abrasive particles of the brazing grinding wheel can reach 70% to 80%, thus increasing the chip-containing space, making the grinding wheel difficult to block and making full use of the abrasive.
On the premise of the same processing as electroplating grinding wheel, the grinding force, power loss and grinding temperature of single-layer high-temperature brazing superhard grinding wheel are lower, which means that it can reach higher working speed, which is of special significance for ultra-high-speed grinding of 300500m/ above. Single-layer high-temperature brazing uncoated diamond grinding wheel and Cr silver-based filler metal Single-layer brazing grinding wheel adopt high-frequency induction brazing method, with CrA g-Cu alloy as filler metal, air brazing at 780℃ and natural cooling, which can realize the firm connection between diamond and steel matrix. X-ray energy spectrum and X-ray diffraction analysis show that Cr3c2 is formed between Cr and diamond and steel matrix (Fexcric is confirmed by the comparative experiment with or without Cr filler metal, which is the main factor to achieve high bonding strength between alloy layer and diamond and steel matrix, and grinding experiment proves that diamond does have high holding strength.
This process has the advantages of low brazing temperature and little damage to diamond. Disadvantages: Low melting point and poor high-temperature wear resistance of silver-based solder limit the application of high-efficiency heavy-load grinding.
External diamond brazing technology of Ni-Cr alloy single-layer brazing grinding wheel: firstly, a layer of Ni-Cr alloy is flame sprayed on the steel matrix of oxyacetylene torch, which can be used as brazing filler metal to directly braze diamond abrasive particles, and then argon induction brazing is carried out at 1080℃ for 30 minutes.
In the process of flame spraying alloy layer, it is difficult to effectively control the consistency of bonding layer thickness and abrasive arrangement after brazing because the surface of steel substrate is easily oxidized.
Wu Zhibin et al. arranged diamond abrasive grains directly on Ni-Cr alloy sheet or powder, pressed diamond abrasive grains with ceramic blocks, and then brazed on a vacuum high-frequency induction machine for 30℃ at1080℃; Or brazing in a radiation heating furnace under the protection of argon, and appropriately controlling the brazing temperature, holding time and cooling speed.
The analysis of X-ray energy spectrum and X-ray diffraction structure of scanning electron microscope shows that during the brazing process, chromium element in Ni-Cr alloy is precipitated, and a chromium-rich layer is formed on the diamond surface. The chromium-rich layer reacts with elements on the diamond surface to form an alloy layer of Cr3C2Cr7C3, which is well infiltrated with diamond and reacts with steel matrix to form ferrous carbide. Therefore, this brazing process can ensure high bonding strength between alloy layer, diamond and steel matrix.
Heavy-load grinding experiments show that the diamond wears normally and no whole diamond falls off. Advantages of this process: the strength of Ni-Cr alloy is high, and the bonding strength can be higher than that of silver-based alloy after brazing; Nickel-chromium alloy has high melting point and good wear-resistant cutting performance at high temperature. However, it still has some limitations, because higher brazing temperature (1080℃) is easy to cause thermal damage to diamond and reduce its strength. Vacuum brazing or argon shielded brazing can minimize the thermal damage and oxidation of diamond.
Ma Chufan et al. selected NiCr 13P9 alloy as active filler metal, and added a small amount of Cr powder, which was heated to 950℃ under pressure in a vacuum furnace for brazing, and developed a single-layer high-temperature brazed diamond grinding wheel for dentistry.
Scanning electron microscope observation shows that the diamond is surrounded by silver-white alloy, and X-ray diffraction analysis proves the birth of Cr3C2, which is the carbide layer that realizes the high bonding strength between diamond and steel matrix. Grinding experiments also prove that diamond does have a high holding strength, and the life and grinding efficiency of single-layer high-temperature brazed diamond grinding wheel are significantly improved compared with electroplating grinding wheel.
Single-layer high-temperature brazing coated diamond grinding wheel has poor thermal stability, graphitization will occur at 800℃, and higher brazing temperature will inevitably cause thermal damage to diamond and reduce its strength. At the same time, the harmful elements in the binder will corrode and graphitize the diamond, so a layer of active metal and its alloy can be plated on the diamond surface before brazing. Ultra-hard abrasive electroplating technology mainly includes chemical vapor deposition, ion plating, thermal evaporation plating, vacuum micro-evaporation plating and so on.
Chemical vapor deposition of chromium and vacuum micro evaporation of titanium can effectively improve the surface function of diamond. In the brazing process, it is easier to achieve strong metallurgical and chemical bonding between diamond and binder with the help of coating. Because of the barrier effect of coating on hot air and oxygen, the reaction speed of carbon atoms and oxygen on diamond surface is greatly reduced. At the same time, the strong carbide-forming elements in the coating react with carbon atoms on the diamond surface to generate carbides, which blocks the dangling bonds on the diamond surface and increases the anti-oxidation reaction ability, thus inhibiting the graphitization process of diamond itself from being eroded by elements such as bonding agent FeCoNi, so that the brazed abrasive can still maintain its original strength crystal form.
During 5-5ming brazing, a layer of Ni-P alloy (melting point 880℃) was pre-plated on the steel substrate, then the plated diamond was placed on the alloy layer, and the alloy layer was brazed in argon at 1050℃ for 5 minutes, and then cooled to room temperature. The grinding test shows that due to the good wettability of coated diamond and binder, the abrasive particles are effectively prevented from falling off, the grinding function of the grinding wheel is greatly improved, and the machining efficiency of the grinding wheel is greatly improved.
However, it should be pointed out that due to the adaptability between coated diamond and binder, only a suitable binder process can make coated diamond achieve the best physical and mechanical properties. Problems and countermeasures of single-layer high-temperature brazing grinding wheel Although good experimental results have been obtained in the research of single-layer high-temperature brazing grinding wheel at home and abroad, its manufacturing technology needs to be further improved. At present, the main problems are: firstly, what brazing process can produce chemical metallurgical bonding with high bonding strength on the diamond bonding interface; Proper thickness and uniformity control of adhesive layer; The arrangement of the three abrasives is fair and orderly. In order to improve the bonding strength between diamond and solder, chemical metallurgical bonding can occur between diamond solder and metal matrix in the key brazing process, so alloy solder should contain strong carbide forming elements (such as TiCrV, etc.). ) and try to braze at a lower temperature to minimize the damage to diamond.
The development of reasonable brazing alloy formula and single-layer brazing grinding wheel should first solve this problem. In the industrial production of brazed grinding wheels, the thickness and uniformity of bonding layer must be strictly controlled. Before brazing, the metal substrate surface should be treated with oxide film, and the diamond solder should be degreased and decontaminated. Solder contains strong carbide forming elements, and adding appropriate BSi can reduce the melting point of solder and improve the wettability of solder. Powder solder is used for brazing under vacuum (or inert gas protection).
The ordered arrangement of abrasives before brazing is also very important to improve the thickness uniformity of bonding agent layer after brazing. The fair and orderly arrangement of abrasives on the grinding wheel working surface has always been the goal pursued by the abrasive industry, and it is expected to be realized on a single-layer superhard grinding wheel.
In the development process of brazing grinding wheel, firstly, the best shape of grinding wheel is optimized according to the requirements of machining premise, and then the grinding performance of the developed brazing grinding wheel reaches a higher level according to the optimization results. The template is proces with regularly arranged hole, that diameter of the holes is equivalent to the diameter of diamond abrasive particles, the depth of the holes is 70% of the height of the diamond, and the thickness of the diamond alloy sol arranged according to the holes is about 30% of the height of the melted diamond. The brazing process realized by using hole template can not only ensure the orderly arrangement of abrasive particles (good contour), but also ensure the exposed height of diamond 70%. However, its industrial production and popularization and application need further study.