Polycrystalline diamond (PCD) has the hardness, wear resistance and impact resistance of natural diamonds. The application range of PCD tools has been extended to the finishing of non-ferrous and non-metallic materials, especially for Precision and ultra-precision machining of aluminum alloy materials. However, due to the high hardness, high wear resistance and low fracture toughness of PCD tool materials, the material removal rate of PCD tools is extremely low, which seriously affects production efficiency. In this paper, ultrasonic vibration is introduced into the grinding process of PCD tool material by analyzing the removal mechanism of PCD material. Tests have shown that ultrasonic vibration grinding of PCD tool materials can achieve high processing efficiency while ensuring surface roughness.
2. Ultrasonic vibration grinding test and result analysis
The ultrasonic vibration grinding test is carried out on a modified high-precision static pressure grinder with adjustable speed; the grinding disc is made of high-phosphorus cast iron disc with diameter D=300mm; the abrasive is W7 granular diamond grinding powder; the test piece material is Zhengzhou Xinya JFER1308DF-IV (specification Ø13.44×0.8mm, particle size W40) produced by Composite Superhard Materials Co., Ltd. and PC1308DA (measured by Ø13×0.8mm, particle size W20) produced by GE Company of the United States; ultrasonic vibration grinding system by J923025 type Electron tube ultrasonic generator (maximum rated output power is 250W), self-developed magnetostrictive ultrasonic transducer and corresponding fixture; the material removal required for grinding processing is extremely small, and PCD material has high wear resistance Therefore, the ES120G-4 electronic balance (weighing can be accurate to 10-4g) is used to measure the quality change of the PCD specimen, and the grinding efficiency is evaluated.
Test method and parameter grinding, the test piece is vibrated in the horizontal direction of the grinding disc (perpendicular to the direction of the grinding disc line speed), the working frequency is 19KHz, the static load is 15N, and the grinding time is 60min. During the test, the grinding discs were selected at three different speeds (80r/min, 320r/min and 500r/min). The ultrasonic generators output different powers, respectively grind PCD samples of different particle sizes, and measure the test results separately.
Analysis of test results
The ultrasonic vibration grinding process can significantly improve the material removal rate of the ground PCD. The test data also shows that as the output power of the ultrasonic generator increases, the removal rate of PCD material is greatly improved (when the output power of the ultrasonic generator is 250W, the removal rate of PCD material is more than double that of ordinary grinding), that is, Ultrasonic vibration grinding can achieve higher processing efficiency than ordinary grinding. This is because the increase of the output power of the ultrasonic generator increases the amplitude of the PCD test piece, indicating that the amplitude of the PCD material removal rate is greatly affected in the ultrasonic vibration grinding; as can be seen from Fig. 1, under the same test conditions, As the rotational speed of the grinding disc increases, the PCD material removal rate also increases accordingly.
Under the same test conditions, the material removal rate of ultrasonic vibration grinding of coarse-grained PCD is higher than that of ground fine-grained PCD, which indicates that the diamond grain size of synthetic PCD has great wear resistance and strength to PCD materials. influences.
3 Ultrasonic vibration grinding PCD removal mechanism analysis PCD is made of pre-treated diamond powder with certain particle size as raw material, with nickel, cobalt, titanium, silicon, boron as the binder, sintered at high temperature and high pressure. . PCD is a diamond compact in which diamond particles are randomly arranged, and has a "concrete" type microstructure. This structure makes the PCD material have isotropic characteristics, so that it can obtain higher wear resistance and higher resistance than natural diamond. Impact strength.
3. Removal mechanism of ground PCD material
The removal mechanism of the ground PCD material mainly includes mechanical action and thermochemical action. In the initial stage of grinding, due to the large surface roughness of the PCD, the diamond grains protrude more from the surface of the PCD, so that the high-speed moving abrasive particles are easy to force, and the diamond crystal grains are easily stressed at the bottom indirect bonding portion. The bonding site between the PCD grains is its weak link, and its binding energy is smaller than the binding energy of the most easily cleaved (1 1 1) crystal plane of the diamond single crystal constituting PCD. When the abrasive particles are impacted at high speed, the stress gradually concentrates, and the microcracks expand along the joint between the diamond grains. Under the repeated high-speed impact of the abrasive grains, the intergranular fatigue fracture occurs, causing the diamond particles to loosen and fall off as a whole. . As the grinding continues, the surface roughness of the PCD gradually decreases, and the occurrence of the overall loss of the diamond grains is gradually reduced.
According to Griffith's theory of fracture mechanics, some solids have high covalent and ionic bond strengths, and their defects are relatively bound, so they are prone to fracture under relatively low stress (relative to their strength). Brittle solid. If the resistance to fracture is primarily dependent on the inherent bond strength of the solid, the solid may be referred to as a highly brittle solid. Since the proportion of diamond grains in the PCD material is as high as 80% to 90%, the removal resistance mainly depends on the strength of the diamond grains themselves constituting the PCD, so the PCD material can be classified as a highly brittle solid. PCD is a highly brittle polycrystalline solid material, and the crystal orientation of each diamond grain is different. When the crack encounters the boundary of two grains, or continues to expand in the second grain through it (for small angle grain boundaries and High-strength grain boundaries are prone to transgranular fracture, or cracks extend along grain boundaries. Since the (1 1 1) crystal plane of diamond particles has the highest probability of exposure, it is easier to work with high-speed moving abrasive grains, and the high-speed moving abrasive grains generate tensile stress perpendicular to the (1 1 1) crystal plane when the tensile stress is applied. When the critical value is exceeded, a crack is generated, and a crack occurs in the direction of the (1 1 1) crystal plane, and a smooth cleavage fracture plane and a cleavage step are formed on the surface of the PCD (as shown in FIG. 5). The cleavage of diamond grains is the result of crack propagation under tensile stress. Due to the influence of the twinning band and the random arrangement of the diamond, the fracture surface of the diamond particles tends to have a zigzag shape, that is, a tiny cleavage step is formed on the cleavage surface. This is because many twin ribbons cross each other, causing the crack propagation direction to change, thus preventing cracks from penetrating the entire diamond. The cleavage of diamond grains is one of the main ways to remove PCD.
During the grinding process, the sharp abrasive grains pass through the PCD at a high speed and appropriate pressure, and a large number of scratches are formed on the surface of the diamond grains. These scratches can be classified into plastic scratches and brittle scratches. Mechanical removal is also a major material removal method for grinding PCD. In addition, there are many kinds of thermochemical removal methods such as stone grinding removal, thermal erosion removal, diffusion removal, and oxidation removal during the grinding process of PCD, but the thermal chemical removal method does not occupy the main position in PCD material removal.
Removal Mechanism of Ultrasonic Vibration Grinding PCD Material
In ultrasonic vibration grinding, an ultrasonic vibration system consisting of an ultrasonic transducer and a corresponding fixture applies a high-frequency vibration of amplitude A (usually A=10-20 μm) and frequency f (usually f=20 KHz) to the PCD material. The PCD material is caused to generate a high-frequency reciprocating motion of amplitude A along the radial direction of the grinding disc. When grinding, the reciprocating motion is combined with the grinding disc linear velocity V, and the grinding trajectory is changed from a normal grinding circle to a superimposed circle. f/V is the frequency and the high frequency harmonic with amplitude A, so that the grinding granules are lengthened by the grinding trajectory of the surface of the PCD material in unit time, and the grinding efficiency is correspondingly improved; at the same time, the lengthening of the grinding trajectory per unit time also makes the grinding The increased speed allows the abrasive particles to have higher impact kinetic energy than under normal grinding conditions, thereby increasing the material removal rate of the ground PCD. For material destruction, the magnitude of energy is an important factor, and the gradient of energy versus time is more important in some sense. Introducing ultrasonic vibration into PCD grinding is to increase the energy gradient and energy concentration by the energy shock of the ultrasonic wave, so that the abrasive particles can overcome the intergranular bonding energy in a short time, and the grain will be fatigue-destroyed (especially the rational brittle fracture and The probability of microscopic damage removal increases. Some scholars have pointed out that under the cyclic load of the impact load, the stress value of the crack generated is much lower than the required static stress. In ultrasonic vibration grinding, the diamond grains in PCD are always under the action of high-frequency alternating impact load, which accelerates the fatigue damage of PCD materials, especially the rational and brittle fracture of diamond grains.
According to the characteristics of the ultrasonic vibration grinding trajectory, the grinding speed direction is within a certain angle (up to 60°), and the ultrasonic vibration frequency f is oscillated at a high frequency, so that a large number of staggered damaged scratches can be seen on the surface of the diamond crystal grain. (See Figure 6.) The changing direction of the grinding speed makes the easy-to-machine crystal orientation of the diamond grains often in the direction of the grinding speed, making the diamond easier to remove, thereby increasing the material removal rate of the PCD.
From the perspective of macroscopic energy, the grinding process is essentially an energy consuming process. In the ordinary grinding, the input of energy is all from the motor that drives the grinding disc to rotate; in the ultrasonic vibration processing, the energy input of the system is not only from the motor that drives the grinding disc, but also from the ultrasonic generator. Therefore, under the same processing conditions, the ultrasonic vibration grinding system has more input power to the grinding zone per unit time than the ordinary grinding, that is, the input power is increased; and the friction coefficient of the ultrasonic vibration grinding is higher than that of ordinary grinding. With the decreasing trend, the energy consumed in the frictional heat does not change greatly due to the introduction of vibration. Due to the energy accumulation and stress waves in the ultrasonic vibration grinding, the energy consumption of the system in crack propagation and new surface formation is increased, so that the material removal rate of the ultrasonic vibration grinding is increased.
In addition, ultrasonic vibration grinding also exacerbates the thermochemical action in the grinding process, which increases the role of various thermochemical removal methods, which is one of the reasons for the improvement of ultrasonic vibration grinding efficiency.
Under the same conditions, the material removal rate of coarse-grained (W40) PCD is significantly higher than that of fine-grained (W20) PCD, because the coarse-grained PCD has a large surface roughness value and the corresponding friction coefficient during grinding. Larger, the abrasive particles collide with the diamond on the surface of the PCD to produce a larger pulse force. At the same time, because the fine-grained PCD is more dense than the coarse-grained PCD, it can better suppress the micro cracks and the internal cracks generated in the PCD during the manufacturing process. The influence of regular holes on its strength makes the diamond particles less likely to fall off and cleave brittlely during ultrasonic vibration grinding, so the material removal rate of fine-grained PCD is low.
4 Conclusion
Applying ultrasonic vibration technology to the grinding of PCD tool materials can significantly improve the processing efficiency of grinding PCD. Compared with ordinary grinding, ultrasonic vibration grinding has the characteristics of longer grinding trajectory per unit time, increased impact kinetic energy, and ultrasonic energy shock wave, which accelerates and enhances the two kinds of cleavage and micro-damage removal of PCD materials in grinding. The main removal method enhances the grinding efficiency. At the same time, ultrasonic vibration grinding also accelerates the role of thermal chemical removal in PCD removal, which significantly improves the processing efficiency of grinding PCD. Grinding tests prove that ultrasonic vibration grinding PCD technology has a good production and application prospects.
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