1 Introduction The polycrystalline diamond (PCD) composite sheet combines the hardness, wear resistance and impact resistance of hard alloys with natural diamonds. It is an ideal tool material. PCD cutters have excellent cutting performance in high-speed machining of non-ferrous metals and their alloys, non-metallic materials and other processing applications, and are therefore widely used in automotive, aerospace and other processing areas. However, the high hardness and high wear resistance of PCD materials also cause difficulties in its processing. At present, PCD materials are usually processed by a diamond wheel grinding process. The grinding effect of diamond abrasives in grinding wheels on PCD material is essentially the interaction between two substances with similar hardness and properties, and it is obviously different from the ordinary grinding process (hardness of abrasive is much higher than the hardness of the material being milled). The sharpening process of PCD composites has its own law of variation. At present, the study of this law is not enough, and the understanding is not uniform enough. Some viewpoints and conclusions are also lacking sufficient basis. In this paper, the influence of the grinding speed vs. the influence of the PCD material's removal rate Q and the wear ratio G, etc., has been systematically studied, and the mechanism of action has been analyzed in depth. The results have been used to optimize the PCD sharpening process. With theoretical guidance. Method 2 Test conditions for
the grinding process and the grinding wheel swing speed
parameter table (times / min) Feed (mm / min) holder static stiffness (N) 40 0.08 387 FC-200D produced Grinding Test Type PCD & PCBN special tool grinder in Taiwan Carried on. A rectangular (25 mm×5 mm) 1300 PCD blank (PCD layer cross-sectional area of ​​2.5 mm 2 ) manufactured by GE of the United States was ground using a domestic diamond wheel model 6A2 150×40×15×5 W20 M100. The Mitutoyo digital micrometer (accuracy 0.001mm) was used to measure the length of the PCD blank. The positioning block was mounted on the grinding machine feed system. The wear depth of the diamond wheel was measured using the feed digital display system (accuracy 0.001mm). The PCD material removal rate, the wear rate of the grinding wheel, and the wear ratio were determined by calculation. Water-based coolant was used in the test. The grinding process parameters are shown in the above table. During the grinding process, the grinding force is measured synchronously with a Kistler dynamometer. The ground PCD specimens were placed on a Japanese JSL-5600LV scanning electron microscope to observe its microscopic appearance. 3 test results and analysis
Fig. 1 Relationship between PCD removal rate, wear ratio and grinding speed
Figure 2 Micro-topography of PCD grinding surface at low speed grinding
Figure 3 Microstructure of PCD Grinding Surface During High Speed ​​Grinding
Fig. 4 Relationship between wear rate of grinding wheel and grinding speed
Fig. 5 Relationship between PVD grinding force Ft, Fn and grinding speed
Grinding test results are shown in Figure 1. It can be seen from the figure that the PCD removal rate Q increases with the increase of the grinding speed vs; the wear ratio G curve has a hump, that is, when the medium speed grinding (7m/s≤vs≤14m/s), the G value is the highest, while the high, The G value is low at low speeds. The author believes that the reason for this result is that as the grinding speed increases, the grinding mechanism of PCD and the wear patterns of diamond grinding wheels have changed. There are four main removal methods for the grinding mechanism of PCD: impact brittleness removal, creep fatigue brittleness removal, fatigue pitting brittleness removal, thermochemical and mechanical heat removal. Brittleness removal occurs mainly at the point where the abrasive is cut (ie at the cutting edge) and will occur at any grinding speed; other removal methods will change their primary and secondary positions at different grinding speeds. With the increase of grinding speed, the temperature in the grinding zone will gradually increase. It can be known from the physical and chemical properties of PCD that the proportion of thermochemical and mechanical heat removal in grinding will gradually increase. Figures 2 and 3 show the microscopic topography of the grinding surface when the test specimens are ground at low speed (vs=3.92m/s) and high speed (vs=25.17m/s), respectively. As can be seen from FIG. 2 , PCD grinding surfaces have uneven pits and crisscross micro cracks near the crystal surface and on the diamond surface, and no obvious smooth areas and mechanical scratches are observed on the grinding surface. This shows that the removal of PCD material during low-speed grinding is mainly based on the removal of crystal brittleness and the removal of fatigue pitting brittleness, supplemented by local thermal chemical and mechanical heat removal. As can be seen from Fig. 3, there are many smooth areas on the grinding surface of PCD. There are obvious mechanical scratches on the smooth area, and there are basically no pits and micro-cracks on the surface; at the same time, the PCD grinding surface is low and the PCD is There are still pits of unequal size near the grains and on the surface of the diamond grains. This shows that the removal of PCD material during high-speed grinding is mainly based on the removal of thermal chemical and mechanical heat and the removal of the creep fatigue of the crystallites, supplemented by the removal of fatigue pitting brittleness, which is consistent with the views of related literature. Therefore, as the grinding speed increases, although the cutting thickness of a single abrasive particle decreases, the PCD grinding mechanism changes, but the PCD removal rate Q increases with the increase of the grinding speed vs. The normal wear of abrasive particles also increases at the same time, so the Q value does not increase much. There is a hump in the grinding range of medium speed (7 m/s ≤ vs ≤ 15 m/s) in the PCD wear specific ratio G curve. The author believes that this is due to a change in the form of abrasive wear. Fig. 4 shows the relationship between the wear rate Qs of the corresponding grinding wheel and the grinding speed vs. From the figure, it can be seen that in the grinding speed range corresponding to the G curve hump, the wear rate of the grinding wheel is the smallest, while at the other grinding speed, the wear rate of the grinding wheel is large. From the relationship between the tangential grinding force Ft and the grinding speed vs shown in FIG. 5 , it can be seen that during grinding, the tangential grinding force Ft decreases as the grinding speed vs increases. In low-speed grinding, the Ft acting on a single abrasive grain is large, and the abrasive grain is likely to produce premature total wear and tear. Therefore, the wear rate of the grinding wheel is large; in medium-speed grinding, Ft acting on a single abrasive grain is affected. Reduced, and the grinding zone temperature is not too high, so the abrasive particles are not easy to produce early overall shedding and rapid thermal passivation wear, but mainly to micro-crushing wear, abrasive grains in the best working condition, so the grinding wheel The wear rate is the minimum; in the high-speed grinding, although the Ft acting on a single abrasive grain is further reduced, the grinding zone temperature is increased due to the high grinding speed, so that the abrasive grain will generate excessive heat-passivation wear. Therefore, the abrasive grains lose their grinding ability quickly, resulting in a sharp increase in the wear rate of the grinding wheel. Since the wear pattern of the abrasive grain undergoes the above-mentioned change as the grinding speed increases, and the PCD removal rate increases with the increase of the grinding speed, the grinding speed within the optimum operating state of the abrasive grain is within the range. The abrasion ratio G of the PCD material is maximum, that is, the hump curve is present in the wear ratio curve. In the grinding process, from the relationship between the normal grinding force Fn and the grinding speed vs shown in Fig. 5, it can be seen that when the middle-low speed grinding (vs≤15m/s), Fn increases with the increase of the grinding speed vs. When high-speed grinding (vs>15m/s), Fn decreases with the increase of vs. Although this is inconsistent with the traditional grinding force and grinding speed, the conclusion that the PCD grinding mechanism is changing with the increase of grinding speed can convincingly explain the test results. It is well known that under normal operating conditions, the passivation wear of diamond grains in grinding wheels increases with the increase of VS; under the same conditions of other grinding conditions, the grinding zone temperature increases with the increase of grinding speed vs. . During low-speed grinding, the PCD material removal method is mainly based on the removal of crystal brittleness and fatigue pitting brittleness, supplemented by local thermal chemistry and mechanical heat removal, the average temperature of the grinding zone is low, and the hardness of the PCD surface varies with temperature. The (speed) rises and the decrease is small, and the passivation wear of the diamond abrasive grains in the grinding wheel increases with the increase of VS. Therefore, at the low speed grinding, Fn increases with the increase of VS; at the time of high-speed grinding , PCD material removal methods are mainly based on thermal chemical and mechanical heat removal and crystallographic fatigue brittleness removal, supplemented by fatigue pitting brittleness removal, the average temperature in the grinding zone is high, and the hardness of the PCD surface increases with temperature (speed). However, the Fn decreases with increasing Vs at high speed grinding. In summary, when grinding a PCD material with a diamond grindstone under the condition of adding coolant, for the two objective functions of the material removal rate Q and the wear ratio G, the G value is the largest in the medium speed grinding range, Q The value is also larger and the grinding wheel is in optimal working condition. This shows that there is an optimal range of grinding speed vs when grinding PCD material. 4 Conclusions With the increase of grinding speed vs, the removal mechanism of PCD material will change, that is, the removal of brittle removal control PCD material to remove by brittleness, thermochemistry and mechanical heat removal while controlling the PCD material removal transformation, and with the The increase in the proportion of thermochemical and mechanical heat removal has gradually increased. With the increase of grinding speed vs, the removal rate Q of PCD material increases gradually, but the growth rate is small. For the PCD material wear ratio G and the removal rate Q, there is an optimum grinding speed range. In this range, the G value is the largest, the Q value is also large, and the wear rate Qs of the grinding wheel is the smallest, that is, the grinding wheel is in the best working condition. Under the test conditions in this paper, the best grinding speed range is: 7m/s≤vs≤14m/s. When grinding at low speed, the normal grinding force Fn increases with the increase of the grinding speed vs; at the high speed grinding, Fn decreases with the increase of the vs. The tangential grinding force Ft decreases as the grinding speed vs increases.
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