3.1 Cutting forces
As shown in the Figure 5 is a table of the three-axis cutting force of the four tools under the cutting conditions of v=160 m/min, f=0.2 mm/r and ap=0.4 mm. It can be seen from the figure that the three-axis cutting force of the bionic tools T8, T20 and T25 is lower than that of the ordinary non-textured tool NT, especially T20, and the three-axis cutting force is smaller than that of the conventional non-textured tool NT. The main cutting force, radial force and feed force were reduced by 16.45%, 31.90% and 25.31% respectively. In addition, generally speaking, the cutting force of the four tools for cutting CFRP is relatively small, but the cutting force oscillates greatly during the cutting process. This is because CFRP is a difficult-to-cut material, and its bar material structure is laminated fibres. The process of turning CFRP is accompanied by the breakage of fibres and the cutting of new fibre layers, so the cutting force is in a continuous oscillation cycle. This continuous oscillation has a considerable impact on the tool, that is, to improve the tool performance is critical for cutting CFRP materials.
Figure 5 Three-axis cutting forces of four tools (v=160 m/min, f=0.2 mm/r, ap=0.4 mm): (a) main cutting force, (b) radial force, (c) feed force
Figure 6 shows the change curve of the three-axis cutting force of the four tools at a cutting speed of 80-180 m/min (f=0.2 mm/r, ap=0.4 mm) to cut CFRP. Judging from the changing trend of cutting force, it generally shows a trend of decreasing first and then increasing, the reason being. In general, the cutting force of the four tools in cutting CFRP in the tested cutting speed range is relatively small, and it can be clearly seen that the three-axis cutting forces of the bionic tools T8 and T20 are the smallest. When the cutting speed is 80-120 m/min, the main cutting force of T8 and T20 is reduced by 5.70%~11.78% and 4.29%~11.42% respectively compared to the conventional non-textured tool NT, and the radial force is reduced by 5.66%~13.08% and 2.94%~18.74%, the feed force is reduced by 2.06%~24.13% and -1.87%~16.36%; when the cutting speed is large, in the range of 140-180 m/min, T8 and T20 main The cutting force is reduced by 9.46%~17.79% and 10.56%~16.45%, the radial force is reduced by 21.38%~28.51% and 28.88%~33.92%, and the feed force is reduced by 15.58%~23.22% and 13.51%~26.48%. It can be seen that the three-axis cutting force of the bionic tools T8 and T20 drops significantly when the cutting speed is large, indicating that these two bionic tools have a more prominent role in reducing the cutting force at higher cutting speeds.
Figure 6 Under the conditions of different cutting speeds, the change of the three-axis cutting force experienced by the four tools: (a) main cutting force, (b) radial force, (c) feed force
In addition, the T25 bionic tool also has the effect of reducing the cutting force of the tool in the higher cutting speed range of 140-180 m/min. Compared with the conventional non-textured tool, the NT main cutting force is reduced by 8.77%, 7.34% and 10.23%, radial force decreased by 15.07%, 14.83% and 19.09% respectively, and feed force decreased by 9.17%, 13.85% and 12.80% respectively. It can be seen that the bionic microstructure is suitable for cutting at a higher cutting speed, and can improve the cutting performance of the tool during high-speed cutting.
3.2 Cutting temperature
During the cutting process, FLUKE TI32 portable infrared camera was used to measure the cutting temperature of the tool. Figure 7 is the infrared thermal image of four tools cutting CFRP when the cutting speed is 140 m/min. It can be seen from the figure that under this cutting condition, the maximum temperature of the tool tip of the conventional non-textured tool is 119.1ºC, while the maximum temperature of the tip of the bionic tool T8 is 77.0ºC, which is 35.35% lower than that of the conventional non-textured tool.
Figure 7 Infrared thermography of conventional non-textured tool NT and bionic tool T8 in dry cutting of CFRP (v=140 m/min, f=0.2 mm/r, ap=0.4 mm): (a) conventional non-textured tool NT, (b) bionic tool T8
Figure 8 shows the cutting temperature changes of four different tools in the cutting speed range of 80-180 m/min. On the whole, the cutting temperature of the four tools shows a trend of decreasing first and then increasing with the increase of cutting speed. The minimum temperature appears at 120 m/min and 140 m/min. It can be clearly seen from the figure that at any cutting speed, the cooling effect of the bionic tools T8 and T20 is obvious, and the bionic tool T25 has a better cooling effect when the cutting speed is higher. T8 and T20 in the test cutting speed range, compared with conventional non-textured cutting tools, the cutting temperature was reduced by 10.25%~20.42% and 11.82%~32.67% respectively. The temperature reduction effect of T25 at a higher cutting speed of 140-180 m/min also reached 6.31%~13.43%. It can be seen that the effects of the three bionic tools at lower cutting speeds are more significant. It can be seen that the presence of microstructures on bionic cutting tools based on surface microstructures of blood clams can effectively improve the heat dissipation efficiency of the friction interface, so that the temperature rise of the tool tip during cutting can be significantly lower than that of the non-textured texture. Therefore, when cutting a hard-to-cut material such as CFRP with a micro-structured tool with surface microstructures of blood clams, the tool surface will have a lower temperature than the conventional tool, and the purpose of extending the tool life is achieved.
Figure 8 Cutting temperature of four kinds of tool at different cutting speeds (f=0.2 mm/r, ap=0.4 mm)
3.3 Average friction coefficient
According to the theory of metal cutting, in the process of metal cutting, the relationship among the cutting tool geometry angles, the radial force FY and the main cutting force FZ satisfies Formula (1), and then the calculation formula of the average friction coefficient μ of the tool-chip contact surface on the tool rake face can be derived.
Where μ-average friction coefficient, β-friction angel, γ0- tool rake angel, FY-radial force and FZ-main cutting force.
The data obtained by the cutting test and the Formula (1) give the average friction coefficient of the tool-chip contact surface of the four tools at different cutting speeds, and the curve of its change with cutting speed is shown in Figure 9. On the whole, the average friction coefficient on the rake face of the tool shows a downward trend with the increase of cutting speed. It can be seen from the figure that the bionic tool T8 has a certain friction reduction effect at the cutting speed v=180 m/min, and the average friction coefficient at other cutting speeds is not much different from that of conventional non-textured tools. The bionic tool T20 has a good friction reduction effect when the cutting speed v≥120 m/min, especially when the cutting speed v=140 m/min, the friction coefficient is reduced by 21.02% compared to NT, and the friction reduction effect is significant. In contrast, the bionic tool T25 not only does not have a good anti-friction effect, but even increases the friction between the rake face and the chip, making the average friction coefficient increase. It can be seen that the proper microstructures of the surface of the bionic tool can play the anti-friction effect under certain cutting conditions and play a positive role in improving the cutting performance of the tool.
Figure 9 Average friction coefficient of four kinds of tools turning CFRP at different cutting speeds (f=0.2 mm/r, ap=0.4 mm)
3.4 Chip adhesion and wear
The main reasons for tool wear are excessive contact pressure between the tool and the workpiece and excessive temperature on the contact surface. In high-speed cutting, a large amount of cutting heat is generated between the workpiece and the tool. The temperature of the contact area between the workpiece and the tool is high and the pressure is high. In this state, the fresh chip surface formed by cutting often has a strong chemical activity and atomic adsorption force, it is easy to cause adhesion on the contact surface between the tool and the workpiece. The adhesion material will take off part of the tool material, causing damage to the tool and causing bond wear. This is the main form of wear in cutting CFRP materials.
Figure 10 shows the two-dimensional bonding morphology of the rake face of conventional non-textured tool NT and bionic tool T8 and T20 dry cutting CFRP and the distribution diagram of H element in epoxy resin under the cutting conditions of v=180 m/min, f=0.2 mm/r, ap= 0.4 mm. It can be seen from the figure that these two kinds of cutters have a more obvious black bond near the cutting edge of the rake face area, which is in sharp contrast with the tool material. The cutter mainly focuses on bond wear. Observing Figure 10 (a)-(f), it can be seen that the size and scope of the non-textured tool NT block bond are significantly larger than the bionic tools T8 and T20, indicating the bond wear of the conventional non-textured tool NT more seriously, the bionic tool can effectively reduce the size of the worn area on the rake face of the tool. Because T8 and T20 have a lower degree of bond wear, the average friction coefficient of T8 and T20 rake faces is somewhat lower than that of NT, thereby increasing the tool life.
Figure 10 Two-dimensional bonding morphology and the H element distribution on the rake faces of three kinds of tools (v=180 m/min, f=0.2 mm/r, ap=0.4 mm): (a) NT two-dimensional bonding morphology, (b) H element distribution of NT, (c) T20 two-dimensional bonding morphology, (d) H element distribution of T20, (e) T8 two-dimensional bonding morphology, (f) H element distribution of T8