# Effect of Surface Micro-Hardness Change in Multistep
## 3 Experimental Results and Analysis
### Micro-Hardness  
In the high-speed machining process, the micro-hardness changed significantly within a certain depth of the surface layer due to the cutting force, cutting temperature, the extrusion and friction of the tool and the workpiece surface. This was especially true the subsurface, which was softened due to poor thermal conductivity, and the surface layer within a certain depth was subjected to work hardening due to the influence of the cutting force [20]. In this experiment, roughing micro-hardness at the depth of 100 \(\upmu\)m below the machined surface and the finishing surface micro-hardness were measured along the feed direction, respectively. Figure 6 showed the variation curve of roughing surface layer micro-hardness in the unit feed, Figure 7 showed the variation curve of finishing surface micro-hardness in the unit feed. According to the micro-hardness value of each measurement point, Figure 8 was the standard deviation of the roughing surface layer micro-hardness and the finishing surface micro-hardness. It could be seen from Figures 6 and 7 that the micro-hardness value of the roughing surface layer or the finishing surface in the unit feed did not show a significant change along the feed direction. Figure 5: Cross-sectional schematic diagrams of the unit feed periodic machining along the feed direction. (**a**) Cross-sectional schematic diagram in ideal machining conditions. (**b**) Cross-sectional schematic diagram in actual machining conditions. Due to the poor thermal conductivity of titanium alloy, the cutting heat generated during the cutting process only acted on the contact area of the tool-chip-part, and the influence on surface layer was not too deep [21]. So, the micro-hardness was not much affected by the cutting heat, and it was more affected by the cutting force during machining. It can be seen from Figures 6 and 8 that the larger the roughing cutting speed or depth of cut was, the more greatly the roughing surface layer micro-hardness in the unit feed fluctuated along the average micro-hardness value, and the standard deviation was larger. This was mainly because, during the roughing process, the larger the cutting speed or depth of cut was, the higher the cutting force and temperature were, the greater the gradients changed in unit feed (especially the stress gradients), the greater the micro-hardness changed along the feed direction. At the same time, different degrees of tool flank wear affected by cutting force and temperature caused also uneven stress in the cutting area, which in turn led to the difference of surface layer micro-hardness in the local area. As could be seen from Figure 7a-e, when the roughing cutting speed \(v\) was 70 m/min or depth of cut \(a_{p}\) was 1.5 mm, the finishing surface micro-hardness in the unit feed fluctuated more greatly along the average micro-hardness value, and the standard deviation is larger. At the same time, it could be seen that when the roughing depth of the cut was 0.5 mm, the finishing surface micro-hardness in the unit feed had a relatively small fluctuation along the average micro-hardness value, and the standard deviation was relatively small. This was mainly because there was a difference in the surface layer micro-hardness after roughing, which caused the cutting vibration when finishing, in turn resulting in the fluctuations of the finishing surface micro-hardness in the unit feed. Additionally, the greater the difference of the roughing surface layer micro-hardness was, the greater the fluctuation of the finishing surface micro-hardness in the unit feed was, and the larger the standard deviation was. Figure 7f was the finishing surface micro-hardness in the three-step machining. It could be seen that the finishing surface micro-hardness in the unit feed fluctuated little along the average micro-hardness value, and the standard deviation was 8.39, which was  
significantly lower than the standard deviation in the two-step machining.