# Effect of Surface Micro-Hardness Change in Multistep
## 2 Cutting Materials and Experimental Preparation
The micro-hardness measurement results showed that there was a difference in the micro-hardness of the local area along the feed direction after roughing. For example, there was a difference in the micro-hardness at points \(i\) and \(j\). Since _gh_ was the tool path curve, the micro-hardness was the same at points \(j\) and \(k\) in theory, and _ef_ was the circumferential curve. Therefore, there was a difference in the micro-hardness at points \(i\) and \(k\). Additionally, in addition to the tool path curve, there was a difference for micro-hardness in the local area of machined surface. By measuring the micro-hardness at 100 \(\upmu\)m below the surface after roughing along the feed direction, it was found that the difference existed not only in the surface but also in the range of the depth \(\Delta l\). Because the finishing depth of cut _a\({}_{p}\)_ was 0.1 mm, within the range of the roughing micro-hardness affected layer depth \(\Delta l\). Therefore, the surface layer micro-hardness after roughing would affect the finishing machining, and affecting the finishing surface micro-hardness, thus affecting the friction and wear characteristics of the titanium alloy. Figure 3 were schematic diagrams of the roughing surface layer micro-hardness and the finishing surface micro-hardness, respectively. Five points (Marked as point 1, point 2, point 3, point 4, and point 5, respectively) were measured in the unit feed (The length of unit feed was 250 \(\upmu\)m, which is the roughing feed rate). When measuring, the first measurement point could be freely selected, and then each point was measured every 50 \(\upmu\)m in turn. Additionally, the total length was measured to be 1250 \(\upmu\)m (5 times the roughing feed fate). When measuring, the applied load was 100 g and the loading time was 15 s. \begin{table}
\begin{tabular}{c c} \hline \hline
**Cutting Tool** & **SNMG120408-EF and SNMG120404-EM** \\ \hline Cutting speed, \(v\) (m/min) & 30, 50, 70 \\ Depth of cut, \(a_{p}\) (mm) & 0.5, 1.0, 1.5 \\ Feed rate,\(f\) (mm/r) & 0.25 \\ Tool geometry & rake angle 14\({}^{\circ}\) and clearance angle 11\({}^{\circ}\) \\ \hline \end{tabular}
\end{table} 
Table 3: Roughing cutting parameters and cutting conditions for the experimental work. Figure 2: Schematic diagram of surface layer micro-hardness analysis. In the friction process, the friction tests were carried out using a UMT-2 friction tester, and the reciprocating test of the ball disc contact was adopted. Figure 4 was a schematic diagram of friction and wear, and Figure 4a was the experiment equipment. For the convenience of the experiment, the finishing surface was subjected to wire cutting, and Figure 4b was a wire-cut sample to be rubbed. Additionally, Figure 4c was a front view and Figure 4d was a left side view. The experiment was carried out at room temperature, and the pair of grinding balls were bearing steel (GCr15, hardness was 36-40 HRC), and the hardness was higher than that of titanium alloy. The load applied on the contact surface was 5 N, the reciprocating sliding speed was 8 mm/s, the sliding distance was 4 mm, and the friction time was 15 min or 30 min, respectively. Finally, the surface wear microtopography and subsurface microstructure were observed by scanning electron microscope. Considering that the width of the wear band is relatively small, less than 1 mm, when observing the worn surface layer, the observation was made along the cutting direction.