# Effect of Surface Micro-Hardness Change in Multistep
## 2 Cutting Materials and Experimental Preparation  
The titanium alloy Ti-6 Al-4V was used as the experimental material, and the chemical composition and mechanical properties of titanium alloy are shown in Tables 1 and 2, respectively. The microstructure of Ti-6 Al-4V alloy is exhibited in Figure 1, and the grey equiaxed \(\alpha\)-Ti phase (hcp) and white \(\beta\)-Ti phase (fcc) shown in Figure 1a composes the \(\alpha+\beta\) two-phase titanium alloy. The titanium alloy workpiece used in turning experiments was a bar dimension of \(\mathcal{O}\) 100 mm \(\times\) 25 mm. The experiments were conducted on a CKD6150H turning lathe under dry cutting conditions, and the experimental setup is shown in Figure 1b. A MSBNR2525M12 tool holder and the TiAlN-coated ultrafine grain WC-Co cemented carbide tools made by Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., Zhuzhou, China was used in the multistep turning experiments. The type of roughing tool was SNMG120408-EF, and the type of semi-finishing and finishing tools were both SNMG120404-EM. The two kinds of tool inserts are both square shaped inserts with a TiAlN coating (type YBG202), double-side chip breaking groove, a 12.7 mm length and width, and a thickness of 4.76 mm. The corner radii of SNMG120408-EF insert and SNMG120404-EM insert are 0.8 mm and 0.4 mm, respectively. The YBG202 type TiAlN coating is combined with the high strength and toughness tool substrate of ultrafine grain WC-Co carbide, which is suitable for finish and semi-finish turning of all kinds of processed materials and rough turning of superalloys. After the tool was installed in the tool holder, the working rake angle was 14\({}^{\circ}\) and the clearance angle was 11\({}^{\circ}\). The final machined surface was obtained by two-step machining (firstly roughing, and finally finishing) and three-step machining (firstly roughing, then semi-finishing, and finally finishing). In the two-step machining, the roughing cutting parameters were variable, as shown in Table 3, the finishing cutting parameters were fixed, the machining was conducted at the cutting speed of 100 m/min, feed rate of 0.05 mm/r, and depth of cut of 0.1 mm. In the three-step machining, the cutting parameters were all fixed, and the roughing machining was carried out at the cutting speed of 50 m/min, feed rate of 0.25 mm/r, and depth of cut of 1.0 mm, the semi-finishing cutting parameters were set at the cutting speed of 80 m/min, feed rate of 0.15 mm/r, and depth of cut of 0.3 mm. In the three-step machining, the finishing cutting parameters were the same as those in the two-step machining. The experimental trials under all cutting conditions were repeated and verified on the condition that fluent chip breaking effect and stable chip formation process was acquired. At the same time, in order to reduce the influence of tool wear on the machining, a new tool would be used each time the machining was performed. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Element** & **Ti** & **Al** & **V** & **Fe** & **C** & **N** & **H** & **O** & **Other** \\ \hline Weight (wt.\%) & base & 5.5–6.75 & 3.6–4.5 & 0.3 & 0.08 & 0.05 & 0.15 & 0.10 & 0.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of titanium alloy Ti-6 Al-4V. Figure 1: Workpiece material and experimental setups. (**a**) Microstructure of Titanium alloy. (**b**) The cutting experiment equipment. Using the HVS-1000 Vickers micro-hardness instrument, the micro-hardness at 100 \(\upmu\)m below the surface after roughing and the finishing surface micro-hardness was measured along the feed direction. Figure 2 was a schematic diagram describing the change of micro-hardness in the local area of surface layer. Among them, _abcd_ was roughing surface, _ef_ was circumferential curve, and _hg_ was roughing tool path curve. Meanwhile, ABCD was the obtained finishing surface when the depth of cut _a\({}_{p}\)_ was 0.1 mm, EF was circumferential curve, and HG was finishing tool path curve.