# Cutting Speed and Feed Influence on Surface Microhardness of Dry-Turned UNS A97075-T6 Alloy
## 2 Materials and Methods  
Several dry-turning tests were carried out in order to evaluate the cutting parameter influence on the surface microhardness of the aluminum alloy UNS A97075-T6. The tested alloy composition (% mass), obtained by arc atomic emission spectroscopy (AES), is shown in Table 1.  
Cylindrical bars (diameter, \(D\) = 20 mm; length, \(L\) = 110 mm) were used as stock geometry to manufacture the specimens. The sample final geometry is shown in Figure 1a. A step (25-mm length) was machined at the end of the specimen, in order to ensure a proper grip to the chuck (Figure 1b). A length of 65 mm was turned as the microhardness measurement area. The starting diameter was reduced to 17 mm in two stages. Firstly, a roughing operation was performed to reach 18 mm; then, a finishing operation was carried out to reduce the diameter to 17 mm. A new tool was used in this last operation, to ensure the same initial conditions. The machining operations were carried out in a CNC turning center. Different cutting parameter values were used to perform the finishing step. Their values are shown in Table 2. The cutting depth (\(a_{p}\)) remained constant in every test. To evaluate the thermo-mechanical effects of the cutting parameters on the surface micro-hardness, several values of cutting speed (\(v_{c}\)) and feed (\(f\)) were used. As previously commented, it is necessary to emphasize that, although this alloy is not usually machined with low cutting speed values, its use hybridized with other materials forces the use of low values. The application of this cutting speed range, together with the analysis of the feed influence, is one of the main novelties of this work. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Zn** & **Mg** & **Cu** & **Cr** & **Si** & **Mn** & **Al** \\ \hline
6.03 & 2.62 & 1.87 & 0.19 & 0.09 & 0.07 & Balance \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Evaluated aluminum–zinc alloy UNS A97075-T6 composition (% weight). Figure 1: (a) Sample geometry; (b) measurement area (all dimensions in mm). Hence, 12 samples were machined (one for each cutting parameter combination). The used cutting tools were uncoated WC-Co inserts, with ISO reference DCMT 11T308-14 IC, a tool-tip angle of 55\({}^{\circ}\), and a tip radius of 0.8 mm. In order to analyze the surface roughness influence on microhardness (before and after a corrosion process), the roughness profile was obtained. It is necessary to point out that the evolution of the microgeometrical deviations as a function of the cutting parameters, in the dry machining of this alloy, was widely studied. Nevertheless, in this work, the microgeometrical deviations were measured with a double purpose. On one hand, they may be used as an element of control and comparison with previous research. On the other hand, they could be useful to analyze possible synergistic actions between cutting parameters, hardness, surface roughness, and corrosion behavior. For this purpose, a Mitutoyo portable roughness tester, model Mitutoyo SURFEST SJ-210, was used. The average roughness (\(Ra\)) and the maximum height of the profile (\(Rz\)) were used as parameters to evaluate the surface quality (Equations (1) and (2)). \[Ra=\frac{1}{lr}\int_{0}^{lr}|Z(x)|dx, \tag{1}\]  
\[Rz=Zp+Zv, \tag{2}\]  
where \(lr\) is the sampling length, \(Z(x)\) is the profile height, \(Zp\) is the peak height, and \(Zv\) is the peak valley of the roughness profile. Two measures were obtained along two different areas. The measurement was repeated along four specimen generatrix (G1-G4, 90\({}^{\circ}\) apart; Figure 2). Therefore, eight roughness profiles were obtained for each specimen. The surface roughness measurement was carried out according to the UNE-EN ISO 4288: 1998 standard [42]. The used parameters are indicated below. * Filter: normal Gaussian distribution;
* Measurement or scan length (\(ls\)): 2.5 \(\upmu\)m;
* Basic or contact length (\(lc\)): 0.8 mm. A Vickers hardness test was performed on each specimen, according to the UNE-EN ISO 6507-1: 2018 standard [43].