# Cutting Speed and Feed Influence on Surface Microhardness of Dry-Turned UNS A97075-T6 Alloy
## 3 Results and Discussion  
Figures 4 and 5 plot the evolution of \(Ra\) and \(Rz\), respectively, as a function of the cutting parameters, \(v_{c}\) and \(f\), before (Figures 3a and 4a) and after (Figures 3b and 4b) the corrosion process. These values were calculated as the average values of the experimental results, measured in four different lines along the specimens. Additionally, the \(Ra\) and \(Rz\) values of the non-machined sample were identified in the figures, in order to analyze the induced alterations by the machining process. The results revealed that the feed was the most influential variable on surface roughness. Both parameters, \(Ra\) and \(Rz\), showed a general trend to increase with \(f\), regardless of \(v_{c}\). This trend was more noticeable from \(f\) = 0.10 to 0.20 mm/r. For samples before corrosion, \(Ra\) was three times higher for  
Figure 3: (**a**) Hardness test set-up; (**b**) generated mark in the test point. \(f\) = 0.10 mm/r than for 0.20 mm/r. For samples after corrosion, the effect of \(f\) was amplified, reaching four-fold higher values for 0.20 mm/r than for 0.1 mm/r. The feed influence was less evident in the range of low \(f\) applied (0.05-0.10 mm/r). For these low \(f\) values, \(Ra\) and \(Rz\) showed similar values to samples before machining. With regard to \(v_{c}\), no significant modifications were observed in \(Ra\) or \(Rz\) when this parameter was modified. Therefore, the effect of the natural roughness, induced by the feed, was predominant over the thermal effects that cutting speed variations may have caused during the cutting process, such as the appearance of built-up edge (BUE), which may have affected the surface quality [5; 44]. In the dry machining of this alloy, this indirect adhesion tool wear is usually a consequence of the thermo-mechanical effects at high feed. Nevertheless, the effect of cutting speed becomes more relevant for low feed, giving rise to BUE with higher intensity for higher cutting speed values. However, the low cutting speed applied in this research made this effect less noticeable and, as a result, the cutting speed effect on surface roughness was almost negligible [45; 46; 47]. It is necessary to point out that all these observations are in good agreement with previous research, in connection with the parametric analysis of surface roughness in the dry turning of the 2000 and 7000 aluminum alloys series [45; 47; 48]. Regarding the corrosion process, no significate changes in \(Ra\) were observed. However, the dispersion in the results seemed to be a little bit higher. This fact was more evident for \(Rz\). These results may be explained considering that \(Ra\) was calculated as an average value of peaks and valleys of the roughness profile, while \(Rz\) was obtained as the maximum height of the profile. The appearance of pitting and intergranular corrosion, mainly in the inter-metallic elements (Al\({}_{23}\)CuFe\({}_{4}\)), resulted in surface irregularities that may have affected the values of the microgeometrical deviations at one point (Figure 6). This effect was softened in the calculation of average values to obtain \(Ra\). In addition, an increment in \(Rz\) was observed after the corrosion process, regardless of \(v_{c}\) and \(f\). Only for the highest \(f\) (0.20 mm/r) was this increment slightly higher. A poorer surface quality, at high \(f\), resulted in a higher corrosion effect, and vice versa. Hence, \(f\) seemed to be a little more influent only in the highest range of the tested values. Figure 4: Average roughness (\(Ra\)) as a function of feed (\(f\)) and cutting speed (\(v_{c}\)), (**a**) before and (**b**) after a corrosion process. Figure 7 shows the average microhardness values as a function of \(v_{c}\) and \(f\), after and before the corrosion process, for the machined samples. In addition, the microhardness values for the non-machined samples are also plotted, as reference values.