# Cutting Speed and Feed Influence on Surface Microhardness of Dry-Turned UNS A97075-T6 Alloy
## 3 Results and Discussion
In general, a surface microhardness (\(HV\)) increment was observed in all samples, after machining and before corrosion (Figure 7a), regardless of \(v_{c}\) and \(f\). Its value was increased by 25% for a certain cutting parameter combination (\(f=0.15\) mm/r; \(v_{c}=60\) m/min), regarding the non-machined sample. This fact can be explained taking into account that machining resulted in very aggressive conditions (high pressures and temperatures) in the cutting area, both in the tool and the part. The compression forces gave rise to a plastic deformation of the machined surface [49], as well as microstructural alterations (finer grain structure), which resulted in strain hardening [29] (Figure 8). Regarding the cutting speed influence on microhardness, Figure 7a shows very similar \(HV\) mean values for \(v_{c}=40\) and 80 m/min, whereas its mean value seemed to increase for \(v_{c}=60\) m/min, for the intermediate range of \(f\) (0.10 and 0.15 mm/r). In addition, higher dispersion in the experimental results was observed for this cutting parameter combination. With regard to \(f\), the best results (the highest \(HV\)) were obtained for \(f=0.10\) and 0.15 mm/r. When these values were combined with the intermediate \(v_{c}\) tested value (60 m/min), \(HV\) reached a difference of 25% compared to other cutting speed values (40 and 80 m/min). For other \(f\) (0.05 and 0.20 mm/r) and \(v_{c}\) (40 and 80 m/min), a clear \(HV\) trend as a function of the cutting parameters was not observed. Figure 5: Maximum height of the profile (\(Rz\)) as a function of feed (\(f\)) and cutting speed (\(v_{c}\)), (**a**) before and (**b**) after a corrosion process. Figure 6: (**a**) SEM image of the microstructural composition of the UNS A97075 alloy tested (before machining and corrosion processes); (**b**) SEM image of the pitting corrosion effect on the machined surface. Therefore, the highest \(HV\) results were obtained when the intermediate range of cutting speed and feed values was applied. These results may be explained considering the effect that \(f\) and \(v_{c}\) produce on the surface microstructure. On one hand, \(f\) increments usually produce a higher plastic deformation on the machined surface due to mechanical effects (strain hardening), whereas the thermal effects are usually lower [49]. Therefore, a finer grain microstructure is obtained, giving rise to \(HV\) increments. On the other hand, \(v_{c}\) increments result in higher temperature in the cutting area [28]. The produced heat is removed from the cutting area by the chip, the tool, and the part. At high cutting speeds, most of the energy is dissipated trough the chip. For low cutting speeds (such as those used in this work), the dissipated energy by the chip becomes lower; thus, the dissipated heat through the tool and the part is increased. This fact results in a grain size increment and an \(HV\) reduction [29]. When \(v_{c}\) is increased (within this low range), this effect becomes more relevant. Hence, for low \(f\) (0.05 mm/r) and \(v_{c}\) (40 m/min), mechanical effects predominated over thermal effects. The grain microstructure was finer than in the non-machined part (from 18.35 to 15.36 \(\upmu\)m average grain size), and \(HV\) was higher (Figure 8c). For the intermediate range of \(f\) (0.10-0.15 mm/r) and \(v_{c}\) (60 m/min), the mechanical effects were stronger (Figure 8b). Thermal effects were increased but were not high enough to compensate for the mechanical effects. As a result, the grain microstructure was finer (14.04 \(\upmu\)m average grain size) than for low \(f\) and \(v_{c}\), and \(HV\) was increased. Finally, for the highest \(v_{c}\) (80 m/min), the thermal effects became more relevant and compensated for the mechanical effects. Therefore, the grain size was greater and \(HV\) was reduced, reaching a  
Figure 8: SEM images of the surface microstructure: (**a**) non-machined; (**b**) \(v_{c}\) = 60 m/min and \(f\) = 0.15 mm/r; (**c**) \(v_{c}\) = 40 m/min and \(f\) = 0.05 mm/r.