# Sustainability-Based Analysis of Conventional to High-Speed Machining of Al 6061-T6 Alloy
## 5 Analysis of Experimental Data: High-Speed Machining (HSM)
### Specific Cutting Energy (SCE)  
A general trend of reduced SCE values was observed for HSM. In order to assess the effect of cutting parameters on SCE, analysis of variance has been performed (results shown in Table 13). ANOVA results depict a trend similar to CSM and TSM. Feed has been observed to be the most significant cutting parameter affecting SCE. Depth of cut and cutting speed have been observed to have a lesser effect (around 14-17%) on SCE. A holistic picture of the impact of cutting parameters on SCE in HSM can be obtained by main effect plots as shown in Figure 8.  
The impact of cutting speed on SCE in HSM was seen to be fairly different as compared to CSM and TSM (Figure 8). SCE consumption was seen to reduce initially, but it increased after the cutting speed exceeded 2000 m/min. The initial drop in SCE can be attributed to the high plastic strain associated with HSM [48]. This high strain generates heat in the primary shear zone that is not completely dissipated through the formed chip due to less contact time in HSM. This leads to a quasi-adiabatic condition, resulting in the thermal softening of the workpiece. Thermal softening is the major reason for reduced cutting forces and specific cutting energy in high-speed machining [42].  
The cutting forces gradually decrease in HSM with increase in cutting speed. This gradual reduction in cutting forces continues up to a certain cutting speed. Beyond that cutting speed, cutting forces tend to increase [42; 49; 50]. The rise in SCE beyond the cutting speed of 2000 m/min can be attributed to the increase in cutting forces beyond this speed. Thus, the SCE consumption trend presents a parabolic curve in HSM, signifying the presence of an optimal cutting speed at which SCE consumption is minimum. Chip morphology was also studied to investigate any change in the form of the chips with an increase in cutting speed. Figure 9 shows microscopic images of chips at different cutting speeds, with feed fixed at 0.4 mm/rev.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Variable** & **DOF** & **Seq. SS** & **Adj. MS** & **F-Value** & **_p-Value** & **CR (\%)** \\ \hline _v (m/min)_ & 3 & 0.100391 & 0.033463667 & 35.72 & 0.000 & 14.09\% \\ _f (mm/rev)_ & 3 & 0.4375 & 0.145833333 & 155.67 & 0.000 & 62.77\% \\ _d (mm)_ & 3 & 0.119032 & 0.039677333 & 42.35 & 0.000 & 16.78\% \\ Error & 38 & 0.035598 & 0.000936789 & & & 6.36\% \\ Total & 47 & 0.692521 & & & 100.00\% \\ \hline \hline \end{tabular}  
* Std. dev = 0.0306070; R\({}^{2}\) = 94.86%; R\({}^{2}\) predicted = 91.8%.  
\end{table}
Table 13: Analysis of variance and percentage contribution results for SCE in HSM.  
Figure 8: Main effect plots showing the influence of cutting parameters on SCE in HSM.  
A noticeable change in the form of the chips can be observed when cutting speed is increased beyond 1750 m/min. The saw-tooth form of the chip starts disappearing at 2000 m/min, and when the cutting speed is further increased to 2250 m/min, serration in the chips appears. Though completely serrated chips are not produced even at the cutting speed of 2500 m/min, it is expected that complete serration might occur if the cutting speed is further increased. High feed rates combined with high cutting speeds promote serration in chips [51]. It can be deduced from the chip morphology study (Figure 9) that serration in chips initiates during the turning of Al 6061-T6 alloys when the cutting speed is increased beyond 1750 m/min at a feed rate of 0.4 mm/rev. Overall, the chip morphology study revealed no plausible justification for high SCE at cutting speeds greater than 2000 m/min.