## 4 Results and Discussion
Hence, surface roughness is reduced from 22.03% to 314.21% for the latter composition compared with the former one. Particularly, the surface roughness is evidently higher in Sets 3, 5, and 7 regardless of the x variations. Furthermore, in these  
Figure 5:4 Surface morphology of the HEA for x = 1.0 using the Set 2 process parameters in the first phase experiment subjected to different ultrasonic-assisted milling techniques. (**a**) Without assistance. (**b**) Single-axis ultrasonic. (**c**) Dual-axis ultrasonic. (**d**) Triple-axis ultrasonic. Figure 5:5 Surface morphology of the HEA for x = 1.0 using the Set 1 process parameters in the first phase experiment subjected to different ultrasonic-assisted milling techniques. (**a**) Without assistance. (**b**) Single-axis ultrasonic. (**c**) Dual-axis ultrasonic. (**d**) Triple-axis ultrasonic. sets, the surface roughness is increased with a decrease in the x value with higher radial depths of cut. Among these sets, the most significant variation is in Set 3, where the surface roughness for x = 0.5 and x = 0.1 increases by 96.02% and 314.21%, respectively, as compared to x = 1.0, which indicates that higher radial depths of cut may lead to a significant deterioration in surface roughness. Moreover, as the spindle speed is increased, the surface roughness for different x variations shows a gradual decreasing trend. This suggests that increasing the spindle speed has a positive effect on improving the surface quality. At the higher cutting speed, the cutting processes tend to be stable, helping the surface roughness to be reduced. Additionally, it can be observed that higher feed rates result in poorer surface roughness. For example, when the feed rate is high, material removal rate and cutting force are increased accordingly, and larger surface roughness is deduced since chip removal from the workpiece becomes less effective. Variance analyses of surface roughness, Ra, for different elemental ratios are performed in this study and the factor response diagrams are illustrated in Figure 21, in which the A, B, C factors represent the spindle speed, feed rate and radial depth of cut, respectively. The factor that most significantly affects machined surface roughness is the radial depth of cut, followed by feed rate, with spindle speed having the least impact. Additionally, it can be noted that as x was varied from 0.1 to 1.0 the contribution rates of these factors, i.e., 45.08-70.51% of A, 31.52-17.26% of B, and 23.40-12.17% of C, are also changed accordingly since the presence of Mn in HEAs has a significant influence on the mechanical properties and machining behavior of the workpiece materials. Therefore, the contribution rates of spindle speed and feed rate show a slight decreasing tendency while that of the radial depth of cut shows an increasing trend as the x value is increases. The workpiece material exhibits a greater resistance to deformation when the Mn content is increased, which makes the effect of the radial depth of cut on surface roughness more pronounced. However, with the decrease in Mn content, the material becomes ductile, and the influences of spindle speed and feed rate on surface roughness are more evident. These factor response diagrams also indicate that the best process parameter combination, i.e., A\({}_{3}\), B\({}_{1}\), C\({}_{1}\), for surface roughness is higher spindle speed and lower feed rate and radial depth of cut regardless of the elemental compositions. Figure 5:6 Comparison of surface roughness using different cutting parameter combinations of the L\({}_{9}\) orthogonal array in the second phase experiment for different elemental compositions. Figure 22 shows the comparison of cutting force using different cutting parameter combinations of the L\({}_{9}\) orthogonal array in the second phase experiment for different elemental compositions. It can be seen that among all experimental sets in this phase, the cutting force is varied from 230 to 980 N for x = 0.1 while it is distributed between 180 to 700 N for x = 1.0.