## 4 Results and Discussion
They all increased for various Mn ratios in the dual-axis situation and it increased only for the Mn ratio of 1.0 with triple-axis assistance, while for higher cutting conditions (Set 2), the cutting tool wear decreased with all Mn elemental ratios using single-axis ultrasonic assistance but exhibited an increasing trend for all ratios with dual-axis and triple-axis assistance, except for the Mn elemental ratio of 0.1 in triple-axis assistance which showed a decrease, as compared with milling without assistance. Figures 16-19 show the machined surface morphology of the HEAs with Mn elemental ratios of x = 0.1 and x = 1.0, respectively, using the Set 1 and 2 process parameters in the first phase experiment subjected to different ultrasonic-assisted milling techniques. It can be observed that using different cutting parameters and with different elemental ratios, the workpiece surface morphology in milling without assistance is coarser and exhibits poorer surface quality, indicated by red boxes in Figures 16a, 17a, 18a and 19a, as compared to those with ultrasonic assistance which demonstrate different extents of improvement in surface quality. The introduction of high-frequency vibrations in ultrasonic-assisted milling effectively reduces residual chips on the workpiece surface and minimizes heat generation, thereby mitigating the effects of plastic deformation and thermal stresses. Due to the effects  
Figure 5:0 Comparisons of cutting tool wear of HEAs with different elemental ratios using the Set 1 process parameters in the first phase experiment subjected to different ultrasonic-assisted milling techniques. Figure 5:1 Comparisons of cutting tool wear of the HEAs with different elemental ratios using the Set 2 process parameters in the first phase experiment subjected to different ultrasonic-assisted milling techniques. of ultrasonic vibration, the cutting marks generated at the tool-workpiece interface may become more uniform, resulting in fewer surface defects and an improved surface finish, surface roughness is reduced ultimately. Under lower cutting conditions (Set 1), the cutting force and heat generated during the milling processes are relatively low, resulting in less plastic deformation and thermal stresses on the workpiece surface. Therefore, under these circumstances, the workpiece exhibits higher surface finish and lower surface roughness. Moreover, tool wear, chip adhesion, and machined surface scratches caused by secondary cutting may be reduced with these machining parameters, resulting in further enhancement of surface quality. In contrast, under higher cutting conditions (Set 2), larger cutting force and heat are generated during the milling processes, leading to greater plastic deformation and thermal stresses  
Figure 5:2 Surface morphology of the HEA for x = 0.1 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. Figure 5:3 Surface morphology of the HEA for x = 0.1 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. on the workpiece surface. Under these high stresses and temperature conditions, the chips easily tend to adhere to the cutting tool face which in turn scratches the machined surface resulting in greater surface roughness and poorer surface finish as indicated by the red boxes in Figures 17c and 19a. Figure 20 shows the comparison of surface roughness, Ra, 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 sets in this phase, the surface roughness varies from 0.40 to 1.85 \(\upmu\)m for x = 0.1 while it is distributed between 0.32 to 0.44 \(\upmu\)m for x = 1.0.