## 3 Experimental Planning and Approach  
This study investigates the cutting performance of HEAs with different elemental ratio workpieces using different process parameters subjected to various ultrasonic-assisted milling techniques. The relevant side milling experiments are divided into two phases. In the first phase, milling experiments are conducted aiming to investigate the cutting performance and identify the better ultrasonic-assisted milling technique, in which milling without assistance, single-axis ultrasonic (z-axis), dual-axis ultrasonic (xy-axis), and triple-axis (xyz-axis) assistances are combined with two designated sets of process parameters. The second phase adopts the most effective ultrasonic assistance based on the results from the first phase and conducts additional milling experiments by using an L\({}_{9}\) orthogonal array to arrange the process parameter combinations. Experiments are performed with three elemental ratios in conjunction with each parameter combination from the L\({}_{9}\) orthogonal array, during which the surface roughness, cutting force, surface morphology, and cutting tool wear of HEAs are investigated. Table 1 shows the HEAs with different elemental ratios and Table 2 shows the specifications of the end mill employed in experiments. The axial depth of cut, a\({}_{p}\), is set as 10 mm in these two phases. Figures 2 and 3 show photos of the high-entropy alloy workpiece and cutting tool employed in the experiments, respectively. Figure 1: Schematic diagram of the ultrasonic-assisted cutting model. The investigations of the cutting performance of HEAs with different elemental ratios subjected to different ultrasonic-assisted milling techniques using two designated set process parameters, Set 1 and Set 2, are presented in the first phase, in which the levels of radial cutting depth and feed rate for Set 1 are all less than those of Set 2, namely the lower and higher cutting conditions are constituted accordingly. Each HEA is subjected to various ultrasonic-assisted milling techniques under lower and higher conditions constituting a total of eight milling experiments, resulting in a total of 24 set experiments in the first phase finally. This process parameter planning is shown in Table 3.  
Through the L\({}_{9}\) orthogonal array, side milling experiments were conducted on three different elemental ratio workpieces of HEAs subjected to an effective ultrasonic assistance, and the milling performances with different process parameter combinations are thus investigated. Each HEA undergoes nine sets of milling experiments, resulting in a total of 27 experiments. The related process parameter planning in the second phase is shown in \begin{table}
\begin{tabular}{c c c c c c} \hline
**at.\%** & **Fe** & **Co** & **Ni** & **Cr** & **Mn** \\ \hline FeCoNiCrMn\({}_{\approx 0.1}\) & 24.39 & 24.39 & 24.39 & 24.39 & 2.44 \\ \hline FeCoNiCrMn\({}_{\approx 0.5}\) & 22.22 & 22.22 & 22.22 & 22.22 & 11.11 \\ \hline FeCoNiCrMn\({}_{\approx 1.0}\) & 20 & 20 & 20 & 20 & 20 \\ \hline \end{tabular}
\end{table} 
Table 1: The HEAs with different elemental ratios. \begin{table}
\begin{tabular}{c c c c c} \hline
**Material** & **Flute** & **Diameter** & **Cutting Length** & **Overall Length** \\ \hline Tungsten carbide steel & 3 & 6 mm & 15 mm & 50 mm \\ \hline \end{tabular}
\end{table} 
Table 2: Specifications of end mill. Figure 3: Photo of the end mill. \begin{table}
\begin{tabular}{c c c} \hline
**Set** & **Spindle Speed, \(n\) (rpm)** & **Feed Rate, f (mm/min)** & **Radial Depth of Cut, a\({}_{\text{e}}\) (mm)** \\ \hline
1 & 9000 & 7000 & 0.3 \\ \hline
2 & 9000 & 8000 & 0.5 \\ \hline \end{tabular}
\end{table} 
Table 3: Process parameter planning in the first phase experiment. Figure 2: Photo of the HEA workpiece.