## 4 Experimental Results and Discussion  
### Analysis of the Surface Roughness and Hardness  
In this study, the experimental scheme of ultrasonic extrusion process based on L\({}_{16}\) (3\({}^{\text{d}}\)) was designed [29], as shown in Table 2. Ultrasonic extrusion tests were carried out in 32 groups, including 16 groups of composite process experiments and 16 groups of conventional ultrasonic vibration extrusion experiments. The surface roughness Ra and hardness of 304 stainless steel substrates were 0.792 \(\upmu\)m and 164.84 Hv before the experiments. The hardness is measured with a Vickers hardness tester (HVS-1000Z Vickers hardness tester). The applied loading force is 20 g and the holding time is 15 s. After unloading, the Vickers hardness value of the measured point is calculated by measuring the diagonal length of the diamond indentation. Jimtec TR200 surface roughness measuring instrument is used for surface roughness. For each experimental group, the surface roughness and hardness were measured five times along the circumferential direction and displayed the averaged value as the final result. Table 2 shows the test results of the composite process and ultrasonic extrusion process. As shown in Table 2, the ultrasonic extrusion can effectively reduce the surface roughness and enhance the surface hardness of 304 stainless steel. The lowest surface roughness value and the highest surface hardness value are obtained at the spindle speed of 300 rpm, the feed of 0.04 mm/r and the static extrusion of 40 \(\upmu\)m, which are 0.065 \(\upmu\)m and 212.49 Hv, respectively. Compared with the 304 stainless steel substrates, the roughness underwent a substantial improvement, and the hardness has increased by about 28%. The average value of processed surface hardness can reach 615 Hv, and the maximum can reach 754.15 Hv after the composite process. Compared with the initial substrate surface hardness (164 Hv), the surface hardness of 304 stainless steel increased by 2.75 times. The average surface roughness value is 0.310 \(\upmu\)m, compared with the initial substrate surface roughness (0.792 \(\upmu\)m) reduced by 60.86%, indicating that the composite process can effectively improve the surface hardness and reduce the surface roughness. During the extrusion process, the extrusion tool head impacts the surface at the ultrasonic frequency and causes micro-separation while feeding along the workpiece axially at certain working parameters. The high-frequency energy is channeled into the material from the surface, causing the surface material of the extruded workpiece to be elastically and plastically deformed. The micro surface depression on the surface will be  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**No. **} & \multirow{2}{*}{**Spindle Speed \(n\)**} & \multirow{2}{*}{**Feed \(f\)**} & \multicolumn{2}{c}{**Roughness Ra (\(\upmu\)m)**} & \multicolumn{2}{c}{**Hardness (Hv)**} \\ \cline{4-9}  & & & & & & \\ \cline{4-9}  & & & & & & \\ \hline
1 & 100 & 0.04 & 10 & 0.247 & 0.067 & 520.92 & 201.58 \\
2 & 100 & 0.06 & 20 & 0.524 & 0.096 & 486.59 & 197.03 \\
3 & 100 & 0.08 & 30 & 0.373 & 0.116 & 601.00 & 206.28 \\
4 & 100 & 0.1 & 40 & 0.292 & 0.131 & 578.12 & 189.77 \\
5 & 160 & 0.04 & 20 & 0.297 & 0.111 & 612.99 & 204.82 \\
6  
filled up by its plastic deformation after ultrasonic extrusion, causing the metal part to achieve a more desirable surface [30]. The surface roughness and hardness of the two processes are compared, as shown in Figure 8. The surface roughness values of the two processes remain at a low level, as shown in Figure 8a. The surface roughness of the composite process is higher than ultrasonic extrusion; there are two reasons for this. One is that the high hardness after low-temperature chromizing penetration decreases the effect of ultrasonic vibration extrusion and plastic deformation requires more energy. Another is that chromizing will increase the roughness of the unfinished surface.