## 4 Experimental Results and Discussion
### Analysis of Machined Surface Topography  
A comparison of the surface morphology is shown in Figure 12, where Figure 12a shows the result of the chromizing surface, Figure 12b-d shows the surfaces after the ultrasonic extrusion and the composite process. Here, the processing parameters of Figure 12b are \(n=160\) rpm, \(f=0.06\) mm/r and \(P=10\) um, the processing parameters of Figure 12c are \(n=240\) rpm, \(f=0.08\) mm/r and \(P=10\) um, and the processing parameters of Figure 12d are \(n=240\) rpm, \(f=0.1\) mm/r and \(P=20\) um.  
The surface after the chromizing treatment is extremely uneven and rough with prominent gullies, as shown in Figure 12, and the surface of the stainless steel is smoother after the ultrasonic extrusion process. It can be seen from Figure 12a,b that there are micro surface protrusions and depressions on the rough surface, as well as many microdefects, including microscopic cavities, microscopic pull marks and microscopic double skin. Although the  
Figure 12: Comparison of the surface morphology. (**a**) Chromizing surface. (**b**) Ultrasonic extrusion. (**c**) Composite process. (**d**) Composite process. Parameters: (**b**) \(n=160\) rpm, \(f=0.06\) mm/r and \(P=10\) μm; (**c**) \(n=240\) rpm, \(f=0.08\) mm/r and \(P=10\) μm; (**d**) \(n=240\) rpm, \(f=0.1\) mm/r and \(P=20\) μm.  
micro surface protrusions and depressions in Figure 12b are not obvious, the surface still regularly arranges the micro surface protrusions and depressions because the stainless steel matrix displays low strength and high plastic fluidity compared with the chromizing matrix.  
The surface obtained by the composite process is shown in Figure 12c,d. Compared with the chromizing surface, its microscopic surface is much smoother, but there are still many bulges although the distribution of chromizing layer is not uniform, causing the uneven distribution of the skin thickness formed by the extrusion. As shown in Figure 12c,d, the microdefects on the surface machined by the composite process will also be reduced, because the plastic flow will eliminate the microdefects.  
Compared with the results shown in Figure 12a,c,d, the micro surface protrusion on the chromizing surface is plastic-deformed by ultrasonic extrusion and flattened and filled into the micro surface depression, leading to the slight surface roughness after ultrasonic extrusion. The ultrasonic vibration energy not only promotes the plastic flow of the machined surface, but also reduces the defects of the machined surface, e.g., the cavity. Comparing the results depicted in Figure 12b-d, the effect of ultrasonic vibration on eliminating micro surface protrusion and filling micro surface depression of the chromizing surface is not as good as that of the stainless steel due to the high hardness of the chromizing surface, leading to high surface roughness. Compared with the results of Figure 12c,d, the increase of \(P\) and \(f\) leads to more plastic deformation, and the surface flatness decreases. In addition, some of the micro surface protrusion on the surface is extruded higher by the extrusion of the micro surface depression. With the decreasing \(f\), the same length of the machined surface receives more ultrasonic impact, which makes the microscopic surface morphology smoother, and the microscopic defects are reduced accordingly.