## 5 Discussion  
From the aluminum foil experiments, it can be seen that the higher driving voltage caused the larger damaged area. The reason was that the higher driving voltage resulted in a higher output power, leading to the generation of more cavitation bubbles on the inner wall of the ring sonotrode. Meanwhile, more cavitation bubbles were generated on the surface of the metallic rod. On the other side, between the driving voltages of 150 and 190 V, the damaged area increased first to a threshold and then decreased. The maximum damaged area corresponding to the diameter of 16 mm was obviously larger than that at other diameters. At the diameter of 16 mm, the gap between the ring sonotrode and the metallic rod was 2.75 mm. At this gap, most of the cavitation bubbles were generated on the rod surface, even at different driving voltages. At large gaps (smaller rod diameter), although the number of the generated cavitation bubbles were the same as the gap of 2.75 mm, the number of the available cavitation bubbles on the rod surface was smaller. This was because the attenuation of the sound pressure increased due to the bubble liquid and there was not enough sound pressure to generate more cavitation bubbles on the surfaces of the rods. At a larger rod diameter, the small gaps limited the dynamics of cavitation bubbles, leading to the change of impacts on the treated surfaces compared to the collapse of bubbles without limitation. When the gap became smaller, the cavitation bubble was no longer spherical if other operation conditions were not changed. Due to this limitation, the impact amplitude on the treated surfaces was reduced.  
The average light intensity increased with the increase of the driving voltage, since more power caused more violent cavitation intensity. The strongest light intensity occurred at a diameter of 16 mm. Similar to the descriptions in the experiments of damaged aluminum foils, at the optimal working gap, there was a boundary point between the attenuation of the sound pressure and the limitation of the bubble growth and collapse. Hence, for this novel ultrasonic cavitation peening process, the working gap of about 2.75 mm can be said to be optimal for the treatment of the cylindrical workpieces. Thus, the rod diameter of 16 mm and the driving voltage of 190 V were selected as a set of optimal parameters for the cavitation processing. After treatments, the hardness with the diameter of 16 mm increased by about 36% without significant increase of the surface roughness. From previous investigations [13; 14], we can deduce that the residual compressive increased as well.  
\begin{table}
\begin{tabular}{c c} \hline
**Diameter** & **Average \(R_{a}\) (\(\mu\)m)** \\ \hline
15 mm & 2.24 \(\pm\) 0.56 \\
16 mm & 2.86 \(\pm\) 0.68 \\ \hline \end{tabular}
\end{table}
Table 2: The average surface roughness after ultrasonic cavitation peening.  
\begin{table}
\begin{tabular}{c c c} \hline
**Diameter** & **Original Micro Hardness (HV)** & **Average Micro Hardness (HV)** \\ \hline
15 mm & 40.05 \(\pm\) 2.56 & 50.43 \(\pm\) 3.53 \\
16 mm & 41.45 \(\pm\) 2.68 & 56.46 \(\pm\) 4.32 \\ \hline \end{tabular}
\end{table}
Table 3: The micro hardness of the surface rods before and after cavitation peening.