# A Novel Ultrasonic Cavitation Peening Approach Assisted by Water Jet
## 4 Results and Discussion
### Surface Properties after Treatment  
The capability of the impacts during the novel ultrasonic cavitation can also be evaluated from the volume loss of the treated workpiece. The volume loss can be obtained utilizing the top cover mode of the microscope. The volume loss is caused by the multi-impact from the shock waves or micro-jets. During ultrasonic cavitation peening, much plastic deformation and small mass loss are generated. The plastic deformation and the mass loss compose volume loss. Therefore, the volume loss can reflect the cavitation intensity. Figure 4 shows the volume loss by increasing the standoff distance at different vibration amplitudes. It can be seen that most volume loss occurs at standoff distances of 0.5 mm and 0.6 mm, respectively. Most volume loss increases with increasing the vibration amplitude, since a larger vibration amplitude leads to higher cavitation intensity. As a result, the volume loss is larger with a larger vibration amplitude. The difference of volume loss between the cases at vibration amplitude of 20 \(\upmu\)m and 25 \(\upmu\)m is about 0.01 mm\({}^{3}\) whereas between the cases at vibration amplitude of 25 \(\upmu\)m and 30 \(\upmu\)m is about 0.04 mm\({}^{3}\). Even as the vibration amplitude increases with a fixed interval, the volume loss increases much more. This is because the energy caused by cavitation bubbles increases in the order of three with the increase in the bubble size. Apart from the standoff distance with the most volume change, the volume loss at other standoff distance decrease when the vibration amplitude decreases. Figure 3: Load peak distribution of at the standoff distances of (**a**) 0.2 mm, (**b**) 0.6 mm and (**c**) 1.0 mm. The surface roughness is one of the important characteristics for the evaluation of the surface properties of the specimen after ultrasonic cavitation peening. Figure 5a,b show the surface parameter \(\mathrm{R_{a}}\) and \(\mathrm{R_{q}}\) after ultrasonic cavitation peening at different vibration amplitudes and standoff distances, respectively. The measurement path is along the diameter of the circular area. The values of the roughness parameters shown in Figure 5 are average values from four measurements for each specimen. It can be seen that the roughness parameter of \(\mathrm{R_{a}}\) increases with increasing standoff distance up to 0.6  mm and then decreases again. \(\mathrm{R_{a}}\) also increases with increasing vibration amplitude. The maximum value of \(\mathrm{R_{a}}\) is less than 0.9 \(\mathrm{\upmu m}\). The roughness parameter of \(\mathrm{R_{q}}\) almost keep the similar tendency as the parameter of \(\mathrm{R_{a}}\). Therefore, it can be deduced that a low impact load will cause low roughness. Microhardness can be utilized to evaluate the surface properties as well. When an impact is generated on metal grain, the grain is plastically deformed, which results in the dislocation movement and multiplication. The effect causes an increase in hardness of the grain. With the intersection of dislocations, jogs are produced and pin the dislocations. In general, it is supposed that the hardness in the plastic deformation area is greater than that in the original surface due to dislocations. During microhardness measurements, the imprinted quadrangular shape produced by the indenter is not clear enough to measure. Therefore, the load-displacement method were used. The original microhardness of the untreated specimen surface was measured as approximately HV 50. The variation of surface microhardness with error bar after different treatment conditions is shown in Figure 6. The maximum microhardness for the vibration amplitude of 20 \(\mathrm{\upmu m}\) occurs at a standoff distance of 0.6 mm, whereas the maximum hardness at the vibration amplitude of 30 \(\mathrm{\upmu m}\) occurs at a standoff distance of 0.5 mm. This slight difference is caused by the variations of the impact.