# A Novel Ultrasonic Cavitation Peening Approach Assisted by Water Jet
## Working Theory  
During ultrasonic cavitation peening, shock wave and micro-jet are generated on the treated surfaces due to the collapse of bubbles. Generally, small standoff distance is beneficial for the treatment process as a result of less attenuation. When the standoff distance is very small, the water stays in the small gap and surrounds the tip of the sonotrode due to surface tension of the water. If the sonotrode vibrates up and down, a great many cavitation bubbles are generated in the gap. Based on this theory, the workpieces no longer need to be put in a water container during the novel ultrasonic cavitation peening process. To compensate for the water loss during cavitation, a water jet is utilized to provide the necessary water. With the constant supply of the water, the small gap will be full of water and the tip end of the sonotrode is surrounded by a water film even when the sonotrode works for a long treatment time. Since the water jet is only utilized for assist, slow velocity and low pressure of the water jet is required during the novel ultrasonic cavitation peening process. Here, tap water can be used directly, which means that no additional equipment is required to generate the water jet. During the novel ultrasonic cavitation peening process, many cavitation bubbles are excited in the small gap by the vibration of the sonotrode. After the collapses of the bubbles, micro jets and shock waves are produced, which leads to the generation of impact loads on the specimen surfaces. Due to the impact loads, plastic deformation occurs on the treated surfaces. Consequently, hardness and compressive residual stress are significantly enhanced as desired.  
To the best of our knowledge, this is the first time that ultrasonic cavitation assisted by water jet has been utilized to treat workpiece surfaces. This novel process has two main advantages: there are no limitations on workpiece size and geometry since the sonotrode can treat any surface that it can reach; a temperature control is not necessarily due to the continuous supply of water. The temperature should be kept nearly the same temperature as that of the injected water. Since the speed of the injected water is very slow, the tap water can be used directly.  
In order for the ultrasonic cavitation bubbles to be confirmed visually, photos of the small gaps were recorded during the process. The gaps can be considered as the working areas which are marked in red frames. As shown in Figure 1a, when the sonotrode does not vibrate, the small gap is full of tap water and very bright. When the sonotrode vibrates up and down, the working area is very dark due the reflection and refraction of light, which is shown in Figure 1b. This is because that there are thousands of cavitation bubbles as the barriers of light propagation in the gap. Moreover, Figure 1c,d show the cases of no vibration loading and under vibration loading assisted by water jet, respectively. It can be seen clearly that the working area without vibration loading is much brighter than that under vibration loading. Therefore, it can be deduced that a great many cavitation bubbles are generated in the gap under vibration loading.