# A Novel Ultrasonic Cavitation Peening Approach Assisted by Water Jet
## 1 Introduction  
In industrial applications, highly stressed components are easily damaged by cyclic loads if there is no surface hardening [1]. Utilizing the peening process, the residual tensile stress of manufactured workpiece can be removed and the surface hardness can be increased [2], leading to the extension of the working life of the workpiece. Peening processes can be mainly divided into two types: shot peening and shotless peening. Shot peening has been widely used in industrial applications [3] due to inexpensiveness and easy operation. However, after shot peening the high surface roughness and the surface deterioration are generated. To avoid these disadvantages, shotless peening processes, such as laser shock peening [4], water cavitation jet peening [5] are developed. The shotless peening processes require skilled operators and complicated equipment. To achieve good surface properties after peening treatment with simple equipment, traditional ultrasonic cavitation peening process has been developed and reported in recent decades. In 1987, [PERSON] et al. [6] first attempted to develop an ultrasonic device for metal working and surface treatment. Each piece was exposed to ultrasonic cavitation with the standoff distance (gap width between sonotrode and specimen) of 0.3 mm and at a resonance frequency of 15 kHz. As a result, both the hardness and the erosion resistance were improved. At the same time, the fatigue limit increased by about 11%. [PERSON] et al. [7] found that the plastic deformation of the material is mainly introduced by shock waves. They also found that the generation of residual stress caused by ultrasonic cavitation is not significantly different from the changes caused by flow cavitation. The effect of water temperature on residual stress was studied with the temperature interval of 15 \(\,{}^{\circ}\)C. The results show that when the temperature is less than 65\(\,{}^{\circ}\)C, the values of residual stress are more or less identical [8]. [PERSON] et al. [9] studied the effects of temperature and vibration amplitude on the damage behaviour in seawater using ultrasonic cavitation as well. They presented that the higher temperatures were associated with higher corrosion. [PERSON] [10] tried to address the burrs and induce residual stress after machining utilizing ultrasonic cavitation peening. It was found that sonication at 40 kHz has the most significant influence on reducing the formation of burr height. However, the best surface qualities are time dependent and the time has to be determined experimentally for every workpiece [11]. To investigate effects of the fatigue properties, a stainless-steel sheet with the thickness of 0.1 mm was treated by ultrasonic cavitation peening [12]. During this peening process, the fatigue limit increased by about 10% while the surface roughness slightly decreased as a result of the peening. Later, they used a piezoelectric force sensor to study the effect within the standoff distance range of 0.5-4 mm [13]. It was found that the impact force caused by the collapses of cavitation bubbles was independent of the vibration amplitude and the sonotrode with the concave of 4 mm radii generated the largest impact. A block type horn designed to peen stainless steel workpiece surface was developed by [PERSON] et al. [14]. The area of the compressive residual stress on the stainless steel workpiece is larger but the stress distribution was not homogenous. [PERSON] et al. [15] found that with the vibration amplitude of approximately 20\(\,{}^{\circ}\)m the hardness of stainless steel and nickel alloy surfaces increases by -18% and -20%, respectively. [PERSON] et al. [16; 17] found that when the standoff distance is smaller than 1 mm, the influence of cavitation bubbles on transducer is less than that at other standoff distances and there is an optimal standoff distance corresponding to the input power. The improvements of the surface hardness of stainless steel and aluminium alloy by ultrasonic cavitation peening were investigated as well [18].