Theoretical and experimental investigations of ultrasonic sound fields in thin bubbly liquid layers for ultrasonic cavitation peening.

Ultrasonic cavitation peening is a potential surface enhancement process. During this process a high input power is necessary to obtain an effective process result. A small gap, usually less than 1 mm, between the sonotrode tip and the treated surface is also required to avoid substantial energy loss. Due to the high vibration of the sonotrode, many cavitation bubbles are generated, forming a thin bubbly liquid layer in the small gap. The cavitation bubbles in the layer seriously disturb the sound wave propagation and interact with each other. The disturbances and interactions change the intensity and the spatial distribution of cavitation bubbles, resulting in the different interactions between cavitation bubbles and workpiece surfaces. The variations of the interactions cause different surface properties of the workpieces after ultrasonic cavitation peening. Therefore, quantifying the ultrasound field in different conditions is of great important to improve the ultrasonic cavitation peening process. A current model of the sound propagation in the bubbly liquid was already developed but did not include the bubble interactions. In this work, the bubble interactions are taken into account to improve the current model. The calculated results of the sound field with the improved model are validated by sonochemiluminescence experiments in various standoff distances and vibration amplitudes. Both of the experimental and the calculated results show that the highest sound pressure is generated when the vibration amplitude is around 25 µm. The strongest cavitation intensity occurs at the gap width of 0.5-0.7 mm.