Shock wave and microjet produced by bubble collapse in cavitation ultrasonic surface rolling

The introduction of the cavitation effect in ultrasonic surface rolling can further improve the strengthening effect. However, the mechanism of the time-space bubble collapse in cavitation ultrasonic surface rolling on the material wall is still uncertain. Therefore, in this study, both the shock wave produced by spherical bubble collapse and the microjet generated by non-spherical bubble collapse in cavitation ultrasonic surface rolling were investigated. A dynamic model of spherical bubble collapse in the ultrasonic surface rolling area was established, and the shock wave pressure on the wall surface produced by the collapse was analyzed numerically. The numerical results reveal that the shock wave pressure is significantly influenced by the ultrasonic amplitude. As the ultrasonic amplitude increased from 2 to 5 μm, the maximum collapse pressure of the bubble rose from 4700 to 38690 MPa, while the wall pressure increased from 75.07 to 416.07 MPa. Additionally, the pressure distribution on the wall surface caused by near-wall non-spherical bubble collapse was determined using the computational fluid dynamics (CFD) analysis method. The wall pressure generated by microjets peaked with a slight delay compared to that generated by shock waves, reaching a maximum value of 20.25 MPa when the normalized standoff distance was 0.6. Finally, cavitation erosion experiments were conducted. The results showed that the wall pressure was approximately between 340 and 382 MPa when the amplitude was 5 μm, which is generally consistent with previous numerical calculation results. This indicates that the shock wave generated by the collapse of the spherical bubble plays a dominant role in cavitation ultrasonic surface rolling. These research findings are crucial for further studies on the formation of residual stresses and microstructure evolution in cavitation ultrasonic surface rolling.