Abstract: Water cavitation peening is a new surface strengthening method with the energy released from cavitation clouds collapse, which are generated in cavitation water jet to impact the surface of metal parts. However, there is currently no basis for the standoff distance selection. The cavitation water jet experiment is performed with the high-speed camera for photographing to analyze the dynamic evolution law of cavitation cloud by FDM (frame-difference method). The concentrated collapse range of cavitation is obtained. The water cavitation peening test for 2A12 aluminum alloy is carried out to study the effect of different standoff distances on the surface properties of the materials with different nozzles diameters d. The results show that the cavitation cloud has obvious periodicity including incubation, developing, shedding and collapse phases in one cycle. The concentrated collapse range of cavitation bubbles is 60d-75d. The optimal standoff distance for water cavitation peening should be 60d-67.5d, but over peening occurs at a standoff distance of 120 mm while d=1.8 mm. The cavitation water jet can be divided into the droplet action, the mixed action, and the cavitation action zones according to the standoff distance.

Key words: cavitation cloud, dynamic change, FDM, nozzle diameter, standoff distance, water cavitation peening