New water jet cavitation technology to increase number and size of cavitation bubbles and its effect on pure Al surface

Currently, a wide variety of alloying elements are added to Al to strengthen it, which complicates the recycling process at the end of the product life. In this study, we focused on water jet cavitation (WJC) as a means to harden pure Al. When this treatment is performed, a peening effect can improve the hardness near the surface and apply a compressive residual stress, thus improving the fatigue strength. However, WJC processing requires a high-water pressure, which generally means the use of large, expensive pumps. In this study, we developed a supplemental nozzle that increases the number and size of cavitation bubbles in the discharge from a water jet nozzle. In addition, we evaluated the effect of longer WJC processing time on the pure Al surface. Conventional WJC nozzles produced erosion-forming air bubbles at the jet center. When WJC processing was performed with the new swirling flow nozzle, the increased cavitation weakened the influence of the erosion-forming bubbles. In fixed-point processing of pure Al with the new nozzle, the specimen surface underwent various cavitation erosion processes that formed a sponge-like structure, a surrounding layered structure, and detached particles. When the processing time was increased, the affected area was extended and it had more of a peened appearance.