Enhancement of cavitation aggressivity around a cavitating Jet by injecting low-speed water jet for cavitation shotless peening

Abstract

Cavitation impact at cavitation bubble collapse can be utilized for surface enhancement in the same way as shot peening. A peening method using cavitation impact is called cavitation peening. In the case of cavitation peening, cavitation bubbles were produced by injecting a high-speed water jet into water, i.e., a cavitating jet. In order to improve effect of cavitation peening, enhancement of cavitation aggressivity around a cavitating jet is required. In the present paper, a lowspeed water jet was injected around a cavitating jet to increase cavitation aggressivity, as the low-speed water jet eliminate residual bubbles after cavitation bubble collapse. The residual bubble causes cushion effect. The cavitation aggressivity was evaluated by an erosion test using aluminum specimen to assume that large mass loss revealed large aggressivity. The injecting condition of the low-speed water jet was optimized by the erosion test. The arc height which was height of convex curve of Almen strip was also evaluated, as the arc height was normally used to evaluate peening intensity of shot peeing. The convex curve was produced by peening as the peened surface was stretched due to plastic deformation. The peening effect was investigated by measurements of residual stress and a plate bending fatigue test using stainless steel specimen. It was revealed that a maximum cumulative erosion rate was increased about 70 % by injecting the low-speed water jet around the cavitating jet at optimum condition. The increasing rate of the arc height induced by the cavitating jet with the low-speed water jet was about five times larger than that of the cavitating jet without the low-speed water jet. The fatigue strength of stainless steel specimen was increased 29 % and 17% peened by the cavitating jet with and without the low-speed water jet comparing to that of non-peened specimen, respectively.