Effect of jet velocity in co-flow water cavitation jet peening

Abstract

Water cavitation jet peening (WCP) uses cavitation caused by the shear layer created by two concentric co-flowing jets with large velocity difference to introduce compressive residual stresses in the surface layers of metal components subjected to fatigue loading or a corrosive environment. Mass loss and surface alteration in WCP have been shown to be minimal compared to other mechanical surface enhancement techniques, such as shot peening (SP). This paper investigates the effect of concentric jet velocities in cavitation jet peening in a co-flow configuration on cavitation intensity and peening performance, which are characterized by accelerated erosion on Al 1100-O and Al 7075-T6 and a strip curvature test on Al 7075-T6. Accelerated erosion tests reveal that cavitation intensity and associated erosion (measured by mass loss) are greatly affected by the combination of the inner (Vin) and outer (Vout) jet velocities and the normalized standoff distance (sn). Two characteristic erosion patterns are found depending on the relative magnitudes of the jet velocities: one that is focused at the jet center (termed center regime) and another that is concentrated in the surrounding annular region (termed ring regime). Erosion tests on Al 1100-O and Al 7075-T6 give unexpectedly different results in terms of the maximum mass loss as a function of the jet velocities and standoff distance. When compared to strip curvature tests, it is found that the accelerated erosion tests on Al 1100-O do not capture the influence of inner jet velocity Vin and imply misleading trends with regard to outer flow velocity Vout. Erosion and curvature tests on Al 7075-T6 are found to be in good agreement and therefore are believed to be better suited to identify the optimum process conditions in WCP. Notwithstanding the higher mass loss density values observed in the center regime, the resultant strip curvature is found to be higher in the ring regime for a higher inner jet velocity Vin, potentially leading to higher and deeper compressive residual stresses.