Effect of nozzle size scaling in co-flow water cavitation jet peening

Water Cavitation Peening (WCP) employs cavitation to introduce compressive residual stresses in metals while limiting the surface roughening typical of similar surface enhancement processes such as shot peening and laser shock peening. As shown by a number of authors, cavitation intensity is greatly affected by the jet velocity as well as by the nozzle dimensions and shape. This paper reports on an investigation how nozzle dimensions affect cavitation intensity and peening performance in co-flow WCP of Al 7075-T651. Scalability of the process is investigated by comparing co-flow nozzles of increasing size but the same diameter ratios. The results show a substantial increase in cavitation intensity with nozzle size and a considerable decrease in the processing time required for saturation of the strip curvature and residual stress. The observed trends are explained by means of high-speed video imaging analysis and pitting tests, which show that the increase in cavitation intensity is due to an increase in the amount of cavitation when nozzle dimensions increase, while the pit depths and pit shape factor, which are measures of the strength of the cavitation events occurring at the workpiece surface, are largely unaffected. The process yields compressive residual stresses as high as 400 MPa and as deep as 350 μm below the surface, which are both a significant improvement upon previous results reported for shot peening.