An experimental and numerical analysis of waterjet peening of 7075-T6 aluminum alloy

Compressive residual stresses have been recognized for enhancement of high cyclic fatigue life of engineering components. One method of inducing compressive residual stresses is by bombarding the material surface with high-velocity solid shots, through a process called shot peening. Recently, the application of waterjet as an alternative to shot peening for improvement of the surface integrity by waterjet peening has been introduced. The high-pressure flow of water at a jet pressure less than 100 MPa has been used to induce compressive residual stresses at the material's surface. This method of surface treatment has produced similar or superior performance to conventional shot peening, but with greater ease in working with complex geometrical or critical sections, and significant cost savings. In this study, an experimental and numerical evaluation of the effects of waterjet peening on material properties of a 7075-T6 aluminum alloy (Al7075-T6) was explored. In contrast to earlier works in the waterjet peening area, this research employed ultrahigh pressure waterjet (jet pressure ≥100 MPa) to induce compressive residual stresses. This research characterized, optimized, and modeled the effects of waterjet peening on material properties. An analytical model was also developed to describe the relations between the waterjet peening parameters and the resulting surface/subsurface modifications. An elastic-plastic nonlinear finite element analysis was performed to predict the evolution of the induced stresses by the impact of the waterjet. In addition, a series of experiments relating the influence of high-pressure waterjet on the surface modifications and high cyclic fatigue properties of this aluminum alloy were conducted. The study showed that waterjet peening induced compressive residual stress and subsurface hardening with a comparable degree to shot peening, but with less change in surface roughness. The mathematical and numerical models first developed in this study to predict an initial range for effective waterjet peening and compressive residual stress showed good agreement between the predictions and the experimental results. In addition, waterjet peening using the optimized peening conditions improved the fatigue life of Al7075-T6 test specimens by about 25%. Results from this study will help establish ultrahigh pressure water jets as another viable application in the area of surface treatment in addition to their success in the fields of cutting, cleaning, machining, and burr removing, etc.