LASER INDUCED SHOCK WAVES AS SURFACE TREATMENT FOR 7075–T7351 ALUMINIUM ALLOY

Abstract

A novel mechanical surface treatment using laser induced shock waves has been applied to the wrought aerospace aluminium alloy 7075–T7 351 to improve its fatigue behaviour. Optimisation of the shock parametersand mechanical effects produced by the treatment have been investigated and fatigue performance is compared with that of conventionally shot peened specimens. It is shown that use of a one-dimensional Lagrangian hydrodynamic shock wave analysis code allows the attenuation of shock waves to be simulated and the depth of the plastically affected zone to be predicted. An analytical model developed to predict residual stress levels gave good agreement with experimental values. The fatigue limit of laser shocked specimens was found to be higher than that following shot peening. This large increase (22%) appeared to result from the high levels of residual stress and the greater extent of the stress field in the laser treated specimens and, most importantly, from the preservation of the surface condition. All these factors greatly increased the duration of the crack initiation stage of fatigue, as was confirmed by ac potential drop measurements.