Laser power density driven residual stress and surface quality of AA7075 alloy under laser shock peening

Abstract Laser Shock Peening (LSP) is an advanced surface treatment technique capable of inducing deep compressive residual stresses, thereby improving fatigue life, wear resistance, and stress corrosion cracking resistance in high-strength aluminium alloys. However, the maximum useful compressive residual stress that can be safely introduced in AA7075, and its dependence on laser shock pressure and overlap strategy, remain insufficiently understood. If laser power density is raised above a critical threshold, surface damage, stress relaxation, or reverse straining may occur, which could limit the effectiveness of LSP. Finding the maximum compressive residual stress limit in AA7075 and the optimal LSP processing window while taking the Hugoniot elastic limit (HEL) into account are the goals of this study. For LSP, a nanosecond Yb:YAG laser with pulse energies between 0.1 and 5 J was employed. This led to shock pressures of up to 3.36 GPa and laser power densities of 0.25–12.5 GW/cm². Two peening strategies, namely: no overlap and 50% overlap, were investigated. Samples treated with the laser power density 12.5 GW/cm2 with the 50% overlap strategy exhibit ~35.31% higher microhardness compared to the base material and a maximum compressive residual stress of -302 MPa at a depth of 0.2 mm. The results imply that cumulative strain from pulse overlaps within an optimal shock pressure range largely determines the effectiveness of laser shock peening in AA7075 rather than peak shock pressure alone. This provides a mechanistic foundation for selecting the LSP parameter window in order to maximize residual stress and minimize negative surface effects.