The evolution of strain gradient and surface microstructure of aluminum alloy during multiple laser shock peening cycles

The ultra-high strain rates process of laser shock peening (LSP) offers a novel approach to tailor surface-to-core gradient structures in metallic materials. In this study, the strain gradient characteristics and the corresponding microstructure evolution of the Al-Mn alloy under cyclic shock loading were investigated by combining experimental analysis and multiscale simulation. It was found that strain accumulation from multiple LSP treatments predominantly occurs in the top surface region (<50 μm) of the Al-Mn alloy, leading to a 67 % increase in surface hardness. LSP increases from one to three cycles continuously deepens the severe plastic deformation (SPD) and minor plastic deformation (MPD) layers but their corresponding maximum strain levels remain constant, due to the competition between shock wave pressures and dynamic yield stresses of material. The repeated LSP treatment produces a more pronounced transverse action of the plastic wave, compared to that of a single LSP, which weakens the strain localization under applied pressure with a Gaussian distribution. The accumulated high dislocation density by multiple LSP cycles locally triggers the transformation of substructures into new grains during room temperature deformation. These results provide new insights into the precise construction of gradient structures in aluminum alloys through multiple LSP treatments.