Effect of low-energy laser shock peening on surface integrity, residual stress, and plastic deformation behavior of AZ31B magnesium alloy

Laser shock peening was investigated as a surface modification technique to improve the surface integrity of AZ31B magnesium alloy. Laser shock peening was performed using an Nd: YAG laser at beam energies of 2, 2.5, and 3 J with a constant beam diameter of 3 mm and 50% overlap. The influence of beam energy on microhardness, surface roughness, residual stress, surface morphology, and microstructural evolution was evaluated experimentally. The cross-sectional microhardness increased from ∼51.38 HV in the untreated alloy to ∼72.75 HV after laser shock peening at 3 J, while the magnitude of cross-sectional compressive residual stress increased with beam energy, reaching ∼−34.3 MPa at 3 J. Laser shock peening increased surface roughness because of severe plastic deformation and localized surface ablation induced by high-pressure plasma shock waves. Scanning electron microscopy revealed plastically deformed surface layers and characteristic shock-induced surface features in the treated specimens. X-ray diffraction confirmed the retention of the single-phase α-Mg (P6 3 /mmc) structure under all processing conditions, with no evidence of secondary phase or oxide formation, indicating that the surface modification was governed by mechanical shock-induced deformation. The Mg (002) basal reflection exhibited a modest peak shift and pronounced peak broadening, with the maximum broadening observed at 2.5 J, consistent with the fast fourier transform-derived heterogeneity trends. Surface roughness showed a non-monotonic response to beam energy, with the lowest Sa values of 16.9 μm on the top surface and 10.1 μm on the cross-section at 2.5 J. These results indicate that higher beam energy promoting greater hardening and compressive residual stress, while an intermediate beam energy provides improved surface finish.