Effect of laser power density and scanning speed on residual stress in laser-peened aluminum alloy

Laser peening has recently been developed as a surface modification technique. When a high-energy laser pulse irradiates a material surface submerged in water, high-pressure plasma is generated immediately on the surface. The plasma induces an impulse wave that penetrates into the surface layer. In the present study, a laser beam of diameter about 10 mm is converged by the converging lens with focal length of 250 mm, and irradiates the aluminum surface. The second harmonic generation of Yttrium-Aluminum-Garnet (YAG) laser was used at a pulse frequency of 10 Hz. The effect of laser power density and scanning speed on residual stress in the laser peened aluminum alloy surface was investigated by X-ray diffraction. The following results were obtained: (1) compressive residual stress was induced in the surface of all samples; (2) many fused traces and many micro-cracks were formed in laser peened aluminum surface; (3) for the series of experiments with the fixed scanning speed of 2.0 mm/s, the maximum compressive residual stress was induced at the laser power density of 31 MW/mm2; and (4) for the fixed laser power density of 61 MW/mm2, that was induced at scanning speed of 1.0 mm/s. (author)