Self-Limiting Hardness Changes in Laser Peened 6061-T6 Aluminium

Laser peening uses shock waves to modify and work harden the surface of metal components and weldments to improve mechanical properties such as fatigue and wear resistance, in particular with aluminium alloys. The literature has reported on a variety of property changes from laser peening aluminium alloys, which varied with the initial metallurgical state and laser generated shock wave pressures. This paper analyses the effect of laser peening repetitions (one and five repetitions) on hardness and elastic modulus versus depth on a 6061-T6 specimen for two laser generated shock wave pressures of 3·5 and 6 GPa. The higher shock wave pressure increased the hardness with both one and five repetitions, however, the lower shock wave pressure only increased the hardness with one repetition and not with five. The two mechanisms responsible for the self-limiting hardness changes are increases in the metal's Hugoniot elastic limit and changes in the metal's dynamic yield strength. These mechanisms limit further shock processing and surface hardness changes in the metal unless the laser generated shock wave pressure remains significantly greater than the increased Hugoniot elastic limit and dynamic yield strength.