Energy feedthrough and microstructure evolution during direct laser peening of aluminum in femtosecond and picosecond regimes

Ultrafast laser was recently used to modify the surface integrity and peen the surface region of aluminum based alloy 2024-T351 without a sacrificial layer prior to the process. We show that controllable laser parameters such as fluence and pulse duration have a significant influence on peening qualities, such as the compressive residual stress, hardness, and surface roughness of peened parts. The residual stress profile was analyzed by x-ray diffraction. By controlling the laser fluence and pulse duration, it was possible to obtain 200 MPa of compressive residual stresses close to the surface and 100 MPa of compressive residual stresses at 50  μm depth. Moreover, micro-hardness was increased from 2.1 to 2.5 GPa in the near-surface region. In addition, the dislocation densities were evaluated from high-resolution x-ray diffraction peaks. The increase of the dislocation density indicates that plastic deformation occurred, which generated compressive residual stresses and hardness enhancement. Plastic deformation is considered to be created by an ultrafast laser-induced pressure wave. The correlation between laser parameters and modified surface properties is interpreted by the complex interplay between laser excitation, material relaxation, and pressure waves. A pulse duration in the picosecond range and a relatively low fluence is possibly the optimal condition for a best peening quality with small surface roughness, which could potentially be used to reduce surface cracking and associated failures of additively manufactured parts.