How to improve surface integrity in discrete laser spot hardening of AISI 4140 when using a fiber laser with the Gaussian beam: A dynamic multi-pass approach based on time-domain energy modulation

Improving the surface quality of discrete laser spot hardening when using a fiber laser with the Gaussian beam has been a major concern in industry applications. A dynamic multi-pass approach based on time-domain energy modulation was proposed and successfully implemented using an innovative motion system. Surface quality characteristics, including geometrical dimensions, topography, residual stress state, microhardness, and metallographic structure, were evaluated to compare the effects of multi-pass. Additionally, the heat transfer in solid module was employed to simulate the temperature histories. The result shows the diameter of hardened zone keeps about 1000 mu m with the increase of multi-pass. It indicates that there was no notable additional thermal effect observed as the number of passes in the temperature histories increased. Meanwhile, the maximum microhardness of the multi-pass processed samples increases by more than 5 % in cross-sectional direction. This increase can be attributed to the occurrence of multiple rapid re-austenitization processes, which lead to a significant refinement in grain sizes and a more uniform distribution. Therefore, this particular method can be recognized as an effective approach for improving surface quality, i.e., reducing remelting, regulating geometrical parameters, ameliorating the 3D topography of the hardened area, increasing the uniformity of the hardened layer, and reducing the tensile residual stress.