A thermo-kinetic phase transformation model for multi-pass laser heat treatment by using high power direct diode laser

Abstract

When a larger surface area is heat treated, multiple scans by laser beam with slightly overlaps are applied. Under this condition, a tempered zone is formed in overlapped regions that will affect the uniformity of heat treated depth of material. In this study, an experimentally-based finite element (FE) thermal model coupled with thermo-kinetic equations is developed to predict the cross-sectional as well as surface temperature history, phase transformations, and hardness of multi-pass laser heat treatment (MPLHT) process. The temperature-dependent material properties and phase change kinetics are taken into account in the model. As-used experimental boundary conditions are incorporated in this model. A tool steel AISI S7 is heat treated by changing the laser power (1400-1800 W) and scanning speeds (15-25 mm/s) while keeping a constant size of overlap (25% of length of laser beam). The tempering effect of the MPLHT process is studied for different lengths of scan (10-35 mm). The thermo-kinetic phase transformation model results are verified with experimental ones to optimize the processing parameters. The optimized processing parameters, including laser power, scanning speed, size of overlap, and the length of scan are used to achieve a uniform hardness distribution and an even depth of heat treatment in the multi-pass laser heat treated area.When a larger surface area is heat treated, multiple scans by laser beam with slightly overlaps are applied. Under this condition, a tempered zone is formed in overlapped regions that will affect the uniformity of heat treated depth of material. In this study, an experimentally-based finite element (FE) thermal model coupled with thermo-kinetic equations is developed to predict the cross-sectional as well as surface temperature history, phase transformations, and hardness of multi-pass laser heat treatment (MPLHT) process. The temperature-dependent material properties and phase change kinetics are taken into account in the model. As-used experimental boundary conditions are incorporated in this model. A tool steel AISI S7 is heat treated by changing the laser power (1400-1800 W) and scanning speeds (15-25 mm/s) while keeping a constant size of overlap (25% of length of laser beam). The tempering effect of the MPLHT process is studied for different lengths of scan (10-35 mm). The thermo-kinetic phase trans...