A molecular dynamics study of laser melting of densely packed stainless steel powders

Understanding the microstructure evolution mechanism of certain alloys during the laser based directed energy deposition (L-DED) process is key to reaching the goal of adjusting the proceeding parameters and controlling the properties of deposited structures. However, it is significantly difficult to capture the corresponding microstructure changes explicitly. In this study, a large-scale molecular dynamics (MD) simulation model, consisting of a densely packed powder bed and a cracked substrate, is built to show the grain growth and crack repairing process of the stainless steel by L-DED. A continuous laser track is employed, while the localized heating and solidification of meltpools are emulated directly by controlling the temperature distribution inside the meltpool areas temporally. By investigating the forming mechanism of the deposited layer, the simulation results show that the epitaxial growth of the grain stems from the substrate region. The defects, such as vacancies, stacking faults, and twin boundaries are usually formed near the cracked sites due to the partial dislocation and the glide of the atom. It is also found that the number of equiaxed grains is significantly larger than the number of columnar grains in the deposited layer, which results in the fact that the average grain size by the L-DED deposition is closely associated with the equiaxed grain size. Furthermore, the MD model (nanoindentation tests, uniaxial tensile tests) also demonstrates qualitative consistency with the experiments. It is found that the enhanced hardness and ultimate strength surface are attributed to the grain refinement on the energy-deposited layer.