Revealing melt pool dynamics during laser temporal shaping directed energy deposition of 316L stainless steel

A three-dimensional thermofluidic coupling transport model of laser temporal shaping directed energy deposition (LTS-DED) for 316L stainless steel is established to investigate the influences of three laser temporal modes on heat transport and fluid flow, and the mechanisms of remelting and macro/microstructural formation of the melt track are elucidated. In three laser temporal modes, an inward vortex flow pattern is observed in the melt pool due to a positive thermocapillary coefficient induced by the oxygen from the oxidation-deoxidation reaction. In a given continuous wave (CW) mode, the melt pool only undergoes the constraint solidification process; and the evolution processes of the heat transport, fluid flow, and pool morphology are stable. The as-deposited sample exhibits the coarsest microstructure dominated by columnar grains. In the given two pulsed wave (PW) modes, the melt pools alternately undergo constraint solidification and free solidification processes, as-deposited samples are predominated by free solidification zones. The periodic remelting phenomenon leads to the periodic fish-scale structures and discrete bands on the surface and interior of the melt track, respectively. The discrete bands are determined by the maximum contour of the regions scanned by the solidification interface during single-pulse remelting. The high-frequency PW mode produces relatively smooth fish-scale structures and low-angle discrete bands. Additionally, the high-frequency PW mode inhibits the constraint solidification zone and CET behavior and results in cross-band columnar grains at the discrete bands. However, in the low-frequency PW mode, constraint solidification zones dominated by coarse grains appear near the discrete zone.