Sulfur-induced transitions of thermal behavior and flow dynamics in laser powder bed fusion of 316L powders

Abstract

A three-dimensional (3D) powder-scale model is developed to analyze the melt pool dynamics in laser powder bed fusion (L-PBF) of 316L powders considering the transition of melt pool dynamics induced by sulfur element. In sulfur-free situation, thermal behavior and fluid flow are modeled involving 3D details. In sulfur-considered situation, sulfur-effect on melt pool dynamics is simulated with the surface tension described by the function of local temperature and the sulfur content. It is concluded that heat transfer is dominated by convection flow, and surface tension is the most important driving force. When the sulfur-effect is absent, surface tension increases from center to the periphery, inducing a centrally outward Marangoni convection. In the situation of 0.03% sulfur, surface tension first increases then decreases from center to the boundary, resulting in the novel flow pattern of combined outward-inward flow. Probing the underlying physical details induced by sulfur-effect, it makes melt pool dynamics more complex with more vortexes, and the sulfur-induced backward branch flow at the transverse view is benefit for the reduction of surface roughness. Furthermore, more vortexes, more branch flow, and more mixing positions of branch flow (MPBF) are observed in the sulfur-considered situation, which induces the increasing flow complexity, the drop of temperature gradient and driving force, as well as the decreasing flow intensity. Moreover, the variation of sulfur content leads to significant transitions of thermal behavior, driving force, and dynamic characteristics. Fundamental conclusions contribute significantly to the comprehensive understanding of the physical process in L-PBF of 316L powders.