Effects of sulfur concentration and Marangoni convection on melt-pool formation in transition mode of selective laser melting process

Highlights: • A 3-D CFD model is used to study the effects of sulfur level of the powder material in the Selective Laser Melting process. • Both simulations and experiments are utilized to reveal the effects of Marangoni convection on the melt-pool formation. • Defining a new processing zone for producing high density parts with SLM based on the concept of normalized enthalpy. -- Abstract: A three-dimensional thermal-fluid model is constructed to investigate the effects of Marangoni convection on the melt-pool formation during the Selective Laser Melting of SS316 powder. The model takes account of the heat-and-mass transfer effects and the melt-pool flow dynamics. In the model, the surface tension gradient in the melt-pool is estimated as a function of the sulfur content of the metal powder based on the reported experimental data. The validity of the model is demonstrated by comparing the simulated melt-pool dimensions under a wide range of laser powers and scanning speeds with the experimental results presented for CL20ES stainless steel powder in the literature and SUS420 stainless steel powder obtained in the present study. The results show that besides the conduction mode, in which the melt-pool formation is dominated by thermal conduction, and the keyhole mode, in which the melt-pool formation is determined mainly by the recoil pressure, an additional transition mode exists between these two modes, in which the melt-pool formation is driven mainly by the Marangoni convection effect. In particular, for stainless steel powders with a higher sulfur content, an inward Marangoni flow occurs, which results in a deeper melt-pool and a lower porosity of the built part.