Surface-active element transport and its effect on liquid metal flow in laser-assisted additive manufacturing

During laser-assisted additive manufacturing, the transport phenomena, solidification behavior, and melt pool geometry are affected by base-metal sulfur content and oxygen present in the atmosphere. The role of these surface-active elements during metal-based additive manufacturing is not well understood. In this study, the mass transport of sulfur and its effect on liquid metal convection (Marangoni flow) are examined by using an improved 3D transient heat transfer and fluid flow numerical model. An improved surface tension model is proposed to take into account the influence of sulfur content and temperature at the surface of melt pool. The sulfur mass transport equation is combined with the other conservation equations. The results show that the sulfur redistribution leads to transitional Marangoni flow. The powder addition into the melt pool results in the dilution of sulfur in the melt pool. Increase in mass flow rate and energy input results in decreased temperature coefficient of surface tension. When temperature coefficient of surface tension changes sign from positive to negative, the flow motion transition from inward flow to outward flow occurs. Two predominant types of flow pattern lead to two types of fusion boundaries of deposited track.