Abstract: According to the multi-region feature (coexist of solid, liquid and mushy regions ) of solid/liquid transformation of alloys, a unsteady, axisymmetric, one phase continuum model, based on enthalpy-porosity fixed grid method, is developed. A computational analysis of heat and fluid flow occurring in weld pools during GTA welding of 1Cr18Ni9Ti stainless steel is carried out. The energy equation considers the latent heat, weld metal vaporization, and heat loss by radiate and convection through weld pool surface tension. The momentum equations consider buoyancy, electromagnetic and Marangoni forces. The influence of Cr and Ni and temperature on the activity of sulfur is included while calculating the coefficient of surface. Great emphasis is put on looking into the effect of sulfur concentration on the fluid flow patterns and weld pool geometry. The predicted results indicate that when the welds with low mass fraction of S, the flow is characterized by the formation of two opposite rotating vortexes, one at the periphery, driven by positive surface-tension temperature coefficient, and the other at the weld pool center, driven by negative surface-tension temperature coefficient. With increasing of mass fraction of S, the former flow develop and eventually occupy the entire weld pool and deepens, as expected, the weld pool.

Key words: Activity element, GTA welding, Marangoni flow, Vaporization Temperature field