Abstract: A 3-D mathematical model is developed to calculate temperature and velocity distributions in a moving TIG weld pool with different sulfur concentrations. It has been showed that，sulfur, which change the sign of surface tension coefficient, can cause significant changes in depth/width ratio. When the mass fraction of sulfur increases, the depth/width increases sharply. When the mass fraction of sulfur increased beyond 80×10–6, the increased sulfur did not have an appreciable difference on the weld pool size and shape. As the mass fraction of sulfur exceed 120×10–6, maximum surface-tension temperature increases beyond the calculated surface temperature, and the sign of surface tension coefficient is positive. Depending upon the sulfur concentrations, three, one, or two vortexes that have different positions, strength, and directions may be found in the weld pool. The contrary vortexes can efficiently transfer the thermal energy from the arc, creating a deep weld pool.

Key words: Depth/width ratio, Flow patterns, Sulfur, Surface-active elements, Welding pool