Abstract: Based on a set of partial differential equations and reasonable boundary conditions, a unified three dimensional transient model including the tungsten electrode, arc plasma and base metal (weld pool) is developed for tungsten inert gas welding. By solving the equations, the temperature, velocity, arc pressure, current density and electromagnetic force of the whole domain are obtained with an anode of SUS 304 stainless steel. The calculated results are compared with the existing experimental and theoretical data in other literature. By investigating the anodic heat input, the welding thermal efficiency is calculated to be 86.3%. The effects of the buoyance, electromagnetic force, plasma drag force and Marangoni stress on the flow of the molten pool are studied respectively. The relative importance of these forces are investigated by using Grashof number Gr, magnetic Reynolds number Rm, and Marangoni number Ma. The relative importance of heat conduction and heat convection in the weld pool is highlighted with the help of Peclet number Pe. It is found that the effects of buoyance and electromagnetic force on the flow of the weld pool is less important compared with plasma drag force and Marangoni stress. Plasma drag force and Marangoni stress are of the same order of magnitude, while the Marangoni stress is larger than plasma drag force, and they together drive the flow from the center to the periphery of the weld pool. Compared with the heat conduction, the heat convection caused by the outward flow determines the heat transfer in the weld pool, and result in a shallow and wide weld pool profile in TIG welding.

Key words: flow, heat transfer, relative importance, tungsten inert gas(TIG) welding, unified model