This article presents a numerical investigation on the transient transport phenomena in the arc which include the arc plasma generation and interactions with moving droplets and workpiece for pure argon and three argon-helium mixtures (75% Ar + 25% He, 50% Ar + 50% He, and 25% Ar + 75% He) during the gas metal arc welding (GMAW) process. The results indicate that the arcs in various shielding gases behave very differently due to the significant differences in thermophysical properties, including the ionization potential and the electrical conductivity, thermal conductivity, specific heat, and viscosity at high temperatures. For the same welding power input, it was found the increase of helium content in the mixtures results in (1) the change of plasma arc shape from bell-like to cone-like and (2) the change of arc pressure distribution along the workpiece surface from Gaussian-like to flat-top with decreasing peak value. Detailed explanations to the physics of the very complex but interesting transport phenomena are given. (C) 2010 Elsevier Ltd. All rights reserved.