This article studies the transient weld pool dynamics under file Periodical impingement of filler droplets that carry mass, momentum, thermal energy, and species in a moving 3D gas metal arc welding. The complicated transport phenomena in the weld pool arc caused by the combined effect of droplet impingement, gravity, electromagnetic force, plasma arc force, and surface tension force (Marangoni effect). The weld pool shape and the distributions of temperature, velocity, and species in the weld pool are calculated as a function of time. The phenomena of "open and close-up" for a crater in the weld pool and the corresponding weld pool dynamics are analyzed. The commonly observed ripples at the Surface Of a solidified weld bead are, for the first time, predicted by the present model. Detailed mechanisms leading to the formation of ripples are discussed. (c) 2007 Elsevier Ltd. All rights reserved.