A three-dimensional mathematical model and numerical techniques were developed for simulating a moving gas metal arc welding process. The model is used to calculate the transient distributions of temperature and velocity in the weld pool and the dynamic shape of the weld pool for aluminum alloy 6005-T4. Corresponding experiments were conducted and in good agreement with modeling predictions. The existence of a commonly observed cold-weld at the beginning of the weld, ripples at the surface of the weld bead, and crater at the end of the weld were all predicted. The measured microhardness around the weld bead was consistent with the predicted peak temperature and other metallurgical characterizations in the heat-affected zone.