Based on the finite difference method and the enthalpy model of Shamsundar, a computer model was developed to describe the steady state, two-dimensional heat flow during the welding of thin plates. In order to allow accurate computations of the weld pool configuration, the size of the mushy zone and the temperature distribution near the heat source, a grid mesh of variable spacings was used. The heat of fusion, the size and distribution of the heat source, the temperature dependence of thermal properties, the heat conduction in the welding direction and the surface heat loss during welding were considered. The model was first checked with Rosenthal’s analytical solution of welding heat flow using pure aluminum for examples. Experimental results of 6061 aluminum, including the width of the fusion zone and the thermal cycles at positions in both the fusion and the heat affected zones, were then compared with the calculated results of the heat flow model. The agreement was very good. Finally, in order to demonstrate systematically the quantitative effect of welding parameters such as the heat input, the welding speed and the preheating of the workpiece, a series of computations were made based upon 6061 aluminum.