The evolution of grain morphology during solidification of a moving aluminum alloy pool is simulated by considering heat transfer, flow of liquid metal in the molten pool and solidification parameters. The computationally efficient model consists of a 3D coupled heat transfer and fluid flow simulation to predict the molten pool shape and temperature field, and a 2D model of grain formation in the molten pool. The results demonstrate that columnar grains grow in a curved pattern rather than along straight lines from the fusion boundary towards the center of the molten pool. The calculated results are validated with independent experimental data. The computed ratio of local temperature gradient to solidification rate, G/R, is used to model the columnar to equiaxed transition during solidification. The simulated results show that only curved columnar grains are formed when the scanning speed is low (2.0 mm/s). In contrast, a transition from curved columnar to equiaxed morphologies occurs at the higher scanning speeds of 8.0 mm/s and 11.5 mm/s, with higher equiaxed grain fraction at higher speed. The similarities between the physical processes governing fusion welding and additive manufacturing (AM) make the model capable of predicting grain orientation in both processes. Display omitted