Multi-physics multi-scale simulation of unique equiaxed-to-columnar-to-equiaxed transition during the whole solidification process of Al-Li alloy laser welding

In this study, a novel multi-physics multi-scale model with the dilute multicomponent phase-field method in three-dimensional (3D) space was developed to investigate the complex microstructure evolution in the molten pool during laser welding of Al-Li alloy. To accurately compute mass data within both two and three-dimensional computational domains, three efficient computing methods, including central processing unit parallel computing, adaptive mesh refinement, and moving-frame algorithm, were utilized. Emphasis was placed on the distinctive equiaxed-to-columnar-to-equiaxed transition phenomenon that occurs during the entire solidification process of Al-Li alloy laser welding. Simulation results indicated that the growth distance of columnar grains that epitaxially grew from the base metal (BM) decreased as the nucleation rate increased. As the nucleation rate increased, the morphology of the newly formed grains near the fusion boundary (FB) changed from columnar to equiaxed, and newly formed equiaxed grains changed from having high-order dendrites to no obvious dendrite structure. When the nucleation rate was sufficiently high, non-dendritic equiaxed grains could directly form near the FB, and there was nearly no epitaxial growth from the BM. Additionally, simulation results illustrated the competition among multiple grains with varying orientations that grow in 3D space near the FB. Finally, how equiaxed grain bands develop was elucidated. The equiaxed band not only hindered the growth of early columnar grains but also some of its grains could grow epitaxially to form new columnar grains. These predicted results were in good agreement with experimental measurements and observations.