Orientation selection in alloy dendritic evolution during melt-pool solidification

Abstract

Investigations of directionally solidifying melt pools during metal additive manufacturing (AM) reveal that the resulting subgrain cellular structures often grow along crystalline orientations different from the temperature gradient direction, some of which are not even along preferred crystallographic directions. It is well-known that dendrite orientation results from the growth competition between the heat flow direction and preferred crystallographic orientation. Specifically, the competition between interfacial anisotropy and process anisotropy (thermal gradient and growth velocity) during directional solidification leads to rich morphological diversity of the resulting dendritic structures, including tilted dendrites and seaweed patterns. The orientation selection mechanisms of such patterns remain unexplored at high velocity in the frame of AM. This study examines the tilted growth of cellular-dendritic arrays as a function of the misorientation angle ($\theta_R$) between the thermal gradient and crystal lattice directions and other relevant control parameters. We use a phase-field model to explore dendritic evolution in a binary alloy during high-velocity solidification in 2D. We find marked effects of thermal gradient, growth velocity, alloy composition, and anisotropy parameters on the possible growth directions and the primary arm spacing, constitutional undercooling, microsegregation, and secondary phases that arise during dendritic solidification. Our work provides a detailed yet concise presentation on the tilted growth and morphological transition for a broad range of thermal conditions in the high-velocity regime and the full range of $\theta_R$ for establishing orientation selection maps. These results should have qualitative relevance for controlling the subgrain structure, chemical segregation, and texture randomization in commercial dendrite materials relevant to AM.