Coupled PF and LB simulation study of morphology evolution in columnar dendrite growth induced by natural convection

• A coupled LB-PF scheme was employed to investigate the growth of both single dendrite arrays and converging dendrite arrays under natural convection conditions. • Both the solute expansion coefficient and dendrite inclination angle significantly influence the molten flow strength. • Changes in molten flow impact both dendrite morphology and dendrite competition behavior. In this study, we present an investigation of microstructure evolution behaviors of both single dendrite arrays and converging dendrite arrays with different crystallographic orientations under natural convection conditions, utilizing phase-field (PF) and lattice-Boltzmann (LB) coupled simulations. It is found that in single dendrite arrays, the primary spacing decreases as the solute expansion coefficient increases. When this coefficient reaches a critical threshold, stable dendrite morphology becomes unstable and begins to degenerate. This critical point is influenced by the inclination angle, as a steeper angle enhances the blocking effect of the inclined dendrite trunk on liquid flow. Flow strength rises with the absolute value of the solute expansion coefficient for both negative (downward) and positive (upward) flows; however, it is significantly weaker for negative coefficients. This discrepancy causes dendrite morphology to degenerate more readily at high positive coefficients. In scenarios of downward flow for converging dendrite arrays, favorably oriented (FO) and unfavorably oriented (UO) grains coexist. Conversely, in upward flow situations, FO and UO grains also coexist at small solute expansion coefficients, but with larger coefficients, the degeneration of FO dendrites allows them to overgrow and occupy the space of UO dendrites.