The effect of natural convection on the dendritic tip stability during directional solidification: insights from phase-field-lattice Boltzmann simulations

In this paper, the effect of natural convection on the stability of the dendritic tip during directional solidification under various gravitational field conditions is numerically investigated using the phase-field lattice-Boltzmann method. The coupled equations were implemented for parallel computing on multi-graphics processing units using in-house code written in modular Compute Unified Device Architecture. In the single-crystal case, downward buoyancy transports solute from the dendrite roots toward the tips, leading to solute enrichment near the tips. This enrichment reduces the local undercooling, slows dendrite growth, and eventually triggers tip splitting at high convection intensities. In contrast, the upward buoyancy moves rejected solutes from tips to interdendritic regions, barely affecting solute distribution along the crystal symmetry axis and stabilizing tips. In bi-crystalline systems, downward convection induces tip splitting and plume formation in converging grain boundaries, and drives solute flow to promote sidebranching in diverging grain boundaries, while upward convection has a negligible impact on grain boundaries. This work offers quantitative insights into the dynamic mechanism by which natural convection regulates dendritic tip stability, thereby elucidating the role of natural convection in microstructure evolution during directional solidification.