The impact of thermocapillary on equiaxed/columnar microstructure evolution in laser powder bed fusion: A high-fidelity ray-tracing based finite volume and cellular automaton study

The thermocapillary effect plays a critical role in the formation of molten pool during laser powder bed fusion (LPBF) process. The resulting temperature history governs the subsequent microstructure evolution of the manufactured component, thereby exerting a substantial influence on its mechanical properties. However, the precise relationship between the thermocapillary effect and the resulting microstructure remains elusive during LPBF, particularly when considering the diverse melting modes. In this study, an innovative computational framework that integrates ray-tracing based finite volume method with cellular automaton modelling is established to investigate the influence of thermocapillary effects on equiaxed/columnar microstructure evolution during LPBF. It is found that in conditions of a conduction-mode molten pool, thermocapillary effects exert a great influence on the size characteristics of the molten pool, and the average laser beam absorption demonstrates a gradual increase as the coefficient of surface tension (CST) rises. With the introduction of keyhole, the influence of thermocapillary on both the molten pool dimensions and laser beam absorption diminishes. A positive correlation is observed between grain sizes and CST under low laser power conditions; this relationship becomes less distinct with the formation of keyhole. Higher laser powers yield a larger proportion of equiaxed grains in the central top region due to bulk nucleation with low G/R. Elongated grain structures appear with higher laser power, but the morphology-CST relationship is inconclusive. Furthermore, variations in grain structure within specific regions of the scanned track are observed, which can be attributed to the disparity in temperature histories experienced by the material.