The formation mechanism of special globular surface grain during the solidification of laser surface remelted near β titanium alloys

During laser materials processing, the cooling rate (R) and thermal gradient (G) near the molten pool surface region are much higher than those in the traditional casting conditions, which may result in special solidification microstructures. In this study, the surface microstructure of near β titanium alloys was investigated by conducting both laser surface remelting experiments and phase-field simulations. It was found that the grains nucleated near the surface region of the molten pool exhibit near globular shape with 〈1 1 0〉-oriented side branches, which are very different from the ordinary dendrite morphology in casting conditions. Phase-field simulations were used to detect the formation mechanism of such special surface grain morphology. It was illustrated that the surface grain morphology is determined by the combined effect of high cooling rate (R) and high vertical thermal gradient (G). The high R results in the morphology transition from needle-like dendrite to near globular shape by increasing the undercooling and reducing the impact of anisotropy of interface energy. The high G promotes the grain contour to be more close to globular and facilitates the fins-like side branches to develop along 〈1 1 0〉 direction. Furthermore, a selection map of surface grain morphology at various thermal conditions (R and G) was established, which can be applied to predict the surface grain morphology.