Phase-field simulations of dendritic morphologies in hot-dip galvanized Zn-Al coatings

The growth morphologies of dendrites in thin Zn-0.2 wt% Al coating layers are investigated as a function of the inclination axis and angle of the Zn basal plane with respect to the coating plane using three-dimensional (3D) phase-field simulations under realistic process conditions. In addition to the well-known growth direction families of 〈101¯0〉 and 〈0001〉, we incorporate a third preferred growth direction family into interface kinetic anisotropy based on the recent suggestion by Kim et al. [Metall. Mater. Trans. A 50 (2019), 3186–3200]. When this interface kinetic anisotropy is combined with the isotropic interface energy in the phase-field model, the 3D simulations realistically reproduce most of the morphological characteristics of the dendrites observed in experiments. These include the asymmetric morphologies about the inclination axis, morphological changes with the inclination angle, and angular variations between the primary aims. However, simulations using the experimental estimates of the interface energy anisotropy fail to reproduce characteristic morphologies, such as the four- and eight-fold dendrites, even when combined with strong interface kinetic effects. The present study establishes the existence of a third preferred growth direction family, which is close to the directions normal to the 12¯11planes, and the importance of the interface kinetic anisotropy over the interface energy anisotropy in the dendritic solidification of Zn-rich alloys.