A MULTISCALE APPROACH FOR THE SIMULATION OF DENDRITIC MICROSTRUCTURES IN AL-ZN-SI COATINGS

摘要

The dendrite structure and the surface appearance of hot-dip metallic coatings are strongly influenced by the interaction of the dendrite tips with the boundaries confining the melt. In order to develop a better understanding of microstructure formation in these systems, a multiscale simulation approach has been developed. The approach consists first of a geometrical model that describes the trajectories of the dendrite tips based on a series of criteria for arm branching and for the interaction with the melt boundaries. The model permits to predict the development of the dendrite skeleton within the confined volume of the coating as a function of the crystallographic orientation of the nucleus.rnA dendrite tip interacting with a boundary has a different velocity and growth direction as compared to growth in the bulk. A phase-field model was used to study these phenomena. It allowed for the determination of a relationship between the tip velocity and the incidence angle of the dendrite tip on the boundary. This relationship was then incorporated into the geometrical model.rnThe model was applied to Al-Zn—Si coatings deposited on steel sheets by hot dipping. It allowed for the description of dendritic growth along crystallographic directions of the < 320 > family, which have been identified by electron back-scattered diffraction as being the preferred growth directions in this system. Optical microscopy of Al-Zn-Si coatings reveals two types of surface structures: shiny areas where the dendrite growth directions are clearly visible and dull areas where the surface is darker and more dimpled. The simulation results showed that the areas exhibiting a shiny surface appearance are due to growth along the free surface, whereas dull areas result from growth along the substrate/coating interface. The results of the geometrical model were confirmed by optical microscopy, laser profilometry of surface topography and composition measurements by WDS.