Effects of Micro-Alloying Elements on the Microstructure and Solidification Behavior of Hot-Dip Al-Zn Coatings

This study investigates the 55%Al-Zn-Si coating system. Using microstructural characterization and thermodynamic simulation, we systematically analyzed its microstructure formation, solidification behavior, and the regulatory effects of Cr, Nb, and V micro-alloying elements. The results show that the typical coating consists of a primary α-Al dendritic skeleton and an interdendritic Zn-rich eutectic phase, exhibiting a characteristic spangle morphology. The addition of Si is crucial. By participating in the formation of a Fe-Al-Si ternary compound layer, it effectively suppresses the intense reaction at the Fe/Al interface, providing essential conditions for the sufficient growth of the outer Al-rich dendrites and the formation of a continuous transition layer. Thermodynamic analysis further clarifies that the coating solidification follows three distinct stages: precipitation of the primary α-Al phase, an Al-Si binary eutectic reaction, and a final Al-Zn-Si ternary eutectic transformation. Regarding micro-alloying, this study reveals the specific roles of different elements: Cr significantly refines the transition layer structure, promoting its transformation from coarse lamellae into a fine and uniform morphology; V tends to combine with Al to form high-melting-point enriched regions, inhibiting the growth of Fe-Al intermetallics and reducing the thickness of the brittle transition layer by approximately 50%; conversely, the addition of Nb disrupts the normal solidification sequence, inducing abnormal segregation of Al-rich and Si-rich phases, which compromises the homogeneity and integrity of the coating structure. Through an in-depth analysis of the fundamental solidification mechanism and micro-alloying effects, this research provides an important theoretical basis for optimizing the microstructure of hot-dip Al-Zn sheets via precise composition design and micro-alloying strategies.