Effect of Si Addition on Formation of Intermetallic Phases at the Interface Between Solid Fe and Liquid Al-Zn Alloy

Hot-dip Zn-55mass%Al alloy galvanized steel sheets are generally called “galvalume” steel sheets and are widely used in structural materials because of their good corrosion resistance, durability, and heat resistance. The commercial Zn-55%Al alloy coating exhibits a composition of Al-25Zn-2Si (at%). In the hot-dip galvanized process for steel sheets, diffusional interfacial reactions occur on the steel surface in the Al-Zn-Si alloy melt, resulting in the formation of various intermetallic compounds in the coating of the hot-dip galvanized steels. In order to achieve good formability and workability of hot-dip galvanized steels, it is necessary to ensure high adhesion of the coating with steel components. The adhesion properties depend on the distribution, morphology, and deformability of the intermetallic compounds in the galvanized coating and its interface with steel substrates. It is generally known that the Si addition to the Al-Zn alloy melt is required to reduce the interfacial reaction between the Al-Zn alloy melt and the steel sheets for controlling the coating thickness. However, there are few studies investigating the formation process of intermetallic compounds in the hot-dip galvanizing process for the production of “galvalume” steel sheets., whereby the details of the role of added Si in the interfacial reaction and its related formation of intermetallic phases remains unclear. In this study, we have systematically characterized microstructures of pure Fe sheets hot-dipped in Al-25Zn binary alloy and Al-25Zn-2Si ternary alloy at an elevated temperature of 600 °C for different dipping times of 2~3600 s. These results were utilized to discuss the effect of the added Si element on the interfacial reaction and its related formation of intermetallic phases.<br/>Significantly thick coatings were formed on Fe sheets hot-dipped in the Al-25Zn binary alloy melt for a longer time than 10 s. The coating thickness became several millimeters after 30 s, resulting in a delamination of the coating. The microstructural characterizations revealed that a significant Fe dissolution occurred in the Al-Zn binary alloy melt, accompanied by the growth of η-Fe₂Al₅ phase toward the solid Fe. The Zn enrichment was found on the surface of the pure Fe sheet, which was indicative of the diffusion of Zn into the solid Fe. Such a Zn enrichment was often localized inside/around the growing η phase in the solid Fe. The measured total compositions of Fe-Al intermetallic phases containing the Zn-rich regions corresponded to two-phase regions of the Zn-rich liquid phase with η-Fe₂Al₅or θ-Fe₄Al₁₃ phases in the calculated isothermal section at 600 °C of the Al-Fe-Zn ternary phase diagram, indicating the significant growth of η phase promoted by the Zn-rich liquid phase with a lower melting temperature. In the case of hot-dipping in the Al-25Zn-2Si ternary alloy melt, uniform coatings were formed on the Fe sheets regardless of the Fe dissolution and growth of Fe-Al intermetallic phases. A continuous layer of Si-rich T₅ (Fe₂Al_7.4Si) phase was formed at the interface of solid Fe with the Al-25Zn-2Si alloy melt. The continuous T₅ phase layer would play a role in a diffusion barrier at the interface of solid Fe with liquid Al-Zn alloy, resulting in the suppressed interfacial reaction.