The behavior of silicon in the solidification of Zn-55Al-1.6Si coating on steel

Silicon is an essential element in the Zn-55Al-1.6Si coating. It is added to promote the formation of an adherent coating and prevent the excessive growth of an intermetallic alloy layer at the steel/coating interface. The addition of silicon also results in the formation of a silicon phase distributed in the interdendritic region of the overlay, having a flowery pattern on the surface, and appearing needlelike when observed inside the overlay. The behavior of silicon during the solidification process of the Zn-55Al-1.6Si coating is examined in the current study. It is found that the coating solidification proceeds in three stages. At stage I, primary α-Al dendrites develop at about 566 °C to 520 °C, forming the framework of the coating structure. This is followed by stage II at about 520 °C to 381 °C, where the binary Al-Si eutectic reaction takes place, with the majority of the silicon phase forming at about 520 °C to 480 °C. At stage III the remaining molten phase undergoes a ternary Al-Zn-Si eutectic reaction forming the interdendritic zinc-rich network. The ternary Al-Zn-Si eutectic reaction is essentially equivalent to the binary Al-Zn eutectic reaction because of the very low level of silicon at the Al-Zn-Si eutectic point.