Examination and Analysis of 2 Pct Mg-55 Pct Al-1.6 Pct Si-Zn Coating on Steel

The current study was conducted to examine the microstructure of the 2 pct Mg-55 pct Al-1.6 pct Si-Zn (wt pct) coating on steel in detail and analyse the formation mechanisms of various reaction products in the coating. The coating microstructure was comprised of an α-Al dendrite framework, and an interdendritic network, occupied by a number of solidification products, which were fine Al-Zn mixtures, Mg₂Si, Si, MgZn₂, and lamellar MgZn₂/Zn mixtures. It was found that the Al-Zn mixtures observed in the interdendritic region were the decomposition products of the high Zn α-Al components of various eutectic reactions taking place during solidification. A method was developed to differentiate the Si and Mg₂Si phases optically with colour contrasts. It was found that, when Si and Mg₂Si formed, they occupied the entire interdendritic spaces at the locations where they formed. Judging from the Al-Mg-Si liquidus projection, the Mg₂Si phase should form first during solidification, followed by the simultaneous formation of Mg₂Si and Si. On the other hand, the MgZn₂ phase formed into two different morphologies, with a large blocky shape and fine precipitates embedded in the lamellar Zn/MgZn₂ mixtures, both surrounded by Al-Zn mixtures. It was observed that all eutectic reactions involved the formation of an α-Al component, which formed in a divorced manner via either growing on existing dendrite arms, or sometimes as isolated islands embedded in blocky MgZn₂.