The effect of silicon on spangle size in hot-dipped 55Â wt%Al-Zn coatings

The effect of various silicon levels on spangle size in hot-dipped 55 wt%Al-Zn-xSi (x = 0.5, 1.0, 1.6, 2.0, 4.0, all in wt%) coatings was studied. The results showed that as silicon content was increased from 0.5 to 4.0 wt%, spangle size increased gradually from the minimized range to the normal range. Spangle range transition occurred in silicon content between 1.0 and 1.6 wt%. Correlation between intermetallic species and spangle sizes under various silicon contents was investigated. It was found that in the process of spangle size from the minimized to the normal, intermetallic species of the alloy layer were also subject to a regular change of phase transformation from FeAl3 to τ5 (also refers to α-AlFeSi), especially remarkable in the range of silicon content where spangle range transited. Phase evolution of the intermetallic layer in various silicon levels was quantitatively analyzed by thermodynamic modelling using Pandat software package, which provided a deep understanding of how the silicon content affect the formation of intermetallic species and controlled the change of intermetallic layer underneath the overlay. First-principles calculations were performed to evaluate the lattice mismatch between intermetallic species and primary α-Al, which gave an interpretation of how the intermetallic species influenced the nucleation behavior of primary α-Al during solidification and then controlled the spangle size.