Role of silicon in solidification microstructure in hot-dipped 55 wt% Al–Zn–Si coatings

An experimental study has been carried out into the effect of silicon content on 55 wt% Al–Zn–Si metallic coating solidification. Immersion experiments were conducted on a metal coating simulator, and the resulting solidification structure was characterised in terms of spangle size and secondary dendrite arm spacing (SDAS). It was found that at bath temperatures of 600 °C as silicon content was increased from 1.05 to 1.5 wt% there were an increase in spangle size without any change in SDAS. Thermodynamic modelling was utilised to probe the phase stability of intermetallic species in the baths. Two competing intermetallic phases, α-AlFeSi and Fe3Al, are stabilised dependant upon bath silicon content and temperature. Literature data for the lattice mismatch between α-Al, the primary solidification phase in this study, with these two intermetallic phases indicates that FeAl3 has much better crystallographic fit than does α-AlFeSi. This supports the proposal that FeAl3 has a higher nucleation site density for solidification, which results in a reduced spangle size. The thermodynamic modelling has been used to provide compositional and temperature ranges for spangle size control.