Surface alloying of AZ91E alloy by Al–Zn packed powder diffusion coating

A new surface coating technique, namely packed powder diffusion coating (PPDC), for AZ91E magnesium alloy is reported. This new technique uses a powder mixture of aluminium and zinc as diffusion source and produces uniform and thick coatings at temperatures below 420 °C. Experimental results showed that zinc in the powder mixture significantly promotes the formation of intermetallic layers on the surface of the magnesium alloy at process temperatures between 350 °C and 413 °C, which is more than 50 °C lower than the previously reported processes. Depending on the temperature and the Zn-content in the powder, X-ray diffraction analysis identified three intermetallic phases and Mg(Al, Zn) solid solution that consist of the surface alloyed layer. The intermetallic compounds are τ-Mg32(Al,Zn)49, φ-Al5Mg11Zn4 and β-Mg17(Al,Zn)12. The hardness of the over 500 μm thick surface alloyed layers is up to four times higher than the substrate. Both the β-Mg17(Al,Zn)12 phase and the τ-Mg32(Al,Zn)49 phase show one to two order magnitude higher corrosion resistance than the α-phase (solid solution) and the φ-Al5Mg11Zn4 phase in 5% NaCl solution. A process parameter window for the layer thickness as well as a schematic model for the formation of the layer is proposed. The PPDC process is a promising technique that provides effective protection of AZ91E alloy from both wear and corrosion.