Microstructure and properties of Al-coating on AZ31 magnesium alloy prepared by pack-cementation

Abstract

Al-containing coatings were prepared on AZ31 magnesium alloy by pack-cementation technology. X-ray diffraction (XRD), backscattered electron imaging (BSEI) and energy dispersive spectroscopy (EDS) were jointly employed to characterize the phases, microstructure and composition of the coated samples. The results show that the feedstock composition has a significant impact on the phases, microstructure and thickness of the coatings. For the sample with AlCl3 powder as the activator, the coating is very thick and composed of gradient phases and structures from surface to inside, including small amount of β-Mg2Al3, coarse eutectic-like structure of γ-Mg17Al12 + δ-Mg, and fine γ-Mg17Al12 precipitations. In contrast, for the sample with AlCl3 and pure Al composite powders as the activator, the coating is relatively thin and contains a thin Al2O3 layer and a small amount of fine γ-Mg17Al12 precipitates. For the pack-cementation aluminizing that is not protected by high-vacuum or inert gas, the addition of pure Al powders can easily introduce the Al2O3 layer into the coating to prevent active Al ions further penetrating into the magnesium matrix, resulting in the thin Al-containing coating. The microhardness and corrosion behavior of the two kinds of aluminized coatings were also studied and discussed.