Effect of solution heat treatment on mircoarc oxidation coatings of AZ91D magnesium alloy

The effects of solid solution heat treatment on micro-arc oxidation (MAO) coatings on AZ91D magnesium alloys were investigated. The results show that the homogenous element distribution in AZ91D substrate resulted from the solution heat treatment enhances the growth of MAO coatings, and the coatings on the solution-treated samples are about 40% thicker than that on the as-cast samples. With the oxidation process going and the coatings growing, the discharging hole and the ejecting deposition on the coating surface become bigger and coarser, which in turn results in the increase of surface roughness of coatings. At the beginning of the oxidation process, the roughness of the coatings on the solution-treated samples is lower than that on the as-cast samples, while it goes opposite after about 100 s later. However, the coating surface on the samples treated by solid solution is always rougher than that on the as-cast samples if based on the same coating thickness. A few cracks featured with shallow and short morphology are observed on the coatings of the solution treated samples, but more cracks with deep and long features occur in the as-cast samples. Also the even element distribution in the substrates due to the solution treatment contributes to the energy saving if producing MAO coatings with the same thickness.