Fabrication and Characterization of Multiscale Graded SMAT-MAO Composite Coating Formed on the Surface of 2024 Al Alloy

In order to improve the fatigue property of microarc oxidation (MAO) coated aluminum alloy, a new multiscale graded coating named SMAT-MAO graded coating, consisting of a bottom nanocrystalline layer covered by a top ceramic coating, was designed and fabricated on the surface of 2024 Al alloy by a duplex process with surface mechanical attrition treatment (SMAT) method prior to microarc oxidation process. Experimental results show that surface crystal size was refined to 52.8nm after SMAT for 15min. A 20μm thick nanocrystalline layer, with gradient changing crystal size from dozens of nm in the surface to 200∼500nm deep into the substrate, was successfully obtained. The microarc oxidation ceramic coating with 5μm thickness grew by consuming part of the nanocrystalline layer. Since the local heat produced in the microarc discharge channels was not enough to lead to the growth of the substrate grain, the alloy substrate near to the coating/substrate interface still maintained a nano-grained size. Therefore SMAT-MAO coating, which consists of an outer ceramic coating and an inner nanocrystalline layer, was obtained.