Study on microstructure and properties of AA7075 coated by micro arc oxidation

Effect of ageing temperature on the properties of 7075 aluminum alloy coated by micro arc oxidation (MAO) technology has been investigated. The precipitates formed at various ageing temperatures are observed by transmission electron microscope (TEM), which would significantly influence the properties of the MAO coating. Coherent distortion, size and volume fraction of precipitates are related to the changing of electrical conductivity during ageing treatment. Full coherent and dense Guinier Preston zones (GP zones) formed at 90 °C can act as scattering centers to impede electron mobility, so decrease the electrical conductivity. η and η′ phases produced at 120 °C and 150 °C are semi-coherent and incoherent, respectively, meanwhile, the precipitate size increases and the volume fraction decreases with increasing the ageing temperature, therefore, the electrical conductivity increases. Higher electrical conductivity contributes to thicker coating with better surface morphology. Double cantilever beam (DCB) tests show that the thicker and less porous coatings prepared at 150 °C aged specimens own the best stress corrosion cracking (SCC) resistance. After pores are broke and coatings are ruptured, high Cu and low Mg segregation at the grain boundary of 150 °C aged specimens founded from high-angle annular dark field scanning transmission electron microscopy (HADDF-STEM) are beneficial to anodic dissolution and hydrogen assisted crack.