Growth mechanism of micro-arc oxidation film on 6061 aluminum alloy

The 6061 aluminum alloy was disposed using bipolar pulse micro-arc oxidation power supply in Na₂SiO₃-CH₃COONa-Na₅P₃O₁₀ electrolyte system, where the surface spark discharge of aluminum alloy during micro-arc oxidation was observed by long distance working microscope, and the micro-structure of molten pool in micro-arc oxidation film were studied by 3D video microscope, SEM, XRD, combined with film/substrate separation technology and layer by layer thinning technique. Also, the mechanism of micro-arc oxidation breakdown and discharge of aluminum alloy was investigated thoroughly. The results showed that duration time of spark discharge increased with the progress of micro-arc oxidation reaction, which was 67 ms when oxidation reaction lasted 1 min, however it reached 532 ms when reaction lasted 90 min. With increase of oxide film thickness, the size of spark and weld pool increased, where the size of spark was exponentially related to thickness of film, and size of molten pool was linear to the film thickness, while two sizes were approximately equal. The profile of molten pool was trumped-like shape, and part of the pool was connected with each other in the oxide film, leading to a formation of three-dimensional network structure. At the bottom of oxide film, there were discharge micropores with a diameter of ∼150-200 nm, and α-Al₂O₃ phase content in the film increased from the surface to bottom. Micro-arc discharge occurred in 'interlock' form at bottom of the oxide film, forming 'fire flower clusters', which melted and vaporized around metals, such that spraying concurred, while new oxides was formed at the surface and bottom of oxide film, thus making the oxide film thicker.