Stage division and discharge mechanism characterization of micro-arc oxidation based on acoustic emission

Acoustic emission monitoring was conducted on the micro-arc oxidation process of the D16AT aluminum alloy plate with double-sided rolled pure aluminum in a silicate system electrolyte. Using the t-SNE algorithm, the recorded signals were classified based on the parameters of the acoustic emission signals. The analysis examined the regularities of the micro-arc oxidation stages under different current density conditions and their correspondence with signal characteristics and categories. Further discussion was held on the passivation and film formation mechanisms during the early, middle, and late stages of micro-arc oxidation. The effective film formation process of MAO was divided into four main stages: initial stage, weak micro-arc discharge, stable microarc discharge, and large arc discharge. Additionally, it included five sub-stages: conventional anodizing, weak glow discharge, transition from weak to strong glow discharge, transition from strong glow discharge to weak micro-arc discharge, and weak micro-arc discharge. The transition moments of these stages can be identified and determined by the frequency distribution of AE signals. As the current density increases, the discharge mechanism undergoes stage-wise changes. At different current densities, type-a signals primarily originate from gas glow discharge, while type-b signals are caused by breakdown at the bottom of the passivation film pores. Type-c signals mainly result from stable micro-arc discharge, and the increase in type-d signals marks the transition to a penetration-type strong discharge mechanism.