Micro-discharge evolution in constant voltage bipolar PEO process on ZL101 Al-Si alloy monitored by acoustic emission...

The plasma electrolytic oxidation (PEO) process on the ZL101 Al-Si alloy under bipolar pulse mode was monitored by acoustic emission (AE) technique, and the influence of cathodic pulse on the AE signals was discussed. It was found that the PEO process could be divided into six stages based on the waveforms and parameters of AE signals. The introduction of cathodic pulse suppressed the increase of the peak amplitude and the hit numbers of AE signals in the anodic pulse time during the late stage of the PEO process, which resulted in the soft spark phenomenon. The thickness of the compact layer in the PEO coating increased to 22 μm at 3600 s; meanwhile, the PEO coating mainly consisted of γ-Al₂O₃ and mullite (3Al₂O₃·2SiO₂) phases. On the other hand, the number and peak amplitude of detected AE hits on the Al-Si alloy sample were higher than those of underwater AE hits. Because the AE signals on the sample surface are affected by the intensity and density of all micro-discharges, while the underwater AE signals mainly depend on the strong discharge sparks. In addition, for anodic pulse time in one pulse period of 13300 μs, AE signals on the sample resulted from the evolution of plasma bubbles, the inner-surface friction of discharge channels and micro-crack propagation. During cathodic pulse time, the release of H₂ gas and vapor bubbles were the sources of AE signals.