Acoustic emission characteristics of micro-discharges in initial pulsed PEO process on 60% SiCp/Al composite.

The initial discharge process of pulsed plasma electrolytic oxidation (PEO) on the 60% SiCp/2009 aluminum metal matrix composite (Al MMC) in silicate solution was monitored by acoustic emission (AE) technique. Parameters and correlations of AE signals on the Al MMC sample and under water were analyzed, and their generation mechanism was discussed. It was found that the peak amplitudes of AE signals and AE hits during the pulse time quickly increased with the increase of micro-discharge intensity, and the absolute energy of AE signals improved several orders of magnitude. Moreover, different from the peak amplitude, duration and rise time, the duration and count had a strong correlation. Elastic stress waves resulted from the microjet of plasma bubble collapse, the inner-surface friction inside discharge channel, the expansion-shrinkage process of plasma bubbles and micro-crack propagation during rapid solidification of melt are sources of AE signals on the Al MMC sample during the pulse time. However, the expansion-shrinkage process of plasma bubbles plays a key role in the generation of underwater AE signals. In the pause time of one pulse period, the bursting and moving of vapor bubbles result in weak AE signals. It is demonstrated that the AE technique can effectively characterize the features of micro-discharges within a pulse period.

In situ monitoring of initial plasma electrolytic oxidation process on 60 vol. % SiCp/2009 aluminum matrix composite by sound and vibration measurement techniques.

The initial discharge process of plasma electrolytic oxidation (PEO) on the 60 vol. % SiCP/2009 aluminum matrix composite in silicate solution was in situ monitored by sound and vibration measurement techniques. The underwater sound, airborne sound, and sample vibration signals were detected in the initial 120 s of the PEO process, and their generation mechanism was discussed. In terms of waveforms and spectrograms of the sound and vibration signals, the initial PEO process can be divided into five stages: conventional anodizing stage (I), glow discharge stage (Ⅱ), tiny spark discharge stage (Ⅲ), large spark discharge stage (Ⅳ), and strong spark discharge stage (Ⅴ). The sound and vibration signals during the PEO process are attributed to the evolution of bubbles, which are from the plasma discharge, electrochemical reactions, and vaporization of electrolyte under Joule heat. In stage I, these signals completely come from the bubbles produced by the evaporative electrolyte and electrochemical reactions. In stages Ⅱ-Ⅴ, the bubbles from the plasma discharge gradually become the main source of these signals with increasing discharge intensity. In addition, the spike peaks on the waveforms of these signals at stage Ⅴ are related to the strong discharge sparks. These results demonstrate that sound and vibration measurement techniques can effectively monitor the PEO discharge process.