Technical Study On Micro-arc Oxidation Of Magnesium Alloy With Different Power Supply Modes

The micro-arc oxidation is one surface treatment technology which can make themetal be oxidized in the local high temperature which was produced by micro-arcdischarge, and generate ceramic membrane in situ surface of the metal. It was thedouble-polarity pulse power supply mode that has been widely used, however therehas not yet been a exact answer what power supply mode is most suitable formagnesium alloy MAO process. In order to determine the optical power supply modeand loading parameters of power supply of the micro-arc oxidation, with AZ91Dmagnesium alloy as the substrate material, using the experimental platform withmultiple output power pulse modes, a large number of experiments were carried outwith the different power supply modes of direct current, single-polarity pulse,double-polarity pulse and the pulse power supply with a discharge loop. Based on thephenomenon and the results of the experiments, micro-arc discharge mechanism andthe load characteristics of MAO, the impacts of different power supply modes onmicro-arc oxidation were studied, and it was determined that what loading parameterof power supply is most optimal.For evaluating the film-forming efficiency and energy utilization rate, the outputelectric energy that correspond to unit membrane thickness of different conditionswas calculated, which was called the average energy consumption. The only effect ofnegative voltage in double-polarity pulse power supply is to inhibit the tendency oflarge-arc discharge, both theoretical analysis and experimental results show nosufficient sign for the existence of negative voltage. Under the power supply modewith a discharge loop，with the same preinstalled pulse parameters，the current and theenergy consumption of the process of MAO are all reduced when the value of thedischarge resistor increases, but the performance of the film of MAO are all most thesame. Comparative study shows that the pulse power supply with discharge loop ismore suitable for magnesium alloy MAO process than the double-polarity one. Mainlybecause of good consistency and stability in the process, excellent controllabilityparameters, uniform arc spots; the quality of coating surface is obviously better thanthe double-polarity one; the corrosion resistance that was obtained under the samecondition is little better than the latter.Based on the study of loading modes of the power supply, the optimal loading parameters of power supply are determined. When the loading mode is that differentvoltage increment is adopted in different stage of the micro-arc oxidation experiment.10V/min is usually set in the beginning stage, when the voltage reaches350V, changethe increment to5V/min until the terminal voltage, or constant current in5A/dm~2~15A/dm~2, the process is the best overall performance. The film quality will bedeteriorated when the value is too large; the efficiency will be low when the value istoo small.