Optimization And Mechanism Of Micro-arc Oxidation Technology Of Low Energy Consumption For AZ91D Magnesium Alloys

Magnesium and its alloy have many excellent physical and mechanical properties, which make it valuable in a number of applications including aeronautics and astronautics, automobile manufacture, electronic communications, military affairs and nuclear power, et al. However, their corrosion resistance, heat resistance and wear resistance are rather poor, which seriously hinder their further application. Microarc oxidation (MAO) is a novel surface treatment technique developed from the common anodic oxidation. By use of this technique, a ceramic coating can be in situ formed on Al, Mg, Ti and their alloys.Thus, this method has been generally recognized as the most promising surface treatment technique for magnesium alloy. However, High energy consumption and low current efficiency limits industrial applications of the technology. It is of importance to research the Micro-arc Oxidation technology of low energy consumption, which is also an important development direction of MAO technique. Aimed at reducing the energy consumption and hencing current efficiency of MAO, through the optimal electrolyte parameters and electrical parameters.NaAlO2,KF·2H2O,Na2HPO4·12H2O electrolyte system with additives in the mode of constantly applied current densities. Through the orthogonal tests, worked out an optional electrolyte system of low energy consumption MAO. The influences on micro-arc oxidation process were investigated, such as applied current densities, concentration and constituents of the electrolyte as well as treatment time, respectively. By this MAO technique, it was found that sparks occurred in 30s, the breakdown voltage was 80V, and the working voltage below 110V. the average thickness of ceramic coatings is over 30μm , the consumption of energy is reduced to 18.08 w·h /(m2·μm),far lower than the energy consumption of the normal MAO. Surface morphologies and phase structures were analyzed using SEM and XRD. Potentiodynamic polarization curves were used to evaluate the corrosion resistance of ceramic coatings formed on magnesium alloys. A few circular pore and micro-cracks were found on the coating surface. XRD analyses indicated that the ceramic coatings fabricated on the surface of magnesium alloys by MAO are mainly composed of spinel phase MgAl2O4 and MgO.Corrosion current density is reduced to 0.134μA/cm2, which indicated that ceramic coating formed on magnesium alloys surface by MAO provide effective corrosion resistance for the substrate in containing Cl- solution and strengthen the corrosion resistance considerably, about 100 times of the naked Mg alloy. The comparing experiment results showed that negative impulse voltage impacted the distribution of elements within the film. The recoil effect is conducive to infiltration of elements to the base body to form a compact layer.Micro-arc Oxidation technology studied on this paper is low energy consumption, in particular. The properties of film made by this technology can be compared with the MAO of high-energy.