| The applications of magnesium alloy are limited by the easily corrosive, lower hardness and weariness. Now, there are many way to improve the ability of against corrosion and wear resistance of the magnesium alloy. Among them, the Micro-arc oxidation (MAO) is a relatively new surface treatment technology of magnesium alloy developed from the traditional anodization, which has promising prospects in the surface treatment of magnesium alloy. The MAO can produce a ceramic oxide coating with higher wear resistance and corrosion resistance by anodic discharge deposition under higher applied voltage and current density. However, it is necessary to research on the complex oxidation growth process and the corrosion process of MAO coating under harsh conditions for further improving its corrosion resistance and eliminating the impact of the porous structure of MAO coating.In the MAO by the asymmetric parameter mode, the influences of frequency ratio of the pulse power and the parameters of negative pulse on the MAO are grateful. First, the transferring process of anions can be changed. Second, the electric parameters are related with the thickness, structure and surface morphologies of MAO coatings. In the current study, the bipolar constant-current pulse mode was used to produce the MAO coating of the magnesium, and the influence of frequency ratio and negative pulse parameters on the MAO of AZ91D magnesium alloy were studied. Furthermore, the influence of TiO2 sol on the MAO of magnesium was also be studied. The main works and results are as follows:The influence of frequency ratio on the MAO was studied by the experiment of frequency ratio 1,5,25, respectively. The results indicate that frequency ratio has influence on the morphology, thickness, uniformity, composition and corrosion resistance of the MAO coating. The porosity of the outer film increased and the thickness, uniformity of coating decreased with the increase of the frequency ratio. But the frequency ratio has no significant impact on the internal structure, composition of the MAO coatings.The influence the pulse current density (ip) and duty cycle (dn) of negative pulse on the MAO were also studied. The thickness, structure, uniformity, composite and morphology of coatings were remarkably influenced by ip and dn. The thickness and porosity of the MAO coating increased, and the uniformity and protectiveness decreased, but the content of Mg2SiO4 decreased and the content of MgO increased with the increase of ip under fixed dn of 10%. The content of MgO decreased and the content of Mg2SiO4 increased in the MAO coating with the increase of dn with fixed ip of 300mA·cm-2. The coating was divided locally with substrate and the applied voltage dropped suddenly in the MAO process in the case of dn more than 10%. The larger of dn, the earlier of the suddenly decline of voltage.Based on the above research, the influence of TiO2 sol on the AZ91D MAO was studied. In this paper, the influence of TiO2 sol on the morphology, thickness, uniformity, composition and corrosion resistance of the MAO coating was studied by the different content of TiO2 sol added. The results indicate that TiO2 sol can improve the performance of the MAO coating produced and the MAO coating with best protectiveness can be got under the content 4 Vol% of TiO2 sol. |
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