Investigation On TEchnology Of Micro-arc Oxidation Of Magnesium Alloy Modified By Nano-TiO₂

There is a large number of Magnesium in the crust and the ocean, magnesium and its alloy offers some advantages, such as low density, higher specific strength and rigidity, good shock absorption, cutting performance, conductivity and heat conductibility, and Magnesium and its alloy as an environmental-friendly material is easy to be casted, it has already became a excellent modern industrial material and been widely used in various fields, such as the aerospace, the military, the nuclear, the automotive and other industries, it's known as the"21st Century Green Engineering Material". However, its chemical property is very actively, and it's easy to occur severe corrosion in the humid environment and Cl--existing conditions, the extensive of its application as structural materials is limited severely, so, it's very necessary to improve the corrosion resistance of magnesium alloy by surface treatment.Today there are many kinds of surface treatment technology for magnesium alloys, but most of them can cause environmental pollution, and the film prepared by the method is thinner. But micro arc oxidation (MAO), a relatively new technique, is very simple and its electrolyte is friendly to environment, it's more important that the requirement for the shape and complexity of material is not high for this process. During the micro arc oxidation process, there are electrochemical oxidation, chemical oxidation in the high temperature, anodic oxidation and plasma oxidation reaction, the combination strength between film and substate is high, at the same time, the film can improve the corrosion and wear resistance of the magnesium alloys, therefore, the technique has a wide prospects for development.At present, although the film prepared by micro arc oxidation has certain characteristics, we can also find a lot of pores on the morphology of the film, and these unequal pores can affect the performance of oxide film seriously. Correlation studies showed that nano-TiO₂ particles offer some advantages, such as quantum effects, color effects, photocatalysis and ultraviolet light shielding function, the coating prepared by using nano-TiO₂ particles has high strength, good toughness, corrosion resistance and excellent wear properties. so, nano-TiO₂ particles are widely used in the modification of multi-coating and the preparation of coating. In the study, we add some nano-TiO₂ particles in the aluminate electrolyte to improve the composition, structure and organizatio of the film, develop the micro-arc oxidation film with excellent corrosion protection, and finally obtain a integritly and systematic preparation process. The substate is die-casting AZ91 magnesium alloy, basing on the current research, the parameter of micro arc oxidation is that: NaAlO2 (20g/l) as the main component, frequency is 700Hz and the duty in view of the present MAO equipment is 30%. The following aspects in research and certain results were achieved:In the study, according to the surface morphology, compactness and corrosion resistance of nano-TiO₂ particles modified micro-arc oxidation film, we optimize the technological parameters and the concentration of electrolyte containing nano- TiO₂ particles, finally, we analysis the effect of additives NaOH, nano-TiO₂ particles and current density on film, results are as follows:(1) Adding certain NaOH in the aluminate electrolyte containing nano-TiO₂ particles (0.4g/L) can reduce the oxidation voltage and the pores in the layers, at the same time, it can improve the corrosion resistance of the coating. We found that it's better to prepare the coatings when the concentration of NaOH in the electrolyte is 5 g/L, the current density is 4A/dm2, and the time of oxidation is 30 min.(2) The morphology and organization of film are influenced by nano-TiO₂ particles in the electrolyte, with the increasing of nano-TiO₂ particles, the color of layer gradually changed from the gray to dark green, and the porosity of film is decreased, the oxidation voltage, compactness and corrosion resistance of film is increased, but if the concentration of TiO₂ in the electrolyte is too much, which can reduce the arc stability and cause partial discharge of the film, thus, we obtained the better parameters is that nano-TiO₂ particles4-5g/L, current density 4A/dm2, oxidation time 30 min.(3) The change of current density can affect the quality of the film. When current density is less than 6A/dm², the oxidation voltage is smoothly increased, the arc stability is better. Surface roughness, thickness and corrosion resistance of the MAO coating is increased with current density increasing, at the same time, the color of layer gradually changed from the gray to dark green, the higher the current density, the darker the color. But if the current density is too large, partial discharge of the film can be caused and the arc stability can be reduced, so, when current density is 6 A/dm2 and oxidation time is 30 min, we obtained excellent MAO film in the aluminate electrolyte containing 4.8g/L nano-TiO₂ particles on AZ91magnesium by Micro-arc oxidation.Finally, we studied the composition and structure of nano-TiO₂ modified micro-arc oxidation film and found that the coating is made of MgAl₂O₄, MgO and Mg phase, which has not significant stratification, the main composition of elements are Al, Mg, O and Ti, diffracted intensity of MgAl2O4 was strengthened, diffracted intensity of Mg was weakened and content of Ti was also increased with concentration of TiO₂ increasing, but diffraction peak of Ti was not founded in the XRD.