Effects Of Technique Parameters On Microstructure And Performances Of Micro-arc Oxidation Coating On Magnesium Alloy

Magnesium alloy is used widely by virtue of it's advantages in property. Especially, with the rapid development of mechanical product and acoustics computer communications and with the progress of digital technique, the applications of magnesium alloy in electronic equipment are increasing at the speed of 25% every year , and show attractive developmental foreground. However, the high chemical activation of magnesium alloy renders the surface of magnesium alloy inferior resisting corrosion, which brings serious obstacles to the development of magnesium alloy. A number of surface modification and coating methods have been developed over years. In virtue of the application of theses methods haven't got an ideal effect, Micro-arc Oxidation treatment emerged as the times require. With the characteristics of high efficiency, nontoxic and simple which can meat the requirements is one of the most promising methods for magnesium alloys.Microarc oxidation is a new material surface treatment technology, which developed on the basis of anodic oxidation. Microarc oxidation is also entitled microplasma oxidation or anodic spark deposition. Al,Mg,Ti or their alloys are placed in electrolyte, spark discharge occurs under high voltage in the microholes of the material surface, and ceramic coating can be formed on the effect of thermochemistry, plasma chemistry and electrochemistry.It is of great significance to take a deep study of the stable composition of the electrolyte,the optimization of the processing parameters and the composition and microstructure of the oxidized film and so the forth, although the study of the micro-arc Oxidation treatment has got a good result in recent years. The magnesium alloys used in this paper was AZ91. Our researching work was concluded as followings:First, a stable electrolyte was developed successfuly as well as the optimization of the processing parameters on the basis of measuring the thickness of the oxidized films, the morphology of the oxidized film and the corrosion resistance. Our experiment adopted the aluminates as the basic electrolyte, the optimized frequency and duty are 700Hz and 30%, respectively.Second, in the aluminate electrolyte, a serial of samples were prepared. Range analysis was employed to observe the effects of the processing parameters to the microstructure of the film. Some conclusions were drawn as the followings:1. A great many of pores existed in the film under different conditions. Some micro- cracks existed around the big pores. The big pores were formed by the repeated eruption from the point when the alloys discharged. The micro-cracks formed due to the heat stress when melting substance meet the electrolyte.2. With the concentration of the sodium hydroxide increased, the porosity of the film decreased, while dimension and fractal dimension decreased, the thickness of the film descended while the thickness of the oxidized film increased when the concentration increased. However, the quality of the film decreased while the porosity, the dimension and the fractal dimension increased.3. The current density and oxidation time influenced the micro-structure of the film remarkably. With the current density increased, the porosity, dimension, fractal dimension and thickness increased. When the current density increased continuously, the surface of the sample discharged tempestuously, the dimension,porosity and fractal dimension decreased.Finally, according to the research results of the contact angle and the corrosion resistance of the oxidized film we acquired the effect laws of the processing parameters to the contact angle and the corrosion resistance of the oxidized film.The contact angle and the corrosion resistance of the oxidized film increased with the increase of the concentration of the sodium aluminates respectively. A high current density could improve the corrosion resistance while it was not favor of the improvement of the contact angle. In a range of time, with the oxidation time increased, the corrosion resistance of the film improved and the contact angle turned bigger. The accession of sodium hydroxide increased the contact angle and improved the corrosion resistance of the film.