Investigation On Microstructures And Properties Of Az91D Magnesium Alloy Surface Layer

Magnesium alloy is the lightest engineering metal materials, known as the "21st Century Green Engineering Materials". It has a series of advantages,like less density,highly specific strength and stiffness,good damping capacity,superior electromagnetic shielding,good machining performance,excellent processing performance and good casting performance advantages.In recent years,it becomes the important lightweight materials in automotive,electronics,aerospace industry. With the further expansion of the scope of application of magnesium alloy, and its poor corrosion resistance problems have become increasingly prominent. Therefore, improving the corrosion resistance of magnesium alloys has become a key issue to expand its applications.In this thesis, AZ91D magnesium alloy by vacuum solid powder thermal diffusion Al-Zn and Al-Zn-Nd treatment was conducted. The microstructure and phase composition of the diffusion layer was analysed, Tafel curves and full immersion of magnesium alloy substrateand diffusion samples in3.5%NaCl solution was measured.The results show that385℃and410℃thermal diffusion10h and6h, can get a certain thickness coating which is uniform and dense on the sample surface. Due to the rare earth can refine the grain and strengthen the grain boundary, so Al-Zn-Nddiffusion layer’s tissue is more denser and smaller than Al-Zn diffusionlayer’s.And the Al-Zn-Nd permeation layer thickness has a slight increase than the Al-Zn diffusion layer thickness at the same diffusion temperature and diffusion time.The Al-Zn diffusion layer is mainly composed of Mg17Al12, Al5Mg11Zn4and a-Mg, and Al-Zn-Nd diffusion layer consists of Mg17Al12, Al5Mg11Zn4, NdAl3and a-Mg. Mg-Al-Zn intermetallic compounds, the newly generated tissue by thermal diffusion, which’s electrode potential is higher than the electrode potential of AZ91D magnesium alloy substrate material.And it has a great impact on improving the corrosion resistance of the material surface.Therefore, diffusion alloy layer significantly improves the corrosion resistance of AZ91D magnesium alloy and Al-Zn-Nd diffusion layer’s corrosion resistance is better than Al-Zn diffusion layer’s.Compared with magnesium alloy sampleswithout thermal diffusion, the thermal diffusion samples significantly improved the corrosion potential, and their corrosion current density also declined. Therefore, vacuum solid powder thermal diffusion obviously improved the corrosion resistance of AZ91D magnesium alloy and Al-Zn-Nd thermal diffusion sample’s corrosion resistance is better than Al-Zn thermal diffusion sample’s.In summary, it can get a diffusion layer on AZ91D magnesium alloy surface by vacuum powder thermal diffusion method, and the layer improved the corrosion resistance of magnesium alloy obviously and widening its field of application.