| Magnesium alloy, as the lightest materials, is available for use in the structure of metals. Magnesium alloy has some advantageous properties, including low density and high strength/weight ratio, good machinability and casting ability, high thermal conductivity, very good electromagnetic features and being easily recycled. At the present time, magnesium alloys have been used in a number of industrial fields including automobile, electronic, aeronautical and aerospace components. However, the wide application of magnesium alloys has been limited due to their undesirable properties including poor corrosion and low wear resistance. Therefore, it is essential that a more effective surface treatment can be commonly applied to magnesium alloys in order to increase the corrosion resistance and wear resistance.The main objective of the research is to search new methods of surface treatment and to develop advanced technology for magnesium alloy on the base of available research results. The microstructure and properties (including hardness, elastic modulus, wear and corrosion) of surface layer on magnesium alloys treated by micro-arc oxidation and high energy beam (electron beam and ion beam) have been studied systematically. The effect of technical parameter on the morphology and microstructure of the treated layer on magnesium alloy has been studied, and the relationship between microstructure and properties has also been discussed. The detailed contents and results include three parts in the following.1. Micro-arc oxidation (MAO) of magnesium alloy: MAO was employed to treat the surface of magnesium alloy, the effect of composition of electrolyte and technical parameter on the structure and properties of ceramic layer was studied. And mechanism of MAO of magnesium was also discussed. It has been indicated that a ceramic-like coating formed on magnesium alloy has been obtained by MAO technology, the thickness of which increases with an increasing MAO treating time. The ceramic coating contains a loose outer layer and a compact inner layer. The thickness of each layer changes with treating time and the concentration of the electrolyte. The ceramic coating obtained in silicate is compact, consisting of MgO, Mg2SiO4 and MgSiO3 phases. And the coating formed in phosphate is loose, consisting mainly of MgO and MgAlO4 phases. The growth rate of ceramic layer in phosphate electrolyte is faster than that in silicate electrolyte. The results of corrosion tests show that the thicker the compact layer in ceramic coating is, the better the corrosion resistance of which is. The ceramic coating obtained in silicate has good corrosion resistance. Nano-hardness and elastic modulus of ceramic coating have been improved significantly than the substrate alloy. The highest hardness is obtained in the compact layer, which is five times as large as that of magnesium alloy. The wear resistance of ceramic coating on magnesium alloy has also been enhanced. And the wear resistance of the MAO coatings is different as a result of different treating conditions, which is correlated with microstructure of ceramic coating. Among the MAO coating, the smallest wear volume is one thirtieth as large as that of magnesium alloy2. High current pulsed electron beam (HCPEB) treatment of magnesium alloy: The surface of magnesium alloy was treated by high energy electron beam device with a pulsed voltage of 15kV. Effect of technical parameter on the microstructure and properties of the treated layer have been studied, and the interaction between electron beam and alloy has also been discussed. Experimental results show that the melted layer was found on the surface of magnesium alloy, and the thickness of the melted surface layer varied with electron beam current and numbers of pulses. The large thickness of the melted layer may be a few microns. The treated surface layer exhibited higher hardness than magnesium alloy. The largest hardness was two times as large as that of the substrate. The friction coefficient and wear volume of magnesium alloy after electron beam treatment decrease obviously. The wear resistance of the treated samples got obviously increased, which may be attributed to high hardness as a result of grain refinement. XRD results show that the intensity of the diffraction peak assigned to Mg17Al12 increases with an increase in the energy of electron beam. And a metastable AlMg phase has also been found from the treated surface of magnesium alloy.3. High intensity pulsed ion beam (HIPIB) treatment of magnesium alloy: An ion accelerator with a voltage of 450 kV was employed to irradiate the surface of magnesium alloy. Effect of pulsed high-energy electron beam on the surface modification of AZ91 magnesium alloy has been studied. The status of surface layer and wear resistance were also investigated. It has been found that the cross-sectional microstructure of treated magnesium alloy consisted of refined layer and re-melted layer along the direction of ion beam irradiation. The thickness of the treated layer increases with the increase of the numbers of pulses and the ion beam density. The surface of magnesium alloy undergone rapid heating, melting and vaporization by ion beam irradiation, there were a lot of craters on the surface when pulsed energy was larger than 100 A/cm2. The hardness of treated magnesium alloy decreased from the surface to the interior. The highest hardness on the surface was two times as high as that of magnesium alloy. Hardening may be attributed to impact and grain refinement, which leads to obviously enhanced wear resistance of the modification layer compared to as-received magnesium alloy. The corrosion resistance of the modified layer was significantly improved because of surface cleaning and inverse absorption, homogeneousness of alloy element and grain refinement.
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