| Magnesium is a low density metal. The density of magnesium is1/1.5of the aluminum,1/4.1of the zinc. But the state of elemental magnesium can not meet the requirements of performance, so it need improve the performance fo all aspects by alloying. Magnesium alloy is the lightest structure materials with high specific strength, specific stiffness, good damping shock absorption, good electromagnetic shielding, as well as good machinability and dimensional stability. China has abundant magnesium with the first production and exportion of primary magnesium in the word which laid a solid resource base for in-depth study of magnesium alloy.Rare earth as the main alloying elements or micro-alloying elements are being widely used in magnesium alloys. In the casting process, magnesium alloy is easy to form micro pores which made the castings poor heat resistance and corrosion resistance. So it can not meet the requirements at high temperature and corrosive environments. According to the study shows that adding rare earth elements can effectively improve the microstructure and the mechanical properties. La is an effective element to improve the mechanical properties of magnesium alloy. In addition, the price of La is not high which can not increase the cost of magnesium alloy.In this paper, high-energy ultrasound was used to prepared Mg-15%La master alloy and use it to add in magnesium alloy. The study showes that when the rare earth La add into AZ61magnesium alloy, its microstructure became much finer. Because when the La add into magnesium alloy, La would make reaction with Al priority to form needle phase Al11La3. Al11La3phase enriched in the interface and formed composition undercooling layer, which inhibited a-Mg phase to continue to grow up, so that it can refine the grain. when the content of rare earth La up to1.0%, the mechanical properties of AZ61magnesium alloy is best. Regardless by way of pure La or the way of Mg-15%La alloy, when the mass fraction of La in AZ61magnesium alloys reached to1.0%is the best value of alloying. In this paper, we study some factors of pouring temperature, holding time, rare earth content by orthogonal experiment to find the effect of AZ61+XLa magnesium alloy. We obtained when the pouring temperature reached to720℃, holding time reached to30min, rare earth content reached to1.0% are the best value of microstructure.Under the isothermal heat temperature process of holding temperature(570℃,580℃,590℃), holding time(5min,10min,15min), it is found that by the isothermal heat-treated temperature increased, the phenomenon of serious dendritic in the microstructure of initial cast alloy turned into semi-solid non-dendritic tissue through the method of isothermal heat-treatment. Its tissue evolution included three process of coarsening, tissue separation and spheroidizing. At the process of isothermal heat-treatment, the element of La was very important to refine alloy’s organization.It could hinder the particles of solid phase from aggregating and associating, which could inhibit the particles of solid phase from further growing to make the microstructure of non-dendritic tissue more smaller. Throughing different time of isot-hermal heat-treatment, the irregular of large block tissue turn to spheroidizing and forming many nearly spherical particles. But the time of isothermal heat-treatment over a certain range, the spherical particles began to coarsen and many liquid islands formed in the particles. The spheroidizing of dendritic microstructure included the disappearance of secondary dendrite wall and Ostwald ripening process. |
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