Study On Microstructural Heredity Of Mg-Sr Master Alloy In AZ91D Magnesium Alloy

As one kind of lightweight structural material, magnesium has many distinct advantages, such as lower dentity, specific strength and specific stiffness, higher damping, strongly shielding ability of electromagnet, easier recycling and etc. The magnesium alloy has been developed rapidly in field of the Aero-Space where large number of die-cast components are used, the vehicle industry, the 3C industry, especially in field of automobile industry.The size of crystalline grains affect the mechanics properties of the magnesium alloy, greatly. Adding modification into the magnesium is one of the methods of refining the grains. Mg-Sr master alloy replaced Al-Sr master alloy, become one of the most important modification in the magnesium industry. There hasn't research literature about the microstructural heredity of the Mg-Sr master alloy, yet.The Mg-4%Sr master alloys with the same composition but different microstructure were prepared by direct mixing method. These different Mg-4%Sr master alloys were added into the AZ91D magnesium alloy under the same process conditions. The microstructural heredity of the Mg-4%Sr master alloy were validated by comparing the modifying effect of the Mg-4%Sr master alloys with each other, on the base of which, the effect of the process parameters on the modifying effect of the master alloy were researched. In addition, the melting characteristic of the Mg-4%Sr master alloy were summarized by analyzing the DSC heating thermogram of the Mg-4%Sr master alloy. The principles of the microstructural heredity of the Mg-4%Sr master alloy were analyzed in view of the melting behavior. Besides that, the physical model for the microstructural heredity and the application of the microstructural heredity of the master alloy were introduced. Moreover, in this thesis, the experimental result were studied by atomic absorption spectrophotometry, Scanning electron microscope(SEM), X ray diffraction appearance(XRD), Optics microscope(OM), etc.The research result shown that: (1)the Mg-4%Sr master alloy with the fine crystalline grains and the microstructural dispersion has better modifying effect on AZ91D magnesium alloy. (2)the process parameters, such as overheat-temperature, solidifying rate and stirring time, can effect the microstructural heredity of the Mg-4%Sr master alloy. The lower the overheat-temperature, the more obvious the microstructural heredity of the Mg-4%Sr master alloy is. The microstructural heredity of the Mg-4%Sr master alloy become weakened untill disappear, as the overheat-temperature raised; the faster the magnesium alloy solidify, the more unobvious the microstructural heredity of the Mg-4%Sr master alloy is. The slower the magnesium alloy solidify, the more obvious the microstructural heredity of the Mg-4%Sr master alloy is; The microstructural heredity of the Mg-4%Sr master alloy become weakened untill disappear, as the stirring time increased. (3)The Mg-4%Sr master alloys with the same composition but different microstructure have different DSC heating thermogram. Both of the melt temperature and the melt enthalpy of the Mg-4%Sr master alloys with thinner microstructure, are lower than that of the Mg-4%Sr master alloys with crassitude microstructure. The melting condition of the Mg-4%Sr master alloys changed, when the microstructure of the Mg-4%Sr master alloys changed, therefor, the modifying effect changed. This is the essential reason why the microstructural heredity of the Mg-4%Sr master alloy can act.