| Magnesium alloys are widely used in automotive,electric and aeronautical industries due to their low density,good electromagnetic shielding characteristics, high damping characteristics,excellent machinability and so on.However,the application of magnesium alloys is far less than other metal materials because of its inherent properties,,such as.lower strength,poor corrosion resistance and high-temperature performance and so on.Therefore,it is important to explore active approach of improving the mechanical properties and formability through grain refinement,thus to push the application of magnesium alloys and develop theirs performance,advantages.In this thesis,the effect of combined electromagnetic stirring and Ce,Sr,Ca on microstructure and performance of AZ91 magnesium alloy,which is the most widely used magnesium alloy,was studied.As well as grain refinement mechanism of alloys was discussed.Microstructures of alloys were observed by means of optical microscope(OM),scanning electron microscope(SEM)and X-ray diffract analyzer (XRD)and so on.Meanwhile,the grain size and hardness of alloys was measured,the linear relation was presented between grain size and hardness by regression analysis. The experimental results show that the effect of combined electromagnetic stirring and Ce,Sr,Ca caused a large decrease of grain size.In the case of 100V electromagnetic stirring voltage,when the content of Ce,Ca,Sr was 0.8%,0.2wt%,0.2wt%,respectively,the refine effect of AZ91D alloy was best as well as the highest hardness vaule after the addintion of 0.2%～0.8%Ce,0.1%～0.3%Sr,0.1%～0.3%Ca. Regression analysis reveals that the relation of between grain size and hardness is in coincidence with Hall-petch formual.Besides,the grain refinement mechanism of combined electromagnetic stirring and Ce,Sr,Ca was attributed to the decrease of undercooling degree of alloys according to differential thermal analysis results。The effect of combined electromagnetic stirring and Ce,Sr,Ca on corrosion perfomance in 3.5%NaCl solution were studied by means of static weight-loss test, X-ray diffract analyzer and energy spectrum analysis.The experiment indicates that the additions of 0.2%～0.8%Ce and 0.1%～0.3%Sr were benefit to improve the corrosion resistance of AZ91 alloy with electromagnetic stirring,which the improvement of corrosion resistance of the addition Sr is inferior to that of Ce.And when the content of Ce was 0.5%,the corrosion resistance was best.However,the additions of Ca deteriorated its corrosion resistance.
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