Effect Of Nano ZnO On Solidification Structure Refinement Of Commercially Pure Magnesium And AZ31Magnesium Alloy

Lower strength is a factor that limits the wide application of magnesium alloy, and the important method to improve its strength is grain refinement. However, there is still no effective grain-refiner of AZ series at present, so it’s really significant to develop an effective grain-refiner and study on the mechanism of it. According to the domestic and foreign research, we choose commercially pure magnesium and AZ31magnesium alloy as study object. By observing their structures and testing their performance. We acquire some achievements and the conclusions are as following:The structure of commercially pure magnesium has been refined by adding different amounts of nano ZnO powder. The structure of commercially pure magnesium with nano ZnO powder is transformed from the coarse columnar grains into equiaxed ones. With the increasing amount of nano ZnO being added, the average grain size could be further refined but refining efficiency decreases. As the amount of nano ZnO was4%added at720℃, the average grain size was refined from the millimeter-scale to the micron-scale as small as170μm.The solidification structure is significantly different as nano ZnO powder was added at different temperatures. It is unbeneficial for the refining effects to add nano ZnO at excessive high or low temperatures. The commercially pure magnesium melt with2%nano-ZnO powder at750℃and holded for40min, has the smallest average grain size of158μm.After nano ZnO were added, the grain size of commercially pure magnesium varys with different holding time. As nano ZnO was added at low temperature, the average grain size firstly decreases and then increases with increasing holding time. The variation on average grain size of commercially pure Mg is small with the increasing holding time, when the nano ZnO was added at the medium temperature. In the present study, the solidification structure with nano ZnO is small and uniform after holded for30～40min.Nano ZnO also has great efficiency on refining AZ31magnesium alloy, the average grain size of AZ31decreases with adding the content of nano ZnO. However, the efficiency of refining goes smooth, when adding0.7%ZnO and holding for10min at690℃, we could got smaller grain, the average grain size of AZ31decreases from313μm to203μm, at the same time, the maximum tensile strength increases from113MPa to204MPa. Holding time has low efficiency on the variation of grain size and tensile strength.By comparing the refine effect and performance of AZ31magnesium alloy after adding equal ZnO and Zn. The results show that the influence of addition is similar, while ZnO has better refinement, and the strength of AZ31after adding ZnO is higher at the same time. So it indicates that the refinement of AZ31is not only related to the substitutional Zn but also the effect of ZnO itself.