| Magnesium and its alloys are widely used in automotive, electrical appliances,transportation and other industrial areas due to their some advantages such as lowdensity, high strength-to-weight, good anti-vibration, strong electromagnetic shieldingand radiation resistance, et al. Mechanical performance can be effectively improvedafter adding grain refiners, Both MnCO3refiner and Al-Sr-La mixed grain refiner wereadded to AZ91and AM60respectively. The effects of grain refiner on as-cast and solidsolution treatment microstructure and mechanical properties of AZ91and AM60alloyswere studied. Besides, the refining mechanism was investigated.The results show that the MnCO3and Al-Sr-La composite refiner can effectivelyrefine AZ91and AM60magnesium alloys. Magnesium matrix was changed from bulkyisometric into tiny petals shape and Mg17Al12phase was changed from the originalnetwork to diffuse intermittent shape. AZ91and AM60’s grain became very small,the mechanical properties of solid-solution alloy is better than the as-cast alloy. It wasfound that the cooling rate of Mg-Al alloy had great effect on the grain size, the fastercooling rate, the bigger of super-cooling, the more nucleus, the smaller grain size. Theresult showed that most Mn, Sr, La highly concentrated on the grain boundary by doingenergy spectrum analysis in SEM. So we inferred the refinement mechanism of MnCO3and Al-Sr-La is mainly derived from the composition undercooling of Mn，Sr and La.Some present model for predicting grain size of the magnesium alloy weresummarized. A mathematic modle for predicting as-cast grain size of magnesium alloywere deduced based on composition super-cooling theory of grain nucleating. Theprocessing parameters such as cooling rate and temperature gradient in front ofsolidification interface on grain size were concerned by the modle. The experimentalresults showed that the model can effectively predict the grain size of as-castmagnesium alloy. |
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