Establishment of Grain Refining Technique of Magnesium Alloy Castings

研究了半固态铸造细化AZ91镁合金的晶粒度，并进行了半固态连铸以获得细小、球化的铸坯。此外，还对磁场除铁进行了研究，认为磁场除铁可以提高晶粒细化的可靠性。这种半固态工艺的特点是采用带有浇注单元的倾斜冷却板提供半固态浆料。当熔体通过冷却板时，在冷却板的冷却作用下，大量细小的一次粒子结晶出来，以半固态浆料的形式相继落入设置在板端的预热中间包中。这样的浆料在中间包中保存一段时间，然后冷却到水中。对铸态组织的微观分析表明，铸态温度为878K，板长为200 mm，板角为60°时，铸态组织最理想(平均晶粒度为60μm)。采用连铸的方法可以获得长600 mm、直径40 mm的铸坯。通常，铁在镁熔体中的溶解度很小，在973K时，铁在镁中的溶解度约为0.01%，在镁熔体中析出细小的颗粒。为此，对电磁铁除铁进行了研究。在973K下，将电磁铁磁化的纯铁棒插入熔化的镁-0.05Fe合金中，然后在炉内凝固。金相分析表明，细小的铁颗粒倾向于相互磁聚在一起，并在镁中形成一些团簇。

Grain refining of magnesium alloy AZ91 by semi-solid casting was investigated, and semi-solid continuous casting to produce fine-and globular-grained billet was performed. In addition, iron removal by applying of magnetic field was also examined because it could be attributed to improve the reliability of grain refining.The feature of this semi-solid process is that inclined cooling plate equipped with casting unit gives the semi-solid slurry. When the melt pass through the cooling plate, it makes a numerous number of fine primary particle crystallize out by the cooling effect of the plate and successively dropped as semi-solid slurry into the pre-heated tundish set at the plate end. Such slurry was kept for a given time in the tundish and then quenched into water. Microscopic examination of the as-solidified structure shows that the most desirable structure (60μm in average grain diameter) arises from the following conditions ; casting temperature of 878K, plate length of 200mm and plate angle of 60deg. Further, when continuous casting was applied to the slurry, the billet which is the length of 600mm and the diameter of 40mm, could be successfully obtained.Generally, iron has precipitated as fine particles in molten magnesium due to its small solubility, e.g., about 0.01% in magnesium at 973K. Therefore, iron removal by electromagnet was examined. A pure iron bar magnetized by electromagnet was inserted into molten Mg-0.05Fe alloy at 973K, and then the melt was as-solidified in the furnace. Metallographic examination suggests that fine iron particle tend to magnetically gather with each other and made some cluster in magnesium.