Establishment of Grain Refining Technique of Magnesium Alloy Castings

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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.