Effects of processing parameters on microstructure of semi-solid slurry of AZ91D magnesium alloy prepared by gas bubbling

The semi-solid slurry of AZ91D magnesium alloy was prepared by gas bubbling process. The effect of processing parameters, including gas flow rate, cooling rate and stirring end temperature, on microstructure of AZ91D semi-solid slurry was investigated. With increasing the gas flow rate from 0 to 5 L/min, the average size of primary alpha-Mg particles decreases from 119.1 to 77.2 mu m and the average shape factor increases continuously from 0.1 to 0.596. The formation of non-dendritic primary alpha-Mg particles during gas bubbling is the result of combined effects of dendrite fragmentation and copious nucleation. With increasing the cooling rate from 3.6 to 14.6 degrees C/min, the average particle size of primary alpha-Mg phase decreases from 105.0 to 68.1 mu m while the average shape factor peaks at 9.1 degrees C/min. Both high and low cooling rates can induce dendritic growth of primary alpha-Mg particles. Changing the stirring end temperature from 590 to 595 degrees C has little effect on the average size and shape factor of primary alpha-Mg particles in AZ91D semi-solid slurry. The insensitivity of semi-solid microstructures to the stirring end temperature is attributed to the sufficient quantity of primary particles formed in the melt.