Production of grain-refined AC7A Al-Mg alloy via solidification in ultrasonic field

The objective of the current study was to investigate the use of ultrasonic melt treatment technology in the production of grain-refined billets of the AC7A alloy, which was intended for subsequent use as a feedstock in forming operations. The experiments included the application of ultrasonic vibrations to the molten alloys via direct and indirect techniques. Several process parameters such as pouring temperatures (several temperatures between 740 and 660 degrees C), and treatment time (from 12 min down to 2 min) were investigated. The experiment included continuous ultrasonic treatment from the liquid to the semisolid states. The results showed that both treatment techniques were viable for producing billets with the desirable microstructural characteristics. The optimum treatment conditions were the short treatment time (2 to 3 min), from about 660 degrees C down to 615 degrees C for the indirect treatment technique, and from 660 degrees C to 635 degrees C for the direct treatment technique. The resulting microstructures, at three positions along the height of the ingot, were characterized by fine, non-dendritic alpha(Al) grains in the order of a hundred microns, as compared to few thousands of microns for the conventional cast ingots. The intermetallic particles were also refined in size and modified in morphology by the ultrasonic treatment. The operating mechanisms by which the ultrasonic vibrations altered the ingot microstructures were discussed and analyzed.