Ultrasonic assisted reduction of hot-tearing during high-speed DC casting of 6000 series aluminum alloys

This work presents results of preliminary investigations concerning the effect of ultrasonic vibrations on the solidification structure and hot-tearing susceptibility of 6000 series aluminum alloys in high-speed direct chill casting processes. A pilot DC caster was used to produce billets of 82–97 mm in diameter. Ultrasonic vibrations were introduced directly into the mold through a high-amplitude ceramic sonotrode, the tip of which was positioned at different distances from the melt crystallization front. The cast billets were then investigated for the microstructure and hot tearing susceptibility. It is shown that the ultrasonic treatment leads to a significant reduction in hot tearing susceptibility, and at the same time to a rise in mechanical properties of the alloys. The results suggest that at least two ultrasonic effects contribute to these improvements. The first one is cavitation which results in forming more refined and uniform microstructure of alloys. The second one is acoustic streaming which is responsible for macro agitation of melt in the sump. This causes the liquid-solid system to approach an equilibrium state that results in increasing the fraction of eutectic phase solidified at the grain boundaries of α-Al phase.