Controlled Solidification to Obtain Equilibrium Microstructure in DC Cast Rolling Ingot

Abstract

In 6xxx Al alloys, most of the secondary constituent phases that are formed at the primary aluminum (α-Al) grain boundary or interdendritic region during direct chill casting are brittle “metastable” phases, which complicates the downstream processes of the cast ingot. To overcome this, the cast ingot is usually homogenized and heat-treated before hot rolling. In the current study, we have shown the possibility of casting 6xxx Al alloy ingot with near-equilibrium solidification structure, thereby eliminating the homogenization process. This in turn increases productivity by reducing downstream processing steps. Novel jet stirring and in situ homogenization techniques control the solidifying microstructure, and the microstructural evidences show that the cast ingot has equilibrium structures. The thermal history of the solidifying ingots is found to play a significant role in producing equilibrium microstructure. The setup of a modified DC casting process with a mechanism for the observed microstructure changes as compared to the standard process has been discussed.