Solidification structure and casting defect in a high-speed twin-roll cast A6022 aluminum alloy with various Fe contents

Closed-loop recycling should be promoted for wrought aluminum alloy scraps including impurity iron in order to use natural resources effectively. Fabrication of the alloy strip using a high rate of cooling is a promising method for reducing the detrimental effects of impurity iron. In the present study, strips of A6022 aluminum alloy with various Fe contents of up to 1.5 mass% were fabricated by a vertical-type, high-speed, twin-roll caster. The caster used in the study was equipped with a pair of water-cooled, pure copper rolls, and a cooling slope upstream of the rolls. The strips exhibited a common microstructural feature. Columnar grains grew from both surfaces of the strip, and they were gradually replaced by equiaxed grains. In the mid-central region, a band of fine grains was observed. Such fine grains seemed to originate from the crystallized alpha-Al dendrites on the cooling slope or the roll surfaces. Internal cracks were observed in the A6022 alloy strip that was subjected to the highest cooling rate. The cracks were located between equiaxed grains and fine grains at the mid-central region. The cracks were reduced with increasing Fe content. No cracks were observed for the alloy strips with 1.0 mass% Fe or greater. It was found that both a slight reduction in the cooling rate and addition of Fe were effective for obtaining sound A6022 strips.