Center Segregation and Cross-Hatched Surface Pattern of Vertical-Type High-Speed Twin-Roll Cast Al–Si–Mg Alloy Strip Formed by Wide Rolls

Abstract

Al–7%Si–0.4%Mg alloy was cast using a vertical-type high-speed twin-roll caster equipped with commercial scale large rolls. The cast strip had a distinct cross-hatched surface pattern on both sides, with matte and glossy regions periodically distributed throughout the surface. Surface characterization revealed that the difference in appearance making up the pattern was correlated with the difference in surface profile, roughness, and microstructure of the outermost surface. In addition, the pattern was in harmony with the discontinuous distribution of the center segregation band. An interrupted casting confirmed that the cross-hatched surface patterns started forming at the site where solidified shells contacted with each other and strip was deformed by the roll separating force. Therefore, a mechanism is proposed to explain that the squeezing of liquid from mushy layer in a cyclic manner forms the cross-hatched pattern, which corresponds to the periodic distribution of eutectic-rich and α-Al-rich regions in the mid-thickness of the strip.