Solidification Structure and Casting Defects in High-Speed Twin-Roll Cast Al–2 mass% Si Alloy Strip

Abstract

Al–2 mass% Si alloy, which has wide freezing temperature range, was cast using a laboratory-scale vertical-type high-speed twin-roll caster. The solidification structure and several kinds of casting defects were examined using OM, SEM and SEM-EDS. The results showed that the unstable melt pool condition could cause the surface defects such as the ripple mark, “un-shiny” zone and the inverse segregation. In the present nozzle type, constructing a stable high melt pool level was seemed to be essential to obtain a sound strip with no defect on the strip surface. A large-scale internal cracking along the casting direction was also observed in the high-speed twin-roll casting of aluminum alloy. The present results revealed that the cracking was related to distribution of the residual liquid in the central band region, which could be controlled by the roll separating force near the roll nip. It is considered that the roll separating force caused the shear localization in the central band region and promoted a formation of continuous liquid film in the shear localized region. The liquid film was a cause of the internal cracking when the strip passed through the roll nip.