Surface quality and microstructure of Al-Mg alloy strips fabricated by vertical-type high-speed twin-roll casting

Abstract

Al-Mg alloy strips were fabricated by the vertical-type high-speed twin-roll casting. Periodic patterns on the strip surface were observed in the casting direction consisting of the shiny region and un-shiny region with cracks. The un-shiny region and cracks remain even after successive cold rolling. Therefore, formation of the periodic patterns during casting should be suppressed. In this study, the formation mechanism of the periodic patterns of the JIS-AC7A strips was examined by microstructural observation and compositional analysis. The cross-sectional observation in the un-shiny region revealed the inverse segregation of Mg and Fe from the mid-thickness area of the strip toward the surface along the grain boundaries. It is considered that the residual liquid is squeezed out from the mid-thickness area of the strip toward the surface through the grain boundaries, and results in the formation of the un-shiny region. Temperature measurement was performed at the gap formed between the roll surface and the nozzle tip by using thermocouples. The strips were also fabricated by using the nozzles with various tip shapes in order to change the contact conditions between the melt and roll surface. The oscillation of the molten metal meniscus in the gap between the roll surface and the nozzle tip is considered to be one of the possible reasons of the formation of the periodic patterns on the strip surface.