Effects of casting conditions on periodic surface patterns and internal cracks of Al-3%Si-1%Fe alloy twin-roll cast strips

High-speed twin-roll casting is expected to achieve highly productive upgrade recycling of aluminum alloys. In this study, Al-3%Si-1%Fe prepared from 6022 and ADC12 aluminum alloys was used as the model alloy for upgrade recycling. This alloy is based on Al-3%Si and has a wide semi-solid temperature range, which often causes periodic surface patterns and internal cracks depending on the casting conditions. These defects degrade mechanical properties and appearance. The surface patterns are thought to be caused by the vibration of the molten metal in the gap at the contact point between the nozzle tip and roll surface. The internal cracks are considered to result from a decrease in fluidity in the semi-solid temperature range. Insufficient supply of molten metal to the center of the casting strips causes shrinkage porosity. In this study, the effects of casting conditions, such as casting speed and solidification length, on the formation of surface patterns and internal cracks were investigated, and optimal conditions for suppressing these defects were discussed. Scanning electron microscopy and X-ray computed tomography were used to study surface patterns and internal cracks in the cast strips to examine the optimal casting conditions. Internal cracking tended to appear in shiny areas of the surface. Increasing the solidification length decreased the periodic patterns on the strip surface, and the entire surface was un-shiny. No internal cracks were observed in the un-shiny strips without periodic patterns.