Effect of Casting Speed on Floating Grains in Direct-Chill Casting of Aluminum Alloys

抄録

The effects of casting speed on macrosegregation and grain structure during the direct-chill casting of an Al–Mg–Si alloy are experimentally investigated. Results show that increasing the casting speed yields a more pronounced negative centerline segregation. Regard this mechanism, apart from the increased contribution of solidification shrinkage, the variation in floating grains is newly observed. The increased casting speed significantly increases the size and fraction of floating grains at the central portion of the billet, thus resulting in a more severe negative segregation. To clarify the experimentally observed phenomena, the solidification and distribution of floating grains in billets cast at different casting speeds are numerically simulated. The increased casting speed deepens the slurry zone significantly, thus providing more time for the floating grains to freely float and develop before settling into the mushy zone. Additionally, the greater slope of the coherency isotherm resulting from the increased casting speed promotes the movement and sedimentation of the floating grains toward the central portion of the billet. Consequently, more floating grains are detected at the billet center.