Floating Grain Formation and Macrosegregation in a 2024 Al Alloy Prepared by Hot-Top DC Casting with a 2024 Al Alloy Insert

For the first time, the method of inserting a 2024 Al alloy rod into a melt of the same alloy during hot-top direct chill casting was used to verify the origin of floating grains and changes in the shape of the sump. Based on the as-cast structure and measured temperature distribution observed, we found that the 2024 Al alloy insert and the grains formed from melt surfaces and the interior walls of the launder or hot-top melted in the high-temperature region of the upper part of the hot-top; additionally, most of the dendrite fragments were captured by feathery grains. We confirmed that the nuclei in the center part of the liquidus isotherm region were able to form coarse-cell dendritic structures in the center part of the slurry zone due to the broad vertical width of this zone. Then, the coarse dendritic structures settled in the mushy zone, and fine dendrites formed in the periphery. In addition, with the insertion of the 2024 Al rod, a hillock-shaped liquidus isotherm was formed in the center part of the slurry zone, and the size of the floating grains in the billet processed with the insert was larger than that in the billet processed without the insert. However, surprisingly, the application of the 2024 Al rod reduced negative centerline segregation because it reduced the horizontal component of shrinkage flow.