Dong Qipeng, Yin Yanbin, Zhu Zhen, Nagaumi Hiromi
High-Performance Metal Structural Materials Research Institute, School of Iron and Steel, Soochow University, Suzhou 215021, China.
State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China.
Materials (Basel). 2020 Nov 26;13(23):5379. doi: 10.3390/ma13235379.
Sedimentation of free-floating grains is the main origin of the negative centerline segregation in direct-chill casting of aluminum alloys. This study examines the motion and distribution of the floating grains during casting using experimental measurements and numerical modeling. The typical floating grains consisting of interior solute-lean coarse dendrites and periphery fine dendrites were experimentally observed only in the central region of the billet along with the negative segregation. The billet exhibits the strongest segregation at the center where the most floating grains are found. In simulations, under the action of the convection and the underlying forces, the grains floating in the transition region exhibit different motion behaviors, i.e., settling to the mushy zone, floating in the slurry zone, and moving upward to the liquid zone. However, most grains were transported to the central region of the billet and then were captured by the mushy zone and settled. Therefore, the floating grains comprise the largest share of the grain structure at the center of the billet, in agreement with the experimental results. Moreover, the simulation results indicate that the increased size of the grains promotes the sedimentation of the floating grains. These results are important for the future alleviation of negative centerline segregation in direct-chill casting of aluminum alloys.
自由漂浮晶粒的沉降是铝合金半连续铸造中负中心线偏析的主要成因。本研究通过实验测量和数值模拟，考察了铸造过程中漂浮晶粒的运动和分布。典型的漂浮晶粒由内部溶质贫化的粗大枝晶和外围细小枝晶组成，实验观察到其仅在铸坯中心区域出现，并伴有负偏析现象。铸坯在发现最多漂浮晶粒的中心处表现出最强的偏析。在模拟中，在对流和潜在力的作用下，过渡区域中漂浮的晶粒呈现出不同的运动行为，即沉降到糊状区、在浆体区漂浮以及向上移动到液相区。然而，大多数晶粒被输送到铸坯的中心区域，然后被糊状区捕获并沉降。因此，漂浮晶粒在铸坯中心的晶粒结构中占比最大，这与实验结果一致。此外，模拟结果表明，晶粒尺寸的增大促进了漂浮晶粒的沉降。这些结果对于未来减轻铝合金半连续铸造中的负中心线偏析具有重要意义。
