Analysis of Macrosegregation in Laboratory-Scale Experiment Using Al-Cu Alloy

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Macrosegregation is directly related to the performance of steel products. So many research works have been performed to tackle this problem. Quality of steel products have been increased remarkably, though, we have still some problems of macrosegregation. In this study, in order to investigate the relationship between solidified structure and macrosegregation, model experiments using Al-10 mass%Cu alloy have been carried out. A mold, which can form macrosegregation in the central region of the small ingot, has been newly developed. About 520 g of Al-10 mass%Cu alloy was used for experiment, changing casting temperature ranged from 650 °C to 850 °C. Areal fraction of eutectic structure was characterized for macrosegregation. When this value exceeded 0.23, which was evaluated using Scheil equation, it was defined that macrosegregation formed in the solidified shell. Primary crystals bridged together and free liquid flow was prevented near the chill plate. Therefore, the negative pressure increased and relatively high suction force acted through the mushy zone near the chill plates. Thus, remained liquid in the interdendritic region, which had high solute content, flew into the bottom area. After complete solidification, there remained large shrinkage and macrosegregation below the chill plated region. On the other hand, near or above the chill plate region, negative segregation and/or small porosity formed.