Effect of Size and Morphology of Equiaxed Grains on Macroscopic Segregation

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It is well-known that the degree of macrosegregation decreased with increasing the amount of equiaxed zone. However, the effect of size and morphology of equiaxed grains on the macrosegregation has not been understood. Thus, laboratory-scale experiments using Al-10 mass%Cu alloy have been carried out. The equiaxed structures can be easily obtained in aluminum alloy using modifier, which contains Ti and B, without changing the solidification condition, such as fluid flow. A mold, which can form macrosegregation in the central region of the ingot, has been developed and used in this study. Changing casting temperature and the amount of modifier, the central regions of the ingot have been metallographically analyzed. The degree of macrosegregation has been characterized by the areal fraction of eutectic structure.It was found that the diameter of equiaxed grains decreased with increasing the amount of modifier. Furthermore, the shrinkage and macrosegregation decreased with decreasing the diameter of equiaxed grains. Complexity of the equiaxed grain has been also characterized using fractal dimension. In case of the same diameter of equiaxed grains, the degree of macrosegregation in the ingot decreased with increasing the fractal dimension of the equiaxed grains. The reason of these may be that the flow of liquid phase in the mushy zone decreases when the equiaxed grains are fine and/or complex. Since the liquid flow in the final stage of solidification is inadequate, small shrinkages form in the interdendritic region and consequently the suction force becomes small. Therefore, neither large shrinkage nor macrosegregation forms.

Effect of Size and Morphology of Equiaxed Grains on Macroscopic Segregation

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