Application of a Pore Fraction Hot Tearing Model to Directionally Solidified and Direct Chill Cast Aluminum Alloys
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Hot tearing is strongly linked with the applied semi-solid strain rate. This defect is commonly qualitatively predicted using a pressure drop equation in the mushy zone that includes the effects of both tensile deformation perpendicular to the thermal gradient and shrinkage feeding. In this study, the effect of strain rate parallel to the thermal gradient is additionally introduced in order to assess its effect on hot tearing predictions. The deformation and shrinkage pore fractions are obtained on the basis of the dimensionless Niyama criterion and a scaling variable method. This Pore Fraction hot tearing model is first applied to the binary Al-Cu system under conditions of directional solidification. It is shown that for the same Niyama criterion, a decrease in the cooling rate increases both the deformation and shrinkage pore fractions because of an increase in the time spent in the brittle temperature region. Then, using a finite element simulation, the pore fraction distributions during Direct Chill casting of the AA5182 aluminum alloy are obtained. It is shown that including the strain rate parallel to the thermal gradient significantly improved the predictive quality of hot tearing criteria based on the pressure drop equation. Further, an increase in the casting speed increases the deformation and shrinkage pore fractions and causes the maximum point of pore fraction to move towards the base of the casting.
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