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Macrosegregation in solidification of A356 Khadivinassab, Hatef

Abstract

A combined experimental / numerical approach has been applied to investigate the bulk transfer of solute due to liquid metal feeding during shape casting of aluminum alloy A356 (Al-7Si-0.3Mg). A series of dumbbell-shaped experimental casting geome- tries have been developed, which promote solute redistribution due to liquid metal feeding. Three of the castings were produced in small moulds with natural cooling, forced cooling and insulated conditions and one casting was made in a large mould with natural cooling. The redistribution of solute in the castings has been evaluated using a novel image processing technique based on the area fraction of silicon. The results show that the casting with the forced cooling configuration exhibited a larger degree of macrosegregation. In the numerical model, silicon segregation during solidification is calculated as- suming the Scheil approximation, and is coupled with a macro-scale transport model that considers resistance in the mushy zone and feeding flow. The model has been implemented within the commercial CFD software, FLUENT, which simultaneously solves the thermal, fluid flow fields and species segregation on the macro-scale. The results from the simulation agree with the experimental results, except for the cases where significant liquid encapsulation occurs. The model predicts high levels of enrich- ment when liquid encapsulation is present in the joint section of the dumbbell-shaped castings. Finally, a constitutive behaviour relationship was developed based on the Ludwik- Hollomon equation to predict the flow stress of Al-Si-Mg alloys with varying silicon composition and Dendrite Arm Spacing (das) in the as-cast (ac) or T6 condition with high accuracy. This model was then used with the results of the segregation model to predict yield strength distribution in the aforementioned dumbbell-shaped casting. The results show that silicon segregation has a more significant effect on the yield strength than das.

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