A model for macrosegregation and its application to Al-Cu castings |
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Authors: | S Chang D M Stefanescu |
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Affiliation: | (1) Departement of Solidification Laboratory, the university of Alabama, 35487 Tuscaloosa, AL |
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Abstract: | A macrosegregation model has been developed to evaluate solute redistribution during solidification of casting alloys. The
continuum formulations were used to describe the macroscopic transports of mass, energy, and momentum, associated with the
microscopic transport phenomena, for two-phase systems. It was assumed that liquid flow is driven by thermal and solutal buoyancy,
as well as by solidification contraction. The movement of free surface was also considered to ensure volume con-servation.
In numerical calculations, the solidification event was divided into two stages. In the first stage, the liquid containing
freely moving equiaxed grains was described through the relative vis-cosity concept. In the second stage, when a fixed dendritic
network formed after dendrite coherency, the mushy zone was treated as a porous medium. After validation of the proposed model
for the case of segregation in a bottom-chilled unidirectionally solidified casting of Al-Cu alloys, the nu-merical model
was applied to the study of three different castings with simple geometry. It was found that solutal convection tends to decrease
the macrosegregation generated by thermal convec-tion. When shrinkage-driven convection was also considered, segregation was
again increased, with highly segregated areas forming away from the riser and next to the mold wall. It was demonstrated that
solidification contraction has a stronger effect on the liquid flow in the mushy region than buoyancy. The model also was
applied to assess the probability of pore formation based on the pressure drop concept. While in the absence of experimental
data for the critical pressure drop it was not possible to uniquely predict the formation of porosity, it was possible to
indicate the regions where porosity may form preferentially. |
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