Abstract
The influence of the β-Al5FeSi intermetallic phase on permeability evolution during solidification in an Al–Si–Cu alloy with a columnar dendritic microstructure has been numerically studied at solid fractions between 0.10 and 0.85. The fluid flow simulations were performed on a semisolid microstructure extracted directly from a single solidifying specimen, enabling the first study of permeability variation on an individual microstructure morphology that is evolving in solid fraction. The 3-D geometries were imaged at the TOMCAT beamline using 4-D (3-D+time) in situ synchrotron-based X-ray tomographic microscopy. The results illustrate the major effect of intermetallic particles on flow blockage and permeability. Intermetallics that grow normal to the flow direction were found to have a greater impact on the flow field in comparison to intermetallics in the parallel flow direction. An analytical expression, based on the anisotropic Blake–Kozeny model, was developed with a particle blockage term that takes into account the effects of intermetallic particles on permeability. In the regime of primary-phase solidification, a good fit between the analytical expression and the simulation results is found.
Highlights
Defect formation during industrial metal alloy casting processes is closely linked with fluid flow in the mushy zone since the resistance to flow through the solid network causes a pressure drop in the liquid that can contribute to porosity formation [1], macrosegregation [2] and hot tearing [3,4]
Unlike previous numerical studies of permeability that utilized quenched specimens in combination with post-mortem tomographic imaging [25,26,27,28,29], this work has extracted the geometry directly from a microstructure captured during solidification. This is the first use of in situ 4-D tomographic microscopy for such studies, and the results provide a novel, detailed permeability assessment of an individual microstructure morphology that is changing both in solid fraction and in intermetallic phase evolution rather than assessing a post-mortem set of snapshots typically given by quenched experiments
A novel method for coupling in situ 4-D synchrotron X-ray tomographic microscopy with numerical simulations was developed to quantify the permeability of solidifying microstructures
Summary
Defect formation during industrial metal alloy casting processes is closely linked with fluid flow in the mushy zone since the resistance to flow through the solid network causes a pressure drop in the liquid that can contribute to porosity formation [1], macrosegregation [2] and hot tearing [3,4]. One parameter used to characterize the fluid flow in the mushy zone is permeability, i.e. a tensor measure of the ease of fluid flow through the solid network [5]. Iron is a common impurity element in aluminium alloys that is difficult to remove during processing. In Al–Si and Al–Si–Cu foundry alloys, the iron impurities may form a b-Al5FeSi intermetallic phase that can be detrimental to the mechanical properties of these alloys [8,9].
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