Permeability calculations on real Al-Cu microstructures assessed by 3D microtomography

The formation of defects like hot tearing, porosity, and macrosegregation in industrial casting processes depends critically on the mushy zone permeability. Due to the lack of experimental validation, there is still a large uncertainty associated with the prediction of permeability in metal alloy solidification: Some permeability measurements reported in literature are subjected to systematic errors induced by the method for displacing the interdendritic liquid in the mushy sample. Other measurements are performed after remelting of the samples, which results in significant changes in the microstructures due to coarsening and globularization. Reliable permeability measurements on industrial alloys with microstructures similar to those in industrial castings are not available. The purpose of this article is to present initial results from a new and alternative procedure to assess the permeability. Samples of Al-Cu alloys, for which the permeability has been measured in a previous work (Nielsen et al., Metall. Mater. Trans., Vol. 30A, 1999, pp. 2455-2462), were mapped in 3D using microtomography at the European Synchrotron Radiation Facility (ESRF), Grenoble. The tomograms were binarized and the permeability tensor was calculated for each sample by solving a set of generalized Stokes problems in a domain constituted by the microtomography mapping. The calculated permeability tensors are nearly isotropic and the equivalent permeability values compare favorably with the values measured by Nielsen et al. The method is promising for assessing the permeability in alloys and microstructures for which permeability measurements are presently not available.