Transition of the mushy zone from continuous liquid films to a coherent solid

Abstract

While studies of solidification microstructures have focused mainly on the tips of the dendrites, the last stage solidification is equally important from the point of view of defect formation (porosity, hot tearing), mechanical strength build-up and precipitation of phases. In particular, the transition from continuous liquid films to a coherent solid in low concentration alloys is of crucial importance for hot tearing formation, and more generally speaking for liquid feeding ability and coherency development. Based on a fairly recent theoretical model of coalescence, which will be recalled briefly, new results obtained for a population of equiaxed grains will be presented. A granular-type model based on a Voronoi tessellation has been used for the description of the gradual disappearance of liquid films and the clustering of equiaxed grains. This percolation-type approach has been then used to calculate the pressure drop in the mushy zone on the assumptions of a Poiseuille flow in between the grains and a Kirchhoff model for the connectivity of the liquid films including the Losses associated with solidification shrinkage (i.e., KPL model). Comparison with a standard average pressure drop calculation based on Carman–Kozeny's relationship will be presented.

Acknowledgements

The authors would like to acknowledge the financial support of Alcan-Pechiney, Research Center of Voreppe (CRV), France. They would also like to thank Ph. Jarry, from the CRV-Alcan at Voreppe, and V. Mathier, from EPFL for very fruitful discussions.