3-DIMENSIONAL SIMULATIONS OF FRECKLES IN BINARY-ALLOYS

A tridimensional finite-element model was developed to calculate the t hermosolutal convection and macrosegregation during the solidification of dendritic alloys. A single set of conservation equations is solved in the mushy zone, all-liquid, and all-solid regions without internal boundary conditions. The model is applied to simulate the directional solidification of a Pb-Sn alloy in cylinders of square and circular c ross section. The calculations are started from an all-liquid state an d the evolution of convection, solute and energy transport, and the mu shy zone growth are followed in time. The results show details of the channels, which result in freckles, that are not observable in existin g two-dimensional simulations. Several qualitative features of channel s and freckles previously observed in experiments with transparent sys tems, like chimney convection, preference of channels to be on surface s, and enhanced solid growth at the channel mouth (''volcanoes'') are successfully reproduced. (C) 1998 Elsevier Science B.V. All rights res erved.