Influence of Bulk Convection on Freckle Formation in Castings

Abstract

In recent years, there has been an increased usage of single crystal superalloys in high temperature components because of the requirements for high creep resistance. For these complex alloys, it is necessary to understand the molten alloy convective phenomena that can cause deleterious grain defects. In particular, it is believed that the penetration of the bulk convection into the mushy zone can assist in the initiation of upward flowing liquid jets, i.e. channels, which sweep dendritic fragments out of the mushy zone and into the bulk liquid. These fragments can then serve as nuclei for growth of spurious grains that lead to defects called freckles. In the present work, a detailed quasi-steady state, axisymmetric FIDAP model is utilized to simulate the thermal buoyancy convection that occurs during directional solidification. The governing equations for the alloy and the crucible are solved subject to appropriate boundary conditions to determine the velocity and temperature distributions. The results obtained show how the major process parameters such as solidification velocity, and crucible diameter influence convection in the mushy zone. Finally, the numerical results are shown to agree with existing experimental data on freckle formation in metal alloy systems.