Stability of dendrite growth during directional solidification in the presence of a non-axial thermal field

The liquid–metal-cooling (LMC) directional-solidification process offers refinement of dendritic structure due to the increased cooling rate from enhanced heat extraction. However, under some conditions in the LMC process substantial lateral heat extraction occurs that leads to a change in dendrite morphology, resulting in grain nucleation or lateral growth – the formation of long secondary dendrite arms overgrowing favorably aligned primary dendrites. The conditions under which lateral growth occurs during solidification of alloys CMSX-486 and René-N4 have been studied experimentally and via solidification modeling. Solidification experiments have been conducted in a LMC furnace that utilizes liquid tin as the cooling medium and a floating ceramic baffle. A mold geometry was designed to evaluate a range of thermal conditions during solidification and assess the tendency for lateral growth. Correlations between dendritic structure, solidification-front curvature, solidification rate and thermal gradients have been analyzed. A criterion for predicting the onset of lateral growth based on the inclination of the solidification front at the casting surface is demonstrated.