An experimental investigation into the formation of an equiaxed zone in ingot casting: Pb-Sb alloy system

Abstract

An experimental investigation was undertaken to obtain a better understanding of the phenomenon of equiaxed zone formation in ingot castings with particular attention to the thermal conditions during dendritic growth and the mechanism of columnar-to-equiaxed transition. Thermal conditions during dendritic growth were measured in the laboratory-scale Pb-Sb castings. Solidification was also directly observed in castings of the cyclohexanol-phenol red system. A close correlation was noted between the general behaviour of the thermal history curves, especially when taken on the centreline of the ingots, and the resulting macrostructure. The dendritic solidification front was found to grow with considerable solute undercooling and also with a slight, but positive, temperature gradient ahead of the front in the bulk liquid. Moreover, the dendritic front temperature was also noted to increase with distance from the mould wall surface, the nominal rate of advance of the front being sensitive to the solute content and the cooling rate. Heterogeneous nucleation ahead of the columnar front was believed to have played a major role in equiaxed zone formation. However, columnar-to-equiaxed transition was observed not to take place immediately when the equiaxed crystals formed, but some time later, equiaxed dendrites growing at a somewhat higher temperature than the columnar dendritic front. Finally, combining the information gained from thermal analysis data with the observations made upon non-metallic castings, a novel picture of how columnar-to-equiaxed transition takes place has been proposed.