Directional solidification is a type of solidification within castings. In other words, it is when the liquid metal solidifies from the farthest end of the casting to closest end, which is towards the sprue. Directional solidification can also be used as a purification process. Most impurities are more soluble in the liquid phase than in the solid phase during solidification.
As a result, the impurities will be pushed by the solidification. This causes the finished casting product to have a lower concentration of impurities than the feedstock material. The last solidified metal, however, will be filled with impurities. Thus, that last piece can be recycled. Downward directional solidification is a novel process that encompasses heat transfer, fluid flow, and solute transport mechanisms. These in turn affect the development of the microstructure and morphology of the phases. Effects on the direction of heat extraction, either downwards or upwards, during solidification of metallic alloys are rarely studied.
This is a new approach to the solidification. Burden and Hunts were two scientists who carried out experiments with non-metallic alloys. They used NH4Cl/H2O in order to investigate the effects of downward and upward directional solidification on the dendritic growth of primary branches. These scientists have observed through their results that small differences in the solidification thermal parameters do not affect the primary dendrite arm spacing significantly. Instead, the primary dendritic arm spacing is more influenced by the fluid flow generated due to solute rejection.