"Directional solidification" is a metal casting term. It refers to the process of controlled feeding of the molten metal into a temperature-controlled mold to produce a part that is free of hollow spots, called shrink defects. Directional solidification is also used to refine the metal during the casting process because the impurities found in the molten metal will continue to rise to the surface of the pool, following the path of least resistance as they are pushed up by the solid materials below.
In the directional solidification process, the molten metal at the far end of the mold begins to cool and solidify before the rest of the mold does. As the metal on the bottom of the mold cools, this line of solidification moves steadily upward toward the molten metal feed. By controlling the rate of flow for the molten metal feed and introducing thermal variations in the mold, shrink defects can be eliminated, because the liquid metal will naturally run into these dips and vacant areas.
The process of directional solidification is not to be confused with progressive solidification, also called parallel solidification. Although these processes share some similar traits, in progressive solidification, the cooling and solidifying process begins at the walls of the casting and works its way inward. With directional solidification, the process of solidification begins at the bottom of the casting and works its way to the top.
Parallel solidification in a casting is the underlying cause of defects. As the molten metal cools too quickly in some areas or remains heated for too long in other areas, it creates defects as a result of solidification, thermal expansion and contraction. For example, if molten metal is poured into an L-shaped mold, the metal at the corner of the mold might cool too quickly, causing a bottleneck and trapping an air pocket in the lower leg of the mold. This air pocket creates a hollow spot in the finished metal part, thus weakening the overall structure.
To control parallel solidification and encourage directional solidification in the casting process, several techniques are employed. Thermal variations are introduced into the mold by using risers or chills to control hot or cold spots that might create problems with the cast part. Insulated sleeves also are used to ensure a steady, controlled temperature for the mold. Finally, the rate of flow and temperature of the molten metal feed are carefully controlled to ensure directional solidification.