Directional solidification is a controlled metal melting process that eliminates shrinkage defects and refines the metal during casting. It starts at the bottom of the die and works its way up, unlike parallel solidification, which causes defects. Techniques like thermal variations and controlled flow rate are used to ensure directional solidification.
“Directional solidification” is a term for metal melting. It refers to the process of controlled feeding of molten metal into a temperature-controlled mold to produce a part free of hollow spots, called shrinkage defects. Directional solidification is also used to refine the metal during the casting process because impurities 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 underlying solid materials.
In the directional solidification process, the molten metal at the opposite end of the die begins to cool and solidify before the rest of the die. As the metal at the bottom of the die cools, this solidification line moves steadily upwards towards the molten metal feed. By controlling the flow rate for the molten metal feed and introducing thermal variations in the mold, shrinkage defects can be eliminated, as the liquid metal will naturally flow into these depressions and void areas.
The directional solidification process is not to be confused with progressive solidification, also called parallel solidification. While these processes share some similar traits, in progressive solidification, the cooling and solidification process starts at the walls of the casting and works its way inward. With directional solidification, the solidification process starts at the bottom of the casting and works its way up.
Parallel solidification in a casting is the underlying cause of defects. As molten metal cools too quickly in some areas or stays heated too long in others, it creates defects through solidification, thermal expansion and contraction. For example, if molten metal is poured into an L-shaped die, the metal at the corner of the die may cool too quickly, causing a bottleneck and trapping a pocket of air at the bottom of the die. This pocket of air creates a void 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 using risers or chills to control hot or cold spots that could create problems with the cast part. Insulated sleeves are also used to ensure a constant and controlled temperature for the mould. Finally, the flow rate and temperature of the molten metal feed are carefully controlled to ensure directional solidification.
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