Directional solidification
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Directional solidification is a series of measures applied to control the feeding of castings. As most metals and alloys solidify, changing from the liquid state to the solid state they will undergo an appreciable volume contraction. Without attention to control principles, objects cast will contain internal voids commonly called "shrink defects".
Some of the measures applied are the use of chills, risers, control of pouring rate, pouring temperature, and the use of exothermic materials.
With proper use of the measures, as the metal solidifies the interface for the boundary between the liquid and solid metal moves towards a source of additional feed metal and away from the region where solidification began.
Directional solidification can be used as a purification process. Since most impurities will be more soluble in the liquid than in the solid phase during solidification, impurities will be "pushed" by the solidification front, causing much of the finished casting to have a lower concentration of impurities than the feedstock material, while the last solidified metal will be enriched with impurities. This last part of the metal can be scrapped or recycled. The suitability of directional solidification in removing a specific inmpurity from a certain metal depends on the partition coefficient of the impurity in the metal in question, as described by the Scheil equation. Directional solidification is frequently employed as a purification step in the production of multicrystalline silicon for solar cells.
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