Directional & Progressive Solidification

Directional & Progressive Solidification

Directional solidification (DS) and progressive solidification are types of solidification within castings. Directional solidification is solidification that occurs from farthest end of the casting and works its way towards the sprue. Progressive solidification, also known as parallel solidification, is solidification that starts at the walls of the casting and progresses perpendicularly from that surface.

                                                                                       Directional solidification

                                                                            Progressive solidification

As material is changed from liquid to solid state, most metals and alloys shrink. When liquid content is thus not sufficient to compensate for this shrinkage, a deficiency in shrinking forms may develop. As gradual solidification replaces directional solidification, a deficiency in shrinkage is created.

The mold cavity's geometrical form influences progressive and lateral solidification. Differently heat streaming happens at the end of tunnel-type geometries, which enable the casting region to cool faster than the surrounding areas. Because there is less heat flow, large cavities are not cooling as rapidly as the surroundings, this is called a rising effect. Remember also that corners may produce separate or convergent heat-flow areas (often called hot spots).[4] Chills, rise-ups, isolating sleeves, regulation of the flow rate and flow temperature may also be employed in order to induce directional solidification.

The washing method can be used with directional solidification. Since most contaminations will be more soluble in liquid than in solids during solidification, they "push" impurities through the solidification front, which leads to a lower concentration of impurities for many of the finished casting than the feedstock, and to the impurities for enrichment of the last solidified metal. It can be replaced and reused for this last portion of steel. The appropriateness for directional solidification to remove certain impurities from certain metals depends, as described in the Scheil equation, on the partition coefficient for the impurity in that metal. Driven solidification (zone melting) is frequently used in the production of multicrystalline silicone for solar cells as a purification step.

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