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Theoretical Considerations

In all casting processes, the solidification of the metal in the mold is a major fundamental issue.

Brief Review on Solidification

Solidification occurs by the nucleation of minute grains or crystals. These grains or crystals grow under the influence of the crystallographic and thermal conditions that prevail. The size and character of these grains are controlled by the composition of the alloy and the cooling rate. Growth ceases when all the available liquid has solidified.

During the freezing process, heat is being extracted from the molten metal as soon as the metal enters the mold.This heat must be removed before solidification can begin. Thus, it is called superheat. Moreover, the latent heat of fusion must be transferred to the surrounding mold before complete solidification can be achieved. Finally, the solid metal transfers heat to the mold, and then to the atmosphere as it cools to room temperature.

During the three stages of cooling: liquid, liquid-solid, and solid, shrinkage is occuring. Thus, the metal contracts in all three stages.

Therefore, three major points are to be considered when a metal solidifies:

1. Growth of solid grains
2. Heat evolution and transfer
3. Dimensional changes

Many variables contribute to the complexity of the solidification process. We list:

• The mold material and its thickness.
• The mold geometry.
• The metal thickness.
• Some metal properties as its thermal conductivity and solidification temperature range.
• The heat transfer from the mold to the atmosphere.
• The control of grain size by the use of inoculants or vibration, and others.