How does molten metal solidify in casting foundry Part Two

5. Formation of Shrinkage Cavities

As the metal cools and solidifies, it contracts. Since metals tend to shrink when they cool, this can create shrinkage cavities or voids inside the casting, particularly if the metal solidifies too quickly or unevenly.

To prevent this, foundries use risers, which are reservoirs of extra molten metal attached to the mold. The riser allows the last part of the metal to solidify in a controlled manner and supplies additional metal to compensate for shrinkage.

6. Complete Solidification and Cooling

Eventually, the entire casting will solidify, and the mold can be removed. The casting itself will still be quite hot at this point and needs to be allowed to cool completely before it’s handled or undergoes further processing.

The cooling rate affects the mechanical properties of the cast part. Slower cooling rates typically lead to larger, coarser grains in the metal, which might be desirable for certain applications, while faster cooling rates produce finer grains that may improve strength.

7. Mold Removal and Final Processing

Once the casting has cooled and solidified, the mold is broken away, and the metal part is extracted. The casting is then cleaned up by removing any excess material (like gates, risers, and any rough edges) and possibly undergoing heat treatment or machining for final shaping.

Factors Affecting Solidification in Foundries:

Mold Material: The material of the mold affects how quickly heat is drawn from the molten metal. For example, sand molds cool the metal more slowly compared to metal molds.

Cooling Rate: The cooling rate determines the size of the crystals formed during solidification. Faster cooling rates can produce a finer microstructure, which may be stronger, while slower cooling rates can lead to coarser grains.

Alloy Composition: Different metals and alloys solidify at different temperatures, and their solidification behavior can vary widely. For example, cast iron has a different solidification process than aluminum.

Risers and Gating: Proper riser and gating design ensures that molten metal reaches all parts of the mold and compensates for shrinkage as the metal solidifies.

In summary, the process of molten metal solidifying in a casting foundry involves controlled cooling, nucleation, crystal growth, and the strategic use of risers and gating systems to manage shrinkage and ensure a high-quality final product.