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Materials processing and joining

What is the main cause of a "shrinkage cavity," a void that tends to form inside a casting, especially at the center of a thick section that solidifies last?

  1. AGas dissolved in the melt is released during solidification
  2. BAfter solidification, the solid contracts as it cools and cracks because it is restrained by the mold
  3. CThe volume shrinkage on going from liquid to solid cannot be compensated by melt from the surroundings
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Answer: C. The volume shrinkage on going from liquid to solid cannot be compensated by melt from the surroundings

Most metals shrink by a few percent in volume on solidification (about 7% for pure aluminum). Solidification proceeds inward from the mold wall, so when a thick section that solidifies last becomes isolated, surrounded by solid, its shrinkage cannot be made up by melt, and a shrinkage cavity with a rough inner surface forms. Countermeasures are to make the riser solidify after the casting so that it can feed melt, and to use chills and similar means so that solidification proceeds progressively toward the riser. Release of dissolved gas produces gas porosity (blowholes) with smooth, round inner surfaces, a different defect. Restrained contraction after solidification produces cracks (hot tears or cold cracks), which differ from a shrinkage cavity, which is a void.

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