Shrinkage and gas porosity are common in aluminum-alloy castings. Porosity due to shrinkage is particularly likely to occur in long freezing-range alloys.

In such alloys, solidification progresses with a mushy zone, the solidified layer, and liquid metal at either side. Since the pockets of liquid metal that solidify last are trapped in the mushy zone near the solidified layer, and thus are deprived of liquid metal that would compensate for the liquid and solidification shrinkage, fine dispersed porosity forms in the casting.

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