In casting, which defect can reduce density uniformity and create weak spots?

• A. Porosity.
• B. Grain refinement.
• C. Surface gloss.
• D. High heat resistance.

Answer: (A)

Porosity creates voids inside the metal as it solidifies, so the part isn’t the same density everywhere. Those voids lower local density and act as weak spots because they’re stress concentrators—load can’t be carried uniformly, and cracks can start at the pores or grow under service. In casting, porosity can come from trapped gas, moisture in the mold, or poor venting, and it may also relate to shrinkage as the metal solidifies and leaves gaps if feeding isn’t adequate. That’s why it directly affects density uniformity and mechanical strength.

Grain refinement, on the other hand, is about making smaller grains to improve strength and toughness rather than creating voids. Surface gloss describes the surface appearance, not internal density. High heat resistance is a material property, not a defect.