Which statement best defines lack of fusion?

• A. Weld metal fails to fuse with the base metal or previous weld bead.
• B. Weld fully penetrates beyond the joint.
• C. Weld metal has excessive inclusions.
• D. Weld reinforcement is too small.

Answer: (A)

Lack of fusion means the weld metal does not bond to the base metal or to a previously deposited weld bead, leaving an unbonded area at the joint. This creates a weak spot because the weld isn’t actually fused to the metal it must join, so it can’t reliably transfer load across the joint. It often happens when heat input is insufficient, when joint fit-up or cleanliness is poor, or when travel speed is too fast. Detecting it visually or with non-destructive testing shows an interface that isn’t fused. The statement that describes this defect is the one where the weld metal fails to fuse with the base metal or a prior bead. The other options describe different problems—over-penetration beyond the joint, excessive slag inclusions, and an inadequately reinforced weld—rather than lack of fusion.