Which of the following is a visual defect that indicates a weld does not fuse properly with the base metal?

• A. Adequate penetration
• B. Lack of fusion
• C. Even bead width
• D. No undercut

Answer: (B)

Fusion between the weld metal and the base metal is what you’re assessing. A visual defect that shows the weld metal didn’t fuse with the base metal is lack of fusion. You’d look for a clear line or gap where the weld bead ends and the base metal remains unjoined, which can appear as un-welded areas at the root or along the interface. This happens when heat input, travel speed, or joint preparation isn’t right, so the weld never actually fuses the base metal together. That’s why it’s the telltale sign of improper fusion.

By contrast, adequate penetration means the weld has fused through the thickness where it’s supposed to; an even bead width indicates consistent deposition; and no undercut means the weld toe isn’t grooved. None of those point to a lack of fusion.