Define lack of fusion and its significance in a weld.

• A. A localized gap between weld passes due to incomplete penetration of the base metal.
• B. Inked
• C. A weld that has too much filler metal.
• D. A weld that has uniform color.

Answer: (B)

Lack of fusion is a welding defect where the molten weld metal does not fuse with the base metal or with a previously deposited weld bead. It shows up as a separation at the bonding line, leaving a gap or notch along the joint where bonding didn’t occur.

This defect matters because it creates a weak spot in the weld. The bonded area carries the intended load, so a lack of fusion reduces the weld’s effective strength and can act as a stress riser, making cracks or failures more likely under normal service or during fatigue and corrosion. In critical welds, lack of fusion is typically considered unacceptable and requires repair, often by removing the defective area and re-welding with proper heat input, cleaning, and technique.