What is a weld undercut and what typical causes?

• A. Lack of fusion at the weld root due to insufficient heat.
• B. Porosity along the weld bead due to gas entrapment.
• C. A groove missing at the base metal adjacent to the weld; caused by too high current, travel speed, or incorrect technique.
• D. Crack along the weld toe due to thermal shock.

Answer: (C)

Undercut is a groove melted into the base metal right at the weld toe that the weld metal does not fill. It leaves a sharp edge or valley along the edge of the weld and reduces the cross‑sectional area that bears load, which can weaken the joint and promote crack initiation under stress.

This occurs when heat input at the toe is not properly controlled—often from too high current or from technique issues that prevent adequate filler metal from depositing into the groove. If the arc is too aggressive or the weld bead isn’t deposited along the toe with enough contour, the base metal melts away faster than it can be filled, creating that characteristic groove. Proper adjustments to current, travel speed, and technique help ensure the toe is built up with filler metal rather than melted away.

Phrases like lack of fusion, porosity, or cracks along the toe describe different defects with distinct causes and appearances, so undercut is specifically that edge groove along the base metal adjacent to the weld.