What is undercut and how can it be prevented?

• A. A groove melted into the base metal at the weld toe; prevent by adjusting travel speed, electrode angle, and ensuring proper filler metal deposition.
• B. A smooth, rounded weld toe that reduces stress concentration.
• C. A surface oxide layer that can be removed with a solvent wipe.
• D. A type of porosity caused by shield gas contamination.

Answer: (A)

Undercut is a groove melted into the base metal at the weld toe, where the edge of the base metal is left thinner because the weld bead didn’t fill the edge properly. This weakens the joint by reducing the cross‑sectional area and creates a stress concentration that can lead to cracking under load. It often occurs when heat input is not enough to fuse the toe, or when travel speed is too high, or the electrode angle and deposition are off so the filler metal doesn’t adequately fill the edge.

To prevent undercut, adjust how you weld to ensure the weld toe fuses properly: use the right heat input so there’s enough molten metal at the toe, moderate your travel speed to give the pool time to fill the edge, and maintain a correct electrode angle and technique to promote proper filling rather than riding over the toe. Ensure sufficient filler metal deposition with appropriate filler size and current, and keep the joint clean and well fit up. In short, promote proper fusion at the toe and adequate filler deposition to eliminate the groove.

The other descriptions refer to different conditions or defects—like a smooth weld toe, surface oxide, or porosity from shielding gas—so they don’t describe undercut.