In a MIG weld, how is an undercut identified and corrected?

In a MIG weld, how is an undercut identified and corrected?

• A. Undercut is a weld porosity in the center of the weld; fix by adding shielding gas.
• B. Undercut is a metal oxidation at the edge; fix by cleaning base metal only.
• C. Undercut is a type of spatter; fix by reducing voltage.
• D. Undercut appears as a groove at the weld toe; correct by increasing heat input, adjusting travel speed or weaving, and ensuring proper root gap.

Answer: (D)

Undercut shows up as a groove or notch along the weld toe where the base metal has been melted away but not filled by the weld metal. You identify it by looking at the edge of the weld bead—the toe will have a slight valley rather than a smooth, raised edge. Correcting it means adding more metal and letting the weld fuse properly into the edge of the base metal. The best approach is to increase the heat input and adjust technique: raise the voltage or current, or slow the travel speed a bit so more heat is delivered to the toe. A slight weaving or multi-pass approach can help fill the toe area more evenly. Also make sure the root gap is appropriate so the weld can penetrate and fill along the edge without leaving a groove. Other described issues—porosity, edge oxidation, or spatter—are different defects with their own fixes, so they don’t describe undercut.

Undercut shows up as a groove or notch along the weld toe where the base metal has been melted away but not filled by the weld metal. You identify it by looking at the edge of the weld bead—the toe will have a slight valley rather than a smooth, raised edge.

Correcting it means adding more metal and letting the weld fuse properly into the edge of the base metal. The best approach is to increase the heat input and adjust technique: raise the voltage or current, or slow the travel speed a bit so more heat is delivered to the toe. A slight weaving or multi-pass approach can help fill the toe area more evenly. Also make sure the root gap is appropriate so the weld can penetrate and fill along the edge without leaving a groove.

Other described issues—porosity, edge oxidation, or spatter—are different defects with their own fixes, so they don’t describe undercut.