Undercut is a groove along the edge of the weld where the parent metal has melted but not been filled with weld metal, creating a weak point. Overlap occurs when weld metal flows beyond the weld toe without fusing to the parent material, creating stress concentrations. Both are weld profile defects caused by incorrect welding current, travel speed or torch angle — and both are prevented by correct parameters and technique.
This guide covers what each defect is, what causes it and how to eliminate it.
What is undercut in welding?
Undercut is a groove that forms along the edge of the weld where the parent metal has melted but has not been filled with weld metal. This creates a weak point that can reduce the strength and fatigue resistance of the joint.
Undercut is often caused by excessive welding current, incorrect torch angle or travelling too quickly. Using the correct welding parameters and technique helps produce a smooth weld profile and minimise undercut.
What causes undercut?
Undercut is commonly caused by excessive welding current, high travel speed, poor torch angle or incorrect weaving technique. These factors prevent enough weld metal from filling the edge of the joint.
Reducing travel speed, adjusting welding current and maintaining the correct torch position usually eliminates the problem.
How do I stop undercut?
To prevent undercut, reduce welding current if necessary, slow your travel speed and maintain the correct work angle. Ensure sufficient filler metal is deposited to fill the edges of the joint.
Good joint preparation and consistent welding technique are equally important for producing strong, defect-free welds.
What is weld overlap?
Overlap occurs when weld metal flows beyond the weld toe without properly fusing to the parent material. Although the weld may appear large, overlap is considered a welding defect because it creates stress concentration points.
Correct travel speed, welding current and torch angle help prevent overlap.
How do I prevent weld overlap?
Overlap is usually prevented by increasing travel speed slightly, reducing weld size where appropriate and maintaining the correct torch angle.
Avoid depositing excessive weld metal and ensure the weld pool remains properly controlled throughout the weld.
A controlled weld pool is the common fix
Undercut and overlap are essentially opposite symptoms of an uncontrolled weld pool — too much current or speed for undercut, too much deposited metal or too slow for overlap. Dialling in the correct current, travel speed and torch angle for the joint produces a smooth, well-fused profile that avoids both.
For settings and technique by process, see our cross-category guides for MIG welding and MMA welding, or contact the Weldability SIF technical team for consumable advice.