MIG Weld Undercut: Causes, Fixes, and How to Prevent It

Undercut is one of the most frustrating defects you’ll run into with MIG welding. It looks like the weld worked, but there’s a groove or notch running along the edge of the bead — and that groove is actually weakening the joint. MIG weld undercut is a groove or channel that forms along the toe of a weld bead where the base metal has melted away but wasn’t filled by the weld deposit. It reduces the effective cross-section of the parent material, creates a stress concentration point, and can lead to joint failure under load. The most common causes are excessive voltage, travel speed that’s too fast, and incorrect torch angle.

What Undercut Actually Looks Like

Undercut appears as a narrow valley or recess running parallel to the weld bead, right at the edge where the weld meets the base metal. It can be subtle — just a slight depression — or severe enough to see clearly with the naked eye. There are two types worth knowing: – Continuous undercut — a consistent groove running the full length of the weld, usually caused by a systematic settings problem – Intermittent undercut — isolated notches or dips, often from inconsistent travel speed or torch angle Both types are considered weld defects under AWS D1.1 and most other structural welding codes. Depending on severity, they can result in weld rejection during inspection.

Why Undercut Weakens the Weld

The base metal along the weld toe acts as the transition zone between the weld deposit and the surrounding material. When that zone melts away without being replaced, the parent material is now thinner than it was before welding. That thin edge becomes a natural stress riser. Under cyclic loading, fatigue cracks tend to initiate exactly at stress concentrations — which is why even shallow undercut is taken seriously in structural or load-bearing applications. In non-structural work, minor undercut may be acceptable. But if you’re welding anything that carries load, experiences vibration, or needs to pass inspection, it needs to be corrected.

The Main Causes of MIG Weld Undercut

Most undercut traces back to one of these root causes: Voltage too high Excess arc voltage increases the heat input and widens the arc cone. This melts more base metal at the edges than the wire feed can fill. High voltage is one of the most common causes of undercut in MIG welding. Travel speed too fast Moving the torch too quickly doesn’t give the molten pool enough time to fill the edges of the joint. The arc digs in, melts the base metal at the toes, and the pool moves on before it fills back in.

Incorrect torch angle If the torch is angled too far to one side, the arc force pushes the molten metal away from the weld toe instead of filling it. This is a common issue when welding horizontal fillet welds or lap joints. Wire feed speed too low Low wire feed speed means there isn’t enough filler metal being deposited to compensate for the base metal that’s being melted at the edges. Weaving too aggressively Wide weave patterns push the molten metal to the center of the bead. The edges cool faster, leaving a depression where the metal was melted but never filled. Wrong shielding gas mixture Higher CO₂ content increases penetration and arc aggressiveness. In some cases, a gas mix that’s too CO₂-heavy can promote undercutting — particularly at higher amperages. For context on how gas selection affects weld quality, the MIG welding shielding gas selection guide covers practical gas choices for different materials and applications.

How to Fix Undercut in MIG Welding

If you’ve already got undercut on a completed weld, the repair approach depends on severity. Shallow undercut (under 1mm deep) On non-structural work, very shallow undercut may be blended with a grinder and left if it doesn’t affect service conditions. However, this is removing material, not adding it — use judgment carefully. Moderate to severe undercut The correct repair is to run a light cover pass directly over the undercut area. Keep the heat input lower than the original pass to avoid making things worse. The goal is to deposit a small bead that fills the groove and ties into the base metal cleanly. Steps for a repair pass: 1. Clean the undercut groove with a wire brush to remove any spatter or oxidation 2. Set voltage slightly lower than your original settings 3. Position the torch so the arc is aimed directly at the undercut groove, not the center of the weld 4. Travel slowly and watch the pool fill the groove 5. Verify the repair is flush or slightly proud of the base metal, with no new undercut introduced

Adjusting Settings to Prevent Undercut

Dialing in the right parameters is the most reliable prevention. Here’s a quick reference for the adjustments that matter most:

Cause	Adjustment
Voltage too high	Reduce voltage in small increments (0.5–1V steps)
Travel speed too fast	Slow down and watch the pool fill the toes
Torch angle off	Keep drag angle 5–15° from vertical; adjust side angle for joint type
Wire feed too low	Increase wire speed to match heat input
Aggressive weave	Narrow the weave or switch to stringer beads
CO₂ content too high	Try 75/25 Ar/CO₂ instead of straight CO₂

For most mild steel MIG welding, a 75% Argon / 25% CO₂ mix gives a smoother, less aggressive arc than straight CO₂, which reduces the tendency to undercut at higher amperages.

Torch Angle and Technique Tips

Technique matters as much as machine settings. In practice, these habits help prevent undercut from developing: – Maintain a consistent travel speed. Speeding up even briefly over a tack weld or wider gap can cause localized undercut. – Watch the toes, not the top of the bead. If you focus only on the bead crown, it’s easy to miss the edges pulling away. – On horizontal fillet welds, angle the torch slightly upward toward the vertical plate (roughly 45° or slightly above). This uses gravity to help the pool fill the lower toe. – Pause slightly at the edges when weaving. This gives the pool a moment to wet in before moving back to center. A common issue in beginners is holding the torch too far from the work and compensating with more voltage. Keeping a consistent contact-tip-to-work distance — typically 10–13mm for standard MIG — makes voltage control far more predictable. If you’re frequently running into MIG weld quality problems beyond undercut, it’s worth reviewing a broader MIG welding troubleshooting process to systematically identify root causes.

When Undercut Is More Likely to Occur

Certain situations create higher undercut risk, and it’s worth being deliberate when you encounter them: – Overhead and horizontal positions — gravity works against you, making the pool want to sag away from the toes – High-amperage passes on thick material — more heat means more base metal melting at the edges – Fillet welds on thin plate — the edge of the flange is already narrow; too much voltage melts it away quickly – Long, fast stringer beads — especially when the welder is rushing to stay ahead of warping On thin material specifically, the margin between good penetration and undercutting is very narrow. MIG welding thin metal without burning through requires similar parameter discipline — lower voltage, controlled speed, and consistent technique.

FAQ

What is the acceptable level of undercut in MIG welding? AWS D1.1 structural welding code generally permits undercut up to 1/32 inch (0.8mm) for statically loaded members and effectively zero for dynamically loaded members. Many fabrication shops apply stricter standards regardless. When in doubt, fill it — a quick repair pass costs far less than a failed weld or rejected part.

Can undercut be prevented just by slowing down travel speed? Slowing down helps in many cases, but it’s rarely the only fix. If voltage is too high, slowing down may actually worsen the undercut by putting more total heat into the joint. The best approach is to balance voltage and wire feed first, then adjust travel speed to control bead size. Does undercut affect weld strength significantly? Yes. Even moderate undercut reduces the effective throat of a fillet weld and creates a stress concentration at the weld toe. Under static loads the effect may be minor, but under fatigue or dynamic loading, undercut is one of the most common crack initiation sites. For structural work, it’s never acceptable to ignore it. Is undercut more common with certain wire sizes? Larger diameter wires run at higher amperages, which generally increases the risk of undercut if voltage isn’t matched carefully. Smaller wire — like 0.030-inch (0.8mm) for thinner material — is more forgiving because the heat input is lower. Matching wire diameter to material thickness is a basic step that reduces several defects, including undercut. Why does undercut happen on only one side of the bead? Single-sided undercut almost always points to torch angle. If the arc is pushed toward one toe, that edge gets melted away while the other side receives less heat. Check your work angle and make sure the torch is positioned symmetrically for the joint type you’re welding. Can a cover pass always fix undercut, or is full removal sometimes needed? For most undercut less than 2mm deep, a properly executed cover pass is an acceptable repair. If the undercut is deeper, if the surrounding weld is already questionable, or if the application is safety-critical, full removal and re-welding may be required by the applicable code or engineer. Does using a Lincoln Electric WELD-PAK 140HD make undercut easier to prevent for beginners? Machines like the Lincoln Electric WELD-PAK 140HD include voltage and wire speed settings charts inside the door, which helps beginners start closer to a balanced parameter set. Undercut prevention still comes down to technique and consistent settings — the machine provides a solid starting point, but good torch angle and travel speed are still on the operator.

Undercut in MIG welding is almost always a correctable problem. It comes from too much heat at the weld toes with not enough filler to compensate — address voltage, travel speed, and torch angle together rather than chasing one variable at a time. If you already have undercut on a finished weld, a low-heat fill pass done carefully will fix most cases cleanly. The habit of watching the weld toes — not just the bead crown — is one of the fastest ways to catch it before it runs the full length of a joint.