Welding Undercut: 5 Causes, Prevention, and Repair

A narrow groove along the edge of a weld may look like a minor cosmetic flaw, but it can be welding undercut—a loss of base-metal thickness at the weld toe that the deposited weld metal did not fill. Depending on its depth, length, location, loading, and the governing acceptance standard, undercut can reduce fatigue resistance and trigger rejection.

Written by dxk | JTCLASER

Quick answer: Welding undercut occurs when the arc melts a groove at the weld toe faster than molten filler metal can fill it. The most common causes are excessive current, excessive voltage or arc length, high travel speed, incorrect torch angle, and poor weave timing. Stop, identify the cause, correct one variable at a time, and inspect the repaired area.

This guide explains the physics behind welding undercut, how to diagnose it in manual and robotic processes, and when a repair requires more than simply adding another bead. It is written primarily for arc welding on steel, including gas metal arc welding, but the diagnostic logic applies to several processes. The approved welding procedure specification (WPS), equipment instructions, drawings, and applicable code always take priority over general starting points.

What Is Undercut in Welding?

Welding undercut is an irregular groove melted into the parent metal at the toe of a weld and left insufficiently filled by weld metal. It may appear on one side or both sides of a bead, at the face of a groove weld, at a fillet-weld toe, between passes in a multi-pass weld, or at the root side of some joints.

The key distinction is missing cross-section. A normal weld toe blends smoothly into the base metal. An undercut leaves a notch. That notch can concentrate stress, particularly under cyclic or impact loading. However, not every visible groove automatically fails inspection. Permitted dimensions vary by material, thickness, service, joint type, loading, and construction code. Do not invent an acceptance limit or judge it only by appearance.

Undercut Symptoms and Likely Causes

What you observe	Likely cause	First check
Continuous groove on both toes	Excess heat, long arc, high travel speed, or insufficient fill	Compare actual parameters and speed with the WPS
Groove mainly on one side	Incorrect work angle, poor joint access, or arc deflection	Center the arc and verify torch orientation
Undercut appears at weave edges	Crossing the center too slowly or not pausing at the toes	Reduce weave width and balance edge timing
Thin, deeply washed bead	Voltage or arc length too high, travel too fast	Shorten the arc within the approved range
Defect starts after a corner or path change	Robot speed, angle, or coordinate transition problem	Review the path at the exact defect location
Irregular groove with unstable arc	Poor wire feeding, grounding, contamination, or arc blow	Correct the mechanical or electrical instability first

Why Does Undercut Form?

The main welding undercut causes are excessive current, excessive voltage or arc length, high travel speed, incorrect torch angle, and insufficient filling at the weave edges. At the weld toe, arc energy melts the edge of the base metal. The liquid weld pool must then wet that edge and deposit enough metal to replace what was melted. Undercut forms when melting and displacement exceed filling.

Understanding this “melt versus fill” balance is more useful than memorizing one parameter. Several different combinations can produce the same groove. For example, high current can create aggressive melting, while normal current combined with excessive travel speed can leave too little filler per unit length. Good MIG welding undercut diagnosis therefore considers current, voltage, wire feed, speed, angle, gas, and joint geometry together.

Cause 1: Excessive Welding Current

High current increases melting and arc force. If travel speed, wire deposition, and pool control do not compensate, the arc can wash out the weld toe and create welding undercut. Excess current can also make the pool more difficult to control in vertical, overhead, and thin-material applications.

Do not automatically reduce current by a fixed 10%–15% on every job. That may be a useful trial on a noncritical practice coupon, but it is not a universal rule. In constant-voltage wire welding, current is closely related to wire-feed speed, so changing wire feed also changes deposition. Start from the approved range, make a small documented adjustment, and retest on matching scrap.

When checking welding current settings, verify the actual machine output, wire diameter, electrode classification, polarity, contact-tip-to-work distance, and joint position. A long stickout can change current and deposition behavior even when the front-panel value has not moved.

Cause 2: Excessive Voltage or Arc Length

Higher voltage generally lengthens and broadens the arc. A wide arc can spread heat toward the bead edges, flatten the profile, and melt a groove that the available filler does not replace. In processes where the operator controls arc length directly, holding too long an arc can create a similar result.

Review welding voltage settings together with wire-feed speed and transfer mode. If the bead looks excessively wide and flat, the arc sounds harsh, and both toes are washed out, reduce voltage or arc length in small increments within the procedure range. A low voltage is not automatically better: too little voltage can cause wire stubbing, poor wetting, overlap, or lack of fusion.

Cause 3: Travel Speed Is Too High

At excessive travel speed, the arc reaches new base metal before enough filler has been deposited behind it. The molten pool becomes narrow and cannot flow back to fill the toes. The result may be a thin bead, incomplete fill, welding undercut, or lack of fusion.

Slowing down can increase fill at the edge, but excessive slowing raises heat input and may cause a large pool, burn-through, overlap, distortion, or poor penetration behavior. Travel speed must be balanced with current, voltage, wire feed, position, and joint size. Mark a known distance on a practice coupon and time the weld if consistent speed is difficult to judge by eye.

Cause 4: Incorrect Torch Angle

The arc follows the torch. If the work angle points too far toward one plate, energy becomes concentrated on one side while filler is delivered away from the opposite toe. This frequently creates one-sided welding undercut in fillet welds or asymmetric joints.

The correct welding torch angle is process- and joint-specific. For a simple equal-leg fillet weld, the work angle normally aims into the joint to distribute heat between both members, while the travel angle is kept modest and consistent. Joint access, position, unequal thickness, transfer mode, and push-versus-pull technique may require a different orientation. Follow the WPS rather than forcing the gun perfectly vertical in every situation.

Support the gun and keep the contact-tip-to-work distance stable. Watch both weld toes rather than staring only at the arc center. If undercut occurs on alternating sides, inconsistent hand position or joint tracking may be more important than the machine settings.

Cause 5: Weave Width and Toe Timing Are Wrong

A weave can distribute metal across a wider joint, but an overly wide or fast pattern may wash out the edges. Moving quickly across a toe does not allow the pool to wet and fill it. Spending too much time in the center can overheat the joint while starving the sides.

Use the narrowest weave permitted by the procedure, keep the pattern symmetrical, move smoothly across the center, and allow controlled filling at each edge. A slight toe pause may help, but it must not be treated as a universal fixed time. A suggested robotic dwell of 0.1–0.2 seconds can be a trial value for a specific qualified setup, not a setting that applies to every material, current, wire size, position, robot, and joint.

How to Prevent Undercut in Welding

For anyone asking how to prevent undercut in welding, use this controlled sequence:

1. Stop and locate the pattern. Record whether the groove is continuous, intermittent, one-sided, or linked to a start, stop, corner, or weave edge.
2. Confirm the WPS essentials. Check process, material, filler, wire size, polarity, shielding gas, joint preparation, position, and preheat requirements.
3. Measure the actual parameters. Compare current, voltage, wire-feed speed, travel speed, and contact-tip distance with the approved range.
4. Check the physical setup. Inspect grounding, cable condition, wire feeding, contact tip, nozzle, joint fit-up, and access.
5. Correct technique. Center the arc, stabilize work and travel angles, shorten an excessive arc, and control the pool at both toes.
6. Make one change at a time. Test on scrap of the same material, thickness, position, and joint geometry whenever practical.
7. Verify the result. Inspect the entire weld and confirm that the correction did not create overlap, lack of fusion, excess reinforcement, or inadequate penetration.

Diagnosing Robotic Welding Undercut

A robot can repeat an error with exceptional consistency. Robotic welding undercut should therefore be investigated at both the process and path levels. Record the exact joint coordinates where the defect starts and ends, then review speed, torch orientation, work distance, weave amplitude, edge timing, seam tracking, part location, and transition behavior at those points.

Check whether the defect moves when the part is repositioned. If it stays at the same robot coordinate, programming, cable dress, reach, or torch orientation may be responsible. If it follows a part feature, investigate fit-up, dimensional variation, tack placement, joint access, contamination, and seam-tracking response.

When evaluating intelligent robotic welding systems, stable fixtures and repeatable part presentation are as important as the power source. A program optimized for one ideal coupon cannot compensate indefinitely for changing gaps or misplaced components. Validate any dwell-time, weave, speed, or parameter change through the approved qualification and inspection process before applying it across production.

How to Fix an Undercut Weld

The answer to how to fix undercut weld depends on whether the welding undercut is acceptable under the applicable requirements. First measure and document it. If repair is required, use an approved repair procedure; do not simply cover the groove and assume it is sound.

1. Identify and correct the root cause before repair.
2. Remove contaminants, cracks, slag, and unacceptable metal by an approved grinding, machining, or gouging method.
3. Blend the excavation smoothly and confirm that sound material remains.
4. Apply required preheat and temperature controls.
5. Deposit the repair bead using the qualified procedure and suitable filler.
6. Blend only as permitted; avoid reducing the base-metal thickness.
7. Repeat the required visual or nondestructive examination.

Minor acceptable undercut may require no repair, while some shallow discontinuities can be corrected by controlled blend grinding or an additional qualified bead. Critical, fatigue-loaded, pressure-retaining, structural, or repeatedly repaired components may require engineering approval. Repairing without understanding the acceptance criteria can make the component worse.

How to Inspect and Measure Undercut

Clean the weld before inspection and provide adequate lighting. View the toe from more than one angle because spatter, scale, and bead ripple can hide the groove. A straightedge and suitable weld inspection gauge can help measure depth, but the user must understand the gauge geometry, calibration, access limitations, and governing standard.

Record the maximum depth, total or continuous length, location, joint type, material thickness, and proximity to starts, stops, transitions, or other imperfections. Visual inspection may be supplemented by other examination methods when required. Never declare a weld acceptable solely because the groove “looks small.”

Common Corrections That Can Create New Defects

• Reducing current too far: may reduce penetration or fusion.
• Lowering voltage too far: may cause wire stubbing, a convex bead, or poor wetting.
• Slowing travel excessively: may increase heat input, distortion, overlap, or burn-through.
• Adding a long toe pause: may overheat the edge or create an oversized bead.
• Grinding without measurement: may deepen the section loss.
• Adding metal over an unprepared groove: may trap contamination or hide lack of fusion.

Frequently Asked Questions

Does undercut always mean the weld must be rejected?

No. Acceptance depends on the applicable code, drawing, contract, material thickness, loading, location, and measured dimensions. Some standards allow limited undercut; others impose stricter limits. The inspector must use the requirements specified for the job.

Can high voltage cause welding undercut?

Yes. Excessive voltage can produce a long, wide arc that melts the weld edges without enough filler to replace the lost metal. High travel speed, low deposition, or poor technique can amplify welding undercut.

Why is the undercut only on one side?

One-sided undercut commonly indicates an incorrect work angle, poor joint access, arc deflection, uneven weave timing, or unequal heat distribution between materials of different thicknesses.

Should current always be reduced by 10%–15%?

No. That range may be a troubleshooting trial on a suitable coupon, but it is not universal. Stay within the WPS, make small documented changes, and evaluate penetration, fusion, profile, and heat input after each adjustment.

Is a 0.1–0.2 second robotic toe pause always correct?

No. Dwell time depends on wire feed, current, voltage, travel speed, weave width, material, joint, position, robot motion, and required bead profile. Establish it through testing and procedure qualification.

Safety Before Adjustment or Repair

Stop the process before inspecting, cleaning, measuring, or servicing the gun. Apply the required energy-isolation procedure for automated equipment. Wear appropriate eye, face, hand, body, hearing, and respiratory protection. Remove combustibles, provide fire controls, inspect leads and grounding, and use suitable local exhaust ventilation without disturbing shielding gas.

Coatings, stainless alloys, galvanized material, confined spaces, and repair grinding may introduce additional fume, dust, fire, and atmospheric hazards. Current workplace ventilation requirements emphasize controlling fumes at their source and providing specific protections in confined spaces and when welding metals of toxic significance.

Final Takeaway

Welding undercut is a balance problem: the arc melts more of the toe than the weld pool fills. Diagnose the pattern before changing controls. Confirm the procedure, then check current, voltage or arc length, travel speed, torch angle, and edge timing. In robotic welding, connect the defect to a precise path location and part condition. Finally, measure against the actual acceptance criteria and repair only through an approved process.

Technical review note: This educational article was prepared by dxk for JTCLASER from a practical welding lesson and cross-checked against a current technical review of weld shape imperfections and applicable safety guidance. It is not a substitute for an approved WPS, qualified personnel, code interpretation, inspection plan, or site-specific risk assessment.