How to Solve Undercut Defects in Laser Welding
Laser welding undercut is corrected by restoring a stable, adequately filled weld toe before polishing the symptom. Verify the defect and acceptance limit, then inspect joint fit-up, beam tracking, focus, optics, surface condition and gas delivery. Rebalance power density, travel speed, oscillation and filler supply through controlled trials, changing one variable at a time.
Do not assume that more power, slower travel or a fixed torch angle is the universal answer. An edge groove may come from high-speed melt-flow instability, excessive local energy, beam misalignment, insufficient molten volume, gap variation or a combination of these conditions. The correction must fit the material, joint and welding system.
Image: Krorc / Wikimedia Commons, CC BY-SA 3.0.
Start with evidence, not a random parameter change.
Undercut is a groove at a weld toe or edge. In laser welding it often indicates that molten metal did not wet and refill the edge consistently. The efficient response is to locate the pattern, identify what changed, run one controlled adjustment and confirm the cross-section as well as the top surface.
Distinguish a toe groove from underfill, incomplete fusion, burn-through and an optical shadow.
A gap, offset beam, dirty cover glass or changing standoff can defeat parameter tuning.
Rebalance the process window; do not lower power and slow travel at the same time without evidence.
Use the specified quality level, measurement method and representative production joint.
Undercut is an edge geometry problem, not a synonym for every bad bead.
A useful diagnosis begins with the location and profile of the imperfection. Photograph the joint from above and at a low angle, mark travel direction, record whether the groove is continuous or intermittent, and examine a cross-section when the application requires it.
Look for a groove next to the fusion zone.
Undercut removes section at a weld boundary. It may appear on one side or both sides, run continuously, repeat as an intermittent wave, or develop near the start and stop. These patterns are diagnostic clues:
Both sides, continuous: examine speed, energy distribution, bead width and molten-volume balance.
One side only: inspect beam tracking, head orientation, wire placement and part height.
Intermittent or wavy: investigate keyhole/melt-flow instability, motion fluctuation and inconsistent fit-up.
Start or stop only: review ramping, acceleration, deceleration, dwell and termination strategy.
A smooth top surface can hide lack of fusion, porosity or an unfavorable cross-section. Conversely, a visible line may be discoloration or lighting rather than missing metal. Use measurement and the required inspection method before assigning a cause.
The adjacent base material is locally recessed. Determine depth, length and location against the applicable acceptance criteria.
The overall weld reinforcement or fill volume is low. Undercut and underfill can occur together but require different observations.
This is an internal or interfacial problem. A cosmetic top-bead correction does not prove that fusion has been restored.
Build a defect map before touching the controls.
Record the material and batch, thickness, joint type, seam length, gap range, fixture, laser power command, travel speed, focus setting, wobble pattern, wire alloy and feed speed, shielding gas, flow arrangement, head angle, standoff, cover-glass condition and production time.
Then map where undercut begins and ends. A defect that follows a fixture transition points to a different cause than one that appears after the cover glass has accumulated contamination. A one-sided defect that reverses when travel direction reverses can reveal asymmetry in beam, wire or gas delivery.
The objective is not to collect paperwork. It is to prevent several variables from changing invisibly while the operator adjusts only one number on the controller.
Define what acceptable means.
Undercut is not automatically accepted or rejected by its name alone. The drawing, application standard, contract, responsible designer and qualified welding procedure determine the permissible condition and the required inspection.
If the part is pressure-bearing, cyclically loaded, safety-critical, sealed or corrosion-sensitive, the responsible engineering authority may require additional acceptance limits and inspection beyond visual appearance.
Six mechanisms can produce a similar edge groove.
The correct adjustment depends on the mechanism. High travel speed is a documented undercut limit in some laser-welding conditions, but speed is not the only cause and simply slowing down can introduce excessive heat, distortion or burn-through.
Travel is faster than the pool can refill the edge.
At high speed, recoil pressure, surface tension and melt flow may leave a persistent groove. Intermittent undercut can precede humping in some regimes.
- Check whether the defect begins above a repeatable speed.
- Compare the bead and cross-section across a controlled speed series.
The interaction is too concentrated or poorly balanced.
Power, focus, beam profile, oscillation width and travel speed work together. Excessive local intensity can displace melt; insufficient lateral melting can prevent toe wetting.
- Do not evaluate power without speed and focus.
- Confirm actual focus and optical condition.
The beam or filler misses the intended joint center.
One-sided undercut often points to seam-tracking error, part-height variation, head orientation, wire position or an asymmetric wobble path.
- Mark beam center relative to the joint.
- Check direction-dependent results.
Gap or joint geometry demands more molten metal.
Autogenous welding cannot create material. Variable gaps, edge mismatch or an oversized preparation may require better fit-up, a different joint design or qualified filler-wire addition.
- Measure gap and edge mismatch.
- Match wire volume and placement to the pool.
Oxide, coating, oil or alloy behavior disturbs wetting.
Contamination can generate gas, spatter and unstable flow. Reflectivity, thermal conductivity, viscosity and vapor-forming elements change the usable process window.
- Identify the exact alloy and coating.
- Use a repeatable preparation method.
Gas, motion, optics or utilities vary during the seam.
Poor gas coverage, plume interaction, dirty protection glass, wire-feed pulsation, loose motion or changing standoff can turn a stable setting into an intermittent defect.
- Inspect hardware before retuning.
- Trend the defect against time and position.
Choose the observed pattern to set a first test.
This planner organizes a diagnostic sequence. It does not generate a qualified welding procedure or a universal power, speed, focus, gas or wire-feed setting.
Map the operating window before changing hardware.
A symmetric groove on a clean, repeatable joint suggests an energy-distribution or melt-flow limit. Confirm the baseline, then run a small controlled power-density and speed matrix.
Fix undercut in six disciplined steps.
A good trial separates equipment stability, joint preparation and process parameters. Skipping directly to random power changes makes a temporary improvement difficult to reproduce.
Define and measure the defect.
Confirm that the indication is undercut, identify its location and measure it using the specified method. Record the applicable drawing, quality level and service requirement before deciding whether repair is necessary.
Freeze a complete baseline.
Record material, batch, thickness, joint, fixture, gap, offset, power command, speed, focus, wobble, gas, wire, head geometry and optical condition. Produce several coupons to determine whether the defect is repeatable.
Remove mechanical variation.
Clean the joint, measure fit-up, check clamping and part height, confirm seam tracking, inspect the cover glass and nozzle, and verify motion and wire delivery. Parameters cannot compensate reliably for uncontrolled hardware.
Rebalance energy and travel.
Run a small designed matrix around the baseline. Test power density, travel speed, focus and wobble systematically. The goal is a stable pool with enough lateral wetting and refill, not maximum penetration or minimum cycle time alone.
Restore molten-volume balance.
If the joint has a real gap or edge-volume demand, improve fit-up or evaluate a qualified filler route. Match wire diameter, feed speed, angle and landing position to the actual molten pool; excess wire can create new defects.
Validate the production window.
Repeat across representative parts, orientations, shifts and consumable conditions. Inspect the cross-section and any required mechanical, leak or NDT evidence. Release a documented window rather than a single successful coupon.
Use direction-of-change logic, not universal numbers.
The table below describes controlled experiments. It is not a welding procedure. A change that reduces undercut can also reduce penetration, increase porosity, widen the heat-affected zone or change cycle time.
| Observation | Likely mechanism to test | Controlled experiment | What else can get worse |
|---|---|---|---|
| Undercut appears only above a repeatable travel speed | Melt cannot wet and refill the edge within the available time; high-speed flow instability. | Hold all else constant and compare a narrow speed series. If throughput must remain high, evaluate beam shaping, oscillation, joint design or filler rather than only slowing down. | Slower travel can raise local heat input, widen the bead, increase distortion or cause burn-through. |
| Deep, narrow bead with sharp toe grooves | Energy density is concentrated and lateral wetting is inadequate. | Verify actual focus first; then compare a small focus/oscillation-width series while holding material, speed and total power constant where appropriate. | Excessive defocus or oscillation can reduce penetration and create incomplete fusion. |
| Wide hot bead with undercut or sagging | Excessive thermal input, dwell or oscillation area for the thickness and position. | Rebalance power, speed and path; inspect start/stop ramps and gravity-sensitive orientations. | Too little energy can create lack of fusion or an unstable shallow weld. |
| One-sided undercut | Beam, wobble, wire or head is offset; part height or joint line changes. | Measure centerline, standoff and wire landing. Reverse travel direction if safe and compare which side is affected. | Moving the beam without confirming the true seam can trade undercut for incomplete fusion on the opposite side. |
| Low bead plus toe groove on gapped joints | Insufficient molten volume or inconsistent fit-up. | Measure gap distribution, improve fixturing, then evaluate qualified filler volume and placement. | Excess filler or poor placement can cause cold lap, spatter, incomplete melting or wire stubbing. |
| Intermittent groove with spatter or plume fluctuation | Keyhole/melt-pool instability, contamination, gas-plume interaction, wire pulsation or optical degradation. | Inspect surface, optics, gas arrangement, motion and wire feed. Use high-speed or coaxial monitoring when available. | Changing gas flow blindly can disturb the pool or alter material response without correcting the actual cause. |
A simple travel-based energy calculation can compare conditions, yet undercut also depends on beam profile, focus, absorptivity, oscillation, keyhole behavior, joint geometry and melt flow. Use the calculation as a planning metric, then qualify the actual joint.
Do not ask the laser to create missing metal.
Autogenous laser welding performs best when joint location and gap are controlled. If the edge volume is inconsistent, the process may alternate between good fill, underfill and undercut even while every displayed parameter remains unchanged.
Reduce geometric variation.
Measure joint gap, edge mismatch, flange angle, part height and clamping. Improve cut-edge consistency and fixtures before increasing oscillation or wire feed. A larger process window is valuable, but it is not a substitute for uncontrolled assembly.
Use oscillation with a purpose.
Beam oscillation can redistribute energy, widen interaction and improve gap bridging in suitable systems. Pattern, frequency, amplitude, focus and speed interact; an arbitrary larger wobble can sacrifice penetration or create edge heating.
Deliver filler into the right pool zone.
Wire must arrive at a stable angle and position, melt completely and supply the required volume. Match alloy, diameter, feed speed and travel speed. Confirm that the wire does not push the pool, shadow the beam or intermittently contact the work.
Joint geometry changes the available correction.
A butt joint, lap joint, fillet and edge weld do not share one universal beam angle or filler position. Program and qualify the actual access geometry.
Laser-welded fillet image: LaserTherm / Wikimedia Commons, CC BY-SA 4.0.
The production seam is the real test.
Curves, corners, changing heat sinks and start-stop overlap can shift the process window. Validate the complete path, not only a straight laboratory coupon.
Image: Phiarc / Wikimedia Commons, CC BY-SA 4.0.
Gas can influence the process, but it is not a generic cure.
Shielding composition, flow, nozzle direction and interaction with the plume can affect weld geometry and stability. The best route depends on alloy, laser wavelength and power, joint and acceptance criteria. “Use argon” is not a sufficient troubleshooting plan.
Make gas delivery repeatable.
Confirm gas identity and purity, inspect hoses and nozzles, check for leaks and drafts, and document flow at the actual setup. Too little protection can permit oxidation; poorly directed or excessive flow can disturb the plume or molten pool. Gas selection must also consider metallurgy and porosity behavior.
If a gas change reduces visible undercut, still section and inspect the joint. The change may have altered penetration or internal porosity rather than simply improving the toe.
Prepare the surface consistently.
Remove oil, moisture, oxide, paint, plating debris and cutting residue using a method compatible with the material and procedure. Identify coatings and elements that can vaporize. On reflective and highly conductive alloys, verify that the selected source and beam delivery provide a stable absorption and melt-flow window.
Cleaning should be controlled, not improvised. Aggressive grinding can change edge geometry and create a new gap, while solvent residue can create another contamination source.
The same defect may need a different control strategy.
Handheld laser welding
First stabilize operator path, travel speed, standoff, head orientation and wire placement. Use guides, fixtures or seam followers where practical. Training coupons should include starts, stops, corners and realistic access positions. A displayed speed setting does not control hand travel unless the system physically governs motion.
Automated laser welding
Interrogate motion logs, acceleration zones, robot accuracy, seam tracking, focus position, part height and fixture repeatability. Map undercut to path coordinates. Monitoring of back reflection, plume, coaxial images or acoustic signals can help distinguish a process instability from a mechanical offset.
Laser heads, wobble optics, wire nozzles and joint geometries differ. Use the equipment manufacturer's permitted geometry, then qualify the orientation that gives safe access, stable shielding, correct beam placement and repeatable wire delivery.
A smooth bead is not enough to prove the repair worked.
The inspection plan should match the joint function and specified quality requirements. A visual check may measure the toe groove, but critical joints can require cross-sections, penetrant testing, radiography, ultrasonics, leak testing or mechanical tests selected by the responsible engineering authority.
Visual and dimensional: undercut depth and length, bead width, underfill, reinforcement, start-stop condition, distortion and alignment.
Cross-section: penetration, fusion boundaries, toe profile, root condition and internal porosity on representative samples.
Functional evidence: leak, pressure, peel, tensile, fatigue, corrosion or other tests required by the product and service.
Process evidence: repeatability across parts, starts, stops, orientations, shifts, consumable life and the approved operating window.
Do not grind first and investigate later.
Grinding may remove a shallow visual indication, but it also changes section thickness, toe radius and surface condition. It cannot restore incomplete fusion or replace missing weld metal. On thin or critical parts it can make the joint worse.
Use the drawing, repair procedure and responsible engineering approval to decide whether the joint can be blended, locally rewelded, completely removed and rewelded, or must be rejected. After any repair, repeat the required inspection.
Match the visible pattern to the next investigation.
| Pattern | Inspect first | Process trial | Confirmation |
|---|---|---|---|
| Continuous undercut on both sides | Focus, optics, actual speed, bead width and material identity. | Controlled power-density, speed and oscillation matrix. | Cross-section plus toe measurement at steady-state seam. |
| Undercut on one side | Beam center, wire landing, head orientation, part height and wobble symmetry. | Center the process; compare direction safely. | Both toe profiles and fusion boundaries. |
| Intermittent groove or humping tendency | Motion stability, surface condition, gas/plume, keyhole signals and wire pulsation. | Reduce instability within a documented speed/energy window. | Longer seam across realistic production duration. |
| Defect at start or stop | Power ramp, speed ramp, dwell, overlap, crater termination and wire timing. | Program coordinated ramps and termination sequence. | Start-stop sections and repeated cycle test. |
| Groove varies with gap | Cut-edge quality, fixture, mismatch, gap map and filler delivery. | Reduce variation; then test qualified wobble/filler strategy. | Worst-case permitted gap and edge mismatch. |
| Defect worsens during the shift | Cover glass, nozzle, contamination, cooling, gas supply, wire path and optics temperature. | Restore hardware condition before changing recipe. | Consumable-life and time-based repeatability test. |
Send the joint, the groove pattern and the production target.
Oceanplayer can review the laser-welding application and help define a machine and sample-test route. Include material grade, thickness, joint drawing, gap range, defect photos, current parameters, wire and gas details, cycle target and acceptance method.
Related laser welding resources
Laser welding undercut FAQ
What is undercut in laser welding?
Undercut is a groove or recession at a weld toe or edge. It represents missing section adjacent to the fusion zone. Its acceptability depends on the measured dimensions, specified quality level, joint function and applicable engineering requirements.
Does excessive laser power cause undercut?
It can. Excessive local power density may drive unstable vapor pressure and melt displacement. However, low lateral melting, excessive speed, focus error, beam offset, gap variation and insufficient fill can produce a similar appearance. Evaluate power together with speed, focus, beam profile and joint condition.
Should I slow down the laser welding speed?
Only as a controlled trial. High speed can impose an undercut limit in some processes because the pool cannot refill the edge. But slower travel increases energy per unit length and may cause a wider heat-affected zone, distortion, sagging or burn-through. Rebalance speed with the other process variables.
Can wobble welding eliminate undercut?
Wobble can redistribute energy, widen the interaction and improve edge wetting or gap tolerance in a suitable process. It is not automatic: excessive width, unsuitable pattern or incorrect frequency can reduce penetration or overheat edges. Qualify the specific pattern and joint.
When should filler wire be used?
Filler deserves evaluation when joint gap, edge volume, alloy control or bead profile cannot be met reliably by an autogenous process. First reduce avoidable fit-up variation. Then qualify wire alloy, diameter, feed speed, angle and landing position with the welding parameters.
Can shielding gas fix laser welding undercut?
Gas delivery can influence plume behavior, oxidation, weld geometry and process stability, but it is not a universal cure. Confirm gas identity, purity, flow arrangement and metallurgy, then compare gas routes while holding other variables constant and inspecting the complete joint.
Why is undercut only on one side of the weld?
One-sided undercut often suggests asymmetry: beam tracking, wobble center, wire position, head orientation, part height, gas flow or fixture offset. Measure the actual joint center and compare travel direction before changing the entire energy recipe.
Can a small undercut simply be ground smooth?
Not automatically. Grinding changes section and toe geometry and cannot repair incomplete fusion. Use the applicable acceptance criteria and an approved repair procedure. Confirm that blending will not reduce minimum thickness, then repeat the required inspection.
How should laser welding undercut be inspected?
Start with visual and dimensional examination using the specified method. Depending on joint function, add cross-sections, penetrant testing, radiography, ultrasonics, leak or mechanical testing selected by the responsible engineering authority. Surface appearance alone may not demonstrate internal quality.
What information should I send for an undercut troubleshooting review?
Send material grade and thickness, coating, joint drawing, gap and mismatch range, seam orientation, photos with travel direction, power, speed, focus, wobble, gas, wire data, head geometry, optics condition, cycle target and acceptance standard. A representative sample is more useful than a generic material name.
Sources used for this guide
- ISO 13919-1:2019 — quality levels for imperfections in electron- and laser-beam welded joints in steel, nickel, titanium and their alloys.
- ISO 13919-2:2021 — quality levels for aluminum, magnesium, copper and their alloys.
- ISO 17637:2016 — visual testing of fusion-welded joints.
- ISO 15609-4:2009 — welding procedure specification requirements for laser beam welding.
- Gook et al., Physics Procedia 2014 — melt-flow mechanisms and undercut behavior in laser-arc hybrid welding.
- Process limit imposed by occurrence of undercuts during high-speed laser welding — high-speed transition from a sound bead toward undercut and humping.
- Capillary geometry and process-limit study, Procedia CIRP 2022 — relationship among welding speed, capillary geometry and undercut onset.
- Beam oscillation and filler-wire study, Journal of Manufacturing and Materials Processing — energy distribution, melt-pool behavior and gap-bridging considerations.
- Argon versus helium shielding study in AA2024 laser welding — evidence that shielding choice is alloy- and process-specific.