Laser Welding Defects: Cracks, Spatter, and Undercut — Causes and Fixes
Laser welding delivers deep, narrow, low-distortion joints at speeds traditional arc processes cannot match. However, it is less forgiving than MIG or TIG. Because the high-energy beam concentrates heat into a microscopic spot and the molten pool cools rapidly, minor errors in parameters, shielding gas, or joint preparation quickly manifest as defects.
Three main defects account for the vast majority of rejected laser welds in modern fabrication shops: cracks, spatter, and undercut. (Note: Porosity — the fourth major defect — is covered separately in our dedicated guide on gas pores in laser welding seams).
This troubleshooting guide helps production managers, welding supervisors, and operators of both handheld and automated laser welding systems quickly identify defects on the shop floor, correct the root cause, and keep production moving.
Why These Three Defects Cost Fabrication Shops Money
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Rework and Scrap: Defects caught at final QC mean expensive re-cutting, manual grinding, and re-welding.
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Failed Structural & Leak Tests: Cracks and deep undercuts reduce the effective cross-section and create critical stress risers, leading to field failures.
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Aesthetic Penalties: Heavy spatter and undercut ruin cosmetic seams that would otherwise bypass post-processing.
Catching and stabilizing defects at the machine — rather than at final inspection — is where true shop floor profitability lives.
1. Cracks
What You See
A crack appears as a sharp discontinuity either directly in or immediately adjacent to the weld bead — running along the centerline, at the weld toe, or within the heat-affected zone (HAZ). It may form immediately upon solidification or hours/days later (delayed cold cracking is common in hardenable carbon steels).
Why It Happens & How to Fix It
The primary culprit is the extreme cooling rate inherent to laser welding. Rapid solidification traps low-melting impurities along the centerline (hot cracking), while high restraint, fast cooling, and hydrogen cause cold cracking in susceptible steels.
| Common Cause | Corrective Action |
| Excessive travel speed or cooling rate | Reduce travel speed or rebalance the power-to-speed ratio to slow down thermal gradients. |
| Crack-sensitive material (High carbon, sulfur, phosphorus, or certain hardenable alloys) | Introduce a compatible filler wire to alter weld chemistry; preheat base materials within recommended ranges. |
| Poor fit-up or gap variation | Tighten joint preparation; maintain a consistent, zero-to-minimal gap. |
| Joint contamination (Oil, moisture, lubricants, coolants) | Clean and degrease edges thoroughly; surface oils introduce hydrogen and impurities. |
| High joint restraint | Adjust physical clamps or change the welding sequence to allow natural movement during cooling. |
| Wrong filler chemistry | Ensure the filler wire metallurgy precisely matches or complements the base material. |
Shop Floor Tip: When cracks appear intermittently rather than continuously, check your material batch numbers or incoming sheet coatings. Frequency often tracks changes in material chemistry rather than machine drift.
2. Spatter
What You See
Spatter consists of molten metal droplets ejected violently from the weld pool, landing on the part surface or baking onto the protective cover lens. Light spatter is sometimes manageable; heavy, continuous spatter signals a severely unstable keyhole process.
Why It Happens & How to Fix It
Spatter means the keyhole is opening and collapsing too rapidly, or metal vapor pressure is bursting through the top of the pool.
| Common Cause | Corrective Action |
| Power too high for the travel speed | Lower laser power or increase travel speed to find the stable operating window. |
| Surface contamination (Rust, mill scale, heavy oils) | Clean the joint zone; surface oxides alter beam absorption and cause localized boiling. |
| Improper shielding gas flow or coverage | Adjust flow rates. Too low leaves the pool exposed; too high creates turbulence that sucks air in. Check nozzle standoff distance. |
| Focus position off the joint | Verify focal point height and optical beam alignment relative to the seam. |
| Wobble / oscillation settings too aggressive | Reduce the amplitude or frequency of the beam scan pattern. |
| Excessive joint gap or edge mismatch | Improve fit-up. Feeding a thin filler wire can successfully stabilize a gapped joint. |
Shop Floor Tip: A minimal amount of transient spatter during setup is normal. Spatter that steadily increases over a production run usually points to optics degradation (dirty protective glass) or thermal lens shift.
3. Undercut
What You See
An undercut is a distinct groove or gouge melted into the base material right at the weld toe, leaving the central weld bead looking "proud" of a recessed edge. It drastically reduces the effective load-bearing cross-section and acts as a classic stress concentrator for fatigue failure.
Why It Happens & How to Fix It
Undercut occurs when energy input at the joint edges overwhelms the available fill material — the laser vaporizes or displaces base material faster than the surrounding molten pool can flow back to refill it.
| Common Cause | Corrective Action |
| Excessive heat input on thin sections | Reduce laser power or increase travel speed. On thin sheet metal, lower power paired with steady speed prevents edge gouging. |
| Beam misaligned relative to the joint line | Re-center the beam precisely. Utilizing beam wobble helps distribute energy evenly across joint walls. |
| Focus spot too tight on the edge | Adjust defocus or focal position slightly; a marginally larger spot reduces aggressive edge gouging. |
| Insufficient filler material on gapped joints | Add filler wire and match wire feed speed to the width of the gap. |
| Shielding gas turbulence pushing the pool | Verify gas delivery angle and flow rate; turbulent gas can physically push molten metal away from the weld toes. |
Shop Floor Tip: Undercut is easily measured using a standard weld gauge. Measure depth across the seam: localized undercut usually indicates beam misalignment, while uniform undercut across a whole run points to excessive heat input.
A Systematic Framework for Fixing Laser Defects
When a defect suddenly appears on the line, resist the urge to change multiple variables at once. Follow this disciplined troubleshooting flow:
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Stabilize the Inputs: Document the exact material grade, thickness, surface condition, joint type, and gap consistency.
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Change One Parameter at a Time: Modify power, travel speed, focus, gas flow, or wobble — one change, one test weld.
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Test on Scrap First: Always run verification coupons using exact matching production material before touching live parts.
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Log Your Working Window: Keep records. Parameters naturally drift as protective lenses age, nozzles wear, and ambient conditions shift.
FAQ
Q: Can the same laser welding parameters produce both spatter and undercut simultaneously?
A: Yes. Both are fundamentally heat-input issues. Excessive power relative to travel speed can violently eject droplets (spatter) while gouging out the joint edges (undercut) in a single pass. Correct your power-to-speed balance first, then evaluate remaining defects.
Q: Is filler wire always required to prevent laser cracks?
A: No. On tight, ultra-clean joints with metallurgically compatible materials, autogenous (wire-free) laser welding produces flawless structural welds. Filler wire becomes mandatory when dealing with gapped joints, crack-sensitive alloys (like certain stainless or high-carbon steels), and dissimilar metal combinations where it acts as a chemistry regulator.
Q: How do I know if a defect is caused by the machine/parameters or the operator?
A: Run a controlled isolation test: use the same operator, the exact same parameters, on pre-cleaned test coupons. Defects that persistently repeat on clean, prepped coupons point to machine, optic, or parameter issues. Defects that follow a specific operator (such as hand-held gun angle drift) point to technique.
Achieve Defect-Free Laser Welds on Your Production Parts
Most laser welding defects are entirely preventable with optimized parameters, proper joint prep, and correct technique — but the "right" setup depends entirely on your specific alloy, joint configuration, and production volume.
Our in-house application engineering team regularly runs sample weld trials on customer components to establish robust, production-ready parameter windows. (Looking for help with gas pores? Read our guide on resolving porosity in laser welding).
[Request a Free Weld Test] — Send us your material specs, thickness, and joint details. Our team will return optimized parameter recommendations and sample evaluation results for your shop.
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