Laser welding undercut is a groove melted into the base material along a weld toe that is not adequately filled by weld metal. It can reduce the effective section and act as a stress concentrator. The correction depends on why molten metal failed to fill the edge; reducing power without checking beam position, speed, joint gap and pool flow may trade undercut for incomplete fusion.
Confirm that the feature is undercut
Undercut occurs at a weld toe. It should be distinguished from overall bead concavity, root concavity, a misaligned joint edge, a pre-existing machining groove and surface erosion caused during preparation. View the seam from multiple angles and use a calibrated profile or cross-section when the feature is near the acceptance limit.
| Feature | Location | Diagnostic clue |
|---|---|---|
| Undercut | Groove beside the weld toe | Base material is melted away and not refilled |
| Face concavity | Depression across the weld face | Center of the bead is low relative to adjacent surfaces |
| Root concavity | Depression at the back or root | Requires root-side viewing or sectioning |
| Joint mismatch | Step between the two members | Follows the original joint rather than the molten boundary |
| Incomplete fusion | Unfused interface within or below the bead | May not be visible from the face |
Acceptance must come from the drawing, qualified welding procedure or applicable standard. ISO 13919-2 provides quality-level concepts for imperfections in electron- and laser-beam welded joints, but the contract and component function determine which criteria apply.
Common causes of undercut
Excessive or poorly distributed energy
High power density can eject or displace molten metal. Excessive travel speed may stretch the pool and prevent metal from wetting the toes before solidification. Focus position, spot shape, beam angle and oscillation can concentrate energy at one edge. Analyze delivered conditions together; power alone does not define the weld pool.
Beam and joint misalignment
On a butt, lap or fillet joint, a small offset can place too much energy on one member. Part variation, fixture wear, seam-tracking error or thermal movement may produce undercut on one side only. A repeated left-or-right pattern is therefore valuable evidence.
Gap, mismatch and edge condition
An excessive or changing gap leaves too little material to bridge the joint. Edge rounding, burrs or inconsistent thickness alter the local volume that must be filled. Measure actual assembled parts from both conforming and nonconforming groups rather than relying only on fixture settings.
Unstable keyhole and molten-metal flow
Vapor pressure, shielding-gas flow, surface tension and gravity all affect how liquid metal circulates and returns behind the laser. Spatter or intermittent undercut may point to an unstable keyhole, contamination or plume interaction. This problem can occur together with laser weld porosity.
Insufficient filler or unsuitable joint design
Many precision laser welds are autogenous, so the original edges must supply all weld metal. If the design requires more volume than the joint provides, parameter optimization may have a narrow or nonexistent window. Where filler is permitted, its form, placement and composition require qualification rather than ad hoc addition.
Inspection and measurement
Visual inspection should use controlled illumination, magnification and a defined viewing angle. A surface-profile instrument can quantify depth and length where access permits. Metallographic sections show the relationship between the toe groove, fusion boundary and penetration, but several sections may be required because undercut can be intermittent.
Record:
- which side of the seam is affected;
- maximum depth and continuous or accumulated length;
- relationship to starts, stops, corners and thickness transitions;
- joint gap and mismatch at the same location;
- bead width, penetration and root condition;
- spatter, porosity, discoloration or cracking present at the same time.
This combined record prevents an isolated cosmetic judgment and supports root-cause comparison.
Corrective-action sequence
- Verify the measurement and criterion. Confirm that the feature is undercut and that the inspection method resolves the specified limit.
- Check beam-to-joint position. Inspect tooling, calibration, seam path and thermal movement before changing energy.
- Measure joint fit-up. Compare gap, mismatch, edge geometry and thickness against qualified limits.
- Inspect optics and shielding delivery. A dirty protective window, focus drift or disturbed gas flow can change pool behavior.
- Review the complete parameter set. Consider power, speed, focus, waveform, angle and oscillation. Change one variable or use a structured experiment.
- Section representative trials. Confirm that the apparent improvement preserves fusion width and penetration.
- Recheck product risks. Perform the surface, mechanical and hermetic tests required for the assembly.
Potential adjustments include reducing excessive power density, modifying speed, repositioning the focus, centering the beam, stabilizing the joint gap, revising oscillation or pulse shape, and controlling the end condition. The direction and amount of change depend on the mechanism demonstrated by evidence; none is a universal setting.
Prevent recurrence
Lock the qualified program, track fixture condition, verify beam position and focus, and measure critical joint dimensions at a frequency based on process risk. Include starts, stops and seam intersections in qualification samples. If process monitoring is used, correlate its limits with measured undercut and other defects rather than assuming a stable signal guarantees an acceptable surface.
Undercut is only one part of weld quality. A complete inspection plan for hermetic laser welds should address cracks, porosity, fusion, penetration, dimensional effects and leakage according to the component's function.