Porosity in a laser weld is a cavity created when gas or vapor is trapped as the molten metal solidifies. The correct response depends on the pore mechanism. Surface contamination, an unstable keyhole, trapped joint gas and volatile coatings can produce different pore populations, so a single adjustment such as increasing shielding-gas flow is not a dependable cure.
Read the pore pattern before changing the process
Inspection should describe pore size, shape, number, distribution and position through the weld depth. One polished section is useful but may miss an intermittent three-dimensional defect; radiography or computed tomography can provide broader spatial evidence when the part geometry and required resolution make those methods suitable.
| Observation | Possible mechanism | Evidence to check next |
|---|---|---|
| Rounded pores near the upper bead | Surface contamination, moisture or shielding disturbance | Cleaning record, gas purity, gas path, surface analysis |
| Pores concentrated near the root | Trapped gas, poor venting, gap condition or unstable full penetration | Joint geometry, root condition, fit-up and section series |
| Irregular cavity in the fusion zone | Keyhole collapse or unstable energy coupling | Power trace, focus, speed, beam position and high-speed monitoring if available |
| Pores associated with coating or plating | Vaporized layer trapped in the pool | Coating thickness, removal strategy, vent path and supplier data |
| Pores along an unfused interface | Incomplete fusion rather than simple gas porosity | Fusion boundary, penetration and joint alignment |
The last distinction is important: a dark indication in a radiograph or section is not automatically a gas pore. Classifying the imperfection correctly prevents a parameter change that makes fusion worse.
Main cause families
Surface contamination and preparation
Oil, fingerprints, moisture, oxide, polishing compound and cleaning residue can decompose or vaporize under the laser. The resulting gas may not escape before solidification. Define the cleaning chemistry, rinse and drying steps, clean-to-weld time and protected handling. A visually bright surface is not proof that it is chemically clean.
For aluminum laser welding, the oxide layer and volatile alloying elements deserve particular attention. For reactive titanium, preparation and atmosphere control interact: oxidation and gas pickup can change both appearance and mechanical behavior.
Keyhole instability
In deep-penetration welding, laser energy creates a vapor cavity called a keyhole. If recoil pressure, surface tension, vapor flow and molten-metal flow are not stable, the cavity can close and trap gas. Power density, focus position, travel speed, beam angle and oscillation all influence this balance. An unstable optical path or dirty protective window can also change the delivered spot without changing the programmed power.
Shielding and chamber conditions
Shielding gas protects the molten pool and can influence plume behavior, but more flow is not always better. Excessive velocity or poor nozzle placement can entrain surrounding air and disturb the pool. Check gas identity, purity, delivery pressure, leaks, nozzle condition, coverage and drafts.
For highly reactive materials or assemblies with inaccessible surfaces, a controlled-atmosphere glove-box welding system may offer more complete coverage. A vacuum laser welding system changes the process environment again and requires its own qualified window. Neither environment removes the need for surface preparation or fit-up control.
Joint design, gap and venting
Gas can be trapped between overlapping sheets, beneath plating, inside a blind cavity or at a poorly vented interface. Gap variation also changes energy coupling and molten-metal flow. Measure the joint; do not assume nominal CAD dimensions describe the assembled condition.
A controlled troubleshooting sequence
- Define the acceptance requirement. Use the drawing, contract or qualified welding specification. A pore that is acceptable in one product may be unacceptable in a thin hermetic enclosure.
- Map the defect. Record where pores occur relative to the start, stop, seam overlap, root, coating and fixture contact.
- Confirm the method's capability. Ensure the inspection technique can resolve the pore size and orientation that matter. Use complementary methods when necessary.
- Audit preparation and atmosphere. Verify material storage, cleaning, time before welding, gas purity, chamber cycle and oxygen or moisture readings where monitored.
- Measure the joint. Check gap, mismatch, thickness, vent path and beam-to-joint alignment on failed and conforming parts.
- Verify delivered laser conditions. Inspect optics and protective glass; confirm focus, spot position, speed and power stability.
- Run bounded trials. Change one factor or use a planned experiment. Section or image multiple locations across each condition.
- Confirm all critical outputs. A porosity reduction is not sufficient if the new condition causes undercut, excessive penetration, cracking or distortion.
Prevention checklist for production
- Approve a repeatable cleaning and protected-handling method.
- Control gap, alignment and coating condition with measurable limits.
- Monitor the atmosphere variables that are meaningful for the material and enclosure.
- Maintain beam-delivery optics and verify focus after relevant service events.
- Correlate process signals with destructive sections and leak results before using them for production decisions.
- Treat starts, stops and overlap regions as distinct risk locations in the inspection plan.
- Maintain traceability between material lot, machine program, chamber cycle and inspection result.
For hermetic products, porosity and leakage are related risks but not interchangeable measurements. A part may contain internal pores without a continuous leak path, or it may leak through an imperfection that is difficult to see in a section. Use weld inspection together with the specified hermetic leak-testing method.