Porosity in welds
Gas cavities in fusion welds: morphology, material-specific sources, shielding checks, inspection limits and acceptance decisions.
Tags: production, welding, porosity, inspection
On this page
Start with shielding and contamination
Check shielding, joint cleanliness, consumables and preparation when pores appear. Increasing gas flow or heat without identifying the cause can worsen porosity. TWI's illustrated porosity guide.
A sound-looking cap can conceal internal pores. Appearance alone leaves cause and acceptance unresolved. TWI, surface and subsurface examination
In the illustrative image, three non-overlapping circular indications of 1.0, 1.5 and 2.0 mm equivalent diameter occupy 5.69 mm² of a 100 × 10 mm evaluation window, or 0.569% projected area. This cannot establish void volume or acceptance.
ISO 17636-1:2022 covers film technique and points to the ISO 10675 series for evaluation. Apply the governing acceptance basis.
Check material condition, use the investigation table and choose an inspection method. Revisit the cost estimate if repair changes the route.
Meaning and boundaries
Gas trapped during solidification can form dispersed or surface-breaking pores and elongated wormholes. Assess a shrinkage-related crater pipe separately. TWI’s overview first appeared in July/August 1999 and has since been updated. TWI, porosity
A pore observation identifies an imperfection, not its unique source or an automatic rejection. Morphology and location affect the interpretation. Porosity is distinct from lack of fusion or hydrogen embrittlement.
For process and equipment context, read GMAW wire, gas and transfer-mode selection.
Investigating the source
| Observation or process context | Evidence to collect | Candidate mechanism and scope |
|---|---|---|
| Pores follow a gas-shielded welding setup change | Gas identity, delivered flow, hoses, fittings, torch condition and local draughts | Inadequate protection can admit air; excessive flow can also entrain it. Raising flow indiscriminately is not a reliable correction. Linde shielding-gas guide, FAQ 1 |
| MIG welds become porous intermittently | Nozzle size and spatter, contact-tip recess, gun angle, wire extension, gas equipment and surface condition | Several coverage and contamination problems can produce the same symptom. Use the equipment maker's setup guidance rather than a universal nozzle distance. Miller, February 19, 2024 |
| Aluminium fusion welds show internal pores | Parent/filler cleanliness, hydrated oxide, handling and moisture exposure | Hydrogen enters the liquid readily and is rejected as aluminium solidifies; contamination control precedes parameter experimentation. TWI, aluminium porosity |
| Wormholes occur near coated joint roots | Coating specification and thickness, cleaning records and joint cavities | Gas from contamination or coatings can become trapped, especially in confined joint features. A coating described as weldable still has application limits. TWI, porosity |
Linde also identifies gas composition and moisture entry through the delivery system as possible contributors. A gas mixture useful for one material can cause porosity in another; retain the qualified material/process pairing. Its linked brochure has no issue date verified here. Linde, FAQ 1
For aluminium, TWI explains that more current can increase hydrogen absorption, while greater time before freezing can help bubbles escape. These competing effects rule out a general instruction to add heat or slow travel. Cleaning-to-welding time also depends on the environment and required quality; use the approved procedure for the actual alloy, joint and conditions. TWI, aluminium process discussion
Inspection and acceptance
TWI notes that radiography generally characterizes pores better than ultrasound, with small pores still difficult to detect in thick sections. Repair requires an agreed welding procedure and renewed assessment, not merely covering visible holes with another pass. TWI, detection and remedial action
Inspection technique and acceptance criteria answer different questions. ISO 17636-1:2022, edition 2, July 2022, covers film radiography of metallic fusion-welded joints; its public scope explicitly supplies no indication acceptance levels. It points to the ISO 10675 series for evaluation. ISO 17636-1
ISO 5817:2023, edition 4, February 2023, provides workmanship quality levels B, C and D for specified fusion welds in steel, nickel, titanium and their alloys, excluding beam welds. Its scope begins at 0.5 mm thickness; B has the highest requirements. This scope does not cover aluminium or establish a universal fitness-for-service decision. Assessment depends on the governing product requirements, edition and quality level. No numerical pore allowance is reproduced from an unread acceptance table. ISO 5817 public scope
Worked example: projected area is only one descriptor
Assume an illustrative calibrated image has three non-overlapping circular indications with equivalent diameters 1.0, 1.5 and 2.0 mm in a defined 100 × 10 mm evaluation window. These are invented measurements, not an acceptance rule.
- Combined projected area = π/4 × (1.0² + 1.5² + 2.0²) = 5.69 mm².
- Window area = 1,000 mm²; projected indication fraction = 0.569%.
- Largest equivalent diameter = 2.0 mm.
This calculation assumes the stated calibration, segmentation and window. It does not give three-dimensional void volume, distinguish overlapping pores at different depths or establish detectability. Three aligned indications can have the same summed area as three scattered ones. Individual sizes, spacing, distribution, weld thickness and the governing evaluation method remain relevant; 0.569% alone cannot establish acceptance.
Evidence and applicability
The same pore pattern can occur with different alloys, consumables, shielding conditions and joint preparations. An explanation is strongest when it accounts for the actual material condition, process and changes in the affected weld. Examination coverage, calibration and the indication map limit what a result establishes. Observed imperfections, a proposed cause and an accepted repair are distinct conclusions.
Reviewed 2026-11-02 PST. Research draft; the cited public standard scopes establish coverage, not access to their complete normative tables.
Related: Manufacturing process references.