EN ISO 10042 is the acceptance criteria standard for arc-welded aluminium — the direct structural counterpart of ISO 5817 for steel. It defines three quality levels (B, C, D), each with specific dimensional limits for every class of weld imperfection. Applying ISO 5817 limits to aluminium welds — or assuming that “the same weld quality standard” applies regardless of base material — is a compliance error with real consequences under EN 1090-3, EN 15085, and aerospace supply chain audits.
This guide covers what EN ISO 10042 requires, how its levels compare side by side, where it diverges from ISO 5817, and how real-time process monitoring supports conformity documentation.
Scope and applicable processes
EN ISO 10042:2005+A1:2013 covers arc welding of wrought and cast aluminium and aluminium alloys, including:
- Gas Metal Arc Welding (GMAW / MIG)
- Gas Tungsten Arc Welding (GTAW / TIG)
- Plasma Arc Welding (PAW)
The standard applies from 0.5 mm thickness for butt welds and 1.5 mm for fillet welds, covering sheet, extrusions, and castings across structural, transport, and industrial manufacturing contexts.
Processes not covered by EN ISO 10042: friction stir welding (FSW — refer to ISO 25239-5), resistance welding, electron beam welding, and standalone laser welding. For laser-MIG hybrid or laser-arc combinations, supplementary quality requirements agreed between manufacturer and customer are common practice alongside this standard.
The three quality levels
EN ISO 10042 uses the same level notation as ISO 5817:
| Level | Designation | Typical application context |
|---|---|---|
| B | Stringent | Safety-critical, fatigue-loaded joints, aerospace, rail CP A/B, EN 1090-3 EXC3/EXC4 |
| C | Intermediate | General structural fabrication (EXC2), moderate fatigue loading |
| D | Moderate | Non-critical assemblies, static loading only, EN 1090-3 EXC1 |
Level B imposes the smallest permitted imperfection sizes. Level D is the most permissive. The sequence deliberately starts at B — following ISO 5817 convention — to leave room for a future Level A, which has not been published for aluminium arc welding.
The quality level is specified, not chosen. EN 1090-3, EN 15085-3, and EN 9100 each define which level applies based on execution class or weld class. The manufacturer must demonstrate conformity to the specified level — not elect the most convenient one.
Acceptance limits: B vs C vs D
The table below summarises the most commonly inspected imperfection types in aluminium arc welds. References are to EN ISO 10042:2005+A1:2013. For thickness-dependent limits, t = nominal material thickness; d = pore diameter; h = height or depth of the imperfection; b = weld width.
| Imperfection type | ISO 6520-1 code | Level B | Level C | Level D |
|---|---|---|---|---|
| Crack (all types) | 100 | Not permitted | Not permitted | Not permitted |
| Lack of fusion | 401 | Not permitted | Not permitted | Not permitted |
| Incomplete root penetration (full-pen butt) | 4021 | Not permitted | Not permitted | Not permitted |
| Oxide inclusions | 3042 | Not permitted | h <= 0.1t, max 1 mm | h <= 0.2t, max 2 mm |
| Single gas pore | 2011 | d <= 0.2 mm or 0.025t | d <= 0.4 mm or 0.05t | d <= 0.5 mm or 0.1t |
| Field porosity (projected area) | 2012 | <= 1% | <= 2% | <= 4% |
| Wormhole / tubular pore | 2016 | Not permitted | Not permitted | h <= 0.2t, max 2 mm |
| Continuous undercut | 5011 | h <= 0.05t, max 0.5 mm | h <= 0.1t, max 1 mm | h <= 0.2t, max 2 mm |
| Overlap (cold lap) | 506 | Not permitted | Not permitted | h <= 1 mm |
| Linear misalignment (butt) | 507 | h <= 0.05t, max 2 mm | h <= 0.1t, max 3 mm | h <= 0.15t, max 4 mm |
| Excess weld metal (butt, face) | 502 | h <= 1 mm + 0.1b | h <= 1 mm + 0.15b | h <= 1 mm + 0.25b |
| Sagging / burn-through | 509/510 | Not permitted | h <= 0.1t, max 1 mm | h <= 0.2t, max 2 mm |
| Crater pipe | 2025 | Not permitted | h <= 0.05t, max 0.5 mm | h <= 0.1t, max 1 mm |
| Root concavity | 515 | h <= 0.05t, max 0.5 mm | h <= 0.1t, max 1 mm | h <= 0.2t, max 2 mm |
The values above are indicative reference figures based on EN ISO 10042:2005+A1:2013. Always verify acceptance limits against the current edition of the standard, any overriding customer specification, sector code, or supplementary requirement. Some limits vary with weld type (butt vs fillet) and material thickness sub-range.
Key differences from ISO 5817
EN ISO 10042 and ISO 5817 share the same structural framework — both use levels B, C, D, both reference ISO 6520-1 imperfection codes, and both classify cracks and lack of fusion as non-acceptable at every level. The divergences are material-specific and non-trivial.
Oxide inclusions
Aluminium forms a tenacious Al₂O₃ oxide film on its surface at room temperature. This film must be removed by mechanical cleaning, chemical etching, or cathodic arc cleaning during GTAW before welding. If inadequately cleaned, or if shielding gas coverage fails, oxide inclusions remain trapped in the solidified weld. ISO 10042 includes a dedicated category for oxide inclusions (code 3042) that has no direct equivalent in ISO 5817.
At Level B, oxide inclusions are not permitted. At Level C and D, limited inclusions are accepted depending on their depth relative to thickness — but their presence typically signals a process control failure that should be investigated rather than accepted on dimensional grounds alone.
Tighter porosity limits
Aluminium alloys absorb hydrogen from moisture, lubricants, oxide films, and contaminated filler wire. As the weld pool solidifies, hydrogen comes out of solution as gas pores. Single-pore diameter limits in EN ISO 10042 are significantly tighter than the equivalent levels in ISO 5817: a Level B pore limit of d ≤ 0.2 mm compares to d ≤ 0.5 mm in ISO 5817 Level B. Field porosity area limits are also reduced by roughly half.
The tighter limits reflect that:
- Gas porosity is far more prevalent in aluminium than in structural steel under comparable process conditions.
- Hydrogen pores can be sub-surface and cluster in ways that are difficult to detect by visual testing alone.
- Porosity in fatigue-loaded aluminium structures acts as a crack initiation site with a lower threshold than in steel.
Sagging and burn-through
Aluminium melts at ~660 °C — less than half the solidus temperature of structural steel — and has roughly three times higher thermal conductivity. These properties make sagging (excess root penetration from gravity-driven pool drop) and burn-through significantly more common, especially in thin extrusions and sheet below 3 mm. ISO 10042 treats sagging and burn-through as specific, separately coded imperfections with their own Level B/C/D limits. ISO 5817 includes the same codes but with limits calibrated for materials that do not approach burn-through under normal weld conditions.
Standard mapping: EN 1090-3, not EN 1090-2
ISO 5817 integrates with EN 1090-2 (execution of steel structures). EN ISO 10042 integrates with EN 1090-3 (execution of aluminium structures). A weld drawing for an aluminium structural component must reference EN ISO 10042 quality levels — using ISO 5817 level designations on an aluminium-structure drawing is a normative error that will be flagged in a CE marking or third-party audit.
Selecting the quality level: standard mappings
| Governing code | Execution / weld class | Required EN ISO 10042 level |
|---|---|---|
| EN 1090-3 | EXC1 | D |
| EN 1090-3 | EXC2 | C |
| EN 1090-3 | EXC3 | B |
| EN 1090-3 | EXC4 | B |
| EN 15085-3 | CP A | B + supplementary requirements |
| EN 15085-3 | CP B | B |
| EN 15085-3 | CP C | C |
| EN 15085-3 | CP D | D |
| EN 9100 / AS9100 | — | B (minimum; customer may require supplementary) |
| NADCAP welding | — | B (or customer-defined above B) |
When no governing code applies, apply the consequence-of-failure logic from ISO 3834: safety-critical or fatigue-loaded welds → Level B; general structural service → Level C; non-critical, statically loaded assemblies → Level D.
Inspection methods and NDT requirements
Visual testing (VT) per ISO 17637 is mandatory for all three quality levels. Additional NDT methods are specified by the applicable product standard and weld class:
- Penetrant testing (PT): ISO 3452-1 — used for surface-breaking defects in aluminium. More reliable than magnetic particle testing, which is not applicable to non-ferromagnetic aluminium.
- Radiographic testing (RT): ISO 17636-1 — used for volumetric inspection (porosity, inclusions). Applicable to butt welds; film or digital detector.
- Ultrasonic testing (UT): ISO 17640 — applicable to thicker sections (typically above 8 mm); phased array techniques improve coverage for aluminium’s high acoustic attenuation.
Level B welds under EN 1090-3 EXC3/EXC4 and CP A/B welds under EN 15085 typically require 100% NDT coverage across all applicable methods. Level C welds may require partial sampling depending on the joint category.
Real-time thermal monitoring as a process quality record
Aluminium-specific defects — porosity, oxide inclusions, and sagging — are strongly correlated with arc parameter deviations during welding. Hydrogen porosity spikes when arc energy drops below the value needed to drive off surface moisture; oxide inclusions appear when shielding gas coverage fails or pre-cleaning is inadequate; sagging develops when heat input exceeds the material’s thermal tolerance in thin sections.
Real-time thermal monitoring for aluminium welding captures the thermal signature of each weld pass in-cycle and ties it to weld position coordinates. This supports the ISO 3834-2 traceability requirement by providing a continuous process record without adding inspection steps. For high-volume aluminium production — automotive body-in-white panels, EV battery enclosures, rail bogie frames — thermal monitoring reduces the cost of post-weld destructive sampling while building a digital evidence trail that directly supports EN ISO 10042 conformity declarations.
Monitor aluminium welds against EN ISO 10042 in real time
Therness thermal monitoring detects arc parameter deviations linked to porosity, oxide inclusions, and sagging before visual inspection. Supports ISO 3834-2 traceability and EN ISO 10042 conformity documentation.
Book a demoStandards and further reading
- EN ISO 10042:2005+A1:2013 — primary reference for aluminium arc weld quality levels
- ISO 5817 — equivalent standard for steel and nickel alloy arc welds
- ISO 3834 quality requirements — comprehensive, standard, and elementary quality requirements for fusion welding
- EN 1090-3 — execution of aluminium structures; maps execution classes to EN ISO 10042 levels
- EN 15085-3 — welding of railway vehicles and components; assigns weld class (CP A–D) quality levels
- ISO 6520-1 — classification and terminology of imperfections in metallic materials with fusion welding
- ISO 17637 — visual testing of fusion-welded joints
- ISO 3452-1 — non-destructive testing: penetrant testing (method applicable to aluminium)
Frequently Asked Questions
What is EN ISO 10042 and how does it differ from ISO 5817?
EN ISO 10042 defines quality levels (B, C, D) for imperfections in arc-welded aluminium and aluminium alloys — the direct counterpart of ISO 5817 for steel. Both share the same level notation but have different acceptance limits and imperfection categories. ISO 10042 includes aluminium-specific discontinuities such as oxide inclusions (from the Al2O3 surface layer) and applies tighter porosity limits than ISO 5817, reflecting aluminium's higher susceptibility to hydrogen-induced gas pores from moisture and oxide contamination.
What are the three quality levels in EN ISO 10042?
Level B (stringent), Level C (intermediate), and Level D (moderate). Level B is the most demanding, with the smallest tolerated imperfection sizes; Level D is the most permissive. The sequence starts at B rather than A following the same convention as ISO 5817 — leaving space for a stricter Level A that does not yet exist in the standard. Cracks, lack of fusion, and incomplete root penetration are not acceptable at any level.
Which EN ISO 10042 quality level is required for structural aluminium?
EN 1090-3 (aluminium structural components) maps execution classes to required quality levels: EXC1 requires Level D, EXC2 requires Level C, EXC3 and EXC4 require Level B. For rail vehicles, EN 15085-3 assigns levels by weld class: CP A and CP B require Level B, CP C requires Level C, CP D requires Level D. Aerospace applications per EN 9100 typically require Level B as a minimum and may add supplementary requirements beyond the standard.
