Welding Imperfections and Classification - Part 3: Solid inclusions

This article continues the series on Welding Imperfections and Classification in accordance with ISO 6520-1:2007, focusing on Group 3 – Solid Inclusions, following earlier discussions on Cracks (Part 1) and Cavities (Part 2).

According to ISO 6520-1, solid inclusions are non-metallic or metallic foreign bodies entrapped in the weld metal or at the fusion boundary. They originate either from the welding consumables, the base material, the environment, or from improper manipulation of the welding process.

Among these imperfections, solid inclusions are particularly significant because they introduce non-metallic or foreign material into the weld metal, which can degrade strength, ductility, and corrosion resistance. Their detection, classification, and prevention are therefore critical to welding quality management.

Inclusions can be broadly divided into:

Slag inclusions – trapped flux or slag from flux-shielded processes.
Oxide inclusions – formed by oxidation of alloying elements during welding.
Tungsten inclusions – fragments of non-consumable electrode in GTAW (TIG).
Copper inclusions – transfer from contact tips in GMAW.
Metallic inclusions – entrapped foreign metallic material or spatter.

Mechanisms and Formation of Solid Inclusions

Solid inclusions occur when molten weld metal fails to expel foreign matter before solidification. This typically results from:

Improper slag removal between passes.
Incorrect electrode manipulation, leading to slag entrapment at weld toes or side walls.
Insufficient fusion or penetration, creating voids where slag or oxides are trapped.
Contaminated joint surfaces, such as mill scale, rust, or grease.
Inadequate shielding or excessive arc length, promoting oxidation.
Damaged TIG electrodes that shed tungsten particles into the weld pool.

Once formed, inclusions act as stress concentrators and may become initiation points for fatigue cracks or corrosion attack. Solid inclusions are normally classified under Group No. 3 in ISO 6520-1, with reference numbers beginning at 300

Designation of Solid Inclusions (ISO 6520-1)

Solid inclusions are represented under Group No. 3 in the ISO 6520-1 classification system. The following are the primary reference numbers and their sub-categories.

300 – Solid Inclusion (General Term)

A general term for any solid material entrapped in the weld metal that is not part of the intended composition.

301 – Slag Inclusion – A non-metallic inclusion derived from slag that becomes trapped in the solidified weld metal.

Typical processes: SMAW, FCAW, SAW.

Forms: Elongated stringers, isolated pockets, or planar films.

Subcategories:

3011 – Linear or longitudinal slag inclusion – aligned parallel to the weld axis.
3012 – Isolated slag inclusion
3013 – Clustered slag inclusion

We can also have

Layered slag inclusion – multiple thin layers trapped between weld beads.
We can have transverse slag inclusion – oriented across the weld axis.

Common Causes Solid Inclusion:

Poor slag removal between passes.
Incorrect electrode angle or travel speed.
Inadequate interpass cleaning or undercut grooves.
Excessive weaving, trapping slag behind the arc.

Detection: Radiographic testing (RT) or ultrasonic testing (UT) are most effective.

302 – Flux inclusion – solid inclusion in the form of flux

Flux inclusions can be:

3021 – linear
3022 – isolated
3023 – clustered

303 – Oxide Inclusion – Entrapment of metallic oxides formed by oxidation during welding.

Common metals affected: Aluminium, titanium, chromium, and manganese.

Causes:

Exposure of molten pool to atmosphere.
Inadequate shielding gas coverage.
Excessive arc length or turbulent gas flow.

Effect: Reduces weld toughness and corrosion resistance.

Detection: Often detected through metallographic examination or microstructural analysis.

Subcategories:

3031 – linear
3032 – isolated
3033 – clustered
3034 – puckering; in certain cases, especially in aluminium alloys, gross oxide film enfoldment can occur due to a combination of unsatisfactory protection from atmospheric contamination and turbulence in the weld pool

304 – Metallic inclusion – solid inclusion in the form of foreign metal

Metallic inclusions can be:

3041 – tungsten,
3042 – copper
3043 – other metal

Tungsten Inclusion – Small particles of tungsten electrode material trapped in the weld metal.
Process involved: Gas Tungsten Arc Welding (GTAW).

Causes:

Electrode dipping into the molten pool.
Excessive current leading to electrode tip erosion.
Contaminated or poorly ground tungsten tip.

Significance: Tungsten inclusions are very hard and brittle, acting as severe stress risers. They may lead to cracking under fatigue or dynamic loading.

Detection: Radiography or metallography.

Copper Inclusion – Entrapment of copper fragments originating from GMAW contact tips or backing bars.

Causes:

Excessive contact tip wear.
Arc striking outside the weld groove.
Molten metal adhesion to copper tooling.

Effect: Copper inclusions can cause localised embrittlement or cracking.

Metallic Inclusion (Other metal) – Inclusion of metallic particles not belonging to the intended weld composition, such as fragments of wire.

Causes: Improper welding technique, poor cleaning, or spatter adhesion during deposition.

Consequences of Solid Inclusions

Reduced ductility and toughness due to the brittle nature of inclusions.
Initiation sites for cracks under cyclic or impact loading.
Corrosion initiation points in aggressive environments.
Impaired ultrasonic inspection results, since inclusions reflect and scatter sound waves irregularly.

Prevention Measures

Clean joint surfaces and interpass areas thoroughly.
Maintain correct electrode angle and travel speed.
Avoid excessive weaving and undercut formation.
Use proper current and polarity for good slag detachability.
Ensure adequate shielding and gas coverage.
Inspect and maintain TIG electrodes and GMAW contact tips.
Follow qualified welding procedures (WPS) with proper interpass cleaning requirements.

Designation Example

To identify a slag inclusion using ISO 6520-1:

Imperfection ISO 6520-1-3011 → Linear or longitudinal slag inclusion.
Imperfection ISO 6520-1-302 → Flux inclusion
Imperfection ISO 6520-1-303 → Oxide Inclusion.

Letter Codes for Solid Inclusions (Sa–Sf)

(Please note that this is proposition and not in ISO 6520-1)

Letter Code	Inclusion Type	Typical Mechanism
Sa	Slag inclusion	Slag entrapment in weld metal.
Sb	Flux inclusion	Entrapment of flux particles.
Sc	Oxide inclusion	Oxide film entrapment.
Sd	Tungsten inclusion	Tungsten pickup from electrode.
Se	Copper inclusion	Transfer of copper fragments.
Sf	Other metallic inclusion	Entrapment of foreign metal.

Full designation format:
Imperfection ISO 6520-1-[Reference Number] S[Letter]

Examples:

Slag inclusion, linear: Imperfection ISO 6520-1-3011 Sa
Oxide inclusion, isolated: Imperfection ISO 6520-1-3032 Sc
Tungsten inclusion: Imperfection ISO 6520-1-3041 Sd

Summary Memory Cues

Reference	Description	Common Process	Detection Method
301	Slag inclusion	SMAW, SAW, FCAW	RT, UT
302	Flux inclusion	SMAW, SAW	RT
303	Oxide inclusion	GTAW, GMAW	Metallography
3041	Tungsten inclusion	GTAW	RT, Metallography
3042	Copper inclusion	GMAW	RT
3043	Metallic, other	All	Visual, RT

Solid inclusions illustrations

(a) Interpass slag inclusions (b) Linear (longitudinal) slag inclusion (c) **Tungsten inclusions on radiographic examination.**

Closing Note

Solid inclusions represent a group of imperfections where foreign materials are trapped within the weld metal, degrading integrity and reliability. Understanding their formation, identification, and classification under ISO 6520-1 ensures accurate defect assessment and promotes high-quality weld fabrication

Frequently Asked Questions (FAQs)

Welding Imperfections and Classification – Part 3: Solid Inclusions

Q1. What are solid inclusions in welding?

Solid inclusions are foreign solid materials trapped in the weld metal or at the fusion boundary. They may be metallic or non-metallic and originate from slag, flux, oxides, or fragments of equipment such as tungsten electrodes or copper contact tips.

Q2. Why are solid inclusions considered harmful?

They reduce weld toughness, ductility, and corrosion resistance. Solid inclusions also act as stress concentrators and may initiate cracks under cyclic, impact, or dynamic loading, compromising weld reliability.

Q3. Which welding processes are most associated with slag inclusions?

Slag inclusions commonly occur in SMAW, FCAW, and SAW because these processes use flux that produces slag. Improper cleaning, poor manipulation, or incorrect technique can cause slag to remain trapped in the weld.

Q4. How do oxide inclusions form?

Oxide inclusions form when molten metal reacts with oxygen due to inadequate shielding, excessive arc length, turbulence in the weld pool, or contamination by rust, mill scale, or oxide films. They are common in aluminium and stainless steel welding.

Q5. What causes tungsten inclusions in GTAW?

Tungsten inclusions occur when the non-consumable tungsten electrode touches the weld pool, is overheated by excessive current, or becomes damaged. Small tungsten particles then break off and become trapped in the solidifying metal.

Q6. How do copper inclusions occur in GMAW?

Copper inclusions originate from worn or overheated contact tips, copper backing bars, or accidental arc strikes. Molten weld metal can pick up copper fragments, which later become entrapped in the weld.

Q7. Are solid inclusions visible to the naked eye?

Most solid inclusions are internal and not visible externally. Detection usually requires radiographic testing, ultrasonic testing, or metallographic examination. Some metallic inclusions may be visible if they occur near the surface.

Q8. Can solid inclusions cause weld failure?

Yes. Inclusions reduce load-carrying capacity and may serve as crack initiation points. Under fatigue, vibration, or impact conditions, inclusions significantly increase the risk of fracture or sudden failure.

Q9. What is the difference between slag inclusion and oxide inclusion?

Slag inclusion: Entrapped flux residue from flux-based welding processes.
Oxide inclusion: Metallic oxides formed by oxidation of alloying elements during welding.

Slag comes from the welding consumable, while oxides are chemical reaction products.

Q10. How can welders prevent slag inclusions?

Remove slag between passes.
Use correct electrode angle and travel speed.
Avoid excessive weaving.
Maintain clean joint surfaces and proper interpass cleaning.
Follow procedure requirements for current, polarity, and technique.

Q11. How can oxide inclusions be avoided?

Ensure proper shielding gas coverage.
Maintain correct arc length.
Avoid contamination (rust, oxides, oil, paint).
Prevent turbulence around the weld area by avoiding excessive gas flow.

Q12. Which inspection methods are best for detecting solid inclusions?

Radiographic Testing (RT): Highly effective for slag and metallic inclusions.
Ultrasonic Testing (UT): Good for elongated inclusions or planar slag.
Metallographic examination: Required for micro-scale oxide and tungsten inclusions.

Q13. How are solid inclusions classified in ISO 6520-1?

Solid inclusions belong to Group 3, with reference numbers beginning at 300. Subcategories include:

301: Slag inclusions
302: Flux inclusions
303: Oxide inclusions
304: Metallic inclusions (tungsten, copper, other)

Q14. What designation is used when reporting inclusions?

Imperfections are reported using the ISO 6520-1 format:
Imperfection ISO 6520-1-[Reference Number]
For example:

Slag inclusion (linear): Imperfection ISO 6520-1-3011
Oxide inclusion (isolated): Imperfection ISO 6520-1-3032
Tungsten inclusion: Imperfection ISO 6520-1-3041

Q15. Are solid inclusions always unacceptable?

Acceptance depends on the code or standard applied (for example ISO 5817 or ASME Section IX), material type, weld category, and service conditions. However, inclusions are typically restricted because they significantly affect structural integrity.

Q16. How do solid inclusions affect ultrasonic inspection?

Inclusions scatter and reflect ultrasonic waves irregularly, causing diffuse or noisy indications. This can make UT interpretation more difficult, especially for clustered or oxide inclusions.

Q17. What steps can fabricators take to reduce inclusion risk?