Lack of Sidewall Fusion in Welds: Causes, Detection and Prevention

Lack of sidewall fusion is a welding discontinuity in which weld metal does not fuse completely with the side of the prepared joint. It can reduce the effective continuity of the joint and may be rejectable under the applicable welding code or project specification.

Terminology note for this revision: The existing page title says “slug welds.” That is not the usual generic term for the second discontinuity described in the article. This draft uses slag inclusion when the intended meaning is non-metallic material trapped in weld metal. A welding engineer should confirm the project terminology before publication.

What is lack of sidewall fusion?

In a groove weld, the deposited metal must fuse with the prepared sidewalls and, where applicable, the previous weld run. Lack of sidewall fusion is a planar or linear region where that bond is incomplete. It may be surface-breaking or subsurface, and its visibility depends on joint geometry, access, weld process, surface condition and inspection method.

It is different from:

• Lack of root fusion/penetration: incomplete fusion or penetration at the root region.
• Slag inclusion: non-metallic material retained in the weld metal, often associated with flux processes or poor inter-run cleaning.
• Porosity: gas cavities in the weld metal.
• Undercut: a groove at the toe or edge of a weld caused by melting that is not properly filled.

The current page combines lack of sidewall fusion with “slug/slag welds.” This final proposal treats sidewall fusion as the primary topic and flags the SEO-19 overlap for an owner decision before publication. Separating the terms makes the inspection and prevention advice more useful and reduces the risk of applying the wrong acceptance criterion.

Common causes

Joint preparation and access

A joint that is too narrow, poorly aligned, contaminated or difficult to access can prevent the arc, electrode or torch from reaching the sidewall. An incorrect bevel angle, land, root gap, mismatch or excessive reinforcement can force the operator to deposit metal without adequately melting the sidewall.

Welding parameters

Insufficient current or excessive travel speed can reduce penetration into the sidewall. The opposite combination can flood the joint and make penetration non-uniform. Arc length, voltage, polarity, wire-feed rate, shielding and heat-input limits must be controlled through the qualified welding procedure specification (WPS), not a generic number copied from another job.

Technique and torch/electrode angle

An incorrect work or travel angle can direct the arc toward one side of the groove. Poor manipulation, inadequate dwell at the sidewall, excessive weaving or failure to maintain the specified travel speed can leave an unfused edge. Magnetic arc blow can also deflect the arc in ferromagnetic material.

Cleaning between runs

Slag, oxide, paint, oil, moisture or other contamination can interfere with fusion. For multi-pass welding, the previous run should be cleaned and inspected as required by the WPS and inspection plan before the next pass.

Slag inclusion: a separate discontinuity

A slag inclusion is a non-metallic inclusion retained in the weld. It can result from inadequate cleaning between passes, unfavourable bead shape, incorrect electrode angle, poor manipulation, low heat input, or slag being trapped in a groove or pocket. It is not the same as lack of sidewall fusion, although the same poor preparation or technique may contribute to both.

Acceptance depends on the applicable code, material, joint type, quality level, service and project specification. Do not call an indication a rejectable defect solely because it appears in a photograph; the inspection report and governing criteria control.

How is lack of sidewall fusion detected?

The inspection method should be selected by a qualified person under the approved inspection plan:

1. Visual testing (VT): checks joint preparation, fit-up, bead profile, arc strikes, visible undercut, overlap, surface-breaking indications and workmanship. VT cannot reliably prove that a subsurface planar discontinuity is absent.
2. Magnetic particle testing (MT): can detect relevant surface or near-surface indications in suitable ferromagnetic materials when the procedure and sensitivity are appropriate.
3. Penetrant testing (PT): can reveal surface-breaking discontinuities on suitable non-porous surfaces; it does not detect a fully subsurface lack of fusion.
4. Ultrasonic testing (UT): can be useful for planar internal reflectors, subject to joint geometry, calibration, access and qualified procedure. TWI notes that UT is often more effective than radiography for some inter-run fusion imperfections, but applicability is case-specific.
5. Radiographic testing (RT): can reveal some volumetric and planar features depending on orientation, thickness, technique and sensitivity. A negative radiograph is not a universal guarantee that every type of lack of fusion is absent.

The report should identify the weld, method, procedure, extent, indication location, interpretation, evaluator and acceptance standard. NDT personnel must be qualified under the project and governing requirements.

Prevention checklist

• Use the approved WPS and qualified welder for the material, process, position and joint.
• Verify bevel, root gap, land, alignment, cleanliness and access before welding.
• Select electrode, wire, shielding, polarity and parameters from the WPS.
• Keep travel speed, angle, arc length and manipulation within the qualified range.
• Clean each inter-run surface and remove slag where the procedure requires it.
• Control preheat, interpass temperature and heat input where specified.
• Check for magnetic arc blow and reposition the return connection or use the approved mitigation.
• Stop and raise a technical query when fit-up or access cannot meet the WPS.
• Perform the required VT and NDT at the specified hold points.
• Record repair location, removal method, re-welding procedure and reinspection.

What happens when a defect is found?

The inspector should identify the weld and issue a nonconformance or repair record under the project quality system. The responsible welding engineer should determine whether the indication is a discontinuity, whether it exceeds the applicable acceptance level, and whether repair is permitted.

A typical controlled repair may involve local removal by grinding or gouging, verification that the indication is removed, re-welding under an approved procedure, and repeat inspection. The exact sequence, extent and acceptance criteria are not universal. Do not grind or weld over an indication without an approved repair method.

Acceptance standards are not interchangeable

ISO 5817 provides quality levels for relevant fusion-welded joints in steel, nickel, titanium and their alloys; ISO 10042 addresses arc-welded aluminium and its alloys. AWS D1.1/D1.1M is used for many structural-steel applications. These are examples, not a universal list. The contract, material, joint, service condition, design authority and project specification identify the governing code and edition.

An “imperfection” becomes a rejectable “defect” only when it fails the applicable acceptance requirement. The same indication may have different disposition rules in different applications.

Example: a sidewall indication in a groove weld

During VT, an inspector notices an irregular toe and the weld map shows a narrow groove preparation. The team should not assume that the visible profile proves internal fusion. The weld is placed on hold, the WPS and fit-up records are checked, and the specified NDT method is selected. If the report confirms lack of sidewall fusion above the project limit, engineering determines the approved removal, re-weld and reinspection sequence. If it does not exceed the criterion, the report should document the basis for acceptance.

FAQ

Is lack of sidewall fusion always visible?

No. It may be subsurface or hidden by later weld metal. Visual inspection is important but may need to be supplemented by an appropriate NDT method.

Is lack of fusion the same as slag inclusion?

No. Lack of fusion is a failure of metallurgical bonding; slag inclusion is trapped non-metallic material. They can have related causes but require different interpretation.

Is a radiograph enough to accept a weld?

Only if the approved procedure, coverage, sensitivity, qualified interpreter and governing acceptance criteria make it sufficient for that joint. No single test is universally adequate for every discontinuity.

Related EngineersBlog reading

• Root concavity in welding
• Cluster porosity in welds
• Tungsten inclusions in welding
• Poor start and stop welds

Technical limitation

This is general welding-quality guidance, not a weld acceptance decision. The qualified welding engineer, approved WPS, material specification, inspection procedure, applicable code, contract and project quality plan govern the work. Inspection and repair must be performed by appropriately qualified personnel.