Lack of fusion

For the 400 mm weld-axis example, indications span 100–112 mm and 110–118 mm. If the 2 mm overlap maps the same projected feature, the union is 18 mm, or 4.5% of the weld axis, rather than 20 mm of unique length. Indications can overlap in projection while lying at different depths. This calculation gives no allowable fraction, flaw height or remaining ligament.

TWI distinguishes unfused sidewalls and interfaces between weld runs. Examine access, settings, technique and magnetic arc blow. A deposited bead can cover an unfused interface. TWI, Lack of sidewall and inter-run fusion

Classify the imperfection by the fusion-welding process and the required interface.

Distinguish fusion from penetration and profile

Sidewall fusion concerns union with the prepared parent-metal boundary; inter-run fusion concerns union between deposited runs. Root fusion concerns the joint root. Incomplete penetration concerns failure to achieve the required penetration into the joint. TWI treats incomplete root fusion and penetration together in a separate discussion because preparation and access can affect both. TWI, Incomplete root fusion or penetration

A partial-penetration joint can be intentional when specified by design. A remaining root face is not necessarily an imperfection when it belongs to the specified joint geometry. Conversely, achieving the intended penetration depth does not prove that every sidewall or inter-run interface fused.

Cracks, slag inclusions, undercut, overlap and lack of fusion describe different discontinuities. An external profile may suggest a problem, but classification depends on evidence of the interface condition. Location, orientation, extent and surface connection are more informative than a photograph or an informal description such as “cold weld.”

Access and weld-pool behavior

Preparation angle, root geometry, fit-up and electrode or torch clearance determine access to the required interfaces. A groove that appears adequate in a drawing can become inaccessible after assembly or after earlier passes. Nearby obstructions and fixtures can restrict the arc direction or prevent it reaching both sidewalls.

TWI explains that an excessively narrow preparation can favor one wall, while too much current or too little speed can flood weld metal ahead of the arc. It also warns that weaving may be restricted by the welding procedure's energy limits. These interacting effects make “increase heat” an incomplete corrective instruction. TWI, preparation and parameter effects

SSAB's product-specific troubleshooting likewise lists both excessive and insufficient travel speed, long stickout, worn contact tips and inadequate inter-run cleaning as possible causes. Its page explicitly limits its recommendations to original SSAB products; the list is useful diagnostic context, not a universal parameter recipe for unrelated alloys or processes. SSAB, Lack of fusion

Fusion behavior depends on the combination of procedure, consumable, transfer mode, polarity, current/voltage behavior, travel, electrode extension and manipulation. Surface contamination and retained inter-run slag can interfere with joining. Magnetic deflection introduces another mechanism: current-return arrangement and material magnetism can redirect the arc even when nominal power settings appear appropriate.

What inspection can demonstrate

Visual examination can assess accessible surfaces and preparation, but it cannot exclude a buried unfused interface. TWI's detection guidance distinguishes surface-breaking examination from volumetric methods and says ultrasonic inspection is normally more effective than radiography for inter-run fusion imperfections, with effectiveness affected by the material. That is a general comparison, not a guarantee for a particular component or inspection setup. TWI, Detection and remedial action

Inspection capability depends on material, thickness, geometry, likely flaw orientation and available access. Penetrant examination requires an accessible surface-breaking discontinuity; magnetic-particle examination also requires a suitable magnetizable material and procedure. A buried planar interface is not ruled out by a clean surface result.

The meaning of an examination result depends on its coverage, calibration, detection and sizing limitations, and the applicable examination procedure. An indication's displayed length is not automatically its true three-dimensional size. A destructive section shows the sectioned plane; it does not establish every unsectioned portion of a production weld.

Acceptance basis and scope

An imperfection description and an acceptance decision are separate. Acceptance depends on the drawing, application code, quality level, edition and examination requirements. A suspected indication is also different evidence from a characterized discontinuity.

ISO 5817:2023, edition 4, February 2023, specifies workmanship-related quality levels for fusion-welded joints in steel, nickel, titanium and their alloys, excluding beam welding. Its published abstract applies from 0.5 mm material thickness, identifies levels B, C and D and excludes metallurgical aspects such as hardness and grain size. It is not a blanket rule for aluminum, beam welds or service-life assessment. ISO, ISO 5817:2023 scope

TWI's Job Knowledge 41 acceptance table names older standards, including BS EN ISO 5817:2007. That historical table does not establish limits for a job governed by another edition. This page does not reproduce current clause-level lack-of-fusion acceptance limits. TWI, historical acceptance table

A workmanship assessment also does not establish fitness for service under arbitrary fatigue, fracture or environmental loading. Any engineering assessment needs the authorized method, relevant material properties, loads and characterized flaw geometry.

Original inspection-length example

Assume a hypothetical 400 mm weld coordinate runs from 0 to 400 mm. An examination report maps one indication over 100–112 mm and another over 110–118 mm. If both reports represent the same projected weld-axis feature where they overlap:

• Individual projected lengths are 12 mm and 8 mm.
• Their overlap is 2 mm.
• The union spans 100–118 mm, giving 18 mm, rather than 20 mm of unique projected length.
• That union is 18 / 400 = 4.5% of the stated weld-axis length.

This original calculation only reconciles a coordinate map. It supplies no permissible percentage, flaw height, remaining ligament or strength reduction. If the indications lie at different depths or interfaces, overlapping projection does not establish that they are one physical discontinuity. Physical grouping, sizing and acceptance remain questions for the governing evaluation rules; projected-length bookkeeping alone cannot resolve them.

Repair and recurrence

Repair involves more than covering the visible weld surface: the unfused interface can remain beneath additional deposited metal. TWI describes localized gouging or grinding followed by rewelding under the agreed procedure. Excavation, confirmation of removal and subsequent examination address different parts of the repair problem. TWI, remedial action

Excavation changes local geometry, remaining thickness and access for repair welding. Material-specific thermal and consumable requirements still apply. Recurring indications suggest that preparation, setup, procedure or execution remains unresolved; repeated cosmetic repair does not establish that the mechanism has been removed.

Explanatory synthesis: prevention, detection and acceptance answer different questions. A process change can reduce the likelihood of recurrence without proving that an existing interface is fused. Conversely, a conforming examination result on a repaired region does not by itself establish that the original production cause has been eliminated.

Source versions

Reviewed 2026-11-02 PST. TWI identifies Job Knowledge 41 as originally published in Connect, May/June 1999, and explicitly says the live page has since been updated; 1999 is not a verified date for every present paragraph. ISO's actual edition is identified above. No separate issue date was verified for the cited FAQs or SSAB page. The coordinate calculation and explanatory synthesis are original and supply no universal acceptance limit.

Related reference

Manufacturing process references