Incomplete fusion — also called lack of fusion (LOF) — is a planar discontinuity that occurs when weld metal fails to bond to the base metal or to a previously deposited weld pass. Unlike porosity (a volumetric defect) or undercut (a surface-accessible notch), LOF is a subsurface planar flaw that concentrates stress like a crack. In cyclically loaded or tension-zone structural welds, it is among the most dangerous discontinuities that can slip through a fabrication process.
Understanding what drives LOF, how AWS D1.1:2025 addresses it, and which WPS parameters control it is essential for any CWI responsible for procedure review or production inspection.
What incomplete fusion is and where it forms
An LOF discontinuity is a cold lap between weld metal and the adjacent fusion boundary — either the groove sidewall, the root face, or the surface of a previous pass. The weld metal solidified in physical contact with the adjacent metal but without the atomic bonding that constitutes fusion.
LOF forms in three characteristic locations:
Sidewall LOF. The weld bead does not penetrate to the groove wall and instead bridges across the gap, leaving an unfused plane running parallel to the groove face. This is especially common in narrow-gap groove welds, where the arc energy is concentrated in the center of the joint and the sidewall does not get enough heat input to melt.
Inter-pass LOF. The heat of the current pass is insufficient to remelt the surface of the previous pass, particularly at the toe of a convex bead. The incoming weld metal wets the convex crown but leaves the toes in cold-lap condition.
Root LOF. At the root of the joint, insufficient penetration combines with the geometry of the joint to leave the root face unfused. Root LOF is a first-pass problem: once the root pass is completed, the defect is buried and can only be found by volumetric NDE.
AWS D1.1:2025 acceptance criteria
AWS D1.1:2025 does not set a size threshold for incomplete fusion in complete joint penetration (CJP) groove welds — any LOF in a CJP weld is rejectable. This reflects the structural function of a CJP weld: it must carry the full cross-sectional stress without reduction, and any unfused plane reduces the effective throat and creates a stress concentration.
For partial joint penetration (PJP) groove welds, LOF in the deposited weld metal is also rejectable, though the effective throat calculation already accounts for the unfilled root — the issue is LOF within the deposited weld volume itself, not the designed-for root condition.
Fillet welds must be free from LOF with the base metal. The AWS D1.1:2025 visual acceptance criteria for fillet welds prohibit cracks and incomplete fusion, though the detection of subsurface LOF in a fillet weld requires NDE methods.
For visual inspection, LOF that breaks the surface (typically at the toe of a bead) is detectable with MT or PT. Subsurface LOF requires UT or RT, and UT is generally preferred for planar defects. For more on UT acceptance limits, see UT acceptance criteria for AWS D1.1 structural welds.
Root causes of incomplete fusion
Inadequate heat input
The arc must transfer enough energy to melt both the deposited wire or electrode and a layer of the adjacent base metal. If the arc energy is too low — either because amperage is below the qualified range, voltage is low causing a short arc, or travel speed is too fast — the groove wall does not reach its melting point before the weld pool passes.
In prequalified WPS procedures, the heat input range is implied by the amperage and voltage ranges recorded in the WPS. In qualified-by-test procedures, the PQR records the actual parameters, and the WPS allows ± tolerances around the qualified values. Dropping outside the lower bound without amending the WPS is a nonconformance; running at parameters inside the range but at the low end still carries LOF risk if technique is poor.
Electrode or wire angle
For SMAW, FCAW, and GMAW, the electrode or wire angle relative to the groove face directly controls where the arc energy is directed. When welding a groove weld in the vertical or overhead position with a beveled sidewall, the welder must angle the electrode toward the fusion face on each side alternately to ensure both walls receive arc energy. Maintaining a steep work angle (perpendicular to the joint axis) without oscillating toward the walls produces a bead that bridges the groove without bonding to the sidewalls.
WPS documents typically specify travel angle (the drag or push angle in the direction of travel) but less often specify work angle (the angle toward the groove face). For wide grooves or multi-pass work, the WPS should call out oscillation technique or weave pattern — specifically that the arc should be directed to the groove wall momentarily at the end of each weave to ensure sidewall fusion.
Root face geometry and fit-up
At the root, the gap between the two groove faces and the land thickness (root face height) determine how much the arc can penetrate before the weld pool freezes. A root face that is too tall prevents the arc from reaching the bottom of the joint and produces root LOF. A root gap that is too tight produces the same result by limiting penetration depth.
AWS D1.1:2025 prequalified joint details in Clause 5 specify root openings and root face tolerances for each joint type. Field fit-up must be confirmed to these tolerances before welding starts. Tight joints that don't meet the minimum root opening should be rejected and re-fit — a CWI who accepts a tight joint to avoid fit-up rework and then encounters root LOF in production NDE is facing a repair problem that is both more expensive and more complicated than the fit-up correction would have been.
For fit-up tolerances and their effect on the procedure, see groove weld fit-up tolerances under AWS D1.1.
Cold starts and stop-and-restart locations
When an electrode is restarted after a stop, the arc strikes cold metal before the crater of the previous weld is remelted. If the welder does not pause long enough at the restart point to remelt the crater and establish fusion with both the previous bead and the sidewall, LOF occurs at the restart. AWS D1.1:2025 requires restarts in CJP groove welds to be made such that the previous pass is remelted and fusion is established before forward travel resumes.
High-restraint joints — thick flanges, box columns, heavy splices — are particularly susceptible to restart LOF because the adjacent base metal acts as a large heat sink that cools the stop crater rapidly.