Incomplete Fusion Weld — D1.1:2025 Acceptance Criteria & Causes
D1.1:2025 Table 8.1 item (2) requires complete fusion between all adjacent weld layers and between weld metal and base metal. Unlike porosity or undercut, incomplete fusion has no quantitative threshold — any detected lack of fusion is rejectable in both statically and cyclically loaded connections.
Table 8.1 Item (2) — Complete Fusion Requirement
The rule is stated in a single sentence: Complete fusion shall exist between adjacent layers of weld metal and between weld metal and base metal. The “X” mark in both the statically loaded and cyclically loaded columns of Table 8.1 indicates this requirement applies unconditionally to both connection types.
There is no minimum detectable size threshold, no length exemption, and no repair allowance without removing the discontinuity. Any gap, cold lap, or unbonded interface at the fusion line is a rejectable defect.
| Location | Statically Loaded | Cyclically Loaded |
|---|---|---|
| Between adjacent weld layers (interpass) | Complete fusion required | Complete fusion required |
| Between weld metal and base metal (fusion line) | Complete fusion required | Complete fusion required |
| Minimum size for rejection | No threshold — any lack of fusion | No threshold — any lack of fusion |
Incomplete Fusion vs. Incomplete Joint Penetration
These two terms describe different failure modes and are often confused. Incomplete fusion (also called lack of fusion, or LOF) is a bonding failure at an interface — the weld metal did not metallurgically bond to the adjacent surface, even if the geometry appears filled. The interface is present but unfused.
Incomplete joint penetration (ICP) is a depth failure — the weld metal did not extend to the required depth through the joint thickness. For a double-groove weld, ICP means the root pass on each side did not meet at the center. For a single-groove weld with a backing bar, ICP means the root pass did not reach the backing.
Both are rejectable under D1.1:2025 Table 8.1, but they are separate items. ICP is addressed under item (2) as a fusion failure when the root of the weld did not fuse to the backing or opposite joint face. The distinction matters for repair: LOF requires removing the unbonded zone; ICP requires excavating to the root and completing penetration.
Why Incomplete Fusion Is Dangerous
A lack-of-fusion plane is a pre-existing crack in the weld. It has a sharp, planar geometry with no crack-tip blunting — the stress intensity factor at its tip is essentially the same as a crack of identical depth. Under cyclic loading, it will propagate.
LOF at the weld toe or fusion line is particularly dangerous because it sits at the highest stress point in the weld joint — exactly where fatigue cracks initiate in properly fused welds. An unfused toe means the fatigue life is zero: the crack is already there.
Inspector scenario: A UT examination of a CJP groove weld in a bridge girder bottom flange detects a planar reflector along the fusion line at mid-length. The indication is evaluated against D1.1:2025 Table 8.2 and classified as Class A. Under Table 8.2 Note 1, any Class A indication shall be rejected regardless of length. The weld is rejected. Under Clause 7.25, the fabricator gouges out the defective zone to sound metal, inspects by MT to confirm removal, and completes the repair weld with an approved WPS. The repair area is re-tested by UT before acceptance.
What Causes Incomplete Fusion
Low arc energy at the fusion line. Travel speed too fast or arc current too low means the weld pool does not have enough energy to melt the base metal at the joint face. The filler metal deposits on top of a partially melted surface rather than into a fully liquid pool. This is the most common cause in mechanized or semi-automatic GMAW where wire feed speed (and therefore heat input) is underestimated for the joint geometry.
Incorrect travel angle. In groove welds, directing the electrode toward the center of the joint rather than toward the joint wall means the arc is heating the prior pass rather than the base metal fusion face. The result is a smooth-looking weld with no bonding to the sidewall — cold lap. A 5–15 degree work angle toward each joint wall is typically required for sidewall fusion in groove welds.
Joint face contamination. Mill scale, rust, paint, or moisture on the fusion faces create a thermal barrier and a chemical barrier to bonding. The weld metal solidifies before the contaminant layer is fully disrupted. D1.1 Clause 7.14 requires the joint faces to be clean before welding.
Slag inclusions at interpass. In SMAW and FCAW-S, incomplete slag removal between passes leaves an insulating layer on the prior bead surface. The next pass cannot fuse through the slag. Proper wire brushing and slag removal between each pass is essential for multi-pass welds.
Joint geometry too tight. An included groove angle below the minimum prequalified value (e.g., 30 degrees for single-V, 20 degrees for double-V per D1.1 Clause 5) prevents the electrode from reaching the joint sidewall. The arc reflects off the near wall instead of penetrating to the far wall. This is a joint design and fitup issue, not purely a welding parameter issue.
Frequently Asked Questions
No. D1.1:2025 Table 8.1 item (2) states that complete fusion shall exist between adjacent layers of weld metal and between weld metal and base metal. This requirement applies to both statically loaded and cyclically loaded nontubular connections. There is no minimum size threshold — any detected lack of fusion is a rejectable discontinuity requiring repair per Clause 7.25.
Incomplete fusion (lack of fusion) is the failure of weld metal to fuse with the base metal or a preceding weld layer — a bonding failure at the interface. Incomplete joint penetration (ICP) is the failure of weld metal to extend through the joint thickness — a depth failure. Both are rejectable under D1.1:2025 Table 8.1, but they are separate discontinuity categories and have different root causes. Incomplete fusion typically results from low heat input, incorrect travel angle, or contaminated surfaces. Incomplete joint penetration typically results from joint geometry that the selected process and parameters cannot fill.
Visual inspection (VT) can only detect incomplete fusion that is open to the weld surface — a gap or notch at the weld toe or along the weld face where the weld metal did not bond to the base metal. Subsurface lack of fusion between weld layers is not detectable by VT alone. D1.1:2025 Clause 8 requires ultrasonic testing (UT) for full volumetric examination of groove welds where subsurface discontinuities must be evaluated. When UT detects a planar reflector at a fusion line, it is classified as lack of fusion and evaluated against Table 8.2.
The four most common causes are: (1) travel speed too fast — insufficient time for the weld pool to wet out and fuse to the base metal or prior layer; (2) heat input too low — the arc does not generate enough energy to melt the base metal at the fusion line; (3) incorrect travel angle — the arc is directed away from the joint wall, depositing weld metal without melting the base metal beneath it (cold lap); (4) surface contamination — mill scale, paint, or oxide films on the joint face prevent bonding even when the temperature is adequate. For multi-pass welds, insufficient interpass cleaning allows slag or oxide to remain at the layer interface.
Yes. D1.1:2025 Clause 7.25 permits repair of incomplete fusion. The defective area is removed by gouging or grinding to sound metal, the cavity is inspected to confirm all unfused material is removed, and the repair weld is made following an approved WPS. The repaired weld must be re-inspected using the same methods that detected the original discontinuity — VT against Table 8.1 item (2) for surface indications, and UT against Table 8.2 for subsurface indications.