Complete Fusion — Detecting Lack of Fusion

What is Lack of Fusion (LOF)?

Lack of fusion is a separation or unbonded area between the weld metal and base metal, or between passes in multi-pass welds. The base metal was not melted or the molten weld metal did not properly wet and bond to the surface.

• Root LOF — Unbonded area at the weld root, visible from the back side or via radiography. Base metal at root did not melt.
• Toe LOF — Separation at the weld toe (edge of the weld bead on base metal). Weld metal did not wet the HAZ properly.
• Pass-to-pass LOF — Unbonded area between two passes in a multi-pass weld. One pass did not reflow or bond to the prior pass.
• Interior LOF — Subsurface lack of fusion, visible only via ultrasonic testing (UT) or radiography. Usually not detected by visual inspection alone.

AWS D1.1 Table 6.1 Acceptance Criterion

Common Lack of Fusion Causes

• Insufficient preheat or interpass temperature — Base metal too cold to melt and accept molten weld metal. Increases surface tension, prevents wetting.
• Too fast travel speed — Arc does not dwell long enough to melt the base metal. Weld metal flows over the surface without bonding.
• Dirty base metal surface — Mill scale, rust, or oil prevents wetting. Clean base metal and HAZ with wire brush before welding each pass.
• Excessive contact tip-to-work distance (CTWD) — In GMAW, long stick-out reduces arc heat at base metal. Shorten CTWD per WPS.
• Misaligned electrode angle — Electrode not perpendicular or properly angled relative to joint. Weld puddle does not flow into the gap.
• Wrong shielding gas or gas coverage — Porosity or oxidation at surface prevents fusion. Check gas purity and flow rate.

How to Inspect & Document for the Report

• Visual examination at high magnification — Use 10x magnifying glass to examine weld toes and root (if visible). Look for dark lines or gaps at the junction of weld and base metal.
• Cross-section examination (optional) — Cut a section perpendicular to the weld and etch to reveal internal structure. LOF appears as a bright unbonded line.
• Ultrasonic testing (UT) — If visual examination is inconclusive or interior LOF is suspected. UT is most sensitive method for subsurface LOF.
• Record the location — Note whether LOF is at root, toe, or between passes. Measure approximate length if possible.
• Photograph if visible — Close-up photo under bright light shows the separation or dark line characteristic of LOF.

CWI Inspection Tips

• Toe LOF is the most common type detected by visual inspection. Inspect both toes (each side of the bead) separately.
• Root LOF is harder to see if the back side is not accessible. Consider radiography or UT for critical structural welds.
• Pass-to-pass LOF in multi-pass welds can indicate that interpass cleaning (wire brush) was skipped or temperature dropped between passes.
• If LOF is detected, the entire weld section may be suspect. Radiograph adjacent sections to ensure no hidden discontinuities.