Planar discontinuity where weld metal lies against the groove face, base metal or an earlier bead without melting into it. Causes include low heat, fast travel, poor electrode angle, too wide a weave, mill scale and short-circuit transfer on thick sections. It is hard to see from the surface, shows on radiographs only when lined up with the beam, is found more reliably with UT, and is rejected outright in most critical welds.
UT picked up lack of fusion on the bevel face between fill passes, mark it at seven inches and gouge it out.
Lack of fusion hides under good-looking caps, which is why codes lean on UT and bend tests rather than appearance.
Also heard as
- LOF
- incomplete fusion
- IF
Do not mix it up with
- Incomplete penetration: Lack of fusion is weld metal that did not melt into a surface it touched, while incomplete penetration is weld metal that never reached through the joint.
Related terms
Cold lap
Shop term for weld metal that rolls over onto the base metal or a previous bead without fusing to it.
Incomplete penetration
A root left unfused because the weld never reached all the way through a joint meant to be fully penetrated.
UT
Ultrasonic testing: finding internal flaws by sending high-frequency sound into a weld and reading echoes.
Burn-in
Shop word for how deeply a bead melts into the base metal or sidewalls, especially at the root and toes.
Short-circuit transfer
GMAW mode in which the wire touches the pool and shorts many times a second, giving a cool, controllable arc.