Lack of fusion and lack of penetration
The two defects that look like nothing and behave like a crack. A weld can pass a visual inspection with either of them running its whole length.
Why they are the dangerous ones
A pore is round; stress flows around it. Lack of fusion is a flat discontinuity with a sharp edge, which concentrates stress exactly the way a crack does. Mechanically it is a crack that was built in rather than grown.
ISO 5817 therefore does not give it a permitted size at any quality level. There is no measurement to argue about — found means repaired.
Lack of fusion: causes
Not enough heat. Current too low for the thickness, or the travel too fast. Short-arc MIG is the classic offender: it is a cold mode, and on anything above about 6 mm it will lay a bead that sits on the joint rather than into it.
The arc pointed at the pool instead of the leading edge. If molten metal runs ahead of the arc, the arc is melting metal that is already molten and never touches the cold side wall. This is what "the pool got ahead of me" means in practice.
Slag or oxide between runs. Whatever is left on the previous run stops the next one fusing. Grinding and wire-brushing between runs is not cosmetic.
Groove too narrow. A tight preparation with a steep side wall gives the arc no angle to reach it. If the joint has to be filled from the bottom of a narrow slot, the geometry is wrong before anyone strikes an arc.
Lack of penetration: causes
Almost always fit-up rather than welding. The root gap is too small, the root face too thick, or the included angle too tight for the electrode to reach the bottom. Then: current too low for the root run, an electrode too large in diameter for the gap, or a travel speed that carries the arc past before the root melts.
On single-sided joints where the root must be full penetration, this is where a backing bar, a consumable insert or a back-gouged and re-welded second side earns its keep.
Preventing both
- Prepare the joint to the WPS sketch and check the gap with a gauge, not by eye. The root gap tolerance is there for this reason.
- Enough current for the thickness — the current calculator for MMA, and for MIG a mode with enough heat: spray or pulse rather than short arc above 6 mm.
- Keep the arc on the leading edge of the pool, never behind it.
- Clean between every run.
- On a critical single-sided joint, expect to back-gouge. Planning for it is cheaper than discovering it at inspection.
Frequently asked questions
What is the difference between the two?
Lack of fusion is metal that touched but never melted together — between a run and the parent metal, or between two runs. Lack of penetration is a root that was never reached at all, so part of the joint thickness was never welded. Different causes, same verdict: both are rejectable at every ISO 5817 quality level.
Why does radiography sometimes miss it?
Because both are planar. A tight, flat discontinuity lying at an angle to the X-ray beam absorbs almost nothing extra, so it leaves no contrast on the film. Ultrasonics reflects off exactly that kind of flat face, which is why UT is specified where these defects are the concern.
Updated: