Troubleshooting guide › Weld defects › Lack of fusion
Lack of fusion (cold lap) is weld metal that lies on the base metal or on the previous pass without actually melting into it. The bead can look fine from outside, which is why it is dangerous: the joint fails in a bend test or in service. It is almost always a heat problem: not enough energy, or energy pointed at the wrong place.
Reviewed by the Cruxweld technical team, October 2026. Settings are starting points to confirm on a test piece; acceptance is decided by the code or drawing the job is built to. Checks marked “call” are for our technical team.
From outside, a bead that looks piled up and rounded, with a sharp edge where it meets the plate instead of a smooth blend. In a cut section, a dark line between the weld and the plate, or between passes. A bend test opens up along that line.
Illustration, not a photo. Orange marks the defect.
The arc cannot melt the plate surface, so the filler just sits on it.
The pool runs ahead of the arc and the arc heats the pool instead of the plate.
Arc pointed at the pool or at one plate only.
The arc has to get through the layer before it can fuse.
The arc cannot reach the root or the side walls.
Edges of the weave are reached only by the cooler part of the arc.
| Process | Typical reason on this process | What to do |
|---|---|---|
| MIG / MAG | The most common MIG defect on thick plate. Current too low, arc pointed at the pool instead of the joint, travel too slow so the pool runs ahead of the arc, mill scale not removed. | Raise current and voltage together, keep the arc on the leading edge of the pool, speed up, grind the joint. |
| ARC / MMA | Current too low, electrode too large for the position, slag running ahead of the pool, bad weaving. | Current in the upper half of the range, smaller electrode in vertical, keep the slag behind. |
| TIG | Not enough current, too much filler dumped at once, too fast. | Make the pool wet both plates before adding filler. |
| Submerged arc | Wire off the joint centre, voltage too high for the groove, root gap closed. | Align the wire, lower voltage, open the root or use a backing. |
| Spot welding | A “stick weld” with no nugget: current too low, time too short, worn or mushroomed tips, sheets not clean. | Raise the tap one step or add weld cycles, dress the tips to the right diameter. |
MIG / MAG, 1.2 mm solid wire on 10 mm mild steel, fillet. A starting point only; confirm on a test piece of the same plate.
| Gas | Ar/CO2 mix for spray, or CO2 at higher current |
| Current / voltage | about 240-300 A / 26-30 V |
| Travel | arc at front of pool, 25-35 cm/min |
| Torch angle | 10-15 degrees push, aimed at the joint line |
| Multi-pass | clean each pass; overlap by one-third |
Typical values for mild steel. Your wire, plate condition and machine will move them. Our technical team can set them with your operator on a video call.
Lack of fusion hides under a good-looking bead. Checks that find it:
A bead that only sits on cold plate can look smooth and even. Fusion needs the arc to melt the plate, not just the wire. Low current with a high wire speed gives exactly this: a fine-looking bead that peels off in a break test.
Yes. A pore is a round void; lack of fusion is a flat crack-like gap that concentrates stress. Most codes reject it at any size.
On plate over 20 mm, or on cold plate in winter, a light preheat of 50-100 degrees C helps the pool wet the plate. It does not replace correct current.
This is the defect that a small machine causes on big plate. Check the current at 60 % duty cycle on the spec table, not the peak current, when choosing a machine for thick work.
Send a photo of the weld or cut, your machine model, consumable and plate thickness. Our technical team replies on WhatsApp with the likely causes and the next checks to make.
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