How to weld thin sheet without burning through
Below about 2 mm the problem stops being penetration and becomes heat: the sheet cannot get rid of it fast enough, and the pool falls through. Everything below is about putting in less heat, or taking it away faster.
Fit-up decides the outcome
On thick plate a root gap is a feature. On 1 mm sheet it is a hole waiting to happen: there is no metal at the edges to bridge it, so the arc goes straight through. Clamp the sheets edge to edge along the whole seam before striking an arc — most burn-through blamed on settings is really a fit-up problem.
Tack every 20–30 mm, and alternate: middle first, then one side, then the other. Tacking sequentially from one end pushes all the shrinkage to the far end and the panel finishes bowed.
Settings
| Thickness | Process | Current | Filler |
|---|---|---|---|
| 0.8 mm | TIG DC− | 25–40 A | 1.0 mm rod |
| 1.0 mm | TIG or MIG short arc | 35–60 A | 1.0 mm rod / 0.8 mm wire |
| 1.5 mm | MIG short arc | 60–90 A | 0.8 mm wire |
| 2.0 mm | MIG short arc | 80–110 A | 0.8–1.0 mm wire |
These are starting points. Test on an offcut of the same material and the same thickness, never on the job — and never on a different grade, because stainless holds heat where aluminium throws it away.
Technique
Stitch, do not run. Weld 20–30 mm, stop, wait until the metal loses its colour, then start the next stitch overlapping the cold end of the last one. It feels slow and it is faster than repairing a blow-out.
Keep the torch moving and upright. A steep drag angle concentrates the arc into one spot. Around 10° from vertical, moving steadily, puts the heat where you want it and nowhere else.
Back the joint. A copper bar clamped behind the seam is the single most effective trick on this list. Copper conducts heat away fast and does not fuse to steel, so it holds the pool and then simply comes off. On stainless and titanium, backing doubles as a gas purge.
Point at the thicker side. When the two parts differ in thickness — sheet to angle, sheet to tube — aim the arc at the heavier one and let the pool wash onto the thin one.
Distortion
Heat that goes in has to come out, and while it does the metal shrinks. On thin sheet the shrinkage shows as waving. Less heat in means less waving: shorter runs, more of them, alternating along the seam, and the panel clamped to something rigid until it is cold. Skip welding — a stitch here, a stitch a hand's width along, then back to fill the gaps — spreads the shrinkage instead of concentrating it.
Frequently asked questions
MIG or TIG for thin sheet?
TIG below about 1 mm, where separating the heat from the filler is the only way to keep control. MIG from roughly 1 mm upwards, because it is far faster and short arc mode is cold enough. Stick welding below 1.5 mm is a fight not worth having.
The sheet warps into a wave. What now?
That is shrinkage, and it is prevented rather than cured. Fewer and shorter runs, stitch welding, alternating ends, and clamping to something rigid. Once the panel has waved, straightening means heat again — usually flame straightening, which is its own skill.
Why does it burn through at the end of the seam?
Because the plate is hot by then. The settings that were right on cold metal are too much for metal that has been absorbing heat for two minutes. Either drop the current as you go — a foot pedal on TIG — or work in stitches with cooling time between them.
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