Burn-through (melt-through; ISO 6520-1 code 510) is the most binary of welding defects: the pool, overheated beyond what the section can support, simply falls through the joint. One moment a weld, the next a hole.
It’s the signature failure of thin-sheet and root-pass welding — automotive, tube, battery trays and enclosures — where the margin between “fused” and “gone” can be a few amps or a tenth of a millimetre of fit-up gap.
The mechanism: a support problem
A stable weld pool is held in place by surrounding solid metal and surface tension. Burn-through happens when the molten zone grows beyond what that support can carry:
- Heat input raises pool size locally
- The solid bridge under the pool thins
- Surface tension can no longer hold the liquid against gravity (and arc pressure)
- Collapse — instantaneous once the threshold is crossed
The critical insight from high-speed footage: the collapse has a visible precursor. The pool widens and sags measurably before it lets go. We filmed the exact moment on a tee joint — frame by frame: watch burn-through under a high-speed camera.
What causes it
Parameters vs gauge
- Current too high / travel too slow for the thinnest member — the thin sheet sets the budget, not the average
- Voltage excess widening the arc onto the thin side of asymmetric joints
Fit-up and geometry
- Root gap above tolerance: less metal supporting the pool — the dominant cause on automated lines, where parameters are fixed but fit-up varies part to part (see fit-up verification before welding)
- Edge misalignment concentrating the arc on one edge
- Missing or shifted backing
Thermal history
- Heat accumulation: later welds in a sequence land on pre-heated metal — the same parameters that were safe on weld 1 burn through on weld 12
- Repairs and re-welds on already-thinned sections
Acceptance: none
Under ISO 5817, burn-through (510) is not permitted at any quality level — B, C, or D. There’s no dimensional discussion: a hole removes the load path. Sentencing context for the levels: ISO 5817 acceptance criteria guide.
Repair means grinding back to sound metal and re-welding under a qualified procedure — on thin sheet, often the hardest welding task in the shop, since the repair zone is now even thinner and pre-sensitized.
Detection and the case for real-time monitoring
Post-weld, burn-through is trivially visible (VT per ISO 17637). The production question is different: how many parts did you scrap before someone looked?
On an automated line, a fixture wear or a blanking-die drift that opens the root gap produces burn-through on every part until detected. The economics favour catching the precursor:
- Thermal imaging sees pool temperature and width exceed the stable envelope seconds before collapse — and on every weld after, until the fit-up cause is fixed
- High-speed vision resolves the sag and oscillation that precede failure (the dynamics are striking at 480 fps: weld pool and droplet transfer in slow motion)
- Either signal can alarm or stop the cell mid-weld, converting scrap into an interrupted seam and a fit-up correction
This is the textbook case for real-time weld monitoring: a defect that is 100% detectable after the fact, but only preventable during the weld.
Prevention checklist
- Budget heat input to the thinnest member; on sheet, prefer pulsed or controlled short-circuit transfer.
- Hold fit-up tolerance — root gap control is burn-through control. Measure it, don’t assume it.
- Use backing (ceramic, copper) where the joint allows.
- Sequence for heat: distribute welds so thin zones don’t accumulate temperature; monitor interpass on multi-weld parts.
- Monitor the pool: on high-value or high-volume thin work, per-weld thermal monitoring turns the precursor into an actionable signal.
Burn-through is one of the geometry-failure defects in the welding defects guide with ISO 5817 acceptance criteria.
Frequently Asked Questions
What is burn-through in welding?
Burn-through (melt-through, ISO 6520-1 code 510) is the collapse of the weld pool through the joint: heat input locally exceeds what the section can support, the pool loses its solid support, and molten metal falls through, leaving a hole or a heavily sagged root. It is most common on thin sheet, root passes, and thickness transitions.
Is burn-through acceptable under ISO 5817?
No — burn-through (510) is not permitted at any ISO 5817 quality level. A hole through the joint eliminates the load path entirely, so unlike porosity or undercut there are no dimensional allowances. Every burn-through is a repair: typically grinding to sound metal and re-welding under a qualified procedure. Verify the current edition of the standard for your case.
What causes burn-through?
Excess local heat input relative to section thickness: current too high or travel too slow for the gauge, excessive root gap or poor fit-up reducing the metal available to support the pool, repeated passes overheating a thin area, and tack or fixture gaps that change heat sinking mid-seam. On automated lines, burn-through often appears exactly where fit-up varies — not where parameters were wrong on paper.
How can burn-through be prevented in production?
Match heat input to the thinnest section (pulsed or short-circuit transfer on sheet), control fit-up so root gaps stay in tolerance, use backing where geometry allows, and sequence welds to avoid heat accumulation. In automated production, thermal monitoring detects the overheating precursor — pool temperature and width rising beyond the stable envelope — in time to flag or stop the weld before collapse.
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