Squeeze-Type Spot Welds Shrinking Through a Three-Thickness Stack
A 2019 unibody SUV is having its left rear rail replaced where it joins the floor pan. The joint is a three-thickness stack: 0.8 mm coated mild steel (floor pan), 1.4 mm DP590, and 1.8 mm HSLA rail flange. The OEM procedure specifies squeeze-type resistance spot welds with a minimum nugget diameter of 4√t (t = thinnest sheet), electrode force 3.5–4.0 kN, weld time 10–14 cycles, and welds spaced at least 25 mm apart. The technician sets 3.2 kN and 8 cycles, and adds two extra welds 10 mm from the first pair "for safety". The electrodes started at 8 mm face diameter but have run three prior jobs and now measure about 10 mm. Structural adhesive was applied between the flange faces 20 minutes earlier and has not fully squeezed out. A chisel test on the first weld tears a 5.6 mm button; the two closest-spaced welds tear at 3.1 mm and 2.9 mm; one weld threw a shower of sparks at the flange edge.
1. Explain why nugget size drops so sharply within a single sequence of welds on the same joint.
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Every deviation in the setup points the same direction - less current actually reaching the faying surface.
Current shunting (dominant). A weld placed 10 mm from a fresh nugget does not have to push all its current through the interface; an adjacent previously formed nugget offers a low-resistance parallel path through the sheet. Current splits, so the interface sees a fraction of the set value and the nugget shrinks. The shrinking trend tracks distance from the first weld exactly, which is the diagnostic signature. Adding welds "for safety" therefore produced weaker welds, not more strength.
Electrode face growth. Faces grew from 8 mm to about 10 mm, roughly a 56% increase in contact area. Current density falls by the same proportion at unchanged current, so heating at the interface drops. Dressing the tips back to 8 mm restores density.
Low electrode force. 3.2 kN against a 3.5–4.0 kN specification reduces contact and fit-up, especially across a three-thickness stack with an 0.8/1.8 mm mismatch, and raises resistance at the tip-to-sheet interface where the heat is wasted rather than at the joint.
Trapped adhesive. Uncured adhesive holds the sheets apart and acts as an insulator, so contact resistance is erratic and the stack does not consolidate under the force.
Short weld time. 8 cycles versus the specified 10–14 cycles truncates nugget growth before it reaches diameter.
2. State the corrected setup and a verification plan that would catch this failure before the vehicle leaves the shop.
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Corrected setup
- Dress or replace electrodes to an 8 mm face; log face diameter at the start of each shift and re-dress at the OEM wear limit.
- Electrode force 3.5–4.0 kN, verified with a force gauge, not by reading the regulator dial.
- Weld time 10–14 cycles per the OEM chart for this stack; set current from the OEM schedule for the 0.8/1.4/1.8 combination.
- Restore the 25 mm minimum spacing and delete the two added welds. Extra welds inside the shunt window do not add strength.
- Allow the adhesive to squeeze out under force, or use the OEM's adhesive-compatible current schedule; do not weld through a wet, uncured bead unless the procedure permits it.
- Check flange edge distance - the spark shower indicates expulsion, often from a weld too close to the edge or a tip misaligned over the flange.
Verification plan
- Before production: build a coupon of the identical stack (0.8 / 1.4 / 1.8, same coatings), weld, then peel or chisel. Measure the nugget at its longest and shortest axis and average; the acceptance criterion here is 4√0.8 ≈ 3.6 mm, with shop QA typically adding a margin.
- In process: chisel test the first weld of each shift and every 10th weld; any weld that does not tear a button, or tears undersized, triggers re-setup of the whole sequence.
- Visual: no expulsion, no surface cracks, electrode indentation within the OEM limit (commonly 30% of sheet thickness).
- Record current, cycles, force, tip diameter, and nugget measurements on the repair file.
3. The 5.6 mm button passed. Would you accept the joint on that basis, and does adding more welds fix a marginal one?
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No on both counts.
The 5.6 mm button is measured against 4√t where t is the thinnest sheet, 0.8 mm, giving 3.6 mm. So 5.6 mm is comfortably acceptable - but only for that one weld, and only if the nugget was measured at the interface of the thinnest sheet. A 3.1 mm and 2.9 mm pair in the same sequence are below the minimum and represent a genuine strength deficit in a rear rail load path. Acceptance is per-weld, not per-joint: one good weld does not certify the row, and the two undersized welds would have to be removed and replaced after the surrounding material is allowed to cool and the spacing corrected.
Adding welds does not fix a marginal joint in resistance spot welding because of shunting. Closely spaced welds steal current from one another, so the additional welds are individually weaker and the net joint gain is negligible or negative - while adding heat, distortion, and coating damage. The correct remedy is to restore the OEM parameters and spacing so each specified weld achieves its nugget, not to increase the count.
Also worth flagging: the joint is only as good as the post-weld corrosion protection. The heat-affected zone burns coating off both faces, so the OEM-required primer/seam sealer application has to be part of the acceptance, not an afterthought.