Undersized STRSW Nuggets in a Three-Thickness Adhesive-Bonded Flange
A 2020 SUV quarter panel is being replaced at the lower B-pillar flange, a three-thickness stack of 1.2 mm DP590, 1.5 mm HSLA and 2.0 mm mild steel joined with structural adhesive and 12 squeeze-type resistance spot welds per the OEM schedule. The shop's 14 kA welder is fitted with 8 m cables, 6 mm dome-radius electrodes, electrode force set to 4.0 kN, squeeze 20 cycles, weld 12 cycles, hold 5 cycles. After dressing the tips, the first five welds show light expulsion and peel tests give average nugget diameters of 2.8 mm; the OEM minimum is 4.5 mm. Three welds make an audible click when pried and separate at the faying surface. The adhesive in the flange has been spread full width and is still uncured.
1. Diagnose the most likely causes of the small nuggets, expulsion, and click-separating welds, and give a prioritized inspection sequence.
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A three-thickness 4.7 mm stack on a 14 kA welder with 8 m cables is already near the edge of capability: long cables cost current through resistance, so the actual secondary current at the tips may be well below the display. The 2.8 mm nuggets with expulsion are consistent with low current plus low force, not too much current - expulsion at the edge of a nugget that never grew means the force is allowing the sheets to separate. Check in this order: (1) secondary current with a calibrated weld checker at the tips; (2) electrode force with a force gauge (4.0 kN is low for a 4.7 mm stack; many OEM schedules call for 4.5–5.5 kN); (3) tip condition and diameter after dressing (worn tips raise resistance and reduce current density); (4) the uncured adhesive, which insulates the faying surface, squeezes out under force and contaminates the tips; (5) shunt paths from adjacent welds if spacing is tight. The click-and-separate welds confirm incomplete fusion, not a cosmetic issue.
2. Describe the corrective parameter and process plan, including what must be verified before production welding resumes.
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First cure or remove the adhesive per the OEM; welding through uncured structural adhesive is not a production condition and produces porosity and variable resistance. Dress or replace tips, set force to the OEM value for the stack and verify it with a gauge, then confirm secondary current with a calibrated meter. If the 8 m cables cannot deliver schedule current at the tap, shorten the leads or move the welder closer. Run a test coupon of the exact 1.2/1.5/2.0 mm stack and peel it: the nugget must be at least 4.5 mm and failure should be button pull, not interfacial. Increase weld time before increasing current to avoid expulsion. Weld from the centre of the flange outward to reduce shunting and maintain OEM spacing. Destructive test the first and last weld of each shift and record diameters, current and force.
3. The technician proposes adding extra welds to compensate for the small nuggets so the vehicle can be delivered on time. Evaluate that proposal against OEM compliance requirements.
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Adding welds does not fix the underlying nonconformance. The OEM schedule specifies 12 welds at a stated spacing; extra welds change load distribution and may violate the procedure, and the nugget size is still below minimum on the welds that remain. If the OEM repair procedure permits supplemental welds after a documented nugget failure, follow that allowance exactly and re-test; otherwise the flange must be reworked or the panel replaced. Do not deliver with click-separating welds. The documentation package should include machine calibration, force verification, coupon peel results, a weld map with measured diameters, adhesive lot and cure record, and the OEM procedure revision. A repair that cannot meet nugget diameter after correct settings indicates the equipment is not capable of the stack, not that the technician should weld longer or hotter without approval.