Spot Weld Quality When Joining Coated and Thin-Gauge Steel
2019 unitized sedan is having its left rear rail extension replaced. The new extension is 1.2-mm HSLA 350, and it is joined to the existing 0.8-mm galvanized wheelhouse with squeeze-type resistance spot welds. The technician sets the welder to 8.5 kA and 20 cycles, installs 6-mm tips, and begins welding. The first production welds show slight discoloration. A destructive coupon made from the same 1.2-mm plus 0.8-mm stack shows a 2.8-mm nugget with interfacial failure. The OEM specification calls for a minimum 4.0-mm nugget and a button pullout. The tips have not been changed in 180 welds, and the parts have a 1.5-mm gap closed by the gun force. The shop is at 18°C. No weld-through primer is used.
1. Evaluate the likely causes and decide whether the production welds are acceptable.
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The production welds are not acceptable. The coupon nugget is 2.8 mm versus a 4.0-mm minimum, and interfacial failure means the weld did not form a proper button. Likely causes include current or time too low, worn tips after 180 welds, insufficient or excessive electrode force, coating resistance, poor fit-up with a 1.5-mm gap, or shunt path through adjacent welds. Discoloration alone is not a quality indicator. Because the coupon failed, every production weld made with that setup is suspect.
2. What corrective actions and validation should be taken before continuing production?
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Replace or dress the tips, clean the mating surfaces, close the gap with clamps, and reset the welder to the OEM schedule for the 1.2-mm plus 0.8-mm stack. Run a new destructive coupon before production and after any setting or tip change. Adjust current and time in small steps and verify nugget diameter and failure mode. If the coupon still fails, check tip force and coating; consider a different tip or weld schedule only if the OEM allows it. Do not continue production until the coupon passes.
3. How should already-made suspect welds be handled and documented?
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Mark and rework suspect welds. For STRSW, verify by peel or pry test where accessible, or replace the section per OEM. Do not simply add a second weld beside a suspect one unless the OEM procedure allows it. Document welder settings, tip change, coupon results, and rework. Perform post-repair inspection and corrosion protection. If the rail extension is structural, confirm the final weld count and locations match the OEM. A second destructive test at the end of the run confirms the process stayed stable.