Stubbing, Start Porosity, and Burn-Through on a 1.0/1.2 mm Quarter-Panel Joint
A 2021 unit-body pickup needs the outer quarter panel replaced at the factory seam. The joint is a 1.2 mm DP780 upper tie-in overlapped by a 1.0 mm mild-steel outer panel. The technician sets a 240 V synergic GMAW machine to 0.8 mm ER70S-6, 100% CO2 at 20 cfh, 17.5 V, 3.8 m/min wire-feed speed, 15 mm contact-tip-to-work distance and a 10° push angle. Fit-up leaves a 1.5 mm gap along the overlap. On the first 12 stitch welds the arc stubs and sputters at every start, porosity clusters in the first 8 mm of each stitch, the 1.0 mm panel burns through on four welds, and spatter covers the surrounding coating. The shop stocks C25 (75% Ar/25% CO2) and 100% CO2, 0.8 mm and 0.9 mm ER70S-6, 4 m and 6 m torch leads, and a flowmeter but no adjustable gas pre-flow.
1. Rank the probable causes of the stubbing, start porosity, and burn-through, and describe a step-by-step check that separates machine and consumable faults from technique and fit-up.
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Work from the arc outward, because the symptoms split into a start-up gas problem and a steady-state heat problem. Stubbing at every start with 100% CO2 at 17.5 V and 3.8 m/min is a cold condition: CO2 needs roughly 1–2 V more than C25 at the same wire-feed speed, so the wire reaches the puddle before a stable arc forms. The clustered start porosity points to gas delivery - check pre-flow (many machines have a fixed 0.3–0.5 s; a surge or blocked nozzle will gulp air), then nozzle spatter, hose leaks, actual flow at the nozzle with a flowmeter (20 cfh at the regulator can be much lower at the torch), and finally liner kinks, drive-roll tension and contact-tip bore. Burn-through on the 1.0 mm side follows from the technician slowing travel to control the stubbing arc, which raises heat input per millimetre. Only after gas and feed are verified should voltage be changed.
2. Recommend specific process changes - gas, wire size, voltage/wire-feed speed, travel technique, and gap management - for this dissimilar-thickness joint, and explain the heat-input trade-off on DP780.
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Switch to C25 and 0.8 mm ER70S-6 for the thin outer panel. C25 stabilizes the low-current arc and reduces spatter, and 0.8 mm wire is the right size for 1.0 mm sheet. Start near 17.5–18.5 V at 2.8–3.2 m/min and tune on a coupon of the same 1.0/1.2 mm stack, not on the vehicle. Keep contact-tip-to-work at 10–12 mm, use a 5–10° push, and make 10–15 mm stitches with a crater fill; do not slow travel to bridge the 1.5 mm gap. Close the gap with clamps or a copper backing bar, because a gap forces the arc to search and concentrates heat on the thin panel. On the DP780 side, heat input is the constraint: the heat-affected zone softens and OEM weld schedules cap current and weld length, so a fast stitch with adequate fusion is preferred to a hot, slow bead. If the gap cannot be closed, stop and use the OEM's alternate joint design.
3. The OEM procedure shows squeeze-type resistance spot welding as the specified method for this flange and permits GMAW only where a spot welder cannot reach. What should the technician do, and what documentation is needed?
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The OEM procedure controls. If STRSW is specified for that flange, the technician must first confirm the spot welder cannot reach the joint - not assume it - and document the reach test, electrode configuration and the reason GMAW was substituted. GMAW is normally allowed only where a squeeze-type welder cannot physically reach, and then only with the OEM-approved wire, gas and weld length or pitch. Substituting GMAW for specified spot welds without authorization can reduce crash performance because spot welds and MIG stitches have different load paths and heat-affected zones. Document the procedure number and revision, VIN, technician certification, weld map, machine settings and destructive test results, and obtain supervisor or engineer sign-off. If the OEM does not permit GMAW at that location, the repair must be redone to the procedure rather than signed off as equivalent.