CuSi3 Brazing on a Galvanized Door Aperture with Collapsed Gap
A door aperture on a 2022 vehicle uses a 0.8 mm galvanized outer panel joined to a 1.2 mm boron-steel inner reinforcement. The OEM procedure permits MIG brazing with CuSi3 only on this non-structural outer joint and calls for a 0.8–1.0 mm gap. The technician uses 0.8 mm CuSi3, C25 gas at 15 cfh, 65 A, 3.2 m/min wire-feed speed, 40 cm/min travel and a 5 mm stick-out. Fit-up closes the gap to 0.3 mm in the middle of the joint. The finished braze shows scattered porosity, blue-grey heat tint extending 10 mm, incomplete fusion at the root on the lower half, and two small burn-through points. Zinc fume is visible and the fume extractor is positioned 300 mm from the arc.
1. Evaluate whether the process window and technique are appropriate, and identify the dominant causes of the porosity, incomplete fusion, and burn-through.
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Most defects trace to the collapsed gap, not the machine. CuSi3 brazing relies on capillary flow into a 0.8–1.0 mm gap; at 0.3 mm the filler bridges the surface and the arc concentrates on the edges, which explains the burn-through and the incomplete root fusion on the lower half. The blue-grey tint extending 10 mm indicates excessive heat input for a braze - CuSi3 should wet at roughly 900–1,000°C, well below steel melting, but 65 A with slow travel and a tight gap overheats the panel. Porosity has two likely sources: zinc vapor from the coating, and shielding gas disturbed by the extractor position or by inadequate cleaning of the weld zone. The 5 mm stick-out and 15 cfh are reasonable; 3.2 m/min is at the high end for 0.8 mm wire but is not the primary fault. The gap is the first variable to correct.
2. Explain why the OEM restricts brazing to this non-structural joint, and what would change if the procedure specified the inner reinforcement joint instead.
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Brazing is restricted to non-structural outer joints because CuSi3 is a low-temperature filler that wets the steel without fusing the base metal. The joint has lower static and fatigue strength than a fusion weld and does not restore a crash load path. Its advantages - preserving the galvanized coating, low heat input, less heat-affected-zone softening in boron and AHSS - matter on cosmetic outer panels where those properties are the priority. If the OEM specified the inner reinforcement joint, brazing would generally not be acceptable; the procedure would call for STRSW, or GMAW with an approved steel filler and a specified weld length or pitch, because the reinforcement is structural. The technician must follow the joint-specific method, not transfer the outer-panel brazing technique to a structural joint. If the procedure is unclear, stop and obtain the OEM position in writing.
3. Specify the corrective actions for gap control, cleaning, heat input, fume control, and verification of the finished joint.
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Close the gap to 0.8–1.0 mm with clamps and spacers; do not try to fill a 0.3 mm gap by slowing down. Clean the zinc only in the weld zone with an approved method and solvent, and keep the fume extractor at the source but far enough not to disturb shielding - typically around 100–200 mm and behind the torch. Tune on a coupon: start around 60–70 A, 2.8–3.2 m/min and 40–50 cm/min with a short arc, then adjust until the bead wets both edges without burn-through. Use a slight push angle and keep stick-out at 5–8 mm. Verify with a peel or chisel test on the coupon and visual inspection of the production joint for porosity, blue overheating and complete root wetting. Record the final parameters, gap measurement and fume-control setup, and require the technician to use the correct respiratory protection for zinc fume.