Flat Position Lap Joint on Galvanized Sheet
A maintenance crew is repairing a guardrail assembly made from 2 mm galvanized steel sheet lap-joined to a 5 mm galvanized channel. The joint is in the flat position and will be welded with GMAW short-circuit transfer using ER70S-6 wire and 75/25 argon-CO2 shielding gas. The welder grinds the coating from the top surface but cannot reach the underside of the lap. During welding, the arc sputters, the puddle is erratic, and the weld shows porosity and a white powdery deposit around the toe. The welder wants to increase wire feed speed and voltage to push through the coating and get a smoother bead.
1. Explain what is happening at the arc and why increasing wire feed speed and voltage is not a safe correction.
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The white deposit and erratic arc are characteristic of welding over zinc coating. Zinc boils at a temperature far below the steel melting point, so as the puddle forms, the coating vaporizes and expands, disrupting the arc and pushing gas through the molten metal. That produces porosity and spatter. The underside of the lap is unground, so zinc is still present where the root forms and where the vapor can escape into the puddle.
Increasing wire feed speed and voltage adds heat, which vaporizes more zinc and creates more fume and porosity, not less. It also raises fume exposure for the welder. The correct approach is to remove the coating from both faying surfaces where accessible, improve ventilation and respiratory protection, and use a technique that lets the zinc vapor escape rather than trapping it. Pushing more heat into a coated lap joint makes the problem worse.
2. The welder cannot reach the underside of the lap. What practical options should be considered, and what are their limits?
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The welder should first consider redesigning the joint so both faying surfaces can be cleaned, for example by repositioning the lap or using a different joint configuration. If that is not possible, the welder can remove coating from the accessible side and use a slight gap or a sequence that allows vapor to escape ahead of the puddle. Grinding the underside before assembly is the most reliable option, so the crew should plan for it rather than trying to weld through the coating.
Welding through zinc is not a solution. The limits are that any remaining coating at the faying surface will produce porosity and fume, and the welder cannot verify fusion under the lap by visual inspection. The crew should consult the engineer about whether the joint can be reworked, and should not accept a porous weld simply because the coating could not be removed.
3. What health and safety controls apply to this repair, and how do they interact with weld quality?
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Welding galvanized steel produces zinc oxide fume, which can cause metal fume fever. The crew should use local exhaust ventilation at the arc, respiratory protection rated for metal fume, and work in a ventilated area. The welder should also keep their head out of the plume and avoid welding in a confined space without monitoring.
These controls interact with quality because the same conditions that remove fume also remove the shielding disruption. Good ventilation reduces the concentration of zinc vapor at the arc and lowers porosity, while a welder working in visible fume is likely welding under poor conditions. The welder should not treat fume control as separate from weld quality; both depend on removing the coating and controlling the arc environment.