Porosity and lack of fusion in trailer chassis welds
While repairing a 6 mm mild steel trailer chassis, a welder makes butt joints in the flat position using GMAW. The joint is a single-V with a 60° included angle, 1.5 mm root face and 3 mm root gap. Settings are 0.8 mm wire, 180 A, 20 V, CO2 shielding gas at 8 L/min, and 18 mm wire stick-out. Travel speed is fast enough to leave a convex bead. The workshop door is open and a fan is running 3 m away. A bend test on a test plate shows clustered porosity near the root and lack of fusion on one sidewall. The welder says the machine was serviced last year, and the wire spool has been open in the damp store for two months.
1. Diagnose the likely causes of the porosity and the lack of fusion, linking each defect to the settings and practice described.
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The clustered root porosity points to shielding failure or contamination. A flow of 8 L/min is low for CO2 in an open workshop, and the fan and open door can blow the gas away. An 18 mm stick-out is long for 0.8 mm wire, so gas coverage at the arc is poor. Wire left open in a damp store can be rusty or damp, and mill scale or condensation in the joint can release gas. The lack of fusion on one sidewall is a heat-input and technique problem: 180 A and 20 V with a fast travel speed and a convex bead suggest the arc is not dwelling at the sidewall. A 3 mm root gap with a 1.5 mm root face can also let the root pass sit high without penetrating the sidewall. The gun angle may be pointing across the joint rather than into both fusion faces.
2. State a corrected welding procedure, including preparation, gas, electrical settings and technique. Include how you would repair the failed weld.
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Grind the joint and surrounding 25 mm to bare metal, remove rust and mill scale, and dry the area. Replace the damp wire with a clean, dry spool. Screen the work from the fan and door draught. Set CO2 flow to 12–15 L/min and check for leaks at the regulator and torch. Reduce stick-out to 10–12 mm. For the root, use about 160–180 A and 20–21 V with a short arc and a slight pause at each sidewall; for fill and cap, 180–200 A and 21–22 V, with travel slow enough to produce a flat-to-slightly-convex bead, not a high crown. Use a 10–15° push angle and watch the puddle wet into both fusion faces. Repair the failed weld by gouging or grinding out the porosity and lack of fusion to sound metal, re-welding with the corrected procedure, and re-testing a coupon.
3. Explain how you would verify the repair and prevent the same defects on the next chassis.
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Verify the repair visually first: no visible porosity, undercut, overlap or high crown, and the bead should blend into both sidewalls. Then make a test plate with the same joint preparation and procedure and bend it; the bend test should show no open defects. For a chassis repair, additional non-destructive testing such as dye penetrant or magnetic particle inspection may be required by the repair scheme, and the welder should record the settings used. To prevent recurrence, write a short welding procedure specification for the joint, calibrate the gas flow meter, store wire in a heated cabinet, fit welding screens, and position the fan so it does not remove shielding gas. Check the first production weld and then at intervals, because a change in draught, wire condition or gas flow can reintroduce porosity even when the machine setting looks correct.