GMAW Fillet Porosity: Draft, Worn Tip, and a Voltage 'Fix'
A fabrication shop is making 6 mm fillet welds on 10 mm mild steel brackets using manual GMAW. Parameters are 1.2 mm ER70S-6, 75% Ar/25% CO2, 210 A, 23 V, wire-feed speed 7.5 m/min, travel speed 350 mm/min, contact-tip-to-work distance 15 mm, and gas flow 15 L/min. The shop door is open to a 20 km/h crosswind. The contact tip is worn oval and the liner was replaced last month. Welds show clustered porosity in the first 20 mm of each run and heavy spatter; the rest of the bead looks acceptable. The welder increases gas to 25 L/min and the porosity improves slightly but does not disappear. The supervisor wants to switch to 100% CO2 and raise voltage to 28 V to burn through the problem. The bracket is a 6 mm fillet in the horizontal-vertical position and cannot be turned.
1. Rank the likely causes and propose a diagnostic sequence before changing the process.
Read the worked answer
Most likely sequence: (1) loss of shielding at the start due to crosswind and insufficient preflow; (2) worn contact tip causing poor current transfer and spatter; (3) contact-tip-to-work distance of 15 mm is at the high end and makes the arc less stable; (4) possible liner or gas-hose leak. The clue is that increasing gas to 25 L/min improved but did not eliminate porosity: that supports a draft or coverage problem, but excessive flow can create turbulence and entrain air. Do not start by changing gas composition. Check with a flowmeter at the nozzle, inspect the tip and liner, set preflow of 0.5-1 s and postflow, and shield the area. Clean mill scale and oil from the start area.
2. Calculate the heat input at the stated parameters and at 28 V, and evaluate the supervisor's proposal.
Read the worked answer
HI = (23 x 210 x 60) / (1000 x 350) = 0.83 kJ/mm. At 28 V, HI = (28 x 210 x 60) / (1000 x 350) = 1.01 kJ/mm. The increase is not dangerous for 10 mm steel, but it does not address shielding. Switching to 100% CO2 changes transfer, increases spatter and penetration, and raising voltage widens the arc, which can increase spatter and undercut. It may make porosity worse by increasing turbulence. Do not use voltage as a porosity cure; the defect is at the start of the run, which points to gas coverage and start technique, not insufficient heat.
3. Specify corrected settings and first-piece checks.
Read the worked answer
Use 1.2 mm ER70S-6, 75% Ar/25% CO2, 200-220 A, 22-24 V, wire-feed speed about 7-8 m/min, travel 300-400 mm/min, contact-tip-to-work distance 12-15 mm, and gas 12-15 L/min measured at the nozzle. Replace the oval tip, set preflow, erect wind screens, and clean the start area. Make a first-piece weld and inspect the start and stop for porosity and spatter; if porosity persists after coverage is proven, check gas purity, hose leaks, and wire condition. Keep the parameter card at the station so the correction is not lost at shift change.