Porosity and Spatter in CO2 Gas-Shielded Welding
A workshop is MAG welding 6 mm mild steel plate in the flat position with 1.2 mm ER70S-6 solid wire and CO2 shielding gas. The machine is set at 180 A, 22 V, DCEP, wire feed speed 6.0 m/min, gas flow 8 L/min, and contact-tip-to-work distance about 22 mm. The workshop door is open and a fan is blowing air across the bench to keep the welder cool. The first welds show scattered porosity, heavy spatter, and lack of fusion at the toes. The gas cylinder was changed that morning and the regulator was refitted by a trainee. The plate was wire-brushed but not degreased, and there is a light oil film from the guillotine. The welder wants to increase current to 230 A to 'burn through the dirt'.
1. Rank the likely causes of the porosity and lack of fusion, and explain why each fits the symptoms.
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- Gas coverage: 8 L/min is low for CO2, especially with a 22 mm stick-out and a fan blowing across the bench. The draft blows the shield away, so air enters the arc. Scattered porosity and lack of fusion at the toes fit this best.
- Long stick-out: 22 mm is excessive; it reduces effective shielding and changes the current path, increasing spatter.
- Regulator or hose leak: a trainee refitted the regulator, so a loose connection, wrong flow setting or leaking hose is possible. A cylinder change can also introduce moisture or air.
- Surface contamination: the light oil film is not removed by wire brushing. Oil cracks in the arc and produces porosity.
- Polarity or electrical connection: check DCEP and the work return. Wrong polarity affects penetration and spatter. Lack of fusion may also come from travel speed that is too fast or current that is too low for the joint, but gas coverage and contamination should be corrected first.
2. State the corrected settings and checks you would make before welding the next test piece, including gas, stick-out, surface preparation and electrical polarity.
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Close the door or move the work away from the fan; shielding gas must not be blown away. Set CO2 flow to about 12–16 L/min using a flowmeter at the nozzle, not just the regulator gauge. Check the regulator, hose and torch for leaks with soap solution, and purge the line after the cylinder change. Reduce contact-tip-to-work distance to about 10–12 mm. Clean the plate with a degreaser and then grind or wire-brush it; wire brushing alone does not remove oil. Check that the machine is DCEP, the work return is firmly connected, and the contact tip and liner are the correct size and not worn. Start with about 180–200 A and 22–24 V, adjust wire feed to give a stable short arc, and run a test piece before welding the frame. Confirm the settings are within the WPS.
3. Evaluate the welder's proposal to increase current to 230 A. Will it fix the problem? What risks would it add?
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Increasing current to 230 A will not fix porosity caused by poor gas coverage or oil contamination. It may increase penetration and make the arc less sensitive to small disturbances, but air and oil will still enter the weld pool. The risks are higher heat input, more spatter, a greater chance of burn-through on 6 mm plate, more distortion, and a wider heat-affected zone. A higher current can also worsen turbulence and draw more air into the shield if gas flow is already inadequate. Correct the shielding gas, stick-out, leaks and surface preparation first, then adjust current within the qualified range. If lack of fusion remains after those corrections, check travel speed and joint preparation rather than simply raising current.