GMAW (MIG) Equipment Setup, Shielding Gas, and Consumables
Power source and wire feed
For steel GMAW, use a constant-voltage power source with solid wire on DCEP (electrode positive). Wire-feed speed sets current; voltage sets arc length; inductance shapes the arc. Set drive-roll tension so the wire feeds without crushing or slipping, install the correct liner and contact tip, and trim the wire before striking an arc.
Gas and consumables
Common shielding gases are 75% argon/25% CO2 and 100% CO2. Flow is typically 15–25 CFH (about 10–15 L/min): too low causes porosity; too high causes turbulence and porosity. ER70S-6 is a common mild-steel filler with higher silicon and manganese for dirty or galvanized steel; ER70S-2 is cleaner. Match wire diameter to thickness-small wire for thin sheet, larger wire for heavier sections.
Worked example
A technician welds 1.2 mm galvanized steel. The reasoning: use 0.023–0.030 in. ER70S-6, 75/25 gas, lower voltage and wire speed, a slight push angle, and fume extraction. A 100% argon bottle is wrong because it gives an unstable arc on steel; 100% CO2 may be used but increases spatter.
Traps
Reversing polarity; setting gas flow by sound alone; over-tightening drive rolls and causing birdnesting; using the same schedule for thin sheet and structural sections; forgetting to purge the line after a bottle change.