MIG-MAG welding (GMAW in American terminology, processes 131, 135 and 136 of ISO 4063) is a gas-shielded arc welding process with a consumable wire electrode fed continuously from the power source. The wire acts as both electrode and filler metal. In MIG (Metal Inert Gas), the gas is inert, argon: it is used mainly for aluminium. In MAG (Metal Active Gas), the gas is active, a mix of argon and CO2 or oxygen: this is the case for steel.
The welder holds the torch, the wire advances on its own and the trigger starts the arc and the gas, hence the term semi-automatic process. Depending on the settings, metal transfer takes place by short circuit (thin sections, out-of-position welding), axial spray (high currents, thick sections) or in pulsed mode, which limits spatter and suits stainless steel and aluminium. The wire may be solid or flux-cored. The torch can be hand-held or mounted on a robot or cobot for repetitive runs. Wind disperses the shielding gas, so welding is done sheltered.
MIG-MAG is productive, with a high deposition rate, and suits medium to thick sections. It is used for structures, frames, trailers, machine bases, tanks and ordinary plate fabrication in structural steel. The weld bead is less fine than that of TIG welding, with spatter to grind off. Parts cut by plasma or laser are then assembled this way. To compare the processes, read the guide TIG, MIG and MAG welding, and the welding and plate fabrication page for workshops.
In a quote request. Specify the steel grade (for example S235JR or S355J2), the thicknesses, the joint types and the length of welds, and say whether they must be continuous or intermittent. State the inspections expected (visual, magnetic particle, radiography), the grinding of visible welds and the final treatment: hot-dip galvanising, for example, requires vent holes in hollow sections.