The MIG process is a versatile welding technique which is suitable for both thin sheet and thick section components. It is capable of high productivity but the quality of welds can be called into question. To achieve satisfactory welds, welders must have a good knowledge of equipment requirements and should also recognise fully the importance of setting up and maintaining component parts correctly.
In MIG the arc is formed between the end of a small diameter wire electrode fed from a spool, and the workpiece. Main equipment components are:
wire feed system
The arc and weldpool are protected from the atmosphere by a gas shield. This enables bare wire to be used without a flux coating (required by MMA). However, the absence of flux to 'mop up' surface oxide places greater demand on the welder to ensure that the joint area is cleaned immediately before welding. This can be done using either a wire brush for relatively clean parts, or a hand grinder to remove rust and scale. The other essential piece of equipment is a wire cutter to trim the end of the electrode wire.
MIG is operated usually with a DC power source. The source is termed a flat, or constant voltage, characteristic power source, which refers to the voltage/welding current relationship. In MIG, welding current is determined by wire feed speed, and arc length is determined by power source voltage level (open circuit voltage). Wire burn-off rate is automatically adjusted for any slight variation in the gun to workpiece distance, wire feed speed, or current pick-up in the contact tip. For example, if the arc momentarily shortens, arc voltage will decrease and welding current will be momentarily increased to burn back the wire and maintain pre-set arc length. The reverse will occur to counteract a momentary lengthening of the arc.