The filler wire (2) is unwound from a reel, steadily and automatically, by means of a motor with variably adjustable speed. The wire is guided directly to the welding point by means of a contact sleeve (1). The filler wire melts immediately. During the MIG welding process, shielding gas (3) is added through a nozzle.
This prevents oxygen from penetrating the weld pool (4) and therefore protects against oxidation. Oxidation would cause weakening of the weld and therefore to reduced quality. The finished weld is also called a weld run or bead (5).
While MAG welding works with active gases and is primarily used to make steel joints, MIG welding uses inert gases to weld non-ferrous metals. The inert gas used is argon, in rare cases helium (is more expensive) or mixtures of these are used. As the gases used do not react with the parent metal and additive materials, MIG welding is mainly used to join copper, aluminium or aluminium alloys as well as for other non-ferrous metals.
Inert gases enable welding at much higher temperatures. The possibility of oxidation of the weld occurring is therefore excluded, which in turn is a very advantageous effect. As the shielding gas is driven away due to weather, e.g. wind, MIG welding can only take place indoors. Outdoors the welding process would therefore take place without the so useful oxidation protection. A reduced quality weld would result, which would be far more susceptible to rust and would have less load-bearing capacity.
This type of welding is used to make vessels and pipes, in mechanical engineering, in precision engineering and in nuclear engineering.
In MIG welding a differentiation is made between different arcs:
- Welding with short arc
In low performance ranges, root welds and welding in uncomfortable positions or positional welding is carried out. It is primarily used to join thin metal sheets. An almost smooth material crossover results, as only a few splashes (sputter) and fine drips are produced.
- Spray arc welding
This is used to join thicker metal sheets. The low-sputter, fine-drop and short-circuit-free material transition is created by the high degree of melting and high speeds of welding with argon-based mixed gases.
- Pulsed arc welding
This type of arc welding is used for all sheet and plate thicknesses. Argon-rich mixed gases are used. This welding procedure tends to be used in the medium performance range. A pulsed current is applied via the background current. Depending on the requirements, it is possible to control the thickness or size of the droplets produced during welding. In this way, short-circuit-free transition of the materials is achieved with minimum spatter and uniform fine droplets.
Advantages of MIG welding:
No slag is produced by MIG welding. The shielding gas screens the weld from the effect of oxygen and therefore prevents the risk of oxidation. Due to the high working rate there is little heat effect on the materials to be welded. Only minor deformations are to be expected. This welding procedures has also established itself most due to its positional welding capabilities (i.e. working in constrained or confined positions). It has developed into the most used welding procedure. The high strength of the weld reduces the intensity of the rework.
Disadvantages of this welding method:
If MIG welding were to be used outdoors, the shielding gases would inevitably drift away. Therefore, this metal shielding gas welding can only be carried out in enclosed rooms, i.e. indoors. It is not easy to control and needs lots of experience. Particular attention must be paid to thorough weld preparation.