When, where and why to use MIG welding

Mick Andrews, superintendent of the Murex Welding Process Centre, explains MIG welding and the advantages offered by this versatile process.

MIG (metal inert gas) welding is the most widely used of the arc welding processes, suitable for everything from hobbies and small fabrications or repairs, through to large structures, shipbuilding and robotic welding.

MIG can be used on a broad range of materials and thicknesses.

The concept underlying MIG welding is simple, with the arc being struck between the tip of the reel-fed wire as it emerges from the torch and the workpiece.

A shielding gas prevents oxidation forming.

The use of some flux-cored wires can avoid the need for a shielding gas.

With most MIG welding sets, the wire feed speed is set to provide the current; then the voltage is set to suit that current.

Ease of use is a major benefit of MIG in many applications, but speed is also important.

It is far quicker to lay down weld metal with MIG than TIG (tungsten inert gas), MMA (manual metal arc - or 'stick' welding) or gas welding.

Typical applications for MIG welding include automotive sub-assemblies and bodywork, ranging from conventional MIG for most bodywork, to heavy-duty MIG for lorry chassis and off-highway vehicles.

MIG is also used extensively in automotive and general repair workshops.

Other applications include the assembly of white goods and office furniture, structural steelwork for the construction industry, as well as bridges and wind turbine towers.

MIG welding is widely used in shipbuilding and ship repair yards.

It is also used extensively in the process industry for pipework and the fabrication of vessels.

The development path for MIG has, in simple terms, progressed from dip transfer to globular transfer (which is avoided today, due to the spatter it produces) and then to spray transfer (although suitable only for downhand or horizontal welding, spray transfer enables the use of high voltages and currents for rapid deposition rates).

Pulse transfer gives the advantages of spray transfer but with lower currents, making this process suitable for positional welding and thin materials.

But the latest development is SuperPulse MIG welding, which gives the advantages of pulse welding combined with the finish that is normally associated with TIG welding.

With the level of control available in SuperPulse machines, the operative can effectively manage the welding parameters to 'mix and match' the modes of transfer to suit the material and the required combination of speed and quality.

As the MIG process utilises a filler wire, this creates the opportunity to use a wire of a considerably different composition to that of the parent metal, for applications such as hard facing or building up the surface of worn track in the rail industry.

By judicious selection of the filler wire and weld parameters, MIG can also be used to join dissimilar metals.

Another application for which MIG is increasing in popularity is brazing.

The main advantage here is that it enables good strength to be achieved when joining thin materials - even down to 0.5mm - and an example of MIG brazing is found in the automotive industry.

As automotive manufacturers seek to reduce weight, they are designing vehicle bodies in thinner grades of stronger steel.

Welding such materials can lead to cracking in the heat affected zone, whereas brazing puts less heat into the joint.

The result is a joint with satisfactory strength and no undesirable affects in the parent metal.

In the automotive industry today almost every welded (or brazed) joint is automated using robots.

Robotic welding (or brazing), whether in the automotive industry or elsewhere, almost invariably uses MIG.

Although it may not sound like a technological leap forward in the same way as, say, the advance from spray transfer to pulse transfer MIG welding, bulk reels of wire can make a substantial difference to a plant's productivity.

It can be very economical to semi-automate or automate MIG welding in straight lines or arcs to eliminate any variability associated with operative skill levels or fatigue.

Anywhere that a tractor unit can be installed, MIG welding can be automated; specialised units are also available for making pipe joints.

In welding applications the most expensive element is usually the welding operative.

MIG welding is more widely used than any other type of arc welding, largely due to its speed and versatility.

However, it has to be recognised there are applications for which MIG is not the ideal.

TIG is the preferred method for very thin material or when joint integrity is critical - such as the root weld in pipes for the offshore industry - and the shielding gas required for MIG can be affected by drafts and wind.

To a certain extent, however, this latter limitation can be overcome when working outside, typically by erecting a tent around the working area or by switching to a self-shielding flux-cored wire.

Users of MIG welding also need to be careful that they select the correct shielding gas, as the argon/CO2 mix used on carbon steels will not give good results on aluminium (which needs pure argon or helium) or stainless steel (for which other gases are necessary).

Other materials will also need different gases.

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