Process Encyclopaedia

Other names / variants: MMA, TIG, MIG, spot welding, seam welding

There are several types of arc welding - MMA (Manual Metal Arc) is probably the most well known. Automated arc processes include TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas). All arc processes use a filler to join the two pieces - in MMA and MIG the filler also serves as the electrode which makes the electric arc.
There are other more specialist arc welding processes such as spot welding or seam welding which work without a filler.
Safety precautions must be taken to protect the welder from the bright arc and the noxious fumes.
Good welding requires a lot of skill, and in industry a welder must have special qualifications.

[Pictures courtesy of TWI]

Although many metals can be joined with MMA, it is most commonly used for steel. Other materials, such as aluminium, are usually joined by more sophisticated arc welding processes (e.g. MIG, TIG).
MMA is portable and so suitable for repair or on-site work.
Thin plates may require only one pass for a successful join. For thicker plates, multiple passes may be required to fill the gap.
For thin plates, the edges may be square. For greater thicknesses, the edges need to be bevelled to allow the gap to be filled more easily.
In the area that has been affected by heat, the properties of the material may change greatly.

The cost of MMA equipment can be less than Ł100. However, the production rate is slow so it is only economic for one-off jobs, repair work and difficult access situations.
MIG and TIG are available as manual processes, but they are often automated to improve quality and production rate.
For joining thick metals, arc welding has few serious competitors.
Where reliable joints are essential (e.g. aeroplane wings) mechanical fasteners such as rivets are used instead of welding.
Joining of sheet (e.g. car body panels) is usually more economic by other welding processes such as spot welding.

TWI have good introductory guides to various welding processes.