overview sub head
The patent for what we now know as the MIG process was granted on the 21st January 1949 to Muller, Gibson and Anderson of the Air Reduction Company (AIRCO), now BOC Gases America, for their invention of a process they termed SIGMA (Shielded Inert Gas Metal Arc) welding.
The term most commonly used to describe this process is MIG (Metal Inert Gas) welding, even though the shielding gas used may not be totally inert. In some countries a differentiation to the naming convention is made based on the shielding gas used. In Europe for example, MIG is used only when a totally inert gas shield e.g. Argon is used, and the MAG (Metal Active Gas) being preferred when the shielding gas also contains active constituents, such as carbon dioxide or oxygen.
A shielding gas of 100%CO2 may also be referred to as 'MAG', although some customers may refer to the process as 'CO2 welding' or semi automatic welding.
In the USA, the term GMAW (Gas Metal Arc Welding) is often used, and covers use of both inert and active shielding gases.
Only the term MIG welding will be used throughout the rest of this paper.
Originally, the process was designed for welding aluminium and its alloys, using pure argon or helium shielding gas, but has been developed into what we know today as a process capable of welding a wide range of ferrous and non-ferrous metals and alloys. (IMAGE = 289x160)
How it Works
MIG welding uses the heat generated by a DC electric arc to fuse the metal in the joint area, the arc being struck between a continuously fed, solid, consumable filler wire and the workpiece, melting both the filler wire and the workpiece in the immediate vicinity. The entire arc area is covered by a shielding gas, which protects the molten weld pool from the atmosphere.
Direct current power sources with constant voltage output characteristics are normally employed to supply the welding current, the filler wire being connected to the positive output terminal, the work return lead being connected to the negative terminal. The output characteristics of the power source can have an effect on the quality of the welds produced.
how it works
how it works sub-head
Circuit diagram of MIG process
The wire feed unit takes the filler wire from a spool or bulk pack, and feeds it through the welding gun, to the arc at a predetermined and accurately controlled speed.
The consumable filler wire and the shielding gas are directed into the arc area by the welding gun. In the head of the gun, the welding current is transferred to the wire by means of a copper alloy contact tip, and a gas diffuser distributes the shielding gas evenly around a shroud which then allows the gas to flow over the weld area. The position of the contact tip relative to the gas shroud may be adjusted to limit the minimum electrode extension.
Schematic of MIG process in operation
The characteristics of the MIG process depend on the way in which metal is transferred from the filler wire to the weld pool, referred to as the 'mode of transfer'. The mode of transfer depends on current, voltage and shielding gas composition, and it has a significant affect on welding speed and weld quality and, therefore, the economics of the process.
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The most common modes of transfer are:
Dip transfer, also called 'short circuiting' or 'short arc' transfer
Spray transfer, also called 'free flight' transfer
Pulsed arc transfer