This article covers the basic process and equipment used for gas metal-arc welding (GMAW MIG welding)
Gas metal arc welding (GMAW or MIG welding) is an electric arc welding process that uses a spool of continuously fed wire. It can be used to join long stretches of metal without stopping. The weldor, or apparatus, holds the wire feeder and a wire electrode is fed into a weld at a controlled rate of speed, while a blanket of inert argon gas shields the weld zone from atmospheric contamination. Shielding the arc and molten weld pool is done by “externally” supplying gas or a gas mixture.
MIG welding can be used on all thicknesses of steels, on aluminum, nickel, and even on stainless steel, etc. However, it is most typically utilized in manufacturing and in commercial fabrication settings.
Advantages of MIG welding are:
quality welds can be produced much faster
a flux is not used, there is no chance for the entrapment of slag in the weld
metal resulting in high quality welds
gas shield protects the arc so that there is very little loss of alloying elements.
Only minor weld spatter is produced
welding is versatile and can be used with a wide variety of metals and alloys
MIG process can be operated several ways, including semi and fully automatic
Two Disadvantages are:
MIG welding cannot be used in the vertical or overhead welding positions because
of the high heat input and the fluidity of the weld puddle
equipment is complex.
MIG equipment consists of a welding gun, a power supply, a shielding gas supply, and a wire-drive system which pulls the wire electrode from a spool and pushes it through a welding gun. A source of cooling water may be required for the water cooled welding gun. There are also Mig rod ovens for proper storage of electrodes.
The majority of MIG welding applications require direct current, reverse polarity. This type of electrical connection yields a stable arc, helps to smooth the metal transfer, has relatively low spatter loss, and gives good weld bead characteristics. Direct current straight polarity (electrode negative) is seldom used, since the arc can become unstable and erratic even though the electrode-melting rate is higher.
Alternating current has found no commercial acceptance with MIG welding because the arc is extinguished during each half cycle as the current reduces to zero and it may not re-ignite if the cathode cools sufficiently.
MIG guns are available for manual manipulation, semiautomatic welding, and for machine or automatic welding. Because the electrode is fed continuously, a welding gun must have a sliding electrical contact to transmit the welding current to the electrode. The gun must also have a gas passage and a nozzle to direct the shielding gas around the arc and the molten weld pool. An electrical switch is used to start and stop the welding current, electrode feed, and the shielding gas flow.