In the TIG welding process, the arc is drawn between a water-cooled non-consumable tungsten electrode and the plate. An inert gas shield is provided to protect the weld metal from the atmosphere and filler metal may be added to the weld pool as required. Ignition of the arc is obtained by means of a high-frequency discharge across the gap since it is not advisable to strike an arc on the plate with the tungsten electrode. Normally the inert gas shield used for welding aluminum and steel is argon. The only plate thickness of less than 6mm would normally be welded by this process, and in particular aluminum sheet, a skilled operator is required for manual work.
METAL INERT GAS WELDING
This is an extension of TIG welding and electrode in this process becoming a consumable metal wire.
Basically, the process consists of a wire feed motor supplying wire via guide rollers through a contact tube in the torch to the arc. An inert gas is supplied to the torch to shield the arc, the electrical connections are made to the content tube and workpiece. Welding is almost always done with a D.C. source and electrode positive for regular metal transfer and when welding aluminum to remove the oxide film by the action of the arc cathode. Although the process may be fully automatic, semi-automatic processes as illustrated with a handgun are now in greater use and are particularly suitable in many cases for application to shipyard work.
Initially, aluminum accounted for most of the MIG welding, the argon is used as the inert shielding gas. Much of the welding undertaken on aluminum deckhouses, and liquid methane gas tanks of specialized carriers, has made use of the process. Generally larger wire sizes and heavier current have been employed in this work, metal transfer in the arch being by means of a spray transfer, that is metal droplets being projected at high speed across the arc.
Early work on the welding of mild steel with the metal inert gas process made use of argon as a shielding gas; but as this gas is rather expensive, the satisfactory welding could only be accomplished in the downhand position, an alternative shielding gas was sought. Research in this direction was concentrated on the use of CO2 as the shielding gas and the MIG/CO2 process is now widely used for welding mild steel. Using higher current values with thicker steel plate a fine spray transfer of the metal from the electrode across the arc is achieved, with a deep penetration. Wire diameters in excess of 1.6mm are used and currents above about 350 amps are required to obtain this form of transfer. Much of the higher current work is undertaken with automatic machines, but some semi-automatic torches are available to operate in this range in the hands of skilled welders. Welding is downhand only.