METAL INERT GAS (MIG) OR GAS METAL ARC WELDING (GMAW)

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This process too may be manual or automatic, the difference being that the electrode is consumable, being wound on a reel and fed at a constant rate through powered wire feed rollers, The torch is special, containing the power cable, a contact tube for the wire electrode and a nozzle at the end to direct the gas shield The inert gas is contained in cylinders under pressure and piped to the torch. It may be CO2 (for mild steel), argon for non-ferrous metals, and a C02-02-argon mixture for high strength, corrosion resistant ferrous alloys.

The arc length is controlled by the power source which has variable inductance in the circuit to prevent current surges. The operator has thus to only concentrate on weld deposit. In the automatic process powdered flux may be added to the gas and the electrode wire magnetised, so that the flux clings to it as it enters the weld Pool )

This method is suitable for thin plates in the down hand position with bare wire electrodes upto 1.6 mm diameter. For welding in the ‘overhead and vertical positions a pulsed arc or ‘dip transfer method is used. In this method, as the wire is continuously fed, the arc first is established, the electrode then comes into contact with the plate, the tip melts, gets detached and deposited as a drop on the plate after which the arc re-establishes itself and the process repeats. For thick plates a higher current is used with which a fine spray transfer of the electrode across the arc is achieved with deep penetration.

This method is continuous, gives a good quality weld with no need of finishing and the equipment is portable and suitable for open air work.