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Gas Shielded Metal Arc Welding


GMAW Gas Metal Arc Welding US
MIG Metal Inert Gas UK
MAG Metal Active Gas UK
CO2 Carbon Dioxide + Oxygen UK

GMAW is an American description of the process. MIG welding means the use of an inert (i.e. Non Active) gas; MAG welding requires the use of an active gas (i.e. carbon dioxide and oxygen). CO2 is a more commonly used abbreviation of MAGS welding gas.

The process consists of a DC arc burning between a thin bare metal wire electrode and the workpiece. The arc and weld area are enveloped in a protective gas shield. The wire electrode is fed from a spool, through a welding torch which is connected to the positive terminal into the weld zone.

Fig. 1

The arc is self adjusting; any variation in the arc length made by the welder produces a change in the burn off rate of the electrode, and the arc re-establishes its original length.

Application

It is used on all thicknesses of steels, aluminium, nickel, stainless steels etc. Due to the process being semiautomatic, low operator skill is required compared to other welding processes. It is used in most metal forming/working environments.

Safety

Good ventilation is essential and personnel protection such as overalls, mask, gloves, screens etc. are required to prevent burns from the molten metal and arc radiations.

Equipment

Click to see complete photograph

Fig. 2 MIG/MAG Welding

The basic equipment consists of :

Power source A direct current unit of design and capacity according to particular usage, i.e. motor generator for site use or a rectifier for shop use.
Wire drive unit Containing feed rolls connected to a variable speed motor.
Shielding gas Correct gas for material, thickness, etc. required.
Torch/gun Connected to the wire feed unit to allow both gas and wire to travel to weld area.

Electrical conditions

Welding Current and Wire Feed Speed

These are generally set by the same control; higher wire speeds mean higher amperages and vice-versa.

Voltage

Open circuit voltages can be either varied or stepped.

Metal Transference Across the Arc

Dip Transfer

Also known as short circuiting arc welding, is carried out using currents below 200 amps and 25 volts. Under these conditions the arc is so short that the molten globules at the electrode (wire) tip short circuit to the work piece at rapid time intervals. The rise in the current melts the electrode tip and this re-establishes the arc. This cycle occurs approximately 100 times per second. Dip transfer is therefore suitable for all positions welding, thin material, open butt type joint etc.

Spray Transfer

Using high currents and voltages, i.e. 250-500 amps and 25+ volts, the metal is transferred across the arc in the form of fine droplets in a spray. It is used for high deposition rates and deeper penetrating welds. Spray, due to the high current and voltages involved, is mainly used on thick materials in the flat position only; (except when welding aluminium, where spray transfer is used on all positions).

Inductance Settings

This control can only be used on dip transfer, and it fine-tunes the arc by either making the arc hotter or cooler. To rapid a rise during short circuiting will cause globules to explode out of the arc at the current peak, thus creating excessive spatter. Too slow a current rise will cause the electrode to freeze in the weld pool. Therefore :-
  1. Low inductance setting:- Gives higher short circuiting frequency and relatively cold welding
  2. High inductance setting:- Gives lower short circuiting frequency and longer arcing periods between short circuiting giving hotter welding.

Applications and Advantages of Spray Transfer

Pulsed Arc (Controlled Spray Transfer)

This combines two power sources into one unit, one side supplies a back-ground current; keeping the electrode in a molten condition, while the other unit produces pulses of higher current at regular intervals, which detach and accelerate the droplets of metal into the weld pool.

This type of transfer allows positions welding and higher deposition rates.

Electrodes

Mainly consisting of a bare wire to allow compatibility with parent plate. Metal powder and flux coated wires are also used with CO2 shielding gas to allow high deposition rates and to improve weld metal quality.

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