Gas Shielded Metal Arc Welding
Terminology
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.
Description of the Process
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
Fig. 2 MIG/MAG Welding
The basic equipment consists of :
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 :-
Low inductance setting:- Gives higher short circuiting frequency and relatively cold welding
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.
| GMAW | Gas Metal Arc Welding | MIG | Metal Inert Gas | MAG | Metal Active Gas | CO2 | Carbon Dioxide + Oxygen |
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.
Description of the Process
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
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 :-
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.