Know About MIG Welding

The welding equipment consists of mainly welding power source, cables, wire feed mechanism having pair of wire driving rolls and electric motor, welding gun and wire electrode coiled spool, inert shielding gas cylinder, and pressure regulator, flow meter controls to regulate the current, electrode wire and gas.

The welding gun consists of two current conductors for shielding gas and a wire guide tube which is passed through welding nozzle. Unlike TIG welding where non-consumable filler rod is used, MIG welding uses consumable filler material on the metal work piece. Also, welding torch is replaced by welding gun in this type of welding to feed direct current to the metal work piece. The welding gun which holds and feeds the electrode and gas may be air-cooled or water-cooled. The nozzle used can be either straight or inclined for convenience to weld complex shapes and intricate joints.

The power source used can be AC transformer or DC generator but DC generator is recommended because with AC transformer, the unequal burn off rates during negative and positive half cycles is evident. DCRP (DC with reverse polarity) provides deep penetration. The power source of 400 Amps with flat or drooping characteristics can be applied for MIG welding. Arc length is better with flat characteristics weld source. For thin metal work piece, electrode is made negative in DC and for welding aluminum and magnesium, DC electrode is positive.

To feed the wire consistently to the weld metal work piece, wire feeding mechanism is installed which delivers the electrode wire at constant speed or different desired speeds. Electrodes come in the sizes ranging from 0.8 mm, 1.2 mm, 1.6 mm etc and different quantity from 1 kg spool and so on. The steel electrodes are copper coated. While welding, one need to ensure that the electrode and metal work piece should match in chemical composition in addition to deoxidizers. The available electrodes in the market are for welding of aluminum, magnesium, nickel, stainless steel and their alloys and low carbon steel.

A number of shielding gases and the mixtures are used in MIG welding for different metals and applications like argon and helium for welding aluminum, magnesium, copper and nickel. Carbon dioxide gas finds application in welding mild steel, nitrogen for copper again etc. Argon, helium, carbon dioxide and oxygen independently or in mixtures are mainly used gases in MIG welding. CO2 and helium gas encourage penetration and argon controls spatter.

Controls and solenoid valves are integrated in the equipment for regulating gas, water and wire feed mechanism.

The gas and water flow is put on. The desired current is regulated, wire feed speed is set and then arc is ignited and established by two methods. In normal practice, he current and shielding gas flow is switched on and the electrode is rubbed against the metal work piece for striking the arc. In the other method to establish the arc, electrode is made to tap the work piece is retracted and welding is followed by moving further. The length of electrode projected out of the welding gun is about 15 mm which before welding process is carried out and is also referred as electrode extension. Metal work piece is about 10-12 mm distance from the weld gun and arc length is retained between 1.5 to 4 mm. A stabilized arc length is maintained by applying the principles of self-adjusted arc and self controlled arc in semi-automatic and automatic welding machines respectively. This is the reason; MIG is also brought under semi-automatic welding process.

The shielding gas shields the arc and molten weld pool and also helps in smooth transfer of metal from electrode wire to the weld pool. The transfer of the metal is carried out by following modes.

1. Spray transfer – This method of transfer was initially used compared to all other transfers. Aluminum and stainless steel used spray type transfer. The weld quality is much better because of stable arc and absence of spatter. Metal from the electrode is passed through an electric arc and goes to the metal work piece.

2. Short circuit transfer – The electrode wire is made to touch the metal work piece and go along with the metal ensuring that metal doesn’t transfer across the arc which also avoids weld puddling as the droplets are formed on the electrode but do not fall on the metal work piece and create a short circuit. Although, the arc is put off, the same is resumed due to the surface tension which pulls and separates the metal goblet from the tip. Since lower current is used in this type, only thin metals can be welded. The gas used is carbon dioxide. Droplets on the electrode.

3. Globular transfer – Carbon dioxide is used in this type of transfer and lot of spatter is a problem because the welding gun gets heated without notice. The molten metal is shaped like a ball which is developed by electrode and the size of ball seem to be bigger than electrode and falls on the work piece causing spatter.

4. Pulse spray – This method is developed off late and latest of all other methods. Current used is constant that pulses and melts the filler wire. Small droplets are formed in each pulse. In this type, the welder can use lower currents to achieve a stable arc which eliminates spatter and short circuiting. Argon gas can be used instead of carbon dioxide to attain better speed in welding.