GAS - METAL - ARC WELDING ( MIG ) AND ITS PROCESS

Although Gas-Metal-Arc Welding (GMA) is the official description of this welding method, The earlier name of metal inert gas (MIG) is still widely used, especially in the shops.

Gas-metal-arc welding is a gas shielded metal arc welding process which uses the high heat of an electric arc between a continuously fed , consumable electrode wire and the material to be welded. Metal is transferred through protected arc column to work.

In this process, the wire is fed continuously from a reel through a gun to constant surface which imparts a current upon the wire. A fixed relationship exists between the rate of wire burn-off and the welding current so that the welding machine at a given wire feed rate will produce necessary current to maintain the arc. The current ranges from 100 to 400 A depending upon the diameter of the wire, and the speed of melting of the wire may be up to 5 m/min. The welding machine is dc constant voltage, with both straight and reverse polarities available.

The welding gun can be either air-or water cooled depending upon the current being used. With the higher amperages, a water-cooled gun is used. The welding wire (continuous electrode) is very often bare. Very lightly coated or flux-cored wire is also used. The wire is usually in diameters 0.09 to 1.6 mm, however, sizes up to 3.2mm, are made.

In gas-metal arc welding, the welding area is flooded with a gas (an inert gas) which will not combine with metal. The rate of flow of this gas is sufficient to keep oxygen of the air away from the hot metal surface while welding is being done. Carbon dioxide (Co2) is used for working with steel, as GMA is a clean, faster method for welding steel. Carbon dioxide is used principally because it is inexpensive. For welding aluminum or copper, argon or argon-helium mixtures are used. For stainless-steel, MIG welding is done with either argon-helium mixtures are used. For stainless-steel, MIG welding is done with either argon-oxygen or helium-argon gas mixtures. Titanium requires pure argon gas shielding, and the copper-nickel and high-nickel alloys use argon-helium mixture.

Some advantages of this processes are :

1. No flux required.

2. High Welding speed.

3. Increased corrosion resistance.

4. Easily automated welding.

5. Welds all metals including aluminum and stainless steel.

6. High economy.

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