Gas Metal Arc Welding

Gas metal arc welding (GMAW) is a fusion welding process. Gas metal arc welding (GMAW) is a high-speed, economical process that is sometimes referred to as metal inert gas (MIG) welding. Gas metal arc welding is a group of arc welding process in which the workpieces are joined by the heat obtained from an electric arc struck between a bare (uncoated) consumable electrode and the  workpiece in the presence of an inert gas atmosphere. The electrode is bare, containing no coating or core. The shielding is provided to protect the molten metal from reacting with constituents of the atmosphere. The shielding gases in gas metal arc welding process, is supplied by an external gas, usually containing one of a mixture of the following: helium, argon, or carbon dioxide.

The consumable electrode acts as a filler metal to fill the gap between the two  workpieces.

Details of the equipments used in Gas Metal Arc Welding

1. The equipment consists of a welding torch in which a bare consumable electrode in the form of a wire is held and guided by a guide tube.
2. The electrode material used in MIG welding is of the same material or nearly the  same chemical composition as that of the base metal.
3. Its diameter varies from 0.7 -2.4 mm.The electrode is fed continuously at a constant rate through feed rollers driven  by an electric motor.
4. MIG makes use of shielding gas to prevent atmospheric contamination of the  molten weld pool.
5. Mixture of argon and carbon dioxide in a order of 75% to 25% or 80% to 20% is  commonly used.The shielding gas flow from the cylinder, through the passage in the electrode  holder and then impinges on the workpiece.
6. AC is rarely used with MIG welding; instead DC is employed and the electrode is  positively charged. This results in faster melting of the electrode which increases  weld penetration and welding speed.

Also read: Gas Tungsten Arc Welding,  Electron Beam Welding

Procedure carried out during Gas Metal Arc Welding

1. The workpieces to be joined are cleaned to remove dust, grease and other oxides chemically or mechanically to obtain a sound weld. The tip of the electrode is also cleaned with a wire brush.
   
2. The control switch provided in the welding torch is switched ON to initiate the  electric power, shielding gas and the wire (electrode) feed.
3. An arc is struck by touching the tip of the electrode with the workpiece and  instantaneously the electrode is separated from the workpiece by a small distance  of 1.5-3 mm such that the arc still remains between the electrode and the  workpiece.
4. The high intensity of the arc melts the workpiece metal forming a small molten  pool.
5. At the same time, the tip of the electrode also melts and combines with the  molten metal of the workpieces thereby filling the gap between the two  workpieces.
6. The deposited metal upon solidification bonds the joint to form a single piece of  metal.

Advantages of Gas Metal Arc Welding

1. MIG welding is fast and economical.
2. The electrode and inert gas are automatically fed, and this makes the operator  easy and to concentrate on the arc.
3. Weld deposition rate is high due to the continuous wire feed
4. No flux is used. Hence, no slag formation. This results in clean welds.  Thin and thick metals can be welded.
5. Process can be automated

Disadvantages of Gas Metal Arc Welding

1. Equipment is costlier.
2. Porosity (gas entrapment in weld pool) is the most common quality problem in  this process. However, extensive edge preparation can eliminate this defect.