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Arc welding encompasses a range of fusion welding processes. These processes can only be used to join certain metals because they rely on the formation of an electric arc between the workpiece and electrode to generate heat. The most common types of arc welding are manual metal arc (MMA), metal inert gas (MIG) and tungsten inert gas (TIG). The joint interface and electrode melt to form a weld pool, which rapidly solidifies to form a bead of weld metal. A shielding gas and layer of slag (in some cases) protects the molten weld pool from the atmosphere and encourages the formation of a 'sound' joint.
MIG welding accounts for about half of all welding operations and is used in many industries, because it is rapid and produces clean welds. Whereas the quality and slower speeds of TIG welding make it ideally suited to precise and demanding applications. The quality of manual welds depends largely on the skill of the operator and nature of the metals being joined. As with all thermal metal fabrication techniques, arc welding produces a heat affected zone (HAZ). All welding produces residual stress in HAZ weld area and beyond. The problems are caused by a change in structure leading to a change in properties and a susceptibility to cracking.