The primary function of the shielding gas is to exclude the atmospheric air from the arc zone. However, the shielding gas has significant influence on the arc characteristics, mode of metal transfer, speed of welding, cleaning action and bead geometry. Pure argon is used only for welding of non ferrous metals. However, pure argon shielding on ferrous alloys causes an erratic arc and a tendency for undercut to occur. Hence, for welding of ferrous alloys, either CO2 or argon + CO2 gas mixture is used. Addition of 1 to 5 % O2 or 3 to 25 % CO2 produce a noticeable improvement in arc stability and freedom from undercut. Argon rich gas mixtures produce a finger shape penetration whereas CO2 shielding gas produces wider and deep penetration.