A shielding gas is used to prevent the molten metal from the harmful effects of the air. Even small amounts of oxygen in the air will oxidize the alloying elements and create slag inclusions. Nitrogen is solved in the hot melted material
but when it solidifies the solubility decreases and the evaporating gas will form pores. Nitrogen can also be a cause of brittleness. The shielding gas also influences the welding properties and has great importance for the penetration
and weld bead geometry. The most common shielding gases are presented here,
Argon is one of the most popular shielding gases thanks to its suitable properties. As an inert gas, it has no chemical interaction with other materials. Therefore it is suitable for materials such as aluminum and stainless steel. At
MIG welding of mild steel an addition of CO2 or a small amount of oxygen will increase the welding properties, especially for short arc welding. Contents of up to 20% CO2 improves the penetration (limits the risk of lack of fusion) while 5± 8% will give reduced spatter.
Helium, like argon, is an inert gas. It gives more heat input to the joint. Mixed with argon it increases welding speed and is advantageous for the penetration in thick-walled aluminum or copper where it compensates for the high heat
conduction. Drawbacks with helium are the high cost and the low density.
Carbon dioxide (CO2)
Pure carbon dioxide can be used for short arc welding. It is cheap gas, has good properties for welding galvanized steel, and gives better safety against lack of fusion than argon-based gases. Drawbacks are a higher amount of spatter and the fact that the gas cannot be used for spray arc welding.
Small additions of hydrogen can be used to increase heat input and welding speed in the same manner as helium, but it is much cheaper. Because of the risk of cracks, hydrogen can only be used for welding austenitic stainless steel. It actively reduces the oxides and is therefore also used in root gases.
Oxygen is also used as a small addition to stabilizing the arc in MIG welding.
Nitrogen can be used as an alloying element in ferritic-austenitic stainless steels.
A small additive of nitrogen in the shielding gas compensates for the losses