MIG Weld How To - SHIELDING GAS

General. Most metals exhibit a strong tendency to combine with oxygen (to form oxides) and to a lesser extent with nitrogen (to form metal nitrides). Oxygen will also react with carbon to form carbon monoxide gas. These reaction products are all a source of weld deficiencies in the form of: fusion defects due to oxides; loss of strength due to porosity, oxides and nitrides; and weld metal embrittlement due to dissolved oxides and nitrides. These reaction products are easily formed since the atmosphere is more or less composed of 80 percent nitrogen and 20 percent oxygen. The primary function of the shielding gas is to exclude the surrounding atmosphere from contact with the molten weld metal.

The shielding gas will also have a pronounced effect upon the following aspects of the welding operation and the resultant weld:

1. Arc characteristics
2. Mode of metal transfer
3. Penetration and weld bead profile
4. Speed of welding
5. Undercutting tendency
6. Cleaning action



The Inert Shielding Gases — Argon and Helium. Argon and helium are inert gases. These gases and mixtures of the two are necessarily used in the welding of nonferrous metals and also widely used to weld stainless steel and low alloy steels.

Basic differences between argon and helium are:

1. Density
2. Thermal conductivity
3. Arc characteristics

Various shielding gases

Relative effect of O2 versus CO2 additions to the argon shield.