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Metal inert gas (MIG) welding uses a wire electrode that’s consumed as the weld is produced. This process also requires a shielding gas to prevent the nitrogen and oxygen in the atmosphere from contaminating the weld while it's being formed. The composition of the shielding gas depends primarily on the specific welding process and the composition of the metal you want to fuse.
Pure Shielding Gases
Inert gases such as argon or helium may be used for metals that don’t contain iron. For example, aluminum is almost always MIG welded by using 100 percent argon as the shielding gas. For steel, however, pure argon doesn’t provide enough weld penetration and helium tends to cause excess spatter.
Pure carbon dioxide allows for a deep penetration and is inexpensive. But it also produces excess spatter and oxide formation, which weakens the weld. Pure carbon dioxide is therefore rarely used as a shielding gas in MIG welding.
Argon and Carbon Dioxide Mixtures
A shielding gas composed primarily of argon with a smaller amount of carbon dioxide is used occasionally for some types of steel. The argon prevents excess oxide formation and the carbon dioxide allows for deeper penetration. Greater carbon dioxide content generally increases the weld heat and also increases spatter, especially when it comprises more than 20 percent of the shielding gas. Argon/carbon dioxide shielding gasses typically contain 10 to 25 percent carbon dioxide.
Additional Mixtures with Argon
Most steel is MIG welded with a shielding gas that contains mostly argon with a small amount of another gas such as hydrogen, helium, nitrogen or oxygen. A mixture that contains a maximum of 5 percent oxygen can provide greater weld penetration. But this mixture also requires you to use electrodes that contain deoxidizers, so that the oxygen doesn't form excess oxides on the new metal.
Argon-helium mixtures are very inert and a greater concentration of helium will increase the voltage needed and the temperature of the arc. A mixture that contains up to 75 percent helium may be used to weld alloys that don’t contain iron. A mixture of up to 5 percent hydrogen may sometimes be used to weld nickel and 25 percent hydrogen may be used to weld copper. A shielding gas that contains from 25 percent to 50 percent nitrogen also may be used to weld copper on rare occasions.