Various shielding gases used in MIG welding
In MIG welding, a wire electrode is consumed as the weld is produced. During the welding process, in order to prevent the nitrogen and oxygen in the atmosphere from contaminating the weld, a shielding gas is required. The contamination can lead to porosity, fusion defects and embrittlement of the weld metals etc. This is a common problem with all welding process. Based on the specific welding process and the composition of the metal that needed to be fused, the composition of the shielding gas would be determined.
For metals that don't contain iron, inert gases such as argon or helium may be used. While MIG welding aluminum, always 100 percent argon is used as shielded gas. On the other hand, for steel pure argon is not favorable as it is doesn't provide enough weld penetration. Even pure form of carbon dioxide is not used in MIG welding even though it allows deep penetration and is inexpensive. This is because it produces excess spatter and oxide formation.
For some types of steel welding, argon with a smaller amount of carbon dioxide is used. The argon prevents excess oxide formation and the carbon dioxide enables deep penetration. The weld heat and spatter is considerably increased if the carbon dioxide content is more. This happens when the carbon dioxide content is more than 20 percent in the shielding gases. However in the argon carbon dioxide shielding gases, the CO2 content ranges from 10 to 25 percent only.
Usually for MIG welding steel, a shielding gas that consists of argon with small amount of other gases like hydrogen, helium, nitrogen or oxygen is used. For getting maximum penetration during the welding, a mixture that contains 5 percent oxygen is used. While using this mixture, you need to use electrodes that contain deoxidizers. This is to avoid the formation of excess oxides on the new metals by oxygen.
However the mixture of argon and helium are very inert and this leads to the greater concentration of helium thereby increasing the required voltage and temperature of the arc. If the mixture contains up to 75 percent helium, it can be used to weld alloys that don't contain iron. For welding nickel, a mixture of up to 5 percent hydrogen may be used and the hydrogen content should be 25 percent for welding copper. On rare instances, a shielding gas that contains 25 to 50 percent nitrogen may be used to weld copper.