They protect the molten metal in the weld from oxygen and nitrogen in the atmosphere.
Unprotected molten metal can be weakened by oxygen and nitrogen causing a range of problems including weld failure, defects in the weld metal and decreased corrosion resistance.
The choice of shielding gas is determined by the material, the process being used and the performance and properties required by the customer. Shielding gases can be individual gases or mixtures.
Argon and Argon based mixtures predominate the joining of metals. Shielding gases are used with arc welding and cutting process such as MIG, TIG and PAC.
They can strongly influence the properties weld metal and even the shape and size of the weld bead.