Shielding gases are commonly used in welding processes. Shielding gases are used to protect the weld area from atmospheric gases. The selection of the proper gas or gas mixture will depend on the materials being welded.  The best obtainable weld is one which has the same chemical, metallurgical, and physical properties as the base metal itself. To obtain such conditions, the molten weld puddle must be protected from the atmosphere during the welding process, otherwise atmospheric gases such as oxygen and nitrogen will mix in with the weld and cause a porous weak weld.  Also, the wrong choice of a welding gas can leave you with a weak weld.