In addition to the specific alloy being welded, the choice of electrode depends on the joint design, the conditions of the metal's surface and the specific process that will be used. The weld metal should generally be similar to the base metal. The welding wire should also contain small concentrations of metal with deoxidizing metals such as aluminum, manganese, silicon and titanium. This will prevent the oxygen in the air from entering the weld and corroding it from the inside. The electrode's diameter depends primarily on the specific welding process, but is generally in the 0.7- to 2.4-mm range. They can be as large as 4 mm, and the smallest electrodes and the short-circuiting metal transfer process uses electrodes that are less than 1.14 mm in diameter.
A shielding gas is needed in MIG welding to protect the weld from chemically active gases in the atmosphere while it's being formed. Nitrogen and oxygen can react with the electrode to cause defects and porosity in the weld metal. The shielding gas may be pure argon or helium for aluminum and other nonferrous metals. Steel, on the other hand, requires a specific mixture of argon and carbon dioxide to achieve the required weld penetration without excessive spatter. The shielding gas for steel generally contains 10 to 25 percent carbon dioxide.
The basic technique for MIG welding is fairly simple and only requires the operator to maintain the welding gun in the proper orientation and position within the weld area. The distance between the tip of the welding gun and the surface of the metal is known as the stick-out distance and should be kept in a narrow range. The stick-out distance varies according to the application, but is generally in the 1/4-inch to 1/2-inch range. The orientation of the gun is another important factor in MIG welding and should be between the two pieces being welded. The welding gun should generally be kept vertical to the welding surface, although this can vary somewhat according the composition of the shielding gas.