TIG Welding, also known as Gas Tungsten Arc Welding (GTAW), is a process that joins metals by heating them with an arc between a tungsten electrode (non-consumable) and the work piece. The process is used with a shielding gas and may also be used with or without the addition of filler metal. The primary variables in TIG Welding are arc voltage (arc length), welding current, travel speed and shielding gas composition. The amount of energy the arc produces is proportional to the current and the voltage. The amount of energy transferred per unit length of weld is inversely proportional to the travel speed. Shielding gases are typically inert to protect the electrode from contamination. The use of helium shielding provides more penetration than argon. The arc, established between the tip of the electrode and the work, generates heat to melt the base metal. Once the arc and weld pool are established, the torch is moved along the joint, and the arc progressively melts the surfaces to be joined. If used, filler wire is usually added to the leading edge of the weld pool to fill the joint. The tungsten electrode can be alloyed with small amounts of active elements to increase emissivity of the electrode; this provides quicker arc starting, greater arc stability, and longer electrode life.