Gas Tungsten Arc Welding (GTAW), often called TIG welding, is a precise method that uses an electric arc to join metals. The process relies on a non-consumable electrode to establish and maintain this arc, which generates the intense heat needed to melt the base material and any added filler metal. Because the weld zone is shielded by an inert gas, the resulting weld is exceptionally clean and high-quality. The electrode’s ability to survive the extreme temperatures of the arc depends entirely on its material composition.
Tungsten’s Refractory Nature
Tungsten is the material of choice because it belongs to the class of refractory metals, defined by their extraordinary resistance to heat and wear. Tungsten has the highest melting point of any known metal, approximately 3,422°C (6,192°F). This unparalleled thermal resistance is the primary reason the electrode is classified as non-consumable; it remains solid while the base metals are liquified beneath it.
This high melting point ensures the electrode tip does not vaporize or melt into the weld pool, preventing contamination. The structural integrity of tungsten is further supported by its high density (about 19.3 grams per cubic centimeter). This density contributes to its stability under the thermal stresses of the welding environment.
Tungsten also possesses the lowest coefficient of thermal expansion among all pure metals. This means the electrode resists significant size changes when subjected to the rapid heating and cooling cycles inherent to the welding process. This stability minimizes the risk of the electrode warping or failing prematurely, ensuring a consistent arc path and a longer service life.
Facilitating the Welding Arc
Beyond surviving the heat, tungsten must efficiently generate and sustain the electric arc, a function governed by its electrical properties. The arc is maintained through thermionic emission, which is the release of electrons from the heated electrode surface. As the electrode heats up, electrons gain enough thermal energy to overcome the material’s surface energy barrier and escape into the surrounding gas, forming the current path for the arc.
The ease with which a material releases electrons is quantified by its “electron work function,” the minimum energy required for an electron to escape the surface. Pure tungsten has a work function low enough to facilitate electron emission at high temperatures without melting the electrode. This balance between a high melting point and a manageable work function is unique to tungsten.
Efficient thermionic emission leads directly to better arc stability and easier arc starting, both crucial for precision welding. A stable flow of electrons prevents the arc from wandering or becoming erratic, which would result in a poor-quality, uneven weld. This reliable electron path ensures the welding power is concentrated precisely on the workpiece.
Enhancing Performance with Alloys
While pure tungsten performs the basic function of the GTAW electrode, its performance is often improved significantly through the addition of alloying elements, or dopants, typically metal oxides. These dopants are added to further reduce the electrode’s electron work function. By lowering this energy barrier, the electrode releases electrons more readily and sustains the arc at a lower temperature than pure tungsten.
The practical benefits of these additions are substantial, including easier arc starting, improved stability, and a lower rate of electrode consumption. Common alloying elements include rare earth oxides such as lanthanum (lanthanated), cerium (ceriated), and thorium (thoriated).
Alloying allows the electrode to operate at lower internal temperatures while maintaining a high current-carrying capacity, extending the overall life of the electrode. Ceriated tungsten is valued for its exceptional arc starting capability at low amperages, making it suitable for welding thin materials and precision work. The choice of alloy depends on the specific welding parameters, such as the type of base metal and the required amperage.