Tungsten is a metal recognized for its extreme heat resistance, boasting the highest melting point of all known elements. This makes it the standard material for electrodes in high-temperature applications, such as arc welding. To improve the performance of pure tungsten, it is often alloyed with small amounts of thorium oxide, a process known as thoriation. This addition enhances the electrode’s ability to emit electrons, allowing it to function efficiently at high current levels without overheating.
Defining Thoriated Tungsten
Thoriated tungsten is a composite material consisting of pure tungsten alloyed with a small percentage of thorium dioxide, also called thoria. The most common formulations are WT10 and WT20, which contain a nominal 1% or 2% thorium oxide by weight, respectively. This oxide is dispersed uniformly throughout the tungsten matrix using powder metallurgy techniques during the manufacturing process.
The primary function of the thoria addition is to lower the electrode’s work function. The work function is the minimum energy required to remove an electron from the surface of a solid material, a process known as thermionic emission. By lowering this value, the electrode requires less heat energy to release the electrons needed to sustain an electrical arc. This allows the tungsten to operate at a lower temperature while still maintaining a high current capacity, which in turn reduces the electrode’s consumption rate and extends its operational life.
Primary Use in TIG Welding
Thoriated tungsten is primarily utilized as a non-consumable electrode in Tungsten Inert Gas (TIG) welding, also known as Gas Tungsten Arc Welding (GTAW). In this process, the electrode generates a highly stable electric arc that melts the base metal and filler material to form a clean, precise weld. Thoriated electrodes are particularly preferred for use with direct current (DC) in the electrode negative (DCEN) polarity.
The unique properties of the thoriated material allow it to maintain a sharply pointed tip during DC welding. This sharpened tip focuses the arc, which is essential for achieving deep, narrow penetration and precise control. The stability and high current density of thoriated tungsten make it the historical choice for welding materials such as carbon steel, stainless steel, nickel alloys, and titanium.
The addition of thoria ensures exceptionally easy arc ignition, allowing the arc to start reliably with minimal voltage. Once started, the arc remains highly stable and resists wandering, which is a major advantage for welders performing delicate or long-duration welds. Furthermore, thoriated tungsten has a higher current-carrying capacity, estimated to be up to 20% greater than pure tungsten. This permits the welding of thicker materials or the use of higher amperage without risking electrode melting or weld contamination.
Other High-Performance Applications
While TIG welding represents its largest market, thoriated tungsten is employed in other specialized applications requiring stable electron emission and high temperature resistance. For instance, thoriated tungsten is sometimes used in the filaments and cathodes of high-power microwave tubes and specialized X-ray tubes. These components rely on a stable and continuous stream of electrons to function effectively. The material is also utilized in plasma processes, such as plasma spraying, where it acts as an electron source to generate the superheated plasma required for surface coating.
Handling, Safety, and Modern Alternatives
The safety concern with thoriated tungsten stems from the radioactivity of thorium, an alpha particle emitter with a long half-life. Although the electrode itself poses a negligible external radiation risk during storage or welding, a hazard arises when the electrode is prepared for use.
To maintain arc stability and precision, the tip of the electrode must be sharpened frequently on a grinding wheel. This grinding process releases fine dust particles containing thorium oxide into the air. If inhaled, these radioactive particles can pose an internal health risk due to the alpha radiation they emit. Consequently, safety protocols mandate the use of a dedicated grinder equipped with a local exhaust ventilation system and often a respirator to prevent inhalation.
Due to these handling and disposal concerns, many industries and regulatory bodies are moving away from thoriated tungsten. Non-radioactive alternatives have been developed that offer comparable or superior performance for most welding tasks. These modern alternatives include lanthanated tungsten, ceriated tungsten, and zirconiated tungsten. Lanthanated electrodes, for example, have become a popular replacement, providing excellent arc stability in DC applications without the associated health risks of thorium.