Identifying, selecting, and preparing tungsten electrodes
March 7, 2006
Choosing one of the six commonly available tungsten electrodes is a crucial first step in successful gas tungsten arc welding (GTAW). In addition, tip preparation is critical. The electrode choices are pure tungsten, 2 percent thoriated, 2 percent ceriated, 1.5 percent lanthanated, zirconiated, and rare earth. The end preparations are balled, pointed, and truncated.
Tungsten is a rare metallic element used for manufacturing gas tungsten arc welding (GTAW) electrodes. The GTAW process relies on tungsten's hardness and high-temperature resistance to carry the welding current to the arc. Tungsten has the highest melting point of any metal, 3,410 degrees Celsius.
These nonconsumable electrodes come in a variety of sizes and lengths and are composed of either pure tungsten or an alloy of tungsten and other rare-earth elements and oxides. Choosing an electrode for GTAW depends on the base material type and thickness and whether you weld with alternating current (AC) or direct current (DC). Which one of three end preparations you choose, balled, pointed, or truncated, also is crucial in optimizing the results and preventing contamination and rework.
Each electrode is color-coded to eliminate confusion over its type. The color appears at the tip of the electrode.
Pure tungsten electrodes (AWS classification EWP) contain 99.50 percent tungsten, have the highest consumption rate of all electrodes, and typically are less expensive than their alloyed counterparts.
These electrodes form a clean, balled tip when heated and provide great arc stability for AC welding with a balanced wave. Pure tungsten also provides good arc stability for AC sine wave welding, especially on aluminum and magnesium. It is not typically used for DC welding because it does not provide the strong arc starts associated with thoriated or ceriated electrodes.
Thoriated tungsten electrodes (AWS classification EWTh-2) contain a minimum of 97.30 percent tungsten and 1.70 to 2.20 percent thorium and are called 2 percent thoriated. They are the most commonly used electrodes today and are preferred for their longevity and ease of use. Thorium increases the electron emission qualities of the electrode, which improves arc starts and allows for a higher current-carrying capacity. This electrode operates far below its melting temperature, which results in a considerably lower rate of consumption and eliminates arc wandering for greater stability. Compared with other electrodes, thoriated electrodes deposit less tungsten into the weld puddle, so they cause less weld contamination.
These electrodes are used mainly for specialty AC welding (such as thin-gauge aluminum and material less than 0.060 inch) and DC welding, either electrode negative or straight polarity, on carbon steel, stainless steel, nickel, and titanium.
During manufacturing, thorium is evenly dispersed throughout the electrode, which helps the tungsten maintain its sharpened edge—the ideal electrode shape for welding thin steel—after grinding. Note: Thorium is radioactive; therefore, you must always follow the manufacturer's warnings, instructions, and the Material Safety Data Sheet (MSDS) for its use.
Ceriated tungsten electrodes (AWS classification EWCe-2) contain a minimum of 97.30 percent tungsten and 1.80 to 2.20 percent cerium and are referred to as 2 percent ceriated. These electrodes perform best in DC welding at low current settings but can be used proficiently in AC processes. With its excellent arc starts at low amperages, ceriated tungsten has become popular in such applications as orbital tube and pipe fabricating, thin sheet metal work, and jobs involving small and delicate parts. Like thorium, it is best used to weld carbon steel, stainless steel, nickel alloys, and titanium, and in some cases it can replace 2 percent thoriated electrodes. Ceriated tungsten has slightly different electrical characteristics than thorium, but most welders can't tell the difference.
Using ceriated electrodes at higher amperages is not recommended because higher amperages cause the oxides to migrate quickly to the heat at the tip, removing the oxide content and nullifying its process benefits.
Lanthanated tungsten electrodes (AWS classification EWLa-1.5) contain a minimum of 97.80 percent tungsten and 1.30 percent to 1.70 percent lanthanum, or lanthana, and are known as 1.5 percent lanthanated. These electrodes have excellent arc starting, a low
burnoff rate, good arc stability, and excellent reignition characteristics—many of the same advantages as ceriated electrodes. Lanthanated electrodes also share the conductivity characteristics of 2 percent thoriated tungsten. In some cases, 1.5 percent lanthanated can replace 2 percent thoriated without having to make significant welding program changes.
Lanthanated tungsten electrodes are ideal if you want to optimize your welding capabilities. They work well on AC or DC electrode negative with a pointed end, or they can be balled for use with AC sine wave power sources. Lanthanated tungsten maintains a sharpened point well, which is an advantage for welding steel and stainless steel on DC or AC from square wave power sources.
Unlike thoriated tungsten, these electrodes are suitable for AC welding and, like ceriated electrodes, allow the arc to be started and maintained at lower voltages. Compared with pure tungsten, the addition of 1.5 percent lanthana increases the maximum current-carrying capacity by approximately 50 percent for a given electrode size.
Zirconiated tungsten electrodes (AWS classification EWZr-1) contain a minimum of 99.10 percent tungsten and 0.15 to 0.40 percent zirconium. A zirconiated tungsten electrode produces an extremely stable arc and resists tungsten spitting. It is ideal for AC welding because it retains a balled tip and has a high resistance to contamination. Its current-carrying capability is equal to or greater than that of thoriated tungsten. Under no circumstances is zirconiated recommended for DC welding.
Rare-earth tungsten electrodes (AWS classification EWG) contain unspecified additives of rare-earth oxides or hybrid combinations of different oxides, but manufacturers are required to identify each additive and its percentage on the package. Depending on the additives, desired results can include a stable arc in both AC and DC processes, greater longevity than thoriated tungsten, the ability to use a smaller-diameter electrode for the same job, use of a higher current for a similar-sized electrode, and less tungsten spitting.
After selecting a type of electrode, the next step is to select an end preparation. The three choices are balled, pointed, and truncated.
Typical current ranges for electrons with argon shielding.
A balled tip generally is used on pure tungsten and zirconiated electrodes and is suggested for use with the AC process on sine wave and conventional square wave GTAW machines. To ball the end of the tungsten properly, simply apply the AC amperage recommended for a given electrode diameter (seeFigure 1), and a ball will form on the end of the electrode. The diameter of the balled end should not exceed 1.5 times the diameter of the electrode (for example, a 1/8-in. electrode should form a 3/16-in.-diameter end). A larger sphere at the tip of the electrode can reduce arc stability. It also can fall off and contaminate the weld.
Preparing tungsten for DC electrode negative welding and AC with wave shaping power sources.
A pointed and/or truncated tip (for pure tungsten, ceriated, lanthanated, and thoriated types) should be used for inverter AC and DC welding processes. To grind the tungsten properly, use a grinding wheel specially designated for tungsten grinding (to prevent contamination) and one that is made of Borazon® or diamond (to resist tungsten's hardness). Note: If you are grinding thoriated tungsten, make sure you control and collect the dust; have an adequate ventilation system at the grinding station; and follow the manufacturer's warnings, instructions, and MSDS.
Grind the tungsten straight on the wheel versus at a 90-degree angle (seeFigure 2) to ensure that the grind marks run the length of the electrode. Doing so reduces the presence of ridges on the tungsten that could create arc wandering or melt into the weld puddle, causing contamination.
Generally, you will want to grind the taper on the tungsten to a distance of no more than 2.5 times the electrode diameter (for example, for a 1/8-in. electrode, grind a surface 1/4 to 5/16 in. long). Grinding the tungsten to a taper eases the transition of arc starting and creates a more focused arc for better welding performance.
When welding with low current on thin material (from 0.005 to 0.040 in.), it is best to grind the tungsten to a point. A pointed tip allows the welding current to transfer in a focused arc and helps prevent thin metals, such as aluminum, from becoming distorted. Using pointed tungsten for higher-current applications is not recommended, because the higher current can blow off the tip of the tungsten and cause weld puddle contamination.
For higher-current applications, it is best to grind a truncated tip. To achieve this shape, first grind the tungsten to a taper as explained previously, then grind a 0.010- to 0.030-in. flat land on the end of the tungsten. This flat land helps prevent the tungsten from being transferred across the arc. It also prevents a ball from forming.