The selection and preparation of tungsten electrodes for GTAW is essential to optimize results and prevent contamination and rework. Getty Images
Tungsten is a rare metal element used to make gas tungsten arc welding (GTAW) electrodes. The GTAW process relies on the hardness and high temperature resistance of tungsten to transfer the welding current to the arc. The melting point of tungsten is the highest among all metals, at 3,410 degrees Celsius.
These non-consumable electrodes come in a variety of sizes and lengths, and are composed of pure tungsten or alloys of tungsten and other rare earth elements and oxides. The choice of electrode for GTAW depends on the type and thickness of the substrate, and whether alternating current (AC) or direct current (DC) is used for welding. Which of the three end preparations you choose, spherical, pointed, or truncated, is also crucial for optimizing results and preventing contamination and rework.
Each electrode is color coded to eliminate confusion about its type. The color appears on the tip of the electrode.
Pure tungsten electrodes (AWS classification EWP) contain 99.50% tungsten, which has the highest consumption rate of all electrodes, and is generally cheaper than alloy electrodes.
These electrodes form a clean spherical tip when heated and provide excellent arc stability for AC welding with balanced waves. Pure tungsten also provides good arc stability for AC sine wave welding, especially on aluminum and magnesium. It is usually not used for DC welding because it does not provide the strong arc start associated with thorium or cerium electrodes. It is not recommended to use pure tungsten on inverter-based machines; for best results, use sharp cerium or lanthanide electrodes.
Thorium tungsten electrodes (AWS classification EWTh-1 and EWTh-2) contain at least 97.30% tungsten and 0.8% to 2.20% thorium. There are two types: EWTh-1 and EWTh-2, containing 1% and 2%, respectively. Respectively. They are commonly used electrodes and are favored for their long service life and ease of use. Thorium improves the electron emission quality of the electrode, thereby improving arc starting and allowing higher current carrying capacity. The electrode operates far below its melting temperature, which greatly reduces the consumption rate and eliminates arc drift, thereby improving stability. Compared with other electrodes, thorium electrodes deposit less tungsten in the molten pool, so they cause less weld pollution.
These electrodes are mainly used for direct current electrode negative (DCEN) welding of carbon steel, stainless steel, nickel and titanium, as well as some special AC welding (such as thin aluminum applications).
During the manufacturing process, thorium is evenly dispersed throughout the electrode, which helps tungsten to maintain its sharp edges after grinding-this is the ideal electrode shape for welding thin steel. Note: Thorium is radioactive, so you must always follow the manufacturer’s warnings, instructions and material safety data sheet (MSDS) when using it.
Cerium tungsten electrode (AWS classification EWCe-2) contains at least 97.30% tungsten and 1.80% to 2.20% cerium, and is called 2% cerium. These electrodes perform best in DC welding at low current settings, but can be skillfully used in AC processes. With its excellent arc start at low amperage, cerium tungsten is popular in applications such as rail tube and pipe manufacturing, sheet metal processing, and work involving small and precise parts. Like thorium, it is best used for welding carbon steel, stainless steel, nickel alloys and titanium. In some cases, it can replace 2% thorium electrodes. The electrical properties of cerium tungsten and thorium are slightly different, but most welders cannot distinguish them.
The use of a higher amperage cerium electrode is not recommended, because higher amperage will cause the oxide to quickly migrate to the tip heat, remove the oxide content and invalidate the process advantages.
Use pointed and/or truncated tips (for pure tungsten, cerium, lanthanum and thorium types) for inverter AC and DC welding processes.
Lanthanum tungsten electrodes (AWS classifications EWLa-1, EWLa-1.5 and EWLa-2) contain at least 97.30% tungsten and 0.8% to 2.20% lanthanum or lanthanum, and are called EWLa-1, EWLa-1.5 and EWLa-2 Lanthanum Department of elements. These electrodes have excellent arc starting ability, low burnout rate, good arc stability and excellent reignition characteristics-many of the same advantages as cerium electrodes. Lanthanide electrodes also have the conductive properties of 2% thorium tungsten. In some cases, lanthanum-tungsten can replace thorium-tungsten without major changes to the welding procedure.
If you want to optimize the welding ability, lanthanum tungsten electrode is the ideal choice. They are suitable for AC or DCEN with tip, or they can be used with AC sine wave power supply. Lanthanum and tungsten can maintain a sharp tip very well, which is an advantage for welding steel and stainless steel on DC or AC using a square wave power supply.
Unlike thorium tungsten, these electrodes are suitable for AC welding and, like cerium electrodes, allow the arc to be started and maintained at a lower voltage. Compared with pure tungsten, for a given electrode size, the addition of lanthanum oxide increases the maximum current-carrying capacity by approximately 50%.
The zirconium tungsten electrode (AWS classification EWZr-1) contains at least 99.10% tungsten and 0.15% to 0.40% zirconium. The zirconium tungsten electrode can generate an extremely stable arc and prevent tungsten spatter. It is an ideal choice for AC welding because it retains a spherical tip and has high contamination resistance. Its current carrying capacity is equal to or greater than thorium tungsten. It is not recommended to use zirconium for DC welding under any circumstances.
The rare earth tungsten electrode (AWS classification EWG) contains unspecified rare earth oxide additives or a mixed combination of different oxides, but the manufacturer needs to indicate each additive and its percentage on the package. Depending on the additive, the desired results may include generating a stable arc during AC and DC processes, a longer life than thorium tungsten, the ability to use smaller diameter electrodes in the same job, and the use of electrodes of similar size Higher current, and less tungsten spatter.
After selecting the electrode type, the next step is to select the end preparation. The three options are spherical, pointed and truncated.
The spherical tip is usually used for pure tungsten and zirconium electrodes and is recommended for AC processes on sine wave and traditional square wave GTAW machines. To correctly terraform the end of the tungsten, simply apply the AC current recommended for a given electrode diameter (see Figure 1), and a ball will be formed at the end of the electrode.
The diameter of the spherical end should not exceed 1.5 times the diameter of the electrode (for example, a 1/8-inch electrode should form a 3/16-inch diameter end). A larger sphere at the tip of the electrode reduces arc stability. It may also fall off and contaminate the weld.
Tips and/or truncated tips (for pure tungsten, cerium, lanthanum and thorium types) are used in inverter AC and DC welding processes.
To grind tungsten properly, use a grinding wheel specifically designed for grinding tungsten (to prevent contamination) and a grinding wheel made of borax or diamond (to resist the hardness of tungsten). Note: If you are grinding thorium tungsten, please make sure to control and collect dust; the grinding station has adequate ventilation system; and follow the manufacturer’s warnings, instructions and MSDS.
Grind the tungsten directly on the wheel at a 90 degree angle (see Figure 2) to ensure that the grinding marks extend along the length of the electrode. Doing so can reduce the presence of ridges on tungsten, which may cause arc drift or melt into the weld pool, resulting in contamination.
Generally, you want to grind the taper on tungsten to no more than 2.5 times the electrode diameter (for example, for a 1/8-inch electrode, the ground surface is 1/4 to 5/16 inches long). Grinding tungsten into a cone can simplify the transition of arc starting, and produce a more concentrated arc, so as to obtain better welding performance.
When welding on thin materials (0.005 to 0.040 inches) at low current, it is best to grind the tungsten to a point. The tip allows the welding current to be transmitted in the focused arc and helps prevent deformation of thin metals such as aluminum. It is not recommended to use pointed tungsten for higher current applications because the higher current will blow away the tip of the tungsten and cause contamination of the weld pool.
For higher current applications, it is best to grind the truncated tip. To obtain this shape, the tungsten is first ground to the taper described above, and then ground to 0.010 to 0.030 inches. Flat ground at the end of tungsten. This flat ground helps prevent tungsten from transferring through the arc. It also prevents the formation of balls.
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Post time: Aug-23-2021