Types of Tungsten Welding Electrodes
Pure tungsten has a number of properties that make it an excellent non-consumable electrode for the GTA welding process. These properties can be improved by adding cerium, lanthanum, thorium, or zirconium to the tungsten.
For TIG welding (GTAW), tungsten electrodes are classified as the following:
■ Pure tungsten, EWP
■ 1% thorium tungsten, EWTh-1
■ 2% thorium tungsten, EWTh-2
■ 1/4% to 1/2% zirconium tungsten, EWZr
■ 2% cerium tungsten, EWCe-2
■ 1% lanthanum tungsten, EWLa-1
■ Alloy not specified, EWG
Types, Color Coding, ISO 6848 & AWSA5.12 Designations of Tungsten electrodes for TIG Welding
Thoriated Tungsten, EWTh-1 and EWTh-2
Thorium oxide (ThO2), when added in percentages of up to 0.6% to tungsten, improves its current-carrying capacity. The addition of 1% to 2% of thorium oxide does not further improve current-carrying capacities. It does, however, help with electron emission. Thorium also increases the serviceable life of the tungsten. The improved electron emission of the thoriated tungsten allows it to carry approximately 20% more current. This also results in a corresponding reduction in electrode tip temperature, resulting in less tungsten erosion and subsequent weld contamination.
Thoriated tungstens also provide a much easier arc starting characteristic than pure or zirconiated tungsten. Thoriated tungstens work well with DCEN. They can maintain a sharpened point well. They are very well suited for making welds on steel, steel alloys (including stainless), nickel alloys, and most other metals other than aluminum or magnesium.
Thoriated tungsten does not work well with alternating current (AC). It is difficult to maintain a balled end, which is required for AC welding.
Pure Tungsten, EWP Welding Electrode
Pure tungsten has the poorest heat resistance and electron emission characteristics of all the tungsten electrodes. It has limited use with AC welding of metals, such as aluminum and magnesium.
Lanthanum Tungsten, EWLa-1.5 & 2
Lanthanum oxide (La2O3) in about 1%-2% concentration is added to tungsten. Lanthanum oxide tungstens are not radioactive. They have current-carrying characteristics similar to those of the thorium tungstens, except that they have a slightly higher arc voltage than thorium and cerium tungstens. This does not normally pose a problem for manual arc welding; however, it will usually require that new test plates be produced to recertify weld procedures.
Cerium Tungsten, EWCe-2
Cerium oxide (CeO2) is added to tungsten to improve the current-carrying capacity
in the same manner as does thorium. These electrodes were developed as replacements for thoriated tungstens because they are not made of a radioactive material. Cerium oxide electrodes have a current-carrying capacity similar to that of pure tungsten; however, they have an improved arc starting and arc stability characteristic, similar to that of thoriated tungstens. They can also provide a longer life than most other electrodes, including thorium.
Cerium tungsten electrodes have a slightly higher arc voltage for a given length than does thoriated tungsten. This very slight increase in voltage does not cause problems
for manual welding. The higher voltage, however, may require that a new weld test be performed to requalify welding procedures. Cerium tungsten may be used for both AC and DC welding currents. Cerium electrodes contain approximately 2% of cerium oxide.
Zirconium Tungsten, EWZr-8
Zirconium oxide (ZrO2) also helps tungsten emit electrons freely. The addition of zirconium to the tungsten has the same effect on the electrode characteristic as thorium, but to a lesser degree. Because zirconium tungstens are more easily melted than thorium tungsten, ZrO2 electrodes can be used with both AC and DC currents. Because of the difficulty in forming the desired balled end on thorium versus zirconium tungstens, they are normally the electrode chosen for AC welding of aluminum and magnesium alloys.
Alloy Not Specified, EWG
The EWG classification is for tungsten whose alloys have been modified by manufacturers. Such alloys have been developed and tested by manufacturers to meet specific welding criteria. Specific alloy compositions are not normally available from manufacturers; however, they do provide welding characteristics for these electrodes.
The current range for different types of Tungsten electrode
Current carrying capacity of tungsten electrode is depends upon:
- Types of tungsten electrode
- Type of shielding gas
- Type of welding polarity
- Type of equipment.
The below figure shows the various types of tungsten electrodes and their current carrying capacity.
Characteristics of Tungsten Electrode & color-coding of tungsten electrode
Color codes of tungsten electrodes are shown in the below figure.
Here Green color is for pure tungsten electrode, Gray for 2% Ceriated, Red for 2% thoriated, Gold & blue are for Lanthanated, White for Zirconated as shown below.