Gas Tungsten Arc Welding (GTAW also referred to as TIG) uses a non-consumable tungsten electrode to create an arc between the electrode and the parent material. This arc creates a molten pool to which a filler wire is added. The success of TIG welding depends on a number of factors, one of which is using the correct type and size of the electrode.
Tungsten alloys are used because of their hardness and resistance to high temperatures. Tungsten can withstand temperatures of up 3,400° C, which makes it suitable for arc welding. Pure tungsten does not perform that well and is susceptible to contamination. For this reason, tungsten electrodes are alloyed with other elements to improve their performance for specific applications. In particular current type and polarity. TIG typically uses DC- or AC.
Tungsten electrodes are colour coded to make it easier for welders to identify them and ensure the correct type is being used. This blog is written to help TIG welders to ensure they have the correct type and size.
Pure Tungsten, Green
Pure tungsten is suitable for use with AC but not DC. Pure tungsten electrodes are 99.5% tungsten, and that gives them the capability to ball or round easily. The ball tip shape is what makes the electrodes good with arc stability. Pure tungsten is also suitable for application of low to medium amperages with magnesium and aluminium alloys. Pure tungstens are cheap but are rarely used in the industry.
Zirconiated tungsten electrodes consist of 99.1% pure tungsten and 0.15-0.4 Zirconium. This type of electrode is often used in place of pure tungsten as they have the ability to retain the balled tip, resist contamination and splitting better than pure tungsten. These characteristics make them perfect for AC welding of aluminium and magnesium. Zirconiated tungsten has high current-carrying ability resulting in a more stable arc. It also handles high amperages very well. Zirconiated tungsten electrodes are not suitable to use with DC.
Thoriated tungsten electrodes contain 97.3% pure tungsten and approximately 2% thorium oxide, which has low radioactivity. These electrodes are used for DC welding of steels and other metals. Thoriated electrodes have an increased current-carrying ability and is the main reason it suits arc welding. The electrodes are easy to use and last a long time. They provide a low consumption rate because they function below their melting point.
They are ideal for welding steel because they maintain a sharp point. Caution is, however, recommended when sharpening the points of thoriated tungsten electrodes.
Welders also prefer thoriated tungsten electrodes for the following reasons:
- Greater resistance to contamination during the welding process
- Suitable for use in medium to high amperage ranges depending on the diameter
- Medium erosion rate
- Tendency to split is average compared to other materials
Lanthanated tungsten electrodes are used with direct current. They typically contain approximately 2% lanthanum and the remainder is tungsten depending on the manufacturer. The addition of lanthanum alloy increases the current carrying capacity of the electrodes. It is claimed that they have improved arc starting and stability, better re-ignition properties with a relatively low consumption rate.
Ceriated tungsten electrodes are alloyed with cerium oxide of volumes of approximately 2%, the remainder is pure tungsten composition. These types of electrodes are often used for low-current AC but can also be used for DC applications.
Because they are intended for low amperages, ceriated tungsten electrodes are ideal for welding thin and intricate parts, in particular, thin sheet metal work.
Some of the compounds that can be used with ceriated electrodes include:
- non-corroding steels
- copper alloys
Ceriated electrodes are preferred in place of pure tungsten because they have improved current-carrying ability at the same diameters and better resistance to contamination.
The benefits of using ceriated electrodes include:
- excellent arc stability
- low erosion rate
- good ignition and re-ignition
Rare-earth tungsten is the newest type of alloyed tungsten and comes with various additives such as rare-earth or hybrid combinations of oxides. They are used for DC current and are not suitable for AC. They are claimed to have the highest current capacity when compared to thoriated, lanthanated and ceriated. This tungsten provides better resistance to contamination, allowing the use of smaller diameter electrodes at a given current. How the electrodes perform will depend on the additives each manufacturer uses to create the electrode. They typically suffer less tungsten splitting when compared to other types of electrodes.
When compared to thoriated tungsten, this type of electrode lasts longer, offsetting the higher initial cost. With that said, for DC welding Rare earth is by far the best choice, followed by throiated and Zirconiated for AC applications in our option.
Size of the Electrode
The size of the electrode you chose comes down to the current you are primarily using. It is not a matter of ‘one size suits all’ circumstances, although a 2.4mm diameter electrode is most probably the most versatile size.
Using too large an electrode for the current, particularly on AC, can create arc starting and stability issues. As a guide, the following diameter electrode should be selected based on the amps/current used.
Sharpening and the tip shape is another important factor when it comes to the performance of the tungsten electrode. This subject will be covered in a future blog.
Now that you have all this information, you may need to review your procedures to ensure your weld process is optimised. Luckily, we can help! Technoweld develops welding procedures with these factors in mind to ensure they are practical and optimised.