Gas Tungsten Arc Welding (GTAW) is also known as Tungsten Inert Gas Welding (TIG Welding). It is an arc welding process that uses a non-consumable electrode made up of Tungsten (or alloy of tungsten). Arc is generated between the tungsten electrode and work piece to establish the weld pool. Inert gas (Argon or Helium) is used to shield the molten weld pool during welding. Additional filler metal may be added in this process.
The tungsten (or tungsten alloy) electrode is held in a torch. Shielding gas is fed through the torch to provide an inert atmosphere that protects the electrode and the molten weld pool. The arc is produced by the passing of electric current from the tip of the electrode to the workpiece through conductive ionised shielding gas. If required, filler metal is also added to the weld pool to fill the joint. The Gas Tungsten Arc Welding (GTAW) process is illustrated in the following figure:
Welding current: GTAW can be used with direct current (DC) as well as alternating current (AC), Choice of the current largely depends upon the metal to be welded. Direct current with electrode negative (DCEN) also known as “straight polarity” is commonly used in GTAW since it facilitates deeper penetration and fast welding speed. Direct current with electrode positive (DCEP) also known as “reverse polarity” is avoided in GTAW because it causes overheating of electrodes.
Components of GTAW: The basic components of GTAW process are:
- Power source
- Welding Torch
- Shielding Gas
Power Source: Constant current power source is most common for GTAW. A transformer-rectifier power source is used to supply the required power.
Electrode: Either pure tungsten or tungsten alloys are used as GTAW electrode. The reason for using tungsten as electrode is it’s high melting point (34100C or 61700F). In GTAW the electrode acts as an electrical terminal and generates the required heat for melting the base metal, unlike the other conventional welding process such as SMAW/GMAW the electrode doesn’t get consumed during the welding. Additional filler metal is fed, if required.
GTAW electrodes come in a variety of sizes and lengths. Classification of tungsten electrodes are done on the basis of chemical composition. These electrodes are color coded to distinguish it from others. The color appears at the tip of the electrode. Different types of TIG welding electrodes are as follows;
- Pure Tungsten (Color Code: Green)
- Thoriated (Color Code: Red)
- Ceriated (Color Code: Orange)
- Lanthanated (Color Code: Gold)
- Zirconiated (Color Code: Brown)
- Rare Earth (Color Code: Gray)
Welding Torch: The GTAW welding torch holds the tungsten electrode and also provides the means for conveying shielding gas to the molten weld pool through a nozzle. A clamping device called as collet is provided in the torch for holding the tungsten electrode. Collets are generally made up of copper alloys. Nozzles are provided onto the head of the torch to provide direction for the flow of the shielding gas. Nozzles are generally made up of ceramic materials.
Shielding Gas: Shielding gas is supplied through the nozzle of torch to prevent the atmospheric contamination of molten weld pool. Inert gases such as Argon or Helium or a combination of both can be used as shielding gas.
Argon provides smoother and quieter arc with reduced penetration, apart from this cost of argon is less as compared to Helium. Due to its reduced penetration Argon is used for joining of thinner metals. On the other hand, Helium provides higher heat penetration hence useful for thicker materials and materials with high heat conductivity such as Aluminium and Copper.
Please watch the following video for a better understanding of GTAW/TIG Welding: