Gas–Tungsten arc welding (GTAW)

GTAW

The Process

Gas–tungsten arc welding (GTAW) is a process that melts and joins metals by heating them with an arc established between a non consumable tungsten electrode and the metals, as shown in Figure. The torch holding the tungsten

electrode is connected to a shielding gas cylinder as well as one terminal of the power source, as shown in Figure a. The tungsten electrode is usually in contact with a water-cooled copper tube, called the contact tube, as shown in Figure b, which is connected to the welding cable (cable 1) from the terminal. This allows both the welding current from the power source to enter the electrode and the electrode to be cooled to prevent overheating.The workpiece is connected to the other terminal of the power source through a different cable (cable 2). The shielding gas goes through the torch body and is directed by a nozzle toward the weld pool to protect it from the air. Protection from the air is much better in GTAW than in SMAW because an inert gas such as argon or helium is usually used as the shielding gas and because the shielding gas is directed toward the weld pool. For this reason, GTAW is also called tungsten–inert gas (TIG) welding. However, in special occasions a non inert gas can be added in a small quantity to the shielding gas. Therefore, GTAW seems a more appropriate name for this welding process. When a filler rod is needed, for instance, for joining thicker materials, it can be fed either manually or automatically into the arc.

GTAW

Electrodes:-

Tungsten electrodes with 2% cerium or thorium have better electron emissivity, current-carrying capacity, and resistance to contamination than pure tungsten electrodes. As a result, arc starting is easier and the arc is more stable. The electron emissivity refers to the ability of the electrode tip to emit electrons. A lower electron emissivity implies a higher electrode tip temperature required to emit electrons and hence a greater risk of melting the tip.

Shielding gas for GTAW:-

1  Argon: -   

•         Less expensive
•         Better blanket over the weld
•         Weld is narrower with a smaller HAZ
•         Better control of weld in vertical & overhead welding 
•         Low arc voltage advantageous in thin materials welding (up to 1/8" thk.)

2    Helium: -

•         Higher arc voltage & higher welding speed
•         Heavy materials and metals having high heat conductivity, helium is required

3.     Argon-Helium mixture: -

•         If higher heat input is required, than mixture of Ar-He is used
•         The rate of flow required depends on the thickness of the metal to be welded

Advantages and Disadvantages of GTAW: -

Gas–tungsten arc welding is suitable for joining thin sections because of its limited heat inputs. The feeding rate of the filler metal is somewhat independent of the welding current, thus allowing a variation in the relative amount of the fusion of the base metal and the fusion of the filler metal. It can also be used to weld butt joints of thin sheets by fusion alone, that is, without the addition of filler metals or autogenous welding. Since the GTAW process is a very clean welding process, it can be used to weld reactive metals, such as titanium and zirconium, aluminium, and magnesium. 

However, the deposition rate in GTAW is low. Excessive welding currents can cause melting of the tungsten electrode and results in brittle tungsten inclusions in the weld metal.

Advantages: -

• Separate control of current and wire feeding hence, excellent control on root pass
• Produces superior quality welds
• Free from spatters

Limitations: -

• Depositions rates are slower 
• Skilled welder is required
• Difficulty in shielding the weld zone in drafty environments
• Accessibility of torch 

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