TUNGSTEN INERT GAS WELDING (TIG)
In this process, an arc is struck between a non consumable tungsten electrode and the base metal. The tungsten electrode is called non consumable electrode as it melts only at around 2800 ° C and in the arc heat, any other metal melts at lower temperatures. The TIG arc is shielded by inert gases like argon, helium or a mixture of both. A filler wire may or may not be used. AC power supply is used for aluminium alloys and DC for all other metals. Normally, when an arc is struck between an anode and cathode, more heat is observed at the anode and less heat at the cathode. Hence, in TIG welding the tungsten electrode is always connected to the negative polarity of the power source so that the electrode does not melt. Since, the tungsten has high melting point, it is brittle and breaks easily. Hence, generally the electrode is not touched with the plate to strike arc but a High Frequency (HF) unit is used to initiate the arc. The HF unit is actually a high voltage, high frequency unit that generates around 3 kV voltage and 3 MHz frequency which ionises the gap between the electrode and the plate thus enabling the arc to jump from the electrode to the plate. This is an ideal process for welding of non-ferrous metals and stainless steel in limited thickness. It is used for root pass in pressure vessels where welding from inside is not possible.