Tungsten Inert Gas Welding (TIG) is one of the two types of gas shielded arc welding process as the other one is Metal Inert Gas Welding (MIG). Also in dual-shield mode ofFlux Core Arc Welding (FCAW), an additional gas supply is provided along with self-shielding of core flux, due to which FCAW is also listed in gas shielded arc processes. Working principle of TIG welding is based on the production of arc using high melting and non-consumable tungsten electrode that is why this process is referred to as (TAGS) tungsten arc gas-shielded which is also known by other names; argon arc welding, tungsten inert gas and gas tungsten arc welding. Gas Shielding is used to protect weld puddle from atmospheric contamination. These gases are transparent so the weld puddle is made visible to a welder.Some distinct features of TIG process are In TIG no flux is used and there is no fear of corrosion due to flux entrapment. No slagging that contributes to elimination of post weld cleaning. Since there is no spark and fumes, and that is why sound weld with perfect definition is obtained.

A non-consumable tungsten electrode is used to produce an arc of considerable high temperature that assists to melt work metal. Air is driven out due to the envelope of inert gas that prevents the weldment, tungsten electrode and heat affected zone from oxidation. Contrary to MIG, tungsten electrode is not consumed rather it provides arc to melt the additional filler metal. Likewise oxyacetylene welding, filler metal is fed to the weld puddle. Normally there are two modes in TIG; semi-automatic or automatic. In semi-automatic, operator has to configure current and gas flow settings and then manually deals with the torch and filler road. However in automatic the operator sets the travel speed, length of arc, gas flow rate, filler rod position and then observes the operation and controls whereby it is needed. In early times TIG was first introduced to weld magnesium to minimize rapid corrosion problems. The technique was successfully applied to achieve best results with no resulting corrosion. This process was introduced with distinct features as compared to other arc processes. For shielding, gas flow rate can be controlled by flow rate and manifold. During welding, gas flow not only shields the weld and filler metal but also protects the torch. There is also a gas delay feature that permits gas to flow for preset time, after the welding current has been shut off. TIG is applied in both welding of ferrous and non-ferrous metals with wide range of thickness not above 8mm. Both alternating (AC) and direct (DC) current supplies are used with current of range 15 to 350 amps. With the use of small diameter electrodes and suitable current range, the process is applied in both repair work and new manufacturing. Although it is comparatively slower than other arc processes but it produce high quality welds with perfect definition in aluminum, carbon steel and stainless steel family. DC source is used for welding of stainless steel, carbon steel, nickel and copper alloys. On the other hand AC is suitable for the welding of Aluminum and its alloys. EquipmentA. TIG Torch Different size of torches are available for different current carrying capacities. Its assembly includes a switch for different welding operations. The torch casing holds gas nozzles and electrode. B. Additional Features 1. Water cooling system Welding with high amperages means cooling is required. A water cooling system is installed in which water is circulated through the torch to keep it cool. Older systems were connected with additional cooling system but now a days it is installed inside the modified equipment. The water flow is controlled either by foot pedal or torch. 2. DC suppression In Aluminum and magnesium alloys welding, formation of surface oxide is major concern. In Aluminum welding as the arc is formed between the tungsten and aluminum (dissimilar metals) chances are that AC is converted to DC mode. So a DC suppressor is used to prevent the conversion of AC to DC.3. Contactor This feature puts the arc to be extinguished when delay shielding is required for cooling purposes. In this way it not only protects weld but also provides means of safety to the welder. This process is also controlled by torch and foot pedal. 4. Arc Initiation Arc initiation by touching the electrode with the base metal can cause serious problems of contamination and may lead to arc un-stability. Obviously tungsten inclusion can cause localized hard spots. On the other hand the transfer of base metal to tungsten electrode can also cause contamination and consequently un-stabilize the arc. For the electrode not to touch with the work piece, high frequency spark is produced that causes some sort of ionization in spark gap. The welding current continues to flow due to this ionized spark gap. This feature also contributes to enhance the life span of tungsten electrode. Listed below are the arc initiation methods that are normally being used in modern equipment.a. Scratch start In this method the electrode is brought down to the work piece and gently scratched to initiate the arc. As it is stated above that the slight contact can cause the problem of contamination. This method is applied only in those applications where contamination is less and no serious problems of weld are generated.

b. Lift up In this method the electrode is brought down to make contact with work at the initial point of welding. Torch switch is pressed that creates contact and still no current is produced. When the electrode is lifted up making a gap, the current continues to flow and rapidly increases to the pre-set value. This rapid increase in current is referred to as popular term slope up.c. Slope up Slope up is the rapid rise of welding current to preset current on the power source. By the use of switch the welder can set the time required for the rise of current to the selected amperages. This feature offers less contamination and reduces the danger of burning thin sheets at the initial point of welding.d. Slope down Slope down is the gradual decrease of welding current to zero. It is referred to as the time taken to decrease the current until the arc is extinguished. Well this feature is used to fill the crater (weld imperfection) at the end of the weld. Normally this function is operated with the help of torch switch and foot pedal.C. Electrode used in TIG Tungsten is high melting point metal which is about 3000 degree Celsius. In TIG process pure tungsten with its best thermal and electrical conductivity is used. Now a days alloyed tungsten with 1-2% thorium and zirconium is applied for better arc stability and improved electrode life. Two types of alloyed tungsten electrodes are; thorated electrode for DC mode and zirconated for AC mode. It is also noticeable that electrode diameter depends on current polarity. Electrode Grinding Electrode grinding is one of the major concern in achieving consistent performance in welds. Electrodes are ground according to the application (joint). There are two ways for electrode grinding; machine grinding and manual grinding. Machine grinding results in consistent grinding. However care must be take while manual grinding.Electrode shape 1. Electrode is usually ground to required vertex angle. For DC usage the electrode is ground to the length of 1-3 times the diameter of electrode. Point length of electrode = 1-3 x Diameter of electrode.In this way the lower the current the longer will be the point length. Electrodes are available normally within the diameter range of 0.8, 1.6, 3.2, 4.0 and 6.4 mm.2. For AC usage the side of electrode end is ground to 45 degree, remaining the front side to be flat. This gives the spherical shape as shown in the figure.

Electrode installation in torchWell this one is simple. The electrode is installed in torch and held tightly with the help of grippers knows as sockets. They can grip and hold the electrodes of various diameters. D. Gas Nozzles These nozzles are of high temperature ceramic materials to bear high welding temperature. As it is stated above some nozzles are equipped with water cool system. Only the right choice of nozzle size gives better protection of weldment, weld pool and filler material. Various shapes of nozzles are available for different kinds of jobs. Small size nozzles are available for confined areas. Long shapes are available for deep groove joints. E. Gas lens Gas lens is provided to prevent the turbulence of gas flow. Poor gas flow may not give good shielding service hence gas lens is used to provide consistent and smooth gas flow to shield the weld puddle. With the help of gas lens, electrode extension can also be achieved for access of complicated areas.