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Top1. Introduction
Welding plays an essential role in manufacturing industries for its ability to fabricate large products in ship building, bridges, and complex structures in aerospace and automotive industries. For the foundations of a sustainable global environment, newer welding techniques are desirable which enhance the productivity while minimizing the cost of production. Tungsten inert gas (TIG) welding is the most established fusion welding process which finds wide application in joining similar and dissimilar metal combinations of stainless steel, aluminium, nickel and titanium-based alloys and other non-ferrous materials. In Tungsten inert gas (TIG) welding, a tungsten electrode is used to heat the metals to be welded. The process is thermo-electric in nature and the melting and fusion occurs by the formation of an arc produced between the cathode and anode surfaces separated by a small distance when they are touched to establish a flow of current. A temperature of around 6000ﹾC is generated at the anode. To avoid contamination during the welding process a shielding gas is used which is an inert gas of helium, argon, or a mixture of both. The heat input may be increased or decreased which produces a clean, precise, and superior welded appearance (Roy, 2015). TIG is different from other arc welding processes since it uses non consumable electrodes. This process can weld with less spatter and produce smooth and sound welded joints. If tungsten moves to molten weld pool, it contaminates the pool as this inclusion is hard and brittle (Karadeniz et al., 2007). Sometimes there is a possibility of contamination of the weld metal by filler rod. The conventional TIG welding possesses some demerits like high input current, distortion and slag inclusion, tungsten inclusions, porosity, heat affected zone cracks, undercuts which can be minimized by Pulse TIG welding.
Pulse TIG welding is an advancement to the conventional TIG welding process (Indira Rani & Marpu, 2012). It operates with two current values viz. base current and peak current. The higher value of pulse current helps to melt the metal at regular time interval and background current keeps the arc ionized. Weld metal starts to freeze when the peak current ceases. The process is used for welding of thin sections of ferrous and nonferrous metals. It produces greater bead width in low pulse frequency. The process has widespread use for better arc stabilisation, improvement of penetration, better weld-bead geometry, spatter reduction and for achieving better control on heat input. The joints produced demonstrate lower residual stress and smaller heat affected zone in comparison to conventional TIG welding (Baghel & Nagesh, 2016). Pulse TIG welding is a difficult process than the other conventional welding techniques. Short arc length must be maintained for improvement of penetration. The contact between electrode and workpiece must be avoided to minimize the contamination of tungsten inclusion to the weld pool. Inclusion of foreign particles prevents contamination of tungsten and helps in achieving deeper penetration in a single pass.
A spectacular improvement in weld penetration depth is observed in a modified TIG welding process known as Activated TIG (A-TIG) welding. A-TIG welding process was invented at Paton Institute of electric welding in 1960. In this process, a thin layer of coating (10 to 15µm) of activated flux is used on the joint area before welding by brush or aerosol spray. The paste used in this process is mainly a mixture of oxides, chlorides, and fluorides (binder) in a suitable solvent like acetone or ethanol. These binders help the flux paste to stick to the workpiece as a coated layer. Flux has a vital role in increasing penetration of the welded samples by flow of the liquid metal through convection. There is no influence of chemical composition in welded samples due to the use of flux, but the mechanical strength of the welded joints increases. The weld penetration of thick components like pipes and plates is increased in a single pass welding without edge preparation. Compared to conventional TIG welding, this welding has more advantages in achieving better weld penetration, increasing the depth to width ratio, increasing heat input, achieving higher mechanical properties, and improving the metallurgical characteristics. The weld penetration can be enhanced twice to thrice than the traditional TIG welding process as suggested by Singh et al. (2017).