International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol.1, Issue.2, pp-690-699 ISSN: 2249-6645 www.ijmer.com 690 | P a g e Jyoti Prakash 1 ,S.P.Tewari 2 , Bipin Kumar Srivastava 3 1 (Senior research scholar,Mechanical Engineering Department,IT- BHU, Varanasi (U.P.), India) 2 ( Professor, Mechanical Engineering Department,IT- BHU, Varanasi (U.P.), India) 3 (Senior research scholar,Mechanical Engineering Department,IT- BHU, Varanasi (U.P.), India) Abstract: The shielding gas is used to protect the finished weld from the effects of oxygen and nitrogen in the atmosphere. Although the weld metal properties are primarily controlled by the composition of the consumable ,the shielding gas can influence the weld‘s strength, ductility, toughness and corrosion resistance. In general, for a given weldi ng wire ,the higher the oxidation potential of a shielding gas, the lower the strength and toughness of the weld. This occurs because the oxygen and carbon dioxide in the shielding gas increase the number of oxide inclusions and reduce the level of materials such as manganese and silicon in the weld metal. When welding thick aluminium sections with pure argon as the shielding gas, porosity, lack of penetration and fusion defects can occur. The addition of helium to the argon shielding gas can significantly reduce these defects. This is because the high thermal conductivity of helium results in more energy being transferred into the weld. This in turn produces a hotter weld pool, resulting in improved fusion and slower freezing times, allowing any trapped gas more time to escape. This paper deals with the detailed study of shielding gas used for aluminium welding. Key Words: Aluminium alloy, MIG/TIG welding,Shielding gas 1.Introduction Aluminium may be welded by either gas or arc processes, but arc welding is more satisfactory as the area over which the heat is generated is smaller and the speed of welding can thus be increased. The thermal conductivity of aluminium is high, being five times greater than that of steel, and hence with arc welding distortion and any tendency to crack are reduced.[1] Aluminum alloys are typically welded on AC with the gas tungsten arc welding (GTAW) process. Many power sources have ―max penetration‖ indicated when more than 50% of the AC cycle is spent on electrode negative polarity and ―max cleaning‖ when more than 50% of the cycle is on electrode positive polarity.[2] In aluminum welding hydrogen porosity is often a problem. The high stability of aluminum oxide creates an instantaneous oxide film on the wire and base metal surface. Ambient humidity reacts with the surface oxide forming chemical compounds containing water. Due to this natural process, improperly stored welding wire or non-wire brushed weld groove prior to welding may contribute to moisture in the welding arc. The moisture dissociates under the arc and hydrogen dissolves in the weld metal to create porosity upon cooling.[3] Using clean consumables and increasing weld-freezing time through modulated or interrupted spray transfer, aluminum welds have shown reduction in weld bead porosity. Experiments have also demonstrated that additions of Oxygen to Argon and Argon-Helium mixtures greatly stabilize the aluminum weld puddle permitting higher travel speeds. Fig. 1 shows the effect of additions of oxygen through the contact tip, while welding aluminum. The researchers observed a 20% improvement in travel speed. [4]While GMAW welding of high nickel alloys such as Inconel 625 and 600 series, difficulties are often encountered with the sluggishness of the weld puddle. Helium or hydrogen additions to Argon generally help, adding heat to the weld pool and improving the fluidity of the weld. Recent experimental work on micro-additions of CO 2 to Argon, Argon-Helium and Argon-Hydrogen mixtures has shown beneficial effect of stabilizing the arc [5]. Optimized micro additions of CO 2 in the ppm range improve not only the bead appearance and shape (Fig.2), but also the arc speeds. Shielding Gas for Welding of Aluminium Alloys by TIG/MIG Welding-A Review