Effect of GTAW and EBW on Titanium Alloy - Ti6Al4V

摘要

Titanium and its alloy present in a range of high specific strength materials with good fatigue and corrosion resistance. The density of Titanium is half of that of Steel. Titanium exists in two allotropic phases i.e. α-phase and β- phase. The HCP structured a is stable up to 882°C and transforms to BCC - β thereafter. Elements like Aluminium, Oxygen, Nitrogen, and Carbon stabilizes α phase whereas β stabilizers are Molybdenum, Vanadium, and Hydrogen. By properly alloying the above stabilizing elements, α - β alloys also can be made. Ti 6Al 4V is such a type of α - β alloy with stabilized α and β phase at room temperature. It contains 6% Aluminium as a stabilizer 4% Vanadium as β stabilizer. The welding technology of Titanium is complicated due to the fact that appreciable contamination by Oxygen, Nitrogen and Hydrogen can occur above 650°C causing severe embrittlement and at its melting point; the metal rapidly dissolves these gases and Carbon. Therefore welding of Titanium alloys are invariably done either in vacuum or in inert atmosphere. Due to these reasons, the suitable welding techniques for Titanium Alloys are limited. While doing GTAW of Titanium, Argon must be supplied to every heated surface as shielding, purging and trailing gas in the case of open air welding or by operating in totally enclosed argon filled welding cabinet. The weld edge preparation, pre-weld cleaning/pickling, welding speed, current, voltage, gas flow rate, etc are to be controlled very closely to achieve a defect free weld in GTAW. In the case of EBW, the welding is carried out in a chamber maintained at a high vacuum level of 10-4 torr. or better. Due to this, absolutely contamination free sound weld can be produced. Faster rate of welding, deep penetration characteristics, precise heat control, lower energy input to the weld etc are some of the salient features of EBW. However perfect joint fit up, pre-weld cleaning, proper joint design, beam alignment, vacuum level; adequate fixtures etc are some of the pre-requisites in EBW for a good weld. Due to introduction of welds, mechanical properties vary and microstructure changes. Welds can also alter the Fracture and Fatigue properties. Also weld repairs can cause significant changes in the strength and properties. Taking Titanium alloys for aerospace application, studying about the variation of these properties on the weld is of utmost importance. The variation of the above aspects with respect to various welding techniques is also different. This paper brings out an effective comparison on the weldabilty, mechanical properties and microstructural properties of this alloy with specific reference to the two important welding processes - EBW and GATW.