Effect of Pulsed and Non-pulsed current on welding characteristics of AA6061 Aluminium alloy welded joints using Tig welding
Abstract
In this experimental work tests were conducted on the weldments of AA6061 aluminium alloy using TIG/GTAW with pulsed current at constant frequency of 6HZ and non-pulsed current on 3mm thick sheets. Destructive and nondestructive tests like Hardness test, tensile test, microstructure, radiography and liquid penetrate tests conducted, evaluated and compared with pulsed and non-pulsed current welding. Aim of the experimental work is to see the effect of pulsed current on characteristics of weldments. The experimental results pertaining to different welding parameters for the above material using pulsed and non-pulsed current GTAW are discussed and compared.
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Introduction
Reduction of mass is a prime concern for many industries like the automobile industry, aerospace applications; hence the focus on light materials like aluminium has became predominant. The demand is increasing for aluminium alloy welded products where high quality is required. Aluminium alloy can be welded easily by conventional arc welding methods like Metal Inert Gas Welding (MIGW) and Tungsten Inert Gas Welding (TIGW/GTAW).Among these two methods, The Gas Tungsten Arc Welding (GTAW) process has proved for many years to be suitable for welding aluminum alloy since it gives best quality welds. GTAW process (AC) is used in this study for welding of AA6061aluminium alloy.
Further development has been Pulsed current GTA welding, developed in 1950s, is a variation of GTA welding which involves cycling of the welding current from a high level to a low level at a selected regular frequency. The high level of peak current is generally selected to give adequate penetration and bead contour, while the low level of background current is set at a level sufficient to maintain stable arc. Pulsed current welding process has many advantages over non-pulsed current (constant current) welding, including enhanced arc stability, increased weld depth/width ratio, narrower HAZ range, reduced hot cracking sensitivity, refined grain size, reduced porosity, low heat input, lower distortion of gas by weld pool and better control of the fusion zone. All these factors will help to improve mechanical properties. Pulsed current welding technology has been widely used in Fabrication of high pressure air bottles, high pressure gas storage tanks, rocket motors, structures in aerospace applications such as aircrafts, rockets and missiles. Switching between predetermined high and low level of welding current can be used to produce pulsed current gas tungsten arc welds.
Some progress is done on pulsed current GTAW of aluminium alloy. So far the pulsed current welding is used to study the effect of pulse current, shielding gas composition, weld speed and bead shape, the incidence of welding defects, joint strength, using alloy sheets of 5083type, angular distortion in SS310 type, to study the microstructure and weld bead geometry. Usually the pulsed waves are in rectangular shape and the parameters used for pulsed GTA welding are in shown in figure 1. The main characteristics of Pulsed Current Welding are determined by peak current Ip, base current Ib, peak time tp and base time tb. The present study was carried out to understand the effect of pulsed current welding technique on the hardness, the microstructure and the tensile properties of gas tungsten arc welded AA6061 aluminium joints
Conclusion
The main conclusions of the present study can be summarized as follows,
1) The pulsed current welding technique records lower peak temperatures and lower magnitude of residual stresses compared with non-pulsed current welding, which is highly preferable for thin sheet welding.
2) The tungsten inert gas or gas tungsten arc welded aluminium alloy joint fabricated by pulsed current welding technique exhibits superior tensile properties compared with non-pulsed current welding technique.
3) The formation of finer grains caused by pulsed current is the main reason for enhanced tensile and hardness properties of the joints