Technical ReportOptimization of pulsed TIG welding process parameters on mechanical properties of AA 5456 Aluminum alloy weldments
Introduction
Al–Mg alloys are extensively used in defence and aerospace applications. Tungsten inert gas (TIG) welding is an arc welding process that produces coalescence of metals by heating them with an arc between a non-consumable electrode and the base metal. TIG welding process [1] is generally used for welding of Al–Mg alloys. The initial strength of the non-heat treatable aluminum alloys depends primarily upon the hardening effect of alloying elements such as silicon, iron, manganese and magnesium [2]. These elements increase the strength either as dispersed phase or by solid solution strengthening. The welding of non-heat treatable aluminum alloys typically have distinct effects when the heat input is increased, i.e. the width of the heat affected zone (HAZ) is increased and the minimum reduction in the mechanical properties are observed. Alloys 5XXX series with more than 3.0% magnesium are not recommended for elevated temperatures above 150 °F because of their potential for sensitization and subject susceptibility to stress corrosion cracking [3]. The minimum HAZ strength approximates to that of the annealed parent metal regardless of the starting temperature.
During welding, vaporization of alloying elements like magnesium can occur and this vaporization loss of any alloying elements can influence the mechanical properties of the welded joints by affecting the chemistry of the weld pool. The gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) welding processes are very often used for welding of these alloys. However, GTAW process is generally preferred because it produces a very high quality welds. Distortion is the major problem in welding of thin sections. However, the distortion is controlled in pulsed and magnetic arc oscillation GTAW process. Metallurgical advantages of pulsed and magnetic arc oscillation welds that are frequently reported in the literature includes grain refinement in the fusion zone, reduced width of HAZ, less distortion, control of segregation, reduced hot cracking sensitivity and reduced residual stresses [4], [5], [6].
The purpose of the present investigation is to optimize the pulsed TIG welding process parameters for increasing the mechanical properties using Taguchi method. Taguchi method is a systematic approach to design and analyze experiments for improving the quality characteristics. Taguchi method [7], [8], [9], [10] permits evaluation of the effects of individual parameters independent of other parameters and interactions on the identified quality characteristics, i.e. ultimate tensile strength, yield strength, hardness, etc. Nowadays, Taguchi method has become a practical tool for improving the quality of the output without increasing the cost of experimentation by reducing the number of experiments.
Welds are made with the use of obtained optimum condition, and these welds are subjected to cold planishing process. The roll planishing is an effective process in which weld is passed between two steel rollers. During the planishing operation, the internal stresses which are induced during welding are relived and the grains are deformed. Hence, the mechanical properties of the welds have been improved.
Section snippets
Scheme of investigation
In order to maximize the quality characteristics, the present investigation has been made in the following sequence.
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Selection of base material and filler material.
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Identify the important pulsed welding process parameters.
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Find the upper and lower limits (i.e. range) of the identified process parameters.
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Select the orthogonal array (design of matrix).
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Conduct the experiments as per the selected orthogonal array.
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Record the quality characteristics (i.e. mechanical properties).
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Find the optimum
Planishing
The roll planishing is an effective process in which weld is passed between two steel rollers. The optimum pressure is applied between the rollers is about 2 bar and the optimum speed of 30 mm/min so that weld bead is pressed and there is no top and bottom reinforcement. During the planishing operation, the internal stresses which are induced during the welding are relived and the grains are deformed [17]. Hence, mechanical properties are improved. The schematic sketch of roll planishing process
Discussion
During tensile tests, all the welded specimens were failed within the weld region. Hence, ultimate tensile strength is equal to the strength of the weld. Pulsed welds have shown fine grain structure compared to the continuous welds is due to thermal disturbances and decrease in heat input.
In general, hardness in the fusion zone is lowest due to the as-cast nature of the microstructure, which is characterized by the coarse dendrite grains, interdendritic segregate phases, and the lack of
Conclusions
The influence of pulsed welding parameters such as peak current, base current, welding speed, and frequency on mechanical properties such as ultimate tensile strength (UTS), yield strength, percent elongation and hardness of AA 5456 Aluminum alloy weldments have been studied and the following conclusions are obtained.
The same optimum combination (i.e. P2B1S2F2) is observed in all the mechanical properties of welds. The behavior of the welded joints at the optimum condition (i.e. P2B1S2F2) of
Acknowledgements
The authors would like to thank the authorities of DRDL, Hyderabad and NIT, Warangal for providing the facilities to carryout this work.
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