International Journal of Mechanical Engineering and Research. ISSN No. 2249-0019, Volume 3, Number 5 (2013), pp. 431-438 © Research India Publications http://www.ripublication.com/ ijmer.htm
Effect of Welding Current on the Mechanical and Structural Properties of TIG Welded Aluminium Alloy AA-5083 Mayur.S1, Pavan.K.M1, Sachin.L.S1, Chandrashekar.A2 and B.S. Ajay Kumar3 1
Student, Dept. of Mechanical Engg. Bangalore Institute of Technology, Bangalore-560 004, India. 2 Dept. of Mechanical Engg. Bangalore Institute of Technology, Bangalore-560 004, India. 3 Dept. of Mechanical Engg. Bangalore Institute of Technology, Bangalore-560 004, India.
Abstract Purpose: In this study, structural and mechanical properties evaluation of AA-5083 alloy after single pass Tungsten Inert Gas(TIG) welding were investigated to reveal the weld strength, hardness of welded joints by using weld current as varying parameter. Design/Methodology/Approach: In this research, AA-5083 alloy plates were joined by TIG welding technique to examine optimal welding current. Welded specimens were investigated using optical microscopy, tensile and Vickers’s micro-hardness tests. Optical microscopy was used to characterize transition sites of welded zone, HAZ and base metal. Tensile test was conducted to characterize weld strength by determining ultimate tensile strength and micro-hardness test was conducted to characterize the homogeneity of welding in terms of mechanical properties. Findings: Results have shown that optimum weld current out of the three weld currents used (70A, 75A and 80A) is 75A. Better micro structures and mechanical properties were found in the welded joints for the weld current 75A. Practical Implications: AA-5083 has excellent resistance to general corrosion and is used in marine applications. Resistance is excellent in aqueous solutions in the pH range 4 – 9. The corrosion resistance of
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Mayur.S et al aluminium alloys relies on a protective surface oxide film, which when damaged is readily repaired by the rapid reaction between aluminium and oxygen. It is commonly used in the manufacture of unfired, welded pressure vessels, marine, auto aircraft cryogenics, drilling rigs, TV towers and transportation equipment such as building of railroad cars, tip truck bodies and vehicle bodies and in missile components. Originality: TIG welding technique was successfully carried out on AA-5083 and better structural and mechanical properties were obtained using weld current as varying parameter. Keywords: AA-5083, Arc Voltage, Welding current, TIG Welding.
1. Introduction TIG welding is the process of joining different materials with high quality weld bead by electric arc generation between electrode and work piece in the presence of inert gas. The welding came into existence from “Bronze Age” about 2000 years ago [1]. But Egyptian people learned to weld iron pieces together during Iron Age. TIG welding technique was demonstrated first by Russell Meredith in 1930 during Second World War for welding aluminium and magnesium in aircraft industry [1]. TIG welding has been used in modern industry, especially for aluminium, stainless steel, titanium alloys and other materials for high quality weld. TIG welding process has some advantages, including high quality weld, easy and precise control of welding parameters [2]. Aluminium, possess high thermal expansion co-efficient, high heat transfer, specific heat and high electrical transfer due to which fusion welding processes are not suitable for aluminium alloys. High co-efficient of thermal expansion results in deformation during fusion welding. Because of high heat transfer in aluminium, laser welding technique is not feasible and due to high electrical conductivity, use of resistance welding is not preferable [3]. Today's aluminum alloys together with their various tempers, comprise a wide welding procedure development. It is important to understand the differences between various aluminium alloys available and their various performances and weldability characteristics. When developing arc weld procedures for these alloys, consideration must be given to the specific alloy being welded. It is often said that arc welding of aluminum is not difficult, it is just different. It is believed that an important part of understanding in differences is to become familiar with the various alloys [4]. TIG welding is preferred mainly for aluminium alloys because it starts to spread out from weld pool during the welding operation when compared to other processes [5]. Usually, the microstructure and the mechanical properties of an aluminium alloy will change after the welding because of the melting of the base material during the welding process [6]. Pulsing the current in gas tungsten arc welding and gas metal arc welding technique has been investigated to obtain refined grain in fusion zone and enhance the weld mechanical
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properties, results shows significant enhancement of welded aluminium alloys [8]. Types of joint and shielding gases also affect the mechanical properties of joints. The most dominant mechanism for increased penetration is considered to be arc constriction rather than a change in the surface tension of the molten pool [9]. The present work deals with evaluation of structural and mechanical properties of welded joints by adopting TIG welding technique with varying current.
2. Material and Methodology Aluminium alloy AA-5083 (Composition shown in Table 1) plates of the dimension 125*60*3 mm were taken for TIG welding technique. These plates are cleaned from dirt, grease and other foreign materials by using cleansing agents, dirt removers and other re-agents. Edge preparation is carried out where double V edge is prepared for an angle of 450. The Aluminium plates are placed on welding table where the welding process is carried out.
Table 1: Chemical Composition of the 5083 aluminum alloy (in wt %).
In this process, all the various welding parameters such as the welding speed, flow rate, inert gas used and the number of passes is kept constant for all the trails and the welding current is used as varying parameter to study the effect of welding current on the structural and mechanical properties of weldments. The inert gas used in this investigation is 99.9% pure argon keeping the flow rate constant. The filler metal selected for the process is AA-5356 which is the standard filler rod to be used for AA5083 alloy (According to AWS Standards). In this study, TIG welding technique was adopted with three different welding currents for the Aluminum plates i.e., 70A, 75A and 80A respectively and these welded joints were further subjected to the following mechanical tests and metallographic analysis.Vickers’s micro hardness test was conducted by applying load of 50g. The hardness is measured at an interval of 10mm from the welded zone and 4 readings were taken on either side of the welded zone. The tests were conducted according to Indian standards 1501 - 2002. AA-5083 plates were subjected to tensile test following ASTM E-08 standards. In order to determine the structural aspects welded region was subjected to microstructure andmacrostructure analysis.
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3. Results and discussion 3.1 Micro hardness Traverse
3.2 Tensile Test
From the values of UTS obtained for 80A, 75A and 70A, it is observed that 75A weldment depicted maximum ultimate tensile strength when compared to weldments of 70A and 80A. There are no significant changes in the value of percentage in elongation.
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3.3 Macrostructure analysis Weld portion revealed fine porosities on the weld zone (Fig.3.a andFig.3.b). No macro defects are observed at the weld portion (Fig.3.c). 3.4 Microstructure analysis Network of coarse silicon particles and needles which formed in the interdendritic Aluminium - silicon eutectic (Fig.4.a). Network of finer particles of silicon in the interdendritic aluminium - silicon eutectic (Fig.4.b). Network of silicon particles which formed in the interdendritic aluminium alloy eutectic (Fig.5.a). Network of finer particles of silicon in the interdendritic aluminium alloy eutectic (Fig.5.b). Network of coarse silicon particles and needles which formed in the interdendritic aluminium silicon eutectic (Fig.6.a). Network of finer particles of silicon in the interdendritic aluminium - silicon eutectic (Fig.6.b). Cast dendritic structure with primary precipitates of silicon (Fig.4.c, Fig.5.c and Fig.6.c).
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4. Conclusions From this investigation, following conclusion has been outlined: The tensile strength has been increased by an amount 34% and 37% at weld current 75A in comparison with weld carried out at 70A and 80A respectively. It may be due to the presence of finer particles of silicon which has formed in the interdendritic aluminium alloy eutectic solution. In any case, eutectic is the point which changes the physical form of the material. Fine particles of silicon might have added contribution to the increase in tensile strength. Increase in hardness at centre of weld zone for weld current 75A has lessened the elongation by an amount of 3% in comparison with welding process carried out for AA-5083 alloy at weld current of 70A and 80A respectively. Microstructure analysis exhibits the presence of α and β dendritic structures in the welded zone due to chilling effect. This corresponds to the increase in hardness of the weld zone. Further the HAZ is a blend of welded and base metal regions which has a partial re-crystallized structure and hence the hardness decreases when compared to the weld zone. The base metal is an alloy of various elements like Mg, Si etc. which gives hardness and therefore the hardness again increases with respect to HAZ. The result reveals that blend of the elements is optimum and possess finer grains in the microstructure at 75A when compared to 70A and 80A, which in turn enhances the structural and mechanical properties.
5. Acknowledgements The authors are grateful to the Management and Dept. of Mechanical Engineering, Bangalore Institute of Technology, Bangalore, India, for extending the facilities to carry out this investigation and are also personally thankful to Dr. P.G. Mukunda, Nitte Meenakshi Institute of Technology for his valuable suggestions and guidance.
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