Comparative Study of Corrosion, Mechanical and Electrical Characteristics of Tungsten inert Gas and Friction Stir Welded Joints
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Abstract Aluminium alloys are welded using both tungsten inert gas welding (TIG) and friction stir welding (FSW). FSW doesn’t need any filler material and shielding gas which results in reduced degradation of the environment. In the present study, joints with FSW and TIG processes were compared in terms of microstructure, corrosion resistance, mechanical, and electrical properties. With a microstructural study, the average grain size of various regions was determined. Coarse grain increases the rate of exfoliation corrosion. The fine grain structure in the FSW stir zone increases the hardness. The coarse grains are a prerequisite for enhanced electrical conductivity. The coarse grains in the heat-affected zone and thermomechanically affected zones of FSW joints increase electrical conductivity. In the case of friction stir welded joints, there is an inverse relationship between hardness and electrical conductivity. As hardness increases, electrical conductivity decreases, and vice versa. Hence, in the case of FSW hardness test can be substituted by an electrical conductivity test. In the case of TIG, no relation was found between hardness and electrical conductivity. High Silicon content at the weld region of TIG significantly reduces electrical conductivity.Keywords:
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Gas tungsten arc welding is one of the widely used techniques for joining ferrous and non ferrous metals. GTAW offers several advantages like joining of dissimilar metals, absence of slag, low heat affected zone etc. Gas Tungsten Arc Welding is an electric arc welding process, in which the fusion energy is produced by an electric arc burning between the work piece and the tungsten electrode. During the welding process the electrode, the arc and the weld pool are protected against the damaging effects of the atmospheric air by an inert shielding gas. By means of a gas nozzle the shielding gas is lead to the welding zone where it replaces the atmospheric air. The welding parameters are selected based on experience or from a handbook by operator. However, this does not ensure that the selected welding process parameters can produce the optimal or near optimal weld metal properties for that environment a particular welding machine. The aim of this paper is to effect on mechanical and metallurgical properties of dissimilar metal (304 stainless steel and CP-copper) weld by
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AISI 304 stainless steel plates were welded with activated flux tungsten inert gas(A-TIG) method by utilizing self-developed activated flux. It is indicated from the experimental results that for 8 mm-thick AISI 304 stainless steel plate, weld joint of full penetration and one-side welding with good weld appearance can be obtained in a single pass without groove preparation by utilizing A-TIG welding. Moreover, activated flux powders do not cause significant effect on the microstructure of TIG weld and the mechanical properties of A-TIG weld joints are also superior to those of C-TIG(conventional TIG) welding.
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The aim of the present study was to investigate the effect of specific oxide fluxes on the surface appearance, weld morphology, retained δ ferrite content, hot cracking susceptibility, angular distortion and mechanical properties obtained with the tungsten inert gas (TIG) process applied to the welding of 5 mm thick austenitic stainless steel plates. An autogenous gas tungsten arc welding process was applied to stainless steels through a thin layer of activating flux to produce a bead on plate welded joint. The MnO 2 and ZnO fluxes used were packed in powdered form. The experimental results indicated that the 80% MnO 2 –20% ZnO mixture can give full penetration and also a satisfactory surface appearance for type 304 stainless steel TIG flux welds. TIG welding with MnO 2 and/or ZnO can increase the measured ferrite number in welds, and tends to reduce hot cracking susceptibility in as welded structures. It was also found that TIG flux welding can significantly reduce the angular distortion of stainless steel weldments.
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The in-cell stainless steel piping and erection works require extensive welding. In many instances the approach for gas tungsten arc welding (GTAW) is limited and it is not possible to provide a cover of high purity inert Argon gas (backing shield) and in some instances, oxidation of the weldment takes place. The oxide forms over the weld fusion zone (root pass) as well as a heat tint forms over the surfaces of the adjacent base material. In reprocessing and waste management plants, the welded pipes come in contact with the process fluid which is nitric acid of concentration up to 6 M and at temperatures up to boiling point. The present study was focused on preparing induced oxidized welds of type 304L using filler wire of type 308L, using gas tungsten arc welding (GTAW) process and studying their corrosion behavior in nitric acid environments. Sample welds were prepared under proper welding conditions and also with conditions in which deliberately Argon gas was not purged or partially purged. The weldments with no oxides, partial oxides and excess oxides on the weld root pass were used for corrosion and characterization studies. Micro Laser Raman spectroscopy established the oxide to be hematite. Metallographic examination of the cross-section of the weldment showed the thickness of oxide to be 200-300 mm. Corrosion tests of the weldments as per practice C, A262, ASTM were done for five periods. Metallographic examination was done after the practice C exposures and showed absence of oxides on the weld root pass. Type 304L specimens were heat treated at 500 – 900 °C for 5 minutes to generate heat tints. These specimens were tested as per practice C, A262, ASTM for 5h and four periods of 48 h each. The corrosion rate in the first five hours exposure was high for the specimen heat treated at 900 °C but it came down to normal values in subsequent exposures. To confirm the corrosion behavior of hematite and magnetite in boiling nitric acid, powders of pure Fe 2 O 3 and Fe 3 O 4 were tested in boiling 65% nitric acid. The results are analyzed to establish the behavior of oxides on the stainless steel welds in nitric acid.
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Active flux for austenitic stainless steel TIG(tungsten inert-gas) welding was studied using the orthogonal method. With the resulting flux and heat input of 1.01 kJ/mm ,the austenitic stainless steel of 5 mm thickness was penetrated by TIG welding without beveling.When increasing heat input to 1.95 kJ/mm, the steel of 8mm thickness was penetrated. The arc was steady during welding,and appearance of weld is good. The flux has a function of suppressing alloy elements to escape from weld pool, so that the weight percentages of Cr and Ni in the A-TIG(active-tungsten inert-gas welding) weld metal are descended very slightly. The microstructure of the A-TIG welds is almost the same as that of the TIG welds. The hardness of the former is somewhate higher than that of the later.
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