Experimental Investigations into Immersed Friction Stir Welding of AA2014-T6

Abstract

The recent advancement in various technological fields demands the development and use of new materials and manufacturing processes. The aluminium alloy is light weight and high strength which finds wide spread use in the automobile and aerospace industry. The welding of heat treatable aluminium alloys has always represented a great challenge for designers and technologists since heat provided by welding process is responsible for the decay of mechanical properties. This decay is due to phase transformations and induced softening in the alloy. This can be overcome by employing friction stir welding (FSW) process. The advantages of FSW method have encouraged research in welding of aluminum alloys using FSW. Air FSW has been widely used for the joining of the aluminium alloy but in air FSW, the HAZ is wider compared to immersed FSW. Considerable improvement in mechanical properties of FSW joint of various aluminium alloys has been achieved by FSW in immersed condition. In immersed FSW, the workpiece is completely immersed in the water environment during welding. This work has attempted to fill some of the gaps in the contemporary research in improving the FSW performance of AA2014-T6. Due to a large number of factors and their interactions influencing the immersed FSW process, the process control becomes complex. The present work correlates the inter-relationship of various immersed FSW parameters, namely, welding speed, rotational speed and shoulder diameter on the tensile strength. Response surface methodology (RSM) has been used to develop the second order model that can be used to predict the tensile strength in terms of the significant parameters under consideration. The significance of immersed FSW parameters on the selected response has been evaluated using analysis of variance. The developed statistical model further utilized to find out the optimum parameters for maximum tensile strength using a Genetic Algorithm (GA). Immersed FSW has multi performance characteristics, namely tensile strength, power consumption and hardness at NZ during welding. An improvement of one performance characteristic may degrade other performance characteristic. Hence, optimization of the multi performance characteristic is more complicated than optimization of the single performance characteristic. In the present work, experiments have been planned using Taguchi’s L9 orthogonal array in immersed condition. Experiments have been performed by considering welding speed, rotational speed and shoulder diameter as a typical process parameter. Optimum immersed FSW parameters have been determined by grey relational grade obtained using the grey relational analysis for multi performance characteristics, namely, tensile strength, power consumption and hardness at the nugget zone (NZ). The ANOVA of grey relational grade has revealed that the shoulder diameter is the most influential parameter. It is found that the shoulder diameter has a maximum contribution of 53% and the rotational speed and welding speed has a contribution of 18% and 15% respectively on grey relational grade. The confirmation tests have been conducted to validate the optimum process parameters obtained and the performance characteristic power consumption found to be improved. The temperature profile of FSW joint is important as it affects the microstructure, residual stresses developed, the distortion produced and quality of the fabricated structure. Thermal numerical modeling of FSW using AA2014-T6 plates for air and immersed conditions have been carried out at a rotational speed of 1000 rpm and welding speed of 100 mm/min. Various backing plates, namely, asbestos, mild steel and copper have been considered for this study to check the effect on the temperature profile developed during welding. Commercially available software ANSYS has been used to develop thermal numerical model using temperature dependent material properties of AA2014-T6. It is found that the peak temperature obtained differs slightly from the temperature obtained experimentally. This may be due to the variation in the coefficient of friction between shoulder and plate with temperature, which is considered constant for the simplification. The simulation results obtained for temperature profiles are in good agreement with those obtained experimentally. It has showed that FE simulation could be used to predict the effects of the various FSW parameters on the temperature profile. The temperature profiles, thus obtained for air and immersed FSW of AA2014-T6 are used to understand their effect on microstructure developed. In the present work, an attempt has also been made to study the influence of welding speed, rotational speed and backing plate materials on microstructure and mechanical properties of FSW joints obtained using AA2014-T6 in air and immersed conditions. Optical and Scanning Electron Microscopy (SEM) images have been used for the detail microstructure study. Hardness measurement across the weldment is carried out using a Vickers micro hardness tester. Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) analysis is used to know the type of precipitate formed during the welding. The fracture feature of the samples is also analysed by SEM to study the fracture behaviour after the tensile strength test. It has been observed that the dissolution of strengthening precipitates is increased and decreased with an increase in rotational speed and welding speed respectively. This trend of dissolution of precipitates is common for both air and immersed FSW. It is also observed that diffusivity of backing plate affects the temperature of weld region. It is found that the plate having lower diffusivity has a higher welded region temperature. Therefore, higher dissolution of strengthening precipitates of weld joint is observed when the asbestos backing plate is used for FSW. The ductility of FSW joint obtained using asbestos as backing plate is the highest among the joints obtained using various backing plates. It is also observed that higher tensile strength and micro hardness attained for the joints produced using mild steel backing plate in immersed FSW compared with other backing plates for same weld parameters. Maximum tensile strength obtained for immersed FSW joint is around 17% higher compared with a maximum tensile strength obtained by air FSW because of lesser dissolution of strengthening precipitates in immersed condition. In summary, this work has, thus contributed to the knowledge of air and immersed FSW of AA2014-T6 by bringing out the influence of process parameters on some aspects of weldability, microstructure and mechanical properties.