Investigations on Post-Weld Heat Treated Friction Stir Welds of High Strength Aluminum Alloy
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Investigations on Post-Weld Heat Treated Friction Stir Welds of High Strength Aluminum Alloy
T. Ramakrishna 1, K.Lallit Narayan 2 , T.Srinivasa Rao 3 ,
1 Department of Mechanical Engineering, Sir CRR College of Engineering, Eluru -534 007, Andhra Pradesh, India
2 Department of Mechanical Engineering, Sir CRR College of Engineering, Eluru -534 007, Andhra Pradesh, India
3Department of Mechanical Engineering, Chirala Engineering College Chirala,Chirala-523157, Andhra Pradesh, India
Abstract - High strength aluminum alloys such as 7075 are extensively used in variety of applications and they are considered unweldable using traditional fusion welding processes. Friction stir welding, being a solid state process, is considered to be a suitable technique to successfully weld these high strength alloys. However, as thickness of the plates to be welded increases, the welds exhibit degraded mechanical properties due to dissolution and over aging of strengthening precipitates. In this study, with an intention to improve mechanical properties, 16 mm thick AA7075-T651 aluminum alloy friction stir welds were subjected to a special heat treatment process. It includes a cyclic solution treatment, repeated heating between 400 and 480 ºC for 15 minutes and water quenching, followed by artificial aging at 130 ºC for 24 hours (CST+AA). The mechanical properties were evaluated using hardness, tensile and impact tests. The weld microstructures were also investigated by optical microscopy. The CST+AA treatment was found to significantly improve the hardness of the weld and homogenize the hardness profile across the joint. However, the tensile and impact tests were found to result in lower values than that of the welds without post-weld heat treatment. The post weld heat treatment also caused notable grain growth in the weld nugget which could be a reason for lower strength values.
Keywords: Thick section aluminum alloy, Friction stir welding, Post weld heat treatment, Optical microscopy, Mechanical properties.