Comparative Study of Mechanical and Microstructural Properties of AA6082-T6 Joints Produced by FSW and Underwater FSW

Abstract

This study compares conventional Friction Stir Welding (FSW) and Underwater Friction Stir Welding (UWFSW) applied to AA6082-T6 aluminum alloy plates. The welds were produced at a constant traverse speed of 80 mm/min. For conventional FSW in air, the tool rotational speed was fixed at 1500 rpm, while underwater welding was performed at 1200, 1500, and 1800 rpm in order to evaluate the influence of rotational speed under enhanced cooling conditions. The welded joints were examined by scanning electron microscopy, Vickers microhardness measurements, tensile testing, and Charpy impact testing. The results indicate that the underwater environment modifies the thermal cycle by increasing heat extraction during welding, which limits excessive grain growth and promotes a finer and more homogeneous microstructure in the weld region. This microstructural refinement is associated with improved hardness distribution and enhanced mechanical behavior compared with the joint welded in air. Among the investigated conditions, UWFSW at 1500 rpm provided the best balance between material flow and heat input, leading to the highest tensile strength, improved joint efficiency, and higher absorbed impact energy. In contrast, the lower rotational speed produced less uniform mixing, whereas the higher rotational speed promoted additional heat input and slight grain coarsening. Overall, the findings demonstrate that underwater cooling can be used as an effective process modification to improve the microstructure-property relationship of friction stir welded AA6082-T6 joints.