Influence of Joint Angle on Residual Stress, Microstructural Evolution, and Mechanical Performance of Friction Stir Welded AA6061 Aluminum Alloy Using a Novel Adjustable Angled-Joint Fixture

Abstract

Friction stir welding (FSW) is a solid-state joining technique capable of producing high-quality joints through frictional heating and severe plastic deformation below the melting point. In this study, the effect of joint angle on the residual stress evolution and integrated mechanical response of AA6061 aluminum alloy was experimentally investigated. Rolled plates (185 × 100 × 4 mm) were annealed prior to welding to improve ductility and material flow. A novel adjustable fixture enabled the fabrication of joints at 90°, 150°, 165°, and 180°, while welding was performed at 1400 rpm and 16 mm/min using angle-specific tools. Residual stresses were quantified using a hybrid digital image correlation–incremental hole drilling (DIC–IHD) method. Microstructural characterization and through-thickness microhardness measurements were conducted to elucidate thermo-mechanical behavior, and tensile tests were performed to evaluate joint performance. Increasing the joint angle progressively reduced tensile residual stresses from 189.5 and 177.3 MPa at 90° to 159.3 and 150.3 MPa at 180° in longitudinal and transverse directions, respectively. The 165° configuration exhibited the most homogeneous microstructure, uniform hardness distribution, and superior tensile strength. The results demonstrate that joint angle governs material flow symmetry and recrystallization behavior, leading to an optimal thermo-mechanical condition at 165° for enhanced structural performance.