Effect of material position and process conditions on defect formation in dissimilar friction stir welding: A CFD-based study

This study investigates the thermal and flow properties of AA5052 and AA6061 aluminum alloys during friction stir welding (FSW) using three-dimensional computational fluid dynamics (CFD) simulations with ANSYS Fluent. The process was performed at rotational speeds ranging from 600 to 1000 rpm and traverse speeds of 60 to 180 mm/min. A coupled thermal-flow model including temperature-dependent material properties and the volume of fluid (VOF) approach was applied to predict temperature profiles, velocity fields, and strain rates. A key novelty of this work is the specific investigation into how the placement of dissimilar alloys on the advancing side (AS) versus retreating side (RT) influences flow behavior and defect formation mechanisms, directly correlating flow dynamics with mechanical strength. The reliability of the model was validated against experimental metallographic data. At a rotational speed of 600 rpm, the peak temperatures decreased by approximately 3-4 K for every 30 mm/min increment, ranging from 688 K at 60 mm/min to 675 K at 180 mm/min. Traverse speeds of 60 and 90 mm/min produced welds without observable tunnel or surface-breaking defects in the examined cross-sections, whereas traverse speeds of 120 and 180 mm/min produced tunneling defects. Increasing rotational speed from 600 to 1000 rpm increased the peak temperature in the nugget zone from 684 to 753 K and the maximum material flow velocity from 0.25 to 0.447 m/s. Because of the higher flow stress and poorer thermal diffusivity of AA5052, placing it on the advancing side increased the peak temperature by 10 K and widened the nugget zone. Ultimately, the study identifies the optimal processing window as rotational speeds of 600–1000 rpm and traverse speeds of less than 90 mm/min.