Impact of Tool Velocity Ratio on Welding Loads and Mechanical Properties in Friction Stir-Welded AA7075/AA2024 Plates

The emphasis on dissimilar joining of aluminum alloys has increased due to the growing need for lightweight, highly durable structures in the transportation and aerospace industries. For these applications, friction stir welding (FSW), a solid-state joining technology that offers better structural integrity than traditional fusion techniques, has proven very successful. The force-torque behavior and mechanical characteristics of friction stir welded dissimilar aluminum alloys, AA7075 and AA2024, with and without titanium diboride (TiB₂) reinforcement, are investigated in this work in relation to the tool velocity ratio (ω/v). With a constant rotational speed of 1000 rpm and a 1.5° tilt angle, a cylindrical taper tool (3 mm tip, 6 mm length) was used. The traverse speeds were varied to 1, 2, 3, and 4 mm/s, yielding velocity ratios of 1000, 500, 333, and 250, respectively. To evaluate the impact of the TiB₂ powder on joint performance, it was injected via machined grooves at the faying surfaces. The microstructural improvement, primarily grain refinement through dynamic recrystallization and Zener pinning effects from TiB₂ particles, significantly enhanced the hardness and tensile strength of the welds. Enhanced particle dispersion and metallurgical bonding were responsible for the superior mechanical response. Because of better metallurgical bonding, grain refinement, and particle dispersion, reinforced welds demonstrated superior characteristics in microstructural, tensile, and hardness tests, particularly at higher velocity ratios (lower traverse speeds). At a velocity ratio of 1000 (1 mm/s), the reinforced samples showed the highest tensile strength (219.5 MPa), elongation (6.9%), and improved microhardness, resulting in peak joint performance. Conversely, unreinforced welds with coarser microstructures and worse mechanical properties were found at lower velocity ratios. These results provide practical advice for dissimilar alloy FSW applications in advanced engineering systems and validate that a high tool velocity ratio in conjunction with TiB₂ reinforcement is essential for maximizing weld integrity and mechanical behavior.