<p>This study presents a comparative analysis between conventional friction stir welding (CFSW) and flexible ultrasonic-enhanced friction stir welding (FLEX-USE-FSW) applied to a 3-mm-thick AA6082-T6 aluminum alloy. A novel ultrasonic system integrated into the tool holder was employed to introduce axial ultrasonic vibrations during the welding process. Mechanical testing, microstructural characterization, electron backscatter diffraction (EBSD), process force monitoring, and laser vibrometry were conducted to evaluate the influence of ultrasonic excitation on weld quality and process behavior. The results show that ultrasonic assistance reduces axial and traverse forces by up to 20%, broadens the operational window for tensile strength and elongation, and enhances material plasticization. EBSD analysis revealed a higher fraction of low-angle boundaries and modified grain texture in the ultrasonic-assisted stir zone (SZ), suggesting a change in recrystallization dynamics. Cross-sectional imaging revealed a larger and more homogeneous stirred zone in FLEX-USE-FSW, accompanied by reduced defect formation at elevated welding speeds. Additionally, laser vibrometry measurements showed improved energy transmission in specimens oriented parallel to the rolling direction. The findings demonstrate that ultrasonic excitation has a positive effect on weld morphology, mechanical properties, and microstructural evolution, particularly under high-speed and low-heat input conditions.</p>