<![CDATA[Friction stir welding (FSW) performance depends on the interaction among processing parameters rather than individual operating conditions. This study examined the effects of rotational speed (800, 1000, and 1200 rpm), travel speed (40, 60, and 80 mm/min), and axial force (4, 6, and 8 kN) on the mechanical properties of 6-mm AA6061-T6 butt joints. A 3 × 3 × 3 full factorial design with three reported replicates per condition yielded 81 mechanical-test observations, covering ultimate tensile strength (UTS), yield strength, elongation, and hardness. Factorial analysis of variance (ANOVA) and coded quadratic regression were performed using IBM SPSS Statistics 31. All three parameters showed significant main effects across the four mechanical responses (p < .001), while the Rotation × Travel interaction was significant for each response. Rotational speed exhibited the largest partial effect size for UTS (partial η² = .807). The quadratic regression models explained 72.1%–84.6% of the response variance and identified 1200 rpm, 80 mm/min, and 8 kN as the highest-predicted UTS setting among the 27 evaluated combinations, with a predicted UTS of 280.39 MPa and a calculated joint efficiency of 90.45%. These findings demonstrate the importance of considering parameter interactions when selecting FSW operating conditions, while limiting the model-based recommendation to the investigated material, tool configuration, and experimental range.]]>
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