In this study, the optimization of Friction Stir Welding (FSW) parameters for butt welding of dissimilar AA6061-T6 and pure copper was performed using the Taguchi method, with the aim of reducing grain size (refinement) in various weld regions. The interplay of mechanical processes, including severe plastic deformation caused by tool agitation, and thermal parameters resulting from friction, was comprehensively examined regarding their effects on microstructure in the three main weld zones: SZ, TMAZ and HAZ. Welding experiments were conducted by varying rotational speeds (1500, 1800, and 2000 rpm) and travel speeds (50, 80, and 120 mm/min), and microstructural properties were analyzed using metallographic methods and numerical modeling. Experimental results and statistical analysis of variance (ANOVA) showed that rotational speed with a contribution of about 45% and feed rate with a contribution of about 30%, are the most important parameters affecting grain size reduction, while the weld zone showed a moderate effect and the material type had no statistically significant effect on grain size (P=1.000). The SZ exhibited the smallest grain size due to the high concentration of plastic deformation. Optimization using the Signal-to-Noise (S/N) ratio with the "smaller is better" strategy revealed that a combination of a rotational speed of 2000 RPM and a travel speed of 120 mm/min provided optimal conditions for microstructure refinement and ultimate grain size reduction. These parameters ultimately led to a significant increase in mechanical properties, including tensile strength up to 177 MPa and improved fatigue life of the welded joint.