The inherent properties of aluminum alloys often result in defects like deformation and porosity in joints and welds created through conventional fusion welding. Our paper investigates the impact of varying rotational speeds in the friction stir welding of 4mm thick 6063-T6 aluminum alloy, focusing on microscopic grain refinement and mechanical property alterations in T-joints. Using mechanical stretching and numerical simulation methods, we observed that the initial weld joint is robust, with minimal flying edges and a distinct fish-scale pattern on the surface. Moreover, the grain sizes in the weld’s core area are notably smaller than those in the region affected by the upper axial shoulder. The mechanical properties of the joint experienced a first increase and then a decrease in mechanical properties when the friction stir welding rotational speed, was increased from 600r/min to 1500r/min. Moreover, in 1000r/min rotational speed, the material welded tensile strength and yield strength of the best tensile strength compared to the raw material to enhance the 21MPa, yield strength of 30MPa, elongation close to the raw material of 89.86%.