Bobbin tool friction stir welding (BT-FSW), or self-reacting tool friction stir welding (SR-FSW), refers to a solid-state welding process which that uses two opposing rotating shoulders (top and lower of the workpiece) connected with a fully penetrated pin. In fact, the bottom shoulder in the BT-FSW design replaced the backing plate used in the conventional tool friction stir welding (CT-FSW) to promote symmetrical solid-state joints. Compared to CT-FSW, the BT-FSW process has many advantages over the use of a conventional tool such as the welded structure is symmetric in thickness, low distortion of weld joint can be obtained, the elimination of root for welds, a backing plate is not required, and high force is not required for fixing the weld plates and possibility welding a closed or a hollow section (U and H shapes). The welding parameters of BT-FSW, such as tool pin profile, rotational speed, welding speed, and axial force, have a considerable effect on the microstructure and the mechanical properties of the resulting assembly. In the current study, two extrusions of aluminum alloy 6061-T6 with 8 mm were joined by the BT-FSW technique with a tool pin with threads and eight different welding parameters (tool rotation speed and welding speed). The maximum value of tensile strength was achieved using optimum welding conditions of a tool rotation speed of 850 rpm/min and a welding speed of 650 mm/min. The study also investigated the joint efficiency of the friction stir welded joint, defects at the weld zone, and fatigue life of BT-FSW samples at the optimized level.