Solid-state welding of aluminum has become increasingly popular in recent years due to its ability to create strong joints without the need for fillers or external heat source. The use of a Bobbin tool can increase the efficiency and productivity of friction stir welding. The geometry of the tool plays a significant role in improving the mechanical and microstructural properties of the welded joint. To study the effect of Bobbin tool geometry, four different shoulder-to-probe diameter ratios were tested with a constant probe diameter of 5mm and pure aluminum sheets 3mm thick were used for the welded specimens. The welding conditions included a constant tool rotational speed of 653 rpm and a constant tool traverse speed of 44 mm/min. The thermal cycles, microstructure, and mechanical properties of the welded specimens were evaluated through various tests including microstructural analysis, fracture morphology examination via OM and SEM images, tensile tests, and microhardness tests. The experimental results showed that the fracture for the stronger specimen obtained when the shoulder to probe ratio was 2 and occurred far away from the welding nugget on the advancing side. On other hand, the fracture for the weakest specimen is produced when the shoulder to probe ratio was 3.5 and occurred in the stir zone center. The maximum joint strength and hardness were obtained at the minimum tool ratio due to dynamic recrystallization, fine particles, and homogenous structure present in both welding nugget and base metal. The weakest specimen showed voids, shallow and deep dimples, as well as many precipitates and cavities due to non-equilibrium material flow. Finally, it was observed that ductile fracture mode was present in optimum samples while brittle fracture mode was present in weakest samples.