In the work described in this paper, the 10 mm-thick 5A05-H112 aluminum alloy plates were welded successfully by bobbin-tool friction stir welding with two pins of the same length but different geometries. Results show that better mechanical properties of weld joints can be obtained when a pin with no plane is used with the same weld parameters. The shape of the pin influences the flow behavior of the plastic metal and could change weld contour. The macro-appearance is ‘dumbbell’-shaped when a pin with three planes is used, while the macro-appearance is ‘drum’-shaped when a pin with no plane is used. Using a pin with no plane, the tensile strength of the weld joint is 95% of the base metal and liquid phase formation was found in the local area of the axial shoulder. The weld joint is broken between the heat-affected zone and thermo-mechanical affected zone of the retreating side, the joint exhibits ductility fracture.