The joining of aluminum alloy sheets with thickness less than 2.0 mm is difficult via conventional friction stir welding owing to the defects in the joint, such as root flaw, keyhole and lazy S. In the present research, a newly designed pinless tool with involute grooves on its shoulder surface was applied to weld 1.5 mm thick AA2024-T3. The effects of the rotating speed and welding speed on the microstructure and mechanical properties of the joints were analyzed. The experimental results showed that the root flaw and keyhole were successfully eliminated. The lazy S was also eliminated under the optimized welding parameters. The maximum tensile strength of the joints was 326 MPa, which is about 74.1% that of the base material. Moreover, all the tensile samples fractured from the retreating side.Two fracture modes were observed during the tensile tests, which are related with the lazy S. The joining of aluminum alloy sheets with thickness less than 2.0 mm is difficult by conventional friction stir welding due to the defects in the joint, such as root flaw, keyhole and lazy S. In the present research, a newly designed pinless tool with involute grooves The effects of the rotating speed and welding speed on the microstructure and mechanical properties of the joints were analyzed. The experimental results showed that the root flaw and keyhole were successfully eliminated. The maximum tensile strength of the joints was 326 MPa, which is about 74.1% that of the base material. Moreover, all the tensile samples fractured from the retreating side. Two fracture modes were observed during the tensile tests, which are related with the lazy S.