Abstract: In the present research, a newly designed pinless friction stir welding tool with involute grooves on its shoulder surface was applied to weld 1.5 mm thick AA2024-T3 alloy in lap configuration. Effects of the rotating and welding speeds on the formability and mechanical properties of joints were analyzed. Experimental results show that rotating and welding speeds have significant impact on the morphology of the Hook defect which in return affects the joint mechanical properties. When the rotating speed is constant, with increasing the welding speed, the tensile-shear strength of joint increases gradually. When the welding speed is constant, the tensile-shear strength of joint decreases first and then increases as increasing the rotating speed. The microhardness of both plates presents a "W" shape with the lowest hardness in the HAZ.