Abstract: Convex-vortex pin was firstly proposed and adopted to join the 2024-T4 aluminum alloy by friction stir diffusion welding. The effect of different stir pins on the cross-section appearance, material flow behavior and mechanical properties was investigated. The results showed that no obvious bending generated in the lap interface under the influence of convex-vortex pins. With the increment of the convex-vortex pins, the width of bottom of nugget zone also increased. The lap joint obtained by four convex-vortex pins presented a higher effective lap width and fracture loads than that of the lap joint obtained by six convex-vortex pins. And the corresponding values were 11.74 mm and 17 531.83 N, respectively.