Friction stir spot welding, a solid-state metal joining process has been utilised to join dissimilar aluminium alloys using pinless tools. The optimal welding conditions were predicted using the Multi-start algorithm with 25 starting points and were validated by confirmation tests. The Multi-start algorithm predicted the maximum tensile shear force of 6.28 kN at the tool rotational speed of 841.83 r/min, plunge depth of 0.618 mm, shoulder diameter of 24.99 mm, plunging speed of 0.04 mm/s and dwell time of 8 s. The scanning electron microscopy analysis revealed the formation of flatter hooks in the weld zone. The electron backscatter diffraction scans revealed a decrease in grain size at the lower sheet (Al 5754) due to the action of 30 kN axial force during pinless friction stir spot welding. Two different modes of fracture were observed: (i) nugget pull-out with necking; (ii) shearing. The fracture modes were observed to be dependent on the process parameters like shoulder diameter and plunge depth. © IMechE 2020.