The current study investigates the mechanical and microstructure properties of 2024 aluminium alloy welded by protrusion friction stir spot welding as a novel method to produce keyholefree welds. Tool rotation speed and anvil protrusion height are used as effective variables of the process to obtain optimum conditions. Results illustrate that the keyhole-free welds with the joint show superior mechanical properties in protrusion friction stir spot welding compared to conventional friction stir spot welding. Failure mode changes from interfacial mode to circumferential mode by increasing the nugget zone depth and joint length, while the effect of nugget zone is considerable. Finally, welding at a rotation speed of 1600 rev min −1 and a protrusion height of 0.4 mm presents significant mechanical properties with more joint length.

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