A pinless friction stir spot welding tool with a scrolled convex shoulder is used to create spot welds on aluminum alloy plates. The effects that the rotation rate, the dwell time, the plunge depth, and the plunge rate have on the spot welding process are investigated. A strong correlation was found to exist between the joint strength versus the tool rotation rate and the joint strength versus the dwell time. Low rotation rates and short dwell times resulted in significantly stronger welds and, conversely, higher rotation rates and longer dwell times resulted in spot welds with hooking defects and lower joint strengths. Using shear tests and macrosection analysis on the spot welds, the dependences of the joint strength, the spindle torque, the temperature and the axial force on the identified process parameters are discussed. A computational fluid dynamics model is created to simulate the effect that the tool rotation rate has on the heat generated and the material flow during spot welding.