Refill Friction Stir Spot Joining (RFSJ) developed by Kawasaki Heavy Industries (KHI) is a derivative technology of friction stir spot welding for joining aerospace aluminum alloys. The aerospace aluminum alloys were previously considered not weldable using conventional fusion welding methods. RFSJ does not consume any filler materials so that no additional weight is introduced to the assembly. As the solid-to-liquid phase transition is not involved in RFSJ in general, there is no lack of fusion or material deterioration caused by liquefaction and solidification. Unlike the conventional friction stir spot welding, RFSJ produces a spot joint with a perfectly flush surface finish without a key/exit hole. KHI has advanced the original friction stir spot welding concept and developed a robotic system that is capable of producing refill friction stir spot joints. The goal of this study is to demonstrate process parameter optimization of RFSJ for baseline aerospace aluminum alloys.