The Refill Friction Spot Joining (RFSJ) is an emerging solid-state spot welding technology that thermo-mechanically creates a molecular-level bond between the work-pieces. RFSJ does not consume any filler or foreign 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 or exit hole. Currently, the aerospace industry employs solid rivets for fastening the primary structures as they meet the baseline requirements and have well-established standards and specifications. However, the riveting process consists of tedious resource-intensive steps, such as hole-drilling, deburring, fastener insertion, and fastener clinching, which reduce the cost efficiency of the entire assembly process. Furthermore, some rivets produce uneven joint surfaces due to their protruding ends that cause not only air turbulence issues but also surface appearance issues in the aircraft structures. Kawasaki Heavy Industries has advanced the basic friction stir spot welding concept and developed a robotic system that is capable of producing refill friction-stir-spot joints in aluminum structures. The ultimate goal of this study is to investigate whether RFSJ is capable of replacing solid rivets in joining the primary aluminum structures of aircraft. The presented results demonstrate a process development methodology as well as mechanical and metallurgical properties of the joint produced by an optimized RFSJ process.