Friction Spot Welding (FSpW) is considered to be a competitive technique for producing high quality welds, especially in lightweight alloys. The geometric defects and features originating at the interface between the two welded sheets have an important influence on the mechanical performance of the joint. This work investigates the weld microstructure for FSpW in AA 5052 sheets produced using tool rotational speeds of 900, 1400 or 1900 rpm. OM and SEM were used to analyse the microstructural features of the weld. The mechanical performance was investigated using shear tensile testing, and the fracture mechanisms were assessed by SEM. Three distinct weld interfaces were observed and named as follows: primary bonding, partial bonding and secondary bonding. For welds processed at a lower tool rotational speed (900 rpm), the equivalent bonding ligament is increased, resulting in better joint mechanical performance.