Age-hardenable alloys of the AA7xxx series are widely used in industrial applications such as aerospace and automotive industry. Their excellent mechanical properties are achieved by strengthening precipitates, created by artificial ageing processes. However, these alloys are usually difficult to weld due to porosity or liquation cracking. Therefore, solid-state joining processes are especially interesting for these alloys, because the maximum temperature stays below the melting point of the material. Refill friction stir spot welding (refill FSSW) is a cost- and weight efficient solid-state joining process that could substitute single-joint processes like riveting. However, the rotating tool in refill FSSW introduces severe plastic deformation and high heating and cooling rates. This has a strong influence on precipitation kinetics and leads to severe microstructural changes in the material, whereby different microstructural zones are formed. In the stir zone (SZ), the hardening precipitates might completely dissolve. In the thermo-mechanically affected zone (TMAZ), they partly recrystallize or recover, while they coarsen or transform into equilibrium phases in the heat-affected zone (HAZ). Since the matrix is highly supersaturated in the SZ after the process, a targeted application of post-weld heat treatments can lead to nucleation of precipitates, and therefore an increase in hardness after the welding process.

High-energy Small Angle X-ray Scattering (SAXS) was used in this work to study the precipitate size distribution in refill FSSW AA7050 samples. 2D SAXS mapping was done on slices cut as cross-sections of the weld.  The influence of welding parameters and post-weld heat treatments was studied, and numerical modelling was done using the commercial Pandat^TM software. This comprehensive study leads to a deeper understanding of precipitation kinetics during thermo-mechanical processing.