Analysis of precipitation kinetics during refill friction stir spot welding and post-weld heat treatments in AA7050 using SAXS and numerical modeling

The impact of the refill friction stir spot welding process (refill FSSW) on precipitates in a high-strength AA7050-T7651 alloy is studied. Cross-sections of the welded samples are analyzed via small-angle X-ray scattering (SAXS), yielding 2D maps of spatial distributions of precipitate volume fraction and mean radius. The welding plunge times are varied during refill FSSW, leading to an increase in the width of the heat-affected zone (HAZ) with increase in energy input. The mean radius of precipitates increases above 25 nm at the edges of the stir zone (SZ) and thermo-mechanically affected zone (TMAZ). Dissolution and growth of η-precipitates in the HAZ are measured in-situ and are successfully modeled with the PanPrecipitation software. Moreover, the effect of various post-weld heat treatments (PWHT) on the precipitate distribution in the weld is analyzed. Reprecipitation in the HAZ is seen already after short aging times. After 20 min, the volume fraction increases all over the HAZ and nearly reaches the volume fraction of the base material. In the SZ/TMAZ, GP zones are formed through natural aging after welding and the PWHT lead to phase transformations and an increase in volume fraction and precipitate growth. Precipitates grow to a size of 2.5 nm after PWHT at 120 ℃ and 6.5 nm at 163 ℃. Thus, the formation of η^′+η-phase is expected after PWHT. Hardness measurements show that the PWHT lead to a hardness increase in the SZ and outer HAZ, but a minimum remains in the TMAZ/HAZ, where precipitates coarsened during welding. No significant changes in the weld zone features and the corresponding microstructure are observed between the as-welded and PWHT joints, indicating that the time–temperature conditions do not induce transformations such as grain growth and/or static recrystallization.