In-situ investigation of precipitation kinetics and microstructural evolution during friction extrusion of the aluminium alloy AA7075

Microstructural evolution under severe thermo-mechanical conditions is challenging to characterize, particularly in precipitation-hardenable aluminium alloys where high strain rates and elevated temperatures promote precipitate dissolution, nucleation, and growth. Capturing these dynamic processes requires in-situ characterization techniques. In this work, a novel experimental setup for in-situ investigation of friction extrusion (FE) is introduced using the FlexiStir device, a portable friction stir unit designed for operation at high-energy synchrotron beamlines. Time-resolved measurements during friction extrusion of AA7075-T651 were successfully performed, enabling direct observation of precipitation and microstructural evolution during processing. The combined effects of frictional heating and severe plastic deformation on precipitation kinetics and grain refinement were analyzed, supported by an analytical model describing the temperature rise due to frictional heating. The results reveal a strong coupling between processing conditions and microstructural evolution. In particular, precipitates exhibit an approximately four-fold increase in size during processing, as confirmed by scanning transmission electron microscopy and small-angle X-ray scattering. In addition, recrystallization-driven grain refinement was characterized through detailed microstructural analysis. Lower applied forces were found to produce higher processing temperatures and longer thermal exposure, promoting grain growth and reducing precipitate retention after cooling. These findings establish a direct relationship between applied force, temperature evolution, and dynamic precipitation behavior during friction extrusion.