A Comparative Study on Microstructure and Mechanical Properties of Friction Stir Welded Butt Joints Produced Under Different Cooling Conditions in a High-Strength Al–Cu–Li Alloy and an Al–Zn–Cu–Mg Alloy Modified with Ag, Zr, and Sc

This study provides a comprehensive and novel comparative investigation into the evolution of microstructure and mechanical properties of FSW butt joints in a high-strength Al–Cu–Li alloy and an Al–Zn–Cu–Mg alloy modified with Ag, Zr, and Sc, processed under different cooling conditions (ambient air and forced water cooling). Mechanical performance was evaluated via transverse tensile testing and microhardness profiling. The scientific novelty of this work lies in revealing the distinct phase-stability and grain-refinement mechanisms governing each alloy under altered thermal cycles. The experimental results reveal that forced water cooling effectively restricts the peak welding temperature and curtails thermal exposure time, thereby significantly suppressing dynamic recrystallization grain growth in the stir zone (SZ) for both alloys. For the first time, a critical divergence in the post-weld hardening response was established: in the Al–Cu–Li alloy, accelerated cooling partially prevents the complete dissolution of the metastable T1-Al2CuLi phase, driving higher joint recovery. Conversely, the Ag-, Zr-, and Sc-modified Al–Zn–Cu–Mg alloy demonstrates superior structural stability; the thermally stable Al3(Sc, Zr) nanoprecipitates were found to exert a powerful Zener pinning effect, maintaining an ultra-fine grain structure (≤1 µm) even under ambient air conditions and completely preventing grain boundary shearing during rapid cooling. Consequently, the water-cooled modified Al–Zn–Cu–Mg joints achieved an exceptional joint efficiency of over 85%, shifting the typical HAZ failure location toward the thermomechanically affected zone (TMAZ) boundary.