Constitutional Liquation and Cavity Formation during FSW of AA2024 and AA7075 Alloys

In this study, constitutional liquation and cavity formation during friction stir welding (FSW) of AA2024 and AA7075 aluminum alloys were investigated through combined FSW experiments, heat treatment experiments and diffusion-controlled transformations (DICTRA) simulations. FSW and heat treatment experiments were conducted to examine microstructural evolution and the mechanisms responsible for cavity formation. Cavity defects were observed in both alloys at rotation speeds greater than 750 rpm, and Cu and Mg rich eutectic films were found along grain boundaries in the stir zone of welds containing cavities. Heat treatment experiments further confirmed that eutectic structures associated with constitutional liquation formed around S phase particles (Al 2 CuMg ) when the maximum temperature exceeded 509 ∘ C. DICTRA simulations showed that S phase particles do not completely dissolve before reaching 509 ∘ C, enabling localized liquation at particle-matrix interfaces. The resulting solute-rich liquid will promote hot working instability, ultimately leading to cavity formation in FSW. • Cavity defects in AA2024 and AA7075 occur at high FSW rotation speeds ( > 750 rpm) and intensify with increased travel speed. • Heat treatment experiments and DICTRA simulation confirm that constitutional liquation will occur around S phase particles (Al 2 CuMg ) when the maximum temperature exceeds 509 ∘ C, leading to eutectic structure formation. • With constitutional liquation, solute-rich liquid will promote hot working instability, resulting in cavity formation in FSW. Cu and Mg rich films will also form around grain boundary after solidification.