Microstructure Evolution and Mechanical Properties of Friction Stir Welded Al–Cu–Li Alloy

The investigation concentrates on friction stir welded (FSW) Al–Cu–Li alloy concerning its local microstructural evolution and mechanical properties. The grain features were characterized by electron back scattered diffraction (EBSD) technology, while precipitate characterization was conducted by using transmission electron microscopy (TEM) aligned along [011]_Al and [001]_Al zone axes. The mechanical properties are evaluated through micro-hardness and tensile testing. It can be found that nugget zones exhibit finely equiaxed grains evolved through complete dynamic recrystallization (DRX), primarily occurring in continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). In the thermal–mechanically affected zone (TMAZ), numerous sub-structured grains, exhibiting an elongated morphology, were created due to partial DRX, signifying the dominance of CDRX, DDRX, and geometric dynamic recrystallization (GDRX) in this region. T₁ completely dissolves in the nugget zone (NZ) leading to the formation of Guinier–Preston zones and increase of δ′, β′ and S′. Conversely, T₁ partially solubilizes in TMAZ, the lowest hardness zone (LHZ) and heat affected zone (HAZ), and the residual T₁ undergoes marked coarsening, revealing various T₁ variants. The solubilization and coarsening of T₁ are primary contributors to the degradation of hardness and strength. θ′ primarily dissolves and coarsens in NZ and TMAZ, whilst this precipitate largely coarsens in HAZ and LHZ. σ, T_B, grain boundary phases (GBPs) and precipitate-free zone (PFZ) are newly generated during FSW. σ exists in the TMAZ, LHZ and HAZ, whereas T_B nucleates in NZ. GBPs and PFZ mostly develop in LHZ and HAZ, which can cause strain localization during tensile deformation, potentially leading to LHZ joint fracture.