Effect of heat treatment on microstructure and strengthening mechanisms of friction stir additive manufacturing 2060 Al-Li alloy

Friction stir additive manufacturing (FSAM) is an efficient solid-state processing method for aluminum alloys. However, during multi-layer FSAM, repeated thermal cycling and severe plastic deformation induce microstructural heterogeneity and the dissolution of strengthening precipitates, leading to significant degradation of mechanical properties. To address these issues, this study fabricated a 10-layer 2060 Al-Li bulk via FSAM and systematically examined the microstructure and mechanical properties along the building direction (BD). Furthermore, a post-heat treatment was applied to restore precipitation strengthening and homogenize the microstructure. With decreasing thermal cycles from the bottom to the top region, the fraction of deformed grains gradually increased. Meanwhile, the fraction of high-angle grain boundaries (HAGBs) and geometrically necessary dislocation (GND) density show an increasing trend along the BD. The as-deposited alloy contained Al₆Mn and θ (Al₂Cu) precipitates, whereas the original strengthening phases had been dissolved or coarsened due to repeated thermal exposure, resulting in limited mechanical performance (YS: 200.1 MPa, UTS: 237.6 MPa, EL: 11.1%). After heat treatment, static recrystallization (SRX) induced grain growth and raised the HAGB fraction to 92.9%. Massive fine intragranular T1 (Al₂CuLi) precipitates newly formed, effectively restoring the precipitation strengthening effect. Consequently, the mechanical properties of heat-treated samples were greatly optimized, with YS of 404.3 MPa and UTS of 434.3 MPa. Compared with the as-deposited alloy, its tensile strength was increased by 82.79%, while the ductility was reduced to approximately 2%. These findings confirm that heat treatment effectively homogenizes the microstructure and markedly enhances the mechanical performance of FSAM-processed 2060 Al-Li alloy.