Abstract: Additive manufacturing (AM) high-strength aluminum (Al) alloys like AA7075 are challenging due to poor printability. The inoculation treatment has proven as a feasible approach to improve printability and enhance the performance of the AM Al alloys. Unlike existing AM-processed Al matrix composites reinforced by ex-situ ceramics addition in literature, this work investigates the laser-directed energy deposition (LDED) AM of AA7075 inoculated by in-situ formed trace TiC-TiB₂ nanoparticles embedded in Al powder feedstock. AA7075 composite powder was fabricated by the master alloy method leveraging a 3Ti+B₄C→2TiB₂+TiC chemical reaction, followed by ultrasonic gas atomization. The effects of trace nanoparticles on formability, solidification behavior, solid-state phase transformations, grain recrystallization, precipitation kinetics, and mechanical properties are studied. Trace nanoparticles can significantly improve printability (with a wide process window) and eliminate the thermal crack, attaining a relative density above 99 %. During the solidification, trace nanoparticles cause significant grain refinement and more equiaxed grain formation, while weakening segregation and promoting dynamic recrystallization. After heat treatment, the particle-reinforced AA7075 achieved the co-precipitation of GP zone, η', and T' phases, exhibiting an outstanding strength-ductility synergy with the ultimate tensile strength ∼480 MPa and elongation of ∼10.5 %. This study solves the multiple limitations (e.g., poor dispersion and poor matrix-particle interface) faced by reinforcement particles in AM. Furthermore, the application of trace nanoparticles offers notable advantages in both strengthening efficiency and cost-effectiveness. The findings of this work could provide significant guidance for multilevel microstructure tuning and performance enhancement in AM-processed Al alloys.

Key words: Laser-directed energy deposition, 7075 aluminum alloy, Al-Zn-Mg-Cu, Embedded nanoparticles, High-strength aluminum alloy