Enhancing strength and toughness of 7xxx aluminum alloys via TiC nanoparticles in electron beam rapid additive manufacturing

TiC nanoparticles play an essential role in controlling the microstructure and phase morphology of additively manufactured aluminum alloys. However, the microstructure evolution and behavior of TiC nanoparticles during the solidification of large-scale and complex-shaped nano-TiC/7xxx aluminum alloys fabricated via electron beam rapid additive manufacturing (EBRM) remain insufficiently understood. In this study, TiC nanoparticles were introduced into the molten pool via metal wires, and the solidification microstructure and distribution of TiC nanoparticles were systematically characterized using SEM-EBSD and TEM-STEM. Results reveal that TiC nanoparticles nucleate as clusters approximately 100 nm in size during solidification. Models for nucleation efficiency and effective nucleation rate were developed, and the mechanisms governing the formation and size evolution of TiC nanoclusters were analyzed. Non-nucleating TiC nanoparticles were found to interact with solutes, suppressing grain growth. The mechanism by which non-nucleating TiC nanoparticles segment the solute boundary layer and form curved nanophase interfaces to drag grain boundaries was investigated. The critical solid-liquid interface velocity required to trap non-nucleating nanoparticles within the solute boundary layer was calculated. Partial trapping of non-nucleating TiC nanoparticles within grains facilitated the adsorption of solute elements, resulting in the formation of additional precipitated phases and second-phase particles after T6 aging, which effectively hindered dislocation motion. This approach enabled the production of a fully equiaxed 7xxx aluminum alloy with a grain size of 11 μm, an ultimate tensile strength of 538 MPa, and an elongation of 6 %. The alloy's strengthening was primarily attributed to the synergistic effects of grain boundary strengthening, precipitation strengthening, and solid solution strengthening. These findings offer valuable insights into improving alloy strength and toughness by regulating nanoparticle distribution within grains through solute boundary layer interactions with TiC nanoparticles.