Achieving ultra-uniform microstructure and isotropic properties in wire-arc additively manufactured high-strength aluminum alloys with TiC nanoparticles

Wire arc additive manufacturing (WAAM) of high-strength Al-Zn-Mg-Cu alloys often suffers from coarse columnar grains, pronounced mechanical anisotropy, and inadequate high-temperature performance. This study demonstrates that introducing TiC nanoparticles (NPs) during WAAM, with optimized deposition parameters, enables an ultra-uniform fine equiaxed grain structure in both horizontal and vertical directions. The uniform microstructure effectively eliminates strong crystallographic texture and minimizes mechanical anisotropy. Systematic investigations reveal that TiC NPs act as potent heterogeneous nucleation sites for the α-Al matrix due to the formation of semi-coherent to coherent interfaces (lattice misfit: 4.7-6.5%), which significantly enhances nucleation efficiency and promotes columnar-to-equiaxed transition. The TiC NPs also restrict grain growth by interacting with the solid-liquid interface front, further contributing to microstructural refinement. After T6 heat treatment, the alloy achieves a superior ultimate tensile strength (UTS) of ∼534 MPa with exceptional isotropy (anisotropy index<0.4% in UTS-the lowest value reported for WAAM-processed high-strength Al alloys). The enhanced mechanical properties are attributed to the synergistic strengthening from grain boundary refinement, solid solution, dislocation density, and most dominantly, precipitation strengthening (∼274 MPa) from η′ and GP zones, as revealed by quantitative analysis. High-temperature tensile tests at 200-300 °C further demonstrate retained strength exceeding that of conventional wrought 7075 alloys, with the advantage persisting even at 300 °C, attributed to the thermal stability of TiC, Al₃Ti, and Al₁₈Mg₃Ti₂ phases that impede dislocation and grain boundary motion at elevated temperatures. This work underscores that the ultra-uniform microstructure and superior properties stem from the incorporation of TiC NPs and the tailored WAAM process, offering a viable pathway for fabricating high-performance, isotropic aluminum components via additive manufacturing.