Understanding Microstructural Gradients and Mechanical Anisotropy in 7075 Aluminium Alloy Processed by Additive Friction Stir Deposition

Additive friction stir deposition (AFSD) has emerged as a promising solid-state additive manufacturing technique for high-strength aluminum alloys; however, the through-thickness microstructural gradients and their correlation with mechanical anisotropy in bulk 7075 aluminum alloy remain insufficiently quantified. In this study, the spatial microstructural evolution and anisotropic mechanical response of AFSD-fabricated 7075 aluminum deposits were systematically investigated using optical microscopy, electron backscatter diffraction (EBSD), kernel average misorientation (KAM)-based geometrically necessary dislocation (GND) analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), and multi-directional tensile testing. The results reveal a largely homogeneous and defect-free deposit with subtle microstructural gradients along the build direction, where the grain size increases slightly from the top (≈1.9 μm) to the bottom (≈2.5 μm) due to cumulative thermal exposure and partial static grain growth. Electron backscatter diffraction (EBSD) analysis indicates a high fraction of high-angle grain boundaries (80–93%), confirming extensive dynamic recrystallization, while texture analysis shows stronger crystallographic orientation in the central region compared to the advancing and retreating sides. KAM-based evaluation reveals spatial variations in GND density ranging from 3.13 × 10 14 to 5.12 × 10 14 m -2 , reflecting heterogeneous thermomechanical conditions during deposition. Mechanical characterization demonstrates a hardness gradient along the build direction and pronounced anisotropic tensile behavior, with the highest ultimate tensile strength (∼490 MPa) observed along the longitudinal direction and elongation ranging from ∼10% to ∼12.5% depending on orientation. The observed anisotropy is attributed to the combined effects of texture intensity, dislocation density distribution, and precipitation evolution. These findings provide comprehensive insight into the microstructure–property relationships governing AFSD-processed 7075 aluminum and contribute to the optimization of processing–structure–performance correlations in solid-state additive manufacturing.