Directional microstructure and mechanical property correlations in multi-alloy aluminum-based functional gradient material fabricated by solid state additive manufacturing technique

Abstract Aluminium alloys are essential in aerospace and automotive structures, yet conventional assembly of dissimilar alloys through welding or riveting introduces weak joints and reduced durability. Aluminium-based functionally gradient materials (AFGMs) produced by Friction stir additive manufacturing (FSAM) offer a promising alternative. This study aims to fabricate and evaluate a five-layer aluminium-based functionally graded material by integrating AA7075, AA2024, AA6061, AA5083, and AA1100 in a tensile-strength-guided stacking sequence, thereby creating a continuous strength-to-ductility gradient within a single structure. The multilayer builds were produced using FSAM with two different tool rotational speeds (600 and 700 RPM), followed by extraction, and systematically microstructural and mechanical characterization along longitudinal (LD), transverse (TD), and through-thickness (TTD) directions. Microstructural analysis revealed refined grains with defect-free interfaces at 600 RPM, while 700 RPM induced coarser grains with localized interfacial softening. EDX analysis confirmed the enrichment of strengthening elements (Cu, Zn, and Mg) in the upper layers, which correlates with the observed hardness distribution. Mechanical testing showed that TTD samples at 600 RPM achieved the highest tensile strength (UTS: 410 MPa, YS:220 MPa), whereas the TD samples of 700 RPM exhibited maximum ductility (i.e. 29.5% elongation). All samples maintained UTS to YS ratio greater than 1.5, confirming ductile behavior with strain hardening. The finding demonstrates that strength-based alloy stacking, using FSAM, enables the fabrication of anisotropic and structurally robust AFGM. This approach provides a scalable pathway to replace multi-alloy assemblies in aerospace fuselage and outer skin structure with a single graded material, reducing joining defects.

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