Solid-State Upcycling of End-of-Life Al–Mn–Cu Alloy via Additive Friction Stir Deposition

The increasing emphasis on sustainable materials recovery has driven research efforts to explore solid-state upcycling routes as a low-energy alternative to remelting and casting, thereby enabling the circular economy. This study employed additive friction stir deposition (AFSD), a solid-state, friction-based additive manufacturing process in which solid feedstock rods are deposited layer by layer. In the present investigation, an end-of-life, nonheat-treatable aluminum alloy (Al–1.2Mn–0.2Cu, wt%) from end of use was mechanically shredded into chips and cold-compacted into square-section rods (∼11.7 mm × 11.7 mm) with a length of 151 mm. The relative density of compacted rods was ∼85%. The rods were subsequently deposited using AFSD with a zig-zag deposition strategy using a flat-faced tool. The mechanical properties of the deposits were evaluated through indentation and uniaxial tensile testing. The deposit exhibited an average Vickers hardness of 46.9 ± 3.5 HV0.1, a yield strength (YS) of 76 ± 15 MPa, an ultimate tensile strength of 162 ± 18 MPa, and an elongation to fracture of 12.6 ± 1.0%. Microstructural analysis revealed effective interlayer metallurgical bonding and significant grain refinement due to continuous dynamic recrystallization (cDRX), resulting in an average grain size of 4.4 ± 0.6 μm. No apparent porosity was observed in the build, indicating near-full densification. Overall, this study demonstrates a resource-efficient upcycling strategy for end-of-life aluminum waste via AFSD, aiming to minimize the energy demand associated with solid-state recycling routes and supporting the transition toward closed-loop circular manufacturing. A quantitative assessment of the energy consumption will be addressed in future studies.