Microstructural Characterization of Recycled Al–Cu Chips Processed by Friction Stir Consolidation in a Cylindrical Die

Abstract

In this work, the microstructural evolution and hardness response of aluminum (AA2024) and high-purity copper machining chips consolidated by the friction stir consolidation (FSC) process using a cylindrical die and pinless tool configuration were studied. Microhardness tests and microstructural analysis were used to assess the effect of three essential processing parameters: tool rotation, preheating time, and chip weight. The hardness measurements showed a clear dependence on heat input and plastic deformation conditions, with regions rich in aluminum reaction showing the highest level of strengthening at optimal consolidation conditions, whereas zones rich in copper only show improvement to the hardness when a sufficient amount of thermal-mechanical energy has been put into the system. The intermediate hardness in the Al–Cu interfacial region indicated an order of magnitude change with a corresponding variation in material flow, chip breakage, and solid-state bonding. SEM observation demonstrated less porosity, better chip dispersion, and stronger microstructure at the higher rotating speed. EDS elemental maps exhibited localized Al–Cu diffusion, but no evidence of the continuous intermetallic layers was identified, while XRD patterns showed only FCC-Al and FCC-Cu, confirming that the FSC thermal cycle was still below the level needed for equilibrium phase formation of Al–Cu compounds. The consolidated disks obtained were dense, well-bonded, and fine-grained, indicating that the FSC has the potential to serve as an energy-efficient, eco-friendly approach for converting metallic machining waste into utility-based solid parts.