Friction stir processing of aluminum machining waste: carbon nanostructure reinforcements for enhanced composite performance - a comprehensive review

Climate change represents a worldwide environmental issue, prompting extensive research and innovation aimed at advancing sustainability within the aluminum manufacturing sector. A significant source of aluminum waste, particularly in the aerospace sector, comes from machining chips, which are crucial for recycling efforts that support a circular economy. The increasing demand for advanced, reliable materials has highlighted the potential of composites, with carbon nanostructures like carbon nanotubes and/or graphene emerging as exceptional reinforcement materials in Metal Matrix Composites (MMCs). Solid-state processing, particularly Friction Stir Processing gained recognition as a sustainable technique for converting aluminum machining waste into high performance carbon-reinforced composites. This review addresses the sustainability challenges of aluminum recycling within the circular economy framework, emphasizing the consolidation mechanisms of machining chips and factors influencing their density and microstructure. Additionally, it explores the significance of CNTs/graphene in enhancing the mechanical, thermal, electrical and tribological properties of aluminum composites, considering key factors such as reinforcement content, agglomeration, and interfacial reactions. The review summarizes and critically analyses the major research findings and discusses future prospects for synthesizing graphene/CNT-reinforced aluminum composites via FSP, underscoring their potential to meet contemporary demands for sustainable, high-performance materials.