An improved upcycling approach for producing bimetallic tube via friction stir extrusion of aluminium chips

Abstract Bimetallic tubes are employed in applications where a single component must satisfy multiple performance requirements, for instance, combining high strength with corrosion resistance. In literature, manufacturing processes like rotary piercing, forward/backward extrusions, and tube cladding have been commonly used to produce high-performance bimetallic tubes, typically starting from bulk materials. Moreover, recently, new sustainable processes belonging to the solid-state recycling (SSR) category, namely friction stir extrusion (FSE), have also been adopted for tube manufacturing. However, both conventional and SSR-based approaches generally rely on multi-step routes involving pre-heating, homogenization, or pre-consolidation to obtain workable billets, which increases energy consumption. This study goes beyond the issues of the existing extrusion-based recycling processes, proposing a single-step FSE approach able to directly convert AA7075 and AA2024 aluminum chips into bimetallic tubes, offering a sustainable upcycling pathway that does not involve any pre-heating or pre-consolidation stages. Three combinations of rotational speed and axial load were investigated to assess their influence on tube quality. The resulting bimetallic tubes were characterized through microstructural and macrostructural analyses, which revealed the absence of voids and inclusions at the bonding interface, the material composition, a grain refinement (avg. 4.6 µm), and an enhanced hardness (up to 175 HV) under optimal processing conditions. As a matter of fact, this process opens new opportunities for the fabrication of bimetallic tubular components, which can be used in electrical, structural, lightweight, and corrosion-resistant applications.