Recent developments in friction-based recycling techniques (FBRTs) for aluminum alloy chips: a comprehensive review

ABSTRACT Friction-based recycling techniques (FBRTs), including friction stir extrusion (FSE), friction stir back extrusion (FSBE), shear assisted processing and extrusion (ShAPE), friction stir consolidation (FSC), continuous friction stir extrusion (CFSE), and solid stir extrusion (SSE), have attracted attention for the solid-state recycling of aluminum alloy chips. These techniques utilize frictional heat and severe plastic deformation generated by the relative motion between the tool and the chips, enabling direct recycling without remelting or separate billet preheating. Despite their rapid development over the past decade, FBRTs have not been systematically reviewed as a distinct class of recycling technologies. This paper reviews the state of the art in FBRTs for recycling aluminum alloy chips and discusses the operating principles, tool configurations, material flow behavior, microstructural evolution, and mechanical properties of the FBRTs. The energy performance of FBRTs and solid bonding criteria for chip consolidation are comparatively evaluated. The mechanisms governing interfacial bonding, dynamic recovery, recrystallization, and grain growth are summarized. FBRTs offer an efficient route for recycling aluminum alloy chips, cutting energy consumption by 45%–75% relative to remelting. The resulting recycled products possess mechanical properties that are comparable to, or even surpass, those of some conventionally extruded or rolled materials. Future research should prioritize three fronts: (i) industrial-scale automation for continuous mass production; (ii) tailored post-recycling heat-treatment protocols; and (iii) integration with additive manufacturing to realize closed-loop material cycles. Achieving these goals will not only consolidate the energy-saving advantages of FBRTs, but also unlock new possibilities for on-demand, low-carbon metal forming.