Industrial symbiosis in aluminium parts manufacturing: Towards 100 % material efficiency by combining friction stir extrusion and molten metal deposition

Aluminium chips resulting from subtractive manufacturing form an unattractive material flow for recycling. Their high surface to volume ratio leads to a higher concentration of oxides compared to other types of scrap, thus exacerbating inefficiencies in recycling and resulting in significant material losses. The field of solid state recycling has seen various technological developments trying to mitigate this problem, but struggles with its industrial adoption due to the rigorous quality demands of finished high-strength aluminium products. In this study, Friction Stir Extrusion (FSE) was used as an intermediary process to directly recycle aluminium chips into a wire suitable as feedstock for the Molten Metal Deposition (MMD) process, a recent development within the category of additive manufacturing. Despite the use of low-quality feedstock and the inclusion of dispersed oxide particles, it was found that the use of FSE wire as feedstock for the MMD process was possible without process modifications, and generated parts with increased hardness (+ 15 %). A life cycle assessment study indicated that the environmental impact was significantly lowered (− 52 %) for the feedstock production for MMD, generating an overall reduction of 6 kg equivalent CO2 emissions per kg of aluminium chips exchanged between the subtractive manufacturing source and the MMD sink. Obtained results demonstrate that selectively combining generated waste and processing options according to the principles of industrial symbiosis opens up opportunities to reduce the environmental impact of aluminium part production processes. This approach practically eliminates waste to achieve full use of the sourced materials.