Microstructural and tribological characterisation of aluminium-bronze bi-materials obtained by solid-state recycling of machining chips

High demand for aluminium and copper generates large quantities of shavings. Currently, these are recycled by melting, a process that requires a lot of energy and is not ideal for machining chips. Solid-state recycling (SSR) techniques are therefore being developed more and more and can be seen as a potential alternative to the traditional method of recycling chips. In this study, the friction stir consolidation (FSC) process, which involves compacting chips from machined parts and rotating a cylindrical tool at high speed, was used to produce aluminium-bronze bi-materials. The microstructural characterisation and tribological performance of these bi-materials were studied and compared to those of the reference material. It was found that recycled aluminium undergoes dynamic recrystallisation, resulting in a refinement of the microstructure. In contrast, no such recrystallisation has been observed in recycled bronze. The latter only undergoes a reduction in the size of the δ phase. The bi-materials consist of aluminium chips that mix without amalgamating. However, Al-Cu intermetallics have been observed at the aluminium-bronze interface, with their quantity increasing in proportion to the amount of bronze incorporated. This results in an improvement in hardness of between 10 and 100%, depending on the proportion of bronze incorporated in the bi-material. Analysis of the friction coefficient shows that aluminium and aluminium-bronze bi-materials tend towards a similar quasi-stationary value, approximately 35% lower than that of the two pure bronze samples. Finally, aluminium-bronze bi-materials exhibit the lowest mass losses, which decrease by 28 to 75% as the amount of bronze increases.