A review of principles and options for the re-use of machining chips by solid, semi-solid or melt-based processing

Machining chips are a potentially versatile material feedstock to be used in a range of solid-state and melt based manufacturing processes. The poor current understanding of the behaviour of chips in such processes limits the likelihood of practical adoption of such materials on an industrial scale. The re-use (as opposed to recycling or landfilling) of machining waste offers both economic and environmental savings. Iron and steel production causes up to 9 %, and aluminium up to 1.5 % of global process and energy CO ₂ emissions. It is proposed that between 0.24 % and 0.5 % (117 Mt CO ₂) of global CO ₂ emissions could be saved by reusing steel machining chips and between 0.13 % and 0.22 % (51 Mt CO ₂) via re-use of aluminium chips, through mitigation of recycling associated emissions or emissions from generation of new steel. Re-use of large manufacturing scrap, i.e. the majority of such scrap, increases these values to savings of 1 % and 0.86 % of global CO ₂ emissions for steel and aluminium respectively in the best scenario. This work gives a perspective on the potential for chips to retain value and be re-used in manufacturing process streams. The paper guides the reader from the generation and control of machining chips, to the principles of material consolidation. The principles of material densification by both solid state and laser deposition based methods are described, and current and potential manufacturing processes which can use chips as a feedstock are critically reviewed in terms of their practicalities and final properties. A perspective on potential CO ₂ savings is described, providing the underpinning motivation behind this work. It is shown there is significant potential for versatile manufacturing using machining chips by both solid-state and laser-based processing of chips into finished products. An argument is also presented for a new approach to processing large-scrap, e.g. from the automotive manufacturing industry, as well as end-of-life automotive scrap, into smaller chips ready for re-use.