Manufacturing the Future with Supercritical CO2 and Minimum Quantity Lubrication

Currently the dominant approach for cooling and lubricating machining processes, such as drilling, milling and turning, is to use emulsion-based coolants (otherwise known as metalworking fluids) at high flow rates. There are many serious environmental, financial and health and safety reasons for reducing industry's reliance on emulsion coolants - an estimated 320,000 tonnes/year in the EU alone, up to 17% of total production costs, and over 1 million people are exposed regularly to the injurious effects of its additives which can cause skin irritation and even cancers. Serious environmental problems are also caused by the up to 30% of coolant that is lost in leaks and cleaning processes and which eventually ends up polluting rivers.

These issues have motivated extensive research efforts to identify more sustainable machining processes. There is growing and compelling evidence from preliminary studies that cryogenic machining with supercritical CO2 (scCO2) with small amounts of lubricant (Minimum Quantity Lubrication, MQL, referred to as scCO2+MQL machining) can provide a high-performing and more sustainable alternative. Current knowledge gaps in the relationships between key input and output variables, the reasons for variations in performance and concerns over the release of CO2, are preventing a major uptake of this technology by UK manufacturers.

This project aims to test the hypothesis that optimising combinations of CO2 with small amounts of the appropriate lubricant can provide reliable, step-change improvements in the performance and sustainability of machining operations. It will carry out a systematic investigation into the effect of scCO2+MQL on cutting forces, heat and tool wear mechanisms during machining of titanium, steels and composite stacks. It will develop: (a) advanced experimental methods in combination with full-scale machining trials to explore how lubrication and heat transfer affect machining performance; (b) lifecycle assessment and scavenging methods for sustainable re-use of CO2; (c) machine learning methods to predict the relationships between process inputs and outputs and (d) develop an effective and efficient optimisation methodology for balancing competing financial, performance and sustainability objectives in scCO2+MQL machining. These will deliver the knowledge, experimental and modelling methods and software tools that UK industry needs to exploit this enormous as-yet untapped potential.

The project will involves staff and postdoctoral research assistants from the Universities of Leeds and Sheffield and the Advanced Manufacturing Research Centres in Sheffield, with advice and guidance from a Project Steering Group comprised of leading international academic and industrial experts. Collectively, the team has the expertise in (a) manufacturing systems and tribology; (b) energy systems and lifecycle assessment; (c) fluid mechanics and heat transfer, and (d) machine learning and optimisation, needed to provide the 'how' and 'why' UK industry needs to reliably achieve or exceed the performance improvements seen in preliminary studies, namely doubling of tool life. We will work with our industrial and business sector collaborators to drive transformations in machining rate, process cost and accompanying safety, environmental and quality metrics for the benefit of the UK's defence, civil nuclear and medical manufacturing industries through the 2020s and beyond.