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Cutting fluid has cooling and lubricating properties and is an important part of the field of metal machining. Owing to harmful additives, base oils with poor biodegradability, defects in processing methods, and unreasonable emissions of waste cutting fluids, cutting fluids have serious pollution problems, which pose challenges to global carbon emissions laws and regulations. However, the current research on cutting fluid and its circulating purification technique lacks systematic review papers to provide scientific technical guidance for actual production. In this study, the key scientific issues in the research achievements of ecofriendly cutting fluid and waste fluid treatment are clarified. First, the preparation and mechanism of organic additives are summarized, and the influence of the physical and chemical properties of vegetable base oils on lubricating properties is analyzed. Then, the process characteristics of cutting fluid reduction supply methods are systematically evaluated. Second, the treatment of oil mist and miscellaneous oil, the removal mechanism and approach of microorganisms, and the design principles of integrated recycling equipment are outlined. The conclusion is concluded that the synergistic effect of organic additives, biodegradable vegetable base oils and recycling purification effectively reduces the environmental pollution of cutting fluids. Finally, in view of the limitations of the cutting fluid and its circulating purification technique, the prospects of amino acid additive development, self-adapting jet parameter supply system, matching mechanism between processing conditions and cutting fluid are put forward, which provides the basis and support for the engineering application and development of cutting fluid and its circulating purification.

The application of cutting fluid in the field of engineering manufacturing has a history of hundreds of years, and it plays a vital role in the processing efficiency and surface quality of parts. Among them, water-based cutting fluid accounts for more than 90% of the consumption of cutting fluid. However, long-term recycling of water-based cutting fluid could easily cause deterioration, and the breeding of bacteria could cause the cutting fluid to fail, increase manufacturing costs, and even endanger the health of workers. Traditional bactericides could improve the biological stability of cutting fluids, but they are toxic to the environment and do not conform to the development trend of low-carbon manufacturing. Low-carbon manufacturing is inevitable and the direction of sustainable manufacturing. The use of nanomaterials, transition metal complexes, and physical sterilization methods on the bacterial cell membrane and genetic material could effectively solve this problem. In this article, the mechanism of action of additives and microbial metabolites was first analyzed. Then, the denaturation mechanism of traditional bactericides on the target protein and the effect of sterilization efficiency were summarized. Further, the mechanism of nanomaterials disrupting cell membrane potential was discussed. The effects of lipophilicity and the atomic number of transition metal complexes on cell membrane penetration were also summarized, and the effects of ultraviolet rays and ozone on the destruction of bacterial genetic material were reviewed. In other words, the bactericidal performance, hazard, degradability, and economics of various sterilization methods were comprehensively evaluated, and the potential development direction of improving the biological stability of cutting fluid was proposed.

Metal cutting fluids in flood condition does not meet the urgent needs of emission cutting and carbon reduction, minimum quantity lubrication (MQL) is an effective alternative to flood lubrication. Nevertheless, the micro-droplets produced by pneumatic atomization MQL have poor infiltration and wetting properties, which cannot fully exert the film-forming and heat transfer performance of lubricant. The unique empowering mechanism of electrostatic atomization can not only solve the above technical bottleneck, but also realize parametric control of the droplets. Previously, the improvement of tool wear and surface integrity of electrostatic atomization have been preliminarily verified by experimental studies. However, the application principle of electrostatic atomization in the manufacturing process is ambiguous, which limits its further development and industrial application. Especially the in-depth understanding of the action mechanism of the high temperature and high pressure contact interface is difficult and meaningful. More importantly, electrostatic atomization-assisted MQL machining has not been systematically reviewed. In this review, firstly, an analysis of the critical devices, common media (nano-biolubricants) and empowering mechanisms is presented. Subsequently, the excellent machining performance of nano-biolubricants was quantitatively evaluated by comparing with flood, revealing its advanced lubrication and heat transfer mechanism. Furtherly, the excellent machining performance was assessed based on the enhanced penetration, infiltration and film formation properties of electrostatic atomization, with a 42.4% reduction in tool wear and a 47% improvement in machined surface Ra compared to pneumatic atomization. Finally, the limitations of current electrostatic atomization and green lubrication technologies are analyzed and the future trends (e.g. new processes and multi-process coupling) are foreseen. The aim of this paper is to provide a comprehensive review and critical assessment of the existing understanding, which can be used by scientists to gain insight into the mechanism of action, the theoretical basis, machining performance and development direction of new electrostatic atomization processes.

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