In many machining operations, metalworking fluids (MWFs) play an invaluable role. Often, proper application of an intelligent MWF strategy allows manufacturing processes to benefit from a multitude of operational incentives, not least of which are increased tool life, improved surface integrity and optimised chip handling. Despite these clearly positive implications, current MWF strategies are often unable to accommodate the environmental, economic and social conscience of industrial environments. In response to these challenges, CO₂ coolants are postulated as an operationally viable, environmentally benign MWF solution. Given the strong mechanistic rationale and historical evidence in support of cryogenic coolants, this review considers the technological chronology of cryogenic MWF’s in addition to the current state-of-the-art approaches. The review also focuses on the use of CO₂ coolants in the context of the machining of a multitude of material types in various machining conditions. In doing so, cryogenic assisted machining is shown to offer a litany of performance benefits for both conventional emulsion (flood) cooling and near dry strategies, i.e., minimum quantity lubrication (MQL), as well as aerosol dry lubrication (ADL).

Whilst subtractive manufacturing has been de-risked significantly over recent decades, the emergence of new unfamiliar materials is proving to be a significant challenge for social sustainability. Given this rapidly evolving landscape, this review serves to outline the current available data on the occupational health implications of various existing and emerging material species, ranging from radioactive metals to composite materials. A structured search of sources up to January 2025 was conducted using databases such as Google Scholar, PubMed and Web of Science in addition to various authoritative occupational health reports, prioritising the literature directly pertaining or analogous to machining-related hazards. Evidence highlights the complexity of the machining environment, with occupational hazards ranging from toxicological factors to fire risks (i.e., due to swarf pyrophoricity). Case studies outline both relatively benign pathologies (e.g., dermatitis and sensitisation) and much more severe health complications (e.g., carcinogenicity, systemic organ damage and death), underscoring the need for continuous assessment and updating of exposure controls, even for materials traditionally regarded as safe.

As manufacturers seek to improve cutting process sustainability, alternative metalworking fluid (MWF) strategies are increasingly being explored. For the first time, this study characterises tool wear when shoulder milling both Grade 5 titanium (Ti-6Al-4V) and Grade 2 commercially pure titanium (CP-Ti) with a novel through-spindle Aurion Machining Technologies ionised air (IA) MWF setup. Preliminary results show that during CP-Ti milling trials, the IA strategy led to a tool life of between 85% and 91% of that which was achieved with soluble oil emulsion MWF, whilst during Ti-6Al-4V milling the observed tool life with IA was between 158% and 278% greater than with emulsion coolant at analogous cutting conditions (2.58 and 3.78 times respectively). In addition, IA generated a 26.2% reduction in surface roughness after Ti-6Al-4V milling, potentially indicating a change in tool–surface interaction behaviour. These benefits are compounded as IA returns to its original condition rapidly after utilisation, meaning low environmental and health impact with no waste MWF liquids/gases/mist, clean metal cuttings and low delivery power. Beyond these promising results, cooled and dried but non-ionised air was also shown to perform strongly (regarding tool wear in Ti-6Al-4V milling), relative to emulsion MWF, such that at 190 m/min cutting speed it generated 90% of the tool life which was achieved by the IA strategy. Whilst these preliminary findings require confirmation through repeat testing, IA remains of clear interest for further experimental investigation across a range of subtractive processes. Moreover, this work highlights the potential benefits, niches and configurations for air-based MWFs in general, with further mechanistic and tribological exploration warranted.

Due to recent developments across the aerospace, power generation and defense sectors, the demand for flat-surfaced components with extremely high surface quality is rapidly increasing. In this regard, although abrasive machining processes often produce fine, contaminated swarf that is frequently relegated to landfill, these processes remain critical for the engineering sector. Motivated by increasing sustainability and circularity pressures, this narrative review examines the current state of the art in recycling and repurposing the chips, tooling and cutting fluids that are typically generated or consumed within grinding processes. In doing so, a number of methodologies for extracting useful materials from swarf slurries are identified, including pyrometallurgical routes (applied successfully to Ni–Co alloys, for example), hydrometallurgical strategies (e.g., iron leaching from ferrous swarf) and, in the case of non-metallic materials such as CMCs and CFRPs, chemical processing methods. Various means of separating abrasive constituents and removing contaminants from grinding swarf are also highlighted, within which centrifugation and heat treatment are found to be particularly useful for non-ferrous materials such as titanium alloys or composites, whilst ferrous materials are largely magnetically separated. Prospective applications for spent abrasive tooling are also explored, including reuse as shot, waterjet machining feedstock, road surface additives, or mortar in the context of cement production. Likewise, heat- and radiation-based strategies for prolonging cutting-fluid life are highlighted, and their associated sustainability benefits and limitations discussed, despite ultimate disposal still being relegated to fuel usage or landfill. Ultimately, this review identifies the scarcity of grinding-specific recycling process data and highlights the need for robust, publicly accessible recycling strategies for novel material systems.