Metalworking fluids (MWF): types and applications

Metalworking fluids (MWF) are liquid multicomponent systems designed for lubricating and cooling metalworking tools and workpieces. They are one of the most common process media in mechanical engineering and are indispensable in modern manufacturing. The use of MWF in metalworking increases equipment productivity, improves the accuracy of machined surfaces, reduces roughness and rejects, and improves working conditions, in some cases reducing the number of technological operations. The correct choice of fluid composition and type directly affects tool life, product quality and the economic efficiency of the entire metalworking operation.

Chemical Nature and Classification

Modern MWFs are divided into two main classes: oil-based (neat) and water-based (water-miscible). Oil-based MWFs are used neat without adding water. Their base consists of mineral, synthetic (polyalphaolefins, esters) or vegetable oils. To improve performance, anti-wear (AW) and extreme-pressure (EP) additives, antioxidants, corrosion inhibitors and antifoam agents are introduced.

Water-miscible MWFs are supplied as concentrates and are diluted with water before use. They are divided into three main groups:

• Soluble oils (emulsifiable oils) — the most common type. They contain 30 to 85% petroleum oil, as well as emulsifiers, surfactants, biocides and corrosion inhibitors. When mixed with water, they form a milky-white emulsion. They have good lubricating properties but are less effective at heat removal than synthetic fluids and are more prone to biological contamination.

• Semi-synthetic fluids — contain less oil (typically 5–50%), along with emulsifiers, lubricants and corrosion inhibitors. They combine the advantages of soluble oils and synthetic fluids: they provide good cooling and lubrication, are stable in hard water and have a longer service life.

• Synthetic fluids — contain no mineral oils. Their base consists of water-soluble polymers, glycols and special additives. They provide the best cooling, high stability and resistance to biological contamination, but are inferior to soluble oils in lubricity during heavy operations.

Key Properties and Quality Indicators

MWF quality is determined by a set of physicochemical and performance indicators. Density at 20 °C is measured according to GOST 3900. Emulsion stability during storage and in the working solution is a key indicator determining fluid service life. Corrosion aggressiveness is assessed by the cast iron chip test (DIN 51360-2): a quality MWF should not cause corrosion on test specimens. The pH of the working emulsion is usually in the range of 8.0–10.0, which provides corrosion protection and suppresses bacterial growth. Foaming, free alkali content and dilution stability are also monitored.

Applications

MWFs are used in all types of metal machining: turning, milling, drilling, grinding, broaching and thread cutting. The choice of fluid type is determined by the type and mode of operation, the composition and properties of the materials being machined, surface quality requirements and the method of fluid supply. For high-load cutting operations on ferrous metals, aluminium and titanium alloys, semi-synthetic and oil-based MWFs with anti-wear additives are used. For grinding operations, synthetic fluids with high cooling properties are preferred. Water with added corrosion-inhibiting salts is used when only cooling is required.

Technological Aspects and Maintenance

Water-miscible MWFs are prone to biological contamination: bacteria, fungi and algae can multiply in the working solution, especially in the presence of vegetable oils, leading to unpleasant odours and loss of functional properties. Biocides are added to suppress biological activity, and regular filtration, removal of tramp oils and concentration adjustment are carried out to extend service life. Modern MWF supply systems include equipment for automatic solution preparation, concentration control and removal of chips and contaminants.

Market and Trends

The global metalworking fluids market demonstrates steady growth, driven by the development of mechanical engineering, aerospace and automotive industries. In recent years, there has been a migration from traditional soluble oils to semi-synthetic and synthetic fluids, which offer longer service life, better hygiene characteristics and lower environmental impact. Requirements for the disposal of spent MWF and the reduction of volatile organic compound emissions are tightening. Amid the transformation of logistics chains, the role of local suppliers and Asian manufacturers offering competitive solutions is growing.

Conclusion

Metalworking fluids are a critically important component of modern metalworking production. The choice of optimal MWF type — oil-based, soluble oil, semi-synthetic or synthetic — is determined by specific technological tasks, materials being machined and product quality requirements. Organising MWF supplies, including selecting the composition for a specific operation, logistics and financial support, requires an individual approach that takes into account operating conditions and compatibility with equipment.