In modern metalworking, semi-synthetic microemulsion cutting fluids have emerged as a core auxiliary medium in industries such as mechanical manufacturing, automotive, and aerospace, thanks to their unique combination of the lubricity of emulsions and the cooling performance of synthetics. The quality of these fluids directly determines machining efficiency, workpiece accuracy, and tool life. A semi-synthetic microemulsion cutting fluid is a thermodynamically stable oil-in-water dispersion system formulated from base oils, water, surfactants, and various additives. The judicious selection of microemulsion cutting-fluid additives and semi-synthetic cutting-fluid additives is a critical prerequisite for ensuring that the fluid’s overall performance meets required standards; only through the synergistic interaction of these three components can the stringent demands of complex machining conditions be satisfied.

Lubricity is one of the most critical performance requirements for semi-synthetic microemulsion cutting fluids. Intense friction between the cutting tool and workpiece, as well as between the chip and the tool/workpiece interface, can lead to tool wear, increased surface roughness of the workpiece, and even workpiece burnout or catastrophic tool failure. A high-quality semi-synthetic microemulsion cutting fluid relies on the synergistic action of base oils, microemulsion cutting fluid additives, and semi-synthetic cutting fluid additives to form a uniform, robust lubricating film on metal surfaces, thereby effectively reducing the coefficient of friction. Oil-based additives in the microemulsion cutting fluid formulation—such as fatty acids and fatty alcohols—can form a primary lubricating film through physical adsorption, making them suitable for low-speed, light-load machining. Extreme-pressure additives—such as sulfur- and phosphorus-containing compounds—on the other hand, can undergo chemical reactions with metal surfaces under high temperature and high pressure to generate a protective film with low shear strength, thus meeting the demands of high-speed, heavy-load machining. Semi-synthetic cutting fluid additives further optimize the lubrication system by addressing the performance limitations of individual additives, ensuring that the semi-synthetic microemulsion cutting fluid delivers stable lubrication protection across a wide range of machining conditions, prolongs tool life, and enhances the quality of the machined workpiece surface.

Cooling performance is another core requirement for semi-synthetic microemulsion cutting fluids, directly impacting process stability and workpiece accuracy. During machining, substantial heat is generated; if this heat is not dissipated promptly, it can lead to thermal deformation of the workpiece and annealing-induced softening of the cutting tool, severely compromising machining accuracy and production efficiency. Semi-synthetic microemulsion cutting fluids use water as the primary carrier, and water’s high specific heat capacity gives them excellent cooling capability, enabling rapid removal of heat from the cutting zone and reduction of machining temperatures. Meanwhile, the surfactants in the microemulsion formulation optimize oil droplet dispersion, ensuring uniform distribution of oil droplets in the aqueous phase and preventing oil-phase aggregation that could impair cooling efficiency. In addition, the hard-water-resistant additives in the semi-synthetic formulation help maintain emulsion stability by inhibiting the destabilizing effects of calcium and magnesium ions in hard water, thereby ensuring consistently stable cooling performance. Compared with fully synthetic cutting fluids, semi-synthetic microemulsions strike a balance between cooling and lubricating properties thanks to their moderate base-oil content, making them well-suited to a wide range of materials and machining processes.

Rust-prevention performance is an indispensable performance indicator for semi-synthetic microemulsion cutting fluids, as it protects workpieces and equipment from corrosion. During machining, metal workpieces are exposed to air, moisture, and various constituents in the cutting fluid, making them susceptible to oxidative corrosion—particularly those made of steel and cast iron. Once rust forms, product quality and subsequent machining operations can be severely compromised. The rust-prevention capability of semi-synthetic microemulsion cutting fluids is primarily achieved through rust-inhibiting additives contained in both the microemulsion formulation and the semi-synthetic formulation. These additives function via chemical adsorption or chelation to form a dense protective film on the metal surface, effectively blocking the ingress of corrosive agents such as water and oxygen. Microemulsion formulations focus on delivering short-term rust protection, ensuring that workpieces remain free from rust during machining; in contrast, semi-synthetic formulations enhance long-term rust resistance, preventing the formation of red rust and white rust during short-term storage after machining, while also safeguarding the metal components of machining equipment and reducing maintenance costs.

Stability is the foundation for the long-term effective use of semi-synthetic microemulsion cutting fluids and is a key performance requirement in industrial applications. The stability of such fluids encompasses three main aspects: chemical stability, physical stability, and microbial stability. Chemical stability requires that, during prolonged service, the fluid does not undergo oxidation or decomposition and that its active ingredients are not lost; this necessitates the addition of appropriate antioxidants to both microemulsion cutting-fluid formulations and semi-synthetic cutting-fluid formulations to inhibit oxidative reactions. Physical stability demands that the emulsion remain homogeneous—without layering, sedimentation, or demulsification. In microemulsion cutting-fluid formulations, surfactants adjust the oil–water interfacial tension to ensure stable dispersion of oil droplets in water, thereby forming a microemulsion with uniform particle size; in semi-synthetic cutting-fluid formulations, the formulation is optimized for compatibility to ensure synergistic interactions among various additives and to prevent ingredient conflicts that could lead to emulsion instability. Microbial stability, on the other hand, requires that the semi-synthetic microemulsion cutting fluid effectively inhibits the growth of bacteria and fungi, thereby preventing spoilage and malodor. The isothiazolinones and triazine-based preservatives added to both microemulsion and semi-synthetic cutting-fluid formulations disrupt microbial cell structures, thus extending the service life of the semi-synthetic microemulsion cutting fluid.

Semi-synthetic microemulsion cutting fluids can rapidly remove metal chips, abrasive particles, and other contaminants generated during machining, preventing these impurities from adhering to the tool and workpiece surfaces and thereby maintaining machining accuracy and lubrication performance.

In terms of environmental performance, in line with the trend toward green manufacturing, semi-synthetic microemulsion cutting fluids are required to be free of harmful substances such as nitrites, non-irritating to humans, and environmentally benign. The eco-friendly upgrade of microemulsion cutting fluid additives and semi-synthetic cutting fluid additives can effectively reduce the environmental impact of semi-synthetic microemulsion cutting fluids and ensure compliance with both domestic and international environmental standards.

In summary, the performance requirements for semi-synthetic microemulsion cutting fluids revolve around key dimensions such as lubrication, cooling, rust prevention, stability, and environmental friendliness. As critical constituents, microemulsion cutting fluid additives and semi-synthetic cutting fluid additives directly determine the upper limit of a semi-synthetic microemulsion cutting fluid’s performance. In practical applications, it is essential to select these additives scientifically based on the workpiece material and process requirements, optimize the formulation of the semi-synthetic microemulsion cutting fluid, and achieve a balanced performance across all parameters. Only in this way can the fluid fully leverage its auxiliary role in metalworking, enhancing processing efficiency and product quality while reducing production costs. Looking ahead, as manufacturing evolves toward greater precision and sustainability, the performance demands for semi-synthetic microemulsion cutting fluids will continue to rise, driving research and development of microemulsion and semi-synthetic cutting fluid additives toward multifunctionality and enhanced environmental compatibility. This will further promote the widespread adoption of semi-synthetic microemulsion cutting fluids in high-end manufacturing sectors.