## 4 Nanofluid Application in Machining  
The cutting fluid has three main functions in metal cutting operations: lubricates the tool/workpiece interface, cools the workpiece surface and the cutting tool, and removes the chips from the cutting from the cutting region. The use of cutting fluids during machining enables the economy of tools, provides tight tolerances, and improves and protects surface properties from damages [64]. Nevertheless, the use of cutting fluids negatively affects human health and the environment, both through their use and their disposal [65]. As well, the cutting fluid occupies 16-20% of the cost of production in the manufacturing industry. Therefore, excessive use of cutting fluids (flood lubrication) should be limited [66] and substituted by dry machining or minimum quantity lubrication (MQL), which can be adopted to spray cutting fluid over the tool/workpiece interface [67]. The MQL is also known as clean manufacturing [68]. There is a classic classification that indicates three types of cutting fluids: neat cutting oil, water-soluble fluids, and gases [69]. The most used are the water-soluble fluids in which it is possible to add nanoparticles, thus obtaining the nanofluids. The nanofluid is a colloidal mixture of nanometer-sized, smaller than 100 nm, metallic and non-metallic particles in conventional cutting fluid [70]. This new class of cutting fluids can be synthesized by mixing metallic, non-metallic, or carbon nanoparticles in a conventional cutting fluid which compared to the isolated fluid with the nanofluids shows better stability, good thermal conductivity, rheological properties, and no effect on pressure drop [71]. Nanofluids possess a high level of heat extraction capabilities (thermal conductivity) over conventional cutting fluids [2; 72]. Some researchers concluded that this enhanced thermal conductivity might be an important factor for better performance in various applications [2]. The ratio of surface area and volume of nanoparticles is the main factor that influences the thermal conductivity of nanofluids [73] and it was also discovered that the thermal conduction increased with the growth of particle concentration and with decreasing particle size [74]. Aside from thermal conductivity, the friction between workpiece and tool can be a crucial element to generate high temperature at the cutting region, which affects the surface quality, the workpiece dimensional accuracy, and the tool life during the cutting process. [PERSON] et al. [75] observed that the addition of graphite nanoparticles increases the lubricating property of standard lubricants because it lowers the friction coefficient. Because of their low friction behavior, the graphite and MoS\({}_{2}\) solid lubricants can reduce the surface roughness and decrease the cutting force in the machining process [76]. Figure 1: Different features that may affect the stability of nanofluids, adapted from [37]. Nanofluids are potential facilitators to reduce cost, improve tool life, and optimize the machining process. The most common machining processes in which nanofluids are used are turning, milling, drilling, and grinding. In former years, different formulations have been developed for all these machining processes. Regarding turning, [PERSON] et al. [77] employed two kinds of formulated nanofluids to machine Ti-6 Al-4V ELI, and concluded canola-based Al\({}_{2}\)O\({}_{3}\) nanofluid with loading concentration of 0.5 vol.% led to lower surface roughness. For the high-speed turning process, [PERSON] et al. [78] used 3 vol% of alumina and 1 vol.% of MWCNT, resulting in a severe reduction of cutting force and specific energy. Still, MWCNT presents better efficiency related to cutting zone temperature and tool life. [PERSON] et al. [79] evaluated the effect of utilizing an Al\({}_{2}\)O\({}_{3}\)-graphene nanoplatelets hybrid nanofluid as the cutting liquid on the tribological features in turning of AISI 304 steel.