1 Introduction
With the rapid development of manufacturing in today’s world, the mechanical properties of core components have become increasingly stringent requirements in aerospace, military, medical, and infrastructure fields. New materials with high strength, toughness, high-temperature resistance, and other characteristics have emerged to meet the needs of the core component use [1–3]. However, in addition to the excellent properties of these materials, an element with poor processability also exists. Therefore, these materials, such as titanium alloys, nickel-based alloys, and high-strength steels, are called difficult-to-machine materials.
In the metal cutting process, choosing the right cutting fluid can improve the cooling and lubrication performance, further reduce the friction in the machining process, and thus improve tool life, machining efficiency, and surface quality. Currently, the common approach of using cutting fluid in the machining process is still flooding, and the use of flooding cutting fluid accounts for 7%–18% of the total cost of the machining process [4–6]. Moreover, traditional flooded metalworking fluids are usually made from nonrenewable mineral oils and scarce water resources. Mineral oils are toxic and nondegradable, and the splashing of cutting fluids during the machining process generates and triggers a variety of respiratory illnesses, which pose a threat to the health of workers. Simultaneously, the use of metalworking fluids easily causes environmental pollution; thus, the subsequent processing requirements have become strict. However, this approach is not in line with the current green manufacturing concept of sustainable development. The sustainable transformation and upgrading of the traditional manufacturing industry is imminent [7,8]. Dry machining in the 1990s has gained increasing attention. Dry machining refers to the absence of the use of cutting fluid; as a clean and environmentally friendly process, the use of cutting fluid, which reduces the cost of processing, has been gradually promoted. However, this machining process is characterized by high demands on machine rigidity and accuracy as well as machining parameters, and the lack of cutting fluid leads to severe friction at the tool–workpiece interface and heat buildup, which does not ensure geometric tolerances and surface integrity of the part [9]. Near dry machining, minimum quantity lubrication (MQL) is a green cooling lubrication method in which a small amount of lubricating medium is atomized by a gas at a certain pressure into micron-sized droplets delivered to the tool–workpiece contact interface [10–12]. MQL is between the dry and flood types; compared to flooding, the droplets flow faster and more efficiently under high-pressure gas, which reduces costs, and dry machining provides some cooling and lubrication, which improves machining quality [13–18]. In addition, the lubricating medium is typically a vegetable oil that has nontoxic, renewable, and biodegradable advantages, hence the name biolubricant. However, numerous studies have revealed that biolubricants suffer from insufficient cooling capacity due to their poor extreme pressure properties, such as high viscosity. Particularly in the case of difficult-to-machine materials, the high temperatures and pressures generated in the cutting zone make it impossible for biolubricants to meet the heat transfer and antiwear properties of machining, thereby limiting their application [19,20]. Nanoparticle-enhanced coolants (NPECs) have been widely used as new cooling and lubricating media in electronic microchannels, engines, spacecraft, nuclear energy, and solar energy [21]. NPECs are obtained by adding nano-enhanced phases to base oils (vegetable oils) via certain ultrasonic vibration, mechanical mixing, and the addition of dispersants. Thus, NPEC is an efficient and clean cooling and lubrication method that overcomes the insufficient heat exchange capacity and poor extreme pressure performance of MQL technology. Fig.1 shows the development route of the NPEC.
The presence of nano-enhanced phases not only efficiently enhances the heat transfer properties but also reduces the tribological behavior of the tool–workpiece interface. Therefore, the heat source is effectively limited during machining, heat is further reduced, and surface quality is substantially improved. NPECs have considerable application potential in new industrialized green cutting technologies. Based on a Web of Science literature search and a metrological analysis of the literature in this field from 2013 to 2022, the results show that the number of publications on NPECs is increasing annually (Fig.1). Some representative review papers that argue in terms of mechanism, device, technology, and application are available in these publications. For example, Sharma et al. [22] compared the effectiveness of the application of conventional cutting fluids and NPECs. They later reviewed the effect of using NPECs in MQL-related methods for commonly used materials in turning, milling, drilling, and grinding [23]. Sidik et al. [24], Said et al. [25], and Singh et al. [17] provided further summaries. Sen et al. [26] demonstrated the advanced performance of ecofriendly NPEC cutting fluids. Wickramasinghe et al. [27] focused on the sustainable application of vegetable oil-based NPEC in machining process optimization, whereas Wang et al. [28] reviewed the application of vegetable oil-based NPEC trace lubrication in turning. Chinchanikar et al. [29] examined the thermophysical properties and applications of NPECs. Kulkarni and Chinchanikar [30] assessed the research progress of NPECs for machining nickel-based high-temperature alloys with minimal lubrication. Baldin et al. [31], Zhang et al. [32], Liu et al. [33], and Cui et al. [34] have also made remarkable contributions. These scholars have reviewed MQL versus conventional flooding and NPEC trace lubrication machining performance. However, the thermophysical properties of NPECs, such as infiltration, film formation, cooling, and lubrication occurrences during processing, as well as the effects of molecular structures and physicochemical properties, remain unclear. This phenomenon limits the ability of technicians to parameterize the NPEC micro-lubricated cutting process and restricts the development of this highly promising field of clean and precision manufacturing. This paper reveals the stabilization mechanism of NPEC and the advanced film-forming and heat-transfer mechanisms based on the technical understanding of NPEC to fill this literature gap and provide a scientific foundation. Comprehensive quantitative assessments of material removal characteristics during machining in turning, milling, and grinding applications and investigations on the effect of NPEC are also conducted. This article is committed to providing a scientific basis and theoretical foundation for the further development of this technology and promoting its practical industrial application. Fig.2 shows the research roadmap of the article.
2 Nanoparticle-enhanced coolants
The performance of NPECs during machining is influenced by the physical properties of the nano-enhanced phase, the type of base fluid, the preparation method, and the dispersion stabilization. This section briefly reviews the commonly used base fluids and nano-enhanced phases for MQL, reveals the mechanism of action of the molecular structure cooling and lubricating properties of vegetable oils and nano-enhanced phases, and examines the commonly used NPEC preparation methods. The main challenge of NPECs lies in their poor stability. This section concludes by describing the kinetic behavior and dispersion-stabilizing mechanism of nano-enhanced phases in NPECs.
2.1 Base fluids
In the preparation of NPECs, water and oil are excellent base fluids for dispersing nano-enhanced phases. However, a large difference in the effect of the two base fluids is observed in the process. Oil-based NPECs have the advantage of ensuring lubrication, whereas water-based NPECs increase the availability of machinists because water is readily available and inexpensive. Kumar et al. [35] found that the use of water-based alumina NPEC can replace conventional cutting fluids in the micromachining of Ti‒6Al‒4V. Mao et al. [36] analyzed water- and oil-based fluids using NPEC and found that water-based fluids have poorer lubrication properties than oil-based fluids but demonstrate superior cooling. In addition, water-based NPECs are unsuitable for machining rusty parts. However, water-based NPECs alone cannot meet the lubrication requirements of strong friction interfaces [37]. Najiha et al. [38] showed that the cutting of aluminum alloy materials can be achieved using water-based NPECs. Compared with traditional oil-based trace lubrication, water-based NPECs have excellent heat dissipation performance but technical drawbacks of insufficient lubrication.
NPEC oil-based fluids typically contain vegetable, mixed vegetable, and mineral oils. Traditional mineral oil-based cutting fluids are widely used. However, the toxicity of the oil mist emitted due to elevated temperatures in the machining process has a considerable impact on the ecological environment and the health of workers. The application of vegetable oil cutting fluids in MQL did not reduce the machining quality compared with that of mineral oil cutting fluids [39]. Yıldırım et al. [40] investigated the machining performance of vegetable oil-based biolubricants compared with conventional mineral oil micro-lubrication for milling high-temperature nickel-based alloys and found that vegetable oil-based cutting fluids have higher tool life and lower cutting forces than mineral oils. In addition, from the perspective of environmental protection, vegetable oil is considered an environmentally friendly biolubricant due to its degradability and sustainable processing [41]. As a renewable resource, the application of vegetable oil in machining avoids the negative impact on the environment and is an important direction for sustainable development [42].
Sustainable cutting fluids, such as vegetable oils and water, are often referred to as biolubricants. As biolubricants, vegetable oils can simultaneously be lubricating, degradable, and environmentally friendly. These oils have all the necessary properties of a lubricant, including a high viscosity index, low volatility, and good lubricity. As cooling media in MQL, vegetable oils can reduce cutting fluid consumption by more than 90% and have made progress in their application in general material cutting [43–45]. Xavior and Adithan [46] used coconut oil, emulsion, and pure cutting oil immiscible with water as cutting fluids for cutting operations on AISI 304 steel using carbide tools. The results of the study showed that coconut oil performed better than the two other cutting fluids in reducing tool wear and improving surface finish, mainly due to its thermal and oxidative stability. Khan et al. [47] conducted an experimental study on the turning performance of an AISI 9310 material using vegetable oil-based cutting fluid as the MQL base fluid. The results showed a direct relationship between the cutting speed, feed rate, and cutting temperature. Compared with dry and wet machining, the use of vegetable oil MQL-assisted machining reduced surface roughness, tool wear, and cutting temperature. This reduction is mainly attributed to the decrease in wear and damage to the tool tip and the favorable chip–tool interactions due to the application of MQL, which eliminates the formation of traces of uniformly built-up edges (BUEs).
However, Tab.1 [48] shows considerable differences in lubrication properties among vegetable oils and large performance gaps in machining. For example, Ojolo et al. [49] investigated the effect of vegetable oils (i.e., peanut, palm kernel oil, shear butter, and coconut oil) on cutting forces during the machining of mild steel, copper, and aluminum. The experimental results showed that palm kernel and peanut oils were the best choices for reducing cutting forces. Wang et al. [48] used the nickel-based alloy GH4169 as the workpiece material and seven vegetable oils as the base oils to experimentally evaluate the tribological properties of the grinding wheel–workpiece interface. The lubricating properties of these vegetable oils were generally evaluated in terms of abrasive force, specific abrasive energy, coefficient of friction (COF), abrasive ratio, and surface morphology. Based on the experimental results, the lubricating properties were ranked as follows: corn oil < canola oil < soybean oil < sunflower oil < peanut oil < palm oil < castor oil.
Tab.1 Machinability of common vegetable oils when grinding nickel-based alloys [48], reproduced with permission from Elsevier |
| Base oil | Friction coefficient | Specific grinding energy/(J·mm‒3) | G-ratio |
|---|---|---|---|
| Pure soybean oil | 0.41 | 91.02 | 26.50 |
| Pure peanut oil | 0.45 | 98.62 | 22.92 |
| Pure maize oil | 0.34 | 80.90 | 29.15 |
| Pure rapeseed oil | 0.39 | 80.94 | 29.13 |
| Pure palm oil | 0.33 | 78.85 | 28.63 |
| Pure castor oil | 0.30 | 73.47 | 26.89 |
| Pure sunflower oil | 0.36 | 86.54 | 28.19 |
The differences in processing properties between different vegetable oils are due to their molecular structures, as shown in Fig.3. Singh et al. [50] observed the effect of the molecular structure of vegetable oils on their tribological properties through microanalysis. They found that the type of fatty acid affects the tribological properties and that sunflower oil is prone to autoxidation due to its high content of polyunsaturated fatty acids (linolenic and linoleic acids). By contrast, rapeseed and olive oils with high a content of monounsaturated (oleic) fatty acids are less susceptible to oxidation. Therefore, the poor oxidative stability of sunflower oil leads to reduced tribological properties compared with the two other types of vegetable oils. In addition, Dong et al. [51] and Liu et al. [39] found that when vegetable oils were processed as MQL base oils, their fatty acid contents affected the processing performance. Zhang et al. [32] systematically summarized the main parameters affecting the properties of vegetable oils. Natural vegetable oils comprise a variety of saturated and unsaturated fatty acids, which can be classified according to the length of the carbon chain, number of C=C, and polar groups. Among them, oils containing large amounts of unsaturated fatty acids (soybean, rapeseed, sunflower, and sesame oils) exhibit a good balance between cooling and lubrication. However, their strength and the susceptibility of the C=C bonds to oxidation result in poor thermal stability. By contrast, oils with high saturated fatty acid contents have higher viscosities and superior lubricating properties. The absence of C=C bonds results in large load-carrying capacities in the cutting zone. However, high viscosity is not conducive to effective heat transfer and dissipation, which can cause heat buildup in the cutting zone and affect machined surface quality. In addition, polar atoms or groups, which can produce strong physical adsorption, create a saponification effect with the metal surface and form a chemical oil film to improve the machining performance.
However, the easy oxidization property of vegetable oils limits their application in the industry. Scholars have attempted to improve the shortcomings of single vegetable oils by blending them to achieve a balance between cooling and lubricating properties. Guo et al. [52] used castor oil blended with six vegetable oils (1:1) for grinding and machining the nickel-based alloy GH4169. Their experimental results showed that the combined lubricating properties of the soybean–castor oil blend were better than those of castor oil and exhibited the best lubricating properties, reducing the specific tangential and specific normal grinding forces by 27.03% and 23.15%, respectively, and obtaining the best surface quality compared with castor oil. In another study, they also analyzed the mixed vegetable oil at a volume ratio of 1:2 and found that the mixed base oil has relatively less ricinoleic and unsaturated fatty acid components and relatively more saturated fatty acid components, and the oil film formed during the grinding process is relatively dense, which provides superior antiwear performance [53]. Jia et al. [54] conducted similar experiments and showed that soybean–castor blended base oil provided the best lubrication and that the addition of soybean oil improved the fluidity, atomization, heat exchange, and wetting properties of castor oil.
The addition of extreme pressure additives is also another way to improve the antioxidant properties of vegetable oils. Ozcelik et al. [55] conducted comparative experiments by adding different ratios of extreme pressure additives to sunflower oil, rapeseed oil, and traditional cutting fluids. The best surface quality was obtained for canola oil and 8% extreme pressure additive during processing. Sani et al. [56] added phosphorus and ammonia particles to vegetable oil to configure a new cutting fluid and compared it with conventional cutting fluids. Their experimental results showed that the new vegetable oil cutting fluid has superior tribological properties. Padmini et al. [57] added MoS2 as an extreme pressure additive to three vegetable-based oils (coconut, sesame, and rapeseed) for turning AISI 1040 steel and compared their properties. They found that coconut oil had the best processing performance with the addition of MoS2 nano-enhanced phases, and tool wear was reduced by 31.58% compared with canola oil. The nano-enhanced phase can be used as an extreme-pressure anti-wear agent due to its good extreme-pressure antiwear and green and environmentally friendly characteristics. Thus, this agent is an ideal substitute for traditional extreme-pressure antiwear agents. Therefore, the addition of chemical extreme pressure additives according to the processing needs is an option.
Overall, the selection of a suitable lubricant base oil should be preceded by an evaluation of the cooling and lubrication needs of the processing conditions. Oil-based NPECs are mainly used to regulate interfacial friction, whereas water-based NPECs are the opposite. Thus, the base oils of oil-based biolubricants should prioritize environmentally friendly and renewable plant-based lubricants. Vegetable oil biolubricants are easy to oxidize due to their fatty acids and polar groups, which improves the lubricity of the process. However, the modification of vegetable oils can help improve their shortcomings; for example, a combination of characteristics of vegetable oil blends and the addition of extreme pressure additives can enhance the physical properties of vegetable oil biolubricants.
2.2 Nano-enhanced phases
Biolubricants have exceptional lubricating film-forming properties, but the grinding process for difficult-to-machine materials can easily lead to lubricating film failure, and the performance of biolubricants can be substantially limited [58]. Thus, the nano-enhanced phase is an effective way to solve the above difficulties, and the incorporation of this phase considerably improves the extreme-pressure performance of pure vegetable oils. Therefore, the synergistic effect of the two can be effectively utilized. The surface effect of nano-enhanced phases is shown in Fig.4. In addition, nano-enhanced phases can reduce the degree of friction in the cutting zone, uniformly withstand compressive forces, improve heat dissipation, and reduce stress concentrations. When milling TC4 steel using graphene-based NPEC, Li et al. [59] demonstrated that graphene (GNP) flakes enhanced the cooling and lubricating properties of the fluid film formed in the cutting region. When turning Ti‒6Al‒4V with a textured carbide tool, Singh et al. [50] found that GNP NPEC reduced COF by 39%. During the milling of Al6061-T6, the application of an enhanced coolant based on MoS2 [60] and SiO2 [61] nano-enhanced phases improves the tribological properties of the tool–workpiece–chip and reduces the cutting forces and temperatures due to the nano-enhanced phase rolling action, leading to superior surface quality of the workpiece. Paturi et al. [62] found that after turning Inconel 718, the surface quality of the workpiece was improved by approximately 35% with the addition of 0.5 wt.% of tungsten disulfide (WS2) NPEC compared with pure MQL (PMQL). Yıldırım et al. [63,64] increased tool life by 105.9% compared with dry cutting after machining nickel-based high-temperature alloys using hexagonal boron nitride (HBN) NPEC with a mass fraction of 0.5 wt.%. The effect of NPECs is influenced by the type, size, shape, and concentration of the nano-enhanced phases. Scholars have conducted numerous experimental studies on the above variables.
1) Shapes
Minor differences in the roles of different shapes of nano-enhanced phases are observed. Tab.2 shows the commonly used nano-enhanced phases. Stratiform structure nano-enhanced phases produce laminar exfoliation during friction, extrusion, and shear. For stratiform nano-enhanced phases, Tevet et al. [65] conducted tribological characterization of nano-enhanced phases with different structures and found their rolling, sliding, and exfoliation behaviors under boundary lubrication conditions. This characterization can improve the machined surface quality and reduce cutting forces and temperatures. HBN has weak van der Waals forces between the molecular layers due to its structure, which reduces friction by sliding through these molecular layers against each other during machining [66]. In addition, GNPs have the advantages of high thermal conductivity and strength, making them the most promising material for cooling and lubrication. However, GNP is restricted by its high price. For spherical nano-enhanced phases, Musavi et al. [67] found that spherical copper oxide NPEC in A286 high-temperature alloy turning machining has superior lubrication performance, which resulted in improved surface quality because the spherical nano-enhanced phases remained in a steady state during the process. Therefore, the friction and wear behavior of the tool–workpiece–chip interface could be improved by increasing the sphericity of the nano-enhanced phases. In addition, nano-enhanced phases provide rolling and surface repair and increase lubricant viscosity. Kao and Lin [68] investigated TiO2 nanoparticle-reinforced coolant and found that the near-spherical nano-enhanced phase reduced rolling resistance. Therefore, spherical nano-enhanced phases are suitable for tribological and lubrication applications and are highly desirable for mechanical manufacturing processes. Threadiness nano-enhanced phases are also widely used in NPECs due to their excellent thermal conductivity, which can markedly reduce the temperature during processing. However, the unavoidable agglomeration of NPECs during their use reduces their tribological properties in the machining region.
Tab.2 Commonly used nano-enhanced phases for machining |
| Categories | Nano-enhanced phases |
|---|---|
| Spherical nano-enhanced phases | Al2O3, SiO2, ZrO2, SiC, ND |
| Threadiness nano-enhanced phases | CuO2, MnO2, TiO2, ZnO, CNTs |
| Stratiform nano-enhanced phases | CBN, GNP, MoO3, HBN, MoS2 |
Notes: CBN, cubic boron nitride; ND, diamond. |
2) Size
NPECs are colloids of particles that are at least less than 100 nm in dimension with a base solvent, and the nano-enhanced phases offer the following advantages over conventional oil emulsion suspensions used for heat dissipation. i) High specific surface area yields effective heat transfer performance. ii) Brownian motion in the NPEC allows for a more stable dispersion of the nano-enhanced phases with less clogging compared with conventional fluid particles. iii) Wettability, concentration, and thermal conductivity can be adjusted by varying the concentrations of NPECs [69]. Small nano-enhanced phases have been previously proposed to provide a smooth surface under NPEC trace lubrication conditions [70]. However, some scholars have discovered experimental phenomena that contradict these conclusions. Dubey et al. [71] used two different particle sizes in the formulation of NPECs for turning and machining AISI 304 steel. Their experimental results showed that the experimental values of surface roughness obtained for the 40 nm diameter were lower for the 40- and 30-nm diameter Al2O3 NPEC conditions. Khajehzadeh et al. [72] investigated the turning of AISI 4140 hardened steel with TiO2 NPEC and showed that the average reduction in tool back face wear decreased from 46.2% to 34.8% as the size of the nano-enhanced phases increased from 10 to 50 nm. Similarly, Lee et al. [73] investigated the machining performance of NPEC airflow-assisted electrostatic atomization lubricated titanium alloy micro-grinding. The results showed that large nanodiamond particles (80 nm) were highly favorable for obtaining low surface roughness values to fully explore the mapping relationship between the cutting and grinding properties and nano-enhanced phases. Nano-enhanced phases with large sizes can be selected in conjunction with high-viscosity vegetable oil biolubricants to address the unfavorable size effect of nano-enhanced phases in NPEC MQL processing and reduce this effect [74].
Therefore, the mechanism of action of different sizes of nano-enhanced phases at the friction interface is also different, and the scale of the roughness peak at the friction interface should be considered. In the case of a large cutting dosage, the large size of the nano-enhanced phases should be first considered when reducing the machining process parameters, such as the cutting force. Meanwhile, in the case of finishing, the use of small nano-enhanced phases is recommended when ensuring the surface integrity of the parts and tool life. Simultaneously, combining different sizes of nano-enhanced phases with base oils of corresponding viscosities can improve the machining quality.
3) Concentration
Nano-enhanced phases lead to superior results in vegetable oil biolubricants; however, the gain effect is also affected by the concentration. Mao et al. [36] found that the lubrication and cooling properties in the abrasive zone improved as the concentration of nano-enhanced phases increased. Talib et al. [75] found that HBN particles provided a lubricating film in Fig.5, which reduced friction and prevented wear. However, the remaining particles in the rough valleys restricted the movement of nearby particles as the HBN concentration increased, resulting in large forces that led to frictional wear. Additional damage areas are formed when 0.1 and 0.5 wt.% HBN particles slide along the contact surface, which leads to abrasive wear. Thus, 0.05 wt.% HBN particles are the optimal concentration. Agglomeration occurs when the concentration of NPEC exceeds certain conditions, subsequently affecting the stability of the NPEC and leading to deterioration of the processing quality.
Pal et al. [76] prepared NPEC with different concentrations (0.5, 1.0, and 1.5 wt.%) by mixing Al2O3 nano-enhanced phases with sunflower oil. The results showed that compared with the flood condition, the optimal concentration of 1.5 wt.% reduced thrust, torque, surface roughness, and drill tip temperature by 44%, 67%, 56%, and 26%, respectively, at the 30th hole. In addition, different concentrations lead to viscosity variations of the NPEC. The thin lubrication film formed by low viscosity cannot withstand the stress from the extra loading of particles, which increases friction and damages the sliding surfaces [77]. Sen et al. [78] found that the maximum viscosity was reached at 3% Al2O3 concentration with superior lubricating properties. However, the nano-enhanced phases can agglomerate and affect the thermal conductivity of the Al2O3 nano-enhanced phases in palm oil. The concentration of the nano-enhanced phases has a direct relationship with the contact angle, which decreases sharply with increasing NPEC concentration when the wetting area increases. However, the contact angle also increases further when a certain concentration is exceeded because the excess nano-enhanced phases aggregate and then deposit after losing dynamic stability. Therefore, the wettability of the NPEC decreases with excessive concentration, and the surface quality of the workpiece deteriorates [79].
The results of numerous studies have shown that optimal concentration values exist and produce optimal effects with the application of different nano-enhanced phase processes. Modulation of the NPEC performance can be achieved by optimizing the concentration of the nano-enhanced phases. Overall, the shape, size, and concentration of the nano-enhanced phases will affect the machining performance. The good or poor machining performance is the result of their joint action, and no complete parameter is available for the nano-enhanced phases. Therefore, future exploration should further optimize the parameters of the nano-enhanced phases to provide a complete parameter for the processing of different workpiece materials.
2.3 Preparation
The preparation of the NPEC is the most important step in the experimental study, that is, “combining” the base fluid with the nano-enhanced phases. The preparation stage is crucial for enhancing the thermophysical properties of NPECs. Therefore, fine preparation is required to avoid chemical changes, agglomeration, and the inhomogeneous suspension of nano-enhanced phases. The two main methods of synthesizing NPECs are as follows: one-step and two-step.
Physical (e.g., vapor deposition, laser ablation, and ultrasound-assisted submerged arc) and chemical (via reducing agents) methods are commonly used in a one-step process. Simultaneous preparation and dispersion of nano-enhanced phases in the base fluid prevent oxidation of the nano-enhanced phases, disperse them homogeneously, enhance stability, and avoid storage, drying, and NPEC transportation in the process, which reduces agglomeration and thus improves stability [80].
The two-step preparation involves the preparation and dispersion of nano-enhanced phases into the base fluid [81]. First, nano-enhanced phases are prepared via vapor deposition, chemical reduction, or mechanical milling and then dispersed into the base liquid via the mechanical dispersion method, ultrasonication, and addition of dispersants [82–85]. The mechanical dispersion method distributes nano-enhanced phases in the liquid medium using external mechanical energy such as shear or collision force, ball milling, mechanical stirring, and other methods. The ultrasonic dispersion method places the NPEC directly in the ultrasonic field and uses ultrasonic waves of suitable frequency to facilitate the movement of the nano-enhanced phases. This approach overcomes the mutual attraction between the nano-enhanced phases and destroys the equilibrium between the original particles as well as between the particles and molecules of the substrate, dispersing the nano-enhanced phases into the substrate liquid. Mechanical dispersion and sonication disrupt the original kinetic behavior of the nano-enhanced phases (van der Waals and Brownian forces) and reduce the interactions between the nano-enhanced phases. The addition of the dispersant increases the repulsive force between the nano-enhanced phases; a greater repulsive force between the particles than the attractive force prevents the agglomeration between the nano-enhanced phases to improve stability. Fig.6 shows the two-step method used to prepare NPECs.
Overall, the one- and two-step methods have advantages and disadvantages. The one-step method has the advantage of good dispersion and high suspension stability but suffers from the limitations of high cost and difficulty in controlling the size of nano-enhanced phases. Thus, this method is only applicable to low-pressure base fluids. The two-step method is the most cost-effective method for obtaining superior particle size control and is suitable for mass production. However, these steps are highly complex and require additional time and experimental equipment. In specific applications, choosing the appropriate preparation method according to different experimental requirements and the use of NPEC conditions is necessary. Thus, a two-step method should be used in the field of machining to prepare NPECs.
2.4 Stability
NPECs are complex mixtures of nano-enhanced phases and biolubricants. They tend to aggregate due to their high surface activity, and the lack of suspension stability leads to increased cluster formation and deposition rates. Agglomeration causes the nano-enhanced phases to settle and clog the nozzles, thus reducing the thermophysical properties of the NPEC. The stability of NPEC depends on the properties of the nano-enhanced phases, the type of base fluid, the preparation method, and the subsequent treatment [86]. Therefore, the stability of NPECs should be carefully considered.
2.4.1 Destabilizing factors
Microscopically, NPECs belong to the liquid–solid two-phase system. Thus, a distinct phase interface exists between the liquid molecules and the nano-enhanced phases, and a buoyant force of the base liquid is observed on the particles. Brownian motion of nano-enhanced phases refers to the irregular way in which microscopic particles move in liquids or gases due to the thermal motion of molecules [87]. The nano-enhanced phases are small and light; thus, their Brownian motion is fast, enabling their distribution over a large area in a short time. The nano-enhanced phases in the base fluid are in irregular motion and are thus subjected to Brownian forces. Van der Waals force potential exists for mutual attraction. Numerous atoms between neighboring particles are attracted to each other due to their large percentages on the surface of nano-enhanced phases; therefore, van der Waals forces emerge between particles. This force varies according to the distance between two particles; it usually increases as the distance decreases and is directly related to the geometry, size, and surface properties of the particles. Van der Waals gravity and electric double-layer repulsion depend on the size of the nano-enhanced phases. Numerous atoms are present on the particle surface when the particle size is small [21], which increases the density of active sites. Agglomeration occurs when van der Waals attraction exceeds electric double-layer repulsion. Several researchers have reported that NPECs are less stable at high particle concentrations [88,89]. The increase in particle concentration results in large particle cluster sizes, which is due to low interparticle distances and increased van der Waals gravity, directly affecting the settling velocity. In addition, the nano-enhanced phases have gravity. Under the joint action of these forces, the metal particles are prone to collision, agglomeration, and settlement. Therefore, NPECs should reduce the frequency and efficiency of collision to minimize the impact of agglomerates and maintain good stability.
2.4.2 Stability strategies
NPECs are usually treated using physical or chemical methods to achieve stability. Physical methods aim to disperse the nano-enhanced phases in the liquid medium using external mechanical energy, such as shear or collision force, which destroys the original kinetic behavior of the nano-enhanced phases and reduces the interaction between nano-enhanced phases. Chemical methods change the surface properties of suspended nano-enhanced phases and inhibit the formation of particle clusters to obtain a stable suspension.
Commonly used physical stabilization methods include mechanical mixing and ultrasonic treatment. The device is shown in Fig.7. Mechanical stirrers are important tools for promoting NPEC stability. Mansour et al. [90] homogeneously dispersed TiO2 nano-enhanced phases in base oil using a magnetic stirring process. However, the stabilization time of NPECs obtained by mechanical stirring is short. Ultrasonication is one of the methods used to improve the stability of NPECs and shows better stability than magnetic stirring. Ultrasonication places the NPEC directly into the ultrasonic field and uses ultrasonic waves of suitable frequency to facilitate the movement of nano-enhanced phases. Therefore, this method overcomes the mutual attraction between the nano-enhanced phases and destroys the balance between the original particles as well as between the particles and molecules of the base liquid, dispersing the nano-enhanced phases into the base liquid. Nguyen et al. [91] reported the effectiveness of sonication for the production of monodispersed Al2O3 by ultrasonication at 30% amplitude for 500 s. The size of the nano-enhanced phase clusters was reduced from 230 to 130 nm. Sadeghi et al. [92] also used ultrasonication and found that the cluster size of nano-enhanced phases decreased with increasing ultrasonication time within 150 min. Stability was verified using ζ potential and dynamic light scattering. In addition, sonication improves stability; however, excessive sonication leads to shortening of the carbon nanotube (CNT) length, and optimized sonication is necessary to prevent such defects [93]. Elsheikh et al. [94] added CuO nano-enhanced phases to rice bran vegetable oil at a volume concentration of 1 vol.% and stirred the mixture under a magnetic stirrer for 4 min. The prepared solution was then sonicated for 2 h using an ultrasonic generator to ensure uniform dispersion. Therefore, the results reveal the optimal ultrasonic treatment time for each NPEC. However, no accurate range of treatment times exists for enhanced coolants of different nano-enhanced phases to obtain NPECs with the desired optimum properties. Multiple physical methods can be used to synergize the treatment to disperse the NPEC and obtain improved processing properties.
The influence of the nature of the nano-enhanced phases (hydrophilic or hydrophobic) should be considered during the selection of the base fluid, which is either polar or nonpolar. Hydrophilic nano-enhanced phases such as metal oxide particles (CuO, SiO2, and Al2O3) are readily dispersed in polar solvents such as water, whereas hydrophobic nano-enhanced phases (CNT, GNP) are readily dispersed in nonpolar base fluids. In these cases, the use of additional stabilizers is no longer necessary because the stability can be sufficiently improved using physical methods. However, different chemical stabilization techniques are required to form stable NPECs by mixing hydrophilic nano-enhanced phases in nonpolar solvents or hydrophobic nano-enhanced phases in polar solvents. Fig.7 shows the chemical stabilization methods, which include electrostatic stabilization (surfactants and covalent surface functional groups) and spatial stabilization (ionic polymers or ionic liquids).
The electrostatic stabilization technique is mainly based on providing a surface charge to the particles. Ion adsorption creates a double electric layer around the nano-enhanced phases, and electrostatic stabilization is achieved when electrostatic repulsive forces play a dominant role and balance the van der Waals attraction between two similarly charged particles. Surfactants with different charges are added to the NPEC suspension to improve electrostatic stability [95]. Tab.3 shows common surfactants, where the amphoteric surfactant includes cationic and anionic hydrophilic groups that can form cations and anions based on the pH of the medium. Mao et al. [96] found that NPEC containing sodium dodecylbenzene sulfonate (SDBS) surfactant prevented the aggregation of nano-enhanced phases and significantly improved the cutting performance. However, supersaturated adsorption on the surface of nano-enhanced phases occurred when the SDBS concentration exceeded 0.5 wt.%, at which point the suspension stability of the NPEC deteriorated with increasing SDBS concentration [67]. Behera et al. [97] investigated the diffusion behavior of Al2O3 NPEC with different surfactants on WC tools and Inconel 718 workpieces. Their results showed that the optimum wetting behavior was observed when nonionic surfactant was added to NPEC, and the lowest COF, tool wear, and radius of chip curl were obtained in Inconel 718 machining. Shukla et al. [98] investigated the effect of MoO3 NPEC mixed with six different surfactants on the properties of turned AISI 304 steel. The optimum mixing ratio and volumetric concentration were obtained from ζ-potential and thermal conductivity tests, respectively. The results showed that SPAN20 surfactant provided the best processing performance at a mixing ratio of 3:2 and 0.45 vol.%. The average cutting force and tool wear were significantly reduced by 32.05% and 53%, respectively, with a minimum surface roughness of 1.21 μm. Şirin et al. [99] used sodium dodecyl sulfate (SDS) and gum arabic (GA) as surfactants added to a hybrid NPEC to improve the homogeneity by decreasing the surface tension, thus enhancing the thermal conductivity.
Tab.3 Surfactants commonly used in machining |
| Type | Surfactant |
|---|---|
| Nonionic | Alkylphenol ethoxylates, octyl phenol ethoxylate, polyvinylpyrrolidone |
| Cationic | Sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, sodium oleate |
| Anionic | Gum arabica, dodecyl trimethyl ammonium bromide, cetyltrimethylammonium bromide, cetyltrimethylammonium chloride |
| Hermaphroditic | Lecithin, hydroxy sulfobetaine |
However, the exact type of surfactant to be added has not been specifically demonstrated, while most surfactants (SDBS, SDS, GA, cetyltrimethylammonium bromide (CTAB), and polyvinylpyrrolidone) are organic in nature and have some biodegradability issues. These surfactants may deteriorate over time due to biological activity and operating conditions. Therefore, the stability of surfactants may be a major challenge when considering long-term applications. Spatial stabilization of NPECs can be achieved using nonionic surfactants and polymers. This stabilization technique involves noncovalent functionalization [100]. Spatial stabilization can stabilize NPECs with high particle concentrations, but electrostatic stabilization cannot. Furthermore, this stabilizing effect cannot be predicted using the ζ-potential method because the polymer does not affect the charge of the nano-enhanced phases. Currently existing limitations include the sensitivity of spatial stability to temperature changes, stabilizer concentrations, and absorption spectra.
Overall, the plant-based oil, molecular movement, and external factors of NPECs affect their stability. The selection and preparation methods for NPEC should consider their application properties and stability. A broad study of hybrid stabilization methods, that is, a combination of different physical and chemical methods, is needed to improve the stability of NPECs under different operating conditions. The above methods of stability enhancement can help improve the availability of NPECs in additional applications.
3 Thermophysical properties
NPECs are widely used in machining applications due to their excellent cooling and lubricating properties. In the last decade, exponential growth in scientific research on NPECs has been observed. However, the properties of NPECs vary depending on their production, particle size, dispersion, and concentration [101]. The presence of nano-enhanced phases modifies the thermal conductivity, viscosity, and wettability of the base fluid and can produce a lubricating film at the processing interface [102]. This lubricant film acts differently at the machining interface. In some cases, the nano-enhanced phases fill microcracks and pores, act by rolling between surfaces, and cause the surface to be polished during the formation process, thus improving surface integrity [61,103–105]. This section focuses on the thermophysical properties of NPECs, such as their thermal conductivity, viscosity, wettability, and tribological properties.
3.1 Thermal conductivity
NPECs are widely used because of their excellent thermal properties, and their thermal conductivity reflects the heat transfer capability of NPECs in machining. Previous studies found that the addition of nano-enhanced phases to the base fluid can further improve heat transfer because solid media typically have better heat transfer properties than liquids. Titanium alloys are commonly used in the manufacture of aerospace structural components; however, they have become a current research hotspot due to their high strength and low thermal conductivity. One study found that at 0.1 wt.% volume fraction of GNP, the addition of GNP can further reduce the grinding temperature by approximately 21.1% compared with biolubricants [106]. The results show that nano-enhanced phases are crucial in difficult-to-machine materials. The excellent lubricating properties of NPECs can reduce the degree of friction and temperature in the cutting zone. The main heat transfer mechanism of NPECs in the cutting zone can be analyzed from two perspectives. i) A thermal boundary layer, including laminar and turbulent thermal boundary layers, is formed during heat transfer in the cutting zone, and the laminar thermal boundary layer mainly serves for heat transfer. NPECs may cause thermal damage to the workpiece due to heat accumulation in the turbulent layer and lowering its viscosity facilitates heat transfer, as shown in Fig.8. ii) Mao et al. [96] proposed four stages of boiling heat transfer in a lubricating medium as a function of temperature: non-boiling heat transfer, nucleation boiling, transition boiling, and stable film boiling. In the nucleation boiling region, the heat transfer coefficient increases rapidly with temperature and reaches a maximum when the critical heat flow density is reached. An extremely high surface temperature can lead to liquid film boiling and bubble generation, which further accelerates the heat transfer. As shown in Fig.8, the binding of nano-enhanced phases results in the formation of an adsorption layer around them, and the presence of this layer reduces the thermal resistance between the interior of the droplets, which, in turn, facilitates heat transfer [107]. In addition, the nano-enhanced phases in the NPEC have irregular diffusion and Brownian motion, and they will adsorb the surrounding biolubricant to form a fluid adsorption layer, which can eliminate part of the heat in contact with the workpiece and enhance the heat transfer between the workpiece and the NPEC.
Therefore, the study of the thermal conductivity of NPECs is of considerable importance for processing performance. The nature of the base fluid, concentration of nano-enhanced phases, particle size, shape, and temperature affect the thermal conductivity of NPECs. The transient hot-wire technique is highly accurate and stable and is commonly used to measure the thermal conductivity of NPECs.
3.1.1 Effect of temperature and concentration
The addition of nano-enhanced phases to the base fluid increases the thermal conductivity of the NPEC. Furthermore, increasing the solid concentration and temperature of the nano-enhanced phases similarly affects the thermal conductivity of NPECs. Fig.9 [50,108–110] shows the variation of NPECs with temperature and thermal conductivity. Ramya et al. [111] investigated the relationship between the thermal conductivity and temperature of enhanced coolants containing ZnO nano-enhanced phases and found that the thermal conductivity increases with temperature. This finding was also demonstrated by Duangtongsuk and Wongwises [112] in their experimental study of the thermal conductivity of TiO2 NPECs with volume concentrations of 0.2–2 vol.%. Fig.9(a) shows the thermal conductivity variation of six concentrations of GNP nanosheets as a function of test temperature, where the NPEC thermal conductivity further increases with temperature, regardless of the concentration [108]. Kole and Dey [113] explored the effect of temperature and nano-enhanced phase concentration on thermal conductivity. The results showed that 2.5 vol.% of CuO nano-enhanced phases increased the thermal conductivity of the base fluid by 10.4% at room temperature and 11.9% at 80 °C. Saeedinia et al. [114] investigated the thermal conductivity of CuO NPECs with particle weight fractions of 0.2‒2 wt.% at different temperatures. The results showed that the thermal conductivity increased with the addition of nano-enhanced phases to the base fluid and further increased with particle concentration. The best thermal conductivity was observed at a volume fraction of 2% with an increase of 6.2%. As shown in Fig.9(c), the thermal conductivity of all three NPECs was enhanced as the GNP concentration increased from 0 to 2.5 wt.%. The 0.5, 1, 1.5, 2, and 2.5 wt.% concentrations of GNP resulted in 3.6%, 7.2%, 10.8%, 14.4%, and 18% enhancements in the thermal conductivity of the base canola oil [50]. Wang et al. [115] investigated the effect of temperature and solid concentration on the thermal conductivity of graphite NPEC. They found that the addition of graphite with a volume fraction of 1.36 vol.% to the base fluid increased the thermal conductivity by 36%.
Fig.9 Thermal conductivity as a function of temperature and concentration: (a) variation of graphene (GNP) nanoparticle-enhanced coolants (NPECs) thermal conductivity with temperature; (b) variation of thermal conductivity of Al2O3 and Al‒GNP NPEC with temperature [109]; (c) variation of GNP thermal conductivity with concentration in three different base oils [50]; (d) variation of thermal conductivity of GNP with concentration [110]. Reproduced with permissions from Refs. [50,109,110] from Elsevier. |
First, the thermal conductivity increases with concentration and temperature, probably because the NPEC reduces the agglomeration of nano-enhanced phases with increasing temperature, thus raising the homogeneity of NPEC in the mixture and the thermal conductivity. Second, the thermal conductivity of the nano-enhanced phases is substantially larger than that of the base fluid, leading to an increase in the thermal conductivity with the nano-enhanced phases. In addition, the random motion of the nano-enhanced phases increases with temperature; therefore, energy is rapidly transferred inside the fluid. The Brownian motion and aggregation of the nano-enhanced phases are affected by temperature changes and show variations in the thermal conductivity of the NPEC.
3.1.2 Influence of other factors
In addition to temperature and nano-enhanced phase concentration as factors influencing the thermal conductivity of NPECs, the size, shape, and type of nano-enhanced phases also affect the thermal conductivity of NPECs.
1) Size
The size of the nano-enhanced phases also affects the thermal conductivity of NPECs [116]. Researchers investigated the effect of particle size variation (5–100 nm) on the thermal conductivity of synthesized NPECs. Sharifpur et al. [117] prepared Al2O3 NPECs of various sizes (31, 55, and 134 nm diameter) and found that a low diameter leads to a superior degree of thermal conductivity enhancement. Teng et al. [118] dispersed nano-enhanced phases of different nominal diameters (20, 50, and 100 nm) into NPECs of four different concentrations (0.5, 1, 1.5, and 2 wt.%). The results show a correlation between small-sized nano-enhanced phases and high temperatures; a small diameter leads to improved thermal conductivity. This phenomenon is attributed to the enhancement of Brownian diffusion at small nano-enhanced phase sizes [119]. Xie et al. [120] measured the thermal conductivity of four different particle sizes of NPECs. The current study found that instead of high thermal conductivity due to the small particle size, the thermal conductivity is a result of the combined effect of Brownian diffusion and its surface area.
Therefore, the size of the nano-enhanced phases affects the thermal conductivity of the NPEC. Small nano-enhanced phases lead to high surface areas and weak van der Waals forces, resulting in low agglomeration and improved stability. This phenomenon increases the thermal conductivity of the NPEC; however, its size should not be too small. The results suggest that the diameters of nano-enhanced phases between 20 and 40 nm are optimal for thermal conductivity.
2) Shapes
The shape of nano-enhanced phases is a crucial parameter that affects the thermal conductivity of NPECs because the surface area to volume ratio (A/V) of these phases affects the thermal conductivity of NPECs. Wei et al. [121] found that NPECs with rod-like particles exhibit superior thermal conductivity due to their larger A/V compared with spherical nano-enhanced phases. Maheshwary et al. [122] investigated the thermal conductivity of NPECs with various nano-enhanced phase shapes (spherical, cylindrical, and cubic). Regarding the three types of nano-enhanced phases, cubic has the best heat transfer performance due to high A/V. Spherical nano-enhanced phases exhibit poor performance; however, their low fabrication cost and excellent tribological properties promote further applications in heat transfer.
3) Types
Different types of nano-enhanced phases may affect the thermal conductivity of NPECs. However, this parameter is not the only one that determines the thermal conductivity of NPECs [123–125]. The results show that TiO2 NPECs exhibit high thermal conductivity based on the thermal conductivities of Al2O3, TiO2, WO3, and Fe NPECs [126]. Pang et al. [127] synthesized Al2O3 and SiO2 NPEC with volume fraction of 0.005‒0.5 vol.% in methanol. The results show that the thermal conductivity in SiO2 NPEC is higher than that in other NPECs, probably because the agglomeration of Al2O3 nano-enhanced phase affects the stability, further influencing the thermal conductivity. Five types of NPECs were synthesized by adding nano-enhanced phases (TiO2, ZnO, Al2O3, SiO2, and MgO) to the ethylene glycol (EG)-based fluid, and the thermal conductivity was measured. Xie et al. [128] found that the thermal conductivity of the NPEC not only depended on the intrinsic thermal conductivity of the nano-enhanced phases but also necessitated considering a combination of influencing factors.
Overall, excellent thermal conductivity was obtained using exceptionally small sizes (average size of 20–40 nm) and relatively large surface areas and volume ratios in the nano-enhanced phases. In addition, the dispersion of nano-enhanced phases (ZnO/GNP/Al2O3/CuO/TiO2/MoS2/SiO2/Fe) in the base fluid (water/vegetable oil) with approximately 1‒5 wt.% increased the thermal conductivity of NPECs by 10%–30%. Incorporation of nano-enhanced phases improves the thermal conductivity of NPECs. However, their importance for sustainability and human health should be considered when using these phases for machining.
3.2 Viscosity
Viscosity is the flow resistance of the fluid. It is another important factor in the application of NPECs in heat transfer applications and has a direct effect on pressure loss and pumping power. Viscosity has a direct effect on the internal COF and influences the formation of the lubricant film. High-viscosity cutting fluids can form a lubricating film on high-temperature friction surfaces, increasing the load-carrying capacity and further improving the lubrication effect. As shown in Fig.10, the different viscosity values of each vegetable oil are mainly due to its fatty acid molecular structure. At high viscosity, the friction conditions between the tool and the workpiece and those between the tool and the chip are improved. This trend is particularly evident in castor oil, which has a substantially higher viscosity than other base oils. Therefore, castor oil has a low COF [52]. The incorporation of nano-enhanced phases further increases the viscosity of the biolubricant and improves its lubricating properties. Cui et al. [129] investigated the tribological properties of GNP NPECs at the wheel–workpiece interface through frictional wear tests. GNP nano-enhanced phases have a large specific surface area, which improves the viscosity and lubrication properties of the NPECs. Zhang et al. [130] suggested that the weak intermolecular shear strength of layered nano-enhanced phases in MoS2 also reduces the friction in the cutting zone. The extreme pressure in the cutting zone extrudes the nano-enhanced phases out of the film to enhance the extreme pressure properties of the biolubricant [131]. As shown in Fig.10(b), nano-enhanced phases with an appropriate volume fraction help in lubrication. However, as the concentration increases, aggregation occurs due to the strong van der Waals gravity of the molecules. The agglomerated nano-enhanced phases eventually lead to the cessation of oil droplet viscosity [132]. This phenomenon can be explained by the aggregation of nano-enhanced phases, which causes some degree of precipitation. In addition, the decrease in the number of effective active nano-enhanced phases in the droplets is unsuitable for mechanical processing.
Therefore, studying the viscosity of NPECs for processing performance is crucial. Various parameters affect the effective viscosity of NPECs, such as the viscosity of the base fluid, concentration of nano-enhanced phases, particle size, shape, and temperature. A piston rheometer, rotational rheometer, and capillary viscometer were used to determine the viscosity of the NPECs.
3.2.1 Effect of temperature and concentration
The temperature and solid concentration of NPECs are two important factors that affect viscosity. Many studies have shown that the addition of nano-enhanced phases increases the viscosity of the base fluid. In addition, the agglomeration behavior increases with the concentration of nano-enhanced phases, increasing the fluid viscosity [133]. Saeedi et al. [134] synthesized CeO2 nano-enhanced phases in an EG to investigate the effect of nano-enhanced phase volume fractions (0.05–1.2 vol.%) on NPEC viscosity. Their results showed that viscosity was enhanced with an increase in the volume fraction. The viscosity of the 1.2 vol.% NPEC increased by 95% compared with that of the base fluid. Krishnakumar et al. [135] synthesized Al2O3-EG NPECs with volume fractions ranging from 0.1 to 1 vol.% and found that the viscosity increased with the volume fraction of NPECs.
Duangtongsuk and Wongwises [136] experimentally investigated the dynamic viscosity of NPECs. In this study, TiO2 nano-enhanced phases dispersed in water with a volume concentration of 0.2–2 vol.% and a temperature range of 15–35 °C were used. The results show that the viscosity of the NPEC increases with particle concentration and decreases with increasing temperature of the NPEC [137]. Singh et al. [50] investigated the effect of temperature on the viscosity of GNP NPECs. Fig.11 [50,104,138,139] shows the relationship between the viscosity of NPECs and the temperature and concentration. As shown in Fig.11(a), the dynamic viscosity decreases with the increase in temperature [104]. Soltani and Akbari [139] investigated the effect of temperature change on the dynamic viscosity of multi-walled carbon nanotube (MWCNT)‒MgO hybrid NPECs. Increasing the temperature from 25 to 50 °C leads to a decrease in the dynamic viscosity of the NPEC by approximately 77%. Fig.11(c) shows the increase in viscosity of the NPEC with the addition of HBN, graphite, and MoS2 [138]. This increase is proportional to the increase in nano-enhanced phases, which is attributed to the increase in the thickness of the fluid layer between the friction pair. Fig.11(d) shows the dynamic viscosity of MWCNT–MgO hybrid NPECs as a function of the nano-enhanced phase volume fraction at different temperatures. The viscosity of the hybrid NPEC increases with the nano-enhanced phase volume fraction at different temperatures with a similar trend [139].
Fig.11 Viscosity as a function of temperature and concentration. (a) Variation of graphene nanoparticle-enhanced coolant (NPEC) viscosity with temperature [50]; (b) variation of viscosity with temperature for six NPECs [104]; (c) viscosity variation with concentration for three different NPECs [138]; (d) variation of MWCNT‒MgO hybrid NPECs viscosity with solid volume fraction [139]. CNTs: carbon nanotubes; HBN: hexagonal boron nitride; MWCNT: multi-walled carbon nanotube; ND: diamond. Reproduced with permissions from Refs. [50,104,138,139] from Elsevier. |
Therefore, the viscosity of the NPEC decreases with increasing temperature, probably due to the effect of Brownian diffusion motion, particle cluster formation, and drag caused by the sliding motion of nano-enhanced phases and the underlying fluid, and the interaction between fluid molecules decreases with increasing temperature. Therefore, the temperature increases and the viscosity of the NPEC decreases. In addition, the formation of particle clusters and the high number of nano-enhanced phases impede the intermolecular motion when the concentration increases, thereby increasing the viscosity of the NPEC.
3.2.2 Influence of other factors
1) Size
The size of the nano-enhanced phases is an important parameter that affects the viscosity of NPECs. Several researchers have investigated the viscosity of NPECs based on the size of the nano-enhanced phases and have reported inconsistent and opposite results [141–147]. Hu et al. [142] synthesized NPECs with Al2O3 and ZnO nano-enhanced phases with particle sizes ranging from 20 to 100 nm. The viscosity of Al2O3 NPECs increased with particle size at a volume concentration of 7.5 vol.%. However, the trend was reversed after the particle size reached 50 nm at the same concentration. The viscosity increases with particle size at volume fractions higher than 7.5 vol.%. The viscosity of the ZnO NPEC tends to increase with particle size. The viscosity enhancement is negligible at low volume concentrations of the ZnO nano-enhanced phases. Minakov et al. [143] investigated the size effect of nano-enhanced phases on the viscosity of NPECs. The authors reported that the viscosity of all types of NPECs decreased with increasing particle size. Nithiyanantham et al. [145] studied the effect of SiO2 particle size on the viscosity of NPECs. The results show that the viscosity increase is high at low particle sizes due to small interparticle distances and high van der Waals forces between the two molecules [148–150]. By contrast, Li et al. [151] investigated the size effect of nano-enhanced phases on the viscosity of NPECs and found that the viscosity increased significantly with particle size. In another study, Abdelhalim et al. [147] reached similar conclusions.
2) Shapes and types
Timofeeva et al. [152] investigated the effect of particle shape on the viscosity of Al2O3 NPECs. Various shapes of nano-enhanced phases were considered in the experimental study, and the results showed that NPECs with cylindrical and lamellar nano-enhanced phases exhibited higher viscosity than other shapes, mainly due to their surface-based volume ratio effects [153,154]. Studies on the effect of the particle shape of SiO2 and ZnO on the viscosity of NPECs revealed that the particle shape does not vary disproportionately [154]. The factors affecting the viscosity of NPECs require further systematic studies before conclusions can be drawn. Cui et al. [129] found that the effect of nano-enhanced phases on the viscosity of base oil varied by comparing four types of nano-enhanced phases (HBN, GNP, MoS2, and MoO3) with pure base oil (palm oil), and the viscosity of the four NPECs was higher than that of pure oil. HBN NPECs showed the lowest viscosity (72.4 mPa·s), which was 9.5% higher than the viscosity of pure oil (66.1 mPa·s). By contrast, GNP NPECs had the highest viscosity (82.2 mPa·s), which was 24.4% higher than the viscosity of pure oil.
Overall, the temperature concentration of the NPEC, as well as the size, shape, and type of the particles, affect its viscosity, thus yielding effects. In the process of selecting nano-enhanced phases, the type of base fluid and nano-enhanced phases should be balanced to obtain optimal processing performance. In addition, the viscosity of the NPEC increases with particle size, probably due to the intermolecular van der Waals forces, which increase fluid resistance, leading to an increase in viscosity.
3.3 Wetting performance
Surface tension has a crucial effect on the NPEC trace lubrication wetting of the tool–workpiece interface. It is an important parameter that affects droplet breakup and tearing, and the state of the droplets on the workpiece determines the wetting effect. Surface tension typically has a significant effect on the boiling heat transfer process, wetting behavior, and spray characteristics of NPECs. Bertolini et al. [155] measured the surface tension of NPECs and found that the addition of nano-enhanced phases to the base fluid decreases the surface tension. In addition, surface tension determines the boiling point, bubble departure field, and interfacial equilibrium of the fluid [156]. Zhang et al. [157] discussed the effect of base oil viscosity and surface tension on friction and found that vegetable oils with high viscosity and low surface tension exhibited excellent cooling lubrication properties. A negative correlation between surface tension and heat transfer coefficient was observed for NPECs. The superior heat transfer coefficient of NPECs may be attributed to the formation of additional bubbles and their expansion due to weak binding. Therefore, the presence of additional bubbles and high boiling point heat transfer activity helps obtain superior performance, such as lowering the temperature in the grinding zone, avoiding burns on the workpiece, and improving the machining quality of the workpiece [104]. Jia et al. [158] reported that lowering the surface tension of the liquid facilitates the improvement of the atomization characteristics. Therefore, lubricant use and migration permeability can be further improved.
Adhesion between the liquid and solid causes droplet distribution onto the surface. The viscosity of the liquid prevents the droplet from contacting the surface. When a drop of liquid (cutting fluid) rests on a solid surface to form an interface, the angular profile between the droplet and the contacting surface is called the wetting (contact) angle. Small contact angles are considered to have superior wetting properties [103]. By contrast, when the contact angle is large, the droplet wetting capability is poor. With the addition of nano-enhanced phases, the contact angle gradually decreases and the wetting performance improves. Li et al. [104] obtained the contact angles of various NPECs on nickel-based workpieces, as shown in Tab.4. They observed additional bubbles on the surface of the workpiece containing NPEC, indicating a high boiling heat transfer coefficient. A high heat transfer coefficient indicates that the NPEC has a high heat-carrying capacity, which may be a good option for effective cooling and lubrication during machining. Zhang et al. [159] found that the contact angle significantly affects the surface quality of the workpiece. A small contact angle leads to a low surface roughness of the workpiece, resulting in good cooling and lubrication. A small contact angle results in a large penetration area of the NPEC, which significantly affects the lubrication and cooling performance [130].
Tab.4 NPEC contact angles on a Ni-based workpiece |
| Nanoparticle-enhanced coolants | Contact angle/(° ) |
|---|---|
| SiO2 | 49.2 |
| PCD | 43.5 |
| CNT | 47.5 |
| MoS2 | 46.0 |
| Al2O3 | 45.5 |
| ZrO2 | 41.5 |
Notes: PCD, polycrystalline diamond; CNT, carbon nanotube. |
A strong relationship exists between boiling heat transfer and wettability, especially during bubble formation, expansion, separation, and movement. The low surface tension of the NPEC suggests a weak binding force for bubble formation and expansion. The presence of additional bubbles and high boiling point heat transfer activity helps in obtaining excellent properties, such as lowered temperature in the grinding zone, prevention of workpiece burning, and improvement of the workpiece machining quality [104]. In addition, according to the theory of thermal convection, the droplets during thermal convection can be divided into the thermal boundary layer and the main flow zone. The thickness of the thermal boundary layer remains constant. However, the fluid in the main flow zone moves quickly from the cut zone before absorbing sufficient heat. In other words, the fluid in the main flow region does not provide satisfactory heat exchange. As shown in Fig.12 [157], when the surface tension decreases, the thermal boundary layer expands and the proportion of abrasive fluid in the main flow region decreases. This result explains the high cooling efficiency of MQL droplets with small contact angles.
Thus, the effectiveness of cooling lubrication depends on the wetting properties of NPECs. Surface tension influences the boiling heat transfer process, and contact angle provides an inverse measure of wettability. When the contact angle decreases, the area of the thermal boundary and the fluid involved in heat transfer both increase. Meanwhile, the high thermal conductivity and heat transfer coefficient of the NPEC indicate improved heat dissipation performance. NPECs can effectively improve the heat transfer efficiency and reduce the thermal load introduced to the tool–workpiece interface during the cutting process. In addition, CNT NPECs with high heat transfer coefficients and significant cooling and lubrication capabilities may be a superior choice for the machining of MQL cemented carbides (high-temperature alloys).
3.4 Tribological properties
Friction plays a vital role in machining. A considerable amount of heat is generated due to the presence of friction between the tool and the chip and between the tool and the workpiece, which affects the tool life and the surface quality of the workpiece [160–163]. Therefore, understanding the tribological behavior of the chip–tool–workpiece interface during machining is crucial [164]. Several factors influence the COF in the machining interface, such as type of machining, material, geometry, and tool wear, as well as lubrication and cooling conditions [165]. Adding coolant to the machining process can reduce the cutting temperature. In addition, scholars have proven that the addition of nano-enhanced phases in the base fluid can effectively reduce the machining temperature, friction, and wear based on the original [166,167]. As shown in Fig.13 [168], scholars have proposed that NPECs have four effects of cooling and lubrication in machining.
Sikdar et al. [169] stated that the addition of solid lubricants to the cutting fluid helps fill the concave and convex surface gaps and promotes sliding on the friction subsurface. Spherical or quasi-spherical nano-enhanced phases can form a ball-bearing effect on the friction surface. In this case, sliding friction is transformed into rolling friction with a low COF. Nano-enhanced phases in NPECs improve tribological performance by eliminating direct tool–workpiece contact and filling surface gaps to reduce frictional wear [61,170–172]. Ouyang et al. [173] evaluated the performance of wear tests containing 0.5 wt.% GNP and 0.7 wt.% MWCNTs in castor oil and wear traces obtained by scanning electron microscopy, energy dispersive spectroscopy, and Raman spectroscopy on steel plates and balls. The results of the wear analysis of steel balls and plates were highly consistent, indicating that graphene CNTs reduced the COF and surface wear by 26% and 48%, respectively, at low speeds and under loaded conditions. Samuel et al. [174] observed that the nanolayer increases with the concentration of nano-enhanced phases, and wear on the surface decreases due to the nanolayer between the surfaces. This NPEC layer supports large loads; in the case of spherical particles, this layer induces a rolling effect, which reduces the COF and cutting forces [168]. Kumar et al. [35] verified the tribological performance tests of four water-based NPECs (HBN, Al2O3, MoS2, and WS2), and alumina NPECs showed optimum tribological performance. The COF was reduced by 53.89% compared with dry friction. HBN, MoS2, and WS2 NPECs may be due to the layered structure, where the layers are bonded together by weak van der Waals forces and the lubrication mechanism involves sliding of the layers.
As indicated in the studies of Gulzar et al. [175] and Ali et al. [176], Al2O3 and TiO2 nano-enhanced phases can form a friction film that protects the machined material from crack propagation and reduces the surface roughness by filling the surface grooves and bumps. Meanwhile, Peña-Parás et al. [177] demonstrated that TiO2-filled surface grooves reduced friction due to their hemispherical shape and small size. However, airflow variations in MQL applications can affect the formation of protective films. High pressure transports the NPEC close to the cutting edge and removes substantial heat [61]. Wang et al. [28] revealed that some nano-enhanced phases of solid lubricating materials, such as GNP, can form a smooth and dense friction film on the workpiece surface during the machining process, which helps further reduce friction and surface wear. Talib et al. [178] found that COF increased at concentrations higher than 0.05 wt.% of HBN because the agglomeration of nano-enhanced phases led to the deterioration of the tribological behavior of the NPEC, possibly due to the high hardness of HBN nano-enhanced phases, which made HBN an abrasive material at high concentrations. However, only some nano-enhanced phases can improve the processing performance of vegetable oil-based lubricants. Alves et al. [179] investigated the effect of adding nano-enhanced phases to vegetable oil-based lubricants and found that the addition of CuO nano-enhanced phases to sunflower and soybean oils increased the COF by approximately 20% and 7.5%, respectively. They found that the addition of oxide nano-enhanced phases to plant-based lubricants may deteriorate film formation. The authors explained that this phenomenon may be related to the third-body behavior of these oxides, which increases friction and reduces the electrical conductivity between the sample surfaces [180]. In addition, Azman et al. [181] revealed that an increase in the concentration of GNP and nano-enhanced phases in the cutting fluid proportionally increased the COF. In this case, the increase in the COF was caused by the substantial aggregation of high concentrations of GNP nano-enhanced phases. The augmented concentration of NPECs and the degree of chemical interaction between the particles and the newly formed surface increase the protective film between the tribological pairs, which improves the surface quality and reduces the COF [61].
In addition, some high hardness nano-enhanced phases can be considered as precision polishing materials. After polishing, the roughness of the friction pair decreases, the contact area increases, and the COF decreases. The compressive stresses on the contact surfaces are also reduced, thus increasing the load-bearing capacity of the lubricant, which is referred to as the polishing mechanism of nano-enhanced phases. Thus, the physicochemical properties of different nano-enhanced phases determine their role at the friction interface. With the combination of relatively low cutting zone friction and heat flux density, in terms of specific energy, NPEC conditions can reduce specific energy by 40%–42%, 25%–30%, and 17%–19% compared with dry cutting, flooded, and MQL, respectively [182]. Sayuti et al. [61] used NPEC produced from SiO2 nano-enhanced phases ranging from 5 to 15 nm with nozzles located at 15°, 30°, 45°, and 60° in MQL applications during end milling and observed that the nano-enhanced phases acted as a polishing effect. In addition, the impregnation of sheared nano-enhanced phases in the surface pores shows shearing of the particles, surface peaks, and pore filling, leading to the formation of a thin protective film comprising sheared whole nano-enhanced phases. Such replicated conditions at the processing interface lead to the removal of surface roughness of the material, contributing to the increase in chemical bonding of the NPEC to the newly formed surface [131,183]. Rahman et al. [184] used nano-enhanced phases of Al2O3, MoS2, and TiO2 in vegetable oils of rapeseed oil and extra virgin olive at concentrations of 0.5, 2, and 4 vol.%, respectively, as a cutting fluid for turning Ti‒6Al‒4V ELI alloys. This study demonstrated improved surface integrity through the polishing effect induced by canola oil NPEC containing 0.5 vol.% by volume Al2O3 particles. The temperature is also reduced at the chip–tool–workpiece interface and at the COF due to the rolling mechanism of the existing spherical Al2O3 nano-enhanced phases in the friction film formed at the machining interface. In addition, NPECs containing 0.5 vol.% volumetric MoS2 were effective in reducing the machining temperature, which may indicate a reduction in cutting forces and COF due to the rolling effect. Similarly, Al2O3 and TiO2 nano-enhanced phases acted as a rolling mechanism between the interacting surfaces [185].
Cui et al. [129] found that the laminar structure allows the lubricant to effectively fill the pits on the surface of the workpiece and increase the surface smoothness, thus improving the lubrication performance, as shown in Fig.14(a). The average value of the COF reaches its maximum (0.449) under palm oil lubrication conditions. As shown in Fig.14(b), the COF of the GNP condition is 34.3% lower than that of the MQL condition of pure palm oil. The experimental results of the average COF proved that GNP nano-enhanced phases exhibit excellent anti-friction properties. This finding may be due to the hexagonal honeycomb structure of GNP molecules. Therefore, GNP NPECs exhibit the strongest lubrication stability and wear resistance. In addition, Singh et al. [109] observed that Al–GNP hybrid NPECs had the lowest COF at 1 vol.% nano-enhanced phase concentration. As shown in Fig.14(c), the tool wear rate and COF of Al–GNP hybrid and Al2O3 NPECs were reduced with the increase of nano-enhanced phase concentration, respectively, compared with the base fluid. In addition, at 1 vol.% nano-enhanced phase concentration, the Al–GNP hybrid NPECs exhibited the best performance. An experimental study by Kalita et al. [186] revealed improved lubrication with increasing concentrations of MoS2 nano-enhanced phases. The COF of soy-based NPECs (8 wt.%) was reduced by almost 14% compared with NPECs containing 2 wt.% nano-enhanced phases.
Fig.14 (a) Macroscopic lubrication mechanism model of stratiform nano-enhanced phases; (b) effect of different types on the coefficient of friction (COF) [129], reproduced with permission from Elsevier; (c) effect of nano-enhanced phase concentration on COF [109], reproduced with permission from Springer Nature. GNP: graphene; HBN: hexagonal boron nitride. |
The application of NPEC markedly reduces friction at the interface, further decreasing the frictional heat source and the generation of cutting heat, subsequently reducing the cutting temperature. Simultaneously, the temperature reduces the thermal load of the friction interface and the thermal stress of the friction vice and suppresses the formation of residual tensile stress and microcracks. Thus, the surface quality of the workpiece is significantly improved. In addition, the reduction in friction reduces tangential force loads, stripping between interfering bumps at the interface of the friction pair and mechanical wear. NPECs provide excellent heat transfer, film-forming, wetting, antiwear, and friction reduction properties, improving machine efficiency and reliability. The analysis shows that the NPEC can easily penetrate the machining interface and increase the heat transfer capacity, facilitating its promotion in the MQL machining field.
4 Applications in machining
Titanium alloys [187], high-temperature alloys [188], high-strength steels [49], composite materials [189], and other difficult-to-machine materials [190,191] are widely used in numerous fields, such as medical, aerospace, and military applications, due to their excellent physicochemical properties. However, these difficult-to-machine materials are characterized by their low thermal conductivity, fracture toughness, and poor machinability, which allows for a significant increase in temperature during machining to produce thermal damage [192]. NPECs have been used as a method to improve the machinability of difficult-to-machine materials. This section provides a comprehensive quantitative assessment of the material removal characteristics during machining for turning, milling, and grinding applications using machining temperature, tool wear, surface quality, and specific grinding energy as evaluation tools.
4.1 Turning
The workpiece in turning is rotated relative to the tool to continuously remove materials, with the cutting edge continually in contact with the workpiece during the process. However, the high temperatures generated in the cutting zone during this process result in chip adherence to the cutting edge of the tool, which leads to deterioration in the quality of the cut and affects tool life. Therefore, improving tool life and workpiece surface quality is crucial. As a new green machining method, the NPEC has a good cooling–lubricating superposition effect in machining.
Fig.15 shows a lubrication exclusion zone during continuous cutting, where the tool and chip have a certain contact length of continuous intense contact [193]. Therefore, continuous contact friction at high temperatures, pressures, and speeds can lead to workpiece surface adhesion and tool wear. In continuous cutting, NPEC wetting performance is a major consideration, and droplet wetting, diffusion, and transport are crucial. Numerous capillary tubes between the tool and the chip are widely recognized when the cutting fluid is wetting the cutting zone. In addition, capillary permeation has three stages: liquid-phase permeation, droplet evaporation, and gas-phase filling. After the three stages, the lubricant is adsorbed on the solid surface, forming a solid boundary lubrication layer. The boundary lubrication layer has a certain load-bearing capacity. Capillary interactions in the tool–chip contact zone directly affect friction on the front surface of the tool and subsequently influence the friction and shear angles. Therefore, heat generation is suppressed due to friction reduction [194].
4.1.1 Cutting temperature
When cutting heat during the removal of difficult-to-machine materials, considerable heat is generated during the cutting process and concentrated in the tool–workpiece contact area due to the low thermal conductivity, resulting in an increase in the cutting temperature [195,196]. The main source of cutting heat is the friction between the front face and the chip, followed by the friction between the rear face and the workpiece. High temperatures in the cutting zone can deteriorate the surface quality of the workpiece and cause its deformation [197]. Sharp temperature changes at the tool–workpiece interface are limited to a region of 1–2 mm on the surface [198]. NPEC not only reduces the use of cutting fluid but also fully immerses the cutting area and suppresses the generation of cutting heat.
Yi et al. [199] investigated the effect of MQL NPEC on the turning temperature of Ti‒6Al‒4V. During the turning process, the lowest temperature at the tool–chip interface (TCIT) was observed at a feed rate of 0.05 mm/r, an air pressure of 105 Pa, and graphene oxide concentration of 0.5 wt.%. The temperature at TCIT was reduced by 50.53 °C compared with the PMQL condition. In addition, a heat transfer model for the turning process using NPEC was created on the basis of the thermal conductivity and specific heat of graphene oxide NPEC as well as their heat transfer and friction coefficients. Comparing the simulation and experimental measurements, the simulation results were close to the experimental results, indicating the reliability of the thermal model. Singh et al. [200] investigated the effect of turning Ti‒6Al‒4V on the cutting temperature at different cutting speeds and found that the cutting temperature under NPEC micro-lubrication was 31%–42% lower than that under dry conditions. The temperature reduction achieved under NPEC trace lubrication is mainly due to the good thermal conductivity and lubrication properties of the tool–chip/tool–workpiece contact surfaces.
Gupta et al. [201] evaluated the performance of Al2O3, MoS2, and GNP NPECs in turning Ti alloy (grade II) using cubic boron nitride (CBN) tools. An increase in the cutting speed and feed rate increases local temperature. Cutting speed has a greater effect on cutting temperature than feed rate. GNP shows a lower cutting temperature than Al2O3 and MoS2 NPEC. This finding is mainly due to the low viscosity and high thermal conductivity of GNP NPECs, which removes the heat released on the cutting zone. Meanwhile, NPEC improved the frictional properties of the chip–tool contact surface, and the enhanced wetted surface could promote improved heat removal [109]. Therefore, GNP NPECs are effective in reducing the cutting temperature. As shown in Fig.16, the cutting temperature T varies in different environments and increases with Vc. NPEC provided effective heat dissipation when canola oil was combined with Al2O3, MoS2, and TiO2 (at low cutting speeds). However, Al2O3 nano-enhanced phases with olive oil exhibited superior heat dissipation at medium and high speeds. The lowest cutoff temperature of 875 °C was obtained for 0.5 wt.% MoS2−canola oil NPEC, again confirming that the percentage of nano-enhanced phases has a considerable influence on the lubrication and cooling properties during machining [184].
4.1.2 Tool wear
Tool wear, which is described as the loss of material that occurs in the area where the tool and the work material are in contact, is the most basic indicator of tool life. An increase in cutting speed typically affects tool wear and tool life. Tool wear influences the workpiece surface integrity, efficiency, cutting force, energy consumption, and other indicators during machining. As shown in Fig.17(a), ISO 3685 indicates that the degree of tool wear is determined by the depth/width of the crater on the front face of the tool, the wear on the back face of the tool (average or maximum), and the notch wear on the back face of the tool [202]. The reduction in cutting forces decreases the mechanical load during material removal, which effectively reduces tool wear. Simultaneously, the reduction in cutting temperature decreases the thermal softening of the workpiece, weakening the material adhesion and effectively reducing tool adhesion. The researchers compared the effect of NPEC on tool wear under different lubrication conditions. The flank wear (VB) of Ti‒6Al‒4V was reduced by 47.37% and 31.58% under 0.1 wt.% MoS2 NPEC and pure biolubricant MQL, respectively, compared with the results of dry cutting [203]. Biolubricant MQL increased tool life by 50%, whereas flake GNP NPEC increased it by 100% [204].
Fig.17 (a) Tool wear types based on ISO 3685 [202], reproduced with permission from Elsevier; (b) flank wear based on machining time [192], reproduced with permission from Springer Nature; (c) tip wear as an evaluation of different lubrication–cooling strategies [205], reproduced with permission from Elsevier; (d) tool life at different cutting speeds [115], reproduced with permission from Elsevier. MQL: minimum quantity lubrication; MQC: minimal quantitative cooling; PTFE: polytetrafluoroethylene. |
Jamil et al. [192] investigated the effect of low-temperature CO2 and the MQL technique based on hybrid NPECs on Ti‒6Al‒4V turning. Fig.17(b) shows the variation of main flank wear at different rotational speeds, feed rates, and cooling environments. The machining time under low-temperature CO2 was 236 s, whereas the tool failure machining time was 292 s for mixed NPECs. The lower COF under Al2O3–MWCNT NPECs compared with cryogenic cooling resulted in a longer tool life. Fig.17(c) shows that the highest VB was measured in the dry and flooded cases at 83.9 and 65.3 μm, respectively. Variations in the graphite content exhibited insignificant differences, with an average VB value of approximately 47 μm. By contrast, a 29% reduction in VB was observed compared with the PMQL technique. In addition, no buildup was observed on the tool back face, and MQL with additional polytetrafluoroethylene (PTFE) particles significantly reduced tool wear [205]. By studying the wear behavior of the texture tool of Ti‒6Al‒4V, as shown in Fig.17(d), T3 (NPEC) has the maximum tool life at all cutting speeds, followed by T2 (rapeseed oil MQL), and finally T1 (dry) [115]. Furthermore, Hegab et al. [206] found that the addition of 2 wt.% MWCNT NPECs reduced tool flank wear by 45% compared with tests without any nano-enhanced phases. Therefore, using NPEC helps improve tool life under the same cutting parameters.
Gupta et al. [201] analyzed some micrographs of cutting tool tips after the machining of Ti alloy using (1) Al2O3, (2) MoS2, and (3) graphite MQL, as shown in Fig.18. Fig.18(a) depicts the major tip fracture and the final damage to the tool edge. Fig.18(b) emphasizes tool tip wear during machining with MoS2 NPECs. During the machining of graphite NPECs, some wear marks appear on the front tool surface, as shown in Fig.18(c). When Al2O3 particles impact the tool–workpiece surface, they may become tool surface crack initiators, which leads to crack expansion due to cutting forces and ultimately to the failure of the outer layer of the tool. The tool wear process was significantly reduced when graphite NPECs were used. This phenomenon is mainly due to the low viscosity of the graphite nano-enhanced phases, which allows the smooth movement of the tool edge over the working surface. The improved tribological behavior resulted in an improved tool surface. Using NPECs to reduce tool wear is better than using MQL alone. Nguyen et al. [204] investigated the effect of dry, MQL, and NPEC conditions on VB values under different cutting parameters. The results showed that the use of MQL and the addition of 0.5 wt.% of GNP nano-enhanced phases significantly reduced flank wear by approximately 40% and 100%, respectively, compared with dry conditions. Singh et al. [200] compared and investigated the effects of dry, MQL, microtextured tools filled with GNP particles, and NPEC on tool wear when turning Ti‒6Al‒4V. The results showed that tool life increased by 178%–190% at different cutting speeds under NPEC conditions compared with dry conditions. Revuru et al. [207] investigated that under different cooling and lubrication conditions, when machining at high feeds and cutting speeds, tool wear was reduced by approximately 85% in graphite soybean oil-based cutting fluids compared with dry machining and by approximately 20% compared with pure cutting fluids. This finding is attributed to the lower cutting temperature of graphite NPECs than that of pure soybean oil cutting fluids. Therefore, the capability to inhibit thermal softening and chemical reaction of active elements is strong. MQL significantly reduces tool wear, which also proves the superiority of NPECs.
4.1.3 Surface quality
NPECs can effectively reduce temperature, cutting forces, and tool wear, thereby improving the surface quality of the workpiece. Hegab et al. [206] found that 2 wt.% MWCNT NPECs reduced power consumption by 11.5% while improving surface quality compared with pure vegetable oil biolubricant. Sahu et al. [208] found that the surface roughness was reduced by 7% and 6.1% at 150 and 90 m/min, respectively, when turning Ti‒6Al‒4V workpieces using MWCNTs as NPECs. Khan et al. [209] found that turning of Ti‒6Al‒4V alloy using Al‒GNP NPECs at four cutting speeds increased the surface finish by 15.7%, 11%, 14.7%, and 15.2%. In addition, no BUE was observed when textured tools were applied under MQL conditions. In another study, compared with dry milling and the use of pure biolubricant, the Ra obtained with NPEC with a volume fraction of 0.1 vol.% was reduced by 40.67% and 10.3%, respectively [210]. Thus, the use of NPECs improves the machined surface integrity and facilitates further use in turning processes.
The incorporation of nano-enhanced phases reduces the mechanical and thermal loads on the workpiece and effectively ensures the surface integrity of the machined part. Compared with the results obtained for dry cutting conditions, the surface roughness values of the workpieces obtained by Ti‒6Al‒4V turning increased by 40.67% and 10.3% for NPEC and MQL conditions, respectively [203]. Singh et al. [200] investigated the effect of dry, MQL, and NPEC on surface roughness when turning Ti‒6Al‒4V using average roughness Ra as a basis for determining surface quality. Fig.19(a) shows the Ra values for different cutting conditions. In T3 environment, Ra at different cutting speeds is 41%–53% and 58%–68% lower than the T2 and T1 conditions, respectively. The low Ra values in the NPEC (T3) environment are mainly due to the GNP NPECs providing superior cooling–lubrication, which reduces the tool adherence tendency of the chips, thus decreasing the wear of the tool and ultimately improving the quality of the machined surface. Fig.19(b) shows the variation in surface roughness at different cutting speeds for the two studied cooling environments. At lower cutting speeds (100 m/min), Ra in the hybrid NPEC environment was reduced by 15.49%, 12.99%, and 8.72% compared with that in the low-temperature CO2 environment. At a high cutting speed (150 m/min), Ra was reduced by 26.2%, 18.18%, and 11.64%. The reduction in Ra may be related to the superior wetting area per unit volume of fluid on the workpiece–tool surface. In addition, the tribological properties of homogenized nanomaterials containing nano-enhanced phases (Al2O3‒MWCNTs) acted as isolators, limiting friction at the tool–workpiece interface [192]. Fig.19(c) shows scanning electron microscope (SEM) micrographs produced by the machining of Ti alloy (grade II) under different NPEC-assisted cutting conditions [201]. In the presence of Al2O3 NPECs, the machined surface is not completely smooth, possibly due to the prolonged residence of nano-enhanced phases on the surface of titanium alloys. A remarkably smooth surface was observed under MoS2 NPECs compared with Al2O3 NPECs. However, in the case of MoS2 machining, tiny micro-burrs and surface pitting were observed. Considerably smooth surfaces with small micro-burrs were observed for graphite NPECs under the same machining conditions. This phenomenon is mainly due to the excellent performance of the graphite NPEC as a spacer and ball bearing at the tool–workpiece contact interface. In addition, the wetting area decreases due to an increase in the contact angle between the workpiece and the NPEC. Elsheikh et al. [94] investigated the application of environmentally friendly vegetable oil NPECs in MQL turning of the AISI 4340 alloy and compared the two coolants (Al2O3 and CuO). CuO NPECs had high thermal conductivity and enhanced the cooling process in the cutting zone. They also demonstrated low viscosity. Thus, the low contact angle and surface tension enhance the wettability and distribution of the CuO coolant at the workpiece–tool interface compared with the Al2O3 NPEC, resulting in a superior surface topography. However, literature guiding the selection of nano-enhanced phases under different operating conditions is lacking. In addition to improving the integrity of the machined surface, NPECs are also effective in controlling residual stresses on the machined surface, which can be attributed to their excellent heat transfer properties.
Fig.19 (a) Variation of surface roughness Ra with cutting speed [110], reproduced with permission from Elsevier; (b) variation of surface roughness for two different environments at different cutting speeds [192], reproduced with permission from Springer Nature; (c) machined surfaces under different working conditions [201], reproduced under the terms of the CC BY license. |
4.2 Milling
In milling, the workpiece remains stationary relative to the milling cutter. In contrast to the continuous cutting of turning, each cutting edge of the milling cutter is processed intermittently, and the actual cutting area changes at any time. As shown in Fig.20, for a single tool insert, the two wedge-shaped friction subsets of the chip on the front face and the workpiece on the back face are not always present but appear and disappear with some frequency [191]. Penetration of individual edges over a short time is detrimental to film formation. The strength of the lubricant film is affected by its high frequency impact. However, the cooling–lubricating media supply modes are different due to the various structures and motions of milling and turning tools, and this difference has an impact on the milling performance. Therefore, the viscosity and heat transfer characteristics of the NPEC trace lubrication must be considered. In addition, the load-bearing capacity of nano-enhanced phases and the oil film strength of NPECs are important features in the milling process.
4.2.1 Milling temperature
In metal removal, a high-speed cutting tool induces considerable plastic deformation of the workpiece material in the primary shear zone and friction in the secondary shear zone on the front tool surface and in the tertiary shear zone on the back tool surface [211]. More than 95% of the energy consumed by plastic deformation and friction losses in the machined metal is converted into heat, resulting in remarkably high temperatures in the machining zone. Therefore, milling temperatures directly affect the integrity of the workpiece surface and tool life.
Jamil et al. [212] investigated the effect of hybrid NPEC (Al2O3–MWCNT) on the temperature of the Ti‒6Al‒4V milling process. At 0.6 MPa air pressure, NPEC concentration of 0.24 wt.%, and flow rate of 120 mL/h, the maximum temperature under drying conditions was 372 °C, while the minimum temperature was 148 °C with MQL mixed NPEC. The hybrid NPEC significantly reduces the cutting temperature by minimizing the generation of shear and friction heat and simultaneously prevents direct contact between the cutting tool and the workpiece, thus reducing the cutting temperature. Kulkarni et al. [213] significantly outperformed dry milling and PMQL conditions in terms of cutting temperature when using copper-coated alumina NPECs for micro-lubricated milling of the A17075-T6 aluminum alloy plate surface. This finding proved that the addition of nano-enhanced phases to the coolant effectively reduces the tool interface temperature without affecting the milling performance characteristics. The addition of nano-enhanced phases to the selected base fluid results in Brownian motion of the particles. The continuously moving nano-enhanced phases have a large surface area, which contributes to enhanced and rapid heat transfer at the tool–workpiece interface [23]. Fig.21 shows the main effects on the maximum cutting temperatures of nickel alloy X-750 with different cooling lubrication environments and machining parameters. The lowest cutting temperature of 325 °C was obtained under HBN NPEC conditions, which reduced the maximum cutting temperature by approximately 23.1% and 27.8% for MQL and HBN coolant conditions, respectively, compared with the dry environment.
Li et al. [59] measured the surface and subsurface temperatures in a milling experiment of TC4 under four cooling–lubrication conditions. The peak surface and subsurface temperatures under dry conditions were 247.01 and 224.13 °C, respectively. The peak surface and subsurface temperatures under gas, PMQL, and graphene MQL (GMQL) conditions were (2.23% and 1.84%), (17.17% and 15.49%), and (29.91% and 26.05%) smaller than those under dry conditions, respectively. Dong et al. [214] prepared an NPEC with a volume fraction of 1.5% for milling Ti‒6Al‒4V using cottonseed oil as a base with six different nano-enhanced phases and 0.3 wt.% CTAB dispersant. The experimental results show that the peak temperatures of SiO2, graphite, Al2O3, MOS2, SiC, and CNT NPEC during processing are 54.9, 64.5, 65.1, 65.8, 87.2, and 294.1 °C, respectively, and the temperature of the workpiece increases in the order of SiO2 < graphite < Al2O3 < MOS2 < SiC < CNTs. The use of SiO2 NPEC can effectively improve the cooling capacity and thus reduce the milling temperature.
4.2.2 Tool wear
As one of the key evaluation parameters in machining, tool wear depends to a large extent on the workpiece material and the cooling and lubrication conditions. Tool wear is directly related to the surface quality of the workpiece during milling. Therefore, investigating milling cutter wear under different operating conditions is crucial.
Zhou et al. [215] found a 63.3% reduction in tool wear compared with conventional cutting fluids and microtextured tools when milling Ti‒6Al‒4V ELI using microtextured tools and Fe3O4 NPECs. Roushan et al. [216] showed a significant reduction in tool wear compared with conventional cutting fluids and microtextured tools by combining CuO NPECs with PVD-coated (AlTiN/TiAlN) and uncoated WC micro end mills hybridized to extend tool life and improve surface quality. The results showed a reduction in tool wear and BUE formation at high concentrations (1 vol.%) of aqueous CuO NPECs. Jamil et al. [69] investigated the tool wear condition of Ti‒6Al‒4V alloy milling based on Al2O3–MWCNT NPECs and found that the hybrid NPEC acted similarly to a spacer, which reduced the friction of the cutting edge on the surface of the workpiece and further decreased tool wear. Kim et al. [217] observed and analyzed the tool wear of micro end mills after milling 10 flutes in each experimental case. In the case of ND NPECs with cold CO2 gas cooling, the tool wear values were substantially lower than those in the other cases. Moreover, a low concentration (0.1 wt.%) of ND coolant was favorable for tool wear reduction. However, in the case of a high weight concentration (1.0 wt.%), the additional scraping effect due to many ND particles may lead to increased tool wear.
Li et al. [59] investigated the effect of four dry cooling–lubrication conditions on the life of milling tools made of Ti‒6Al‒4V alloy. Fig.22(a) compares the maximum width of the tool flank wear zone under these conditions. The maximum width of the tool flank wear zone was 187 μm in dry conditions and 175, 140, and 129 μm in gas, PMQL, and GNP NPEC conditions, respectively, which was reduced by 31.02% using GNP NPECs compared with dry conditions. Fig.22(b) shows the milling cutter wear curves. Tool wear undergoes a period of sharp increase before entering a relatively steady state and then proceeds into a phase of sharp deterioration until tool failure. GNP NPECs increased tool life by 77.78% and 33.34% compared with dry and PMQL conditions, respectively. Fig.22(c) shows the evolution of milling cutter flank wear under four conditions when the milling length reaches 5280 mm. Adhesion, edge fragmentation, and stacked edges were observed under dry and gaseous conditions. However, fewer adhesions were observed under the two other conditions. Edge fragmentation, stacking edges, and large adhesion were almost absent under PMQL and GNP NPEC conditions. GNP NPECs have significantly improved the machining performance of cutting tools.
Fig.22 Tool wear and life evolution patterns under different cooling–lubrication strategies [59]: (a) maximum width of the tool flank wear zone; (b) milling cutter wear curve; (c) cutting edge in the final stage. MQL: minimum quantity lubrication; GNP: graphene. Reproduced with permission from Springer Nature. |
4.2.3 Surface quality
The surface quality of the workpiece determines its performance during application, and NPEC reduces the milling temperature and tool wear, which further improves surface quality of the workpiece.
Li et al. [59] investigated the effect of dry, gas, PMQL, and GNP NPEC conditions on the surface quality of milled titanium alloys. As shown in Fig.23(a), the surface roughness obtained a maximum value of 0.653 μm in drying conditions, and that of the workpiece was 0.647, 0.425, and 0.311 μm in the three other conditions. A minimal difference was observed between the pure gas and dry processing conditions, and the use of GNP NPECs reduced the surface roughness by 52.37% and 17.45%, respectively, compared with the drying and PMQL conditions. The lubricant film formed at the tool–workpiece contact interface under PMQL conditions reduced the milling temperature and tool wear, and the effect was further enhanced by adding GNP NPECs. High milling forces and temperatures may lead to surface craters and adhesion, and high tool wear leads to large furrows and reduced surface quality. Simultaneously, the lubricant film formed in the cutting zone separates the surface of the workpiece from the side of the cutting edge and reduces the generated adhesion, surface craters, and large grooves, as shown in Fig.23(b). GNP NPECs further improve the surface quality of the workpiece compared with PMQL conditions because the excellent tribological properties of the GNP nano-enhanced phase improve the cooling performance of the lubricant film [218].
Roushan et al. [216,219] investigated the utilization of CuO NPECs in an MQL environment with uncoated and AlTiN-coated WC micro end mills to improve the machining performance of Ti‒6Al‒4V titanium alloy micro milling. Minimum average burr widths of 9.93 and 10.58 μm were obtained for up- and down-milling, respectively, with AlTiN-coated cutters under 0.25 vol.% CuO NPEC conditions. In addition, the surface roughness (Ra) was reduced by 81.54% when compared with that of the uncoated WC cutter under dry conditions. CuO nano-enhanced phases are spherical; thus, they can easily penetrate and embed in the fabrication of microchannels and enhance lubrication by forming a thin film between the tool and chip interface. Edelbi et al. [220] milled Ti–3Al–2.5V alloy pairs of Ti–3Al–2.5V using a two-nozzle MQL system with PVD-coated carbide. The results show that the surface roughness obtained under ZnO and Al2O3 NPECs is in the range of (0.312–1.032 µm) and (0.374–1.124 µm), respectively, and the ZnO NPEC is more effective than that observed under Al2O3 NPEC with an average Ra reduction of approximately 18.49%.
Yin et al. [221] investigated the effect of several typical nano-enhanced phases (Al2O3, MoS2, SiO2, CNTs, SiC, and graphite) on the surface quality of milled Ti‒6Al‒4V using cottonseed oil as the base. The SiO2 NPEC treatment had the lowest surface roughness (Ra = 0.594 μm), which was 62.78% lower than that of the MQL treatment, followed by Al2O3 NPEC (Ra = 0.633 μm), which was 60.34% lower than that of the MQL treatment. Fig.24 shows the surface quality of the workpiece surface under different machining environments. Fig.24(d) shows that under the NPEC condition of CNTs, many scratches appeared on the surface of the workpiece. The burrs not only affect the roughness of the machined surface but also act on the tool surface, leading to additional grooves on the tool surface and exacerbating tool wear, which creates a vicious circle. Fig.24(b) shows that the workpiece surface under MoS2 NPEC condition exhibited furrows, while that under Al2O3 NPEC condition revealed numerous scratches, as shown in Fig.24(a). By contrast, the workpiece surface under SiO2 NPEC condition has shallow scratches and the lowest surface roughness (Fig.24(c)).
4.3 Grinding
Grinding is a basic form of machining and an important part of modern precision machining. The final accuracy and surface quality of most parts are ensured through the grinding process. The contact area of the grinding wheel with the workpiece is significantly larger than that of turning and milling, which is mainly characterized by abrasive cutting with negative rake angle, severe friction, and high specific energy [222]. Therefore, high temperatures are observed in the grinding zone, and most of the heat flows into the workpiece [223]. As the main technological bottleneck in grinding processes, heat dissipation induces a substantially short contact time between each abrasive grit and the workpiece due to the speed of the grinding wheel [224]. Simultaneously, the heat removed by the grinding debris is remarkably small. The high energy density in the grinding zone significantly affects the surface quality and performance of the workpiece. Therefore, effective measures must be taken to reduce or even eliminate the effect of grinding heat on workpiece machining accuracy and surface quality. The use of NPEC improves the heat dissipation performance. Simultaneously, the antiwear and friction reduction effects of nano-enhanced phases reduce the heat flux at the friction interface. Fig.25 shows that the NPEC can form a lubricating film with large coverage and wear resistance via self-diffusion osmosis. The lubricating film is easily bonded to the surface of the friction subsurface, which improves the stability of the oil film [225].
4.3.1 Grinding temperature
Zou et al. [226] analyzed the output response of belt grinding of titanium alloys and found that the use of MQL provided low abrasive wear and improved machined surfaces. In addition, the MQL-assisted belt grinding process resulted in good fatigue strength and compact microstructure. CNTs were applied to the grinding fluid to form a lubricant film on the surface of the grinding wheel and workpiece, reducing the generation of grinding heat to further improve the cooling effect of MQL. The addition of CNTs significantly reduced the grinding temperature by 32% and further increased the grinding rate by 48% with significant cooling effect.
Li et al. [106] measured the grinding temperature of workpiece Ti‒6Al‒4V under six operating conditions and presented the experimental results. Fig.26(a) shows the measurement setup for surface grinding temperatures. A type K thermocouple with a diameter of 0.30 mm was mounted in a thermocouple mounting slot with a diameter of 0.30 mm for measuring the surface grinding temperature. Fig.26(b) presents the average temperature peaks for each of the six working conditions. For grinding temperatures, the highest temperature peak of 278.932 °C was obtained under dry conditions. Under five other conditions, the Tmax was 229.175, 203.41, 183.612, 194.914, and 200.723 °C, which were 17.838%, 27.075%, 34.173%, 30.121%, and 28.039% lower than that of the dry condition, respectively. The peak grinding temperatures of PMQL, as well as four different concentrations of NPEC conditions, were significantly lower relative to the dry conditions. This finding may be due to the superior cooling capacity provided by the oil film generated by the metal saponification reaction in the grinding zone [227]. As shown in Fig.26(b), as the concentration increased from 0.0 to 0.2 wt.%, the peak grinding temperature Tmax first decreased and then increased, reaching a minimum at a concentration of 0.1 wt.%. This result may be attributed to the enhanced cooling capability of the oil film using a moderate amount of GNP. However, when the concentration is too low, the reinforcing effect of GNP will be reduced due to insufficient cooling. When the concentration is too high, excess GNP tends to clog the abrasive region and impedes the formation of lubricant film, thus reducing the strengthening influence of GNP due to insufficient cooling [73].
In addition, some scholars have analyzed the heat transfer behavior under the coupling of low temperature and NPECs. Zhang et al. [228] simulated the temperature field in the grinding zone under different cooling conditions. The results showed that the cooling effect of the combined process of cold air and NPEC was the best, followed by low-temperature cold air and NPECs. In addition, Zhang et al. [229] established a convective heat transfer coefficient (CHTC) model for cold air NPEC in a vortex tube based on boiling heat transfer conduction theory and numerically simulated the finite difference and temperature fields in the milling zone in a vortex tube with different cold air fractions. The simulation results show that the maximum temperature decreases and then increases with the increase of cold air fraction. The validity of the CHTC model was verified by experiments on Ti‒6Al‒4V.
4.3.2 Specific grinding energy
Specific grinding energy is the energy required to remove a unit volume of material and can characterize the lubrication effect at the wheel–workpiece interface [230]. Ibrahim et al. [231] prepared palm oil-based NPECs containing different percentages of GNP and evaluated the performance of Ti‒6Al‒4V machining using MQL. Fig.27 shows the typical values of specific cutting energy and percentage reduction for different lubrication modes. As shown in Fig.27(a), the specific grinding energies were 193.551, 53.929, 79.709, and 75.423 J/mm3 for dry machining, flooding, LB2000 mode, and pure palm oil–MQL lubrication modes, respectively. As shown in Fig.27(b). The optimal value of specific energy consumption was achieved by utilizing NPEC MQL conditions with a concentration of 0.1 wt.%. GNP NPEC at a concentration of 0.1 wt.% significantly reduces specific energy by 91.87% and 80.25% compared with flood and LB2000/MQL lubrication technologies, respectively. In addition, NPEC at concentrations of 0.2, 0.3, and 0.4 wt.% saved 72.39%, 69.74%, and 66.35% of cutting energy, respectively, compared with the LB2000/MQL lubrication model. Singh et al. [50] comparatively analyzed the effect of GNP in three vegetable oils (canola, olive, and sunflower) mixed with different concentrations of GNP on the grinding force during Ti‒6Al‒4V grinding process. The graph in Fig.27(d) reveals that canola oil causes a reduction in specific grinding energy by 9.72% compared with conventional cutting fluids. When GNP nano-enhanced phases were not added, the specific grinding energy was in the following order: sunflower oil < olive oil < rapeseed. However, when used with GNP nano-enhanced phases, the same order of performance was changed to olive < sunflower oil < rapeseed. A GNP concentration of 1.5 wt.% resulted in a relative reduction of 33.83% in grinding energy compared with conventional cutting fluids.
Fig.27 (a) Effect of each lubrication method on specific grinding energy; (b) percentage reduction in specific grinding energy [231]; (c) specific grinding energy at six operating conditions [106]; (d) variation in specific grinding energy with different concentrations of graphene (GNP) in different vegetable oils [50]. MQL: minimum quantity lubrication; PMQL: pure MQL; GMQL: graphene MQL. Reproduced with permissions from Refs. [50,106,231] from Elsevier. |
Li et al. [106] evaluated the effect of GMQL on specific grinding energy through grinding experiments. Fig.27(c) shows the specific grinding energies for six operating conditions (dry, PMQL, and four GMQLs). For specific grinding energy, a maximum of 598.384 J/mm3 was obtained under dry conditions. In the five other conditions, the specific grinding energy was reduced by 42.978%, 59.606%, 67.375%, 62.039%, and 57.912%, respectively, considering the dry condition. The oil film exhibited excellent lubrication properties after the addition of GNP nano-enhanced phases, which improved the lubrication condition in the cutting area and reduced the cutting force and COF, thus decreasing the specific grinding energy. As the concentration of NPEC increases, the specific grinding energy first decreased and then increased, reaching the minimum value at a concentration of 0.1 wt.%. This phenomenon is due to the enhanced oil film lubrication performance, which is attributed to the moderate GNP. However, when the concentration is too low, the reinforcing effect of GNP is weakened due to insufficient lubrication in the grinding area. However, when the concentration is too high, excess GNP tends to clog the grinding area and impedes the formation of the lubricant film, reducing the strengthening effect of GNP due to insufficient lubrication.
4.3.3 Surface quality
Ibrahim et al. [231] prepared palm oil-based NPEC containing different percentages of GNPs and evaluated the performance of Ti‒6Al‒4V processing using MQL. Under dry conditions, severe delamination can be observed on the contact surfaces due to lack of lubrication and heat dissipation in the contact zone. By contrast, under LB2000 MQL lubrication conditions, relatively smooth surfaces and minimal surface damage can be observed due to the lubricating effect of the LB2000 droplets and the effective cooling characteristics. Palm oil can provide adequate lubrication because it has highly polar molecules that adhere tightly to the contact surfaces and are used as an anti-friction layer [232]. The synergistic interaction between the GNP nano-enhanced phase and the molecular layer of palm oil yielded the best results at GNP 0.1 wt.%. In addition, GNP exhibits self-healing and crack bridging effects. Increasing GNP to 0.4 wt.% leads to an increase in the contact angle of the lubricant droplets, which reduces the wettability of the NPEC. In addition, high GNP concentrations result in high viscosity, which limits aerosol penetration into the cutting zone. Zhang et al. [130] investigated the lubrication properties of MoS2 nano-enhanced phase palm oil-based NPEC under MQL conditions. When 2% nano-enhanced phase jet MQL milling was performed with paraffin and palm oil liquids, the corresponding Ra and Rz values perpendicular to the direction of the milled particles were (50.0% and 61.1%) and (41.1% and 48.2%) lower than that of dry milling, respectively. Compared with the MQL of paraffin and palm oil liquids without nano-enhanced phases, Ra was reduced by 25% and 30%, and Rz by 15.4% and 23.7%, respectively. This reduction is due to the strong absorption capacity and high film-forming strength of MoS2, which increases the difficulty in breaking the oil film. MoS2 nanofibers have minimal effect on the wear resistance of the nanofibers because they mainly wear on the surface of the friction partner to form a lubricating film. Fig.28(a) shows the surface roughness measured under the six operating conditions; in terms of surface quality, significant adhesion, grinding burns, and large furrows were observed under dry conditions. Under the PMQL and the four NPEC MQL conditions, almost no grinding burns were observed, and adhesion was significantly reduced. Especially under the NPEC condition (0.1 wt.%), almost no adhesion phenomenon was also observed. For surface roughness, the surface roughness Ra was reduced by 51.797% under the optimum condition of NPEC compared with the dry condition [106].
Fig.28 (a) Surface roughness for six operating conditions; (b) dry, (c) PMQL, (d) GMQL with lower cutting fluid concentration, (e) GMQL with appropriate cutting fluid concentration, (f) GMQL with higher cutting fluid concentration [106]. GMQL: graphene minimum quantity lubrication; PMQL: pure minimum quantity lubrication. Reproduced with permission from Elsevier. |
Setti et al. [233] investigated the grinding properties of Ti‒6Al‒4V, where Al2O3 NPEC improved the surface quality of Ti‒6Al‒4V by reducing the friction and temperature generation between the contacting surfaces. Fig.29 reveals the difference in surface morphology due to pure rapeseed oil and rapeseed oil with GNP additives. In the case of pure rapeseed oil, rough surfaces and sticky debris due to lack of heat transfer were observed on the specimen surfaces. However, in the case of rapeseed oil-based MQL, the GNP additive produced a remarkably fine and defect-free surface. The GNP additive significantly improved the tribological properties of vegetable oil. Relatively rough surfaces were observed on specimens cooled by overflow based on synthetic cutting fluids. The surface quality of the specimens was worst under dry conditions, and major surface defects such as surface burns were observed. Thus, multilayer graphene nanosheets have superior tribological potential as additives for green tribological vegetable oils [50]. Another study revealed that the best cooling lubrication was achieved at 0.1 wt.% GNP NPEC with 51.797% reduction in Ra compared with dry conditions. This finding may be attributed to the improved lubrication and cooling status of the grinding zone due to the oil film, thus reducing the cutting heat, COF, and cutting force and improving the surface quality. The results show that moderate GNP can enhance the lubrication and cooling capability of the oil film, which enhances the surface quality and reduces the surface roughness. In addition, the grinding process produces the work-hardening phenomenon due to the extrusion and friction of the grinding wheel and the workpiece, which accelerates the tool wear and thus reduces the surface processing quality [234]. The high temperatures generated in the grinding zone reduce work hardening but are not conducive to machining [224]. The introduction of NPECs reduces the grinding heat, while the synergistic effect of the nano-enhanced phases and the biolubricant reduces the friction and extrusion phenomena at the contact interface of the grinding wheel workpiece, avoiding the generation of work-hardening phenomena [106].
Fig.29 Scanning electron microscope images of the surface of specimens processed under different conditions [50]: (a) graphene nanoparticle-enhanced coolant, (b) cooling based on synthetic cutting fluid, (c) pure rapeseed oil-based minimum quantity lubrication, and (d) dry grinding. Reproduced with permission from Elsevier. |
4.4 Application effects
1) Turning
Overall, the application of NPEC MQL in turning achieves the desired cutting performance, and the end result is superior to the single application of pure vegetable oil-based biolubricant MQL. The NPEC in turning titanium alloys can reduce the cutting temperature by 31%–42%, increase the tool life by 178%–190%, and improve the surface quality by 58%–68% relative to dry machining. However, the application of NPEC in the turning process should still focus on the following points.
First, an optimal solution for matching the selection of different NPECs with the cutting parameters exists. Under specific NPEC conditions, additional heat is generated at the tool–workpiece contact interface when the cutting speed is high, leading to surface integrity deterioration. In this case, nano-enhanced phases with strong heat transfer properties, such as GNPs, should be selected to effectively reduce the above evaluation indexes and obtain the ideal surface quality of the workpiece. Therefore, the machining quality will be improved according to the corresponding level of cutting parameter evaluation indexes by choosing a reasonable cooling medium and its flow parameters to establish a quantitative correspondence. Second, the application of NPEC should be comprehensively explored, and the addition of hybrid nano-enhanced phases or the use of low-temperature media coordinated with textured tools can effectively enhance cutting performance and show superior results. However, if a single NPEC can obtain the workpiece surface quality required to address engineering applications, then the addition of multiple nano-enhanced phases will result in resource wastage and increased cost. Therefore, the nano-enhanced phase is accordingly rationalized for MQL-assisted machining.
2) Milling
Overall, the application of NPEC in milling achieved the desired cutting performance. The cutting temperature was reduced by 26.05%–29.91% relative to dry machining. Tool life and surface quality were increased by 31.02%–77.78% and 52.37%–81.54%, respectively. Similar to turning, milling also suffers from vegetable oil biolubricant and nano-enhanced phase selectivity. Therefore, further research is necessary to investigate the optimal combination of different vegetable oils and nano-enhanced phases under various machining parameters. Internal jet supply is gradually becoming mainstream based on the special characteristics of milling equipment, such as the structure and motion forms of tools and tool holders, but this method is mainly applicable to end mills. In addition, the use of cryogenic gases such as LN2 can further improve the cooling effect and machining quality. However, the use of cryogenic media is highly demanding for milling cutter manufacturing and the supply system with NPEC, which increases the cost outlay. Research on the use of hybrid nano-enhanced phases for milling titanium alloys is still relatively small, but the hybrid nano-enhanced phases can synthesize their excellent properties to play a crucial role in coordination with each other.
3) Grinding
The heat generated by the same workpiece material is substantially higher than that generated by turning and milling due to the special characteristics of the grinding wheels. Surface burns still exist due to the low thermal conductivity of titanium and nickel-based alloys. Compared with dry machining, NPECs can reduce the cutting temperature by 17.8%–32% and improve the surface quality by 50.0%–61.1%. Scholars have focused on increasing the cooling capacity of cutting fluids to reduce the proportion of energy flowing into the workpiece. The use of NPEC addresses heat buildup in difficult-to-machine materials, thereby reducing surface burns and thermal softening of the workpiece. Recommended biolubricants should have high viscosity and fatty acid saturation and contain polar groups and other properties (e.g., palm and castor oils) that favor the formation of strongly adsorbed lubricant films. Nano-enhanced phases have correspondingly high hardness and spherical properties (e.g., Al2O3), providing excellent rolling friction reduction and anti-wear in high-pressure grinding interfaces. However, efficient matching of the grinding parameters to the type of nano-enhanced phase is also required for optimal results. Tab.5 summarizes the performance of the MQL with NPEC during machining.
Tab.5 NPEC processing properties |
| References | Cutting fluid | Working condition | Conclusion (compared with MQL) |
|---|---|---|---|
| [115,200] | Canola oil + GNP | Turning | COF: ↓ 16%‒39%, Ra: ↓ 41%–53%, cutting temperature: ↓ 50.53 °C |
| [206] | Vegetable oil + MWCNTs | Turning | Energy consumption: ↓ 11.5%, tool wear: ↓ 45% |
| [204] | Canola oil + GNP | Turning | Tool life: ↑ 50%, tool wear: ↓ 60% |
| [209] | Vegetable oil + Al‒GNP | Turning | Energy consumption: ↓ 1.5%, surface quality: ↑ 11%‒15.7% |
| [192] | Distilled water + Al2O3‒CNT | Turning | Ra: ↓ 8.72%, tool life: ↑ 23% |
| [207] | Soybean + GNP | Turning | Tool wear: ↓ 20% |
| [205] | Vegetable oil + GNP | Turning | Surface quality: ↑ 36% |
| [210] | Jojoba oil + MoS2 | Turning | Ra: ↓ 34.56%, tool wear: ↓ 16% |
| [235] | Cottonseed oil + Al2O3 etc. | Milling | Ra: Al2O3 < SiO2< MoS2 < CNTs < graphite < SiC |
| [236] | Vegetable oil + CuO/GNP | Milling | (CuO) Ra: ↓ 14.7%, (GNP) Ra: ↓ 21.96% |
| [59,218] | Vegetable oil + GNP | Milling | Ra: ↓ 17.45%, tool wear: ↓ 5.9% |
| [214] | Cottonseed oil + Al2O3 | Milling | Ra: ↓ 66.7% |
| [215] | Olive oil + Fe3O4 | Milling | Ra: ↓27.75%, tool wear: ↓ 63.3% |
| [35] | Distilled water + Al2O3 | Milling | COF: ↓ 53.9% |
| [208] | Water + MWCNTs | Milling | Ra: ↓ 27%, tool wear: ↓ 34% |
| [171] | Vegetable oil + carbon onion | Milling | Ra: ↓ 46.32% |
| [237] | Cottonseed oil + Al2O3 | Milling | Ra: ↓ 48.12% |
| [238] | Vegetable oil + HBN | Milling | Ra: ↓ 8.2%, milling temperature: ↓ 4.7%, tool wear: ↓ 5.4% |
| [50] | Canola oil + GNP | Grinding | Ra: ↓ 16.9%, specific grinding energy: ↓ 33.83% |
| [129] | Palm oil + GNP | Grinding | COF: ↓ 71.2% |
| [226] | Castor oil + CNT | Grinding | Cutting temperature: ↓ 32°C |
| [239] | Canola oil + GNP | Grinding | Specific grinding energy: ↓ 15.7%, Ra: ↓ 36.4%, COF: ↓ 40.9% |
| [106] | Vegetable oil + GNP | Grinding | Grinding temperature: ↓ 16.3%, specific grinding energy: ↓ 24.4%, Ra: ↓ 23.4% |
| [233] | Water + Al2O3−CuO | Grinding | Ra: ↓ 18% |
| [231] | Palm oil + GNP | Grinding | Specific grinding energy: ↓ 80.3% |
| [208] | Water + MWCNTs | Grinding | Tool wear: ↓ 34% |
Overall, GNP NPECs exhibit extraordinary machining performance in turning, milling, and grinding. However, optimal volume fractions for different machining environments are lacking. In addition, the application of hybrid nano-enhanced phases can compensate for some of the cooling and lubrication capacity deficiencies that exist under the conditions of the use of single nano-enhanced phases. Meanwhile, coupling with a low-temperature medium and a multi-energy field can further improve the processing performance. However, the current research is still relatively small and should be further explored in the future.
5 Conclusions and prospects
5.1 Conclusions
The excellent heat transfer and film-forming properties of NPECs can improve direct tool–workpiece contact, reduce machining temperatures and tool wear, and improve surface quality, thereby conforming to the contemporary concept of green manufacturing for sustainable development. The following conclusions were drawn by revealing the thermophysical properties of NPEC for MQL and their mechanism of action and verifying the effectiveness of NPEC in machining.
1) High fatty acid saturation and polar groups are key properties of vegetable oil-based fluids that improve the lubricant film adsorption capacity and strength in the cutting zone. The base fluid can be water- or vegetable oil-based depending on the cooling process and lubrication guidance. Hard or nano-enhanced phases with a layered structure can be selected depending on the interfacial load. One- and two-step methods for the preparation of NPEC have advantages and disadvantages. Thus, the two-step method is recommended for the preparation of NPECs in the industrial field. The stability of NPECs is an important bottleneck that limits their use in machining, and their stability can be achieved by physically/chemically dispersing and modulating the nano-enhanced phase in the base fluid.
2) Compared with traditional cutting fluids, the Brownian motion and heat transfer channels of NPECs markedly improve the high-temperature environment in the cutting zone. The infiltration film-forming properties of droplets are affected by surface tension and viscosity; a small surface tension leads to the high utilization and permeability of NPECs and extends the thermal boundary layer to improve heat transfer performance. NPECs with fast droplet penetration rate should be selected in the turning process, while those with large droplet diffusion area and high viscosity should be selected in the milling process.
3) NPEC is a high-performance cooling lubricant medium with cutting fluid reduction supply. The friction performance of the cutting interface is dramatically improved, with the COF reduced by 34.3% under GNP NPEC lubrication compared with palm oil lubrication, resulting in enhanced tool life and surface integrity. Compared with conventional MQL, this technology reduces cutting temperatures by approximately 26.05%–42%, increases tool life by approximately 17.8%–190%, improves tool wear by approximately 25%–42.4%, and enhances machined surface quality, including Ra reduction, by approximately 50.0%–81.54%.
5.2 Prospects
NPEC technology is an environmentally friendly alternative to traditional cutting fluid-flooded machining. Future development trends should focus on the following points.
1) NPECs show excellent processing performance. However, the oxidization problem of vegetable oil biolubricants in the cutting zone at non-cold temperatures limits its promotion. Considerable potential for chemical modifications, such as epoxidation, esterification, and hydrogenation, currently exists. Thus, these modifications may also be an important area of research in the future.
2) A hybrid NPEC lubrication method is proposed and validated for the coordination of cooling and lubrication properties during the machining of difficult-to-machine materials. Excellent tribological and heat transfer properties are guaranteed to improve the surface quality of the workpiece and tool wear. However, the synergistic mechanism of the particle size ratio and the proportioning concentration of the hybridized nano-enhanced phases for heat dissipation from anti-friction must be further analyzed. Simultaneously, the sustainable assessment of processing must be further improved by creating a database of NPEC and different materials and processes to provide guidance for sustainable processing in the future.
3) As the most commonly used atomization method for MQL, pneumatic atomization has several limitations. The multi-energy field atomization intelligent supply system will be a key to solving the bottleneck of the current atomization technology. The NPEC technology can then be further improved in terms of processing performance. The current experimental research and equipment development in this area still have room for improvement.
4) The current application mechanism of ultrasonic vibration, textured tool, and magnetic field empowerment-assisted NPEC needs further in-depth study. Simultaneously, the key technology of intelligent supply should be combined to establish the mapping relationship between machining and optimal empowerment parameters, which can realize the large-scale application of intelligent technology in industrialization at an early date.