Minimum quantity lubrication machining of aeronautical materials using carbon group nanolubricant: From mechanisms to application

Xn CUI, Cn LI,*, Wnn DING, Yun CHEN, Con MAO,Xun XU, Bo LIU, Dzon WANG, Ho Nn LI, Ynn ZHANG,Zr SAID, Sun DEBNATH, Mumm JAMIL, Hz Mumm ALI,Sum SHARMA

a School of Mechanical and Automotive Engineering, Qingdao University of Technology, Qingdao 266520, China

b College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

c Chengdu Tool Research Institute Co., Ltd. Chengdu 610500, China

d College of Automotive and Mechanical Engineering, Changsha University of Science and Technology, Changsha 410114, China

e Key Laboratory of Special Purpose Equipment and Advanced Processing Technology, Ministry of Education & Zhejiang Province,Zhejiang University of Technology, Hangzhou 310032, China

f Sichuan Future Aerospace Industry LLC., Shifang 618400, China

g School of Mechanical and Automotive Engineering, Shanghai University of Engineering Science, Shanghai 201620, China

h School of Aerospace, University of Nottingham Ningbo China, Ningbo 315100, China

i College of Engineering, University of Sharjah, Sharjah 27272, United Arab Emirates

j Mechanical Engineering Department, Curtin University, Miri 98009, Malaysia

k Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

l Department of Mechanical Engineering and Advanced Materials Science, Council of Scientific and Industrial Research (CSIR) -

Central Leather Research Institute (CLRI), Regional Center for Extension and Development, Jalandhar 144021, India

KEYWORDS Aerospace materials;Carbon nanoparticles;Grinding;Lubrication mechanism;Milling;Minimum quantity lubrication;Turning

Abstract It is an inevitable trend of sustainable manufacturing to replace flood and dry machining with minimum quantity lubrication (MQL) technology. Nevertheless, for aeronautical difficult-tomachine materials,MQL couldn’t meet the high demand of cooling and lubrication due to high heat generation during machining. Nano-biolubricants, especially non-toxic carbon group nanoenhancers (CGNs) are used, can solve this technical bottleneck. However, the machining mechanisms under lubrication of CGNs are unclear at complex interface between tool and workpiece,which characterized by high temperature, pressure, and speed, limited its application in factories and necessitates in-depth understanding.To fill this gap,this study concentrates on the comprehensive quantitative assessment of tribological characteristics based on force,tool wear,chip,and surface integrity in titanium alloy and nickel alloy machining and attempts to answer mechanisms systematically. First, to establish evaluation standard, the cutting mechanisms and performance improvement behavior covering antifriction, antiwear, tool failure, material removal, and surface formation of MQL were revealed. Second, the unique film formation and lubrication behaviors of CGNs in MQL turning, milling, and grinding are concluded. The influence law of molecular structure and micromorphology of CGNs was also answered and optimized options were recommended by considering diverse boundary conditions. Finally, in view of CGNs limitations in MQL,the future development direction is proposed,which needs to be improved in thermal stability of lubricant,activity of CGNs,controllable atomization and transportation methods,and intelligent formation of processing technology solutions.

1. Introduction

The continuous development of the aerospace field proposes higher requirements for the mechanical properties of important components and materials. Difficult-to-machine materials, especially titanium alloy and nickel alloy, have become an irreplaceable and most popular option of key aerospace components.1–3Titanium alloy, which became popular in the 1950 s, is characterized by high specific strength, good heat resistance, low and ultra-low temperature stability, and corrosion resistance.4,5However, Ding et al.6found that bad machinability of titanium alloy puzzles the industry because of its small thermal conductivity (50% lower than pure titanium) and elastic modulus. A similar problem occurs in the machining of nickel alloy, especially nickel-based superalloys,as reported in 2007 by Kamata and Obikawa.7Pusavec et al.8reviewed the research status and proposed predictive performance models of nickel alloy manufacturing. Through longterm experimental research, Suarez et al.9found that fatigue life is also a key parameter, that depends on the processing technology. Ding10has emphasized this aerospace research hotspot until today.

According to M’Saoubi et al.11,the high surface integrity of key aerospace components is the main requirement that depends on machining processes including turning, milling,and grinding.However,da Silva et al.12reported that adhering to tool surface, and then aggravating tool wear is a common phenomenon in titanium alloy machining. Zou et al.13also reported that surface temperature increase, poor surface quality, and the tendency to cause burns are limitations in nickel alloy machining due to low thermal conductivity. To solve these problems, the heavy usage of traditional metal-working fluids (MWFs) is a forced solution that plays a cooling, lubrication,and chip removal role under a strong thermal mechanical coupling interface. Nevertheless, traditional MWFs has become an obstacle to sustainable manufacturing nowadays.Jia et al.14reported that a mass of oil mist and PM 2.5 is generated from traditional MWFs around the high speed rotating tool and high temperature cutting zone, thereby causing lung diseases such as mild respiratory illness, asthma, and cancer.The discharge of traditional MWFs to the natural environment after use will also cause a series of environmental pollution problems, which couldn’t meet carbon peak, carbon neutral development needs. The manufacturing power countries have introduced some tough new laws or standard to tackle above issues, such as carbon emission standard (ISO 14067–2018), ‘cleaner production promotion law’, Integrated wastewater discharge standard (GB 8978–1996), and so on.Therefore, the cost of traditional MWFs is increased greatly.Sanchez et al.15and Tai et al.16reported that traditional MWFs accounts for 10–21% of the total cost from purchase to bio-safety disposal,which is 3–5 times the tool cost.According to a report prepared by the German Social Accurant Insurance by Benedicto et al.17, costs related to traditional MWFs account for approximately 16% of total manufacturing expenses based on data from some major european companies.Dragicevic18concluded that the cost of purchase,maintenance and disposal of traditional MWFs is twice that of machining,accounting for 7%-17% of the total manufacturing cost.

Shokrani et al.19predicted that sustainable manufacturing will become the main method for aerospace materials, such as dry, minimum quantity lubrication, and cryogenic cooling machining. From the economic, ecological, and machining performance aspects, researchers have evaluated the strengths and limitations of the aforementioned three methods. As an extreme green machining technology without traditional MWFs,dry machining was first used in the automobile industry,which was defined as a green machining method by Klocke and Eisenbla¨tter.20However, serious friction, adhesion, and heat accumulation occurs in difficult-to-machine material machining, especially grinding, and then exacerbate tool failure and cause bad surface quality. Cryogenic cooling, instead of traditional MWFs, is widely used in metal processing by spray cryogenic gas and liquid media into the cutting/grinding zone. The advantage lies in improved heat transfer and low temperature,which improves tool life and surface quality compared with dry machining. However, several limitations have to be considered:i)The cost of cryogenic cooling is high:Generating of low-temperature gas depends on the use of expensive cryogenic refrigerating machines.ii)Cryogenic media are dangerous: Cryogenic media are directly sprayed on the cutting/-grinding zone, posing the risk of choking for humans when the concentration of nitrogen or carbon dioxide in the air is extremely high.

Minimum quantity lubrication(MQL),a promising way to solve lubrication and cooling problems for difficult-to-machine materials machining,21introduces bio-lubricant with flow rate of 10–100 mL/h into tool/workpiece interface through high pressure airflow. Sharma et al.22reviewed MQL technology,which produces surfaces better than dry machining and similar to machining with traditional MWFs.Furthermore,MQL presents unique advantages in terms of cost (because of an extremely small amount of coolant usage and large disposal saving),environmental protection, and human safety. Encouragingly,constantly increasing attention and studies have pushed the development of MQL over the past 20 years. Maruda et al.23observed chip shape and surface roughness in X10CrNi18-8 turning, and found better effects of MQL compared with dry machining. Dhar et al.24investigated tool wear and surface roughness in MQL turning and obtained significant improvement compared with traditional MWFs. Tasdelen et al.25found that MQL present lower tool wear and shorter chip length than traditional MWFs in precipitation hardened steel drilling. Khan et al.26reported improved tool life and surface quality of MQL compared with traditional MWFs in AISI 9310 turning. In AISI 4340 grinding, Silva et al.27observed better lubrication performance of MQL compared with traditional MWFs, by observing surface integrity and wheel wear. Okafor and Nwoguh28found that soybean oil is better than mineral oil in high speed milling of Inconel 718 with MQL.Rodriguez et al.29and Bianchi et al.30found that MQL could clean grinding wheels and keep them in good sharpening condition. Subsequently, Ge et al.31emphasized the significance of MQL technology for aerospace material machining.

However,Zhang et al.32observed surface burning in MQL grinding of nickel alloy,and then solved this problem by using nanolubricant, the method of which was proposed by Mao et al.33and called nanolubricant minimum quantity lubrication (NMQL). Setti et al.34applied NMQL in titanium alloy grinding with aluminium oxide (Al2O3) and cupric oxide(CuO) nano-enhancers and observed reduced force, temperature,wheel wear,and shorter C-type chips.Wu et al.35studied force, specific energy and G ratio in YG8 grinding and confirmed the superiority of NMQL.Zhang et al.36reported that nanolubricant concentration plays a significant role in the material removal of nickel alloy.Yuan et al.37have confirmed the good compatibility of diamond nano-enhancers with lubricants in NMQL according to reduced cutting force and surface roughness.In addition,Marques et al.38reported that NMQL showed improved tool life,wear,and surface quality in Inconel 718 turning compared with MQL. Mishra et al.39obtained similar results in Ti-6Al-4V turning. Duan et al.40used 7050 aluminum alloy and conducted NMQL milling experiment with Al2O3nanofluid. The results indicated that milling parameters based on NMQL is also a very important factor.Yan et al.41verified the feasibility of nano-enhancers in diamond turning of reaction-bonded silicon carbide (SiC) and explored the influence of the kinds and concentrations of dispersed nanoparticles on the lubrication performance.

From previous studies, it could be concluded that NMQL technology is a feasible method for difficult-to-machine materials due to excellent tribological characteristics in the tool/-workpiece interface. Many types of nano-enhancers have been tried in recent studies.Li et al.42used molybdenum disulfide (MoS2), zirconium oxide (ZrO2), Carbon nanotubes(CNTs), polycrystalline diamond, Al2O3, and silicon oxide(SiO2) in NMQL and obtained different grinding temperatures. Dong et al.43reported variant cooling and lubrication performance by using different nano-enhancer types in NMQL milling of Ti-6Al-4V. Yildirim et al.44found lower tool wear,roughness, and higher tool life with 0.5 vol% hBN based nanolubricant in Inconel 625 turning. In addition, ManojKumar and Ghosh45and Ondin et al.46found that CNTs can effectively reduce surface roughness, flank wear, and the size of the built-up edge. Gutnichenko et al.47studied 0.2% vol.graphite (GNP) in high-strength steel turning, which significantly improved tool life and surface integrity. Hosseini et al.48confirmed that GNP reduced the friction, force,and roughness in the NMQL grinding of YG8.

With the continuous development of material science, carbon group nano-enhancers(CGNs)provide new opportunities for NMQL.According to the‘green additive standard of lubricants’drawn up by German Blue Angel,CGNs are almost the best option among the various nano-enhancers mentioned.Furthermore, CGNs with different dimensions have unique molecular structure, antifriction, and antiwear performance,as well as higher chemical stability and thermal stability.Based on the preceding analysis, CGNs have the potential to be excellent lubricants for NMQL because of their excellent environmental protection, chemical inertness, antifriction, antiwear, and heat transfer characteristics. Early in 2008,scholars tried to reveal the lubrication mechanism of CGNsbased nanolubricant by tribological tests. Joly-Pottuz et al.49analyzed the tribological performance of diamond and speculated on the formation of a diamond-like tribofilm. Europe scholar Sa′nchez Egea et al.50first confirmed the excellent antifriction and antiwear properties of CGNs through friction and wear tests.However,the cutting/grinding zone interface is more complex, being characterized by high temperature, pressure, and speed. Previous studies do not apply to this condition, thereby limiting application.

Some studies involving CGNs application in NMQL,antifriction, antiwear, tool failure, material removal mechanism,and surface formation mechanisms remain blank.Particularly for the typical turning, milling, and grinding conditions with different boundary conditions, the application of CGNs also shows various levels of processing performance. At present, no in-depth study has been published on the influence of CGNs parameters,processing types,and lubrication performance.Thus,references on production practice are limited.To fill this gap,the present study aims to provide a comprehensive review and periodic critical assessment of existing knowledge.

As shown in Fig. 1, review starts with the physicochemical properties of common CGNs (section 2), followed by the antifriction and antiwear mechanisms of NMQL and its influence on processability, which establishes a standard for machining evaluation (section 3). The application in turning,milling and grinding, and lubrication property of CGNs are evaluated, and selection standard of CGNs for various conditions are summarized(section 4–6).The conclusion and future challenges are presented in section 7 and 8.

2. Carbon group nano-enhancers

As common types of CGNs, fullerene, nano-diamond (ND),Carbon nanotubes (CNTs), and Graphene (GR) have become research hot-spots due to their unique structure and excellent properties.Fullerene and ND are spherical,CNTs are tubular,and GR is layered.According to previous studies,CGNs have specific self-lubricating properties and the potential to become‘molecular ball (shaft)’ lubricating additives, which are widely considered in the field of fluid and solid lubrication. Physicochemical properties, micromorphology, and molecular structure of CGNs are presented in Table 1.51–55

2.1. Spheroidal nano-diamond (zero-dimensional)

Zero-dimensional CGNs refer to the three dimensions of materials below 100 nm, such as spherical fullerene, spherical ND,and others.Tiwari et al.56reviewed the types and applications of zero-dimensional CGNs in detail and described experimental techniques involved in the synthesis and fabrication of processes. Not surprisingly, if spherical CGNs could act like ‘ball rollers in the bearings’ at nanoscale in NMQL, friction can be significantly reduced to improve tool life, temperature, force,and other characteristics.The antifriction and anti-wear mechanism of spherical nano-enhancers was explained intuitively by Khanna et al.57.

ND,formed by a single crystal diamond‘core’has a unique spherical shape and abundant surface functional groups.Each carbon atom forms covalent bonds with the surrounding four carbon atoms. ND presents maximum hardness, high thermal conductivity, high wear resistance, good chemical stability,and corrosion resistance.

2.2. Tubular carbon nanotubes (one-dimensional)

One-dimensional CGNs are materials with two scales below 100 nm,such as tubular CNTs.Kuchibhatla et al.58proposed the definition of one-dimensional nano-enhancers. After the discovery of C60, with the increase of n value, C(n) crystal gradually develops into ellipsoid. When n is large enough,the shape of C(n) becomes tubular in the middle and hemispherical at both ends, and thus they are called CNTs. Kharlamova59reported that CNTs can reduce coefficient of friction (CoF) and effectively improve the friction and wear resistance as an additive, and further summed up the advantage of CNTs in NMQL. CNTs have low cost and higher theoretical Young’s modulus (1.8 TPA), bending strength (14.2 GPA), and lower density. Peng et al.60discovered as early as in 2007 that CNTs could improve antiwear and antifriction of water as lubricant additive.In 2011,Liu et al.61also found that CNTs display better tribological performance in paraffin oil. Thereafter, Akbarpour et al.62found that the mixture of CNTs and Cu exhibited excellent self-lubricating properties.

2.3. Layered graphite/graphene/graphene oxide (twodimensional)

Two-dimensional CGNs are to materials with a scale of less than 100 nm, such as GNP, GR, and GO. Tao et al.63reported detailed classification information on twodimensional nano-enhancers that have the characteristics of atomic thickness. GR, the thinnest layered material (only 0.335 nm),is a new carbonaceous material formed by the close arrangement of single-layer carbon atoms, with a single-layer honeycomb lattice structure.

Although GR was discovered in 2004,Geim and Novoselov 64felt that the tribological property research on GR was still at the initial stage until 2009.In 2010,Dreyer et al.65mentioned the unique properties of GR and its potential for use as a lubricant. Loh et al.66reported that GR has excellent mechanical and thermal conductivity, as well as electrical properties due to its special microstructure.Samuel et al.67preliminarily verified the good machinability of GR for synthetic traditional MWFs. Furthermore, Rummeli et al.68proposed only GR films because composite and lubricant additives have been studied. Rao et al.69reviewed the synthesis, characterization,structure, and properties of GR, which provided support for the application of GR in NMQL.

Fig. 1 Structure of this study.

GO, as an oxide of GR, still maintains the layered structure.Zhu et al.70considered that GO is more likely to achieve better performance due to its polar functional groups.Kinoshita et al.71verified the excellent tribological properties of GO in a water-based lubricant.Senatore et al.72provided a comprehensive review of the tribological results of GO-based nanolubricant. Abundant oxygen-containing functional groups(hydroxyl,carboxyl,epoxy.)are in the molecular structure, which interact with the metal surface through electrostatic interaction, hydrogen bonding, and dispersion force.This condition enables GO to have a strong adsorption on the metal surface and good dispersion in the lubricant.

GNP presents high temperature resistance, corrosion resistance, and self-lubricating characteristics. Huang et al.73reviewed in detail the application of GNP in solid and fluid lubricants. Hou et al.74demonstrated the improvement of the tribological properties by GNP.Li et al.75analyzed the tribological properties of GNP as lubricant additive. The results showed that the expanded GNP additives improved the loadcarrying capacity and antiwear ability, and the optimal concentration of the nanolubricant was proposed to be approximately 0.2%. The carbon atoms in the middle layer and between the layers in the crystal structure are combined by a weak Л electronic bond. The common understanding is that the low CoF of GNP is due to a weak binding force.

3. Cutting mechanism of nanolubricant minimum quantity lubrication

By introducing nano-enhancers into the interface between the tool and the workpiece, the antifriction and antiwear charac-teristics are changed by altering the boundary condition. A variation of the material removal constitutive relation also happens under the action of thermal softening effect, thereby affecting the force, CoF, tool life, tool wear, surface integrity,and other features. Chan et al.76observed the changes of the aforementioned phenomenon through experiments and discussed the criteria for the molecular structure of biolubricants for higher tribological performance. Nevertheless, no mechanism can explain the change rules,and no unified index system has been established for comprehensive quantitative evaluation of NMQL machinability. Therefore, this section is set to fill the preceding gaps and provide a basis for the follow-up evaluation (see Fig. 2(a))76.

Table 1 Physicochemical properties, micromorphology and molecular structure of different CGNs.51–55

3.1. Antifriction and antiwear mechanism

Compared with MQL, the lubrication state between the tool/-workpiece interface changes when the nano-enhancer is involved. The lubrication state changes from boundary lubrication under MQL to oil-film lubrication state under NMQL due to the following mechanisms:i)The space between the tool and workpiece is increased with the support action of nanoenhancers, and the direct contact between the tool and workpiece is avoided,thereby reducing friction.ii)A larger and continuous space is generated with nano-enhancer support,facilitating the infiltration of the nanolubricant into the tool/-workpiece interface. Therefore, a continuous lubricant film is formed, which improves the lubrication property (see Fig. 2(b) and 2(f)).

3.2. Tool failure mechanism

Fig. 2 Cutting mechanism of NMQL.

Tool wear (such as spalling and abrasion) and material adhesion (such as built-up edge, diffusion, and adhesion) under poor lubrication are main forms of tool failure in aerospace material machining, which not only increases cost but also reduces the machining accuracy and surface quality.Tool failure mainly occurs at the tool nose and main cutting-edge,thereby increasing friction. After addition of nano-enhancers,a dynamic or static protective oil film is formed in this contact zone, therefore delaying the tool failure and increasing the sharpness of the tool nose significantly (see Fig. 2(c)).

(1) Tool wear: The tool bears a higher mechanical load when cutting difficult-to-machine materials, thereby leading to stress concentration and crack under a high pressure interface.Then,material spalling occurs after the crack expands on the tool surface. In addition, abrasion is a common problem under poor lubrication. After nano-enhancers are added, the tool cracks are filled at the initial stage of crack formation and further crack propagation is restrained. More significantly, the nano-enhancers form a dynamic and static oil film on the tool as a protective layer,which effectively reduces CoF and delays the tool wear.

(2) Material adhesion: Due to the extremely low thermal conductivity and high strength of difficult-to-machine materials,the tool bears a high thermal load during cutting.The large amount of heat produced by friction, which is difficult to disperse through the workpiece, rapidly accumulates in the first deformation zone and increases the plasticity of the material workpiece. Therefore, the material is bonded to the tool by plastic flow under poor lubrication. After adding nanoenhancers, a formed lubricant film reduces the friction force and temperature in the processing area.Thus,the thermoplastic flow and accumulation of the workpiece material on the tool are reduced,and the formation of built-up edge,diffusion,and adhesion are avoided.

3.3. Material removal mechanism

With addition of nano-enhancers, the cutting force and interfacial CoF are changed significantly, thereby leading to the radical change of the material removal mechanism and chip formation.Dhar et al.77found that chip morphology is different between MQL and other conditions,but did not provide a detailed explanation. As a main basis for material removal analysis, chip morphology could be conducted from four aspects: thickness, sawtooth, chip curl diameter and fresh surface (see Fig. 2(e)).

(1) Thickness: The chip thickness decreases with the addition of nano-enhancers due to the significant improvement of lubrication properties and reduction of friction heat.The plasticity of the workpiece in the material removal process decreases, and the generation of built-up edge on the tool is weakened. Therefore, the tool maintains a sharp edge that reduces the chip thickness.

(2) Sawtooth: When the lubrication condition is poor, the chip breaks under the cutting action of the main cutting-edge,which easily causes the production of sawtooth. This condition further aggravates the main cutting-edge wear and reduces the surface quality. The sawtooth mainly appears on the side and free surface of chip. The side sawtooth is mainly due to the tearing of the workpiece material, while the free surface sawtooth is formed by the extrusion of the rake face,which is related to the force direction of material removal.When the material is removed, the higher the deformation is, the higher is the sawtooth height (or even fracture) formed. With the addition of nano-enhancers and accumulation in the wear pits of the main cuttingedge,the lubrication state is improved and the wear rate of the main cutting-edge decreases.At the same time,the interface CoF and friction angle decrease,thereby weakening the deformation degree of the material removal.As a result, the material breaks near the sharper main cutting-edge, resulting in sawtooth free chips. Jadam et al.78confirmed this analysis by clearly observing the sawtooth shape in experiments.

(3) Chip curl diameter: With the decrease of CoF, friction angle in the second deformation zone, and deformation degree in the first deformation zone, the chip becomes thinner and the curl diameter decreases. In this case,the chip extrusion force, friction area, and friction heat of the rake face in the second deformation zone decrease. The heat required for material removal in the first deformation zone also decreases. This is result is meaningful for difficult-to-machine materials.

(4) Fresh surface: The roughness of the fresh surface represents the tribological characteristics between the rake face and chip. When the chip outflows along the tool,the larger friction and shear force formed at the tool/chip interface leads to adhesion of the rake face. Therefore,the adhesion scratches the workpiece material, leaving a certain degree of scratch on the fresh surface.In addition, a large amount of heat generated by the tool/chip interface increases the number of scratches, which indicates the wearing of the tool. The addition of nanoenhancers protects the tool and reduces scratches.

3.4. Surface formation mechanism

The surface integrity of the workpiece is mainly affected by force, temperature, and tool wear degree. Surface damage is the primary to avoid for difficult-to-machine materials79.The most important method to improve the surface integrity is to restrain the surface defects formed by machining. Yin et al.80found that NMQL significantly inhibited the formation of defects in the surface formation process but did not provide a specific explanation. For titanium alloy and nickel alloy, the common surface defects are adhesion, burn, plastic uplifts, and scratch (see Fig. 2(d)).

(1) Adhesion:The adhesion and micro-burrs formed on the machined surface are from two aspects: i) The material at the tool nose is rolled and welded to the machined surface under high cutting heat. ii) In the process of material removal, friction heat is generated between flank surface and machined surface. The workpiece material peels off under the thermal softening effect,while it is rolled and welded as adhesion, and pitting is formed in the material spalling place.

(2) Burn: Due to the low thermal conductivity of the workpiece,the cutting/grinding heat generated on the surface cannot be diffused quickly through the workpiece.Therefore, the heat accumulation in the surface layer results in burns and hardening layer.

(3) Plastic uplifts:The plastic uplifts of the workpiece material are serious under the thermal softening, which reduces the material removal rate and irregularity of the feed-mark shape.

(4) Scratch: The scratch is caused by the decrease of the straightness of the main cutting-edge of the tool. At the same time, with the continuous processing of the tool, the scratch continues to expand and deepen, and even the material exhibits a plastic uplift.

The reason for the aforementioned phenomenon is the insufficient cooling and lubrication performance of the tool/-workpiece contact area.The addition of nano-enhancers leads to significant changes. The formation of a nano-lubricating film between the flank and workpiece surfaces,reduces the friction and heat generation will protecting the newly formed surface of the workpiece. The nano-lubricating film has been confirmed through the energy dispersive spectrometer (EDS)analysis of the workpiece surface.

4. Application in turning

Turning is indispensable machining method for aerospace rotational parts such as aeroengine casings, turbines, combustors, cabin sections and others, which were manufactured by titanium alloy and nickel alloy. Peng et al.81stated that turning of difficult-to-cut materials is common in the manufacturing of cases for gas turbine components due to their excellent mechanical properties, and traditional MWFs is important in machining. From another aspect, the usage of traditional MWFs in turning the aforementioned large rotational parts has brought a recycling problem for open-type machinery.NMQL not only solves above perplex but also realizes good machining performance. CGNs have been confirmed to play a significant role in tribological properties by single-factor experiments in NMQL, but influence laws and application specifications are unclear. To provide a basis for application,this section proposes a comprehensive review for processability and identifies the unique mechanisms of CGNs in turning.

4.1. Turning force

Compared with MQL, a large force reduction of 45.65% was directly observed by Li et al.82when using GO in NMQL for Ti-6Al-4V, whereas 22.22% force reduction was reported by Nguyen et al.83with employment of GNP and GR for Ti-6Al-4V. Marques et al.38used GNP and MoS2nanolubricant in Inconel 718 turning and obtained the maximum cutting force decreased by 10.64% and 8%, respectively (see Fig. 3).With the increase of cutting speed, the cutting force fluctuates greatly under the condition of MoS2nanolubricant, while it changes smoothly under the condition of GNP nanolubricant.

The results show that the turning force of titanium alloy and nickel alloy decreased obviously after layered nanoenhancers are added. The decreasing order of turning force was MoS2< GNP = GR < GO. On the one hand, layered CGNs show better lubrication performance than MoS2. The reason is that their smaller intermolecular bond energy results in better antifriction and antiwear behavior,which is analyzed in a later part of this paper. On the other hand, among the three CGNs, GO has the largest reduction because it is rich in functional group content. Therefore, GO facilitates adsorption on the surface of the workpiece and tool,which results in the formation of a stable oil film.

Fig. 3 Analysis of experimental data in turning.

Furthermore, Singh et al.84used a hybrid nanofluid containing the Al2O3and GR for turning of AISI 304 steel, and the cutting force was decreased by about 9.94% compared to Al2O3nanolubricant. Sharma et al.85researched AISI 304 steel turning experiment with the hybrid of Al2O3and multiwalled CNTs. Compared with Al2O3nanofluid, the force of hybrid nanofluid lubrication condition was decreased 20.2%.The improved performance of hybrid nano-enhancers might due to ‘physical synergistic effect’.

4.2. Coefficient of friction (CoF)

CoF is the evaluator of lubrication at the tool/workpiece interface and can be calculated based on measured normal and tangential forces for different workpiece material (WPM)machining. The reduction of CoFs in NMQL turning of Ti-6Al-4V were observed,including 27.54%as reported by Singh et al.86using GR,43.55%reported by Li et al.82using GNP,and 68.63% reported by Hegab et al.87using CNTs, respectively (see Fig. 3).

The results show that the decreasing degree of CoF is GR < GNP < CNTs. Compared with layered nanoenhancers, tubular nano-enhancers exhibit better friction reduction performance, because of the different film-forming and lubrication mechanisms of various nano-enhancers in the cutting area. The details are presented in section 4.6.

In addition, Khan et al.88used Al-GNP hybrid nanofluid to conduct the turning experiments of AISI 52100 Steel.Compared with the Al2O3nanofluid, the CoF was decreased by 58.8%. Compared with Al2O3nanofluid, Sharma et al.89found that the CoF of Al2O3and multi-walled CNTs hybrid nanofluid decreased 73.5%.

4.3. Chip morphology

Compared with MQL,Hegab et al.87found that adding CNTs as nano-enhancer(NE)reduces the chip thickness,the chip morphology was more uniform than that of MQL,and the sawtooth height was reduced.Moreover,the chip thickness of the 4wt%CNTs lubricant was smaller than that of 2wt%, which eliminated the nonhomogeneous serrated chips. In another study,

Li et al.82confirmed that adding GO can effectively reduce the area/degree of built-up edge on the rake face. Singh et al.86analyzed sawtooth chip morphologies under different lubrication conditions.When GR with 1.0wt%was added,the chip curl diameter decreased to the minimum value(see Fig.4(a)).90–91According to the analysis of the experimental results of the aforementioned chip morphology, adding GR and CNTs improve the interfacial lubrication characteristics,and reduces the CoF and material removal friction angle, which shows the strengths of CGNs. In addition, when the concentration of nano-enhancers increases, the lubrication performance also changes.

4.4. Tool

(1) Tool life: In Ti-6Al-4V turning with GNP nanoenhancers as additive,Nguyen et al.83reported that tool life increased by 22.22% compared with MQL. When the concentration of GNP particles was further changed,the tool life first increased and then decreased. Furthermore the tool life reached the maximum when the mass fraction was 0.5%. When the concentration of GNP nano-enhancers increased to 2wt%, the lifetime decreased greatly, to only 23.96% of 0.5wt%. When GR was used in Ti-6Al-4V turning, the tool life increased by 27.27% compared with MQL, as reported by Singh et al.92. Marques et al.38used GNP and MoS2nanolubricant to conduct turning Inconel 718 experiments. The results showed that tool life under GNP nanolubricant lubrication was 2.38% and 16.21% longer than that under MQL and MoS2nanolubricant, respectively (see Fig. 3).

According to tool life analysis, the inhibition effect of various types of nano-enhancers on the tool wear in the turning process is MoS2< GNP < GR. This result is the same as the conclusion obtained in section 4.1, which confirms the advantages of layered CGNs. At the same time, GR also has better lubrication performance than GNP, which is due to the better film-forming performance of GR with fewer molecular layers in turning.

(2) Tool wear: the tool flank wear (VB) is an important parameter. For turning of Ti-6Al-4V, compared with MQL, obviously improvement of the tool wear at the flank surface was observed by Singh et al.92, where VB values were decreased by 54.55% with employment of GR. When CNTs were used as additives, Hegab et al.90found that VB decreased by 63.64%. A similar conclusion was obtained by Marques et al.38when turning Inconel 718; VB values were slightly reduced by 2.38% when GNP was used, compared with MQL.Hegab et al.93studied CNTs used as additives, and found that VB decreased by 51.72% (see Fig. 3).According to VB analysis, CNTs showed better friction reduction performance than GR, and both of them are much better than GNP. This result is also consistent with those of CoF, which shows that the tubular nano-enhancers can obtain lower interfacial CoFs, thus greatly reducing tool wear. Further, Compared with Al2O3nanofluid, Sharma et al.89found that VB decreased 11% for Al2O3and CNTs hybrid nanofluid,while 12.29% for Al2O3and GR hybrid nanofluid94.

(3) Tool wear and material adhesion:When turning Inconel 718,the phenomena of abrasion,diffusion,and attrition were improved after GNP was added.The same rule was obtained by Gutnichenko et al.95. In the process of turning titanium alloy by Hegab et al.90, the addition of CNTs significantly reduced abrasion. In another group of experiments, GR was used to avoid chipping and attrition. Marques et al.38found a lubricant film on the tool surface through EDS analysis, and the C content increased to 14.38% (the content of C element in the titanium alloy itself was 0.1%). This lubricating film may come from workpiece material or GNP.Gupta et al.91prevented a slight adhesion under MQL conditions by adding GNP. Li et al.82found a built-up edge in MQL, which disappeared after GO was added.Revuru et al.96found that the tool seriously failed due to plastic deformation under MQL condition,which was effectively avoided by adding GNP particles. Furthermore, due to the different micro-morphologies and physical properties of nano-enhancers, the tool wear form also changed. Gupta et al.91found that, different from the layered nano-enhancers, the tool wear was characterized by attrition when spherical Al2O3was used (see Fig. 4(b)).

Fig. 4 Analysis of NMQL turning using CGNs.

Observation of the tool surface morphology showed that the tool wear and surface adhesion improved after nanoenhancers were added. In addition, a large amount of C element was found on the tool surface by EDS analysis, which also confirmed that nano-enhancers can form lubricating film on the tool surface. However, current studies did not analyze the film-forming mechanism of various nano-enhancers.

4.5. Surface integrity

(1) Surface roughness: the arithmetical mean deviation of pro-file (Ra) of Ti-6Al-4V was reduced by 29.17% in study of Singh et al.86and 28.54% in research of Li et al.82when GR nanolubricant was used. For turning Inconel 718, Hegab et al.93conducted an experiment on CNTs and Al2O3nanolubricant turning. Compared with MQL, the surface roughness Ra of NMQL was reduced by 79.03% and 69.35%, respectively. Marques et al.38used GNP for Inconel 718 turning and found that Ra decreased by 23.68%(see Fig.3).Based on these results, the influence order of different shape nanoenhancers on the surface roughness is GNP

(2) Morphology of machined surface: Gupta et al.97used Al2O3,MoS2,and GNP to conduct titanium alloy turning experiments. After adding GNP, the workpiece surface produced finer, more regular, and clearer cutting marks than MQL and the other two kinds of nanoenhancers, which showed that GNP has a more stable friction-reducing performance. Therefore, the vibration caused by variable cutting force is avoided and results in a reduction of furrow width. Gupta et al.91found that, compared with Al2O3and MoS2, the feed marks on the workpiece surface under GNP was more regular and obvious. Adhered microparticles of cutting material, various surface and subsurface layers disappeared,small micro burrs and surface pitting were significantly reduced, and the surface was smooth and clear (see Fig.4(c)).The reason is that GNP has higher heat transfer coefficient than the other two nano-enhancers,which makes the heat transfer from the cutting zone faster and reduces the temperature. Therefore, the microstructure changes slightly, and the cutting force and tool wear are reduced.

4.6. Turning mechanism of NMQL using CGNs

From the preceding research, we can observe that different morphologies of CGNs present various processing properties(see Table 2)98.In particular,tubular CNTs show best antifriction and antiwear performance, which is related to the wettability and film-forming mechanism in the turning zone (see Fig. 5).

4.6.1. Infiltration and film formation mechanisms in turning

In turning, the tool is in a continuous work state of material removal process (see Fig. 5(a)) and, therefore, the infiltration and film formation of CGNs-based nanolubricant is also a continuous process in wedge-shaped interfaces between the rake face/chip and flank/fresh surface. From the tool nose to the non-contact area, mechanical load is reduced from the maximum to zero so that the tool nose is the most serious wear area.99In the MQL condition,the contact surfaces of the rake face/chip and the flank/fresh surface are in the boundary lubrication state,in which the lubricant has difficulty infiltrating the tool nose, because of the weak load-carrying capacity of the lubricant.

The load-carrying capacity and wettability are greatly improved by adding nano-enhancers,but are essentially different with employment of different CGNs. The layered CGNs self-spread and infiltrate from the entrance of the wedgeshaped area to the tool nose, which can easily to form a film at the entrance of the wedge-shaped area and has a filling effect on the micro pits of the tool surface. However, the space for deeper distance infiltration is weakened due to the ‘plugging effect’, resulting in fast falling speed and depth of nanolubricant infiltration (see Fig. 5(f)-5(g)).

A favorable turn is presented when using tubular and spherical CGNs, in which two wettings at interfaces occur through rolling movement and,at the same time,provide space for the subsequent infiltration of the lubricant.Under the supporting role of CGNs, the tribological state is more similar to oil film lubrication than boundary lubrication. Tubular and spherical nano-enhancers have not only high infiltration speed but also large infiltration depth (see Fig. 5(b) and 5(d))100-101.

4.6.2. Antifriction and antiwear mechanisms in turning

Furthermore,various degrees of processability are also caused by the different antifriction and antiwear mechanisms of various CGNs morphologies.

Table 2 Conclusion of processing properties in turning.

Fig. 5 Turning mechanism of NMQL using CGNs.

Spherical CGNs are in point contact state with the tool and workpiece surface. Although friction reduction by CGNs rolling is available, the tool surface easily to produces stress concentration and cracks when rolling at a high pressure interface due to high hardness of spherical CGNs (see Fig. 5(h)). Thus block spalling of tool failure form was observed in previous studies.

Layered CGNs are in surface contact state with the tool and workpiece surface. Different from spherical CGNs, layered CGNs play a solid lubricant role by sliding behavior and,therefore,avoid spalling wear.However,because of poor wettability, the tool/workpiece interface is in a heterogeneous lubrication state.This condition easily causes repeated changes in friction force and fluctuation of turning force, and then the tool may fall into a destabilizing or vibration state and failure in the form of diffusion or adhesion. At the same time, workpiece marks are broadened and Ra is increased (see Fig. 5(c)).

Tubular CGNs are in line state with the tool and workpiece surface. Compared with layered CGNs, tubular CGNs have stronger wettability and can easily enter the tool nose area.Compared with spherical CGNs, tubular CGNs present a larger bearing area and more stable lubrication state. Furthermore, they are advantageous because the rugged surface of the tool and workpiece become smooth under hard tubular rolling, thereby resulting in better tool life and machining surface integrity (see Fig. 5(e)).

5. Application in milling

In the milling of aero-engine blades, frame structural parts play an important role in the aerospace industry, which is characterized by a high rate of material removal. Except for the recycling problem of traditional MWFs, the chip removal function is insufficient, which results in chip accumulation,workpiece scratch, and even tool failure as well as workpiece thermal damage. The preceding issues not only increase work intensity but also decrease the qualified rate of parts.Application of NMQL solves these problems with the help of high pressure gas and minimum quantity lubricant. Furthermore,different from turning, milling is an intermittent and higher speed material removal process with multiple cutting-edges.In this situation, the infiltration, film formation and tribological characteristics of CGNs-based lubricants are completely changed; therefore, a comprehensive review and analysis is necessary in this section.

5.1. Milling force

Milling force is an important indicators for aerospace materials machining.102Compared with MQL,the large force reduction of 18.22%and 20.83%direction was observed when using GR for TC4 by Li et al.101and TC21 milling by Li et al.103.Bai et al.104used CNTs and GNP in NMQL milling of Ti-6Al-4V and found that milling forces were reduced by 22.4% and 8.43%,respectively.In this study,CNTs present better performance with force reduction of 15.35% compared with GNP.S?irin and Kivak105used GNP in Inconel X-750 milling, which reduced the force by 16.23%compared with MQL(see Fig.6).According to the preceding literature, the order of decreasing milling force by adding different CGNs is GNP

5.2. Chip morphology

Scholars set chip morphology for key investigations in their NMQL studies on titanium alloy by employing GR, GNP,CNTs, and other nano-enhancers. For instance, Bai et al.104used Al2O3, SiO2, MoS2, CNTs, SiC, and GNP. Park et al.106used GNP and Yin et al.107used SiO2and Al2O3.

Chip thickness is decreased with the addition of nanoenhancers,compared with MQL.When comparing CNTs with GNP,we found that CNTs can obtain thinner chips,but both are larger than spherical Al2O3, which remains basically unchanged.

Chip shapes and fresh surface are also different when various morphologies of CGNs are used.For layered GNP,the Cshaped chips and rough fresh surface with irregular scratches were observed, which is similar to MQL. Moreover, obvious cracks and fractures appear on the fresh surface, which indicates that the chip has experienced a large shear plastic deformation under the worst lubrication. For tubular CNTs, the spiral chips and smooth fresh surface with obvious scratches were observed, which indicates better lubrication effect than the layered GNP. For spherical Al2O3, chip parameters are the best with a long spiral shape, that is more slender than other nano-enhancers. Furthermore, the fresh surface is smoother without any obvious scratch, the metal deformation of the cutting layer is more uniform,and the free surface has a small degree of sawtooth. Therefore, we can conclude that lubrication performance is significantly improved by spherical Al2O3(see Fig. 7(a)).

According to the previous studies, the order of lubrication effect is GR

Fig. 6 Analysis of experimental data in milling.

Fig. 7 Chip and tool analysis of NMQL milling using CGNs.

5.3. Tool

(1) Tool life: S?irin and Kivak105used GNP in NMQL milling of Inconel X-750 and found that tool life increased by 20.5% compared with MQL. When VB = 300 μm was defined as the threshold of tool failure, Li et al.101concluded that the tool life of TC4 was prolonged by 23.07% when using GR compared with MQL (see Fig. 6).

(2) Tool wear(VB):Park et al.106found that the flank wear was reduced by 52.17%in Ti-6Al-4V milling when GNP is used,compared with MQL.Li et al.101also found that the flank wear decreased by 8.57%when GR was used in TC4 milling.Sirin and Kivak105studied maximum VB in Inconel X-750 milling, and VB=0.32 mm under MQL was decreased significantly after GNP was added. Furthermore, with the increase of GNP concentration, the tool wear first decreased and then increased (see Fig. 7(b)). The reason is that nano-enhancers aggregate when the concentration is increased,which leads to poor stability of lubricants. Lv et al.108conducted a comparative study on the processing characteristics of GO/SiO2hybrid nanofluid through milling experiments. The reduction of VB were 33.3% and 46.7% compared to SiO2and GO nanofluid, respectively.

By comparing different lubrication conditions including dry, high pressure gas, MQL, and NMQL, Li et al.101identified four stages of tool wear:wearing in,normal wearing,rapid wear, and tool failure. Compared with MQL, the tool wear curve with GR had a smaller slope and longer length in the first three stages, which indicated that the wear rate was slower.The tool surface adhesion under GR condition was significantly reduced, and cutting-edge and build-up edge disappeared. Moreover, when GR was used, the cutting-edge maintained better straightness (see Fig. 7(b)).

5.4. Surface integrity

Fig. 8 Workpiece analysis of NMQL milling using CGNs.

(1) Surface roughness: Li et al. used GR in NMQL milling of TC4101and TC21103and obtained Ra reduction of 29.88% and 26.19% compared with MQL, respectively.The layer thickness of micro-hardness (HV) of Ti-6Al-4V decreased by 8.84% when using GR (see Fig. 8(b)).The micro-hardness is an important indicators for surface integrity109.Bai et al.104conducted NMQL milling experiments of Ti-6Al-4V using CNTs and GNP. Compared with MQL,the Ra values were reduced by 51.07%and 39.84%,respectively.S?irin and Kivak105used GNP in Inconel X-750 NMQL milling and obtained Ra reduction of 30.51%compared with MQL,when nanolubricant concentration was 0.5 vol% (see Fig.6). Based on the preceding results, the usage of tubular CNTs could ensure better surface quality than those with layered GNP and GR. Further, in the study of Lv et al.108, the Ra values of GO/SiO2 hybrid nanofluid were reduced by 4.88% and 9.3%, respectively, compared to SiO2and GO nanofluid for milling AISI 304 stainless steel.

(2) Morphology of machined surface: Li et al.101,103used GR in NMQL milling of TC21 and TC4, where the workpiece surface significantly improved compared with MQL, in which adhesion, pitting, and large fuse disappeared. In the work of Bai et al.104, the Ti-6Al-4V surface machined by different nano-enhancers showed that GNP-based NMQL obtained the deepest furrow and plastic uplift, while CNTs had only a slight scratch.The different surface qualities were also caused by various film-forming and lubrication performance of nanoenhancers. Yin et al.107analyzed the element composition of Ti-6Al-4V surface by EDS and found that the content of carbon element deposited on the workpiece surface machined by CNTs and GNP based NMQL was the highest,reaching 4.72%and 5.31%,respectively(see Fig. 8(a)). However, the content of the carbon element was less than that detected in turning, which indicated that the wetting film-forming performance of nano-enhancers in milling was worse than that in turning.

5.5. Milling mechanism of NMQL using CGNs

Based on the preceding results, the order of processability of different CGNs is layered CGNs

5.5.1. Infiltration mechanisms in unique milling zone

The milling process is the intermittent cutting of several edges to remove material (see Fig. 9(a)). For a single cutting-edge,the two wedge-shaped friction pairs of rake face/chip and flank surface/fresh surface do not exist all the time, but appear and disappear at a certain frequency (see Fig. 9(b)).

Compared with turning, the wetting boundary of CGNs is essentially changed,which is the infiltration of a single edge in an extremely short time. This situation is not conducive to infiltration and film formation due to two reasons: i) When the cutting-edge is not cutting, nano-enhancers are easily lost under the ‘impact cleaning effect’ of high pressure gas from the nozzle. ii) Forming a more stable lubricant film quickly at an extremely short wetting time is difficult. iii) Some nano-enhancers flow out with the cutting tool before they infiltrate the cutting area. Other nano-enhancers do not enter the cutting area and cannot play a cooling and lubricating role.

Thus the reduction percentage of the milling force with the same nano-enhancers is less than that of the turning force,and the content of C element on the surface of the milling workpiece is much lower than that of turning.

5.5.2. Film formation, antifriction, and antiwear mechanisms in milling

As the cutting boundary conditions of milling are different from those of turning, the film formation, antifriction, and antiwear mechanisms of various CGNs are also changed.For layered nano-enhancers, the self-spreading behavior is not conducive to film formation at the interface of rapid cutting.In addition,layered nano-enhancers sustain greater resistance when moving in the lubricant due to its larger specific surface area and plate shape. Therefore, they encounter difficulty in entering the cutting area and spreading as a largearea lubrication film in a very short time. On the contrary,spherical nano-enhancers sustain less resistance when moving in the lubricant and present better wettability. More significantly, with the relative sliding of the tool rake face and fresh surface, spherical nano-enhancers are more likely to roll into the interface depth and play a rolling lubrication role.Although tubular CNTs also roll in the cutting zone, they easily easy to get entangled with each other, which increases the wetting resistance. In a very short time, the infiltration speed and depth become smaller than those of the spherical nano-enhancers.

Table 3 Conclusion of processing properties in milling.

Fig. 9 Milling mechanism of NMQL using CGNs.

Fig. 10 Analysis of experimental data in grinding.

Therefore,the spherical nano-enhancers are the best for use in the milling process (see Fig. 9(c)). However, in previous research, few scholars have conducted NMQL processing experiments for spherical diamond or fullerene.The preceding rules are based on the experiment of Al2O3.Further research is necessary on the application of diamond or fullerene in milling.

6. Application in grinding

Grinding is an important finish machining method for aeroengine blade mortise, tenon, and other parts, which could satisfy high precision and surface quality requirements110,111.The main concern of titanium alloy and nickel alloy grinding is thermal burns (caused by bad tribological characteristics)due to negative rake angle cutting of abrasives,which is different from turning and milling112,113. The usage of CGNs in NMQL is an effective method due to its better lubrication performance.However,the mechanisms and application specifications under grinding boundary conditions differ from cutting,which are discussed in detail in this section.

6.1. Grinding force

The NMQL grinding experiments were conducted with different CGNs,and various conclusions were obtained(see Fig.10).

(1) Layered CGNs:When GR is used in NMQL grinding of Ti-6Al-4V,compared with MQL,normal grinding force(Fn) and tangential grinding force (Ft) in the work of Singh et al.114were reduced by 11.1% and 26.71%,respectively. Meanwhile, Ibrahim et al.115found that Fnand Ftdecreased by 19.39% and 79.13%, respectively. Li et al.116used GR in grinding TC4, and Fnand Ftdecreased by 22.28% and 42.79%, respectively.The experiment by Pavan et al.117on Inconel 718 grinding with GR nanolubricant showed that Fnand Ftdecreased by 25.26% and 27.22%, respectively.

(2) Tubular CGNs: Compared with MQL grinding of Inconel 718, the Ftdecreased by 14.15% in the study of Jia et al.118with the use of CNTs.Wang et al.119performed similar experiment, where Fnand Ftdecreased by 12.23% and 24.31%, respectively.

(3) Spherical CGNs: The experiment of Inconel 718 grinding with ND by Jia et al.118showed that Ftdecreased by 20.28% compared with MQL.

According to the preceding results, the order of grinding force reduction is tubular CGNs

Moreover, the effect of various CGNs concentrations on grinding force is also significant. In TC4 grinding with GR nanolubricant, Li et al.116found that the grinding force first increased, and then decreased with the rise of mass fraction.Force was reached at a minimum value of 0.1wt% due to the enhancement of the lubricating property of the oil film by an appropriate amount of GR. However, the GR function is weakened with a lower concentration due to insufficient lubrication. On the contrary, when the concentration is extremely high, excessive GR easily blocks the grinding zone and hinders the film formation process, thereby resulting in poor lubrication performance.

6.2. Coefficient of friction (CoF)

The index of CoF is very important in grinding. Compared with turning and milling,90%of the grinding heat comes from large area friction between the abrasive and workpiece surface.Therefore, reducing CoF is important to reduce the grinding heat. The NMQL grinding experiments were conducted with different nano-enhancers, and different conclusions were obtained (see Fig. 10).

(1) Layered CGNs: Researchers conducted experimental studies on Ti–6Al–4 V grinding with GR,and CoFs decreased by 26.3% as reported by Li et al.116and 75.24% by Ibrahim et al.115.The experimental study of Inconel 718 grinding with GR by Pavan et al.117showed that the CoF was reduced by 0.72% compared with MQL.

(2) Tubular CGNs: A comparative experiment on Inconel 718 grinding with CNTs nanolubricant was conducted, and results showed that CoF decreased by 4.35% as reported by Jia et al.118, 14.29% by Zhang et al.121,122, and 10.75% by Wang et al.119compared with MQL.

(3) Spherical CGNs: CoF was reduced by 5.23% as reported by Jia et al.118and 6.31% by Wang et al.119in Inconel 718 grinding with ND nanolubricant, compared with MQL.

(4) Hybrid nano-enhancers: Zhang et al.123used MoS2/CNT and synthetic bio-lubricant to prepare NEBL and carried out NMQL grinding research of Inconel 718. The CoF for composite MoS2/CNT NEBL was decreased separately by 8.8% and 15.3% when using mono MoS2and CNT. They attribute the improved performance to ‘physical synergistic effect’ of composite nano-enhancers.

The layered GR had more potential to reduce CoF,that is,to improve the lubrication performance compared with the spherical ND and tubular CNTs. Different layered CGNs still had different lubricating properties. Singh et al.114reported that the CoF order was MoS2> GNP > GR. The usage of GR presented better lubrication performance with CoF reduction of 21.9% compared with MoS2due to the different interlayer bond energy of various nano-enhancers.As explained by Cui et al.124(see Fig.11(a)),the atomic binding force between the layers of the nano-enhancers is weak, thereby forming a low shear plane. When the upper and lower layers move each other, the nano-enhancers extend along the fracture layer of the low shear plane and form a film.The weaker the interlayer bonding force(i.e.,the weaker the interlayer bond energy),the easier it is to spread the film. The interlayer bond energy of nano-enhancers is proportional to the mass fraction. Among the three nano-enhancers, the order of mass fraction is MoS2>GNP>GR.Therefore,the lubrication performance of GR is proved to be better than that of the other two.

6.3. Chip morphology

The change rule of chips observed in grinding is similar to that in cutting. In the work of Wang et al.119, compared with MQL, a smoother fresh surface, and longer and finer shape were observed by lubrication with ND nanolubricant (see Fig. 11(b)). The ND has excellent lubricating properties and reduces the friction angle of material removal, which plays an important role in reducing abrasive wear and force.

6.4. Tool

(1)Abrasive wear:Ibrahim et al.115characterized the abrasive wear by friction and wear experiments using GR NMQL as lubrication condition. The wear rate of ZrO2abrasive was reduced by 90% compared with that of MQL. Therefore, the abrasive can maintain good shape and size (i.e., sharpness)so that it can be processed with less grinding force and CoF.

(2) Wear of grinding wheel: Wang et al.119conducted a comparative experiment with usage of ND and CNTs-based nanolubricant in Inconel 718 grinding.The results showed that grinding ratio(G-ratio)of CNTs and ND increased by 18.43%and 27.22%, respectively, compared with MQL. This result also showed the excellent properties of CGNs in antifriction and antiwear, which was initially confirmed by Kalita et al.125(see Fig. 10). Their group found a lubricant film mainly

composed of nano-enhancers by observing the surface of abrasive and scanning electron microscope and energy dispersive spectrometer.

6.5. Surface integrity

(1) Surface roughness: Researchers conducted experimental studies on Ti–6Al–4 V grinding with GR, and Ra values were reduced by 32.65% as reported by Ibrahim et al.115, 14.46%by Li et al.116,and 48.8%by Singh et al.120.At the same time,Li et al.116observed the 10.81% decreased layer thickness of micro-hardness under GR-based nanolubricant compared with MQL.Pavan et al.117reported that Ra value was reduced by 41.67% in Inconel 718 grinding with usage of GR nanolubricant (see Fig. 10). In comparison with other two kinds of mono nano-enhancero, the employment of MoS2/CNT in NEBL grinding of Inconel 718 resulted in good machined surface morphology with no workpiece burns phenomena.Zhang et al.123found when the mix ratio of MoS2and CNT is 2:1,The roughness value Ra was reduced to 0.294 μm, 13% and 38.9% lower than that of mono MoS2and CNT, respectively.

(2) Morphology of machined surface: For different layered nano-enhancers,Singh et al.120found that the order of surface quality was GR>GNP>MoS2.The chip/burr welding phenomenon is disappeared and the most improved surface quality was obtained through GR (see Fig. 12c). Singh et al.114used GR nanolubricant in Ti–6Al–4 V grinding and eliminated the surface burn,which was obvious in MQL.Li et al.116performed grinding test of Ti–6Al–4 V,where large furrow,adhesion, and surface burn disappeared with the use of GR nanolubricant. Furthermore, surface quality showed a trend of first improving and then decreasing with concentration increasing (see Fig. 12a), the reason for which has been explained in section 6.1. In addition, economic feasibility has worth to be considered for optimization of concentration.

To verify the film-forming phenomenon on the workpiece surface, the SEM image and element distribution were analyzed by Ibrahim et al.115.The C content on the Ti-6Al-4V surface increased from 0.03% before grinding to 71.7% after grinding with the GR-based nanolubricant (see Fig. 12b).Therefore, a large amount of GR lubrication film was found on the workpiece surface,which plays a role in antifriction and antiwear.

Fig. 11 Chip analysis of NMQL grinding using CGNs.

6.6. Grinding mechanism of NMQL using CGNs

Based on the preceding results, the order of antifriction and antiwear characteristics is tubular CGNs < spherical CGNs < layered CGNs, which is different from the rules obtained in turning and milling (see Table 4). The infiltration,film formation, and antifriction and antiwear mechanisms in grinding is shown in Fig. 13.

6.6.1. Infiltration and film formation mechanisms in grinding

Grinding is similar to milling, in which a single abrasive intermittently cuts and removes material(see Fig.13(a)).However,many abrasives and pores are on the grinding wheel surface,and abrasives have a negative rake angle in the material removal process. This condition results in essentially different infiltration and film formation mechanisms from three aspects:

In terms of cutting characteristics,the negative rake angle is favorable to film formation. In the grinding process, sliding and ploughing stages occur before material removal and,therefore, nano-enhancers are extrusion squeezed on the surface to form a lubrication film by abrasives. However, milling is a positive front-angle cutting, and no obvious sliding and ploughing stage occurs in the cutting process. In this case,the self-spreading infiltration of layered CGNs can form a lubricant film with larger coverage and stronger antifriction and antiwear ability, and this lubricant film is easier to combine with the surfaces of friction pairs to improve the stability of oil film (see Fig. 11(c)).

Considering the material removal behavior, this study shows that the lubricant film on the workpiece surface can play a greater role in the sliding and ploughing stages.In the grinding process,the machining track of the latter abrasive is based on the former one, that is, the surface to be processed by the latter abrasive is the fresh surface processed by the former abrasive.As a stable lubricant film is formed on the workpiece surface after the former abrasive cutting, the latter abrasive benefits from this stable lubricant film and is in a good lubrication state.Therefore,friction heat in the sliding and ploughing stages is greatly reduced. However, for milling, the latter cutting-edge does not cut along the track of the former one,so whether the cutting-edges form an oil film on the workpiece surface is not important (see Fig. 13(b)).

Fig. 12 Workpiece analysis of NMQL grinding using CGNs.

In the tool structure,the grinding wheel is beneficial to infiltration and film formation process. For the milling tool, the distance between the cutting-edges is large, which is not conducive to the storage and full infiltration of the lubricant. In the grinding zone with large cutting arc length,dozens or even more than 100 abrasives exist at the same time. Owing to thepores on the wheel surface,the capillary and microchannel network is formed in the grinding process. On the one hand, the lubricant infiltrates with enough space and power by a ‘dynamic pumping effect’. On the other hand, the lubricant can be stored in pores and always be filled; thus, sufficient nanolubricant is rolled into the film repeatedly during the repeated cutting process of abrasives.

Table 4 Conclusion of processing properties in grinding.

Fig. 13 Grinding mechanism of NMQL using CGNs.

Therefore, under this favorable boundary condition, the infiltration speed and depth are no longer the standard to measure various types of CGNs, and the spreading area becomes the key ability. Thus, layered CGNs are more advantageous than the other two.

6.6.2. Antifriction and antiwear mechanisms in tool/workpiece interface

Compared with spherical and tubular CGNs, the interlayer atomic binding force of the layered CGNs is weak, thereby forming a low shear plane. When the upper and lower layers move mutually, the CGNs extend along the fracture layer and form a lubrication film. Owing to the repeated grinding of the wheel, CGNs spread repeatedly and the coverage area increases. Thus, the lubrication performance is enhanced and continuous and the stable lubricant film forms easily. Moreover, the weaker the interlayer bonding strength is, the easier it is to spread the film.It is difficult for the spherical and tubular CGNs to form a stable lubricant film under sufficient wetting condition, and they always work in rolling action for friction reduction, thereby resulting in poor antifriction and antiwear performance (see Fig. 13(b)).

7. Conclusions

In this study, we reviewed the recent advancements of CGNs in NMQL turning, milling, and grinding of titanium alloy and nickel alloy in the aerospace industry by detailing the processability and mechanisms. The key findings are the following:

(1) The antifriction and antiwear mechanism of NMQL is complex and different for difficult-to-machine materials.Compared with MQL, the unique lubrication state avoids direct contact and reduces the friction of the tool/workpiece interface with the use of CGNs. As the mechanical and thermal loads in the cutting zone are reduced, the adverse thermal softening effect in the material removal process are not apparent. This condition results in improved processability in terms of tool failure, material removal, and surface formation.

(2) The high lubrication requirement of titanium alloy and nickel alloy in machining result in limited types of nano-enhancers for NMQL. CGNs (e.g., ND, CNTs,GNP, GR, and GO) meeting the processing performance and sustainable development at the same time.The largest force reduction of 45.65%, CoF reduction of 68.63%, VB reduction of 63.63%, tool life improvement of 27.27% and Ra reduction of 79.03% were obtained by CGNs in turning comparing with MQL.For milling, the largest force reduction of 22.4%, VB reduction of 52.17%, tool life length of 23.07%, Ra reduction of 51.07% were observed by CGNs comparing with MQL. When it comes to grinding, the largest force reduction of 79.13%, CoF reduction of 75.24%,tool wear rate reduction of 71.92%, and Ra reduction of 48.8% were concluded comparing with MQL.

(3) The machining of titanium alloy and nickel alloy encounters severe challenges such as premature tool failure, surface burn, and others, which can be solved with the use of CGNs. The microstructures of CGNs are important in application specification due to their comprehensive impact on film formation, antifriction, and antiwear behaviors. More interestingly, CGNs with different microstructures present various machining performance for different processing forms. Tubular CNTs,spherical ND or fullerene, layered CGNs (such as GNP, GR, and GO) are suggested for application in turning, milling, and grinding, respectively. Hybrid nano-enhancer may present better performance than mono used, however, necessitates more researches for application.

(4) The processing form and boundary condition significantly affect the film-formation principles and machining behaviors, which have been revealed in text. With the help of CGNs, milling benefits less than turning and grinding due to its unique intermittent material removal process of cutting-edges. The obvious phenomenon is reflected in the cutting force, in which 22.4% largest reduction of milling was obtained compared with MQL, whereas 45.65% was obtained for turning and 79.13% for grinding.

Global collaborative efforts are necessary to promote the advancement and application of CGNs for difficult-tomachine materials. This review is expected to provide fundamental guidance and offer potential insights that can be used as a reference by NMQL and aerospace manufacturing communities as well by potential researchers and consumers.

8. Possible future directions

The number of references relating to the application of CGNs in NMQL has exhibited an increasing trend in recent years and some limitations have also been identified. Based on the preceding summary,future investigations may focused on the following subjects:

(1) Design and improvement of CGNs-based nanolubricant:A bio-lubricant is an ideal base oil for preparation of CGNs-based nanolubricant when considering ecofriendly and sustainable access to resources. However,the thermal stability of the nanolubricant under high temperature needs to be improved by modifying the C = C bond to deal with failure. In addition, an active design scheme of nanolubricant preparation is needed to be studied considering the synergistic effect between CGNs and oil under different thermal and mechanical conditions. In other words, an intelligent matching system or database is needed to achieve intelligent parameter selection including oil type, CGNs type and concentration under different working conditions.

(2) Modification of CGNs for high-efficiency infiltration and lubrication: The infiltration, antifriction, and heat transfer performance of CGNs still need to be improved because of the processability limitation caused by complex boundary conditions of milling and grinding. The performance may be improved by modification. For example, the insufficient infiltration capacity of CGNsbased nanolubricant in the cutting/grinding zone limits its full antifriction effect. This issue can be addressed by applying an auxiliary energy field (magnetic, electric,or ultrasonic) to pull the modified CGNs. For instance,through the composite of CGNs or CGNs with other nano-enhancers, the cooling and lubrication can be improved at the same time by the physical synergy effect.

(3) New supply methods to restrain dispersion of nanolubricant droplets: Electrostatic atomization method, by completely controlling the atomization and supply process of microdroplets, is expected to be a future NMQL solution.The reason is that existing devices may cause a dispersion of nanolubricant droplets and pose a health threat to workers due to high pressure gas atomization and transportation process. Electrostatic atomization and other methods to control the atomization and supply process of microdroplets is a future research hotspot.An energy field (electrostatic or other forms) can help restrain dispersion in the atomization and supply process.

(4) Processing technology solutions considering multifactor influence: Processing technology solutions for NMQL machining are required in the aerospace industry, which should include process parameters such as machining form, cutting consumption, workpiece material,and tool parameters.The existing references are not enough to support database build-up. Further studies may focus on multi-parameter coordinated control of the specific process system and a quantitative characterization mathematical model for optimal processability to guide the production practice.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Nos. 51975305 and 51905289), the Major Research Project of Shandong Province (No.2019GGX104040),the Major Science and Technology Innovation Engineering Projects of Shandong Province (No.2019JZZY020111), the Natural Science Foundation of Shandong Province (Nos. ZR2020KE027 and ZR2020ME158)and the Applied Basic Research Youth Project of Qingdao science and technology plan (No. 19-6-2-63-cg).