Investigation of the Lubrication Performance of γ -Al 2 O 3 /ZnO Hybrid Nanoﬂuids for Titanium Alloy

: Titanium alloys are difﬁcult to machine and have poor tribological properties. This paper investigates the lubricating performance of γ -Al 2 O 3 /ZnO hybrid nanoﬂuids for Ti-6Al-4V. Pure and hybrid nanoﬂuids are compared, and the effects of γ -Al 2 O 3 /ZnO ratios are studied. The results show that γ -Al 2 O 3 /ZnO hybrid nanoﬂuids outperform pure nanoﬂuids in terms of lower friction coefﬁcients and better surface quality. Moreover, the hybrid nanoﬂuid with a mass ratio of Al 2 O 3 to ZnO of 2:1 demonstrates the best lubrication performance with a reduced friction coefﬁcient of up to 22.1% compared to the base solution, resulting in improved surface quality. Al 2 O 3 nanoparticles can adhere to the surface of ZnO nanoparticles and work as a coating, which further enhances the lubrication performance of the water-based nanoﬂuid.


Introduction
Titanium alloys are difficult to machine due to their low thermal conductivity, low elastic modulus, and high chemical reactivity.High cutting temperatures and severe built-up edge formation during machining lead to excessive tool wear and poor surface quality [1].Traditional lubricants such as mineral oil, vegetable oil, and grease fail to lubricate titanium alloys well [2].Effective water-based lubricants for titanium alloys have been investigated by researchers [3,4].Castor oil sulfated sodium salt (CSS) solution can decrease friction coefficient and adhesive wear and can be used as a good water-based lubricant base stock for the design of cutting fluid for titanium alloys [5].However, the lubricating and cooling performance of CSS solutions for titanium alloys still needs to be improved for efficient application.
Nanoparticles are added to lubricants to create nanofluids that possess excellent frictional properties and high load-carrying capacity.Rahmati et al. [6] investigated the effects of MoS 2 nanoparticles suspended in nanolubricants on machined surface morphology.Experimental results showed that MoS 2 nanoparticles with a concentration of 0.5 wt% led to superior machined surface quality compared to pure oil or other nanoparticle concentrations.Marcon et al. [7] reported that the addition of graphite nanoplatelets significantly reduced cutting forces and friction in the micro-milling of hardened steel.As to the mechanism, nanoparticles have a small size but a large surface area, and their thermal conductivity is much higher than that of a millimeter-or micrometer-sized particles of the same volume.Nanoparticles alter the structure of the liquid, turning it into a liquid-solid two-phase suspension, which affects the energy transfer process within the nanofluid [8,9].The randomly dispersed nanoparticles in the liquid promote micro-disturbances within the fluid, thereby enhancing the rate of energy transfer between the nanoparticles and the base liquid [10].Due to the increased thermal conductivity and other mechanisms, such as the thermal Brownian motion of nanoparticles, the use of nanofluids can significantly improve heat transfer performance [11].
Metals 2023, 13, 1701 2 of 14 Hybrid nanofluids, which are mixtures of two or more types of nanoparticles, exhibit better lubrication and heat transfer properties than single nanoparticle additives due to the variations in molecular structures and shapes.The synergistic effect of different nanoparticles contributes to better performance [12].Therefore, for difficult-to-machine metals, hybrid nanofluid lubrication has unique advantages in cutting efficiency, machining surface quality, and tool life.Zhang [13] prepared a hybrid nanofluid consisting of MoS 2 with a good lubrication effect and CNTs with a high heat conductivity coefficient.The effects of grinding force, coefficient of friction, and workpiece surface quality for Ni-based alloy grinding were investigated, and MoS 2 /CNT hybrid nanofluids achieved better lubrication than single nanoparticles.Kalita [14] found that MoS 2 /Al 2 O 3 hybrid nanofluid provided better lubrication than dry grinding and soybean oil pouring lubrication methods, and its processing efficiency and surface roughness have been improved.Setti [15] investigated the surface grinding process of Ti-6Al-4V under different Al 2 O 3 /CuO hybrid nanofluid lubrications and found that the tangential force and grinding temperature were reduced with the hybrid nanofluid.
As to the environmental impact, oil-based cutting fluid, which is difficult to biodegrade, may lead to serious environmental pollution [16].Compared with traditional pouring lubrication, only a small amount of nanofluid is required, which significantly reduces the economic costs and environmental hazards [17,18].Although the misuse and improper disposal of nanoparticles may lead to their accumulation in different segments of the environment [19][20][21], the small additive amount of the nanoparticles and the big performance improvement make the water-based nanofluid a valuable research direction for metal cutting.
In summary, while scholars have achieved significant progress in the field of hybrid nanofluid lubrication, there is limited research on the application of hybrid nanofluids in lubricating titanium alloys and on the micro-scale mechanisms of hybrid nanofluids.Therefore, the objective of this study is to evaluate the effectiveness of hybrid nanofluid lubrication for Ti-6Al-4V and to analyze the lubrication mechanisms of hybrid water-based nanofluids.Using Al 2 O 3 as a lubricant additive in nanofluid can enhance the lubrication performance, effectively reducing the friction coefficient and wear rate.Eltaggaz [22] investigated the impact of Al 2 O 3 nanofluid on tool wear, surface finish, and power consumption in the Ti-6Al-4V machining process.The results showed that nanofluid minimal lubrication had a positive effect on tool life and surface finish quality.However, existing research on the lubrication performance of hybrid nanofluids containing Al 2 O 3 mostly focuses on the more common α-Al 2 O 3 , rarely discussing γ-Al 2 O 3 for its lower thermal conductivity.Studies have shown that ZnO exhibits good thermal conductivity in aqueous solutions [23,24].Furthermore, the hybrid nanofluid is expected to yield better thermal conductivity compared to individual nanofluids due to its synergistic effect [25].Therefore, a hybrid nanofluid is prepared in this paper by mixing γ-Al 2 O 3 and ZnO at certain mass fractions, which combines the advantages of both and results in superior lubrication performance.The lubricating properties of γ-Al 2 O 3 /ZnO hybrid nanofluids for Ti-6Al-4V with CSS aqueous solution as the base stock are investigated.Finally, a new water-based hybrid nanofluid with good lubricity and thermal conductivity is developed to improve the working performance of the cutting fluid for Ti-6Al-4V.

Experimental Setup and Materials
Cutting fluids are mixtures of various functional additives such as fungicides, defoaming agents, and rust inhibitors, except for lubricants and extreme-pressure additives.It is difficult to clearly investigate the lubricating mechanism for the complex components.As to the titanium alloy, due to its bad tribological properties, lubricating performance is a high priority.Therefore, this paper focuses on the lubricant additives, and only the CSS solution is chosen as the base stock.Nanoparticles are added to improve the lubricity of the solution.Considering that the aqueous solution without other additives is corrosive to Metals 2023, 13, 1701 3 of 14 machine tools, friction, and wear tests are carried out in the laboratory to investigate the lubricating characteristics, which is a convenient and fast way to develop new lubricants.
Friction tests were conducted using a ball-on-disc device (CFT-I, Licp, Lanzhou, China) under various lubrication conditions.The schematic diagram and photo of the testing instrument are shown in Figure 1.The lower specimen was a Ti-6Al-4V disc with a hardness of HRC 35.Table 1 displays the chemical composition of the Ti-6Al-4V titanium alloy, while Table 2 shows its mechanical parameters.Before the friction tests, all samples underwent automated polishing and grinding to achieve a surface roughness (Sa) of less than 40 nm.Throughout the frictional test, the friction force was continuously measured by a 2D built-in load cell, and the measurement range is 0-100 N with a measurement accuracy of 0.01 N. The friction coefficient was determined by the ratio of the measured friction force to the normal force.
ponents.As to the titanium alloy, due to its bad tribological properties, lubricating performance is a high priority.Therefore, this paper focuses on the lubricant additives, and only the CSS solution is chosen as the base stock.Nanoparticles are added to improve the lubricity of the solution.Considering that the aqueous solution without other additives is corrosive to machine tools, friction, and wear tests are carried out in the laboratory to investigate the lubricating characteristics, which is a convenient and fast way to develop new lubricants.
Friction tests were conducted using a ball-on-disc device (CFT-I, Licp, Lanzhou, China) under various lubrication conditions.The schematic diagram and photo of the testing instrument are shown in Figure 1.The lower specimen was a Ti-6Al-4V disc with a hardness of HRC 35.Table 1 displays the chemical composition of the Ti-6Al-4V titanium alloy, while Table 2 shows its mechanical parameters.Before the friction tests, all samples underwent automated polishing and grinding to achieve a surface roughness (Sa) of less than 40 nm.Throughout the frictional test, the friction force was continuously measured by a 2D built-in load cell, and the measurement range is 0-100 N with a measurement accuracy of 0.01 N. The friction coefficient was determined by the ratio of the measured friction force to the normal force.Cemented carbide is the optimal tool material for machining Ti-6Al-4V titanium alloy [4].The upper specimen is a YG8 (WC-Co) ball with a hardness of 89HRA, a diameter of  Cemented carbide is the optimal tool material for machining Ti-6Al-4V titanium alloy [4].The upper specimen is a YG8 (WC-Co) ball with a hardness of 89HRA, a diameter of 10 mm, and a surface roughness (Sa) of 25 nm.Each specimen was cleaned with acetone and ethanol, followed by ultrasonic cleaning with deionized water for 10 min.The upper ball is reciprocated on the stationary disc with a 5 mm amplitude and a 5 Hz frequency for 10 min.Before the reciprocating motion, 0.2 mL of lubricant was dropped onto the disc surface.A normal load of 50 N was applied, and the maximum Hertz contact pressure was 7.5 GPa.Each experiment was repeated more than three times, and the friction coefficient curves were the average of the tested data.The relative errors of the friction coefficients were in the order of ±1%.
After the frictional test, the worn specimens were cleaned with acetone for 30 min and then dried.The wear volume was calculated by the LEXT™OLS5100 laser scanning confocal microscope (Olympus, Tokyo, Japan).Each test was repeated three times, and the average value was calculated.The microtopography of the worn surface was observed using a scanning electron microscope (FEI, Hillsboro, OR, USA) with Energy-dispersive X-ray spectroscopy (FEI, Hillsboro, OR, USA).

Nanofluids Preparation
Aluminum oxide (Al 2 O 3 ) and zinc oxide (ZnO) are selected as nano-additives for lubricating titanium alloys, with castor oil-sulfated sodium salt (CSS) as the base stock.Table 3 presents the relevant parameters of the nanoparticles, and their microscopic morphology is depicted in Figure 2. It is worth noting that although the crystal structures of both nanoparticles are cubic, γ-Al 2 O 3 has a unique porous structure and therefore has a high specific surface area.The nanofluids were prepared using a two-step method to disperse the nanoparticles in a water-based lubricant.The components used were γ-Al 2 O 3 and ZnO nanoparticles and a 10 wt% CSS aqueous solution.The sum mass fraction of the two types of nanoparticles remained constant at 1 wt%.The mixture was ultrasonically vibrated for 20 min.The mass fraction of Al 2 O 3 and ZnO nanoparticles was given by mix(x:y), and seven groups of hybrid nanofluids were prepared to investigate the effects of the nanoparticles' content ratio, including pure Al 2 O 3 , pure ZnO, mix(1:1), mix(1:2), mix(2:1), mix(1:4), and mix(4:1).10 mm, and a surface roughness (Sa) of 25 nm.Each specimen was cleaned with acetone and ethanol, followed by ultrasonic cleaning with deionized water for 10 min.The upper ball is reciprocated on the stationary disc with a 5 mm amplitude and a 5 Hz frequency for 10 min.Before the reciprocating motion, 0.2 mL of lubricant was dropped onto the disc surface.A normal load of 50 N was applied, and the maximum Hertz contact pressure was 7.5 GPa.Each experiment was repeated more than three times, and the friction coefficient curves were the average of the tested data.The relative errors of the friction coefficients were in the order of ±1%.
After the frictional test, the worn specimens were cleaned with acetone for 30 min and then dried.The wear volume was calculated by the LEXT™OLS5100 laser scanning confocal microscope (Olympus, Tokyo, Japan).Each test was repeated three times, and the average value was calculated.The microtopography of the worn surface was observed using a scanning electron microscope (FEI, Hillsboro, OR, USA) with Energy-dispersive X-ray spectroscopy (FEI, Hillsboro, OR, USA).

Nanofluids Preparation
Aluminum oxide (Al2O3) and zinc oxide (ZnO) are selected as nano-additives for lubricating titanium alloys, with castor oil-sulfated sodium salt (CSS) as the base stock.Table 3 presents the relevant parameters of the nanoparticles, and their microscopic morphology is depicted in Figure 2. It is worth noting that although the crystal structures of both nanoparticles are cubic, γ-Al2O3 has a unique porous structure and therefore has a high specific surface area.The nanofluids were prepared using a two-step method to disperse the nanoparticles in a water-based lubricant.The components used were γ-Al2O3 and ZnO nanoparticles and a 10 wt% CSS aqueous solution.The sum mass fraction of the two types of nanoparticles remained constant at 1 wt%.The mixture was ultrasonically vibrated for 20 min.The mass fraction of Al2O3 and ZnO nanoparticles was given by mix(x:y), and seven groups of hybrid nanofluids were prepared to investigate the effects of the nanoparticles' content ratio, including pure Al2O3, pure ZnO, mix(1:1), mix(1:2), mix(2:1), mix(1:4), and mix(4:1).

Lubricating Properties of the Nanofluids
The friction coefficient curves for seven groups of nanofluids and one control group (CSS aqueous solution without nanoparticles) were presented in Figure 3.During the running-in period, the friction coefficients of pure Al 2 O 3 and ZnO nanofluids increased to 0.38 and then decreased with fluctuations due to factors such as lubricating film rupture.Then, the friction coefficients reached a steady state after about six minutes.The COF of pure Al 2 O 3 nanofluid in the steady state was about 0.162, while it was 0.168 for pure ZnO nanofluid.When the mass fraction ratio of Al 2 O 3 to ZnO nanoparticles was 4:1, the friction coefficient rapidly increased to 0.38 during the running-in period and then gradually decreased.For the other hybrid nanofluids, the maximum friction coefficient during the running-in period decreased to below 0.35.
The friction coefficient curves for seven groups of nanofluids and one control group (CSS aqueous solution without nanoparticles) were presented in Figure 3.During the running-in period, the friction coefficients of pure Al2O3 and ZnO nanofluids increased to 0.38 and then decreased with fluctuations due to factors such as lubricating film rupture.Then, the friction coefficients reached a steady state after about six minutes.The COF of pure Al2O3 nanofluid in the steady state was about 0.162, while it was 0.168 for pure ZnO nanofluid.When the mass fraction ratio of Al2O3 to ZnO nanoparticles was 4:1, the friction coefficient rapidly increased to 0.38 during the running-in period and then gradually decreased.For the other hybrid nanofluids, the maximum friction coefficient during the running-in period decreased to below 0.35.After six minutes, the stabilized COF values were compared and listed in Figure 4.The average friction coefficients at different stages for various nanofluids are shown in Table 4.All the nanofluids significantly reduced the friction coefficients at different stages for the titanium alloy.Combining Figure 4 with Table 4, it can be concluded that mix(1:2) and mix(2:1) showed the most significant friction reduction for water-based nanofluid lubrication of titanium alloy.The lowest average friction coefficient was 0.144, which was 22.1% lower than the CSS solution without nanoparticles.The friction coefficients of mix(4:1), mix(1:1), and mix(1:4) solutions were similar to those of pure Al 2 O 3 nanofluid Metals 2023, 13, 1701 6 of 14 but all lower than those of pure ZnO nanofluid.Compared with the control group, their friction coefficients could be reduced by up to 16.7%, 17.3%, and 14.1%, respectively.When using traditional oil-based lubricants, the friction coefficient of titanium alloy is about 0.34 [18], while in this experiment, the lowest friction coefficient of Al 2 O 3 /ZnO hybrid nanofluids can reach 0.154.Therefore, the lubrication performance of hybrid nanofluids has significantly improved.Moreover, the Al 2 O 3 and ZnO nanoparticles used in this study are easy to prepare and cost-effective.Due to its small dosage, its cost increase does not exceed 20% compared to the base solution.Because of the excellent lubrication performance of nanofluids, the actual amount of lubricant used for titanium alloy processing is likely to be less.Therefore, this hybrid nanofluid has potential applications in the cutting fluid.
The average friction coefficients at different stages for various nanofluids are shown in Table 4.All the nanofluids significantly reduced the friction coefficients at different stages for the titanium alloy.Combining Figure 4 with Table 4, it can be concluded that mix(1:2) and mix(2:1) showed the most significant friction reduction for water-based nanofluid lubrication of titanium alloy.The lowest average friction coefficient was 0.144, which was 22.1% lower than the CSS solution without nanoparticles.The friction coefficients of mix(4:1), mix(1:1), and mix(1:4) solutions were similar to those of pure Al2O3 nanofluid but all lower than those of pure ZnO nanofluid.Compared with the control group, their friction coefficients could be reduced by up to 16.7%, 17.3%, and 14.1%, respectively.When using traditional oil-based lubricants, the friction coefficient of titanium alloy is about 0.34 [18], while in this experiment, the lowest friction coefficient of Al2O3/ZnO hybrid nanofluids can reach 0.154.Therefore, the lubrication performance of hybrid nanofluids has significantly improved.Moreover, the Al2O3 and ZnO nanoparticles used in this study are easy to prepare and cost-effective.Due to its small dosage, its cost increase does not exceed 20% compared to the base solution.Because of the excellent lubrication performance of nanofluids, the actual amount of lubricant used for titanium alloy processing is likely to be less.Therefore, this hybrid nanofluid has potential applications in the cutting fluid.The worn surface of the titanium discs after frictional tests was observed, and the wear volume was shown in Figure 5.It can be seen that several hybrid nanofluids increased the wear volume to some extent compared to pure Al 2 O 3 nanofluid.What is more, the increase in wear volume was positively correlated with the content of ZnO nanoparticles.Compared to pure ZnO nanoparticles, mix(4:1), mix(2:1), mix(1:1), and mix(1:2) showed a reduction in wear volume of 23.5%, 18.9%, 21.2%, and 9.6%, respectively.It is worth noting that the wear volume of the mix(1:4) even exceeded that of pure ZnO nanofluid, which is related to the interaction between nanoparticles.
On one hand, pure Al 2 O 3 nanofluid exhibits a better lubricating effect than pure ZnO nanofluid due to its different physical properties and shape characteristics.On the other hand, the friction coefficients of γ-Al 2 O 3 /ZnO hybrid nanofluids are generally lower than those of pure nanofluids, indicating that the hybrid nanofluids have better anti-friction effects compared to pure nanofluids under the experimental conditions.This indicates that different nanoparticles interact with each other to improve the lubrication performance of hybrid nanofluids.Furthermore, the lubrication performance of different hybrid nanofluids varies.Mix(1:2) and mix(2:1) demonstrate the lowest friction coefficients.Based on the comparison among multiple hybrid nanofluids, the wear rate of hybrid nanofluids is higher than that of pure Al 2 O 3 nanofluids, and the ZnO content affects the wear rate of hybrid nanofluids' lubrication.Generally, a higher ZnO content leads to poorer lubrication performance for the hybrid nanofluids.These findings indicate that the synergistic effect of hybrid nanoparticles can be influenced by adjusting the proportion of hybrid nanofluids.The worn surface of the titanium discs after frictional tests was observed, and the wear volume was shown in Figure 5.It can be seen that several hybrid nanofluids increased the wear volume to some extent compared to pure Al2O3 nanofluid.What is more, the increase in wear volume was positively correlated with the content of ZnO nanoparticles.Compared to pure ZnO nanoparticles, mix(4:1), mix(2:1), mix(1:1), and mix(1:2) showed a reduction in wear volume of 23.5%, 18.9%, 21.2%, and 9.6%, respectively.It is worth noting that the wear volume of the mix(1:4) even exceeded that of pure ZnO nanofluid, which is related to the interaction between nanoparticles.On one hand, pure Al2O3 nanofluid exhibits a better lubricating effect than pure ZnO nanofluid due to its different physical properties and shape characteristics.On the other hand, the friction coefficients of γ-Al2O3/ZnO hybrid nanofluids are generally lower than those of pure nanofluids, indicating that the hybrid nanofluids have better anti-friction effects compared to pure nanofluids under the experimental conditions.This indicates that different nanoparticles interact with each other to improve the lubrication performance of hybrid nanofluids.Furthermore, the lubrication performance of different hybrid nanofluids varies.Mix(1:2) and mix(2:1) demonstrate the lowest friction coefficients.Based on the comparison among multiple hybrid nanofluids, the wear rate of hybrid nanofluids is higher than that of pure Al2O3 nanofluids, and the ZnO content affects the wear rate of hybrid nanofluids' lubrication.Generally, a higher ZnO content leads to poorer lubrication performance for the hybrid nanofluids.These findings indicate that the synergistic effect of hybrid nanoparticles can be influenced by adjusting the proportion of hybrid nanofluids.

Microtopography of the Worn Titanium Discs under Various Lubrication Conditions
The microtopography of the worn titanium discs was observed using scanning electron microscopy (SEM).The SEM images under different lubricant conditions are shown in Figure 6.As seen from Figure 6a-c, among the various nanofluids, the surfaces of three

Microtopography of the Worn Titanium Discs under Various Lubrication Conditions
The microtopography of the worn titanium discs was observed using scanning electron microscopy (SEM).The SEM images under different lubricant conditions are shown in Figure 6.As seen from Figure 6a-c, among the various nanofluids, the surfaces of three nanofluids, mix(4:1), mix(2:1), and pure Al 2 O 3 , exhibit the smoothest surfaces without obvious grooves and scratches.The worn surface under mix(2:1) hybrid nanofluid lubrication shows particularly excellent surface quality, which is likely attributed to the synergistic effect of Al 2 O 3 and ZnO nanoparticles.As shown in Figure 6e, the worn surface of the mix(1:2) lubrication is relatively smooth; however, there are some plow furrows on the surface, indicating that the lubricating effect under the separate action of ZnO is relatively poor compared to the other hybrid nanofluids.As shown in Figure 6d,f, obvious scratches, and plow furrows can be observed on the worn surfaces under the mix(1:1) and mix(1:4) lubrication, and their surface quality is even worse compared to mix(1:2) and pure ZnO nanofluid.The possible reason is that the excess ZnO nanoparticles plowed the surface of the titanium alloy workpiece.However, due to their limited cutting ability, they cannot cut undivided chips from the disk completely and effectively, resulting in significant agglomeration on the friction surface.Compared to the surface under the lubrication of pure ZnO nanofluid, the relatively smooth worn surface is not conducive to the high storage capacity of the nanofluid, thus being unable to form a friction film and instead reducing the surface quality of the titanium alloy.The surface roughness (Ra) of the wear marks was measured, and the results are shown in Table 5.It can be seen that the surface roughness data are basically consistent with the SEM images.Mix(4:1) and mix(2:1) have the ideal surface roughness (0.832 and 0.867), and the surface roughness of mix(1:2) and mix(1:4) is relatively poor.Due to the poor heat transfer performance of titanium alloys, adhesive wear often occurs during friction.Many scholars have studied oil-based lubricants for titanium alloys, and machined surface quality is not ideal, with a surface roughness ranging from 1.14 µm to 1.328 µm [5].In this experiment, the surface after frictional tests is smooth and flat with fewer adhesive wear scars, which indicates that the hybrid aqueous nanofluid has good lubrication for titanium alloys.EDS analysis was conducted on the worn surface to determine the elemental distribution of the product.The data in Figure 7 and Table 6 show that the elemental content does not significantly change with the proportion of hybrid nanofluids, suggesting that  EDS analysis was conducted on the worn surface to determine the elemental distribution of the product.The data in Figure 7 and Table 6 show that the elemental content does not significantly change with the proportion of hybrid nanofluids, suggesting that the synergistic effect between Al 2 O 3 and ZnO nanoparticles is predominantly physical while the frictional chemical reactions are not dominant.The physical synergistic effect of Al 2 O 3 and ZnO nanoparticles provides better lubrication than the individual nanoparticles.In addition, there are dark accumulations on the friction surfaces of mix(1:1), mix(1:2), and mix(1:4).The accumulated portion on the friction surface contains some Zn and O elements that were not previously detected, as seen in Figure 7d2,f2,g2.This suggests the presence of residual nanoparticles and intermediate products from the cutting process.The content of Ti and Al elements significantly decreased compared to previous results, indicating the presence of many other impurities besides undivided chips.Additionally, there is a significant fluctuation in the O element content among the three EDS analyses.ZnO and Al 2 O 3 nanoparticles aggregate in some form, and when the content of ZnO is high, it has a certain impact on its machined surface and remains on machined surface.The aggregation effect of nanoparticles may have contributed to this result, considering the relatively small sampling range.

Lubrication Mechanisms of Al 2 O 3 /ZnO Hybrid Nanofluids
Nanoparticles can fill in micro-pits and damage the surface, thus playing a repairing role.Some particles embed into the machined surface after collision.Their shape is changed due to shear compression, and the fragments continue to assist in cutting [26].Part of the embedded particles is plowed off by new nanoparticles, and some continue to polish the surface.The rolling nanoparticles generate a lubricating film that is easy to shear.As a result, the surface is polished, leading to better surface quality [26][27][28].
Figure 2a depicts the shape and structure of Al 2 O 3 nanoparticles.In our experiment, γ-Al 2 O 3 is utilized, which has uniform particle size, high purity, excellent dispersion, a high specific surface area, inertness to high temperatures, and high reactivity.γ-Al 2 O 3 is classified as active alumina with porosity, high hardness, and good dimensional stability.The oxygen anions in γ-Al 2 O 3 have the same cubic-close-packing arrangement as in spinel (e.g., MgAl 2 O 4 ), and the cations have an averaged cubic spinel structure [29].The spinel structure can be described as a sequence of layers of close-packed oxygen anions stacked in ABC order (cubic close packing, ccp) [30].Therefore, porous Al 2 O 3 nanoparticles expand the coverage area of the lubricating oil film formed between the interfaces of nanofluids.Al 2 O 3 nanoparticles can repair the oil film through their porous structure [31].This leads to a more stable development of the lubricating oil film and consequently produces a good lubrication effect.Due to the different shape characteristics of Al 2 O 3 and ZnO nanoparticles, hybrid nanofluids with corresponding mass fraction ratios have different lubrication effects.ZnO nanoparticles act as rolling balls at the workpiece interface.When there are relatively few ZnO nanoparticles, it provides only limited ZnO nanoparticles to take effect, while the lubricant with a high ZnO concentration indicates the ZnO nanoparticles to agglomerate.On the other hand, the agglomeration of ZnO nanoparticles operates as a barrier to hinder the perpetual supply of fine nanoparticles to the contact zone for lubrication [32].
According to the experimental results above, the friction coefficients of the five hybrid nanofluids are generally lower than those of pure Al 2 O 3 and pure ZnO nanofluids.Due to the synergistic effect between nanoparticles, hybrid nanofluids exhibit better lubrication performance.Figure 8 shows the lubrication mechanism diagram of Al 2 O 3 /ZnO hybrid nanofluids.Al 2 O 3 nanoparticles have a porous structure and excellent adsorption ability.As a result, they can adhere to the surface of ZnO nanoparticles, creating a "physical coating."The adhesion of Al 2 O 3 nanoparticles to the surface of ZnO nanoparticles improves the dispersion stability of nanoparticles in water-based lubricants.When the proportion of ZnO in the hybrid nanofluids is very small, local agglomeration occurs, and Al 2 O 3 nanoparticles coat the surface of the ZnO nanoparticles.Each group of such locally agglomerated nanoparticles forms a "big bearing," which further enhances the lubrication performance of the water-based nanofluid.Compared with pure Al 2 O 3 nanofluids, as the content of ZnO in the hybrid nanofluids increases, the number of adhered local agglomerations also increases.This is reflected in the experimental results, which show that mix(2:1) achieves an excellent friction coefficient.However, as the content of ZnO in the hybrid nanofluids increases, Al 2 O 3 nanoparticles are unable to cover all ZnO nanoparticles.The unadhered ZnO nanoparticles act independently and improve the lubricating effect of the water-based lubricants to some extent, achieving an excellent friction coefficient for mix(1:2) nanofluid.Nevertheless, ZnO nanoparticles act as independent bearings for lubrication, which may cause some degree of plowing on the surface of the titanium alloy workpiece.Therefore, as the mass fraction of ZnO nanoparticles increases, the wear of the titanium alloy under water-based hybrid nanofluids shows an upward trend.In summary, a certain amount of local agglomeration or a certain number of ZnO nanoparticles can significantly reduce the friction coefficient.The lower content of ZnO and the higher content of Al 2 O 3 improve the lubrication of water-based hybrid nanofluids.The synergistic effect of nanoparticles improves the lubrication effect of hybrid nanofluids [33,34].Al2O3 and ZnO nanoparticles provides better lubrication than the individual nanoparticles.In addition, there are dark accumulations on the friction surfaces of mix(1:1), mix(1:2), and mix(1:4).The accumulated portion on the friction surface contains some Zn and O elements that were not previously detected, as seen in Figure 7d2,f2,g2.This suggests the presence of residual nanoparticles and intermediate products from the cutting process.The content of Ti and Al elements significantly decreased compared to previous results, indicating the presence of many other impurities besides undivided chips.Additionally, there is a significant fluctuation in the O element content among the three EDS analyses.ZnO and Al2O3 nanoparticles aggregate in some form, and when the content of ZnO is high, it has a certain impact on its machined surface and remains on the machined surface.The aggregation effect of nanoparticles may have contributed to this result, considering the relatively small sampling range.synergistic effect of nanoparticles improves the lubrication effect of hybrid nanofluids [33,34].

Conclusions
The lubrication performance of Al2O3/ZnO hybrid nanofluid in water-based lubrication for titanium alloys is investigated, and several conclusions are drawn as follows: At present, the use of water-based hybrid Al2O3/ZnO nanofluid in the milling of Ti-6Al-4V alloy has not yet been reported.This article gives inspiration for the water-based hybrid nanofluid application of difficult-to-machine materials to improve cutting efficiency and surface quality.Compared with traditional flood lubrication, hybrid nanofluid lubrication has significant advantages in terms of cost, environmental protection, health, and efficiency and is the direction of the modern cutting fluid industry.It is expected to be used in cutting-edge fluid development in the future.

Conclusions
The lubrication performance of Al 2 O 3 /ZnO hybrid nanofluid in water-based lubrication for titanium alloys is investigated, and several conclusions are drawn as follows: At present, the use of water-based hybrid Al 2 O 3 /ZnO nanofluid in the milling of Ti-6Al-4V alloy has not yet been reported.This article gives inspiration for the water-based hybrid nanofluid application of difficult-to-machine materials to improve cutting efficiency and surface quality.Compared with traditional flood lubrication, hybrid nanofluid lubrication has significant advantages in terms of cost, environmental protection, health, and efficiency and is the direction of the modern cutting fluid industry.It is expected to be used in cutting-edge fluid development in the future.

Figure 1 .
Figure 1.(a) The schematic diagram and (b) the photo of the CFT-I tribo-tester.

Figure 1 .
Figure 1.(a) The schematic diagram and (b) the photo of the CFT-I tribo-tester.

Figure 2 .
Figure 2. The microtopography of the two kinds of nanoparticles is (a) Al2O3 and (b) ZnO.

Figure 2 .
Figure 2. The microtopography of the two kinds of nanoparticles is (a) Al 2 O 3 and (b) ZnO.

Figure 3 .
Figure 3. Friction coefficient of seven nanofluids and the control group.

Figure 3 .
Figure 3. Friction coefficient of seven nanofluids and the control group.

Figure 4 .
Figure 4. Comparison diagram of friction coefficients in steady state.

Figure 4 .
Figure 4. Comparison diagram of friction coefficients in steady state.

Figure 5 .
Figure 5.The wear volume of the discs lubricated by different lubricants.

Figure 5 .
Figure 5.The wear volume of the discs lubricated by different lubricants.

Figure 8 .
Figure 8.The schematic diagram of the physical synergy of Al2O3 and ZnO.

1 .
Pure Al2O3 nanoparticle nanofluid exhibits a lower friction coefficient and wear volume, indicating better lubrication performance than pure ZnO.It can be attributed to the unique lamellar structure and high porosity of Al2O3 nanoparticles.2. The Al2O3/ZnO hybrid nanofluid outperforms pure nanofluid lubrication for titanium alloys.Hybrid nanofluids with different ratios consistently achieve lower friction coefficients and better surface quality.The coating of Al2O3 nanoparticles on the surface of ZnO nanoparticles improves the dispersion stability of ZnO nanoparticles, thus enhancing their lubrication performance.3. The five hybrid nanofluids with different Al2O3/ZnO ratios exhibit different lubrication effects.The hybrid nanofluid with a mass ratio of Al2O3 to ZnO of 2:1 demonstrates the best lubrication performance with a reduced friction coefficient of up to 22.1% compared to the base solution, resulting in improved surface quality.The mix(2:1) hybrid nanofluid can be used for the cutting fluid of titanium alloys.

Author
Contributions: Y.Y.: methodology, study design, writing-review & editing and funding acquisition; H.L.: investigation, data analysis and writing-original draft; F.L.: validation and writing-review & editing; L.S.: formal analysis and supervision; H.Y.: methodology and data curation; C.Z.: methodology, validation and supervision.All authors have read and agreed to the published version of the manuscript.Funding: The research was funded by the National Key Research and Development Program of China (Grant No. 2022YFB3403801), the National Natural Science Foundation of China (Grant No. 52005010), and the Project of Cultivation for Young Top-Most Talents of Beijing Municipal Institutions (Grant No. BPHR202203035).Data Availability Statement: Not applicable.

Figure 8 .
Figure 8.The schematic diagram of the physical synergy of Al 2 O 3 and ZnO.

1 .
Pure Al 2 O 3 nanoparticle nanofluid exhibits a lower friction coefficient and wear volume, indicating better lubrication performance than pure ZnO.It can be attributed to the unique lamellar structure and high porosity of Al 2 O 3 nanoparticles.2. The Al 2 O 3 /ZnO hybrid nanofluid outperforms pure nanofluid lubrication for titanium alloys.Hybrid nanofluids with different ratios consistently achieve lower friction coefficients and better surface quality.The coating of Al 2 O 3 nanoparticles on the surface of ZnO nanoparticles improves the dispersion stability of ZnO nanoparticles, thus enhancing their lubrication performance.3. The five hybrid nanofluids with different Al 2 O 3 /ZnO ratios exhibit different lubrication effects.The hybrid nanofluid with a mass ratio of Al 2 O 3 to ZnO of 2:1 demonstrates the best lubrication performance with a reduced friction coefficient of up to 22.1% compared to the base solution, resulting in improved surface quality.The mix(2:1) hybrid nanofluid can be used for the cutting fluid of titanium alloys.

Author
Contributions: Y.Y.: methodology, study design, writing-review & editing and funding acquisition; H.L.: investigation, data analysis and writing-original draft; F.L.: validation and writing-review & editing; L.S.: formal analysis and supervision; H.Y.: methodology and data curation; C.Z.: methodology, validation and supervision.All authors have read and agreed to the published version of the manuscript.Funding: The research was funded by the National Key Research and Development Program of China (Grant No. 2022YFB3403801), the National Natural Science Foundation of China (Grant No. 52005010), and the Project of Cultivation for Young Top-Most Talents of Beijing Municipal Institutions (Grant No. BPHR202203035).

Table 2 .
The mechanical parameters of the Ti-6Al-4V titanium alloy.

Table 2 .
The mechanical parameters of the Ti-6Al-4V titanium alloy.

Table 3 .
Properties of the nanoparticles as provided by the manufacturer.

Table 3 .
Properties of the nanoparticles as provided by the manufacturer.

Table 4 .
Average friction coefficient of seven nanofluids.

Table 4 .
Average friction coefficient of seven nanofluids.

Table 5 .
Average surface roughness of the worn surfaces.

Table 6 .
The element content of the above EDS analysis.

Table 6 .
The element content of the above EDS analysis.