Tribological and Machinability Performance of Hybrid Al2O3 -MWCNTs MQL for Milling Ti-6Al-4V

: Recent burgeoning development in nanotechnology unfold an avenue in the manufacturing industry. Owing to the superior heat transfer potential of nanoadditives mentioned recently, it could be interesting to improve the heat transfer and tribological capability of metal cutting fluids by mixing nanofluids in emulsions properly. In order to attain high-performance cutting of difficult-to-cut alloys, hybrid nanofluids assisted Minimum Quantity Lubrication (MQL) system is applied with the anticipation of efficient lubrication and heat transfer. Taguchi based L 16 (4 3 ) orthogonal array is used involving nanofluids concentrations of alumina-multiwalled carbon nanotubes (Al 2 O 3 -MWCNTs) air pressure and cooling flow rate at constant cutting conditions in the milling of Ti-6Al-4V. The resultant cutting force (F R ), cutting temperature (T), and surface roughness (Ra) is considered as key machining responses. Besides, tool wear, chip analysis, and surface topography are also analyzed under the effect of hybrid nanofluids. Findings have shown the minimum resultant force, cutting temperature and surface roughness of 24.3N, 148.7 o C, and 0.67µm respectively at nanofluids concentration of 0.24vol%, 120ml/h of flow rate at 0.6MPa of air pressure. The microscopic analysis of the end-mill depicted minor thermal damage, chip-welding, and coating peeling. Also, chip analysis depicts the clean back surface and less melting of saw-tooth chip edges. The surface topography confirms the less micro-adhesion of chips and material debris. The summary showed that appropriately chosen MQL parameters have improved the lubrication/cooling performance by providing oil film and enhancing the milling performance measures. The outcomes of the proposed study are useful for the manufacturing industry for the enhancement of process performance.


Introduction:
Ti-6Al-4V (α-β alloy) has become an attractive alternative due to exceptional performance at hightemperature owing to strength, resistance to corrosion, and high toughness. That is why, it has enormous usage in shipbuilding, automobile, aerospace, and medical industries. Ti-6Al-4V is hard-to-cut, reactive to tool material at elevated temperature, prone to the built-up edge (BUE) formation, welding of chips during cutting, with a high coefficient of friction [1]. Thermal conductivity of Ti-6Al-4V is also extremely less (6.7W/mK), accumulates heat into the workpiece material leading to surface burning, fatigue, and thermal shock.
Surface burning leads to poor surface quality, and chemical reactivity accelerating the tool wear as reported by numerous researchers [2][3][4]. To cope up with the challenge, it is necessary to use low cutting conditions to achieve better surface quality, low temperature, and gradual tool wear. In the meantime, low cutting conditions reduce productivity, which is unacceptable from the industrial point of view. The application of mineral-based metal cutting fluids is very common in the industry to control the heat generation and to provide adequate lubrication. However, the emerging sustainable approach to reduce consumption of cutting fluids (i.e., 785 billion tons in 2018) [5], 15~17% of the coolant cost, and associated disposal cost have put forward to adopt dry or near-to-dry sustainable cooling techniques in machining.
A near to dry minimum quantity lubrication (MQL) is used to improve the machining characteristics during the process of medium-hard materials [6]. MQL reduced 40~60% of the lubrication cost, fluid quantity, and pressurized fine coolant spray to penetrate at the cutting zone effectively [6]. However, MQL effectiveness decreases significantly during machining of Ti-6Al-4V due to high heat generation during cutting. Due to the poor thermal conductivity, this heat accumulates at primary cutting zone enabling ease in reactivity towards cutting tool material, chip welding, and BUE formation that accelerates tool wear. The nickel-based superalloy was machined under MQL based CNC milling through uncoated carbide tools. A comparison between mineral and vegetable oil assisted MQL was applied at the varying flow rate, nozzle distance, nozzle type, and type of cutting process. The performance measures were tool life and cutting forces. The findings have indicated that vegetable oils provided extended tool life at a flow rate of 100 ml/h in up milling process [7]. In a nutshell, MQL has the potential to limit the heat generation, improve production quality and material removal rate. Considering the importance of improving the material removal rate in the industry, the addition of nanoparticles of 1-100nm size particles dispersed in vegetable oil-based MQL has been adopted widely to improve the lubrication and thermal conductivity of a nanofluid [8]. Numerous types of nanoparticles are available in the market for various applications such as iron oxide (Fe2O3), aluminum nitride (AlN), zinc oxide (ZnO), alumina (Al2O3), single, and multi-walled carbon nanotubes (MWCNTs). The addition of a tiny volume of nano-additives in base oil significantly increases the surface area of cooling medium, heat transfer, and stable Brownian motion of particles [9,10]. The effect of MWCNTs assisted MQL in the machining of Ti-6Al-4V is investigated at varying cutting conditions. A significant decrease in surface roughness was observed due to the formation of stable tribological thin film and filled micro-pores on newly generated machined surface and the tool cutting edge [11]. Another study has considered the comprehensive comparison of silver and alumina nanoparticles assisted MQL in the machining of Nickle alloy. The comparison has provided exciting results about the turning performance measures. The alumina-based nanoparticles have provided less cutting force, wear, and chip curling due to small contact angle, fine droplet, and spreadability of particles. Besides, the exceptional ball-bearing effect of silver nanofluids provided good surface quality and limited abrasive wear [12]. Fig.1  MWCNTs nano-additives [16]. Furthermore, MWCNTs nanoadditives based MQL has been applied in the machining of titanium dispersed in vegetable oil in different concentrations to optimize the turning process.
Results have concluded that 2 wt% MWCNTs reduced power by 11.5% compared to MQL without additives [17].
To improve the thermal and tribological characteristics of nanofluids, Al2O3-MWCNTs have been applied in different concentrations. The study showed a significant reduction in wear and friction coefficient due to excellent lubrication, wettability, and dispersion. Besides, hybrid nanofluids have reduced flank wear by 11%, and nodal temperature by 27.36% as compared to only Al2O3 based MQL cooling/lubrication [18]. nanofluids [19].
The principal objective of this research study is to investigate the effect of hybrid nanofluids dispersed in vegetable oil-based MQL parameters to model and optimize simultaneously the MQL associated parameters in milling of Ti-6Al-4V to improve the machinability. Taguchi based-L16 orthogonal array was applied to design the experiments. The fluid concentration, flow rate, and air pressure were varied to investigate their effect on cutting forces, temperature, and surface roughness.

Experimental Details
In this experimental study, milling experiments were performed on Ti-6Al-4V alloy under hybrid nanofluids based MQL technology. Besides, the effect of cutting conditions and concentration of fluid (wt.%), flow rate (mL/h), and pressure (MPa) are considered as critical parameters. The performance measures such as milling force, temperature, and surface quality were investigated under MQL technology.

Design of experiment
In order to investigate the performance of hybrid nanofluids under machining Ti-6Al-4V, constant cutting conditions of cutting speed (100 m/min), feed per tooth (0.02 mm/tooth), a width of cut (0.4 mm) and depth of cut (0.5 mm) were used through the experimentation to remove the uncertainty. The levels of the variable parameters were determined through a series of trial tests and recommendations from the tool manufacturer. The workpiece is Ti-6Al-4V alloy with dimensions of 150x80x50 mm 3 and is prepared for experimentation in Fig.2a. The EDS analysis was done on workpiece material to determine the elemental distribution Fig.2 (b). Before experimental runs, the grinding process was performed to remove the outer oxide layer. The solution 20% and 5% of nitric acid and hydrofluoric acid were used to take off the layer of the heat-affected zone. The chemical composition and thermo-physical properties of Ti-6Al-4V are provided in Table 1.  [11]. The TiAlN coating was used due to high oxidation temperature and hardness, good thermal resistivity, and adhesion with poor frictional coefficient and thermal conductivity [20]. Fig.3 (Table 2).

Hybrid nanofluids preparation and MQL system
Initially, Blasocut (Vascomill MMS FA-1, Blaser-Swisslube, Switzerland) oil having 5vol% of distilled water was used as base fluid. The hybrid Al2O3-MWCNTs is finally achieved by mixing 25% of Al2O3 nanoadditives (diameter: 20~30 nm) with the multiwalled carbon nanotubes MWCNTs (diameter: 30~50nm) in 90:10 at different concentration in the base fluids. There was no surfactant or detergent used in this preparation of hybrid nanofluids, keeping the sustainability perspective during preparations. These nanofluids were ultrasonicated in Digital Ultrasonic heater for 3hrs to prepare stable and homogeneous suspension [18]. (Fig.4). The process was repeated in cycles to ensure the fine dispersion of particles without agglomeration and aging after some time or during experimentation. Figure 4: Two-step method to prepare hybrid nanofluids The MQL system comprises a compressor, nanofluids reservoir, and an adjustable nozzle. MQL chamber mixes the oil and air to imping on the cutting zone. In MQL, an atomizing nozzle is fixed near the cutting zone to supply air-oil (containing hybrid nanofluids) mixture at the tool-chip interface. The two adjustable nozzles, 20 mm far from the tool-workpiece contact at an angle of 40 o from the workpiece surface.

Responses measurement
Cutting forces in milling are critical characteristics of a process that has a direct relation with the cutting tool, material hardness, and precision of the machine tool. The Kistler dynamometer was utilized to measure 3-dimensional forces such as cutting force, radial force, and feed force. This system amplifies the multi-channel signals, processes the data acquisitions, and displays the force signals (Fx=radial force,

Results and discussion
In this section, the experimental findings are collected and discussed in detail under different input variables. Based on Taguchi-method, milling characteristics such as cutting force, cutting temperature, and surface quality were evaluated under the effect of concentration, flow rate, and air pressure. According to Taguchi L16 orthogonal array, the experimental results have listed in Table. 3. Moreover, the S/N ratio was provided for each response value.  Table 3. In addition, ANOVA for S/N ratio, having each performance measure, was determined to confirm the model significance as well as an individual variable contribution (Table 4). parameter at different levels is exhibited in Table 5.

Effect of hybrid Al2O3-MWCNTs MQL on heat transfer
In the process of material removal, most of the heat generates at the primary and secondary shearing zone.
The most of heat generated in the workpiece material is due to shear and rubbing, while in the tool material is due to friction, rubbing, and distribution of heat at rake and flank face. The quasi-steady-state heat model is normally used to define the heat distribution in the workpiece [21].
Where is the frictional or shear heat, ( ) = (1 + (2 / )), 0 is the Bessel function, is the thermal diffusivity of the workpiece, = ℎ ℎ / . Therefore, heat generation can be evaluated through this relation. In this place, it is assumed that total heat generation in the workpiece and the tool remains equal. The following relation depicts this assumption [22].
As MQL is an air-oil mixture impinged on the tool-chip interface, it dissipates heat from the tool and workpiece. Due to oil-mixture, the heat losses in X, Y, and Z-directions can be defined as follows [22];  (wt%), flow rate, fr (mL/h), and air pressure, Ap (MPa). As the criteria defined for temperature are 'smaller the better,' that is why the flow rate has provided less S/N ratio of temperature at high and low levels of temperature. It is valuable to emphasize that the flow rate was the most significant parameter affecting temperature, followed by concentration and air pressure, respectively. This temperature reduction can be associated with the appropriate lubrication of hybrid nanoparticles that increased the cooling capacity, and efficiently dissipated heat from the cutting zone and controlled the cutting temperature. Due to the high wettability of multiwalled nano additives, it formed a shielding layer of nanoparticles and reduced the friction. In addition, various sized nanoparticles having high heat-carrying capacity lowered the cutting temperature [23]. That is why a high flow rate of MQL has reduced the temperature efficiently. Fig.7(b) shows the 3D surface plot of temperature under simultaneous variation of concentration and flow rate. The temperature was decreased with the increase of the concentration and flow rate at all the levels of the parameters. It is worthy of mentioning that the lowest temperature was 148 o C at the highest levels of the concentration and flow rate. The key reason behind the significant reduction of temperature is that high flow rate and concentration of nanofluids have extended capacity to absorb the tool-chip heat through Al2O3-MWCNTs particles. The high heat absorption of hybrid nano-additives also dissipates heat quickly from the cutting region to chip or fluid. As only MWCNTs have almost 150% higher thermal conductivity than the base fluid. So, MWCNTs show better performance regarding the heat transfer properties [24]. can be attributed to the better morphology, and different sized nano-additives penetrated well at the cutting zone, dispersed in the base fluid due to weak Vander Waals forces and higher wettability has extracted heat well. Also, due to high pressure, the evaporation phenomenon to dissipate heat was prominent. This phenomenon had effectively prevented the elevation of temperature in the cutting zone. Fig.7(d) shows the 3D surface plot of temperature under the simultaneous effect of flow rate and air pressure. The temperature was reduced by increasing flow rate and air pressure. The higher flow rate added more nanofluids assisted fine mist at high air pressure. In the process of the fine penetration of hybrid nanofluids, the spherical and cylindrical shapes of nanoparticles having taken advantage to behave like spacers at the tool-chip interface to directly touch the tool to workpiece surface ultimately reducing the frictional heat generation.
Considering advantages of vegetable oils, fine droplets of vegetable oil having a polar nature, align themselves on the surface of the workpiece, forming a thin lubrication film providing adequate lubrication compared to non-polar nature of cutting oils. Therefore, MQL with vegetable oil has reduced frictional temperature by applying hybrid lubri-cooling.

MQL parameters effect on resultant force
The cutting force is very useful in engineering for machine designs and machining settings, although the resultant average force is not the maximum value in the milling process [25]. As the end-mill contains several teeth to cut the material simultaneously, the average cutting force per teeth per cut in x, y, and zdirection (refer Fig.8) are: = ∑ (i).  decreases at the highest level of the air pressure. As the S/N ratio of FR is smaller, the better, so by increasing the concentration and flow rate, the resultant force FR was decreased. While increasing air pressure, the resultant force FR was slightly increased; however, the effect of air pressure was not so high on the effect of the resultant force. Fig.9(b) depicts the 3D response surface plot of underscoring the simultaneous effect of concentration and flow rate on resultant force FR. The resultant force FR decreased with the increase of concentration and flow rate. However, the effect of flow rate on the reduction of resultant force FR was significantly higher than in concentration.
The highest resulting force FR was 24N at the highest level of concentration and flow rate. The effectiveness of nanofluids concentration and flow rate towards the reduction of resulting force FR can be associated with the addition of hybrid nanoparticles provided lubrication due to a ball-bearing effect at the cutting zone.
The suspended hybrid nano-additives of spherical and cylindrical shapes in Blaser oil form a thin layer of lubrication that reduces friction between the tool-chip interface. The high flow rate has more ability to prevent direct contact of end-mill with the workpiece surface from lowering down the cutting forces [26].

MQL parameters effect on surface roughness
The arithmetic average height parameter (Ra), also named CLA (center-line average), is the most frequently used roughness parameter for quality control. For a specific length of the sample, the average irregularities/deviations from the mean line are named as an average surface roughness (Ra). The is the measurement length, is the necessary length for measurement, ℎ . . . . . ℎ are the absolute distance of each point from the mean line. The equation for the arithmetic-mean height can be defined as follows: Ra=average height(μm); l=number of intersections of the profile. Fig.10(a) shows the surface plots of the S/N ratio of surface roughness corresponding to the simultaneous effect of concentration, flow rate, and air pressure. The S/N ratio of surface roughness increased with the increase of concentration, flow rate, and air pressure; however, the concentration of nanofluids affected the S/N ratio of surface roughness considerably at the high level. Fig.10

Tool wear mechanism
The cutting forces and temperature are mostly influenced by the wear of the cutting tool. The wear of cutting tools depends upon tool materials, type of coatings, cutting conditions, and cooling/lubrication. In this section, the effect of nanofluids assisted MQL associated parameters (concentration, flow rate, air pressure) were studied to explore the effect of hybrid nanofluids MQL on tool wear behavior. The % concentration, flow rate, and air pressure are fixed as 0.24 wt%, 120 mL/h, and 0.6 MPa, respectively. The state of the cutting tool was highlighted through the scanning electron microscopy (Fig.11). The dominant wear patterns under MQL are the abrasive wear, coating peeling, and chip adhesion. From Fig. 11, it can be determined that less severe abrasive wear was observed between the cutting tool and the workpiece.
Besides, coating peeling was appeared due to chip sliding on the tool surface and formation of saw tooth chips and BUE during cutting. Ezugwu et al. [28] have reported BUE formation on the cutting tool in the machining of hard-to-cut materials is one of the issues encountered during cutting. It can be said that when the cutting temperature reaches a critical stage, the tendency of chip welding becomes high enough owing to the chemical closeness between workpiece material and cutting tool material.

Flank wear
Coating Peeling Edge Chipping Adhesion Figure 11: The SEM analysis of the milling tool under 0.24 wt% concentration, 120 mL/h flow rate, and 0.6 MPa air pressure As the cutting tool starts cutting, due to poor thermal conductivity of Ti-6Al-4V, cutting tool scratches leading to thermal cracks initiation on the cutting edge due to imbalanced heat and fatigue on different portions of the cutting tool. As the cutting tool continues to cut, thermal crack energy starts accumulating.
When the thermal crack energy exceeds the crack propagation threshold, enabling thermal cracks initiation.
As a result, this phenomenon lowers the coating thickness, reduces its strength, and accelerates coating peeling [29]. The mist carrying hybrid nano-additives penetrated well at the tool-chip interface due to air pressure, formed a tribo-film. It significantly reduced the severe edge chipping, coefficient of friction, and adhesion from the tool cutting edge. These advances with superior lubrication/cooling allowed chip adhesion and coating peeling to occur in a smaller area of the cutting edge. The reduction of such sever chipping may occur due to nano-additives behaving as spacers between tool-workpiece interface and prevented direct tool contact, helped to slip the chip on the cutting edge rather than adhesion.

Surface topography and chip morphology
The surface topography of the machined workpiece is highlighted in Fig.12. It is obvious that MQL conditions have generated smooth, with fewer peaks and valleys. It can be attributed to the smooth wear of the cutting tool and less chip adhesion and welding under hybrid nanofluid conditions. The feed marks were appeared on the surface owing to the interaction of the current tool path with the neighboring tool paths.
Thus, the feed marks were blurred, and an increase in feed marks with increasing the workpiece length.
However, very fewer chips sticking on the newly generated surface were observed because of the pressured mist has flown away from the chips.  The energy-dispersive X-ray spectroscopy (EDS) analysis has underscored Ti, V, and Al components of chip analysis. To prevent the heat or transfer heat from the tool-chip interface, hybrid nanofluids MQL is expected to dissipate heat from the cutting region. In this way, it limits the thermal softening of the workpiece material and eases in chip flow by the presence of nano-additives at the tool-chip interface. In addition, the pressured nano-mist dissipated the generated heat and prevented chip welding which facilitates the chip flow, and that is owing to the presence of MWCNTs nano-additives. Also, due to the uniform strain on the material during cutting (i.e., low and high strain leads to the formation of saw-tooth chips), wider and lower in depth saw-tooth were observed under MWCNTs assisted MQL.

Tribology mechanism of hybrid (Al2O3-MWCNTs) nanoadditives
Several researchers have reported the superior thermal, physical, and rheological properties of hybrid nanofluids than single nanofluids in turning, milling, drilling, and metal forming [14,30]. The improved cutting conditions for the machining of difficult-to-cut materials are attributed to the hybrid nanofluids owing to thermal conductivity, lubrication to limit the heat and wear of the cutting tool. The variables sizes and shapes of the nanoparticles suspended in base fluid provide significant advantages during the cutting process. Also, understanding the possible mechanisms is highly essential (Fig.14).

Figure 14:
The hybrid nanofluids mechanism and effect on machining Ti-6Al-4V The possible mechanisms of hybrid nanofluids are summarized as follows:  A very fine mist emits from MQL nozzle as compressed air is mixed with hybrid nanofluids suspended fluids in 60:40 in a mixing chamber.
 The fine mist droplet carries nanoparticles surrounded by thin oil-film. This high-velocity mist penetrates well since it impinges and fills the tool pores and groves.
 Thus, mist droplets spread and form a dipolar thin film on the tool, and the workpiece surface enhances tribological characteristics and lowers the friction. Also, superior thermal conductivity and lubrication function of Al2O3-MWCNTs dissipates heat from the cutting zone.
 Increasing concentration of hybrid nanoadditives increased nanoparticles at the tool-chip interface, behave as spacers, and ball-bearing to limit the tool contact as well as chip contact length.
 Variables sizes and shapes of hybrid nanoadditives have advantages to fill the wide/close contact between the tool-workpiece. Also, fills micro-voids on workpiece surface due to external pressure, results in improving the workpieces surface roughness.

Conclusion and Future Work
In this research, the milling characteristics of Ti-6Al-4V under hybrid nanofluids assisted MQL conditions are investigated experimentally. The influence of Al2O3-MWCNTs on the milling force, temperature, and surface roughness are analyzed. The following conclusions are drawn: 1. When hybrid nanofluids are applied to the milling, it is observed the flow rate was the most significant parameter influenced resultant force, followed by concentration and flow rate. It could be associated with the tribo-film formation due to the high concentration of nanofluids showed excellent lubrication, regarding load-carrying, wear resistance at 0.24 wt%, and 160 mL/hr.

2.
Regarding the cutting temperature, hybrid nanofluids assisted MQL reduced the shearing as well as frictional heat generation. The MQL flow rate played a significant role in the reduction of cutting temperature, followed by concentration and air-pressure. The hybrid nanofluids prevented the direct contact of the cutting tool with the workpiece to control the heat generation.
3. In respect of surface roughness, a significant improvement in surface quality was observed due to a reduction in chip adhesion, severe BUE formations, and a smooth machined surface. A considerable reduction in surface roughness was noticed under the increase of nano-additives concentrations, followed by flow rate and air pressure. It can be associated with the capability of different shape hybrid nanofluids behaved as ball-bearings and able to fill the micro-voids on the surface.
The SEM analysis of tool flank-wear depicted less sever tool edge damage, chip-welding, and coating peeling. It principally can be attributed to the excellent penetration of nanofluids and forming a protective layer on the tool surface to slide the chip. Also, chip analysis depicts the clean back surface and less melting of saw-tooth chip edges. The surface topography confirms the less micro-adhesion of chips and material debris.

Conflict of Interest:
It is declared that authors have no conflict of interest Figure 1 Application of hybrid nano uids through the minimum quantity lubrication system  The coated carbide power-mill with geometric characteristics Figure 4 Two-step method to prepare hybrid nano uids      The SEM analysis of the milling tool under 0.24 wt% concentration, 120 mL/h ow rate, and 0.6 MPa air pressure Figure 12 Surface topography of the machined surface at nano uids concentration of 0.24vol%, 120ml/h of ow rate at 0.6MPa of air pressure Figure 13 The SEM analysis of chip and EDS of the chip produced under MQL machining at a nano uids concentration of 0.24vol%, 120ml/h of ow rate at 0.6MPa of air pressure Figure 14 The hybrid nano uids mechanism and effect on machining Ti-6Al-4V