A Novel Study on The Effects of Minimum Quantity Lubrication and Machining Parameters in Turning of Ti-6Al-4V Alloy By Applying Al2O3-Graphene Hybrid Nanoparticle Enriched Cutting Fluid

In Minimum Quantity Lubrication (MQL) very small amount of cutting uids are used. Currently, nanoparticles are added into cutting uids to magnify the cooling and lubricating properties. Several studies are available on MQL to check the machining performance in terms of cooling and lubrication using nanouids like Ag, SiO 2 , MoS 2 , Al 2 O 3 , Cu and MWCNT. However, limited evidences are available in applying hybrid nanoparticles in machining processes. Present research investigates the effect of hybridization of two different nanouids on machining performance in turning operation of Ti-6Al-4V alloy. Moreover, machineability was evaluated and analyzed by performing turning using minimum quantity lubrication (MQL) cooling technique. Cutting temperature and surface roughness of machined surface were taken as technological performance parameters to evaluate the machinability of Ti-6Al-4V alloy. Hybridization was performed by mixing alumina based nanouid into graphene nanoparticles in a xed volumetric proportion 80:20 using vegetable oil as base uid. Additionally, machining performance was evaluated by preparing hybrid nanouid in different concentrations like (0.25,0.50,0.75 and 1.00vol%) and tested for thermophysical properties before experimentation. Signicant improvements in thermophysical properties were observed during hybridization of Al 2 O 3 and Graphene. For parametric optimization and design of experiment, Taguchi orthogonal array has been employed. Machining performance of vegetable oil base alumina-graphene hybrid nanouid was compared with monotype alumina based nanouid and a signicant reduction cutting temperature and surface roughness was observed respectively.


Introduction
In manufacturing industry during machining of metals especially machining of super alloys large amount of heat is generated at the machining zone due to friction between tool tip and workpiece interface. This generated heat has lot of negative thermal effects So, cutting uids are used to reduce the friction between tool tip and the workpiece by providing e cient lubricity and cooling during machining operations. Cutting uids are used to reduce the negative thermal effects and chips are washed away from the machining zone. However, when cutting uids are used in excessive amount it causes environmental pollution and effects the workers' health. Therefor dry machining could be the alternative option to restrict the excessive usage of cutting uids. Krolczyk et al. and Grzesik et al [1,2] have adopted dry machining and observed encouraging results regarding machining performance.
However dry machining is suitable at low cutting speed and preferred for easily machinable materials.
Dry machining is not preferred at high depth of cutting and high-speed machining of super alloys because during dry machining of superalloys build up edges are produced on tool which reduced the tool life and effects the dimensional accuracy of the workpiece [3] Therefor to reduce the consumption of cutting uids and negative effects of dry machining a novel technique MQL can be chosen.
Minimum Quantity Lubrication (MQL) also known as near dry machining (NDM). In this technique minimum amount of any cutting uid is sprayed optimally into the cutting zone at high pressure for better penetration into the machining zone and improve tool chip interaction. Reddy et al. [4] studied the machining performance of MQL and the results were compared with dry machining and concluded that machining temperature, surface roughness, tool wear and the cutting forces were reduced signi cantly as compared to dry machining. Padmini et al. [5] observed the machining performance using MQL and concluded that lower cutting forces and tool wear were produced as compared to dry machining. Khan et al. [6] studied the machining performance using MQL and compared the results with ood cooling and concluded that by using MQL machining temperature was reduced by 10% and tool wear, surface roughness was also reduced.
Meanwhile, Maruda et al. [7] evaluated the machining performance using MQL turning of AISI 1045 carbon steel they observed a signi cant reduction of 40% in tool wear compared to dry machining.
Maruda et al. [8] studied the machining performance of MQL and observed that by using MQL surface roughness of machined stainless steel was reduced more than three times as compared to dry machining. Behera et al. [9] proposed a model for local coe cient of friction as a function of MQL parameters and cutting conditions that predicts cutting forces, contact length and chip thickness under MQL environment.
Furthermore, Singh et al. [10] and Kishawy et al. [11] studied the machining performance using MQL and concluded that using MQL technique surface nish and tool life improved and machining temperature, cutting forces are reduced. Ganguli et al. [12] investigated the machining performance of Automized cutting uid spray system during machining of Titanium Alloy. They concluded that lower cutting forces, lower surface roughness and observed enhancement of tool life as compared to wet machining. In their opinion MQL might be a viable alternate to dry and wet machining. Therefor MQL can minimize the machining cost without effecting the machining performance, by restricting the excessive use of cutting uids and environmental hazards.
The conventional cutting uids possess good lubrication properties but limited cooling properties due to poor thermal conductivity associated with them, restrict their use as cutting uid during high speed machining of super alloys. To overcome this problem nano uids are used, nano uids are suspension of nanometer size particles into conventional cutting uids or vegetable oil, which leads to improve heat extraction rate from machining zone due to enhanced thermal conductivity [13].
Sen Gupta et al. [14] observed in their researches that by adding nanoparticles into conventional cutting uids enhanced their thermal conductivity compare to base uids. An improvement up to 22.4% in thermal conductivity of conventional uid at room temperature could be achieved by adding 6% Al 2 O 3 at room temperature. Esfe et al. [15] developed the Al2O3 ethylene glycol based nano uid and studied the effect of temperature and Al2O3 nanoparticles on thermal conductivity. They observed signi cant enhancement in thermal conductivity with the increase in concentration and temperature. Choi et al. [16] prepared the nano uid by mixing multi walled carbon nanotubes [MWCNT] into base uid and observed signi cant improvement in its thermal conductivity up to 200% and 150% respectively compared to its base uid.
Ghosh et al. [17] prepared the nano uid by mixing Al 2 O 3 and MWCNT separately into base uid and evaluated the machining performance during high speed turning of steel using MQL. They concluded that nano uid containing MWCNT nanoparticles have better lubricity and cooling effect as compared to Al 2 O 3 nanoparticles. Saravana Kumar et al. [18] evaluated the machining performance of MQL by using Ag nanoparticles. Turning operation was performed at different cutting speed and depth of cut using nano uid containing Ag nanoparticles and compared the results with conventional uid cooling and concluded that cutting forces and surface roughness was reduced to 8.8% and 7.5% with the use of nano uid.
MingLi et al. [19] evaluated the machining performance of Graphene nanoparticles enriched cutting uid in milling operation and observed signi cant reduction in cutting forces and cutting temperature up to 18.13% and 13.59% respectively. Sirikant et al. [20] achieved a signi cant reduction of 71% and 25% in tool wear and machining temperature with the use of nano uid containing graphite nanoparticles in MQL Turning compare to conventional ood machining. Krishna et al. [21] prepared the nano uid by using boric acid in coconut oil and evaluated the machining performance during MQL Turning of AISI 1040 steel and achieved a signi cant reduction in cutting temperature, surface roughness and tool wear.
Sharma et al. [22] prepared the nano uid by mixing TiO 2 nanoparticles in DI water and evaluated the machining performance during Turning operation and observed enhance machining performance over conventional ood cooling. Sidik et al. [23] reviewed the research work carried out in various research works based on application of Al 2 O 3 , CNT, MoS 2 and diamond nanoparticles with MQL in machining operations. They observed that signi cant improvement could be achieved in all researches.
Many researches have been conducted in manufacturing processes with cutting uids containing mono type of nanoparticles. However, to the best of author's knowledge very few researches have been carried out in machining using nano uids containing hybrid nanoparticles (i.e. a colloidal suspension having two different type of nanoparticles ) Sarkar and Gosh [24] reviewed the research works available on hybrid nano uids and observed that proper hybridization might be helpful in making hybrid nano uids very useful for heat transfer enhancement in machining processes and other heat transfer applications. Suresh et al. [25] prepared the water-based hybrid nano uid containing Al2O3 and Cu nanoparticles made it a potential heat transfer uid. Tansen et. al [26] prepared the Nano cutting uid enriched with MWCNT and alumina, yielded superior machining performance compared to single nanoparticle enriched cutting uid. Nine et al. [27] prepared a nano uid containing MWCNT and alumina nanoparticles and observed signi cant enhancement in thermal conductivity compared to nano uid containing single nanoparticle.
Furthermore, Zhang et al. [28] evaluated the machining performance of Al2O3-SiC enriched hybrid nano uid and concluded that hybrid nano uid yielded better surface quality compared to mono type nano uid. Moreover, Ahammad et al. [29] prepare the alumina-graphene enriched hybrid nano uid and achieved 88.62% enhancement in convective heat transfer and a reduction of 4.7 0 C in cutting tool temperature. Zhang et al. [30] prepared the MoS2-CNT enriched hybrid nano uid and evaluated machining performance by MQL grinding operation and achieved lower G ratio and surface roughness (Ra=0.328um) compared to mono type nano uid of MoS 2 and CNT.
Moreover, few researches have been conducted on enhancement in thermophysical properties [31] and Tribological properties [32] of base nano uids by using different types of nanoparticles. However, not much signi cant work could be reported in the available literature regarding the application of hybrid nano uids in manufacturing processes, especially in milling operation.
In the present research, the Al 2 O 3 -Graphene enriched hybrid nano uid is prepared by mixing Aluminum

Experimentation
The Turning of Ti-6Al-4V was carried out using MQL technique under the mist having different concentrations of Al 2 O 3 -Graphene enriched nano uid. However, the nano uid samples having various concentration were prepared and tested for their thermophysical properties, before machining.

Nano Cutting uids: Preparation and Characterization
The nano uids were prepared by dispersing various concentration of Al 2 O 3 and graphene in soybean oil.
To make homogeneous mixture of hybrid nanoparticles and soybean oil the cutting uids were rst stirred using a magnetic stirrer (for 40 mins) and then dispersed using an ultrasonic dispersion instrument (for 3 h) shown in g. 1 This process repeated several times until the nanoparticles were uniformly dispersed in soybean oil. The Al 2 O 3 -Graphene enriched cutting uids were stable and no agglomeration was observed during the entire experimentation process. A fresh sample of nano uid was developed for each experiment and used immediately to avoid possible sedimentation/agglomeration of nanoparticles. The nano uids were tested for thermal stability at various temperatures. Moreover, the effect of nanoparticles concentration on its thermophysical properties and heat transfer enhancement was investigated. The Al 2 O 3 -Graphene hybrid nano uid shows an enhancement of 9.8% in thermal conductivity and thermal stability also improved [33]. Furthermore, with the rise in temperature all samples of Al 2 O 3 -Graphene enriched nano uids showed reduction in viscosity [34].
Moreover, with the increment in volumetric concentration (0.25,0.5,0.75 and 1.00 etc.) vol% of nanoparticles with Al 2 O 3 -graphene hybrid nano uid increment in viscosity also occurs. Investigations reveal that with the rise in nanoparticles volumetric concentration increase both the thermophysical properties and viscosity. The increment in thermal conductivity affects the machining performance positively due to enhancement in heat transfer from machining zone to the cutting uid provide cooling and lubrication to the tool workpiece interface, while higher viscosity causes a problem (pressure drop) in spraying mist of Nano lubricants to the machining zone with the MQL technique. To stabilize the advantages of greater thermal conductivity and disadvantages of higher viscosity (loss in pumping power), the authors selected the particle volumetric range from 0.25 vol% to 1.00 vol%.

Experimental Setup
Turning of Ti-6Al-4V alloy was accomplished on Lathe machine (Institute of Space Technology, Pakistan) by applying aerosol of hybrid nano-cutting uid using various concentrations of Al 2 O 3 and Graphene nanoparticles under MQL technique. The experimental setup is shown in g.4. The coated cemented carbide tool tip was used for experimentation. In addition, an Accu-Lube MQL system imported from China was used to applying the Al 2 O 3 -graphene dispersed cutting uid to the cutting zone. This Accu-Lube MQL system consists of a pulse pump, air pump, pulse generator, lter, oil tank and oil gas mixing tube as shown in g.4. The nano cutting uid ow rate and air supply pressure is adjustable for this MQL system and nano uid ow rate was set at 40mL/h. A nozzle having 1mm discharge dia. was used and set up 6cm above the rake face of the cutting tool, capable of impinging aerosol vertically downward on the workpiece tool tip interface in order to provide e cient cooling and lubrication.
To evaluate the effects of MQL and machining parameters on machining performance during turning of Ti-6Al-4V alloy each experiment were performed in triplicate and taken average of the values. The Machining temperature T(C 0 ) was measured with an Arduino based data acquisition system which includes K type thermocouple which was connected with an electrical circuit through MAX 6675. The electric circuit connects the sensors through MAX 6675, with an Arduino Mega 2560, which displays the thermocouple temperature readings through serial monitor in Arduino IDE. Thermocouple was inserted in to a 4mm diameter hole in the tool tip (as shown in below g.) to measure the surface temperature. Moreover, the average surface roughness (Ra) was measured by 3D optical pro ler, which can measure curved, at or stepped, polished or rough surfaces e ciently.

Selection of workpiece material and Tool
In this research work Ti-6Al-4V alloy was used as workpiece material. The dimension of the workpiece was 40mm in diameter and 0.61m in length. Grinding process was used to remove the oxide layer from the workpiece material earlier to experimentation. Chemical composition and mechanical properties workpiece material are shown in table 1 and table 2   Additionally, due to large coe cient of friction, low thermal conductivity, and high chemical reactivity Ti-6Al-4V alloy is di cult to machine and high heat generation occurs. So, coated cemented carbide insert clamped by a screw on a rigid tool holder was used as a cutting tool. It is perfect due to its high toughness, high wear resistance and low chemical reactivity for machining Ti-6Al-4V alloys. Ghani et al.
[35] Therefore, coated cemented carbide tool was adopted in this research work.

Experimental Design
The technique of investigation and de ning all possible conditions in experimentation, involving various factors is known as design of experiments (DOE). Taguchi method was selected to design the experimental array L 16 (4 3 ) OA in order to minimize the machining temperature, and surface roughness.
The standard OA consist of 16 experiments with four control factors and four experimental state levels for each control factor. The parameters such as nanoparticles concentration, air pressure, cutting speed and depth of cut were chosen as control factors and machining characteristics such as cutting temperature and surface roughness were recognized as response factors. The detail of experimental conditions for each control factors and orthogonal array are shown in table 5 and table 6. In order to remove any other invisible factors that may also affect the cutting temperature and surface roughness all the experiments were performed randomly.

Results And Discussion
Turning operation on Ti-6Al-4V was performed as per experimental design by varying the MQL and machining parameters such as concentration of nanoparticles, air pressure, cutting speed and depth of cut to evaluate the machining performance in terms of machining temperature and surface roughness using alumina based nano uid and Al-GnP hybrid cutting uid.

In uence on cutting temperature
A signi cant reduction in machining temperature was observed during the turning process of Ti-6Al-4V alloy by using Al-GnP hybrid cutting uid over monotype alumina based cutting uid. This is due to the greater thermophysical properties processed by Al-GnP hybrid cutting uid compared to alumina mixed nano uid [36]. Moreover, the attained results regarding thermophysical properties were found to be in good agreement with previous investigation [37]. They observed during their research work that graphene based cutting uid could be used in high speed metal removal processes as potential heat transfer nano uid. Moreover, they observed enough thermal conductivity enhancement up to 45.2% with graphene dispersed nano uid over base uid.
It is concluded from the table 5 that concentration of Al-GnP nanoparticles (W) and its interaction with cutting speed and depth of cut (d) have prominent effect on the cutting temperature compared to air pressure (P). The most signi cant parameter for the machining zone temperature was nanoparticles concentration. It was observed during experimentation that when the nano uid concentration was increased from 0.25 vol.% to 1.00 vol.% machining temperature rst increased and then decreased and when Al-GnP concentration was 1.00 vol.% cutting temperature was again higher as shown in below graph. This is due to relatively large quantity of nanoparticles that blocked the machining zone and prevent the formation of oil lm thereby weakening the rate of heat transfer from machining zone. Meanwhile, when the nanoparticles concentration was too low the cutting temperature was maximum as shown in table 5 due to less rate of heat transfer and insu cient cooling.
It was investigated that during the turning of Ti-6Al-4V alloy when the air pressure increased from 0.3MPa to 0.6MPa initially the cutting temperature was high however gradually decreased and was minimum at 0.5 vol % nano uid concentration, 0.5MPa air pressure, 1670mm/min cutting speed and 1.2mm depth of cut. The variation of milling temperature also shown in above graph ( g. 7) It is observed from the table 5 that there is signi cant %age change in cutting temperature Tc while using Al-GnP hybrid nano uid compared to monotype alumina based nano uid.
So, the lower machining temperature was observed during turning operation of Ti-6Al-4V alloy by using Al-GnP hybrid nano uid compared to monotype alumina based nano uid due to greater thermophysical properties of hybrid nano uid. It can be observed from the table 5 that the optimal values of MQL parameters for minimum milling temperature can be designed namely 0.5 vol.% hybrid nanoparticles concentration W, 0.5MPa air pressure, 1670 mm/min cutting speed and 1.2 depth of cut.

In uence on surface roughness
In the case of Al-GnP hybrid nano uid coe cient of friction is reduced due to synergic effect, polishing and rolling effect so, the better surface quality was achieved compared to monotype type alumina enriched cutting uid [38]. It was investigated during the turning of Ti alloy that air pressure P, depth of cut d, cutting speed v and Al-GnP hybrid nanoparticles concentration signi cantly affect the surface roughness. Moreover, Al-GnP hybrid nano uid concentration (W) and depth of cut (d) were the most signi cant parameter, followed by cutting speed (v) and air pressure (P).
Furthermore, during the turning operation of TC4 alloy when the concentration of alumina based nano uid and Al-GnP hybrid nano uid was increased from 0.25 vol.% to 1.00 vol.% initially surface roughness was higher but decreased gradually and was minimum at 0.50 vol.% nano uid concentration. However, if the concentration (W) increased further then surface roughness increased and reaching the maximum at 1.00 vol.% nano uid concentration as shown in table 5 and below graph as well.
Meanwhile, when the air pressure was increased from 0.3 MPa to 0.6 MPa surface initially surface roughness was higher then gradually decreased and was minimum at 0.5 MPa. However, if the air pressure increased further then surface roughness increased again and was maximum at 0.6 MPa as shown in table 5. Similarly, when the cutting speed was 470 mm/min the surface roughness was maximum and decreased gradually as the cutting speed increased, was minimum at 1670mm/min as shown in table 5. Moreover, when the depth of cut was higher the surface roughness was also higher while with the decrease in depth of cut the surface was also reduced and minimum at 1. With the use of Al-GnP hybrid nano uid lower value of surface roughness has obtained. Moreover, nano uid concentration, air pressure, cutting speed and depth of cut have signi cant effect on surface roughness. However, nano uid concentration and depth of cut have greater effect on surface roughness followed by air pressure has least signi cant effect as shown in table 5.
The Al-GnP hybrid vegetable oil based nano uid is effective for improving machining performance compared to monotype alumina based nano uid in terms of machining temperature and surface roughness through better cooling and lubrication.
The improved thermophysical properties due to blending of two different nano uids graphene with Al 2 O 3 in xed volumetric ratio (80:20) proved to be the effective lubricant. However, the optimization of mixing ratio may improve the performance of lubricant in heat transfer applications.
Most of the publish work on MQL have given more attention to nano uids containing single nanoparticles in machining processes. In present work the researcher has investigated the hybridization of two different types of nanoparticles and evaluated the machining performance. The blending of alumina with graphene in a xed volumetric ratio improved its thermophysical properties (heat transfer enhancement). However, optimization of mixing ratio may further improved machining performance. Meanwhile, the present work can further be extended by performing machining at different nanoparticles volumetric fraction their shape, size and employing different nanoparticles. This would be helpful in developing cutting uids with improved thermophysical properties for the machining of metals and different alloys. Furthermore, with the usage of MQL technique and vegetable oil-based cutting uids with improved thermophysical properties may reduce the machining cost with effecting the machining performance and environmental pollution.

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