ORIGINAL ARTICLE

On machining of Ti-6Al-4V using multi-walled carbonnanotubes-based nano-fluid under minimum quantity lubrication

H. Hegab1,2& H. A. Kishawy1 & M. H. Gadallah2

& U. Umer3 & I. Deiab4

Received: 5 January 2018 /Accepted: 9 April 2018# Springer-Verlag London Ltd., part of Springer Nature 2018

AbstractTitanium alloys are the primary candidates in several applications due to its promising characteristics, such as high strength toweight ratio, high yield strength, and high wear resistance. Despite its superior performance, some inherent properties, such aslow thermal conductivity and high chemical reactivity lead to poor machinability and result in premature tool failure. In order toovercome the heat dissipation challenge during machining of titanium alloys, nano-cutting fluids are utilized as they offer higherobserved thermal conductivity values compared to the base oil. The objective of this work is to investigate the effects of multi-walled-carbon nanotubes (MWCNTs) cutting fluid during cutting of Ti-6Al-4V. The investigations are carried out to study theinduced surface quality under different cutting design variables including cutting speed, feed rate, and added nano-additivepercentage (wt%). The novelty here lies on enhancing theMQL heat capacity using nanotubes-based fluid in order to improve Ti-6Al-4V machinability. Analysis of variance (ANOVA) has been implemented to study the effects of the studied design variableson the machining performance. It was found that 4 wt%MWCNTs nano-fluid decreases the surface roughness by 38% comparedto the tests performed without nano-additives, while 2 wt% MWCNTs nano-fluids improve the surface quality by 50%.

Keywords Multi-walled carbon nanotubes (MWCNTs) . Average surface roughness . Ti-6Al-4V alloy . Nano-cutting fluids .

Analysis of variance (ANOVA)

1 Introduction

Titanium alloys are broadly used in different industrial appli-cations within the military, aerospace, power generation, au-tomotive, and other fields due to their promising mechanical,physical, and chemical characteristics; for example, high yieldstrength, high strength to weight ratio, high toughness andhigh creep, corrosion, and wear resistance [1]. These materialsalso retain their hardness and strength at high temperatures[2], which make them one of the primary candidates for

aerospace, nuclear, power generation, and automotiveapplications.

However, despite the abovementioned superior character-istics, titanium alloys are inherently difficult-to-cut materialsdue to high stresses and high cutting temperatures generatedwhen they are being machined. This is mainly attributed tolow thermal conductivity of titanium that adversely affects thetool life and can lead to premature tool failure. Because oftheir low thermal conductivity, the generated heat during ma-chining titanium alloys is mainly dissipated through the cut-ting tool and cooling media other than the workpiece or chip.In addition, titanium alloys become chemically reactive at ahigh cutting temperature and react with some tool materials,which also deteriorate the cutting tool and fasten the tool fail-ure. Moreover, the resultant chip shape in titanium machiningis serrated or saw-toothed as localized adiabatic shear bendingand intense shear strain rate exist in the primary shear zonedue to the high temperature at the chip–tool interface. Someother properties that make titanium alloys difficult-to-cut andimpose barriers towards their widespread applications are lowelastic modulus, strain hardening, tendency to adhesion, andforming built-up edge [3, 4].

* H. HegabHussien.Hegab@uoit.ca

1 Machining Research Laboratory, UOIT, Oshawa, Ontario, Canada2 Mechanical Design and Production Engineering Department, Cairo

University, Giza, Egypt3 Advanced Manufacturing Institute, King Saud University,

Riyadh, Saudi Arabia4 Advanced Manufacturing Laboratory, Guelph, Ontario, Canada

The International Journal of Advanced Manufacturing Technologyhttps://doi.org/10.1007/s00170-018-2028-4

Different attempts [5–7] have been performed to study,optimize, and improve the excessive heat generation problemduring cutting processes to achieve better machinability espe-cially for difficult-to-cut materials.

Major advances in the lubrication and cooling techniqueshave been presented through several literature studies, such asminimum quantity lubrication (MQL), cryogenic cooling,compressed air cooling, dry machining, and nano-cutting fluidlubrication (NCFL) [8–10]. As a result, machining difficult-to-cut materials must be thoroughly studied from a sustain-ability point of view to achieve the most feasible cutting strat-egy. A promising technology in the application of cutting fluidis known as minimum quantity lubricant (MQL). MQL is acooling/lubrication method in which an optimal amount ofcutting fluid is forced to penetrate into the cutting zone bymeans of compressed air. Among the lubrication techniquesnoted above, the MQL technique with vegetable oil offers thebest environmental solution. Not only does this techniqueprovide the optimum amount of lubricant, utilizing vegetableoil presents a promising alternative to overcoming harmfulenvironmental impacts of commercial cutting fluids (e.g. min-eral oil). The MQL system consists of an air compressor, flowcontrol system, gas-based coolant lubricants (CLs) container,tunings, and spray nozzles [11]. The major important factor toimprove the cooling and lubricant functions in theMQLmeth-od is utilizing certain cutting fluids, such as nano-fluids whichwould develop its wettability aspects [12, 13]. Carbon nano-tubes (CNTs) are among the nano-additives that have superiorproperties. The diameters of these nano-additives vary fromfew nanometers to hundreds of nanometers, and their lengthsrange from tens of nanometers to several centimeters [14].

With a remarkable combination of mechanical propertiesand an attractive price, it is no surprise that such nano-additives have a very wide range of implementations. Thesesuperior properties include hardness, wear resistance, excel-lent thermal conductivity, high strength, and stiffness [15].Proposing new nano-cutting fluids is contributing to over-come the heat dissipation challenge during the cutting pro-cesses as it offers a highly observed thermal conductivity val-ue in comparison with the base lubricants. Additionally, it isshown that nano-cutting fluids have superior cooling proper-ties due to its good heat extraction capabilities [16].

Regarding the use of carbon nanotubes as nano-additivesinto the conventional cutting fluids, some studies have present-ed promising results in terms of improving the machining qual-ity characteristics. Sharma [17] applied the MQL system usingnano-cutting fluid based on multi-wall carbon nanotubes(MWCNTs) through turning high carbon high chromiumAISI D2 using tungsten carbide insert (CNMG 120408). TheTaguchi method has been implemented to investigate the effectsof cutting parameters on the surface finish and cutting zonetemperature. In comparison with the MQL technique basedon conventional cutting fluid, promising results have been

observed through using the proposed technique (MQL nano-flu-id). Huang et al. [18] investigated the effects of the MWCNT/MQL system during high-speed milling of AISI 1050 and AISIP21. The results have been compared to dry and wet cutting. Ithas been noticed that the MWCNT nano-fluid has an importantrole in reducing the tool wear and accordingly improving theinduced surface quality because of the excellent heat conductivityof MWCNT nano-fluids. In addition, Roy and Ghosh [19] con-ducted several experiments through high-speed turning of AISI4140 steel with a TiN-top coated multi-layered carbide insertusing small quantity lubrication (SQL) technology. Using of3 vol% alumina and 1 vol% MWCNT nano-fluid instead ofsoluble oil have shown significant reduction in the cutting forcesand tensile residual stresses. Furthermore, an improvement ofsurface quality has been observed due to the improvement inmaintaining the cutting tool edges sharpness.

Regarding the nanoparticle effects, Setti et al. [20], focusedon controlling the friction behavior in the grinding process,which occurs as a result of the interaction between the work-piece and abrasive grains and which influences the machiningquality characteristics, such as the generated forces and toolwear. Different volume concentrations of Al2O3 and CuOnanoparticles were added to water as a base fluid. The grind-ing process was applied to Ti-6Al-4Valloy using MQL strat-egy with and without nano-cutting fluid. The wheel morphol-ogy, surface integrity of ground surface, grinding forces, thecoefficient of friction, and chip formation were investigated.They concluded that MQL nano-fluid technique showed bet-ter results compared to the regular MQL technique in reducingthe coefficient of friction and tangential forces. Moreover, thechip morphology examination exhibited promising resultswhich indicate the significant heat transfer and lubricationcharacteristics of the nano-mist.

Nano-cutting fluids have shown promising effects on thecutting performance characteristics through different cuttingoperations, such as turning, milling, and grinding as has beenpresented in some previous studies [21–26]. Also, MQL nano-cutting fluid is one of the suggested techniques to further im-prove the performance of MQL particularly when machiningdifficult-to-cut materials. The major important factor to im-prove the cooling and lubricant functions in the MQL methodis utilizing certain cutting fluids, such as nano-fluid, whichwould develop its wettability, convection, and conduction as-pects as has been discussed in several previous studies [12, 13,27–29]. It is found from the open literature that MWCNTs haveachieved promising results as a nano-additive into the conven-tional cutting fluid; however, only few studies have studied itseffects on different machining operations. From the literaturereview, there is a research gap in investigation of the nano-fluids technology effects through cutting of titanium alloys.The proper dispersion of nanoparticles into the base cuttingfluid shows a promising advantage in cutting processes as ithelps to reduce the induced friction between the workpiece

Int J Adv Manuf Technol

and cutting tool [30]. Also, nano-cutting fluids can contribute infacing the heat dissipation challenge during cutting processes.

On the other hand, one of the most important machiningoutputs when cutting Ti-6Al-4V is surface quality, since it hassignificant effects on different functional characteristics (e.g.surface friction, load bearing, coating). Surface quality ismainly depending on the used feed rate, tool nose radius,and cutting speed. Furthermore, applying an adequate cuttingfluid plays an important role in improving the induced surfacequality. In order to achieve the desired surface quality,selecting the optimal design variables levels and studying theirinteractions is significantly required. Thus, several studies[31–34] have focused on modeling and optimization of sur-face roughness when machining different materials by apply-ing various techniques, such as analysis of variance, multiple-variables regression, artificial neural network, and fuzzy logic.

The main objective in this research is to study the effects ofdispersed MWCNTs into the conventional fluid (vegetableoil) using minimum quantity MQL technique during turningof Ti-6Al-4V. Investigations are carried out to study, analyze,and model the machined surface quality when employingMWCNT nano-fluids.

2 Materials and methods

In order to investigate the effects of dispersed MWCNT-nano-fluid on the cutting quality performance, experiments are car-ried out by turning of Ti-6Al-4V. Ti-6Al-4V (UNS R56400) isutilized as the experiments workpiece. The tests are performedon CNC lathe machine using standard carbide turning insertsCNMG 432MMH13A (ANSI standard). Cutting tests are per-formed under MQL strategy, using different levels of cuttingspeed, feed rate, and weight percentages of added nano-addi-tives. The depth of cut for each cutting pass is 0.2 mm. Thecutting tool information is listed in Table 1. Regarding theMQL system, the air-oil mixture was supplied by the stand-alone booster system (Eco-Lubric) installed on the machinetool with a nominal oil flow rate of 40 ml/h and air pressure of0.5 MPa. In addition, ECOLUBRIC E200 is the vegetable oilused (base cutting fluid). The used oil is environmentallyharmless, suitable for industrial application, and has a biode-gradability of 90% in 28 days [30]. The angle and position ofMQL nozzle were adjusted by experimental observations toavoid blocking by chips. The average surface roughness pa-rameter is used to evaluate the machined surface quality using

a surface roughness tester (Mitutoyo SJ.201). Ra is the arith-metic average height of surface component (profile) irregular-ities from the mean line within the measuring length used todescribe the vertical dimension of roughness. Ra is commonlyrecognized, and the most used parameter to evaluate the ma-chined surface roughness. The used cut-off length is 5 mm.After each cutting test, the surface roughness tester is used infive random regions along the machined surface, and the av-erage value is considered. The cutting tests are replicated threetimes and average response value is calculated.

MWCNTs with a 13–20-nm average diameter, 95% purity,10–30-μm length, and 110-m2/g specific surface area are usedfor nano-cutting fluid preparations. Dispersion ofMWCNTintothe vegetable oil is an important aspect, which affects the resul-tant nano-cutting fluid thermal conductivity and viscosity. Thedispersion of MWCNT into the base cutting fluid is performedusing an ultrasonic machine (AQUASONIC-50HT) for 3 h at60 °C followed by a stirring step using a magnetic stirrer (HotPlate Stirrer-3073-21) for 30 min to ensure fully dispersion ofMWCNTs into the resultant nano-cutting fluid. The experimen-tal setup schematic is provided in Fig. 1.

The nano-fluids usage when employing flood coolant canbe an environmental concern; however, when using MQLtechnique, an optimal amount of oil is used resulting in a veryfine mist, where certain procedure is followed to eliminate anyconcern of using the nano-additives. Also, during the experi-mentation phase, certain safety procedures (i.e., standardnano-additives safety data sheets) have been applied to main-tain a standard health and safety level in the workshop to avoidany harmful impacts for the machine operator. Regarding thedisposal method, the nano-fluids have been carefully filteredbefore being released to the sewer according to a standardmaterial safety data sheet [35]. Thus, the MQL nano-fluidtechnique offers two main advantages: (a) enhancing the ma-chining process performance as the employed nano-additivesimprove the thermal and friction behavior and (b)accomplishing a sustainable process as using vegetable oilsbased on MQL provide effective environmental benefits.

In terms of nano-cutting fluid performance indicators, zetapotential for stability analysis is performed to characterize thenano-cutting fluid suspension stability. The Zetasizer nano-device is used to determine the zeta potential absolute valuesfor the resultant nano-fluids using different weight percent-ages of added MWCNTs. Also, sodium dodecyl sulfate(SDS) has been used in preparing the nano-fluid as a surfac-tant (i.e., 0.2 g). The purpose of using a surfactant is enhancing

Table 1 The used cutting insertand tool holder during machiningof Ti-6Al-4V

Cutting insert Coated carbide insert (CNMG 120416MR (ISO))

Tool holder SANDVICK SCLCR-2525M12 with clearance angle 5o and rake angle 0o, nose radiusof 1.58 mm

Int J Adv Manuf Technol

the nano-additives performance to be more hydrophilic, andincreasing the nano-additives surface charges; hence, the re-pulsive forces between the nano-additives are increased asmentioned in previous studies [36, 37].

In this work, three design variables (i.e., cutting speed,cutting feed, and added nano-additives percentage) are usedat three levels each. Table 2 indicates the design variablesstudied and the assignment of the corresponding levels. L9orthogonal array (L9OA) based on the Taguchi method isemployed as shown in Table 3 [38]. The full factorial arrayin this work is L27OA (33); however, fractional factorial arrayL9OA based on the design of experiments methodology hasbeen employed to save time and cost. The plan consists ofnine experiments in which the first column is assigned to thecutting speed, the second column to the feed rate, and the thirdcolumn to the weight percentage of added nano-additives. Inthis study, only main effects of the three process variables areconsidered. No interaction effects are studied due to limita-tions of L9OA. In fact, L9OA only studies the main and in-teraction effects of design variables A and B. The effect of C(added MWCNTs) is partially blocked with the interactioneffect of (A × B). In the future, it is intended to extend thisstudy to include the main effects as well as the interactioneffects using L27OA.

3 Results and discussions

The zeta potential results for the nano-cutting fluids used inthis study are provided in Table 4. It can be found that all

values are in the range of moderate stability, according to thesuspension stability evaluation criteria which has been men-tioned in a previous study [39].

The average surface roughness results are provided asshown in Fig. 2. It is clear that MWCNT nano-fluid showspromising results to improve the resultant surface quality;however, cutting tests three and nine did not show effectiveresults compared to the other tests performed using nano-additives as these tests have been performed at the highervalue of feed rate level (i.e., 0.4 mm/rev). Cutting test fourshowed the best surface quality. The cutting conditions of thistest include cutting speed of 170 m/min, feed rate of 0.1 mm/rev, and 2 wt% of added MWCNTs.

ANOVA results are listed as shown in Table 5. Both cuttingfeed rate and wt% of added nano-additives are the main sig-nificant design variables affecting the resultant surface qualityat 90% confidence level. In addition, the plot of design vari-ables effects is provided as shown in Fig. 3. The cutting feedrate of 0.1 mm/rev, cutting speed of 120 m/min, and 2 wt%added nano-additives are the optimal design variables levels.

Fig. 1 Experimental setup schematic

Table 2 The levels assignment to design variable for Ti-6Al-4V cuttingtests

Design variables Symbol Level 1 Level 2 Level 3

Cutting speed (m/min) A 120 170 220

Feed rate (mm/rev) B 0.1 0.15 0.2

Nano-additives (wt%) C 0% 2% 4%

Table 3 The studied design variables levels and plan of experiments(L9OA)

Test no. Levels ofcutting speed

Levels offeed rate

Levels ofnano-additivespercentage (wt%)

1 1 1 1

2 1 2 2

3 1 3 3

4 2 1 2

5 2 2 3

6 2 3 1

7 3 1 3

8 3 2 1

9 3 3 2

Int J Adv Manuf Technol

An improvement of 38% is noticed for 4 wt% of addedMWCNTs in comparison with the tests performed withoutnano-additives, while 2 wt% of added MWCNTs enhancedthe surface quality by 50%. Increasing the added percentageof nano-additives usually results in higher thermal conductiv-ity and better heat transfer performance; however, 2 wt%MWCNTs offered better cutting performance than 4 wt%.Thus, more investigations are needed in order to investigatethe effect of nano-additives concentration on the tribologicaland heat transfer characteristics when machining with nano-fluids. Also, the nano-additives concentration and size effectson the thermal and frictional behavior should be studied andanalyzed.

In order to verify the developed ANOVA results, Eq. (1)has been employed to estimate the optimal predicted re-sponses values as follows:

Ypredicted ¼ Ymean þ ∑n

i¼1Yi‐Ymeanð Þ ð1Þ

Where Yi is the average response at the optimal designvariable level and Ymean is the overall mean value. Theverification results were calculated as shown in Table 6.Good agreement can be noticed between the predicted andexperimental optimal results. In addition, the verificationaccuracy results were calculated and acceptable resultshave been obtained.

Also, response surface methodology (RSM) techniquehas been applied to model the studied cutting process withMQL (with and without MWCNT nano-fluid). The

developed model using MWCNTs nano-fluid for surfaceroughness is provided as shown in Eq. (2).

Ra3 ¼ 8:6*10−6A2 þ 58:32 B2

þ 1480:65 C2−0:002 AB−0:21 AC−268:55 BC

þ 0:27 ð2Þ

The design variables quadratic and interaction effects areused to express the developed model as all studied designvariables have three levels (2nd degree of freedom). The av-erage model accuracy for the surface roughness under usingMWCNTs nano-fluid is about 93.71%. The 3-D surface plotsfor the measured surface roughness is provided as shown inFig. 4. In all 3-D surface plots, the third design variable is heldconstant at its second level. As can be seen in Figs. 4b, c,increasing MWCNTs percentage results in decreasing the sur-face roughness. Also, Figs. 4b, c showed an agreement withthe results obtained in Fig. 3 as both of them confirmed that2 wt% MWCNTs showed the lowest surface roughnessvalues. In addition, Fig. 4a confirmed the previous findingsin Table 5 and Fig. 3 since changing the feed rate levelsshowed significant effects on the surface roughness results.From Fig. 4b, c, it was found that 2 wt% provided the lowestpower consumption values.

Table 4 Zeta potential results

Added MWCNTs (wt%) Zeta potential absolute value

2 wt% 31

4 wt% 34

0

0.5

1

1.5

2

2.5

3

1 2 3 4 5 6 7 8 9

Av

erag

e su

rfac

e ro

ug

hn

ess

(µm

)

Cutting test no.

Fig. 2 Average surface roughness results

Table 5 ANOVA results for average surface roughness (MWCNTsnano-fluid)

Source Statistical sum Variance F (calculated) P value

A 0.1049 0.0524 0.664 0.608

*B 2.8528 1.4264 18.059 0.052

*C 1.4131 0.7065 8.94 0.1

Error 0.1579 0.0789

Total 4.5289

*Significant at 90% confidence level

0

1

2

3

4

5

6

7

A1 A2 A3 B1 B2 B3 C1 C2 C3

Sum

mat

ion o

f av

erag

e su

rfac

e ro

ughnes

s at

cer

tain

level

Design variables levels

Fig. 3 The plot of control variables effects on the measured surfacequality

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The cooling and lubrication properties of the nano-fluidsimprove the rake and flank regions’ lubrication and wettingproperties so that better heat dissipation could be achieved.Consequently, partially smoothed cutting processes is accom-plished and better average surface roughness values are ob-served in comparison with the processes performed withoutany nano-additives as discussed previously [40, 41]. In addi-tion, applying an appropriate lubrication and cooling systemto the tool-workpiece interface area provides a viable role toreduce the coefficient of friction. Introducing the nano-cuttingfluid system would decrease the induced friction and the ma-chined surface roughness. This is mainly attributed to the

significant tribological properties of the resultant nano-mistwhich decreases the friction at the tool-chip interface. Also,the nano-additives work as a spacers reducing the rubbingbetween the cutting tool and workpiece, as discussed previ-ously [42]. The schematic of the lubrication/cooling system(MQL nano-fluid) is provided in Fig. 5. It can be seen that thenano-cutting fluid is atomized into the MQL nozzle and itresults in a very fine mist. The resultant mist represents thenano-additives surrounded by a thin base fluid film. Thus, thedroplets of the nano-cutting-fluid are formed on the workpieceand cutting tool surfaces and a tribo-film is also formed (seeFig. 5), which significantly enhances the tribological charac-teristics and reduces the induced friction.

In addition, increasing the added percentage of nano-additives results in higher thermal conductivity for the resul-tant nano-fluid and improves the heat transfer performanceaccordingly. However, in the current study, 2 wt%MWCNTs nano-fluid offers better cutting performance thanusing 4 wt%. This is mainly attributed to the plowing mech-anism (see Fig. 5). When there is an abundance of nano-

Table 6 ANOVA verification results

Optimal surface roughness values (μm) Accuracy %

Experimental value Predicted value

0.51 0.45 89.3%

Fig. 4 3-D surface plots for the surface roughness model

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additives in the resultant nano-fluid, they collide with and areimpeded by the asperities on the work surface and hence gen-erate stronger cutting forces. As a result, the nano-additiveinduced wear is increased with increasing the nano-additives

concretion (see Fig. 6) as similarly discussed in a previouswork [43]. As a result, the resultant flank tool wear wouldincrease and accordingly would affect the surface quality.Also, as can be seen in Fig. 5, increasing the nano-additives

Nozzle

MQL nano-mist

Cutting tool

Workpiece

Base fluid film

Nano-additive

Thin exfoliated film

Ploughed off nano-

additives

Fig. 5 Schematic of the rolling and plowing MQL nano-cutting fluid mechanism

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0

4E-07

8E-07

1.2E-06

1.6E-06

2E-06

0 2 4 6 8

Coff

icie

nt

of

fric

tion

Ind

uce

d n

ano

-ad

dit

ive

wea

r p

er s

lid

ing

dis

tan

ce

(m2)

Nano-additive volume concentration %

Nano-additive wear

Coefficient of friction

Fig. 6 Coefficient of friction and induced wear versus nano-additives content [43]

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concentration means more nano-additives in the workpiece-interface area, and therefore the resultant friction would de-crease (see Fig. 6). Thus, the nano-additives concentrationshould be carefully selected and optimized in order to strikea balance between all previous considerations.

Zeta potential index expresses the repulsive forceamong the nano-additives, and it is considered as a worthydispersion stability indicator [44, 45]. Thus, higher valuesof zeta potential index mean the employed nano-fluid pre-sents better performance, which could prevent any possi-ble obstacles, such as nano-additives agglomeration, clog-ging, or sedimentation. In terms of the measured zeta po-tential results, 2 wt% MWCNTs nano-fluid offers highervalues than 4 wt% MWCNTs nano-fluid as previouslypresented in Table 4, which attributes to the better im-provements obtained when using 2 wt% MWCNTs.

In order to obtain the nano-cutting fluids significant im-provements on the cutting performance, understanding theMQL nano-fluid mechanisms is highly important. The MQLnano-fluid mechanism has been summarized as follows:

& By applying a source of compressed air, the nano-cuttingfluid is atomized into the MQL nozzle and it results in avery fine mist.

& The resultant mist represents the nano-additivessurrounded by a thin base fluid film. This mist could pen-etrate the cutting zone as its velocity is significantly largerthan cutting tool velocity since it can pass through the toolpores and grain fractured groves.

& Thus, the droplets of the nano-cutting-fluid are formed onthe workpiece and cutting tool surfaces and a tribo-film isalso formed which significantly enhances the tribologicalcharacteristics and reduces the induced friction [46–48].Also, these droplets have an important role in dissipatingthe generated heat because of the improved thermal con-ductivity of the resultant mist.

& Due to the increasing of nano-additive concentration,which increases the number of nano-additives at thetool-workpiece interface; these nano-additives perform avital role as spacers, eliminating the contact between thetool and workpiece as previously discussed [46, 49].

& Due to the high compression with increasing the nano-additives concertation, the nano-additive shape is changed

and the shearing would be more intense (see Fig. 7). Someof the nano-additives are partially ejected by other addi-tives that left the nozzle into the cutting zone as has beensimilarly discussed in a previous work [47].

& Therefore, due to the nano-additive extreme pressure inthe resultant nano-cutting fluid and existence of a gapbetween tool–workpiece interfaces, nano-additives pro-vide high contact resistance which helps in forming achemical reaction film on the workpiece surface (seeFig. 7). The increase of nano-additives concentration in-creases the growth of the thin protective film on the ma-chined surface. The thin protective film also enhances theheat transfer performance since it prevents dissipating thegenerated heat into the cutting tool and workpiece.

Also, the deformed chip thickness was measured for differ-ent cases during cutting Ti-6Al-4V alloy with/withoutMWCNTs nano-fluid, and the results were plotted at differentfeed rate levels as provided in Fig. 8. The deformed chipthickness has been measured by analysis of micrographs cap-tured using a digital optical microscope (KEYENCE VHX-1000) and an ImageJ software under different feed rate levels.The cutting tests performed without nano-additives showed

A protective nano-additive thin film Cutting tool

Workpiece

Fig. 7 The formation of a protective nano-additives thin film

0.1

0.12

0.14

0.16

0.18

0.2

0.22

0 0.1 0.2 0.3 0.4 0.5

Def

orm

ed c

hip

th

ick

nes

s (m

m)

Feed rate (mm/rev)

Without nanoadditves

MWCNT - 2 wt.%

MWCNT - 4 wt.%

Fig. 8 Effect of feed rate on the deformed chip thickness duringmachining of Ti-6Al-4V with/without MWCNTs nano-fluids

Int J Adv Manuf Technol

the highest chip thickness due to the chip welding tendency tothe top surface layers of the tool rake face. Based on the resultsprovided in Fig. 8, lower deformed chip thickness has beenobtained when using either 4 wt% or 2 wt% MWCNTs. It ismainly attributed to the effective cooling and lubrication prop-erties upon usingMQL nano-fluid, which help in reducing theseverity of the chip welding tendency. The chip thicknessreduction would lead to shorter shear plane and larger shearangle. Thus, lower cutting force [50] and induced frictioncoefficient [51] would occur as similarly discussed in previousstudies. Furthermore, the chip breakability mechanism wouldbe enhanced [52]. On the other hand, the chip thickness resultsshowed that 4 wt% offered lower deformed thickness than2 wt%; however, 2 wt% offered better surface quality. Thus,studying and analyzing the nano-cutting fluid tribological andheat transfer mechanisms are required in order to physicallyemphasize this concern, and understand the machining pro-cesses with MQL nano-fluid.

Also, some micrographs have been captured for the gener-ated chips with/without MWCNT nano-fluids. Regarding thetests performed without nano-additives, serrated chips or saw-tooth-like appearance (see Fig. 9a) are generated due to thenon-uniform strain on the material during the cutting process(i.e., existence of low and high shear strain zones). In addition,that is usually observed when machining low thermal conduc-tivity materials, such as Ti-6Al-4V. In the majority of cuttingtests done using MWCNT nano-fluids, wider saw-toothspaces have been observed compared with the tests performedwithout nano-additives as shown in Figs. 9b, c. It is mainlyattributed to the formation of a hydrodynamic layer betweenthe chip and tool rake face, which enhances the heat dissipa-tion performance. During cutting test 7, the non-homogenousserrated chips have not been observed as shown in Fig. 9d,and it could be attributed to employing a higher percentage ofadded MWCNTs (i.e., 4 wt%), which increases the inducedheat convection coefficient and improves the heat dissipationperformance.

Finally, it can be concluded that the proper dispersion ofnano-additives into vegetable oil using theMQL system could

achieve two main important objectives: enhancing the ma-chining quality characteristics as nano-additives improve thethermal and friction behavior, and accomplishing a sustain-able process as using vegetable oils based on MQL provideeffective environmental benefits.

4 Conclusions

Minimum quantity lubricant (MQL) has been successfullydemonstrated as an acceptable coolant strategy; however, itsheat capacity is much lower than the one achieved using floodcoolant. Adding nano-additives is one of the most effectivetechniques to increase the heat capacity of the MQL systemsas they enhance the convection, conduction, and wettabilitycharacteristic of the resultant nano-mist. The focus of thisresearch was mainly to study the effects of dispersed multi-wall carbon nanotubes (MWCNT) into the conventional fluid(vegetable oil) under MQL during bar turning of Ti-6Al-4Valloy. MWCNTs fluid with MQL showed promising results incomparison with the cutting tests performed without nano-additives. ANOVA results showed that percentage of addednano-additives and feed rate are the signification design vari-ables affecting the surface quality. A mathematical model hasbeen developed to express the measured surface roughnessbased on the studied design variables range. An acceptableaverage model accuracy has been obtained. Also, the devel-oped model trends were in agreement with the findings pre-sented in ANOVA results. The combination of MQL nano-fluid at cutting speed of 120 m/min, feed rate of 0.1 mm/rev,and 2 wt% added MWCNTs showed the best machined sur-face quality, while test 6 which includes cutting speed of170 m/min and feed rate of 0.2 mm/rev using classical MQLprovided the worst surface roughness. Due to the improvedcooling and lubrication properties of the resultant nano-mist,the tool wear behavior has been significantly enhanced be-cause of the tribo film formation along the chip-tool interfacezone and accordingly better surface quality has been accom-plished. Also, lower chip thickness has been observed upon

Fig. 9 Micrographs of the generated chips; a without nano-additives, b at cutting test 2, c at cutting test 4, and d at cutting test 7

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using MWCNTs nano-fluid which confirms the superiorcooling and lubrication properties of the employed nano-fluid.Finally, additional investigations are still required in order tounderstand the tribological and heat transfer mechanismswhen machining with nano-fluids.

Funding information The authors acknowledge the support of theNatural Sciences and Engineering Research Council of Canada(NSERC) and the International Scientific Partnership Program ISPP atKing Saud University for funding this research work through ISPP#0059.

Publisher’s Note Springer Nature remains neutral with regard to jurisdic-tional claims in published maps and institutional affiliations.

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