**2.1. Thermal properties of nanofluids**

such as ISO 14000 and Green Round, have become much stricter for promoting green manufacturing approaches. The necessity of reducing environmental loads should be increasingly con-

According to the statistics of cost distribution in manufacturing shown in **Figure 1**, the coolant expense for usage and disposal represents about 15% of total production costs, depending on the workpart and the types of cooling system, as well as machining location [3]. In contrast, tooling cost contributes only a small value of 4%. On the other hand, health and environmental issues associated with the airborne cutting fluid particles on factory shop floors motivate manufacturing enterprises to drastically reduce coolant consumption and, if possible, elimi-

As a result, the conception of dry cutting has been first considered to achieve environmental friendliness. Eliminating the cutting fluids in machining processes means that there is no cooling lubricating media, which has three essential functions (i.e., reduction of friction, absorption of the generated heat, and chip evacuation). Hence, these following problems must be

• Workpart: deteriorate surface texture and need additional works (cleaning or deburring) • Cutting tool: difficulty in chip formation, reduction of tool life or change to expensive ones • Machine tool: high rigidity, equipment specialized in pushing chips away from the cutting

Especially in cases of machining difficult-to-machine materials like high-strength and highhardness steels, solving the mentioned problems has strong influence on leading industrial branches as automotive, roller bearing, hydraulic, and die and mold sectors. The term "hard machining" is a recent technology that can be defined as the machining operation of a workpiece that has a hardness value typically in the 45–70 HRC range, using directly tools with geometrically defined cutting edges [5]. Hard-cutting operations are capable of replacing, in some cases, grinding operations and produce comparable surface finish. Various machining operations in hard machining include milling, boring, broaching, hobbling, and others. Together

sidered, and many green manufacturing processes have been developed and studied.

nate it altogether.

162 Microfluidics and Nanofluidics

considered:

zone as well as controlling temperature

**Figure 1.** Distribution of manufacturing costs for wet machining [2].

From previous investigations, nanofluids have been found to possess enhanced thermophysical properties such as thermal conductivity, thermal diffusivity, viscosity, and convective heat transfer coefficients. The thermal conductivity of more than 50 various nanofluids based on water, ethylene glycol, and engine oil containing particles of SiO<sup>2</sup> , Al<sup>2</sup> O3 , TiO<sup>2</sup> , ZrO<sup>2</sup> , CuO, and diamond was experimentally measured [8]. The obtained results had shown that the thermal conductivity coefficient of nanofluids enhances with increasing particle sizes. It has been confirmed that the lower the thermal conductivity of the base fluid, the higher the relative thermal conductivity coefficient of the nanofluids. The different researches have made to investigate convective heat transfer of nanofluids [10]. Based on the results, augmenting nanofluid volume fraction, Rayleigh and Magnetic numbers lead to improve of the temperature gradient, while it reduces with augment of Lorentz forces. Heat transfer improvement augments with increase in Kelvin forces, while it reduces with augment of Lorentz forces at high Rayleigh number, but different manners are detected for low Rayleigh number. The active method for nanofluid heat transfer enhancement by means of EHD was also studied [10]. The obtained results suggest that influence of electric field on forced convection improvement is more sensible for lower *Re* number. Temperature gradient enhances with rise of voltage supply. Moreover, throughout the experimental results, the convective heat transfer increases with the presence of nanoparticles in the base fluids [11, 12, 21, 27]. Based on the newest publications, the deeper understanding about thermal properties of NFs is studied. The shapes of NPs are proven to influence on the rate of heat transfer, and the effect of thermal radiation on CuO nanofluid behavior is successfully modeled via Control Volume–based Finite Element Method (CVFEM). Platelet shape nanoparticles reveal the highest heat transfer rate [22, 25, 28, 29]. Nanofluid motion, as well as flow circulation and thermal energy transport, enhances by increasing the volume fraction of NPs [23, 24, 26]. A novel research of melting temperature of CuO-water NF heat transfer enhancement is simulated by CVFEM. The highlighted results include (1) flow velocity of NF increases due to the presence of CuO nanoparticles; (2) heat transfer enhancement of NF improves at higher nanoconcentrations; (3) melting temperature rises with the increment of nanovolume fraction [30, 31, 33]. The same observations were obtained from the study of Fe<sup>3</sup> O4 -water nanofluid [32, 34]. They contribute a very good understanding of nanofluid behavior as cutting fluid in various cutting processes.

nose. Dry turning of AISI 304 stainless steel (85 HRB) at v = 75÷265 m/min; f = 0.1÷0.3 mm/rev; a<sup>p</sup> = 0.8÷1.6 mm with two different groups of inserts: uncoated and TiN coated carbide inserts (rake angle γ<sup>o</sup> = 15°, relief angle α<sup>o</sup> = 8°, inclination angle λs = − 4°, and side cutting-edge angle

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091 165

**Figure 2.** Machining operations and their needs for cooling and lubricating functions [3].

From **Figure 3**, the cutting temperatures of two kinds of cutting tools increase with increasing cutting speed. The reason was that frictional heat generated from the contact zone of the bottom of chip and tool rake face was too late to transfer and was accumulated at the bottom of chip. Therefore, the cutting temperature increased. The comparison of cutting temperature is made among dry, wet, and MQL turnings of AISI 4140 steel (340 HV) at v = 50.2÷141.4 m/min; f = 0.09÷0.22 mm/rev; and ap = 0.5÷1.5 mm with HSS tools [15]. The tool-chip interface temperature in which MQL fluid is supplied from both nozzles to the rake and flank faces is approximately

**Figure 3.** Evolution of cutting temperature with cutting speed in dry turning of AISI 304 stainless steel [14].

k<sup>r</sup> = 75°) [14].

#### **2.2. The effects on cutting temperature**

The cutting fluid can be useless if not delivered efficiently to contact zone, and so the methods of supplying the coolant in machining are the critical parameter. However, the effectiveness of supplying cutting fluids in wet cutting can help to dissipate relatively small amount of the generated heat. It is well known that only very small amount of cutting fluid can penetrate to contact zone although large amount is delivered. On the other hand, costs, as well as health and environmental issues, motivate manufacturing enterprises to drastically reduce consumption of cooling fluids.

Dry machining processes face the serious difficulties in heat dissipation and chip transportation though eliminating the use of cutting fluids. From **Figure 2**, nongeometrically defined machining processes, such as grinding, honing, etc., are considered cooling function the most important factor. When some of these processes can be replaced by geometrically defined hard machining methods (for instance hard turning, hard milling), successful machining with minimization or without fluids can be achieved [13].

Stainless steel, for instance, belongs to the difficult machining material, which is easy to stick tool leading to increasing the cutting temperature and intensifying the abrasion of the tool

**Figure 2.** Machining operations and their needs for cooling and lubricating functions [3].

conductivity coefficient of nanofluids enhances with increasing particle sizes. It has been confirmed that the lower the thermal conductivity of the base fluid, the higher the relative thermal conductivity coefficient of the nanofluids. The different researches have made to investigate convective heat transfer of nanofluids [10]. Based on the results, augmenting nanofluid volume fraction, Rayleigh and Magnetic numbers lead to improve of the temperature gradient, while it reduces with augment of Lorentz forces. Heat transfer improvement augments with increase in Kelvin forces, while it reduces with augment of Lorentz forces at high Rayleigh number, but different manners are detected for low Rayleigh number. The active method for nanofluid heat transfer enhancement by means of EHD was also studied [10]. The obtained results suggest that influence of electric field on forced convection improvement is more sensible for lower *Re* number. Temperature gradient enhances with rise of voltage supply. Moreover, throughout the experimental results, the convective heat transfer increases with the presence of nanoparticles in the base fluids [11, 12, 21, 27]. Based on the newest publications, the deeper understanding about thermal properties of NFs is studied. The shapes of NPs are proven to influence on the rate of heat transfer, and the effect of thermal radiation on CuO nanofluid behavior is successfully modeled via Control Volume–based Finite Element Method (CVFEM). Platelet shape nanoparticles reveal the highest heat transfer rate [22, 25, 28, 29]. Nanofluid motion, as well as flow circulation and thermal energy transport, enhances by increasing the volume fraction of NPs [23, 24, 26]. A novel research of melting temperature of CuO-water NF heat transfer enhancement is simulated by CVFEM. The highlighted results include (1) flow velocity of NF increases due to the presence of CuO nanoparticles; (2) heat transfer enhancement of NF improves at higher nanoconcentrations; (3) melting temperature rises with the increment of nanovolume fraction [30, 31, 33]. The same observations were


obtained from the study of Fe<sup>3</sup>

164 Microfluidics and Nanofluidics

consumption of cooling fluids.

minimization or without fluids can be achieved [13].

**2.2. The effects on cutting temperature**

O4

standing of nanofluid behavior as cutting fluid in various cutting processes.

The cutting fluid can be useless if not delivered efficiently to contact zone, and so the methods of supplying the coolant in machining are the critical parameter. However, the effectiveness of supplying cutting fluids in wet cutting can help to dissipate relatively small amount of the generated heat. It is well known that only very small amount of cutting fluid can penetrate to contact zone although large amount is delivered. On the other hand, costs, as well as health and environmental issues, motivate manufacturing enterprises to drastically reduce

Dry machining processes face the serious difficulties in heat dissipation and chip transportation though eliminating the use of cutting fluids. From **Figure 2**, nongeometrically defined machining processes, such as grinding, honing, etc., are considered cooling function the most important factor. When some of these processes can be replaced by geometrically defined hard machining methods (for instance hard turning, hard milling), successful machining with

Stainless steel, for instance, belongs to the difficult machining material, which is easy to stick tool leading to increasing the cutting temperature and intensifying the abrasion of the tool nose. Dry turning of AISI 304 stainless steel (85 HRB) at v = 75÷265 m/min; f = 0.1÷0.3 mm/rev; a<sup>p</sup> = 0.8÷1.6 mm with two different groups of inserts: uncoated and TiN coated carbide inserts (rake angle γ<sup>o</sup> = 15°, relief angle α<sup>o</sup> = 8°, inclination angle λs = − 4°, and side cutting-edge angle k<sup>r</sup> = 75°) [14].

From **Figure 3**, the cutting temperatures of two kinds of cutting tools increase with increasing cutting speed. The reason was that frictional heat generated from the contact zone of the bottom of chip and tool rake face was too late to transfer and was accumulated at the bottom of chip. Therefore, the cutting temperature increased. The comparison of cutting temperature is made among dry, wet, and MQL turnings of AISI 4140 steel (340 HV) at v = 50.2÷141.4 m/min; f = 0.09÷0.22 mm/rev; and ap = 0.5÷1.5 mm with HSS tools [15]. The tool-chip interface temperature in which MQL fluid is supplied from both nozzles to the rake and flank faces is approximately

**Figure 3.** Evolution of cutting temperature with cutting speed in dry turning of AISI 304 stainless steel [14].

350°C lower than that in dry turning, and if supplied only to rake face, the tool temperature is about 200°C lower than that in dry turning. Additionally, the tool-chip interface temperature in wet turning is about 300°C lower than that in dry turning. The difference in cutting temperatures under dry, wet, and MQL conditions is closely related to the difference in cutting forces. The greater the cutting forces, the more heat and higher cutting temperatures are generated. Accordingly, the application of dry cutting processes is limited. It can also be observed that MQL techniques effectively provide oil mist directly to cutting zone to improve lubricant characteristics, but the main drawback of this technique is cooling character. It has a significant meaning for machining hard materials with the hardness range of 45 ÷ 70 HRC. Therefore, the application of nanofluids in MQL machining, an up to date research topic, brings out a novel substitution for dry and wet cutting, as well as the development of semi-dry machining (MQL technique).

The thermal conductivity of nanofluids has been found to be higher than that of the base fluid by using KD2 Pro Thermal analyzer to measure at room temperature (25°C) to note down the increased conductivity value (seen in **Figure 4**) [16]. It is also observed that the thermal conductivity of nanofluids enhances when increasing the nanoparticle concentration.

The comparison of six types of nanoparticles, namely molybdenum disulfide (MoS<sup>2</sup> ), zirconium dioxide (ZrO<sup>2</sup> ), carbon nanotube (CNT), polycrystalline diamond, aluminum oxide (Al<sup>2</sup> O3 ), and silica dioxide (SiO<sup>2</sup> ), mixed with palm oil to formulate nanofluids is made and used for MQL grinding of Ni-based alloys [17]. The grinding temperatures of six nanofluids are shown in **Figure 5**.

From **Figure 6**, it can be clearly seen that six different nanofluids help to effectively reduce MQL grinding temperature compared to the base fluid due to the presence of nanoparticles with hard property, as well as good heat transfer. The use of nanoparticles has a significant meaning in improving cooling and lubricating characteristics. CNT nanofluid shows the best cooling performance, presumably it has good heat transfer properties. On the other hand, the viscosity of cutting fluids is an important influencing factor of lubrication performance. **Figure 7** shows the relationship between six different nanofluids' viscosity and temperature.

**Figure 5.** Grinding temperature of six nanofluids with respect to a dimensionless grinding distance *x/l* [17].

The viscosity of all nanofluids decreases with the rise of temperature, especially before 70°C. SiO<sup>2</sup>

**Figure 6.** Grinding temperatures of six different nanofluids [17].

, and CNT nanofluids have higher viscosity than other ones. High viscosity allows the cutting fluids to stay in the cutting area for a longer time. This phenomenon improves the cooling

Al<sup>2</sup> O3 ,

167

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091

It can be clearly observed from **Figure 5** that the grinding temperatures sharply increase at the initiation of grinding process but decrease gradually to reach a stable temperature when six different nanofluids are supplied to contact zone.

**Figure 4.** Thermal conductivity variation for silver and zinc oxide nanofluids with different volume fractions [16].

350°C lower than that in dry turning, and if supplied only to rake face, the tool temperature is about 200°C lower than that in dry turning. Additionally, the tool-chip interface temperature in wet turning is about 300°C lower than that in dry turning. The difference in cutting temperatures under dry, wet, and MQL conditions is closely related to the difference in cutting forces. The greater the cutting forces, the more heat and higher cutting temperatures are generated. Accordingly, the application of dry cutting processes is limited. It can also be observed that MQL techniques effectively provide oil mist directly to cutting zone to improve lubricant characteristics, but the main drawback of this technique is cooling character. It has a significant meaning for machining hard materials with the hardness range of 45 ÷ 70 HRC. Therefore, the application of nanofluids in MQL machining, an up to date research topic, brings out a novel substitution for dry and wet cutting, as well as the development of semi-dry machining (MQL technique).

The thermal conductivity of nanofluids has been found to be higher than that of the base fluid by using KD2 Pro Thermal analyzer to measure at room temperature (25°C) to note down the increased conductivity value (seen in **Figure 4**) [16]. It is also observed that the thermal con-

used for MQL grinding of Ni-based alloys [17]. The grinding temperatures of six nanofluids

It can be clearly observed from **Figure 5** that the grinding temperatures sharply increase at the initiation of grinding process but decrease gradually to reach a stable temperature when six

**Figure 4.** Thermal conductivity variation for silver and zinc oxide nanofluids with different volume fractions [16].

), carbon nanotube (CNT), polycrystalline diamond, aluminum oxide

), mixed with palm oil to formulate nanofluids is made and

), zir-

ductivity of nanofluids enhances when increasing the nanoparticle concentration.

conium dioxide (ZrO<sup>2</sup>

166 Microfluidics and Nanofluidics

are shown in **Figure 5**.

), and silica dioxide (SiO<sup>2</sup>

different nanofluids are supplied to contact zone.

(Al<sup>2</sup> O3

The comparison of six types of nanoparticles, namely molybdenum disulfide (MoS<sup>2</sup>

**Figure 5.** Grinding temperature of six nanofluids with respect to a dimensionless grinding distance *x/l* [17].

From **Figure 6**, it can be clearly seen that six different nanofluids help to effectively reduce MQL grinding temperature compared to the base fluid due to the presence of nanoparticles with hard property, as well as good heat transfer. The use of nanoparticles has a significant meaning in improving cooling and lubricating characteristics. CNT nanofluid shows the best cooling performance, presumably it has good heat transfer properties. On the other hand, the viscosity of cutting fluids is an important influencing factor of lubrication performance. **Figure 7** shows the relationship between six different nanofluids' viscosity and temperature.

The viscosity of all nanofluids decreases with the rise of temperature, especially before 70°C. SiO<sup>2</sup> , Al<sup>2</sup> O3 , and CNT nanofluids have higher viscosity than other ones. High viscosity allows the cutting fluids to stay in the cutting area for a longer time. This phenomenon improves the cooling

**Figure 6.** Grinding temperatures of six different nanofluids [17].

**Figure 7.** Relationship between nanofluids' viscosity and temperature [18].

lubrication of the contact area. In addition, the use of vegetable oils as the base nanofluids not only improves their cooling, lubricating, and viscous characteristics but also is the step toward sustainable manufacturing.

#### **2.3. The effects on cutting forces**

The tribological characteristic of cutting fluids has a significant meaning for investigating cutting forces. The nanofluids' tribological characteristics are improved by using Al<sup>2</sup> O3 and TiO<sup>2</sup> nanomaterials. The kinematic viscosity of Al<sup>2</sup> O3 and TiO<sup>2</sup> nanolubricants decreased slightly due to the presence of nanoparticles between the lubricant layers leading to an ease of relative movement with the nanoparticles acting as "rollers." On the other hand, the viscosity index increased with the use of nanolubricants [9]. Accordingly, nanoparticles in MQL fluid play an important role in converting sliding into rolling contact. That is the reason why the friction coefficient in cutting zone is much reduced, and the cutting temperature, cutting forces, and tool wear decrease. MQL hard milling of 60Si<sup>2</sup> Mn steel (50÷52HRC) was done by using Al<sup>2</sup> O3 nanofluid (0.5 wt%) with carbide inserts at v = 110 m/min; f = 0.12 mm/tooth; ap = 0.2 mm [19]. The cutting forces were directly measured during cutting process by Kistler quartz, threecomponent dynamometer (9257BA). **Figures 8**–**10** show the cutting force components Fx , Fy , and Fz of MQL hard milling process with/without Al<sup>2</sup> O3 nanoparticles. It is clearly observed that, compared to the case of MQL fluids without nanoparticles, all the cutting force components are much reduced when cutting with nanofluids. Interestingly, it is revealed that during the first 20 min, all the cutting forces Fx , Fy , and Fz in both cases are low; therefore, in this time, the performance of Al<sup>2</sup> O3 nanoparticles in MQL hard milling is not really clear. After the first period, the rapid tool wear occurs, and wear land reaches some extent, which allows nanoparticles to penetrate to cutting zone. The formulation of oil film with nanoparticles in contact zone plays an important role in creating "roller effect." Rolling friction instead of sliding one occurs between flank face and machined surface, rake face and chip surface, and so forth. Hence, the cutting forces significantly reduce and the tool life extends much.

The use of soybean oil–based nanofluids in MQL hard milling was less effective than that of emulsion. However, both of them allow the normal APMT 1604 PDTR LT30 carbide inserts to use effectively for hard milling, and the economic and technological characteristics of cutting performance are achieved. Another promising research investigated lubrication properties of the wheel/workpiece interface in MQL nanofluids grinding compared with flood and MQL grinding without nanoparticles. The experiments were conducted at wheel speed *vs* = 30 m/s, feed speed *vw* = 3000 mm/min, and cutting depth *ap* = 10 μm for machining the high-temperature nickel base alloy GH4169 [18]. **Figures 11** and **12** show

under MQL conditions with/without nanofluids [19].

under MQL conditions with/without nanofluids [19].

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091 169

the grinding forces obtained.

**Figure 9.** Cutting force component Fy

**Figure 8.** Cutting force component Fx

**Figure 8.** Cutting force component Fx under MQL conditions with/without nanofluids [19].

lubrication of the contact area. In addition, the use of vegetable oils as the base nanofluids not only improves their cooling, lubricating, and viscous characteristics but also is the step toward

The tribological characteristic of cutting fluids has a significant meaning for investigating cut-

O3

due to the presence of nanoparticles between the lubricant layers leading to an ease of relative movement with the nanoparticles acting as "rollers." On the other hand, the viscosity index increased with the use of nanolubricants [9]. Accordingly, nanoparticles in MQL fluid play an important role in converting sliding into rolling contact. That is the reason why the friction coefficient in cutting zone is much reduced, and the cutting temperature, cutting forces, and

nanofluid (0.5 wt%) with carbide inserts at v = 110 m/min; f = 0.12 mm/tooth; ap = 0.2 mm [19]. The cutting forces were directly measured during cutting process by Kistler quartz, threecomponent dynamometer (9257BA). **Figures 8**–**10** show the cutting force components Fx

that, compared to the case of MQL fluids without nanoparticles, all the cutting force components are much reduced when cutting with nanofluids. Interestingly, it is revealed that during

period, the rapid tool wear occurs, and wear land reaches some extent, which allows nanoparticles to penetrate to cutting zone. The formulation of oil film with nanoparticles in contact zone plays an important role in creating "roller effect." Rolling friction instead of sliding one occurs between flank face and machined surface, rake face and chip surface, and so forth.

, and Fz

, Fy

Hence, the cutting forces significantly reduce and the tool life extends much.

and TiO<sup>2</sup>

O3

nanoparticles in MQL hard milling is not really clear. After the first

O3

nanolubricants decreased slightly

nanoparticles. It is clearly observed

in both cases are low; therefore, in this time,

Mn steel (50÷52HRC) was done by using Al<sup>2</sup>

and TiO<sup>2</sup>

O3

, Fy ,

ting forces. The nanofluids' tribological characteristics are improved by using Al<sup>2</sup>

sustainable manufacturing.

168 Microfluidics and Nanofluidics

and Fz

**2.3. The effects on cutting forces**

nanomaterials. The kinematic viscosity of Al<sup>2</sup>

**Figure 7.** Relationship between nanofluids' viscosity and temperature [18].

tool wear decrease. MQL hard milling of 60Si<sup>2</sup>

the first 20 min, all the cutting forces Fx

O3

the performance of Al<sup>2</sup>

of MQL hard milling process with/without Al<sup>2</sup>

**Figure 9.** Cutting force component Fy under MQL conditions with/without nanofluids [19].

The use of soybean oil–based nanofluids in MQL hard milling was less effective than that of emulsion. However, both of them allow the normal APMT 1604 PDTR LT30 carbide inserts to use effectively for hard milling, and the economic and technological characteristics of cutting performance are achieved. Another promising research investigated lubrication properties of the wheel/workpiece interface in MQL nanofluids grinding compared with flood and MQL grinding without nanoparticles. The experiments were conducted at wheel speed *vs* = 30 m/s, feed speed *vw* = 3000 mm/min, and cutting depth *ap* = 10 μm for machining the high-temperature nickel base alloy GH4169 [18]. **Figures 11** and **12** show the grinding forces obtained.

**Figure 10.** Cutting force component Fz under MQL conditions with/without nanofluids [19].

**2.4. The effects on tool wear and tool life**

spherical morphology of Al<sup>2</sup>

Hard milling process of 60Si<sup>2</sup>

cutting temperature.

(Al<sup>2</sup> O3

With the presence of nanoparticles between rake face and fresh chip, as well as flank face and machined surface, the mechanism of the tribological effect takes many forms, such as "roller effect," third body effect, chemical mechanical protective film effect, mending effect, and polishing effect [2, 3]. For instance, during gear hobbing process of AISI 4118 steel (spindle speed = 200 rev/min, depth of cut =4.375 mm, feed rate = 1.27 mm/rev), using nanofluid

**Figure 12.** Specific normal sliding grinding forces in the cases of flood, pure MQL, and nanofluids.

significant increment in tool life when machining under nanolubrication.

O3

On the other hand, the tool wear is much reduced under nanolubrication, which leads to achieve higher gear profile accuracy (shown in **Figure 15**). It could be said that nanolubrication is the main factor contributed to preserve the tool profile accuracy. In addition, the

Mn steel (50÷52HRC) was done by using Al<sup>2</sup>

with carbide inserts at v = 110 m/min; ft = 0.12 mm/tooth; and ap = 0.2 mm. **Figures 16** and **17** illustrate the difference of tool wear between MQL hard milling with nanofluids and pure MQL. In **Figure 16**, the wear on cutting edge including rake and flank faces is dominant.

nanoparticles takes part in the decrease of friction force and

O3

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091 171

nanofluid (0.5wt %)

 with the size 80 nm suspended in ISO VG46 lubricant oil with volume fraction of 0.1÷ 0.2%) shows many promising results. Nanoparticles in the base oil effectively improve the heat transfer capability and reduce the friction by "roller effect" in cutting zone, leading to the reduction of tool wear, the much extension of tool life, and the enhancement of gear profile accuracy and gear surface roughness [20]. **Figures 13** and **14** show the flank wear of hob tools at different time of machining the 50th, 300th gears. It is clearly seen that during the period of machining first 50 gears, gear hobbing under nanolubrication exhibits the reduction of tool wear, but at the period of machining the 300th gear, the significant reduction of hob wear is observed. Moreover, the wear land at the time of machining the 300th gear with nanolubrication (39.93 μm) is nearly equivalent to that of the tool at the period of machining the 50th gear under flood lubrication (36.29 μm). Accordingly, it is clarified that there is a

**Figure 11.** Specific tangential sliding grinding forces in the cases of flood, pure MQL, and nanofluids.

Both normal and tangential grinding forces increased with an increased number of passes. The flood grinding process shows the largest sliding grinding force. MQL grinding with pure palm oil achieves smaller sliding force because it effectively increases the lubrication effect of the grinding area due to oil-mist formation. Nanofluids are superior to pure palm oil in lubrication improvement. When making the comparison among six different nanofluids, the sliding grinding forces under MQL nanofluid (Al<sup>2</sup> O3 , MoS<sup>2</sup> , SiO<sup>2</sup> , and ND) are lowest due to hard characteristic and small sliding friction coefficient. The effectiveness of nanofluids on reduction of cutting forces becomes a novel observation and has an important influence on tool wear and tool life, which directly affect the surface quality and manufacturing cost.

**Figure 12.** Specific normal sliding grinding forces in the cases of flood, pure MQL, and nanofluids.

#### **2.4. The effects on tool wear and tool life**

Both normal and tangential grinding forces increased with an increased number of passes. The flood grinding process shows the largest sliding grinding force. MQL grinding with pure palm oil achieves smaller sliding force because it effectively increases the lubrication effect of the grinding area due to oil-mist formation. Nanofluids are superior to pure palm oil in lubrication improvement. When making the comparison among six different

under MQL conditions with/without nanofluids [19].

**Figure 11.** Specific tangential sliding grinding forces in the cases of flood, pure MQL, and nanofluids.

are lowest due to hard characteristic and small sliding friction coefficient. The effectiveness of nanofluids on reduction of cutting forces becomes a novel observation and has an important influence on tool wear and tool life, which directly affect the surface quality and

O3 , MoS<sup>2</sup>

, SiO<sup>2</sup>

, and ND)

nanofluids, the sliding grinding forces under MQL nanofluid (Al<sup>2</sup>

manufacturing cost.

**Figure 10.** Cutting force component Fz

170 Microfluidics and Nanofluidics

With the presence of nanoparticles between rake face and fresh chip, as well as flank face and machined surface, the mechanism of the tribological effect takes many forms, such as "roller effect," third body effect, chemical mechanical protective film effect, mending effect, and polishing effect [2, 3]. For instance, during gear hobbing process of AISI 4118 steel (spindle speed = 200 rev/min, depth of cut =4.375 mm, feed rate = 1.27 mm/rev), using nanofluid (Al<sup>2</sup> O3 with the size 80 nm suspended in ISO VG46 lubricant oil with volume fraction of 0.1÷ 0.2%) shows many promising results. Nanoparticles in the base oil effectively improve the heat transfer capability and reduce the friction by "roller effect" in cutting zone, leading to the reduction of tool wear, the much extension of tool life, and the enhancement of gear profile accuracy and gear surface roughness [20]. **Figures 13** and **14** show the flank wear of hob tools at different time of machining the 50th, 300th gears. It is clearly seen that during the period of machining first 50 gears, gear hobbing under nanolubrication exhibits the reduction of tool wear, but at the period of machining the 300th gear, the significant reduction of hob wear is observed. Moreover, the wear land at the time of machining the 300th gear with nanolubrication (39.93 μm) is nearly equivalent to that of the tool at the period of machining the 50th gear under flood lubrication (36.29 μm). Accordingly, it is clarified that there is a significant increment in tool life when machining under nanolubrication.

On the other hand, the tool wear is much reduced under nanolubrication, which leads to achieve higher gear profile accuracy (shown in **Figure 15**). It could be said that nanolubrication is the main factor contributed to preserve the tool profile accuracy. In addition, the spherical morphology of Al<sup>2</sup> O3 nanoparticles takes part in the decrease of friction force and cutting temperature.

Hard milling process of 60Si<sup>2</sup> Mn steel (50÷52HRC) was done by using Al<sup>2</sup> O3 nanofluid (0.5wt %) with carbide inserts at v = 110 m/min; ft = 0.12 mm/tooth; and ap = 0.2 mm. **Figures 16** and **17** illustrate the difference of tool wear between MQL hard milling with nanofluids and pure MQL. In **Figure 16**, the wear on cutting edge including rake and flank faces is dominant.

**Figure 13.** Flank wear after machining the 50th gear using: (a) flood lubrication; (b) nanolubrication [20].

In **Figure 17**, the friction between rake face and chip reduces due to "roller effect" of Al<sup>2</sup> O3 nanoparticles. Wear abrasion is not concentrated in cutting edge, and the abrasive area is formed on rake face (marked area shown in **Figure 17a**). On flank face, the formation of small wear land helps to form oil mist and contains nanoparticles to create "roller effect". The pressure on cutting edge decreases due to the reduction of friction. Therefore, the uniform wear occurs on cutting edge, which is different from the case without nanoparticles (**Figure 17b**). Moreover, the tool wear is reduced (about 26.4–33%) with the use of Al<sup>2</sup> O3 nanofluids.

The nanoparticles suspended in cutting fluids bring out the new trend in machining industries, which not only suggests many alternative solutions for conventional problems in metal cutting but also suits with green manufacturing industries, especially used as the base fluids of MQL techniques. Many publications have shown that the vegetable oils as the base fluids with MQL method, inherently nontoxic as well as biodegradable, can be effectively applied for machining processes, but their cooling characteristics is the main problem when cutting hard materials. During hard machining, the enormous amount of heat generated from cutting zone and strong adhesive wear between the tool and the work material will cause the reduction of hardness of cutting tool, increase the wear rate, and decrease the tool life. The occurrence of nanomaterials in MQL fluids has a strong meaning to overcome this problem. The difficulty of heat dissipation from cutting zone has been solved by the reduction of friction coefficient caused by "roller effect" of nanoparticles. Besides, MQL nanofluids also broaden

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091 173

**Figure 14.** Flank wear after machining the 300th gear using: (a) flood lubrication; (b) nanolubrication [20].

the applicability of carbide tools in hard cutting with economic characteristic.

From **Figure 18**, in case of soybean oil with Al<sup>2</sup> O3 nanoparticles, tool life is about 80 minutes (increase almost 177% compared to pure soybean oil). In case of emulsion 5% coolant with Al<sup>2</sup> O3 nanoparticles, tool life is about 115 minutes (increase almost 230% compared to pure emulsion 5% coolant). The promising results are supported to prove the explanation of "roller effect" of nanofluids.

**Figure 14.** Flank wear after machining the 300th gear using: (a) flood lubrication; (b) nanolubrication [20].

In **Figure 17**, the friction between rake face and chip reduces due to "roller effect" of Al<sup>2</sup>

**Figure 13.** Flank wear after machining the 50th gear using: (a) flood lubrication; (b) nanolubrication [20].

Moreover, the tool wear is reduced (about 26.4–33%) with the use of Al<sup>2</sup>

From **Figure 18**, in case of soybean oil with Al<sup>2</sup>

Al<sup>2</sup> O3

effect" of nanofluids.

172 Microfluidics and Nanofluidics

nanoparticles. Wear abrasion is not concentrated in cutting edge, and the abrasive area is formed on rake face (marked area shown in **Figure 17a**). On flank face, the formation of small wear land helps to form oil mist and contains nanoparticles to create "roller effect". The pressure on cutting edge decreases due to the reduction of friction. Therefore, the uniform wear occurs on cutting edge, which is different from the case without nanoparticles (**Figure 17b**).

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 nanoparticles, tool life is about 115 minutes (increase almost 230% compared to pure emulsion 5% coolant). The promising results are supported to prove the explanation of "roller

(increase almost 177% compared to pure soybean oil). In case of emulsion 5% coolant with

O3

O3

nanoparticles, tool life is about 80 minutes

nanofluids.

The nanoparticles suspended in cutting fluids bring out the new trend in machining industries, which not only suggests many alternative solutions for conventional problems in metal cutting but also suits with green manufacturing industries, especially used as the base fluids of MQL techniques. Many publications have shown that the vegetable oils as the base fluids with MQL method, inherently nontoxic as well as biodegradable, can be effectively applied for machining processes, but their cooling characteristics is the main problem when cutting hard materials. During hard machining, the enormous amount of heat generated from cutting zone and strong adhesive wear between the tool and the work material will cause the reduction of hardness of cutting tool, increase the wear rate, and decrease the tool life. The occurrence of nanomaterials in MQL fluids has a strong meaning to overcome this problem. The difficulty of heat dissipation from cutting zone has been solved by the reduction of friction coefficient caused by "roller effect" of nanoparticles. Besides, MQL nanofluids also broaden the applicability of carbide tools in hard cutting with economic characteristic.

**Figure 15.** Measuring the gear profile error of the 300th machined gear by OSAKA SEIKI KIKAI gear measuring machine: (a) flood lubrication; (b) nanolubrication [20].

#### **2.5. The effects on surface integrity**

The surface integrity has become unquestionably a crucial parameter of any product in the past, present, and future. Quality characteristics must be tested during and after the manufacturing processes. With regard to components, the distinction is often made between macrogeometrical parameters and the surface quality. Macro-geometrical parameters refer to deviations of dimension, form and position. The surface quality is defined by roughness parameters. **Figures 19** and **20** show the surface roughness values of hard milling of 60Si<sup>2</sup> Mn steel (50÷52HRC) under different MQL conditions. The values of surface roughness obtained from MQL nanofluids are better than those of MQL pure fluids. Furthermore, the good surface quality of hard milling under MQL nanofluid condition achieves and remains stable during longer cutting time. The best performance of nanoparticles is obtained when the flank wear land reaches to some extent called "appropriate wear land." This can be explained that the profile of machined surface of hardened steel reflects that of flank face of cutting tool with

reasonable accuracy. As long as the flank wear profile is remained smooth, the flank wear to some extent not only deteriorates the surface finish but also somehow keeps or increases the surface quality [19]. This feature makes MQL hard machining utilizing nanofluids very differ-

**Figure 17.** Tool wear under MQL nanofluid with soybean oil (cutting time at 80 minutes): (a) rake face wear; (b) flank

**Figure 16.** Tool wear under pure MQL cutting fluid with soybean oil (cutting time at 45 minutes): (a) rake face wear; (b)

The surface roughness of grinding the high-temperature nickel base alloy GH4169 is shown in **Figure 21**. The comparison of different lubricating conditions reveals that the amount of surface quality improvement in the nanofluid MQL grinding is much higher. It is attributed

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nanofluid MQL grinding achieved

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091 175

ent from other types of machining processes.

the best surface roughness.

flank face wear [19].

face wear [19].

to the more effective lubrication of nanofluids. The Al<sup>2</sup>

**Figure 16.** Tool wear under pure MQL cutting fluid with soybean oil (cutting time at 45 minutes): (a) rake face wear; (b) flank face wear [19].

**Figure 17.** Tool wear under MQL nanofluid with soybean oil (cutting time at 80 minutes): (a) rake face wear; (b) flank face wear [19].

**2.5. The effects on surface integrity**

(a) flood lubrication; (b) nanolubrication [20].

174 Microfluidics and Nanofluidics

The surface integrity has become unquestionably a crucial parameter of any product in the past, present, and future. Quality characteristics must be tested during and after the manufacturing processes. With regard to components, the distinction is often made between macrogeometrical parameters and the surface quality. Macro-geometrical parameters refer to deviations of dimension, form and position. The surface quality is defined by roughness parameters. **Figures 19** and **20** show the surface roughness values of hard milling of 60Si<sup>2</sup>

**Figure 15.** Measuring the gear profile error of the 300th machined gear by OSAKA SEIKI KIKAI gear measuring machine:

steel (50÷52HRC) under different MQL conditions. The values of surface roughness obtained from MQL nanofluids are better than those of MQL pure fluids. Furthermore, the good surface quality of hard milling under MQL nanofluid condition achieves and remains stable during longer cutting time. The best performance of nanoparticles is obtained when the flank wear land reaches to some extent called "appropriate wear land." This can be explained that the profile of machined surface of hardened steel reflects that of flank face of cutting tool with

Mn

reasonable accuracy. As long as the flank wear profile is remained smooth, the flank wear to some extent not only deteriorates the surface finish but also somehow keeps or increases the surface quality [19]. This feature makes MQL hard machining utilizing nanofluids very different from other types of machining processes.

The surface roughness of grinding the high-temperature nickel base alloy GH4169 is shown in **Figure 21**. The comparison of different lubricating conditions reveals that the amount of surface quality improvement in the nanofluid MQL grinding is much higher. It is attributed to the more effective lubrication of nanofluids. The Al<sup>2</sup> O3 nanofluid MQL grinding achieved the best surface roughness.

**Figure 18.** Tool life under MQL conditions with or without nanofluids [19].

**Figure 19.** Surface roughness R<sup>a</sup> under MQL conditions with or without nanofluids [19].

The nanofluid MQL grinding leads to a smoother surface and is better than either pure palm oil MQL or flood lubrication (seen in **Figure 22**). Interestingly, MQL grinding with Al<sup>2</sup> O3 nanofluid offers significant reduction in the sliding friction coefficient, specific sliding grinding energy, and best surface quality [18]. In addition, the SiO<sup>2</sup> and diamond nanofluids show relatively good lubrication effect.

layer formed by nanoparticles has the same characteristics as nanopowder, and so through cutting processes by nanofluids, we can make further improvement for tribological effect on part surfaces by using proper nanofluids and cutting condition. These topics will be discussed

**Figure 21.** Surface roughness of grinding process at different lubrication conditions [18].

under MQL conditions with or without nanofluids [19].

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An inclusive review on the application of nanofluids in various machining processes has been made. The nanofluid has achieved significant attention due to its capability to enhance the heat transfer and lubrication performance in cutting zone. The effects of nanofluids were proven to reduce the coefficient of friction and wear effect to enhance the cutting performance,

and confirmed in many further researches.

**2.6. Conclusion**

**Figure 20.** Surface roughness Rz

The application of nanolubrication led to the formation of a tribo-film (seen in **Figure 23**) as a solid lubricant [9]. This observation can be made in machining field due to extremely high contact pressure and temperature in cutting zone, and so many nanoparticles are deformed and remained in the machined surface. The occurrence of tribo-film on the machined surface may lead to many new research topics needed to study. The deposition of a tribo-film on the surfaces could help to improve the operating function of the machined part. The very thin

**Figure 20.** Surface roughness Rz under MQL conditions with or without nanofluids [19].

**Figure 21.** Surface roughness of grinding process at different lubrication conditions [18].

layer formed by nanoparticles has the same characteristics as nanopowder, and so through cutting processes by nanofluids, we can make further improvement for tribological effect on part surfaces by using proper nanofluids and cutting condition. These topics will be discussed and confirmed in many further researches.

#### **2.6. Conclusion**

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and diamond nanofluids show

The nanofluid MQL grinding leads to a smoother surface and is better than either pure palm oil MQL or flood lubrication (seen in **Figure 22**). Interestingly, MQL grinding with Al<sup>2</sup>

under MQL conditions with or without nanofluids [19].

nanofluid offers significant reduction in the sliding friction coefficient, specific sliding grind-

The application of nanolubrication led to the formation of a tribo-film (seen in **Figure 23**) as a solid lubricant [9]. This observation can be made in machining field due to extremely high contact pressure and temperature in cutting zone, and so many nanoparticles are deformed and remained in the machined surface. The occurrence of tribo-film on the machined surface may lead to many new research topics needed to study. The deposition of a tribo-film on the surfaces could help to improve the operating function of the machined part. The very thin

ing energy, and best surface quality [18]. In addition, the SiO<sup>2</sup>

**Figure 18.** Tool life under MQL conditions with or without nanofluids [19].

relatively good lubrication effect.

**Figure 19.** Surface roughness R<sup>a</sup>

176 Microfluidics and Nanofluidics

An inclusive review on the application of nanofluids in various machining processes has been made. The nanofluid has achieved significant attention due to its capability to enhance the heat transfer and lubrication performance in cutting zone. The effects of nanofluids were proven to reduce the coefficient of friction and wear effect to enhance the cutting performance,

tool life, and surface quality. Moreover, MQL technique with NFs makes a big improvement for some hard machining processes like hard turning, hard milling in term of surface quality, which is equivalent to that of finish grinding. Together with MQL using nontoxic fluids like water and vegetable oils, nanofluids have opened the new trend in machining and exhibited a wide range of application in different cutting processes. The promising results obtained definitely ensure the success of MQL machining with nanofluids. However, the performance and behavior of nanofluids may be affected by many parameters, such as the base fluid, nanoparticle type, nanoparticle size, nanoconcentration, and so on. Further research is necessarily

cross-section, (b) EDS element mapping on tribo-boundary film, (c) EDS spectrum on tribo-boundary film [9].

tribo-boundary film on worn surface of the piston ring surface. (a) FE-SEM imaging on the

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Each type of nanoparticles has with different structures, shapes, and sizes, which will vary in physical and morphological features, demonstrating diverse tribological performances [35].

made to optimize these parameters.

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**Figure 23.** Formation Al<sup>2</sup>

**3. The effects of parameters of nanofluids**

**Figure 22.** SEM micrographs of workpiece surface of nickel base alloy GH4169 at different lubricating conditions [18].

**Figure 23.** Formation Al<sup>2</sup> O3 tribo-boundary film on worn surface of the piston ring surface. (a) FE-SEM imaging on the cross-section, (b) EDS element mapping on tribo-boundary film, (c) EDS spectrum on tribo-boundary film [9].

tool life, and surface quality. Moreover, MQL technique with NFs makes a big improvement for some hard machining processes like hard turning, hard milling in term of surface quality, which is equivalent to that of finish grinding. Together with MQL using nontoxic fluids like water and vegetable oils, nanofluids have opened the new trend in machining and exhibited a wide range of application in different cutting processes. The promising results obtained definitely ensure the success of MQL machining with nanofluids. However, the performance and behavior of nanofluids may be affected by many parameters, such as the base fluid, nanoparticle type, nanoparticle size, nanoconcentration, and so on. Further research is necessarily made to optimize these parameters.
