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INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING-GREEN TECHNOLOGY Vol. 5, No. 2, pp. 317-326

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REGULAR PAPER

DOI: 10.1007/s40684-018-0034-5 ISSN 2288-6206 (Print) / 2198-0810 (Online)

Effects of Machining and Oil Mist Parameters on Electrostatic Minimum Quantity Lubrication–EMQL Turning Process Shuiquan Huang1, Tao Lv1, Minghuan Wang1, and Xuefeng Xu1,# 1 Key Laboratory of Special Purpose Equipment and Advanced Processing Technology, Ministry of Education & Zhejiang Province, Zhejiang University of Technology, 18, Chaowang Road, Hangzhou, Zhejiang, 310014, China # Corresponding Author / E-mail: [email protected], TEL: +86-571-8832-0729 ORCID: 0000-0002-1663-1291 KEYWORDS: MQL, Electrostatic minimum quantity lubrication, Machining parameters, Oil mist parameters, Cutting performance, Green machining

A cost-effective and eco-friendly alternative to conventional flood cooling-lubrication and minimum quantity lubrication (MQL) is electrostatic minimum quantity lubrication (EMQL). EMQL, which is a novel green machining technology that utilizes the synergetic effects between electrostatic spraying (ES) and MQL, has been successfully shown a potential in milling process. However, the effective application of EMQL is not only connected with machining parameters, such as cutting speed and feed rate, but also related to oil mist parameters including charging voltage, lubricant flow rate, air pressure, and nozzle position and distance. This paper investigated the effect of the above parameters on the cutting performance of EMQL turning stainless steels in comparison with completely dry and conventional wet and MQL cutting. The results suggested that cutting speed and voltage were important factors affecting the effectiveness of EMQL, and found that there were the optimum air pressure and nozzle position and distance when EMQL turning AISI 304 stainless steel. Properly selecting these parameters, a viable alternative to wet and MQL cutting could be achieved by promoting lubricants into cutting interface to reduce friction and adhesion of work-piece materials on the interface. Manuscript received: August 31, 2016 / Revised: April 10, 2017 / Accepted: June 21, 2017

NOMENCLATURE

machining performance such as low thermal conductivity, high work hardening coefficient, and strong material adhesion on the cutting edge, thus reducing tool life and surface quality. Cutting fluids in flood form have conventionally been used in the process to decrease the tool wear and lower the heat on the machining area, but has raised major environmental and economic concerns. An alternative to the established flood cooled-lubrication application is dry cutting, i.e., machining without the use of cutting fluids. Dry cutting is preferred in the field of environmentally friendly manufacturing. However, dry cutting stainless steels was not an option because it easily caused high friction force, followed by accelerating tool wear and poor surface finish owing to the absent lubrication and cooling. Instead of the dry cutting, a promising approach to these requirements was to implement near-dry machining and minimum quantity lubrication (MQL). MQL involved the application of a very small amount of biodegradable lubricants, which were dispensed into the cutting zone by compressed air flow. Compared with dry cutting, MQL was found to lower the friction coefficient and cutting 1-3

Ra = Surface roughness VB = Maximum flank wear FR = Resulting cutting force Fx = Radial thrust force Fy = Tangential force Fz = Feed force

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1. Introduction Stainless steels are widely used in the aerospace and automotive industries, medical and other related fields because of their high corrosion resistance and excellent metallurgical and mechanical characteristics. These properties, however, resulted in a relatively poor

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temperatures. The reduction in the consumption of cutting fluids and the minimal lubricant residue on the chips, work-piece, and tool holder resulted in an economical benefit and industrial hygiene. Nevertheless, Su et al. reported that MQL was unable to effectively cool the machining area, because the heat removal was attained mainly by the convection of compressed air and partially by the evaporation of lubricant droplets. Therefore, MQL did not work very well in a machining operation in which many thermal issues occur, such as machining difficult-to-machine materials. Leppert investigated the effects of dry, flood cooling, and MQL machining on the surface layer properties in turning AISI 316L stainless steel. It was found that MQL did not significantly influence the cutting force in comparison with both the wet and dry machining. Meanwhile, the investigation in surface finish during the finish turning of AISI 420B stainless steel under MQL, wet, and dry cutting also reported that MQL did not present any advantages over dry cutting in terms of Ra values. A new machining technology called “electrostatic minimum quantity lubrication” (EMQL) has been innovated and developed as a solution that greenly machines difficult-to-machine materials. With EMQL, a very small quantity of lubricants (5-20 mL/h), negatively charged by the electrostatically contact charged method, is fed onto the machining area in the form of a fine, uniform, and highly penetrable and wettable aerosol. Previous studies reported that EMQL improved the tribological and milling performances considerably, showing up ~ 73% improvement in the tool life and a 67% reduction in the lubricant A further study also found consumption in comparison with MQL. that because of the electrostatic adsorption of lubricant droplets, the use of EMQL led to a reduction of around 10% in the concentration of droplets floating in the working space compared with MQL, which thus decreased the number of droplets affecting operators’ health. In this study, the cutting performance of EMQL in turning AISI 304 stainless steel was systematically investigated using a PVD coated cemented carbide tool. Firstly, the effect of machining parameters on the EMQL cutting performance wzere studied. Subsequently, the influences of spray delivery parameters such as charging voltage, air pressure, lubricant flow rate, nozzle position, and nozzle distance on the cutting forces, tool wear, and surface roughness were experimentally evaluated to determine an appropriate EMQL spray parameters. For comparison, cutting tests were also performed under dry, wet and MQL conditions. 9,10

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Fig. 1 (a) Photographic view of EMQL equipment, (b) structure of nozzle for EMQL, and (c) schematic diagram of EMQL equipment

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Fig. 2 Distribution of lubricant droplets on silicon wafers under (a) MQL, (b) -5 kV EMQL, (c) -10 kV EMQL, and (d) -15 kV EMQL

2. Experimental Procedure 2.1 EMQL Equipment Fig. 1 shows the photographic view and schematic diagram of the EMQL equipment. The nozzle used in EMQL was made of acetal copolymer, which is one of the typically high insulation materials, and its schematic diagram is also shown in this figure. The equipment mainly consisted of a negative high-voltage electrostatic power supply (EST705, Beijing Huajinghui Technology Ltd., China), a precision lubricant applicator (Accu-Lube, Illinois Tool Works Inc., USA) and an external air compressor. As shown in Fig. 1(b), lubricants were initially charged at the outlet of the oil hose through a needle-like copper electrode, which was attached to the negative

high-voltage electrostatic power supply using a voltage-supply wire, and were then atomized into charged lubricant droplets by the compressed air. To achieve a fine chargeability, trial tests have been conducted to select the minimum value of charging voltage, and concluded that -5 kV was effective. In EMQL mode, as the lubricant droplets were charged, the surface adhesive force and activity of lubricants could be increased, as well as molecules at the lubricant surface could be directionally arranged, which would result in a reduction in the surface tension and contact angle of lubricant droplets. As seen in Table 1, the EMQL of -15 kV shown up 28% and 31% reductions in the surface tension and contact 16

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Table 1 Performance parameters of lubricant droplets Lubrication condition EMQL EMQL MQL (-5 kV) (-15 kV) 2.9 2.6 2.1 50.7 45.2 34.8 34.5 27.4 22.3

Property o

Surface tension, 25 C, N/m Contact angle, Ave. diameter, μm o

Table 2 Experimental conditions Machine tool Work-piece material Hardness Dimension Cutting tool Tool holder Turning parameters Environment Wet turning fluid

CAK6150D precision lathe AISI 304 austenitic stainless steel (C-0.08%, Mn-2.00%, P-0.045%, S-0.03%, Si-1.00%, Cr-18.00~20.00%, Ni-8.00%~11.00%) 150~160 HB ϕ 70 × 160 mm Coated cemented carbide, PVD-TiAlN, CCMT09T304N-SU, Sumitomo SCLCR2020KO9C Cutting speed: 77, 138, and 175 m/min; feed rate: 0.1, 0.15, 0.2 mm/rev; depth of cut: 1.0 mm; cutting length: 640 mm Dry, Wet, MQL, -5 kV EMQL, -10 kV EMQL, and -15 kV EMQL Emulsified mineral liquid in a 5% concentration Flow rate: 5 L/min LB-2000 vegetable oil, ITW Inc., USA Density: 0.92 g/cm at 25 C Kinetic viscosity: 37 mm /s at 40 C Flash point: 320 C Oil flow rates: 10 and 20 ml/h; air pressures: 4 and 5 bars; nozzle positions: 45 toward the rake face, 45 toward the principal flank face, and 45 toward the auxiliary flank face nozzle distance: 10, 20, and 40 mm 3

MQL/EMQL oil

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MQL/EMQL Supply

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angle, respectively, in comparison with MQL. Meanwhile, the reduction in the surface tension could also decrease atomization resistance, and the secondary atomization could be improved when the droplets were electrostatically charged, which would lead to an improvement in their average diameter and distribution. It is seen in Fig. 2 that the increase in charge voltage from 0 to -15 kV resulted in significant improvement in both droplet diameter and distribution. These results above indicate that the use of EMQL considerably increased the penetrability and wettability of lubricant droplets, as well as reduced their average diameter with improved droplet distribution. Eventually, these lubricant droplets were sprayed and quickly attracted to the surfaces of cutting tool and work-piece based on an effect of as shown in Fig. 1(c). electrostatic adsorption, 18,19

2.2 Turning Tests The cylindrical turning experiments were performed on a CAK6150D precision lathe, as shown in Fig. 3. The work-piece material used in this investigation was an AISI 304 austenitic stainless steel with a diameter of 70 mm and a length of 160 mm. Before turning, the work-piece was pre-machine with a cut depth of 2 mm to

Fig. 3 Experimental system configuration: (1) work-piece, (2) EMQL nozzle, (3) dynamometer, (4) charge amplifier, (5) DAQ system, (6) PC, and (7) EMQL/MQL equipment

remove any surface irregularities and ensure similar surface properties for all tests. PVD-TiAlN coated cemented carbide cutting tools (CCMT09T304N-SU, Sumitomo Electric, Japan) with 8 rake angle and 0.4 mm radius were used for the cutting experiments. To maintain the same experimental context, each set of cutting tests was performed using fresh tool edge and work-piece. The experimental conditions are summarized in Table 2. The ranges of the cutting speed and feed rate were selected based on the tool manufacturer’s recommendation and industrial practices. The depth of cut was fixed to 1.0 mm. Each test was repeated three times, and average values and standard deviations (error bars) were given. During the experiment, the tool wear, cutting forces, and surface roughness were measured after the 4th pass. The tool wear was examined in terms of maximum flank wear using a microscope (VW6000, Keyence, Japan). The cutting forces were measured using a three-component dynamometer (9129A, Kistler, Switzerland). The work-piece surface roughness was measured at three specific points along the cutting direction (i.e. along the 160 mm direction) using a portable surface roughness tester (SJ-210, Mitutoyo, Japan). The progressions of tool wear, cutting forces, and surface roughness against cutting length were measured at the end of each pass. The tool life was defined as the flank wear of 0.3 mm. The worn tool was analyzed using a field-emission scanning electron microscope (FESEM, Hitachi S4700, Japan) to identify the main type of wear mechanisms. o

3. Results and Discussion 3.1 Effect of Cutting Speed Fig. 4 shows the effect of cutting speed on tool wear, resulting cutting forces, and surface finish when increasing the cutting speed from 77 to 175 m/min under all cutting environments. The resulting cutting force was calculated using the formula FR = (Fx2 + Fy2 + Fz2) , where Fx is the radial thrust force, Fy is the tangential force, and Fz is the feed force. It is seen in Fig. 4(a) that the lowest tool wear was achieved when using the EMQL of -5 kV, with its respective reductions being 39%, 44%, and 37% compared with the dry cutting and turning with emulsion and MQL. EMQL could promote a more efficient entry of the lubricant and oxygen 1/2

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into the cutting zone owing to the improved droplet performance, which would lower the friction and heat on the cutting tool and thus retain its strength and wear resistance, decreasing the tool wear considerably. The fact is confirmed by the cutting temperatures under various lubricating conditions. As seen in Fig. 5, the -5 kV EMQL shown 24% and 8% reductions in cutting temperatures, in comparison with those of the dry and MQL cutting, respectively. Further increase in charging voltage led to a slight increase in the rate of tool wear. Such an increase might be attributed the fact that the increased voltage led to a larger spray angle of the lubricant mists with much smaller droplet diameter, which would degrade the distribution and deposition of lubricant droplets on the small cutting zone, and thus When MQL was used, the tool wear gradually reduce the lubrication. increased as the cutting speed increased from 77 to 175 m/min. A higher cutting speed could produce higher pressures and lower cycle time among the tool, work-piece and chips, which would lead to fewer lubricants penetrating and lubricating the interfaces. Therefore, the increase of cutting speed to 175 m/min resulted in the insufficient lubrication and thus increased the tool wear. Fig. 4(b) shows the cutting force achieved with different cutting environments, plotted as a function of the cutting speed. The cutting force clearly decreased as the cutting speed increases from 77 to 138 m/min in all cases. This is as expected because a higher cutting speed could reduce the chip thickness and result in lower cutting force. The effect of the cutting speed on the surface roughness is shown in Fig. 4(c). The mean Ra values were generally lower in all cases of EMQL technologies. When the voltage was -5 kV, the mean R value of the EMQL was 28% lower than that of the dry cutting, 8% lower than that of the wet mode, and 10% lower than that of the MQL mode, respectively. This is attributed to the fact that because of the better lubricating effect, EMQL lowered the tool wear and thus improve the surface quality. Meanwhile, the better lubrication allowed the chips to slide more easily over the tool surface, which resulted in less adhesion, and hence improved the surface quality too. 20-22

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Fig. 4 Effect of cutting speed on (a) tool wear, (b) cutting force, and (c) surface finish under different cutting environments (depth of cut: 1.0 mm; feed rate: 0.1 mm/rev; air pressure: 4 bars; flow rate: 10 mL/h; nozzle position: 450 toward the rake face; nozzle distance: 20 mm)

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3.2 Effect of Feed Rate Fig. 6(a) shows the effect of feed rate in the tool wear using different cutting environments. The tool wear was clearly lower in all cases of EMQL technologies in comparison with other three cutting modes, confirming its effective lubrication. With the -5 kV EMQL, it was seen to result in 45%, 40%, and 28% reductions in mean VB value compared with those of the dry, wet and MQL cutting. The reason is because that EMQL improved the performance of lubricant droplets, which allowed them to adhere and lubricate the cutting interface quickly, thus decreasing the tool wear. When dry cutting used, the tool wear increased rapidly as the feed rate increased from 0.10 to 0.20 mm/rev, suggesting the tool failure. For wet cutting, the highest tool wear was achieved when the feed rate was 0.20 mm/rev. The reason is because that the increased feed rate might result in a significant thermal shock on the cutting edge, which reduced the strength of the cutting edge, and thus caused the rapid tool wear and failure. With MQL cutting, lubricant droplets with small diameters and high velocities provided more effective lubrication because they wetted and penetrated deeper into the cutting interface, which thus produced a slightly lower tool wear value in comparison with the wet cutting.

Fig. 5 Cutting temperature at different cutting speeds using EMQL and MQL lubrication modes, compared with dry cutting


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Fig. 6 Effect of feed rate on (a) tool wear, (b) cutting force, and (c) surface finish under different cutting environments (depth of cut: 1.0 mm; cutting speed: 77 m/min; air pressure: 4 bars; flow rate: 10 mL/h; nozzle position: 450 toward the rake face; nozzle distance: 20 mm)

Fig. 7 Effects of air pressure and flow rate on (a) tool wear, (b) cutting force, and (c) surface finish under different cutting environments (depth of cut: 1.0 mm; cutting speed: 77 m/min; feed rate: 0.1 mm/rev; nozzle position: 450 toward the rake face; nozzle distance: 20 mm)

Figs. 6(b) and 6(c) show the effect of the feed rate on the cutting force and surface finish using all cutting environments. The cutting force and surface finish increased with the increasing feed rate at all cutting modes. During the cutting processes, the increase in feed rate could reduce the penetration rate of the lubricants into the cutting interface, which resulted in the increased friction at the tool–chip interface. For the dry cutting, the highest cutting force and surface finish Ra were achieved primarily because of the more intensive friction and temperature resulting from the absence of lubricants. It is seen in Figs. 6(b) and 6(c) that EMQL achieved a better cutting performance under low voltage and feed rate conditions. This indicated that excessive voltage and feed rate might degrade the lubrication on the cutting interface. Taking both tool wear and surface quality into account, the EMQL with -5 and -10 kV was selected in the further tests.

3.3 Effect of Air Pressure and Lubricant Flow Rate Fig. 7 shows that the effects of air pressure and lubricant flow rate on the tool wear, resulting cutting force, and surface finish under all cutting environments. It is seen that the cutting performance of EMQL was much better than that of wet and MQL cutting. When compared against the wet cutting (flow rate: 30000 mL/h), -10 kV EMQL at 20 mL/h and 4 bars caused a decrease in the tool wear by 47% and a reduction in the surface finish by 26%, respectively. When the EMQL was used, the increase of air pressure from 4 to 5 bars led to an increase in tool wear. As the lubricant was charged, the spray angle of droplets was increased and their size was decreased. Meanwhile, the increased air pressure could also increase the spray angle and reduce the droplet size. These results might reduce the deposition rate of droplets on the cutting interface and thus increased the tool wear.

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As seen in Fig. 7, at the air pressure of 5 bar the tool wear increased with the increasing flow rate for both the MQL and EMQL cutting, whereas their surface finish values decreased. This suggests that the surface roughness did not depend on the tool wear alone. As the lubricant flow rate increased, the deposition rate of droplets on the cutting zone increased, which resulted in a reduction in the friction force, thus decreasing the surface roughness.

3.4 Effect of Nozzle Position To determine the effect of nozzle position on the cutting performance of EMQL, three different EMQL nozzle positions shown in Fig. 1(b) were compared: 45 toward the rake face, 45 toward the principal flank face, and 45 toward the auxiliary flank face. Fig. 8 shows the variation in tool wear, resulting cutting force, and surface finish as a function of nozzle position under MQL and EMQL environments. The cutting performance of EMQL was better than that of MQL for different nozzle position conditions. As the lubricating effect improved, there was a corresponding decrease in friction force and heat formation, as well as an improvement in chip formation and flow, which thus decreased the tool wear and enhanced the surface quality. As shown in Fig. 8, EMQL had the minimum tool wear value when the nozzle position was 45 toward the rake face, which was attributed to the more efficient lubrication. This result also indicates that when the nozzle position was 45 toward the principal flank face, due to the increased spray angle, there was a possibility that the deposition rate of droplets on the cutting interface decreased, consequently led to a higher tool wear rate. When MQL was used, the nozzle position of 45 toward the principal flank face achieved the lowest tool wear and cutting force values, and at the nozzle position of 45 toward the rake face, the cutting force and surface finish decreased significantly as the flow rate increased from 10 to 20 mL/h (see Fig. 7). These suggest that because of the insufficient wetting and penetration of lubricant droplets into the contact layer, there was a possibility that the quantity of the lubricant was not sufficient when MQL turning AISI 304 stainless steel at the condition of 10 mL/h and 45 toward the rake face. Fig. 8(c) shows the effect of the nozzle position on the surface finish of EMQL. The results show that the Ra value of EMQL was clearly lower than that of MQL in all cases. The lowest Ra value was found for -5 kV EMQL at 45 toward the auxiliary flank face, with its value being ~20.0% lower than that of the wet mode. In addition, as shown in Fig. 8(c), the lowest Ra values of the work-piece were acquired when the nozzle was positioned 45 toward the auxiliary flank face for both MQL and EMQL cases. The lubricant droplets, with smaller physical diameters and higher velocities, could adhere and penetrate quickly into the interface between the auxiliary flank face and the newly machined surface, which would result in a reduction in the friction force, consequently enhance the surface quality. o

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3.5 Effect of Nozzle Distance Fig. 9 shows the effect of nozzle distance on the tool wear, resulting cutting force, and surface finish under MQL and EMQL environments. The better cutting performance was acquired with EMQL for all three nozzle distances tested. The mean tool wear of EMQL was 34.3% lower than that of MQL. The above results suggest that EMQL

Fig. 8 Effect of nozzle position on (a) tool wear, (b) cutting force, and (c) surface finish under different cutting environments (depth of cut: 1.0 mm; cutting speed: 77 m/min; feed rate: 0.1 mm/rev; air pressure: 4 bars; flow rate: 10 mL/h; nozzle distance: 20 mm)

outperformed MQL in reducing friction at the tool-chip interface. As shown in Fig. 9, the tool wear and surface finish first decreased and then increased as the nozzle distance increased from 10 to 40 mm in both MQL and EMQL cases. The variation in the cutting performance with nozzle distance is attributed to the fact that a change in nozzle distance would affect the lubrication effect by altering the velocities of lubricant droplets and carrier gas. At lower nozzle distances, droplet velocity was lower than that of the carrier gas, whereas with increasing the distance, it increased first and then was higher compared with the carrier gas velocity. As a result, droplet deposition rate and penetration capacity into the cutting interface increased, resulting in decreased friction at the contact area. Meanwhile, the increased distance could also lead to smaller droplet diameter because of the secondary atomization, which would improve the wetting ability of the lubricant droplets and thus increase lubrication efficiency. However, the further increase in the nozzle 20


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Fig. 9 Effect of nozzle distance on (a) tool wear, (b) cutting force, and (c) surface finish under different cutting environments (depth of cut: 1.0 mm; cutting speed: 77 m/min; feed rate: 0.1 mm/ rev; air pressure: 4 bars; flow rate: 10 mL/h; nozzle position: 450 toward the rake face)

Fig. 10 Progress of (a) tool wear, (b) cutting force, and (c) surface finish with cutting length under different cutting environments (depth of cut: 1.0 mm; cutting speed: 77 m/min; feed rate: 0.1 mm/rev; air pressure: 4 bars; flow rate: 10 mL/h; nozzle position: 450 toward the rake face; nozzle distance: 20 mm)

distance would lower the wetting area resulting from the increased spray angle and excessively small droplet size, which thus reduced the lubrication on the cutting zone.

highly sensitive to temperatures, and reduced the abrasion wear by maintaining tool hardness, consequently prolonged the tool life. With MQL cutting, the lubricant droplets with smaller diameters and higher velocities could provide more effective lubrication because they wetted and penetrated deeper into the cutting interface compared with the flood lubricant, thus resulting in the better tool life. For dry cutting, the shortest tool life was because of the high friction and temperature in the cutting zone, which indicated the effectiveness of the cutting fluid in machining difficult-to-machine materials. Fig. 10(b) shows the cutting force progression under all cutting environments. Taking the whole period of cutting into account, the lowest cutting force was achieved using the -5 kV EMQL. The lubrication effectiveness of EMQL in reducing friction force at the tool–chip interface was evident from the decreased resulting cutting force. The reduction in the cutting force caused an equivalent reduction

3.6 Progression of Tool Wear, Cutting Force and Surface Finish Fig. 10(a) shows the growth of the tool wear with cutting length using different lubrication conditions. It is seen that EMQL remarkably enhanced the tool life compared with other three cutting modes, especially when using -5 kV EMQL. When the tool wear reached 0.3 mm, the cutting length of -5 kV EMQL was 4200 mm, followed by 3640 mm of MQL, 3080 mm of wet, and dry cutting of 1700 mm. With EMQL cutting, the fine and uniform lubricant droplets with improved performance were directly applied to the cutting zone, which could contribute to reducing the friction force and cutting temperatures between the tool and chip. This lowered the adhesion wear that is

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of the specific energy requirement and thus improved the cutting performance. When the dry cutting used, the cutting force increased drastically after the cutting length of 840 mm, as shown in Fig. 10(b), indicating the tool failure. Compared with the wet cutting, the effect of MQL on the cutting force was not noticeable along the whole duration of cutting. According to Yuan et al., this might be attributed to the lower cooling ability of compressed air in comparison with the flood lubricant. Fig. 10(c) shows the effect of the cutting distance on the surface finish under all cutting environments. The lowest Ra value was achieved with the -5 kV EMQL among the four cutting modes being used. As is the case with tool life and cutting force, this result suggests a better surface finish with EMQL, which reiterated its effectiveness. The ability to improve the surface finish in the EMQL mode was mainly attributed to the better lubricating effect, which resulted in lower friction force and reduced cutting temperatures. Meanwhile, the better lubrication helped the chips to slide more easily on the tool surface, leading to a less adhesion. These prevented tool wear and prolonged the tool life, resulting in the improved surface quality. In addition, as shown in the figure, MQL showed a higher Ra value compared with EMQL, the cause of the higher surface roughness might reasonably be attributed to an insufficient lubrication at the tool-workpiece interface because of the limited penetration of the lubricant. 5

3.7 Analysis of Tool Wear Mechanisms The optical images of the worn rake faces achieved after the cutting length of 640 mm using various cutting environments are shown in Fig. 11. A lighter area on the rake face was observed under all cutting conditions, indicating the adhesion wear resulting from the high content of work-piece materials. When EMQL was used, little work-material adhesion was observed on the rake faces, as shown in Figs. 11(d)-11(f), indicating the effective lubrication. It can be clearly seen in Figs. 11(a) and 11(c) that a severe adherence of work-piece materials occurred on the rake faces lubricated with the dry and MQL conditions. This is because that when the dry cutting used, the high friction force and intense heat generation involved in the process, which thus led to the poor chip removal on the rake face. With the MQL cutting, the lubricant droplets were unable to penetrate and wet the chip–tool interface quickly enough to lubricate the contact layer, thus leading to the thermally related wear mechanism. With wet cutting, a much lighter color was observed on the contact interface shown in Fig. 11(b) compared with EMQL, indicating a lower cutting area temperature. This is because that the cooling achieved by the convection of compressed air and evaporation of lubricant droplets in EMQL application was still lower than that provided by the convection of flood coolants only. SEM analysis of the worn flank faces was performed to explore the tool wear mechanisms. Fig. 12 shows that adhesive wear and abrasive wear were the main tool wear mechanisms when turning AISI 304 austenitic stainless steel using coated cemented carbide inserts under all cutting conditions. A typical adhesive wear occurred on the flank surfaces under all the conditions. This is attributed to the pressure welding between the work-piece and the tool flank. During the cutting processes, the cutting edge was subjected to high pressure, temperature, and heat concentration, which accelerated the adhesion or

Fig. 11 Optical images of the worn rake faces using (a) dry, (b) wet, (c) MQL, (d) -5 kV, (e) -10 kV, and (f) -15 kV cutting (depth of cut: 1.0 mm; cutting speed: 77 m/min; feed rate: 0.1 mm/ rev; air pressure: 4 bars; flow rate: 10 mL/h; nozzle position: 450 toward the rake face; nozzle distance: 20 mm)

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Fig. 12 SEM images of the worn flank faces using (a) dry, (b) wet, (c) MQL, (d) -5 kV, (e) -10 kV, and (f) -15 kV (depth of cut: 1.0 mm; cutting speed: 77 m/min; feed rate: 0.1 mm/rev; air pressure: 4 bars; flow rate: 10 mL/h; nozzle position: 450 toward the rake face; nozzle distance: 20 mm)


welding of the work-piece materials on the flank face, and thus caused a strong bonding between the tool and work-piece interface. This restrained the flow of work-piece materials at the interface, consequently led to the generation of the adhesive wear. The SEM observations in Figs. 12(d)-12(f) also show that because of the effective friction and temperature control, EMQL achieved the lowest adhesion intensity of the work-piece materials, followed by wet machining. For dry cutting, the severe adhesive wear on the principal flank face was attributed to the high cutting force and cutting temperature during the cutting process. For MQL cutting, a slightly lower adhesion intensity was observed, as shown in the insert in Fig. 12(c), in comparison with the dry cutting. This is because that the lubricant droplets were unable to reduce the friction and heat effectively. With wet cutting, the worn flank face shown in Fig. 12(b) also appears clear grooves, which indicated the abrasive. This is mainly attributed to the fact that the cutting fluid failed to lubricate the cutting interface, but a low cutting temperature generated owing to the cooling action of the flood coolant, which frequently caused the occurrence of abrasive wear because the chips were hard enough to plough into the tool. In addition, the worn surface lubricated with EMQL showed almost no evidence of the abrasive wear, especially when using -5 kV EMQL (Fig. 12(d)). This phenomenon indicates that through facilitating the better penetration of the lubricant into the cutting zone, EMQL could produce better lubrication and lower heat because of friction, followed by reducing the growth of abrasion wear on the main cutting edge. Overall, the low friction and heat generation, good chip flow, and low shear resistance with EMQL reduced the principal flank wear via controlling the degradation of the cutting edge by adhesion and abrasion.

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(4) The superior machining performance of EMQL was attributed to the increased deposition and penetration capabilities of the lubricant droplets, as well as the decreased physical size, which facilitated a more efficient entry of the lubricant into the cutting interfaces, resulting in lower friction and heat generation. The excellent cutting performance over the dry, wet and MQL cutting, and the drastic reduction in lubricant consumption result in the conclusion that EMQL can be successfully applied to the turning process of AISI 304 austenitic stainless steel. The technology is effective as an alternative to flood cooling-lubrication and MQL cutting.

ACKNOWLEDGEMENT This study was financially supported by National Natural Science Foundation of China (Grant No. 51375454) and Public Projects of Zhejiang Province (2016C31G2020038).

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REFERENCES 1. Naves, V. T. G., Da Silva, M. B., and Da Silva, F. J., “Evaluation of the Effect of Application of Cutting Fluid at High Pressure on Tool Wear During Turning Operation of AISI 316 Austenitic Stainless Steel,” Wear, Vol. 302, Nos. 1-2, pp. 1201-1208, 2013. 2. Deng, J. X., Zhou, J. T., Zhang, H., and Yan, P., “Wear Mechanisms of Cemented Carbide Tools in Dry Cutting of Precipitation Hardening Semi-Austenitic Stainless Steel,” Wear, Vol. 270, Nos. 78, pp. 520-527, 2011.

4. Conclusions

3. Habak, M. and Lebrun, J. L., “An Experimental Study of the Effect of High-Pressure Water Jet Assisted Turning (HPWJAT) on the

To explore the potential of electrostatic minimal quantity lubrication (EMQL) in the green cutting of difficult-to-machine materials, and investigate the effects of machining and oil mist parameters on the cutting performance of EMQL, a series of EMQL turning AISI 304 austenitic stainless steel experiments were performed. Based on the experimental results, the following conclusions can be drawn. (1) Under the same cutting conditions, EMQL remarkably reduced the tool wear and cutting forces, as well as improved the surface quality compared with the dry and conventional wet and MQL cutting. Particularly, the -5 kV EMQL gave the lowest tool wear and the smallest surface roughness, up to 37% and 10% reductions, respectively, in comparison with those of the MQL cutting. (2) Dry cutting was not recommended for turning stainless steel using coated cemented carbide, which produced maximum tool wear and surface roughness. Wet cutting provided the lowest cutting force, whereas it showed higher tool wear and surface roughness. (3) Adhesive wear and abrasive wear were the main tool wear mechanisms responsible for the failure modes under various cutting environments. The lowest adhesion strength of the work-piece materials was achieved with EMQL. Abrasive scratch marks could be clearly observed on the flank surfaces under wet condition, whereas the flank surface of EMQL did not exhibit the abrasion marks.

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