Effectiveness of Minimizing Cutting Fluid Use when ... - Science Direct

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ScienceDirect Procedia CIRP 29 (2015) 341 – 346

The 22nd CIRP conference on Life Cycle Engineering

Effectiveness of minimizing cutting fluid use when turning difficult-to-cut alloys Paolo C. Priarone a,*, Matteo Robiglio a, Luca Settineri a, Vincenzo Tebaldo b a

Politecnico di Torino, Department of Management and Production Engineering, Corso Duca degli Abruzzi 24, 10129 Torino, Italy b National Research Council (CNR), Imamoter, Strada delle Cacce 73, 10135 Torino, Italy

* Corresponding author. Tel.: +39-011-0907206; fax: +39-011-0907299. E-mail address: [email protected]

Abstract The environmental impact of lubricants is a key issue towards sustainable manufacturing. Even if dry cutting can be identified as the ultimate goal to achieve, lubrication is still a hardly surmountable industrial standard when machining difficult-to-cut alloys. In order to reduce the pollutant emissions and the problems related to the workers’ health, alternative systems as Minimum Quantity Lubrication (MQL) or Cooling (MQC) have been emerging over the years. This research aims to investigate the machinability of a Ti-48Al-2Cr-2Nb (at. %) alloy, applying low cutting fluid (water and emulsion) volumes to the cutting area in the form of a precision-metered droplets mist. The results in terms of tool wear/life, surface quality, lubricant consumption, and environmental impact are discussed with reference to those of MQL, wet and dry cutting. The experimental evidences show that, as far as tool life is concerned, the use of an emulsion mist is an advantageous strategy in comparison to MQL and dry cutting. Moreover, the flow rate and the type of cutting fluid are variables significantly affecting the process performance. © 2015 The Authors. Published by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the International Scientific Committee of the Conference “22nd CIRP conference on Life Cycle Engineering. Peer-review under responsibility of the scientific committee of The 22nd CIRP conference on Life Cycle Engineering Keywords: MQCL; lubrication; cutting; titanium aluminide.

1. Introduction The changes in laws against pollution, together with the increased consciousness concerning the environmental impact of industrial activities, are forcing companies to increase the sustainable performance of products and processes. For instance, if this challenge is faced from the products’ viewpoint, new lightweight structural materials have been developed over the last few decades, in order to improve efficiency and performance of aerospace and automotive engines. In this context, titanium aluminides, intermetallic alloys showing an advantageous combination of thermal, physical and mechanical properties, have proved to be a viable solution to substitute Ni-based superalloys in advanced applications [1]. However, such materials are difficult-to-cut, since they present many drawbacks related to machinability, as excessive tool wear, heat and cutting forces, as well as poor surface quality and difficulties in chip formation [2]. The research activities discussed in this paper aim to analyze the

influence of more sustainable lubrication/cooling strategies on cutting performance when turning a titanium aluminide. Nowadays cutting fluids are used in machining processes for cooling and lubrication purposes, since the reduction in cutting temperatures increases tool life [3, 4]. On the other hand, cutting fluids negatively affect the environmental impact of a process. As an example, common soluble oils are highly diluted in water, nevertheless the 5% (by volume) of the cutting fluid is a mixture of oil, emulsifiers (as sodium sulfonate, nonylphenol ethoxylates, PEG esters), and additives (as calcium sulfonate, alkanoamides, and blown waxes) [5]. Many chemical additives are pollutant for the environment, as well as hazardous substances for workers’ health. Exposures to the airborne particles of the mist consequent to cutting fluid vaporization might lead to occupational diseases [6, 7]. In addition, the costs for cutting fluid supply and disposal are not negligible with respect to the overall production costs. Therefore, strategies focusing to the reduction of cutting fluid use are indeed required [8].

2212-8271 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of The 22nd CIRP conference on Life Cycle Engineering doi:10.1016/j.procir.2015.02.006

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Paolo C. Priarone et al. / Procedia CIRP 29 (2015) 341 – 346

Dry machining is the optimum solution, when applicable. A near-dry cutting condition can be achieved by implementing the Minimum Quantity Cooling Lubrication (MQCL). A small amount of lubricant and/or cooling medium is supplied to the cutting area in the form of precision-metered droplets. The kind of fluid distinguishes the two main systems: oils are used for Minimum Quantity Lubrication (MQL), whilst emulsions (made of soluble oil in water), water, or air are exploited for Minimum Quantity Cooling (MQC). A MQL system provides a better lubricating effect, with the reduction of friction between tool, chip, and workpiece. A MQC system allows a better heat exchange due to the higher specific heat capacity of water (4.18 kJ/kgK versus 1.92 kJ/kgK for the oil). Therefore, MQC appears to be suitable for continuous cutting processes, and for machining difficult-to-cut workpieces characterized by low thermal conductivity [4, 9]. The application of MQL, mainly coupled with the use of vegetable oils, has been investigated by many authors [10, 11]. MQC, even if promising for some processes, has been relatively unexplored [12]. For this latter case, it is worth pointing out that the typical values for the normal MQC/L medium consumption (ranging between 10-50 ml/h, according to Weinert et al. [9]) are exceeded in some researches, while retaining a flow rate two orders of magnitude less than conventional flood cooling. Machado and Wallbank [13] designed a venturi-based system in which compressed air was mixed with a small quantity of fluid (either water or soluble oil). Five different lubrication conditions were compared (dry, air, air plus water, air plus soluble oil at the concentration of 5%, and conventional flood cooling) when turning an AISI 1040 steel bar with P40-grade uncoated carbide inserts. Air pressure was set to 2.3 bar. The flow rate was 294 ml/h and 196 ml/h when using water and soluble oil, respectively. The flow was directed through a nozzle to the rake face of the cutting tool. The experimental tests proved that, the application of air plus water and air plus soluble oil mixtures led to force components values comparable (or, in some cases, even lower) to those of wet cutting. Moreover, as far as surface finish and chip thickness are concerned, better results were achieved with respect to dry or wet cutting, but only for low cutting speeds and high feed rates. Sukaylo et al. [14] developed a FEM-based process simulation analyzing the workpiece surface temperatures and the thermally-induced deformations when turning a C45 steel workpiece, with HM-P30 coated tools, under three different lubrication conditions: dry, MQC and MQL. MQC was performed by using an air-water mixture supplied at a pressure of 5 bar and with a flow rate of 600 ml/h. Global and local heat transfer coefficients were approximately equal to 2500, 1000 and 50 W/m2K for MQC, MQL and dry conditions, respectively. Due to the differences between the thermal properties of water and rapeseed oil (used by the MQL system), the cooling effect of MQC noticeably exceeded that of MQL. Both MQC and MQL caused a reduction in temperatures and deformations with respect to dry machining. Furthermore, this effect became even more marked when a longer machining cycle took place. Nath et al. [15] designed an Atomized Cutting Fluid (ACF) spray system based on an ultrasonic atomizer able to produce

a uniform fluid droplet size of about 50 μm at a pressure of 10-20 bar and a flow rate of 600-1200 ml/h. A water-soluble cutting fluid (at 10% dilution) was applied as coolant, and pure N2 or air-CO2 mixture were used as droplet carrier gases. When turning a Ti-6Al-4V alloy, the results showed that the ACF spray system (for a specific setting of spray system parameters) improved tool life up to 40-50% over wet lubrication. Air-CO2 mixture droplet carrier gas allowed to reduce cutting temperatures, due to the lower dispensing temperature (about 2°C) with respect to that of pure N2. Furthermore, the use of air-CO2 mixture produced broken chips allowing a better penetration of cutting fluid at the toolchip contact area. For aluminum turning under MQC conditions, Hadi [16] adopted four different flow rates (80, 200, 400, 600 ml/h) of propylene glycol ester solution (oil mixed in water). In terms of surface roughness (Ra index), MQC performed better than dry cutting, and the results were comparable with those obtained with conventional flood cooling. No benefits were achieved by increasing the MQC flow rate. Marksberry and Jawahir [17] developed a tool-wear/tool-life model, including mist spray delivery parameters (as MWF flow rate, MWF type and nozzle position). For the best case, by using NDM, an improvement of 400% over dry machining was achieved. Five cooling and lubrication methods with different media (water-soluble cutting oil, vegetable oil mist, oil-water combined mist, oil-water combined mist plus water mist, and dry cutting) were compared in terms of tool wear, surface roughness and cutting temperature by Chen et al. [18, 19]. Built-up-edge formation during stainless steel cutting was prevented by using oil-water combined mist, under a flow rate of 17 ml/h (oil) and 150 ml/h (water). Consequently, surface roughness was better than that of dry cutting, oil mist or water-soluble oil application. Adopting low cutting speed, oilwater combined mist plus water mist allowed reaching the lowest tool wear. Temperature measurements showed that wet cutting was able to retain the lowest temperature on the cutting tool. Overall, the abovementioned researches highlight that, the application of Minimum Quantity Cooling or, more in general, the reduction of cutting fluid flow rate allows to obtain capable technological and environmental performance. In this paper, the effectiveness of cooling with low cutting fluid flow rates, down to 390 ml/h, is investigated in turning of a γ-TiAl difficult-to-cut alloy. Two cutting fluids are considered, and results are compared to those of dry cutting, MQL and conventional flood cooling. 2. Experimental procedure Longitudinal external turning operations were carried out, by means of a Graziano 101 SAG CNC lathe, on a bar made of a Ti-48Al-2Cr-2Nb (at. %) γ-TiAl alloy produced via vacuum arc remelting by GfE Metalle und Materialien GmbH (Germany). The microstructure of the workpiece is presented in Figure 1. For this specific alloy, the authors formerly investigated the effects of dry, near-dry and conventional lubrication conditions [20]. In order to compare the actual results with those previously obtained, the same experimental set-up was kept.

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and 390 ml/h, respectively) were considered as process variables. Two media were used: pure water, and a 5% emulsion of Vasco 5000 ester-based oil (by Blaser Swisslube Inc.) in water. For the cutting tests, at fixed feed (f = 0.1 mm/rev) and depth of cut (ap = 0.3 mm), the range of cutting speed vc was varied for each condition, according to Table 1. 500 μm

Fig. 1. Workpiece (bar) microstructure. W orkpiece

Table 1. Experimental plan for cutting tests, as a function of the cutting fluid. Cutting fluid

Flow rate Q (ml/min)

Cutting speed vc (m/min)

Emulsion of (5%) oil in water

115

50; 55; 60; 70

51

40; 50; 55; 60

6.5

45; 50; 55

115

40; 50; 60; 70

6.5

40; 50; 60

Water

Insert: R C MT 12 04 M0-S M S 05F T ool holder: S R D C N 3225-12M -E B

Pictures of flank and rake faces of worn tools were periodically acquired by means of an optical microscope, at 50× magnification. Tool wear was measured according to ISO 3685 standard, and the maximum flank wear land VB Bmax = 0.2 mm was assumed as tool wear limit to estimate the tool life TL. Worn inserts were also observed, at the end of their tool life, by using a Zeiss EVO 50 XVP scanning electron microscope (SEM) fitted out with an energy dispersion spectroscopy (EDS) analyzer for elemental composition detection. A LaB6 filament was employed, using secondary electrons to acquire the micrographs. A Form Talysurf 120 contact-type profilometer, coupled with the Talymap software for the analysis, was used to acquire and evaluate the 3D roughness profiles. 3. Results and discussion

G raziano S A G 101 C NC lathe

S NS S ystem (by A uges)

Fig. 2. Experimental set-up for cutting tests.

The experimental set-up is shown in Figure 2. CVD-coated RCMT 1204 M0-SM S05F round carbide inserts (from Sandvik Coromant), suitable for high-speed finishing of heat resistant and superalloys, were clamped in a Mircona SRDCN 3225-12M-EB tool holder designed to direct the lubricoolant flow on both flank and rake face of the tools by means of two nozzles. In order to perform the cutting operations with the reduced amounts of cutting fluid, the machine tool was equipped with a SNS system (model SNS03IDR) provided by Auges Srl (Italy). Such apparatus nebulizes the cutting fluid stored in a tank by using compressed air. The lubricoolant micro-mist is formed into the internal SNS mixer, and then supplied through the tool holder to the tool and the cutting area. The air supply pressure was fixed to 3 bar, while the cutting fluid flow rate was adjusted by setting the flow regulator of the SNS system. Three different values for the flow rate, namely Q = 115, 51 and 6.5 ml/min (or 6900, 3060

Images showing the tool wear evolution when using both emulsion and water as cutting fluid (and compressed air as droplet carrier gas) are presented in Figure 3. The wear mechanism is mainly abrasive in the first few minutes of cutting time. Flank wear progressively increases until reaching a catastrophic tool failure with the breakage of cutting edge. High-magnification pictures obtained via SEM observations reveal that chipping, coating delamination or breakage, and tool cracks appeared at the end of tool life. Moreover, a moderate adhesion of workpiece material on the wear scar was detected, as confirmed by the EDS analysis in Figure 4. Similar results were achieved in previous tests, even under different lubrication conditions [20, 21]. As far as the emulsion mist was used, typical tool wear curves as a function of flow rate are plotted in Figure 5. Tool wear increases by increasing cutting speed, as expected, and there is a clear correlation between flow rate and tool wear. Moreover, the type of cutting fluid is another variable heavily affecting the results. Tool life values are summarized in Figure 6, showing the Taylor’s curves. Each point in the graphs corresponds to a single experimental test. At fixed cutting conditions, the spread between tool life results of repeated tests is coherent with previous research studies [20]. The results obtained when applying the emulsion mist and the water mist are compared in Figure 6a and 6b, respectively.

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Paolo C. Priarone et al. / Procedia CIRP 29 (2015) 341 – 346 R C MT 1204 M0-S M S 05F coated ; vc = 50 m/min; f = 0.1 mm/rev; ap = 0.3 mm

E mulsion mist Q emulsion = 115 ml/min

tc = 20 min V B B max = 0.11 mm

F lank face



E mulsion mist, Q emulsion = 115 ml/min vc = 60 m/min, f = 0.1 mm/rev, ap = 0.3 mm, tc = 5.1 min

E mulsion mist Q emulsion = 6.5 ml/min

100 μm


1 mm

1 mm

1 mm

1 mm

1 mm

1 mm

R ake face

50 μm




1 mm

1 mm

1 mm

1 mm

1 mm

1 mm



2

1


1 mm

1 mm

1 mm

1 mm

1 mm

1 mm

S pectrum 1

(a) S pectrum 2

W ater mist

W ater mist

W ater mist

Q water = 115 ml/min vc = 40 m/min

Q water = 115 ml/min vc = 50 m/min

Q water = 6.5 ml/min vc = 50 m/min


F lank face

1 mm


1 mm


1 mm

R ake face

Fig. 4. SEM observation and EDS analyses carried out on a worn tool.

1 mm tc = 8.2 min V B B max = 0.35 mm

1 mm tc = 4 min V B B max = 0.26 mm

1 mm tc = 3 min V B B max = 0.28 mm

1 mm

1 mm

1 mm

1 mm

1 mm

1 mm

(b)

Fig. 3. Pictures of worn tools, referring to emulsion (a) and water (b) mist. Compressed air was used as droplet carrier gas.

When using the emulsion (of soluble oil in water) mist, and reducing the flow rate of the cutting fluid from 115 ml/min to 6.5 ml/min, the Taylor’s curves shift in parallel from the right to the left side of the graph, as shown in Figure 6a. As a result, at fixed process conditions, and particularly for the same cutting speed, an increase in cutting performance is obtained at higher flow rates. This evidence is likely due to the benefits deriving by the increase of the cooling effect [22]. Moreover, when changing the cutting fluid to pure water, and even retaining the same flow rate, tool life significantly decreases (Figure 6b). The different slope of Taylor’s curves accentuates the differences at lower cutting speeds. The lubricating effect of the emulsion mist has a non-negligible positive effect on the cutting mechanics. In addition, trials carried out with the lowest water flow rate (6.5 ml/min) did

345


0 0.25


0.20 0

0 0.15

(a)

50 m/min

200

55 m/min

0 0.10

vc

(a) E mulsion mist

100

60 m/min 70 m/min

0 0.05

Q = 115 ml/min

f = 0.1 mm/rev; ap = 0.3 mm 0

0

10

20

30

40

50

60

70

80

90

C utting time, tc (min)

M ax. flank wear land, V B B max (mm)

then supplied to the cutting area. The flow rate of both the cutting fluids was varied from 6.5 to 115 ml/min. Such range is higher with respect to the values assigned to MQCL systems by Weinert et al. [9]. However, it is comparable to other research studies. Results proved that, as far as tool life is concerned, the use of emulsion mist gave better outcomes than those of pure water mist. Moreover, when applying both the cutting fluids, the increase of flow rate improved tool life, due to the higher cooling effect.

0 0.25 E mulsion mist Q emulsion = 51 ml/min

0 0.20

(b)

Q = 51 ml/min

T ool life, TL (min)


not allowed to achieve a stable cutting process. Catastrophic tool failure was reached after few minutes of cutting time, and maximum tool life was less than 3 minutes, regardless of test conditions.

10 Q = 6.5 ml/min

1

0 0.15 40 m/min

30

50 m/min

0 0.10

vc

f = 0.1 mm/rev; ap = 0.3 mm

0.1 40

60

60 m/min

200

f = 0.1 mm/rev; ap = 0.3 mm

80

100

90

100

(b)

0 0

10

20

30

40

50

60

70

80

90





70

C utting speed, vc (m/min)

55 m/min

0 0.05

50

0.25 0 E mulsion mist Q emulsion = 6.5 ml/min

0 0.20

(c)

0 0.15

10

W ater mist Q water = 115 ml/min

1

45 m/min

0 0.10

50 m/min

vc 0 0.05

55 m/min

f = 0.1 mm/rev; ap = 0.3 mm

0.1

f = 0.1 mm/rev; ap = 0.3 mm

30

40

0

5

10

15

20

25

30


Experimental turning tests were performed on a Ti-48Al2Cr-2Nb γ-TiAl alloy. Low cutting fluid (either emulsion or water) volumes were nebulized by using compressed air, and

70

80

90

100

vc = 40 m/min; f = 0.1 mm/rev; ap = 0.3 mm E mulsion mist, Q emulsion = 6.5 ml/min

E mulsion mist, Q emulsion = 115 ml/min 0

4. Conclusions

60

Fig. 6.Taylor’s curves for emulsion mist (a) and water vs. emulsion mist (b)

Fig. 5. Typical tool wear curves applying emulsion mist. Flowrate: 115 ml/min (a), 51 ml/min (b), and 6.5 ml/min (c).

Roughness was measured on surfaces machined with fresh tools by using a Form Talysurf 120 profilometer. 3D roughness parameters Sa and Sz were calculated on rectangular areas of 3.8 × 3.9 mm, as shown in Figure 7. Sa and Sz indices were generally lower than 0.8 μm and 4.5 μm, respectively. The flow rate of cutting fluid did not seem to affect the results in a statistically significant way [16].

50


0

0.5

1

1.5

2

2.5

3

3.5 mm

0

0

0

0.5

0.5

1

1

1.5

1.5

2

2

2.5

2.5

3

3

3.5

3.5

mm m

mm m

6.2 μm

0.5

1

1.5

2

2.5

3

3.5 mm

7.8 μm

3.8 mm

3.9 mm

3.8 mm

Fig. 7.Surface roughness observations.

3.9 mm

346



200 100

MQ L Q oil = 0.3 ml/min

E mulsion mist (a) Q emulsion = 6.5 ml/min E mulsion mist Q em. = 51 ml/min E mulsion mist Q em. = 115 ml/min

10 DRY 1

0.1

WE T W ater mist Q flood = 10 l/min Q water = 115 ml/min

f = 0.1 mm/rev; ap = 0.3 mm 20

30

40

50

60

70

80

90 100

1.2 1,2

6 6.0 E mulsion mist

1,0 1.0

5 5.0

0,8 0.8

4 4.0

0,6 0.6

3 3.0

0,4 0.4

2 2.0

0,2 0.2

1 1.0

0,00

00 D R DY RWE L ET Y T M QW

MQ L

6.5 ml/min 51 ml/min 115 ml/min

Sz (μm)

Sa (μm)


(b)

Fig. 8.Influence of cutting conditions on tool life (a) and surface finish (b).

The comparison of the actual results with those formerly achieved [20] is shown in Figure 8. The tool life results with the emulsion mist are better than those of MQL with vegetable oil, comparable (or even better in some cases) to conventional flood cooling. The surface roughness indices allow to classify the emulsion mist in an intermediate position between standard wet lubrication and MQL. Results in [20] were achieved performing MQL and wet cutting by means of external nozzles. For the actual tests, the use of a tool holder designed to supply the lubricoolant to rake and flank face of the tool, together with the use of compressed air as droplet carrier gas, allowed a better penetration of the cutting fluid into the wedge between tool and chip. Some differences might also be addressed to the different kind of cutting fluids used in the trials. From the environmental point of view, when spraying cutting fluids, mist is dispersed in the surrounding environment [13, 23]. Close to the machine tool, an exhaust extraction system is needed for respecting allowed pollution limits inside of workplaces. Moreover, when using sprayed cutting fluids, the choice of the oil is critical. MQL exploits vegetable oils that are typically non-toxic, biodegradable, and manufactured from renewable raw materials. Soluble oils used in emulsions should have similar eco-properties. For Minimum Quantity Cooling, water is an environmentalfriendly cutting fluid. However, a reduction in process parameters is needed to achieve an acceptable tool life, penalizing the process productivity. The presented data might be useful to further studies aimed at assessing the environmental impact for different combinations of processes, workpiece materials, and lubricoolant strategies.

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