- Email: [email protected]
Micro-liter lubrication machining of Inconel 718
ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Micro-liter lubrication machining of Inconel 718 Toshiyuki Obikawa a,, Yasuhiro Kamata a, Yuki Asano b, Kousuke Nakayama b, Andrew W. Otieno c a b c
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-ku, Tokyo 153-8505, Japan Department of Mechanical and Control Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan Department of Technology, Northern Illinois University, Dekalb, IL 60115, USA
a r t i c l e in fo
abstract
Article history: Received 18 January 2008 Received in revised form 29 July 2008 Accepted 31 July 2008 Available online 3 August 2008
Minimum quantity lubrication (MQL) machining is one of the promising solutions to the requirement for decrease in cutting fluid consumption. This paper describes MQL machining in a range of oil consumption o1.0 ml/h, which is 10–100 times smaller than the consumption usually adopted in industries. MQL machining in this range is called micro-liter lubrication machining in this paper. A specially designed nozzle was used for concentrating small amounts of oil mist onto the cutting interface. The performance of concentrated spraying of oil mist in micro-liter lubrication machining of Inconel 718 was investigated and compared with that of ordinary spraying. This proved that the concentrated spraying of oil mist with a specially designed nozzle was quite effective in increasing tool life in the micro-liter lubrication range. & 2008 Elsevier Ltd. All rights reserved.
Keywords: MQL machining Inconel 718 Control of oil-mist flow
1. Introduction These days, it is widely known that cutting fluids have an impact on the environment and human health. Therefore, it is required to reduce the consumption of these fluids without decreasing tool life. The use of biodegradable oils that do not contain harmful elements and compounds such as extremepressure additives is also recommended. In addition, the power consumption and maintenance for operating a traditional cutting fluid supply system increase machining cost. Thus, if possible, it is desirable to remove such equipment from a machine tool. The most promising solution to these requirements is minimum quantity lubrication (MQL) machining, which has been applied to milling [1–3], drilling [4,5], turning [6,7] and so on. Studies on MQL show very promising results although not all of them have been compared to those of wet cutting [4–7]. In MQL machining, a small amount of biodegradable lubricant is sprayed to the cutting point with compressed air, typically at consumptions of 10–100 ml/h. Accordingly, the method used to spray oil mist to the cutting point affects the performance of MQL machining. For a certain amount of oil consumption, there are three important factors associated with the spray of oil mist: pressure of compressed air, spray direction and distance from the nozzle to the cutting point. Recently, it was found that tool wear decreased with increasing pressure of compressed air when the oil mist was sprayed to a semi-closed space as in the case of
Corresponding author. Tel.: +81 3 5452 6771; fax: +81 3 5452 6773.
E-mail address: [email protected] (T. Obikawa). 0890-6955/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2008.07.011
grooving [8]. On the other hand, minimal tool wear occurred at an optimum pressure when oil mist was sprayed into an open space, as in the case of turning [9,10]. It was also found that appropriate control of spray direction using a controlled oil-mist direction (COD) tool decreased tool wear effectively [8]. With respect to the position of the nozzle relative to the cutting point, it is natural that the concentration of oil mist decreases with the distance. Thus, a large fraction of oil sprayed may be waste, without lubricating the surface of the tool if the nozzle is not placed close to the cutting point. According to computational fluid dynamics (CFD) analysis of oil-mist flow, only a small fraction of oil mist contributes to the lubrication at an interface between the tool flank face and the finished workpiece surface [11]. Typically, the value of the fraction is approximately 0.01%, but depends on conditions for MQL machining [11]. By contrast, if the nozzle is very close to the cutting point, and oil mist is sprayed as a pinpointed jet from the side of the tool flank, the sprayed oil would enter the interface between the tool flank face and machined surface, thus act more effectively. However, it is still not easy to optimize the conditions of MQL because of insufficient information on effective method to spray oil mist. For the above reasons, specially designed nozzles were set very close to the tool tip in this study. The workpiece used was a nickel-base superalloy Inconel 718, a difficult-to-machine material characterized by unfavorable machining properties such as high temperature strength, hot hardness, low thermal conductivity and high chemical affinity to most tool materials [12,13]. These characteristics cause severe and rapid tool wear in high-speed machining. For this reason, finish machining of Inconel 718 has been carried out at low cutting
ARTICLE IN PRESS 1606
T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
speeds with flood coolant recommended for cooling the tool and workpiece. Nevertheless, a new trend arose in the machining of nickel-base superalloys from an environmental point of view [14] and MQL machining has been applied to the superalloys [6,9,10]. In this paper, the specially designed nozzles were applied to MQL turning of Inconel 718 and their performances were investigated based on tool life and surface finish. The ranges of oil consumption were set from 0.2 to 15 ml/h, in which machining at oil consumption of o1.0 ml/h is called micro-liter lubrication (mLL) machining.
2. Experimental details 2.1. Oil-mist supply system and three types of nozzles The schematic diagram of an oil-mist supply system is shown in Fig. 1. Cutting oil, a kind of biodegradable ester with properties given in Table 1, was supplied from a graduated cylinder gauge with a plunger pump at oil consumption rates Q of 0.20, 0.50, 1.1, 3.0 and 15 ml/h. Because the inner diameter and the minimum scale of the cylinder gauge were 2.45 mm and 0.02 ml, respectively, it was possible to measure the oil consumption accurately even if Q ¼ 0.2 ml/h. The ester and compressed air were supplied through inner and outer spaces of a double-walled pipe, respectively, to an inlet bore of a tool shank. They were mixed there to form oil mist. The mist was sprayed to the cutting point from only a nozzle on the tool flank. The pressure of compressed air P was 0.40 MPa. This pressure setting was chosen because it had been found to bring about the longest tool lives in previous studies [9,10].
Three types of nozzles were applied to MQL turning: an ordinary type, and cover types for normal and oblique spraying (Figs. 2–4, respectively). The angle of oblique spraying is 451. The effectiveness of oblique spraying was suggested by CFD analysis of oil-mist flow. Fig. 5 shows a calculated flow pattern of oil mist sprayed from the ordinary nozzle [11]. In this figure, oil mist is sprayed downward as in the case of practical CNC machining. The workpiece is taken into account in CFD analysis, but is neglected in flow pattern presentation in Fig. 5. The depth of cut, feedrate, air pressure, and position and shape of the nozzle are the same as in this study. It was seen that oil mist is likely to flow to the side cutting edge more than the end cutting edge. Hence, the covertype nozzle for oblique spraying was prepared to spray oil mist obliquely from the side of the end cutting edge so that sufficient oil mist is sprayed to the end cutting edge. A tool shank with the ordinary type of nozzle, 1.0 mm in diameter, was on the market. The distance from the nozzle to the tool tip l0 was 14.7 mm for the ordinary nozzle as shown in Fig. 2. A cover type of nozzle for normal spraying was prepared by putting three pieces of polyvinylchloride (PVC) sheet on the flank face to make a straight oil-mist guide from the ordinary nozzle, then covering the flank face with a sheet of cupper and leaving a nozzle for concentrating the oil mist to the tool tip. The distance from the nozzle to the tool tip was 4.8 mm, the same as the thickness of the insert, and the thicknesses of PVC and cupper sheets were 0.4 and 0.1 mm, respectively. This is illustrated in Fig. 3. Thus, the cover would not interfere with either the rotating workpiece bar or the insert indexing. The cross-sectional area of the nozzle was 3.90 mm2, approximately five times larger than the cross-sectional area of the ordinary-type nozzle. A cover type of nozzle for oblique spraying was prepared similar to that for normal spraying, but its oil-mist guide is bent sharply as shown in Fig. 4. The covers did not affect the oil consumption, but affected the flow rate of compressed air. The flow rate decreased as the nozzle was extended and became more complicated: 73.8, 72.7 and 66.0 Nl/min for an ordinary nozzle and cover-type nozzles for normal and oblique spraying, respectively. 2.2. Variation of oil consumption during pumping interval The plunger pump in Fig. 1 dispensed oil intermittently. Interval of pumping oil was not short: 1.0, 3.4, 3.4, 5.5 and 15.0 s for Q ¼ 15.0, 3.0, 1.1, 0.50 and 0.20 ml/h, respectively. Hence, change in the amount of oil mist sprayed from an ordinary nozzle during an interval was measured with an apparatus shown in
Fig. 1. Oil-mist supply system.
Table 1 Properties of biodegradable ester Density (15 1C) (g/cm3) Kinematic viscosity (40 1C) (mm2/s) Flash point (COC) (1C) Viscosity index Pour point (1C) Acid number (mg KOH/g) Biodegradability OECD301B (%)
0.95 19 250 137 o45.0 0.02 72
Fig. 2. Ordinary-type nozzle.
ARTICLE IN PRESS T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
1607
Fig. 3. Cover-type nozzle for normal spraying. (a) Tool without a cover of copper sheet and (b) complete tool.
Fig. 4. Cover-type nozzle for oblique spraying. (a) Tool without a cover of copper sheet and (b) complete tool.
Fig. 6. Four filter papers, A, B, C and D, 70 mm in diameter, which were each weighed with an electronic balance a priori, were placed on four quadrants of a round acrylic resin plate 184 mm in diameter and separated with four partitions of acrylic resin 60 mm high. The oil mist was sprayed from the ordinary nozzle normally to the round plate at three different oil consumptions of 0.22, 0.60 and 1.1 ml/h. The period of the plate rotation must be the same as that of pumping oil so that the pump could always dispense oil when the nozzle came to a position 67 mm right above the center of a filter paper on quadrant A. Thus, the interval of oil pumping was always fixed at 15.0 s for the three oil consumptions while the amount of oil pushed at a time was changed. After 120 turns, each sheet of filter paper was weighed again to note an increase in the paper’s weight due to oil absorption. Fig. 7 shows the weight increments normalized with respect to the value of filter paper A for each rate of oil consumption. It is natural that filter paper D showed the smallest increase in weight among the four filter papers. From the result, decrease in absorbed oil during an interval was 20% at worst. Therefore, even if the pumping interval increases to 15 s, oil mist was found to be supplied to the cutting point continuously and at a nearly constant rate. The inlet bore and unused piping holes in the tool shank must have worked as storages of oil and oil mist, and contributed to the continuous oil spraying.
2.3. Workpiece, tool and cutting conditions Table 2 shows the chemical composition of the forged workpiece bar of nickel-base superalloy Inconel 718. It was heat treated by solution annealing at 1000 1C for an hour followed by precipitation hardening at 721 1C for 8 h and at 620 1C for another 8 h. Vickers hardness, 0.2% proof stress, tensile strength and elongation of the workpiece are 440 HV, 1189, 1363 MPa and 22%, respectively. Before the cutting experiment, the outer part of the specimen over-hardened by heat treatment was machined, resulting in a reduction of diameter by 15 mm. Coated carbide inserts with multiple CVD coatings of TiCN/Al2O3/TiN were used in this study, but they are different from CVD inserts used in a previous study [10]. The types of tool insert and holder were DNMG150404 and PDJNR2525, respectively. Depth of cut d was set at 0.1 mm, feedrate f was 0.1 mm/rev and cutting speed was 1.3 m/s (78 m/min), a rather high cutting speed for Inconel 718. The small depth of cut and feedrate were selected on the supposition that a near-net-shaped workpiece would be finish-turned on a CNC lathe. Under these conditions, corner wear was dominant. This is because the nose radius of the insert R was 0.4 mm and ratio d/R ¼ 0.25 was less than the critical value for the development of large notch wear: 1sin(p/4)E0.29. Hence, tool life was defined as cutting distance or cutting time at which the corner wear reached a value of 0.20 mm. In wet cutting,
ARTICLE IN PRESS 1608
T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
Normalized mass of adherent oil
1.5 0.22ml/h
0.60ml/h
1.1ml/h
1.0
0.5
0 A
B
C
D
Fig. 7. Normalized weight increments of filter papers on four quadrants after 120 turns.
Table 2 Chemical composition of the workpiece, Inconel 718 (wt%)
Fig. 5. Calculated oil-mist flow pattern for the ordinary nozzle.
Ni
Fe
Cr
Mo
Al
Ti
Mn
Si
Nb
Co
C
52.3
19.35
18.5
2.98
0.51
0.99
0.04
0.05
5.02
0.2
0.02
meters, arithmetic mean roughness Ra and maximum height of the profile Rz were measured in the feed direction of the workpiece with a profilometer after the corner wear of a tool exceeded the tool life criterion of 0.2 mm, the smallest value for machining studies of Inconel 718 [6,15]. It should be noted that the surface measured was finished with a worn-out cutting edge, but not with a sharp one. Finally, the corner wear of worn-out tools were surveyed with a SEM for inspecting the adhesion of the work material and other phenomena peculiar to this study.
3. Results and discussion Changes in corner wear with cutting length obtained for MQL machining with the ordinary nozzle and cover-type nozzles for normal and oblique sprayings are shown in Figs. 8–10, respectively. Results for dry and wet cutting are also shown in these figures. It is seen that tool lives longer than 40 min were obtained for MQL and wet cutting. Now let eMQL denote the effectiveness of MQL, which is defined as eMQL ¼
Fig. 6. Experimental apparatus for measuring change in oil supply during pumping interval.
a flood of an emulsion type of cutting fluid with an oil concentration of 6.7% in volume was supplied to the cutting point at 3.7 l/min.
2.4. Measurement Tool wear was measured with a CCD digital microscope at a magnification of 100. The cross-sectional area of the cover-type nozzle was also measured with the CCD microscope using a function of closed area measurement. Two surface finish para-
T T dry T wet T dry
(1)
where T, Tdry and Twet are tool lives for MQL, dry and wet machining. The relationship between the effectiveness of MQL and oil consumption rate obtained from Figs. 8–10 is shown in Fig. 11. Unity and zero lines for the effectiveness of MQL correspond to the tool lives for wet and dry cuttings: 3752 and 1772 m in cutting length, and 48 and 23 min in cutting time, respectively. It is seen in Fig. 8 that when cutting oil was supplied at 15 ml/h, MQL machining with the ordinary-type nozzle exhibited a good tool life, 3459 m in cutting length and 44 min in cutting time, only 3.8 min shorter than wet cutting. An oil consumption of 15 ml/h for Inconel 718 is not too sufficient in comparison to recently reported values of 90 [6] and 16.8 ml/h [10]. Thus, MQL machining can be an alternative to wet cutting in case of finish turning of Inconel 718. However, decrease in oil consumption led to substantial reduction of tool life as was observed in the case of
ARTICLE IN PRESS T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
0
Tool life min 20 30
10
40
0
50
Tool life min 20 30
10
40
50
0.2 Corner wear mm
0.2 Corner wear mm
1609
0.1 Wet MQL ordinary type 0.2ml/h 3.0ml/h
0
1000
Dry 0.5ml/h 15.0ml/h
2000 Cutting length m
Dry Wet MQL cover type (Oblique spraying)
1.1ml/h
3000
0.2ml/h
4000
Fig. 8. Change in corner wear with cutting length for MQL machining with the ordinary nozzle. Cutting conditions: cutting speed, 1.3 m/s (78 m/min); depth of cut, 0.1 mm; feedrate, 0.1 mm/rev.
0
10
20
1000
1.1ml/h
0.5ml/h
2000 Cutting length m
3000
4000
Fig. 10. Change in corner wear with cutting length for MQL machining with the cover-type nozzle for oblique spraying.
1
Tool life min 0
0.1
30
40
50 0.8
0.2 eMQL
Corner wear mm
0.6
0.4
0.1 Ordinary type Cover type (Normal spraying) Cover type (Oblique spraying)
0.2 Wet Dry MQL cover type (Normal spraying) 0.2ml/h 0.5ml/h 3.0ml/h 15.0ml/h
1.1ml/h
0
0
1000
2000 Cutting length m
3000
4000
Fig. 9. Change in corner wear with cutting length for MQL machining with the cover-type nozzle for normal spraying.
intermittent turning of carbon steel [16], in which sprayed oil mist lubricated all the tool faces sufficiently when the tool was disengaged from cutting. Value of eMQL decreased to 0.50, 0.47 and 0.36 as oil consumption decreased to 1.1, 0.5 and 0.2 ml/h, respectively. From the results, it is seen that the application of mLL to MQL machining with an ordinary nozzle is not practical. The cover-type nozzle for normal spraying provided longer tool lives than the ordinary nozzle for the same oil consumption. Value of eMQL was 0.82, 0.90 and 0.92 for Q ¼ 1.1, 3.0 and 15 ml/h, respectively. MQL cuttings at oil consumption rates of 3 and 15 ml/h were approximately 2 min shorter in tool life than the wet cutting. Therefore, this nozzle can be applied to finish turning of Inconel 718 as long as Q43.0 ml/h. The concentration of oil mist to the cutting point must have caused better lubrication conditions between the worn tool flank and machined surface. In mLL machining range, change from the ordinary-type nozzle to this cover-type nozzle increased eMQL by 0.22 and 0.18 for Q ¼ 0.50 and 0.20 ml/h, respectively. However, these improvements were not sufficiently large to cause any significant changes in tool life.
5
10 Oil consumption ml/h
15
Fig. 11. Relationship between the effectiveness of MQL and oil consumption rate.
The cover-type nozzle for oblique spraying exhibited better performance than that for normal spraying, resulting in significant improvement of eMQL. Value of eMQL for oblique spraying was 0.80, 0.94 and 0.97 for Q ¼ 0.2, 0.5 and 1.1 ml/h, respectively. It should be noted in Fig. 11 that the cover-type nozzle for oblique spraying with Q ¼ 0.5 ml/h provided longer tool life than that for normal spraying with Q ¼ 15 ml/h and the ordinary nozzle with Q ¼ 15 ml/h. The improvement in tool life using oblique flow of oil mist is consistent with the results of the CFD analysis mentioned above. These results proved that MQL machining with quite a small amount of cutting oil, which is truly close to dry machining, can be accomplished by controlling the flow of oil mist. The relation between the mean concentration of oil mist at the cutting edge qe and the effectiveness of MQL eMQL may be expressed as eMQL ¼ f(qe). When oil mist is sprayed directly to the open air, the specific flow rate of oil mist per unit area and unit time q decreases substantially with the inverse of the square of the distance; thus, q¼
Q ðl þ lnozzle Þ2
(2)
ARTICLE IN PRESS 1610
T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
where l is the distance from the nozzle and lnozzle is the equivalent distance from a virtual point source of oil mist to the exit of nozzle. Similarly, in the case of turning with ordinary-type nozzle and cover-type nozzle for normal spraying, the specific flow rate would be approximated by Q ðl þ lnozzle Þa
0.6
(3)
where a is an exponent. Substitution of l ¼ 0 and q ¼ Q/Anozzle into Eq. (3) results in lnozzle ¼ ðAnozzle Þ
0.8
1=a
eMQL
q¼
1
0.4
(4)
Normal spraying
0.2
where Anozzle is the cross-sectional area of nozzle exit. The specific flow rate at the cutting edge qt is given by Q qt ¼ ðlt þ lnozzle Þa
Q ðlt þ lnozzle Þa
-3
1
0.8
eMQL
0.6
0.4
0.2
Ordinary type Cover type (Normal spraying)
0 -2
-1
0
log qt Fig. 12. Relationship between the specific flow rate at the cutting edge and the effectiveness of MQL for the ordinary-type and cover-type nozzles for normal spraying.
-2
-1
0
log qt Fig. 13. Relationship between the specific flow rate at the cutting edge and the effectiveness of MQL for the two cover-type nozzles.
(6)
Relationships between log qt and eMQL for the nozzles of ordinary-type and normal-spraying cover-type are shown in Fig. 12. It was assumed that data points obtained for the two nozzles except two data at an oil consumption of 15.0 ml/h had a linear relation eMQL ¼ a+b log qt. Then, the value of a was calculated at 1.85 by maximizing correlation coefficient between log qt and eMQL for the eight data points. The fact that this value of a is close to two suggests that sprayings of compressed air to both the wedge-shaped space and the open air may cause similar volume expansion. Substitution of a ¼ 1.85 into Eq. (5) revealed that changing from the ordinary nozzle to the cover-type nozzle would increase the value of qt by a factor of 4.26. Therefore, shortening the distance from the nozzle to the cutting edge has a direct affect on improving tool life. For the two cover-type nozzles, relationship between log qt and eMQL is shown in Fig. 13, where a ¼ 1.85 as obtained above, and k ¼ 1 and 10. Value of k moves the line for oblique spraying to the right direction by log k. When k ¼ 10, the two lines for the two cover-type nozzles are closest. Then, from Figs. 12 and 13, difference in value of log qt between the shifted line for oblique
-3
0
2 MQL Ordinary type MQL Cover type (Normal spraying) MQL Cover type (Oblique spraying)
Surface roughness Ra µm
qt ¼ k
Oblique spraying (k = 10)
(5)
where lt is the distance from a nozzle to the cutting edge. For the cover-type nozzle for oblique spraying, which could increase the spray efficiency of oil mist by a factor of k, the following equation may hold:
Oblique spraying (k = 1)
Wet
1 Theory Dry
0
5 10 Oil consumption ml/h
15
Fig. 14. Comparison of arithmetic mean surface roughness for the different lubrication conditions.
spraying and the line for ordinary nozzle is found to be 1.45 for the same oil consumption. Namely, the efficiency of oil spraying is increased by a factor of 101.45 ¼ 28.4. Therefore, short distance lt and control of oil-mist flow are quite effective for enhancing the cutting performance of MQL machining at smaller amounts of oil consumption. Surface finishes Ra and Rz measured after the end of tool service lives are shown in Figs. 14 and 15, respectively. Surface finishes with worn-out tools were fairly good and some data were close to or better than the theoretical values Ra ¼ 0.0321 f2/r and Rz ¼ f2/8r. It was seen that the surface finishes obtained by MQL cutting were always better than those by wet cutting. Similar results were reported in intermittent turning of alloy steel [16]. These results also proved that MQL machining can be an alternative to wet cutting in case of finish turning of Inconel 718. Reasons for good surface finishes obtained above can be found in the SEM micrographs of a worn-out flank face. Fig. 16(a) shows the whole view of corner wear of a tool insert used in dry cutting. As seen in Figs. 14 and 15, this tool generated one of the best surfaces with quite a small roughness though no lubricant was applied to the interface between the tool flank face and finished surface. Three-dimensional display of a part of the wear land in
ARTICLE IN PRESS T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
8
Surface roughness Rz µm
MQL Ordinary type MQL Cover type (Normal spraying) MQL Cover type (Oblique spraying)
6 Wet
4
1611
worn area would be about 50 mm in the cutting direction. Hence, the outer area, which was quite smooth and free from adhesion of the workpiece, must be the worn coating layer. This area finished the machined surface finally; therefore, one of the reasons for good surface finish should be attributed to the very smooth surface of the worn coating layer. The other reason may be attributed to no visible notch and scratch on the flank face, especially on the worn coating layers.
Dry
Theory
4. Conclusions
2
0
5 10 Oil consumption ml/h
15
Fig. 15. Comparison of the maximum height of surface profile for the different lubrication conditions.
MQL finish turning of Inconel 718 in a range of mLL, oil consumption o1.0 ml/h, was investigated. Cover-type nozzles were specially designed and used for concentrating small amounts of oil mist to the cutting point and increasing the effectiveness of MQL. The performance of concentrated spraying was quantitatively compared with that of ordinary spraying. As a result, it was found that the control of the flow of oil mist increases the efficiency of oil spraying drastically and makes the MQL machining more stable. When the cover-type nozzle for oblique spraying was used, the efficiency of oil spraying was increased by a factor of 28.4, and the tool life at an oil consumption of 0.5 ml/h and a higher cutting speed of 1.3 m/s was extended to 47 min, only 1 min shorter than that for wet cutting. From the above results, it is concluded that control of oilmist flow and shortening of the distance from the nozzle to the tool tip are quite effective for enhancing the cutting performance of MQL machining, especially in the mLL range.
Acknowledgments The authors acknowledge Mitsubishi Materials Corporation for coated carbide tool inserts and Nippon Oil Corporation for biodegradable cutting oil used in this research. This research was partially supported by the Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B) (2), 17360060, 2005 and that for Young Scientists (B) 18760094, 2006. References
Fig. 16. SEM micrographs of tool wear. (a) Whole view of corner wear and (b) 3-D display of the lower part of wear land.
Fig. 16(b) shows that there was inappreciable adhesion of the workpiece in the outer worn area. Because the coating layer of TiCN/Al2O3/TiN was about 5-mm thick in total, its width on the
[1] Y.S. Liao, H.M. Lin, Y.C. Chen, Feasibility study of the minimum quantity lubrication in high-speed end milling of NAK80 hardened steel by coated carbide tool, International Journal of Machine Tools & Manufacture 47 (11) (2007) 1667–1676. [2] M. Rahman, A.S. Kumar, M.U. Salam, Experimental evaluation on the effect of minimal quantities of lubricant in milling, International Journal of Machine Tools & Manufacture 42 (5) (2002) 539–547. [3] H.A. Kishawy, M. Dumitrescu, E.G. Ng, M.A. Elbestawi, Effect of coolant strategy on tool performance, chip morphology and surface quality during high-speed machining of A356 aluminum alloy, International Journal of Machine Tools & Manufacture 45 (2) (2005) 219–227. [4] R. Heinemann, S. Hinduja, G. Barrow, G. Petuelli, Effect of MQL on the tool life of small twist drills in deep-hole drilling, International Journal of Machine Tools & Manufacture 46 (1) (2006) 1–6. [5] H. Hanyu, S. Kamiya, Y. Murakami, M. Saka, Dry and semi-dry machining using finely crystallized diamond coating cutting tools, Surface & Coatings Technology 173–174 (2003) 992–995. [6] Y. Su, N. He, L. Li, A. Iqbal, M.H. Xiao, S. Xu, B.G. Qiu, Refrigerated cooling air cutting of difficult-to-cut materials, International Journal of Machine Tools & Manufacture 47 (6) (2007) 927–933. [7] N.R. Dhar, M.T. Ahmed, S. Islam, An experimental investigation on effect of minimum quantity lubrication in machining AISI 1040 steel, International Journal of Machine Tools & Manufacture 47 (5) (2007) 748–753. [8] Toshiyuki Obikawa, Yasuhiro Kamata, Jun Shinozuka, High speed grooving with applying MQL, International Journal of Machine Tools & Manufacture 46 (14) (2006) 1854–1861. [9] Toshiyuki Obikawa, Yasuhiro Kamata, MQL cutting of nickel base superalloy with a super lattice coating tool, Key Engineering Materials 291 (2005) 433–438.
ARTICLE IN PRESS 1612
T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
[10] Y. Kamata, T. Obikawa, High speed MQL finish-turning of Inconel 718 with different coated tools, Journal of Materials Processing Technology 192 (2007) 281–286. [11] Yasuhiro Kamata, Oil mist flow and its tool wear suppression mechanisms in MQL cutting, PhD thesis, Tokyo Institute of Technology, 2006. [12] W.F. Smith, Structure and Properties of Engineering Alloys, McGraw-Hill, New York, 1981. [13] T.H.C. Childs, K. Maekawa, T. Obikawa, Y. Yamane, Metal Machining—Theory and Application, Arnold, London, 2000.
[14] D. Dudzinski, A. Devillez, A. Moufki, D. Larrouquere, V. Zerrouki, J. Vigneau, A review of developments towards dry and high speed machining of Inconel 718 alloy, International Journal of Machine Tools & Manufacture 44 (2) (2004) 439–456. [15] C. Ducros, F. Sanchette, Multilayered and nanolayered hard nitride thin films deposited by cathodicarc evaporation, Surface & Coatings Technology 201 (2006) 1045–1052. [16] T. Wakabayashi, H. Sato, I. Inasaki, Turning using extremely small amounts of cutting fluids, JSME International Journal, Series C 41 (1) (1998) 143–148.
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Micro-liter lubrication machining of Inconel 718 Toshiyuki Obikawa a,, Yasuhiro Kamata a, Yuki Asano b, Kousuke Nakayama b, Andrew W. Otieno c a b c
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-ku, Tokyo 153-8505, Japan Department of Mechanical and Control Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan Department of Technology, Northern Illinois University, Dekalb, IL 60115, USA
a r t i c l e in fo
abstract
Article history: Received 18 January 2008 Received in revised form 29 July 2008 Accepted 31 July 2008 Available online 3 August 2008
Minimum quantity lubrication (MQL) machining is one of the promising solutions to the requirement for decrease in cutting fluid consumption. This paper describes MQL machining in a range of oil consumption o1.0 ml/h, which is 10–100 times smaller than the consumption usually adopted in industries. MQL machining in this range is called micro-liter lubrication machining in this paper. A specially designed nozzle was used for concentrating small amounts of oil mist onto the cutting interface. The performance of concentrated spraying of oil mist in micro-liter lubrication machining of Inconel 718 was investigated and compared with that of ordinary spraying. This proved that the concentrated spraying of oil mist with a specially designed nozzle was quite effective in increasing tool life in the micro-liter lubrication range. & 2008 Elsevier Ltd. All rights reserved.
Keywords: MQL machining Inconel 718 Control of oil-mist flow
1. Introduction These days, it is widely known that cutting fluids have an impact on the environment and human health. Therefore, it is required to reduce the consumption of these fluids without decreasing tool life. The use of biodegradable oils that do not contain harmful elements and compounds such as extremepressure additives is also recommended. In addition, the power consumption and maintenance for operating a traditional cutting fluid supply system increase machining cost. Thus, if possible, it is desirable to remove such equipment from a machine tool. The most promising solution to these requirements is minimum quantity lubrication (MQL) machining, which has been applied to milling [1–3], drilling [4,5], turning [6,7] and so on. Studies on MQL show very promising results although not all of them have been compared to those of wet cutting [4–7]. In MQL machining, a small amount of biodegradable lubricant is sprayed to the cutting point with compressed air, typically at consumptions of 10–100 ml/h. Accordingly, the method used to spray oil mist to the cutting point affects the performance of MQL machining. For a certain amount of oil consumption, there are three important factors associated with the spray of oil mist: pressure of compressed air, spray direction and distance from the nozzle to the cutting point. Recently, it was found that tool wear decreased with increasing pressure of compressed air when the oil mist was sprayed to a semi-closed space as in the case of
Corresponding author. Tel.: +81 3 5452 6771; fax: +81 3 5452 6773.
E-mail address: [email protected] (T. Obikawa). 0890-6955/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2008.07.011
grooving [8]. On the other hand, minimal tool wear occurred at an optimum pressure when oil mist was sprayed into an open space, as in the case of turning [9,10]. It was also found that appropriate control of spray direction using a controlled oil-mist direction (COD) tool decreased tool wear effectively [8]. With respect to the position of the nozzle relative to the cutting point, it is natural that the concentration of oil mist decreases with the distance. Thus, a large fraction of oil sprayed may be waste, without lubricating the surface of the tool if the nozzle is not placed close to the cutting point. According to computational fluid dynamics (CFD) analysis of oil-mist flow, only a small fraction of oil mist contributes to the lubrication at an interface between the tool flank face and the finished workpiece surface [11]. Typically, the value of the fraction is approximately 0.01%, but depends on conditions for MQL machining [11]. By contrast, if the nozzle is very close to the cutting point, and oil mist is sprayed as a pinpointed jet from the side of the tool flank, the sprayed oil would enter the interface between the tool flank face and machined surface, thus act more effectively. However, it is still not easy to optimize the conditions of MQL because of insufficient information on effective method to spray oil mist. For the above reasons, specially designed nozzles were set very close to the tool tip in this study. The workpiece used was a nickel-base superalloy Inconel 718, a difficult-to-machine material characterized by unfavorable machining properties such as high temperature strength, hot hardness, low thermal conductivity and high chemical affinity to most tool materials [12,13]. These characteristics cause severe and rapid tool wear in high-speed machining. For this reason, finish machining of Inconel 718 has been carried out at low cutting
ARTICLE IN PRESS 1606
T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
speeds with flood coolant recommended for cooling the tool and workpiece. Nevertheless, a new trend arose in the machining of nickel-base superalloys from an environmental point of view [14] and MQL machining has been applied to the superalloys [6,9,10]. In this paper, the specially designed nozzles were applied to MQL turning of Inconel 718 and their performances were investigated based on tool life and surface finish. The ranges of oil consumption were set from 0.2 to 15 ml/h, in which machining at oil consumption of o1.0 ml/h is called micro-liter lubrication (mLL) machining.
2. Experimental details 2.1. Oil-mist supply system and three types of nozzles The schematic diagram of an oil-mist supply system is shown in Fig. 1. Cutting oil, a kind of biodegradable ester with properties given in Table 1, was supplied from a graduated cylinder gauge with a plunger pump at oil consumption rates Q of 0.20, 0.50, 1.1, 3.0 and 15 ml/h. Because the inner diameter and the minimum scale of the cylinder gauge were 2.45 mm and 0.02 ml, respectively, it was possible to measure the oil consumption accurately even if Q ¼ 0.2 ml/h. The ester and compressed air were supplied through inner and outer spaces of a double-walled pipe, respectively, to an inlet bore of a tool shank. They were mixed there to form oil mist. The mist was sprayed to the cutting point from only a nozzle on the tool flank. The pressure of compressed air P was 0.40 MPa. This pressure setting was chosen because it had been found to bring about the longest tool lives in previous studies [9,10].
Three types of nozzles were applied to MQL turning: an ordinary type, and cover types for normal and oblique spraying (Figs. 2–4, respectively). The angle of oblique spraying is 451. The effectiveness of oblique spraying was suggested by CFD analysis of oil-mist flow. Fig. 5 shows a calculated flow pattern of oil mist sprayed from the ordinary nozzle [11]. In this figure, oil mist is sprayed downward as in the case of practical CNC machining. The workpiece is taken into account in CFD analysis, but is neglected in flow pattern presentation in Fig. 5. The depth of cut, feedrate, air pressure, and position and shape of the nozzle are the same as in this study. It was seen that oil mist is likely to flow to the side cutting edge more than the end cutting edge. Hence, the covertype nozzle for oblique spraying was prepared to spray oil mist obliquely from the side of the end cutting edge so that sufficient oil mist is sprayed to the end cutting edge. A tool shank with the ordinary type of nozzle, 1.0 mm in diameter, was on the market. The distance from the nozzle to the tool tip l0 was 14.7 mm for the ordinary nozzle as shown in Fig. 2. A cover type of nozzle for normal spraying was prepared by putting three pieces of polyvinylchloride (PVC) sheet on the flank face to make a straight oil-mist guide from the ordinary nozzle, then covering the flank face with a sheet of cupper and leaving a nozzle for concentrating the oil mist to the tool tip. The distance from the nozzle to the tool tip was 4.8 mm, the same as the thickness of the insert, and the thicknesses of PVC and cupper sheets were 0.4 and 0.1 mm, respectively. This is illustrated in Fig. 3. Thus, the cover would not interfere with either the rotating workpiece bar or the insert indexing. The cross-sectional area of the nozzle was 3.90 mm2, approximately five times larger than the cross-sectional area of the ordinary-type nozzle. A cover type of nozzle for oblique spraying was prepared similar to that for normal spraying, but its oil-mist guide is bent sharply as shown in Fig. 4. The covers did not affect the oil consumption, but affected the flow rate of compressed air. The flow rate decreased as the nozzle was extended and became more complicated: 73.8, 72.7 and 66.0 Nl/min for an ordinary nozzle and cover-type nozzles for normal and oblique spraying, respectively. 2.2. Variation of oil consumption during pumping interval The plunger pump in Fig. 1 dispensed oil intermittently. Interval of pumping oil was not short: 1.0, 3.4, 3.4, 5.5 and 15.0 s for Q ¼ 15.0, 3.0, 1.1, 0.50 and 0.20 ml/h, respectively. Hence, change in the amount of oil mist sprayed from an ordinary nozzle during an interval was measured with an apparatus shown in
Fig. 1. Oil-mist supply system.
Table 1 Properties of biodegradable ester Density (15 1C) (g/cm3) Kinematic viscosity (40 1C) (mm2/s) Flash point (COC) (1C) Viscosity index Pour point (1C) Acid number (mg KOH/g) Biodegradability OECD301B (%)
0.95 19 250 137 o45.0 0.02 72
Fig. 2. Ordinary-type nozzle.
ARTICLE IN PRESS T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
1607
Fig. 3. Cover-type nozzle for normal spraying. (a) Tool without a cover of copper sheet and (b) complete tool.
Fig. 4. Cover-type nozzle for oblique spraying. (a) Tool without a cover of copper sheet and (b) complete tool.
Fig. 6. Four filter papers, A, B, C and D, 70 mm in diameter, which were each weighed with an electronic balance a priori, were placed on four quadrants of a round acrylic resin plate 184 mm in diameter and separated with four partitions of acrylic resin 60 mm high. The oil mist was sprayed from the ordinary nozzle normally to the round plate at three different oil consumptions of 0.22, 0.60 and 1.1 ml/h. The period of the plate rotation must be the same as that of pumping oil so that the pump could always dispense oil when the nozzle came to a position 67 mm right above the center of a filter paper on quadrant A. Thus, the interval of oil pumping was always fixed at 15.0 s for the three oil consumptions while the amount of oil pushed at a time was changed. After 120 turns, each sheet of filter paper was weighed again to note an increase in the paper’s weight due to oil absorption. Fig. 7 shows the weight increments normalized with respect to the value of filter paper A for each rate of oil consumption. It is natural that filter paper D showed the smallest increase in weight among the four filter papers. From the result, decrease in absorbed oil during an interval was 20% at worst. Therefore, even if the pumping interval increases to 15 s, oil mist was found to be supplied to the cutting point continuously and at a nearly constant rate. The inlet bore and unused piping holes in the tool shank must have worked as storages of oil and oil mist, and contributed to the continuous oil spraying.
2.3. Workpiece, tool and cutting conditions Table 2 shows the chemical composition of the forged workpiece bar of nickel-base superalloy Inconel 718. It was heat treated by solution annealing at 1000 1C for an hour followed by precipitation hardening at 721 1C for 8 h and at 620 1C for another 8 h. Vickers hardness, 0.2% proof stress, tensile strength and elongation of the workpiece are 440 HV, 1189, 1363 MPa and 22%, respectively. Before the cutting experiment, the outer part of the specimen over-hardened by heat treatment was machined, resulting in a reduction of diameter by 15 mm. Coated carbide inserts with multiple CVD coatings of TiCN/Al2O3/TiN were used in this study, but they are different from CVD inserts used in a previous study [10]. The types of tool insert and holder were DNMG150404 and PDJNR2525, respectively. Depth of cut d was set at 0.1 mm, feedrate f was 0.1 mm/rev and cutting speed was 1.3 m/s (78 m/min), a rather high cutting speed for Inconel 718. The small depth of cut and feedrate were selected on the supposition that a near-net-shaped workpiece would be finish-turned on a CNC lathe. Under these conditions, corner wear was dominant. This is because the nose radius of the insert R was 0.4 mm and ratio d/R ¼ 0.25 was less than the critical value for the development of large notch wear: 1sin(p/4)E0.29. Hence, tool life was defined as cutting distance or cutting time at which the corner wear reached a value of 0.20 mm. In wet cutting,
ARTICLE IN PRESS 1608
T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
Normalized mass of adherent oil
1.5 0.22ml/h
0.60ml/h
1.1ml/h
1.0
0.5
0 A
B
C
D
Fig. 7. Normalized weight increments of filter papers on four quadrants after 120 turns.
Table 2 Chemical composition of the workpiece, Inconel 718 (wt%)
Fig. 5. Calculated oil-mist flow pattern for the ordinary nozzle.
Ni
Fe
Cr
Mo
Al
Ti
Mn
Si
Nb
Co
C
52.3
19.35
18.5
2.98
0.51
0.99
0.04
0.05
5.02
0.2
0.02
meters, arithmetic mean roughness Ra and maximum height of the profile Rz were measured in the feed direction of the workpiece with a profilometer after the corner wear of a tool exceeded the tool life criterion of 0.2 mm, the smallest value for machining studies of Inconel 718 [6,15]. It should be noted that the surface measured was finished with a worn-out cutting edge, but not with a sharp one. Finally, the corner wear of worn-out tools were surveyed with a SEM for inspecting the adhesion of the work material and other phenomena peculiar to this study.
3. Results and discussion Changes in corner wear with cutting length obtained for MQL machining with the ordinary nozzle and cover-type nozzles for normal and oblique sprayings are shown in Figs. 8–10, respectively. Results for dry and wet cutting are also shown in these figures. It is seen that tool lives longer than 40 min were obtained for MQL and wet cutting. Now let eMQL denote the effectiveness of MQL, which is defined as eMQL ¼
Fig. 6. Experimental apparatus for measuring change in oil supply during pumping interval.
a flood of an emulsion type of cutting fluid with an oil concentration of 6.7% in volume was supplied to the cutting point at 3.7 l/min.
2.4. Measurement Tool wear was measured with a CCD digital microscope at a magnification of 100. The cross-sectional area of the cover-type nozzle was also measured with the CCD microscope using a function of closed area measurement. Two surface finish para-
T T dry T wet T dry
(1)
where T, Tdry and Twet are tool lives for MQL, dry and wet machining. The relationship between the effectiveness of MQL and oil consumption rate obtained from Figs. 8–10 is shown in Fig. 11. Unity and zero lines for the effectiveness of MQL correspond to the tool lives for wet and dry cuttings: 3752 and 1772 m in cutting length, and 48 and 23 min in cutting time, respectively. It is seen in Fig. 8 that when cutting oil was supplied at 15 ml/h, MQL machining with the ordinary-type nozzle exhibited a good tool life, 3459 m in cutting length and 44 min in cutting time, only 3.8 min shorter than wet cutting. An oil consumption of 15 ml/h for Inconel 718 is not too sufficient in comparison to recently reported values of 90 [6] and 16.8 ml/h [10]. Thus, MQL machining can be an alternative to wet cutting in case of finish turning of Inconel 718. However, decrease in oil consumption led to substantial reduction of tool life as was observed in the case of
ARTICLE IN PRESS T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
0
Tool life min 20 30
10
40
0
50
Tool life min 20 30
10
40
50
0.2 Corner wear mm
0.2 Corner wear mm
1609
0.1 Wet MQL ordinary type 0.2ml/h 3.0ml/h
0
1000
Dry 0.5ml/h 15.0ml/h
2000 Cutting length m
Dry Wet MQL cover type (Oblique spraying)
1.1ml/h
3000
0.2ml/h
4000
Fig. 8. Change in corner wear with cutting length for MQL machining with the ordinary nozzle. Cutting conditions: cutting speed, 1.3 m/s (78 m/min); depth of cut, 0.1 mm; feedrate, 0.1 mm/rev.
0
10
20
1000
1.1ml/h
0.5ml/h
2000 Cutting length m
3000
4000
Fig. 10. Change in corner wear with cutting length for MQL machining with the cover-type nozzle for oblique spraying.
1
Tool life min 0
0.1
30
40
50 0.8
0.2 eMQL
Corner wear mm
0.6
0.4
0.1 Ordinary type Cover type (Normal spraying) Cover type (Oblique spraying)
0.2 Wet Dry MQL cover type (Normal spraying) 0.2ml/h 0.5ml/h 3.0ml/h 15.0ml/h
1.1ml/h
0
0
1000
2000 Cutting length m
3000
4000
Fig. 9. Change in corner wear with cutting length for MQL machining with the cover-type nozzle for normal spraying.
intermittent turning of carbon steel [16], in which sprayed oil mist lubricated all the tool faces sufficiently when the tool was disengaged from cutting. Value of eMQL decreased to 0.50, 0.47 and 0.36 as oil consumption decreased to 1.1, 0.5 and 0.2 ml/h, respectively. From the results, it is seen that the application of mLL to MQL machining with an ordinary nozzle is not practical. The cover-type nozzle for normal spraying provided longer tool lives than the ordinary nozzle for the same oil consumption. Value of eMQL was 0.82, 0.90 and 0.92 for Q ¼ 1.1, 3.0 and 15 ml/h, respectively. MQL cuttings at oil consumption rates of 3 and 15 ml/h were approximately 2 min shorter in tool life than the wet cutting. Therefore, this nozzle can be applied to finish turning of Inconel 718 as long as Q43.0 ml/h. The concentration of oil mist to the cutting point must have caused better lubrication conditions between the worn tool flank and machined surface. In mLL machining range, change from the ordinary-type nozzle to this cover-type nozzle increased eMQL by 0.22 and 0.18 for Q ¼ 0.50 and 0.20 ml/h, respectively. However, these improvements were not sufficiently large to cause any significant changes in tool life.
5
10 Oil consumption ml/h
15
Fig. 11. Relationship between the effectiveness of MQL and oil consumption rate.
The cover-type nozzle for oblique spraying exhibited better performance than that for normal spraying, resulting in significant improvement of eMQL. Value of eMQL for oblique spraying was 0.80, 0.94 and 0.97 for Q ¼ 0.2, 0.5 and 1.1 ml/h, respectively. It should be noted in Fig. 11 that the cover-type nozzle for oblique spraying with Q ¼ 0.5 ml/h provided longer tool life than that for normal spraying with Q ¼ 15 ml/h and the ordinary nozzle with Q ¼ 15 ml/h. The improvement in tool life using oblique flow of oil mist is consistent with the results of the CFD analysis mentioned above. These results proved that MQL machining with quite a small amount of cutting oil, which is truly close to dry machining, can be accomplished by controlling the flow of oil mist. The relation between the mean concentration of oil mist at the cutting edge qe and the effectiveness of MQL eMQL may be expressed as eMQL ¼ f(qe). When oil mist is sprayed directly to the open air, the specific flow rate of oil mist per unit area and unit time q decreases substantially with the inverse of the square of the distance; thus, q¼
Q ðl þ lnozzle Þ2
(2)
ARTICLE IN PRESS 1610
T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
where l is the distance from the nozzle and lnozzle is the equivalent distance from a virtual point source of oil mist to the exit of nozzle. Similarly, in the case of turning with ordinary-type nozzle and cover-type nozzle for normal spraying, the specific flow rate would be approximated by Q ðl þ lnozzle Þa
0.6
(3)
where a is an exponent. Substitution of l ¼ 0 and q ¼ Q/Anozzle into Eq. (3) results in lnozzle ¼ ðAnozzle Þ
0.8
1=a
eMQL
q¼
1
0.4
(4)
Normal spraying
0.2
where Anozzle is the cross-sectional area of nozzle exit. The specific flow rate at the cutting edge qt is given by Q qt ¼ ðlt þ lnozzle Þa
Q ðlt þ lnozzle Þa
-3
1
0.8
eMQL
0.6
0.4
0.2
Ordinary type Cover type (Normal spraying)
0 -2
-1
0
log qt Fig. 12. Relationship between the specific flow rate at the cutting edge and the effectiveness of MQL for the ordinary-type and cover-type nozzles for normal spraying.
-2
-1
0
log qt Fig. 13. Relationship between the specific flow rate at the cutting edge and the effectiveness of MQL for the two cover-type nozzles.
(6)
Relationships between log qt and eMQL for the nozzles of ordinary-type and normal-spraying cover-type are shown in Fig. 12. It was assumed that data points obtained for the two nozzles except two data at an oil consumption of 15.0 ml/h had a linear relation eMQL ¼ a+b log qt. Then, the value of a was calculated at 1.85 by maximizing correlation coefficient between log qt and eMQL for the eight data points. The fact that this value of a is close to two suggests that sprayings of compressed air to both the wedge-shaped space and the open air may cause similar volume expansion. Substitution of a ¼ 1.85 into Eq. (5) revealed that changing from the ordinary nozzle to the cover-type nozzle would increase the value of qt by a factor of 4.26. Therefore, shortening the distance from the nozzle to the cutting edge has a direct affect on improving tool life. For the two cover-type nozzles, relationship between log qt and eMQL is shown in Fig. 13, where a ¼ 1.85 as obtained above, and k ¼ 1 and 10. Value of k moves the line for oblique spraying to the right direction by log k. When k ¼ 10, the two lines for the two cover-type nozzles are closest. Then, from Figs. 12 and 13, difference in value of log qt between the shifted line for oblique
-3
0
2 MQL Ordinary type MQL Cover type (Normal spraying) MQL Cover type (Oblique spraying)
Surface roughness Ra µm
qt ¼ k
Oblique spraying (k = 10)
(5)
where lt is the distance from a nozzle to the cutting edge. For the cover-type nozzle for oblique spraying, which could increase the spray efficiency of oil mist by a factor of k, the following equation may hold:
Oblique spraying (k = 1)
Wet
1 Theory Dry
0
5 10 Oil consumption ml/h
15
Fig. 14. Comparison of arithmetic mean surface roughness for the different lubrication conditions.
spraying and the line for ordinary nozzle is found to be 1.45 for the same oil consumption. Namely, the efficiency of oil spraying is increased by a factor of 101.45 ¼ 28.4. Therefore, short distance lt and control of oil-mist flow are quite effective for enhancing the cutting performance of MQL machining at smaller amounts of oil consumption. Surface finishes Ra and Rz measured after the end of tool service lives are shown in Figs. 14 and 15, respectively. Surface finishes with worn-out tools were fairly good and some data were close to or better than the theoretical values Ra ¼ 0.0321 f2/r and Rz ¼ f2/8r. It was seen that the surface finishes obtained by MQL cutting were always better than those by wet cutting. Similar results were reported in intermittent turning of alloy steel [16]. These results also proved that MQL machining can be an alternative to wet cutting in case of finish turning of Inconel 718. Reasons for good surface finishes obtained above can be found in the SEM micrographs of a worn-out flank face. Fig. 16(a) shows the whole view of corner wear of a tool insert used in dry cutting. As seen in Figs. 14 and 15, this tool generated one of the best surfaces with quite a small roughness though no lubricant was applied to the interface between the tool flank face and finished surface. Three-dimensional display of a part of the wear land in
ARTICLE IN PRESS T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
8
Surface roughness Rz µm
MQL Ordinary type MQL Cover type (Normal spraying) MQL Cover type (Oblique spraying)
6 Wet
4
1611
worn area would be about 50 mm in the cutting direction. Hence, the outer area, which was quite smooth and free from adhesion of the workpiece, must be the worn coating layer. This area finished the machined surface finally; therefore, one of the reasons for good surface finish should be attributed to the very smooth surface of the worn coating layer. The other reason may be attributed to no visible notch and scratch on the flank face, especially on the worn coating layers.
Dry
Theory
4. Conclusions
2
0
5 10 Oil consumption ml/h
15
Fig. 15. Comparison of the maximum height of surface profile for the different lubrication conditions.
MQL finish turning of Inconel 718 in a range of mLL, oil consumption o1.0 ml/h, was investigated. Cover-type nozzles were specially designed and used for concentrating small amounts of oil mist to the cutting point and increasing the effectiveness of MQL. The performance of concentrated spraying was quantitatively compared with that of ordinary spraying. As a result, it was found that the control of the flow of oil mist increases the efficiency of oil spraying drastically and makes the MQL machining more stable. When the cover-type nozzle for oblique spraying was used, the efficiency of oil spraying was increased by a factor of 28.4, and the tool life at an oil consumption of 0.5 ml/h and a higher cutting speed of 1.3 m/s was extended to 47 min, only 1 min shorter than that for wet cutting. From the above results, it is concluded that control of oilmist flow and shortening of the distance from the nozzle to the tool tip are quite effective for enhancing the cutting performance of MQL machining, especially in the mLL range.
Acknowledgments The authors acknowledge Mitsubishi Materials Corporation for coated carbide tool inserts and Nippon Oil Corporation for biodegradable cutting oil used in this research. This research was partially supported by the Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (B) (2), 17360060, 2005 and that for Young Scientists (B) 18760094, 2006. References
Fig. 16. SEM micrographs of tool wear. (a) Whole view of corner wear and (b) 3-D display of the lower part of wear land.
Fig. 16(b) shows that there was inappreciable adhesion of the workpiece in the outer worn area. Because the coating layer of TiCN/Al2O3/TiN was about 5-mm thick in total, its width on the
[1] Y.S. Liao, H.M. Lin, Y.C. Chen, Feasibility study of the minimum quantity lubrication in high-speed end milling of NAK80 hardened steel by coated carbide tool, International Journal of Machine Tools & Manufacture 47 (11) (2007) 1667–1676. [2] M. Rahman, A.S. Kumar, M.U. Salam, Experimental evaluation on the effect of minimal quantities of lubricant in milling, International Journal of Machine Tools & Manufacture 42 (5) (2002) 539–547. [3] H.A. Kishawy, M. Dumitrescu, E.G. Ng, M.A. Elbestawi, Effect of coolant strategy on tool performance, chip morphology and surface quality during high-speed machining of A356 aluminum alloy, International Journal of Machine Tools & Manufacture 45 (2) (2005) 219–227. [4] R. Heinemann, S. Hinduja, G. Barrow, G. Petuelli, Effect of MQL on the tool life of small twist drills in deep-hole drilling, International Journal of Machine Tools & Manufacture 46 (1) (2006) 1–6. [5] H. Hanyu, S. Kamiya, Y. Murakami, M. Saka, Dry and semi-dry machining using finely crystallized diamond coating cutting tools, Surface & Coatings Technology 173–174 (2003) 992–995. [6] Y. Su, N. He, L. Li, A. Iqbal, M.H. Xiao, S. Xu, B.G. Qiu, Refrigerated cooling air cutting of difficult-to-cut materials, International Journal of Machine Tools & Manufacture 47 (6) (2007) 927–933. [7] N.R. Dhar, M.T. Ahmed, S. Islam, An experimental investigation on effect of minimum quantity lubrication in machining AISI 1040 steel, International Journal of Machine Tools & Manufacture 47 (5) (2007) 748–753. [8] Toshiyuki Obikawa, Yasuhiro Kamata, Jun Shinozuka, High speed grooving with applying MQL, International Journal of Machine Tools & Manufacture 46 (14) (2006) 1854–1861. [9] Toshiyuki Obikawa, Yasuhiro Kamata, MQL cutting of nickel base superalloy with a super lattice coating tool, Key Engineering Materials 291 (2005) 433–438.
ARTICLE IN PRESS 1612
T. Obikawa et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1605–1612
[10] Y. Kamata, T. Obikawa, High speed MQL finish-turning of Inconel 718 with different coated tools, Journal of Materials Processing Technology 192 (2007) 281–286. [11] Yasuhiro Kamata, Oil mist flow and its tool wear suppression mechanisms in MQL cutting, PhD thesis, Tokyo Institute of Technology, 2006. [12] W.F. Smith, Structure and Properties of Engineering Alloys, McGraw-Hill, New York, 1981. [13] T.H.C. Childs, K. Maekawa, T. Obikawa, Y. Yamane, Metal Machining—Theory and Application, Arnold, London, 2000.
[14] D. Dudzinski, A. Devillez, A. Moufki, D. Larrouquere, V. Zerrouki, J. Vigneau, A review of developments towards dry and high speed machining of Inconel 718 alloy, International Journal of Machine Tools & Manufacture 44 (2) (2004) 439–456. [15] C. Ducros, F. Sanchette, Multilayered and nanolayered hard nitride thin films deposited by cathodicarc evaporation, Surface & Coatings Technology 201 (2006) 1045–1052. [16] T. Wakabayashi, H. Sato, I. Inasaki, Turning using extremely small amounts of cutting fluids, JSME International Journal, Series C 41 (1) (1998) 143–148.