- Email: [email protected]
Experimental study on turning of TC9 titanium alloy with cold water mist jet cooling
International Journal of Machine Tools & Manufacture 51 (2011) 549–555
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Experimental study on turning of TC9 titanium alloy with cold water mist jet cooling Q.L. An a,n, Y.C. Fu b, J.H. Xu b a b
School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
a r t i c l e i n f o
abstract
Article history: Received 20 October 2010 Received in revised form 11 March 2011 Accepted 16 March 2011 Available online 23 March 2011
Titanium alloys, as difficult-to-cut materials, have poor machinability due to their superior mechanical properties, heat resistance and corrosion resistance. High cutting temperature that will greatly accelerate tool wear often occurs in titanium alloy cutting process. In this paper, cold water mist jet (CWMJ) cooling method, an eco-friendly cooling method, was used to obtain a lower cutting temperature during TC9 titanium alloy turning process. The effects of CWMJ were mainly discussed as compared with cold air jet and flood cooling methods. A comprehensive evaluation on the cooling effects of CWMJ was carried out by hydrodynamic tests, heat transfer tests and turning tests, respectively. Experimental results indicated that CWMJ had better cooling effects as compared with other two cooling methods. Cutting temperature was greatly reduced, and tool life was improved with CWMJ during TC9 turning process. Machined surface quality and chip morphology were also acceptable. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Titanium alloy Cold water mist jet Turning Cutting temperature
1. Introduction Since the introduction of titanium and titanium alloys in the early 1950s, these materials are used more and more widely in aerospace, energy, and chemical industries. The combination of high strength-to-weight ratio, excellent mechanical properties and corrosion resistance makes them the best material choices for the critical applications [1]. However, titanium alloys machining is hindered basically due to the superior properties, which are mainly displayed as follows: (1) Poor thermal conductivity. It may lead to violent temperature rise in the cutting zone during the cutting process, especially at chip–tool contact area where temperature may reach 1000 1C or more. High cutting temperature will greatly accelerate tool wear and lead to a poor tool life [2]. (2) Maintaining high strength at elevated temperature. It is a bad news for cutting tool whose mechanical strength will decrease rapidly with the increasing of cutting temperature. And it may lead to a stubborn cutting resistance at higher cutting speeds. (3) High resistance to plastic deformation. It may lead to a small chip–tool contact area, which will cause a serious stress concentration and heat generation at the tool nose. (4) Strong chemical reactivity. Tool failure is easily caused due to the strong chemical reactivity with tool materials under high cutting temperature conditions. In order to maintain a reasonable tool life, a high
n
Corresponding author. Tel.: þ862134206824. E-mail address: [email protected] (Q.L. An).
0890-6955/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2011.03.005
efficiency cooling strategy should be employed to reduce the cutting temperature. Ti–6.5Al–3.5Mo–2.5Sn–0.3Si (TC9) is one kind of high-temperature titanium alloy, which can work for a long period under high temperature condition of 5001C. It is often used as the materials for gas compressor disk and blade of aircraft jet engine. TC9 is one kind of a þ b titanium alloy as same as Ti–6Al–4V (TC4). The chemical compositions of TC4 and TC9 are shown in Table 1. Al, Mo and Si are all necessary for high temperature titanium alloys. Si has the function of improving the strength and anti-creep-deformation under high temperature conditions. Al can provide the capacity to obtain a given hardness for the alloy. Mo is favorable for ultimate tensile strength and hardness in the aged condition [3]. TC4 and TC9 have similar tensile strength about 1000 Mpa under ambient temperature condition, while TC9 has higher strength under high temperature conditions. For example, the tensile strength of TC4 at temperature of 400 1C is 630 Mpa, whereas the tensile strength of TC9 at temperature of 500 1C can maintain as high as 800 Mpa [4,5]. Therefore TC9 has worse machinability than TC4 under high temperature conditions. The paper mainly focused on turning machinability of TC9. Cutting temperature is an important factor for the titanium alloy cutting process. High cutting temperature will aggravate the tool wear and influence the machining efficiency. To get a lower cutting temperature, various cooling methods were employed, such as steam/vapor cooling [6,7], cold air cooling [8–12], liquid nitrogen (LN) [13–15], high pressure coolant jet (HPCJ) [14–17], minimum quantity lubricant (MQL)/near dry machining (NDM), etc. [18–20].
550
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
Steam/vapor, compressed air, cold air are all eco-friendly cooling medium, which are applied into the cutting zone with high velocity jet flow by nozzle. CO2, O2, N2 and mixture of gas at normal or cryogenic temperature all can be used as the medium of the jet flow. The limitation for these cooling strategies is the air’s worse cooling capacity as compared with liquid or mist. It makes them unfit for machining of difficult-to-cut materials, where high cutting temperature often happens in the cutting zone. LN and HPCJ are reinforced cooling methods, which can effectively reduce cutting temperature at the cutting zone. They are often employed in cutting process of difficult-to-cut materials to obtain a better surface quality, an improved machining efficiency and a lower production costs in comparison to conventional machining [14,15]. LN is completely clean and has higher sustainability potential. However, LN with 196 1C may result in the overcooling of the workpiece and influence the mechanical characteristic of workpiece material [20]. HPCJ often needs a high pressure supply above 10 MPa and a large mass of coolant with a flow rate as high as or higher than 10 L/min. MQL/NDM means a very little quantity of coolant is used in cutting operations, where the flow rate is often lower than 10 mL/h. It is an eco-friendly cooling method. Vegetable oil or biodegradable synthetic ester is often used as cooling medium. The most important characteristics for MQL are lubrication and vaporization cooling. The vaporization mode is more efficient than convection heat transfer, which is prevalent in steam cooling, cold air cooling, LN, HPCJ or other wet cooling methods. Ambient air quality should be guaranteed during the application of MQL, where the oil particles are harmful for the workers’ health. All the cooling methods motioned above have been tested by researchers for their ability to keep low cutting temperature and reduce tool wear [21]. The paper will try another high efficiency cooling method to obtain a lower cutting temperature in turning process of TC9 titanium alloy. Friction and heat generation in the cutting zone are troublesome problems during titanium alloy turning process. High efficiency cooling strategy should perform both high efficiency cooling and lubricant function simultaneously. Here cold water mist jet impinging cooling (CWMJ) is offered. In this method, a little quantity of 0 1C water is carried by
Table 1 Chemical composition of TC4 and TC9. Alloys
TC4 TC9
Working temperature (1C)
300–400 500
high speed cold air ( 20 1C) and reaches the machining zone in the form of mist jet. CWMJ covers heat convection of three phases (including cold air, droplet and even ice particles), jet impingement and vaporization. The aim of CWMJ is to achieve a high efficiency cooling effect in the cutting zone. The present study will evaluate the cooling effects of CWMJ during turning process of TC9 titanium alloy.
2. Experimental tests Heat transfer tests and turning tests were carried out to evaluate the cooling effects of CWMJ. Before these tests, hydrodynamic parameters of CWMJ, such as droplet diameter and droplet velocity, were measured by a Particle Dynamic Analyzer (PDA) set-up. The measured position is in the spray field identical to the position in the cutting zone. The experimental system consists of a CWMJ set-up, a PDA spray parameter measurement set-up, a heat transfer tester and a turning set-up. 2.1. CWMJ set-up Fig. 1 shows the schematic of CWMJ set-up. The heat exchanger is a cold gun air-coolant system Exair3925, which is mainly made of a vortex tube. The vortex tube is a simple device operating as a refrigerating machine without any moving part, e.g. rotating shafts or piston cylinders. It can provide high speed cold air flow for CWMJ [22]. As shown in Fig. 1, compressed air is first accumulated in a tank, which serves to minimize the pulsating flow of the air. The air supplied from the tank flows through a filter, a drier and Exair3925, and then high speed cold air flow is generated. The filter and drier are used to purify the air, which is helpful for improving the cooling effect of the cold gun. The cold air and water of 0 1C were mixed in the jet nozzle, and then the cold water mist jet comes into being. The inside mixing way offered a good atomizing effect, which was helpful for cooling and lubrication in turning process. CWMJ is mixed by cold air of 20 1C with flow rate of 300 L/min and water of 0 1C with flow rate of 180 mL/min. 2.2. PDA droplet parameters measurement
Chemical composition (%) Al
Mo
6 6.5
3.5
V
Si
Sn
Ti
0.3
2.5
Balance Balance
4
Flow regulating valve
A PDA system manufactured by Dantec Dynamics Company was used to measure the droplet parameters of mist jet, such as droplet size and velocity. Fig. 2 shows the schematic of the PDA set-up, which is based on light-scattering interferometry. It can perform non-intrusive simultaneous measurement and does not
Pressure gauge Filter
Air dryer
Nozzle
Tank H eat exchanger Gate valve Relief valve Flowmeter M Compressor Constant temperature water tank Fig. 1. Schematic of CWMJ set-up.
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
551
600
Nozzle
500
Beam Expander
Laser
Droplet number N
Mist Jet Argon Ion
Transmitting Optics
Receiving Optics
400 300 200 100
Detectors
0 D/A
0
20
Computer Fig. 2. Schematic of the PDA set-up.
Velocity Mean particle size
Velocity (m/s)
32 200
28 24
150
20
100
16 50
Mean particle size (µm)
36
250
12
0 0
20
40 60 80 100 120 Impingement distance (mm)
60
80
Fig. 4. Number concentration and particle size distribution for CWMJ at the impingement distance of 40 mm.
40
300
40 Particle size (µm)
140
8 160
Fig. 3. Variation of particle size and velocity at impingement distance of 10–150 mm.
require calibration when measuring the individual spherical particle’ parameters in liquids or gaseous flows. The lubricating and cooling effect of the mist jet have close relation with its particle size and velocity [23]. In this research, the mist generation mechanism, particle size and distribution of the droplet was experimentally investigated. Dantec 3-Dimensional PDA was used to measure the distributions of particle size and velocity at different cooling distances. The diameter at the exit of the nozzle is 1.2 mm. The air pressure and water pressure were 5 and 4 bar, respectively. Fig. 3 shows the variation of particle size and velocity at different distances on the symmetry axis. It can be seen that the jet velocity attenuates gradually along the jet spraying direction. The attenuation rate is faster in the section of 10–40 mm than that in the section of 40–150 mm. The jet velocity can be kept above 150 m/s in the section of 10–40 mm. The impingement effect of high velocity droplets will help to strengthen the heat transfer effect in the machining zone. It also can be seen from Fig. 3 that the mean particle size of the mist jet becomes bigger with the increasing of the distance. The decrease of jet velocity will result in the decrease of the relative velocity between the mist particles, which will lead to gathering of these mist particles. High velocity and small particle size both help to enhance heat transfer on hot surface [24]. The temperature of mist jet can be
kept at 5 1C or so within the cooling section of 10–40 mm. It will suit for the heat transfer in cutting zone during turning process. Fig. 4 shows the number concentration and particle size distribution at the impingement distance of 40 mm. The horizontal and vertical axis represents the particle size and number concentration, respectively. It can be seen that particle size mainly concentrates in the range 20–40 mm. In real turning process, the particle size will become smaller and the number of particle will be higher due to the centrifugal force of the rotational spindle. The smaller droplet will be helpful for its vaporization and lubrication in the cutting zone [25].
2.3. Heat transfer tests In this section, steady-state heat transfer experiments were performed. The test set-up with CWMJ is shown in Fig. 5. It was developed to simulate the heat input during turning process by using a passive heating unit supplied with electrical power [16,26]. The specimen is one thin chromel sheet with a dimension of 15 mm 2.5 mm 0.2 mm. The specimen was adiabatic with heat resistant epoxy resin on three surfaces except the top surface as the heat transfer surface on which coolant acts. In the heat transfer tests, three cooling methods were evaluated: (1) flood cooling with soluble oil (1:25) at a flow rate of 90 L/min. (2) Cold air jet ( 20 1C) at a flow rate of 150 L/min. (3) CWMJ. The distance between the nozzle exit and hot surface was 40 mm. Fig. 5(a) gives the heating principle. The heating power P ¼IU can be controlled by adjusting the voltage U exerted on the heat source. The area of heat transfer surface was S¼15 2.5 mm2. As shown in Fig. 5(b) and (c), a couple of noncontact chromel–alumel thermocouples brazed on the heat transfer surface were used to measure the temperature of the surface. The heat flux q on the heat transfer surface in the thermal equilibrium can be calculated by Eq. (1). q ¼ IU=S
ð1Þ
Then the steady-state heat transfer curve about the heat flux and surface temperature of the specimen under the different cooling conditions can be obtained. Heat transfer coefficient is an important parameter that indicates the variation of the heat flux and is often used to express the magnitude of heat transfer efficiency. It can be
552
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
Chromel sheet
Variac
Chromel sheet A
Alumel wire
Chromel wire
Emf Thermocouple
Exair 3925 Chromel wire
Chromel sheet Adiabatic base
Mixing nozzle
Water inlet
Power supply terminal Adiabatic base Alumel wire
Fig. 5. Test section for heat transfer experiments. (a) Heating principle; (b) layout of brazed thermocouple; (c) platform of heat transfer experiments with CWMJ.
calculated by Eq. (2). q Tw Tf
ð2Þ
where h is the heat transfer coefficient (W/mm2 1C), q the heat flux (W/mm2), Tw the surface temperature of the specimen (1C) and Tf the temperature of the coolant (1C). The slope of the steady-state heat transfer curve can be considered as heat transfer coefficient, which is often used to express the cooling efficiency for different cooling methods. The higher of the slope of the heat transfer curve, the better of the cooling effects. Fig. 6 illustrates the heat transfer curves obtained. It can be seen that the slope of the heat transfer curve for CWMJ is much higher than that of other two methods. The highest heat transfer coefficient of CWMJ can reach 0.44 W/mm2 1C, 22 and 3 times those of cold air jet (0.02 W/mm2 1C) and flood cooling method (0.14 W/mm2 1C), respectively. The heat transfer efficiency with CWMJ has been improved significantly as compared with other two methods. It is all attribute to its high efficiency cooling mode of three-in-one (forced convection, reinforced jet impingement and vaporization). However, cold air jet mainly depends on forced convection of cold air, which is limited for its low heat transfer coefficient. Flood cooling mainly depends on the convection or boiling heat transfer of the soluble oil. The cooling effect is limited by the contact area between the coolant and hot surface. Moreover, vapor film may form when the coolant contacts high temperature surface. If vapor layer forms between coolant and hot surface, the cooling effect will be greatly reduced [24].
2.4. Turning tests Titanium alloy turning tests were performed on a universal lathe CA6140. The impingement position of the CWMJ in the cutting zone was set nearby the chip–tool interface. The mist jet impinged perpendicularly to the cutting edge with a distance of 40 mm and at an angle of 301 in relation to the rake face. Different
30 25 q (w/mm2)
h¼
35
20 15
Cold air jet Flood cooling
10
CWMJ
5 0 -50
0
50
100
150
200
250
300
Tw (°C) Fig. 6. Heat transfer curves of the different cooling methods.
cutting speeds were chosen to evaluate the cooling effect of CWMJ. Experimental conditions are shown in Table 2. To monitor the real time change of the chip–tool temperature, a tool–workpiece thermocouple technique was employed and the signal was recorded directly by HP3562 dynamic signal analyzer. The calibration of the tool–workpiece thermocouple has been carried out by external flame heating [27]. The thermocouple calibration result of the tool–workpiece pair (workpiece and cutting insert material) is shown as Eq. (3), where T is the temperature (1C), Emf, the voltage of the tool–workpiece thermocouple (mV). The relation of T and Emf is approximately linearity. According to Eq. (3), cutting temperature can be obtained. T ¼ 45:077 Emf þ 13:328
ð3Þ
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
Tool wear measurement system is composed of a CCD camera, a toolmaker’s microscope and tool wear measurement software. During the turning process of titanium alloys, intense rubbing exists between the machined surface and the flank face of the tool. It results in a wear land on the flank face of the tool. Therefore, flank wear is the most common pattern during cutting process of titanium alloys, which would be discussed in the paper [28].
3. Results and discussion 3.1. Cutting temperature Fig. 7 gives the variation of cutting temperatures under different cooling conditions. It can be seen that cutting Table 2 Experimental conditions during turning of TC9 titanium alloy. Machine tool Cutting inserts Work material Insert geometry Cutting speed vc (m/min) Feed rate f (mm/rev) Depth of cut ap (mm) Cooling environment
CA6140 Uncoated carbide, K30 grade TC9 titanium alloy g0 ¼ 101, a0 ¼151, kr ¼ 601, ls ¼ 01, e ¼ 0.3 mm 38, 60, 75, 120 0.1 0.5 Cold air jet, flood cooling, CWMJ
600
temperatures under different cooling conditions all increase with the increasing of the cutting speed. But most important of all, there has been a substantial reduction in cutting temperature for CWMJ as compared with the other two cooling methods, especially at higher cutting speed. The cutting temperature with CWMJ at cutting speed of 120 m/min is below 600, 150 and 100 1C lower than those with cold air jet and flood cooling, respectively. Cold air jet mainly depends on forced convection of cold air whose cooling capacity is limited for its low heat transfer coefficient. Flood cooling method mainly depends on the convection of the soluble oil in the cutting zone. The coolant will boil and vaporize under high temperature above 400 1C in the cutting zone. It will result in a vapor film layer, which may prevent the penetration of the coolant into chip–tool interface or the cutting zone and influence the cooling effect of the coolant. With the rising of cutting temperature, the cooling effect for the flood cooling method will become worse. It is the reason that the cooling effect is less significant at the cutting speed of 120 m/min than that at the cutting speed of 38 m/min. A good penetration for CWMJ can be obtained with the impingement effect of high velocity droplets, which can enter into chip tool interface and reduce the contact length between the chip and rake face of the cutting tool by lifting of the chip. The minute droplets with size of 20 mm will vaporize and produce abundant of water vapor, which is helpful for improvement of lubrication at the chip–tool interface. The lubrication of water vapor has been described in the earlier Ref. [7]. All these are favorable for reducing the cutting heat generated in the cutting zone. On the other hand, the impingement of CWMJ on the chip– tool interface is also a reinforced cooling method. The mixture of 0 1C water and 20 1C compressed air flow brings 5 1C mist jet. Small water droplets with high velocity are apt to penetrate into the cutting zone and vaporize under high temperature conditions. The cooling capability of CWMJ will be effectively enhanced during the turning process because the heat transfer effect of vaporization is greater than that of convection. Therefore cutting temperature with CWMJ can be reduced efficiently.
3.2. Flank wear
Cold air jet 400
Fig. 8 shows the flank wear process under different cooling conditions. It can be seen that cutting speed is an important parameter that influences the flank wear. The amount of flank wear with CWMJ is less than those under other two cooling methods, especially at high cutting speed of 120 m/min. It can be attributed to the good performance of CWMJ during the cutting process, such as lower cutting temperature and better lubrication effect. Cutting temperature plays the most important role for
Flood cooling CWMJ 30
60
90 Cutting Speed (m/min)
120
150
Fig. 7. Cutting temperature under different cooling conditions.
0.4
0.4 Flank wear VB (mm)
Flank wear VB (mm)
Cutting temperature (°C)
800
553
0.3 0.2 0.1
Cold air jet Flood cooling CWMJ
0.3 0.2 Cold air jet Flood cooling CWMJ
0.1 0.0
0.0 0
10
20 30 40 Time (min)
50
60
0
5
10
15 20 Time (min)
25
30
Fig. 8. Flank wear curves of cutting inserts under different cooling conditions. (a) vc ¼38 m/min and (b) vc ¼ 120 m/min.
554
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
anti-wear ability of carbide tools. High cutting temperature will weaken the strength of tool material. Adhesion wear often occurs during titanium alloy cutting process, especially under high cutting temperature conditions. All these will make the tool wear easily [29]. The micro-droplets of CWMJ may vaporize as a blast of water vapor under high temperature condition in the cutting zone. The water vapor can penetrate into the chip–tool interface and improve the lubrication between the chip and rake face. It will greatly reduce the friction heat and improve the abrasion status of the cutting tool. As shown in Figs. 6 and 8, a lower cutting temperature could be obtained at the low cutting speed of 38 m/min when the tool wear is not fast. With the increasing of cutting speed, the substantial amount of cutting heat generated in the cutting zone would increase rapidly. But the cooling capacity of cold air is too limited to reduce the temperature in cutting zone efficiently at this moment. Then the amount of flank wear increases quickly [30]. Though the cooling capacity of flood cooling method is far greater than that of cold air jet, most of the coolant cannot enter the cutting zone due to the vapor film layer formed under high temperature conditions. The coolant cannot play its role of cooling and lubrication effect as CWMJ does, then cutting temperature cannot be reduced effectively. Then tool wear deteriorates and tool life is shortened. Therefore, CWMJ is more suitable for high speed turning of TC9 titanium alloy than other two cooling methods. 3.3. Surface roughness Fig. 9 shows the surface roughness under different cooling conditions. It can be seen that surface roughness under different cooling conditions all decreases with the increasing of cutting
2.5 Cold air jet Flood cooling CWMJ
Ra (µm)
2.0
1.5
speed. But regardless of the cutting speed, both CWMJ and flood cooling methods can lead to a small surface roughness during TC9 turning process comparing with cold air jet. It can be attributed to the lubricative performance of cooling medium. The soluble oil can provide a higher level of lubrication than the other two cooling medium. The observation has been reported by Weinert et al. [31]. The large flow rate of soluble oil is helpful for the lubrication between the machined surface and flank face of cutting tool, which will benefit the surface quality. Cold air can almost provide no lubrication in the cutting zone. The serious friction between the cutting tool and machined surface will lead to the worst surface quality. Although water mist of CWMJ provides little lubrication by itself, the abundant of water vapor formed under high temperature condition in the cutting zone can provide enough lubrication at the tool–workpiece interface as analyzed above. Though the surface quality with CWMJ is not so well as the one with flood cooling, the difference is very small. If another nozzle of CWMJ is applied on the position between the flank face and workpiece simultaneity, a better surface quality would be expected. 3.4. Chip morphology Typical chip morphologies formed under different cooling conditions are shown in Fig. 10. It can be seen that irregular chips were obtained with cold air jet and CWMJ. When the chip is flowing along the rake face, high pressure of cold air jet and CWMJ is put on the chip–tool interface. It will lead to extremely irregular chip under the interference of tool and rotating workpiece. The long twisted chips with cold air jet may scrap machined surface or break the tool. Short coiled chips with CWMJ can be attributed to the better cooling capacity and high impacting effect on the chip–tool contact zone. The high temperature cutting chips will be lifted and cooled down by the high-pressure mist jet, and then break down with short coiled or twisted chips on contacting the chip-breaking groove or high speed rotating workpiece. Stable spiral chips were often obtained with flood cooling method. Short spiral chips are perfect for cutting operations, which may fall along without tangling the tool and work. However, the long spiral chips may be twisted on the cutting tool or workpiece. It may also cause interruptions in the machining process as well as destroy the cutting tool or machined surface.
1.0 4. Conclusions
0.5
0.0 38
60 75 Cutting speed vc (m/min)
120
Fig. 9. Surface roughness under different cooling conditions.
In this paper, CWMJ, a high efficiency cooling method, is offered to reduce high cutting temperature during TC9 turning process. CWMJ is a mixed by a little quantity of 0 1C water and high speed cold air ( 20 1C). The cooling method covers forced convection, reinforced jet impingement heat transfer and high efficiency vaporization cooling. In this research, droplet parameters of CWMJ were first measured by Dantec PDA system.
Fig. 10. Typical chip morphologies under different cooling conditions (vc ¼120 m/min, ap ¼0.5 mm, f¼ 0.1 mm/rev). (a) Cold air jet, (b) flood cooling and (c) CWMJ.
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
Then, steady-state heat transfer experiments were carried out to obtain the heat transfer coefficient of CWMJ, cold air jet and flood cooling methods. Finally, the machining effect of CWMJ during TC9 turning process was evaluated in terms of cutting temperature, flank wear and chip morphology comparing with cold air jet and flood cooling methods. The experimental results can be concluded as follows: (1) The jet velocity can be kept above 150 m/s within the cooling section of impingement distance of 10–40 mm. Small particle size can also be obtained. The high velocity and small particle size both help to enhance heat transfer and lubrication effect in the cutting zone. (2) Forced convection, reinforced jet impingement and vaporization cooling make CWMJ a high efficiency cooling method. The highest heat transfer coefficient of CWMJ can reach 0.44 W/mm2 1C, almost 22 and 3 times those of cold air jet and flood cooling method, respectively. The heat transfer efficiency with CWMJ was improved significantly as compared with other two methods. (3) Cutting temperature can be reduced more effectively with CWMJ than that with cold air jet and flood cooling methods, especially at higher cutting speed. Due to its better cooling and lubrication effects, tool life was improved effectively. Machined surface quality and chip morphology were also acceptable. (4) CWMJ would be a good eco-friendly cooling method that is suitable for turning process of titanium alloys, if the rust problem of the machine tool aroused by water mist is resolved.
References [1] C. Leyens, M. Peters, Titanium and Titanium Alloys: Fundamentals and Applications, WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim, 2003. [2] Shane Y. Hong, Y.C. Ding, Cooling approaches and cutting temperatures in cryogenic machining of Ti–6Al–4V, International Journal of Machine Tools and Manufacture 41 (10) (2001) 1417–1437. [3] P.-J. Arrazola, A. Garay, L.-M. Iriarte, M. Armendia, S. Marya, F. Le Maˆıtre, Machinability of titanium alloys (Ti6Al4V and Ti555.3), Journal of Materials Processing Technology 209 (2009) 2223–2230. [4] Y. Chen, B.Y. Jiang, Z.P. Zhou, J.P. Zhan, Burned titanium alloy aircraft structures, Aerospace Materials and Technology 5 (2007) 75–77. [5] G.D. Xu, F.E. Wang, Development and application on high-temperature ti-based alloys, Chinese Journal of Rare Metals 32 (2008) 774–780. [6] X. Li, J.L. Gaddis, T. Wang, Modeling of heat transfer in a mist/steam impinging jet, Journal of Heat Transfer 123 (6) (2001) 1086–1092. [7] J.Y. Liu, R.D. Han, Y.F. Sun, Research on experiments and action mechanism with water vapor as coolant and lubricant in green cutting, International Journal of Machine Tools and Manufacture 45 (2005) 687–694. [8] S.W. Kim, C.M. Lee, D.W. Lee, J.S. Kim, Evaluation of machinability by cutting environments in high-speed milling of difficult-to-cut materials, Journal of Materials Processing Technology 111 (2001) 256–260.
555
[9] H.Z. Choi, S.W. Lee, A comparison of the cooling effects of compressed cold air and coolant for cylindrical grinding with a CBN wheel, Journal of Materials Processing Technology 111 (2001) 265–268. [10] 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 and Manufacture 47 (2007) 927–933. [11] J. Liu, Y. Kevin Chou, On temperatures and tool wear in machining hypereutectic Al–Si alloys with vortex-tube cooling, International Journal of Machine Tools and Manufacture 47 (2007) 635–645. [12] S. Sun, M. Brandt, M.S. Dargusch, Machining Ti–6Al–4V alloy with cryogenic compressed air cooling, International Journal of Machine Tools and Manufacture 50 (2010) 933–942. [13] K.A. Venugopal, S. Paul, A.B. Chattopadhyay, Growth of tool wear in turning of Ti–6Al–4V alloy under cryogenic cooling, Wear 262 (2007) 1071–1078. [14] F. Pusavec, P. Krajnik, J. Kopac, Transitioning to sustainable production—part I: application on machining technologies, Journal of Cleaner Production 18 (2010) 174–184. [15] F. Pusavec, D. Kramar, P. Krajnik, J. Kopac, Transitioning to sustainable production—part II: evaluation of sustainable machining technologies, Journal of Cleaner Production 18 (2010) 1211–1221. [16] H.J. Xu, Y.C. Fu, F.H. Sun, X.P. Xu, Fundamental studies on enhancing heat transfer in contact zone during high efficiency grinding, Science in China (Series E) 45 (2002) 261–272. [17] Emmanuel O. Ezugwu, John Bonney, Rosemar B.D. Silva, C.O. Akir, Surface integrity of finished turned Ti–6Al–4V alloy with PCD tools using conventional and high pressure coolant supplies, International Journal of Machine Tools and Manufacture 47 (2007) 884–891. [18] F. Klocke, G. Einesblatter, Dry cutting, Annals of the CIRP 46 (2) (1997) 519–526. [19] A. Attanasio, M. Gelfi, C. Giardini, C. Remino, Minimal quantity lubrication in turning: effect on tool wear, Wear 260 (3) (2006) 333–338. [20] Vishal S. Sharma, Manu Dogra, N.M. Suri, Cooling techniques for improved productivity in turning, International Journal of Machine Tools and Manufacture 49 (2009) 435–453. [21] P.W. Marksberry, I.S. Jawahir, A comprehensive tool-wear/tool-life performance model in the evaluation of NDM (near dry machining) for sustainable manufacturing, International Journal of Machine Tools and Manufacture 48 (2008) 878–886. [22] M.H. Saidi, M.S. Valipour, Experimental modeling of vortex tube refrigerator, Applied Thermal Engineering 23 (2003) 1971–1980. [23] M.R. Schumack, J.B. Chung, W.W. Schultz, Analyses of fluid flow under a grinding wheel, Transactions of the ASME 113 (1991) 190–197. [24] Ruey-Hung Chen, Louis C. Chow, Jose E. Navedo, Effects of spray characteristics on critical heat flux in subcooled water spray cooling, International Journal of Heat and Mass Transfer 45 (2002) 4033–4043. [25] Tae Jo Ko, Sung Ho Park, Hee Sool Kim, Experimental verification of the mist generation mechanism in turning, International Journal of Machine Tools and Manufacture 43 (2003) 115–120. [26] T. Howes, Assessment of the cooling and lubricative properties of grinding fluids, Annals of the CIRP 39 (1) (1990) 313–316. [27] N.R. Dhar, S. Paul, A.B. Chattopadhyay, Role of cryogenic cooling on cutting temperature in turning steel, Journal of Manufacturing Science and Engineering 124 (2002) 146–154. [28] Z.Q. Liu, X. Ai, Wear characteristics of cutting tools in high speed machining, Tribology 22 (2002) 468–471. [29] Z.L. Man, N. He, K. Wu, L. Li, C.Y. Jiang, Investigation on the wear of the milling cutter during milling ti-alloy with nitrogen media, Mechanical Science and Technology 22 (2003) 996–998. [30] M. Rahman, A. Senthil Kumar, Manzoor-Ul-Salam, M.S. Ling, Effect of chilled air on machining performance in end milling, International Journal of Advanced Manufacturing Technology 21 (2003) 787–795. [31] K. Weinert, I. Inasaki, J.W. Sutherland, T. Wakabayashi, Dry machining and minimum quantity lubrication, Annals of the CIRP 53 (2004) 511–537.
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Experimental study on turning of TC9 titanium alloy with cold water mist jet cooling Q.L. An a,n, Y.C. Fu b, J.H. Xu b a b
School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
a r t i c l e i n f o
abstract
Article history: Received 20 October 2010 Received in revised form 11 March 2011 Accepted 16 March 2011 Available online 23 March 2011
Titanium alloys, as difficult-to-cut materials, have poor machinability due to their superior mechanical properties, heat resistance and corrosion resistance. High cutting temperature that will greatly accelerate tool wear often occurs in titanium alloy cutting process. In this paper, cold water mist jet (CWMJ) cooling method, an eco-friendly cooling method, was used to obtain a lower cutting temperature during TC9 titanium alloy turning process. The effects of CWMJ were mainly discussed as compared with cold air jet and flood cooling methods. A comprehensive evaluation on the cooling effects of CWMJ was carried out by hydrodynamic tests, heat transfer tests and turning tests, respectively. Experimental results indicated that CWMJ had better cooling effects as compared with other two cooling methods. Cutting temperature was greatly reduced, and tool life was improved with CWMJ during TC9 turning process. Machined surface quality and chip morphology were also acceptable. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Titanium alloy Cold water mist jet Turning Cutting temperature
1. Introduction Since the introduction of titanium and titanium alloys in the early 1950s, these materials are used more and more widely in aerospace, energy, and chemical industries. The combination of high strength-to-weight ratio, excellent mechanical properties and corrosion resistance makes them the best material choices for the critical applications [1]. However, titanium alloys machining is hindered basically due to the superior properties, which are mainly displayed as follows: (1) Poor thermal conductivity. It may lead to violent temperature rise in the cutting zone during the cutting process, especially at chip–tool contact area where temperature may reach 1000 1C or more. High cutting temperature will greatly accelerate tool wear and lead to a poor tool life [2]. (2) Maintaining high strength at elevated temperature. It is a bad news for cutting tool whose mechanical strength will decrease rapidly with the increasing of cutting temperature. And it may lead to a stubborn cutting resistance at higher cutting speeds. (3) High resistance to plastic deformation. It may lead to a small chip–tool contact area, which will cause a serious stress concentration and heat generation at the tool nose. (4) Strong chemical reactivity. Tool failure is easily caused due to the strong chemical reactivity with tool materials under high cutting temperature conditions. In order to maintain a reasonable tool life, a high
n
Corresponding author. Tel.: þ862134206824. E-mail address: [email protected] (Q.L. An).
0890-6955/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2011.03.005
efficiency cooling strategy should be employed to reduce the cutting temperature. Ti–6.5Al–3.5Mo–2.5Sn–0.3Si (TC9) is one kind of high-temperature titanium alloy, which can work for a long period under high temperature condition of 5001C. It is often used as the materials for gas compressor disk and blade of aircraft jet engine. TC9 is one kind of a þ b titanium alloy as same as Ti–6Al–4V (TC4). The chemical compositions of TC4 and TC9 are shown in Table 1. Al, Mo and Si are all necessary for high temperature titanium alloys. Si has the function of improving the strength and anti-creep-deformation under high temperature conditions. Al can provide the capacity to obtain a given hardness for the alloy. Mo is favorable for ultimate tensile strength and hardness in the aged condition [3]. TC4 and TC9 have similar tensile strength about 1000 Mpa under ambient temperature condition, while TC9 has higher strength under high temperature conditions. For example, the tensile strength of TC4 at temperature of 400 1C is 630 Mpa, whereas the tensile strength of TC9 at temperature of 500 1C can maintain as high as 800 Mpa [4,5]. Therefore TC9 has worse machinability than TC4 under high temperature conditions. The paper mainly focused on turning machinability of TC9. Cutting temperature is an important factor for the titanium alloy cutting process. High cutting temperature will aggravate the tool wear and influence the machining efficiency. To get a lower cutting temperature, various cooling methods were employed, such as steam/vapor cooling [6,7], cold air cooling [8–12], liquid nitrogen (LN) [13–15], high pressure coolant jet (HPCJ) [14–17], minimum quantity lubricant (MQL)/near dry machining (NDM), etc. [18–20].
550
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
Steam/vapor, compressed air, cold air are all eco-friendly cooling medium, which are applied into the cutting zone with high velocity jet flow by nozzle. CO2, O2, N2 and mixture of gas at normal or cryogenic temperature all can be used as the medium of the jet flow. The limitation for these cooling strategies is the air’s worse cooling capacity as compared with liquid or mist. It makes them unfit for machining of difficult-to-cut materials, where high cutting temperature often happens in the cutting zone. LN and HPCJ are reinforced cooling methods, which can effectively reduce cutting temperature at the cutting zone. They are often employed in cutting process of difficult-to-cut materials to obtain a better surface quality, an improved machining efficiency and a lower production costs in comparison to conventional machining [14,15]. LN is completely clean and has higher sustainability potential. However, LN with 196 1C may result in the overcooling of the workpiece and influence the mechanical characteristic of workpiece material [20]. HPCJ often needs a high pressure supply above 10 MPa and a large mass of coolant with a flow rate as high as or higher than 10 L/min. MQL/NDM means a very little quantity of coolant is used in cutting operations, where the flow rate is often lower than 10 mL/h. It is an eco-friendly cooling method. Vegetable oil or biodegradable synthetic ester is often used as cooling medium. The most important characteristics for MQL are lubrication and vaporization cooling. The vaporization mode is more efficient than convection heat transfer, which is prevalent in steam cooling, cold air cooling, LN, HPCJ or other wet cooling methods. Ambient air quality should be guaranteed during the application of MQL, where the oil particles are harmful for the workers’ health. All the cooling methods motioned above have been tested by researchers for their ability to keep low cutting temperature and reduce tool wear [21]. The paper will try another high efficiency cooling method to obtain a lower cutting temperature in turning process of TC9 titanium alloy. Friction and heat generation in the cutting zone are troublesome problems during titanium alloy turning process. High efficiency cooling strategy should perform both high efficiency cooling and lubricant function simultaneously. Here cold water mist jet impinging cooling (CWMJ) is offered. In this method, a little quantity of 0 1C water is carried by
Table 1 Chemical composition of TC4 and TC9. Alloys
TC4 TC9
Working temperature (1C)
300–400 500
high speed cold air ( 20 1C) and reaches the machining zone in the form of mist jet. CWMJ covers heat convection of three phases (including cold air, droplet and even ice particles), jet impingement and vaporization. The aim of CWMJ is to achieve a high efficiency cooling effect in the cutting zone. The present study will evaluate the cooling effects of CWMJ during turning process of TC9 titanium alloy.
2. Experimental tests Heat transfer tests and turning tests were carried out to evaluate the cooling effects of CWMJ. Before these tests, hydrodynamic parameters of CWMJ, such as droplet diameter and droplet velocity, were measured by a Particle Dynamic Analyzer (PDA) set-up. The measured position is in the spray field identical to the position in the cutting zone. The experimental system consists of a CWMJ set-up, a PDA spray parameter measurement set-up, a heat transfer tester and a turning set-up. 2.1. CWMJ set-up Fig. 1 shows the schematic of CWMJ set-up. The heat exchanger is a cold gun air-coolant system Exair3925, which is mainly made of a vortex tube. The vortex tube is a simple device operating as a refrigerating machine without any moving part, e.g. rotating shafts or piston cylinders. It can provide high speed cold air flow for CWMJ [22]. As shown in Fig. 1, compressed air is first accumulated in a tank, which serves to minimize the pulsating flow of the air. The air supplied from the tank flows through a filter, a drier and Exair3925, and then high speed cold air flow is generated. The filter and drier are used to purify the air, which is helpful for improving the cooling effect of the cold gun. The cold air and water of 0 1C were mixed in the jet nozzle, and then the cold water mist jet comes into being. The inside mixing way offered a good atomizing effect, which was helpful for cooling and lubrication in turning process. CWMJ is mixed by cold air of 20 1C with flow rate of 300 L/min and water of 0 1C with flow rate of 180 mL/min. 2.2. PDA droplet parameters measurement
Chemical composition (%) Al
Mo
6 6.5
3.5
V
Si
Sn
Ti
0.3
2.5
Balance Balance
4
Flow regulating valve
A PDA system manufactured by Dantec Dynamics Company was used to measure the droplet parameters of mist jet, such as droplet size and velocity. Fig. 2 shows the schematic of the PDA set-up, which is based on light-scattering interferometry. It can perform non-intrusive simultaneous measurement and does not
Pressure gauge Filter
Air dryer
Nozzle
Tank H eat exchanger Gate valve Relief valve Flowmeter M Compressor Constant temperature water tank Fig. 1. Schematic of CWMJ set-up.
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
551
600
Nozzle
500
Beam Expander
Laser
Droplet number N
Mist Jet Argon Ion
Transmitting Optics
Receiving Optics
400 300 200 100
Detectors
0 D/A
0
20
Computer Fig. 2. Schematic of the PDA set-up.
Velocity Mean particle size
Velocity (m/s)
32 200
28 24
150
20
100
16 50
Mean particle size (µm)
36
250
12
0 0
20
40 60 80 100 120 Impingement distance (mm)
60
80
Fig. 4. Number concentration and particle size distribution for CWMJ at the impingement distance of 40 mm.
40
300
40 Particle size (µm)
140
8 160
Fig. 3. Variation of particle size and velocity at impingement distance of 10–150 mm.
require calibration when measuring the individual spherical particle’ parameters in liquids or gaseous flows. The lubricating and cooling effect of the mist jet have close relation with its particle size and velocity [23]. In this research, the mist generation mechanism, particle size and distribution of the droplet was experimentally investigated. Dantec 3-Dimensional PDA was used to measure the distributions of particle size and velocity at different cooling distances. The diameter at the exit of the nozzle is 1.2 mm. The air pressure and water pressure were 5 and 4 bar, respectively. Fig. 3 shows the variation of particle size and velocity at different distances on the symmetry axis. It can be seen that the jet velocity attenuates gradually along the jet spraying direction. The attenuation rate is faster in the section of 10–40 mm than that in the section of 40–150 mm. The jet velocity can be kept above 150 m/s in the section of 10–40 mm. The impingement effect of high velocity droplets will help to strengthen the heat transfer effect in the machining zone. It also can be seen from Fig. 3 that the mean particle size of the mist jet becomes bigger with the increasing of the distance. The decrease of jet velocity will result in the decrease of the relative velocity between the mist particles, which will lead to gathering of these mist particles. High velocity and small particle size both help to enhance heat transfer on hot surface [24]. The temperature of mist jet can be
kept at 5 1C or so within the cooling section of 10–40 mm. It will suit for the heat transfer in cutting zone during turning process. Fig. 4 shows the number concentration and particle size distribution at the impingement distance of 40 mm. The horizontal and vertical axis represents the particle size and number concentration, respectively. It can be seen that particle size mainly concentrates in the range 20–40 mm. In real turning process, the particle size will become smaller and the number of particle will be higher due to the centrifugal force of the rotational spindle. The smaller droplet will be helpful for its vaporization and lubrication in the cutting zone [25].
2.3. Heat transfer tests In this section, steady-state heat transfer experiments were performed. The test set-up with CWMJ is shown in Fig. 5. It was developed to simulate the heat input during turning process by using a passive heating unit supplied with electrical power [16,26]. The specimen is one thin chromel sheet with a dimension of 15 mm 2.5 mm 0.2 mm. The specimen was adiabatic with heat resistant epoxy resin on three surfaces except the top surface as the heat transfer surface on which coolant acts. In the heat transfer tests, three cooling methods were evaluated: (1) flood cooling with soluble oil (1:25) at a flow rate of 90 L/min. (2) Cold air jet ( 20 1C) at a flow rate of 150 L/min. (3) CWMJ. The distance between the nozzle exit and hot surface was 40 mm. Fig. 5(a) gives the heating principle. The heating power P ¼IU can be controlled by adjusting the voltage U exerted on the heat source. The area of heat transfer surface was S¼15 2.5 mm2. As shown in Fig. 5(b) and (c), a couple of noncontact chromel–alumel thermocouples brazed on the heat transfer surface were used to measure the temperature of the surface. The heat flux q on the heat transfer surface in the thermal equilibrium can be calculated by Eq. (1). q ¼ IU=S
ð1Þ
Then the steady-state heat transfer curve about the heat flux and surface temperature of the specimen under the different cooling conditions can be obtained. Heat transfer coefficient is an important parameter that indicates the variation of the heat flux and is often used to express the magnitude of heat transfer efficiency. It can be
552
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
Chromel sheet
Variac
Chromel sheet A
Alumel wire
Chromel wire
Emf Thermocouple
Exair 3925 Chromel wire
Chromel sheet Adiabatic base
Mixing nozzle
Water inlet
Power supply terminal Adiabatic base Alumel wire
Fig. 5. Test section for heat transfer experiments. (a) Heating principle; (b) layout of brazed thermocouple; (c) platform of heat transfer experiments with CWMJ.
calculated by Eq. (2). q Tw Tf
ð2Þ
where h is the heat transfer coefficient (W/mm2 1C), q the heat flux (W/mm2), Tw the surface temperature of the specimen (1C) and Tf the temperature of the coolant (1C). The slope of the steady-state heat transfer curve can be considered as heat transfer coefficient, which is often used to express the cooling efficiency for different cooling methods. The higher of the slope of the heat transfer curve, the better of the cooling effects. Fig. 6 illustrates the heat transfer curves obtained. It can be seen that the slope of the heat transfer curve for CWMJ is much higher than that of other two methods. The highest heat transfer coefficient of CWMJ can reach 0.44 W/mm2 1C, 22 and 3 times those of cold air jet (0.02 W/mm2 1C) and flood cooling method (0.14 W/mm2 1C), respectively. The heat transfer efficiency with CWMJ has been improved significantly as compared with other two methods. It is all attribute to its high efficiency cooling mode of three-in-one (forced convection, reinforced jet impingement and vaporization). However, cold air jet mainly depends on forced convection of cold air, which is limited for its low heat transfer coefficient. Flood cooling mainly depends on the convection or boiling heat transfer of the soluble oil. The cooling effect is limited by the contact area between the coolant and hot surface. Moreover, vapor film may form when the coolant contacts high temperature surface. If vapor layer forms between coolant and hot surface, the cooling effect will be greatly reduced [24].
2.4. Turning tests Titanium alloy turning tests were performed on a universal lathe CA6140. The impingement position of the CWMJ in the cutting zone was set nearby the chip–tool interface. The mist jet impinged perpendicularly to the cutting edge with a distance of 40 mm and at an angle of 301 in relation to the rake face. Different
30 25 q (w/mm2)
h¼
35
20 15
Cold air jet Flood cooling
10
CWMJ
5 0 -50
0
50
100
150
200
250
300
Tw (°C) Fig. 6. Heat transfer curves of the different cooling methods.
cutting speeds were chosen to evaluate the cooling effect of CWMJ. Experimental conditions are shown in Table 2. To monitor the real time change of the chip–tool temperature, a tool–workpiece thermocouple technique was employed and the signal was recorded directly by HP3562 dynamic signal analyzer. The calibration of the tool–workpiece thermocouple has been carried out by external flame heating [27]. The thermocouple calibration result of the tool–workpiece pair (workpiece and cutting insert material) is shown as Eq. (3), where T is the temperature (1C), Emf, the voltage of the tool–workpiece thermocouple (mV). The relation of T and Emf is approximately linearity. According to Eq. (3), cutting temperature can be obtained. T ¼ 45:077 Emf þ 13:328
ð3Þ
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
Tool wear measurement system is composed of a CCD camera, a toolmaker’s microscope and tool wear measurement software. During the turning process of titanium alloys, intense rubbing exists between the machined surface and the flank face of the tool. It results in a wear land on the flank face of the tool. Therefore, flank wear is the most common pattern during cutting process of titanium alloys, which would be discussed in the paper [28].
3. Results and discussion 3.1. Cutting temperature Fig. 7 gives the variation of cutting temperatures under different cooling conditions. It can be seen that cutting Table 2 Experimental conditions during turning of TC9 titanium alloy. Machine tool Cutting inserts Work material Insert geometry Cutting speed vc (m/min) Feed rate f (mm/rev) Depth of cut ap (mm) Cooling environment
CA6140 Uncoated carbide, K30 grade TC9 titanium alloy g0 ¼ 101, a0 ¼151, kr ¼ 601, ls ¼ 01, e ¼ 0.3 mm 38, 60, 75, 120 0.1 0.5 Cold air jet, flood cooling, CWMJ
600
temperatures under different cooling conditions all increase with the increasing of the cutting speed. But most important of all, there has been a substantial reduction in cutting temperature for CWMJ as compared with the other two cooling methods, especially at higher cutting speed. The cutting temperature with CWMJ at cutting speed of 120 m/min is below 600, 150 and 100 1C lower than those with cold air jet and flood cooling, respectively. Cold air jet mainly depends on forced convection of cold air whose cooling capacity is limited for its low heat transfer coefficient. Flood cooling method mainly depends on the convection of the soluble oil in the cutting zone. The coolant will boil and vaporize under high temperature above 400 1C in the cutting zone. It will result in a vapor film layer, which may prevent the penetration of the coolant into chip–tool interface or the cutting zone and influence the cooling effect of the coolant. With the rising of cutting temperature, the cooling effect for the flood cooling method will become worse. It is the reason that the cooling effect is less significant at the cutting speed of 120 m/min than that at the cutting speed of 38 m/min. A good penetration for CWMJ can be obtained with the impingement effect of high velocity droplets, which can enter into chip tool interface and reduce the contact length between the chip and rake face of the cutting tool by lifting of the chip. The minute droplets with size of 20 mm will vaporize and produce abundant of water vapor, which is helpful for improvement of lubrication at the chip–tool interface. The lubrication of water vapor has been described in the earlier Ref. [7]. All these are favorable for reducing the cutting heat generated in the cutting zone. On the other hand, the impingement of CWMJ on the chip– tool interface is also a reinforced cooling method. The mixture of 0 1C water and 20 1C compressed air flow brings 5 1C mist jet. Small water droplets with high velocity are apt to penetrate into the cutting zone and vaporize under high temperature conditions. The cooling capability of CWMJ will be effectively enhanced during the turning process because the heat transfer effect of vaporization is greater than that of convection. Therefore cutting temperature with CWMJ can be reduced efficiently.
3.2. Flank wear
Cold air jet 400
Fig. 8 shows the flank wear process under different cooling conditions. It can be seen that cutting speed is an important parameter that influences the flank wear. The amount of flank wear with CWMJ is less than those under other two cooling methods, especially at high cutting speed of 120 m/min. It can be attributed to the good performance of CWMJ during the cutting process, such as lower cutting temperature and better lubrication effect. Cutting temperature plays the most important role for
Flood cooling CWMJ 30
60
90 Cutting Speed (m/min)
120
150
Fig. 7. Cutting temperature under different cooling conditions.
0.4
0.4 Flank wear VB (mm)
Flank wear VB (mm)
Cutting temperature (°C)
800
553
0.3 0.2 0.1
Cold air jet Flood cooling CWMJ
0.3 0.2 Cold air jet Flood cooling CWMJ
0.1 0.0
0.0 0
10
20 30 40 Time (min)
50
60
0
5
10
15 20 Time (min)
25
30
Fig. 8. Flank wear curves of cutting inserts under different cooling conditions. (a) vc ¼38 m/min and (b) vc ¼ 120 m/min.
554
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
anti-wear ability of carbide tools. High cutting temperature will weaken the strength of tool material. Adhesion wear often occurs during titanium alloy cutting process, especially under high cutting temperature conditions. All these will make the tool wear easily [29]. The micro-droplets of CWMJ may vaporize as a blast of water vapor under high temperature condition in the cutting zone. The water vapor can penetrate into the chip–tool interface and improve the lubrication between the chip and rake face. It will greatly reduce the friction heat and improve the abrasion status of the cutting tool. As shown in Figs. 6 and 8, a lower cutting temperature could be obtained at the low cutting speed of 38 m/min when the tool wear is not fast. With the increasing of cutting speed, the substantial amount of cutting heat generated in the cutting zone would increase rapidly. But the cooling capacity of cold air is too limited to reduce the temperature in cutting zone efficiently at this moment. Then the amount of flank wear increases quickly [30]. Though the cooling capacity of flood cooling method is far greater than that of cold air jet, most of the coolant cannot enter the cutting zone due to the vapor film layer formed under high temperature conditions. The coolant cannot play its role of cooling and lubrication effect as CWMJ does, then cutting temperature cannot be reduced effectively. Then tool wear deteriorates and tool life is shortened. Therefore, CWMJ is more suitable for high speed turning of TC9 titanium alloy than other two cooling methods. 3.3. Surface roughness Fig. 9 shows the surface roughness under different cooling conditions. It can be seen that surface roughness under different cooling conditions all decreases with the increasing of cutting
2.5 Cold air jet Flood cooling CWMJ
Ra (µm)
2.0
1.5
speed. But regardless of the cutting speed, both CWMJ and flood cooling methods can lead to a small surface roughness during TC9 turning process comparing with cold air jet. It can be attributed to the lubricative performance of cooling medium. The soluble oil can provide a higher level of lubrication than the other two cooling medium. The observation has been reported by Weinert et al. [31]. The large flow rate of soluble oil is helpful for the lubrication between the machined surface and flank face of cutting tool, which will benefit the surface quality. Cold air can almost provide no lubrication in the cutting zone. The serious friction between the cutting tool and machined surface will lead to the worst surface quality. Although water mist of CWMJ provides little lubrication by itself, the abundant of water vapor formed under high temperature condition in the cutting zone can provide enough lubrication at the tool–workpiece interface as analyzed above. Though the surface quality with CWMJ is not so well as the one with flood cooling, the difference is very small. If another nozzle of CWMJ is applied on the position between the flank face and workpiece simultaneity, a better surface quality would be expected. 3.4. Chip morphology Typical chip morphologies formed under different cooling conditions are shown in Fig. 10. It can be seen that irregular chips were obtained with cold air jet and CWMJ. When the chip is flowing along the rake face, high pressure of cold air jet and CWMJ is put on the chip–tool interface. It will lead to extremely irregular chip under the interference of tool and rotating workpiece. The long twisted chips with cold air jet may scrap machined surface or break the tool. Short coiled chips with CWMJ can be attributed to the better cooling capacity and high impacting effect on the chip–tool contact zone. The high temperature cutting chips will be lifted and cooled down by the high-pressure mist jet, and then break down with short coiled or twisted chips on contacting the chip-breaking groove or high speed rotating workpiece. Stable spiral chips were often obtained with flood cooling method. Short spiral chips are perfect for cutting operations, which may fall along without tangling the tool and work. However, the long spiral chips may be twisted on the cutting tool or workpiece. It may also cause interruptions in the machining process as well as destroy the cutting tool or machined surface.
1.0 4. Conclusions
0.5
0.0 38
60 75 Cutting speed vc (m/min)
120
Fig. 9. Surface roughness under different cooling conditions.
In this paper, CWMJ, a high efficiency cooling method, is offered to reduce high cutting temperature during TC9 turning process. CWMJ is a mixed by a little quantity of 0 1C water and high speed cold air ( 20 1C). The cooling method covers forced convection, reinforced jet impingement heat transfer and high efficiency vaporization cooling. In this research, droplet parameters of CWMJ were first measured by Dantec PDA system.
Fig. 10. Typical chip morphologies under different cooling conditions (vc ¼120 m/min, ap ¼0.5 mm, f¼ 0.1 mm/rev). (a) Cold air jet, (b) flood cooling and (c) CWMJ.
Q.L. An et al. / International Journal of Machine Tools & Manufacture 51 (2011) 549–555
Then, steady-state heat transfer experiments were carried out to obtain the heat transfer coefficient of CWMJ, cold air jet and flood cooling methods. Finally, the machining effect of CWMJ during TC9 turning process was evaluated in terms of cutting temperature, flank wear and chip morphology comparing with cold air jet and flood cooling methods. The experimental results can be concluded as follows: (1) The jet velocity can be kept above 150 m/s within the cooling section of impingement distance of 10–40 mm. Small particle size can also be obtained. The high velocity and small particle size both help to enhance heat transfer and lubrication effect in the cutting zone. (2) Forced convection, reinforced jet impingement and vaporization cooling make CWMJ a high efficiency cooling method. The highest heat transfer coefficient of CWMJ can reach 0.44 W/mm2 1C, almost 22 and 3 times those of cold air jet and flood cooling method, respectively. The heat transfer efficiency with CWMJ was improved significantly as compared with other two methods. (3) Cutting temperature can be reduced more effectively with CWMJ than that with cold air jet and flood cooling methods, especially at higher cutting speed. Due to its better cooling and lubrication effects, tool life was improved effectively. Machined surface quality and chip morphology were also acceptable. (4) CWMJ would be a good eco-friendly cooling method that is suitable for turning process of titanium alloys, if the rust problem of the machine tool aroused by water mist is resolved.
References [1] C. Leyens, M. Peters, Titanium and Titanium Alloys: Fundamentals and Applications, WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim, 2003. [2] Shane Y. Hong, Y.C. Ding, Cooling approaches and cutting temperatures in cryogenic machining of Ti–6Al–4V, International Journal of Machine Tools and Manufacture 41 (10) (2001) 1417–1437. [3] P.-J. Arrazola, A. Garay, L.-M. Iriarte, M. Armendia, S. Marya, F. Le Maˆıtre, Machinability of titanium alloys (Ti6Al4V and Ti555.3), Journal of Materials Processing Technology 209 (2009) 2223–2230. [4] Y. Chen, B.Y. Jiang, Z.P. Zhou, J.P. Zhan, Burned titanium alloy aircraft structures, Aerospace Materials and Technology 5 (2007) 75–77. [5] G.D. Xu, F.E. Wang, Development and application on high-temperature ti-based alloys, Chinese Journal of Rare Metals 32 (2008) 774–780. [6] X. Li, J.L. Gaddis, T. Wang, Modeling of heat transfer in a mist/steam impinging jet, Journal of Heat Transfer 123 (6) (2001) 1086–1092. [7] J.Y. Liu, R.D. Han, Y.F. Sun, Research on experiments and action mechanism with water vapor as coolant and lubricant in green cutting, International Journal of Machine Tools and Manufacture 45 (2005) 687–694. [8] S.W. Kim, C.M. Lee, D.W. Lee, J.S. Kim, Evaluation of machinability by cutting environments in high-speed milling of difficult-to-cut materials, Journal of Materials Processing Technology 111 (2001) 256–260.
555
[9] H.Z. Choi, S.W. Lee, A comparison of the cooling effects of compressed cold air and coolant for cylindrical grinding with a CBN wheel, Journal of Materials Processing Technology 111 (2001) 265–268. [10] 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 and Manufacture 47 (2007) 927–933. [11] J. Liu, Y. Kevin Chou, On temperatures and tool wear in machining hypereutectic Al–Si alloys with vortex-tube cooling, International Journal of Machine Tools and Manufacture 47 (2007) 635–645. [12] S. Sun, M. Brandt, M.S. Dargusch, Machining Ti–6Al–4V alloy with cryogenic compressed air cooling, International Journal of Machine Tools and Manufacture 50 (2010) 933–942. [13] K.A. Venugopal, S. Paul, A.B. Chattopadhyay, Growth of tool wear in turning of Ti–6Al–4V alloy under cryogenic cooling, Wear 262 (2007) 1071–1078. [14] F. Pusavec, P. Krajnik, J. Kopac, Transitioning to sustainable production—part I: application on machining technologies, Journal of Cleaner Production 18 (2010) 174–184. [15] F. Pusavec, D. Kramar, P. Krajnik, J. Kopac, Transitioning to sustainable production—part II: evaluation of sustainable machining technologies, Journal of Cleaner Production 18 (2010) 1211–1221. [16] H.J. Xu, Y.C. Fu, F.H. Sun, X.P. Xu, Fundamental studies on enhancing heat transfer in contact zone during high efficiency grinding, Science in China (Series E) 45 (2002) 261–272. [17] Emmanuel O. Ezugwu, John Bonney, Rosemar B.D. Silva, C.O. Akir, Surface integrity of finished turned Ti–6Al–4V alloy with PCD tools using conventional and high pressure coolant supplies, International Journal of Machine Tools and Manufacture 47 (2007) 884–891. [18] F. Klocke, G. Einesblatter, Dry cutting, Annals of the CIRP 46 (2) (1997) 519–526. [19] A. Attanasio, M. Gelfi, C. Giardini, C. Remino, Minimal quantity lubrication in turning: effect on tool wear, Wear 260 (3) (2006) 333–338. [20] Vishal S. Sharma, Manu Dogra, N.M. Suri, Cooling techniques for improved productivity in turning, International Journal of Machine Tools and Manufacture 49 (2009) 435–453. [21] P.W. Marksberry, I.S. Jawahir, A comprehensive tool-wear/tool-life performance model in the evaluation of NDM (near dry machining) for sustainable manufacturing, International Journal of Machine Tools and Manufacture 48 (2008) 878–886. [22] M.H. Saidi, M.S. Valipour, Experimental modeling of vortex tube refrigerator, Applied Thermal Engineering 23 (2003) 1971–1980. [23] M.R. Schumack, J.B. Chung, W.W. Schultz, Analyses of fluid flow under a grinding wheel, Transactions of the ASME 113 (1991) 190–197. [24] Ruey-Hung Chen, Louis C. Chow, Jose E. Navedo, Effects of spray characteristics on critical heat flux in subcooled water spray cooling, International Journal of Heat and Mass Transfer 45 (2002) 4033–4043. [25] Tae Jo Ko, Sung Ho Park, Hee Sool Kim, Experimental verification of the mist generation mechanism in turning, International Journal of Machine Tools and Manufacture 43 (2003) 115–120. [26] T. Howes, Assessment of the cooling and lubricative properties of grinding fluids, Annals of the CIRP 39 (1) (1990) 313–316. [27] N.R. Dhar, S. Paul, A.B. Chattopadhyay, Role of cryogenic cooling on cutting temperature in turning steel, Journal of Manufacturing Science and Engineering 124 (2002) 146–154. [28] Z.Q. Liu, X. Ai, Wear characteristics of cutting tools in high speed machining, Tribology 22 (2002) 468–471. [29] Z.L. Man, N. He, K. Wu, L. Li, C.Y. Jiang, Investigation on the wear of the milling cutter during milling ti-alloy with nitrogen media, Mechanical Science and Technology 22 (2003) 996–998. [30] M. Rahman, A. Senthil Kumar, Manzoor-Ul-Salam, M.S. Ling, Effect of chilled air on machining performance in end milling, International Journal of Advanced Manufacturing Technology 21 (2003) 787–795. [31] K. Weinert, I. Inasaki, J.W. Sutherland, T. Wakabayashi, Dry machining and minimum quantity lubrication, Annals of the CIRP 53 (2004) 511–537.