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Design, fabrication and performance evaluation of pulsating heat pipe assisted tool holder
Journal of Manufacturing Processes 50 (2020) 224–233
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Technical Paper
Design, fabrication and performance evaluation of pulsating heat pipe assisted tool holder
T
Ze Wua, Youqiang Xinga,*, Lei Liua,*, Peng Huanga, Guolong Zhaob a b
School of Mechanical Engineering, Southeast University, Nanjing 211189, PR China School of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pulsating heat pipe Tool holder Titanium alloy
Pulsating heat pipe is a kind of highly efficient heat conductor in which the latent heat of evaporation is used to transport great amounts of heat in the presence of only small differences in temperature. The pulsating heat pipe assisted tool holder was designed and fabricated. Dry turning of Ti-6Al-4V alloy was carried out with the pulsating heat pipe assisted tool holder as well as the conventional one for comparison. FEM simulations were also carried out to investigate the influence of the pulsating heat pipe assisted cooling on the cutting performance. Results show that the highest cutting temperatures of the PHP assisted tool holder could be reduced by 10 % as so compared to that of the conventional one. The pulsating heat pipe assisted cooling can also reduce the wear of the cutting tools and improve their lives by 20∼30 %. The FEM simulations show a good agreement with the cutting experiment, as a result, validate the potential value of the pulsating heat pipe assisted cooling in dry cutting operations.
1. Introduction In the last several decades, the usage of cutting fluids has become a common choice for improving machinability of metals. The role of cutting fluids is well known for their contribution in removing heat from the cutting edge, lubricating the tool-chip contact and transporting the metal chips away from the cutting zone [1]. In other words, the application of cutting fluids serves a vital part in machining operations. However, conventional cutting fluids can cause problems associated with the environment as well as people's health. The recycling and disposal of waste cutting fluids have also increased the cost of machining process. In order to address this issue, dry machining is implemented as much as possible. Due to the technical innovations such as advanced tool materials [2–4], surface texturing [5–10] and new tool coatings [11–13], dry machining without cutting fluid is developed. However, in dry machining operations, the friction between tool and chip tends to be severe which causes higher temperature, higher wear rates and consequently shorter longevity of the tool, especially in cutting of difficult-to-cut materials. Moreover, the pure dry machining needs extremely rigid requirement which is difficult to be achieved in current shop floors. All these problems related to machining with conventional cutting fluids and pure dry machining lead to research on innovative techniques for cooling in cutting operations. Heat pipe is a highly efficient heat conductor in which the latent ⁎
heat of evaporation is used to transport great amounts of heat in the presence of only small differences in temperature [14]. It is used to transport heat from one location to another without the need for an external power supply by diffusion. Application of heat pipe in metal cutting process as an alternative method for cooling is an interest area. Some researchers have proven the effectiveness of the heat pipe in improving machinability of metals in different styles of cutting operations. Jen et al. [15] proposed a kind of internally cooled drill with heat pipe embedded in center of the drill stem. Lower wear rates and longer longevity were obtained by using the heat pipe cooled drill compared to the conventional one. Furthermore, based on experimental analysis and numerical simulation, Zhu et al. [16] indicated that heat pipe assisted drilling could reduce the peak temperature and stress on the tool tip. Haq et al. [17] investigated the effect of parameters such as length of heat pipe, diameter of heat pipe, material used for making heat pipe and magnitude of vacuum in the heat pipe on cutting performance in turning operation. Liang et al. [18,19] also studied the tool-chip interface temperature in dry turning assisted by a flat heat pipe cooling and indicated that the temperature could be reduced effectively. It was reported by Gnanadurai et al. [20] that the usage of heat pipe combined with minimal fluid application reduced cutting temperature and tool wear in hard turning process. The feasibility and effectiveness of heat pipe cooling in end milling operations were also verified by Zhu et al. [21]. Qian et al. [22] proposed an environmentally friendly grinding
Corresponding authors. E-mail addresses: [email protected] (Y. Xing), [email protected] (L. Liu).
https://doi.org/10.1016/j.jmapro.2019.12.054 Received 2 April 2019; Received in revised form 16 December 2019; Accepted 26 December 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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wheel which was cooled by a revolving heat pipe and investigated its machinability in dry abrasive-milling operations. Review of literatures has indicated that the usage of heat pipe is a potential and effective approach for cooling in machining process, and allows cutting operations to be carried out in a green fashion mostly. Pulsating heat pipe (PHP) is a new and effective heat transfer device, which is different from the conventional heat pipe in structure and working principle [23,24]. The pulsating heat pipe consists of a long and sealed capillary tube bent into many turns, and the inner cavity of the capillary tube is evacuated and filled partially with working fluid. However, the conventional heat pipe just consists of a single pipe. Compared with the conventional heat pipe, the pivotal feature of pulsating heat pipe is that the wick structure is not required to return the condensate to the evaporator section. The inner diameter of the pipe must be sufficiently small so that vapor bubbles can grow to vapor slugs in the tube. Due to the effect of surface tension, the working fluid would arrange in vapor-liquid plugs in the PHP, and heat is transported from the evaporator to the condenser region by means of local axial oscillations and phase changes in the working fluid. When the evaporator section is heated, the vapor pressure is increased, as a result, the vapor slugs move toward the condenser region. The vapor is condensed to liquid in the condenser region and the liquid would be pushed back to the evaporator region along the inner wall of the pipe. The process is repeated, and the oscillation of the vapor-liquid plugs can be maintained. As the liquid moves, the trailing edge of the liquid leaves a thin liquid film on the pipe wall. The evaporation and condensation over this thin liquid film are the driving forces of pulsation flow in a pulsating heat pipe. We had proposed a kind of PHP assisted cutting inserts and investigated their performance in dry cutting of titanium alloys [25–27]. As a result, the potentiality of them in reducing cutting temperature and improving tool wear resistance was proved completely. In the present work, an attempt was made to investigate the feasibility of pulsating heat pipe assisted tool holders in dry turning operations. The cutting performance of the PHP assisted tool holders was assessed by cutting tests as well as numerical simulations. The objective is to excavate the potentiality of the PHP assisted tool holders in improving cutting performance, and then extend the application of pulsating heat pipe in machining process.
Fig. 1. Schematic diagram of the pulsating heat pipe assisted tool holder.
Fig. 2. Photograph of the prepared pulsating heat pipe assisted tool holder.
2. Experimental procedures Table 1 Composition wt% of Ti-6Al-4V alloy.
2.1. Preparation of pulsating heat pipe assisted tool holders The pulsating heat pipe (PHP) consists of a long and sealed capillary tube which bent into many turns, and the inner cavity of the capillary tube is evacuated and filled partially with working fluid. Due to the effect of surface tension, the working fluid would arrange in vapor-liquid plugs in the PHP, and heat is transported from the evaporator to the condenser region by means of local axial oscillations and phase changes in the working fluid. In the present study, the pulsating heat pipe assisted tool holder was designed and fabricated. The schematic of the pulsating heat pipe assisted tool holder is presented in Fig. 1. There were many grooves below the cutting insert, which were also artificially fabricated on the surface of the tool holder. A long continuous tube was passed through the grooves and bent into many turns to form pulsating heat pipe. A kickstand was also provided to protect the pulsating heat pipe. As shown in Fig. 2, the pulsating heat pipe assisted tool holder was fabricated by copper tube which was filled with 50 % acetone, and the bent tubes could be in contact with the reverse side of the tool insert.
Al
V
Fe
Si
C
H
O
Ti
6
4
0.3
0.1
0.1
0.015
0.15
Balance
The CA6140 lathe was selected as the experimental set-up. Two kinds of tool holders (the PHP assisted one as well as the conventional one) were both used for comparison in the turning operations. However, beside containing pulsating heat pipe or not, the two kinds of tool holders had the same geometry: rake angle γo = 0°, clearance angle αo = 0°, inclination angle λs = 0°, side cutting edge angle Kr = 45°. The used cutting tools were cemented carbide inserts with grade of YG6. All tests were carried out with the following parameters: cut depth ap = 0.5 mm, feed rate f = 0.3 mm/r, cutting speed v = 60∼150 m/min. 2.3. Measurement for cutting performance Under given cutting conditions, each test was replicated three times. Cutting temperatures were measured with a TH5104R infrared thermal imaging system. The parameters of the TH5104R infrared thermal imaging system are as following: measured temperature range of −25∼2000 ℃, temperature resolution of 0.1 ℃, and response frequency of 20 ms/p (20 ms per procedure). The scanning electron
2.2. Workpiece material and cutting conditions The workpiece material used in the present study was Ti6Al4V alloy with hardness of 35 ± 3 HRC. The chemical composition of the workpiece material is listed in Table 1. 225
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Fig. 3. The temperature distributions of tool nose measured by infrared thermal imaging system (v = 60 m/min, ap = 0.5 mm, f = 0.3 mm/r): (a) the PHP assisted cooling, (b) the conventional one.
microscopy (SEM) was used to examine the worn flank faces of the cutting inserts. The flank wears were evaluated and the tool lives were investigated. 3. Results and discussion of cutting tests 3.1. Cutting temperatures The temperature distribution of the chip on the rake face can be measured by infrared thermal imaging system. The temperature distributions in cutting operations with the PHP assisted cooling and the conventional one (v = 60 m/min, ap = 0.5 mm, f = 0.3 mm/r) are presented in Fig. 3(a) and (b), respectively. It is noted that the highest temperature at tool nose position in the measurement for the PHP assisted cooling is 669.1 ℃, while the value is 735.7 ℃ for the conventional one. In the measuring by the infrared thermal imaging system, the tool nose is covered by the flowing chips, and the temperature of the other area on the cutting insert which is away from the tool nose and not covered by the chips is not in the displayable temperature range set by the infrared camera. In other words, the displayed color temperature image in Fig. 3 almost represents the temperature of chips. In the present study, the temperature distribution was measured at 15-second intervals in cutting process. The average values of the highest temperature at tool nose position obtained in each measurement in three time repeated experiments are used as cutting temperatures for comparison. The cutting temperatures with two different kinds of tool holders at different cutting speeds are illustrated in Fig. 4. From this figure, it is found that the cutting temperatures of the pulsating heat pipe assisted tool holder are lower than that of the conventional one. For example, at the cutting speed of 150 m/min, the corresponding cutting temperatures are 1120.4 ℃ for the conventional one, however, just 1003.1 ℃ for the PHP assisted one. As a result, a decreased ratio of 10.47 % is obtained by the calculation ((1120.4-1003.1)/1120.4). According to the comparison of the temperatures illustrated in Fig. 4, it can be calculated that the highest cutting temperatures of the PHP assisted tool holder could be reduced by 10 % as so compared to that of the non PHP one under the same cutting condition.
Fig. 4. Cutting temperatures of the pulsating heat pipe assisted tool holder and the conventional one at different cutting speeds.
depth ap = 0.5 mm, feed rate f = 0.3 mm/r and cutting speed v = 60 m/min. The maximum flank wear lengths after cutting operations with PHP assisted tool holder as well as the conventional one are 392 μm and 514 μm, respectively. It is obvious that the flank wear in cutting operation with the PHP assisted tool holder is smaller than that of the conventional one. In addition, the wear of the main cutting edge with the PHP assisted tool holder is also milder than that of the conventional one. The PHP assisted tool holder could improve the anti-wear ability of the cemented carbide inserts. The variations of flank wear with the PHP assisted tool holder as well as the conventional one at cutting speed of 60 m/min are plotted in Fig. 6. It is obvious that the PHP assisted tool holder could alleviate the wear rates of the cutting inserts. In other words, the tool lives could be improved by the pulsating heat pipe assisted cooling. 3.3. Tool life The tool failure for cemented carbide inserts in cutting of titanium alloy is mainly attributed to nose edge wear as well as severe flank wear [28,29]. In the present study, a wear criterion of maximum flank wear VBmax = 0.6 mm was selected based on ISO 3685 standard. The tool lives with the PHP assisted tool holder as well as the conventional one for comparison at different cutting speeds are shown in Fig. 7. It is clear that all the tool lives decrease sharply with increased cutting speed. It is also obvious that the tool life is improved by the PHP assisted cooling. For example, at the cutting speed of 60 m/min, the average tool lives
3.2. Tool wear The SEM images of the worn flank faces after 15 min cutting operations with PHP assisted tool holder as well as the conventional one for comparison are presented in Fig. 5(a) and (b), respectively. The corresponding cutting conditions for the wear results consist of cut 226
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Fig. 5. The SEM images of the worn flank faces after 15 min cutting operations with the speed of 60 m/min: (a) with PHP assisted tool holder, (b) with conventional tool holder.
conventional one under the same cutting condition. The improved tool life would have important significance for the actual cutting operations. The result in the present study is concurrent with other researchers who have reported similar investigations. 3.4. Discussion As mentioned in the introduction, conventional cutting fluids could cause problems associated with the environment as well as people's health. The recycling and disposal of waste cutting fluids would also increase the cost of machining process. However, the titanium alloys are classified as difficult-to-machine materials due to their high strength to weight ratio as well as low thermal conductivity. The pure dry machining of titanium alloy is difficult to carry out, especially in high-speed cutting operations. As a result, advanced cooling techniques are expected to resolve the problems existed in the machining of difficult-to-machine materials. The pulsating heat pipe assisted tool holder is an innovative device which could promote heat loss in cutting of difficult-to-machine materials. In the present study, it can be seen that the PHP assisted tool holder could reduce the cutting temperature as well as the tool wear, at the same time, increase the tool life. Firstly, it should be discussed that how is the cutting temperature influenced by the PHP assisted tool holder. It has been indicated by Pan et al. that it is impossible to accurately measure the cutting temperature at the cutting edge in realtime, for the cutting temperature at the cutting edge is dynamically changing and the cutting edge is covered by chips [30,31]. However, in the present study, the temperatures obtained by the infrared thermal imaging system could be assumed as the cutting temperatures for comparison, because it is more important to investigate the influence of the PHP assisted device on the cutting temperatures, the accurate value of the temperatures is not essential. It has been demonstrated that the cutting temperatures of the PHP assisted tool holder could be reduced by 10 % as so compared to that of the non PHP one under the same cutting condition. This can be primarily attributed to the excellent heat transfer property of the pulsating heat pipe. The cutting heat in cutting inserts could be transferred quickly by the PHP assisted tool holder, as a result, the temperature in the cutting area become lower. It has been also demonstrated from the experimental results that the tool wear of the PHP assisted tool holder is milder than that of the non PHP one. This can be primarily attributed to the lower cutting temperature. The wear mechanisms mainly consist of adhesive wear and abrasive wear in cutting of Ti6Al4V alloy with cemented carbide tools.
Fig. 6. Variations of flank wear with the PHP assisted tool holder as well as the conventional one at cutting speed of 60 m/min.
Fig. 7. Tool life in cutting operations by using conventional and PHP assisted tool holders at different cutting speeds.
are 18.58 min for the conventional one, however, 23.75 min for the PHP assisted one. As a result, an increased ratio of 27.83 % is obtained by the calculation ((23.75-18.58)/18.58). According to the comparison of the tool lives illustrated in Fig. 7, the tool life with PHP assisted cooling could be improved by 20∼30 % compared to that of the 227
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experimental experience. The most important parameter for evaluating the heat transfer performance of pulsating heat pipe is equivalent thermal conductivity, and it is also a vital parameter in the simulation. The equivalent thermal conductivity λe of the pulsating heat pipe can be expressed as:
The adhesive wear and abrasive wear are both influenced by the cutting temperatures. With the cutting temperature increasing, the tool material and the workpiece material would both become softer. The lower hardness of the cemented carbide tool would increase its abrasive wear. The lower hardness of the titanium alloy as well as lower hardness of the cemented carbide would both promote adhesive wear of cutting tools. The pulsating heat pipe assisted tool holder can reduce the cutting temperatures, as a result, it can also reduce the tool wear. In other words, the wear resistance of cutting tools is improved by the PHP assisted cooling. As a result, the tool life is also increased, which is the reason for that the tool life with PHP assisted cooling could be improved by 20∼30 % compared with that of the conventional one. Based on the present study, it is obvious that the pulsating heat pipe (PHP) assisted tool holders can reduce the cutting temperatures, as a result, prolong the usage of cutting inserts compared to the conventional tool holders. The pulsating heat pipe assisted cooling present significant improvement in dry machining of difficult-to-machine materials. The pulsating heat pipe assisted cooling is an environmentfriendly and effective technology. In general, the pulsating heat pipe assisted cooling could improve the cutting performance, and moreover, it can get rid of the disadvantages in use of conventional cutting fluids.
λ e = QLe [Ac (Th − Tc )]
(1)
Where Q is the heat transfer power of the pulsating heat pipe; Le is the equivalent length of the pulsating heat pipe; Ac is the equivalent crosssectional area of the pulsating heat pipe; Th is the average temperature of the heating end of the pulsating heat pipe and Tc denotes the average temperature of the cooling end. In order to obtain the equivalent thermal conductivity of the pulsating heat pipe, an experimental setup for testing the heat transfer performance was built as shown in Fig. 9. In the experimental setup, the heating end of the pulsating heat pipe is inserted into an insulating sleeve and is heated by a resistance wire which is connected to the power supply. The insulating sleeve has good heat-insulating property, which can prevent heat loss effectively. In a steady state, almost all heat is transferred from the heating end to the cooling end only through the pulsating heat pipe. The temperature of the heating end Th and the temperature of the cooling end Tc are measured by thermocouples which are connected to the data collector, and the final data of temperatures is collected by the computer. In a steady state, the output power of the power supply is considered as the heat transfer power Q, which can be calculated according to the displayed output electrical parameters on the power supply. The equivalent length Le and the equivalent cross-sectional area Ac of the pulsating heat pipe can be got by simple geometric measurement. As a result, the equivalent thermal conductivity λe of the pulsating heat pipe can be calculated based on Eq. (1). The density of the pulsating heat pipe was obtained by a simple calculation of ratio of weight to volume, while the specific heat was measured by an electrothermal method. The young’s modulus and the poisson ratio of the pulsating heat pipe are set to the values corresponding to the copper material, which would almost have no impact on accuracy of the simulation. The mechanical parameters (young’s modulus and the poisson ratio) of the copper can be given according to reference [32]. In the present study, the selected materials for the cutting insert and the tool holder are YG6 cemented carbide and 40Cr steel, respectively. The input parameters for the YG6 cemented carbide can be given according to references [33,34], while the input parameters for the 40Cr steel can be given according to references [32,34]. The mechanical and thermal input parameters for the cutting insert, the tool holder and the pulsating heat pipe used in the simulations are listed in Table 2. According to the actual conditions of the study, the initial temperature is set to a value of 20 °C in accordance with the ambient air temperature of the cutting experiment. The specific setting of the heat transfer coefficient between different objects can be given according to reference [34]. The heat transfer coefficient of the contact surface between the cutting insert and the pulsating heat pipe is set to 4000 W/ (m2°C), the heat transfer coefficient of the contact surface between the cutting insert and the tool holder is set to 2500 W/(m2°C), while the value between the pulsating heat pipe and the tool holder is set to 3000 W/(m2°C). Finally, under natural convection conditions, the heat transfer coefficient of the contact surface between the pulsating heat pipe and the air, between the cutting insert and the air, between the tool holder and the air is set to 5 W/(m2°C) uniformly.
4. Simulation of cutting operations 4.1. Modeling and parameter setting In cutting process, the temperature measuring by the infrared thermal imaging system is difficult for that the view of the tool-chip interface is usually covered by the chip. In order to obtain an explicit temperature distribution in the cutting operation, simulation based on the software of ABAQUS is developed. A finite element model was built under the condition that cutting heat was transferred into tool bit through tool-chip interface. As shown in Fig. 8, the FEM gridding model of the PHP assisted tool holder was established. In the software of ABAQUS, the types of elements for meshing consist of hexahedron, tetrahedron and wedge. In the present study, the hexahedral element was selected to ensure the accuracy of simulation. As shown in Fig. 8, a dense mesh was designed for the cutting insert and the pulsating heat pipe, however, a sparse mesh was arranged for the tool holder. In this way, the accuracy and efficiency of simulation could be improved while saving computational resources. In the present study, the pulsating heat pipe is made up of copper and acetone. The actual status for heat transfer of the pulsating heat pipe is very complicated. However, assuming the pulsating heat pipe as a single medium is an acceptable choice for the simulation, and the equivalent physical parameters of the PHP can be given by
4.2. Solving of heat flux To solve the models mentioned above, the heat flux loaded on the tool-chip interface in the FEM simulation is needed. Under normal circumstances, the heat flux could be predefined by an inverse procedure [35]. In the present study, the inverse procedure is carried out.
Fig. 8. FEM gridding model of the PHP assisted tool holder. 228
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Fig. 9. Experimental setup for testing the heat transfer performance of pulsating heat pipe.
heat flux on tool-chip interface is uniform, the objective function F(w) of the inverse procedure can be written as:
Table 2 Mechanical and thermal input parameters for cutting insert, tool holder and pulsating heat pipe. Properties
Cutting insert
Tool holder
PHP
Density (g/cm3) Young’s modulus (GPa) Poisson ratio Thermal conductivity (W/(m℃)) Specific heat (J/(kg℃))
14.8 640 0.22 79.6 208
7.79 210 0.28 44 590
3.39 115 0.32 2 × 105 1985
ts
F (w ) =
4
∫ ∑ [Ts (n, t, w) − Tm (n, t )]2 dt t=0 n=1
(2)
Where Ts(n,t,w) is the simulated temperature at point n of insert with heat flux loaded time of t, which is solved by the finite element model with respect to the assumed heat flux w on the tool-chip interface; Tm(n,t) is the measured temperature at point n of insert with durative cutting time of t, which is obtained by the thermocouple; n denotes the number of measured temperature extracting points, the total of which is 4 in the present work; ts denotes the final time of cutting. In the present study, the temperatures of four appointed locations at the rake face of the conventional tool are utilized for the inverse solution. A 1 mm × 1 mm squared area on the rake face along the main
Four appointed locations on the rake face of cutting insert are selected as temperature measuring points, and thermocouples are used to measure the temperatures at appointed locations. The specific position of the selected points is depicted in Fig. 10. Assuming the distribution of
Fig. 10. Schematic diagram of the selected points on the rake face for temperature measuring by thermocouples. 229
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Fig. 11. The simulated temperature fields with heat flux loading time of 200 s at cutting speed of 60 m/min: (a) with PHP assisted tool holder, (b) with conventional tool holder.
conventional one. The similar simulated results at cutting speed of 150 m/min are presented in Fig. 12. It can be seen from Fig. 12 that the maximum temperature at the tool tip is 1075 ℃ with PHP assisted tool holder at the cutting speed of 150 m/min, while the value is 1170 ℃ for the conventional one. It is obvious that the PHP assisted cooling can reduce the maximum cutting temperature, which is also approved in the cutting experiment. By loading the heat flux corresponding to different cutting speeds, the temperature fields with the PHP assisted tool holder and the conventional one in all cutting speeds were obtained. The simulated maximum temperatures at the tool tips in different cutting speeds are shown in Fig. 13. It can be seen from Fig. 13 that the maximum temperatures increase along with the increase of cutting speeds for all cutting operations. Under the same cutting condition, the maximum cutting temperature with the PHP assisted tool holder is lower than the value of the conventional one. The PHP assisted cooling can reduce the cutting temperature by 10 % as so. It can be seen that the value of the simulated temperature is a little higher than the value obtained in the cutting experiment in the same
cutting edge as well as the minor cutting edge is selected as the loading area of heat flux. By optimizing the unknown heat flux w on the finite element model to minimize the objective function F(w) in Eq. (2), the effective heat flux loaded on the tool-chip interface for different cutting speeds can be determined. The values of the effective heat flux are 70.2 W/mm2, 78.9 W/mm2, 87.3 W/mm2 and 94.1 W/mm2 for the corresponding cutting speeds of 60 m/min, 90 m/min, 120 m/min and 150 m/min, respectively. 4.3. Results of simulations By loading the heat flux, the temperature fields of the cutting tools can be obtained. The simulated temperature fields with PHP assisted tool holder as well as the conventional one at cutting speed of 60 m/ min are shown in Fig. 11(a) and (b), respectively. It is worth mentioning that the corresponding heat flux loading time is 200 s in the comparative results. It can be seen from Fig. 11 that the maximum temperature at the tool tip is 804.4 ℃ with PHP assisted tool holder at the cutting speed of 60 m/min, while the value is 874.8 ℃ for the 230
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Fig. 12. The simulated temperature fields with heat flux loading time of 200 s at cutting speed of 150 m/min: (a) with PHP assisted tool holder, (b) with conventional tool holder.
cutting speed. This can be attributed to the covering by the chip, i. e. the tool nose is covered by the chip in temperature measuring by the infrared thermal imaging system. However, the influence of the PHP assisted cooling in the cutting experiment as well as the FEM simulation is accordant. The simulated results show a good agreement with the cutting experiment. It can be seen from the present simulations that the cutting temperature in dry turning of Ti-6Al-4V alloy becomes higher than 800 ℃ at a relatively low cutting speed of 60 m/min, and it can be as high as 1170 ℃ at cutting speed of 150 m/min. In machining of titanium alloys, the low thermal conductivity of this particular material is easy to bring high cutting temperature at the cutting edge. Conventional method for reducing the cutting temperature is to use coolant. The coolant is more effective if it penetrates into the tool-chip interface during the cutting process. Generally speaking, as generalized by Pramanik [36], effective cooling of liquid coolant can reduce the cutting temperature by 30 % compared to dry cutting of titanium alloy. Compared to liquid cooling, the PHP assisted cooling which reduces the cutting temperature by 10 % as so does not seem to be dominant. However, it is worth noting that
Fig. 13. The simulated maximum temperature at the tool-chip interface with the heat flux loading time of 200 s as a function of cutting speed.
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the PHP assisted cutting in the present study is completely dry cutting, which could avoid the adverse effects of coolant. In fact, for coolant cooling, the effectiveness is also affected by the cooling medium. For example, Yi et al. [37,38] have investigated the cooling effectiveness of graphene oxide nanofluid and conventional cutting fluid in turning of Ti-6Al-4V alloy by both FEM simulation (ABAQUS software) and experiment. It is reported by Yi et al. that the application of graphene oxide nanofluid can reduce the cutting temperature by 4.2 %, 8.43 % and 5.37 % compared to the application of conventional cutting fluid in cutting speed of 80 m/min, 160 m/min and 240 m/min, respectively. Obviously, the effectiveness of the PHP assisted cooling in the present study also has certain advantages compared to the effectiveness of single modification in cooling medium (graphene oxide nanofluid). The significance of the pulsating heat pipe assisted cooling is to provide a new way to control cutting temperature under dry cutting conditions, and the effectiveness of the innovative method in improving cutting performance is also considerable. The prospect of the pulsating heat pipe assisted cooling in dry cutting operations is worth looking forward to.
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Int J Adv Manuf Technol 2012;58(9–12):861–8. https://doi.org/10.1007/s00170-011-3436-x. [17] Haq AN, Tamizharasan T. Investigation of the effects of cooling in hard turning operations. Int J Adv Manuf Technol 2006;30(9–10):808–16. https://doi.org/10. 1007/s00170-005-0128-4. [18] Liang L, Quan Y, Ke Z. Investigation of tool-chip interface temperature in dry turning assisted by heat pipe cooling. Int J Adv Manuf Technol 2011;54(1–4):35–43. https://doi.org/10.1007/s00170-010-2926-6. [19] Liang L, Xu H, Ke Z. An improved there-dimensional inverse heat conduction procedure to determine the tool-chip interface temperature in dry turning. Int J Therm Sci 2013;64:152–61. https://doi.org/10.1016/j.ijthermalsci.2012.08.012. [20] Gnanadurai RR, Varadarajan AS. Investigation on the effect of cooling of the tool using heat pipe during hard turning with minimal fluid application. Eng Sci Technol, Int J 2016;19(3):1190–8. https://doi.org/10.1016/j.jestch.2016.01.012. [21] Zhu L, Jen TC, Yin C, Kong X, Yen YH. Investigation of the feasibility and effectiveness in using heat pipe-embedded drills by finite element analysis. Int J Adv Manuf Technol 2013;64(5–8):659–68. https://doi.org/10.1007/s00170-0124077-4. [22] Qian N, Fu Y, Zhang Y, Chen J, Xu J. Experimental investigation of thermal performance of the oscillating heat pipe for the grinding wheel. Int J Heat Mass Transf 2019;136:911–23. https://doi.org/10.1016/j.ijheatmasstransfer.2019.03.065. [23] Bastakoti D, Zhang H, Li D, Cai W, Li F. An overview on the developing trend of pulsating heat pipe and its performance. Appl Therm Eng 2018;141:305–32. https://doi.org/10.1016/j.applthermaleng.2018.05.121. [24] Zhang Y, Faghri A. Heat transfer in a pulsating heat pipe with open end. Int J Heat Mass Transf 2002;45(4):755–64. https://doi.org/10.1016/S0017-9310(01) 00203-4. [25] Wu Z, Deng J, Su C, Luo C, Xia D. Performance of the micro-texture self-lubricating and pulsating heat pipe self-cooling tools in dry cutting process. Int J Refract Met Hard Mater 2014;45:238–48. https://doi.org/10.1016/j.ijrmhm.2014.02.004. [26] Wu Z, Yang Y, Luo C. Design, fabrication and dry cutting performance of pulsating heat pipe self-cooling tools. J Clean Prod 2016;124:276–82. https://doi.org/10. 1016/j.jclepro.2016.02.129. [27] Wu Z, Yang Y, Su C, Cai X, Luo C. Development and prospect of cooling technology for dry cutting tools. Int J Adv Manuf Technol 2017;88(5–8):1567–77. https://doi. org/10.1007/s00170-016-8842-7. [28] Ezugwu EO, Wang Z. Titanium alloys and their machinability-a review. J Mater Process Technol 1997;68(3):262–74. https://doi.org/10.1016/S0924-0136(96) 00030-1. [29] Li A, Zhao J, Luo H, Pei Z, Wang Z. Progressive tool failure in high-speed dry milling of Ti-6Al-4V alloy with coated carbide tools. Int J Adv Manuf Technol 2012;58(5–8):465–78. https://doi.org/10.1007/s00170-011-3408-1. [30] Pan W, Ding S, Mo J. Thermal characteristics in milling Ti6Al4V with polycrystalline diamond tools. Int J Adv Manuf Technol 2014;75(5–8):1077–87. https://doi.org/10.1007/s00170-014-6094-y. [31] Pan W, Ding S, Mo J. The prediction of cutting force in end milling titanium alloy (Ti6Al4V) with polycrystalline diamond tools. Proc Inst Mech Eng Part B J Eng
5. Conclusions The pulsating heat pipe assisted tool holder was designed and fabricated. Experiments and FEM simulations were implemented to investigate the influence of the PHP assisted cooling on the cutting performance. Conclusions based on the present study are obtained as follows. 1 The pulsating heat pipe assisted cooling is effective for the cooling in turning of titanium alloys. The highest cutting temperatures of the PHP assisted tool holder could be reduced by 10 % as so compared to that of the non PHP one. 2 The pulsating heat pipe assisted cooling can also reduce the wear of the flank face and the cutting lip. The tool lives with PHP assisted cooling could be improved by 20∼30 % compared with that of the conventional one. 3 The FEM simulations also indicate the effectiveness of the PHP assisted cooling in reduce the cutting temperatures. The simulated results show a good agreement with the cutting experiment, which validates the potential value of the PHP assisted cooling in dry cutting operations. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Natural Science Foundation of China (Grant no.51775109), Natural Science Foundation of Jiangsu Province (Grant no. BK20161203, BK20170676 and BK20181274) and Jiangsu Key Laboratory of Precision and Micro-Manufacturing Technology. References [1] Sharma VS, Dogra M, Suri NM. Cooling techniques for improved productivity in turning. Int J Mach Tools Manuf 2009;49(6):435–53. https://doi.org/10.1016/j. ijmachtools.2008.12.010. [2] Chen Z, Ji L, Guo R, Xu C, Li Q. Mechanical properties and microstructure of Al2O3/ Ti(C, N)/CaF2@Al2O3 self-lubricating ceramic tool. Int J Refract Met Hard Mater 2019;80:144–50. https://doi.org/10.1016/j.ijrmhm.2019.01.006. [3] Chen Z, Ji L, Guo N, Guo R. Mechanical properties and microstructure of Al2O3/TiC based self-lubricating ceramic tool with CaF2@Al(HO)3. Int J Refract Met Hard Mater 2018;75:50–5. https://doi.org/10.1016/j.ijrmhm.2018.04.001. [4] Wu G, Xu C, Xiao G, Yi M, Chen Z, Xu L. Self-lubricating ceramic cutting tool material with the addition of nickel coated CaF2 solid lubricant powders. Int J Refract
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[36] Pramanik A. Problems and solutions in machining of titanium alloys. Int J Adv Manuf Technol 2014;70(5–8):919–28. https://doi.org/10.1007/s00170-0135326-x. [37] Yi S, Li N, Solanki S, Mo J, Ding S. Effects of graphene oxide nanofluids on cutting temperature and force in machining Ti-6Al-4V. Int J Adv Manuf Technol 2019;103(1–4):1481–95. https://doi.org/10.1007/s00170-019-03625-1. [38] Yi S, Mo J, Ding S. Experimental investigation on the performance and mechanism of graphene oxide nanofluids in turning Ti-6Al-4V. J Manuf Processes 2019;43:164–74. https://doi.org/10.1016/j.jmapro.2019.05.005.
Manuf 2017;231(1):3–14. https://doi.org/10.1177/0954405415581299. [32] Liu S. Handbook of practical metal materials. Beijing: China Machine Press; 2017. [33] Deng J, Zhao J. Handbook of NC tool material selection. Beijing: China Machine Press; 2005. [34] Ma Q. Handbook of practical thermal physical properties. Beijing: China Agricultural Machinery Press; 1990. [35] Yvonnet J, Umbrello D, Chinesta F, Micari F. A simple inverse procedure to determine heat flux on the tool in orthogonal cutting. Int J Mach Tools Manuf 2006;46(7–8):820–7. https://doi.org/10.1016/j.ijmachtools.2005.07.030.
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Technical Paper
Design, fabrication and performance evaluation of pulsating heat pipe assisted tool holder
T
Ze Wua, Youqiang Xinga,*, Lei Liua,*, Peng Huanga, Guolong Zhaob a b
School of Mechanical Engineering, Southeast University, Nanjing 211189, PR China School of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pulsating heat pipe Tool holder Titanium alloy
Pulsating heat pipe is a kind of highly efficient heat conductor in which the latent heat of evaporation is used to transport great amounts of heat in the presence of only small differences in temperature. The pulsating heat pipe assisted tool holder was designed and fabricated. Dry turning of Ti-6Al-4V alloy was carried out with the pulsating heat pipe assisted tool holder as well as the conventional one for comparison. FEM simulations were also carried out to investigate the influence of the pulsating heat pipe assisted cooling on the cutting performance. Results show that the highest cutting temperatures of the PHP assisted tool holder could be reduced by 10 % as so compared to that of the conventional one. The pulsating heat pipe assisted cooling can also reduce the wear of the cutting tools and improve their lives by 20∼30 %. The FEM simulations show a good agreement with the cutting experiment, as a result, validate the potential value of the pulsating heat pipe assisted cooling in dry cutting operations.
1. Introduction In the last several decades, the usage of cutting fluids has become a common choice for improving machinability of metals. The role of cutting fluids is well known for their contribution in removing heat from the cutting edge, lubricating the tool-chip contact and transporting the metal chips away from the cutting zone [1]. In other words, the application of cutting fluids serves a vital part in machining operations. However, conventional cutting fluids can cause problems associated with the environment as well as people's health. The recycling and disposal of waste cutting fluids have also increased the cost of machining process. In order to address this issue, dry machining is implemented as much as possible. Due to the technical innovations such as advanced tool materials [2–4], surface texturing [5–10] and new tool coatings [11–13], dry machining without cutting fluid is developed. However, in dry machining operations, the friction between tool and chip tends to be severe which causes higher temperature, higher wear rates and consequently shorter longevity of the tool, especially in cutting of difficult-to-cut materials. Moreover, the pure dry machining needs extremely rigid requirement which is difficult to be achieved in current shop floors. All these problems related to machining with conventional cutting fluids and pure dry machining lead to research on innovative techniques for cooling in cutting operations. Heat pipe is a highly efficient heat conductor in which the latent ⁎
heat of evaporation is used to transport great amounts of heat in the presence of only small differences in temperature [14]. It is used to transport heat from one location to another without the need for an external power supply by diffusion. Application of heat pipe in metal cutting process as an alternative method for cooling is an interest area. Some researchers have proven the effectiveness of the heat pipe in improving machinability of metals in different styles of cutting operations. Jen et al. [15] proposed a kind of internally cooled drill with heat pipe embedded in center of the drill stem. Lower wear rates and longer longevity were obtained by using the heat pipe cooled drill compared to the conventional one. Furthermore, based on experimental analysis and numerical simulation, Zhu et al. [16] indicated that heat pipe assisted drilling could reduce the peak temperature and stress on the tool tip. Haq et al. [17] investigated the effect of parameters such as length of heat pipe, diameter of heat pipe, material used for making heat pipe and magnitude of vacuum in the heat pipe on cutting performance in turning operation. Liang et al. [18,19] also studied the tool-chip interface temperature in dry turning assisted by a flat heat pipe cooling and indicated that the temperature could be reduced effectively. It was reported by Gnanadurai et al. [20] that the usage of heat pipe combined with minimal fluid application reduced cutting temperature and tool wear in hard turning process. The feasibility and effectiveness of heat pipe cooling in end milling operations were also verified by Zhu et al. [21]. Qian et al. [22] proposed an environmentally friendly grinding
Corresponding authors. E-mail addresses: [email protected] (Y. Xing), [email protected] (L. Liu).
https://doi.org/10.1016/j.jmapro.2019.12.054 Received 2 April 2019; Received in revised form 16 December 2019; Accepted 26 December 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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wheel which was cooled by a revolving heat pipe and investigated its machinability in dry abrasive-milling operations. Review of literatures has indicated that the usage of heat pipe is a potential and effective approach for cooling in machining process, and allows cutting operations to be carried out in a green fashion mostly. Pulsating heat pipe (PHP) is a new and effective heat transfer device, which is different from the conventional heat pipe in structure and working principle [23,24]. The pulsating heat pipe consists of a long and sealed capillary tube bent into many turns, and the inner cavity of the capillary tube is evacuated and filled partially with working fluid. However, the conventional heat pipe just consists of a single pipe. Compared with the conventional heat pipe, the pivotal feature of pulsating heat pipe is that the wick structure is not required to return the condensate to the evaporator section. The inner diameter of the pipe must be sufficiently small so that vapor bubbles can grow to vapor slugs in the tube. Due to the effect of surface tension, the working fluid would arrange in vapor-liquid plugs in the PHP, and heat is transported from the evaporator to the condenser region by means of local axial oscillations and phase changes in the working fluid. When the evaporator section is heated, the vapor pressure is increased, as a result, the vapor slugs move toward the condenser region. The vapor is condensed to liquid in the condenser region and the liquid would be pushed back to the evaporator region along the inner wall of the pipe. The process is repeated, and the oscillation of the vapor-liquid plugs can be maintained. As the liquid moves, the trailing edge of the liquid leaves a thin liquid film on the pipe wall. The evaporation and condensation over this thin liquid film are the driving forces of pulsation flow in a pulsating heat pipe. We had proposed a kind of PHP assisted cutting inserts and investigated their performance in dry cutting of titanium alloys [25–27]. As a result, the potentiality of them in reducing cutting temperature and improving tool wear resistance was proved completely. In the present work, an attempt was made to investigate the feasibility of pulsating heat pipe assisted tool holders in dry turning operations. The cutting performance of the PHP assisted tool holders was assessed by cutting tests as well as numerical simulations. The objective is to excavate the potentiality of the PHP assisted tool holders in improving cutting performance, and then extend the application of pulsating heat pipe in machining process.
Fig. 1. Schematic diagram of the pulsating heat pipe assisted tool holder.
Fig. 2. Photograph of the prepared pulsating heat pipe assisted tool holder.
2. Experimental procedures Table 1 Composition wt% of Ti-6Al-4V alloy.
2.1. Preparation of pulsating heat pipe assisted tool holders The pulsating heat pipe (PHP) consists of a long and sealed capillary tube which bent into many turns, and the inner cavity of the capillary tube is evacuated and filled partially with working fluid. Due to the effect of surface tension, the working fluid would arrange in vapor-liquid plugs in the PHP, and heat is transported from the evaporator to the condenser region by means of local axial oscillations and phase changes in the working fluid. In the present study, the pulsating heat pipe assisted tool holder was designed and fabricated. The schematic of the pulsating heat pipe assisted tool holder is presented in Fig. 1. There were many grooves below the cutting insert, which were also artificially fabricated on the surface of the tool holder. A long continuous tube was passed through the grooves and bent into many turns to form pulsating heat pipe. A kickstand was also provided to protect the pulsating heat pipe. As shown in Fig. 2, the pulsating heat pipe assisted tool holder was fabricated by copper tube which was filled with 50 % acetone, and the bent tubes could be in contact with the reverse side of the tool insert.
Al
V
Fe
Si
C
H
O
Ti
6
4
0.3
0.1
0.1
0.015
0.15
Balance
The CA6140 lathe was selected as the experimental set-up. Two kinds of tool holders (the PHP assisted one as well as the conventional one) were both used for comparison in the turning operations. However, beside containing pulsating heat pipe or not, the two kinds of tool holders had the same geometry: rake angle γo = 0°, clearance angle αo = 0°, inclination angle λs = 0°, side cutting edge angle Kr = 45°. The used cutting tools were cemented carbide inserts with grade of YG6. All tests were carried out with the following parameters: cut depth ap = 0.5 mm, feed rate f = 0.3 mm/r, cutting speed v = 60∼150 m/min. 2.3. Measurement for cutting performance Under given cutting conditions, each test was replicated three times. Cutting temperatures were measured with a TH5104R infrared thermal imaging system. The parameters of the TH5104R infrared thermal imaging system are as following: measured temperature range of −25∼2000 ℃, temperature resolution of 0.1 ℃, and response frequency of 20 ms/p (20 ms per procedure). The scanning electron
2.2. Workpiece material and cutting conditions The workpiece material used in the present study was Ti6Al4V alloy with hardness of 35 ± 3 HRC. The chemical composition of the workpiece material is listed in Table 1. 225
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Fig. 3. The temperature distributions of tool nose measured by infrared thermal imaging system (v = 60 m/min, ap = 0.5 mm, f = 0.3 mm/r): (a) the PHP assisted cooling, (b) the conventional one.
microscopy (SEM) was used to examine the worn flank faces of the cutting inserts. The flank wears were evaluated and the tool lives were investigated. 3. Results and discussion of cutting tests 3.1. Cutting temperatures The temperature distribution of the chip on the rake face can be measured by infrared thermal imaging system. The temperature distributions in cutting operations with the PHP assisted cooling and the conventional one (v = 60 m/min, ap = 0.5 mm, f = 0.3 mm/r) are presented in Fig. 3(a) and (b), respectively. It is noted that the highest temperature at tool nose position in the measurement for the PHP assisted cooling is 669.1 ℃, while the value is 735.7 ℃ for the conventional one. In the measuring by the infrared thermal imaging system, the tool nose is covered by the flowing chips, and the temperature of the other area on the cutting insert which is away from the tool nose and not covered by the chips is not in the displayable temperature range set by the infrared camera. In other words, the displayed color temperature image in Fig. 3 almost represents the temperature of chips. In the present study, the temperature distribution was measured at 15-second intervals in cutting process. The average values of the highest temperature at tool nose position obtained in each measurement in three time repeated experiments are used as cutting temperatures for comparison. The cutting temperatures with two different kinds of tool holders at different cutting speeds are illustrated in Fig. 4. From this figure, it is found that the cutting temperatures of the pulsating heat pipe assisted tool holder are lower than that of the conventional one. For example, at the cutting speed of 150 m/min, the corresponding cutting temperatures are 1120.4 ℃ for the conventional one, however, just 1003.1 ℃ for the PHP assisted one. As a result, a decreased ratio of 10.47 % is obtained by the calculation ((1120.4-1003.1)/1120.4). According to the comparison of the temperatures illustrated in Fig. 4, it can be calculated that the highest cutting temperatures of the PHP assisted tool holder could be reduced by 10 % as so compared to that of the non PHP one under the same cutting condition.
Fig. 4. Cutting temperatures of the pulsating heat pipe assisted tool holder and the conventional one at different cutting speeds.
depth ap = 0.5 mm, feed rate f = 0.3 mm/r and cutting speed v = 60 m/min. The maximum flank wear lengths after cutting operations with PHP assisted tool holder as well as the conventional one are 392 μm and 514 μm, respectively. It is obvious that the flank wear in cutting operation with the PHP assisted tool holder is smaller than that of the conventional one. In addition, the wear of the main cutting edge with the PHP assisted tool holder is also milder than that of the conventional one. The PHP assisted tool holder could improve the anti-wear ability of the cemented carbide inserts. The variations of flank wear with the PHP assisted tool holder as well as the conventional one at cutting speed of 60 m/min are plotted in Fig. 6. It is obvious that the PHP assisted tool holder could alleviate the wear rates of the cutting inserts. In other words, the tool lives could be improved by the pulsating heat pipe assisted cooling. 3.3. Tool life The tool failure for cemented carbide inserts in cutting of titanium alloy is mainly attributed to nose edge wear as well as severe flank wear [28,29]. In the present study, a wear criterion of maximum flank wear VBmax = 0.6 mm was selected based on ISO 3685 standard. The tool lives with the PHP assisted tool holder as well as the conventional one for comparison at different cutting speeds are shown in Fig. 7. It is clear that all the tool lives decrease sharply with increased cutting speed. It is also obvious that the tool life is improved by the PHP assisted cooling. For example, at the cutting speed of 60 m/min, the average tool lives
3.2. Tool wear The SEM images of the worn flank faces after 15 min cutting operations with PHP assisted tool holder as well as the conventional one for comparison are presented in Fig. 5(a) and (b), respectively. The corresponding cutting conditions for the wear results consist of cut 226
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Fig. 5. The SEM images of the worn flank faces after 15 min cutting operations with the speed of 60 m/min: (a) with PHP assisted tool holder, (b) with conventional tool holder.
conventional one under the same cutting condition. The improved tool life would have important significance for the actual cutting operations. The result in the present study is concurrent with other researchers who have reported similar investigations. 3.4. Discussion As mentioned in the introduction, conventional cutting fluids could cause problems associated with the environment as well as people's health. The recycling and disposal of waste cutting fluids would also increase the cost of machining process. However, the titanium alloys are classified as difficult-to-machine materials due to their high strength to weight ratio as well as low thermal conductivity. The pure dry machining of titanium alloy is difficult to carry out, especially in high-speed cutting operations. As a result, advanced cooling techniques are expected to resolve the problems existed in the machining of difficult-to-machine materials. The pulsating heat pipe assisted tool holder is an innovative device which could promote heat loss in cutting of difficult-to-machine materials. In the present study, it can be seen that the PHP assisted tool holder could reduce the cutting temperature as well as the tool wear, at the same time, increase the tool life. Firstly, it should be discussed that how is the cutting temperature influenced by the PHP assisted tool holder. It has been indicated by Pan et al. that it is impossible to accurately measure the cutting temperature at the cutting edge in realtime, for the cutting temperature at the cutting edge is dynamically changing and the cutting edge is covered by chips [30,31]. However, in the present study, the temperatures obtained by the infrared thermal imaging system could be assumed as the cutting temperatures for comparison, because it is more important to investigate the influence of the PHP assisted device on the cutting temperatures, the accurate value of the temperatures is not essential. It has been demonstrated that the cutting temperatures of the PHP assisted tool holder could be reduced by 10 % as so compared to that of the non PHP one under the same cutting condition. This can be primarily attributed to the excellent heat transfer property of the pulsating heat pipe. The cutting heat in cutting inserts could be transferred quickly by the PHP assisted tool holder, as a result, the temperature in the cutting area become lower. It has been also demonstrated from the experimental results that the tool wear of the PHP assisted tool holder is milder than that of the non PHP one. This can be primarily attributed to the lower cutting temperature. The wear mechanisms mainly consist of adhesive wear and abrasive wear in cutting of Ti6Al4V alloy with cemented carbide tools.
Fig. 6. Variations of flank wear with the PHP assisted tool holder as well as the conventional one at cutting speed of 60 m/min.
Fig. 7. Tool life in cutting operations by using conventional and PHP assisted tool holders at different cutting speeds.
are 18.58 min for the conventional one, however, 23.75 min for the PHP assisted one. As a result, an increased ratio of 27.83 % is obtained by the calculation ((23.75-18.58)/18.58). According to the comparison of the tool lives illustrated in Fig. 7, the tool life with PHP assisted cooling could be improved by 20∼30 % compared to that of the 227
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experimental experience. The most important parameter for evaluating the heat transfer performance of pulsating heat pipe is equivalent thermal conductivity, and it is also a vital parameter in the simulation. The equivalent thermal conductivity λe of the pulsating heat pipe can be expressed as:
The adhesive wear and abrasive wear are both influenced by the cutting temperatures. With the cutting temperature increasing, the tool material and the workpiece material would both become softer. The lower hardness of the cemented carbide tool would increase its abrasive wear. The lower hardness of the titanium alloy as well as lower hardness of the cemented carbide would both promote adhesive wear of cutting tools. The pulsating heat pipe assisted tool holder can reduce the cutting temperatures, as a result, it can also reduce the tool wear. In other words, the wear resistance of cutting tools is improved by the PHP assisted cooling. As a result, the tool life is also increased, which is the reason for that the tool life with PHP assisted cooling could be improved by 20∼30 % compared with that of the conventional one. Based on the present study, it is obvious that the pulsating heat pipe (PHP) assisted tool holders can reduce the cutting temperatures, as a result, prolong the usage of cutting inserts compared to the conventional tool holders. The pulsating heat pipe assisted cooling present significant improvement in dry machining of difficult-to-machine materials. The pulsating heat pipe assisted cooling is an environmentfriendly and effective technology. In general, the pulsating heat pipe assisted cooling could improve the cutting performance, and moreover, it can get rid of the disadvantages in use of conventional cutting fluids.
λ e = QLe [Ac (Th − Tc )]
(1)
Where Q is the heat transfer power of the pulsating heat pipe; Le is the equivalent length of the pulsating heat pipe; Ac is the equivalent crosssectional area of the pulsating heat pipe; Th is the average temperature of the heating end of the pulsating heat pipe and Tc denotes the average temperature of the cooling end. In order to obtain the equivalent thermal conductivity of the pulsating heat pipe, an experimental setup for testing the heat transfer performance was built as shown in Fig. 9. In the experimental setup, the heating end of the pulsating heat pipe is inserted into an insulating sleeve and is heated by a resistance wire which is connected to the power supply. The insulating sleeve has good heat-insulating property, which can prevent heat loss effectively. In a steady state, almost all heat is transferred from the heating end to the cooling end only through the pulsating heat pipe. The temperature of the heating end Th and the temperature of the cooling end Tc are measured by thermocouples which are connected to the data collector, and the final data of temperatures is collected by the computer. In a steady state, the output power of the power supply is considered as the heat transfer power Q, which can be calculated according to the displayed output electrical parameters on the power supply. The equivalent length Le and the equivalent cross-sectional area Ac of the pulsating heat pipe can be got by simple geometric measurement. As a result, the equivalent thermal conductivity λe of the pulsating heat pipe can be calculated based on Eq. (1). The density of the pulsating heat pipe was obtained by a simple calculation of ratio of weight to volume, while the specific heat was measured by an electrothermal method. The young’s modulus and the poisson ratio of the pulsating heat pipe are set to the values corresponding to the copper material, which would almost have no impact on accuracy of the simulation. The mechanical parameters (young’s modulus and the poisson ratio) of the copper can be given according to reference [32]. In the present study, the selected materials for the cutting insert and the tool holder are YG6 cemented carbide and 40Cr steel, respectively. The input parameters for the YG6 cemented carbide can be given according to references [33,34], while the input parameters for the 40Cr steel can be given according to references [32,34]. The mechanical and thermal input parameters for the cutting insert, the tool holder and the pulsating heat pipe used in the simulations are listed in Table 2. According to the actual conditions of the study, the initial temperature is set to a value of 20 °C in accordance with the ambient air temperature of the cutting experiment. The specific setting of the heat transfer coefficient between different objects can be given according to reference [34]. The heat transfer coefficient of the contact surface between the cutting insert and the pulsating heat pipe is set to 4000 W/ (m2°C), the heat transfer coefficient of the contact surface between the cutting insert and the tool holder is set to 2500 W/(m2°C), while the value between the pulsating heat pipe and the tool holder is set to 3000 W/(m2°C). Finally, under natural convection conditions, the heat transfer coefficient of the contact surface between the pulsating heat pipe and the air, between the cutting insert and the air, between the tool holder and the air is set to 5 W/(m2°C) uniformly.
4. Simulation of cutting operations 4.1. Modeling and parameter setting In cutting process, the temperature measuring by the infrared thermal imaging system is difficult for that the view of the tool-chip interface is usually covered by the chip. In order to obtain an explicit temperature distribution in the cutting operation, simulation based on the software of ABAQUS is developed. A finite element model was built under the condition that cutting heat was transferred into tool bit through tool-chip interface. As shown in Fig. 8, the FEM gridding model of the PHP assisted tool holder was established. In the software of ABAQUS, the types of elements for meshing consist of hexahedron, tetrahedron and wedge. In the present study, the hexahedral element was selected to ensure the accuracy of simulation. As shown in Fig. 8, a dense mesh was designed for the cutting insert and the pulsating heat pipe, however, a sparse mesh was arranged for the tool holder. In this way, the accuracy and efficiency of simulation could be improved while saving computational resources. In the present study, the pulsating heat pipe is made up of copper and acetone. The actual status for heat transfer of the pulsating heat pipe is very complicated. However, assuming the pulsating heat pipe as a single medium is an acceptable choice for the simulation, and the equivalent physical parameters of the PHP can be given by
4.2. Solving of heat flux To solve the models mentioned above, the heat flux loaded on the tool-chip interface in the FEM simulation is needed. Under normal circumstances, the heat flux could be predefined by an inverse procedure [35]. In the present study, the inverse procedure is carried out.
Fig. 8. FEM gridding model of the PHP assisted tool holder. 228
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Fig. 9. Experimental setup for testing the heat transfer performance of pulsating heat pipe.
heat flux on tool-chip interface is uniform, the objective function F(w) of the inverse procedure can be written as:
Table 2 Mechanical and thermal input parameters for cutting insert, tool holder and pulsating heat pipe. Properties
Cutting insert
Tool holder
PHP
Density (g/cm3) Young’s modulus (GPa) Poisson ratio Thermal conductivity (W/(m℃)) Specific heat (J/(kg℃))
14.8 640 0.22 79.6 208
7.79 210 0.28 44 590
3.39 115 0.32 2 × 105 1985
ts
F (w ) =
4
∫ ∑ [Ts (n, t, w) − Tm (n, t )]2 dt t=0 n=1
(2)
Where Ts(n,t,w) is the simulated temperature at point n of insert with heat flux loaded time of t, which is solved by the finite element model with respect to the assumed heat flux w on the tool-chip interface; Tm(n,t) is the measured temperature at point n of insert with durative cutting time of t, which is obtained by the thermocouple; n denotes the number of measured temperature extracting points, the total of which is 4 in the present work; ts denotes the final time of cutting. In the present study, the temperatures of four appointed locations at the rake face of the conventional tool are utilized for the inverse solution. A 1 mm × 1 mm squared area on the rake face along the main
Four appointed locations on the rake face of cutting insert are selected as temperature measuring points, and thermocouples are used to measure the temperatures at appointed locations. The specific position of the selected points is depicted in Fig. 10. Assuming the distribution of
Fig. 10. Schematic diagram of the selected points on the rake face for temperature measuring by thermocouples. 229
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Fig. 11. The simulated temperature fields with heat flux loading time of 200 s at cutting speed of 60 m/min: (a) with PHP assisted tool holder, (b) with conventional tool holder.
conventional one. The similar simulated results at cutting speed of 150 m/min are presented in Fig. 12. It can be seen from Fig. 12 that the maximum temperature at the tool tip is 1075 ℃ with PHP assisted tool holder at the cutting speed of 150 m/min, while the value is 1170 ℃ for the conventional one. It is obvious that the PHP assisted cooling can reduce the maximum cutting temperature, which is also approved in the cutting experiment. By loading the heat flux corresponding to different cutting speeds, the temperature fields with the PHP assisted tool holder and the conventional one in all cutting speeds were obtained. The simulated maximum temperatures at the tool tips in different cutting speeds are shown in Fig. 13. It can be seen from Fig. 13 that the maximum temperatures increase along with the increase of cutting speeds for all cutting operations. Under the same cutting condition, the maximum cutting temperature with the PHP assisted tool holder is lower than the value of the conventional one. The PHP assisted cooling can reduce the cutting temperature by 10 % as so. It can be seen that the value of the simulated temperature is a little higher than the value obtained in the cutting experiment in the same
cutting edge as well as the minor cutting edge is selected as the loading area of heat flux. By optimizing the unknown heat flux w on the finite element model to minimize the objective function F(w) in Eq. (2), the effective heat flux loaded on the tool-chip interface for different cutting speeds can be determined. The values of the effective heat flux are 70.2 W/mm2, 78.9 W/mm2, 87.3 W/mm2 and 94.1 W/mm2 for the corresponding cutting speeds of 60 m/min, 90 m/min, 120 m/min and 150 m/min, respectively. 4.3. Results of simulations By loading the heat flux, the temperature fields of the cutting tools can be obtained. The simulated temperature fields with PHP assisted tool holder as well as the conventional one at cutting speed of 60 m/ min are shown in Fig. 11(a) and (b), respectively. It is worth mentioning that the corresponding heat flux loading time is 200 s in the comparative results. It can be seen from Fig. 11 that the maximum temperature at the tool tip is 804.4 ℃ with PHP assisted tool holder at the cutting speed of 60 m/min, while the value is 874.8 ℃ for the 230
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Fig. 12. The simulated temperature fields with heat flux loading time of 200 s at cutting speed of 150 m/min: (a) with PHP assisted tool holder, (b) with conventional tool holder.
cutting speed. This can be attributed to the covering by the chip, i. e. the tool nose is covered by the chip in temperature measuring by the infrared thermal imaging system. However, the influence of the PHP assisted cooling in the cutting experiment as well as the FEM simulation is accordant. The simulated results show a good agreement with the cutting experiment. It can be seen from the present simulations that the cutting temperature in dry turning of Ti-6Al-4V alloy becomes higher than 800 ℃ at a relatively low cutting speed of 60 m/min, and it can be as high as 1170 ℃ at cutting speed of 150 m/min. In machining of titanium alloys, the low thermal conductivity of this particular material is easy to bring high cutting temperature at the cutting edge. Conventional method for reducing the cutting temperature is to use coolant. The coolant is more effective if it penetrates into the tool-chip interface during the cutting process. Generally speaking, as generalized by Pramanik [36], effective cooling of liquid coolant can reduce the cutting temperature by 30 % compared to dry cutting of titanium alloy. Compared to liquid cooling, the PHP assisted cooling which reduces the cutting temperature by 10 % as so does not seem to be dominant. However, it is worth noting that
Fig. 13. The simulated maximum temperature at the tool-chip interface with the heat flux loading time of 200 s as a function of cutting speed.
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the PHP assisted cutting in the present study is completely dry cutting, which could avoid the adverse effects of coolant. In fact, for coolant cooling, the effectiveness is also affected by the cooling medium. For example, Yi et al. [37,38] have investigated the cooling effectiveness of graphene oxide nanofluid and conventional cutting fluid in turning of Ti-6Al-4V alloy by both FEM simulation (ABAQUS software) and experiment. It is reported by Yi et al. that the application of graphene oxide nanofluid can reduce the cutting temperature by 4.2 %, 8.43 % and 5.37 % compared to the application of conventional cutting fluid in cutting speed of 80 m/min, 160 m/min and 240 m/min, respectively. Obviously, the effectiveness of the PHP assisted cooling in the present study also has certain advantages compared to the effectiveness of single modification in cooling medium (graphene oxide nanofluid). The significance of the pulsating heat pipe assisted cooling is to provide a new way to control cutting temperature under dry cutting conditions, and the effectiveness of the innovative method in improving cutting performance is also considerable. The prospect of the pulsating heat pipe assisted cooling in dry cutting operations is worth looking forward to.
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5. Conclusions The pulsating heat pipe assisted tool holder was designed and fabricated. Experiments and FEM simulations were implemented to investigate the influence of the PHP assisted cooling on the cutting performance. Conclusions based on the present study are obtained as follows. 1 The pulsating heat pipe assisted cooling is effective for the cooling in turning of titanium alloys. The highest cutting temperatures of the PHP assisted tool holder could be reduced by 10 % as so compared to that of the non PHP one. 2 The pulsating heat pipe assisted cooling can also reduce the wear of the flank face and the cutting lip. The tool lives with PHP assisted cooling could be improved by 20∼30 % compared with that of the conventional one. 3 The FEM simulations also indicate the effectiveness of the PHP assisted cooling in reduce the cutting temperatures. The simulated results show a good agreement with the cutting experiment, which validates the potential value of the PHP assisted cooling in dry cutting operations. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Natural Science Foundation of China (Grant no.51775109), Natural Science Foundation of Jiangsu Province (Grant no. BK20161203, BK20170676 and BK20181274) and Jiangsu Key Laboratory of Precision and Micro-Manufacturing Technology. References [1] Sharma VS, Dogra M, Suri NM. Cooling techniques for improved productivity in turning. Int J Mach Tools Manuf 2009;49(6):435–53. https://doi.org/10.1016/j. ijmachtools.2008.12.010. [2] Chen Z, Ji L, Guo R, Xu C, Li Q. Mechanical properties and microstructure of Al2O3/ Ti(C, N)/CaF2@Al2O3 self-lubricating ceramic tool. Int J Refract Met Hard Mater 2019;80:144–50. https://doi.org/10.1016/j.ijrmhm.2019.01.006. [3] Chen Z, Ji L, Guo N, Guo R. Mechanical properties and microstructure of Al2O3/TiC based self-lubricating ceramic tool with CaF2@Al(HO)3. Int J Refract Met Hard Mater 2018;75:50–5. https://doi.org/10.1016/j.ijrmhm.2018.04.001. [4] Wu G, Xu C, Xiao G, Yi M, Chen Z, Xu L. Self-lubricating ceramic cutting tool material with the addition of nickel coated CaF2 solid lubricant powders. Int J Refract
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