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Drilling of AISI 304 Stainless Steel under Liquid Nitrogen Cooling: A Comparison with Flood Cooling
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 4 (2017) 1518–1524
www.materialstoday.com/proceedings
5th International Conference of Materials Processing and Characterization (ICMPC 2016)
Drilling of AISI 304 Stainless Steel under Liquid Nitrogen Cooling: A Comparison with Flood Cooling Pradeep Kumar Ma, *, Shakeel Ahmed La a
Depaertment of Mechanical Engineering, CEG Campus, Anna University, Chennai – 600 025, India
Abstract In this experimental study, investigations were carried out in a drilling operation on AISI 304 stainless steel and TiCN coated carbide indexable insert tool under flood and liquid nitrogen (LN2) cooling separately. Cutting speeds (40 m/min and 50 m/min) and feed rates (0.02, 0.05 and 0.08 mm/rev) at constant depth were selected as input parameters. The cutting temperature, thrust force and surface roughness were analyzed. The microstructure of drilled surface, tool wear and chip morphology were studied. The experimental results indicate a reduction in cutting temperature, increase in thrust force and surface roughness when cryogenic LN2cooling is used. ©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016). Keywords:drilling; cryogenic cooling; temperature; thrust force; surface roughness.
1. Introduction AISI 304 stainless steel finds extensive use in aerospace and automotive products. It is very hard to cut material. It also presents some problems after machining such as poor surface finish, high wear rate, tool failure, Built up Edge (BUE) formation. The BUE formation leads to affect the surface quality of the material and rapid tool wear [1, 2]. Automotive industries aim mainly for high material removal rate, better surface finish, less tool wear and high productivity. The main problem in the achievement of the productivity and surface quality is high cutting temperature developed in the machining zone, leading to tool failure. AISI 304 steel is a poor machinability material due to high tensile strength, toughness, work hardening, ductility and low heat conductivity [3]. The effective way to improve the performance in drilling hard to cut materials is to reduce the cutting zone temperature. Using cutting coolants is the better way to reduce the cutting temperature. Many researchers have reported failure of the conventional cutting fluids in the removal of cutting zone temperature. Currently, cryogenic coolant is being used as a coolant in the machining zone. Cryogenic LN2 cooling is an alternative approach to conventional cutting fluids. The main benefits of using cryogenic coolant in the cutting zone are (i) Improved tool
* Corresponding author. Tel.: +91 99625 62713; fax: +91 44 22357744. E-mail address:[email protected] 2214-7853©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).
Pradeep Kumar M and Shakeel Ahmed L / Materials Today: Proceedings 4 (2017) 1518–1524
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life (ii) Lesser cutting force (iii) Better surface finish (iv) Better chip breaking & chip handling (v) Better dimensional accuracy (vi) Higher productivity (vii) Lower production cost [4-9]. Reduction of cutting temperature is seen through conduction by 60 – 66% in cryogenic LN2 cooling over flood cooling [10]. Hong and Broomer [11] report a 67% tool life improvement using cryogenic LN2 cooling instead of flood cooling. Further, cutting temperature is more favorable at higher cutting velocities. Nalbant and Yildiz [12] have investigated the effect of cryogenic LN2cooling in AISI 304 stainless steel and report increased cutting forces and torque in cryogenic LN2cooling over flood cooling. They also report the absence of any significant advantages in cryogenic machining compared with dry machining. Govindaraju et al [13, 14] have investigated the effect of cryogenic LN2 cooling in drilling and reported that cutting temperature was reduced in cryogenic LN2 cooling as against flood cooling. Shakeel et al [15] and Shakeel and Kumar [16] have reported drastic reduction in cutting temperature in cryogenic drilling of titanium alloys as against flood cooling. Kuram et al [17] have reported that increase in feed rate increased the thrust force and surface roughness. In cryogenic LN2cooling, more shearing action is required to produce deformation of the workpiece material which results in increased in thrust forces and torque compared with flood cooling [13–16]. Surfaces with lower surface roughness can be achieved in cryogenic LN2 cooling on AISI 304 stainless steel [18]. Pusavec et al [19] have investigated the cryogenic LN2cooling effect on microstructure and report absence of any significant changes in microstructure over dry machining. Literature survey shows many researchers reporting that cryogenic LN2 cooling in machining is always beneficial and better alternative cutting fluid for flood coolants.Available literature indicates very few studies in drilling operations using Cryogenic LN2 cooling. The effect of Cryogenic LN2 cooling on temperature, thrust force, surface roughness, tool wear and chip morphology are compared with that of flood cooling. 2. Experimental Procedure The drilling experiments were conducted on ARIX CNC Vertical Machining Centre (VMC) 100. AISI 304 stainless steel was chosen as a workpiece material with dimensions of 164 x 80 x 30 mm. PVD TiCN coated carbide indexable tool was used as a cutting tool. The machining parameters are shown in Table 1. In flood cooling, the commercial grade water soluble oil was uses in the ratio of 1: 20. Large quantities of flood cooling were used for quenching the heat generated in the cutting zone. Flood cooling fails to penetrate the cutting zone and also to reduce the cutting zone temperature. Table 1. Experimental Parameters Constant Conditions Type of operation Workpiece material Cutting Tool Diameter Depth Number of Inserts Type of Insert Cutting Tool Experimental Variables Cutting Speed, vc Feed Rate, f Cooling Method
Drilling AISI 304 Stainless Steel 16 mm 15 mm 2 PVD TiCN Coated Carbide – PR1025 ZCMT 06T204 KYOCERAMAGIC DRILLDRZ 1648 40 and 50 m/min 0.02, 0.05 and 0.08 mm/rev Flood and Cryogenic cooling (LN2)
Cryogenic cooling system has been developedto supply the liquid nitrogen onto the tool – chip interface at a delivery pressure of 3 bar. The experimental setup and LN2 delivery nozzle for the cryogenic LN2cooling is shown in Figure 1.
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Pradeep Kumar M and Shakeel Ahmed L / Materials Today: Proceedings 4 (2017) 1518–1524
Fig. 1. Experimental Setup for Cryogenic LN2 cooling
Six experiments of 16 mm diameter were conducted at a constant depth of 15 mm under cryogenic LN2cooling and flood cooling. Cutting temperature was measured using contact type K type thermocouple which is placed at the side of the workpiece surface [20]. A three component Kistler (9257B type) dynamometer was used for thrust force measurement. Surface roughness of the drilled surface was measured using a contact type TalysurfSurfcoder roughness measuring instrument with a sampling length of 0.8mm. The microstructural behavior of workpiece and wear of cutting tool insert were compared using SEM (Scanning Electron Microscope) images. 3. Results and Discussion 3.1. Effect of cryogenic cooling on Cutting Temperature
Fig. 2.Cutting temperature Vs. Feed rate
The cutting temperature variations in drilling AISI 304 stainless steel under flood and cryogenic LN2 cooling is shown in Figure 2. At a lower feed rate0.02 mm/rev and a lower cutting speed of 40 m/min, the cutting temperature was 48.45oC and 23.95oC for flood and cryogenic LN2 cooling respectively. There was a 51% reduction in cutting temperature under cryogenic cooling as against flood cooling. In general machining, an increase in feed rate resulted in increase in cutting temperature. At a higher feed rate of 0.08 mm/rev, the cutting temperature was 62.86oC and 51.10oC for flood and cryogenic cooling respectively. A 19% reduction of cutting temperature was observed in cryogenic cooling over flood cooling. At a higher cutting speed 50 m/min in all feed rates, the cutting temperature was found reduced by 12 – 40% in cryogenic LN2cooling over flood cooling. At higher feed rates, the cryogenic LN2 cooling effect was less effective due to heat transfer rate and chip tool contact. The results showed a 12 – 51% reduction in cryogenic LN2 cooling as against flood cooling. The main reason for cutting temperature reduction in cryogenic LN2 cooling was quick absorption of heat in the machining zone and increases the thermal gradient between cutting zone and tool.
Pradeep Kumar M and Shakeel Ahmed L / Materials Today: Proceedings 4 (2017) 1518–1524
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3.2. Effect of cryogenic cooling on Thrust Force
Fig. 3.Thrust force Vs. Feed rate
The thrust force in drilling AISI 304 stainless steel under flood and cryogenic LN2 cooling is shown in Figure 3. At a lower cutting speed 40 m/min and lower feed rate of 0.02 mm/rev, the thrust force was 1537 N and 1667 N under flood and cryogenic LN2cooling respectively. An increase in thrust force of about 7.8% under cryogenic cooling over flood cooling was noticed. When the feed rate was increased to 0.05 mm/rev and 0.08 mm/rev, the thrust force kept increasing under both the machining conditions, due to an increase in the chip load. Thrust force increased by 5.92 – 7.80% in cryogenic cooling over flood cooling. At a higher cutting speed of 50 m/min in all feed rates, 4– 12% increase in thrust force was observed in cryogenic LN2 cooling than flood cooling. A constant increase in thrust force was seen. This was due to increase in chip loadingin cryogenic LN2 cooling compared with flood cooling. The results showed that thrust force increased by about 4 – 12% in drilling under cryogenic LN2cooling over flood cooling. The hardness of the material was increasing in the cryogenic LN2 cooling, showing greater need for shearing action to produce deformation on the work material resulting in higher thrust forces compared to flood cooling condition. 3.3. Effect of cryogenic cooling on Surface Roughness The variation of surface roughness in drilling AISI 304 stainless steel under flood and cryogenic LN2cooling is shown in Figure 4. At a lower cutting speed 40 m/min and a lower feed rate 0.02 mm/rev, the surface roughness values were 3.42µm and 3.97µm in flood cooling and cryogenic LN2 cooling respectively. An increase of 13% surface roughness in cryogenic LN2cooling over flood cooling was seen. For higher feed rates of 0.05 mm/rev and 0.08 mm/rev, 21% and 18% increase in surface roughness under cryogenic LN2 cooling as against flood cooling. At a higher cutting speed 50 m/min in all feed rates, a 15 – 23% increase in surface roughness was seen with the application of cryogenic LN2 cooling over flood cooling.
Fig. 4.Surface roughness Vs. Feed rate
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Pradeep Kumar M and Shakeel Ahmed L / Materials Today: Proceedings 4 (2017) 1518–1524
In cryogenic LN2 cooling conditions, the chip was found hardened and cryogenic LN2 coolant evaporated during machining. Hence, the coolant was unable to assist the exit of the chip, resulting in a dragging phenomenon, which in turn had an effect on the hole surface finish resulting in higher roughness value. The results showed about a 15 – 23% increase in surface roughness in drilling under cryogenic LN2 cooling as against flood cooling. An increase in cutting speeds, caused increase in surface roughness values under flood and cryogenic cooling conditions. When the feed rate increased for the same cutting conditions, surface roughness values increased too. In cryogenic LN2 cooling conditions, surface roughness values were higher compared to flood condition for all the feed rates. 3.4. Effect of cryogenic cooling on tool wear Tool wear causes loss of original shape in the tool so that over a period of time the tool causes cut inefficiency and even complete failure.Chip control in drilling stainless steel is critical because it determines the tool wear.In flood cooling, adhesive wear occurs due to high cutting temperature was developed at the cutting zone interface region. This higher cutting zone temperature allows the drilling tool to stick with the work material, causing a high tool wear, and softening the work material. The tool wear is slight lesser in cryogenic LN2 cooling than flood cooling. The cutting temperature is reduced resulting in less thermal shock of the cutting inserts in cryogenic LN2 cooling.Cooling medium is distinct due to sufficient penetration of cryogenic LN2 cooling into the in-depth of the drilled hole during the process. As a result, the tool undergoes abrasive wear and causes removal of a portion of the particles from the cutting tool material despite the employment of LN2. 3.5. Effect of cryogenic cooling on Chip Morphology The chip samples collected during the drilling of stainless steel under both flood and cryogenic LN2 cooling. Chip formation is generally continuous and discontinuous form in drilling operation. The width of the chip formation is large and small. Chip formation is discontinuous at lower cutting speed, width of the chip is less and uniform in flood cooling. It is continuous and of smaller width of the chip with non - uniform pattern being observed in cryogenic LN2 cooling. At a higher cutting speed, chip formation is discontinuous and of smaller width with uniform pattern observed in flood cooling. Continuous and non - uniform larger width of the chip formation was observed at the same condition in cryogenic LN2 cooling. The observations show thatflood cooling provides better chip breakability compared to cryogenic LN2 cooling. 3.6. Microstructural studies Specimens with a lower cutting speed 40 m/min and higher cutting speed 50 m/min were cut in both flood and cryogenic LN2 cooling conditions using Wire Electric Discharge Machining. The specimens were cut cylindrically at a suitable distance from the hole surface. The specimens of the drilled surface layer were rough and of fine polish with the use of different grades of emery papers, diamond paste and water in a disc polisher. Aqua regia that is used as an etchant solution, consists of 3 parts of concentrated Hydrochloric acid and 1 part of concentrated Nitric acid. The etched specimens were finally viewed under a Scanning Electron Microscope (SEM) and the images of microstructures are shown in Figure 5. The microstructures of drilled surface have almost same grain size in both flood and cryogenic LN2 cooling ,but smaller than the bulk material. During the drilling process, cutting temperature increased under both coolant conditions. In comparison with flood and cryogenic LN2 cooling, no drastic changes in the microstructures were observed.
Pradeep Kumar M and Shakeel Ahmed L / Materials Today: Proceedings 4 (2017) 1518–1524
Flood Cooling
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Cryogenic LN2 Cooling
Cutting Speed 40 m/min
Cutting Speed 50 m/min Fig. 5. Microstructure of the drilled hole
4. Conclusion A drilling experimental study was made on AISI 304 stainless steel with PVD TiCN coated carbide indexable insert. The major conclusions are (i) Cryogenic LN2 cooling results in reduction of cutting temperature compared to flood cooling. Cryogenic LN2 cooling reduces the cutting temperature by about 12 to 51% when compared to flood cooling. (ii) Cryogenic LN2 cooling increases the thrust force by about 4 to 12% over flood cooling. (iii) Cryogenic LN2 cooling increases the surface roughness to a range of 15 to 23% more than in flood cooling. (iv) Tool wear is slight lesser in cryogenic LN2 cooling and better chip breakability was observed in flood cooling when compared with cryogenic LN2 cooling. (v) Grain size after drilling was almost same and no drastic changes in the microstructure were observed in both flood and cryogenic LN2 cooling. Based on the experimental results obtained, cryogenic LN2 cooling is found to be beneficial in cutting temperature reduction, improvement in tool life and the other factors affected in the cryogenic environment.
Acknowledgements The authors would like to thank the Anna University, Chennai, India for the financial support under Young Faculty Research Support Scheme – 2015,Ref. No: 1607/CTDT-1/RSS/2015, Dated: 07.09.2015. The authors also thank to Central Workshop Division, Department of Mechanical Engineering, CEG, Anna University, Chennai for providing experimental facilities for carried out the research work. References [1] [2] [3]
C.J. Novak,Handbook of stainless steels, McGraw-Hill, New York, 1977. K. Tetal,Machining of stainless steels handbook, 9th ed. ASM Intl. (1989) 681. M.P. Groover,Fundamentals of modern manufacturing, Materials processes and systems, Englewood Cliffs, NJ, Prentice-Hall, 1990.
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[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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A. Shokrani, V. Dhokia, P. Muñoz-Escalona, S.T. Newman, Int. J Comp. Int. Manuf. 26(7) (2013) 616–648. N.R. Dhar, S. Paul, A.B. Chattopadhyay,J. Mat. Proc. Tech. 123(3) (2002) 483–489. N.R. Dhar, S. Paul, A.B. Chattopadhyay, Wear. 249(10-11) (2001) 932–942. Y. Yildiz, M. Nalbant, Int. J. Mach. Tools Manuf. 48(9) (2008) 947–964. K.K. Mandal, Adv. Mat. Manuf. Charac. 3(1) (2013) 281-284. S. Pradeep Kumar, S. Raviraj, S. Divakara, N. FazalulRehaman, K.J. Tony, Proc. Mat. Sci. 5 (2014) 26052614. M. Dhananchezian, M. Pradeep Kumar,Cryogenics. 51(1) (2011) 34–40. S.Y. Hong, M. Broomer, Clean Tech.Env. Policy. 2 (3) (2000) 157–166. M. Nalbant, Y. Yildiz,Trans. Nonferrous Met. Soc. China. 21 (1) (2011) 72–79. N. Govindaraju, L. Shakeel Ahmed, M. Pradeep Kumar, Mat. Manuf. Proc. 29(11-12) (2014) 1417–1421. N. Govindaraju, L. Shakeel Ahmed, M.Pradeep Kumar, App. Mech. Mat. 592-594 (2014) 316–320. L. ShakeelAhmed, N. Govindaraju, M. Pradeep Kumar, Mat. Manuf. Proc. 0 (2015) 1–5. L. ShakeelAhmed, M. Pradeep Kumar,Mat. Manuf. Proc. 0 (2015) 1–9. E. Kuram, B.Ozcelik, E. Demirbas, E. Şik, I.N. Tansel,Mat. Manuf. Proc. 26(9) (2011) 1136–1146. N. Ben Fredj, H. Sidhom, Cryogenics, 46(6) (2006) 439–448. F. Pusavec, P. Krajnik, C.M. Nicolescu, J. Kopac, The 4th International Swedish Production Symposium, At Lund, Sweden, 2011. R.P. Zeilmann, W.L. Weingaertner, J. Mat. Proc. Tech. 179(1-3) (2006) 124–127.
ScienceDirect Materials Today: Proceedings 4 (2017) 1518–1524
www.materialstoday.com/proceedings
5th International Conference of Materials Processing and Characterization (ICMPC 2016)
Drilling of AISI 304 Stainless Steel under Liquid Nitrogen Cooling: A Comparison with Flood Cooling Pradeep Kumar Ma, *, Shakeel Ahmed La a
Depaertment of Mechanical Engineering, CEG Campus, Anna University, Chennai – 600 025, India
Abstract In this experimental study, investigations were carried out in a drilling operation on AISI 304 stainless steel and TiCN coated carbide indexable insert tool under flood and liquid nitrogen (LN2) cooling separately. Cutting speeds (40 m/min and 50 m/min) and feed rates (0.02, 0.05 and 0.08 mm/rev) at constant depth were selected as input parameters. The cutting temperature, thrust force and surface roughness were analyzed. The microstructure of drilled surface, tool wear and chip morphology were studied. The experimental results indicate a reduction in cutting temperature, increase in thrust force and surface roughness when cryogenic LN2cooling is used. ©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016). Keywords:drilling; cryogenic cooling; temperature; thrust force; surface roughness.
1. Introduction AISI 304 stainless steel finds extensive use in aerospace and automotive products. It is very hard to cut material. It also presents some problems after machining such as poor surface finish, high wear rate, tool failure, Built up Edge (BUE) formation. The BUE formation leads to affect the surface quality of the material and rapid tool wear [1, 2]. Automotive industries aim mainly for high material removal rate, better surface finish, less tool wear and high productivity. The main problem in the achievement of the productivity and surface quality is high cutting temperature developed in the machining zone, leading to tool failure. AISI 304 steel is a poor machinability material due to high tensile strength, toughness, work hardening, ductility and low heat conductivity [3]. The effective way to improve the performance in drilling hard to cut materials is to reduce the cutting zone temperature. Using cutting coolants is the better way to reduce the cutting temperature. Many researchers have reported failure of the conventional cutting fluids in the removal of cutting zone temperature. Currently, cryogenic coolant is being used as a coolant in the machining zone. Cryogenic LN2 cooling is an alternative approach to conventional cutting fluids. The main benefits of using cryogenic coolant in the cutting zone are (i) Improved tool
* Corresponding author. Tel.: +91 99625 62713; fax: +91 44 22357744. E-mail address:[email protected] 2214-7853©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).
Pradeep Kumar M and Shakeel Ahmed L / Materials Today: Proceedings 4 (2017) 1518–1524
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life (ii) Lesser cutting force (iii) Better surface finish (iv) Better chip breaking & chip handling (v) Better dimensional accuracy (vi) Higher productivity (vii) Lower production cost [4-9]. Reduction of cutting temperature is seen through conduction by 60 – 66% in cryogenic LN2 cooling over flood cooling [10]. Hong and Broomer [11] report a 67% tool life improvement using cryogenic LN2 cooling instead of flood cooling. Further, cutting temperature is more favorable at higher cutting velocities. Nalbant and Yildiz [12] have investigated the effect of cryogenic LN2cooling in AISI 304 stainless steel and report increased cutting forces and torque in cryogenic LN2cooling over flood cooling. They also report the absence of any significant advantages in cryogenic machining compared with dry machining. Govindaraju et al [13, 14] have investigated the effect of cryogenic LN2 cooling in drilling and reported that cutting temperature was reduced in cryogenic LN2 cooling as against flood cooling. Shakeel et al [15] and Shakeel and Kumar [16] have reported drastic reduction in cutting temperature in cryogenic drilling of titanium alloys as against flood cooling. Kuram et al [17] have reported that increase in feed rate increased the thrust force and surface roughness. In cryogenic LN2cooling, more shearing action is required to produce deformation of the workpiece material which results in increased in thrust forces and torque compared with flood cooling [13–16]. Surfaces with lower surface roughness can be achieved in cryogenic LN2 cooling on AISI 304 stainless steel [18]. Pusavec et al [19] have investigated the cryogenic LN2cooling effect on microstructure and report absence of any significant changes in microstructure over dry machining. Literature survey shows many researchers reporting that cryogenic LN2 cooling in machining is always beneficial and better alternative cutting fluid for flood coolants.Available literature indicates very few studies in drilling operations using Cryogenic LN2 cooling. The effect of Cryogenic LN2 cooling on temperature, thrust force, surface roughness, tool wear and chip morphology are compared with that of flood cooling. 2. Experimental Procedure The drilling experiments were conducted on ARIX CNC Vertical Machining Centre (VMC) 100. AISI 304 stainless steel was chosen as a workpiece material with dimensions of 164 x 80 x 30 mm. PVD TiCN coated carbide indexable tool was used as a cutting tool. The machining parameters are shown in Table 1. In flood cooling, the commercial grade water soluble oil was uses in the ratio of 1: 20. Large quantities of flood cooling were used for quenching the heat generated in the cutting zone. Flood cooling fails to penetrate the cutting zone and also to reduce the cutting zone temperature. Table 1. Experimental Parameters Constant Conditions Type of operation Workpiece material Cutting Tool Diameter Depth Number of Inserts Type of Insert Cutting Tool Experimental Variables Cutting Speed, vc Feed Rate, f Cooling Method
Drilling AISI 304 Stainless Steel 16 mm 15 mm 2 PVD TiCN Coated Carbide – PR1025 ZCMT 06T204 KYOCERAMAGIC DRILLDRZ 1648 40 and 50 m/min 0.02, 0.05 and 0.08 mm/rev Flood and Cryogenic cooling (LN2)
Cryogenic cooling system has been developedto supply the liquid nitrogen onto the tool – chip interface at a delivery pressure of 3 bar. The experimental setup and LN2 delivery nozzle for the cryogenic LN2cooling is shown in Figure 1.
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Pradeep Kumar M and Shakeel Ahmed L / Materials Today: Proceedings 4 (2017) 1518–1524
Fig. 1. Experimental Setup for Cryogenic LN2 cooling
Six experiments of 16 mm diameter were conducted at a constant depth of 15 mm under cryogenic LN2cooling and flood cooling. Cutting temperature was measured using contact type K type thermocouple which is placed at the side of the workpiece surface [20]. A three component Kistler (9257B type) dynamometer was used for thrust force measurement. Surface roughness of the drilled surface was measured using a contact type TalysurfSurfcoder roughness measuring instrument with a sampling length of 0.8mm. The microstructural behavior of workpiece and wear of cutting tool insert were compared using SEM (Scanning Electron Microscope) images. 3. Results and Discussion 3.1. Effect of cryogenic cooling on Cutting Temperature
Fig. 2.Cutting temperature Vs. Feed rate
The cutting temperature variations in drilling AISI 304 stainless steel under flood and cryogenic LN2 cooling is shown in Figure 2. At a lower feed rate0.02 mm/rev and a lower cutting speed of 40 m/min, the cutting temperature was 48.45oC and 23.95oC for flood and cryogenic LN2 cooling respectively. There was a 51% reduction in cutting temperature under cryogenic cooling as against flood cooling. In general machining, an increase in feed rate resulted in increase in cutting temperature. At a higher feed rate of 0.08 mm/rev, the cutting temperature was 62.86oC and 51.10oC for flood and cryogenic cooling respectively. A 19% reduction of cutting temperature was observed in cryogenic cooling over flood cooling. At a higher cutting speed 50 m/min in all feed rates, the cutting temperature was found reduced by 12 – 40% in cryogenic LN2cooling over flood cooling. At higher feed rates, the cryogenic LN2 cooling effect was less effective due to heat transfer rate and chip tool contact. The results showed a 12 – 51% reduction in cryogenic LN2 cooling as against flood cooling. The main reason for cutting temperature reduction in cryogenic LN2 cooling was quick absorption of heat in the machining zone and increases the thermal gradient between cutting zone and tool.
Pradeep Kumar M and Shakeel Ahmed L / Materials Today: Proceedings 4 (2017) 1518–1524
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3.2. Effect of cryogenic cooling on Thrust Force
Fig. 3.Thrust force Vs. Feed rate
The thrust force in drilling AISI 304 stainless steel under flood and cryogenic LN2 cooling is shown in Figure 3. At a lower cutting speed 40 m/min and lower feed rate of 0.02 mm/rev, the thrust force was 1537 N and 1667 N under flood and cryogenic LN2cooling respectively. An increase in thrust force of about 7.8% under cryogenic cooling over flood cooling was noticed. When the feed rate was increased to 0.05 mm/rev and 0.08 mm/rev, the thrust force kept increasing under both the machining conditions, due to an increase in the chip load. Thrust force increased by 5.92 – 7.80% in cryogenic cooling over flood cooling. At a higher cutting speed of 50 m/min in all feed rates, 4– 12% increase in thrust force was observed in cryogenic LN2 cooling than flood cooling. A constant increase in thrust force was seen. This was due to increase in chip loadingin cryogenic LN2 cooling compared with flood cooling. The results showed that thrust force increased by about 4 – 12% in drilling under cryogenic LN2cooling over flood cooling. The hardness of the material was increasing in the cryogenic LN2 cooling, showing greater need for shearing action to produce deformation on the work material resulting in higher thrust forces compared to flood cooling condition. 3.3. Effect of cryogenic cooling on Surface Roughness The variation of surface roughness in drilling AISI 304 stainless steel under flood and cryogenic LN2cooling is shown in Figure 4. At a lower cutting speed 40 m/min and a lower feed rate 0.02 mm/rev, the surface roughness values were 3.42µm and 3.97µm in flood cooling and cryogenic LN2 cooling respectively. An increase of 13% surface roughness in cryogenic LN2cooling over flood cooling was seen. For higher feed rates of 0.05 mm/rev and 0.08 mm/rev, 21% and 18% increase in surface roughness under cryogenic LN2 cooling as against flood cooling. At a higher cutting speed 50 m/min in all feed rates, a 15 – 23% increase in surface roughness was seen with the application of cryogenic LN2 cooling over flood cooling.
Fig. 4.Surface roughness Vs. Feed rate
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Pradeep Kumar M and Shakeel Ahmed L / Materials Today: Proceedings 4 (2017) 1518–1524
In cryogenic LN2 cooling conditions, the chip was found hardened and cryogenic LN2 coolant evaporated during machining. Hence, the coolant was unable to assist the exit of the chip, resulting in a dragging phenomenon, which in turn had an effect on the hole surface finish resulting in higher roughness value. The results showed about a 15 – 23% increase in surface roughness in drilling under cryogenic LN2 cooling as against flood cooling. An increase in cutting speeds, caused increase in surface roughness values under flood and cryogenic cooling conditions. When the feed rate increased for the same cutting conditions, surface roughness values increased too. In cryogenic LN2 cooling conditions, surface roughness values were higher compared to flood condition for all the feed rates. 3.4. Effect of cryogenic cooling on tool wear Tool wear causes loss of original shape in the tool so that over a period of time the tool causes cut inefficiency and even complete failure.Chip control in drilling stainless steel is critical because it determines the tool wear.In flood cooling, adhesive wear occurs due to high cutting temperature was developed at the cutting zone interface region. This higher cutting zone temperature allows the drilling tool to stick with the work material, causing a high tool wear, and softening the work material. The tool wear is slight lesser in cryogenic LN2 cooling than flood cooling. The cutting temperature is reduced resulting in less thermal shock of the cutting inserts in cryogenic LN2 cooling.Cooling medium is distinct due to sufficient penetration of cryogenic LN2 cooling into the in-depth of the drilled hole during the process. As a result, the tool undergoes abrasive wear and causes removal of a portion of the particles from the cutting tool material despite the employment of LN2. 3.5. Effect of cryogenic cooling on Chip Morphology The chip samples collected during the drilling of stainless steel under both flood and cryogenic LN2 cooling. Chip formation is generally continuous and discontinuous form in drilling operation. The width of the chip formation is large and small. Chip formation is discontinuous at lower cutting speed, width of the chip is less and uniform in flood cooling. It is continuous and of smaller width of the chip with non - uniform pattern being observed in cryogenic LN2 cooling. At a higher cutting speed, chip formation is discontinuous and of smaller width with uniform pattern observed in flood cooling. Continuous and non - uniform larger width of the chip formation was observed at the same condition in cryogenic LN2 cooling. The observations show thatflood cooling provides better chip breakability compared to cryogenic LN2 cooling. 3.6. Microstructural studies Specimens with a lower cutting speed 40 m/min and higher cutting speed 50 m/min were cut in both flood and cryogenic LN2 cooling conditions using Wire Electric Discharge Machining. The specimens were cut cylindrically at a suitable distance from the hole surface. The specimens of the drilled surface layer were rough and of fine polish with the use of different grades of emery papers, diamond paste and water in a disc polisher. Aqua regia that is used as an etchant solution, consists of 3 parts of concentrated Hydrochloric acid and 1 part of concentrated Nitric acid. The etched specimens were finally viewed under a Scanning Electron Microscope (SEM) and the images of microstructures are shown in Figure 5. The microstructures of drilled surface have almost same grain size in both flood and cryogenic LN2 cooling ,but smaller than the bulk material. During the drilling process, cutting temperature increased under both coolant conditions. In comparison with flood and cryogenic LN2 cooling, no drastic changes in the microstructures were observed.
Pradeep Kumar M and Shakeel Ahmed L / Materials Today: Proceedings 4 (2017) 1518–1524
Flood Cooling
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Cryogenic LN2 Cooling
Cutting Speed 40 m/min
Cutting Speed 50 m/min Fig. 5. Microstructure of the drilled hole
4. Conclusion A drilling experimental study was made on AISI 304 stainless steel with PVD TiCN coated carbide indexable insert. The major conclusions are (i) Cryogenic LN2 cooling results in reduction of cutting temperature compared to flood cooling. Cryogenic LN2 cooling reduces the cutting temperature by about 12 to 51% when compared to flood cooling. (ii) Cryogenic LN2 cooling increases the thrust force by about 4 to 12% over flood cooling. (iii) Cryogenic LN2 cooling increases the surface roughness to a range of 15 to 23% more than in flood cooling. (iv) Tool wear is slight lesser in cryogenic LN2 cooling and better chip breakability was observed in flood cooling when compared with cryogenic LN2 cooling. (v) Grain size after drilling was almost same and no drastic changes in the microstructure were observed in both flood and cryogenic LN2 cooling. Based on the experimental results obtained, cryogenic LN2 cooling is found to be beneficial in cutting temperature reduction, improvement in tool life and the other factors affected in the cryogenic environment.
Acknowledgements The authors would like to thank the Anna University, Chennai, India for the financial support under Young Faculty Research Support Scheme – 2015,Ref. No: 1607/CTDT-1/RSS/2015, Dated: 07.09.2015. The authors also thank to Central Workshop Division, Department of Mechanical Engineering, CEG, Anna University, Chennai for providing experimental facilities for carried out the research work. References [1] [2] [3]
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