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Influence of sustainable cutting environments on cutting forces, surface roughness and tool wear in turning of Inconel 718
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 6746–6754
www.materialstoday.com/proceedings
IMME17
Influence of sustainable cutting environments on cutting forces, surface roughness and tool wear in turning of Inconel 718 A Mehta*, S.Hemakumar, A.Patil, S.P.Khandke, P.Kuppan, R.Oyyaravelu, A.S.S.Balan School of Mechanical Engineering, VIT University, Vellore 632014, Tamil Nadu
Abstract Inconel 718 is a potential material for aerospace, nuclear and automobile industries because of its resistance to high temperature and corrosion. It becomes important to ensure good surface quality for better part performance and life. This paper shows a comparative study of the cutting forces, surface roughness and analysis of tool wear while machining of Inconel 718 under different sustainable machining environments such as dry, minimum quantity lubrication (MQL), cryogenic, cold air and MQL, MQL and cryogenic. The cutting speed, feed rate and depth of cut were kept constant for all environments based on the previous studies from literature. Results showed that machining using combination of cold air and MQL tends to reduce the surface roughness by about 86% and cutting force by about 28% when compared to dry machining. The minimum flank wear was obtained under MQL and cryogenic cutting condition (127.8µm) which is about 92% lower when compared to dry condition. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Emerging Trends in Materials and Manufacturing Engineering (IMME17). Keywords: Inconel 718; minimum quantity lubrication; cryogenic; cold air; surface roughness; tool wear.
1. Introduction Nickel based Super alloys finds range of applications in aerospace, nuclear and automobile sector due to their high performance in stringent working conditions. Material like Inconel 718, reported to be high performance material for its high strength at elevated temperatures, with oxidation and corrosion resistance properties [3,5,8,9].
* Corresponding author. Tel.: 09921787797; E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Emerging Trends in Materials and Manufacturing Engineering (IMME17).
Mehta et al., / Materials Today: Proceedings 5 (2018) 6746–6754
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But its high strength, thermal resistivity, work hardenability and presence of abrasive carbide particles, it becomes difficult to machine this material [1,3]. Along with these factors the cutting insert and the tool edge geometry also plays an important role in influencing the cutting forces, temperature, tool wear, surface roughness and chip formation while machining [10]. So, it becomes important to carry out a comprehensive study on its machinability characteristics under different cutting environment. Simultaneously abiding by the governmental laws of preventing pollution makes it important to undertake some steps towards sustainable machining environments. To make machining more sustainable, alternative cooling and lubrication, should be developed which replace the conventional methods. Recently, lot of cooling and lubricating methods have been introduced among which minimum quantity lubrication and cryogenic environment is dominant. These methods not only improve the machinability of Inconel 718 but also helpes in making machining processes more cleaner and environment friendly [1,8]. Kaynak through his work showed that at 60m/min cryogenic cooling increased the radial force, feed force and cutting approximately by 20,36 and 25% respectively in comparison with MQL, whereas at 120m/min cryogenic cooling generated the least forces among all three conditions. The cutting parameters were feed(f)=0.075mm/rev and depth of cut(ap)=0.8mm. Also, lower flank and crater wear rate was observed for both MQL and cryogenic cooling. In addition, the surface roughness was lower in MQL and cryogenic cooling. In addition, the surface roughness was lower for both MQL and cryogenic when compared to dry. Pusavec et al. carried out an array of experiments, investigating the machining characteristics of Inconel 718 under different conditions such as dry, MQL, cryogenic and combination of MQL and cryogenic cooling. On comparing the results, it was found that the combination of MQL and cryogenic cooling produced lowest cutting and feed forces. The MQL and cryogenic cooling demonstrated the minimum flank and rake wear when compared with other conditions. Further the surface quality improved for a range of feed and cutting speed. Literature shows, using MQL and cryogenic cooling tend to improve the machining performance of Inconel 718. But at the same time cryogenic cooing is costly. So, it is important to make the cooling cost effective. Therefore, using cold air can replace cryogenic cooling. Cold air guns can be employed to produce temperatures as low as -34ºC and thus can be helpful in reducing the temperatures while machining. This in turn increases the tool life, forces and gives a good surface finish. In addition, cold air gun uses compressed air which is cheaper as compared to other coolants. Further it is seen that cryogenic cooling tends to increase the hardness of the workpiece before it is cut [4,5,11]. This is not the case with cold air. Also, it is seen that a lot f work has been done on sustainability of MQL and cryogenic [4,11]. So, this paper shows the influence of the cold air and MQL on the different cutting parameters and its comparison with other environments. For each condition the cutting forces, surface quality and tool wear were measured. 2. Experimental details The composition of Inconel 718 is given in Table 1. The work piece used was cylindrical in shape with diameter of 22mm and length of 100mm. The work piece was first rough turned for a depth of cut of 1mm to remove the excess non-uniformities which might have interfered with the turning operation. The insert used for the turning operation was Kennametal KCU25 with AlTiN coating, the geometry of the insert is as shown in Fig 2 and Table 2. The insert with the tool holder was mounted on the dynamometer which was used for measuring cutting force values. The dynamometer used was KISTLER 9275B. The work piece was turned on CNC lathe machine of SIMPLE TURN 5075-SPM. The cutting parameters were kept constant, cutting speed (Vc) = 75 m/min, feed rate =0.125 mm/rev and depth of cut (ap) = 0.5mm. The parameters were decided by referring the literature and were found to be optimum for machining. The machining was carried out under five different cooling environments such as dry, MQL, Cold air + MQL, Cryogenic, Cryogenic + MQL. Under each environment, a cutting length of 35mm was maintained constant. New cutting edge was used for respective environment of machining. The tool post was mounted on the dynamometer and the force was measured using DynoWare software.
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2.1 MQL setup The oil used was Esters with wear resistance additives. The viscosity of this oil is 24cSt at 40˚C. The flow rate used for the machining was 25mm3/rev. MQL was supplied using two nozzles. One nozzle supplied oil to rake face of insert and other nozzle supplied MQL to the flank face of the cutting tool in the direction of cutting. 2.2 Cryogenic The cryogenic fluid used for carrying out the experiment was Liquid Nitrogen. The nozzle used for supplying the cryogenic fluid had diameter of 2mm. The cryogenic fluid was supplied with a pressure of 1.5 bar. The nozzle was focused on flank face of cutting tool in the direction of cutting. Liquid Nitrogen was not supplied to the work piece to avoid surface hardening of the work piece which might lead to bad surface finish. The setup of Cryogenic was decided based on the previous work done on turning of Inconel 718 with cryogenic. 2.3 Cold air + MQL The supplied cold air with flow rate of 1981 SPLM. It was supplied on the flank face of cutting tool in the cutting direction. MQL was being supplied at a flow rate of 25mm/rev and was focused at the rake face and flank face of the cutting tool in the cutting direction. 2.4 Cryogenic + MQL In case of cryogenic + MQL, cryogenic fluid was supplied to flank face of the cutting tool in the cutting direction and MQL was supplied to rake and flank face of the cutting tool at 25mm3/rev. After carrying out the experiments in five different environments, the cylindrical work piece bar was tested for surface roughness using MAHR’s profile meter of MARSURF XCR 20. The Ra and Rt values of the respective machined surfaces were noted. The readings were taken at three different points on machined surface of an environment and respective average was taken. A diamond probe of 3mm was used to measure surface roughness. Specimen of respective diameter of the machined surface were then cut using wire EDM. The thickness of the specimen was 0.5 mm. The wire EDM machine used was EZEEWIN with a brass wire of diameter 2.5mm. The specimen was then tested on Vickers’s micro hardness tester to measure the micro hardness of each of the specimen. For measuring the micro hardness. The indentation was made at 10 different points at 0.75 mm from each other, the first point being very close to the machined surface. A load of 500 grams was applied. The 10 points were taken for comparing micro hardness at machined surface and surface sufficiently away from the machining affected zone.
Fig. 1 Geometry of the insert KCU25
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Table 1. Material composition
Element
C
Mn
Si
P
S
Ni
Cr
Nb + Ta
Composition
0.0301
0.0701
0.0901
0.007
0.001
18.45
52.50
5.321
Element
Co
Al
Cu
Fe
Mo
Nb
B
Mg
Composition
0.251
0.481
0.04
17.65
3.02
5.31
0.005
0.0018
Table. 2 Nomenclature of insert KCU25
ANSI Catalog Number
D(mm)
Rake Angle
L10 (mm)
Re (mm)
D1 (mm)
CNMG430MS
12.7
6°
12.9
0.4
5.67
3. Results and Discussions 3.1. Forces Forces are basic and important factors on which the power consumption and energy utilization in a machining process is dependent. Forces are affected by variety of factors such as friction between tool and chip, properties of work piece material, tool and cutting tool geometry, cutting environments, etc. The influence of the different environments on the cutting forces, forces and radial forces and the comparison of each component under different environments is shown in Fig 2.
FEED FORCES(N)
Fx
250 200 150 100 50 0 DRY (a)
CRYO CRYO MQL
MQL
CUTTING ENVIRONMENTS
COLD AIR MQL
150 100 50 0 DRY
(b)
CRYO CRYO MQL
MQL
COLD AIR MQL
CUTTING ENVIRONMENTS
Fy RADIAL FORCES(N)
CUTTING FORCES(N)
Fz
300 200 100 0 DRY (c)
CRYO
CRYO MQL
MQL
COLD AIR MQL
CUTTING ENVIRONMENTS
Fig.2 comparison of different force components under different environments :( a) cutting forces, (b) feed forces, (c) Radial forces
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3.1.1. Cutting forces Fig.2. (a) show that MQL decreases the cutting forces by about 26.5% whereas combination of cold air and MQL by about 28% when compared with dry. The cutting forces for cryogenic and combination of cryogenic and MQL were higher because the hardness of the material increased probably due to freezing of the work piece before cutting. 3.1.2. Feed forces The comparison of the feed forces is shown in Fig2 (b), it is seen that the feed force for MQL and cold air are least followed by MQL, dry and cryogenic-MQL. The values were highest for cryogenic. 3.1.3. Radial forces The radial forces for different environments are shown in the Fig 2(c). Results showed that the combination of MQL and cold air produced forces which were about 46% lesser than MQL and cryogenic. MQL also showed similar results. Cryogenic and cryogenic-MQL showed larger values of feed forces
SURFACE ROUGHNESS(RA)
Ra 4 3.75 3.5 3.25 3 2.75 2.5 2.25 2 1.75 1.5 1.25 1 0.75 0.5 0.25 0 DRY
CRYO
CRYO MQL MQL CUTTING ENVIRONMENTS
COLD AIR MQL
Fig.3 Comparison of surface roughness under different environments
3.2. Surface roughness One of the important factor that ensures good machining performance of components is surface quality. Surface quality is mainly dependent on surface roughness. Surface roughness should be measured to check whether the expected results are obtained through the chosen cooling and lubricating conditions. Fig3 indicates the comparison of the surface roughness values for different conditions. Results shows, surface quality deteriorate in dry machining whereas tend to improve under other conditions. In comparison to dry cryogenic and MQL-cryo produce lower surface roughness values and the surface quality improves in both cases by about 86%. MQL also shows a similar trend in surface roughness. Machining under combination of MQL and cold air showed satisfactory surface roughness values and was lower by about 85% as compared to dry. 3.3. Tool Wear Adhesion of material was observed during the machining under dry environment. Notch wear is found to be most common in machining of nickel based super alloys. The different types of wear commonly found on the insert are notch wear, chipping of the edge of the tool, flank wear, Built up Edge (BUE). These were found on the insert after machining under the different environments. The turning of Inconel 718 results in high local temperatures due to strain hardening. The thermal conductivity of Inconel 718 is not as high as other materials leading to higher temperatures during machining, this leads to chipping of the tool, as the high local temperature leads to plastic
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deformation of the tool which gets easily chipped off during turning. The above conditions also lead to the notch wear in the tool.
a)Cold air+MQL
c)Cryogenic
b)MQL
d)Dry
e)Cryo+MQL Fig. 4 Flank wear on the insert after machining in different environment
The chips generated during the machining are abrasive in nature, the contact of the tool with these chips also leads to notch wear. Crater wear unusually observed in the tool, crater wear is seen at higher cutting speeds. Mostly the mechanism behind the flank wear and crater wear in Inconel or nickel based alloy turning is diffusion wear. Diffusion wear occurs because at the higher temperatures generated during the machining causes diffusion to accelerate in between the tool and the work piece. One of the reasons for the wear in the tool is that the thermal conductivity of the TiAlN coating is also low. The flank wear was measured using the maximum length of the flank wear observed under the digital microscope, L. The maximum lengths, L of the flank wear under the different environments of machining were as given in the Table 3. From the values of the Length, L obtained we can observe that turning of the work piece under dry environment has produced maximum flank wear. The least value of L is observed under the environment of MQL. The values of L under MQL + Cold air and MQL + Cryogenic were quite like each other, cryogenic machining also produced less value of L and thus smaller amount of flank wear. As can be observed from the Fig 4, dry machining of the work piece leads chipping of tool. Because in dry machining, the temperature generated during machining is more than that generated during other environments. The heat generated is not effectively dissipated under dry machining which leads to the chipping of the tool. Besides flank wear, notch wear is also observed under dry machining. There is formation of BUE under dry machining, due to the plastic deformation of the tool due to the generation of high local temperature. The heat
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affected zone was less in cryogenic machining as compared to any other machining. There is no Built-up Edge formation under cryogenic machining, this is because the heat generation under cryogenic machining is the least amongst all the machining environments. Machining using MQL led to least amount of flank wear. This is because the friction generated between work piece and cutting edge of the tool is reduced by oil as it provides lubrication subsequently reducing the temperature generated during machining and leading to lesser flank wear. But the heat affected zone of the tool in MQL machining is more as compared to that in cryogenic machining.
a) Dry
b)MQL
c)Cold air+ MQL
d)Cryogenic
e) MQL and cryogenic Fig. 5 Crater wear on the insert after machining in different environment
Chipping and notch wear is observed under the environment of cryo(cryogenic) + MQL. This might be due to the ineffective penetration of MQL in the cutting zone which makes it difficult to reduce the friction in the exact machining region. Cryogenic alone is more effective in reducing the temperature of the cutting region, avoiding overheating and thus avoiding thermal cracking. Thermal crack generation leads to micro- chipping as is observed in case of MQL, cryo + MQL and cold air + MQL. Similarly, in case of cold air + MQL, the flank wear is lesser than that in dry machining, but greater than that in MQL and cryogenic. This might be because the temperature of cold air supplied is not as low as that of cryogenic. Cryogenic fluid had been supplied at a higher pressure, which also led to effective removal of the chips produced during machining.
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Table 3. Flank wear under different environments Environments
Dry
MQL + Cold air
MQL + Cryo
MQL
Cryo
Max length of Flank wear, L (µm)
1557.9
505.6
506.7
170.9
199.7
The pressure of cold air was not as high as that of cryogenic fluid, which did not efficiently remove the chips formed during the machining. The abrasive action of these chips can also be a major reason for the flank wear in the machining under cold air + MQL machining. The oil in MQL did not penetrate the cutting zone to provide effective lubrication leading to increase in friction. The tool used was a TiAlN coated insert of PVD insert. The analysis of the tool wear showed that notching and chipping were the common reasons for tool wear leading to high temperature and flank wear. In cryogenic + MQL, the supply of both cryogenic fluid + MQL at the flank face must have led to interference between the effective flow of both the fluids leading to poor action of both the fluids in the cutting zone, giving more flank wear as compared to individual supply of cryogenic fluid and MQL. Crater wear occurs normally due to contact with the chips. Another major reason for crater wear to occur is due to generation of high temperatures during machining. The height of crater wear is normally equal to the cutting depth. Normally, the crater wear occurs if the cutting speed is too low or the feed rate is too high. But in the above experiment, these parameters have been kept constant. As can be observed in the figure 5, the region of crater wear is the highest during dry machining. The least crater wear is observed in cryogenic machining followed by MQL. Cryogenic + MQL and shows crater wear less than dry machining but greater than cryogenic machining and MQL machining. The crater wear during cold air + MQL machining is comparable to that produced by cryogenic machining. Dry machining tends to produce very high temperature, leading to crater wear. MQL effectively reduces friction between tool and work piece leading to reduced temperature and as a result reducing the crater wear. Cryogenic fluid reduces the temperature in the cutting zone very effectively and hence produces least crater wear, also the high pressure of cryogenic fluid removes the chips produced during machining leading to reduced abrasive action of the chips on the rake face of tool and thus reduced crater wear. During cold air + MQL machining, the crater wear observed is low. This might be because of the reduced temperature due to supply of cold air. As the pressure of cold air is lesser than cryogenic fluid, the flow of cold air and MQL might not have interfered in the cutting region leading to better penetration of the MQL oil in the cutting zone leading to effective reduction of friction and hence temperature. But in case of cryogenic + MQL, the pressure of cryogenic fluid being high, the flow of the fluids might have interfered in the cutting zone, leading to poor penetration of both the fluids and increasing crater wear. Also, the cryogenic fluid leads to hardening of surface of the work piece, which increases the forces required by the tool to cut the same depth, which also leads to crater wear, this is not the case with cold air. 4. Conclusion The results show that using MQL and cold air reduce the cutting forces by 28% whereas MQL by 26.5%. The feed forces and radial forces were minimum in case of MQL and cold air. Whereas for MQL and cryo all the force components were higher for work piece material before cutting. The surface roughness in all the cases except dry was lower and the least value was obtained in MQL and cryo. From the above experiment and the results, we conclude that in tool wear, the flank wear is observed to be maximum in dry machining and least in cryo followed by MQL, the flank wear of the tool on machining under cryo + MQL and cold air + MQL lies between the values of dry machining and cryogenic and MQL machining. The crater wear is observed to be maximum in dry machining and least in cryogenic machining, followed by MQL. Similar crater wear is observed during machining in the environment of cold air + MQL and cryogenic + MQL. Thus, we conclude that the tool wear is observed to be least during cryogenic machining and maximum during dry machining. Thus, it can be concluded that employing cold air instead of cryogenic is more efficient and sustainable.
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References [1] [2] [3] [4]
A. Iturbe, E. Hormaetxe, A. Garay, P.J. Arrazola, Procedia CIRP 45(2016)67-70. A. Devillez, G. Le Coz, S. Dominak, D. Dudzinski, Journal of Materials Processing Technology 211(2011) 1590-1598. D. G. Thakur, B. Ramamoorthy, L. Vijayaraghavan, Materials and design 30(2009) 1718-1725. Franci Pusavec , Ashish Deshpande , Shu Yang , Rachid M'Saoubi , Janez Kopac ,Oscar W. Dillon Jr., I.S. Jawahir, Journal of Cleaner Production 81 (2014) 255-269 [5] F. Pusavec, H. Hamdi, J. Kopac, I.S. Jawahir, Journal of Materials Processing Technology 211 (2011) 773–783. [6] J. L. Cantero, J. Diaz. Alvarez, M. H. Miguelez, N. C. Marin, Wear 297 (2013) 885–894. [7] Kyung- Hee Park, Gi-Dong Yang, Dong Yoon Lee, International Journal of Precision Engineering and Manufacturing Vol. 16, No. 7, pp. 1639-1645. [8] N.G. Patil, Ameer Asem, R. S. Pawade Procedia CIRP 24(2014) 86-91. [9] R.S. Pawade, Suhas .S. Joshi, P.K. Brahmankar, International Journal of Machine Tools & Manufacture 48 (2008) 15-28. [10] R.S. Pawade, Suhas .S. Joshi, P.K. Brahmankar, M. Rahman, Journal of Materials Processing Technology 192-193 (2007) 139-146.
ScienceDirect Materials Today: Proceedings 5 (2018) 6746–6754
www.materialstoday.com/proceedings
IMME17
Influence of sustainable cutting environments on cutting forces, surface roughness and tool wear in turning of Inconel 718 A Mehta*, S.Hemakumar, A.Patil, S.P.Khandke, P.Kuppan, R.Oyyaravelu, A.S.S.Balan School of Mechanical Engineering, VIT University, Vellore 632014, Tamil Nadu
Abstract Inconel 718 is a potential material for aerospace, nuclear and automobile industries because of its resistance to high temperature and corrosion. It becomes important to ensure good surface quality for better part performance and life. This paper shows a comparative study of the cutting forces, surface roughness and analysis of tool wear while machining of Inconel 718 under different sustainable machining environments such as dry, minimum quantity lubrication (MQL), cryogenic, cold air and MQL, MQL and cryogenic. The cutting speed, feed rate and depth of cut were kept constant for all environments based on the previous studies from literature. Results showed that machining using combination of cold air and MQL tends to reduce the surface roughness by about 86% and cutting force by about 28% when compared to dry machining. The minimum flank wear was obtained under MQL and cryogenic cutting condition (127.8µm) which is about 92% lower when compared to dry condition. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Emerging Trends in Materials and Manufacturing Engineering (IMME17). Keywords: Inconel 718; minimum quantity lubrication; cryogenic; cold air; surface roughness; tool wear.
1. Introduction Nickel based Super alloys finds range of applications in aerospace, nuclear and automobile sector due to their high performance in stringent working conditions. Material like Inconel 718, reported to be high performance material for its high strength at elevated temperatures, with oxidation and corrosion resistance properties [3,5,8,9].
* Corresponding author. Tel.: 09921787797; E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Emerging Trends in Materials and Manufacturing Engineering (IMME17).
Mehta et al., / Materials Today: Proceedings 5 (2018) 6746–6754
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But its high strength, thermal resistivity, work hardenability and presence of abrasive carbide particles, it becomes difficult to machine this material [1,3]. Along with these factors the cutting insert and the tool edge geometry also plays an important role in influencing the cutting forces, temperature, tool wear, surface roughness and chip formation while machining [10]. So, it becomes important to carry out a comprehensive study on its machinability characteristics under different cutting environment. Simultaneously abiding by the governmental laws of preventing pollution makes it important to undertake some steps towards sustainable machining environments. To make machining more sustainable, alternative cooling and lubrication, should be developed which replace the conventional methods. Recently, lot of cooling and lubricating methods have been introduced among which minimum quantity lubrication and cryogenic environment is dominant. These methods not only improve the machinability of Inconel 718 but also helpes in making machining processes more cleaner and environment friendly [1,8]. Kaynak through his work showed that at 60m/min cryogenic cooling increased the radial force, feed force and cutting approximately by 20,36 and 25% respectively in comparison with MQL, whereas at 120m/min cryogenic cooling generated the least forces among all three conditions. The cutting parameters were feed(f)=0.075mm/rev and depth of cut(ap)=0.8mm. Also, lower flank and crater wear rate was observed for both MQL and cryogenic cooling. In addition, the surface roughness was lower in MQL and cryogenic cooling. In addition, the surface roughness was lower for both MQL and cryogenic when compared to dry. Pusavec et al. carried out an array of experiments, investigating the machining characteristics of Inconel 718 under different conditions such as dry, MQL, cryogenic and combination of MQL and cryogenic cooling. On comparing the results, it was found that the combination of MQL and cryogenic cooling produced lowest cutting and feed forces. The MQL and cryogenic cooling demonstrated the minimum flank and rake wear when compared with other conditions. Further the surface quality improved for a range of feed and cutting speed. Literature shows, using MQL and cryogenic cooling tend to improve the machining performance of Inconel 718. But at the same time cryogenic cooing is costly. So, it is important to make the cooling cost effective. Therefore, using cold air can replace cryogenic cooling. Cold air guns can be employed to produce temperatures as low as -34ºC and thus can be helpful in reducing the temperatures while machining. This in turn increases the tool life, forces and gives a good surface finish. In addition, cold air gun uses compressed air which is cheaper as compared to other coolants. Further it is seen that cryogenic cooling tends to increase the hardness of the workpiece before it is cut [4,5,11]. This is not the case with cold air. Also, it is seen that a lot f work has been done on sustainability of MQL and cryogenic [4,11]. So, this paper shows the influence of the cold air and MQL on the different cutting parameters and its comparison with other environments. For each condition the cutting forces, surface quality and tool wear were measured. 2. Experimental details The composition of Inconel 718 is given in Table 1. The work piece used was cylindrical in shape with diameter of 22mm and length of 100mm. The work piece was first rough turned for a depth of cut of 1mm to remove the excess non-uniformities which might have interfered with the turning operation. The insert used for the turning operation was Kennametal KCU25 with AlTiN coating, the geometry of the insert is as shown in Fig 2 and Table 2. The insert with the tool holder was mounted on the dynamometer which was used for measuring cutting force values. The dynamometer used was KISTLER 9275B. The work piece was turned on CNC lathe machine of SIMPLE TURN 5075-SPM. The cutting parameters were kept constant, cutting speed (Vc) = 75 m/min, feed rate =0.125 mm/rev and depth of cut (ap) = 0.5mm. The parameters were decided by referring the literature and were found to be optimum for machining. The machining was carried out under five different cooling environments such as dry, MQL, Cold air + MQL, Cryogenic, Cryogenic + MQL. Under each environment, a cutting length of 35mm was maintained constant. New cutting edge was used for respective environment of machining. The tool post was mounted on the dynamometer and the force was measured using DynoWare software.
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2.1 MQL setup The oil used was Esters with wear resistance additives. The viscosity of this oil is 24cSt at 40˚C. The flow rate used for the machining was 25mm3/rev. MQL was supplied using two nozzles. One nozzle supplied oil to rake face of insert and other nozzle supplied MQL to the flank face of the cutting tool in the direction of cutting. 2.2 Cryogenic The cryogenic fluid used for carrying out the experiment was Liquid Nitrogen. The nozzle used for supplying the cryogenic fluid had diameter of 2mm. The cryogenic fluid was supplied with a pressure of 1.5 bar. The nozzle was focused on flank face of cutting tool in the direction of cutting. Liquid Nitrogen was not supplied to the work piece to avoid surface hardening of the work piece which might lead to bad surface finish. The setup of Cryogenic was decided based on the previous work done on turning of Inconel 718 with cryogenic. 2.3 Cold air + MQL The supplied cold air with flow rate of 1981 SPLM. It was supplied on the flank face of cutting tool in the cutting direction. MQL was being supplied at a flow rate of 25mm/rev and was focused at the rake face and flank face of the cutting tool in the cutting direction. 2.4 Cryogenic + MQL In case of cryogenic + MQL, cryogenic fluid was supplied to flank face of the cutting tool in the cutting direction and MQL was supplied to rake and flank face of the cutting tool at 25mm3/rev. After carrying out the experiments in five different environments, the cylindrical work piece bar was tested for surface roughness using MAHR’s profile meter of MARSURF XCR 20. The Ra and Rt values of the respective machined surfaces were noted. The readings were taken at three different points on machined surface of an environment and respective average was taken. A diamond probe of 3mm was used to measure surface roughness. Specimen of respective diameter of the machined surface were then cut using wire EDM. The thickness of the specimen was 0.5 mm. The wire EDM machine used was EZEEWIN with a brass wire of diameter 2.5mm. The specimen was then tested on Vickers’s micro hardness tester to measure the micro hardness of each of the specimen. For measuring the micro hardness. The indentation was made at 10 different points at 0.75 mm from each other, the first point being very close to the machined surface. A load of 500 grams was applied. The 10 points were taken for comparing micro hardness at machined surface and surface sufficiently away from the machining affected zone.
Fig. 1 Geometry of the insert KCU25
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Table 1. Material composition
Element
C
Mn
Si
P
S
Ni
Cr
Nb + Ta
Composition
0.0301
0.0701
0.0901
0.007
0.001
18.45
52.50
5.321
Element
Co
Al
Cu
Fe
Mo
Nb
B
Mg
Composition
0.251
0.481
0.04
17.65
3.02
5.31
0.005
0.0018
Table. 2 Nomenclature of insert KCU25
ANSI Catalog Number
D(mm)
Rake Angle
L10 (mm)
Re (mm)
D1 (mm)
CNMG430MS
12.7
6°
12.9
0.4
5.67
3. Results and Discussions 3.1. Forces Forces are basic and important factors on which the power consumption and energy utilization in a machining process is dependent. Forces are affected by variety of factors such as friction between tool and chip, properties of work piece material, tool and cutting tool geometry, cutting environments, etc. The influence of the different environments on the cutting forces, forces and radial forces and the comparison of each component under different environments is shown in Fig 2.
FEED FORCES(N)
Fx
250 200 150 100 50 0 DRY (a)
CRYO CRYO MQL
MQL
CUTTING ENVIRONMENTS
COLD AIR MQL
150 100 50 0 DRY
(b)
CRYO CRYO MQL
MQL
COLD AIR MQL
CUTTING ENVIRONMENTS
Fy RADIAL FORCES(N)
CUTTING FORCES(N)
Fz
300 200 100 0 DRY (c)
CRYO
CRYO MQL
MQL
COLD AIR MQL
CUTTING ENVIRONMENTS
Fig.2 comparison of different force components under different environments :( a) cutting forces, (b) feed forces, (c) Radial forces
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3.1.1. Cutting forces Fig.2. (a) show that MQL decreases the cutting forces by about 26.5% whereas combination of cold air and MQL by about 28% when compared with dry. The cutting forces for cryogenic and combination of cryogenic and MQL were higher because the hardness of the material increased probably due to freezing of the work piece before cutting. 3.1.2. Feed forces The comparison of the feed forces is shown in Fig2 (b), it is seen that the feed force for MQL and cold air are least followed by MQL, dry and cryogenic-MQL. The values were highest for cryogenic. 3.1.3. Radial forces The radial forces for different environments are shown in the Fig 2(c). Results showed that the combination of MQL and cold air produced forces which were about 46% lesser than MQL and cryogenic. MQL also showed similar results. Cryogenic and cryogenic-MQL showed larger values of feed forces
SURFACE ROUGHNESS(RA)
Ra 4 3.75 3.5 3.25 3 2.75 2.5 2.25 2 1.75 1.5 1.25 1 0.75 0.5 0.25 0 DRY
CRYO
CRYO MQL MQL CUTTING ENVIRONMENTS
COLD AIR MQL
Fig.3 Comparison of surface roughness under different environments
3.2. Surface roughness One of the important factor that ensures good machining performance of components is surface quality. Surface quality is mainly dependent on surface roughness. Surface roughness should be measured to check whether the expected results are obtained through the chosen cooling and lubricating conditions. Fig3 indicates the comparison of the surface roughness values for different conditions. Results shows, surface quality deteriorate in dry machining whereas tend to improve under other conditions. In comparison to dry cryogenic and MQL-cryo produce lower surface roughness values and the surface quality improves in both cases by about 86%. MQL also shows a similar trend in surface roughness. Machining under combination of MQL and cold air showed satisfactory surface roughness values and was lower by about 85% as compared to dry. 3.3. Tool Wear Adhesion of material was observed during the machining under dry environment. Notch wear is found to be most common in machining of nickel based super alloys. The different types of wear commonly found on the insert are notch wear, chipping of the edge of the tool, flank wear, Built up Edge (BUE). These were found on the insert after machining under the different environments. The turning of Inconel 718 results in high local temperatures due to strain hardening. The thermal conductivity of Inconel 718 is not as high as other materials leading to higher temperatures during machining, this leads to chipping of the tool, as the high local temperature leads to plastic
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deformation of the tool which gets easily chipped off during turning. The above conditions also lead to the notch wear in the tool.
a)Cold air+MQL
c)Cryogenic
b)MQL
d)Dry
e)Cryo+MQL Fig. 4 Flank wear on the insert after machining in different environment
The chips generated during the machining are abrasive in nature, the contact of the tool with these chips also leads to notch wear. Crater wear unusually observed in the tool, crater wear is seen at higher cutting speeds. Mostly the mechanism behind the flank wear and crater wear in Inconel or nickel based alloy turning is diffusion wear. Diffusion wear occurs because at the higher temperatures generated during the machining causes diffusion to accelerate in between the tool and the work piece. One of the reasons for the wear in the tool is that the thermal conductivity of the TiAlN coating is also low. The flank wear was measured using the maximum length of the flank wear observed under the digital microscope, L. The maximum lengths, L of the flank wear under the different environments of machining were as given in the Table 3. From the values of the Length, L obtained we can observe that turning of the work piece under dry environment has produced maximum flank wear. The least value of L is observed under the environment of MQL. The values of L under MQL + Cold air and MQL + Cryogenic were quite like each other, cryogenic machining also produced less value of L and thus smaller amount of flank wear. As can be observed from the Fig 4, dry machining of the work piece leads chipping of tool. Because in dry machining, the temperature generated during machining is more than that generated during other environments. The heat generated is not effectively dissipated under dry machining which leads to the chipping of the tool. Besides flank wear, notch wear is also observed under dry machining. There is formation of BUE under dry machining, due to the plastic deformation of the tool due to the generation of high local temperature. The heat
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affected zone was less in cryogenic machining as compared to any other machining. There is no Built-up Edge formation under cryogenic machining, this is because the heat generation under cryogenic machining is the least amongst all the machining environments. Machining using MQL led to least amount of flank wear. This is because the friction generated between work piece and cutting edge of the tool is reduced by oil as it provides lubrication subsequently reducing the temperature generated during machining and leading to lesser flank wear. But the heat affected zone of the tool in MQL machining is more as compared to that in cryogenic machining.
a) Dry
b)MQL
c)Cold air+ MQL
d)Cryogenic
e) MQL and cryogenic Fig. 5 Crater wear on the insert after machining in different environment
Chipping and notch wear is observed under the environment of cryo(cryogenic) + MQL. This might be due to the ineffective penetration of MQL in the cutting zone which makes it difficult to reduce the friction in the exact machining region. Cryogenic alone is more effective in reducing the temperature of the cutting region, avoiding overheating and thus avoiding thermal cracking. Thermal crack generation leads to micro- chipping as is observed in case of MQL, cryo + MQL and cold air + MQL. Similarly, in case of cold air + MQL, the flank wear is lesser than that in dry machining, but greater than that in MQL and cryogenic. This might be because the temperature of cold air supplied is not as low as that of cryogenic. Cryogenic fluid had been supplied at a higher pressure, which also led to effective removal of the chips produced during machining.
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Table 3. Flank wear under different environments Environments
Dry
MQL + Cold air
MQL + Cryo
MQL
Cryo
Max length of Flank wear, L (µm)
1557.9
505.6
506.7
170.9
199.7
The pressure of cold air was not as high as that of cryogenic fluid, which did not efficiently remove the chips formed during the machining. The abrasive action of these chips can also be a major reason for the flank wear in the machining under cold air + MQL machining. The oil in MQL did not penetrate the cutting zone to provide effective lubrication leading to increase in friction. The tool used was a TiAlN coated insert of PVD insert. The analysis of the tool wear showed that notching and chipping were the common reasons for tool wear leading to high temperature and flank wear. In cryogenic + MQL, the supply of both cryogenic fluid + MQL at the flank face must have led to interference between the effective flow of both the fluids leading to poor action of both the fluids in the cutting zone, giving more flank wear as compared to individual supply of cryogenic fluid and MQL. Crater wear occurs normally due to contact with the chips. Another major reason for crater wear to occur is due to generation of high temperatures during machining. The height of crater wear is normally equal to the cutting depth. Normally, the crater wear occurs if the cutting speed is too low or the feed rate is too high. But in the above experiment, these parameters have been kept constant. As can be observed in the figure 5, the region of crater wear is the highest during dry machining. The least crater wear is observed in cryogenic machining followed by MQL. Cryogenic + MQL and shows crater wear less than dry machining but greater than cryogenic machining and MQL machining. The crater wear during cold air + MQL machining is comparable to that produced by cryogenic machining. Dry machining tends to produce very high temperature, leading to crater wear. MQL effectively reduces friction between tool and work piece leading to reduced temperature and as a result reducing the crater wear. Cryogenic fluid reduces the temperature in the cutting zone very effectively and hence produces least crater wear, also the high pressure of cryogenic fluid removes the chips produced during machining leading to reduced abrasive action of the chips on the rake face of tool and thus reduced crater wear. During cold air + MQL machining, the crater wear observed is low. This might be because of the reduced temperature due to supply of cold air. As the pressure of cold air is lesser than cryogenic fluid, the flow of cold air and MQL might not have interfered in the cutting region leading to better penetration of the MQL oil in the cutting zone leading to effective reduction of friction and hence temperature. But in case of cryogenic + MQL, the pressure of cryogenic fluid being high, the flow of the fluids might have interfered in the cutting zone, leading to poor penetration of both the fluids and increasing crater wear. Also, the cryogenic fluid leads to hardening of surface of the work piece, which increases the forces required by the tool to cut the same depth, which also leads to crater wear, this is not the case with cold air. 4. Conclusion The results show that using MQL and cold air reduce the cutting forces by 28% whereas MQL by 26.5%. The feed forces and radial forces were minimum in case of MQL and cold air. Whereas for MQL and cryo all the force components were higher for work piece material before cutting. The surface roughness in all the cases except dry was lower and the least value was obtained in MQL and cryo. From the above experiment and the results, we conclude that in tool wear, the flank wear is observed to be maximum in dry machining and least in cryo followed by MQL, the flank wear of the tool on machining under cryo + MQL and cold air + MQL lies between the values of dry machining and cryogenic and MQL machining. The crater wear is observed to be maximum in dry machining and least in cryogenic machining, followed by MQL. Similar crater wear is observed during machining in the environment of cold air + MQL and cryogenic + MQL. Thus, we conclude that the tool wear is observed to be least during cryogenic machining and maximum during dry machining. Thus, it can be concluded that employing cold air instead of cryogenic is more efficient and sustainable.
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