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Experimental Investigations on Multiple Responses in Abrasive Waterjet Machining of Ti-6Al-4V Alloy
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 13413–13421
www.materialstoday.com/proceedings
ICMMM - 2017
Experimental Investigations on Multiple Responses in Abrasive Waterjet Machining of Ti-6Al-4V Alloy A.Gnanavelbabua,*, P.Saravananb, K.Rajkumarc, S.Karthikeyand a
Associate Professor, Department of Industrial Engineering, Anna University Chennai-600025, India b Research Scholar, Department of Industrial Engineering, Anna University Chennai-600025, India c Associate Professor, Department of Mechanical Engineering, SSN College of Engineering, Kalavakkam-603110, India d Professor, Centre for Innovative Manufacturing Research, VIT University, Vellore-632014, India
Abstract In general, Ti-6Al-4V alloys are one of the challenging materials to machine using conventional machining process. Due to its high strength to weight ratio, good thermal stable property and exceptional corrosion resistance, Ti-6Al-4V became an attentiongrabbing material to propose a proper modeling and machining for various types of applications. In this research study, Ti-6Al4V was machined using Abrasive Waterjet Machining under different process parameters such as mesh size, abrasive flow rate, pressure and traverse speed. The Machining approach is based on the Box Behnken method to enhance the Abrasive waterjet machining process parameter for effective machining of Ti-6Al-4V. Then multiple responses were carried out such as kerf taper geometries (θ), surface roughness (Ra) and material removal rate (MRR). The structures of various machining surface regions were examined using Scanning electron microscopy (SEM). The experimental results specify that high pressure, low traverse speed, low abrasive mesh size and high abrasive flow rate were resulted in lower surface roughness in Ti-6Al-4V. And it was found that high pressure, high mesh size leads to minimum kerf taper ratio and whereas high traverse speed produce a maximum kerf taper. It was also evident that high abrasive flow rate, standard traverse speed and low pressure provide high MRR in selected conditions. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
Keywords: Titanium; Abrasive Waterjet machining; Kerf width, MRR, Surface roughness
1. Introduction Titanium alloys are difficult to machine using conventional machining process due to its inherent properties. The mechanical and physical property of Ti-6Al-4V includes high strength to weight ratio and better corrosion resistance [1]. Those exceptional characteristics of Ti-6Al-4V are applicable for marine industries and aircraft industries [2].
* Corresponding author. A.Gnanavelbabu, Tel.: +91-9551133779; E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
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Titanium alloy is also used in production of transplants and entrenched implants in biomedical field [3]. Titanium material is chemically reactive towards cutting tool elements because of its low thermal conductivity and lowelasticity modulus [4]. The known complications of machining Ti-6Al-4V are tool wear concerns and cutting temperature. By these reasons during machining process, lifetime was severely affected. Consequently, proper modeling of machining and its oriented control over parameters increases rapidly [5-7]. The suitable process model to machining Titanium alloy was found to be Abrasive water jet machining, because it has no thermal influence on machining material. Abrasive water jet can machine a various range of materials such as metal matrix composites (MMC), Titanium, alloys, stainless steel, ceramics, and plastics too. The Cutting time is faster but cost per cutting is relatively high. Abrasive water jet machining is easy as friendly and it does not produce any machining dust or air pollutants. High energy of water jet and abrasive particles plays a key role in machining process. The water carrying the garnet abrasive particles acts as a coolant in AWJ technology. A small cutting force is enough for material processing with huge advantages of no thermal distortion, good flexibility and high process versatility [8, 9]. Material defects were eliminated in AWJ technology due to absence of thermal or electrical energy. The cutting performance of an abrasive water jet mainly depends on the abrasive particle’s velocities and impact angles. In AWJ technology erosion process takes place in high speed. Hitting of Abrasive particles with high traverse speed and high impact angle leads to erosion of the machining surface. There are two modes of erosion takes place by abrasive particles in AWJ machining processes. Larger particle impact angle causes deformation wear mode and smaller particle impact angle causes cutting wear mode. The quality of cutting evaluation is based on the surface roughness, waviness, angle of striations and MRR. The ultimate motto is to acquire smooth surface of cut region. This can be achieved by changing process parameters like abrasive flow rate, mesh size, pressure, traverse speed and standoff distance effectively [10-13]. The Kerf taper increases with increases in standoff distance due to widening of water jet at higher standoff distance [14]. The surface morphology was irregular in lower traverse speeds but lowest surface roughness is obtained [15]. Increase in waterjet pressure made smoother surface due to extent kinetic energy of abrasives. The surface seems to smoother at jet entrance and rougher at jet exit. Work feed rate is not a significant parameter as compared to abrasive flow rate and pressure in surface roughness response [16]. The material removal rate (MRR) increases with increase in standoff distance because of abrasive jet particles impacts deeper on work surface which creates craters [17]. Lower traverse speed also influences in higher material removal rate (MRR) and high surface waviness [18]. The depth of cut is high when abrasive flow rate, pressure, traverse speeds were kept constant [19]. By different responses with respective to process parameters, quality of cutting is highly improved by eliminating unstable errors. 2. Experimental Procedure Commercial Ti-6Al-4V was used in this AWJ experiment with a thickness of 5mm. The chemical compositions of Ti-6Al-4V are listed below in table 1. Table 1. Chemical Composition of Ti-6Al-4V Ti
Al
V
Fe
O
C
N
H
90.64
5.48
3.92
0.28
0.17
0.03
0.02
0.00583
The experiments were performed using Precision Abrasive WaterJet Machining Center manufactured by OMAX Corporation (Model: 2626) which is shown in figure 1. Garnet is used as abrasives in AWJ machining. Process parameters such as mesh size, abrasive flow rate, pressure and traverse speed were varied at three different levels using Box Behnken method. The Parameters and factors used in the machining process and its detailed values are listed below in table 2.
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Table 2. Parameters and factors Oriface Diameter Nozzle diameter Focusing tube length Focusing tube Diameter Impact angle Abrasive Type Abrasive mesh size Abrasive flow rate Pump Pressure Traverse Speed Standoff Distance
0.25 mm 0.75 mm 75mm 1mm 90° Garnet 80,100,120 # 240,340,440 g/min 125,200,275 MPa 60,90,120 mm/min 1.5 mm
Fig. 1 Precision Abrasive WaterJet Machining Center manufactured by OMAX Corporation (Model: 2626)
Fig. 2 Typical AWJM of Ti-6Al-4V alloy
The variable parameters are abrasive flow rate, mesh size, pressure and traverse speed. By using each level of process parameters, 29 cuts were executed in the titanium alloy under the appropriate environmental circumstances which is shown in figure 2. After the machining, cut surfaces were examined using Scanning electron microscopy (SEM) for microstructural analysis and Kerf width were measured using Video measurement system for kerf taper angle. Later surface roughness was measured using portable stylus-type contact roughness meter. MRR was calculated using the Kerf width, traverse speed and depth of penetration.
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3. EXPERIMENTAL RESULTS AND DISCUSSIONS Table 3. Measurement of Kerf taper angle, Surface roughness and MRR MRR (mm3/min)
Sl.No
MS (#)
AFR (g/min)
P (MPa)
TR (mm/min)
Kerf Top (mm)
Kerf Bottom (mm)
Ѳ (Degrees)
1
80
240
200
90
0.631
0.484
0.752
2.309
251.1
2
80
240
200
60
0.709
0.571
0.724
2.226
192.1
3
80
340
200
120
0.563
0.446
0.812
2.64
303.1
4
80
340
125
90
0.616
0.494
0.742
2.22
231.65
5
80
340
275
90
0.687
0.558
0.69
2.356
310.55
6
80
240
200
90
0.650
0.535
0.748
2.311
266.85
7
100
340
125
60
0.641
0.531
0.632
2.132
175.95
8
100
340
275
60
0.568
0.511
0.426
2.342
270.82
9
100
340
275
120
0.695
0.591
0.595
2.38
345.8
10
100
340
125
120
0.735
0.602
0.693
2.232
274.26
11
100
340
200
90
0.684
0.569
0.788
2.302
260.4
12
100
340
200
90
0.557
0.452
0.764
2.304
260.8
13
100
340
200
90
0.593
0.494
0.758
2.306
262.45
14
100
340
200
90
0.644
0.559
0.766
2.301
266.98
15
100
340
200
90
0.652
0.54
0.75
2.298
258.66
16
100
240
200
60
0.642
0.556
0.734
2.23
194.26
17
100
240
200
120
0.636
0.552
0.796
2.6
270.46
18
100
240
125
90
0.633
0.554
0.72
2.218
200.75
19
100
240
275
90
0.658
0.567
0.676
2.344
280.86
20
100
440
275
90
0.569
0.463
0.716
2.336
340.75
21
100
440
200
60
0.671
0.572
0.754
2.178
260.6
22
100
440
125
90
0.726
0.619
0.768
2.196
262.32
23
100
440
200
120
0.638
0.513
0.82
2.624
335.6
24
120
440
200
90
0.663
0.550
0.789
2.29
293.225
25
120
340
200
60
0.701
0.600
0.742
2.21
255.2
26
120
340
200
120
0.631
0.491
0.812
2.65
302.6
27
120
340
125
90
0.634
0.529
0.746
2.24
234.78
28
120
340
275
90
0.692
0.604
0.696
2.228
308.88
29
120
240
200
90
0.689
0.594
0.75
2.312
255.34
Ra (µm)
After the machining, cut surfaces were examined using Scanning electron microscopy (SEM) for microstructural analysis and Kerf width were measured using Video measurement system for kerf taper angle. Later surface roughness was measured using portable stylus-type contact roughness meter. MRR was calculated using the Kerf width, traverse speed and depth of penetration. The calculation of kerf taper, MRR and surface roughness was shown in above table 3. Response surface methodology approach is the technique for describing the correlation
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between different process parameters with various cutting conditions and determining the various effects of the process parameters on the mutual responses [20, 21]. 3.1. Kerf Geometry Rational Video Measurement System (VMS) 2010-F was used to measure the kerf width. The typical view kerf width is shown in figure 3. For calculating the kerf taper angle for each cut, both top kerf and bottom kerf of each cut was measured. arctan 2
Where t is the thickness or width of the material t = 5 mm; Wt = Top Kerf; Wb = Bottom Kerf
Fig. 3 Typical view of Lowest Kerf Width
The Top kerf is obviously wider than bottom kerf due to increase in pressure, which is a vital factor in Abrasive water jet machining. Due to the pressure factor, Kerf taper is produced. The largest kerf taper angle ratio affects the straightness of cutting and its results in poor dimensional quality. It is evident that high water pressure (275 MPa) and low traverse speed (60 mm/min) were produced the minimum kerf taper angle of θ = 0.426 which is shown in figure 4. When traverse speed increases, it will increase the slope of kerf wall due to abrasives strike on target which produces narrower slot. Analysis of variance (ANOVA) was used to analyze the results of Kerf taper angle which is shown in table 4. Table 4. ANOVA for Kerf Taper Angle S.No
Source
1
Model
Sum of Squares 0.14
Mean Square 9.688E-003
F Value
df 14
4.22
P- Value Prob > F 0.0055
2
A-Mesh Size
1.498E-004
1
1.498E-004
0.065
0.8021
3 4
B-Abrasive Flow rate C-Pressure
7.207E-003 0.021
1 1
7.207E-003 0.021
3.14 9.14
0.0983 0.0091
5
D-Traverse rate
0.020
1
0.020
8.69
0.0106
The Model F-value of 4.22 implies the model is significant. Values of "Prob > F" less than 0.0500 indicates that model terms are significantly contributed. In this case C (Pressure) and D (Traverse rate) are significant model terms.
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Fig. 4 3D Surface of Kerf Taper angle
3.2. Surface Roughness Roughness measurement was carried out using portable stylus-type contact roughness meter. The profilometer was calibrated with traverse speed 1mm/sec, cutoff length of 0.8 mm, and 3 mm evaluation length. Roughness measurements are taken in the transverse direction of the work pieces and it is repeated twice, and the average of two measurements of Surface Roughness (Ra) was calculated. Analysis of variance (ANOVA) was used to analyze the results of Surface roughness which is shown in table 5.The Regression model for Surface roughness in figure 5 shows the response surface graphs which are plotted to achieve minimum surface roughness with different combinations of AWJM process parameter during machining of Ti-6Al-4V. Table 5. ANOVA for Surface Roughness S.No
Source
Sum of Squares
df
Mean Square
F Value
p-value Prob > F
1
Model
0.98
14
0.070
5.58
0.0014
2
A-Mesh Size
0.010
1
0.010
0.83
0.3767
3
B-Abrasive Flow rate
7.461E-003
1
7.461E-003
0.60
0.4527
4
C-Pressure
0.54
1
0.54
43.02
< 0.0001
5
D-Traverse rate
0.12
1
0.12
9.37
0.0085
The Model F-value of 5.58 implies the model is significant. Values of "Prob > F" less than 0.0500 indicates that model terms are significantly contributed. In this case C (Pressure) and D (Traverse rate) are significant model terms. From the different combinations of process parameters, it was observed that constant increase in the traverse speed constantly decreases the surface roughness which is shown in figure 5. In other hand it is evident that lowest pressure and lowest traverse speed produces lower surface roughness. The lowest Surface roughness Ra of value 2.132µm was achieved at lowest pressure (125MPa) and lowest traverse speed (60 mm/min).
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Fig. 5 3D Surface of Surface Roughness
3.3. Material Removal Rate Material removal rate (MRR) was calculated using Kerf width, traverse speed and depth of penetration. Moreover standoff distance plays a major role in material removal rate (MRR). Material removal rate increases with standoff distance, because of penetration depth creating craters in working surface. Material removal was calculated using following formula, Material Removal Rate (MRR) = ht.W.Vt W= Kerf Width = (Wt + Wb)/2 Vt= Traverse speed (m/min); Wt = Top Kerf; Wb = Bottom Kerf ht= Depth of Penetration (ht=5mm) Analysis of variance (ANOVA) was used to analyze the results of Material removal rate which is shown in table 6. Table 6. ANOVA for MRR S.No
Source
Sum of Squares
df
Mean Square
F Value
p-value Prob > F
1
Model
48854.82
14
3489.63
37.51
< 0.0001
2
A-Mesh Size
16.62
1
16.62
0.18
0.6790
3
7850.98
1
7850.98
84.40
< 0.0001
4
B-Abrasive Flow rate C-Pressure
19036.35
1
19036.35
204.64
< 0.0001
5
D-Traverse rate
16606.34
1
16606.34
178.52
< 0.0001
The Model F-value of 37.51 implies the model is significant. Values of "Prob > F" less than 0.0500 indicates that model terms are significantly contributed. In this case, B (Abrasive flow rate), C (Pressure) and D (Traverse rate) are significant model terms. It is obviously evident that MRR is high at high pressure (275 MPa) and high traverse speed (120 mm/min) which is shown in figure 6.
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Fig. 6 3D Surface of Material Removal Rate
3.4. Surface Morphology The machined surface of samples having lower roughness and higher roughness under 50µm in SEM was shown in figure 7 and figure 8. There are two distinct zones were found as: Smooth machining region (SMR) which is present in lower roughness sample and Rough machining region (RMR) which is present in higher roughness sample. Smooth machining region was obtained at higher cutting angle zone and whereas rough machining region was obtained at reversal jet deflection [22]. The width of the smooth machined region increases with higher traverse rate, because of higher depth of penetration. The cutting wear and deformation wear found as a scar in rough cutting region due to high pressure and low traverse speed.
Fig. 7 Cut Surface of Lower Ra
Fig. 8 Cut Surface of higher Ra
4. Conclusions In this research, Kerf geometry, surface roughness (Ra), microstructural analyses and material removal rate (MRR) of Ti-6Al-4v were investigated using Abrasive water jet machining. By the summarized results from above responses, the following conclusions was made and recommended.
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1. Kerf Geometry shows that kerf width gets narrower with increase in traverse speed. This is due to abrasive particles hitting on jet target to produce a narrower slot. Kerf taper ratio varies for increasing traverse speed and lowest kerf taper was obtained for lowest traverse speed (60 mm/min). 2. Surface Roughness shows that Ra is lower when pressure (125 MPa) is low and traverse speed (60 mm/min) is low. Surface quality was affected at highest pressure, because kinetic energy of jet reduced with material-jet interaction and influence of abrasive particles. 3. Material Removal Rate (MRR) analysis shows that MRR increases when pressure (275 MPa) is high and traverse speed (120 mm/min) is high. 4. Microstructural analyses of machined surfaces revealed two regions: 1. Smooth machining Region (SMR) and Rough machining Region (RMR). Smooth machining region is found at higher cutting angle zone and Rough machining region is found at jet reversal deflection. References [1] Barry, J., G. Byrne, and D. Lennon. "Observations on chip formation and acoustic emission in machining Ti–6Al–4V alloy." International Journal of Machine Tools and Manufacture, Vol. 41, No.7 (2001): pp.1055-1070. [2] Ezugwu, E. O., and Z. M. Wang. "Titanium alloys and their machinability—a review." Journal of materials processing technology, Vol.68, No.3 (1997): pp.262-274. [3] Lütjering, Gerd, and James Case Williams. Titanium. Vol. 2. Berlin: Springer, 2003. [4] Hong, Shane Y., Irel Markus, and Woo-cheol Jeong. "New cooling approach and tool life improvement in cryogenic machining of titanium alloy Ti-6Al-4V." International Journal of Machine Tools and Manufacture, Vol. 41, No.15 (2001):pp. 2245-2260. [5] Arola, D., A. E. Alade, and W. Weber. "Improving fatigue strength of metals using abrasive waterjet peening." Machining science and technology, Vol.10, No.2 (2006): pp.197-218. [6] Rabani, Amir, Iulian Marinescu, and Dragos Axinte. "Acoustic emission energy transfer rate: a method for monitoring abrasive waterjet milling." International journal of machine tools and Manufacture, Vol.61 (2012): pp.80-89. [7] Anwar, S., D. A. Axinte, and A. A. Becker. "Finite element modelling of a single-particle impact during abrasive waterjet milling." Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology, Vol. 225, No.8 (2011): pp.821-832. [8] Van Luttervelt, C. A. "On the selection of manufacturing methods illustrated by an overview of separation techniques for sheet materials." CIRP Annals-Manufacturing Technology, Vol.38, No.2 (1989): pp.587-607. [9] Akkurt, Adnan, et al. "Effect of feed rate on surface roughness in abrasive waterjet cutting applications." Journal of Materials Processing Technology, Vol. 147, No.3 (2004): pp.389-396. [10] Hashish, Mohamed. "A modeling study of metal cutting with abrasive waterjets." Journal of Engineering Materials and Technology, Vol. 106, No.1 (1984): pp.88-100. [11] Hashish, Mohamed. "Optimization factors in abrasive-waterjet machining." Journal of Engineering for industry, Vol.113, No.1 (1991): pp.29-37. [12] Arola, Dwayne, and Mamidala Ramulu. "Mechanism of material removal in abrasive waterjet machining of common aerospace materials." Proceedings of the 7th American Water Jet Conference. 1993. [13] Blickwedel, H., et al. "Prediction of abrasive jet cutting performance and quality." Proceedings of 9th International Symposium on Jet Cutting Technology. 1990. [14] Khan, Ahsan Ali, and M. M. Haque. "Performance of different abrasive materials during abrasive water jet machining of glass." Journal of materials processing technology, Vol.191, No.1 (2007): pp.404-407. [15] Ojmertz, K. M. C. "Abrasive waterjet milling: an experimental investigation." Proceeding of the 1993 American water jet conference, Washington, Paper. Vol. 58. 1993. [16] Khan, Ahsan Ali, Mohd Efendee Bin Awang, and Ahmad Azwari Bin Annuar. "Surface roughness of carbides produced by abrasive water jet machining." Journal of Applied Sciences, Vol. 5, No.10 (2005): pp.1757-1761. [17] Palleda, Mahabalesh. "A study of taper angles and material removal rates of drilled holes in the abrasive water jet machining process." Journal of materials processing technology, Vol.189, No.1 (2007): pp.292-295. [18] Fowler, G., P. H. Shipway, and I. R. Pashby. "Abrasive water-jet controlled depth milling of Ti6Al4V alloy–an investigation of the role of jet–workpiece traverse speed and abrasive grit size on the characteristics of the milled material." Journal of materials processing technology Vol.161, No.3 (2005): pp. 407-414. [19] Selvan, M. Chithirai Pon, Dr N. Mohana Sundara Raju, and R. Rajavel. "Effects of process parameters on depth of cut in abrasive waterjet cutting of cast iron." International Journal of Scientific & Engineering Research, Vol. 2, No.9 (2011): pp. 1-5. [20] Sankar, M., Gnanavelbabu, A. and Rajkumar, K., “Effect of reinforcement particles on the abrasive assisted electrochemical machining of aluminium-boron carbide-graphite composite” Procedia Engineering, Vol.97, (2014): pp.381-389. [21] Sankar, M., Gnanavelbabu, A., Rajkumar, K. and Thushal, N.A., “Electrolytic concentration effect on the abrasive assisted-electrochemical machining of an aluminum–boron carbide composite” Materials and Manufacturing Processes, Vol.32, No.6 (2017): pp.687-692. [22] Hascalik, Ahmet, Ulaş Çaydaş, and Hakan Gürün. "Effect of traverse speed on abrasive waterjet machining of Ti–6Al–4V alloy." Materials & Design, Vol.28, No.6 (2007): pp.1953-1957.
ScienceDirect Materials Today: Proceedings 5 (2018) 13413–13421
www.materialstoday.com/proceedings
ICMMM - 2017
Experimental Investigations on Multiple Responses in Abrasive Waterjet Machining of Ti-6Al-4V Alloy A.Gnanavelbabua,*, P.Saravananb, K.Rajkumarc, S.Karthikeyand a
Associate Professor, Department of Industrial Engineering, Anna University Chennai-600025, India b Research Scholar, Department of Industrial Engineering, Anna University Chennai-600025, India c Associate Professor, Department of Mechanical Engineering, SSN College of Engineering, Kalavakkam-603110, India d Professor, Centre for Innovative Manufacturing Research, VIT University, Vellore-632014, India
Abstract In general, Ti-6Al-4V alloys are one of the challenging materials to machine using conventional machining process. Due to its high strength to weight ratio, good thermal stable property and exceptional corrosion resistance, Ti-6Al-4V became an attentiongrabbing material to propose a proper modeling and machining for various types of applications. In this research study, Ti-6Al4V was machined using Abrasive Waterjet Machining under different process parameters such as mesh size, abrasive flow rate, pressure and traverse speed. The Machining approach is based on the Box Behnken method to enhance the Abrasive waterjet machining process parameter for effective machining of Ti-6Al-4V. Then multiple responses were carried out such as kerf taper geometries (θ), surface roughness (Ra) and material removal rate (MRR). The structures of various machining surface regions were examined using Scanning electron microscopy (SEM). The experimental results specify that high pressure, low traverse speed, low abrasive mesh size and high abrasive flow rate were resulted in lower surface roughness in Ti-6Al-4V. And it was found that high pressure, high mesh size leads to minimum kerf taper ratio and whereas high traverse speed produce a maximum kerf taper. It was also evident that high abrasive flow rate, standard traverse speed and low pressure provide high MRR in selected conditions. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
Keywords: Titanium; Abrasive Waterjet machining; Kerf width, MRR, Surface roughness
1. Introduction Titanium alloys are difficult to machine using conventional machining process due to its inherent properties. The mechanical and physical property of Ti-6Al-4V includes high strength to weight ratio and better corrosion resistance [1]. Those exceptional characteristics of Ti-6Al-4V are applicable for marine industries and aircraft industries [2].
* Corresponding author. A.Gnanavelbabu, Tel.: +91-9551133779; E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).
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Titanium alloy is also used in production of transplants and entrenched implants in biomedical field [3]. Titanium material is chemically reactive towards cutting tool elements because of its low thermal conductivity and lowelasticity modulus [4]. The known complications of machining Ti-6Al-4V are tool wear concerns and cutting temperature. By these reasons during machining process, lifetime was severely affected. Consequently, proper modeling of machining and its oriented control over parameters increases rapidly [5-7]. The suitable process model to machining Titanium alloy was found to be Abrasive water jet machining, because it has no thermal influence on machining material. Abrasive water jet can machine a various range of materials such as metal matrix composites (MMC), Titanium, alloys, stainless steel, ceramics, and plastics too. The Cutting time is faster but cost per cutting is relatively high. Abrasive water jet machining is easy as friendly and it does not produce any machining dust or air pollutants. High energy of water jet and abrasive particles plays a key role in machining process. The water carrying the garnet abrasive particles acts as a coolant in AWJ technology. A small cutting force is enough for material processing with huge advantages of no thermal distortion, good flexibility and high process versatility [8, 9]. Material defects were eliminated in AWJ technology due to absence of thermal or electrical energy. The cutting performance of an abrasive water jet mainly depends on the abrasive particle’s velocities and impact angles. In AWJ technology erosion process takes place in high speed. Hitting of Abrasive particles with high traverse speed and high impact angle leads to erosion of the machining surface. There are two modes of erosion takes place by abrasive particles in AWJ machining processes. Larger particle impact angle causes deformation wear mode and smaller particle impact angle causes cutting wear mode. The quality of cutting evaluation is based on the surface roughness, waviness, angle of striations and MRR. The ultimate motto is to acquire smooth surface of cut region. This can be achieved by changing process parameters like abrasive flow rate, mesh size, pressure, traverse speed and standoff distance effectively [10-13]. The Kerf taper increases with increases in standoff distance due to widening of water jet at higher standoff distance [14]. The surface morphology was irregular in lower traverse speeds but lowest surface roughness is obtained [15]. Increase in waterjet pressure made smoother surface due to extent kinetic energy of abrasives. The surface seems to smoother at jet entrance and rougher at jet exit. Work feed rate is not a significant parameter as compared to abrasive flow rate and pressure in surface roughness response [16]. The material removal rate (MRR) increases with increase in standoff distance because of abrasive jet particles impacts deeper on work surface which creates craters [17]. Lower traverse speed also influences in higher material removal rate (MRR) and high surface waviness [18]. The depth of cut is high when abrasive flow rate, pressure, traverse speeds were kept constant [19]. By different responses with respective to process parameters, quality of cutting is highly improved by eliminating unstable errors. 2. Experimental Procedure Commercial Ti-6Al-4V was used in this AWJ experiment with a thickness of 5mm. The chemical compositions of Ti-6Al-4V are listed below in table 1. Table 1. Chemical Composition of Ti-6Al-4V Ti
Al
V
Fe
O
C
N
H
90.64
5.48
3.92
0.28
0.17
0.03
0.02
0.00583
The experiments were performed using Precision Abrasive WaterJet Machining Center manufactured by OMAX Corporation (Model: 2626) which is shown in figure 1. Garnet is used as abrasives in AWJ machining. Process parameters such as mesh size, abrasive flow rate, pressure and traverse speed were varied at three different levels using Box Behnken method. The Parameters and factors used in the machining process and its detailed values are listed below in table 2.
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Table 2. Parameters and factors Oriface Diameter Nozzle diameter Focusing tube length Focusing tube Diameter Impact angle Abrasive Type Abrasive mesh size Abrasive flow rate Pump Pressure Traverse Speed Standoff Distance
0.25 mm 0.75 mm 75mm 1mm 90° Garnet 80,100,120 # 240,340,440 g/min 125,200,275 MPa 60,90,120 mm/min 1.5 mm
Fig. 1 Precision Abrasive WaterJet Machining Center manufactured by OMAX Corporation (Model: 2626)
Fig. 2 Typical AWJM of Ti-6Al-4V alloy
The variable parameters are abrasive flow rate, mesh size, pressure and traverse speed. By using each level of process parameters, 29 cuts were executed in the titanium alloy under the appropriate environmental circumstances which is shown in figure 2. After the machining, cut surfaces were examined using Scanning electron microscopy (SEM) for microstructural analysis and Kerf width were measured using Video measurement system for kerf taper angle. Later surface roughness was measured using portable stylus-type contact roughness meter. MRR was calculated using the Kerf width, traverse speed and depth of penetration.
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3. EXPERIMENTAL RESULTS AND DISCUSSIONS Table 3. Measurement of Kerf taper angle, Surface roughness and MRR MRR (mm3/min)
Sl.No
MS (#)
AFR (g/min)
P (MPa)
TR (mm/min)
Kerf Top (mm)
Kerf Bottom (mm)
Ѳ (Degrees)
1
80
240
200
90
0.631
0.484
0.752
2.309
251.1
2
80
240
200
60
0.709
0.571
0.724
2.226
192.1
3
80
340
200
120
0.563
0.446
0.812
2.64
303.1
4
80
340
125
90
0.616
0.494
0.742
2.22
231.65
5
80
340
275
90
0.687
0.558
0.69
2.356
310.55
6
80
240
200
90
0.650
0.535
0.748
2.311
266.85
7
100
340
125
60
0.641
0.531
0.632
2.132
175.95
8
100
340
275
60
0.568
0.511
0.426
2.342
270.82
9
100
340
275
120
0.695
0.591
0.595
2.38
345.8
10
100
340
125
120
0.735
0.602
0.693
2.232
274.26
11
100
340
200
90
0.684
0.569
0.788
2.302
260.4
12
100
340
200
90
0.557
0.452
0.764
2.304
260.8
13
100
340
200
90
0.593
0.494
0.758
2.306
262.45
14
100
340
200
90
0.644
0.559
0.766
2.301
266.98
15
100
340
200
90
0.652
0.54
0.75
2.298
258.66
16
100
240
200
60
0.642
0.556
0.734
2.23
194.26
17
100
240
200
120
0.636
0.552
0.796
2.6
270.46
18
100
240
125
90
0.633
0.554
0.72
2.218
200.75
19
100
240
275
90
0.658
0.567
0.676
2.344
280.86
20
100
440
275
90
0.569
0.463
0.716
2.336
340.75
21
100
440
200
60
0.671
0.572
0.754
2.178
260.6
22
100
440
125
90
0.726
0.619
0.768
2.196
262.32
23
100
440
200
120
0.638
0.513
0.82
2.624
335.6
24
120
440
200
90
0.663
0.550
0.789
2.29
293.225
25
120
340
200
60
0.701
0.600
0.742
2.21
255.2
26
120
340
200
120
0.631
0.491
0.812
2.65
302.6
27
120
340
125
90
0.634
0.529
0.746
2.24
234.78
28
120
340
275
90
0.692
0.604
0.696
2.228
308.88
29
120
240
200
90
0.689
0.594
0.75
2.312
255.34
Ra (µm)
After the machining, cut surfaces were examined using Scanning electron microscopy (SEM) for microstructural analysis and Kerf width were measured using Video measurement system for kerf taper angle. Later surface roughness was measured using portable stylus-type contact roughness meter. MRR was calculated using the Kerf width, traverse speed and depth of penetration. The calculation of kerf taper, MRR and surface roughness was shown in above table 3. Response surface methodology approach is the technique for describing the correlation
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between different process parameters with various cutting conditions and determining the various effects of the process parameters on the mutual responses [20, 21]. 3.1. Kerf Geometry Rational Video Measurement System (VMS) 2010-F was used to measure the kerf width. The typical view kerf width is shown in figure 3. For calculating the kerf taper angle for each cut, both top kerf and bottom kerf of each cut was measured. arctan 2
Where t is the thickness or width of the material t = 5 mm; Wt = Top Kerf; Wb = Bottom Kerf
Fig. 3 Typical view of Lowest Kerf Width
The Top kerf is obviously wider than bottom kerf due to increase in pressure, which is a vital factor in Abrasive water jet machining. Due to the pressure factor, Kerf taper is produced. The largest kerf taper angle ratio affects the straightness of cutting and its results in poor dimensional quality. It is evident that high water pressure (275 MPa) and low traverse speed (60 mm/min) were produced the minimum kerf taper angle of θ = 0.426 which is shown in figure 4. When traverse speed increases, it will increase the slope of kerf wall due to abrasives strike on target which produces narrower slot. Analysis of variance (ANOVA) was used to analyze the results of Kerf taper angle which is shown in table 4. Table 4. ANOVA for Kerf Taper Angle S.No
Source
1
Model
Sum of Squares 0.14
Mean Square 9.688E-003
F Value
df 14
4.22
P- Value Prob > F 0.0055
2
A-Mesh Size
1.498E-004
1
1.498E-004
0.065
0.8021
3 4
B-Abrasive Flow rate C-Pressure
7.207E-003 0.021
1 1
7.207E-003 0.021
3.14 9.14
0.0983 0.0091
5
D-Traverse rate
0.020
1
0.020
8.69
0.0106
The Model F-value of 4.22 implies the model is significant. Values of "Prob > F" less than 0.0500 indicates that model terms are significantly contributed. In this case C (Pressure) and D (Traverse rate) are significant model terms.
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Fig. 4 3D Surface of Kerf Taper angle
3.2. Surface Roughness Roughness measurement was carried out using portable stylus-type contact roughness meter. The profilometer was calibrated with traverse speed 1mm/sec, cutoff length of 0.8 mm, and 3 mm evaluation length. Roughness measurements are taken in the transverse direction of the work pieces and it is repeated twice, and the average of two measurements of Surface Roughness (Ra) was calculated. Analysis of variance (ANOVA) was used to analyze the results of Surface roughness which is shown in table 5.The Regression model for Surface roughness in figure 5 shows the response surface graphs which are plotted to achieve minimum surface roughness with different combinations of AWJM process parameter during machining of Ti-6Al-4V. Table 5. ANOVA for Surface Roughness S.No
Source
Sum of Squares
df
Mean Square
F Value
p-value Prob > F
1
Model
0.98
14
0.070
5.58
0.0014
2
A-Mesh Size
0.010
1
0.010
0.83
0.3767
3
B-Abrasive Flow rate
7.461E-003
1
7.461E-003
0.60
0.4527
4
C-Pressure
0.54
1
0.54
43.02
< 0.0001
5
D-Traverse rate
0.12
1
0.12
9.37
0.0085
The Model F-value of 5.58 implies the model is significant. Values of "Prob > F" less than 0.0500 indicates that model terms are significantly contributed. In this case C (Pressure) and D (Traverse rate) are significant model terms. From the different combinations of process parameters, it was observed that constant increase in the traverse speed constantly decreases the surface roughness which is shown in figure 5. In other hand it is evident that lowest pressure and lowest traverse speed produces lower surface roughness. The lowest Surface roughness Ra of value 2.132µm was achieved at lowest pressure (125MPa) and lowest traverse speed (60 mm/min).
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Fig. 5 3D Surface of Surface Roughness
3.3. Material Removal Rate Material removal rate (MRR) was calculated using Kerf width, traverse speed and depth of penetration. Moreover standoff distance plays a major role in material removal rate (MRR). Material removal rate increases with standoff distance, because of penetration depth creating craters in working surface. Material removal was calculated using following formula, Material Removal Rate (MRR) = ht.W.Vt W= Kerf Width = (Wt + Wb)/2 Vt= Traverse speed (m/min); Wt = Top Kerf; Wb = Bottom Kerf ht= Depth of Penetration (ht=5mm) Analysis of variance (ANOVA) was used to analyze the results of Material removal rate which is shown in table 6. Table 6. ANOVA for MRR S.No
Source
Sum of Squares
df
Mean Square
F Value
p-value Prob > F
1
Model
48854.82
14
3489.63
37.51
< 0.0001
2
A-Mesh Size
16.62
1
16.62
0.18
0.6790
3
7850.98
1
7850.98
84.40
< 0.0001
4
B-Abrasive Flow rate C-Pressure
19036.35
1
19036.35
204.64
< 0.0001
5
D-Traverse rate
16606.34
1
16606.34
178.52
< 0.0001
The Model F-value of 37.51 implies the model is significant. Values of "Prob > F" less than 0.0500 indicates that model terms are significantly contributed. In this case, B (Abrasive flow rate), C (Pressure) and D (Traverse rate) are significant model terms. It is obviously evident that MRR is high at high pressure (275 MPa) and high traverse speed (120 mm/min) which is shown in figure 6.
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Fig. 6 3D Surface of Material Removal Rate
3.4. Surface Morphology The machined surface of samples having lower roughness and higher roughness under 50µm in SEM was shown in figure 7 and figure 8. There are two distinct zones were found as: Smooth machining region (SMR) which is present in lower roughness sample and Rough machining region (RMR) which is present in higher roughness sample. Smooth machining region was obtained at higher cutting angle zone and whereas rough machining region was obtained at reversal jet deflection [22]. The width of the smooth machined region increases with higher traverse rate, because of higher depth of penetration. The cutting wear and deformation wear found as a scar in rough cutting region due to high pressure and low traverse speed.
Fig. 7 Cut Surface of Lower Ra
Fig. 8 Cut Surface of higher Ra
4. Conclusions In this research, Kerf geometry, surface roughness (Ra), microstructural analyses and material removal rate (MRR) of Ti-6Al-4v were investigated using Abrasive water jet machining. By the summarized results from above responses, the following conclusions was made and recommended.
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1. Kerf Geometry shows that kerf width gets narrower with increase in traverse speed. This is due to abrasive particles hitting on jet target to produce a narrower slot. Kerf taper ratio varies for increasing traverse speed and lowest kerf taper was obtained for lowest traverse speed (60 mm/min). 2. Surface Roughness shows that Ra is lower when pressure (125 MPa) is low and traverse speed (60 mm/min) is low. Surface quality was affected at highest pressure, because kinetic energy of jet reduced with material-jet interaction and influence of abrasive particles. 3. Material Removal Rate (MRR) analysis shows that MRR increases when pressure (275 MPa) is high and traverse speed (120 mm/min) is high. 4. Microstructural analyses of machined surfaces revealed two regions: 1. Smooth machining Region (SMR) and Rough machining Region (RMR). Smooth machining region is found at higher cutting angle zone and Rough machining region is found at jet reversal deflection. References [1] Barry, J., G. 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