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Machining characteristics of Inconel 718 using ultrasonic and high temperature-aided cutting
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 359–365
journal homepage: www.elsevier.com/locate/jmatprotec
Machining characteristics of Inconel 718 using ultrasonic and high temperature-aided cutting C.Y. Hsu a,∗ , Y.Y. Lin b , W.S. Lee b , S.P. Lo b a
Department of Mechanical Engineering, Lunghwa University of Science and Technology, No. 300, Sec. 1, Wanshou Road, Guishan Shiang, Taoyuan 33306, Taiwan, ROC b Department of Mechanical Engineering, De Lin Institute of Technology, No. 1, Lane 380, Qingyun Road, Tucheng City, Taipei County 236, Taiwan, ROC
a r t i c l e
i n f o
a b s t r a c t
Article history:
This investigation discusses the machining characteristics of Inconel 718 by combining
Received 2 September 2005
ultrasonic vibration with high-temperature-aided cutting. Taguchi experimental design was
Received in revised form
used to clarify the influence of various machining parameters on the machining charac-
29 March 2007
teristics. This study discusses six machining parameters, including cutting tool made of
Accepted 6 July 2007
different materials, depth of cut, cutting speed, feed rate, working temperature and ultrasonic power. The machining characteristics studied include surface roughness and cutting force. According to the experimental results, as Inconel 718 is heated for high-temperature
Keywords:
(190 ◦ C) cutting, cutting becomes increasingly difficult owing to precipitating the hard sec-
Inconel 718
ondary phase of ␥ (Ni3 Nb), thus the cutting force is higher than that at room temperature.
Ultrasonic vibration
Formation of built-up-edge (BUE) when using a NX2525 cermet cutter is far less than that of
Taguchi method
a TN35 tungsten carbide cutter, the surface roughness and the cutting force are lower than
BUE
those for a TN35 tungsten carbide cutter. As a result, the NX2525 cermet cutter provides a more suitable cutting tool for Inconel 718. Furthermore, this investigation also found that when being aided by ultrasonic vibration in the tangential direction, it could obtain a lower surface roughness of the machined workpiece and a reduced cutting force, hence service life of the cutter was lengthened. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Nickel-base superalloy Inconel 718 is a high-strength and thermal-resistant material. Because of its excellent mechanical properties at low and intermediate temperatures (−250 to 700 ◦ C), it plays an important part in recent years in aerospace, petroleum and nuclear energy industries. It is also noted for its excellent corrosion resistance. Nickel-base alloy would appear to be difficult to machine because of a tendency of the maximum temperature of tool face existing at the tip of the tool. Micro-welding at the tool tip and chip interface takes place so
∗
as to lead to the built-up-edge formation (in short, BUE) (Chen and Liao, 2003). Due to precipitating the hard secondary phase of ␥ (Ni3 Nb) during machining, it makes the cutting condition even worse. All these difficulties lead to serious tool wear and less material removal rate (Rahman et al., 1997; Choudhury and El-Baradie, 1998). Ultrasonic vibration has been extensively adopted in manufacturing processes. Weber et al. (Weber et al., 1984) used high-frequency vibration (20 kHz) in the radial and cutting directions to cut steel materials. They found that this approach could increase tool life. Moreover, Wang and Zhao
Corresponding author. Fax: +886 2 82094845. E-mail addresses: [email protected], [email protected] (C.Y. Hsu). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.07.015
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Table 1 – Cutting tools Type
Material
TN35
Fig. 1 – Three directions of the ultrasonic vibration cutting system.
(Wang and Zhao, 1987) applied high-frequency vibrations (16 kHz) to improve surface roughness, reduce micro-cracks on the workpiece surface and increase cutting stability. Additionally, Liu et al. (2002) proposed to apply the ultrasonic-aided vibration to the machining of SiCp/Al thin-wall parts for precision processing. Compared with conventional cutting, cutting force is lower in this method and BUE does not occur. The analytical results presented by Babitsky et al. (2003) demonstrated that ultrasonic vibration is applied to aid the machining of aerospace material of Inconel 718 and it can improve the surface roughness and roundness. The ultrasonic vibration-aided turning was shown in Fig. 1 (Wang and Zhao, 1987; Liu et al., 2002) where three directions were employed. Direction 1 was feed direction of vibration cutting. Although there are effects of tool tip sharpening and cutting force reduction in vibration cutting, the rapid friction of tool tip against the workpiece results in a shorter tool life. Meanwhile, Direction 2 was radial direction of vibration cutting, which was characterized by reduced cutting force and also resulted in a short tool life. Finally, Direction 3 was tangential direction of vibration cutting, which had the advantage of reduced cutting force, lower cutting temperature, lower BUE and enhanced workpiece surface quality. This study considers tangential direction as the direction of ultrasonic vibrationaided cutting. Tests have been made in several laboratories to determine the practicability of artificially heating the surface of the workpiece just before a cut is taken. While the method does not appear to be of general interest, it may prove to be of particular value in the machining of high-temperature alloys where strain hardening is serious (Shaw, 1996). In this study, gas torch
Comparison according to ISO
Insert spec.
P20–P40
CCMT09T304
CT3000
Carbide coating Al2 O3 Cermet
P05–P15 M05–M15 K05–K15
CCMT09T304 FG
NX2525
Cermet
P01–P20 K01–K20
CCMT09T304MW
was employed to heat the workpiece in order to supply enough heat to the surface so as to raise the temperature of a layer of surface being approximately equal to the depth of cut.
2.
Cutting parameters and their levels
This work investigates the machining characteristics of Inconel 718 using ultrasonic and high-temperature-aided cutting by using a low-cost conventional lathe, and uses the information obtained to improve the machining efficiency. Based on probes into the relevant literatures (Liu et al., 2002; Babitsky et al., 2003) and practical cutting tests, this study selected three cutting tools, TN35 (carbide coating Al2 O3 ), CT3000 (cermet, bending-resistance strength is 1.59 GPa) and NX2525 (cermet, bending-resistance strength is 2.0 GPa), as listed in Table 1. A dynamometer of Kistler type 9257A was set below the tool to record the cutting forces of the three axes during the cutting process. Taguchi methods which combine the experiment design theory and the quality loss function concept have been applied to the robust design of products and process and have solved some confusing problems in manufacturing. Orthogonal array is one of the important tools used in the experimental design of Taguchi method. An L18 (36 ) was chosen for the experimental tests because it has a good even distribution of factorial interactions over the control factors (Ross, 1988). This experiment selects six influential cutting parameters, such as cutting tool made of different materials, depth of cut, cutting speed, feed rate, working temperature and ultrasonic power, each of which is assigned high, medium and low levels, as shown in Table 2. The machining characteristics studied include surface roughness and cutting force. By adjusting the experimental combinations and analyzing the experimental results, this work seeks the optimal cutting parameters.
Table 2 – Setting of factors and levels in experiment Symbol T D S F Temp. P
Machining factor
Level 1
Level 2
Level 3
Cutter Depth of cut Cutting speed Feed rate Working temperature Ultrasonic power 20 kHz
TN35 0.1 mm 22 m/min 0.054 mm/rev 25 ◦ C 140 W
CT3000 0.2 mm 43 m/min 0.103 mm/rev 105 ◦ C 160 W
NX2525 0.3 mm 73 m/min 0.147 mm/rev 190 ◦ C 180 W
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Table 3a – Experimental results for surface roughness and S/N ratio Experiment no.
Control factors
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
T
D
S
F
1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 2 3 1 3 1 2 2 3 1 3 1 2
1 2 3 2 3 1 1 2 3 3 1 2 3 1 2 2 3 1
Surface roughness (m) Temp. 1 2 3 2 3 1 3 1 2 2 3 1 1 2 3 3 1 2
P
Ra1
Ra2
Ra3
1 2 3 3 1 2 2 3 1 2 3 1 3 1 2 1 2 3
0.30 0.85 0.52 0.91 0.82 0.34 0.28 0.22 0.32 0.92 0.27 0.35 0.60 0.57 0.35 0.32 0.36 0.17
0.27 0.87 0.60 0.62 0.52 0.32 0.25 0.27 0.30 0.87 0.24 0.30 0.90 0.42 0.48 0.25 0.35 0.20
0.25 0.67 0.50 0.55 0.66 0.25 0.30 0.17 0.35 0.90 0.27 0.60 0.77 0.70 0.45 0.30 0.32 0.15
S/N (dB)
11.24 1.92 5.32 2.97 3.38 10.29 11.14 13.00 9.79 0.95 11.69 7.19 2.31 4.81 7.33 10.71 9.28 15.16
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
Table 3b – S/N response table for surface roughness
3. Analysis and discussion of experimental results 3.1.
Level
Factors
Analysis of the S/N ratio
Taguchi method was used for the plan of experiment as it employs a generic signal-to-noise (S/N) ratio to quantify the present variation. Depending on the particular type of characteristics involved, different S/N ratios may be applicable, including “lower is better” (LB), “nominal is best” (NB), and “higher is better” (HB). The S/N ratios were calculated using the following equations (Wang and Yan, 2000): = 10 log(S/N ratio)
HB :
1 (S/N ratio) = 2 ,
LB :
(S/N ratio) =
1 , 2
(1) 1 = n 2
2 =
1 y21
+
1 y22
+ ··· +
1 2 (y + y22 + . . . + y2n ) n 1
1 y2n
(2)
1 2 3 Level effect
T
D
S
F
Temp.
P
6.39 5.18 11.51 6.33
6.55 7.35 9.18 2.63
8.71 6.85 7.51 1.86
10.72 7.19 5.17 5.55
8.89 5.93 8.26 2.95
7.85 6.82 8.41 1.59
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
To discuss the influence of ultrasonic-aided cutting onto surface roughness of the workpiece, the experimental conditions of No. 10 and No. 18 in Table 3a were chosen, which represent the maximum and minimum surface roughness combinations, respectively. In Table 3c, No. 10 and No. 18, No. 10-1 and No. 18-1 are the experimental conditions with and without ultrasonic-aided cutting, and the resulting average
(3)
where yn is the characteristic property, and n is the repeated number of the experiment, denotes the observed value, i.e., the calculated value of the S/N ratio. The unit of S/N ratio is decibel (dB), which is frequently used in communication engineering. Table 3a lists the experimental results for surface roughness and the corresponding signal-noise ratios (S/N) using Eqs. (1) and (3). The mean S/N ratio for each level of the cutting parameters is summarized and called the S/N response table for surface roughness. The experiment is calculated and illustrated in Table 3b. Fig. 2 shows the S/N response graph for surface roughness and indicates that better surface roughness can be obtained by using NX2525 cermet cutting tools and a smaller feed rate.
Fig. 2 – S/N graph for surface roughness.
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Table 3c – Analysis of surface roughness with and without ultrasonic-aided cutting Experiment no.
10 10-1 18 18-1
Factors T
D
S
F
1 1 3 3
1 1 3 3
3 3 2 2
3 3 1 1
Surface roughness (m) Temp. 2 2 2 2
P
Ra1
Ra2
Ra3
2
0.92 0.98 0.17 0.25
0.87 0.89 0.20 0.37
0.90 1.10 0.15 0.31
3
Mean value (m)
0.90 0.99 0.17 0.31
Improvement rate (%)
9.10 51.61
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
surface roughness values are 0.90 and 0.17, 0.99 and 0.31 m, respectively. It is indicated that as using ultrasonic aid cutting, the surface roughness improves 9.10% and 51.61%. Moreover, Fig. 3(a)–(d) illustrate the surface textures with and without ultrasonic aid cutting. Fig. 3(a) and (c) reveal finer and smoother tool marks on the workpiece surface, and obtain a lower surface roughness of the machined workpiece under the condition of ultrasonic-aided cutting. The result corresponds to the experimental result of Wang and Zhao (Wang and Zhao, 1987), that is, turning with ultrasonic vibration may decreases the bump effect of the side flow, therefore, it can improves the surface finish of the machined surface. During the experiment, a dynamometer is used to measure the cutting force, which is expressed by the following equation (4):
R=
Fx2 + Fy2 + Fz2
(4)
where R denotes the cutting force, and is the resultant force of Fx , Fy and Fz . Meanwhile, Fx , Fy and Fz represent the force components of the tangential, radial and feed directions. Table 4a displays the experimental results for cutting force R and the corresponding S/N ratios. Additionally, the mean S/N
ratio for each level of the cutting parameters is summarized and called the S/N response table for cutting force. The experiment is also calculated and illustrated in Table 4b. Moreover, Fig. 4 shows the S/N response graph for cutting force. By using smaller depth of cut, lower cutting speed and lower feed rate, the cutting force could be reduced. To discuss further the effect of ultrasonic-aided cutting onto cutting force, the experimental conditions of No. 3 and No. 1 in Table 4a were chosen, which represent the maximum and minimum cutting force, respectively. In Table 4c, No. 3 and No. 1, No. 3-1 and No. 1-1 are the experimental conditions with and without ultrasonic-aided cutting, and the resulting mean cutting force values are 704.31 and 24.29, 1041.02 and 32.16 N, respectively. It is indicated that as using ultrasonic-aided cutting, the cutting force reduces by 32.34% and 24.47%. Generally, most materials soften with heating to high temperatures. However, Inconel 718 precipitates a hard second phase of ␥ (Ni3 Nb) at an appropriate temperature to increase the difficulty of cutting. Observing the factor of working temperature in Fig. 4, it reveals that when a workpiece is heated for high-temperature cutting, the cutting force increases with the temperature (190 ◦ C). Since the BUE grow outward and downward, this gives rise to a variation in depth of the cutting surface. It represents a
Fig. 3 – Comparisons of surface roughness with and without ultrasonic-aided cutting.
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Table 4a – Experimental results for cutting force and S/N ratio Experiment no.
Control factors
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
T
D
S
F
1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 2 3 1 3 1 2 2 3 1 3 1 2
1 2 3 2 3 1 1 2 3 3 1 2 3 1 2 2 3 1
Cutting force (N) Temp. 1 2 3 2 3 1 3 1 2 2 3 1 1 2 3 3 1 2
S/N (dB)
P
R1
R2
R3
1 2 3 3 1 2 2 3 1 2 3 1 3 1 2 1 2 3
26.00 92.97 689.42 44.11 123.58 99.65 64.00 132.33 99.78 157.40 54.97 159.20 73.42 73.79 81.61 112.58 61.91 64.35
24.11 88.34 709.20 41.69 116.15 100.07 39.05 135.68 87.87 188.47 59.86 141.96 81.10 75.84 89.70 140.44 64.87 64.62
22.77 93.65 714.31 40.33 134.58 105.24 59.03 148.48 97.40 187.93 72.42 170.72 76.03 83.46 90.15 141.51 67.80 65.06
−27.72 −39.25 −56.96 −32.48 −41.94 −40.15 −34.82 −42.86 −39.57 −45.03 −35.97 −43.96 −37.72 −37.82 −38.81 −42.42 −36.25 −36.22
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
Table 4b – S/N response table for cutting force Level
Factors T
1 2 3 Level effect
D
−41.48 −38.15 −38.69 3.33
S
F
−36.70 −35.13 −39.01 −38.98 −42.61 −44.21 5.91 9.07
Temp.
−35.45 −39.96 −42.91 7.46
−38.11 −38.39 −41.82 3.71
P −38.91 −39.05 −40.37 1.46
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
major component of surface roughness when cutting with a BUE (Shaw, 1996). The phenomenon of BUE occurs at TN35 tungsten carbide tool as shown in Fig. 5(a) and (b), but not found at NX2525 cermet tool as illustrated in Fig. 5(c)–(e). Comparing Fig. 5(a) with (e), they are at same cutting condition, but Fig. 5(a) using cutting tool TN35 (carbide coating Al2 O3 ) and Fig. 5(e) using NX2525 (cermet). No BUE occurred in Fig. 5(e) is because the cermet tool NX2525 has more refractory and higher hardness, also low affinity to Inconel 718. Due to no BUE in the NX2525 cermet tool, it can obtain a low surface roughness as shown in Fig. 2 and reduce the cutting force in Fig. 4. As a result, NX2525 cermet tool is suitable for the machining of Inconel 718.
Fig. 4 – S/N graph for cutting force.
3.2.
Analysis of variance
After the construction of factor response table, an analysis of variance (ANOVA) table can be established based on the sum of the square (SSi ), the degree of freedom (DoFi ), the variance (Vari ) and the experimental error. The variance ratio, F ratio, is
Table 4c – Analysis of cutting forces with and without ultrasonic-aided cutting Experiment no.
3 3-1 1 1-1
Factors T
D
S
F
1 1 1 1
3 3 1 1
3 3 1 1
3 3 1 1
Cutting force (N) Temp. 3 3 1 1
P
R1
R2
3
689.42 1230.56 26.00 29.54.
709.20 980.37 24.11 34.47
1
Mean value (N)
Improvement rate (%)
R3 714.31 912.13 24.77 32.48
704.31 1041.02 24.29 32.16
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
32.34 24.47
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then evaluated from the variance of each control factor (Vari ) and the variance of error factor (Vare ). Finally, the expected value of the sum of square (SSi ) and the total square sum (SSt ) are used to calculate the percent contribution (Pi ) of various control factors and error factors (Pe ). If the percent contribution of the error factor, Pe , falls below 15%, it implies that the quality characteristics of the experiment are under a precise control. In contrast, if the percent contribution of the error factor exceeds 50%, it means that certain significant factor is overlooked and the experiments must be reviewed again (Lin and Chang, 2003). Table 5 is the ANOVA table for surface roughness and cutting force. For the surface roughness of the workpiece, all the F ratio of the six control factors achieved the confidence level of 99% [F(0.01, 2, 41 = 5.2)] in F test. It can be found that cutting tool and feed rate are the significant cutting parameters which affect surface roughness. For the cutting force, the confidence level of the F test of the six control factors also reached 99%. Cutting speed, feed rate, cutting tools, and depth of cut are the significant cutting parameters. From the above analysis, the percent contribution of error factor, Pe , of the two machining characteristics were approximately 15% (11.6% and 15.8%, respectively). This indicated that the experimental plan in this study was feasible, that no important factor has been ignored and that the analysis results were reliable.
3.3.
Confirmation test
Once the optimal level of the design parameters has been selected, the final step is to predict and verify the improvement of the quality characteristic using the optimal level of the design parameters. The estimated S/N ratio Ypredicted using the optimal level of the design parameters can be calculated as
Ypredicted = Ym +
k
(Yi − Ym )
(5)
i=1
where Ym is the total mean S/N ratio, Yi the mean S/N ratio at the optimal level, and k is the number of the main design parameters that affect the quality characteristics. The measuring data and the actual S/N ratio of confirmation experiments are listed in Table 6. The largest S/N ratios in the experiments were 15.16 (Table 3a, surface roughness) and −27.72 (Table 4a, cutting force), respectively. However, the S/N ratios of confirmation experiments are 16.25 and −24.98, and both are larger than the above two. In addition, the S/N prediction, 18.25 and −25.25, are very close to the actual value. This indicated that the experiments in this study possess excellent repetitiveness and great potential for future references.
Fig. 5 – Comparison of the BUE phenomenon in using TN35 (carbide coating Al2 O3 ) and NX2525 (cermet) as the cutting tool for Inconel 718.
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Table 5 – Results of the ANOVA for the surface roughness and the cutting force Factor
T D S F Temp. P Error Total
Surface roughness
Cutting force
SS
DoF
Var
F ratio
(%)
SS
0.9425 0.2607 0.1510 0.7108 0.3799 0.0720 0.3252 2.8422
2 2 2 2 2 2 41 53
0.4712 0.1304 0.0755 0.3354 0.1900 0.0360 0.0079
59.64 16.50 9.55 44.99 24.05 4.55
33.16 9.17 5.31 25.00 13.36 2.53 11.44 100
158325.01 153338.58 255548.71 193616.20 123187.39 82015.65 181247.57 1147279.12
Var
F ratio
(%)
79162.51 76669.29 127774.36 96808.10 61593.69 41007.83 4420.67
17.9 17.3 28.9 21.9 13.9 9.3
13.80 13.37 22.27 16.88 10.73 7.15 15.79 100
DoF 2 2 2 2 2 2 41 53
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power; , contribution. The F ratios according to the F distribution table are F(0.01, 2, 41) = 5.2.
Table 6 – Confirmation experiment for the surface roughness and the cutting force Control factors T Surface roughness Cutting force
NX2525 CT3000
D (mm) 0.3 0.1
S (m/min) 22 22
S/N (dB) prediction
F (mm/rev)
Temp. (◦ C)
P (W)
0.054 0.054
25 25
180 140
Confirmation experiments Measuring data
18.25 −25.25
0.15 19.07
0.17 17.88
0.14 16.16
Mean value 0.15 17.70
S/N (dB) 16.25 −24.98
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
4.
Conclusions
This paper has discussed an application of the Taguchi method for optimizing the cutting parameters in turning Inconel 718. The conclusions of this study may be summarized as follows: 1. NX2525 cermet cutting tool produces almost no BUE and achieves the best surface roughness. Moreover, the measured cutting forces are relatively small. 2. When the workpiece underwent high-temperature cutting (190 ◦ C), owing to the hard secondary phase of ␥ (Ni3 Nb) precipitating, cutting becomes increasingly difficult, and thus the cutting force is higher than that at room temperature. 3. The percentage contributions of the cutting tool, feed rate, working temperature and depth of cut for surface roughness are 33.16, 25.00, 13.36 and 9.17, respectively. 4. The percentage contributions of the cutting speed, feed rate, cutting tool and depth of cut for cutting force are 22.27, 16.88, 13.80 and 13.37, respectively. 5. Ultrasonic-aided cutting improved the surface roughness by 9.10–51.61%, as well as decreasing cutting force by 32.34–24.47%. As a result, ultrasonic-aided cutting can enhance the cutting quality of Inconel 718.
references
Babitsky, V.I., Kalashnikov, A.N., Meadow, A., Wijesundara, A.A.H.P., 2003. Ultrasonic assisted turning of aviation materials. Int. J. Adv. Manuf. Technol. 132, 157–167. Chen, Y.C., Liao, Y.S., 2003. Study on wear mechanisms in drilling of Inconel 718 superalloy. J. Mater. Process. Technol. 140, 269–273. Choudhury, I.A., El-Baradie, M.A., 1998. Machinability of nickel-base super alloys: a general review. J. Mater. Process. Technol. 77, 278–284. Lin, Z.C., Chang, D.Y., 2003. Tool wear investigation on the precision progressive die for the IC dam-bar cutting process. Int. J. Adv. Manuf. Technol. 22, 344–356. Liu, C.S., Zhao, B., Gao, G.F., Jiao, F., 2002. Research on the characteristics of the cutting force in the vibration cutting of a particle-reinforced metal matrix composites SiCp/Al. J. Mater. Process. Technol. 129, 196–199. Rahman, M., Seah, W.K.H., Teo, T.T., 1997. The machinability of Inconel 718. J. Mater. Process. Technol. 63, 199–204. Ross, P.J., 1988. Taguchi Techniques for Quality Engineering. Mcgraw-Hill, New York. Shaw, M.C., 1996. Metal Cutting Principles. Oxford Science Publications. Wang, C.C., Yan, B.H., 2000. Blind-hole drilling of Al2 O3 /6061 Al composite using rotary electro-discharge machining. J. Mater. Process. Technol. 102, 90–102. Wang, L.J., Zhao, J., 1987. Influence on surface roughness in turning with ultrasonic vibration tool. Int. J. Mach. Tools Manuf. 27 (2), 181–190. Weber, H., Herberger, J., Pilz, R., 1984. Turning of machinable glass ceramics with an ultrasonic vibration tool. Ann. CIRP 33 (1), 85–87.
journal homepage: www.elsevier.com/locate/jmatprotec
Machining characteristics of Inconel 718 using ultrasonic and high temperature-aided cutting C.Y. Hsu a,∗ , Y.Y. Lin b , W.S. Lee b , S.P. Lo b a
Department of Mechanical Engineering, Lunghwa University of Science and Technology, No. 300, Sec. 1, Wanshou Road, Guishan Shiang, Taoyuan 33306, Taiwan, ROC b Department of Mechanical Engineering, De Lin Institute of Technology, No. 1, Lane 380, Qingyun Road, Tucheng City, Taipei County 236, Taiwan, ROC
a r t i c l e
i n f o
a b s t r a c t
Article history:
This investigation discusses the machining characteristics of Inconel 718 by combining
Received 2 September 2005
ultrasonic vibration with high-temperature-aided cutting. Taguchi experimental design was
Received in revised form
used to clarify the influence of various machining parameters on the machining charac-
29 March 2007
teristics. This study discusses six machining parameters, including cutting tool made of
Accepted 6 July 2007
different materials, depth of cut, cutting speed, feed rate, working temperature and ultrasonic power. The machining characteristics studied include surface roughness and cutting force. According to the experimental results, as Inconel 718 is heated for high-temperature
Keywords:
(190 ◦ C) cutting, cutting becomes increasingly difficult owing to precipitating the hard sec-
Inconel 718
ondary phase of ␥ (Ni3 Nb), thus the cutting force is higher than that at room temperature.
Ultrasonic vibration
Formation of built-up-edge (BUE) when using a NX2525 cermet cutter is far less than that of
Taguchi method
a TN35 tungsten carbide cutter, the surface roughness and the cutting force are lower than
BUE
those for a TN35 tungsten carbide cutter. As a result, the NX2525 cermet cutter provides a more suitable cutting tool for Inconel 718. Furthermore, this investigation also found that when being aided by ultrasonic vibration in the tangential direction, it could obtain a lower surface roughness of the machined workpiece and a reduced cutting force, hence service life of the cutter was lengthened. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Nickel-base superalloy Inconel 718 is a high-strength and thermal-resistant material. Because of its excellent mechanical properties at low and intermediate temperatures (−250 to 700 ◦ C), it plays an important part in recent years in aerospace, petroleum and nuclear energy industries. It is also noted for its excellent corrosion resistance. Nickel-base alloy would appear to be difficult to machine because of a tendency of the maximum temperature of tool face existing at the tip of the tool. Micro-welding at the tool tip and chip interface takes place so
∗
as to lead to the built-up-edge formation (in short, BUE) (Chen and Liao, 2003). Due to precipitating the hard secondary phase of ␥ (Ni3 Nb) during machining, it makes the cutting condition even worse. All these difficulties lead to serious tool wear and less material removal rate (Rahman et al., 1997; Choudhury and El-Baradie, 1998). Ultrasonic vibration has been extensively adopted in manufacturing processes. Weber et al. (Weber et al., 1984) used high-frequency vibration (20 kHz) in the radial and cutting directions to cut steel materials. They found that this approach could increase tool life. Moreover, Wang and Zhao
Corresponding author. Fax: +886 2 82094845. E-mail addresses: [email protected], [email protected] (C.Y. Hsu). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.07.015
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Table 1 – Cutting tools Type
Material
TN35
Fig. 1 – Three directions of the ultrasonic vibration cutting system.
(Wang and Zhao, 1987) applied high-frequency vibrations (16 kHz) to improve surface roughness, reduce micro-cracks on the workpiece surface and increase cutting stability. Additionally, Liu et al. (2002) proposed to apply the ultrasonic-aided vibration to the machining of SiCp/Al thin-wall parts for precision processing. Compared with conventional cutting, cutting force is lower in this method and BUE does not occur. The analytical results presented by Babitsky et al. (2003) demonstrated that ultrasonic vibration is applied to aid the machining of aerospace material of Inconel 718 and it can improve the surface roughness and roundness. The ultrasonic vibration-aided turning was shown in Fig. 1 (Wang and Zhao, 1987; Liu et al., 2002) where three directions were employed. Direction 1 was feed direction of vibration cutting. Although there are effects of tool tip sharpening and cutting force reduction in vibration cutting, the rapid friction of tool tip against the workpiece results in a shorter tool life. Meanwhile, Direction 2 was radial direction of vibration cutting, which was characterized by reduced cutting force and also resulted in a short tool life. Finally, Direction 3 was tangential direction of vibration cutting, which had the advantage of reduced cutting force, lower cutting temperature, lower BUE and enhanced workpiece surface quality. This study considers tangential direction as the direction of ultrasonic vibrationaided cutting. Tests have been made in several laboratories to determine the practicability of artificially heating the surface of the workpiece just before a cut is taken. While the method does not appear to be of general interest, it may prove to be of particular value in the machining of high-temperature alloys where strain hardening is serious (Shaw, 1996). In this study, gas torch
Comparison according to ISO
Insert spec.
P20–P40
CCMT09T304
CT3000
Carbide coating Al2 O3 Cermet
P05–P15 M05–M15 K05–K15
CCMT09T304 FG
NX2525
Cermet
P01–P20 K01–K20
CCMT09T304MW
was employed to heat the workpiece in order to supply enough heat to the surface so as to raise the temperature of a layer of surface being approximately equal to the depth of cut.
2.
Cutting parameters and their levels
This work investigates the machining characteristics of Inconel 718 using ultrasonic and high-temperature-aided cutting by using a low-cost conventional lathe, and uses the information obtained to improve the machining efficiency. Based on probes into the relevant literatures (Liu et al., 2002; Babitsky et al., 2003) and practical cutting tests, this study selected three cutting tools, TN35 (carbide coating Al2 O3 ), CT3000 (cermet, bending-resistance strength is 1.59 GPa) and NX2525 (cermet, bending-resistance strength is 2.0 GPa), as listed in Table 1. A dynamometer of Kistler type 9257A was set below the tool to record the cutting forces of the three axes during the cutting process. Taguchi methods which combine the experiment design theory and the quality loss function concept have been applied to the robust design of products and process and have solved some confusing problems in manufacturing. Orthogonal array is one of the important tools used in the experimental design of Taguchi method. An L18 (36 ) was chosen for the experimental tests because it has a good even distribution of factorial interactions over the control factors (Ross, 1988). This experiment selects six influential cutting parameters, such as cutting tool made of different materials, depth of cut, cutting speed, feed rate, working temperature and ultrasonic power, each of which is assigned high, medium and low levels, as shown in Table 2. The machining characteristics studied include surface roughness and cutting force. By adjusting the experimental combinations and analyzing the experimental results, this work seeks the optimal cutting parameters.
Table 2 – Setting of factors and levels in experiment Symbol T D S F Temp. P
Machining factor
Level 1
Level 2
Level 3
Cutter Depth of cut Cutting speed Feed rate Working temperature Ultrasonic power 20 kHz
TN35 0.1 mm 22 m/min 0.054 mm/rev 25 ◦ C 140 W
CT3000 0.2 mm 43 m/min 0.103 mm/rev 105 ◦ C 160 W
NX2525 0.3 mm 73 m/min 0.147 mm/rev 190 ◦ C 180 W
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Table 3a – Experimental results for surface roughness and S/N ratio Experiment no.
Control factors
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
T
D
S
F
1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 2 3 1 3 1 2 2 3 1 3 1 2
1 2 3 2 3 1 1 2 3 3 1 2 3 1 2 2 3 1
Surface roughness (m) Temp. 1 2 3 2 3 1 3 1 2 2 3 1 1 2 3 3 1 2
P
Ra1
Ra2
Ra3
1 2 3 3 1 2 2 3 1 2 3 1 3 1 2 1 2 3
0.30 0.85 0.52 0.91 0.82 0.34 0.28 0.22 0.32 0.92 0.27 0.35 0.60 0.57 0.35 0.32 0.36 0.17
0.27 0.87 0.60 0.62 0.52 0.32 0.25 0.27 0.30 0.87 0.24 0.30 0.90 0.42 0.48 0.25 0.35 0.20
0.25 0.67 0.50 0.55 0.66 0.25 0.30 0.17 0.35 0.90 0.27 0.60 0.77 0.70 0.45 0.30 0.32 0.15
S/N (dB)
11.24 1.92 5.32 2.97 3.38 10.29 11.14 13.00 9.79 0.95 11.69 7.19 2.31 4.81 7.33 10.71 9.28 15.16
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
Table 3b – S/N response table for surface roughness
3. Analysis and discussion of experimental results 3.1.
Level
Factors
Analysis of the S/N ratio
Taguchi method was used for the plan of experiment as it employs a generic signal-to-noise (S/N) ratio to quantify the present variation. Depending on the particular type of characteristics involved, different S/N ratios may be applicable, including “lower is better” (LB), “nominal is best” (NB), and “higher is better” (HB). The S/N ratios were calculated using the following equations (Wang and Yan, 2000): = 10 log(S/N ratio)
HB :
1 (S/N ratio) = 2 ,
LB :
(S/N ratio) =
1 , 2
(1) 1 = n 2
2 =
1 y21
+
1 y22
+ ··· +
1 2 (y + y22 + . . . + y2n ) n 1
1 y2n
(2)
1 2 3 Level effect
T
D
S
F
Temp.
P
6.39 5.18 11.51 6.33
6.55 7.35 9.18 2.63
8.71 6.85 7.51 1.86
10.72 7.19 5.17 5.55
8.89 5.93 8.26 2.95
7.85 6.82 8.41 1.59
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
To discuss the influence of ultrasonic-aided cutting onto surface roughness of the workpiece, the experimental conditions of No. 10 and No. 18 in Table 3a were chosen, which represent the maximum and minimum surface roughness combinations, respectively. In Table 3c, No. 10 and No. 18, No. 10-1 and No. 18-1 are the experimental conditions with and without ultrasonic-aided cutting, and the resulting average
(3)
where yn is the characteristic property, and n is the repeated number of the experiment, denotes the observed value, i.e., the calculated value of the S/N ratio. The unit of S/N ratio is decibel (dB), which is frequently used in communication engineering. Table 3a lists the experimental results for surface roughness and the corresponding signal-noise ratios (S/N) using Eqs. (1) and (3). The mean S/N ratio for each level of the cutting parameters is summarized and called the S/N response table for surface roughness. The experiment is calculated and illustrated in Table 3b. Fig. 2 shows the S/N response graph for surface roughness and indicates that better surface roughness can be obtained by using NX2525 cermet cutting tools and a smaller feed rate.
Fig. 2 – S/N graph for surface roughness.
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Table 3c – Analysis of surface roughness with and without ultrasonic-aided cutting Experiment no.
10 10-1 18 18-1
Factors T
D
S
F
1 1 3 3
1 1 3 3
3 3 2 2
3 3 1 1
Surface roughness (m) Temp. 2 2 2 2
P
Ra1
Ra2
Ra3
2
0.92 0.98 0.17 0.25
0.87 0.89 0.20 0.37
0.90 1.10 0.15 0.31
3
Mean value (m)
0.90 0.99 0.17 0.31
Improvement rate (%)
9.10 51.61
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
surface roughness values are 0.90 and 0.17, 0.99 and 0.31 m, respectively. It is indicated that as using ultrasonic aid cutting, the surface roughness improves 9.10% and 51.61%. Moreover, Fig. 3(a)–(d) illustrate the surface textures with and without ultrasonic aid cutting. Fig. 3(a) and (c) reveal finer and smoother tool marks on the workpiece surface, and obtain a lower surface roughness of the machined workpiece under the condition of ultrasonic-aided cutting. The result corresponds to the experimental result of Wang and Zhao (Wang and Zhao, 1987), that is, turning with ultrasonic vibration may decreases the bump effect of the side flow, therefore, it can improves the surface finish of the machined surface. During the experiment, a dynamometer is used to measure the cutting force, which is expressed by the following equation (4):
R=
Fx2 + Fy2 + Fz2
(4)
where R denotes the cutting force, and is the resultant force of Fx , Fy and Fz . Meanwhile, Fx , Fy and Fz represent the force components of the tangential, radial and feed directions. Table 4a displays the experimental results for cutting force R and the corresponding S/N ratios. Additionally, the mean S/N
ratio for each level of the cutting parameters is summarized and called the S/N response table for cutting force. The experiment is also calculated and illustrated in Table 4b. Moreover, Fig. 4 shows the S/N response graph for cutting force. By using smaller depth of cut, lower cutting speed and lower feed rate, the cutting force could be reduced. To discuss further the effect of ultrasonic-aided cutting onto cutting force, the experimental conditions of No. 3 and No. 1 in Table 4a were chosen, which represent the maximum and minimum cutting force, respectively. In Table 4c, No. 3 and No. 1, No. 3-1 and No. 1-1 are the experimental conditions with and without ultrasonic-aided cutting, and the resulting mean cutting force values are 704.31 and 24.29, 1041.02 and 32.16 N, respectively. It is indicated that as using ultrasonic-aided cutting, the cutting force reduces by 32.34% and 24.47%. Generally, most materials soften with heating to high temperatures. However, Inconel 718 precipitates a hard second phase of ␥ (Ni3 Nb) at an appropriate temperature to increase the difficulty of cutting. Observing the factor of working temperature in Fig. 4, it reveals that when a workpiece is heated for high-temperature cutting, the cutting force increases with the temperature (190 ◦ C). Since the BUE grow outward and downward, this gives rise to a variation in depth of the cutting surface. It represents a
Fig. 3 – Comparisons of surface roughness with and without ultrasonic-aided cutting.
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Table 4a – Experimental results for cutting force and S/N ratio Experiment no.
Control factors
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
T
D
S
F
1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 2 3 1 3 1 2 2 3 1 3 1 2
1 2 3 2 3 1 1 2 3 3 1 2 3 1 2 2 3 1
Cutting force (N) Temp. 1 2 3 2 3 1 3 1 2 2 3 1 1 2 3 3 1 2
S/N (dB)
P
R1
R2
R3
1 2 3 3 1 2 2 3 1 2 3 1 3 1 2 1 2 3
26.00 92.97 689.42 44.11 123.58 99.65 64.00 132.33 99.78 157.40 54.97 159.20 73.42 73.79 81.61 112.58 61.91 64.35
24.11 88.34 709.20 41.69 116.15 100.07 39.05 135.68 87.87 188.47 59.86 141.96 81.10 75.84 89.70 140.44 64.87 64.62
22.77 93.65 714.31 40.33 134.58 105.24 59.03 148.48 97.40 187.93 72.42 170.72 76.03 83.46 90.15 141.51 67.80 65.06
−27.72 −39.25 −56.96 −32.48 −41.94 −40.15 −34.82 −42.86 −39.57 −45.03 −35.97 −43.96 −37.72 −37.82 −38.81 −42.42 −36.25 −36.22
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
Table 4b – S/N response table for cutting force Level
Factors T
1 2 3 Level effect
D
−41.48 −38.15 −38.69 3.33
S
F
−36.70 −35.13 −39.01 −38.98 −42.61 −44.21 5.91 9.07
Temp.
−35.45 −39.96 −42.91 7.46
−38.11 −38.39 −41.82 3.71
P −38.91 −39.05 −40.37 1.46
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
major component of surface roughness when cutting with a BUE (Shaw, 1996). The phenomenon of BUE occurs at TN35 tungsten carbide tool as shown in Fig. 5(a) and (b), but not found at NX2525 cermet tool as illustrated in Fig. 5(c)–(e). Comparing Fig. 5(a) with (e), they are at same cutting condition, but Fig. 5(a) using cutting tool TN35 (carbide coating Al2 O3 ) and Fig. 5(e) using NX2525 (cermet). No BUE occurred in Fig. 5(e) is because the cermet tool NX2525 has more refractory and higher hardness, also low affinity to Inconel 718. Due to no BUE in the NX2525 cermet tool, it can obtain a low surface roughness as shown in Fig. 2 and reduce the cutting force in Fig. 4. As a result, NX2525 cermet tool is suitable for the machining of Inconel 718.
Fig. 4 – S/N graph for cutting force.
3.2.
Analysis of variance
After the construction of factor response table, an analysis of variance (ANOVA) table can be established based on the sum of the square (SSi ), the degree of freedom (DoFi ), the variance (Vari ) and the experimental error. The variance ratio, F ratio, is
Table 4c – Analysis of cutting forces with and without ultrasonic-aided cutting Experiment no.
3 3-1 1 1-1
Factors T
D
S
F
1 1 1 1
3 3 1 1
3 3 1 1
3 3 1 1
Cutting force (N) Temp. 3 3 1 1
P
R1
R2
3
689.42 1230.56 26.00 29.54.
709.20 980.37 24.11 34.47
1
Mean value (N)
Improvement rate (%)
R3 714.31 912.13 24.77 32.48
704.31 1041.02 24.29 32.16
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
32.34 24.47
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then evaluated from the variance of each control factor (Vari ) and the variance of error factor (Vare ). Finally, the expected value of the sum of square (SSi ) and the total square sum (SSt ) are used to calculate the percent contribution (Pi ) of various control factors and error factors (Pe ). If the percent contribution of the error factor, Pe , falls below 15%, it implies that the quality characteristics of the experiment are under a precise control. In contrast, if the percent contribution of the error factor exceeds 50%, it means that certain significant factor is overlooked and the experiments must be reviewed again (Lin and Chang, 2003). Table 5 is the ANOVA table for surface roughness and cutting force. For the surface roughness of the workpiece, all the F ratio of the six control factors achieved the confidence level of 99% [F(0.01, 2, 41 = 5.2)] in F test. It can be found that cutting tool and feed rate are the significant cutting parameters which affect surface roughness. For the cutting force, the confidence level of the F test of the six control factors also reached 99%. Cutting speed, feed rate, cutting tools, and depth of cut are the significant cutting parameters. From the above analysis, the percent contribution of error factor, Pe , of the two machining characteristics were approximately 15% (11.6% and 15.8%, respectively). This indicated that the experimental plan in this study was feasible, that no important factor has been ignored and that the analysis results were reliable.
3.3.
Confirmation test
Once the optimal level of the design parameters has been selected, the final step is to predict and verify the improvement of the quality characteristic using the optimal level of the design parameters. The estimated S/N ratio Ypredicted using the optimal level of the design parameters can be calculated as
Ypredicted = Ym +
k
(Yi − Ym )
(5)
i=1
where Ym is the total mean S/N ratio, Yi the mean S/N ratio at the optimal level, and k is the number of the main design parameters that affect the quality characteristics. The measuring data and the actual S/N ratio of confirmation experiments are listed in Table 6. The largest S/N ratios in the experiments were 15.16 (Table 3a, surface roughness) and −27.72 (Table 4a, cutting force), respectively. However, the S/N ratios of confirmation experiments are 16.25 and −24.98, and both are larger than the above two. In addition, the S/N prediction, 18.25 and −25.25, are very close to the actual value. This indicated that the experiments in this study possess excellent repetitiveness and great potential for future references.
Fig. 5 – Comparison of the BUE phenomenon in using TN35 (carbide coating Al2 O3 ) and NX2525 (cermet) as the cutting tool for Inconel 718.
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Table 5 – Results of the ANOVA for the surface roughness and the cutting force Factor
T D S F Temp. P Error Total
Surface roughness
Cutting force
SS
DoF
Var
F ratio
(%)
SS
0.9425 0.2607 0.1510 0.7108 0.3799 0.0720 0.3252 2.8422
2 2 2 2 2 2 41 53
0.4712 0.1304 0.0755 0.3354 0.1900 0.0360 0.0079
59.64 16.50 9.55 44.99 24.05 4.55
33.16 9.17 5.31 25.00 13.36 2.53 11.44 100
158325.01 153338.58 255548.71 193616.20 123187.39 82015.65 181247.57 1147279.12
Var
F ratio
(%)
79162.51 76669.29 127774.36 96808.10 61593.69 41007.83 4420.67
17.9 17.3 28.9 21.9 13.9 9.3
13.80 13.37 22.27 16.88 10.73 7.15 15.79 100
DoF 2 2 2 2 2 2 41 53
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power; , contribution. The F ratios according to the F distribution table are F(0.01, 2, 41) = 5.2.
Table 6 – Confirmation experiment for the surface roughness and the cutting force Control factors T Surface roughness Cutting force
NX2525 CT3000
D (mm) 0.3 0.1
S (m/min) 22 22
S/N (dB) prediction
F (mm/rev)
Temp. (◦ C)
P (W)
0.054 0.054
25 25
180 140
Confirmation experiments Measuring data
18.25 −25.25
0.15 19.07
0.17 17.88
0.14 16.16
Mean value 0.15 17.70
S/N (dB) 16.25 −24.98
Note: T, cutter; D, depth of cut; S, cutting speed; F, feed rate; temp., working temperature; P, ultrasonic power.
4.
Conclusions
This paper has discussed an application of the Taguchi method for optimizing the cutting parameters in turning Inconel 718. The conclusions of this study may be summarized as follows: 1. NX2525 cermet cutting tool produces almost no BUE and achieves the best surface roughness. Moreover, the measured cutting forces are relatively small. 2. When the workpiece underwent high-temperature cutting (190 ◦ C), owing to the hard secondary phase of ␥ (Ni3 Nb) precipitating, cutting becomes increasingly difficult, and thus the cutting force is higher than that at room temperature. 3. The percentage contributions of the cutting tool, feed rate, working temperature and depth of cut for surface roughness are 33.16, 25.00, 13.36 and 9.17, respectively. 4. The percentage contributions of the cutting speed, feed rate, cutting tool and depth of cut for cutting force are 22.27, 16.88, 13.80 and 13.37, respectively. 5. Ultrasonic-aided cutting improved the surface roughness by 9.10–51.61%, as well as decreasing cutting force by 32.34–24.47%. As a result, ultrasonic-aided cutting can enhance the cutting quality of Inconel 718.
references
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