http://pib.sagepub.com/
Manufacture
Engineers, Part B: Journal of Engineering
Proceedings of the Institution of Mechanical
http://pib.sagepub.com/content/224/2/195
The online version of this article can be found at:
DOI: 10.1243/09544054JEM1437
2010 224: 195
Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture
Y-F Chen, Y-J Lin, Y-C Lin, S-L Chen and L-R Hsu
2
ceramic using the Taguchi method
Optimization of electrodischarge machining parameters on ZrO
Published by:
http://www.sagepublications.com
On behalf of:
Institution of Mechanical Engineers
can be found at:
Manufacture
Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering
Additional services and information for
http://pib.sagepub.com/cgi/alerts Email Alerts: http://pib.sagepub.com/subscriptions Subscriptions: http://www.sagepub.com/journalsReprints.nav Reprints: http://www.sagepub.com/journalsPermissions.nav Permissions: http://pib.sagepub.com/content/224/2/195.refs.html Citations:
What is This?
- Feb 1, 2010
Version of Record
>>
Optimization of electrodischarge machining parameters
on ZrO
2
ceramic using the Taguchi method
Y-F Chen1, Y-J Lin1, Y-C Lin2*, S-L Chen3, and L-R Hsu4 1
Department of Mechanical Engineering, Nankai University of Technology, Nantou, Taiwan, Republic of China 2Graduate School of Vehicle and Mechatronic Industry, Nankai University of Technology, Nantou, Taiwan, Republic of China
3
Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan, Republic of China
4
Graduate Institute of Engineering Technology, Chungchou Institute of Technology, Changhau, Taiwan, Republic of China
The manuscript was received on 30 November 2008 and was accepted after revision for publication on 9 July 2009. DOI: 10.1243/09544054JEM1437
Abstract: The purpose of the present investigation was to optimize the electrodischarge machining (EDM) parameters for machining ZrO2ceramic. During the EDM process, the sur-face of the electrically non-conductive ceramic was covered with adhesive conductive copper and aluminium foils to attain the threshold of electrical conductivity for the EDM process. The machining characteristics associated with the EDM process such as material removal rate (MRR), electrode wear rate (EWR), and surface roughness (SR) were explored through the experimental study according to an L18 orthogonal array based on the Taguchi experimental design method. Analysis of variance was conducted to examine the significant machining parameters affecting the machining characteristics. The experimental results show that peak current and pulse duration significantly affected MRR and SR, and the adhesive conductive material was the significant parameter correlated with EWR. In addition, the optimal combi-nation levels of machining parameters were also determined from the response graph of signal-to-noise ratios for each level of machining parameters. The aim of the study was to establish a feasible process and optimize the parameter levels for processing electrically non-conductive ceramics through EDM. A practical and convenient process for shaping electrically non-conductive ceramics was developed in the present work with the features of high effi-ciency, high precision, and excellent surface integrity.
Keywords: electrodischarge machining, Taguchi method, material removal rate, electrode wear rate, surface roughness, non-conductive ceramics
1 INTRODUCTION
Electrodischarge machining (EDM) is a non-conventional machining process which is the most common technique used in the mould and die manufacturing industries. The EDM process removes surplus material by means of consecutive sparks
(discharge columns) produced between the tool electrode and the workpiece, which are separated by a dielectric fluid such as kerosene or deionized water. During the EDM process, if the gap condition is suitable for developing the discharge column, an extremely elevated temperature is generated by con-secutive electrical sparks when electrical power is supplied to the tool electrode and workpiece. Partial amounts of workpiece material and tool electrode on a sparking spot are vaporized and melted due to the high temperature. Then, an impulsive force is devel-oped as a result of dielectric fluid explosion and the melted material is ejected from the machined surface
*Corresponding author: Graduate School of Vehicle and Mechatronic Industry, Nankai University of Technology, No.568 Zhongzheng Road, Caotun Town, Nantou County 54243, Taiwan, Republic of China.
due to the locally impulsive force. It is well known that the material removal effects caused by vaporiz-ing, meltvaporiz-ing, and dielectric explosion are not gov-erned by the mechanical properties of the workpiece like strength, toughness, and hardness. Conse-quently, the EDM process is able to be employed in the machining of difficult-to-machine materials. In general, the EDM process is used mainly to machine electrically conductive materials. There is a threshold of electrical conductivity near 100–300V cm for the EDM process to generate electrical discharges between the tool electrode and workpiece [1]. Kiyak and Cakir [2] studied the influence of EDM para-meters on surface roughness (SR) for machining AISI P20 tool steel. Their experimental results found that SR was obviously affected by discharge current as well as pulse duration, and SR was high if the dis-charge current and pulse duration were set at a higher level. The feasibility of the EDM process for polycrystalline diamond, B4C and SiC ceramics was investigated by using wire EDM and die-sinking EDM [3, 4]. The experimental results indicated that a desirable material removal rate (MRR) with excellent surface finish could be attained. Lauwers et al. [5] explored the machinability of SiC, B4C, and Si3N4/ TiN by milling EDM and die-sinking EDM, and a more efficient strategy for machining ceramic mate-rials was developed to validate ceramic matemate-rials. Lin et al. [6] also conducted experimental work on SKH 57 high-speed steel to investigate the effects of EDM parameters on machining characteristics. Moreover, the machining performance of cemented tungsten carbides using EDM has also been explored in well-designed experimental investigations [1, 7, 8]. Ceramics have diverse applications in a wide range of industrial fields such as machining tools, moulds and dies, vehicle systems, electronic devi-ces, and semiconductor systems. Although ceramics with excellent properties such as high strength, distinguished hardness, excellent dielectric strength, and outstanding corrosion resistance could pro-mote their application in various fields, it also reveals vigorous challenges during machining using traditional processes. Thus, the machining cost will inevitably be increased in machining ceramic materials.
Several researchers have studied ceramics machin-ing usmachin-ing the EDM process and their experimental results confirmed that thermal spalling is one of the main material removal mechanisms in EDM for machining conductive ceramics. Zhang et al. [9] studied the machinability of a hot-pressed alumi-nium oxide-based ceramic. In their work, the machining mechanism of EDM for conductive cera-mics was investigated, and the effects of EDM para-meters on MRR, SR, and diameter of the spark spot were measured and discussed. Hocheng et al. [10]
reported that the MRR was higher when the EDM machining parameters were set at larger peak current and shorter pulse duration for machining SiC/Al composite material. Trueman and Huddleston [11] introduced the concept of a rapid high-machining regime using die-sinking EDM for ceramic materials, and the potential of promoting MRR with thermal-shock spalling was discussed. Liu and Huang [12] investigated the influence of the microstructure and conductivity of conductive TiN/Si3N4ceramic mate-rials in the EDM process. As their experimental results indicated, a higher voltage and current, as well as higher content of TiN, resulted in greater MRR. Luis and Puertas [13] suggested the use of statistical analysis methods to analyse the optimization of EDM parameters to ameliorate the machining char-acteristics for ceramic materials. Although the EDM process exhibits conspicuous performance during the machining of ceramics, the main EDM applica-tions in ceramics machining are confined to elec-trically conductive ceramics. However, the major electrically non-conductive ceramics that are most frequently used in modern industrial fields, such as ZrO2, reveal rigorous limitations associated with choosing a suitable machining process with reason-able efficiency and quality. Therefore, the machining feasibility of electrically non-conductive ceramic materials using the EDM process has attracted extensive attention. Several research works have been conducted to explore the machining characteristics of electrically non-conductive ceramics by conven-tional EDM [14–18]. In these investigations, metal plate, metal mesh, baked colloidal graphite, or a physical vapour deposited (PVD) layer was arranged on the workpiece surface as an assisting electrode, and then the ceramics could be machined very easily in die-sinking EDM or wire EDM. In these approaches for electrically non-conductive ceramic materials, the initial electrical discharges could be constructed in the machining gap between the tool electrode and the assistant conductive material covering the ceramic workpiece, and then the pyrolytic carbon cracked from kerosene was depos-ited on the machined surface of the electrically non-conductive ceramic to reach the threshold of electrical conductivity for EDM to progress. The experimental results showed that the baked colloidal graphite and PVD coating layer methods gave outstanding machining performance for elec-trically non-conductive ceramics. However, the baked graphite and PVD processes inevitably need some additional facilities and intensive operating time, and the operating cost will be high for machining electrically non-conductive ceramics. For each practical approach introduced and employed in the EDM process, the efforts are focused on obtaining better product quality, improving the
capability of machining characteristics, and exploit-ing new techniques to extend applications of the EDM process. The technical challenge in EDM for processing electrically non-conductive ceramics is to develop a robust and efficient electrically conductive layer created on the machined surface to maintain progress in the EDM technique.
The Taguchi method has been adopted broadly in engineering analyses, revealing that it is a powerful approach to design complicated systems with high quality performance. The Taguchi method has been applied in various industrial fields and research areas to solve the optimization parameters in complicated systems. Liao et al. [19] used a Taguchi experimental design approach to determine the optimization of machining parameter levels in wire EDM. The Taguchi method was also adopted to determine the optimal machining parameters of a hybrid process of EDM with ball burnish machining for surface modification [20, 21]. Sundaram et al. [22] studied ultrasonic-assisted micro EDM using the Taguchi method to explore the optimal combination levels of machining parameters. George et al. [23] conducted a type of experiment based on the Taguchi method to deter-mine the optimal setting of machining parameters for EDM of a composite material. Yang and Tarng [24] used the Taguchi method to analyse the optimal cut-ting parameters for a turning operation. These works revealed the Taguchi method is a powerful approach that can be used in the design of experiments and in determining the effect of each machining parameter. Moreover, the Taguchi method uses a specially designed regime called an orthogonal array, which nowadays is a predominant approach in experiment design. It has been proved that the experiment con-ducted with the Taguchi method is more efficient and less costly for investigating the effects of the entire machining parameters. Parameter design via the Taguchi method can also optimize the machining characteristics through setting process parameters and reducing the sensitivity of the system performance to sources of variation. Therefore, high quality of machining characteristics can be achieved without increasing the operating cost.
In the present investigation, an L18 orthogonal array based on the Taguchi experimental design method was employed in determining the effects of essential EDM parameters on MRR, electrode wear rate (EWR), and SR. Moreover, the optimal machining parameters for processing electrically non-conductive ceramics through EDM were established. A sophisticated pro-cess with high efficiency and high quality of surface integrity was achieved by the EDM process for shaping electrically non-conductive ceramics, with practical and convenient features to fit modern industrial requirements.
2 EXPERIMENTAL METHOD 2.1 Experimental materials
High-purity ZrO2ceramic was adopted as workpiece material in this investigation; it is widely used in modern industrial applications owing to its excellent corrosion resistance, hardness, and exceptional strength at high temperature. The electrode material employed was electrolytic copper. The dimensions of the workpiece and electrode were 12 mm· 12 mm · 5 mm and 30 mm · 20 mm · 1.5 mm, respectively. Thus, a machined area of 1.5 mm· 5 mm would be formed on the machined surface of the workpiece. The essential properties of the copper electrode as well as the ZrO2ceramic are listed in Tables 1 and 2. The end face of the electrode against the workpiece was ground on a plate using emery paper with grain sizes of 600#, 800#, and 1200# in sequence, to guar-antee the surface finish and flatness of each electrode to be the same. The experiments were performed under kerosene dielectric (commercial grade) at a depth of 20 mm. Since the workpiece material (ZrO2) is an electrically non-conductive ceramic, the ZrO2 surface to be machined had to be covered with an assistant electrically conductive material, to reach the threshold of electrical conductivity for EDM and to form the electrical discharge columns between the tool electrode and workpiece at the initial stage of the process. Subsequently, pyrolytic carbon cracked from kerosene would be produced and deposited on the machined ZrO2 surface. The effects of cracked carbon adhesion have been discussed by Natsu et al. [25]. Moreover, material from the tool electrode would be transferred to the machined surface during the EDM process, and the migrated tool electrode
Table 2 Essential properties of the ZrO2 workpiece
material
Property Value
Melting point (C) 2720
Thermal conductivity (W/m K) 2
Specific heat capacity (J/g C) 0.4
Electrical resistivity (V cm) 1010
Thermal expansion coefficient (1/C) 7.0· 106
Specific gravity (g/cm3) 5.68
Hardness (Hv) 1270
Table 1 Essential properties of the copper electrode
Property Value
Melting range (C) 1065–1083
Thermal conductivity (W/m K) 388
Specific heat capacity (J/gC) 0.385 Electrical resistivity (V cm) 1.7· 106 Thermal expansion coefficient (1/C) 1.7· 105
where
^h is the estimated S/N ratio for the optimal com-bination level of machining parameters
hmis the total mean S/N ratio
n0is the number of significant parameters hiis the mean S/N ratio at the optimal level. Table 10 displays the results of confirmation experiments on ZrO2 ceramic in the EDM process with adhesive foils covering the workpiece surface. The results indicate that the S/N ratios correlated with MRR, EWR, and SR for the optimal combination levels of the machining parameters are 8.53 dB, 16.73 dB, and 7.87 dB higher than those obtained at the initial experimental conditions A1B2C2D2E2F2. The experimental results confirm that the machining parameters of ZrO2 using EDM with adhesive metal foils will be optimized for MRR, EWR, and SR, so that the observed values will be significantly improved.
4 CONCLUSIONS
Electrically non-conductive ZrO2 ceramic was suc-cessfully machined by EDM when using electrically conductive copper and aluminium foils to cover the workpiece surface. The optimal machining para-meters of the EDM process were estimated based on the Taguchi method. From the experimental results and statistical analysis using ANOVA and F tests, the following conclusions can be drawn.
1. The significant machining parameters associated with MRR were Ip and tp for ZrO2 in the EDM process with conductive foils covering the work-piece surface. In addition, the significant machining parameters associated with EWR were the type of adhesive conductive foil (Type). Moreover, Ip and tp were the significant para-meters affecting SR for ZrO2in the EDM process with adhesive copper foil.
2. The S/N ratios correlated with MRR, EWR, and SR for the optimal combination levels of machining
parameters were 8.53 dB, 16.73 dB, and 7.87 dB, which are higher than those obtained from the initial experimental conditions. The experimental results confirmed that the machining parameters of ZrO2 in the EDM process with adhesive con-ductive foils would be optimized for MRR, EWR, and SR based on the Taguchi method, so the observed values would be significantly improved. ACKNOWLEDGEMENT
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for finan-cial support of this research under Contract No. NSC 94-2212-E-235-001.
Authors 2010 REFERENCES
1 Puertas, I., Luis, C. J., and Alvarez, L. Analysis of the influence of EDM parameters on surface quality, MRR and EW of WC–Co. J. Mater. Process. Technol., 2004, 153–154, 1026–1032.
2 Kiyak, M. and Cakir, O. Examination of machining parameters on surface roughness in EDM of tool steel. J. Mater. Process. Technol., 2007, 191, 141–144.
3 Sanchez, J. A., Cabanes, I., Lopez de Lacalle, L. N., and Lamikiz, A. Development of optimum electrodischarge machining technology for advanced ceramics. Int. J. Adv. Mfg Technol., 2001, 18, 897–905.
4 Liu, Y. H., Guo, Y. F., and Liu, J. C. Electric discharge milling of polycrystalline diamond. Proc. IMechE, Part B: J. Engineering Manufacture, 1997, 211(B8), 643–647. DOI: 10.1243/0954405981516580.
5 Lauwers, B., Kruth, J. P., and Brans, K. Development of technology and strategies for the machining of ceramic components by sinking and milling EDM. Ann. CIRP, 2007, 56(1), 225–228.
6 Lin, Y. C., Cheng, C. H., Su, P. L., and Hwang, L. R. Machining characteristics and optimization of machin-ing parameters of SKH 57 HSS usmachin-ing EDM based on Taguchi method. Mater. Mfg Process., 2006, 21, 922–929. Table 10 Results of the confirmation experiments for EDM of ZrO2
Initial levels of machining parameters
Optimal combination levels of machining parameters
Prediction Experiment MRR Level A1B2C2D2E2F2 A1B3C1D3E3F2 A1B3C1D3E3F2
Observed value (mm3/min) 0.1074 – 0.2869
S/N ratio (dB) 19.38 9.68 10.85
EWR Level A1B2C2D2E2F2 A2B1C2D1E2F3 A2B1C2D1E2F3
Observed value (mm3/min) 0.01799 – 0.0026
S/N ratio (dB) 34.94 45.96 51.67
SR Level A1B2C2D2E2F2 A2B1C2D1E2F1 A2B1C2D1E2F1
Observed value (Ra,mm) 8.34 – 3.37
S/N ratio (dB) 18.42 13.45 10.55
7 Lin, Y. C., Hwang, L. R., Cheng, C. H., and Su, P. L. Effects of electrical discharge energy on machining performance and bending strength of cemented tung-sten carbides. J. Mater. Process. Technol., 2008, 206, 491–499.
8 Kanagarajan, D., Karthikeyan, R., Palanikumar, K., and Sivaraj, P. Influence of process on electric dis-charge machining of WC/30%Co composites. Proc. IMechE, Part B: J. Engineering Manufacture, 2008, 222(B7), 807–815. DOI: 10.1243/09544054JEM925. 9 Zhang, J. H., Lee, T. C., and Lau, W. S. Study on the
electrode-discharge machining of a hot pressed alumi-num oxide based ceramics. J. Mater. Process. Technol., 1997, 63, 908–912.
10 Hocheng, H., Lei, W. T., and Hsu, H. S. Preliminary study of material removal in electrical-discharge machining of SiC/Al. J. Mater. Process. Technol., 1997, 63, 813–818.
11 Trueman, C. S. and Huddleston, J. Material removal by spalling during EDM of ceramics. J. Eur. Ceram. Soc., 2000, 20, 1629–1635.
12 Liu, C. C. and Huang, J. L. Effect of the electrical dis-charge machining on strength and reliability of TiN/ Si3N4composites. Ceram. Int., 2003, 29, 679–687.
13 Luis, C. J. and Puertas, I. Methodology for developing technological tables using in EDM processes of con-ductive ceramics. J. Mater. Process. Technol., 2007, 189, 301–309.
14 Fukuzawa, Y., Tani, T., Iwane, E., and Mohri, N. A new machining method for insulating ceramics with an electrical discharge phenomenon. J. Ceram. Soc. Jap. Int. Edn, 1995, 103, 987–992.
15 Mohri, N., Fukuzawa, Y., Tani, T., Saito, N., and Furutani, K. Assisting electrode method for machining insulating ceramics. Ann. CIRP, 1996, 45(1), 201–204. 16 Mohri, N., Fukuzawa, Y., Tani, T., and Sata, T. Some
considerations to machining characteristics of insulat-ing ceramics – toward practical use in industry. Ann. CIRP, 2002, 51(1), 161–164.
17 Liu, Y. H., Ji, R. J., Li, X. P., Yu, L. L., and Zhang, H. F. Electric discharge milling of insulating ceramics. Proc.
IMechE, Part B: J. Engineering Manufacture, 2008, 222(B2), 361–366. DOI: 10.1243/09544054JEM970. 18 Tani, T., Fukuzawa, Y., Mohri, N., Saito, N., and Okada,
M. Machining phenomena in WEDM of insulating ceramics. J. Mater. Process. Technol., 2004, 149, 124–128. 19 Liao, Y. S., Huang, J. T., and Su, H. C. A study on the machining-parameters optimization of wire electrical discharge machining. J. Mater. Process. Technol., 1997, 71, 487–493.
20 Yan, B. H., Wang, C. C., Chow, H. M., and Lin, Y. C. Feasibility study of rotary electric discharge machining with ball burnishing for Al2O3/6061 Al composite. Int. J.
Machine Tools Mf., 2000, 40, 1403–1421.
21 Lin, Y. C., Yan, B. H., and Huang, F. Y. Surface improvement using a combination electrical discharge machining with ball burnish machining based on the Taguchi method. Int. J. Adv. Mfg Technol., 2001, 18, 673–682.
22 Sundaram, M. M., Pavalarajan, G. B., and Rajurkar, K. P. A study on process parameters of ultrasonic assisted micro EDM based on Taguchi method. J. Mater. Engng Perform., 2008, 17(2), 210–215.
23 George, P. M., Raghunath, B. K., Manocha, L. M., and Warrier, A. M. EDM machining of carbon–carbon composite – a Taguchi approach. J. Mater. Process. Technol., 2004, 145, 66–71.
24 Yang, W. H. and Tarng, Y. S. Design optimization of cutting parameters for turning operations based on the Taguchi method. J. Mater. Process. Technol., 1998, 84, 122–129.
25 Natsu, W., Kunieda, M., and Nishiwaki, N. Study on influence of inter-electrode atmosphere on carbon adhesion and removal amount. Int. J. Electrical Mach., 2004, 9, 43–50.
26 Soni, J. S. and Chakraverti, G. Experimental investiga-tion on migrainvestiga-tion of material during EDM of die steel (T215 Cr12). J. Mater. Process. Technol., 1996, 56, 439–451.
27 Lin, Y. C., Yan, B. H., and Huang, F. Y. Surface mod-ification of Al–Zn–Mg aluminum alloy using combined process of EDM with USM. J. Mater. Process. Technol., 2001, 115, 359–366.
