本論文係為了測量高硬度和高韌性材料之殘留應力，而設計與規劃EDM應變規鑽孔法(簡稱EDM鑽孔法)之測量流程。在此測量法的鑽孔過程，生成於孔壁的變質層會引入額外應力，而產生過大之測量誤差；雖然此誤差可利用鑽孔引入應力值予以校正，但EDM參數中的放電電流和放電持續時間強烈影響引入應力，因而降低鑽孔參數的通用性。故本研究之目的為：首先，找出最佳的EDM參數，以提升鑽孔參數之通用性和減小引入應力，之後再依據實驗結果規劃最佳的測量流程。
經實驗發現，二次放電現象是影響引入應力的重要因素，亦發現利用放電時間效率穩定係數(Crsc)能有效評估二次放電發生率。當Crsc穩定係數維持在0.99~1之間的理想值，可有效抑制二次放電現象。如此，除了可減小鑽孔引入應力外，亦能大幅降低放電電流和放電持續時間對引入應力的影響，而提升此二參數的通用性。本研究尚發現排渣性能和放電休止時間是影響Crsc穩定係數的重要參數。採用較佳排渣能力的空心電極鑽孔，且放電電流和放電持續時間分別在4~12 A與9~23 μs之間時，若 τoff＞2.5τon-12或duty factor＜0.34，則Crsc穩定係數可維持於0.99~1；於此範圍的參數均可用於EDM鑽孔法，而最佳的鑽孔參數建議為8A/16μs/0.3 (Ip/τon/duty factor)。但採用實心電極鑽孔時，因受排渣能力不佳的影響，壓縮了EDM鑽孔法的可用參數範圍，於本研究，僅發現12A/6μs/30μs (Ip/τon/τoff)參數能維持Crsc穩定係數於理想值。
進一步分析引入應力與試件材料性質之間的關係，發現引入應力和熱傳導係數之間有明顯的冪次反比關係，且引入應力和碳當量具有線性正比關係。依據這些經驗關係式，繼而建構用於補償程序的一系列校正方程式，並依EDM鑽孔條件設計完整的測量流程。經準確度評估實驗證實，本論文所提出之校正方程式能有效補償變質層所造成的量測誤差，校正後之量測誤差低於10MPa，已符合應變規鑽孔法之±20MPa精確度的要求；據此顯示，搭配本論文所建立之測量流程的EDM鑽孔法，能提供令人滿意的準確度，可為高硬度和高韌性試件提供另一種測量殘留應力的可行方法。

關鍵字：放電加工, 應變規鑽孔法, 殘留應力, 變質層, 熱傳導係數, 碳當量。

For measuring the residual stress in ferrous component with high hardness and high toughness, an innovative measurement method (EDM hole-drilling strain-gage method) is developed in this study. When utilizing the EDM technique to drill the measurement hole required during the process of the hole-drilling strain gage method, the metallurgical transformation layer formed on the surface of the hole-wall induces an extra stress, which can lead to significant measurement errors. Although a value of hole-drilling induced stress can be used to compensate for the errors, an inappropriate specification of the EDM parameters will increase the extra stress considerably and will therefore limit the effectiveness of the calibration procedure. Accordingly, the first objective of the present work is to explore and determine the optimal EDM hole-drilling parameters with which the magnitude of the induced stress can be effectively minimized.
The experimental results demonstrate that when using the hollow electrode to drill the hole, the occurrence of secondary discharges is suppressed by maintaining the relative stability coefficient of the discharge duty ratio (Crsc) at a ideal value of 0.99 ~ 1, and therefore, the induced stress can be reduced substantially and becomes insensitive to the parameters of the pulse current and pulse-on duration. In addition, the Crsc coefficient is highly sensitive to the parameter of pulse-off duration. As the value of τoff increases, the value of Crsc also increases, while that of the induced stress reduces. Furthermore, it has is revealed that when the value of τoff is increased to the critical value at which Crsc coefficient has a value of more than 0.99, the magnitude of the hole-drilling induced stress becomes insensitive to the value of the pulse-off parameter. Overall, the wide ranges of parameters with which the Crsc value can be kept at the ideal value in EDM drilling process, has been found. Further experimental results have demonstrated that the parameters within these ranges are the optimal EDM hole-drilling parameters. However, when using a solid electrode to drill the measurement hole, the ranges of optimal EDM parameters are narrowed as result of a poor removal of debris. In this study, only one setting of optimal parameters is found.
It has established that provided the Crsc coefficient is retained between 0.99 and 1, the stress induced during the drilling process can be predicted by the thermal conductivity and carbon equivalent of the specimen. Accordingly, these two material properties are used as the basis of calibration equations designed to compensate the residual stress measurement obtained using the EDM hole-drilling method. It is shown that the calibration scheme reduce the errors of measurement results to less than 12 MPa. The precision of the measurement results are well within the limit of ±20 MPa recommended in the handbook of measurement of residual stresses for the accuracy of the hole-drilling strain-gage method. Accordingly, the validity of the EDM hole-drilling method is confirmed. This method offers a potential means to measure the residual stress of metallic materials, including those with high hardness and high toughness properties.

Keywords: Electrical discharge machining, Hole-drilling strain-gage method, residual stress, metallurgical transformation layer, thermal conductivity, carbon equivalent.

1. J. Lu, “Handbook of Measurement of Residual Stresses”, Society for Experimental Mechanics, Lilburn, GA, Fairmont Press, Inc., pp. 229, 1996.
2. F. C. Hsu, “The Calibration Study of EDM Hole Drilling Method for Residual Stress Measurement”, Master Thesis, Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, 2000.
3. ASTM Standard E837-95, “Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain Gage Method”, ASTM, Philadelphia, PA, 1995.
4. J. P. Sandifer and G. E. Bowie, “Residual Stress by Blind-Hole Method with Off-Center Hole”, Experimental Mechanics, Vol. 18, No. 5, pp. 173-179, 1978.
5. H. P. Wang, “Alignment Error of the Hole Drilling Method”, Experimental Mechanics, Vol. 9, No. 1, pp. 23-27, 1979.
6. J. Y. Wang, “Refined Analysis of the Relieved Strain Coefficients for the Off-Center Hole-Drilling Case”, Experimental Mechanics, Vol. 30, No. 4, pp. 367-371, 1990.
7. D. Vangi, “Residual Stress Evaluation by the Hole-Drilling Method with Off-Center Hole: An Extension of the Integral Method”, Journal of Engineering Materials and Technology, Vol. 119, No. 1, pp. 79-85, 1997.
8. J. Mathar, “Determination of Initial Stress by Measuring the Deformation Around Drilled Holes”, Trans. ASME, Vol. 4, pp. 249-254, 1934.
9. M. T. Flaman, “Brief Investigation of Induced Drilling Stresses in the Center-Hole Method of Residual-Stress Measurement”, Experimental Mechanics, Vol. 22, No. 1, pp. 26-30, 1982.
10. M. T. Flaman and J. A. Herring, “SEM/ASTM Round-Robin Residual Stress Measurement Study PhaseⅠ”, Experimental Techniques, Vol. 10, No. 5, pp. 23-25, 1986.
11. M. T. Flaman and J. A. Herring, “Comparison of Four Hole-Producing Techniques for the Center-Hole Residual-Stress Measurement Method”, Experimental Techniques, Vol. 9, No. 8, pp. 30-32, 1985.
12. M. T. Flaman and J. A. Herring, “Ultra-High-Speed Center-Hole Technique for Difficult Machining Materials”, Experimental Techniques, Vol. 10, No. 1, pp. 34-35, 1986.
13. M. T. Flaman and J. M. Boag, “Comparison of Residual-Stress Variation with Depth-Analysis Techniques for the Hole-Drilling Method”, Experimental Techniques, Vol. 30, No. 4, pp. 352-355, 1990.
14. J. A. McGeough and H. Rasmussen, “A Macroscopic Model of Electro-discharge Machining”, International Journal of Machine Tools & Manufacture, Vol. 22, pp.333-339. 1982.
15. H. T. Lee, F. C. Hsu, and T. Y. Tai, “Study of Surface Integrity Using the Small Area EDM Process with a Copper-tungsten Electrode”, Materials Science and Engineering: A, Vol. 364, pp.346-356, 2004.
16. Y. H. Guu, H. Hocheng, C. Y. Chou, and C. S. Deng, “Effect of Electrical Discharge Machining on Surface Characteristics and Machining Damage of AISI D2 Tool Steel”, Materials Science and Engineering: A , Vol. 358, pp.37-43,2003.
17. B. Ekmekci, “Residual Stresses and White Layer in Electric Discharge Machining (EDM) ” , Apply Surface Science, Vol. 253, pp.9234-9240, 2007.
18. S. Das, M. Klotz, and F. Klocke, “EDM Simulation: Finite Element-based Calculation of Deformation, Microstructure and Residual Stresses”, Journal of Material Processing Technology, Vol. 142, pp.434-451, 2003.
19. L. C. Lim, L. C. Lee, Y. S. Wong, and H. H. Lu, “Solidification Microstructure of Electrodischarge Machined Surfaces of Tool Steels”, Materials Science and Technology, Vol. 7, No. 3, pp. 239-248, 1991.
20. H. K. Lloyd and R. H. Warren, “Metallurgy of Spark-machined Surfaces”, Journal of Iron and Steel Institute, Vol. 203, No.3, pp.238-247,1965.
21. J. R. Crookall and B. C. Khor, “Residual Stresses and Surface Effects in Electro Discharge Machining”, in: Proceedings of 13th International Machine Tool Design and Research Conference, Birmingham, pp. 331-338, 1972.
22. J. C. Rebelo, A. Morao Dias, D. Kremer, and J. L. Lebrun, “Influence of EDM Pulse Energy on the Surface Integrity of Martensitic Steels”, Journal of Material Processing Technology, Vol. 84, No. 1-3, pp. 90-96, 1998.
23. B. Ekmekci, A. E. Tekkaya and A. Erden, “A Semi-empirical Approach for Residual Stresses in Electric Discharge Machining (EDM)”, International Journal of Machine Tools & Manufacture, Vol.46, pp.858-868, 2006.
24. H. T. Lee and F. C. Hsu, “Feasibility Evaluation of EDM Hole Drilling Method for Residual Stress Measurement”, Materials Science and Technology, Vol.19, pp.1261-1265, 2003.
25. H. T. Lee, W. P.Rehbach, F. C. Hsu, T. Y.Tai, and E. Hsu, “The Study of EDM Hole-drilling Method for Measuring Residual Stress in SKD 11 Tool Steel”, Journal of Material Processing Technology, Vol.149, pp.88-93, 2004.
26. H. T. Lee, J. Mayer and F. C. Hsu, “Application of EDM Hole-drilling Method to the Measurement of Residual Stress in Tool and Carbon Steels”, Journal of Material Processing Technology, Vol.128, pp.468-475, 2006.
27. F. C. Hsu, “Feasibility Study and Evaluation of Applying EDM Hole-drilling Method for Measuring Residual Stress”, Ph. D. Dissertation, Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, 2005.
28. J. P. Yur, “Surface Integrity and Characteristics of Electro-discharge Machined Surface”, Ph. D. Dissertation, Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, 2002.
29. W. Soete and R. Vancrombrugge, “An Industrial Method for the Determination of Residual Stress”, Proceedings SESA, Vol. 1, pp. 17-28, 1950.
30. R. A. Kelsey, “Measuring Non-uniform Residual Stresses by the Hole-Drilling Method”, Proceedings SESA, Vol. 14, pp. 181-194, 1956.
31. ASTM E837-01, “Standard Test Method for Determining Residual Stresses by the Hole Drilling Strain-Gage Method”, ASTM, Philadelphia, PA, 2001.
32. N. J. Rendler and I. Vigness, “Hole-Drilling Strain-Gag Method of Measuring Residual Stress”, Experimental Mechanics, pp. 577-586, 1966.
33. R. G. Bathgate, “Measurement of Non-uniform Biaxial Residual Stress by the Hole Drilling Method”, Strain, Journal of BSSM, Vol. 4, No. 2, pp. 22-29, 1968.
34. E. M. Beaney and E. M. Procter, “Critical Evaluation of the Centre Hole Technique for the Measurement of Residual Stress”, Strain, Vol. 10, No. 1, pp. 7-14, 1974.
35. E. M. Beaney, “Accurate Measurement of Residual Stress on Any Steel Using the Center Hole Method”, Strain, Vol. 12, No. 3, pp. 99-106, 1976.
36. G. S. Schajer, “Application of Finite Element Calculations to Residual Stress Measurement”, Journal of Engineering Materials and Technology, Transactions of the ASME, Vol. 103, No. 2, pp. 157-163, 1981.
37. M. T. Flaman and B. H. Manning, “Determination of Residual-Stress Variation with Depth by the Hole-Drilling Method”, Experimental Mechanics, Vol. 25, No. 3, pp. 205-207, 1985.
38. G. S. Schajer, “Measurement of Non-Uniform Residual Stresses Using the Hole-Drilling Method. Part I. Stress Calculation Procedures”, Journal of Engineering Materials and Technology, Transactions of the ASME, Vol. 110, No. 4, pp. 338-343, 1988.
39. D. Shaw and H. Y. Chen, “Finite-Element Technique to Analyze the data measured by Hole-Drilling Method”, Experimental Mechanics, Vol. 30, No. 2, pp. 120-123, 1990.
40. H. Wern, R. Cavelius and D. Schlaefer, “New Method to Determine Triaxial Non-uniform Residual Stresses from Measurements using the Hole-Drilling Method”, Strain, Vol. 33, No. 2, pp. 39-45, 1997.
41. G. Petrucci and B. Zuccarello, “A New Calculation Procedure for Non-uniform Residual Stress Analysis by the Hole-drilling Method”, Journal of Strain Analysis for Engineering Design, Vol. 33, No. 1, pp. 27-37, 1998.
42. A. J. Bush and F. J. Kromer, “Simplification of the Hole-drilling Method of Residual Stress Measurement”, ISA Transactions, Vol. 12, No. 3, pp. 249-259, 1973.
43. W. E. Nickola, “Post-yield Effects on Center Hole Residual Stresses Measurements”, Proceedings of the 5th International Congress on Experimental Mechanics, SESA, Brookfield Center, CT, USA, pp. 126-136, 1984.
44. M. Tootoonian and G. S. Schajer, “Enhanced Sensitivity Residual-stress Measurements Using Taper-hole Drilling”, Experimental Mechanics, Vol. 35, No. 2, pp. 124-129, 1995.
45. J. C. Biong, “Feasibility Evaluation of EDM Hole-drilling Method for Residual Stress Measurement”, Master Thesis, Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, 1994.
46. “Measurement of Residual Stresses by the Hole-drilling Strain-gage Method”, Tech Note TN-503-4, Measurement Group, Raleigh, NC.
47. J. Lu and J. F. Flavenot, “The Residual Stresses Distribution and Microstructure Studies in Advanced Coating Materials”, Surface Modification Technologies III, pp. 101-113, 1990.
48. C. C. Weng and S. C. Lo, “Measurement of Residual Stresses in Welded Steel Joints Using Hole Drilling Method”, Materials Science and Technology, Vol. 8, No. 3, pp. 213-218, 1992.
49. A. G. Olabi and M. S. J. Hashmi, “Stress Relief Procedures for Low Carvon Steel (1020) Welded Components”, Journal of Materials Processing Technology, Vol. 56, No. 1-4, pp. 552-562, 1996.
50. N. F. Petrofes and A. M. Gadalla, “Processing Aspects of Shaping Advanced Materials by Electrical Discharge Machining”, Matrrials and Manufacturing Processes, Vol. 3, No. 1, pp.127-157,1988.
51. O. A. Abu Zeid, “On the Effect of Electrodischarge Machining Parameters on the Fatigue Life of AISI D6 Tool Steel”, Journal of Materials Processing Technology, Vol. 68, No. 1, pp. 27-32, 1997.
52. M. Ramulu, M. G. Jenkins, and J. A. Daigneault, “Spark-Erosion Process Effects on the Properties and Performance of a TiB2 Particulate-Reinforced/SiC Matrix Ceramic Composite”, Ceramic Engineering and Science Processings, Vol. 18, No. 3, pp. 227-238, 1997.
53. D. M. Allen, “Demonstrated Micromachining Technologies for Industry (Ref. No. 2000/032) ”, IEE Seminar, pp.6/1 -6/4, 2000.
54. J. L. Lin and C. L. Lin, “The Use of the Orthogonal Array with Grey Relational Analysis to Optimize the Electrical Discharge Machining Process with Multiple Performance Characteristics”, International Journal of Machine Tools & Manufacture, Vol. 42, pp. 237-244, 2002.
55. D. D. Dibitonto, P. T. Eubank, M. R. Patel, and M. A. Barrufet, “Theoretical Models of the Electrical Discharge Machining Process. I. A Simple Cathode Erosion Model”, Journal of Applied Physics, Vol. 66, No.9, pp.4095-4103, 1989.
56. Y. F. Luo, “Investigation into the Actual EDM Off-time in SEA Sachining”, Journal of Materials Processing Technology, Vol. 74, No. 1-3, pp. 61-68, 1998.
57. C. T. Fu, J. M. Wu, and D. M. Liu, “The Effect of Electro-discharge Machining on the Fracture Strength and Surface Microstructure of an Al2O3-Cr3C2 Composite”, Materials Science and Engineering: A, Vol. 188, No. 1-2, pp. 91-96, 1994.
58. H. M. Chow, B. H. Yan, and F.Y. Huang, “Micro Slit Machining Using Electro-discharge Machining with a Modified Rotary Disk Electrode (RDE)”, Journal of Materials Processing Technology, Vol. 91, pp. 161–166, 1999.
59. K. Takahata and Y. B. Gianchandani, “Batch mode micro-EDM for high-density and high-throughput micromachining”, The 14th IEEE International Conference, Micro Electro Mechanical Systems, MEMS, pp. 72-75, 2001.
60. K. Takahata and Y. B. Gianchandani, “Batch Mode Micro-electro-discharge Machining”, Journal of Microelectromechanical Systems, Vol. 11, No. 2, pp.102 -110, 2002.
61. B. H. Yan, A. C. Wang; C. Y. Huang and F. Y. Huang, “Study of Precision Micro-holes in Borosilicate Glass Using Micro EDM Combined with Micro Ultrasonic Vibration Machining”, International Journal of Machine Tools and Manufacture, Vol. 42, No. 10, pp. 1105-1112, 2002.
62. P. S. Heeren, D. Reynaerts and H. Van Brussel, “Three-dimensional Silicon Micromechanical Parts Manufactured by Electro-discharge Machining”, Proceedings of 8th International Conference on Advanced Robotics, ICAR '97, pp. 247-252, 1997.
63. H. T. Lee, W. P. Rehbach, T. Y. Tai, and F. C. Hsu, “Surface Integrity in Micro-hole Drilling Using Micro-electro Discharge Machining”, Materials Transactions, Vol. 44, No. 12, pp. 2718-2711, 2003.
64. T. Y. Tai, “Effect of Electrode Size and Working Parameters upon Tool Steel Surface Integrity by EDM Process”, Ph. D. Dissertation, Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, 2004.
65. K. Takahata, N. Shibaike and H. Guckel, “A Novel Micro Electro-discharge Machining Method Using Electrodes Fabricated by the LIGA Process”, The 12th IEEE International Conference on Micro Electro Mechanical Systems, MEMS '99, pp. 238-243, 1999.
66. T. Masaki, K. Kawata and T. Masuzawa, “Micro Electro-discharge Machining and its Applications”, Micro Electro Mechanical Systems, Proceedings, An Investigation of Micro Structures, Sensors, Actuators, Machines and Robots', IEEE, pp. 21-26.1990.
67. L. Hong and S.D. Senturia, “Molding Of Plastic Components Using Micro-EDM Tools”, Electronics Manufacturing Technology Symposium, 13th IEEE/CHMT International, pp. 145-149, 1992.
68. H. H. Langen, T. Masuzawa, and M. Fujino, “A micro-EDM Assembly System Unit for Microparts Fabrication”, Emerging Technologies and Factory Automation, Design and Operations of Intelligent Factories”, Workshop Proceedings, IEEE 2nd International Workshop, pp. 229-237, 1993.
69. Y. Zuyuan, T. Masuzawa, and M. Fujino, “3D micro-EDM with Simple Shape Electrode Part1: Machining of Cavities with Sharp Corners and Electrode Wear Compensation”, Science and Tech, Proceedings of the 4th Korea-Russia Int'l Symp, Vol, 3, pp. 102-105, 2000.
70. W. H. Tung, “The Relationship between White Layer and Residual Stress Induced by EDM Hole-drilling”, Master Thesis, Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, 1998.
71. T. Y. Tai, “Measurement of Surface Residual Stress in Steel with Hard Layer by Using EDM Hole-drilling Method”, Master Thesis, Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, 1999.
72. Y. H. Chen, “Surface Crack Susceptibility of Electro-discharge Machining Process”, Master Thesis, Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, 2003.
73. H. T. Lee, W. P. Rehbach, T. Y. Tai and F. C. Hsu, “ Relationship between Electrode Size and Surface Cracking in the EDM Machining Process”, Journal of Material Science, Vol. 39, pp. 6981-6986, 2004.
74. H. T. Lee and T. Y. Tai, “Relationship between EDM Parameters and Surface Crack Formation”, Journal of Materials Processing Technology, Vol. 142, No. 3, pp.676-683, 2003.
75. 褚方勳, ”放電加工表面的殘留應力與變質層”, 應用機械工學, pp. 90~95, 1989.
76. 李驊登等, “放電加工參數對加工表面缺陷及粗糙度之影響分析”, 材料科學（Chinese Journal of Material Science）,Vol. 28, No. 4, pp. 270~278, 1996.
77. 向山方世，緒方勳，日原正彥, “WEDM 放電加工後殘留應力研究（第一報）-加工條件和應力分佈”, 精密工學會誌, Vol. 57, No. 1, pp. 144~149, 1991.
78. M. A. E.-R. Merdan and R. D. Arnell, “The Surface Integrity of a Die Steel after Electro-discharge Machining:2 Residual Stress Distribution”, Surface Engineering, Vol. 7, No. 2, pp 154-158, 1991.
79. 莊永旺，顏瑞宏，潘湧川, “放電加工對 SKD61 熱間工具鋼表面性質之影響”, 中國機械工程學會第十三屆學術研討會, pp. 437~443, 1996.
80. S. Timoshenko and J. M. Goodier, Theroy of Elasticity, New York, MCGraw-Hill, 1995.
81. M. Kabiri, “Measurement of Residual Stress by the Hole-drilling Method: Influence of Transverse Sensitivity of the Gages and Relieved Strain Coefficients”, Experimental Mechanics, pp. 252-256, 1984.
82. http://www.vishay.com/brands/measurements_group/guide/tn/tn503/503index.htm
83. P. W. Chen, “Size Effect on the Surface Roughness and Surface Crack of EDMed Surface”, Master Thesis, Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, 2003.
84. L. C. Lee, L. C. Lim, Y. S. Wong and H. H. Lu, “Towards a Better Understanding of the Surface Features of Electro-discharge Machined Tool Steels”, Journal of Materials Processing Technology, Vol. 24, pp. 513-523, 1990.
85. 林楠盛, “放電加工之應用技術”, 機械工業雜誌, 82卷, pp.203~215,1990
86. S. Kher and A. Dua, “EDM Pulse Control: a Design Approach”, In: SICE Annual, Proceedings of 38th Annual conference, Morioka, pp.1047-1052, 1999.
87. L.Ott and M. Longnecker, “Statistical Methods and Data Analysis”, fifth ed. Duxbury, CA, USA, pp.93-96. 2001.
88. http://www.gmtc.com.tw/html/02speciality06_c.htm
89. http://www.matweb.com/search/SearchSubcat.asp
90. ASTM Standard E8M, “Standard test methods for tension testing of metallic material (metric)”, ASTM, Philadelphia, PA, 1999.
91. P. M. Unterweiser, H. E. Boyer, and J. J. Kubbs, “Heat Treatment’s Guide – Standard Practices and Procedures for Steel,”ASM, 2000.
92. D. D. Dibitonto, P. T. Eubank, M. R. Patel, and M. A. Barrufet, “Theoretical Models of the Electrical Discharge Machining Process. I. A Simple Cathode Erosion Model”, Journal of Applied Physics, Vol.66, No.9, pp. 4095-4103, 1989.
93. K. Easterling,“Introduction to the Physical Metallurgy of Welding ”, second ed., Butterworth Heinemann, pp. 132-262, 1997.
94. A. J. Deardo, C. I. Garcia and E. J. Palmiere, “ASM Handbook”, Vol. 4, pp. 237-255, USA, The Materials Information Company, 1991.