Electrical discharge machining of nickel-based super alloy L. Li a, R.G.Hou b, Zh.W. SiMa c

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1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Electrical discharge machining of nickel-based super alloy L. Li a, R.G.Hou b, Zh.W. SiMa c School of Mechanical Engineering of Shandong University of Technology,Zibo, Shandong,China a sdutlili@163.com, b hourongguo@sdut.edu.cn, c smzw0827@163.com Keywords: Electrical discharge machining, nickel-based alloy Abstract. This paper presents an experimental investigation of the machining characteristics of IN718 nickel-based super alloy in wire electrical discharge machining (WEDM) process. During experiments, parameters of discharge current and pulse on time were changed to explore the effect on various aspects of the surface characteristic. Scanning electron microscopy (SEM), surface roughness and micro hardness tests were performed. Experimental results reveal that the surface roughness will increase with the increasing of current and pulse duration. Micro hardness tests show there is no hard phenomenon. Introduction Nickel based super alloys are widely used in aerospace, gas turbine, rocket, nuclear and chemical vessels because of its excellent performance including high melting temperature, high toughness as well as resistance to thermal fatigue, good corrosion and creep resistance[1,2]. The most advantage is its high mechanical and chemical properties at elevated temperatures. It is reported that Nickel-based alloys compose over half of the materials used in the aerospace industry, in particular in the hot sections of gas turbine engines. Among nickel-based alloys, In-718 is one of a family of nickel-based proven high-performance materials which is used extensively in the aerospace industry for the got section of gas turbine engines for components such as turbine disk, blades, combustors, etc[3,4]. It plays an increasingly important part in the aerospace industry. Wire Electrical discharge machining (WEDM) is a non-traditional machining process in which a pulsed voltage difference between electrode and conductive workpiece initiates sparks which erode workpiece material[5,6]. In the process, it does not make direct contact between the electrode and the workpiece, so material with high strength, hardness and toughness could be machined effectively by WEDM [7]. The aim of this study is to study the machined surface characteristics of IN718 after WEDM promoting the quality of the EDM process by developing a thorough understanding of the relationship between the WEDM process and surface characteristics. Experimental procedures The experimental study was carried out on a Sodick Wire EDM machine. The possible more influential machining parameters were selected which including pulse on time and current. The machining cut was carried out in de-ionized water using a brass with 0.01 diameter as wire electrode. Table1 shows the values of current and pulse on time on different samples. Table1 Values of current and pulse on time on different samples Maching paramenter value Pulse on time: ON ( µ s ) 4,8,12,16,20 Pulse current:ip(a) 1,2,3.5,5,6 Ratio of discharge 3:1 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-06/03/16,14:52:45)

2 Advanced Materials Research Vols After completion of Wire EDMed process, following experimental techniques were employed to study surface characteristic of the machined surface. Surface roughness of each sample was measured six times at various positions. Three measurements were parallel to the cutting speed and three measurements were perpendicular to the cutting speed. Scanning electron microscopy (SEM) is employed to examine the surface integrity and the recast layer. Surface hardness was determined with micro-hardness testing machine. Results and Analysis Fig.1 and 2 show surface roughness versus pulse current and pulse on time after WEDM. Roughness was measured along the directions parallel and perpendicular to the cutting direction. Each equivalent roughness was obtained by averaging three measurements at various positions of workpiece surface. Three measurements were parallel to the cutting speed and three measurements were perpendicular to the cutting speed. According to these figures, the cutting speed direction has no effect on surface roughness value and the surface directionality is negligible. Results also show that surface roughness increases as the pulse current and pulse on time increase. This is explained by the fact that as the pulse current increase, discharge strikes the surface more intensely, results in a deeper and larger erosion crater on the surface. Furthermore, as the pulse on time increases, the amount of heat energy transferred to the sample increases, and so more material melts. This molten material is not swept away from the surface by the dielectric material, it will be re-solidified during the rapid cooling process. The effect of pulse on time on surface roughness is more evidence than that of current. Surface rougnness/µm Pulse current/a Fig.1 Surface roughness of machined surface Pa Pe Surface rougnness/µm Pulse on time/ µ s Fig.2 Surface roughness of machined surface Pa Pe Fig.2, Fig.3 show SEM photographs of high discharge energy and low discharge energy. It is clear that the morphology of the EDM was dependent on the applied pulsed current and pulse on duration. During the EDM process, the every discharge produces very high temperatures at the sparking point, causing a part of the specimen to melt and evaporation, a crater is then formed on the machined surface. The size of the crater is influenced by discharge energy. The SEM figures are characterized by discharge craters, droplets, micro-voids and debris. These effects become more pronounced as the current and pulse on time increase. Unlike many EDMed surface in literature, micro cracks can not be seen on the surface at high magnification SEM images. Compared with Fig. 2 and Fig.3, there are many micro-voids and the voids are more when machining energy is low. During discharge, the presence of large volume of gases in the channel will lead to a high super saturation of gas in the molten pool. The energy provided by the plasma channel melts the material, but is insufficient to produce a high exploding pressure which can spray all the molten metal away from the EDM surface. When the remaining molten material solidifies on the surface, the gas bubbles would expel from the molten material, and result in micro-voids. As the current decreases, the discharge energy decreases, more percentage of the molten material is not swept away and solidify on the surface, resulting in more micro-voids.

3 380 Frontiers of Materials, Chemical and Metallurgical Technologies Fig.2 Surface photographs of EDM (ON:16, IP:5) Fig.3 Surface photographs of EDM (ON: 4, IP:2) Micro-hardness results are shown in Fig.4, which indicate that there was no significant difference between the hardness on the surfaces with and without undergone EDM process. Wire-EDM process had no effect on the machined surface below the hardness of the bulk material Microhardness/Hv Distance from surface/mm Fig.4 Microhardness of machined surface Conclusions In this experimental study, the effect of WEDM parameters of current and duration on machining characteristics of IN718 was investigated. Summarizing the main results, the following conclusions may be drawn. (1) The machined surface texture is composed of a random array of overlapping craters, debris, melted drops and micro-voids. No micro cracks on the EDMed surface at high magnification SEM. (2) The surface roughness is increasing with EDM discharge energy. And the cutting speed direction has no effect on surface roughness value. (3) There is no change of hardness with and without undergoing the EDM process.

4 Advanced Materials Research Vols Acknowledgement This project was supported by Natural Science Foundation of Shandong Province(ZR2009FQ030). References [1] Huang-Sung Liu, 2005, A study on the characterization of high nickel alloy micro-holes using micro-edm and their applications, Journal of materials processing technology, 169, [2] A.R.C.Sharman, J.I.Hughes,and K.Ridgway, 2004, Workpiece surface Integrity and Tool Life Issues When Turning Inconel718 Nickel Based Superalloy, Machining science and technology, Vol.8 No.3, [3] Durul Ulutan, Tugrul Ozel, 2011, Machining induced surface integrity in titanium and nickel alloys: A review, International journal of machine tool and manufacture, 51, [4] H.T.Lee, T.Y.Tai, 2003, Relationship between EDM parameters and surface crack formation, Journal of materials processing Technology, 142, [5] I. Puertas, C.J. Luis, 2003, A study on the machining parameters optimisation of electrical discharge machining, Journal of Materials Processing Technology, , [6] B. Lauwers, J.P. Kruth, W. Liu, W. Eeraerts, B. Schacht, and P. Bleys, 2004, Investigation of material removal mechanisms in EDM of composite ceramic material, Journal of Materials Processing Technology, 149(1-3), [7] R.S.Paeade, Suhas S.Joshi, P.K.Brahmankar, and M.Rahman, 2007, An investigation of cutting forces and surface damage in high-speed turning of Inconel 718, Journal of Materials processing technology, ,

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