«Parametric Optimization of Taper Cutting Process using Wire Electrical Discharge Machining (WEDM) A THESIS SUBMITTED IN FULFILLMENT OF THE ...»
Parametric Optimization of Taper Cutting Process
using Wire Electrical Discharge Machining (WEDM)
A THESIS SUBMITTED IN FULFILLMENT OF
THE REQUIREMENT FOR THE AWARD OF THE DEGREE
Doctor of Philosophy
Bijaya Bijeta Nayak
(Roll No. 512ME124)
Department of Mechanical Engineering
National Institute of Technology, Rourkela - 769008, India September–2015
NATIONAL INSTITUTE OF TECHNOLOGYROURKELA-769008, ODISHA, INDIA
CERTIFICATE OF APPROVAL
Dr. Siba Sankar Mahapatra Professor Department of Mechanical Engineering National Institute of Technology, Rourkela Place: Rourkela
This Thesis Dedicated to My Sweet Family Acknowledgement While bringing out this thesis to its final form, I came across a number of people whose contributions in various ways helped my field of research and they deserve special thanks. It is a pleasure to convey my gratitude to all of them.
First and foremost, I would like to express my deep sense of gratitude and indebtedness to my supervisors Prof. Siba Sankar Mahapatra for their valuable guidance, suggestions, support, scholarly inputs and invaluable encouragement throughout the research work that instil confidence in me during research and writing of this thesis. This feat was possible because of the unconditional support provided by him. I specially acknowledge him for his amicable, positive disposition, patience, motivation, enthusiasm, and immense knowledge. I consider it as a great opportunity to do my doctoral research under his guidance and to learn from his research expertise.
Besides my supervisor, I would like to express my heartiest thankfulness to the members of my doctoral scrutiny committee (DSC): Prof. D. R. K. Parhi (Chairman DSC), Department of Mechanical Engineering, Prof. S. K. Sahoo, Department of Metallurgical and Materials Engineering, Prof. S. K. Panda, Department of Mechanical Engineering and Prof. H. B. Sahu, Department of Mining Engineering of our Institute for their kind co-operation and insightful suggestions throughout period of work which has been instrumental in the success of thesis.
I am highly grateful to Prof. Sunil Kumar Sarangi, our Honourable Director and Prof. Banshidhar Majhi, Dean (Academic Affaires) of National Institute of Technology, Rourkela for their academic support and the facilities provided to carry out the research work at the Institute.
I express my thankfulness to the faculty and staff members of the Mechanical Engineering Department for their continuous encouragement and suggestions. Among them, I specially acknowledge Prof. Saurav Datta for his advice and continuous encouragement as and when required during this research. Mr.
Prasanta Kumar Pal and Mr. Sambit Senapati also deserves special thanks for his kind cooperation in non-academic matters during the research work.
Special thanks to Mr. Ambuj Ballav Nayak, Divisional Head (Marketing) and also staffs of Production department, CTTC BBSR for their kind support and co-operation during my experimental work.
I extend my heartfelt thanks to my friends especially Suman Chatterjee, Kumar Abhishek, Sangeeta Gauraha, Rajiv Kumar Yadav and Chhabi Ram Matawale i who worked with me in every difficulty that I have faced; their constant support was the tremendous source of inspiration. I would give thanks to my friends Chinmay Prasad Mohanty, Swayam Bikash Mishra, Pragyan Paramita Mohanty, Shrutee Nigam, Ankit Kumar Pandey, Raviteja Buddala, Rameez Malik, Amit Kumar Mehar and Dilip Kumar Sen for their support and co-operation which is difficult to express in words. The time spent with them will remain in my memory for years to come.
There goes a popular maximum, “Other things may change us, but we start and end with family”. I owe a lot to my parents Mr. Surendra Kumar Nayak and Mrs.
Laxmipriya Nayak for their blessings and ever increasing unconditional love for me.
A stock of loving appreciation is reserved for my brothers Mr. Sangram Bijaya Nayak and Mr. Gouri Sankar Mahapatra for their extreme affection, unfathomable belief and moral support for me. I am also grateful to my parents-in-law Mr. Suresh Samal and Mrs. Meera Samal for their support and cooperation.
The completion of this work came at the expense of my absence from home.
Words fail me to express my appreciation to my husband Mr. Ritesh Samal for his understanding, patience and active cooperation throughout the course of my doctoral dissertation. I thank him for being supportive and caring. Last, but not the least, I thank the one above all of us, the omnipresent God, for giving me the strength during the course of this research work.
BIJAYA BIJETA NAYAK
AbstractSignificant technological advancement of wire electrical discharge machining (WEDM) process has been observed in recent times in order to meet the requirements of various manufacturing fields especially in the production of parts with complex geometry in precision die industry. Taper cutting is an important application of WEDM process aiming at generating complex parts with tapered profiles. Wire deformation and breakage are more pronounced in taper cutting as compared with straight cutting resulting in adverse effect on desired taper angle and surface integrity. The reasons for associated problems may be attributed to certain stiffness of the wire. However, controlling the process parameters can somewhat reduce these problems. Extensive literature review reveals that effect of process parameters on various performance measures in taper cutting using WEDM is also not adequately addressed. Hence, study on effect of process parameters on performance measures using various advanced metals and metal matrix composites (MMC) has become the predominant research area in this field. In this context, the present work attempts to experimentally investigate the machining performance of various alloys, super alloys and metal matrix composite during taper cutting using WEDM process. The effect of process parameters such as part thickness, taper angle, pulse duration, discharge current, wire speed and wire tension on various performance measures such as angular error, surface roughness, cutting rate and white layer thickness are studied using Taguchi’s analysis. The functional relationship between the input parameters and performance measures has been developed by using non-linear regression analysis. Simultaneous optimization of the performance measures has been carried out using latest nature inspired algorithms such as multi-objective particle swarm optimization (MOPSO) and bat algorithm. Although MOPSO develops a set of nondominated solutions, the best ranked solution is identified from a large number of solutions through application of maximum deviation method rather than resorting to human judgement. Deep cryogenic treatment of both wire and work material has been carried out to enhance the machining efficiency of the low conductive work material like Inconel 718. Finally, artificial intelligent models are proposed to predict the various performance measures prior to machining. The study offers useful insight into controlling the parameters to improve the machining efficiency.
Keywords: Wire electrical discharge machining (WEDM); Taper cutting; Multiobjective particle swarm optimization (MOPSO); Maximum deviation method; Bat algorithm (BA); Deep cryogenic treatment (DCT); Artificial neural network (ANN); Support vector regression (SVR); Multi-gene genetic programming (MGGP)
BACKGROUND AND MOTIVATION
1.1 Introduction Wire electrical discharge machining (WEDM) process is a widely accepted nontraditional manufacturing process in industries. Although it is first introduced to the manufacturing industry in late 1960s, its popularity is rapidly increased in 1970s when computer numerical control (CNC) system was incorporated into WEDM (Ho et al., 2004). Die-sinking electrical discharge machining (EDM) process possesses limitations of producing tight corners or intricate patterns but the flexibility of wire in three dimensional axis of WEDM process allows to produce intricate patterns and cuts. Its broad capabilities make it as a potential non-traditional machining process because it provides an effective solution for difficult-to-machine materials applied in tooling, especially in aerospace and defence applications. WEDM provides the best alternative or sometimes the only alternative for machining conductive, high strength and temperature resistant metals, super alloys, composites with scope of generating intricate shapes and profiles (Benedict, 1987; Lok and Lee, 1997). Considering its increasing demand in the field of manufacturing, a significant amount of research has been directed to improve the machining efficiency, surface integrity and eliminate wire breakages (Huang et al., 1999; Rajurkar et al., 1997; Tosun and Cogun, 2003;
Kanlayasiri and Boonmung, 2007).
Moreover, modelling and optimization of process parameters, modelling of wire deformation and wire breakages in straight cutting WEDM has been a topic for research during last three decades (Dauw et al., 1989; Rajurkar and Wang, 1993;
Puri and Bhattacharyya, 2003; Mahapatra and Patnaik, 2007; Mukherjee et al., 2012). Generally, it is difficult to achieve curved surfaces frequently used for molds and dies using traditional processes as well as straight cutting in WEDM process.
However, it is feasible to generate the curved surfaces using WEDM’s tapering capability. Taper cutting is a useful application of the wire electrical discharge machining process used for the production of parts with complex geometry such as extrusion and cutting dies (Huse and Su, 2004; Sanchez et al., 2008; Plaza et al., 2009). During taper cutting, the wire is subject to deformation resulting in deviations in the inclination angle of machined parts. The major concerns of the tool engineers are dimensional errors and loss of tolerances encountered during taper cutting. A limited number of research works deal with performance evaluation in taper cutting using WEDM process (Kinoshita et al., 1987; Huse and Su, 2004; Sanchez et al., 2008; Plaza et al., 2009). Hence, there is an urgent need of research to improve the machining efficiency during taper cutting using WEDM.
1.2 Working principle of wire electrical discharge machining process The working principle of WEDM is similar to conventional die-sinking EDM process involving the erosion effect produced by electric discharges. The material is eroded from the work piece by a series of discrete sparks occurring between work piece and wire electrode separated by a stream of dielectric fluid which is continuously fed to the machining zone with the help of nozzle as shown in Figure 1.1. The WEDM process makes use of electrical energy generating a channel of plasma between the wire electrode (cathode) and work piece (anode) and convert it into thermal energy at a temperature of around 8000-12000°C initializing a substantial amount of melting and vaporizing of material on the surface of the work piece (Ho et al., 2004). When the pulsating direct current power supply occurring between 20,000 and 30,000Hz is turned off, the plasma channel breaks down (Krar and Check, 1997). This causes a sudden reduction in temperature allowing the circulating dielectric fluid to the plasma channel and flushes away the molten particles from the work piece surface in the form of debris.
Basically, there are four basic elements of WEDM machine such as the power supply unit, the positioning system, the drive system and the dielectric system as shown in Figure 1.2. The power supply unit comprises of electric pulse generator, motor driver units for X, Y, U, V axes positioning system and CNC controller. The positioning system comprises of a main work table (X-Y) on which the work piece is clamped, an auxiliary table (U-V) and wire drive mechanism. The main table moves along the X and Y axis and it is driven by the D.C servo motors. The power supply and dielectric system used in WEDM is similar to conventional EDM. The main difference lies only in type of dielectric used. WEDM process uses de-ionized water due to its low viscosity and rapid cooling rate. The filtration of dielectric fluid before recirculation is highly essential so that change in its insulation qualities during the machining process is minimized. In this process, a thin wire electrode continuously fed through the work piece which enables part of complex shapes to be machined with high accuracy. The wire is wound on a spool and is kept at constant tension. The drive system continuously delivers the fresh wire to the work area. The servo system maintains the gap between wire and the work piece which varies from 0.01 to 0.40 mm. WEDM is a stress-free cutting operation as the wire electrode never touches the work piece surface. The dimensional accuracy generally achieved is of the order of ±5 μm or even lower in some cases.
Figure 1.1 Working principle of WEDM process Figure 1.
2 Block diagram of WEDM (Mahapatra et al., 2007)