PERFORMANCE EVALUATION OF WIRE ELECTRO-DISCHARGE MACHINING ON TUNGSTEN CARBIDE MAS AYU BINTI HASSAN

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1 i PERFORMANCE EVALUATION OF WIRE ELECTRO-DISCHARGE MACHINING ON TUNGSTEN CARBIDE MAS AYU BINTI HASSAN A project submitted in partially fulfillment of the requirements for the award of the Master Degree in Mechanical Engineering (Advanced Manufacturing Technology) Faculty of Mechanical Engineering Universiti Teknologi Malaysia NOVEMBER, 2006

2 iii ESPECIALLY DEDICATED TO My beloved husband Md Fauzee Bin Mohamad My supportive parents Hj. Hassan Bin Hj. Sharif Hjh. Juwita Binti Hj. Omar My wonderful brothers and sisters Asmalinda Binti Hj. Hassan Mohd Khamsyari Bin Hj. Hassan Irwan Bin Hj. Hassan Syed Syafiq Bin Hj. Hassan Muhammad Hafiz Bin Hj. Hassan Nurul Hafizah Binti Hj. Hassan And last but not least to all my relatives and my close friends Thank you for all the prayers, courage, confident and trust that you all gave to me. May Allah bless all people that I love and it s my honor to share this happiness with my love ones.

3 iv ACKNOWLEDGEMENTS First and foremost, I would like to express my deepest, sincerest gratitude to my supervisors Associate Professor Hamidon Bin Musa and Associate Professor Dr Safian Bin Sharif for their guidance and advice, without it I would never have been able to accomplish the objectives of my project. Their intelligence and determination have been a source of inspiration, their demands for quality and perfection, a challenge which taught me lessons beyond the reach of my classroom textbook. Secondly, I would like to dedicate my thanks to all technical staffs at Production Lab, Metrology Lab and Material Science Lab especially to En. Ali, En. Mahudin, En. Khalid and En. Ayub for lending me their helps and supports in completing this project. Their time and patience for providing many useful advices and ideas through the trials and tribulations during this project execution are very much appreciated. Lastly, I would also like to express my special thanks to my husband and my family members for believing in me and continuously supporting me throughout this project. Less but not least, I would like to thank for those who have contributed directly or indirectly towards the success of this research study. THANK YOU

4 v ABSTRACT This thesis presents the machining of tungsten carbide (WC-15%Co) using wire electro-discharge machining (WEDM) with brass wire diameter of 0.2 mm used as the tool electrode. The main purpose of this study was to investigate the influenced of various parameters involved in WEDM on the machining characteristics, namely, surface roughness (Ra), sparking gap (Gap), cutting speed (CS), recast layer (RL) and microcracks after undergone WEDM process. The Full Factorial Design of Experiment (DOE) approach with two-level was used to formulate the experimental layout, to analyze the effect of each parameter on the machining characteristics and to predict the optimal setting for each WEDM parameters such as pulse on (ON), pulse off (OFF), peak current (IP) and servo voltage (SV). Confirmation tests were also conducted for the optimum conditions for each machining characteristics in order to verify and compare the results from the theoretical prediction using Design Expert software and experimental confirmation tests. In this investigation, the machining operation for tungsten carbide was performed using a Sodick linear motor WEDM series AQ537L. Meanwhile, for the measurement equipments; Mitutoyo Formtracer CS-5000 was used to measure the Ra, Zeiss Axiotech High Power Optical Microscope was used to measure the Gap and lastly, the thickness of recast layer and the presence of microcracks were examined using the Scanning Electron Microscope XL40. In general, results revealed that pulse on have appeared to be the significant effect to all responses investigated. Overall, the results from the confirmation tests showed that the percentage of performance was acceptable due to all the results obtained were within the allowable values which was less than 10% of margin error.

5 vi ABSTRAK Kajian yang dijalankan ini adalah mengenai pemesinan WEDM terhadap bahan tungsten karbida (WC-15%Co) dengan menggunakan wayar tembaga yang berdiameter 0.2 mm sebagai elektrod. Tujuan utama kajian ini adalah untuk menyiasat kesan beberapa parameter yang terlibat dalam WEDM proses terhadap kriteria pemesinan seperti kekasaran permukaan (Ra), kelajuan pemotongan (CS), jarak percikan api (Gap), ketebalan lapisan tuangan semula (RL) dan rekahan kecil selepas menjalani pemesinan WEDM. Rekabentuk ujikaji dengan pendekatan Full Factorial dua tahap ini digunakan bagi merekabentuk ujikaji, menganalisis kesan setiap parameter terhadap kriteria pemesinan dan untuk menjangkakan penetapan yang optimal bagi setiap parameter WEDM seperti tempoh denyutan (ON), masa rehat (OFF), arus puncak (IP) dan voltan servo (SV). Ujikaji pengesahan juga telah dijalankan untuk setiap jangkaan parameter yang optimum, bagi tujuan pengesahan dan perbandingan keputusan di antara nilai ramalan teori menggunakan perisian Design Expert dengan nilai yang diperolehi dari ujikaji. Dalam kajian ini, pemesinan yang dijalankan ke atas tungsten karbida dilakukan menggunakan Sodick linear motor WEDM series AQ537L. Sementara itu, bagi peralatan pengukuran; Mitutoyo Formtracer CS-5000 digunakan untuk mengukur Ra, Zeiss Axiotech High Power Optical Misroscope pula digunakan untuk mengukur Gap dan akhir sekali, ketebalan lapisan tuangan semula dan kewujudan rekahan kecil diperiksa menggunakan Scanning Electron Microscope XL40. Umumnya, keputusan yang diperolehi menunjukkan, tempoh denyutan memberikan kesan yang utama terhadap kesemua respon yang dikaji. Secara keseluruhan, keputusan pengesahan ujikaji menunjukkan bahawa kesemua peratusan ralat perbezaan yang diperolehi berada di dalam lingkungan nilai yang dibenarkan iaitu peratus ralat kurang daripada 10%.

6 vii CONTENTS CHAPTER TITLE PAGE DECLARATION DEDICATION ACKNOWLEDGEMENTS ABSTRACT ABSTRAK CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF APPENDICES ii iii iv v vi vii xi xiii xvi ONE INTRODUCTION 1.1 Background of the Project Problem Statement Objective Scope Expected Results 5

7 viii TWO LITERATURE REVIEW 2.1 Introduction of Wire EDM (WEDM) WEDM Process EDM Machining Parameter Influence of Machining Parameter to the Machining Characteristics Effect of Machining Parameters on the Cracks Effect of Machining Parameter on Surface Finish Effect of Machining Parameter on Recast Layer Effect of Machining Parameter on Spark Gap Types of Wire Materials and Its Application Copper Wire Brass Wire Coated Wire Tungsten Carbide (WC) as a Work Material 26 THREE METHODOLOGY 3.1 Research Methods and Procedures Research Design and Data Analysis Design of Experiment (DOE) Two-level Full Factorial Design Variables Machining Parameters Machining Characteristics Research Procedures Experimental Set-up Experimental Equipment 42

8 ix FOUR RESULTS AND DATA ANALYSIS 4.1 Experimental Results Analysis of Results Analysis Results for Surface Roughness, Ra Analysis Results for Sparking Gap, Gap Analysis Results for Cutting Speed, CS Analysis Results for Recast Layer, RL Confirmation Tests Comparison of Test Results Verification of Mathematical Models 74 FIVE DISCUSSION 5.1 Introduction Surface Roughness, Ra Sparking Gap, Gap Cutting Speed, CS Recast Layer, RL Microcracks 91 SIX CONCLUSION 6.1 Conclusions Recommendations 96

9 x REFERENCES 97 APPENDIX A 103 APPENDIX B 106 APPENDIX C 119 APPENDIX D 130

10 xi LIST OF TABLES NO. TITLE PAGE 2.1 List of process factors for EDM Previous researches of EDM process on tungsten carbide Previous researches of EDMed surface roughness on tungsten carbide Literature reports on formation of recast layer Brass wire designations and its application The designation of various types of coated wires and their applications Basic carbide grade chart Two-level full factorial experiment with four factors The design machining parameters and their levels List of machine parameters Grade specifications of the work piece Experimental results for surface roughness, Ra Experimental results of sparking gap, Gap Experimental results of cutting speed, CS Experimental results of recast layer, RL Experimental results corresponded to each run ANOVA for surface roughness, Ra ANOVA for spark gap, Gap ANOVA for cutting speed, CS ANOVA for recast layer, RL 65

11 xii NO. TITLE PAGE 4.10 Quality characteristics of the machining performance Confirmation test results for surface roughness, Ra Confirmation test results for spark gap, Gap Confirmation test results for cutting speed, CS Confirmation test results for recast layer, RL Comparison test results for Ra Comparison test results for Gap Comparison test results for CS Comparison results for RL The comparison of setting parameters with previous researchers 92

12 xiii LIST OF FIGURES NO. TITLE PAGE 2.1a Conventional wire EDM 7 2.1b Close-up view of the gap and continuous electrical sparks 7 2.2a Micrograph of the white layer due to the EDM process (200 ) b SEM picture (at 370 ) of an EDM surface showing globules and micro cracks in the white layer Illustration of wire electric discharge machining (WEDM) model a Wire-cut sparking from the electrode side b Wire-cut sparking area The application range of straight grade cemented carbides Various types of blanks and punches made from tungsten carbide The overall research methodology The diagram of spark gap measurement Flow chart outlining the analysis steps undertaken The diagram of work piece after cutting using WEDM The diagram of cutting direction on the work piece WEDM linear motor 5-axes Sodick series AQ537L Formtracer CS (Roughness/Contour Measuring System) Mitutoyo Scanning Electron Microscope (SEM) Philips XL High Power Optical Microscope Zeiss Axiotech 44

13 xiv NO. TITLE PAGE 4.1 Normal probability plot of Ra Main effects plot of Ra Normal probability plot of Gap Main effects plot of Gap Interactions plot of Gap Normal probability plot of CS Main effects plot of CS Interactions plot of CS a Interaction between ON*SV b Interaction between OFF*SV c Interaction between ON*OFF Normal probability plot of RL Main effects plot of RL Interactions plot of RL a Interaction between OFF*IP b Interaction between ON*OFF SEM photograph on smaller Ra of tungsten carbide SEM photograph on larger Ra of tungsten carbide SEM micrographs showing globules of debris and pockmarks on WEDMed surface of tungsten carbide SEM micrographs showing shallow craters and voids on WEDMed surface of tungsten carbide D interaction of ON*SV for Gap D interaction plot of ON*SV for CS D interaction plot of OFF*SV for CS D interaction plot of ON*OFF for CS D interaction plot of ON*OFF for RL D interaction plot of ON*OFF for RL 87

14 xv NO. TITLE PAGE 5.11 SEM of recast layer after undergone WEDM process EDX analysis result for recast layer (after WEDM process) EDX analysis result for tungsten carbide workpiece surface before undergone WEDM process 91

15 xvi LIST OF APPENDICES NO. TITLE PAGE A Master project planning 103 B The overall measurement results for surface roughness 106 C SEM micrographs of the recast layer 119 D The EDX analysis results of tungsten carbide 130

16 1 CHAPTER ONE INTRODUCTION 1.1 Background of the Project Electrical discharge machining, commonly known as EDM, is a process that is used to remove metal through the action of an electrical discharge of short duration and high current density between the tool and the workpiece. There are no physical cutting forces between the tool and the workpiece involved. EDM has proved valuable especially in the machining of super-tough, electrically conductive materials such as the new space-age alloys. It can be used to produce parts with intricate shape that is impossible when using conventional cutting tools. This machining process is continually finding further applications in the metal machining industry. It is being used extensively in the plastic industry to produce cavities of almost any shape in metal moulds. Other applications are also included such as producing critical parts for aerospace, electronics and medical industries [1-2]. Although the application of EDM is limited to the machining of electrically conductive workpiece materials, the process has the capability to cut these materials regardless of their hardness or toughness.

17 2 In this research, tungsten carbide is chosen to be the work material. Tungsten carbide (WC-Co) is an important material for tool and dies mostly because of its high hardness, strength and wear resistance over a wide range of temperature. It has high specific strength and cannot be fabricated easily by conventional machining techniques. Literature reports indicated that EDM can be successfully applied to a single-phase ceramics, cermets and ceramic-matrix composites, as far as they exhibit an electrical resistivity lower than values between 100 and 300Ω.cm [3]. Since EDM has been shown to be a versatile method for machining difficult-to-work materials and suitable in conforming WC-Co cemented carbides, therefore EDM process is chosen as a method to machine tungsten carbide in this study. Although many studies have been conducted on EDM of tungsten carbide [1-4], investigations on the surface integrity on the tungsten carbide are still lacking. Hence, this study was attempts to investigate the effect of wire EDM parameters on the surface integrity of tungsten carbide, namely the surface finish, thickness of recast layer and over-cut of the wire electrode. The main focus in this study is on micro-crack formation. The machining parameters are the input parameters of EDM process, namely voltage, peak current, pulse duration and interval time, which is believed have great influence to EDMed surface. Classical Design of Experiment (DOE) is used to investigate the effect of machining variables and to establish the relationship of certain responses. 1.2 Problem Statement During the EDM process, both the workpiece and the tool undergo surface modification. Many researchers [5-14] have looked at the modification of steel workpieces, but few have examined the modification of the tungsten carbide materials. The plasticity of tungsten carbide is high and in comparison with steel, its modulus of elasticity is three times greater. Its hardness is greater than that of steel, even in very extreme conditions it does not make noticeable changes at high temperature [15].

18 3 However, the problem in machining tungsten carbide with WEDM is difficulty in achieving high accuracy and high productivity with minimum surface damage. This is due to large number of variables and uncertain nature of the process. Although most of WEDM equipment nowadays has advanced process control, selecting and maintaining optimal settings is still an extremely difficult job to do [1]. The lack of machinability data on the conventional as well as advanced materials, precise gap monitoring devices and an adaptive control strategy that accounts for the time-variant and stochastic nature of the process are the main obstacles toward achieving the ultimate goal of unmanned WEDM operation [16]. Former researchers claimed that during machining tungsten carbide, the microstructure of EDMed surface varies significantly under different peak current and pulse duration [17]. It is also reported that even for the cases where optimum surface conditions are achieved, EDM of tungsten carbide usually induces cracks and relevant residual stresses within a thermally affected zone (recast layer and adjacent regions) beneath the shaped surface [18]. Accordingly, about 70% of the punches used within dies for production of high quality are made of carbide (WC-Co) material [19]. These punches usually require high dimensional accuracy and excellent surface finish that is difficult to produce using WEDM process. Therefore, there is a need to understand the important parameters that greatly influence surface integrity when machining tungsten carbide using WEDM. As such, it is the aim of this work to investigate the correlation between the EDM machining parameters and surface integrity including micro-cracks, recast layer and surface finish. In addition, the influence of machining parameters on the spark gap during WEDM process also will be observed. The Classical Design of Experiment (DOE) technique will be employed as the methodology since this approach is powerful in providing various information of machining process.

19 4 1.3 Objective The objectives of this research are: 1. To evaluate the performance of WEDM on tungsten carbide (WC) with respect to various responses such as surface integrity of the machined surface, sparking gap and cutting speed. 2. To establish mathematical model for all responses involved which are sparking gap, recast layer, surface finish, cutting speed and microcracks of the tungsten carbide during WEDM. 3. Full factorial method from DOE used in order to analyze and determine global solutions for optimal cutting parameters of WEDM operation. 1.4 Scope The scope of this research consists of: 1. Machining tungsten carbide (work material) with brass wire using WEDM machine. 2. Surface integrity to be investigated includes surface finish, microcracks and thickness of recast layer. 3. Parameters to be studied include voltage, peak current, pulse duration and interval time. 4. Scanning Electron Microscope (SEM) will be use to evaluate the micro-cracks presence and thickness of recast layer. 5. The Classical DOE and analysis of variance (ANOVA) method will be processed using Design Expert software version

20 5 1.5 Expected Results The expected result from this study is to obtain the optimum conditions in various situations in WEDM using brass wire for machining tungsten carbide. The detail results and discussions of the effect of machining parameters on the microstructure of the machined surface such as the presence of micro-cracks and the depth of recast layer, surface roughness and spark gap of brass wire will be provided. It was hoped that the findings could be used by industry practitioners to select the most suitable cutting parameter for tungsten carbide application to realize its economic potential to the fullest. It was expected that the mathematical models that were developed could help in predicting various responses such as surface roughness, recast layer, micro-crack and spark gap for any conditions within the scope of the study conducted by the author.