T.C.
ALTINBAS UNIVERSITY
Institute of Graduate Studies
Mechanical Engineering
Numerical Investigation of Transient Thermal and
Structural Analysis in Electrical Discharge
Machining (EDM) Process by Using Finite Element
Method
Mohammed Hatem Nassr AL-AZZAWI
Master of Science
Supervisor
Asst. Prof. Dr. Suleyman BASTURK
Numerical Investigation of Transient Thermal and Structural Analysis in
Electrical Discharge Machining (EDM) Process by Using Finite Element
Method
by
Mohammed Hatem Nassr AL-AZZAWI
Mechanical Engineering
Submitted to the Institute of Graduate Studies in partial fulfillment of the requirements for the degree of
Master of Science
ALTINBAŞ UNIVERSITY 2020
The thesis titled “Numerical Investigation of Transient Thermal and Structural Analysis in Electrical Discharge Machining (EDM) Process by Using Finite Element Method ” prepared and presented by “Mohammed Hatem Nassr AL-AZZAWI” was accepted as a Master of Science Thesis in Mechanical Engineering.
Co-Supervisor
Asst. Prof. Dr. Suleyman BASTURK Supervisor
Thesis Defense Jury Members:
Prof. Dr. Osman Ergüven VATANDAŞ Faculty of Engineering and Architecture,
Istanbul Gelişim University __________________ Asst. Prof. Dr. Süleyman BAŞTÜRK Faculty of Engineering and
Natural Science,
Altınbaş University __________________ Asst. Prof. Dr.İbrahim KOÇ Faculty of Engineering and
Natural Science,
Altınbaş University __________________
I certify that this thesis satisfies all the requirements as a thesis for the degree of Master of Science.
Approval Date of Institute of Graduate Studies: ____/____/____
I hereby declare that all information in this document has been obtained and presented in accordance with academic rules and ethical conduct. I also declare that, as required by these rules and conduct, I have fully cited and referenced all material and results that are not original to this work.
DEDICATION
I would like to dedicate this work to my first teacher, my mother, my first supporter and role model, and my companion throughout the journey and my father and my brothers. Without you, this dream would never come true and I give this thesis in a form of appreciation and I may ask Allah to accept this work.
ACKNOWLEDGEMENTS
I might want to offer my thanks to every one of the individuals who have upheld me all through the regularly extended periods of this voyage. I might want to thank my advisor, Asst. Prof. Dr. Suleyman BASTURK for being my compass notwithstanding, when I believed I was lost and being in extraordinary part in charge of the zenith of this work. I might likewise want to thank my supervisor for his supportive exhortation, which incredibly enhanced the nature of this work. Finally, I thank this institution for hosting me during these years, securely earning its place as my home. I am very thankful to my mom and dad, whose values and education motivate me to keep asking questions; to my siblings and family for their infinitely appreciated love and to my country which, although inanimate, keeps me anchored and offers me an example of resiliency.
ABSTRACT
Numerical Investigation of Transient Thermal and Structural Analysis in
Electrical Discharge Machining (EDM) Process by Using Finite Element
Method
Mohammed Hatem Nassr AL-AZZAWI M.Sc., Mechanical Engineering, Altinbaş University,
Supervisor: Asst. Prof. Dr. Suleyman BASTURK Date: November 2020
Pages: (67)
Electrical discharge machining (EDM) can be categorized as unconventional processes for removing materials. EDM is widely used in many applications such as molding and dies fabrication. This thesis research deals with the heat and stress analysis of the production of the EDM machining of titanium and stainless steel sheets. The effect of undercutting velocity on sheet stresses as well as thermal properties by using ANSYS (Transient Thermal and Transient Structural) software has been investigated. The analysis set was achieved with 10 steps with 30, 20 and 10 seconds under the surrounding temperature of 27oC as well as the initial temperature of the electrode and the workpiece as 27oC. Gaussian heat flux profile with heat transfer by conduction, convection, and radiation were modeled. The electrode moves in the x-direction with three various velocity such as 0.667 mm/s, 1 mm/s, and 2 mm/s in the middle of the workpiece. Heat flux distribution is more operative where the heat was distributed quickly, and this will help to protect the electrode. The maximum temperature in the case of the titanium sheet is more than the stainless steel sheet. The equivalent mechanical stresses that resulted from thermal stresses are studied and the distribution of equivalent stresses was performed with the second velocity where the residual stress is small because the solidification layer melts thinly.
Özet
Sonlu Elemanlar Yöntemi Kullanarak Elektro Erozyon (EDM) İşleminin
Geçici Termal ve Yapısal Analizinin Sayısal İncelenmesi
Elektro erozyon ile işleme işlemi (EDM), malzemelerin şekillendirilmesi için geleneksel olmayan işlemlerden biri olarak kategorize edilebilir. EDM işlemi, kalıplama ve kalıp üretimi gibi birçok uygulamada yaygın olarak kullanılmaktadır. Bu tez çalışmasında, titanyum levhanın EDM ile işlenmesi esnasında oluşan ısı ve stres analizi yapılmıştır. ANSYS (Geçici Termal ve Geçici Yapısal) Programı kullanımı ile kesme hızının alt tabaka kesme gerilmeleri üzerindeki etkileri ve termal özellikleri incelenmiştir. Analiz seti, 30, 20 ve 10 saniyelik çevre sıcaklığı 27 oC ile 10
aşamada gerçekleştirilmiştir. İletim, konveksiyon ve radyasyon yoluyla ısı transferi ile Gauss ısı akısı profili modellenmiştir. Elektrot, iş parçasının ortasında üç farklı hızda 0.667 mm/s, 1 mm/s, ve 2mm/s ile x ekseni yönünde hareket etmiştir. Elektrot ve iş parçasının başlangıç sıcaklığı 27
oC'dir. Isı akısı dağılımı, ısının hızlı bir şekilde dağıtıldığı yerde daha etkilidir ve bu, elektrotun
korunmasına yardımcı olmaktadır. Termal gerilmelerden kaynaklanan eşdeğer mekanik gerilmeler incelenmiş ve eşdeğer gerilmelerin dağılımının yeniden katılaştırma tabakasının çok az erimesinden dolayı artık gerilmenin küçük olduğu ikinci hız ile gerçekleştirilmiştir.
TABLE OF CONTENTS
Pages
ABSTRACT ... vii
LIST OF FIGURES ... xii
LIST OF ABBREVIATIONS ... xiv
LIST OF SYMBOLS ... xv
1. INTRODUCTION ... 1
1.1 MACHINING ... 1
1.2 EDM CUTTING ... 1
1.2.1 Advantages and Disadvantages of EDM ... 2
1.3 EDM CUTTING TYPES ... 3
1.3.1 Die-sink EDM ... 3
1.3.2 Wire-cut EDM ... 4
1.3.3 Applications ... 6
1.3.3.1 Prototype production... 6
1.3.3.2 Small hole drilling ... 7
1.3.3.3 Metal disintegration machining ... 8
1.4 MATERIALS OF ELECTRODE ... 8
1.4.1 Types ... 9
1.4.2 Specifications ... 10
1.5 OVERVIEW OF PROCESS MODEL DEVELOPMENT ... 10
2. LITERATURE REVIEW ... 12
2.1 INTRODUCTION ... 12
2.2 THE NOVELTY OF THE STUDY ... 21
3. NUMERICAL STUDY ... 22
3.1 MATERIAL AND GEOMETRY SELECTION ... 23
3.2.1 Physical Model ... 26
3.2.2 Meshing ... 26
3.2.2.1 Governing equations ... 28
3.2.3 Boundary and Initial Condition ... 29
3.3 MODEL VALIDATION ... 29
4. RESULTS AND DISCUSSIONS... 31
4.1 THE TEMPERATURE AND FLUX DISTRIBUTION AT 0.667 MM/S SPEED ... 31
4.2 THE TEMPERATURE AND FLUX AT 2 MM/S SPEED ... 38
4.3 THE EFFECTS OF CUTTING SPEED ON TEMPERATURE ... 44
4.4 THE EFFECTS OF MATERIALS TYPE ON THE TEMPERATURE ... 45
4.5 EQUIVALENT STRESS DISTRIBUTION FOR WORKPIECE AT 2 MM/S SPEED. 50 5. CONCLUSION ... 53
LIST OF TABLES
Pages
Table 3.1: Properties of titanium sheet. ... 24
Table 3.2: Properties of stainless steel. ... 24
Table 3.3: Properties of graphite electrode. ... 25
LIST OF FIGURES
Pages
Figure 1.1: Schematic diagram of experimental set-up for die-cut EDM process [1]. ... 4
Figure 1.2: EDM process model development and optimization approach [4]. ... 11
Figure 2.1: Effect of pulse on-time on the material removal rate at different workpiece speeds [8]. ... 13
Figure 2.2: Temperature distribution versus the radial distance from the electrode axis [14]. ... 15
Figure 2.3: The arithmetical mean roughness values versus the spark time [16]. ... 16
Figure 3.1: Three dimensional model used for simulation. ... 25
Figure 3.2: Mesh generation in 3D dimension and elevation and top views ... 27
Figure 3.3: Explanation of Gaussian heat flux with boundary condition of one pulse. ... 29
Figure 3.4: Comparison between the experimental (Natsu et al [13]) and numerical results. ... 30
Figure 3.5: Simulation results of one spark for validation the results ... 30
Figure 4.1: The relation between the temperature and time during the cutting process. ... 32
Figure 4.2: Temperature distribution for a three-dimensional model at various times: (a) 6 s, (b) 12 s, (c) 24 s, (d) 30 s. ... 34
Figure 4.3: Temperature distribution for elevation view across the thickness of the workpiece at various times: (a) 6 s, (b) 12 s, (c) 24 s, (d) 30 s... 35
Figure 4.4: Heat flux distribution for the top view of the workpiece at various times: (a)6 s, (b)12
s, (c) 24 s, (d) 30 s. ... 37
Figure 4.5: The relation between the temperature and time during the cutting process for 10 seconds. ... 39
Figure 4.6: Temperature distribution for a three-dimensional model at various times: ... 41
Figure 4.7: Temperature distribution for elevation view at various times: ... 42
Figure 4.8: Heat flux distribution for top view of the workpiece at various times: ... 44
Figure 4.9: The effect of cutting speed on the average temperature. ... 45
Figure 4.10: Maximum temperature versus the time for titanium and stainless steel. ... 46
Figure 4.12: Temperature distribution of top view for stainless steel workpiece material (a) t=3 s, (b) t=5 s, (c) t=7 s, (d) t=10 s ... 48
Figure 4.13: Temperature distribution of elevation view for stainless steel workpiece material (a) t=3 s, (b) t=5 s, (c) t=7 s, (d) t=10 s ... 49
Figure 4.14: Equivalent stress distribution for the workpiece at cutting speed 2 mm/s at (a) 3 s, (b) 5 s, (c) 7 s, and (d) 10 s. ... 52
LIST OF ABBREVIATIONS
UDF : User Defined Function
CNC : Computer Numerical Control EDM : Electrical Discharge Machining FEM : Finite Element Method
ANN : Artificial Neural Networks SEM : Scanning Electron Microscope GA : Genetic Algorithm
CAD : Computer-Aided Design MRR : Material Removed Rates
LIST OF SYMBOLS
t : Time (s) T : Temperature (K) k : Thermal conductivity (W/m. K) cp : Specific heat (J/kg. K) V : Cutting velocity (m/s) L : Cutting length (m) qrad : Heat flux (W/m2)Σ : Boltzmann constant FR : Feed-rate (m) s : Second
ρ : Density (kg/m3) Φ : Radiative source term Ε : Emissivity of the surface A : Surface area (m2)
Q : Heat transferred by convection (W/m2)
h : Heat transfer coefficient (W/m2.K)
Ts : Surface temperature (K)
1.
INTRODUCTION 1.1 MACHININGMachining is one of the methods in which a piece of raw material is sliced by a managed material-removal process into the desired final form and size. In comparison to processes of controlled material addition, which are known as additive manufacturing, the processes that have this common theme, controlled material elimination, are now collectively known as subtractive processing. It may differ precisely what the "regulated" part of the concept implies, but it almost always implies the use of machine tools (besides just power tools and hand tools). Machining forms part of the manufacturing of many metal goods but may also be used with materials such as lumber, rubber, ceramics and composites. An individual is called a machinist who specializes in machining. A room, house, or business is called a machine shop where machining is performed. Most of the modern-day machining is carried out by computer numerical control (CNC), in which the movement and operation of mills, lathes and other cutting machines is controlled by computers. This increases productivity as the CNC system works unmanned, thereby reducing system shops' labor costs.
1.2 EDM CUTTING
Electrical discharge machining (EDM), also known as flame machining, flame eroding, die sinking, wire burning or wire erosion, is a method of metal fabrication whereby electrical discharges (sparks) are used to achieve a desired form. A series of quickly repeated current discharges between two electrodes, isolated by a dielectric liquid and subject to an electrical voltage, strip material from the work piece. The instrument-electrode, or simply the instrument or electrode, is one of the electrodes, while the other is considered the workpiece-electrode, or piece of work. The method relies on not making direct contact with the instrument and work piece. Material is removed from the electrodes as a consequence. Once the current stops (or is stopped, depending on the generator type), new dielectric liquid is conveyed into the volume of the inter-electrode, allowing the solid particles (debris) to be carried away and the dielectric's insulating properties to be restored. The addition of new dielectric fluid to the volume of the inter-electrode is usually referred to as flushing. The voltage between the electrodes is returned to what it was
before the breakdown after a current flux, so that a new liquid dielectric breakdown will occur to restart the cycle.
1.2.1 Advantages and Disadvantages of EDM
Advantages of EDM include of:
a- Exceptionally strong material to tolerances that are very similar.
b- Very small work bits where the component can be weakened by unnecessary cutting tool pressure by traditional cutting tools.
c- There is no immediate interaction between the instrument and the piece of work. Therefore, without perceivable distortion, fragile parts and poor materials can be machined.
d- A good surface finish can be obtained
e- Redundant finishing paths can create a very good surface.
f- Complicated forms using traditional cutting methods that would otherwise be hard to make. g- It is possible to achieve very fine holes.
h- Tapered holes can be manufactured.
i- Limited internal contours and internal corners of the pipe or bottle.
Disadvantages of EDM include of:
a- Difficulty seeking competent machinists. b- The slow rate of removal of materials.
c- Potential fire hazard associated with use of combustible oil based dielectrics. d- The extra time and cost used for producing electrodes for ram / sinker EDM. e- Due to electrode wear, reproducing sharp corners on the workpiece is difficult. f- It is very high for particular power usage.
g- The use of electricity is high. h- Overcut shall be created.
i- Undue wear of the instrument happens during machining.
j- Only with a particular set-up of the device will electrically non-conductive materials be machined.
1.3 EDM CUTTING TYPES 1.3.1 Die-sink EDM
Two scientists from Russia, B. R. Lazarenko, as well as N. In 1943, I. Lazarenko was charged with exploring methods of avoiding tungsten electrical touch erosion due to sparking. In this mission, they struggled, but found that if the electrodes were submerged in a dielectric fluid, the erosion was more precisely regulated. This prompted them to create an EDM system that was used to deal with hard-to-machine materials like tungsten. The Lazarenkos' unit, after the resistor-capacitor circuit (RC circuit) used to charge the electrodes, is known as an R-C-type unit. An American team, Harold Stark, Victor Harding, and Jack Beaver, concurrently but separately, created an EDM machine for extracting damaged drills and taps from aluminum castings. In initially constructing their devices from under-powered electric-etching equipment, they were not quite successful. Yet practical devices developed more efficient sparking systems, together with automatic spark replication and fluid substitution with an electromagnetic interrupter arrangement. Machines by Stark, Harding, and Beaver were able to generate 60 sparks per second. Vacuum tube circuits which were able to create thousands of sparks per second were used by later machines based on their architecture, greatly improving the cutting speed. The Schematic Diagram of the experimental set-up for the die-cut EDM method is shown in Figure 1.1.
Figure 1.1: Schematic diagram of experimental set-up for die-cut EDM process [1].
1.3.2 Wire-cut EDM
For producing tools (dies) from hardened steel, the wire-cut form of machine emerged in the 1960s. In Wire EDM, the tool electrode is merely a wire. The wire is wound between two spools to prevent the erosion of the wire that allows it to snap, meaning that the working portion of the wire is continually shifting. Conversions of punched-tape vertical milling machines were the first numerical controlled (NC) machines. In 1967, in the USSR, the first commercially available NC computer designed as a wire-cut EDM machine was produced. Machines that could optically follow lines on a master drawing were developed at Andrew Engineering Company for milling and grinding machines by the group of David H. Dulebohn in the 1960s. For greater precision, master drawings were later produced by computer numerical controlled (CNC) plotters. A wire-cut EDM system was developed in 1974 using the techniques of the CNC drawing plotter and optical line follower.
To directly control the EDM system, Dulebohn later used the same plotter CNC software, and the first CNC EDM system was manufactured in 1976. The Wire electrical discharge machining process is a sophisticated process of machining which is used to create parts with complicated shapes and structures. With great versatility and excellent precision, it is capable of producing 2D and 3D elements. The tolerance is ± 0.0001 mm and the plate thickness can be cut up to 300 mm
by A Wire electrical discharge machining. The thin wire of around 0.2 mm diameter is machined in the Wire electrical discharge. The electrical discharge machining of the wire is based on the electro-thermal mechanism principle. The Wire electrical discharge machining Technology is based on the conventional EDM sparking process. The material is eroded ahead of the wire and there is no contact between the tool and workpiece during machining.
Exotic, high pressure, super alloys and temperature tolerant materials can be machined by the Wire electrical discharge machining technique. As seen in Figure 1. the Wire EDM process is capable of cutting diverse materials such as aluminum, bronze, copper, tungsten, titanium, molybdenum, nickel, stainless steel and composites. Using thin wire, the electrical discharge machining wire discharges the electrified current. A very high temperature of around 8000-12,000 oC is encountered by the Wire EDM process. The dispersed melting and vaporization of working material culminated in the creation of intense sparks. The metal corrosion in Wire EDM by means of fast consequent spark discharges between the electrode of the device and the workpiece. Using mechanical tension systems, the wire is kept under tension, minimizing the propensity of incorrect sections. The discharge of sparks repeats up to 250,000 times per second quickly, and there is no contact between the material and the wire electrode. The discharge sparks are retained in Wire EDM from the gap (0.025 to 0.05 mm) around the workpiece and wire. The WEDM uses deionized water as a dielectric solution, serving as a non-conductive buffer, stopping the electrically conductive channel in the machining region from forming. Wire breakage and wire vibration cause the undesirable features of machining to greatly affect the precision and performance quality of the machining. The breakage of the wire results in an adverse thermal load and high electrode temperature. The Wire EDM is the best choice for dimensional precision and surface finish consistency to manufacture micro-scale components. The most common applications of Wire EDM follows as: Fixtures, Die & mold, extrusion dies, stamping dies, prototype, stripper plates, aircraft components, missile devices and medical parts. Wire EDM 's characteristics include: lower cutting power, no tooling, better utilization, strong surface finish, repeatable length without deformation, and complicated profile machining. The Wire EDM is used in numerous applications: in the aerospace, industrial, pharmaceutical, rocket, chemical, robotics and instrument industries. Wire EDM is used to manufacture special gears, bearing cages, splints and hand tools. Therefore, without losing machining efficiency, the Wire EDM technique is an accurate and effective machining method.
Figure 1.2: Photograph for wire-cut EDM process [2].
1.3.3 Applications
1.3.3.1 Prototype production
In the mold-making, tooling, and die industries, the EDM process is more commonly used, but it is becoming a common method of making prototype and production parts, especially in the aerospace, automotive and electronics industries where production volumes are relatively low. In sinker EDM, the desired (negative) form is machined into a graphite, copper tungsten, or pure copper electrode and fed into the workpiece at the end of a vertical ram. Coinage die making: top master, bottom badge die workpiece, left oil jets have been drained. Initial flat stamping can be compromised to give a curved surface, see sinking metalworking. By the use of a pancake die through the coinage stamping process, the positive master can be made of sterling silver for the creation of dies for the production of jewelers and badges, or blanking and piercing, because the master is greatly eroded and is used only once with appropriate machine settings. In order to produce stamped flats from cutout sheet blanks of bronze, silver, or low proof gold alloy, the resulting negative die is then hardened and used in a drop hammer. For badges, these flats may be
further created by another die on a curved surface. Usually, this form of EDM is carried out immersed in a dielectric based on gasoline. Strong (glass) or soft (paint) enamel, or electroplated with pure gold or nickel, can further refine the finished object. As a refinement, softer materials such as silver can be hand etched.
1.3.3.2 Small hole drilling
In a number of uses, small hole drilling EDM is used. Small hole drilling EDM is used on wire-cut EDM machines to generate a hole in a workpiece from which the wire for the wire-wire-cut EDM process is threaded. On a wire-cut unit, a discrete EDM pate specially for small hole drilling is installed which permits big hardened plates to have finished components eroded from them as necessary and without pre-drilling. To drill rows of holes into the leading and trailing edges of turbine blades used in jet engines, small hole EDM is used. The leakage of gas into these tiny holes allows the engines to work at higher temperatures than otherwise feasible. The high-temperature, very complicated, single crystal alloys used in these blades make it incredibly difficult, if not impossible, to conventionally machine these holes with a high aspect ratio. Small hole EDM is also used for the development of microscopic orifices for parts of the fuel system, spinners for synthetic fibers such as rayon’s, and other applications. There are also stand-alone small hole drilling EDM devices with an x-y axis that can be machined blind or through holes, also known as a super drill or hole popper. EDM drills bore holes with a long brass or copper tube electrode that, as a flushing agent and dielectric, rotates in a chuck with a steady flow of purified or deionized water running through the electrode. The electrode tubes, having a spark gap and wear rate, work like the wire in wire-cut EDM machines. Some small-hole drilling EDMs will drill in less than 10 seconds through 100 mm of soft or hardened steel, averaging a wear rate of 50 percent to 80 percent. It is possible to achieve holes of 0.3 mm to 6.1 mm in this drilling process. Brass electrodes are simpler to machine, but due to eroded brass fragments triggering "brass on brass" wire breakage, copper is also not recommended for wire-cut operations.
Figure 1.3: Many applications of EDM cutting [3]. 1.3.3.3 Metal disintegration machining
For the exact determination of eliminating broken instrument drill bits, taps, bolts and studs from workpieces, numerous constructors produce EDM machines. The method is called metal disintegration machining or MDM in this application. Only the center of the tap, bolt or stud is removed by the metal disintegration process, leaving the hole intact and allowing a portion to be recovered.
1.4 MATERIALS OF ELECTRODE
Products with greater thermal conductivity and melting point are used as a method for the EDM machining operation. Nickel, graphite, nickel tungsten, silver tungsten, copper graphite, and brass are used in EDM as an element of the instrument (electrode). All of them have the properties of fine wearing, better conductivity and advanced machining sparking conditions. Sparking conditions which is typically used in specialized applications where, such as small hole drilling, high wear is appropriate. The factors that affect electrode material selection are metal removal
rate, wear resistance, necessary surface finish, and the cost of manufacturing electrode material, and the consistency and characteristics of the working material to be machined. EDM electrodes consist of materials such as graphite or copper that are extremely conductive and/or resistant to arc erosion. EDM is an acronym for electrical discharge machining, a technique that erodes metal using a regulated electrical spark. Components made from brass, copper and copper alloys, graphite, molybdenum, platinum, and tungsten include EDM electrodes. Electric discharge machining (EDM) makes it possible to deal with metals that are useless in conventional machining techniques. It only works (except by specific design) with materials that are electrically conductive. Tiny, odd-shaped angles and detailed contours or cavities in hardened steel as well as rare metals such as titanium and carbide can be sliced using periodic electric discharge.
1.4.1 Types
EDM electrode materials need to have properties that allow charge easily and yet resist the corrosion encouraged and stimulated by the EDM method in the metals that it machines. Alloys provide properties that offer multiple benefits depending on the application’s needs. Brass is a copper and zinc alloy. For the forming of EDM wire and small tubular electrodes, brass materials are used. Brass, like copper or tungsten, does not resist wear, but is much simpler to machine and can be die-cast or extruded for advanced applications. As new wire is constantly fed during the EDM wiring cutting process, EDM wire does not need to have wear or arc corrosion resistance. Alloys of copper and copper have greater wear resistance to EDM than steel, but are harder to machine than either steel or graphite. It is more costly than graphite as well. However, since it is highly conductive and solid, copper is a common base material. It is useful in tungsten carbide EDM machining, or in applications that need a fine finish. The copper tungsten components are tungsten and copper composites. Using powder metallurgy techniques, they are made. Compared to other electrode materials, copper tungsten is very pricey, but is useful for creating deep slots under bad flushing conditions and in tungsten carbide EDM machining. Often used in resistance welding electrodes and some circuit breaker applications are copper tungsten materials. At low speeds, graphite gives a cleaning effect. Carbon graphite was one of the first produced types of brush material and is used in many older engines and generators. It has a form that is amorphous. For producing EDM wire, molybdenum is used. For small slot work and for applications involving
extremely small corner radii, it is the wire of choice. Molybdenum exhibits high tensile strength and good conductivity, making it suitable for challenging applications where small diameter wire is needed.
Silver tungsten materials are fragments of tungsten carbide spread in a silver matrix. Silver offers high electrical conductivity and, in high-power applications, tungsten has excellent corrosion resistance and good anti-welding characteristics. For EDM electrode applications where optimizing conductivity is key, this composite is thus the ideal alternative. In EDM machining applications that need a fine finish, tellurium copper is useful. Tellurium copper is comparable to brass and stronger than pure copper in its machinability.
1.4.2 Specifications
Besides its shape and purpose, the most important factors when choosing EDM electrodes are the conductivity (or resistivity) of the substrate and its resistance to erosion. Conductivity facilitates the effectiveness of cutting, since the "cutting instrument" is electric current. Erosion resistance (a melting point, hardness, and structural integrity factor) gives a longer service life to the electrode and reduces the replacement frequency. When choosing an electrode, these features, which differ almost entirely by the type of alloy or substrate used, must be the determining factors.
1.5 OVERVIEW OF PROCESS MODEL DEVELOPMENT
Figure 1.4 shows the proposed approach to creating an intelligent process model for EDM using the Finite Element System (FEM), Artificial Neural Networks ( ANN) and Genetic Algorithm (GA). It consists primarily of three stages: computational (FEM) simulation of the process of EDM, taking into account the thermo-physical features of the process. Creation of an ANN-based process model based on data produced by numerical (FEM) model simulation. Optimal collection of process parameters using ANN process model optimization based on GA. The unique merit of this integrated model creation approach (FEM-ANN-GA) is that it is based on reliable FEM interpretation and not on experimental data collection, which may be expensive , time-consuming and vulnerable to error.
2.
LITERATURE REVIEW 2.1 INTRODUCTIONNowadays, electrical discharge machining (EDM) is considered the most vital method for cutting. Titanium alloys have properties of high strength-to-density and corrosion resistance and have a wide variety of applications due to these special characteristics. Titanium and its alloys are classified as hard-to-cut materials using traditional production processes. Sinking EDM is utilized for cutting the titanium alloys.
Arthur and Dicken [5] investigated the potential for manufacture of electro-discharge machining (EDM) electrodes, using rapid prototyping (RP) technologies. Several probable routes were indented to include a combined system from CAD data concluded to manufacture tooling. One such route is the production of RP models by electrodepositing copper on stereolithography (SL). Electrodes may be used at low Material Removal Rates (MRR), generally referred to as semi-rough or finishing cuts. Higher MRR generates higher heat on the cutting face which causes the RP electrodes to fail. This failure is thought to occur by combining deformation, thinning, and electrode distortion. The heat distribution and related failure modes for these electrodes are being studied to determine how to boost the machining efficiency.
Abdel-Khalik et al. [6] carried out in support of the design efforts to manufacture tritium the accelerator production of tritium accelerator., the accelerator production of tritium system designs involves water-cooled microchannel which can experience off-normal boiling. The results showed that the Critical heat flux occurs in all the experiments at very high flow rates (0.36 and higher) and suggests dry out incidence. Besides, the Critical heat flux tends to grow monotonically as mass flex or pressure increases. The Critical heat flux depends on channel cross-section geometry and increases with increasing channel diameter, unlike high mass flux data. The dissolved air for the smaller circular channel raises Critical heat flux marginally and decreases Critical heat flux for the other test pieces. The experimental data were contrasted with the three commonly used empirical correlations predicted.
Ochoa et al. [7] developed small-scale EDM systems and calculated the effect of the electrical arc output voltage and current on the consistency and efficiency of the workpiece machining
operation. The suggested scheme is a resonant converter operated by dc to dc mode to produce current-controlled arc pulses at a constant frequency that erodes the workpiece. The advantages of this proposal are that the device has inherent short-circuit safety, and a simple linear controller results in a highly stable load feedback control that guarantees zero-voltage transistor turn-on at any load value. The output voltage is intended to be adjusted by an external.
Hocheng et al. [8] studied the effects on material elimination level and surface roughness of machining factors for example pulsed current, pulse on-time, and workpiece rotation were analyzed. Such results were compared to those of traditional workpiece-free rotational EDM. Experimental findings showed that the rotation increased the dielectric flow, thereby helping to effectively flush the debris out of the gap between the electrodes. Furthermore, with the rotation speed of the workpiece, the material removal rate increased, whereas traditional EDM only delivers half the material removal rate as seen in Figure 2.1. Surface roughness increases with higher rotational speed, a phenomenon due to the decreased recast layer during EDM.
Figure 2.1: Effect of pulse on-time on the material removal rate at different workpiece speeds
[8].
Lin and Lin et al. [9] The orthogonal collection with the grey hierarchical approach analyzed the optimization of various output characteristics. The specification of machining parameters and the
measurement of machining efficiency are then addressed in the EDM process. A grey relational study of the electrode wear ratio, material removal rate and surface roughness experimental findings can turn the optimization of the multiple performance characteristics into the optimization of a single performance characteristic called the grey relational grade. As a result, the optimization of the complex multiple output features can be simplified greatly by this approach. The performance features of the EDM method, such as the rate of material removal, surface roughness, and electrode, are seen using the solution suggested in this study to jointly improve the wear ratio. Jain et al. [10] studied the EDM thermal stress using the finite-element technique. The primary objective of developing this model is to predict the existence of the thermal stresses that occur during EDM. The temperature distribution in the workpiece must be measured first, to determine the thermal stresses in the workpiece during EDM. Floor temperature is measured using a Temperature model representing the magnitude and distribution of heat flux, pulse time, duty cycle, and current as key parameters. The findings obtained illuminate the dangerous essence of thermal stress during EDM. It is observed that significant compressive and tensile stresses build around the spark position in a thin layer following one spark. The thermal stresses are also found to exceed the yield Workpiece strength mainly in an extremely thin zone close to the spark. Kozak et al. [11] studied the modeling of the surface profile generated with cutting by EDM with considering tool wear effects experimentally and theoretically. It is indicated that the tool wear adversely affects the accuracy of machined micro features. The confirmation of the mathematical formula and the modeling method was done by comparing surface profiles obtained with the experimental results of micro and macro EDM. Ho and Newman [12] reviewed the various research activities of the die-Sinking EDM process. The base of this study is to identify the major academic research field of EDM with the headings of EDM performance measures, EDM operating parameters along with the design and manufacture of electrodes. It is indicated of the basis for regulating the EDM process is mostly based on empirical techniques due largely to the stochastic of the sparking phenomenon involving parameters of both electrical and non-electric processes.
Ojima et al. [13] studied the temperature distribution measurement in EDM arc plasma by spectrographic analysis. It was indicated that the maximum temperature appeared at the center of
the arc and its range from 4000 K to 8000 K as seen in Figure 2.2. Plasma diameter increased with the growth of the discharge current. Gab distance effect the plasma deionize.
Figure 2.2: Temperature distribution versus the radial distance from the electrode axis [14].
Kao et al. [15] studied the Electrical Discharge Machining (EDM) cross-section wire with minimal thickness and compatible mechanisms. It explores the effects of EDM process parameters on thin cross-section cutting of Nd-Fe-B magnetic material, bipolar carbon plate, and titanium, especially the spark cycle time and on-time spark. An envelope of feasible wire EDM process parameters is generated for the commercially pure titanium. It is shown that the implementation of such an umbrella selects the necessary parameters of the EDM system for the generation of micro-features. With scanning electron microscopy (SEM), EDM soil, subsurface, and debris analysis is performed. SEM results lead to a hypothesis that defines the limiting factor for thin-section wire EDM cutting, based on the fracturing induced by thermal and electrostatic stress. For rendering compatible mechanisms, applications of thin cross-section EDM cutting are explored.
Halkacı et al. [16] investigated experimentally the surface roughness parameters in EDM. The data collected for performance measurements were analyzed using an experiment system design. For surface roughness, a significantly deep graph is given using power, pulse time, and spark time. A
stronger technique is assumed to be more useful for calculating the surface properties. This is simply due to the overlapping craters over the matte-looking surfaces, and the nature of the irregularities is quite different from the traditional processes as observed in Figure 2.3. Surface roughness measurements for EDM can be used to compare similar experimental conditions, but it may not be appropriate to compare the surface properties of components machined by one of the conventional methods.
Figure 2.3: The arithmetical mean roughness values versus the spark time [16].
Marafona and Chousal [17] studied the extracted material from the instrument and workpiece as well as the maximum temperature reached in the discharge channel . The surface roughness, the substance collected from both the anode and the cathode, and the highest temperature reached in the discharge channel can be measured for the EDM process in the new FEA model. In the discharge pipe, a measure of the thermal behavior of the system is the optimum temperature. Compared to previous versions, the key benefit of this model is an optimized model, which requires the cathode and anode to be addressed concurrently under the same problem and thus under the same working conditions.
Solomon et al. [18] finished a study on electrical discharge machining for several forms, such as EDM for powder additives and EDM with water, dry and ultrasonic vibration machining and simulation techniques in EDM performance prediction. It can be seen that the micromachining process, the ultrasonic vibration process, is the best, for the cost, prosecco dry machining has been chosen, EDM in water is the more harmless and beneficial working environment, EDM with
powder additives requires more tool wear using dielectric oil and EDM modelling is implemented to predict the performance parameters that lead to the production of accurate and precise.
Plaza et al. [19] studied angular error in wire EDM taper cutting experimentally and numerically. They introduced a comprehensive study on the impact of process parameters on angular error using the central composite rotatable analysis method has been carried out with taking considering of taper angle, thickness, pulse off time, pulse energy, and open-circuit voltage. The results showed that part angle and thickness have more effect on the mechanical behavior of the wire than the effect of the angular error.
Jeong et al. [20] studied a three-dimensional geometry simulation of micro EDM machining process. A Z-map procedure was utilized to represent the machined workpiece geometry and evaluate the effect of wear on the tool shape. The simulation was validated, and it can be concluded that the EDM simulator established could be used for cutting tool depth generation for tool wear compensation and also for tool wear estimation.
Garg et al. [21] augmented an MRR approach that included several interesting experimental ideas that vary from the conventional phenomena of EDM sparking. Despite a number of diverse techniques, all scientific work in this field shares the same aims of achieving more effective removal of material coupled with a decrease in wear of the instrument and increased efficiency of the surface. The paper discusses studies on EDM related to MRR development, along with some insight into the substance removal process. The scope for future theoretical work was discussed at the end of the article.
Guo et al. [22] studied the discharge craters establishing process in three-dimensions and the material removal method in EDM using molecular dynamics. They used vaporization and super-heated metal bubble explosion theories to explain the material removal method. The metal removal rate was also observed to be from 0.02 to 0.05 and that can cause the bulk of the melted pool to be solidified. It can be concluded that the diameter and depth of the melted area increased with the rise of the power density. It was also noticed that the presence of micropores in the material on the workpiece improves the depth of the discharged cavity and the molten area, thus growing the roughness of the machining surface.modeling of the distribution of removed materials in the gap indicated that some portion of the extracted material becomes debris expelled from the gap, while
another portion settles on the opposite electrode surface, and the last portion moves to the electrolyte interface from which it was expelled.
Anderson [23] studied the abrasive grain cutting process of the AISI4340 workpiece and spherical diamond tool numerically and experimentally. The tests were performed for cutting speed equals to 530 m/s and cutting depth changed from 0.3 to 7.5 mm. It was concluded that the strain rate toughening workpiece leads to an increase in the normal force with the rise of cutting speed. As cutting speed increased due to a reduction in tool-workpiece friction and due to a change in cutting dynamics, the tangential forces decreased. The scratch profiles revealed that as cutting speed was increased due to a reduction in material pile-up height, the cutting mechanics changed. As cutting speed was improved, the estimated uncut chip thicknesses for the transitions from elastic, elastoplastic, and fully plastic cutting were established and were observed to increase.
Sutter et al. [24] studied thermal and mechanical parameters that influence the interface cutting temperature with the cavity wear using the Abaqus software at high speed up to 20 m/s. For orthogonal cutting of steel up to a speed of 60 m/s, the correlation between experiments and simulation is validated with a great agreement in measure of cutting force, contract length, and temperature.
Tabar and Mobadersany [25] performed a numerical study of the influence of pressure distribution ultrasonic vibration and the velocity in the dielectric fluid around the bubble in the process of EDM. It is indicated that the evolution of the bubble, fluid behavior, and the machining efficiency was affected by the ultrasonic tool vibration. A liquid jet develops on the bubble at the final phases of the bubble's crash phase which has various forms. Because of the various cases and a high-pressure region appears just off the bubble jet . The liquid particles also have the maximum relative velocity just off the bubble's liquid jet.
Sahoo et al. [26] investigate the temperature, heat flux, and equivalent stress of the copper work-piece. The results showed that the temperature, heat flux, and stress of the work-piece material achieved the highest values close to the sparking region. Gradually, the temperature falls towards the end of the work-piece material. The mesh convergence test was conducted implying that the temperature, heat flux, and stress remains constant at a specific point which meets the model's stabilization test.
Mahdavinejad et al. [27] performed a review study to investigate the temperature distribution in the EDM process numerically, experimentally, and analytically. The review showed that past studies have been developed to improve the precision of temperature estimate, to encourage the selection of the approach for temperature distribution evaluation, to encourage predictive measures of output parameters constructed on temperature distribution, and finally to allocate the requirements for modeling and facilities desired for investigational studies. To create further improvement in the accuracy level of temperature prediction, some beneficial improvements are still needed for future research, hence some essential shortcomings of latest systems and useful suggestions are given in the following section.
Chen et al. [28] addressed the development of a micro-rotating structure that incorporates wire-EDM (micro-reciprocated wire electrical discharge machining) and multi-cutting techniques. To accomplish the rotation of the cylindrical workpiece, a new rotating spindle is created and the multi-cutting technique including rough, semi-finish, and finish machining is attempted to increase the removal rate of machining (MRR) and minimize the micro-rotating structure's surface roughness (SR). The effects of machining parameters such as open voltage, discharge capacitance and rotation rate on MRR and SR was routinely analyzed by experiments planned by single factor design and central composite design (CCD) of Response Surface Methodology (RSM) to determine right parameters for multi-cutting processes. Analysis of variance (ANOVA) is performed to decide the critical factor and affirm the adequacy of the MRR and SR models developed. Compared to the additivity measures, the statistical models are used to forecast MRR and SR within 8.18 and 8.74 percent prediction error respectively. The RSM method of desirability is used to evaluate three sets of multi-cutting parametric configurations, which are confirmed by validation tests with a prediction error of 1.75~7.76 percent for MRR and a prediction error of 2.77~8.59 percent for SR. Finally, a standard micro balloon probe is successfully machined by the proposed process with 0.435 Ra and 98.7-μm diameter, which offers a viable solution for the manufacturing of high-efficiency and smooth surface micro-rotating structures.
Ebisu et al. [29] studied the wire electrode readily splits in wire electric discharge machining (EDM), and when cutting corner shapes, the shape quality tends to decrease. During the process, the wire electrode is exposed to explosive discharge forces, electrostatic force with open application of voltage and jet flushing hydrodynamic force. And the wire still deflects in the
direction of cutting on the other side. In the other side, wire breakage is primarily caused by discharge concentrations in the narrow-machined kerf at the same position due to a lot of debris stagnation. Therefore, for smooth debris exclusion, jet flushing of dielectric fluid with nozzles has been conventionally added. However, broad wire vibration and deflection can be promoted by excessive flow rate. Furthermore, the effect of jet flushing conditions on the removal of debris and wire deflection in the cutting of the corner shape has not yet been studied.
Duongb et al. [30] studied the influence of the electrode diameter on the crater size, phase change, and overcut during micro-EDM numerically. It is identified that the crater sizes decreased, and surface finish improved with reducing the capacity and discharge energy. It is indicated that Capacity is the supreme essential factor in micro EDM which has a major effect on crater sizes, recast layers, and overcut. The voltage provides limited impact and the rotating speed of the electrode has no major impact on crater depth, recast layer, and depth of cut.
Shabgard et al. [31] investigated the influence of the Reynolds number on heat transfer and material removal rate in case of a single discharge pulse with the aid of melting point numerically and experimentally. They used mathematical modeling to indicate the heat transfer distribution for the cutting tool and the workpiece. The numerical results of calculating the EDM performance were validated by comparing with the experimental results. It was concluded that the EDM efficiency augmented by up to 44% with growing of Reynolds number. For numerical and experimental tests, the amount of tool erosion compared to the amount of material extracted from the workpiece is decreased by 5 % and 10 % respectively.
Salvati et al. [32, 33] studied wire EDM is well-known as a metallic sample sectioning technique that creates minimal disruption in terms of macro-scale residual stress initiation and alteration. Indeed, EDM-induced residual stress will generally be ignored at the millimeter scale. However, when the measurements of the machined product are reduced to less than a few millimeters, the structural alteration of the layer affected by the EDM can play an important role in altering its structural integrity by microstructural changes and residual stresses caused. For the proper evaluation of the in-service life of mechanical parts and modules, the surface finish and mechanical properties of the modified surface layer are of considerable importance. The EDM-affected layer created after the primary cut and also after a subsequent trim cut will be analyzed in this analysis. Micro- to nano-scale residual stress measurement and modelling using the FIB-DIC tool, assisted
and validated against non-linear thermo-mechanical FEM analysis of the cutting mechanism, is the main advance recorded in the current research. In the optimization of process parameters, the simulation of such a machining process has important consequences. On the surface of the sample, a thin compressive residual stress layer was observed that was due to the diffusion of Cu and Zn inside the parent material, giving rise to phase transformation. For the exploration of the surface and through-thickness morphology, scanning electron microscopy with Focused Ion Beam milling was used. In terms of elemental concentration and grain orientation, EDS and EBSD techniques were used to interrogate the material microstructure.
Ovsyannikov et al. [34] studied electric discharge micromachining experiment. A wire electrode-tool with a diameter of 7-20 μm was studied to study the characteristics of electroerosive cutting of sections. The findings are represented using the EDM cutting method's evolved mathematical model, collecting usable experimental data with a nonrigid wire electrode on the dynamics of the EDM mechanism. When using the discrete feed of the electrode method, the issue of stabilization of microprocessing with a thin wire is investigated. The ways to achieve the optimum cutting speed are calculated under defined conditions.
2.2 THE NOVELTY OF THE STUDY
According to the above-specified literature review, many studies investigated the temperature and stress parameters for the electrical discharge machining process experimentally, but less numerical papers studied those parameters. Copper and steel materials were studied, while titanium was not investigated. Many papers investigated the steady-state of cutting by single spark electrical discharge, but no one studied the dynamic simulation of EDM where the tool gets to throw the workpiece. It is difficult to study the internal distribution of temperature for the sheet experimentally so that the numerical study can be beneficial for this. Equivalent stress and heat will be studied in this investigation. The simulations have been performed for three speeds such as 0.667, 1, and 2 mm/s. This study contributes to the development of the EDM with heat and stress analysis of the titanium sheet and stainless steel. The effect of speed on the stresses of the sheet undercutting as well as the thermal properties with ANSYS (Transient Thermal and Transient Structural) has been investigated.
3.
NUMERICAL STUDYIn the Finite Element Approach, the underlying principle is to find the solution to a complex problem by replacing it with a simpler one. As the real problem is replaced by a simplified one in finding the solution, most of the functional problems will be able to find just an approximate solution rather than the exact solution (and often, even an approximate solution). Finite Element Analysis is a method of analysis of numerical approximation that splits a complex system into discrete elements, resulting in an idealized system that can be solved mathematically. A systematic technique for deriving the approximation functions over sub-regions of the domain called Finite Elements is given by the Finite Element Method. The solution to the problem is decided by numerically solving the variables at specified points called nodes on each unit and applying the estimated finite solutions to the whole geometry. The advantages of using FEM in studying machining can be seen from the following aspects:
1. Material properties can be managed as functions or experimental curves; as a function of pressure, pressure intensity and temperature, flow stress can be used in FEM, for example. 2. You should model the relationship between the chip and the instrument as sticking and
sliding.
3. Nonlinear geometric boundaries can be represented and used, such as the free surface on the chip.
Apart from global variables such as cutting power, feed power, chip geometry, it is also possible to obtain local stress and temperature distributions.
For a general linear and/or nonlinear static problem the equilibrium equations for a FEA are expressed as
{V} = [K] {U}……….(1) Where V is the vector matrix for the forces on the element, K is the stiffness matrix, and U is the vector of nodal displacements to be determined. However, for time dependent dynamic problems, the equation requires the form for a dynamic analysis as represented by previous equation.
3.1 MATERIAL AND GEOMETRY SELECTION
In this study, workpiece of Titanium and Stainless Steel will be anglicized by Ansys fluent because of the wide use in many industrial applications. Titanium is a chemical element in the periodic table with its symbol Ti and atomic number 22. It is a lightweight, strong, glossy, rust-resistant transition metal (including sea water and chlorine), with a silvery white metallic color. Titanium is used in strong, lightweight alloys (especially iron and aluminum) and its most common compound is titanium dioxide, which is used in white pigments. Titanium has a high density resulting in great tensile strength, high corrosion resistance, fatigue resistance, high crack resistance, and the ability to withstand moderate high temperatures. Titanium alloys are used in aircraft, armor and coatings. Tires and goes into the medical field such as bone implants. Stainless Steel is an iron metallic alloy that contains a mixture of elements where the percentage of iron in it is not less than 50%, the proportion of chromium from 5% to 30%, nickel and molybdenum about 8.5% and the percentage of carbon is a maximum of 2%, and its resistance to rust and corrosion is acquired due to the formation of a thin coherent and invisible layer The chromium oxide adheres to the surface of the metal and protects it from corrosion, and this layer is sufficiently protective whenever the chromium content is high in the steel. It is subject to corrosion and rust under special conditions, and its resistance to rust varies according to the percentage of chromium in it. It is used in industries that need a very high resistance to rust (such as household appliances, drink cans, internal razors, transportation and some parts of machines exposed to moisture). It is used in surgical tools and tableware, which is cheap and resists rust when exposed to water or stored without drying, and can be used continuously for a long time.
The mechanical and thermal properties of the titanium and stainless steel sheets that were applied in this study were explained in Table 3.1 and Table 3.2, respectively. The electrode material was chosen from graphite because of its arc erosion-resistant properties as seen in Table 3.3
Table 3.1: Properties of titanium sheet.
Property Symbol Magnitude Unit
Density ρ 4620 kg/m3 Thermal conductivity k 21.9 W/m.oC Specific heat cp 522 J/ kg.oC Young's Modulus E 9.6*1010 Pa Poisson's Ratio υ 0.36 Bulk Modulus B 1.143*1011 Pa Shear Modulus G 3.529*1010 Pa
Table 3.2: Properties of stainless steel.
Property Symbol Magnitude Unit
Density ρ 7750 kg/m3 Thermal conductivity k 15.1 W/m.oC Specific heat cp 480 J/ kg.oC Young's Modulus E 1.93*1011 Pa Poisson's Ratio υ 0.31 Bulk Modulus B 1.69*1011 Pa Shear Modulus G 7.366*1010 Pa
Table 3.3: Properties of graphite electrode.
Property Symbol Magnitude Unit
Density ρ 1758 kg/m3 Thermal conductivity k 70 W/m.oC Specific heat cp 1350 J/ kg.oC Young's Modulus E 4.1*109 Pa Poisson's Ratio υ 0.21 Bulk Modulus B 2.3 *109 Pa Shear Modulus G 1.7*109 Pa
The dimensions of the plate that was utilized in this study are 20 mm × 16 mm × 1 mm were illustrated in Figure 1.1. The dimensions were chosen according to the previous study of Natsu et al. [13].
3.2 NUMERICAL METHOD
This section explains the physical model and of the assumptions used during simulation, thermal model of EDM, heat transferred to the workpiece and electrodes, mesh generation, and governing equations. Boundary conditions and validation of the simulation results will be described.
3.2.1 Physical Model
The geometry of the model generated using SOLIDWORKS software. Then the model was converted to Ansys workbench the geometry of the model is generated in ANSYS design modular. The geometry and thickness of the model are 20 mm×16 mm and 1 mm respectively. The proper thermal and mechanical properties of the titanium were applied for the model then the model was investigated by Transient Thermal to indicate the temperature distribution and heat flux then Transient Structural was utilized to study the equivalent stresses. Some assumptions were performed for modeling as:
• Gaussian heat flux distribution has been used as a heat source. • Material is considered as uniform, isotropic, and homogeneous. • The thermal analysis was regarded as a transient phase.
• The efficacy of materials flushing is believed to be 100%.
3.2.2 Meshing
The tetrahedron structured mesh were employed for the physical domain. Mesh refinements were applied for the region where the electrode was applied. Figure 3.2 displays the mesh grid used in this study where four grid systems with 185324, 254821, 402157, and 1412758 elements are produced to confirm the mesh dependence check. The error between the finest grid having 1412758 cells and the grid having 1412758 cells is near 0.921% for temperature and 1.14% for the heat flux as seen in Table 3.4. Therefore, the numerical study will use the model of mesh with 1412758 elements for more accuracy and precision.
Table 3.4: Error percentages of temperature and heat flux.
Meshing elements number 185324 254821 402157 1412758
Temperature Error 3.64% 2.31% 1.32% 0.921%
Heat flux Error 5.61% 3.14% 1.641% 1.14%
3.2.2.1 Governing equations
Heat distribution by transient conduction in Cartesian coordinates cross workpiece due to electrode spark is governed by the following thermal distribution differential equation:
∂ ∂x(k ∂T ∂x ) + ∂ ∂y(k ∂T ∂y ) + ∂ ∂z(k ∂T ∂z ) + q = ρcp ∂T ∂t (3) Where
T is the temperature in Kelvin (K) unit, t term is the time in seconds unit,
k is the thermal conductivity in W/moC unit. And (x, y,z) is the Cartesian coordinates.
The radiation heat that occurred at the outer surface of the workpiece can be explained as follow. qrad = ε A σ (Ts4− Tsur4) (4)
Where, ε is the emissivity of the surface, σ Stefan Boltzmann constant and it equals to 5.67 × 10−8 (W/m2 .K4 ). Heat transfer occurred by convection can be expressed as:
qconv = A h (Ts− Tsur) (5)
The heat transfer coefficient is expressed in term h where Ts is the surface temperature and Tsur
is the surrounding temperature.
Cutting speed can be calculated by the following formulae v = l
where l is the cutting length in mm and t is the time in seconds.
3.2.3 Boundary and Initial Condition
The analysis set was performed with 10 steps with 30, 20 and 10 seconds with surrounding temperature is 27 oC. Gaussian heat flux profile with heat transfer by conduction, convection, and
radiation was explained in Figure 3.3 where convection and radiation occurred in the outer surfaces and the conduction inside the workpiece. The electrode moves in the x-direction with three various velocity such as 0.667 mm/s, 1 mm/s, 2mm/s in the middle of the workpiece. The initial temperature of the electrode and the workpiece is 27 oC.
Figure 3.3: Explanation of Gaussian heat flux with boundary condition of one pulse. 3.3 MODEL VALIDATION
For validation of the numerical model, previous experimental study results were compared with the results of this study [13]. Figure 3.4 shows a comparison between temperature versus the distance from the center of the electrode for experimental and numerical results. The average difference between numerical and experiment results around 1.24%. The highest difference between the results is 2.76% and the minimum difference is 0.097%. Those values can be calculated by determining the minimum and maximum difference between the values. Figure 3.5 shows the temperature distribution for one pulse and then comparing with the numerical results [13] was performed and except for the inescapable simulated and experimental errors, the
differences are appropriate. In this research, it is shown that the model of CFD modeling being considered is efficient and precise.
Figure 3.4: Comparison between the experimental (Natsu et al [13]) and numerical results.
4.
RESULTS AND DISCUSSIONS4.1 THE TEMPERATURE AND FLUX DISTRIBUTION AT 0.667 MM/S SPEED
As the electrode passes through the width of the workpiece for 30 seconds the cutting velocity equals to 0.667 mm/s according to the Equation (4) depending on the cutting length and time and near to the velocities in the previous studies. Figure 4.1 explains the maximum, average, and minimum temperatures during the EDM cutting process. The temperature is measured at the contact area between the electrode and plate surface. It can be observed that the maximum temperature increased rapidly during the first two seconds from the beginning of the process until reaching the value of 6550 oC. Then the temperature fluctuated between 4600 oC and 6550 oCuntil the end of the cutting. This fluctuation can be explained as the electrode does not move smoothly with constant speed, but move down until suitable gab with the surface of the plate then move up again then translation in the horizontal axis occurred. For the average temperature, it increased gradually until having the value of 991oC, and it is still constant for the rest of the process. The minimum temperature of the workpiece increased gradually during the half time of the process up to 723oC then the temperature is constant at the other half time of the machining process.
Figure 4.1: The relation between the temperature and time during the cutting process.
The temperature distribution of the workpiece at different times such as. 6 s, 12 s, 24 s, and 30 s is illustrated in Figure 4.2. It is indicated that the maximum temperature occurred at the center of the electrode where the electrode transfer in the middle plane of the workpiece and the maximum temperature region also travel with the movement of the electrode. Figure 4.2 (a) explains the temperature distribution at 6 s where the first half of the workpiece is heated by the electrode and the anther half does not affect by the electrode and has the ambient temperature. Figure 4.2 (b) illustrates the temperature distribution at 12 s where the electrode is near the half distance. More thermal heat is stored in the workpiece and the temperature have maximum value at the center of the workpiece. Figure 4.2 (c) and Figure 4.2 (d) show the temperature distribution at 24 s and 30 s, respectively.
(a)
(b)
(d)
Figure 4.2: Temperature distribution for a three-dimensional model at various times: (a) 6 s, (b)
12 s, (c) 24 s, (d) 30 s.
To show the internal temperature distribution across the workpiece, the cross-section in the middle of the workpiece was applied. Figure 4.3 explains the temperature distribution at 6 s, 12 s, 24 s , 30 s in Figure 4.3 (a), Figure 4.3 (b), Figure 4.3 (c), and Figure 4.3 (d), respectively. The temperature increased until the maximum one near to 7000 oC at the surface of the workpiece that exposed to the heat of the electrode. Then the temperature decreased with getting through the workpiece. The temperature decreased towards radial following the Gaussian distribution heat flux equation. Heat transferred by convection from the outer surface help in decreasing the temperature in axial and radial directions.
Figure 4.3: Temperature distribution for elevation view across the thickness of the workpiece at various
times: (a) 6 s, (b) 12 s, (c) 24 s, (d) 30 s.
Figure 4.4 shows the heat flux distribution for the titanium workpiece at the cutting speed of 0.677 mm/s, where the electrode moves in the front of the workpiece the heat flux transfer in the same direction. Heat flux was applied according to the Gaussian distribution heat flux equation. This process continues for 30 seconds until the electrode reaches the end of the workpiece.
(a)
(c)
(d)
Figure 4.4: Heat flux distribution for the top view of the workpiece at various times: (a)6 s,
4.2 THE TEMPERATURE AND FLUX AT 2 MM/S SPEED
To increase the velocity of the cutting speed, the same workpiece was machined for 10 seconds so that the velocity recorded 2 mm/s. Figure 4.5 shows the variation of temperature versus the time during the cutting process. As seen, the temperature increased speedily in the first second up to having a value of 6350 oC then variation in the temperature during the rest of the time of the process followed. The temperature variation as the average value of the full workpiece enhanced smoothly following the figure of 991 oC near the end of the process. This can be explained as the electrode move quickly the heat can’t transfer to the far region from the center of the electrode so that the average value is less than the value of the case where the velocity is 0.677 mm/s. The high temperature resulted from the moving spark leads to melting the titanium and some liquids evaporated near to the electrode center. The maximum temperature with the velocity of the electrode equal to 2 mm/s is less than the maximum of the temperature in case of 0.677 mm/s velocity.
