Optimization of Process Parameters of Powder Mixed Electric Discharge machining for D2 Steel

CONTENTS

TITLE

DECLARATION

ACKNOWLEDGEMENT

ABSTRACT

LIST OF FIGURES

LIST OF TABLES

ABBREVATIONS

CHAPTER 1: INTRODUCTION
1.1 Introduction
1.2 History of EDM
1.3 Working Principle of EDM
1.4 Process variables
1.5 Performance Measures
1.6 Categories of EDM
1.6.1 Die-sinking EDM
1.6.2 Wire EDM
1.7 Powder mixed EDM
1.8 Advantages of EDM
1.9 Dis-advantages of EDM
1.10 Applications of EDM

CHAPTER 2: LITERATURE REVIEW
2.1 Influence of powder characteristics
2.2 Gap in Literature Survey
2.3 Problem Formulation
2.4 Objectives for Research Work
2.5 Methodology Flow Chart

CHAPTER 3: EXPERIMENTAL DETAILS
3.1 Experimental set-up of EDM
3.2 Selection of materials
3.2.1 Work-piece material
3.2.2 Tool material
3.2.3 Powder material
3.3 Process parameters
3.4 Design of experiments using Taguchi
3.5 Performance Measures
3.5.1 Material removal rate (MRR)
3.5.2 Tool wear rate (TWR)
3.5.3 Surface roughness (SR)
3.5.4 Hole Diameter Variation(HDV)
3.5.5 Scanning electron microscope (SEM)
3.6 Work-piece after machining and testing

CHAPTER 4: OPTIMIZATION OF OUTPUT PARAMETERS
4.1 Introduction
4.2 Experimental results of MRR, TWR, SR and HDV
4.3 S/N Ratios of MRR, TWR, SR and HDV
4.4 Optimization of material removal rate
4.4.1 Results for S/N Ratios of MRR
4.4.2 Results for mean of MRR
4.5 Optimization of tool wear rate
4.5.1 Results for S/N Ratios of TWR
4.5.2 Results for mean of TWR
4.6 Optimization of surface roughness
4.6.1 Results for S/N Ratios of SR
4.6.2 Results for mean of SR
4.7 Optimization of hole diameter variation
4.7.1 Results for S/N ratios of HDV
4.7.2 Results for mean of HDV
4.8 Calculation of optimal design for output responses
4.8.1 Optimal design for MRR
4.8.2 Optimal design for TWR
4.8.3 Optimal design for SR
4.8.4 Optimal design for HDV

CHAPTER 5: GREY RELATIONAL ANALYSIS
5.1 Introduction
5.2 Experimental Data for MRR, TWR, SR and HDV
5.3 Grey Relational Analysis
5.3.1 Normalize the Original Sequence and Deviation
Sequence
5.3.2 Grey Relational Coefficient and Grey Relational Grade

CHAPTER 6: SCANNING ELECTRON MICROSCOPE (SEM) 54-
6.1 Introduction

CHAPTER 7: CONCLUSION AND FUTURE SCOPE 59-
7.1 Conclusion
7.1.1 Material Removal Rate (MRR)
7.1.2 Tool Wear Rate (TWR)
7.1.3 Surface Roughness (SR)
7.1.4 Hole Diameter Variation(HDV)
7.1.5 Grey Relational Analysis (GRA)
7.2 Future Scope

REFERENCES

LIST OF FIGURES

Figure 1.1 A typical Set-up of EDM

Figure 1.2 Material removal mechanism in EDM

Figure 1.3 EDM sparking cycle

Figure 1.4 Layers of an EDM machined surface

Figure 1.5 Schematic Diagram of PMEDM process

Figure 1.6 Series discharging in PMEDM

Figure 2.1 Different forces acting on a powder particle

Figure 2.2 Flow Chart of Methodology

Figure 3.1 Electric Discharge Machine Used

Figure 3.2 Work-piece Material Before Machining

Figure 3.3 Chemical composition of work-piece material

Figure 3.4 Tool Materials Used

Figure 3.5 Electronic weighing machine

Figure 3.6 Surface Roughness Tester used

Figure 3.7 Scanning Electron Microscope Equipment Used

Figure 3.8 Work-piece samples after Machining and Testing

Figure 4.1 Main Effects plot for S/N Ratios of MRR

Figure 4.2 Main Effects plot for Means of MRR

Figure 4.3 Main Effects plot for S/N Ratios of TWR

Figure 4.4 Main Effects plot for Means of TWR

Figure 4.5 Main Effects plot for S/N Ratios of SR

Figure 4.6 Main Effects plot for Means of SR

Figure 4.7 Main Effects Plot for S/N Ratios of HDV

Figure 4.8 Main Effects Plot for Means of HDV

Figure 5.1 Effect of EDM Parameters on the Multiple-Performance

Characteristics

LIST OF TABLES

Table No. Description Page No

Table 3.1 Properties of work piece material

Table 3.2 Properties of Tool Materials

Table 3.3 Properties of powder material

Table 3.4 Parameters considered and their levels

Table 3.5 Experimental Design L16

Table 4.1 Experimental Results for MRR, TWR, SR, HDV

Table 4.2 S/N Ratios of MRR, TWR, SR and HDV

Table 4.3 Analysis of Variance for S/N Ratios of MRR

Table 4.4 Response Table for Signal to Noise Ratios of MRR

Table 4.5 Analysis of Variance for Means of MRR

Table 4.6 Response Table for Means of MRR

Table 4.7 Analysis of Variance for S/N Ratios of TWR

Table 4.8 Response Table for Signal to Noise Ratios of TWR

Table 4.9 Analysis of Variance for Means of TWR

Table 4.10 Response Table for Means of TWR

Table 4.11 Analysis of Variance for S/N Ratios of SR

Table 4.12 Response Table for Signal to Noise Ratios of SR

Table 4.13 Analysis of Variance for Means of SR

Table 4.14 Response Table for Means of SR

Table 4.15 Analysis of Variance for S/N Ratios of HDV

Table 4.16 Response Table for Signal to Noise Ratios of HDV

Table 4.17 Analysis of Variance for Means of HDV

Table 4.18 Response Table for Means of HDV

Table 4.19 Significant Factors for MRR

Table 4.20 Significant Factors for TWR

Table 4.21 Significant Factors for Surface Roughness

Table 4.22 Significant Factors for Hole Diameter Variation

Table 5.1 Design of Experiments (Orthogonal Array L16)

Table 5.2 The sequence of each performance characteristic after data processing

Table 5.3 The calculated GRG and its order in the optimization process

Table 5.4 Response Table for the Grey Relational Grade

Table 5.5 ANOVA of Gray Relation Grade

Table 5.6 Improvement in GRG with optimized EDM machining parameters

ABBREVATIONS

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CHAPTER 1 INTRODUCTION

1.1 INTRODUCTION

The growing trend to use slim, light and compact mechanical components in automobile, aerospace, medical, missile, and nuclear reactor industries has led to the development of high strength, temperature resistant, and hard materials during last few decades. It is almost impossible to find sufficiently strong and hard tools to machine aforesaid materials at economic cutting speeds [1]. Moreover, machining of complex shapes in these materials with low tolerances and high surface finish by conventional methods is even more troublesome. Hence, there is great demand for new machining technologies to cut these ‘difficult-to-machine’ materials with ease and precision. Among modern machining processes, electric discharge machining (EDM) has become highly popular in manufacturing industries due to its capability to machine any electrically conductive material into desired shape with required dimensional accuracy irrespective of its mechanical strength [4].

1.2 HISTORY OF EDM

Joseph Priestley, The English physicist, first noted the erosion of metals by electric sparks in 1770. Furthering Priestly research, the EDM process was invented by two Russian scientists, Dr. B. R. Lazarenko and Dr. N. I. Lazarenko, in 1943. Intermittent arcing in air between tool electrode and work-piece material, connected to a DC electric supply, caused the erosion of material. The process was not very accurate due to overheating of the machining region and may be defined as ‘arc machining’ rather than ‘spark machining’ [2]. During 1980s, the efficiency of EDM raised extraordinarily with the introduction of computer numerical control (CNC). Self- regulated and unattended machining from loading the electrodes into the tool changer to a finished smooth cut was possible with CNC control system. Since then, these emergent virtues of EDM have been vigorously sought after by the manufacturing industries producing tremendous economic advantage and creating keen research interest [6].

1.3 WORKING PRINCIPLE OF EDM

Despite the fact that the material removal mechanism of EDM is not absolutely identified and is still contentious, the most widely established principle is the transformation of electrical energy into thermal energy through a sequence of distinct electric discharges. Fig. 1.1 shows a representative diagram of a typical EDM setup. Build-up of suitable voltage across tool and work-piece (cathode and anode respectively) that are submerged in an insulating dielectric, causes cold emission of electrons from the cathode [9]. These liberated electrons accelerate towards the anode and collide with the dielectric fluid, breaking them into electrons and positive ions. A narrow column of ionized dielectric fluid molecules is established connecting the two electrodes. A spark generates due to the avalanche of electrons. This results in a compression shock wave. Very high temperature (8,000 to 12,000 °C) is developed which induces melting and evaporation of both the electrodes. The molten metal is evacuated by the mechanical blast (of the bubble), leaving tiny cavities on both tool and work-piece [8].

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Figurel.1 A typical set-up of EDM [7]

A step by step description of the material removal process due to sparking is presented in Fig. 1.2. There is no direct contact between the tool and work-piece (held at a small distance) and a high potential is applied between them. The electrode moves towards the work-piece and enhances the electric field in the inter electrode gap, until the breakdown voltage of dielectric is reached. The spot of discharge is normally between the nearest points of the tool and the work-piece. However, the spot location may change depending on the impurities or debris present in the inter electrode gap. Voltage drops and current flows from work-piece to electrode due to ionization of dielectric and formation of plasma channel. The flow of discharge current continues and there is a constant attack of ions and electrons on the electrodes which ultimately lead to intense heating of the work-piece. The temperature rises between 8,000 °C and 12,000 °C [3], resulting in the formation of a small molten metal pool at both the electrode surfaces and some of the molten metal directly vaporizes. During this period, plasma channel widens and radius of the molten metal pool increases.

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Figure1.2 Material removal mechanism in EDM [2]

Towards the end of the discharge, voltage is shut and plasma channel collapses inwards due to the pressure exerted by the neighboring dielectric. As a result, the molten metal pool is powerfully drawn into the dielectric, producing a tiny cavity at the surface of work-piece. The machining process successively removes minute quantities of work-piece material, in the form of molten metal, during discharges. The removed material solidifies to form debris. The flow of dielectric washes away the debris from the discharge zone. The gap increases after material removal at the point of spark, and the position of the next spark shifts to a different place, where inter electrode gap is the smallest. In this manner, thousands of electric discharges take place at different localities of the work-piece surface corresponding to tool-work-piece gap. As a consequence, a negative replica of the tool surface shape is produced in the work-piece [11].

1.4 PROCESS VARIABLES

As per the discharge phenomena explained earlier, some of the key process parameters which influence the EDM process are,

1. Discharge current or peak current (I p): During each pulse-on time, current rises until it attains a certain predetermined level that is termed as discharge current or peak current. It is governed by the surface area of cut. Higher currents produce high MRR, but at the cost of surface finish and tool wear. Accuracy of the machining also depends on peak current, as it directly influences the tool wear [14].

2. Discharge voltage (V): Open circuit voltage between the two electrodes builds up before any current starts flowing between them. Once the current flow starts through plasma channel, open circuit voltage drops and stabilizes the electrode gap. A preset voltage determines the working gap between the two electrodes. It is a vital factor that influences the spark energy, which is responsible for the higher MRR, higher tool wear rate and rough surfaces [21].

3. Pulse-on time or pulse duration (T on): It is the duration of time (ps), the current is allowed to flow per cycle. Dielectric ionizes and sparking takes place during this period. It is the productive regime of the spark cycle during which current flows and machining is performed. The amount of material removal is directly proportional to the amount of energy applied during this on-time. Through MRR increases with T-on, rough surfaces are produced due to high spark energy [29].

4. Pulse-off time or pulse interval (T off): It is the duration of time between two consecutive pulse-on times. The supply voltage is cut off during pulse-off time. Dielectric de-ionizes and regains its strength in this period. This time allows the molten material to solidify and to be washed out of the arc gap. Pulse-off time should be minimized as no machining takes place during this period. However, too short T off leads to process instability [17].

5. Duty cycle (τ): It is a percentage of the on-time relative to the total cycle time. This parameter is calculated by dividing the on-time by the total cycle time (on-time and off-time). At higher τ, the spark energy is supplied for longer duration of the pulse period resulting in higher machining efficiency.

Pulse On Time

Duty factor(T) = Totol Cycle Time [2]

6. Polarity: Polarity refers to the potential of the work-piece with respect to the tool. In straight or positive polarity the work-piece is positive, whereas in reverse polarity work-piece is negative. In straight polarity, quick reaction of electrons produces more energy at anode (work-piece) resulting in significant material removal. However, high tool wear takes place with long pulse durations and positive polarity, due to higher mass of ions. In general, selection of polarity is experimentally determined depending on the combination of work-piece material, tool material, current density and pulse duration [3].

7. Dielectric Fluid: Dielectric fluid carries out three important tasks in EDM. The first function of the dielectric fluid is to insulate the inter electrode gap and after breaking down at the appropriate applied voltage, conducting the flow of current. The second function is to flush away the debris from the machined area, and lastly, the dielectric acts as a coolant to assist in heat transfer from the electrodes. Most commonly used dielectric fluids are hydrocarbon compounds, like light transformer oil and kerosene [28].

8. Inter electrode gap (IEG): The inter electrode gap is a vital factor for spark stability and proper flushing. The most important requirements for good performance are gap stability and the reaction speed of the system; the presence of backlash is particularly undesirable. The reaction speed must be high in order to respond to short circuits or open gap conditions. Gap width is not measurable directly, but can be inferred from the average gap voltage. The tool servo mechanism is responsible for maintaining working gap at a set value. Mostly electro mechanical (DC or stepper motors) and electro hydraulic systems are used, and are normally designed to respond to average gap voltage [15].

9. Tool work time (T w) and tool lift time (T up ): During the working time T w , multiple sparks occur with a pulse on duration T on and pulse off time T off . Then, the quill lifts up for T up duration when impulse flushing is done. The impulse flushing is an intermittent flushing through side jet and is done through a solenoid valve is synchronized with the lifting of tool. The dielectric is directed towards the IEG to accomplish removal of the debris [21]. The sparking cycle consists of T w and T up which are shown in Fig. 1.3.

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10. Flushing Pressure and Type of flushing: Flushing is an important factor in EDM because debris must be removed for efficient cutting. Moreover, it brings fresh dielectric into the inter electrode gap. Flushing is difficult if the cavity is deeper and inefficient flushing may initiate arcing that may create unwanted cavities which are detrimental for surface quality and dimensional accuracy. There are several methods generally adopted to flush the EDM gap: jet or side flushing, pressure flushing, vacuum flushing and pulse flushing. In jet flushing, hoses or fixtures are used and directed at the inter electrode gap to wash away the debris. In pressure and vacuum flushing, dielectric flows through the drilled holes in the electrode, work-piece or fixtures [31]. In pulse flushing, the movement of electrode in up and down, orbital or rotary motion creates a pumping action to draw the fresh dielectric. The usual range of pressure used is between 0.1 and 0.4 kg f/cm2 [23].

1.5 PERFORMANCE MEASURES

Material removal rate (MRR) determines the productivity of any machining process. It can be defined as the volume of the material removed in a unit time. MRR achieved during EDM is quite low (0.1 to 10 mm3/min). Actual value of MRR depends on the machining conditions employed. Overcut determines the accuracy of EDM process. It is the difference between the size of the electrode and the size of the cavity created during machining. Overcut has to be minimized to achieve close tolerances on the machined components. Since the material removal in EDM is achieved through the formation of craters due to the sparks, it is obvious that larger crater size results in a rough surface. So, the crater size, which depends mainly on the energy per spark, controls the quality of the surface [29].

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Figure1.4 Layers of an EDM machined surface [29]

Apart from productivity (MRR), surface integrity of the machined component plays a vital role in the selection of a machining process. Surface integrity deals basically with two issues, i.e., surface topography and surface metallurgy (possible alterations in the surface layers after machining). Surface integrity greatly affects the performance, life and reliability of the component. Typically EDM results in two kinds of surface or sub-surface layers, i.e., recast layer, heat affected zone (HAZ) as shown in Fig. 1.4. If molten material from the work-piece is not flushed out quickly, it will re-solidify and harden due to cooling effect of the dielectric, and gets adhered to the machined surface. This thin layer (about 2.5 to 50 μm) is known as ‘re-cast layer or white layer’. It is extremely hard and brittle and hence often causes micro-cracks to nucleate and proliferate. The layer next to recast layer is called ‘heat affected zone’. Heating, cooling and diffused material are responsible for the presence of this zone.

Thermal residual stresses, weakening of grain boundary and consequent formation of cracks are some of the characteristics of this zone. The application of higher discharge energy results in deeper HAZ and subsequently deeper cracks. Excessive local thermal expansion and subsequent contraction may result in residual tensile stresses in the eroded layer [4]. The surface finish achieved during EDM is also influenced by the chosen machining conditions. Surface finish is primarily governed by the pulse frequency and energy per spark.

1.6 CATEGORIES OF EDM

EDM facilitates the machining in a number of ways, a lot of these operations are similar to conventional machining operation, for instance milling and die sinking. A variety of classifications are possible and recent developments in its technology append new operations owing to increase in various requirements. A simple and general classification can be given in view of standard applications such as: die sinking EDM and wire EDM

1.6.1 Die sinking EDM

Die sinking EDM, comprises a tool electrode and work-piece that are immersed in an insulating dielectric fluid. A pulsating power supply that produces a voltage potential, connects the tool and work-piece. A constant gap between the tool and the work-piece is maintained by a servo motor control of the tool holder. As tool moves towards the work-piece, dielectric breaks down into electrons and ions, creating a plasma column between two electrodes. A momentary flash jumps between the electrodes. Automatic movement of tool, towards work-piece takes place as the spark gap increases due to metal erosion. Thus the process continues without any interruption. As a result, the complementary shape of the tool electrode accurately sinks into the work-piece [19].

1.6.2 Wire EDM

Wire EDM uses a very thin wire of 0.02 to 0.03 mm diameter usually made of brass or stratified copper as electrode and machines the work-piece with electric discharges by moving either the wire or work-piece. Erosion of work-piece by utilizing spark discharges is very same as die sinking EDM. The predominant feature of a moving wire is that a complicated cut can be easily machined without using a forming tool. The wire which is a constantly fed from a spool is held between upper and lower diamond guides. This process is frequently used to machine plates about 300 mm to manufacture dies, punches, and tools from hard materials which are difficult to machine using other processes [22].

1.7 POWDER-MIXED EDM (PMEDM)

In PMEDM, the addition of suitable powder particles to the dielectric leads to superior surface finish combined with better machining rates compared to those for conventional EDM (without powder). A typical dielectric circulation system used in PMEDM is shown in Fig. 1.5. This kind of specially designed system is mounted in the working tank of an EDM setup. A stirrer or a micro-pump is provided to avoid the settling of powder particles at the bottom of dielectric reservoir. It also helps to prevent the stagnation of the powder particles on the work-piece surface. A set of permanent magnets is provided to separate the debris from the powder particles through the filtering system. This separation is possible only when the work-piece is magnetic in nature and the powder material is not [7].

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Figure1.5 Schematic diagram of PMEDM process [8]

Current understanding of the PMEDM is presented here as the process is yet to be fully established. In PMEDM, fine powder particles are suspended in the dielectric oil. An electric field is created in the inter-electrode gap (IEG) when sufficient voltage (about 80 to 320 V) is applied between them. Ionization of dielectric takes place as in the case of conventional EDM. Under the applied electric field, positive and negative charges accumulate at the top and bottom of the powder particles respectively (Work-piece positive and tool negative case). The capacitive effect of the electrodes leads to the formation of chains of powder particles. First discharge breakdown occurs where the electric field density is the highest (between ‘a’ and ‘b’ in Fig. 1.6). This breakdown may be between two powder particles or a powder particle and an electrode (Tool or work-piece). Redistribution of electric charges takes place after the first discharge and electric charges gather at point ‘c’ and ‘d’. Further discharge happens between these powder particles and the other particles where electric field density is highest [16, 19]. Effect of powder particles given below:

1. Enlargement of discharge gap: Size of the discharge gap largely depends on the electrical and physical properties of the powder particles. Under high- temperature machining conditions, the free electrons present in electrically conductive powder particles reduce the overall resistance of the dielectric. The improved conductivity helps the spark to be generated from a longer distance and thus enlarges the discharge gap [21, 22].
2. Widening of discharge passage: After the first discharge, powder particles in IEG get energized and move rapidly along with ions and electrons. These energized powder particles colloid with dielectric molecules and generate more ions and electrons [19].

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Figure1.6 Series discharging in PMEDM [18]

Thus, more electric charges are produced in PMEDM compared to conventional EDM. Also increased discharge gap aids in the reduction of hydrostatic pressure acting on the plasma channel. These two phenomena ensure the widening of the discharge passage. The enlarged and wide discharge column decreases the intensity of discharge energy leading to the formation of large shallow cavities on the work-piece surface.

3. Multiple discharges: Multiple discharge paths are observed in PMEDM due to the rapid zigzag movement of the suspended powder particles ensuring uniform distribution of energy and formation of multiple craters in single pulse duration. Unlike conventional EDM, the discharge waveform in the case of PMEDM is significantly different from the input pulse. Voltage fluctuates rapidly within single pulse duration due to multiple discharges [24].

1.8 ADVANTAGES OF EDM

1. Any electrically conductive material can be machined.
2. Stress free complicated geometries can be produced.
3. Eliminates the necessity of grinding and fine surface finish.
4. Extremely hard material to very close tolerances.
5. Composite materials can be easily machined by EDM process.
6. Complex dies sections and moulds can be produced accurately, faster, and at lower costs.
7. There is no physical contact between tool and work-piece. Therefore delicate sections and weak materials can be machined without any distortion.

1.9 DISADVANTAGES OF EDM

1. Work-piece material must be electrically conductive.
2. Reproducing sharp corners on the work piece is difficult due to electrode wear.
3. Electrical power consumption is very high.
4. Excessive tool wear occurs during machining.
5. Process cannot be monitored during machining. Thus high skilled person are able to operate.
6. Higher machining time, during the machining of very hard materials.
7. “Overcut” is formed, causes easy removal of wear debris particles.

1.10 APPLICATIONS OF EDM

1. EDM has been used in manufacturing of aerospace components such as fuel system, engine, impeller and landing gear components where high temperature and high-stress conditions prevail.
2. Drilling of micro-holes, thread cutting, helical profile milling, rotary forming and curved holes drilling.
3. The EDM process is becoming a common method of making proto-type and production parts, especially in the aerospace, automobile and electronics industries in which production quantities are relatively low.
4. It is used to machine extremely hard materials that are difficult to machine, like alloys, tool steel, tungsten carbides, titanium etc.
5. It is used for machining sharp edges and corners that cannot be machined effectively by other machining processes.
6. EDM has also made its presence felt in the new fields such as sports, medical and surgical instruments, optical, including automotive R & D areas.
7. Other applications: deep, small-diameter holes using tungsten wire as tool, narrow slots, cooling holes in super alloy turbine blades and various intricate shapes.

CHAPTER 2 LITERATURE REVIEW

The invention of powder-mixed EDM (PMEDM) process took place around late seventies and the first publication came in 1980 [31]. In PMEDM, the addition of suitable powder particles to the dielectric leads to a superior surface finish, and better machining rate compared to those for conventional EDM (without powder- mixed dielectric). A novel EDM two-tank system was first developed and marketed by Mitsubishi [32]. One of the tanks consisted of standard dielectric oil and the second one contained powder-mixed dielectric. After completion of initial machining operation in the first tank, the tool head moved to the second tank to perform the finish machining. However, the extensive application of PMEDM in the industry requires a thorough understanding of its mechanism and the influence of different powder characteristics on performance measures.

The emphasis in the current section is given on influence of powder characteristics and machining parameters on various responses. Some of the major application areas, variants of the basic PMEDM process and potential future direction of research are also discussed.

2.1 INFLUENCE OF POWDER CHARACTERISTICS

Jahan et al. [27] presented a comprehensive analytical modeling of PMEDM process. Fig. 2.1 shows the schematic representation of different forces acting on a powder particle present in the inter-electrode gap. In Fig. 2.1, F i, F c, F d, F e, and ‘f are lift, columbic, drag, electric, friction (direction only) forces respectively. W denotes the self-weight of the particle. The derived formula for breakdown energy of powder-mixed dialectic is provided in Eq. (2.1).

(Equation 2.1)

Where Ei = Initial voltage for concentration Ni, Ebr = Breakdown voltage for final concentration Nf, σ = Boltzmann constant, T = Temperature, 81 = Permittivity of dielectric, 8p = Permittivity of powder particle and r = Radius of powder particle.

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Workpiece

Figure2.1 Different forces acting on a powder particle

From Eq. (2.1) it is evident that Ebr depends on particle radius and change in concentration ‘N’, permittivity of the particles and dielectric. For no addition of powder particles or unchanged concentration (Nf = Ni ), the value of Ebr = Ei which means no change in breakdown strength. The derived expression for spark gap during PMEDM is given in Eq. (2.2).

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Where α = Field enhancement factor for small protrusion, gd = Distance between bottom of the particle and micro-peak, hp = Height of the protrusion. d1 = Spark gap without powder suspension. From Eq. (2.2), it is clear that spark gap during PMEDM (d2) is higher than that of conventional EDM process (d1).

Density, size, electrical and thermal conductivities are some of the critical characteristics of the powder particles that significantly affect PMEDM process. Increase in electrical conductivity of the dielectric, and resulting extension of discharge gap in PMEDM, as discussed earlier, enhance spark frequency and facilitate easy removal of debris from the machining zone [27].

High thermal conductivity of powder particles removes a large amount of heat from the discharge gap leading to reduction in discharge density. Therefore, only shallow craters are formed on the work-piece surface. Number of surface cracks developed on the machined surface is also reduced along with their width and depth, as the intensity of discharge energy is less in PMEDM compared to conventional EDM process [29].

Pallavi Chaudhury et al (2017) investigated the multi-response optimization of powder additive mixed electrical discharge machining by Taguchi analysis. The study was carried out on the influence of the parameters such as peak current, powder type powder concentration on EN-19 work-piece. The result for made that higher MRR has been achieved in powder mixed as compared to conventional EDM and tool wear rate is firstly increases and then decreases with increase the pulse on time and discharge current.

Misbah niamat et al (2017) investigated the effect of different dielectrics on material removal rate, electrode wear rate and micro structures in EDM. In this research the effect of pulse on time and current were evaluated for performance measure using kerosene and water as dielectrics. Experiments were designed using Taguchi L9 orthogonal array.

Parveen Goyal et al (2017) investigated the surface properties of EN-31 die-steel after machining with powder metallurgy EDM electrodes. It has been found that Copper-Manganese (70:30) is better composite material electrode as it gives highest surface micro-hardness as compared to Copper-Manganese (80:20) and copper electrodes. It has been observed that the micro hardness is highest for the maximum peak current and maximum pulse on time which may also be due to more heating and rapid quenching of machined surface, so composite Copper-Manganese material electrode may be recommended in industries for machining of EN-31 die steel.

Bryan E.J.Lee et al (2016) overviewed of the potential of using dual-topography EDM on titanium as a surface modification technique for dental and orthopedic implant applications and highlighted the use of novel multi-current machining approach to create dual-topography surfaces with both micron and enhanced sub­micron topographies. After testing it is noted that EDM modified surfaces increased ALP activity compared to untreated titanium.

M.Risto et al (2016) optimized the EDM drilling process to increase the productivity and geometrical accuracy with different electrode diameters, a specified borehole diameter of 2.00 mm was produced. The impact of the used electrode diameter and required discharge energy on the geometrical accuracy and productivity was investigated.

Yinsheng Fan et al (2016) investigated the maintaining voltage of spark discharge in EDM. The observation was made that the generation of spark discharge and its maintaining depends on size of gap voltage in the process of transistor resistor pulse power and transistor inductor pulse. Maintaining voltage of spark discharge is not inevitable phenomenon in EDM.

Modi et al. (2015) studied EDM process parameters so that the whole process is affected by the electrical and non-electrical. The project work rotating equipment metal removal rate (MRR) to improve and to monitor its impact on surface finish is used.RSM and Taguchi method is used to optimization the design.

Shankar P. et al. (2015) Material removal rate gets increased with the increasing current. Tool wear rate slightly gets increased by the increasing current. With increase in pulse on time, material removal rate gets slightly decreased. Metal removal rate is better when using copper electrode at all the values of current, pulse on time and pulse off time when compared with the brass electrode. Tool wear rate of brass electrode is lesser than the copper electrode.

Pradhan et al. (2014) stated that Electrical discharge machining is typically performed based on material removal rate (MRR), tool wear rate (TWR), relative wear ratio (RWR) and surface roughness (SR) is assessed on. EDM machining process performance measures that affect important parameters of the discharging current, Ton time, pulse off time, gap, and are duty cycle. A considerable amount of work MRR, TWR, RWR based on EDM performance measurement, and different materials have been reported by researchers at the SR.

Abedi et al. (2013) analysed the copper and brass tool electrodes effect on material removal rate in nickel titanium smart alloy using Taguchi method. This study investigated that MRR increases with increase in pulse current and pulse duration. This study also concluded that copper tool electrode is better for higher MRR and brass tool electrode is better for good surface finishing for smart NiTi alloy.

Singh et al. (2013) studied the impact on operating parameters such as pulse-time work piece and cryogenic and non-cryogenic DM electrode using steel such as metal removal rate (MRR) and tool wear ratio (TWR) as a response to copper content and pulse time. Cryogenic treatment is increasing material removal rates and tool wear is used to reduce the rate. It was found that the tool wear rate increase with the pulse treatment cryogenic and non-cryogenic copper electrode, both electrodes is decreased. Tool wear rate increases with increasing pulse off time. With the increase in time for the 100 μs pulse of 50 μs and 20 μs to 15 μs pulse of time with increasing material removal rate increased material removal rate has decreased.

Raghuraman et al. (2013) investigated machining effect on mild steel IS 2026 using copper tool electrode using Taguchi method. During this experimental work they found that MRR and TWR are directly proportional to current and pulse on time. They also found that the Taguchi Grey relational Analysis is being effective technique to optimize the machining parameters for EDM process.

Srivastava and Pandey (2012) compared the machining effect of conventional EDM and cryogenic assisted EDM using copper electrode for machining of high speed steel (M2 grade). From this study, found that the EWR increases with increase in discharge current and duty cycle for both conventional and cryogenic EDM and also found that the EWR and SR are lower in cryogenic EDM as compare conventional EDM process.

Beri et. al. (2012) MRR increases with the increase in current with both +ve and -ve polarity and this increase is higher with +ve polarity as compared to -ve polarity. SR increases with the increase in current with both +ve and -ve polarity. SR obtained with -ve polarity is drastically lower as compared with the SR obtained with +ve polarity. Minimum SR is obtained at 4 amps current with -ve polarity.

Singh et al. (2012) MRR achieved material different than the material work equipment, such as copper and brass D3 mainly achieved using electrodes. The parameter chosen for the study/off time pulse to pulse. They concluded that the MRR brass electrodes increase with increasing pulse on time. He also found time to MRR copper electrode decreases significantly with the decrease in pulse.

Pradhan & Jayswal (2011) applied 23 full factorial design for examine the behaviour of copper and aluminum tool electrodes on electrical discharge machining of EN-8 alloy steel. It has been found that for all values of peak current, surface roughness increases with the increase in pulse on time of both electrodes. This experimental work also concluded that copper gives better surface finishing than aluminum.

Khan and Hammedullah (2011) investigated the effect of electrical discharge machining polarity with straight polarity and reverse polarity using work-piece as a silver steel of 28 grade with copper tool electrode has centre flushing system for higher MRR and lower SR. This study shows that the higher metal removal rate and lower electrode wear is achieved with the straight polarity. Lower SR is obtained by reverse polarity.

Reza et al. (2010) analyzed the effect of machining polarity on Alloy Steel (AISI P20 GRADE 1.2738) using copper electrode by DOE using L18 orthogonal array. The results of this study show that, the positive polarity gives optimum result for material removal rate, tool wear rate and surface roughness.

Choudhary et al. (2010) investigated and evaluation of heat affected zones in electrical discharge machining of EN-31 die steel with copper, brass and graphite tool electrodes using reverse polarity. During the machining of EN-31 die steel, it was observed that copper gives higher MRR, brass gives higher surface finishing and graphite have much deeper heat affected zone. It is also found that with the increase of discharge energy, the debris particles become too large which damages both the work-piece and electrode surfaces.

Kumar et al. (2009) reported the results of experimental investigations during EDM of OHNS die steel with Inconel electrode under machining conditions favoring high electrode wear.

Payal et al. (2008) analyzed the electro discharge machined surfaces of EN-31 tool steel using copper, brass and graphite tool electrodes with reverse polarity. They found that for high discharge current, copper electrode gives highest MRR, whereas brass gives higher surface finishing and graphite tool electrode gives deeper HAZ. The SEM of Machined surface indicated that molten mass removed from surface in the form of ligaments, sheets and chunks, which stuck to the surface due to molten.

Beri et al. (2008) performed evaluation of powder metallurgy electrode (CuW) and Cu in Electrical Discharge Machining of AISI D2 Steel Using L18 orthogonal array of Taguchi Method. During electrical discharge machining of AISI D2 steel, it was observed that Cu tool electrode gives better response in term of MRR and CuW powder metallurgy based electrode gives better response in term of surface finishing.

Patel et al. (2008) investigated the electrical discharge machined effect of copper, brass and aluminum electrodes on MRR and surface roughness of mild steel. During the machining, it was observed that MRR increases with increase in discharge current for all three electrodes. Copper gives better response in MRR and brass gives better surface finishing. It is also found that the SR increases with increase in discharge current and SR of aluminum is more than copper followed by brass.

P. Pecas, E. Henriques (2007) studied nevertheless current EDM technologies have major limitations, when dealing with fine surface finish over large process area. EDM with powder-mixed dielectric (PMD-EDM) has been a focus of an intense research work in order to overcome these technological performance barriers. It was found that the sensitivity of the surface quality measures to the electrode area is smaller when mixed powder dielectric is used.

B. Casas et al (2005) investigated that flexural strength of WC-Co cemented carbides (hard metals), under both monotonic and cyclic loading, is strongly affected by EDM. An analysis of the results using a linear-elastic fracture mechanics approach allows establishing a clear connection between surface integrity and mechanical strength.

Shankar Singh et al (2004) investigated the effects of machining parameters such as pulsed current on material removal rate, diametric overcut, electrode wear and surface roughness in electric discharge machining of En-31 tool steel. The investigation indicate that the output parameters of EDM increase with the increase in pulsed current and the best machining rates are achieved with copper and aluminum electrodes.

2.2 GAP IN LITERATURE SURVEY

A review of the literature reveals that most of the research work has been directed towards optimization and modeling of powder EDM process. Limited research work has been reported on the machining of High carbon high chromium alloy D2 steel through powder EDM. From literature review following gaps have been identified.

The effect of input parameter related to work material need to be explode properly (composition of work material and thickness of work sample), in conjunction with other process parameters, e.g. tool based input parameters: hardness, roughness of tool (hollow, solid and taper) for powder EDM process. Some important machining input parameters related to abrasive material have not been investigated. Input parameter like powder concentration, grit size and tool material have not been included in previous research studies. There has been lack of investigation of production in accuracies such as: hole oversize, out of roundness and conicity. Few studies were available in the literature which have been investigated hole accuracy. However, even these studies have missed some productivity based outcomes like MRR and TWR. Hole accuracy and productivity both are deeply correlated and must be optimized simultaneously.

2.3 PROBLEM FORMULATION

Machining of high carbon high chromium alloy steel (D2 steel) is difficult with conventional machining process because of its high hardness, high wear resistant and high toughness. Due to these properties complex shapes and precise dimensions cannot be accomplished by conventional machining methods. When machining of high carbon high chromium alloy steel (D2 steel) by conventional machining process (CNC turning using CBN tools) results in very high cutting forces are acting on the surface and very poor surface finish.

Conventional machining of D2 steel causes a significant amount of cracks spoiling on the machining surface, which results decreased hardness, wear resistance, corrosion resistance of machining surface. This affects the product quality and service life of the machining component. So that powder mixed EDM will be preferred for the machining of high carbon high chromium alloy steel (D2 steel).

2.4 OBJECTIVES FOR RESEARCH WORK

1. The objective of the present work is an attempt to finding feasibility of machining of High Carbon High Chromium Alloy Steel (D2 Steel) using Copper-chrome and Copper electrode.
2. To analyze the responses; MRR, TWR, Surface Roughness and Hole Diameter Variation by using Taguchi design approach with some selected input machining parameters like; discharge current, pulse on time, tool material and grit size.
3. For Industrial application, Gray Relational Analysis has been performed to optimize the whole machining process for obtain the better output response simultaneously.

2.5 METHODOLOGY FLOW CHART

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Figure2.2 Flow Chart of Methodology

CHAPTER 3 EXPERIMENTAL DETAILS

3.1 EXPERIMENTAL SET-UP OF EDM

All experiments were conducted on a die sinking EDM machining setup (make: Electronica, India; model: Expert 1. As shown in figure 3.1. Available at Central Tool Room, A-5 Focal point Ludhiana.

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Figure 3.1: Electric Discharge Machine Used

3.2 SELECTION OF MATERIALS

3.2.1 Work-piece material

High Carbon High Chromium Alloy Steel (D2 Steel) was used as a work-piece material in this work. The size of work-piece material was (65*30*10 mm) used for all the experiments (L16).

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Figure 3.2: Work-piece material before machining

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Figure 3.3: Chemical composition of work-piece material

Table 3.1: Properties of work-piece material

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3.2.2 Tool Material

Two different tool materials copper and copper-chrome with 19 mm diameter were used to carry out the experiments, details of which are given following:

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Figure 3.4: Tool materials used Table 3.2: Properties of tool materials

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3.2.3 Powder Material

The silicon carbide (SiC) which has significant variation in their thermo­physical characteristics as shown in Table 3.3 was used as additive in kerosene dielectric. Two different grit sizes 280 grit and 600 grit of silicon carbide were used for experimentations. Diameter of grit is measured in microns, 280 grit size is 55 micron and 600 grit size is 15.3 micron diameter respectively. SiC was taken from the Pioneer Chemical Company, Tilak Bazar, Delhi.

Table 3.3: Properties of powder material

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3.3 PROCESS PARAMETERS

Four process parameters i.e., pulse on-time, discharge current, tool material and grit size were selected for the present research work. Choice of parameters was influenced by the fact that these parameters have significant impact on various EDM and PMEDM characteristics as evident from Chapter 2. Lateral flushing with a pressure of 0.4 kg/cm2 and positive polarity (work-piece +ve) were used for all the experiments. Tool Material, Discharge Current, Pulse-on-time and Grit Size were chosen as input parameters in my experiments and these had varied during the entire Powder Mixed EDM experiments. Discharge Current, Pulse-on-time and Grit Size are the parameters of the Powder Mixed EDM machine and which are varied from the machine to get varied output in several forms like MRR, TWR, Surface Finish and Hole Diameter Variation.

Table 3.4: Parameters considered and their levels

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3.4 DESIGN OF EXPERIMENS USING TAGUCHI

A number of parameters are taken into account at same time at initial stage while solving a problem. When the number of factors taken in the experiments increases the experiment frequency, also increases in the factorial experiments by all the level conditions. Then, the obtaining necessary information can be done by an experiment frequency that is less than the factorial experiment by providing experimental conditions by using the table that is called an orthogonal array. The effect of various input parameters on the performance measures in a set of experiments can be easily examined by using orthogonal array experimental design proposed by Taguchi. After determining the parameters affecting a process that can be controlled, the levels of these parameters are to be determined. Finding out the levels of the parameter requires an in-depth understanding of the process, including the minimum, maximum, and current value of the parameter. If the difference between the minimum and maximum values of a parameter is large, the values being tested may be more distant and more values can be tested. But if that difference is small, the values being tested may be closer together and therefore lesser values can be tested. The algorithms make sure that the orthogonal array to be constructed in a statistically independent manner that each level should have an equal number of occurrences within each column and for each level within one column, each level within any other column will be occur an equal number of times. Then the columns are called orthogonal to each other. Orthogonal Arrays are available with a range of factors and levels in the Taguchi method. As each column is orthogonal to the others, if the output associated with one level of a particular factor is much different at another level, it is because changing that factor from one level to the next level has strong effect on the performance characteristic being measured. The 16 trial conditions represented by the Taguchi’s L16 are given in Table 3.5.

Table 3.5: Experimental Design L16

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3.5 PERFORMANCE MEASURES

Effect of powder materials and EDM parameters was investigated in terms of a large number of performance measures in the current research work. Productivity of EDM or PMEDM process is typically characterized by MRR while dimensional accuracy is expressed in terms of ROC. The present investigation attempts major emphasis on various aspects of surface integrity in terms of tool wear rate, surface roughness and Hole Diameter Variation. Methodology of assessing these response characteristics is discussed below:

3.5.1 Material Removal Rate (MRR)

Weight of the work-piece before and after the experiment was measured using an electronic balance (Make: Shinko Denshi, Japan; Model: DJ 300S) shown in Fig. 3.5. Time duration of each experimental run was recorded using a digital stop watch. Eq. (3.1) was used to calculate the MRR.

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Figure3.5 Electronic weighing machine

Wi-Wf

MRR = X 1000mm3/min. (Equation 3.1)

3.5.2 Tool wear rate (TWR)

Weight of the tool material before and after the experiment was measured using an electronic balance (Make: Shinko Denshi, Japan; Model: DJ 300S) shown in Fig. 3.5. Time duration of each experimental run was recorded using a digital stop watch. Eq. (3.2) was used to calculate the TWR.

TWR = X 1000 mm3/min (Equation 3.2)

3.5.3 Surface roughness (SR)

Surface roughness was measured using the equipment Roughness Tester; model SM, RT-10 of S.M. Labs Private Limited, Hyderabad, India available in Inspection Lab of Central Tool Room, A-5 Focal Point Ludhiana. The equipment has profile resolution of 0.01 pm & 0.04 pm for horizontal and 10 pm for vertical measurements. The measure roughness up to ±200pm and has traverse length up to 0.20-16 mm.

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Figure 3.6: Surface Roughness Tester used

3.5.4 Hole Diameter Variation (HDV)

Hole Diameter Variation has been measured by difference between actual diameter of hole and required diameter of hole. Hole Diameter Variation has been a measured by Digital vernier caliper. Eq. (3.3) was used to calculate the HDV.

Hole Diameter Variation (HDV) = HDa - HDr (Equation 3.3)

3.5.5 Scanning Electron Microscope (SEM)

Microstructure was carried out of some selected samples on Scanning Electron Microscope; model JSM-6100 of JEOL, USA available in Sophisticated Analytical Instrumentation Facility (SAIF) Laboratories in Punjab University, Chandigarh. The range of magnification from 5X to 3,00,000X. SEM of samples was carried out on four ranges, i.e. 330X and 2000X.

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Figure 3.7: Scanning Electron Microscope equipment used

3.6 WORKPIECE AFTER MACHINING AND TESTING

Work-piece sample of High Carbon High Chromium Alloy Steel (D2 Steel) has been machined according to Taguchi design of experiments (L16) Orthogonal Array. Work-piece samples after Machining and Testing shown in figure 3.8.

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Figure 3.8: Work-piece samples after Machining and Testing

CHAPTER 4 OPTIMIZATION OF OUTPUT PARAMETERS

4.1 INTRODUCTION

The effect of parameters taken into account in the experiment, i.e. Tool Material, Discharge Current, Pulse-On-Time and Grit Size of abrasive are evaluate using Taguchi Technique. A confidence interval of 95% has been used for the analysis. One repetition for each 16 trails is complete to measure the Single and Noise ratio (S/N Ratio).

4.2 EXPERIMENTAL RESULTS OF MRR, TWR, SR AND HDV

The analysis results for MRR, TWR, SR and HDV for each of the 16 experiment conditions are given in Table 4.1. MRR and TWR for each experiment have been calculated from weight difference between before and after machining of work-piece and tool material. Equation 4.1 and Equation 4.2 was used to calculate the MRR and TWR respectively. Hole Diameter Variation has been calculated by Equation 4.3 and Surface Roughness is measured by surface roughness tester.

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