An experimental investigation on surface characteristics of tool steel using powder mixed micro EDM

... of powder EDM process 17 2.9.1 Mechanism of powder EDM process 17 2.9.2 Influence of powder EDM in surface roughness 20 2.9.3 Influence of powder EDM in surface modification 22 2.10 Conclusion... voltage and capacitance in powder mixed scanning micro- EDM 73 Figure 4.44 Cross section profile of maximum Peak to valley distance in (a) powder mixed die sinking, (b) powder mixed scanning micro- EDM. .. normal EDM while only 25 minutes in EDM with Al powder suspension The spark gap distance depends on powder concentration, type of powder and electrical condition In general an increase in powder concentration

AN EXPERIMENTAL INVESTIGATION ON SURFACE CHARACTERISTICS OF TOOL STEEL MACHINED BY

POWDER MIXED MICRO-EDM

MOHAMMED MUNTAKIM ANWAR

NATIONAL UNIVERSITY OF SINGAPORE

2008

AN EXPERIMENTAL INVESTIGATION ON SURFACE CHARECTERISTICS OF TOOL STEEL MACHINED BY

POWDER MIXED MICRO-EDM

MOHAMMED MUNTAKIM ANWAR

(B.Sc in Mechanical Engineering, BUET)

A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING

DEPARTMENT OF MECHANICAL ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2008

Acknowledgements

I would like to express my deepest and heartfelt gratitude and appreciation to my Supervisors, Professor Wong Yoke San and Professor Mustafizur Rahman for their valuable guidance, continuous support and encouragement throughout the tenure They have provided valuable suggestions from the development of my thesis concept to the fulfillment of my research work Without their continuous supervision it would have been impossible to manage the research within this short period

I would like to take this opportunity to thank National University of Singapore (NUS) for providing me with research scholarship and sufficient funds for purchasing different materials to carry out my research work The rich state of the art facilities and support of Advanced Manufacturing Lab (AML) and Micro Fabrication Lab provided the opportunity to carry out experiments and their analysis smoothly

I also would like to take this opportunity to thank the following staff for their sincere help, guidance and advice without which this project would not be successfully completed: Mr Tan Choon Huat, Mr Lim Soon Cheong and Mr Wong Chian Long from Advanced Manufacturing Lab (AML) and Mr Lee Chiang Soon from Workshop

2 Special thanks go to Mr Abu Bakar Md Ali Asad, an M.Eng candidate of NUS, and Atikur Rahman from Mikro tools, a NUS spin off company, for their help with the machine set-up and for their guidance and technical assistance provided during different period of my research I would also like to thank Mr Muhammad Pervej Jahan, a PhD candidate of NUS, for his valuable advice and encouragement during my experimental work

I would like to offer my appreciation for the support and encouragement during various stages of this research work to my labmates and friends My appreciation goes

to Mohammad Ahsan Habib, Chandra Nath, Woon Keng Soon, Sadiq Mohammad Alam, Mohammad Sazedur Rahman, Masheed Ahmad, Angshuman Ghosh, Md Saiful Karim, Saira Sanjida, Md Ershadul Alam, Mohammad Iftekhar Hossain, Abdullah Al Mamun and many more Special thanks to all of them for being so supportive for the past two years

Last but not the least, my heartfelt gratitude goes to my mother, Ms Shirin Akhter, for her loving encouragement and best wishes throughout the whole period and my father,

Mr Manjour Murshed Anwar, for his mental support and encouragement I would also like to convey my sincere gratitude to my loving sister, Ms Nawrin Anwar for her inspiration and my aunty, Ms Rumaisa Samad, for always being there

2.6 Key system components of Micro-EDM 11

3.4 Measurement apparatus 37

3.4.2 Scanning electron microscope (SEM) and Energy

4.1 Introduction 40

4.3.2 Analysis for powder mixed scanning micro-EDM 67

Summary

Electrical Discharge Machining (EDM) is a non traditional machining process that has become a well established machining option in manufacturing industries throughout the world and has replaced drilling, milling, grinding and other traditional machining operations EDM is capable of machining geometrically complex or hard material components that are precise and difficult to machine In recent years numerous developments in EDM focused in the fabrication of micro tools, micro components and parts with micro features Due to negligible forces and good repeatability of the process, micro-EDM has become the best means for achieving high aspect ratio micro features

For the fabrication of complex 3-D molds using tough die materials, micro-EDM is one of the successfully applied machining processes Since the surface finish of the micro mold cavity affects the surface quality of the finished products, it is necessary to perform investigation about how to improve surface roughness using micro-EDM Moreover fabrication of parts smaller than several micrometers requires pulse duration

of several dozen of nanoseconds Since RC type pulse generator can generate such small energy by simply minimizing the capacitance, it is necessary to investigate the effect of such pulse generators in fine finish micro-EDM

In view of this ongoing challenge to improve surface finish by micro-EDM, a series of experiments were conducted using tungsten electrode of 500µm diameter as a tool and SKH-51 tool steel as workpiece to machine blind holes of 5 µm depth on the workpiece using RC type pulse generator The aim is to find the correct combinations

of parameters that result in better surface characteristics The effect of voltage and capacitance on surface topography and surface roughness were investigated

Since most of the study on surface characteristics of EDM has been conducted using die sinking method, it is important to adopt scanning micro-EDM to evaluate if there is any change in surface characteristics In this point of view experiments are conducted using 500 µm tungsten electrodes to machine 1 mm slot at a depth of 5 µm on SKH-51 tool steel The combination of voltage and capacitance settings for best surface finish

is evaluated The results were compared with die-sinking micro-EDM

With the continuous process improvement in EDM, the demand for high machining precision with low surface roughness at relatively high machining rates arise in die, mold and tool manufacturing industries To fulfill this requirement a relatively new advancement in the direction of process capabilities is the addition of powder in the dielectric fluid of EDM The results show that powder mixed EDM (PMEDM) can distinctly improve the surface finish and surface quality Following the results on studies conducted on the use of PMEDM, an extension of this technique to micro-EDM is essential In this aspect graphite powder with average particle size of 55 nm was mixed with EDM oil at a concentration of 2 g/l and die sinking micro-EDM were conducted to find the correct combination of voltage capacitance setting that results in improved surface quality The results were compared with die-sinking micro-EDM conducted without powder

In the final part of the study a combination of powder added dielectric and tool movement were applied to study the effect of powder mixed scanning micro-EDM on the surface roughness The results were compared with scanning micro-EDM without powder additives

Therefore in this study effort has been made to employ the micro-EDM process in surface study considering different types of dielectric using both die sinking and scanning micro-EDM The results show that powder added dielectric and scanning method together generates the best surface finish in micro-EDM Few recommendations for further improvements are also put forward

List of Tables

List of figures

Figure 3.1 Block diagram of Multi-Purpose Miniature

Figure 3.7 Electrode dressing methods (a) first step with plate micro-EDM,

Figure 3.8 SEM images of electrode after dressing (a) electrode surface after

Figure 3.10 Scanning Electron Microscope (SEM) also with

Figure 4.1 Optical images of surface after die sinking micro-EDM

Figure 4.2 SEM images of surface after die sinking micro-EDM

Figure 4.3 EDX analysis of surface after die sinking micro-EDM

Figure 4.4 EDX analysis of surface after die sinking micro-EDM

Figure 4.5 AFM images of surface after die sinking micro-EDM at 10 pf-120

Figure 4.6 AFM images of surface after die sinking micro-EDM at 100 pf-120

Figure 4.7 Variation of surface roughness with voltage and capacitance

Figure 4.8 Peak-to-valley distance across the line drawn on the surface

profile using 100 pf and 120 volt in die sinking micro-EDM

Figure 4.9 Variation of distance between highest peak and lowest valley

Figure 4.10 Optical images of surface after scanning micro-EDM at

Figure 4.11 SEM images of surface after scanning micro-EDM at

Figure 4.12 SEM images of surface after micro-EDM at 47 pf-80 volt

Figure 4.13 SEM images of surface after micro-EDM at 47 pf-120 volt

Figure 4.14 EDX analysis of surface after die sinking micro-EDM

Figure 4.15 EDX analysis of surface after scanning micro-EDM

Figure 4.16 AFM images of surface after die scanning micro-EDM at 10 pf-120

Figure 4.17 AFM images of surface after scanning micro-EDM at 100 pf-120

Figure 4.18 Surface profiles of AFM images of surface at 10 pf-80 volt

Figure 4.19 Variation of surface roughness with voltage and capacitance

Figure 4.20 Comparison of variation of surface roughness with voltage at

Figure 4.21 Variation of distance between highest peak and lowest valley

Figure 4.22 Cross section profile of maximum Peak to valley distance in

Figure 4.23 Optical images of surface after powder mixed die sinking

Figure 4.24 SEM images of surface after powder mixed die sinking

Figure 4.25 SEM images of surface after micro-EDM at 10 pf-80 volt setting

Figure 4.26 EDX analysis of surface after die sinking micro-EDM

Figure 4.27 EDX analysis of surface after powder mixed die sinking

Figure 4.28 Variation of surface roughness with voltage and capacitance

Figure 4.29 AFM images of surface after die sinking micro-EDM at 47 pf-80

Figure 4.30 AFM images of surface after powder mixed die sinking

micro-EDM at 47 pf-80 volt setting (a) surface profile,

Figure 4.31 Comparison of variation of surface roughness with voltage at

47 pf capacitance in die sinking and powder mixed

Figure 4.32 Variation of distance between highest peak and lowest valley

with voltage and capacitance in powder mixed

Figure 4.33 Cross section profile of maximum Peak to valley distance in

Figure 4.34 Optical images of surface after powder mixed scanning

Figure 4.35 SEM images of surface after powder mixed scanning

Figure 4.36 SEM images of surface after micro-EDM at 100 pf-1000 volt

setting using (a) powder mixed die sinking,

Figure 4.37 EDX analysis of surface after powder mixed die sinking

Figure 4.38 EDX analysis of surface after powder mixed scanning

Figure 4.39 Variation of surface roughness with voltage and capacitance

Figure 4.40 AFM images of surface after powder mixed die sinking

micro-EDM at 10 pf-80 volt setting (a) surface profile,

Figure 4.41 AFM images of surface after powder mixed scanning micro-EDM

at 10 pf-80 volt setting (a) surface profile,

Figure 4.42 Comparison of variation of surface roughness with voltage

at 10 pf capacitance at powder mixed die sinking and

Figure 4.43 Variation of distance between highest peak and lowest valley with

Figure 4.44 Cross section profile of maximum Peak to valley distance

in (a) powder mixed die sinking,

Chapter 1

Introduction

1.1 Motivation

Electrical Discharge Machining (EDM) is one of the most extensively used non

conventional material removal processes It uses thermal energy to machine

electrically conductive parts regardless of hardness In addition there is no direct

contact between the electrode and workpiece This characteristics of EDM eliminates

mechanical stresses, chatter and vibration problems during machining

With growing demands for micro parts and the development of the micro electro

mechanical system (MEMS), micro-EDM is becoming increasingly important

Micro-EDM is considered as one of the most promising methods in terms of size and

precision It has advantage over other fabrication process, such as liga, laser, ultrasonic

ion beam etc because of its lower cost Also the majorities of such non conventional

processes are slower and limited in planar geometries There have been several

successful attempts in producing micro parts such as micro pins, micro nozzles and

micro cavities using micro-EDM

In conventional EDM two types of pulse generators are used: RC type pulse generator

and Transistor type pulse generator The fabrication of parts smaller than several

micrometers require minimization of pulsed energy supplied into the gap between the

workpiece and electrode This means that finishing by micro-EDM requires pulse

duration of several dozen nano seconds Since RC type pulse generators can generate

such small discharge energy simply by minimizing capacitance in the circuit, it is

widely used in micro-EDM However most of the surface study on EDM has been

done using transistor type pulse generator Therefore it is important to investigate the

effect of RC type pulse generator in finishing and micro-machining operation since it

is difficult to obtain significantly short pulse duration with constant pulse energy using

transistor type pulse generators While using RC type pulse generators, the variation of

discharge energy can be done by using different gap voltage and capacitance setting

However at lower gap voltage, the gap between electrode and workpiece is very small

Therefore debris produced in EDM can not be flushed out properly, resulting in arcing

and damaging the surface Therefore the correct combination of voltage and

capacitance is required for better surface finish

Two types of EDM can be used in surface finish operation In die sinking

micro-EDM an electrode with micro features is employed to produce its mirror image in the

workpiece In scanning micro-EDM or micro-EDM milling, micro electrodes adopt a

movement strategy similar to conventional milling and produce 3-D cavities It is

required to understand the effectiveness of both these processes in obtaining cavities

with micro features

Surfaces produced by micro-EDM are generally known to have a matt appearance and

required polishing before most practical applications But apart from enormous time

spent in polishing to reach at an acceptable finish, polishing sometimes become

impossible in case of micro parts This drawback can be overcome to a great extent

through various changes in EDM procedures Therefore scanning micro-EDM can be

used to observe any changes in surface finish of micro-parts relative to die-sinking

micro-EDM

A new approach to a practical and efficient finish process comes in the form of

machining in presence of suspended powder dispersed uniformly in the dielectric

medium Powders such as graphite, silicon and aluminium suspended in the dielectric

have been found to generate fine surfaces over large working areas in high machining

rate in case of conventional EDM However the effectiveness of such a method in case

of micro-EDM needs to be investigated

1.2 Objectives of the research

The aim of this project is to make a comprehensive study on the surface of tool steel

and to investigate the parameters that results in better surface finish using die-sinking

micro-EDM Another purpose of this project is to find the feasibility of venturing

variation in the micro-EDM process by employing tool movement to improve the

surface finish While pursuing this, other possibilities, such as the effect of using

suspended powder in the dielectric in modifying surface roughness is also investigated

The following objectives are to be achieved in this study:

• To perform die sinking micro-EDM to find out the parameters that results in

better surface roughness

• To perform scanning micro-EDM to find out the parameters that results in

good surface finish and to compare the results with die-sinking micro-EDM

• To use suspended powder in dielectric and perform die sinking micro-EDM to

investigate the surface characteristics and compare the results with die sinking

micro-EDM without powder additives

• To use suspended powder in dielectric and incorporate tool movement and

investigate whether there is any improvement in surface finish and compare

the results with suspended powder micro-EDM without the tool movement

1.3 Organization of the thesis

There are five chapters in this dissertation In chapter 2, a comprehensive review is

given, which includes historical background of EDM, principles of EDM, micro-EDM

and its types, micro-EDM compared to other micro machining processes, key system

components of micro-EDM, types of pulse generator, surface characteristics after

EDM, characteristics of powder EDM process

Chapter 3 describes the experimental details This is done in thrre parts In the first

part, details of the experimental set-up are first given It also illustrates the details for

the experiments done in micro-EDM, i.e., selection of tool and workpiece materials,

the specifications of the dielectric used, the details for experiments done with powder

suspended dielectrics such as powder specifications and properties and specifications

of magnetic filter The second part illustrates the experimental procedure followed

throughout the course of study It gives a brief description of electrode dressing, a

summary of the different machining parameter settings and machining details used

throughout the experiments The third part gives brief descriptions of the different

measuring equipment used

Chapter 4 describes the results obtained from the experiments done without powder

additives and with powder additives using die-sinking and scanning micro-EDM and

roughness and peak to valley height of machined surfaces This also highlights the

parameters that results in better surface finish

Chapter 5 summarizes the conclusions derived from the experimental analysis and

guides for possible future work that can be incorporated

of EDM with powder mixed dielectric Section 2.2 gives a brief history of EDM In section 2.3, principles of EDM process is illustrated while in section 2.4, the different types of micro-EDM are discussed Section 2.5 compares micro-EDM with other micro-machining processes Key system components of micro-EDM and types of pulse generators have been discussed in sections 2.6 and 2.7 respectively Section 2.8 deals

with the surface characteristics after EDM, while section 2.9 discusses the

characteristics of powder EDM process

2.2 Historical Background of EDM

In 1770, English chemist Joseph Priestly came to discover that electrical discharge or sparks had erosive effects and it is believed to be the basis of EDM (Kalpajian and Schmid, 2003) In 1943, B.R Lazarenko and N.I Lazarenko at the Moscow University were able to use this sparks and developed resistance-capacitance type power supply to use with Lazarenko EDM System, which was able to machine difficult to machine materials in a controlled process by vaporizing material from workpiece surface (Webzell, 2001) This RC type power supply was extensively used at EDM machines

in 1950s At about the same time three American employees used electrical discharges

to remove broken taps and drills from hydraulic valves They were able to use electronic-circuit servo system which maintained space between electrode and workpiece automatically for sparks to occur (Jameson, 2001) In 1980, the introduction

of CNC in EDM has automated the EDM process Therefore after inserting the electrodes in tool changer, there is no requirement to monitor the process till the final product is ready (Houman, 1983) Since then EDM has been used in manufacturing industries and has become an issue of research Through the years, the machines have improved drastically – progressing from RC (resistor capacitance or relaxation circuit) power supplies and vacuum tubes to solid-state transistors with nanosecond pulsing, from crude hand-fed electrodes to modern CNC-controlled simultaneous six-axis machining

2.3 Principles of EDM

Electrical discharge machining (EDM) is based on the material erosion of electrically conductive materials It is given through the series of spatially discrete high-frequency electrical discharges (sparks) between the tool and the workpiece (Llanes et al., 2001) When sparks are generated the electrode materials will erode and in this way material removal is realized Every discharge (or spark) melts a small amount of material from both of them Part of this material is removed by the dielectric fluid and the remaining solidifies on the surface of the electrodes The net result is that each discharge leaves a small crater on both workpiece and tool electrode (Allen et al., 1996)

In the EDM process, as the electrode charged with a high-voltage potential, come close to the workpiece, an intense electromagnetic flux or ‘energy column’ is formed and eventually breakdown the insulating properties of the dielectric fluid (Guitrau, 1997) The voltage then drops as current is produced, and the spark vaporizes anything

in contact with it, including the dielectric fluid The area struck by the spark will be vaporized and melted, resulting in crater being formed Thus metal is predominantly removed by the effect of intense heat locally generated and collapse of the vaporized dielectric Melting and vaporization actions are the causes of removing material in the EDM process

2.4 Micro-EDM and its types

The basic mechanism of the micro-EDM process is essentially similar to that of the EDM process with the main difference being in the size of the tool used, the power supply of discharge energy and the resolution of the X-, Y- and Z-axis movement

Current micro-EDM technology used for manufacturing micro-features can be categorized into four different types (Pham et al., 2004):

ƒ Die-sinking micro-EDM, where an electrode with micro-features is employed

to produce its mirror image in the workpiece

ƒ Micro-wire EDM, where a wire of diameter down to 0.02mm is used to cut through a conductive workpiece

ƒ Micro-EDM drilling, where micro-electrodes (of diameters down to 5–10μm) are used to ‘drill’ micro-holes in the workpiece

ƒ Micro-EDM milling, where micro-electrodes (of diameters down to 5–10μm) are employed to produce 3D cavities by adopting a movement strategy similar

to that in conventional milling

There is another variant of micro-EDM called micro- WEDG (wire electro-discharge grinding) in which grinding is done using EDM mechanism An overview of the capabilities of micro-EDM is provided in the table 2.1 (Masuzawa, 2000):

Table 2.1 Overview of the micro-EDM varieties Micro-EDM

variant

Geometric Complexity

Minimum feature size

Maximum aspect ratio

2.5 Micro-EDM compared to other Micro-machining processes

2.5.1 Advantages of micro-EDM over other micromachining processes

Compared to traditional micromachining technologies micro-EDM has several advantages such as (Reynaerts et al., 1997):

ƒ EDM requires a low installation cost compared to lithographic techniques

ƒ EDM is very flexible, thus making it ideal for prototypes or small batches of products with a high added value

ƒ EDM requires little job overhead (such as designing masks, etc.)

ƒ EDM can easily machine complex (even 3D) shapes

ƒ Shapes that prove difficult for etching are relatively easy for EDM

2.5.2 Compatibility of micro-EDM with other micromachining processes

Another aspect of comparison between the micromachining technologies is of course the compatibility of the machining technology with the material to be machined Micro-EDM has the advantage of machining of all types of conductive materials such

hardness (Masuzawa, 2000) The following table gives an overview of the

compatibility of the different micromachining technologies (Reynaerts et al., 1997)

Table 2.2 Compatibility of machining technologies with different materials

Micro-stereolithography Polymers

2.6 Key system components of micro-EDM

Micro-EDM is a version of the conventional die-sinking EDM Initially, this type of

equipment was used as a slicing machine for thin-walled structure With the help of

computer numerical control, complex shapes can be cut without using special

electrodes The narrow spark gap and dimensional accuracy of the process make it

possible to provide close fitting parts A typical micro-EDM set-up consists of the

following parts:

ƒ Controller circuit

ƒ Main spindle unit

ƒ Workpiece holder and base

ƒ Dielectric fluid circulation unit

2.7 Types of pulse generator

The controller circuit can have two types of pulse generators – Resistance-Capacitance (RC) or Relaxation type and Transistor type pulse generator Based on the research by Han et al (2004) and review by Kunieda et al (2005) a description is given here on these different types of pulse generators Two kinds of pulse generators: relaxation or

RC type pulse generator and transistor type pulse generator shown in Figures 2.1 (a) and (b) respectively

(a)

(b) Figure 2.1 Different types of pulse generators (a) RC type and (b) Transistor type The fabrication of parts smaller than several micro meters requires minimization of the pulse energy supplied into the gap between the workpiece and electrode This means that finishing by micro-EDM requires pulse duration of several dozen nano-

seconds Since the RC pulse generator can generate such small discharge energy

simply by minimizing the capacitance in the circuit, it is widely applied in EDM RC pulse generator results in extremely low material removal rate from its

micro-The transistor type pulse generator is on the other hand widely used in conventional EDM Compared with the RC pulse generator, it provides a higher removal rate due to its high discharge frequency because there is no need to charge any capacitor Moreover, the pulse duration and discharge current can arbitrarily be changed depending on the machining characteristics required

However, the relaxation type pulse generators are used in finishing and machining because it is difficult to obtain significantly short pulse duration with constant pulse energy using the transistor type pulse generator If the transistor type is used, it takes at least several tens of nano-seconds for the discharge current to diminish

micro-to zero after detecting the occurrence of discharge because the electric circuit for detecting the occurrence of discharge, the circuit for generating an output signal to switch off the power transistor and the power transistor itself have a certain amount of delay time Hence, it is difficult to keep the constant discharge duration shorter than several tens of ns using the transistor type pulse generator But with RC type pulse generator, uniform surface finish is difficult to obtain because the discharge energy varies depending on the electrical charge stored in the capacitor before dielectric breakdown

2.8 Surface characteristics after EDM

There has been extensive use of EDM technology in tool, die and mould making industries, which requires tool steels with high precision and better surface finish, since conventional machining cannot machine these difficult-to-cut materials economically (Singh et al., 2004)

Amorim et al.(2003) conducted experiments on AISI P200 tool steel with a view to analyze material removal rate, volumetric relative wear and workpiece surface roughness by varying discharge current, discharge duration, pulse interval time, gap voltage, electrode polarity and generator mode It shows that negatively polarized electrode results in better surface finish

Singh et al (2004) carried out experiments on En-31 tool steel with different electrodes and reported the effects of pulsed current on material removal rate, overcut, electrode wear and surface roughness The results revealed that output parameters of EDM increases with an increase in pulse current Guu (2005) worked on AISI D2 Tool steel to analyze surface roughness (Ra) by varying pulsed current and pulse on duration AFM study of the specimen shows that the higher discharge energy, the poorer the surface finish It was reported that surface roughness varied in a range of 103-172 nm, whereas peak to valley height varied in a range of 1272-1873 nm Puertas and Luis (2003) used a prismatic electrode made of copper with a square cross-section

of 25mm × 25mm to perform EDM on soft steel.(F-1110) It was observed, the pulse

on time (ti) and pulse off time (to) have little influence on the variation of surface roughness (Ra) On the contrary, the discharge current intensity (I) has a great deal of influence on the Ra parameter Moreover, there is a great decrease of Ra when

increasing the I parameter and the same tendency but in a much lower rate can be considered for the ti parameter This behavior is just the opposite of what is expected but considering that the variation range of Ra is a narrow one, it is considered that this effect is due to a better arc stability causing more uniform pulses There is an interaction between the I factor and the ti factor The best Ra value will be obtained when increasing the I factor and maintaining the ti factor at its low value The copper electrode was kept at positive polarity and the specimen was kept at negative polarity during the EDM process of AISI D2 tool steel (Guu et al., 2003) EDM surfaces were measured The surface roughness on the machined surface varied from 1.3 to 11.0 µm The results exhibit that a higher pulse current causes a poorer surface finish An excellent machined finish can be obtained by setting the machine parameters at a low pulse current and small pulse-on duration The pulse current has a more dominant effect on the surface roughness compared to the pulse-on duration It can be attributed

to the fact that a higher pulse current may cause more frequent cracking of the dielectric fluid, there is more frequent melt expulsion resulting in the poorer surface finish Ghanema et al (2003) reported that surfaces obtained by EDM of X155CrMoV12 steel exhibit a characteristic aspect consisting of the superposition of craters due to metal evaporation during machining No evidence of preferential material removal directions was observed (no ridges were seen).The metal ejected by erosion is resolidified and redeposited at the surface as spheroidal particles of different sizes The evolution of the surface roughness is mainly affected by the machining intensity Qualitative energy dispersive X-ray (EDX) analysis, performed in the SEM

on specimen surfaces of hardenable steels before and after EDM Comparison between these profiles clearly shows enrichment in carbon and hydrogen in the outer layer of EDM specimens This phenomenon is due to the diffusion of carbon and hydrogen

atoms produced by the evaporation of the dielectric liquid and the graphite of the tool This evaporation occurs during the electric discharge Lee et al (1988) showed that EDMed surfaces usually have a matt appearance covered by shallow craters, globules

of debris, and pockmarks formed by entrapped gases escaping from the redeposited material After EDM of AISI D2 tool steel it has been found that at low discharge energy, the craters are shallow and the density of global appendages and pockmarks is low Whereas at higher discharge energy, the craters are deeper and global appendages are most evident These global appendages are molten metals which were expelled randomly during the discharge and later solidified on the workpiece surfaces The edge

of some of the pockmarks is thin and sharp suggesting that the molten metal must have solidified at an extremely high rate For otherwise the surface tension of the molten metal would have caused the sharp edges to be rounded off Another common feature

on EDMed surfaces is the abundance of cracks, especially at high discharge energies These cracks are formed as a result of the exceedingly high thermal stresses prevailing

at the specimen surface as the latter was cooled at a fast rate after the discharge It is interesting that under identical discharge conditions, the surface roughnesses of all the steels are quite similar Their values, however, depend on both the pulse energy and current in a complex manner By and large, at a given pulse energy, a poorer surface finish is obtained with a higher pulse current This implies that the pulse current has a more dominant effect on the surface roughness compared to the pulse-on duration It is possible that a higher pulse current (or input power) may cause more frequent cracking

of the dielectric, giving rise to more frequent melt expulsion This in turn will result in

a higher density of global appendages and poorer surface finish

The surface texture in EDM is characterized by discharge craters The surface roughness is therefore mainly determined by the electric pulse parameters: discharge current, discharge duration, and polarity Very fine roughness and shiny surfaces are hard to obtain directly by sinking EDM Smooth or shiny surfaces (with Ra < 1 µm) can only be obtained with low discharge energy and inverse polarity (with the tool electrode as cathode), resulting in smooth surface craters (Bleys et al., 2006).When the tool is kept at negative polarity, it is exposed to more heat than workpiece Optimal pulse duration produces sufficient heat for controlled melting of workpiece and removes fine amounts of material resulting in better surface appearance (Wong et al 1998).Wu et al (2005) reported that discharge current in EDM is composed of ions and electrons With increase in pulse duration time, the proportion of ion flow increases So if the workpiece is kept at negative polarity the ion flow will impact violently on the workpiece resulting in poor surface roughness

2.9 Characteristics of powder EDM process

There have been various attempts to improve the surface finish after EDM by polishing and other means But if it is possible to improve the surface during machining it will shorten the machining time From this viewpoint powder EDM is one of these processes However suspended powder EDM not only imparts fine surface finish but also modifies the surface However, it is necessary to understand the mechanism of powders EDM process

2.9.1 Mechanism of Powder EDM process

Suspended powders increase the spark gap distance due to their presence between tool and workpiece It has two outcomes: firstly, increased spark gap is useful in effective

removal of debris from the gap; secondly, it makes the powder EDM process highly stable with effective discharge dispersion throughout the gap An increase in the distance decreases the electrostatic capacitance of the gap Therefore, it becomes possible to discharge micro-current at potential micro-peaks throughout the work surface To prove this phenomenon, the electrode was divided into two units of equal areas and connected to two current transducers Waveforms were monitored on oscilloscope (CRT).In case of powder EDM, discharge was observed in both electrodes at the same time But in normal EDM almost all discharges occur at only one of the two electrodes at any point in time Efficient discharge dispersion not only forms uniform work surface but also prevents the occurrence of concentrated arc discharge and hence reduces finishing time Suspended powder EDM reduces machining time significantly for fine finishing compared to normal EDM methods It

normal EDM while only 25 minutes in EDM with Al powder suspension The spark gap distance depends on powder concentration, type of powder and electrical condition In general an increase in powder concentration tends to increase the spark gap distance Al powder suspension of 30g/l increases the spark gap by factor of ten than normal EDM (Narumiya et al., 1989) Zhao et al (2002) reported that powder particles in the gap are conducting and the role these powders play causes aberration in electric field Under applied gap voltage, positive and negative charges gather on to the top and bottom of powder particles respectively When the electric field density surpasses the breakdown voltage, discharge breakdown starts at the points which are near to the top or bottom as these points have higher electric charge density It results

in short circuit between two powder particles and causes redistribution of electrical charges More electric charges gather at points which are near to tool and workpiece It

results in series discharge between two powders and other powders and accordingly the discharge breakdown between tool and workpiece This enlarges the discharge gap Moreover electrodes and ions collide with powder particles and release more electrons and ions This results in increase in carriers (ions and electrons) and widening of the discharge passage This widened discharge passage enlarges the area that is influenced

by discharge heat resulting in reduced discharge density This improves the surface finish

Yih-fong and Fu-chen (2005) described the material removal mechanism and mentioned the different factors that affect the surface roughness in powder EDM In case of normal EDM, working fluid evaporates and creates gas explosion that results in mechanical thrust In case of powder EDM the striking impact of powder particles is added with this mechanical thrust to remove materials However the grinding effects of particles are negligible in case of material removal It is reported that smallest particle yields best surface finish as they produce fine cutting effects Low electrical resistivity, high thermal conductivity and low density of the powder particles are essential for better surface finish Due to low resistivity higher spark gap is created and for high thermal conductivity more heats are taken away resulting lower discharge density Lower discharge density generates low gas explosion and shallow craters are produced

on the surface Moreover low density particles result in low explosive impact on the surface resulting better surface finish Narumiya et al (1989) reported that with the increase in powder concentration the discharge gap also increases But after a certain concentration, the gap distance does not increase with further increase in concentration An excessive amount of powder additives is likely to display similar characteristics as an excessive accumulation of debris resulting in short circuit and thus damaging the machined surface and deteriorating machining stability

2.9.2 Influence of Powder EDM in surface roughness

Erden and Bilgin (1980) added powdered form of copper, iron, aluminium and carbon into commercial kerosene oil as impurities Brass-steel and copper-steel pairs were used as electrode and workpiece The results indicated that added powder improves the breakdown characteristics of dielectric fluid Machining rate increases with the increase in powder concentration as there is a decrease in time lags at high purity concentration However excessive powder concentration makes the machining unstable

as there occurs short-circuit Impurity concentrations improved the surface quality and gap size Jeswani (1981) added graphite powder into kerosene oil at a concentration of 4g/l and reported that the interspace for electric discharge initiation was increased while breakdown voltage was lowered An improvement in machining process stability resulted in 60% increase in MRR and 28% reduction in WR Mohri et al (1991) added silicon powder of 10-30µm size uniformly in dielectric fluid and performed machining

at discharge current of 0.5-1 A for discharge duration less than 3 µs using negative polarity It was able to produce surface with roughness less than 2 µm However to achieve this performance even distribution of debris and short discharge time are required Narimuya et al (1989) reported that Aluminium and Graphite powder generates better surface roughness than silicon powder at specific machining conditions Aluminium and graphite powder having diameter less than 15µm and at a concentration of 2 to 15 g/l yield surface roughness of less than 2 µm Kansal et al (2005) reported that the concentration of powder should be 2g/l to obtain better surface finish Kobayashi et al (1992) reported that silicon powder improves the surface finish

of SKD-61 tool steel Yan and Chen (1994) added aluminium and silicon carbide powder with dielectric and performed EDM on SKD-11 and Ti-6Al-4V and reported

improvement in MRR and increase in surface roughness Uno and Okada (1997) reported that silicon powder mixed with dielectric reduces the impact force acting on the workpiece and results in smaller undulation of craters, hence glossy surface Wong

et al (1998) used fine powders of silicon, graphite, molybdenum, aluminium and silicon carbide with dielectric Aluminium powders produce mirror finish on SKH-51 workpiece but failed to produce mirror finish on SKH-54 material Rather semi-conductive silicon and carbon powders were able to produce very fine finish condition

It was observed that electrode polarity (negative), pulse parameter and powder characteristics play a major role in producing very fine finish Rahuman (1994) reported that with low powder feed rate (2 ml/s) graphite powder at a concentration of

2 gm/l was able to yield Ra value of 0.0931 µm on SKH-54 tool steel Wang et al (2001) mixed Al and Cr powder in kerosene fluid which reduces isolation, increases the gap between tool and workpiece and thus stabilize the process As small size particles have higher concentration, they would have higher suspension effect in dielectric fluid Therefore there is higher probability that the gap will be bridged and uniform discharge dispersion will result in good surface finish Chow et al (2000) added SiC and Aluminium powders into kerosene to machine micro-slit on titanium alloy by EDM The addition of powders increased the inter-electrode gap that facilitates debris removal and also causes extension of slit expansion Moreover uniform dispersion of discharge results in several discharge trajectories from single input impulse Therefore several discharging spots are created resulting in increased MRR and better surface finish It was also observed that material removal depth is better using SiC than using aluminium powder Tzeng and Lee (2001) added Al, Cr,

Cu and Sic with working fluid and performed EDM on SKD-11 material It was reported that machining performance is significantly affected by concentration, size,

density, electrical resistivity and thermal conductivity of added powder It was found that for a fixed concentration, highest MRR was achieved with smallest size particles Moreover it was also revealed that due to its higher density Cu powder has almost no effect in EDM Pecas and Henriques (2003) added silicon powder at a concentration of 2g/l and found that operating time and surface roughness decreases It was also reported that average surface roughness depends on machining area and machining time The variation in surface roughness is in the range of 0.09 to 0.57 µm for the area

2.9.3 Influence of Powder EDM in surface modification

Powder EDM also modifies the surface properties These modifications result in high corrosion resistance, high oxidation temperature and high wear resistance EDM with silicon electrode produces fine surface structure in stainless steel workpieces and hence imparts new properties In plastic deformation test it has been revealed that EDMed surface of stainless steel has a strong bonding with the parent surface (Mohri et al., 1988).There have been reports that specimens machined in graphite, silicon and aluminium powder suspensions exhibits low level of corrosion compared to those machined in normal working fluid The results have been confirmed by tests in aqua regia The absence of heat affected areas in powder mixed EDM are mainly responsible for this effect The surfaces generated in powder EDM are found to withstand hard corrosion and wear Uno et al (1998) observed that nickel powder mixed with working fluid deposits a layer on EDMed surface of aluminium bronze components and makes the surface abrasion resistant Okada et al (2000) performed EDM with titanium electrode and silicon powder mixed dielectric and was able to form

a thick and smooth layer of titanium carbide on the workpiece The hardness and wear

resistance of the coated layer tend to be much higher than that of base material Furutani et al (2001) used Titanium powder of size less than 36 µm at a concentration

of 50g/l with EDF-K (Mitsubishi oil).Gap voltage was maintained at 320V with negative polarity A smooth layer of titanium carbide accretion was observed with thickness of 150 µm and hardness of 1600 Hv on surface of carbon steel with an

electrode of 1 mm diameter

2.10 Conclusion

SKH-51 tool steel has high wear resistance compared to conventional mold materials for processing thermoplastic, thermosetting or glass-fiber resins These molds can be used several times compared to conventional products Moreover mold maintenance requirements are reduced and the difference in total cost becomes significant when small parts are fabricated However for micro slots and micro molds in tool steels, the optimum parameters for surface roughness using micro-EDM are not completely investigated yet Moreover most of the works in EDM have been carried out using transistor type pulse generator to find the effect of discharge energy on the surface roughness Since RC type pulse generator can produce very small discharge energy it

is used in finishing operation Therefore it is necessary to investigate the effect of RC type pulse generator on the surface characteristics of SKH-51 tool steel

Moreover in conventional EDM most of the work has been done using die sinking EDM In die sinking EDM, the tool electrode has the complementary form of the finished workpiece and literally sinks into the workpiece On the other hand, scanning EDM, more commonly known as EDM milling, is a newer process which is mainly

used when large and complex geometries are required In contrast to die-sinking EDM, scanning EDM eliminates the need for complex-shape electrodes In this process, usually tubular or cylindrical micro-electrodes are employed to produce the desired complex shape by scanning A cylindrical electrode rotates around its axis (Z-axis) with the scanning movements in X and Y directions The contour of a particular layer

is specified in the part program of CNC However, electrode compensation is an important factor to consider as electrode length is reduced after scanning every layer This study uses a common wear compensation method for layer-by-layer milling EDM indicated as “anticipated wear compensation” as shown in Fig.2.2 (Bleys et al., 2004) Electrode wear was compensated by down-motion (sW) of the electrode When machining with a cylindrical electrode, such a continuous down-motion of the electrode, combined with occurring tool wear, causes a rapid stabilization of the electrode shape thus maintaining steady-state shape of the tool electrode tip Figure 2.2 presents the schematic diagram of the scanning EDM process So the effectiveness of scanning micro-EDM in modifying surface roughness is also investigated

Figure 2.2 Schematic representation of layer-by-layer scanning µEDM

Following the results of studies conducted on the use of powder suspended dielectrics for conventional EDM, which indicated the possibility of reducing surface roughness while ensuring a limited increase in gap distance through the use of selected semi-