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Chapter 1

Introduction

1.1 DRILLING

Machining of holes is one of the most common operations in the

manufacturing industries. Literally no workpiece leaves the machine shop without

having a hole made in it. Drilling is a machining process, which produces or enlarges

holes. A research says approximately 50 to 70 % of all production time is spent in hole

making process in industries (Benes, 2000). Recent progress made in the field of

aviation (cooling holes in jet turbine blades), space, automobile, electronics and

computer, medical (surgical implants), optics, miniature manufacturing and other

created the need for small and micro-size holes with high aspect ratio in extremely hard

and brittle materials (Baker, 1991).

1.1.1 CLASSIFICATION OF HOLES

There are many individual views related to characterization of a hole. A hole

is generally perceived to be a circular opening in an object. It is defined as “an opening

in or through anything; a hollow place; a cavity in a solid body or area; a three

dimensional discontinuity in the substance of a mass or body” (Yeo et al., 1994). In

general, hole may be categorized by considering the cross-section in different planes,

aspect ratio and size. Table 1.1 and Fig. 1.1 show the various classifications of holes.

Table 1.1 Classification of holes

(a) Based on size (Yeo et al., 1994)

Name Bore Large hole Small hole Micro hole Hole size (mm) >25.4 12.7 to 25.4 1.0 to 3.2 <1.0

(b) Based on aspect ratio (Bellows and Kohls, 1982; Benes, 2000)

Name Cavity Pit Hole Deep hole Aspect ratio l/d < 1 1 < l/d < 4 l/d > 4 l/d ≥ 5

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1.2 DEEP HOLE

The term deep hole is defined as the one with a depth equal to five times its

diameter or greater (Benes, 2000). Deep-hole drilling is becoming increasingly more

prominent in a variety of applications, such as weaponry, automobile industries, textile

industries, electronic industries, aerospace industries, medical applications etc. When

the ratio of depth to diameter (aspect ratio) increases it becomes extremely difficult to

manufacture such holes. Some of the typical applications of deep hole drilling are given

in Fig. 1.2.

1.2.1 DEEP HOLE MAKING PROCESSES

Drilling is one of the earliest machining activities of mankind. Machining

high precision small holes usually presents several problems related to dimensional

accuracy, surface roughness and tool life. Today, there are lot of applications that

require the machining of small holes on a production basis. Several techniques are

available for machining small diameter deep holes, including drilling, Electrical

Discharge Machining (EDM), Electro Chemical Machining (ECM), Laser Beam

Machining (LBM), Electron Beam Machining (EBM), Photo Chemical Machining

(PCM) and Ultrasonic Machining (USM).

Fig. 1.1 Nomenclature of holes (Bellows and Kohls, 1982)

d

a

d

d

d

d

l

l

l

l

Pit: 1:1 , : 4 :1l d l d> <

Contoured hole

Circular d

Slot : 4 :1d a >

Geometric d

Irregular or shaped d

Depression

Cavity l: d < 1:1

Hole l: d > 4 : 1

3

1.2.1.1 TRADITIONAL METHODS

According to Heinemann et al. (2006) about 75 % of the deep holes are still

produced by conventional drills, mainly because of their versatility and apparent

low cost. The traditional method of small diameter deep hole drilling needs a suitable

drilling machine and a skilled operator in which drill bit encounters frequent breakage.

Hence the actual cost is usually high while drilling tough materials and small diameter

high aspect ratio deep holes because of broken drills and scrapped parts. Deep hole

drilling is generally accomplished by either gun drilling, ejector drilling or trepanning.

Gun drilling is most commonly used for holes ranging from 3 mm to around 25.4 mm

in diameter. Ejector drilling can be used to drill larger and longer holes than gun

(i) Small holes in hardened steel (Bilgi, 2005), (ii) Hip joint holes in cobalt based

superalloy (Bilgi, 2005), (iii) Cooling holes in turbine blade of nickel based super

alloy (Bellows and Kohls 1982), (iv) Cooling holes in gas turbine nozzles segment

(Raju and Ahmed, 2004)

Fig. 1.2 Applications of deep hole drilling

(i) (ii)

(iii) (iv)

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drilling. The ejector system works well for large depth-to-diameter ratio as high as 50:1

and hole diameters ranging from 20 to 180 mm. Trepanning is often referred to as a

core-drilling process. It is used to produce large diameter holes. This process is some

what more efficient than all other methods because it cuts out a core during drilling

(Schrader et al., 2000). The conventional methods of making deep holes are not always

suitable for machining today’s tough superalloy materials, complex shaped components

and finds its limitation in economic machining. The challenge being faced by

manufacturing is to sustain productivity especially in the case of superalloys. Since

superalloys are extremely difficult to machine, traditional drilling methods don’t

provide any solution. Conventional twist drilling process becomes unsuitable for

drilling small diameter deep holes in superalloys due to high tool wear and tool

breakage (Chen and Liao, 2003). Usually, the advanced machining processes (AMPs)

are the preferred techniques for drilling holes in high strength temperature resistant

(HSTR) alloys where metal removal rate is independent of the toughness of the

material.

1.2.1.2 ADVANCED MACHINING PROCESSES

For machining small holes, several AMPs are available (Table 1.2). However,

the aspect ratio demand is often larger than that which can be easily achieved by these

techniques. If a hole with high aspect ratio (l:d > 10) can be machined in a short time it

would provide a good support for the production of precision machines and aerospace

parts. EDM, ECM, LBM, EBM, PCM, USM can be successfully employed in deep

holes (Bellows and Kohls, 1982). The performance of machining processes can be

evolved in terms of type of workpiece material (conductive or non-conductive), shape

of the hole (circular, slot, geometric, irregular), size of hole, aspect ratio, surface

integrity, production rate and cost of machining. The comparative study indicates that

only four AMPs viz., (i) EDM, (ii) ECM, (iii) LBM, (iv) EBM are suitable for drilling

deep holes. But none of them are capable of producing holes to the fullest satisfaction.

EDM is the most commonly used nontraditional machining processes for

drilling because of the following advantages: Much of its success has come from its

simple tooling and its capability of drilling multiple holes simultaneously. A principal

economic advantage comes from the absence of burrs and the elimination of secondary

deburring operations. When cost of deburring is added to conventional drilling and

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reaming, the cost advantage frequently swings to EDM. The cost advantage is even

more pronounced when shallow surface angles and tough super-alloys or high-strength

steels are involved. The absence of drill wandering with EDM contributes to a reduction

of scrap. Electrochemical processes for drilling small and fine holes by controlled and

anodic dissolution invariably use a weak acidic solution as electrolyte. These include

electrochemical drilling (ECD) and acid based ECM drilling processes: Shaped Tube

Electrolytic Machining (STEM), Capillary Drilling (CD), Electro-Stream Drilling

(ESD) and Jet Electrolytic Drilling (JED). ECM drilling process is applicable only for

non-corrosive materials and it is not suitable for producing micro holes. Both LBM and

EBM can not be effectively used to drill relatively thicker materials, since the

maximum cutting depth being restricted to only about 18 mm. PCM and USM can be

used to drill materials chemically active and harder than HRc 35 respectively. A

comparison of various advanced machining processes in drilling is given in Table 1.2.

Table 1.2 Comparison of different AMPs in drilling operation (Bellows and Kohls, 1982; Uno et al., 1999)

Process EDM ECM STEM LBM EBM PCM USM Types of material Conductive Conductive Conductive Any

material Any

material Chemically

active Harder than

Rc 35 Hole

size(mm) Min/Max

0.009/6.35

3.17/76.2

0.5/6.35

0.002/1.52

0.02/1.27

0.025/no

limit

0.076/3.17

Hole Depth (mm) Max

200

304.8

914.4

17.78

7.62

4.76

25.4 Aspect

ratio (l/d) Typical

Max

10:1 200:1

8:1 20:1

16:1 300:1

16:1 75:1

6:1 100:1

2:1 5:1

2.5:1 10:1

No. of multiple drilling

200

100

100

2

1

No limit

10

Cutting rate

(mm/min) 0.762 7.62 1.524 < 1 s 15.24 0.0254 0.05-25.4

Typical tolerance (± mm)

0.012 0.05 10 % d 5 to 20 % d 5 to 10 % d 0.025 0.025

Finish µm AA 1.6 – 3.2 0.4 – 1.6 0.8 – 3.2 0.8 -6.4 0.8 – 6.4 0.8 – 3.2 0.4 – 0.8

Surface integrity

Heat-affected surface; No burrs

No residual stress ;

Polished surfaces; No burrs

No residual stress;

No burrs

Heat-affected surface

Heat-affected surface

No residual stress; Sides

undercut

Gentle

Special attributes

Irregular piercing or contours; No burrs; Delicate

component

Contoured and

irregular shapes;

No burrs

Shaped and multi-

angled; No burrs

Rapidly adjustable

Rapid positioning

Continuous strip

production

Sharp corners; No burrs

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1.3 ELECTRIC DISCHARGE MACHINING

In electric discharge machining, the removal of material is based upon the

electro-discharge erosion effect of electric sparks occurring between two electrodes that

are separated by a dielectric liquid as shown in Fig. 1.3 and material is removed by

electric erosion and vaporization of the molten metal. The EDM system consists of a

shaped tool (electrode) and the workpiece, connected to a DC power supply and placed

in a dielectric (electrically non-conducting) fluid. When potential difference between

the tool and the workpiece is sufficiently high, a transient spark discharges through the

fluid, removing a very small amount of workpiece surface. The capacitor discharge is

repeated at the rates of between 50 and 500 kHz, with voltages usually raging between

30 and 380 V, currents from 0.1 to 500 A.

Fig. 1.3 Schematic diagram of EDM process (Mishra, 1997)

Fig. 1.4 shows the three important phases in single electrical spark discharge,

preparation phase for ignition (a, b, c), phase of discharge (d, e, f), and interval phase

between discharges (g, h, i). The phases of electrical discharges shown in Fig.1.4 are

briefly explained below.

First, the electrode moves close to the workpiece as shown in Fig. 1.4(a). As

the potential difference increases between the two surfaces, the local dielectric fluid

breaks down and ions are generated. The electrical field is strongest at the point where

the distance between the two surfaces is minimum. Electric discharge then occurs at

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that area. The voltage has increased but no current is flowing because of the presence of

the dielectric fluid. Next, as shown in Fig. 1.4(b), more and more ions being generated,

making the insulating property of the dielectric fluid begins to decrease along a narrow

channel at the point where strongest electrical field occurred. This time the voltage

reaches its peak, while current is still zero. Fig. 1.4(c) shows that current starts to

establish, making the voltage decreases. A discharge channel begins to form between

the electrode and the workpiece.

Fig. 1.4 Phases of electrical discharges (Schumacher, 2004)

Current

Discharge channel Ions

Molten material

Molten material

Debris

Current

Discharge channel Ions

Molten material

Molten material

Debris

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The voltage continues to decrease as seen in Fig. 1.4(d), while current

continues to increase. This will allow the heat to build up rapidly, causing some of the

anode, cathode and dielectric materials to vaporize. Fig. 1.4(e) depicts the expansion of

the discharge channel which is full of vapour was constrained by the rush of ions,

attracted towards it by the intense electromagnetic field. Fig. 1.4(f) shows the situation

near the end of the voltage when the current and voltage have stabilized. The heat and

pressure inside the channel have reached the maximum and some materials have been

melted and removed. The molten material is held in place by the pressure of the vapor.

Figs. 1.4(g, h and i) show the conditions after voltage and current approach to

zero. The temperature and pressure rapidly decreased in the discharge channel and

cause it to collapse, thus allowing the molten material to be expelled from the

workpiece surface. The dielectric fluid flows in, flushes the debris away, and quenches

the surface of the electrode and the workpiece. Unexpelled molten material resolidifies

back to the surface to form a recast layer. At this stage the electrical spark is completed

and the condition is ready for the next spark.

1.4 ELECTRIC DISCHARGE DRILLING

One of the first applications of EDM was drilling holes in injector nozzles of

diesel engines. According to Jeswani (1979), electric discharge drilling (EDD) started

replacing the mechanical drilling since 1948. The electrical discharge drilling (EDD) is

an extremely prominent machining process for small deep hole drilling among the

newly developed non-traditional machining techniques. The EDD process, which

utilizes thermal effect rather than mechanical force to remove material is suitable for

machining of superalloys, which have the highest hardness in reinforcement, creating

cooling channels in aero-engine gas-path components such as turbine blades, guide

vanes, after burners and castings and is free from burr formation (El-Hofy, 2005).

EDD uses a tubular tool electrode where the dielectric is flushed down the

interior of the hole in order to remove machining debris. When solid rods are used, the

dielectric is fed to the machining zone either by suction or injection through the

predrilled holes. Irregular, tapered, curved, as well as inclined holes can be produced by

EDD. The scope of the EDD process ranges from the drilling of micro holes that are

smaller than a human hair to machining large diameter holes. In EDD process the

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Fig. 1.5 Schematic diagram of electric discharge drilling process

electrode is made to rotate with the help of separate rotary attachment. Some times both

tool and the workpiece are made to rotate relatively. Fig. 1.5 shows the schematic

diagram of EDD process.

1.5 SUPERALLOYS

Superalloys perform a major role in meeting space-age materials requirements

in the temperature range between approximately 3000 and 4265 K. Their high strength

at these temperatures coupled with generally good oxidation resistance makes such

alloys prime candidates for many aerospace applications (Freche, 1964). Superalloys

are important in high-temperature applications; hence, they are also known as

heat-resistant or as high-temperature alloys. These alloys are referred to as nickel-, iron-

and cobalt-base superalloys. These superalloys (Ni, Fe–Ni, Co-base) are further

subdivided into wrought, cast, and powder metallurgy alloys (Metals hand book, 1990).

Nickel-base alloys are the most widely used superalloy, accounting for 50 %

of materials used in aerospace engines, mainly in the gas turbine components

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(Miller, 1996). Fig. 1.6 shows the weight percentage of materials used in aerospace

engines. There is an increase in use of nickel-base and titanium alloy, suggesting their

dominant use in aerospace engines. Other applications include marine equipment,

nuclear reactors, petrochemical plants and food processing equipment. Amongst the

commercially available nickel-base superalloys, Inconel 718 stands out as the most

dominant alloy in production, accounting for as much as 45 % of wrought nickel-based

alloy production and 25 % of cast nickel-based products (Choudhury and El-Baradie,

1998).

Nickel-base superalloys have some characteristics that are responsible for its

poor machinability. They have an austenitic matrix, and like stainless steels, work

hardens rapidly during machining. Moreover, localization of shear in the chip produces

abrasive saw-toothed edges which make swarf handling difficult. These alloys also have

a tendency to weld with the tool material at the high temperature generated during

machining. The tendency to form a BUE during machining and the presence of hard

abrasive carbides in their microstructure also deters machinability. These characteristics

of the alloys cause high temperature (1270 K) and stresses (3450 MPa) in the cutting

zone leading to accelerated flank wear, cratering and notching, depending on the tool

material and cutting conditions used (Choudhury and El-Baradie, 1998). The research

on machining of nickel-base alloy, Inconel 718 in the past was mainly on turning and

milling operations and only few investigations are reported on drilling of Inconel

superalloy by conventional twist drill (Ezugwu and Lai, 1995; Lacalle et al., 2000;

Chen and Liao, 2003; Sharman et al. 2008).

Fig. 1.6 Materials used in aero engines (Miller, 1996)

% E

ngin

e w

eigh

t

1960 1970 1980

Metal matrix composites

1990 2000 2010

10

20

30

40

50

60 Steel

Nickel

TitaniumAluminium

Carbon composites

Ceramic matrix composites

Year

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1.6 MOTIVATION FOR THE RESEARCH WORK

EDM has been successfully used to machine hard materials that pose

problems for traditional mechanical cutting, yet aerospace materials such as titanium

and nickel-base alloys are rarely machined using the EDM process. Their low thermal

conductivity and high strength at elevated temperatures complicate mechanical cutting

processes, and pose difficulty in producing high quality products. By applying modern

EDM technology, however, these materials could be effectively and efficiently

machined.

Inconel 718 is the widely used nickel-base superalloy in aerospace at high

temperature regions. These parts require a large number of small diameter (1- 4 mm)

cooling holes with high aspect (40 – 200) to maintain the working temperature to

increase the operational efficiency (Sharma et al., 2002). Drilling of small diameter

deep holes has been a difficult problem, especially in parts made of superalloys such as

Inconel 718 because of the toughness of the material. Deep hole drilling by EDD is one

of the viable advanced machining processes and most economical methods of hole

producing with length-to-diameter ratios greater than five (Bellows and Kohls, 1982).

Electro discharge drilling, as it is currently practiced, is a versatile and

relatively low cost process for small/micro diameter deep holes in difficult to machine

materials. Even though hole making by EDD in aerospace/automotive industries is a

common process, not much work has been published in the field of EDD of Inconel 718

and no standard data on the effect of machining parameters on the output responses

(material removal rate, tool wear rate, electrode wear ratio and surface roughness) are

readily available for reference. Hence, there exists a great need for investigating the

effect of various electrode materials and EDM process parameters on output responses,

and optimization of process parameters.

There is also a great need to study the effect of machining parameters on hole

quality produced by the EDD process and to develop better understanding on the effect

of these process parameters on the hole quality. Such an understanding will solve

quality control problems of the holes when the process parameters are adjusted to

obtain certain characteristics.

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In the present work, response surface methodology and central composite

design is used to develop mathematical models for output responses viz., material

removal rate, tool wear rate, electrode wear ratio and surface roughness, and to analyse

the effects of process parameters on hole quality while machining small diameter deep

holes in Inconel 718 using electric discharge drilling process.

1.7 OBJECTIVES

The objectives of the present work are:

i. Investigating the machinability of a superalloy, Inconel 718 by conducting deep

hole drilling experiments designed based on statistical technique using different

electrode materials viz., copper, copper-tungsten and graphite;

ii. Establishing empirical models sufficiently tuned with experimental data for

predicting responses like Material Removal Rate (MRR), Tool Wear Rate (TWR),

Electrode Wear Ratio (EWR) and Depth Averaged Surface Roughness (DASR);

iii. Finding out the effect of operating parameters like average current, pulse on-time,

duty factor and electrode rotational speed on responses viz., MRR, TWR, EWR

and DASR;

iv. Evaluating the performance of copper, copper-tungsten and graphite electrode

materials on deep hole drilling of Inconel 718;

v. Optimizing the EDM process parameters for the desired average surface

roughness with maximum MRR;

vi. Correlating the geometrical accuracy of high aspect ratio deep holes viz., hole

profile and overcut with the machining parameters.

The ultimate aim of the present research work is to provide guide lines for the

selection of parameters like average current, pulse on-time, duty factor and electrode

rotational speed to make good quality holes with high aspect ratio in nickel-base

(Inconel 718) superalloy.

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1.8 LAYOUT OF THE THESIS

This thesis is written in the following manner:

First chapter of the thesis introduces the engineering applications of deep

hole drilling and problems associated with drilling of superalloy. It also deals with the

application of advanced machining processes for deep hole drilling in general and EDD

in specific. It also includes the motivation for the research work and objectives of the

present work.

Second chapter focuses on the literature review in EDM process with special

reference to electric discharge drilling process for micro/small hole drilling. Also

literature on effect of EDM process parameters and electrode materials on steel,

titanium and nickel-base alloys are presented.

Third chapter presents the experimental set-up, identification of key process

parameters and their levels, characterization of machined hole and design of

experiments.

Fourth chapter discusses the experimental results, regression models and

validation, the effect of parameters and tool electrode materials on output responses,

selection of best tool electrode for deep hole drilling of Inconel 718 and optimization of

process parameters for the desired average surface roughness with maximum MRR. It

also includes results obtained from high aspect ratio deep hole drilling of Inconel 718

using the best electrode (Copper) and the geometrical accuracy of the deep holes.

Fifth chapter presents main conclusions of this research work and scope for

future work.