SURFACE INTEGRITY OF INCONEL 718 DURING DRILLING OPERATION

ALI AKHAVAN FARID

A thesis submitted in fulfillment of the requirement for the award of the degree of Master of Engineering

(Advanced Manufacturing Technology)

Faculty of Mechanical Engineering Universiti Teknologi Malaysia

MAY, 2008

To my parents, Hossein Akhavan Farid and Parvin Sami Zadeh, my brother and my

sweet sister, Amin and Elham.

ACK�OWLEDGEME�T

My foremost thank goes to my thesis supervisor Prof. Dr. Safian Sharif.

Without him, this dissertation would not have been possible. I thank him for his

patience and encouragement that carried me on through difficult times, and for his

insights and suggestions that helped to shape my research skills. His valuable

feedback contributed greatly to this dissertation.

I thank all the students and staffs in department of Manufacturing and

Industrial Engineering, whose presences and fun-loving spirits made the otherwise

gruelling experience tolerable. They are: Mr. Rival, Mr. Denny, Mr. Chia, Mr. Ayub

and Mr. Ali. Not forgetting my friends especially Hamid Reza, Amir and Hesam for

their ideas and involvement during discussion and ideas of the project.

Lastly, I would like to thank my family for all their love and encouragement.

For my parents who raised me with a love of science and supported me in all my

pursuits.

ABSTRAK

Aloi-aloi super seperti Inconel 718 memiliki kekuatan yang tinggi pada suhu

tinggi. Dan ini menjadikan mereka menarik digunakan untuk aplikasi industri

angkasa. Walau bagaimanapun, bahan-bahan ini merupakan bahan yang sukar untuk

dimesin. Keadaan permukaan yang digerudi pastilah dipengaruhi oleh parameter

pemotongan, seperti halaju pemotougan, kadar uluran, jenis dan geometri mata

gerudi. Ujian penggerudian pada berbagai halaju, jenis dan sudut mata gerudi

dilakukan untuk menilai kesan parameter diatas pada kualiti lubang-lubang termesin

dan integriti permukaan Inconel 718. Kualiti lubang-lubang yang dimesin dinilai dari

segi ketepatan geometri dan pembentukan gerigis. Integriti permukaan yang dinilai

melibatkan aspek-aspek kekasaran permukaan, perubahan metalurgi, dan kekerasan

mikro substrat permukaan lubang. Dari kajian yang dilakukan, lubang-lubang yang

dihasilkan memiliki kualiti yang tinggi meskipun digerudi menggunakan mata alat

yang telah haus, jika dinilai dari sudut ukuran, kekasaran permukaan, dan tinggi

gerigis. Walau bagaimanapun, nilai kekerasan mikro dan analisis struktur mikro

menunjukkan perubahan-perubahan struktur mikro yang jelas yang berkait dengan

kemerosotan sifat-sifat mekanikal. Secara umumnya, parameter pemotongan didapati

memberikan kesan-kesan yang signifikan pada kualiti dan integriti permukaan pada

penggerudian Inconel 718 menggunakan mata gerudi karbida tak bersalut.

ABSTRACT

Superalloys such as Inconel 718 have high strength at elevated temperatures,

which make them attractive towards various applications in aerospace industry.

However, these materials are considered difficult to machine materials. The state of a

workpiece surface after machining is definitely affected by cutting parameters, such

as cutting speed, feed rates, drill types and drill geometries. Drilling tests, at different

spindle-speed, feed rates, drills and point angles of drill, were conducted in order to

investigate the effect of the above parameters on the quality of machined holes and

surface integrity of Inconel 718. The quality of machined holes was evaluated in

terms of the geometrical accuracy and burr formation. Surface integrity involved the

aspect of surface roughness, metallurgical alterations and microhardness of the

substrate of the hole surface. High hole quality was observed even at holes produced

using worn tools, in relation to dimensions, surface roughness and burr height.

However, microhardness measurements and microstructural analysis of work-piece

showed significant microstructural changes related with a loss of mechanical

properties. In general the cutting parameters have significant effects on the surface

quality and surface integrity when drilling Inconel 718 using uncoated carbide drill.

TABLE OF CO�TE�TS

CHAPTER TITLE PAGE

STATUS OF THESIS

SUPERVISOR DECLARATIO�

TITLE PAGE i

DECLARATIO� OF ORGI�ALITY ii

DEDICATIO� iii

ACK�OWLEDGEME�TS iv

ABSTRAK v

ABSTRACT vi

TABLE OF CO�TE�TS vii

LIST OF TABLES x

LIST OF FIGURES xii

LIST OF SYMBOLS xv

CHAPTER1 I�TRODUCTIO�

1.1 Introduction 1

1.2 Project scope 3

1.3 Project objective 3

1.4 Problem statement 3

CHAPTER2 LITREATURE REVIEW

2.1 Nickel 4

2.1.1 Production of nickel 4

2.2 Nickel- copper alloys 5

2.3 Nickel-chromium alloys 6

2.4 Nickel-iron-base supperalloys 6

2.4.1 Chemical composition and typical application 6

2.4.2 Microstructure 7

2.4.3 Solid-solution strengtheners 7

2.4.4 Precipitation strengtheners 8

2.5 Inconel 718 8

2.6 High-temperature stress-rupture properties 9

2.7 Machinability 10

2.8 Drilling 13

2.9 Twist drill parts 13

2.9.1 Shank 14

2.9.2 Body 15

2.9.3 Point 16

2.9.4 Drill point characteristics 17

2.9.5 Drill point angle and clearance 18

2.10 Drilling facts and problems 19

2.11 Cause of drill failure 22

2.12 Cutting tool 23

2.12.1 Cutting Tool Materials 24

2.13 Surface Integrity 28

2.14 Surface roughness 28

2.14.1 Quantification of surface roughness 29

2.14.2 Effective parameters on surface roughness

in drilling 32

2.15 Microhardness 36

2.15.1 Effective parameters on microhardness changes

in drilling 38

2.16 Microstructural changes 39

2.17 Burr formation 41

CHAPTER3 METHODLOGY

3.1 Introduction 46

3.2 Research Design Variables 46

3.3 Workpiece Material 47

3.3.1 Analysis the workpiece material 48

3.4 Cutting Tools 51

3.5 Machining Procedure 52

3.6 Selection of independent variables 53

3.7 Investigation of Surface Finish 53

3.8 Dimensional accuracy 53

3.9 Burr height measuring 54

3.10 Preparing the samples 56

3.11 Microstructural analysis 57

3.12 Microhardness measurement 57

CHAPTER4 RESULTS A�D DISCUSSIO�

4.1 Introduction 59

4.2 Tool wear 59

4.3 Surface roughness 60

4.4 Dimensional accuracy 61

4.5 Burr height 62

4.6 Microstructure 64

4.7 Microhardness 68

CHAPTER5 CO�CLUSIO�S

5.1 Conclusions 70

5.2 Future study 71

REFERE�CES 72

LIST OF TABLE

TABLE TITLE PAGE

2.1 Chemical composition and typical application of 7

nickel-iron-base superalloys

2.2 Geometry and coating details 33

3.1 Machining parameters 47

3.2 Mechanical properties of Inconel 718 47

3.3 Chemical composition of Inconel 718 47

3.4 Average hardness of Inconel 718 51

3.5 Drill information 51

3.6 Experimental planning at all levels 53

4.1 Number of holes drilled under different conditions 60

LIST OF FIGURES

FIGURE TITLE PAGE

2.1 Electron micrographs of Inconel 718 10

2.2 The main parts of twist drill 14

2.3 Types of drill shanks 15

2.4 The web of a twist drill 16

2.5 The point of a twist drill 16

2.6 The lip clearance angle of the cutting edge 17

2.7 The drill angle of a twist drill 18

2.8 Wear at outer corners of drill 20

2.9 Breakdown of chisel point 21

2.10 Excessive and insufficient clearance of the cutting edge 21

2.11 Improper web thinning of twist drill 21

2.12 Cutting lips with unequal angles 22

2.13 Cutting lips with unequal length 22

2.14 Cross-section of a surface 29

2.15 Sampling length 30

2.16 Several different elements of a normal finish 31

2.17 Different profile in different directions 32

2.18 Surface roughness measurements 34

2.19 Surface roughness values when different cutting fluids 34

was applied

2.20 Surface roughness of AISI 1045 steel finished by different 35

coated drills

2.21 SEM micrograph showing three microindentation marks 38

2.22 Typical microhardness profile from drilling 39

2.23 (a) Grain boundary deformation and white layer from drilling 40

(b) Microstructure resulting from Mill Boring

2.24 Burr types formed in dry cutting 43

2.25 Formation of a burr with drill cap 44

2.26 Burrs produced in wet cutting 44

2.27 Correlation between the burr formation and the cutting conditions 45

2.28 Correlation between the burr formation and the point angle 45

and the lip relief angle

3.1 Workpiece material 48

3.2 Mounted specimen 49

3.3 Buehler electromet 49

3.4 Grain structure of Inconel 718 at 100X magnification 50

3.5 Grain structure of Inconel 718 at 200X magnification 51

3.6 Image of uncoated tool drill 52

3.7 MAHO MH 700S CNC machining center 52

3.8 Coordinate measuring machine 54

3.9 Samples are separated from the workpiece plate 55

3.10 Optical microscope used to measure burr height 55

3.11 Linear precision saw 56

3.12 Preparations of samples to metallographic studies 56

3.13 Toolmakers’ light optical microscope 57

3.14 Vickers pyramid microhardness tester 58

4.1 Surface roughness measurement at different 60

experiment condition

4.2 Surface roughness measurement comparison in 61

different experimental trials

4.3 Variation of machined hole dimension and tool diameter 62

4.4 Burr heights obtained using an optical microscope 63

4.5 Comparison of burr height at different cutting condition 63

4.6 Burr height versus point angle 64

4.7 Comparison between first and last hole produced in exprement1 65

4.8 Subsurface microstructure in last holes produced using worn tool 67

4.9 Microhardness changes versus distance from machined surface 68

LIST OF SYMBOLS & ABBREVIATIO�S

% percent

°C Degree celsius

mm Millimetre

µm Micrometer

Å Angstrom

MPa Mega Pascal

W Watt

N Newton

wt Weight

HV Hardness Vickers

HK Hardness Knoop

K Degree Kelvin

Ni Nickel

Cu Copper

Fe Iron

Mo Molybdenum

Al Aluminium

Ti Titanium

Mn Manganese

Si Silicon

C Carbon

Cr Chromium

Nb Niobium

N Nitrogen

CW Tungsten carbide

γ Gamma phase

'γ Gamma prime phase

η Eta phase

δ Delta phase

µ Mu phase

FCC Face-centered cubic

HCL Hydrochloric acid

V Cutting speed

PA Point angle

f Feed rate

BUE Built-up edge

MRR Material removal rate

H.S.S High speed steel

ANSI American National Standards Institute

CBN Cubic boron nitride

PCD Polycrystalline diamond

PVD Physical vapor deposition

CVD Chemical vapor deposition

CMM Coordinate measuring machine

CHAPTER 1

I�TRODUCTIO�

1.1 Introduction

Nickel-based alloys account for 80% of the superalloy usage within the

aerospace industry, with the remainder being iron and cobalt based. Approximately

45–50% of the total material requirements for a gas turbine engine are met using

nickel alloys [1]. Other areas of application are within space exploration (main space

shuttle engine, nickel–hydrogen batteries (international space station)), power

generation (industrial gas turbines), chemical industry (cryogenic tanks), etc. [1–3].

The properties that make nickel-based superalloys attractive to industry are: high

yield strength (retained to approximately 750° C), high ultimate tensile strength, high

fatigue strength, retention of corrosion and oxidation resistance up to elevated

temperatures and good creep resistance [1,4,5].

Numerous publications have shown that nickel based superalloys are difficult

to machine regardless of the process being used [6-11]. The properties that make

Inconel 718 an important engineering material are also responsible for its generally

poor machinability.

Low thermal conductivity (11.4 W/mK) leads to high cutting temperature

being developed in the cutting zone. In turning, temperatures of around 900°C have

been reported at the relatively low cutting speed of 30 m/min with over 1300°C found

at 300 m/min [12]. In addition, temperature gradients in the tool are much steeper than

for steels with the maximum temperature being generated in the tool nose region [13].

The materials ability to retain its mechanical properties at elevated temperature results

in high cutting forces being generated, around double that found when cutting

medium carbon alloy steels. This in combination with the relatively short chip tool

contact length means that stress is concentrated on the area of maximum tool

temperature leading to chipping and/or plastic deformation of the cutting edge [10,

13]. Nickel based superalloys have a high chemical affinity for many tool materials

and as such form an adhering layer leading to diffusion and attrition wear[14]. They

are also highly sensitive to strain rate and rapidly work harden causing abrasive wear,

particularly at the depth of cut and leading edge positions. The presence of hard

phases in the microstructure, such as carbides, nitrides, oxides, etc, further

exacerbates tool abrasion.

In contrast to other machining processes drilling has received relatively little

attention and most literature available for nickel base superalloys are related only to

tool wear and productivity [15]. Drilling is one of the most important processes in

aerospace manufacture and being the last operation performed, particular emphasis on

the reliability of the process due to the costs already entailed. In addition a hole

amplifies the stress around it by a factor of two, placing considerable restraints on

dimensional tolerance and hole quality.

1.2 Problem statement

- The metallurgical and mechanical characteristics that give nickel alloys highly

valued properties also make them one of the most difficult-to-machine

aerospace materials.

- The tendency of nickel alloys to accrue surface damage during machining.

- Burr formation during drilling can increase the cost of manufacturing due to

extra time give in removing the burrs.

1.3 Project objective

The objectives of the project are as follows:

� To evaluate the machined hole quality and surface integrity of a

Inconel 718 when drilling using carbide drill with respect to surface

roughness, microhardness, microstructure defects.

� To study the influence of the cutting conditions on the surface

roughness, microstructure defects and burr formation when drilling of

Inconel 718.

1.4 Project scope

This study will be focused on drilling of Inconel 718 using uncoated carbide

tools. This process is conducted under various independent variables which include

cutting speed, feed rate and tool geometries. The surface roughness, microhardness

and microstructural changes of subsurface will be evaluated.

CHAPTER 2

LITERATURE REVIEW

2.1 �ickel

Nickel is an excellent structural metal for many engineering application. It has

the desirable FCC crystal structure, so it is tough and ductile. It also has good high-

and low- temperature strength as well as high oxidation resistance and good corrosion

resistance for most environments. Few metals can match the attractive engineering

properties of nickel. Unfortunately, its greatest disadvantage is its relatively high cost,

and thus its use as a base metal for alloy is greatly limited. Nickel-base alloys are

therefore used when no cheaper material can provide the necessary corrosion- or heat

– resisting properties required for special engineering application [16].

2.1.1 Production of nickel

In general, there are three major types of nickel deposits: nickel-copper

sulfides, nickel silicates, and nickel laterites and serpentines. The sulfide deposits,

which are located mainly in Canada, provide most of the western world’s supply of

the metal. The second most important source is the nickel silicate ores of New

Caledonia. Laterite ores, which have relatively low nickel contents, are located mainly

in tropical and subtropical regions of the world. These deposits have not been

extensively developed because of the high cost of the recovering the nickel. There are

several established processes for the extraction of nickel from its ores with the process

used depending mainly on the type of ore being treated. The Canadian Sudbury,

Ontario, deposits which are controlled by the Inco metals company are processed in

the following manner. After the nickel-copper-iron sulfide ore is crushed and ground,

an iron sulfide (pyrrhotite) concentrate is separated magnetically and processed in an

iron-ore recovery plant. The remaining ore product is subjected to froth flotation

treatment which produces separates nickel and copper concentrates [17].

The copper concentrate is sent to the copper product. The nickel concentrate is

processing to produce copper products. The nickel concentrate is processed separately

and is roasted, smelted in a reverberatory furnace, and converted to a Bessemer matte

which consist mainly of nickel and copper sulfide and a Nickel copper metallic alloy

are formed. After the cooled matte is crushed and ground, the metallic alloy is

separated by forth flotation. The copper sulfide is returned to the copper smelter for

further processing while the nickel sulfide is roasted to produce various grades of

nickel oxides. The purest nickel oxide products are marketed directly and the less pure

oxides are processed further at Inco’s port colborne, Ontario, and clydach, wales,

nickel refineries to produce commercially pure nickel and other nickel-alloy products

[17].

2.2 �ickel- copper alloys

Nickel and copper are completely soluble in each other in all properties.

However, the most important nickel-copper alloys are those containing about 67% Ni

and 33% Cu, which are called Monels [16].

2.3 �ickel-chromium alloys

Chromium is an important alloying element for many corrosion-resistant and

high-temperature-resistance nickel-base alloys. It has a high solid solubility

(approximately 30 wt% at room temperature) in nickel [16].

2.4 �ickel-iron-base supperalloys

Nickel-base superalloys containing substantial amounts of both nickel and iron

form a second important class of supperalloys. In these alloys, lower-cost iron is

substituted in part for nickel. However, because of their lower nickel content, they are

not able to be utilized at as high temperatures as the nickel-base superalloys [18].

2.4.1 Chemical composition and typical application

Knowledge of the stainless steel and the nickel-base supperalloy led to the

development of the nickel-iron-base superalloys. Most of them contain from 25 to

45% Ni and from 15 to 60% Fe. Chromium from 15 to 38 percent is added for

oxidation resistance at elevated temperatures, while 1 to 6% Mo is also added to most

of them for solid-solution strengthening. Titanium, aluminum, and niobium are added

to combine with nickel for strengthening precipitates. Carbon, boron, zirconium,

cobalt, and some other elements are added for various complex effects. Table 2.1

shows the lists of the chemical compositions and typical application for selected

nickel-iron-base superalloys [19].

Table 2.1 Chemical composition and typical application of nickel-iron-base

superalloys

Alloy %Ni %Fe %Cr %Mo %Al %Ti %Mn %Si %C %other Typical

applications

Inconel

706

41.5 40 16 0.5 0.2 1.75 0.2 0.2 .03 2.9 Nb,

0.5 Co

Gas turbine

components

Inconel

718

53 18.5 18.6 3.1 0.4 0.9 0.2 0.3 0.04 5.0 Nb Jet engines,

rocket motores

Inconel

800

32.5 44.5 21 0.4 0.4 0.8 0.5 0.05 0.4 Cu Furnace, heat

exchanger

parts

Inconel

801

32 46 20.5 - - 1.1 0.8 0.5 0.5 0.2Cu Heat exchange

Inconel

901

42.5 36.0 12.5 5.7 0.2 2.8 0.1 0.1 0.05 0.015 B Gas turbine

rotors, blades,

bolts

2.4.2 Microstructure

Most nickel-iron-base superalloys are desired so they have an austenitics FCC

matrix. Since they contain less than 0.1% C and relatively large amounts of ferrite

stabilizers such as chromium and molybdenum, the minimum level of nickel required

to maintain an austenite stabilizers can slightly lower this nickel level. High-nickel

contents are associated with higher useful temperatures and improve malleability, but

also considerably lower the oxidation resistance of these alloys [19].

2.4.3 Solid-solution strengtheners

The solid-solution strengthening elements added to nickel-iron supperalloy are

10 to 25% Cr,0 to 9% Mo, 0 to 5% Ti, 0 to 2% Al, and 0 to 7% Nb. Of these,

molybdenum is the most useful. Chromium is also solid-solution strengthener of the

γ matrix and also enters carbides and 'γ . however, its chief function is to provide

oxidation resistance. Niobium, titanium, and aluminum also provide some solid-

solution strengthening of the austenite matrix, but this is not their primary function in

nickel-iron base alloys. Small amounts of carbon and boron are also potent solid-

solution strengtheners [19].

2.4.4 Precipitation strengtheners

The most important precipitation strength-enters in nickel-iron-base alloys are

titanium, aluminum, and niobium since they combine with nickel to from

intermetallic phases. An important different in the structure of 'γ and γ ′′ -

strengthened nickel-iron-base superalloys from the nickel-base alloys is that the Ni-Fe

alloys are all susceptible to the precipitation of one or more secondary phases such as

η , δ , µ , or laves. These phases can be detrimental or beneficial to rupture

properties, depending on their morphology and distribution. Titanium is major 'γ

forming element in 'γ strengthened nickel-iron superalloy, which in contrast most

nickel-base superalloys are strengthened principally by aluminum-rich 'γ . aluminum

however, does provide some oxidation resistance to nickel-iron alloys. Niobium is the

principal γ ′′ forming element in γ ′′ strengthened nickel-iron-base superalloys [19].

2.5 Inconel 718

Inconel 718 is an example of a nickel-iron-base superalloy that is strengthened

by niobium-rich 'γ ( NB NI3 , FCC) precipitates. Some aluminum and titanium atoms

may substitute for the niobium. This type of precipitate is in contrast to that found in

other nickel-iron-base superalloys in which the 'γ precipitate is NI3 (Al, Ti).

According to barker et al [20]. FCC 'γ is the main phase which is initially present in

the matrix of alloy 718 heat-treated in the standard precipitation-strengthened

naodition. The 'γ particles were found to be 7.5 to 30 nm in size and were both

spherical and dislike in morphology. When the samples of alloy 718 were exposed for

long period of time at elevated temperatures, the 'γ phase transformed into a BCT

phase of uncertain composition designated NB NIx . Upon even longer exposure

times, part of the NB NIx phase transformed into orthorhombic NB NI3 , which is

lamellar (needle like). After prolonged exposure in the 650 to 700 C range, three

distinct structural shapes were identified the spherical precipitates as FCC 'γ . X-ray

diffraction analysis identified the spherical precipitates as FCC X, the BCT NB NIx

as the small plates, and orthorhombic NB NI3 as the large plates [17].

2.6 High-temperature stress-rupture properties

In general, the nickel-iron-base superalloys cannot be used at as high

temperatures as the nickel-base alloys. Nickel-iron-base alloys that are strengthened

by ordered FCC 'γ (such as A-286 and V-57, which contain about 25 to 26 wt% Ni)

can be used to about 650° C, while alloys which have higher nickel contents (such as

860 and 901, with 42 to 43 wt%) can be used to about 815° C. Inconel 706 and 718,

which are strengthened by a niobium-containing 'γ , can be used to about 650° C.

Figure 2.1 shows the electron micrographs of Inconel 718 sample exposed 705 at 37

Ksi for 6,048 hours [17].

Figure 2.1 Electron micrographs of Inconel 718. (a) immersion etched in 20% HCL-

methanol. (b) Electrolytically etched at 2 V in a chormic-phosphoric sulfuric solution.

2.7 Machinability

The properties that make Inconel 718 an important engineering material are

also responsible for its generally poor machinability. Low thermal conductivity (11.4

W/m/K) leads to high cutting temperatures being developed in the cutting zone. These

have been shown to rise from around 900° C at a relatively low cutting speed of 30

m/min up to 1300° C at 300 m/min [21]. The cutting forces generated are also very

high, around double that found when cutting medium carbon alloy steels. Literature

detailing the effects of operating parameters on tool life when machining nickel based

superalloys is comprehensive, however, relatively little of this data refers to the

effects of machining on workpiece surface integrity. The main problems reported are

surface tearing, cavities, cracking, metallurgical recrystalisation, plastic deformation,

microhardness increases and the formation of residual stresses [22–27]. Residual

stress is defined as the stress that persists in the absence of external force [28].

The properties responsible for the poor machinability of the nickel-based superalloys,

especially of Inconel 718, are [29–34]:

- A major part of their strength is maintained during machining due to their

high-temperature properties.

- They are very strain rate sensitive and readily work harden, causing further

tool wear.

- The highly abrasive carbide particles contained in the microstructure cause

abrasive wear.

- The poor thermal conductivity leads to high cutting temperatures up to 1200C’

at the rake face [21].

- Nickel-based superalloys have high chemical affinity for many tool materials

leading to diffusion wear.

- welding and adhesion of nickel alloys onto the cutting tool frequently occur

during machining causing severe notching as well as alteration of the tool rake

face due to the consequent pull-out of the tool materials.

- Due to their high strength, the cutting forces attain high values, excite the

machine tool system and may generate vibrations which compromise the

surface quality.

The microstructure of Inconel 718 is comprised of an austenitic face centred cubic

(FCC) matrix phase, which is a solid solution of Fe, Cr and Mo in nickel together with

other secondary phases. The main strengthening phase is the precipitate gamma

double prime (denoted γ''). This phase consists of uniformly distributed body centred

tetragonal (BCT) disc shaped particles (of compositionNB NI3) that are coherent with

the parent matrix. The diameter of these particles is approximately 600 Å by around

50-100 Å thick. Inconel 718 is often used in a solution treated and aged condition, this

involves a solution treatment at 970-1175° C, followed by a precipitation treatment at

600- 815° C [35]. This results in a microstructure of large grains containing the

NB NI3 precipitated phase and a heavy concentration of carbides at the grain

boundaries. The difficulty of dislocation motion through the γ''/ γ' microstructure is

responsible for high tensile and yield strength of Inconle 718 (approximately 1300

and 1100 MPa, respectively, at temperature up to 600 °C) [35].

In machining Inconel 718 alloy, it is well known that the tool temperature rises

easily due to its poor thermal properties. Micro-welding at tool-tip and chip interface

takes place leading to the formation of built-up edge (BUE). The excellent material

toughness results in difficulty in chip breaking during the process. In addition,

precipitate hardening γ ′′ secondary phase ( NB NI3 ) together with work-hardening

during machining makes the cutting condition even worse. All these difficulties lead

to serious tool wear and less material removal rate (MRR) [32, 36].

The difficulty of machining resolves itself into two basic problems: short tool life

and severe surface abuse of machined workpiece [31, 22]. The heat generation and the

plastic deformation induced during machining affect the machined surface. The heat

generated usually alters the microstructure of the alloy and induces residual stresses.

Residual stresses are also produced by plastic deformation without heat. Heat and

deformation generate cracks and microstructural changes, as well as large

microhardness variations [37]. Residual stresses have consequences on the

mechanical behaviour, especially on the fatigue life of the workpieces [38]. Residual

stresses are also responsible for the dimensional instability phenomenon of the parts

which can lead to important difficulties during assembly [39, 40]. Extreme care must

be taken therefore to ensure the surface integrity of the component during machining.

Most of the major parameters including the choice of tool and coating materials, tool

geometry, machining method, cutting speed, feed rate, depth of cut, lubrication, must

be controlled in order to achieve adequate tool lives and surface integrity of the

machined surface [37, 38].

Field and Kahles [41] summarized the metallurgical alterations that occur in

the surface layer as a function of machining parameters in conventional and

nonconventional machining operations of several alloy systems, including Inconel

718. They concluded that it is highly desirable to develop surface integrity data for

specific situations and only in the absence of specific data should general guidelines

be employed or considered for the manufacture of critical components. Bellows [42]

inferred that the mechanical properties of components made from Inconel 718 are

more sensitive to residual stresses than to surface finish, consequently sharp tools

must be maintained at all times.

2.8 Drilling

Twist drills are end-cutting tools used to produce holes in most types of

material. On standard drills, two helical grooves, or flutes, are cut lengthwise around

the body of the drill. They provide cutting edges and space for cutting to escape

during the drilling process. Since drills are among the most efficient cutting tools, it is

necessary to know the main parts, how to sharpen the cutting edges, and how to

calculate the correct speeds and feeds for drilling various metals to use them most

efficiently and prolong their life.

2.9 Twist drill parts

Most twist drills used in machine shop work today are made of high-speed

steel. High-speed steel drills have replaced carbon-steel drills, since they can be

operated at double the cutting speed and the cutting edge lasts longer. High-speed

steel drills are always stamped with the letters “H.S.” or “H.S.S.” since the

introduction of carbides-tripped drills, speeds for production drilling have increased

up to 300% over high-speed steel drills. Carbide drills have made it possible with

high-speed steel drills. Carbide drills have made it possible to drill certain materials

that would not be possible with high-speed steels.

A drill may be divided into three main parts: shank, body, and point (Figure 2.2).

Figure 2.2 The main parts of twist dill

2.9.1 Shank

Generally drills up to ½ in. or 13 mm in diameter have straight shanks, while

those over this diameter usually have tapered shanks. Straight-shank drills are held in

a drill chuck; tapered-shank drills fit into the internal taper of the drill press spindle. A

tang is provided on the end of tapered-shank drills to prevent the drills from slipping

while is cutting and to allow the drill to be removed from the spindle or socket

without the shank being damaged. Figure 2.3 shows two types of drill shanks.

Figure 2.3 Types of drill shanks: (a) straight; (b) tapered

2.9.2 Body

The body is the portion of the drill between the shank and the point. It consists

of a number of parts important to the efficiency of the cutting action.

1- The flutes are two or more helical grooves cut around the body of the drill.

They form the cutting edges, admit cutting fluid, and allow the chips to escape

from the hole.

2- The margin is the narrow, raised section on the body of the drill. It is

immediately next to the flutes and extends along the entire length of the flutes.

Its purpose is to provide a full size to the drill body and cutting edges.

3- The body clearance is the undercut portion of the body between the margin

and the flutes. It is made smaller to reduce friction between the drill and the

hole during the drilling operation.

4- The web is the thin portion in the center of the drill that extends the full length

of the flutes (Figure 2.4). This part forms the chisel edge at the cutting end of

the drill. The web gradually increases in thickness toward the shank to give the

drill strength.

Figure 2.4. The web is a tapered metal column that separates the flutes.

2.9.3 Point

The point of a twist drill consists of the chisel edges, lips, lip clearance,

and heel (Figure 2.5). The chisel edge is the chisel-shaped portion of the drill

point. The lips (cutting edges) are formed by the intersection of the flutes. The lips

must be of equal length and have the same angle so that the drill will run true and

will not cut a hole larger than the size of the drill.

Figure 2.5 The point of a twist drill

The lip clearance is the relief ground on the point of the drill extending from

the cutting lips back to the heel. The average lip clearance is from 8° to 12°

depending on the hardness or softness of the material to be drilled (Figure 2.6).

Figure 2.6 The lip clearance angle of the cutting edges should be 8° to 12° degree.

2.9.4 Drill point characteristics

Efficient drilling of the wide variety of materials used by industry requires a

great variety of drill points. The most important factors determining the size of the

drilled hole are the characteristics of the drill point [43].

A drill is generally considered a roughing tool capable of removing metal quickly. It

is not expected to finish a hole to accuracy possible with a reamer. However, a drill

can often be made to cut more accurately and efficiently by proper drill point

grinding. The use of various point angles and lip clearance, in conjunction with the

thinning of the drill web, will:

1- Control the size, quality, and straightness of the drilled hole

2- Control the size, shape, and formation of the chip

3- Control the chip flow up the flutes

4- Increase the strength of the drill’s cutting edges.

5- Reduce the rate of wear at the cutting edges

6- Reduce the amount of drilling pressure required

7- Control the amount of burr produced during drilling

8- Reduce the amount of heat generated

9- Permit the use of various speeds and for more drilling

2.9.5 Drill point angle and clearance

Drill point angles and clearance are varied to suit the wide variety of material

that must be drilled. The general drill points are commonly used to drill various

materials; however, there may be variation of these to suit various drilling conditions.

The conventional point (118°) is the most commonly used drill point and gives

satisfactory result for most general-purpose drilling (Figure 2.7). The 118° point angle

should be ground with 8° to 12° lip clearance for best results. Too much lip clearance

weakens the cutting edge and causes the drill to chip and break easily. Too little lip

clearance results in the use of heavy drilling pressure; this pressure causes the cutting

edge to wear quickly because of the excessive heat generated and places undue strain

on the drill and equipment.

Figure 2.7. The drill angle of 118 degree is suitable for most general work; (b) a

drill point angle of 60 to 90 degree is used for soft material; (c) a drill point angle

of 135 to 150 degree is best for hard material.

The long angle point (60° to 90°) is commonly used on low helix drills for the

drills for the drilling of nonferrous metals, soft cast iron, plastics, fibers, and wood.

The lip clearance on long angle point drills is generally from 12° to 15°. on standard

drill, a flat may be ground on the face of the lips to prevent the drill from drawing

itself into the soft material.

The flat angle point (135° to 150°) is generally used to drill hard and tough

materials. The lip clearance on flat angle point drills is generally only 6° to 8° to

provide as much support as possible for the cutting edges. The shorter cutting edge

tends to reduce the friction and heat generated during drilling.

2.10 Drilling facts and problems

The cutting efficiency of a drill is determined by the characteristics and

condition of the point of the drill. Most new drills are provided with a general-purpose

point (118° point angle and an 8° to 12° lip clearance). As a drill is used, the cutting

edges my wear and become chipped, or the drill my break. Drills are generally

resharpened by hand. A properly ground drill should have thee flowing

characteristics:

- The length of both cutting lips should be the same. Lips of unequal length will

force the drill point off center, causing one lip to do more cutting than the

other and producing an oversize hole.

- The angle of both lips should be the same. If the angles are unequal, the drill

will cut an oversize hole because one lip will do more cutting than the other.

- The lips should be free from nicks or wear.

- There should be no sign of wear on the margin.

If the drill does not meet all of these requirements, it should be resharpened. If

the drill is not resharpened, it will give poor service, will produce inaccurate holes,

and may break because of excessive drilling strain.

While a drill is being used, there will be signs to indicate that the drill is not cutting

properly and should be resharpened. If the drill is not sharpened at the first sign of

dullness, it will require extra power to force the slightly dulled drill into the work.

This causes more heat to be generated at the cutting lips and results in a faster rate of

wear.

When any of the following conditions arise while a drill is in use, it should be

examined and reground:

- The color and shape of the chips change.

- More drilling pressure is required to force the drill into the work.

- The drill turns blue because of the excessive heat generated while drilling.

- The top of the hole is out of round.

- A poor finish is produced in the hole.

- The drill chatters when it contacts the metal.

- The drill squeal and may jam in the hole.

- An excessive burr is left around the drilled hole.

Excessive speed will cause wear at outer corners of drill (Figure 2.8); this

permits fewer regrinds of drill due to amount of stock to be removed in

reconditioning. Discoloration is warning sign of excess speed [43].

Figure 2.8 Wear at outer corners of drill

Excessive feed sets up abnormal end thrust, which causes breakdown

of chisel point and cutting lips (Figure 2.9). Failure induced by this cause will

be broken or split drill [43].

Figure 2.9 Breakdown of chisel point

Excessive clearance results in lack of support behind cutting edge with quick

dulling and poor tool life (Figure 2.10-a), despite initial free cutting action. Clearance

angle behind cutting lip for general purposes is 8° to 12° degree. Insufficient

clearance causes the drill to rub behind the cutting edge, it will make the drill work

hard, generate heat, and increase end thrust (Figure 2-10-b). This results in poor holes

and drill breakage.

(a) Excessive clearance (b) Insufficient clearance

Figure 2.10 Cutting angle

Improper web thinning is the result of taking more stock from one cutting

edge than from the other, thereby destroying the concentricity of the web and outside

diameter (Figure 2.11).

Figure 2.11 The web is the tapered central position of the body that joints the

lands

Cutting lips with unequal angles will cause one cutting edge to work harder

than the other this cause torsion strain, Bellmouth holes, rapid dulling, and poor tool

life (Figure 2.12).

Figure 2.12 Cutting lips with unequal angles

Cutting lips unequal in length cause chisel point to be off center with axis and

will drill holes oversize by approximate twice the amount of eccentricity (Figure 2-

13).

Figure 2-13 Cutting lips with unequal length

2.11 Cause of drill failure

Drills should not be allowed to become so dull that they cannot cut. Over

dulling of any metal-cutting tool generally results in poor production rates, inaccurate

work, and the shortening of the tool life [43]. Premature dulling of a drill may be

caused by any one of a number of factors:

- The drill speed may be too high for the hardness of the material being cut.

- The feed may be too heavy and overload the cutting lips.

- The feed may be too light and cause the lips to scrape rather than cut.

- There may be hard spots or scale on the work surface.

- The work or drill may not be supported properly, resulting in springing and

chatter.

- The drill point may be incorrect for the material being drilled.

- The finish on the lips may be poor.

2.12 Cutting tool

Principal categories of cutting tools include single point lathe tools, multipoint

milling tools, drills, reamers, and taps. All of these tools may be standard catalogue

items or tooling designed and custom-built for a specific manufacturing need.

Different machining applications require different cutting tool materials. The ideal

cutting tool material should have all of the following characteristics:

• Harder than the work it is cutting

• High temperature stability

• resists wear and thermal shock

• Impact resistant

• Chemically inert to the work material and cutting fluid

No single cutting tool material incorporates all these qualities. Instead, trade-

offs occur among the various tool materials. For example, ceramic cutting tool

material has high heat resistance, but has a low resistance to shock and impact. Every

new and evolving tool development has an application where it will provide superior

performance over others. Many newer cutting tool materials tend to reduce, but not

eliminate the applications of older cutting tool materials.

2.12.1 Cutting Tool Materials

As rates of metal removal have increased, so has the need for heat resistant

cutting tools. The result has been a progression from high-speed steels to carbide, and

on to ceramics and other superhard materials.

a) High Speed Steel (HSS): Developed around 1900, high-speed steels cut four

times faster than the carbon steels they replaced. There are over 30 grades of

high-speed steel, in three main categories: tungsten, molybdenum, and

molybdenum-cobalt based grades. Since the 1960s the development of

powdered metal high-speed steel has allowed the production of near-net

shaped cutting tools, such as drills, milling cutters and form tools. The use of

coatings, particularly titanium nitride, allows highspeed steel tools to cut faster

and last longer. Titanium nitride provides a high surface hardness, resists

corrosion, and it minimizes friction.

b) Cemented Tungsten Carbides: In industry today, carbide tools have replaced

high speed steels in most applications. These carbide and coated carbide tools

cut about 3 to 5 times faster than high-speed steels. Cemented carbide is a

powder metal product consisting of fine carbide particles cemented together

with a binder of cobalt. The major categories of hard carbide include tungsten

carbide, titanium carbide, tantalum carbide, and niobium carbide. Each type of

carbide affects the cutting tool’s characteristics differently. For example, a

higher tungsten content increases wear resistance, but reduces tool strength. A

higher percentage of cobalt binder increases strength, but lowers the wear

resistance. Carbide is used in solid round tools or in the form of replaceable

inserts. Every manufacturer of carbide tools offers a variety for specific

applications. The proper choice can double tool life or double the cutting

speed of the same tool. Shock-resistant types are used for interrupted cutting.

Harder, chemically-stable types are required for high speed finishing of steel.

More heat resistant tools are needed for machining the superalloys, like

Inconel and Hastelloy. There are no effective standards for choosing carbide

grade specifications so it is necessary to rely on the carbide suppliers to

recommend grades for given applications. Manufacturers do use an ANSI

code to identify their proprietary carbide product line. Two-thirds of all

carbide tools are coated. Coated tools should be considered for most

applications because of their longer life and faster machining. Coating

broadens the applications of a specific carbide tool. These coatings are applied

in multiple layers of under 0.001 of an inch thickness. The main carbide insert

and cutting tool coating materials are titanium carbide, titanium nitride,

aluminum oxide, and titanium carbonitride.

c) Ceramic: Ceramic cutting tools are harder and more heat-resistant than

carbides, but more brittle. They are well suited for machining cast iron, hard

steels, and the superalloys. Two types of ceramic cutting tools are available:

the alumina-based and the silicon nitride-based ceramics. The alumina-based

ceramics are used for high speed semi- and final-finishing of ferrous and some

non-ferrous materials. The silicon nitride-based ceramics are generally used

for rougher and heavier machining of cast iron and the superalloys.

d) Cermet: Cermet tools are produced from the materials used to coat the carbide

varieties: titanium carbides and nitrides. They are especially useful in

chemically reactive machining environments, for final finishing and some

turning and milling operations.

e) Superhard Materials: Superhard tool materials are divided into two categories:

cubic boron nitride, or "CBN", and polycrystalline diamond, or "PCD". Their

cost can be 30 times that of a carbide insert, so their use is limited to well-

chosen, cost effective applications. Cubic boron nitride is used for machining

very hard ferrous materials such as steel dies, alloy steels and hard-facing

materials. Polycrystalline diamond is used for non-ferrous machining and for

machining abrasive materials such as glass and some plastics. In some high

volume applications, polycrystalline diamond inserts have outlasted carbide

inserts by up to 100 times.

For the improvement of tool lives, surface and coating technologies have

developed rapidly to produce several types of coated tools for machining of difficult-

to-machine materials. The cemented carbide tools are still largely used for machining

the nickel-based superalloys, especially for the Inconel 718 [39,40,31]. However, In

order to achieve higher cutting speeds, coated cemented carbides have been developed

[44,45].

In the following, typical results from the literature using coated and uncoated

carbide tools in turning, milling and drilling operations of Inconel 718 will be

presented. . Itakura et al. [46], conducted dry turning experiments to identify clearly

the tool wear mechanisms when a commonly used coated cemented carbide tool cuts

Inconel 718. Jindal et al. [47] studied the relative merits of PVD TiN, TiCN and

TiAlN coatings on cemented carbide substrate (WC—6wt.% Co alloy) in the turning

of Inconel 718. The tested cutting speeds were 46 and 76 m/min, the feed rate and the

depth-of-cut were maintained constant and respectively equal to 0.15 mm/rev and

1.5mm. At both speeds, TiAlN and TiCN coated tools performed significantly better

than tools with TiN coatings. The maximum flank wear was about 0.15mm after a

cutting time of 5 min. In addition the TiAlN tools exhibit lower nose and crater wear

than the TiCN and TiN coated tools. . Panjan et al. [48] studied TiN/AlTiN and

CrN/TiN nanolayer coatings deposited on a K20 cemented carbide and its machining

performance was tested by turning Inconel 718 alloy. The performance of the

nanolayer coated tools was compared with those of classical mono and multilayer

coated and uncoated inserts. Abrasive nose wear and chipping at the cutting edge

were the main failure modes observed. The depth-of-cut notch is considered as

determinant for tool life during machining Inconel 718. The notching is influenced by

burr formation on the uncut diameter; this failure mode is mainly due to the hardening

of the material during machining. This phenomenon appeared for uncoated or

CrN/TiN coated tool and was attenuated with TiN/AlTiN nanolayer coated insert.

According to the authors, this was probably due to better chip sliding and a reduced

cutting temperature with this coating. Abrasive wear is mainly due to carbide particles

in Inconel 718. The high hardness of the TiN/AlTiN nanolayer coating (Hardness

HV0.05 = 3900) provides better abrasion resistance than classical multilayer and

monolayer structures. In addition, TiN/AlTiN nanolayer coating presents a better

resistance to welding. High temperature resistance of AlTiN included in this coating

allows better resistance to the built-up-edge phenomenon than CrN/TiN nanolayer

coating.

Derrien and Vigneau [49] found that TiN coated tools resulted in higher tool

life and lower surface roughness (Ra) than uncoated tools when contour milling

Inconel 718. Gatto and Iuliano [3] suggested that CrN and TiAlN coatings improved

tool performance by acting as a thermal barrier and therefore preventing the high

temperature generated in the cutting process from softening the substrate. Sharman et

al. [29] also examined TiAlN and CrN coated carbide tools in end milling of Inconel

718. They found that TiAlN gave the overall better performance compared to CrN,

due to the lower hardness (lower abrasive wear resistance) and higher chemical

affinity of CrN to Inconel 718. This resulted, based on the wear mechanism proposed

by Liao and Shiue [44], in faster exposure of the carbide substrate and therefore

higher wear.

Sharman et al. [50] studied TiAlN multilayer PVD, TiN/TiAlN multilayer

PVD coated and uncoated cemented carbide tool and its machining performance

during drilling Inconel 718. They have reported that drills, TiAlN multilayer PVD and

TiN/TiAlN multilayer PVD tested failed due to localised wear exceeding 0.5mm at

the drill periphery. Chen [15] used multi-layer TiAlN PVD coated tungsten carbide

twist drill when drilling Inconel 718. He stated that Friction force is found to be the

most important factor governing tool failure. Wear mechanisms can be classified into

four stages. The coated layer is abraded-off first. It is followed by flank wear, and

chipping at the outer cutting edge.

2.13 Surface Integrity

Numerous investigations confirm that the quality and especially the lifetime of

the dynamically loaded parts are very much dependent on the properties of the

material in the surface [51]. Severe failures produced by fatigue, creep and stress

corrosion cracking invariably start at the surface of components and their origins

depend to a great extent on the quality of the surface [52]. Therefore, in machining

any component it is first necessary to satisfy the surface integrity requirements.

Surface integrity refers to residual stress analysis, microhardness

measurements, surface roughness and degree of work hardening in the machined sub-

surfaces and they were used as criteria to obtain the optimum machining conditions

that give machined surfaces with high integrity. Field and Kahles [41] have defined

surface integrity as the relationship between the physical properties and the functional

behaviour of a surface. The surface integrity is built up by the geometrical values of

the surface such as surface roughness (for example, Ra and Rt), and the physical

properties such as residual stresses, hardness and structure of the surface layers.

2.14 Surface roughness

In addition to the more straightforward dimensional characteristics of an

engineering product, its performance, appearance and cost are likely to be strongly

influenced by the quality of the finish on the various surfaces. This may be important

for a variety of reasons. The most oblivious is that the surface has a function which

involves contact with another surface. This can be moving contact, as in the case of a

bearing diameter, or static, as in the case of surfaces required to provide an oil tight

joint. Finish might also be important in the interests of reducing stress on the part and,

particularly if on an external surface, it might be important merely for aesthetic

appearance.

2.14.1 Quantification of surface roughness

Surface finish can be accurately quantified, and several different principles

have been used to achieve the desirable objective of expressing the requirement and

the measuring in terms of only one number. The four main methods are indicated in

the diagrammatic representation of the cross-section of a surface shown in Figure

2.14. These are Center Line Average, referred to as Ra, Root Mean Square, refereed

to as RMS, maximum peak to valley, refereed to as Ra, and maximum peak to mean,

referred to as Rp.

Figure 2.14 Cross-section of a surface

Whilst all these parameters have some relevance, depending on the role the

surface has to play, the most common method is the Center Line Average, and it is

worth describing in some detail the way in witch this figure is derived.

Figure 2.15 Sampling length

In the figure diagram at Figure 2.15 a straight line x-x is drawn by eye

following the general direction of the profile and covering the sampling length L. The

areas of the profile p above, and q below this line are then measured and a distance z

is obtained by dividing the difference between these areas by the sampling length, i.e.

L

q areas - p areasz =

If a new line y-y is now drawn parallel to x-x at a distance z from it, this new

line will be a mean centerline, and the CLA value (Ra) will be the sum of the area

above and below this line divided by the sampling length, i.e.

L

s area r areasR a

+=

The length over which the sample is taken is obviously very important, and in

the example shown in Figure 2.15 the sample length L is adequate to give a measure

of the total surface because it covers a significant number of complete surface finish

cycles. If, however, the combination of sampling length and finish characteristics is

such that the sample contains say, only one or even less total cycles, the result will not

include all the characteristics of the surface. The sampling length is used extensively

in surface measurement to segregate the various characteristic of the surface.

As shown in Figure 2.16 a normal finish consists of several different elements.

These are referred to in this drawing as primary texture, secondary texture and form

errors, but they might also be described as roughness, waviness and flatness

respectively. The term “surface finish” is normally used to describe the first, and

perhaps the second of these elements. Errors of flatness are usually considered, and

measured, separately.

Figure 2.16 Several different elements of a normal finish

The parameter which is necessary to achieve this is the cut-off value. This is

the length of the surface sample to be considered, and all features within this length

will be included to arrive at, foe example, the CLA value. In figure 2.16 the effect of

three cut-off values, L1, L2 and L3, is indicated. It will be seen that if a cut-off value

of L1 were selected, the reading H1 obtained would cover only the primary texture.

The values H2 and H3 obtained from L2 and L3, however, would, in addition, include

the secondary texture and the form respectively. Whilst it is possible to obtain a finish

reading for any cut-off value, there are again preferred values. The imperial units

range, for example, cover six option between 0.003 in and 1.0 in. Since surface for

which measurement of the finish in quantitative terms is required are likely to be fine,

small sampling lengths are appropriate and the normal standard is 0.030 in. such a

sample is not likely to cover all the characteristics of a surface, but experience has

shown that, in practice, it is the most useful. In metric units the figure used is 0.8 mm.

Surface produced by normal engineering methods would, if looked at in cross-section,

generally show a different profile in different directions. Cutting processes, such as

turning and boring, produce a surface which is evenly spaced and unidirectional, as

indicated in Figure 2.17.

Figure 2.17 Different profile in different directions

Grinding generally produces a surface which is unidirectional, but does not

have regular cycles. Operation such as lapping and polishing produce very fine

surface but they are both multidirectional and irregular. The direction in which the

cutting tool moves is known as the “lay”. Normally, surface finish would be measured

in the direction which gives maximum roughness, and this is likely to be in a direction

at right angles to the lay.

2.14.2 Effective parameters on surface roughness in drilling

Sharman et al [50] did some drilling experiments on Inconel 718 using five

different 8mm diameter drills. Each of these drills had slightly different geometries,

substrate grades and coatings although a number of similarities can be seen (Table 2.2).

Table 2.2 Geometry and coating details

Tool Coating Substrate Point angle Helix angle Web width

SS TiAlN multilayer

PVD

90% WC - 10% Co <1µm grain

size

140 35 0.15

DS TiAlN multilayer

PVD

90% WC - 10% Co <1µm grain

size

130 30 0.2

CS TiN/TiAlN multilayer

PVD

90% WC - 10% Co <1µm grain

size

130 30 0.15

SD TiAlN multilayer

PVD

90% WC - 10% Co <1µm grain

size

130 30 0.15

SD2 Uncoated 90% WC - 10% Co <1µm grain

size

130 30 0.15

They reported that the majority of surface roughness results fall between 1 and

1.5 µm Ra with a wide range of scatter around these values and there is little difference

between the values obtained with different drills, only for drill CS was a significant

difference (Figure 2.18). For both new and worn conditions this drill produced the lowest

surface roughness and the lowest scatter in surface roughness measurements. For drills

CS and SS it appears that surface finish has improved with increasing drill wear however

only for drill CS did an independent t-test show this to be statistically significant at the

5% level.

Figure 2.18: Surface roughness measurements

Chen and Lio [15] investigated wear mechanism of the TiAlN coated carbide

tool in drilling Inconel 718. They reported that deterioration of surface roughness is

greatly improved with the application of nano-modifier fluid and the use of uncoated

carbide drill and nano-modifier fluid results in even better drill life than the use of

coated carbide drill and traditional cutting fluid (Figure 2.19).

Figure 2-19. Surface roughness values when different cutting fluids was applied

Nouari and List [53] stated that drilling with the coated and uncoated

carbide drills produced similar surface finishes while higher surface roughness

values, hence poor surface finish, were recorded when drilling with HSS drills

during dry drilling of AA2024 aluminium alloy.

Kao and Yao [54] have studied on thrust forces, torque, flank wear and hole

surface roughness during the drilling of AISI 1045 steel workpieces using Ti-

based (Ti–C:H, Ti–C:H/TiC/TiCN/TiN and TiC/TiCN/TiN) and Cr-based (Cr–

C:H/CrC/CrCN/CrN, Cr–C:H and CrC/CrCN/CrN) Me–C:H coated high-speed

steel drills. They reported that the surface roughness of the hole is affected by

two factors, namely the cutting speed and the coating properties. As the cutting

speed increases, the workpiece material readily adheres to the cutting edge.

This causes the formation of a built-up edge, which increases the surface

roughness (as shown in Figure 2.20). They showed that the hole roughness

generated by the Ti-based coated drills with a top Ti–C:H coating (Ti0025,

Ti0050 and Ti2525) is lower than that produced by the Ti-based coated drill

with no overcoat, or by any of the Cr-based coated drills. Furthermore, the

machined surfaces of the holes produced by the Ti2500, Ti5000, Cr1010 and

Cr4400 coated drills at 310 rpm, or by the Ti2500, Cr0025 and Cr1010 coated

drills at 480 rpm, are of an inferior quality to those produced by an uncoated

drill.

Figure 2.20. Surface roughness of AISI 1045 steel finished by different coated drills

after drilling 12 holes at spindle speed of 310 rpm and 480 rpm, respectively.

Tsann and Lin [55] have investigated the tool life and surface roughness,

tool wear and burr formation during drilling of stainless steel using a Ti-N

coated carbide tool. They stated that the surface roughness increases with feed

rate for different cutting speeds. They found that the optimum cutting speed is

V=75 m/min from the standpoint of the minimum surface roughness. This is

because at high cutting speed (V =85 m/min) there was high vibration, whilst

outer corner wear occurred easily at low cutting speed (V =65 m/min). The

surface roughness produced was less than 1 mm with a cutting speed of V =75

m/min and a feed rate of f =0.1 mm/rev, and the surface produced was

generally very smooth.

2.15 Microhardness

The term "microhardness" has been widely employed in the literature to

describe the hardness testing of materials with low applied loads; however,

microhardness implies that the hardness is very small rather than the load. A more

precise term is "microindentation hardness testing." In microindentation hardness

testing, a diamond indenter of specific geometry is impressed into the surface of the

test specimen using a known applied force (commonly called a “load” or “test load”)

of 1 to 1000 gf. Microindentation tests typically have forces of 2 N and produce

indentations of about 50 µm. Due to their specificity, microhardness testing can be

used to observe changes in hardness on the microscopic scale. Unfortunately, it is

difficult to standardize microhardness measurements; it has been found that the

microhardness of almost any material is higher than its macrohardness. Additionally,

microhardness values vary with load and work-hardening effects of materials.

Regardless, the two most commonly used microhardness tess are the Knoop and

Vickers tests.

In microindentation testing, the hardness number is based on measurements

made of the indent formed in the surface of the test specimen. The hardness number is

based on the surface area of the indent itself divided by the applied force, giving

hardness units in kgf/mm². Microindentation hardness testing can be done using

Vickers as well as Knoop indenters. For the Vickers test, both the diagonals are

measured and the average value is used to compute the Vickers pyramid number. In

the Knoop test, only the longer diagonal is measured, and the Knoop hardness is

calculated based on the projected area of the indent divided by the applied force, also

giving test units in kgf/mm².

The Vickers microindentation test is carried out in a similar manner to the

Vickers macroindentation tests, using the same pyramid. The Knoop test uses an

elongated pyramid to indent material samples. This elongated pyramid creates a

shallow impression, which is beneficial for measuring the hardness of brittle materials

or thin components. Both the Knoop and Vickers indenters require prepolishing of the

surface to achieve accurate results. Scratch tests at low loads, such as the Bierbaum

microcharacter test, performed with either 3 gf or 9 gf loads, preceded the

development of microhardness testers using traditional indenters. In 1925, Smith and

Sandland of the UK developed an indentation test that employed a square-based

pyramidal indenter made from diamond. They chose the pyramidal shape with an

angle of 136° between opposite faces in order to obtain hardness numbers that would

be as close as possible to Brinell hardness numbers for the specimen. The Vickers test

has a great advantage of using one hardness scale to test all materials. ASTM

Specification E384 states that the load range for microhardness testing is 1 to 1000 gf.

For loads of 1 kgf and below, the Vickers hardness (HV) is calculated with an

equation (Equation 2.1), wherein load (L) is in grams force and the diagonal (d) is in

micrometers:

Equation 2.1: Vickers hardness 2

4.1854d

LHV ×=

For any given load, the hardness increases rapidly at low diagonal lengths,

with the effect becoming more pronounced as the load decreases. Thus at low loads,

small measurement errors will produce large hardness deviations. Thus one should

always use the highest possible load in any test. Also, in the vertical portion of the

curves, small measurement errors will produce large hardness deviations.

2.15.1 Effective parameters on microhardness changes in drilling

Canteroa and Tardi [56] evaluated the tool wear, quality of machined holes

and surface integrity of work-piece, in the dry drilling of alloy Ti–6Al–4V using TiN-

coated fine-grain carbide drill. They measured Microhardness (Vickers) in five points

(located at a distance from machined surface ranging from 50 to 600 mm) in two lines

perpendicular to the drill displacement direction (Figure 2-21). They also reported in

the zone close to machined surface (distance 75 mm) value of 420 HV was obtained,

approximately 30% greater to hardness obtained in material before machining. These

phenomena, but less pronounced, were observed at shorter cutting time, because

prolonged machining with nearly worn tools, produced severe plastic deformation and

thicker disturbed layer on the machined surface and the hardness of the disturbed

layer of the machined surface increased significantly.

Figure 2-21 SEM micrograph showing three microindentation marks on a region

approximately 2 mm away from the drill exit, for the 128th hole machined

In other research Sharman [50] stated that in drilling Inconel 718 For all the

drills examined the workpiece surface hardness was increased compared to bulk

(~500 HK0.05) with a return to bulk values within the first 50µm depth below the

surface (Figure 2.22). There was also no difference between the hardness profiles seen

when cutting with a worn or unworn drill. This result was most likely caused by the

fact that it is the flute margins that are responsible for forming the hole surface and in

comparison to the wear encountered at the cutting edges the flutes are relatively

unworn.

Figure 2.22. Typical microhardness profile from drilling (range bars show standard deviation of results)

2.16 Microstructural changes

Sharman [50] showed that in drilling Inconel 718 the subsurface microstructural

damage seen in all the holes produced consisted of deformed grain boundaries and

white layer in the direction of cutting (Figure 2.23).

Figure 2.23 (a) Grain boundary deformation and white layer from drilling. (b)

Microstructure resulting from Mill Boring.

White layer is a resulte of microstructural alteranation. It is called “white”

layer because it resists standard etchants and appears white under an optical

microscope (or featureless in a scanning electron microscope). In addition, the white

layer has high hardness, often higher than the bulk. White layers are found in many

material removal processes such as grinding [57-59], electrical discharge machining

[60] and drilling [61]. Large plastic deformation and or rapid heating cooling are

possible formation mechanisms. White layers seem to be detrimental to product

performance, and therefore require a post-finishing process. White layers seem to be

detrimental to product performance, and therefore require a post-finishing process.

Most noted that white layer occurs when cutting tools wear out to a certain level, but

did not provide an in-depth explanation.

Tonshoff et al [62] studied the influence of hard turning on workpiece

properties and properties and reported that retained austenite is the major composition

of white layer structures. A higher thrust force component seems to accompany white

layer occurrence, as does tensile residual stress. They further showed that the white

layer decreases bending fatigue strength probably due to associated tensile residual

stress. Tool wear was suggested at the most influential parameter on white layer

formation, though frequently it was the only variable studied. However, the

explanation of white layer formation was rather qualitative and, thus, there was no

important that optimization of surface structure or minimization of white layer is

possible. Several factors may limit tool life and therefore affect machining cost. In a

finishing process, surface integrity is often of great concern because of its impact on

product performing; indeed, it may be used as a tool-changing criterion. Thus,

understanding tool wear and cutting parameter effect on surface integrity is of

practical significant.

2.17 Burr formation

Nickel and its alloys are used widely in aerospace, pressure vessels, aircraft

turbine and compressor blades and disks, surgical implants, etc. The alloys are

difficult to machine and, in particular, burr formation due to drilling is troublesome in

aerospace applications due to the difficulty of completely removing the burrs.

Estimates are that up to 30% of the cost of some components is due to deburring

operations. Most drilling processes create a burr on both entrance and exit surfaces.

The exit burr is much larger in size and is the main concern. In multi-layer materials

(for example multi-layer metal-composite laminates) the burr at the exit surface

between layers is a major problem requiring disassembly of the laminate, deburring

and re-assembly.

Preventing, or at least minimizing (or controlling) the formation of drilling

burrs is therefore very important. Burr formation in drilling has not been as

extensively studied as drilling itself. Most studies are concerned with tool wear or

hole quality and don’t consider burr formation. And, the studies usually focus on

conventional materials. Gillespie [63] was one of the first researchers to study burr

formation at an academic level in several machining operations including drilling. For

drilling, he studied the effects of process conditions, tool geometry and material

properties on burr formation over a wide range of test conditions and proposed a basic

model of burr formation in drilling. Gillespie’s studies on titanium alloys covered

hole quality issues and evaluated the influence of drill wear land size on burr size. No

influence was found in that study. Importantly, most of Gillespie’s tests were done

with hand fed drills (hence feed rate is unknown and not controlled) so the influence

of feed rate is confounded with other parameters studied. Sakurai et al [64] noted the

reduction in burr height with the vibratory step feed drilling of Ti-6AI-4V but offered

no explanation. Sofronas [65] made a fundamental study of burr formation in drilling

but for carbon steel.

Stein and Domfeld [66] studied the burr formation of miniature hole drilling in

stainless steel. Increases in feed, cutting speed and drill wear were found to increase

the burr height and thickness. Drill pecking stabilized the burr formation leading to a

process with less variation and uncertainty regarding the exit burrs. Link [67]

concluded that it is necessary to take the temperature dependency of material

properties into account when explaining burr formation phenomena. The nonlinearity

of these properties is the main reason there exists no general model of burr formation.

Dornfeldl and Kim [68] have done some studies on burr deformation during

drilling the Ti-6Al-4V using carbide drills for dry cutting and Cobalt high speed steel

drills for wet cutting. They have reported that four type of burr will be formed during

dry cutting. Figure 2.24 shows four burr types categorized by cross-section shape

created under different machining conditions. Type I is a uniform burr which has

uniform height and thickness. Type II is similar to Type I but has a "leaned-back"

cross section. Type Ill has a severe rolledback shape and Type IV is also rolled-back

but has a relatively small burr height with widened exit. For each cutting condition

and burr type, a drill cap was formed. However, the drill caps were separated from the

workpiece during the process. The shape of the drill caps were different depending on

the drill type due to the difference in point angle. Drill caps can be a problem in

intersecting holes or interior cavities where removal is difficult.

(a) Type I (b) Type II (c) Type 111 (d) Type IV

Figure 2.24: Burr types formed in dry cutting.

The roll back phenomena can be explained through the burr formation

mechanism illustrated in Figure 2.25. As the drill approaches the exit surface, the

material under the chisel edge begins to deform, (b). The distance from the exit

surface to the point where the deformation starts depends on the thrust force of the

drill. As the drill advances, the plastic deformation zone expands from the center to

the edge of the drill, (c). One of the cutting edges advancing will cause separation of

the cap from the hole perimeter, (d). Depending on the point angle and drill point

geometry, initial rupture may occur near the center of the remaining material. At the

final stage, the remaining material at the hole perimeter is pushed out to form a burr

with a drill cap, (e). While the remaining material at hole perimeter is being formed

into a burr no more cutting occurs (no chip formation) which means there is no way to

dissipate the generated heat. The low thermal conductivity of the material inhibits

heat dissipation. Thus, there should be a localized temperature increase at the inner

surface of the burr. This temperature increase and resulting thermal expansion is

believed to be the main cause of the lean back and roll back phenomena observed.

The amount of heat generation and temperature rise is proportional to cutting speed

and feed rate. Higher feed rate and cutting speed will generate more heat, and result in

more rolling-back as observed here.

Figure 2.25: Formation of a burr with drill cap

They have also reported three type burr will be formed during wet cutting.

Figure 2.26 shows three representative types of burrs observed in wet cutting

experiments. They are a standard uniform burr without any attachment, a burr with a

drill cap and a burr with "ring formation". The most common type is the burr without

attachment. A burr with a drill cap occurred at the lowest feed rate of 0.04 mm/rev.

The uniform portion of all the burrs was the Type I burr seen in dry cutting. This is

the burr at the hole perimeter. No rolled-back burr was observed and this supports the

proposed explanation of roll back formation in dry cutting.

Burr with ring formation Burr without attachment burr with a drill cap formation

Figure 2.26: Burrs produced in wet cutting.

The burr with a ring is believed to be an intermediate type between the plain

uniform burr and drill cap formation. The drill continues to remove material from the

workpiece even after breaking through the exit surface. However before the normal

bending process occurs which is responsible for the formation of uniform burr, the

remaining ring-shaped material in front of the drill is not able to sustain the thrust

force of the drilling operation and detaches from the workpiece. This detached

material has the shape of a ring and leaves a small "secondary burr" on the hole

perimeter as often seen in peripheral milling.

Both feed rate and cutting speed seem to have influence on the burr formation. Figure

2.27 shows the correlation between the burr formation and the cutting conditions.

Figure 2.27: Correlation between the burr formation and the cutting conditions

The correlation between burr formation and tool geometry is seen in Figure

2.28 (a) and (b). Lip relief angle seems to have little influence on burr formation. Lip

relief angles used were large enough compared to the feed rate so that contact

between the flank of the drill and the workpiece was minimal. Increasing point angle

produced burrs of decreased height and thickness and increasing helix angle increased

burr size. Concerning the style of the drills, the helical point drill proved to be very

suited to minimize the exit burr formation. The burr height and thickness were

reduced with a helical point drill. This reduction in burr size is due to the smaller

thrust force that is required for the helical drill compared to the split point drill.

(a): Lip relief angle (degree) (b): Point angle (degree)

Figure 2.28: Correlation between the burr formation and the point angle and the lip

relief angle.

CHAPTER 3

RESEARCH METHODOLOGY

3.1 Introduction

Proper experimental plan is necessary to achieve good results in conducting

research. In this chapter all the equipments used in this study are described

accordingly.

3.2 Research Design Variables

The design variables are described into two main groups, which are dependent

variables response variables and independent variables (machining parameters).

The response variables include:

1- Surface integrity which include surface roughness, microhardness changes and

microstructure of drilled surface.

2- geometrical accuracy

3- burr height

The parameters involved in this study are shown in Table 3.1.

Table 3.1 Machining parameters

Machine Tools/Equipment MAHO MH 700S CNC milling

machine

Workpiece Material Inconel 718

Cutting Tool, 2 flute drill 1) Uncoated carbide (WC/Co)

Cutting speed (m/min) 10-20

Feed rate (mm/rev) 0.03-0.012

Depth of cut (mm) 18

Tool diameter (mm) 6

Type of Cutting Through Hole

Coolant 6% concentration

3.3 Workpiece Material

Inconel 718 (nickel alloys) was chosen as the workpiece material for the test

specimens. The mechanical properties and chemical compositions of the Inconel 718

is shown in Tables 3.2 and 3.3 respectively. Figure 3.1 shows the workpiece material

of 105 mm × 105 mm× 18 mm that was prepared for experiments.

Table 3.2 Mechanical properties of Inconel 718

Tensile strength (ksi) 199.9

Yield strength (ksi) 161.1

Elongation (%) 20

Reduction in Area (%) 50

Density (lb/in3) 0.296

Hardness (HRC) 43

Grain size (µm) 6

Table 3.3 Chemical composition of Inconel 718

�i Cr Mo Fe �b Ti

≥54 18 3.0 18.5 5.0 1.0

Figure 3.1 Workpiece material

3.3.1 Analysis the workpiece material

In order to specify the microstructure study of workpiece material, a sample

specimen was prepared using standard metallography techniques. In the first step

metallographic specimen was cold mounted (Figure 3.2) using BUEHLER low

viscosity epoxy that requared 18-20 hour curing time in the temperature of 27° C.

After mounting, the specimen was wet ground to reveal the surface of the metal. The

specimen was successively ground with fine and finer grades of silicon carbide paper

from 100 to 4000 mesh number to remove damage from sectioning and then from

each grinding step. After grinding the specimen, polishing was performed. Typically,

specimen was polished with slurry of alumina on a napless cloth to produce a scratch-

free mirror finish, free from smear, drag, or pull-outs and with minimal deformation

remaining from the preparation process.

Figure 3.2 Mounted specimen

After polishing the specimen was etched using electrolytic etchant by Buehler

electromet (Figure 3.3). The speciment was etched in sulfuric acid (3%) at the

electrical condition of 6 volts and temperature of 24° C for 15 sec. The material of the

cathode used was stainless steel. This etchant method is suitable for showing the

carbides and grain boundaries of Inconel and nickel alloys.

Figure 3.3 Buehler electromet

Prepared specimen should be examined after etching with the unaided eye to

detect any visible areas that respond differently to the etchant as a guide to where the

microscopical examination should be employed. Specimen was examined under an

Olympus toolmakers’ light optical microscope which is connected to Sony Digital

hyper head color video camera. Figure 3.4 and 3.5 show samples grain structure of

specimen at magnification of 100X and 200X.

Figure 3.4 Grain structure of Inconel 718 at 100X magnification

Figure 3.5 Grain structure of Inconel 718 at 200X magnification

The average hardness measurement of the workpiece was performed at three

different places on the specimen using Matsuzawa Seiki microhardness tester with

Vickers pyramid indenter of 10 kg load. The obtained hardness values have been

shown in Table 3.4.

Table 3.4 Average hardness of Inconel 718

3.4 Cutting Tools

In this experiment, uncoated of solid carbide (WC/Co) twist drill with

different geometry were used to drill Inconel 718. Tool geometry information is

presented in Table 3.5. Sample of the tool is shows in Figure 3.6.

Table 3.5 Drill information

Description

No of flute 2

Shank diameter tolerance h6

Shank diameter (mm) 6

Tool diameter (mm) 6

Helix angle (0) 25

LOC (mm) 35

OAL (mm) 65

Point angle (0) 120,125,130

Fluting web (mm) 1.65

Margin width (mm) 0.6

Chisel edge angle (0) 135

Lip relief angle (0) 10

Bevel width (mm) 1.1

Web Thickness (mm) 0.4-0.6

Secondary relief angle (0) 20

Vickers hardness

1 247.5

2 249.2

3 248.4

Figure 3.6 Image of uncoated tool drill (PA: 1250)

3.5 Machining Procedure

The drilling experiments were carried out on a MAHO MH 700S CNC

machining center shown in Figure 3.7. The drills were clamped to the tool holder with

an overhang of 35mm.

Figure 3.7 MAHO MH 700S CNC machining center.

3.6 Selection of independent variables

The independent variables considered in this investigation are of two types:

(1) variables related to machining conditions, and (2) variables related to geometry of

cutting tool. The machining parameters were selected on the basis of the information

available in the literature. The value of independent variables and the values were

selected in different runs are shown in Table 3.6.

Table 3.6 Experimental planning at all levels

Experiment

�o.

Cutting Speed

(m/min)

Feed rate

(mm/rev)

Point Angle

(0)

1 13 0.12 125 2 8 0.1 130 3 8 0.1 120 4 18 0.05 130

3.7 Investigation of Surface Finish

In evaluating the roughness of the drilled hole a Handysurf model E-35A was

used. Two surface roughness readings were taken at four positions spaced at 90 deg

intervals around the hole circumference and approximately mid-way down the depth

of the hole.

3.8 Dimensional accuracy

The hole diameter was been measured at 6 points located at different height

and orientation using of CMM- KN810 Mitutoyo (Figure 3.8).

Figure 3.8 Coordinate measuring machine

3.9 Burr height measuring

The first and last hole of each run are separated from the main plate by

electro-discharge wire cut, AQ537L Sodick. To measure the burr height optical

microscope is used (ZEISS, Figure 3.10), the separated sample are located under the

microscope and the burr was focused and captured digitally. The height of burr was

analyzed using an image analyser. Figure 3.9 show sample preparation for measuring

of burr height

Figure 3.9 Samples are separated from the workpiece plate

Figure 3.10 Optical microscope used to measure burr height

3.10 Preparing the samples

Samples created by wire-cut are sectioned along the holes axis and

perpendicular of hole axis using precision cutter (Figure 3.11) to prepare

metallographic samples for investigating the microstructure of machined surface and

sub-surface and for measuring microhardness.

Figure 3.11 Linear precision saw

Cross-sections of each surface will be prepared using standard metallography

techniques of sample mounting and polishing as like as described previously. These

steps are shown in Figure 3.12.

Figure 3.12 Preparations of samples to metallographic studies

3.11 Microstructural analysis

Subsurface microstructural analysis is conducted with Olympus toolmakers’

light optical microscope (Figure 3.13) which is connected to Sony Digital hyper head

color video camera.

Figure 3.13 Toolmakers’ light optical microscope

3.12 Microhardness measurement

Microhardness (Vickers) was measured at five points (located at a certain

distance from machined surface ranging from 40 to 480 µm) in four line positions

spaced at 90 degree intervals around the hole perpendicular to the drill direction and

2mm away from the drill entrance. Measurement was conducted using SHIMADZU

(Figure 3.14) micro hardness tester under 4.903 N force with Vickers pyramid

indenter.

Figure 3.14 Vickers pyramid microhardness tester

CHAPTER 4

RESULTS A�D DISCUSSIO�

4.1 Introduction

This chapter present the experimental results and discussion. The results from

the surface roughness measurement, geometrical accuracy, burr height,

microstructural changes and microhardness are shown graphically. The effect of

various parameters on the machining response such as tool wear, cutting speed, feed

rate and point angles are investigated. The collected data are analyzed graphically

using Microsoft Excel 2004.

4.2 Tool wear

Tool wear was measured with a toolmakers microscope fitted with a digital

camera and image analysis software. When flank wear reached VB= 0.25mm or

VBmax=0.5mm the trials were stopped. Number of holes created in each run is shown

in Table 4.1.

Table 4.1. Number of holes drilled under different conditions

Experiment

No.

Cutting

speed

(m/min)

Feed rate

(mm/rev)

Point

angle

(deg)

Number

of holes

1 13 0.12 125 10

2 8 0.1 130 29

3 8 0.1 120 24

4 18 0.05 130 27

4.3 Surface roughness

Figure 4.1 shows results of surface roughness values of machined hole of

different conditions. All the drills at various conditions produced irregular results in

the values of surface roughness profile under various positions measured, however no

trend could be noted. This is probably due to presence and absence of built up edge

during drilling trials. Majority of the results fell between 0.6 and 1.1 µm Ra with a

wide range of scatter around these values and there was little difference between the

values obtained at different runs. It can be seen that smoother surface finish has been

obtained at higher cutting speed and lower feed rate.

0.57

0.77

1.041

0.81 0.660.76

0.7

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

machinig time (min)

Ra ( µm ) V=13m/mim, f=0.12mm/rev, A=125deg

V=8m/min f=0.1mm/rev, A=130deg

V=8m/min, f=0.1mm/rev, A=120deg

V=18m/min f=0.05mm/rev A=130deg

Figure 4.1 Surface roughness measurement at different experiment condition

Similar results are reported by Sharman and Ridgway [50] when drilling

Inconel 718. They have reported that surface finish has improved with increasing drill

wear. Figure 4.2 shows the comparison of average surface roughness values produced

with new tools and worn tools at different conditions. Surface roughness values of

worn tool were higher than new tools while these values were lower in experiment

number 3 and 4. This may be described due to presence of built up edge during

drilling of Inconel 718.

Figure 4.2 Surface roughness measurement comparison in different experimental trials

4.4 Dimensional accuracy

All diameter measurements ranged from 6.009 to 6.088 mm, and mean values

were between 6.013 to 6.074 mm. These values corresponded to the dimensional

tolerance reasonable in drilling operation. Recorded values of first and last hole

diameters were subtracted form the actual diameter of drill at each run and results are

shown in Figure 4.3. It can be seen that this variation in the hole diameter produced

using worn tool are higher than those produced using new tools. The heat generated

by the drilling process can lead to thermal expansion of the drill and work-piece

which may affect the size and quality of the holes leading to oversized holes [69].

0.086

0.073

0.054

0.066

0.03

0.057

0.036

0.088

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

1 2 3 4

Experiment number

variation (mm)

new tool

worn tool

Figure 4.3 Variation of machined hole dimension and tool diameter

4.5 Burr height

The burr height of first and last holes of each run was measured (Figure 4.4)

and the maximum values of each specimen are shown in Figure 4.5. It can be seen

that burr height in last holes using worn tool were higher than those obtained using

new tool. This phenomenon was observed in all experiments except experiment

number 1. It could be due to the experimental errors as a result of clamping the plate

during the separation of samples by wire-cut machine.

It can be concluded that burr height increased with increase in tool wear which

may be due to the ploughing of the worn tool. Cantero et al. [56] have reported similar

results in dry drilling of Ti-6Al-4V. They concluded burr formation presented more

sensibility to heat accumulation than the resultant diameter.

Figure 4-4 Burr heights obtained using an optical microscope

0.09 0.1

0.16

0.35

0.07 0.07

0.03

0.08

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

1 2 3 4

Experiment number

burr height (mm)

new tool

worn tool

Figure 4.5 Comparison of burr height at different cutting condition

The burrs created during drilling are burrs with a drill cap. The shape of the

drill caps were different depending on the drill type due to the difference in point

angle. Drill caps can be a problem in intersecting holes or interior cavities where

removal is difficult. Dornfeld et al. [68] has stated that both feed rate and cutting

speed seemed to have very little influence on the burr formation. The correlation

between burr formation and point angle is seen in Figure 4.6. It is observed that

increasing point angle produced smaller burr, as reported by Dornfeld [68] increasing

the point angle may produced burrs of decreased height and thickness and similarly

increasing the helix angle may increase burr size. Larger helix angle and increasing

point angle reduced the burr height and thickness.

0.09

0.07

0.1

0.06

0.07

0.08

0.09

0.1

0.11

115 120 125 130 135

point angle (deg)

burr heigth (mm)

Series1

Figure 4.6 Burr height versus point angle

4.6 Microstructure

The microstructrul changes of the first hole and last hole of run1 and other

runs subsurface are investigated using the optical microscope. Figure 4.7 shows the

comparison of microstructure of the first hole and last hole of run 1 in two 200 and

500 magnification.

No specific changes are observed in the first hole using a new tool while the

subsurface microstructural damage was seen in all holes produced using worn tools at

different runs. The changes involved deformed grain boundaries in the direction of

drilling and formation of white layer as shown in Figure. 4.7 (c,d).

A discontinuous white layer (up to 7µm depth) was present on all drilled surfaces but

not present at the first hole of each condition using new tools (Figure 4.7, a, b). It is

obvious that the holes produced by the drilling alone would not meet the aerospace

standards due to the high levels of white layer presence.

(a) (b)

(c) (d)

Figure 4.7 Comparison between first and last hole produced in exprement1

(V=8m/min, f=0.1 mm/rev, point angle= 130 deg). (a, b) first hole produced using

new tool, (c, d) last hole produced worn tool

Prior researches had stated that white layers are produced due to grain

refinement induced by severe plastic deformation and/or thermally induced phase

transformation [70, 71]. Li et al.[72] showed that when drilling plain carbon steel a

white layer was produced by both thermal and deformation driven phase

transformations acting in combination, with the dominant mechanism defined by the

relative workpiece material properties and cutting conditions used. Osterle and Li [71]

found that the white layer produced in ground IN738LC nickel-based superalloy

contained equiaxed grains of 50–100nm diameter produced by melting and rapid

quenching. They suggested that in cases where the wheel dressing rate was not

sufficient, wheel loading would occur causing chips to be plastically deformed

between the wheel and the workpiece. These chips become pressure welded to the

workpiece surface and are spread across it as the wheel rotates, forming a white layer

due to incipient melting and severe plastic deformation.

It was well understood from the analysis of the hole surface that during

drilling chips were become entrap between the flute margins and the hole wall. These

chips are extruded between the flute margins and the hole wall as the drill rotates and

become pressure welded to the workpiece surface and forming a white layer due to

incipient melting and severe plastic deformation. This process continues as the drill is

fed down the hole and as it is retracted. Prior work on turning has shown that greater

levels of grain boundary deformation are produced when cutting with worn tools (at

least two to three times higher). This may be due to the higher cutting and frictional

forces that would be developed when cutting with a worn tool, due to the increase in

tool/workpiece contact area [27]. Wear on the tool flank reduces the tools clearance

angle leading to greater rubbing of the workpiece surface.

Figure 4.8 shows the microstructural changes during drilling Inconel 718 in

other experiments with different conditions. Results obtained in this study are

compatible with prior studies. It can be observed that white layer depth progressively

increases with flank wear as well increases with cutting speed. Though whit layer

depth increases with increasing speed, it clearly becomes saturated at high speed.

Flank wear land rubbing may be the primary heat source for white layer formation.

Thus it can be suggested tool wear is the most influential parameter on white layer

formation, though it was the only variable under studied.

(a)

(b)

(c)

Figure 4.8 Subsurface microstructure in last holes produced using worn tool. (a)

V=8m/min, f= 0.1 mm/rev, point angle=130 deg, (b) V= 8m/min, f= 0.1 mm/rev,

point angle=120 deg, (c) V= 18m/min, f= 0.05 mm/rev, point angle=130 deg

4.7 Microhardness

Data of microhardness are recorded and the average values versus distance

from machined surface are shown graphically in Figure 4-9.

200

220

240

260

280

300

320

340

360

0 40 80 120 160 200 240 280 320 360 400 440 480 520

depth (microns)

hardness (HV)

run1 new tool, V=13m/min,

f=0.12mm/rev, A=125 deg

run 1 worn tool,V=

13m/min,f=0.12mm/rev, A=125deg

run2 worn tool,V=8m/min,f=0.1

mm/rev,A=130deg

run 3 worn tool,V=8/min,f=0.1 mm/rev,

A=120 deg

run 4 worn tool,V=8m/min,

f=0.1mm/rev, A=120 deg

figure 4.9 Microhardness changes versus distance from machined surface

It was found that the surface hardness values increase as compared to bulk

material (240-250 HK. Figure 4.7 illustrates the average microhardness at the depth

below 120 µm for the first hole of run1 using new tool was lower than that

microhardness produced using worn tools. Therefore, it may be suggested that tool

wear has great influence on the degree of work hardening of material during

machining. On the other word the hardening of work material increases with drilling

force and accelerated tool wear.

In drilling of titanium alloy, Cantero [56] has found that as distance to

machined surface increases microhardness decreases until values to those obtained on

the bulk material before machining. This phenomenon, but less pronounced, were

observed at shorter cutting time, because prolonged machining with nearly worn tools

produced severe plastic deformation and thicker disturbed layer on the machined

surface and the hardness of the disturbed layer of the machined surface increased

significantly. These microstructural changes originated during machining were mainly

because of the elevated temperatures which influenced mechanical properties of

material, decreasing fatigue and stress corrosion resistance [73-75].

CHAPTER 5

CO�CLUSIO�

5.1 Conclusion

This project is focused on the drilling of Inconel 718 in evaluating quality of

machined holes, burr formation and surface integrity after machining at various

conditions. Four different drilling conditions were analysed in order to observe the

effect of tool wear of the drill and work-piece. The most number of holes produced

was in run 2 with 29 holes under condition of 8m/min cutting speed, 0.1 mm/rev feed

rate and 130 degree point angle.

From the obtained results, the following conclusions can be drawn:

- Surface roughness values were between 0.6 and 1.1 µm Ra at all condition

investigated. Smoother surface finish was obtained at higher cutting speed and

lower feed rate (V=18 m/min, f= 0.05 mm/rev, point angle= 130 deg).

- The size of the hole varies between 6.009 to 6.088 mm, and values ranged

from 6.013 to 6.074 mm. Holes with higher accuracy were obtained in first

hole of each experiment using new tools as compared to the last holes

produced using worn tools.

- The burrs created during drilling are burrs with drill caps. The burr height in

last holes were higher than those obtained during the initial cut.

- Increasing point angle tends to reduce the burr height.

- Subsurface microstructural damage was very obvious in the holes produced

using worn tools, consist of deformed grain boundaries in the direction of

drilling and a formation white layer. The white layer progressively increases

with flank wear and cutting speed.

- Subsurface hardness was observed to increase compared as to bulk material

(240-250 HK) within the first 40 to 120µm depth below the surface.

5.2 Future study

It is suggested that further investigation should be focused on finding ways to

improve the surface quality and reducing the whit layer during drilling. The effect

other processes such as reaming are be analyzed on the quality of surface finish and

microstructural changes of the workpiece. In addition studies can be performed on

coated carbide tool when drilling Inconel 718 and also such as TiALN and AlTiN

coatings.

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