iv

MACHINABILITY STUDY OF UNCOATED AND COATED CARBIDE TOOLS

WHEN DRILLING CFRP/METAL STACKS

ISMAIL BIN MAHAMAD HAKIMI

UNIVERSITI TEKNOLOGI MALAYSIA

vi

Dedicated to:

My beloved parents,

Norizan Esa, Mahamad Hakimi Ibrahim

My wife and daughter,

Nur Hazwani Mohamad, Fatimah

My dedicated lecturers,

My endless spirits

and all my friends.

This is for you.

vii

ACKNOWLEDGEMENT

In the name of Allah, the Most Gracious and Most Merciful, all praise be to

Allah SWT, the Almighty, for His love has given me strength, perseverance, diligence

and satisfaction in completing this project.

First and foremost, I would like to express my deepest gratitude especially to

my main supervisor, Prof. Dr. Safian Sharif who had taken a lot of effort to

meticulously go through my work and come out with helpful suggestions. Not

forgotten, appreciations for my co-supervisor Assoc. Prof. Dr. Mohd Yazid Yahya for

his valuable advices.

I would like to express my appreciation to Ministry of Science Technology

and Innovation (MOSTI), UTM Zamalah Programme and Research University Grant

(RUG) for supporting the scholarship along my study.

Finally, I would like to express my greatest gratitude to my beloved family for

their relentless support and prayer. Without the family members support and prayer,

this project would have been difficult at best. Thank You.

viii

ABSTRACT

Carbon fiber reinforced plastic (CFRP) composite is often used in combination

with other materials, requiring it to be machined during fabrication of a structure. In

the aerospace industry, CFRPs are often stacked together with metals to provide

stronger and better performance of aircraft structural components with the advantage

of weight reduction and increasing mechanical strength. Drilling which is the most

common machining process of CFRP is complex and often results in delamination of

the composites. This study presents the findings of an experimental investigation in

drilling of CFRP plate stacked on top of Aluminum 2024 plate. Uncoated carbide and

TiAlN coated carbide tool were used to perform the drilling of the CFRP/aluminum

stack at cutting speeds of 70, 85, 100 mm/min with constant feed rate of 0.1 mm/rev

using dry drilling. The responses that were analyzed include thrust force, torque, hole

quality, CFRP surface delamination, and tool wear. Experimental results showed that

the application of various cutting speeds have no significant effect on all the responses

analyzed when drilling CFRP/aluminum stacks for both tools. In terms of hole quality

and CFRP surface delamination, the uncoated tool displayed better results compared

to coated tool at cutting speeds of 85 and 100m/min. This is due to the sharp cutting

edge of the uncoated tool that enabled efficient cutting in producing holes.

Experimental results also showed that coated tool performed better than uncoated tool

producing lower thrust force and torque, lower surface roughness, and lower tool wear

rate.

ix

ABSTRAK

Komposit karbon bertetulang gentian plastik (CFRP) lazimnya digabungkan

dengan bahan lain yang memerlukan ia dimesin ketika fabrikasi sebuah struktur. Di

dalam industri aeroangkasa, CFRP lazimnya disusun berlapis bersama-sama logam

untuk memberi kekuatan dan prestasi yang lebih baik bagi komponen struktur pesawat

dengan pengurangan berat dan menambah kekuatan mekanikal. Penggerudian CFRP

adalah proses pemesinan yang sukar di mana lekangan komposit seringkali berlaku.

Kajian ini menunjukkan penemuan ujikaji ketika menggerudi kepingan CFRP yang

disusun berlapis di atas kepingan Aluminum 2024. Mata alat karbida tidak bersalut

dan karbida bersalut TiAlN digunakan untuk menggerudi kepingan CFRP/aluminum

pada halaju pemotongan pelbagai iaitu 70, 85, dan 100 m/min serta kadar uluran tetap

pada 0.1 mm/pusingan dalam keadaan kering. Sambutan yang dianalisa adalah daya

tujah, daya kilas, kualiti lubang, lekangan permukaan CFRP, dan kehausan mata alat.

Keputusan ujikaji menunjukkan bahawa penggunaan halaju pemotongan yang

pelbagai tidak memberi kesan yang ketara terhadap semua sambutan yang dianalisa

ketika menggerudi lapisan kepingan CFRP/aluminium bagi kedua-dua mata alat. Dari

segikualitilekangan; iaitu masing-masingdndisebabkancekap Keputusan ujikaji juga

menunjukkan prestasi mata alat bersalut lebih baik daripada mata alat tidak bersalut

dengan menghasilkan daya tujah dan daya kilas yang rendah, kekasaran permukaan

yang rendah, serta kadar kehausan mata alat yang perlahan.

x

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT vii

ABSTRACT viii

ABSTRAK ix

TABLE OF CONTENTS x

LIST OF TABLES xiii

LIST OF FIGURES xiv

LIST OF SYMBOLS AND ABBREVIATIONS xvii

LIST OF APPENDICES xviii

INTRODUCTION 1

1.1 Background 1

1.2 Problem Statement 3

1.3 Objectives 5

1.4 Scope of Study 5

LITERATURE REVIEW 6

2.1 Introduction 6

2.2 Composite-Metal Stack 7

2.2.1 Fiber Reinforced Composites 8

2.2.2 Fiber Reinforced Polymers (FRPs) 8

2.2.3 Carbon Fiber Reinforced Plastic (CFRP) 9

2.2.4 CFRP Applications in Aerospace Industry 10

2

1

xi

2.3 Machining Applications 11

2.3.1 CFRP Machining 11

2.3.2 Aluminum Machining 13

2.3.3 Composite-Metal Stack Machining 14

2.4 Thrust Force and Torque Relationship in Drilling Composite-Metal

Stack 15

2.5 Drilling Process 16

2.5.1 Geometry of Drill Bit 17

2.6 Special Tool for Drilling Composite-Metal Stack 20

2.7 Common Tool Material for Composite-Metal Stack 22

2.7.1 Solid Carbide Drill 23

2.7.2 Diamond Coated Drills 23

2.7.3 Polycrystalline Diamond (PCD) Drill 24

2.7.4 High Speed Steel (HSS) Drill 24

2.8 Tool Wear in Drilling Composite-Metal Stack 25

2.8.1 Flank Wear 26

2.8.2 Crater Wear 27

2.9 Damages in Drilling FRPs 28

2.10 Summary of Literature Review 31

RESEARCH METHODOLOGY 32

3.1 Introduction 32

3.2 Workpiece Material 33

3.3 Experimental Set Up 34

3.4 Cutting Tool 34

3.5 Experiment Details 35

3.6 Cutting Conditions 38

3.7 Measurement of the Machining Responses 39

3

xii

3.7.1 Tool Wear Measurement 39

3.7.2 Surface Roughness Measurement 40

3.7.3 Hole Size Measurement 40

3.7.4 Delamination Measurement 42

3.8 Summary of Research Methodology 42

EXPERIMENT RESULTS AND DISCUSSION 43

4.1 Introduction 43

4.2 Thrust Force 44

4.3 Drilling Torque 49

4.4 Hole Size 53

4.5 CFRP Surface Delamination 56

4.6 Surface Roughness 58

4.7 Tool Wear 61

4.8 Summary of Experimental Results and Discussion 65

CONCLUSION AND RECOMMENDATIONS 66

5.1 CONCLUSION 66

5.2 RECOMMENDATIONS 68

REFERENCES 70

APPENDICES 76

4

5

xiii

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Composite advantages and its applications 7

3.1 Deckel Maho DMC 835 V CNC milling machine specifications 37

3.2 Rotary dynamometer specifications 38

4.1 Dimensional deviation of the hole size for CFRP and aluminum

plates when drilling using uncoated and coated carbide tools 56

xiv

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 (a) An example of a UD ply [14], (b) An example of a

quasi-isotropic laminate stacking composition [16] 10

2.2 Cutting mechanisms in composites machining based on

fiber orientations [19] 12

2.3 The main parts of a twist drill body [44] 17

2.4 Classifications of chisel edge angle [19] 18

2.5 Different tool point angles 20

2.6 Router tool (a), Dagger tool (b) [47] 21

2.7 Amamco modified tool geometry specific for drilling

composites [49] 22

2.8 Sandvik , helix drills ,both with modified geometry [50] 22

2.9 Tool Materials used to drill Polymeric Composites [52] 25

2.10 Flank wear and definitions of ( VB,Max ) and ( VB ) [53] 27

2.11 Potential defects which may occur during drilling FRP [55] 30

2.12 Schema of visualization of damage [57] 31

3.1 Flow of research approach phases 34

3.2 Workpiece CFRP/Aluminum 2024 stack 34

3.3 Experimental set up 35

3.4 Image of the drill taken by Zeiss Stemi 2000C microscope 36

3.5 CNC Maho DMG 835 V 37

3.6 Rotary dynamometer for force and torque measurement 38

3.7 Force and torque data acquisition system from Kistler Rotary

Dynamometer Type 9123C instruction manual 39

3.8 Zeiss Stemi 2000C microscope 41

xv

3.9 Mitutoyo CMM used for measuring hole size 42

3.10 Measuring hole size for CFRP/AL stack using Mitutoyo CMM 42

3.11 Method to measure surface delamination [57] 43

4.1 (a) Thrust force at the 17th hole when drilling CFRP/Aluminum

2024 stack using uncoated carbide tool and 45

(b) TiAlN coated carbide tool at cutting speed of 85 m/min with feed rate of

0.1 mm/rev. 46

4.2 Thrust force when drilling CFRP-Aluminum stack at various

cutting speed with feed rate of 0.1 mm/rev using uncoated

carbide drill (Tool A) 46

4.3 Thrust force when drilling CFRP-Aluminum stack at

various cutting speed with feed rate of 0.1 mm/rev using

TiAlN coated carbide drill (Tool B) 48

4.4 Average thrust force comparison between uncoated carbide drill

(Tool A) and TiAlN coated drill (Tool B) at various cutting speed

with feed rate of 0.1 mm/rev 49

4.5 (a) Torque at the 17th hole when drilling CFRP/aluminum stack

using uncoated and (b) TiAlN coated carbide tool at cutting speed

of 85 m/min with feed rate of 0.1 mm/rev . 51

4.6 Torque of CFRP/Aluminum stack when drilling at

various cutting speeds using uncoated carbide (Tool A) with

feed rate of 0.1 mm/rev 53

4.7 Torque of CFRP/Aluminum stack when drilling at

various cutting speeds using TiAlN coated carbide (Tool B)

with feed rate of 0.1 mm/rev 54

4.8 Average torque comparison between Tool A and B at

various cutting speed with feed rate of 0.1 mm/rev 54

4.9 Average hole size of the CFRP/Aluminum 2024 stack

using uncoated carbide tool at various cutting speed with

feed rate of 0.1 mm/rev 56

4.10 Average hole size of the CFRP/Aluminum 2024 stack

using TiAlN coated carbide tool at various cutting speed

with feed rate of 0.1 mm/rev 57

4.11 Delamination evaluation method around CFRP hole at 20X

xvi

magnification. 58

4.12 Delamination at hole 30 for Tool A and Tool B at various cutting

speed with feed rate of 0.1 mm/rev 59

4.13 Average surface delamination of CFRP plate when drilling at various

cutting speed using uncoated carbide tool and TiAlN coated tool 60

4.14 Surface roughness values of CFRP and aluminium holes when

drilling at various cutting speed using uncoated carbide tool 61

4.15 Surface roughness values of CFRP and aluminium holes when

drilling at various cutting speed using TiAlNcoated carbide tool 62

4.16 Average surface roughness values of CFRP and aluminium

holes when drilling at various cutting speed using uncoated

carbide tool and TiAlN coated carbide tool 63

4.17 Flank wear (Vb avg) of CFRP and aluminium holes when

drilling at various cutting speed using uncoated carbide tool 64

4.18 Flank wear (Vb avg) of CFRP and aluminium holes when

drilling at various cutting speed using TiAlN coated carbide tool 65

4.19 Flank wear (Vb avg) of CFRP and aluminium holes when

drilling at various cutting speed using uncoated carbide tool

and TiAlN coated carbide tool 65

4.20 Average flank wear (Vb avg) of CFRP and aluminium holes

when drilling at various cutting speed using uncoated carbide

tool and TiAlN coated carbide tool 66

4.21 Flank wear images of uncoated carbide and TiAlN coated

carbide tool at 50th hole and at various cutting speed 67

4.22 BUE occurrence at hole 25 when drilling CFRP/aluminium

stack at cutting speed of 85 m/min using TiAlN coated

carbide tool 67

xvii

LIST OF SYMBOLS AND ABBREVIATIONS

FRP Fiber Reinforced Plastic

CFRP Carbon Fiber Reinforced Plastic

BUE Built Up Edge

HSS High Speed Steel

TiAlN Titanium Aluminum Nitride

Wc Tungsten

2p Drill Point Angle

Fz Drilling thrust force

Mz Drilling torque

L Lead length of the helix

γ Chisel edge angle

VBB Average width of flank wear

VBBmax. Maximum width of flank wear

f Feed rate

D Diameter

Ra Arithmetical mean surface roughness

S Spindle rotational speed

Cs Cutting speed

xviii

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Graph of hole diameter when drilling CFRP/Aluminum

stack using uncoated carbide tool at various cutting speed 79

Graph of cutting force (Fz) and torque (Mz) when

drilling CFRP/Aluminum stack using uncoated carbide

tool (Vc= 85 m/min, f= 0.1 mm/rev, α= 800) 80

B Graph of hole diameter when drilling CFRP/Aluminum stack

using TiAlN coated carbide tool at various cutting speed 81

Graph of cutting force (Fz) and torque (Mz) when drilling

CFRP/Aluminum stack using TiAlN coated carbide tool

(Vc= 85 m/min, f= 0.1 mm/rev, α= 800) 82

C Example of hole size data from Mitutoyo CMM 83

D Publication 84

1

CHAPTER 1

INTRODUCTION

1.1 Background

Machining is widely used in the manufacturing industries to produce accurate

and precise components. A material is considered machinable when it can be

machined with relative ease while producing the desired range of surface finish. The

characteristics of a good machinability materials are when less power is consumed in

a short machining time, good surface finish and can be produced without causing the

cutting tool to wear in a short period of usage. It is difficult to determine the

machinability of a material since it involved many criteria such as workpiece material,

type of cutting tool used, the cutting parameters, and also machine tool. Among all

the machining processes, drilling is known to be most used machining process which

is usually done at the end of manufacturing stage [1].

Carbon fiber reinforced plastic is (CFRP) a unique material that is highly used

in industries such as aerospace, automotive, marine, electronics, medical, and others.

This is due to the unique characteristics of CFRP especially in the materials structure

that provides high performance properties such as high strength-to-weight and

stiffness-to-weight ratios [2]. Since the mechanical properties of CFRP is superior and

2

having different structure than the majority of conventional material, a different

approach is needed in order to obtain the best results when drilling CFRP. The

uniqueness of CFRP structure is based on its fiber orientation, fiber and matrix

properties, and relative volume of the fiber and matrix [2]. In the CFRP drilling

process, the tool encounters different layer of fiber materials and matrix thus causing

the tool to experience a variety of cutting mechanisms.

Aluminum or aluminum alloys play a significant role in the world of

manufacturing, especially in the aerospace industry. Aluminum alloy 2024 is chosen

in this research as they are widely used as structural applications of aircrafts around

the world [3]. This highly abundant material is favored due to its properties such as

high durability, high strength, and light weight that are highly suitable for aircrafts.

These advantages are utilised in aircraft structural applications such as fuselage, wing

tension members, shear webs and ribs and in vital places that require stiffness,

outstanding fatigue resistance and good strength-to-weight ratio. There are also

substantial applications of aluminum alloy 2024 in commercial and military aircrafts.

These include fuselage skins, wing skins, and engine parts that usually experience

high temperatures.

Combining the advantages of CFRPs high strength-to-weight ratio with

aluminium in the structural applications of aerospace, a hybrid multi-layer material is

produced. This combination allows the properties structural component of an aircraft

to be heightened to a new level. CFRP is usually selected as a substitute for aluminium

in an aircraft, whereby it provides considerable weight reduction to an aircraft while

maintaining its strength comparable to metals [4]. For instance, the new Boeing 787

Dreamliner and Airbus A350 utilised the benefits of CFRP as aircraft parts as it

comprises more than 60% of the weight of the aircraft when empty [5]. When

composites and metals are joined together in a manufacturing line, the processing time

and productivity can be improved. However, in a drilling process, different materials

have different machining characteristics. Both materials have different set of

3

machining parameter settings as they are of different properties. The stacking of CFRP

with aluminium means that a tight control of drilling procedure must be followed to

avoid errors such as hole diameter tolerances and hole location. A number of studies

in drilling multi-layer materials such as CFRP stacked on top of metals are available;

however, they are still limited as compared to studies that focused on drilling on a

single material only. Thus, various characteristics can be explored in drilling multi-

layer materials. Through the findings of this study, it is expected that a further

apprehension of the machinability and the effect of coating performance and drilling

parameters in drilling stack of CFRP/Aluminium 2024.

1.2 Problem Statement

Drilling a stack of different materials such as CFRP and aluminum plates

posesses great challenges to manufacturing industries. Both materials behave

distinctly during drilling process due to the fact that CFRP has low thermal

conductivity and highly abrasive properties. The abrasiveness nature of CFRP, would

accelerate tool wear and shorten tool life. Meanwhile aluminum is a ductile material

which cause built up edge (BUE) on the cutting tool during machining. The formation

of BUE is caused by the adhesion of aluminum to the cutting tool. This BUE is

responsible for accelerating the tool wear leading to premature tool failure, low quality

of hole, poor surface roughness, and high deviation of hole diameter.

Suitable cutting tool selection must be done in order to allow good finishing

for both materials. In addition, cutting conditions must comply with tool and

workpiece as the response differently towards cutting parameters. The general cutting

parameter recommended for drilling CFRP is to employ relatively high speed

combined with low feed rate [6]. This cutting parameter is most likely selected to

minimize or avoid drilling-induced delamination. As such for aluminum alloys, there

4

is a contrasting effect between drilling at high and low speed [7]. Selection of suitable

cutting parameters to create holes with good quality when drilling aluminum alloy is

not easy because of the ductility and the tendency of this material to cause BUE to the

cutting tool thus causing undesireable continuous chips and poor hole quality [7].

BUE usually occurs frequently at low cutting speed thus one of the solution to reduce

or avoid BUE is to apply higher cutting speed. However, increasing the cutting speed,

increases the drilling temperature. Aluminum alloy is much more ductile, harder, and

having more density than CFRP, thus cutting tools that is suitable to drill aluminum

is also suitable for drilling CFRP/aluminum stack.

Fiber-reinforced materials possess better mechanical properties and its

strength-to-weight ratio is highly advantageous compared to most metal alloys. These

advantages lie in the material structure of fiber-reinforced materials. Due to the

material structure of CFRP, as compared to metal alloys [2]. The different behaviour

of CFRP during drilling process causes defects that are dissimilar compared to drilling

of metal. The process is used to produce holes in the composite’s body before joining

with other materials. In drilling process, it is highly desirable to avoid defects. The

defects that were commonly found in CFRP drilling are matrix cracking, fiber pullout,

fiber fracture, fiber debonding, delamination, fuzzing and spalling. Among the

common defects induced by drilling process, delamination is the most undesirable and

most frequently occurs [3,4,5,7-12].

Delamination is of interest when drilling CFRP since it may result in reduction

of material strength and fracture toughness [10] and the fact that 60% of part rejections

during final assembly in aircraft industry consist of delamination rejects due to drilling

[11]. There are various factors in the process parameters that are directly related to the

defects in drilling CFRP. For example, the most common factors identified were the

cutting tool type, cutting tool material, point angle, and helix angle of the cutting tool,

cutting speed, feed rate, and thrust force [3,4,5,7-12]. These factors in turn can be

5

related to the progress of the tool wear which is used to determine the appropriate tool

life of a drilling tool.

1.3 Objectives

The main objective of this research study is to investigate the machinability of

CFRP/Aluminum 2024 stack during drilling process. The specific objectives of this

study are as follows:

i. To evaluate the machining performance of uncoated and TiAlN coated carbide

tools when drilling of CFRP/Aluminum 2024 stack.

ii. To study the effect of cutting speed on thrust force, torque, hole size, delamination,

surface roughness, and tool wear.

1.4 Scope of Study

This study focuses on the drilling of CFRP/Aluminum 2024 stack using

carbide drill bits, which are uncoated carbide, and TiAlN coated carbide. The cutting

conditions include variation in cutting speed of 70. 80, and 100 m/min and constant

feed rate of 0.1 mm/rev. The workpiece, CFRP plate was supplied by CTRM

Malaysia. All experimental and analysis works are conducted at the Production

Laboratory, FKM, UTM using a CNC machining centre various measuring

equipments.

70

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