INVESTIGATING COOLING AND LUBRICATION STRATEGIES FOR SUSTAINABLE MACHINING OF

TITANIUM ALLOYS: IMPACT ON MACHINABILITY AND ENVIRONMENTAL PERFORMANCE

Salman Pervaiz

Licentiate Thesis

School of Industrial Engineering and Management Department of Production Engineering

KTH Royal Institute of Technology

APRIL 2014

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TRITA- IIP-14-02 ISSN 1650-1888 ISBN 978-91-7595-091-4 Akademisk avhandling som med tillstånd av KTH i Stockholm framlägges till offentlig granskning för avläggande av licentiate fredagen den 4 april kl. 10:00 i sal M311, KTH, Brinellvägen 68, Stockholm.

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ABSTRACT The manufacturing sector is one of the most rapidly growing sectors in the industrialized world today. Manufacturing industry is concerned with being more competitive and profitable. Profit margins are directly related to the productivity of the company, and productivity improvements can be achieved by making manufacturing processes more efficient and sustainable. Knowledge of cutting conditions and their impact on machined surface and tool life enable productivity improvement. These days the main emphasis is not only to increase productivity, but also to make processes cleaner and more environmental friendly. This research aims to study machinability of difficult to cut, titanium alloys, in close reference to the application of different cooling/ lubrication strategies and their environmental impact. Total energy consumed (kWh) and carbon dioxide (CO2) emissions produced in machining are common environmental indicators. In this research project environmental implications of the cutting process were calculated in terms of carbon dioxide (CO2) emissions and energy consumption analysis. The experimental work consisted of controlled machining tests with cutting force, surface roughness, power, and flank wear measurements under dry, mist, combination of vegetable oil mist and cooled air (MQL+CA) and flood cutting environments. The current study provides better understanding of the cutting performance of TiAlN coated and uncoated carbide tools. The study also investigated tool failure modes, tool wear mechanisms, surface roughness and energy consumption to improve machinability of Titanium alloys. The study revealed the promising behaviour of minimum quantity lubrication (MQL) under specific ranges of cutting speed for both coated and uncoated tools. Variation in the cutting force showed close link with built up edge (BUE) formation. MQL based systems have huge potential to improve machinability of Titanium alloys and should be investigated further. Keywords: Titanium alloys, Energy consumption, Wear mechanisms

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PREFACE The work reported in this Licentiate thesis was a collaborative project between Production Engineering Department, Royal Institute of Technology and Mechanical Engineering Department, American University of Sharjah. The presented work has been conducted during the years 2011-2013 at the Manufacturing Laboratory, Department of Mechanical Engineering, American University of Sharjah, UAE. The research study was supervised by Professor Cornel Mihai Nicolescu (KTH Royal Institute of Technology) and Dr. Amir Rashid (KTH Royal Institute of Technology) and also co-advised by Dr. Ibrahim Deiab (American University of Sharjah/ University of Guelph). I would like to express my gratitude towards my all three supervisors for their valuable efforts, continuous support and professional mentoring throughout the research work. I would like to acknowledge the financial support of National Research Foundation (NRF) for this project. I would also like to thanks Accu-Svenska AB for supporting the research work by providing MQL booster system. Finally, I would also like to thank my family and friends for their continuous support.

Salman Pervaiz Stockholm, April 2014

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DEDICATION To my beloved wife, children and my parents.

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Table of Contents Copyright Statement I Abstract III Preface IV Dedication V Table of Contents VI Appended Papers X List of Figures XI List of Tables XV List of Abbreviations XVI List of Nomenclature XVII CHAPTER ONE INTRODUCTION

1.1 Evolution of materials used in aerospace engines 1 1.2 Material requirements for aerospace engines and other

applications 3 1.3 Physical properties 5 1.4 Energy consumption and environmental aspects 7

1.4.1 Titanium production 7 1.4.2 Titanium alloys machining 8

1.5 Challenges in the machining of titanium alloys 9 1.6 Research aim and objectives 10 1.7 Organization of thesis 11

CHAPTER TWO LITERATURE REVIEW 2.1 Specific energy and power consumption 13

2.1.1 Energy of chip formation 14 2.1.2 Stress distribution 16 2.1.3 Power consumption in machining operation 18

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2.1.4 Power consumption in turning operation 19 2.2 Machinability of aero-engine alloys 19 2.3 Classification of titanium alloys 20

2.3.1 Machinability of titanium alloys 22 2.4 Cutting tool materials for titanium alloys 25

2.4.1 Tool wear mechanisms and patterns 28 2.5 Surface integrity 33 2.6 Cutting fluid 37 2.7 Environmental friendly cooling strategies 38

2.7.1 Dry cutting 38 2.7.2 Minimum quantity lubrication (MQL) 39 2.7.3 High pressurized cooling (HPC) 40 2.7.4 Cryogenic cooling 41

2.8 Sustainable manufacturing concepts 41 2.8.1 Energy consumption in machining 43 2.8.2 Environmental implication of energy

consumption 47 2.9 Summary of literature review 48 CHAPTER THREE

EXPERIMENTAL METHODS 3.1 Milling experiments 50 3.2 Turning experiments 55

CHAPTER FOUR MACHINABILITY EVALUATION METHODS 4.1 Surface roughness analysis 60 4.2 Cutting force evaluation 62 4.3 Flank wear assessment 63 4.4 Wear mechanism analysis using scanning electron microscopy 64 4.5 Power and energy consumption analysis 65

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4.6 Minimum quantity cooled air lubrication (MQL+CA) system and supporting tool holder 66

CHAPTER FIVE RESULTS AND DISCUSSION 5.1.1 Milling of Aluminum alloy 6061 68 5.1.2 Energy consumption analysis 69 5.1.3 Total machining time 70 5.2.1 Machining of Titanium alloy (Ti6Al4V) 72 5.2.2 Surface roughness analysis 72 5.2.3 Flank wear measurement 74 5.2.4 Wear mechanisms 75 5.3.1 Surface roughness comparison 78 5.3.2 Cutting force comparison 82 5.3.3 Power and energy consumption 84 5.3.4 Tool wear assessment 86 5.3.5 Wear mechanisms in coated and uncoated cutting tools 88

5.4.1 Surface roughness Vs. Energy curves at dry environment 99

5.4.2 Surface roughness Vs. Energy curves at flood environment 101

5.4.3 Observations for similar material removal rate 103 5.4.4 Complimentary results 105

5.4.4.1 Environmental implications of energy 105 consumption 5.4.4.2 Influence of geographical location of 105 CO2 emissions

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5.4.4.3 Estimation of CO2 emissions using energy consumption used in Paper D 106

5.5 Machinability of MQL+CA 5.5.1 Surface roughness analysis 108 5.5.2 Tool wear measurement 108 5.5.3 Cutting temperature analysis 108 CHAPTER SIX

CONCLUSIONS AND FUTURE WORK 6.1 Energy consumption in milling tool path strategies 116 6.2 PVD-TiAlN coated and uncoated carbide tools 116 6.3 Cutting force behaviour 117 6.4 Role of cutting environment 117 6.5 Minimum quantity lubrication and 117 cooled air (MQL+CA) vegetable oil based mist system investigation 6.6 Energy consumption and environmental 118 implications 6.7 Future work 119 REFERENCES 120

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APPENDED PAPERS Paper A: Pervaiz, S., Deiab, I., Rashid, A., and Nicolescu, M., "An

Experimental Analysis of Energy Consumption in Milling Strategies", 2012 IEEE International Conference on Computer Systems and Industrial Informatics – ICCSII’12, Sharjah, UAE, Dec 18-20, 2012.

Paper B: Pervaiz, S., Deiab, I., Darras, B., Rashid, A., and Nicolescu, M., "Performance evaluation of TiAlN- PVD coated inserts for machining Ti-6Al-4V under different cooling strategies”, Advanced Materials Research, Advanced Materials Research Vol. 685 (2013) pp. 68-75.

Also presented in 3rd International Conference on Advanced Materials Research (ICAMR 2013), Dubai, UAE, Jan 19 -20, 2013.

Paper C: Pervaiz, S., Deiab, I., and Darras, B., "Power consumption and tool wear assessment when machining titanium alloys," International Journal of Precision Engineering and Manufacturing, Vol. 14, No. 6, pp. 1-12, 2013.

Paper D: Pervaiz, S., Deiab, I., Rashid, A., Nicolescu, M., and Kishawy,

H., " Energy consumption and surface finish analysis of machining Ti6Al4V”, International Conference on Manufacturing Systems Engineering ICMSE 2013, Venice, Italy, April 14 - 15, 2013.

Paper E: Pervaiz, S., Deiab, I., Rashid, A., Nicolescu, M., and Kishawy, H., “Performance evaluation of different cooling strategies when machining Ti6Al4V,” International Conference on Advanced Manufacturing Engineering and Technologies - NEWTECH 2013, Sweden, October 27-30, 2013.

Words Count: 48,574

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List of Figures Figure 1.1 Materials and operating temperatures in aerospace engines 2 Figure 1.2 Strength vs density materials selection chart 4 Figure 1.3 Strength/ Density ratio versus operating temperature (F°) 6 Figure 1.4 Production technologies from titanium 8 Figure 2.1 Schematic illustration of orthogonal cutting 13 Figure 2.2 Schematic illustration of chip formation by using “Deck of

Cards” approach 14

Figure 2.3 Schematic illustration of forces acting in the cutting zone 15 Figure 2.4 Schematic illustration of flow pattern in chip formation 17 Figure 2.5 Stress distribution on the rake face during cutting 17 Figure 2.6a Phase diagram of Titanium alloys 20 Figure 2.6b Phase diagram Ti-Al alloys 21 Figure 2.7 Sources of heat generation in machining 23 Figure 2.8 Schematic illustration of segmented chip formation 24 Figure 2.9 Machinability ratings of Titanium alloys 24 Figure 2.10 Schematic illustration tool wear patterns 28 Figure 2.11 Abrasive wear observed for different cutting tool materials

when machining titanium alloys 29

Figure 2.12 Adhesive wear observed for different cutting tool materials when machining titanium alloys

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Figure 2.13 Diffusion wear observed for different cutting tool materials when machining titanium alloys

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Figure 2.14 (a) Schematic illustration of the diffusion couple (b) Cross-sectional view of WC/Co carbide tool after exposing to air for 90 min at 800 °C

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Figure 2.15 Chipping/ Flanking observed for different cutting tool materials when machining titanium alloys

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Figure 2.16 (a) Surface tearing (Ti6Al4V), V=100 m/min, f=0.15 mm/ tooth, DoCa = 2.0 mm, DoCr = 8.8 mm [73], (b) White layer in Ti6Al4V machined at V=95 m/ min, f = 0.35 mm/rev, and DoC = 0.10mm

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Figure 2.17 Microhardness behaviour observed in machining FGH 95 (a) Microhardness region (b) Microhardness measurement

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Figure 2.18 Surface roughness behaviour with cutting speed for turning Ti6Al4V (a) ISO-883-MR4 Tool at feed = 0.35 mm/ rev, (b) ISO-890-MR3 Tool at feed = 0.25 mm/ rev

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Figure 2.19 Schematic illustration of MQL 39 Figure 2.20 Through spindle high pressure coolant delivery systems by

Sandvik 40

Figure 2.21 Cumulative growth of federal environmental laws and amendments

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Figure 2.22 Energy consumption breakdown for milling process 44 Figure 2.23 Characterization of power consumption during cutting

processes 45

Figure 2.24 Global CO2 emissions from fossil fuels 47 Figure 3.1 High speed steel (HSS) 02 flute end milling cutter with

code DIN884 51

Figure 3.2 Experimental setup of the experimental setup 53 Figure 4.1 Arithmetical mean of the profile (Ra) as per ISO 4287

standard 60

Figure 4.2 Mitutoyo surface roughness tester SJ 201P 61 Figure 4.3 Cutting force data evaluation system, (a) Kistler 9257b

Dynamometer, and (b) Kistler 5070 charge amplifier 62

Figure 4.4 (a) Mitutoyo tool maker microscope (Model: TM 510), and (b) sample flank wear measurement

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Figure 4.5 (a) Scanning electron microscope (Model: Philips FEI XL 30), Sample SEM images of uncoated carbide tool from paper C, Vc = 30 m/min and f = 0.1 mm/ rev, (b) Dry (c) Mist (d) Flood

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Figure 4.6 (a) PS 3500 power data logger and (b) Sample calculation from paper C for energy consumption calculated for mist condition, vc = 30 m/min, f = 0.1 mm / rev, Depth of cut = 0.8 mm

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Figure 4.7 MQL (vegetable oil based) system with low temperature cool air

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Figure 4.8 (a) Schematic view of Mircona SCLCR 2525 M12-EB tool holder (b) View of actual tool holder used in experimentation

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Figure 5.1 Schematic illustration of milling strategies 69 Figure 5.2 Zigzag, Constant overlap spiral, Parallel spiral and One-

way milling strategies, 4000 rpm 69

Figure 5.3 Zigzag, Constant overlap spiral, Parallel spiral and One-way milling strategies, 2000 rpm

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Figure 5.4 Machining time of milling strategies at 4000 rpm 71 Figure 5.5 Machining time of milling strategies at 2000 rpm 71 Figure 5.6 Total machining length with Zones A, B, C and D 72

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Figure 5.7 Surface roughness trend under various cooling techniques (a) Vc = 30 m/min, f = 0.1 mm/ rev (b) Vc = 90 m/min, f = 0.1 mm/ rev

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Figure 5.8 Surface roughness trend under various cooling techniques (a) Vc = 30 m/min, f = 0.2 mm/ rev (b) Vc = 90 m/min, f = 0.2 mm/ rev

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Figure 5.9 Maximum flank wear measurement 74 Figure 5.10a Wear mechanisms in PVD TiAlN coated inserts 77 Figure 5.10a Wear mechanisms in PVD TiAlN coated inserts 78 Figure 5.11b Wear mechanisms in PVD TiAlN coated inserts 79 Figure 5.11b Wear mechanisms in PVD TiAlN coated inserts 80 Figure 5.12 Surface roughness (Ra) measurements 81 Figure 5.13 Statistical analysis of surface roughness (a) Half-normal

plot (b) Residuals vs. Run 83

Figure 5.14 Cutting force at different cutting speeds under dry, mist and flood cooling strategies (a) Uncoated inserts, f = 0.1 mm/ rev (b) Coated inserts, f = 0.1 mm/ rev (c) Uncoated inserts, f = 0.2 mm/ rev, and (d) Coated inserts, f = 0.2 mm/ rev.

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Figure 5.15 Power and Energy consumption in Dry cutting, Uncoated tool, f = 0.1 mm/min

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Figure 5.16 Specific energy consumption by uncoated and coated inserts under dry, mist and flood conditions, (a) Cutting speed of 30 m/ min, (b) Cutting speed of 60 m/ min, and (c) Cutting speed of 90 m/ min

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Figure 5.17 Flank wear on coated and uncoated tool (a) Dry, (b) Mist and (c) Flood

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Figure 5.18 Wear mechanisms in uncoated tool@ 30m/ min, (a) Dry (b) Mist (c) Flood

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Figure 5.19 Wear mechanisms in coated tool@ 30m/ min, (a) Dry (b) Mist (c) Flood

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Figure 5.20 Wear mechanisms in uncoated tool@ 60m/ min, (a) Dry (b) Mist (c) Flood

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Figure 5.21 Wear mechanisms in coated tool@ 60m/ min, (a) Dry (b) Mist (c) Flood

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Figure 5.22 Wear mechanisms in uncoated tool@ 90m/ min, (a) Dry (b) Mist (c) Flood

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Figure 5.23 Wear mechanisms in coated tool@ 90m/ min, (a) Dry (b) Mist (c) Flood

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Figure 5.24 Energy consumption and surface finish curves for dry cutting at different material removal rates using five feed

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levels f = 0.1 – 0.5 mm/ rev, (a) Vc = 30 m/ min, (b) Vc= 60 m/ min, and (c) Vc= 90 m/ min

Figure 5.25 Energy consumption and surface finish curves for flood cutting at different material removal rates using five feed levels f = 0.1 – 0.5 mm/ rev, (a) Vc = 30 m/ min, (b) Vc= 60 m/ min, and (c) Vc= 90 m/ min

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Figure 5.26 Energy consumption and surface finish at similar material removal rates using different cutting speeds of 30, 60 and 90 m/ min under dry cutting

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Figure 5.27 Energy consumption and surface finish at similar material removal rates using different cutting speeds of 30, 60 and 90 m/ min under flood cutting

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Fig 5.28 Variation in global energy mix 107 Figure 5.29 Equivalent CO2 emissions (g) produced for different

cutting conditions under dry cutting 109

Figure 5.30 Equivalent CO2 emissions (g) produced for different cutting conditions under flood cutting

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Figure 5.31 Surface roughness trends with respect to dry, MQL+CA, and flood cooling strategies, (a) Vc = 90 m/min, (b) Vc = 120 m/min and (c) Vc = 150 m/min

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Figure 5.32 Flank wear for flood, dry and MQL+CA at cutting speed of 90 m/ min, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25 mm/ rev

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Figure 5.33 Flank wear for flood, dry and MQL+CA at cutting speed of 120 m/ min, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25 mm/ rev

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Figure 5.34 Flank wear for flood, dry and MQL+CA at cutting speed of 150 m/ min, (a) feed = 0.15 mm/ rev, (b) feed = 0.20 mm/ rev and (c) feed = 0.25 mm/ rev

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Figure 5.35 Sample measurements of cutting temperature under dry environment at cutting speed of 150 m/ min, (a) feed =

0.15 mm/ rev (b) feed = 0.25 mm/ rev

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Figure 5.36 Cutting temperature under dry, MQL+CA and flood environment

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List of Tables Table 1.1 Commercially available high performance alloys 2 Table 1.2 Physical properties of different engineering alloys 5 Table 2.1 Shear stresses and specific horsepower of different

materials 18

Table 2.2 Softening points of tool materials 25 Table 2.3 Advantages and disadvantages of different environmental

friendly cooling strategies employed in machining titanium alloys

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Table 3.1 Nominal chemical composition of Al 6061 51 Table 3.2 Mechanical properties of Al 6061 at room temperature 51 Table 3.3 Specificans of the DIN 844 HSS C08 cutter 52 Table 3.4 Design of experiments 52 Table 3.5 Cutting parameters 53 Table 3.6 Nominal chemical composition of Ti6Al4V 55 Table 3.7 Mechanical properties of Ti-6Al-4V at room temperature 55 Table 3.8 Specificans of the turning cutting insert 56 Table 3.9 Cutting parameters and experimental set up in turing

experiments 57

Table 4.1 Specifications of Mitutoyo surface roughness tester 61 Table 4.2 Specifications of Kistler dynamometer 62 Table 4.3 Specifications of Mitutoyo tool maker microscope 63 Table 4.4 Specifications of power logger 66 Table 4.5 Properties of vegetable oil used in mist (ECULUBRIC

E200L) 66

Table 5.1 Results of ANOVA for surface roughness 80 Table 5.2 Lifecycle estimates of gCO2e/ kWh for electricity

generation procedures 108

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List of Abbreviations ANOVA Analysis of variance BUE Built up edge CCNG Compressed cold nitrogen gas CCNGOM Compressed cold nitrogen gas and oil mist CCS Carbon capture and storage CNC Computer numerical control CVD Chemical vapour deposition COF Coefficient of friction GDP Gross domestic product GHG Greenhouse gas emissions HPC High pressurized cooling HSS High speed steel LCA Life cycle assessment MRR Material removal rate MWF Metal working fluids MQL Minimum quantity lubrication PCD Polycrystalline diamond PCBN Polycrystalline cubic boron nitride PVD Physical vapour deposition SEM Scanning electron microscope TiC Titanium carbide WC MQL+CA

Tungsten carbide Minimum quantity lubrication and cooled air

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List of Nomenclature Symbol Parameter Units

α Rake angle degrees β Friction angle degrees Fc Cutting force N Ft Thrust forces N Fs Shear force parallel to shear plane N Fn Normal force perpendicular to shear plane N F Force at tool-chip interface N N Normal force at tool-chip interface N Ø Shear angle degrees i Inclination angle degrees VB Width of flank wear land mm Ra Surface roughness μm Vc Cutting speed m/ min DOC Depth of cut mm f Feed mm/ rev fr Feed rate mm/ min

CHAPTER 1

INTRODUCTION

This chapter describes the background and aim of the research. Organization of the thesis is also elaborated in this chapter

1.1 EVOLUTION OF MATERIALS USED IN AEROSPACE ENGINES

In aerospace industry, titanium and nickel based alloys are preferred over conventional steels and aluminium alloys due to their high strength to weight ratio, fracture toughness, fatigue strength, superior corrosion resistance and ability to operate at higher temperature. The strong point of titanium alloys is that they show higher strength than aluminium alloys and less density than steels, making it suitable for structural applications [1]. Due to these extraordinary properties, titanium and nickel based alloys are used extensively in other demanding sectors like automotive, petrochemical, marine, military, biomedical and nuclear [2, 3].

As titanium and nickel based alloys are well suited to meet the demands of aerospace industry, it is the largest consumer for titanium alloy components. In aerospace industry titanium alloys are used in the construction of air frames, fastening applications, landing gears, jet engine shafts and casings for the front engine fan [4]. An aircraft engine generally consists of three main subassemblies namely compressor, combustor and turbine housed casing. Figure 1.1 shows the cross sectional view of an engine that shows the usage of nickel and titanium alloys in aerospace sector for engine components. Besides aerospace applications, titanium alloys are highly preferred as a composite interface material for advanced aircraft designs [5]. The demand rate of titanium products is increasing rapidly as aerospace industry demands for fuel efficient and light weight aircraft designs. In order to estimate that how extensive titanium alloys are

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being used in an aerospace sector the application areas are presented in Table 1.1.

Figure 1.1: Materials and operating temperatures in aerospace engines [6]

Table 1.1: Commercially available high performance alloys [7, 8, and 9]

Material Alloys Applications

α-alloys/ near α-alloys

T-0.3Mo-0.8Ni Ti-3Al-2.5V Ti-3Al-2.5V-Pd Ti-3Al-2.5V-Ru Ti-5Al-2.5Sn

Chemical processing, desalination, hydrometallurgical extraction Aircraft ducting, tubing, watches, eye glass frames Offshore hydrocarbon production Offshore hydrocarbon production Gas turbine engine parts

α-β alloys Ti-6Al-4V Ti-6Al-7Nb Ti-6Al-6V-2Sn Ti-6Al-2Sn-4Zr-6Mo Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.15Si

Aircraft ducting, Airframe parts, Automotive parts, Consumer products (watches, eye glass frames, etc) Medical implants, surgical instruments Airframe parts Gas turbine engine parts Airframe parts, Space vehicles/ structures

β alloys/ near β- alloys

Ti-10V-2Fe-3Al Ti-3Al-8V-6Cr-4Zr-4Mo Ti-3Al-8V-6Cr-4Zr-4Mo-0.05Pd

Airframe parts, Landing gear parts Geothermal brine energy extraction, navy ship parts, space vehicles Navy ship parts, space vehicles

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1.2 MATERIAL SELECTION REQUIREMENTS FOR AEROSPACE INDUSTRY

In aerospace sector, material selection process is based on high performance and extraordinary thermo-mechanical properties of the engineering materials. There are some driving key factors in the aerospace industry which highly influence the material selection process [10]. These key driving factors and their associated material responses are listed as under; • High engine efficiency, High efficiency in aerospace engines is

generally attained by achieving high pressure ratio in the compressor and high temperature rise in the combustor. High engine efficiency also depends on the 3D aerofoil geometry as well. In financial perspective increasing efficiency results in reduction in the fuel cost.

Material response, In order to achieve high efficiency in the aerospace engine, materials used in the compressor and combustor should be capable of sustaining high operating temperatures. Titanium and nickel based alloys are capable to operate at high temperatures.

• High thrust to weight ratio, Thrust to weight ratio is one of widely

adopted parameters used to evaluate the performance of an aircraft engine. The capability of an engine to generate thrust depends on the operating temperature. If material can operate can operate at high temperature. High thrust to weight combines the requirement of high operating temperature with mass reduction. Generally engineering materials with high strength to weight ratio enhances the performance and reliability of the components.

Material response, Reduction in the structural mass results in an improved aircraft performance by improving the response time, better speed of climb, efficient fuel consumption and survivability. This requirement can be fulfilled by selecting light weight engineering materials with high strength. Titanium and nickel based alloys offer high strength to weight ratio which makes them compatible for aerospace industry. Figure 1.2 shows the plot of all engineering materials on strength (σ) – density (ρ) selection chart. Titanium alloys show higher strength than aluminum alloys and less density than steel, making it suitable for structural applications.

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Fig 1.2: Strength vs density materials selection chart (Adapted from M. F.

Ashby, Materials Selection in Mechanical Design [11]) • Challenging loading conditions, Material selection requirements vary

throughout the aerospace engine. The part of the compressor present at low temperature experiences corrosion, erosion, impact and fatigue as dominant loading conditions. Generally, turbine blades operate at high temperature as a result material experiences creep and corrosion as dominant loads. Whereas at the same time turbine disc operates at low temperature and experiences higher cyclic loads resulting in fatigue load as more critical condition.

Material response, In order to meet the challenges for specific operating conditions, titanium and nickel based superalloys are used

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due to their superior performance characteristics. For high operating temperature derivatives of titanium and nickel based superalloys with tough engineering ceramics are being explored. However to reduce structural mass, derivatives of superalloys, metal composites and ceramics are being investigated.

1.3 PHYSICAL/ MECHANICAL PROPERTIES

Titanium alloys are useful structural material because of their high strength to weight ratio, fracture toughness, fatigue strength, superior corrosion resistance and ability to operate at higher temperatures in the range between 550 – 700 C°. In the Table 1.2 a brief comparison of a titanium alloy (Ti6Al4V) has been presented with conventional steels and aluminium alloy. Table 1.2: Physical properties of different engineering alloys [12]

Properties Ti6Al4V Stainless Steel Carbon

Steel Al 6061

Melting point (K) 1813-1923 1673-1703 1500 855- 925

Density (x103 kg/m3) 4.42 8.02 7.83 2.7 Young’s Modulus (GPa) 113.19 198.94 205.8 68.9

Poisson ratio 0.3-0.33 0.3 0.3 0.33 Fracture toughness (MPa – m1/2) 75 - 50 29

Specific Heat (kJ/(Kg. K)) 0.56 0.5 0.46 0.89 Thermal Conductivity (W/ (m.K))

7.54 16.34 53.5 167

First titanium alloy (Ti6Al4V) was developed in United States in 1954. It is the most commonly used alloy of titanium that is widely utilized in industry due to the better heat resistance, corrosion and erosion resistance, formability, weldability and biocompatibility etc. Consumption of Ti6Al4V is almost 75 – 80% among all different derivatives of titanium alloys.

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Fig. 1.3: Strength/ Density ratio versus operating temperature (F°)

Figure 1.3 represents the plot of strength to weight ratio with respect to the operating temperature range for engineering alloys. It can be observed that titanium alloys offer very impressive strength to weight ratio even at higher temperature making them suitable for the challenging engineering sectors.

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1.4 ENERGY CONSUMPTION AND ENVIRONMENTAL ASPECTS

In the current form, cost involved in the refining, processing and production of titanium is very high as compared to the other metals. It has been observed that cost involved in the phase from melting till refining is five times higher aluminium. If cost is considered for titanium shaped into ingots and finished part, then it has been found ten times higher than the aluminium. The demand rate of titanium products is increasing rapidly as aerospace industry demands for fuel efficient and light weight aircraft designs. In 2009 approximately 21,000 tons of titanium was used in automotive, chemical processing, metallurgical plants, sports, marine, medical and architectural sectors. A careful industrial forecast represents approximately 40 % increase by 2015 [13]. From energy consumption and environmental burden improvement perspectives, in parallel with the optimization of titanium alloys machining, it is important to significantly improve the production process of titanium alloys. 1.4.1 Titanium Production Titanium is manufactured by performing several phases including the extraction, refining, processing and production. In the first phase titanium sponge is extracted from the titanium ore. In the next phase titanium sponge is melted and cast into the ingots shape. Then rolling and forging operations are used to shape ingot into the desired form of plate, billet and rod. At the final stage titanium parts are finished by forging, extrusion, hot and cold forming, machining, and casting processes etc. Figure 1.4 represents the stages and processes involved in the production of titanium. The solid line shows already established technology, whereas dotted line shows that there is potential to improve the process. Titanium parts can be manufactured using titanium in powder form by powder metallurgy based techniques. As shown in the Figure 1.4 that powder can be processed into the final shapes using metal injection molding (MIM), direct powder rolling (DPR) and hot Isostatic pressing (HIP) processes [13].

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Fig. 1.4: Production technologies from titanium (after Chunxiang et al.

[13])

In the conventional titanium production process, melting of titanium sponge is required to cast it into the ingots form. Generally this melting of titanium is performed using electron beam melting (EBM), vacuum arc remelting (VAR) or plasma arc melting (PAM) process. These melting processes consume very large amount of energy and resources. There is a need to develop more energy efficient technologies for the production of titanium alloys. It will reduce the energy consumption and associated environmental burden. 1.4.2 Titanium Machining United Nations world commission on environment and development has defined sustainability as the capability to encounter the need of the present without compromising the capability of future generations to meet their own needs [14]. United States, Department of Commerce has defined sustainable manufacturing as “The creation of manufactured products through economically sound processes that minimize negative environmental impacts while conserving energy and natural resources. Sustainable manufacturing also enhances employee, community, and product safety” [15].

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These days metal cutting sector is under immense pressure to improve environmental performance due to the implementation of strict international environmental regulations. By adopting the sustainable practices in metal cutting sector, environmental performance can be improved significantly under economic conditions. The idea of sustainable manufacturing deals with effective use of material flow, energy, knowledge, health safety, and environmental concerns. In the manufacturing sector, sustainable practices can be employed by minimizing resources (energy, material, and water), improving environmental concerns by reducing the use of toxic and non-biodegradable chemicals, efficiently designing life cycles, and improving working conditions (such as ergonomics and health safety) [16].

The sustainable manufacturing concept aims to reduce the amount of greenhouse gas (GHG) emissions and the ecological footprint. In order to promote sustainable practices in the metal cutting sector it is also important to consider the surface integrity of the machined part to avoid rework. As most of the cutting fluids are environmental hazard in nature. It is also important to limit the usage of cutting fluids. Near dry and minimum quantity lubrication (MQL) techniques are being explored to replace conventional flood cooling method to reduce environmental burden. Machining is the most commonly used operation in the manufacturing sector. In order to reduce greenhouse gas (GHG) emissions and the ecological footprint in machining, processing time and energy consumption also plays an important role. In order to conduct a desired machining operation on a certain machine tool, electrical energy is drawn from an electrical grid system. Electrical energy is generated by using different energy sources such as coal, fossil fuels, and hydraulic, nuclear, solar and wind energy. Each source produces different amounts of greenhouse gas (GHG) emissions, but renewable energy resources (such solar, wind, geothermal and tidal) generate significantly less greenhouse gas (GHG) emissions. Greenhouse gas (GHG) emissions in machining can be reduced by utilizing electricity from renewable sources and by minimizing energy consumption during the machining phase.

1.5 CHALLENGES IN THE MACHINING OF TITANIUM ALLOYS

Despite the increased demand of these alloys in engineering sector, there are difficulties present in the primary and secondary processes [17]. As these alloys maintain high strength and hardness at elevated temperatures, machining is difficult to perform. Properties like low thermal conductivity,

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high strain hardening, high hardness at elevated temperature and high chemical reactivity are responsible of poor machinability rating of high performance alloys. Challenges faced during machining of titanium alloys are mentioned below;

• Titanium has low thermal conductivity of 4 – 16 W m-1 K-1 and high specific heat capacity of 520 J kg-1 K-1. For comparison purpose, thermal conductivity of structural steel is 450 W/ m*K much higher than titanium. Combination of high cutting temperature, high heat capacity and low thermal conductivity of titanium results in poor heat dissipation during the cutting process. As most of the heat stays at the cutting edge because of low thermal conductivity it produces high thermal stresses at the cutting edge resulting in poor tool life.

• Titanium alloys maintain strength at elevated temperatures that makes plastic deformation very difficult during machining phase.

• Titanium alloys have high yield point and low plasticity which gives it good elastic properties. During machining phase cutting tool bounces back like a spring because of high elasticity of titanium resulting in chatter [2].

• Less contact area on rake face between tool and workpiece material causes high magnitude of stresses at cutting edge [2].

• Formation of segmented chips form pulsating load at cutting edge [34,35, 39].

• Due to high chemical reactivity of titanium alloys chips tend to weld at tool tip and cutting edge which results in catastrophic tool failure and severe edge chipping [2].

1.6 RESEARCH AIMS AND OBJECTIVES

Motivation of the research work has been described as under;

These days metal cutting sector is under immense pressure to improve environmental performance due to the implementation of strict international environmental regulations. In the metal cutting sector, utilization of cutting fluids is being questioned due to their negative impact on the environment. Majority of the commonly used cutting fluids are toxic and non-biodegradable in nature. The main drawback of using these cutting fluids is the waste disposal after being used. At the same time energy consumed in

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the machining process has strong connection with the amount of greenhouse gas (GHG) emissions.

The aims of this research are listed as under;

• The main objective of this research is to study the feasibility aspects of shifting from conventional (dry and flood) to the environmental friendly cooling strategies.

• Cutting fluid application has direct influence on the machining performance of the cutting process. The process of moving from one cooling strategy to other was examined by analysing the machining performance (tool life assessment, wear mechanisms, and machined surface quality) and the energy requirements (power and energy).

• The current study investigates energy consumption and its associated environmental implications by considering carbon dioxide CO2 emissions.

1.7 ORGANIZATION OF THE THESIS

The thesis is based on five research papers enclosed in six chapters. Paper A is based on the preliminary experimentation to investigate the behaviour of energy consumption under milling operation using Al-6061 alloy as workpiece material. In paper B, machinability of Titanium alloy (Ti-6Al-4V) is investigated using TiAlN-PVD coated carbides. The study used limited cutting parameters and surface roughness and flank tool wear were used as machinability evaluation criteria. Paper C, further extends the machinability evaluation of Ti6Al4V using uncoated and TiAlN coated carbide tools. The study utilized comprehensive cutting parameters and different cooling strategies during investigation. The machinability was evaluated using surface roughness, tool wear, cutting force and energy consumption criteria. The study also discussed the detected wear mechanisms in detail. In paper D, energy consumption and surface roughness plots are created against material removal rate (MRR). These curves will be helpful in order to optimize the energy consumption and surface roughness at desired material removal rate (MRR). In Paper E, machining performance of vegetable oil based MQL system was investigated experimentally.

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The thesis consists of six chapters:

• Chapter one provides a brief introduction about the topic and describes the aims and objectives of the study.

• Chapter two provides literature review on the machinability of

titanium alloys and sustainability concepts in machining.

• Chapter three describes the methodology and experimental setup adopted to perform the conducted research.

• Chapter four demonstrate the machinability evaluation criteria used

this research.

• Chapter five summarises the important findings and results obtained from all four appended papers.

• Chapter six concludes the research work and recommends

proposals for future work.

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CHAPTER 2

LITERATURE REVIEW

This chapter presents a brief review of the available literature.

2.1 SPECIFIC ENERGY AND POWER CONSUMPTION

In this section, a brief review of the theoretical concepts of metal cutting and specific energy and power consumption is introduced. In the material removal processes, material is removed using shearing operation in the form of small chips. Orthogonal machining process is the basic fundamental model used to understand the machining operation. Figure 2.1 represents the schematic illustration of orthogonal cutting. In orthogonal cutting orientation, the cutting edge position is perpendicular to the direction of cutting speed.

Figure 2.1: Schematic illustration of orthogonal cutting [18]

Metal cutting process is highly complex nonlinear and combined thermo-mechanical operations. The major complications are because of high strain

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and high strain rate in the primary shear zone and high friction at tool- chip interface along secondary shear zone. Chip formation is also very complex in nature because it involves huge plastic deformation [19]. Piispanen [20] illustrated the chip formation process at low and high cutting speed with respect to the shear angle approach. The chip formation process was explained with the help of deck of cards placed at shear angle (Ø) as shown in Figure 2.3. Each parallelogram shaped card represents a chip segment. As the cutting tool moves with certain velocity, each card slides on the neighbouring card representing chip flow action. This approach assumes friction at tool face during the cutting process as elastic instead of plastic. The approach also assumes no built-up-edge (BUE) and chip curl.

Figure 2.2: Schematic illustration of chip formation by using “Deck of

Cards” approach [18, 20]

2.1.1 Energy of Chip Formation

During the machining process, total energy consumed per unit time (power) can be computed by taking the product of primary cutting force component (Fc) and the cutting velocity (Vc). As cutting process is very complex in nature and energy consumed in cutting depends on many cutting parameters. In order to normalize the energy consumption it is generally divided by the material removal rate (MRR). Material removal rate can be computed by multiplying the area being cut with the velocity perpendicular to that area. Area being cut can be calculated by taking the product of uncut chip thickness (t) and width of the sample being cut (w) as shown in Figure 2.3. Thus, the energy per unit time, or specific energy, u, can be calculated as shown in Eq. 2.1:

u = 𝐹𝑐𝑉𝑡 𝑤 𝑉𝑐

= 𝐹𝑐𝑡 𝑤

(2.1) [21]

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Figure 2.3: Schematic illustration of forces acting in the cutting zone [21]

Specific energy consumed during a machining process can be divided into four different portions. These four different portions will be discussed in detail.

• Shear energy per unit volume (us), In order to estimate this component of energy consumption, energy involved in the shearing of material is divided by material removal rate as shown in Eq. 2.2. Where Fs and Vs are shear force and shear velocity respectively.

us = 𝐹𝑠 𝑉𝑠𝑡 𝑤 𝑉𝑐

(2.2) [21]

• Friction energy per unit volume (uf), it is the energy consumed for sliding action of chip on the rake face. It is computed by taking into consideration the sliding velocity of chip (V chip) over the rake face. Eq. 2.3 shows the formula for friction energy calculation.

uf = 𝐹 𝑉𝑐ℎ𝑖𝑝𝑡 𝑤 𝑉𝑐

(2.3) [21]

• Kinetic (momentum) energy per unit volume (um), it is the energy required to accelerate the chip. Generally it is neglected as it very less as compared to over portions of energy. But in case of high speed machining, it is important to take into account this energy as well. Momentum force is represented as Fm. Eq. 2.4 shows the formula for kinetic energy calculation.

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um = 𝐹𝑚 𝑉𝑠𝑡 𝑤 𝑉𝑐

(2.4) [21]

Fm can be computed as Fm = ρV2twγ. Where ρ is density and γ is shear strain.

• Surface energy per unit volume (ua), it is the energy utilized to create a new uncut surface during machining operation. This component can be computed by using surface energy of the material being cut (T) as shown in Eq. 2.5.

ua = 𝑇. 2 𝑉𝑤𝑡 𝑤 𝑉𝑐

= 2𝑇𝑡

(2.5) [21]

2.1.2 Stress Distribution In case of metal cutting operation, high normal and shear stresses are formed in the primary and secondary shear zones. These normal and shear stresses are formed due to presence of high plastic deformation in the primary shear zone and friction in the secondary shear zone. For the primary shear zone, it is assumed that both normal and shear stresses are distributed uniformly over the shear plane. Area at shear plane (As) can be calculated by taking cutting area (t.w) and shear angle (Ø) into account.

As = 𝑡 . 𝑤𝑆𝑖𝑛Ø

(2.6) [21] Taking this shear area into consideration normal (σ) and shear (τ) stresses can be calculated by the formulas mentioned below;

τs = 𝐹𝑠𝐴𝑠

(2.7a) [21] & σs = 𝐹𝑛𝐴𝑠

(2.7b) [21] There are several theories about the stress distribution on the rake face. The classical approach assumes that stress distribution obeys coulombs sliding law and is uniformly distributed. In this case coefficient of sliding friction (µ) is the ratio between friction (F) and normal (N) forces.

µ = 𝐹𝑁

(2.8) [21] However, a large number of experimental evidences are there to negate that stresses are uniformly distributed over the rake face. These experiments

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were conducted using scanning electron microscope, photoelastic tool and quick stop arrangements. These experimental studies revealed that there are different chip flow patterns available in the chip formation. The flow pattern near the tool tip was seized as shown in Figure 2.4. However, on the rest of the tool face the flow patterns shows sliding contact. Due to the seizure at the tool tip, material close to the tool surface is stationary which facilitates relative shearing. Generally seizure is called as sticking region.

Figure 2.4: Schematic illustration of flow pattern in chip formation [21] The actual normal and shear stress distributions were represented by several researchers. The non-linear behaviour of stress distribution by developed by Zorev [22] as shown in Figure 2.5.

Figure 2.5: Stress distribution on the rake face during cutting [22]

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2.1.3 Power Consumption in Machining Operations

Generally machine tools are rated in terms of power. Specific power (Ps) can be computed by dividing the input power (Fc.Vc) with material removal rate (MRR) 1. Specific power provides a measure of difficulty involved while machine a certain material. Total power in cutting process can be computed by the product of specific power with material removal rate. Specific horse power for different materials has been presented in the Table 2.1.

P = Ps · MRR (2.9) [21] Table 2.1: Shear stresses and specific horsepower of different materials [21]

Material Shear stress (psi)

Specific horsepower

(hp/in3./min)

Hardness (HB)

Magnesium

28000

0.17

…

1100 aluminium alloy 16700 … … 6061-T4aluminium alloy 35722 0.35 … 2024-T4aluminuim alloy 50000 0.46 … Copper 44850 0.78 … 60-40 Brass 47000 … … 65-35 Brass 50000 … … 70-30 Brass 56940 0.59 … AISI 1020Steel 61500 0.58 150-175 AISI 1112 Steel 63500 0.5 150-175 Type 304 Stainless steel 105000 1.1 – 1.9 … Titanium 173500 1.9 …

Here it is also important to incorporate the efficiency (ɳ) of machine tool into the consideration. Due to the presence of friction and wear in between different parts of the machine tool there are different losses present. The gross power (Pg) can be represented as under;

1 Nomenclature used in equations was adopted from ASM International: Machining handbook, Volume 16.

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Pg = 𝑃ɳ

(2.10) [21]

2.1.4 Power Consumption in Turning Operation

In order to estimate the power consumption in a turning operation material removal rate (MRR) should be defined with respect to the turning operation for basic power consumption equation (P = Ps · MRR). Cutting speeds are generally selected from literature or metal cutting handbooks for specific workpiece materials. The cutting speed (Vc) is then used to compute spindle rpm (N) utilizing the workpiece diameter (D) as shown in the equation mentioned below;

Vc = πDN (2.11) [21] Similarly the amount of travel for a cutting tool is known as feed and generally represented for one revolution of the workpiece. Feed is recommended by the manufacturer with respect to the tool and workpiece materials.

2.2 MACHINABILITY OF AEROSPACE MATERIALS Machinability is the ease with which a material can be machined [21]. Assessment of machinability can be based on several parameters like tool life/ tool wear, cutting conditions, workpiece/ tool material, power and thrust forces generated during machining, chip formation, cutting temperature, etc. For practical considerations, machinability is judged by using tool life/ tool wear, cutting forces and surface roughness based criteria [23]. However, a material with superior machinability rating in one criterion can show unacceptable rating when observed from other criterion. Aero engine components are generally manufactured by titanium and nickel based alloys. These materials offer high strength to weight ratio, high chemical wear resistance and high hot hardness value. However they offer extreme difficulty while machining. Due to the level of difficulty involved in the machining of these materials, they are termed as difficult-to-cut materials. Difficulties in the machinability of aerospace alloys can be traced backed to the below listed causes;

• Low heat dissipation by the chips and workpiece material produces high thermal stresses at the cutting edge. When machining aerospace alloys, cutting temperature is approximately twice higher than the value observed in machining of conventional steel.

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• Another important consideration is that aerospace alloys specifically titanium alloys are extremely chemically reactive which facilitates adhesion mechanism and built-up-edge formation. Due to this property titanium tends to weld at the cutting edge resulting in poor tool life.

• High cutting pressure loads are formed at the cutting edge of the tool as reduced contact surface area is involved in the cutting process.

• Segmented chip formation during the cutting of aerospace alloys results in cyclic cutting forces. This pulsating nature of cutting load results in vibrations and self-induced chatter.

• Titanium alloys have low modulus of elasticity and high yield stress value. This elastic behaviour supports the spring back action during the cutting process and excites chatter.

2.3 CLASSIFICATION OF TITANIUM ALLOYS Titanium alloys are normally categorized into four main types which are α-alloys, near α-alloys, α-β alloys and β alloys [24]. At ambient temperature titanium contains closed pack hexagonal microstructure that is known as α-phase. The alloy elements that increase transformation temperature are known as α- stabilizers. These elements are aluminum, oxygen, nitrogen and carbon. Figure 2.6a is schematic illustration of two phase diagrams with an α- and a β stabilizing element respectively.

Fig 2.6a: Phase diagram of Titanium alloys [25]

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In general α-alloys are hard and exhibit high hardening affinity and creep resistance. Increase in oxygen and nitrogen content results in higher strength. However decrease in oxygen, aluminum and nitrogen results in better ductility and fracture toughness. These alloys are utilized in cryogenic applications. Near α-alloys act more like α-alloys but they contain small quantities of β-phase as well. Typical examples of near α-alloys are Ti8Al1Mo1V and Ti6Al5Zr0.5Mo0.25Si. Titanium α and near α-alloys are used in manufacturing of steam turbine blades and autoclaves. In α-β alloys both stabilizers are present in high proportions. In industry they are used for high strength applications with temperature range 350 – 400 Co. Most popular alloys in this category are Ti6Al4V and Ti4Al2Sn4Mo0.5Si. Alloying elements such as molybdenum, silicon and vanadium are known as β – stabilizers. The β alloys are denser in nature and offer high strength at low operating temperatures. Aerospace industry is exploring these alloys for structural applications. Table 1.1, in previous chapter 1, shows commercially available titanium based alloys being widely used in industry.

Fig 2.6b: The Ti - Al phase diagram [127]

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The most intensively investigated titanium phase diagram is the Ti-Al system. Besides α and β phases of traditional titanium alloys, there are several intermetallic phases present such as α 2-Ti3Al, γ-TiAl, TiAl2 and TiAl3. Out of these titanium aluminides, α 2-Ti3Al and γ-TiAl are widely used in high temperature applications. However, TiAl2 and TiAl3 are very brittle in nature [127].

2.3.1 Machinability of Titanium alloys Titanium alloys are normally categorized into four main types which are α-alloys, near α-alloys, α-β alloys and β alloys [24]. Titanium and its alloys are difficult to cut materials, especially β - alloys are famous for their complexities during machining. Poor machinability of titanium alloys has been reported due to their inherent properties such as high chemical reactivity and low thermal conductivity result in poor machinability. Presence of high temperature in cutting zone results in poor tool life and accelerated abrasion, adhesion and diffusion wear mechanisms. Trent and Wright [26] revealed that 99% of the work is converted into heat that caused high temperature in cutting tool and workpiece surface. Temperature on tool face changes with change in cutting speed and exposure time [27]. Longer time duration and higher cutting speed results in higher cutting temperature in the cutting zone [28]. Abele and Frohlich [30] reports that titanium has low thermal conductivity of 4 – 16 W m-1 K-1 and high specific heat capacity of 520 J kg-1 K-1. Combination of high cutting temperature, high heat capacity and low thermal conductivity of titanium results in poor heat dissipation during the cutting process. As most of the heat stays at the cutting edge because of low thermal conductivity it produces high thermal stresses at the cutting edge. Combination of high thermal stresses and chemical affinity of titanium alloys facilitate tool failure through diffusion and adhesion wear mechanisms. It has been reported [31] that approximately 80% of the heat generated during machining of Ti6Al4V is transferred to the cutting tool due to low thermal conductivity of workpiece and fast flowing chip removal cannot take heat away from the cutting zone.

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Figure 2.7: Sources of heat generation in machining [29]

Aerospace alloys, especially titanium alloys have very high chemical reactivity [25]. In general all tool materials react with titanium alloys at elevated temperatures. Due to high chemical reactivity of titanium alloys chips tend to weld at tool tip and cutting edge which results in catastrophic tool failure and severe edge chipping. High tendency of built up edge (BUE) formation is present due to high chemical reactivity of these alloys. Aerospace alloys maintain strength at elevated temperatures that makes plastic deformation very difficult during machining phase. Adequate plastic deformation is required to facilitate the chip formation mechanism. For any specific material, selection of cutting speed decides if the chip will be continuous, discontinuous or segmented. For all conventional materials chip morphology changes from continuous to discontinuous as cutting speed and feed rates increases. It has been reported that for titanium alloys segmented chips are formed at all levels of cutting speed. In segmented chip formation, material deforms plastically ahead of the tool. Fig. 2.8 shows schematic illustration of segmented chip formation. Fracture occurs in the form of shear band when certain strain level is reached in the cutting process. The chips formed under these conditions are segment like in shape [32]. The phenomenon of cyclic chip formation generates variable forces during machining phase [33]. It is reported in literature [34, 35] that cyclic nature of cutting forces are generated due to serrated chip formation and low Young modulus which results in excessive chatter on cutting tool.

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Figure 2.8: Schematic illustration of segmented chip formation [34]

The American Iron and Steel Institute (AISI) machined many alloys and compared the normal cutting speed, tool life and surface finish to the one attained when machining B1112. Materials with rating above 1.00 were easier to machine than B1112. Likewise, materials with ratings less than 1.00 were difficult to machine. For example, Inconel is an alloy that is very difficult to machine and it has a rating of 0.09.

Figure 2.9: Machinability ratings of Titanium alloys [36]

If the criterion of machinability is cutting force and energy consumption, α titanium alloys have comparatively less tensile strengths and generate comparatively lower cutting forces in contrast to that produced during machining of α−β alloys, β and near β alloys. Hence machinability of α−β alloys, β and near β alloys is lower than α titanium alloys.

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2.4 CUTTING TOOL MATERIALS FOR TITANIUM ALLOYS Low machinability rating of titanium alloys is responsible of creating high thermal stresses and pulsating loads at the cutting edge. Combination of high stress level at cutting edge and high chemical reactivity of titanium starts tool wear at accelerated rate. Abele and Frohlich [30] revealed that high cutting temperature was obtained at higher cutting speeds. Titanium has low thermal conductivity that results in poor heat dissipation from cutting zone. In order to perform machining at high cutting speeds cutting tool material show exhibit excellent hot hardness to with stand elevated temperatures during cutting phase. Table 2.2 shows the softening point1 temperature for all known cutting tool materials. Table 2.2: Softening points of tool materials [2]

Tool Materials

High Speed Steel

Cemented Carbides (WC/Co)

Aluminum oxide

(Al2O3)

Cubic boron nitride (CBN)

Diamond Based Tools

Softening Point

Temperature (C°)

600 1100 1400 1500 1500

Cemented carbide, Komduri and Reed [37] suggested that titanium alloys are difficult to machine above cutting speed of 60 m/ min using cemented carbide tools because of high temperature and chemical affinity. The study observed segmented chip formation and higher resulting stresses at apex of tool in machining titanium alloys. Combination of higher stresses and chemical reactivity of titanium results in rapid adhesion of workpiece and erosion of tool material. Zhang et al. [28] conducted an experimental study to evaluate diffusion wear in high speed machining of Ti6Al4V using micro-grain straight cemented carbides. Tool -chip interface was analysed in this study. The EDX results revealed that tool particles (WC and Co) were diffused in Ti6Al4V chips. Diffusion of Cobalt into tool results in pulling out of WC from tool that helps in crater wear mechanism. Jiang and Shivpuri [39] developed a wear model based on diffusion rate to address crater wear in WC/ Co tool. They validate the numeral model with published experimental data. Jawaid et al. [40] conducted experiment on Ti

1 The softening point is the temperature at which a material softens beyond some arbitrary softness. It shows the thermal softening tendency of cutting tool materials.

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– 6% Al 2% Sn 4% Zr 6% Mo using two grades of fine grain size straight1 tungsten carbide tools. Experimentation was performed using three levels of cutting speed (60, 75 and 100 m/ min) and two levels of feed (0.25 and 0.35 mm/ rev). The study revealed that fine grain size carbide grade has longer tool life. Chipping was the main reason of tool failure at flank face. Coated cemented carbides, Coatings are performed to improve machining performance of carbide tools. Hardness, thermal and chemical properties of coating increases resistance to abrasion, adhesion, diffusion and oxidation wear mechanisms. Jawaid et al. [41] performed face milling experiments on Ti6Al4V using PVD – TiN and CVD - TiCN/ Al2O3 coated inserts. The study pointed out that CVD tools outperformed PVD tools. Wear starts on flank and rake face by coating delamination and then extends into attrition and diffusion wear. Wang and Ezugwu [42] executed machining tests on Ti6Al4V using TiN and TiN/ TiCN/ TiN - PVD coated tools. The results revealed that flank wear, chipping and flaking on rake face and nose were the major failure modes in PVD coated inserts. The study pointed out that TiN single coated inserts out performed TiN/ TiCN/ TiN multilayer coated tools at higher feed rate. Rahim and Sasahara [43] conducted drilling test on Ti6Al4V using TiAlN coated drills under palm oil (PO) and synthetic ester (SE) based minimum quantity lubrication (MQL) system. The study revealed that MQL (PO) produced less cutting forces pointed at lower friction coefficient. Ceramic tools, these tools attracted researchers for machining applications especially for high speed cutting conditions. Ceramic tools are widely used in industry because of their high hot hardness2. It is reported in literature [44] that mixed ceramics tools can be used for machining nickel base alloys at cutting speeds ten times higher than carbide tools. Choudury et al. [44] reported that alumina whisker tools can machine aero-engine alloys up to cutting speed of 750 m/ min and feed rate of 0.375 mm/ rev. Jianxin et al. [45] performed turning tests on Inconel 718 using Al2O3/ TiB2 /SiCw tools with different proportions of TiB2 particles and whisker. The experiments were conducted at cutting speed range of 50 – 180 m/min. Dominant wear mechanism found on flank face was abrasion and on rake face were adhesion and diffusion. The study revealed that ceramic tools performed with stable wear rate for cutting speed less than 80 m/ min. Lo Casto et al.

1 Straight Tungsten carbide (WC), the hard phase, together with cobalt (Co), the binder phase, forms the basic Cemented Carbide structure from which other types of Cemented Carbide have been developed 2 Hot hardness means ability of material to retain it bulk hardness and geometry at elevated temperatures.

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[46] conducted turning test on nickel based alloys using cemented carbide (WC/ TiC /Co), zirconia toughened alumina (ZTA), silicon nitride, alumina based and alumina whisker tools. The SEM micrographs revealed erosion of tool and chemical wear as dominant wear mechanisms in ceramic tools. Narutaki et al. [47] executed high speed machining experiments on Inconel 718 using whisker reinforced alumina, silicon nitride and TiC mixed alumina. SiC whisker tool showed less notch wear under 300 m/ min cutting speed. TiC mixed alumina showed less comparatively less wear at cutting speeds above 400 m /min. Cubic boron nitride (CBN) tools, these are produced by hexagonal structured crystals of boron nitride in presence of 1400° C temperature and 6000 MPa pressure [7]. Although CBN tools are the second hardest cutting tool material, but due to high cost their utilization is only limited to finishing operations. Zoya and Krishnamurthy [48] evaluated performance of CBN tools for machining titanium alloys. Experimentation was performed using cutting speed range of 150 – 350 m/ min and feed rate of 0.5 mm / rev. The study recommended cutting speed range of 185 – 220 m/ min for machining titanium alloys. The study revealed chipping and notch wear as main failure modes. Diffusion was the mainly found on rake face of the tool. Critical temperature1 for CBN tools was identified to be 700° C Ezugwu et al. [50] executed experiments on Ti-6Al-4V using CBN tools under three different coolant flow rates. Experimentation was conducted using conventional, 11M Pa, and 20.3 M Pa pressurized coolant flow. It was observed that 11 M Pa and 20.3 M Pa strategies enhanced 68% and 150% of tool life respectively. The study revealed that increasing CBN content results in severe chipping and notch wear at cutting edge. Diamond based tools, In general diamond coated tools and poly crystalline diamond tools are used for machining of titanium alloys. Many researchers have investigated machining performance of PCD and diamond coated tools with respect to the conventional carbides, ceramics and CBN tools. Velasquez et al. [51] performed high speed turning experiments on Ti6Al4V using carbide tool with polycrystalline diamond (PCD) tip. Cutting speed used in experiments range from 20 – 660 m/ min. The study exposed white layer formation characteristics with respect to cutting speed. Oosthuizen et al. [52] conducted milling experiments on Ti6Al4V using PCD coated tools. Experimentation was performed using 100 – 500 m/ min cutting speed and 0.025 – 0.050 mm/ z feed rate. It was observed that PCD

1 Critical temperature of the cutting tool corresponds to the value after which tool wear occurs at higher rate [128].

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tools outperformed carbide tools at higher cutting speeds. A slower wear progression was observed in PCD tools at cutting speed of 200 m/ min. Adhesion wear mechanism was found in PCD tool when machining Ti6Al4V. 2.4.1 Tool wear mechanisms and patterns In the machining process, due to the mechanical contact between the cutting tool and workpiece material, gradual wear is observed at the cutting edge of the tool. Tool wear is defined as change in shape of the cutting tool during the cutting process [53]. Wear patterns formed on the cutting tool during machining result from different types of wear mechanisms. Literature has revealed that tool wear is a complex function of workpiece and tool materials, tool geometry, cutting conditions and cutting environment. To understand the tool wear patterns it is essential to investigate the tool wear mechanisms [54]. The type and rate of tool wear depend on the workpiece and tool materials, cutting conditions, cutting environment and dynamic characteristics of machine tool used during cutting. The rate and type of tool wear is deciding factor towards tool life. Tool life of the cutting tool is measured by measuring the flank and crater wear using ISO 3685:1993 standard [57].

Figure 2.10: Schematic illustration tool wear patterns [56]

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Abrasive wear, When the two surfaces are in sliding contact the hard particles of harder material rub against the less hard surface [53]. Similarly in machining hard particles such as carbides and nitrides rub against the cutting tool material during cutting process. The process of abrasion is dependent upon the relative hardness of abrading particles and abrading materials. Abrasion wear rate increases rapidly with increase in temperature at cutting zone. It is due to the fact that increase in temperature lowers material hardness level [57]. Below in Figure 2.11, there are some reported cases of abrasion wear.

Turning, Ti6Al4V (Uncoated Carbides) V=150,200,250 m/min, Ezugzu et al. [50]

Turning, Ti alloy, PCD tool, Corduan et al. [58]

Figure 2.11: Abrasive wear observed for different cutting tool materials when machining titanium alloys

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Adhesive wear, In adhesive wear mechanism, two metals are forced together at high temperature and pressure that result in welding of two materials. This mechanism is also observed in the machining [53, 60]. The particles of workpiece metal tend to weld at the cutting edge of the cutting tool during machining process. This deposition of workpiece material is termed as built-up-edge (BUE). Built up edges (BUE) forms and then break at regular intervals during cutting. Every time built-up-edge (BUE) breaks it also peels off some tool material resulting in adhesive tool wear. Below in Figure 2.12, there are some reported cases of adhesive wear.

Milling, (TA15) alloy, PCBN tool, V=250-350m/min, Honghua et al. [62]

Milling, (TA15) alloy, PCD , V=250-350m/min, Honghua et al. [62]

Figure 2.12: Adhesive wear observed for different cutting tool materials when machining titanium alloys

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Diffusion wear, during the cutting operation, workpiece and tool materials are in contact. It has also been found in literature that atomic particles of workpiece material diffuse to the cutting tool material that weakens the tool [26]. Diffusion wear is highly dependent on the cutting temperature and chemical affinity between tool and workpiece materials. Normally diffusion wear appears on the rake face of the cutting tool. Diffusion wear mechanism is a dominant tool wear mechanism for higher cutting speed conditions [63]. Crater wear is shown below in Figure 2.13. Crater wear can be formed due to the mechanical rubbing action of the hard particles on the rake face or complex atomic diffusion between tool and chip materials. Crater wear generally shows that tool material is diffused into the chip material.

Milling, Ti6Al4V

TiCN/ Al2O3 CVD Coated tool Jawaid et al. [41]

Figure 2.13: Diffusion wear on the rake face of cutting tool materials when

machining titanium alloys

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Chemical wear, during the cutting process high cutting temperature is generated that facilitates chemical reactions like oxidation and creation of other chemical compounds [57]. This oxide layer is too brittle in nature and breaking off this oxide layer sometimes results in chipping of tool material [26]. Jianxin et al. [64] studied the diffusion and oxidation chemical wear by developing a diffusion couple, just like an interface between Ti6Al4V and WC/Co tool as shown in Figure 2.14. The study showed oxidation layer formed by exposing the WC/Co carbide tool in air for 90 min at 800 °C.

Figure 2.14: Schematic illustration of the diffusion couple [64]

Chipping and Flaking, when small amount of material peels off from cutting edge, it is called as chipping [53]. Chipping can lead to gross fracture at later stages as well. It is unpredictable in nature but most commonly found in the tools with low fracture toughness. Chipping is formed due to mechanical and thermal shocks during machining [65]. Chipping is found in uncoated tool. Flaking is similar in concept to the chipping, but it means large amount of material will be peeled off from tool surface. Flaking is generally observed in coated tool materials. Figure 2.15 shows different cases of chipping and flaking observed in different cutting tools when machining titanium alloys.

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Chipping Case, Milling, Ti-6242S (Uncoated tool)

V= 150m/min, Ginting and Nouari. [5]

Flaking Case, Milling, Ti6Al4V TiN PVD Coated tool, V= 55, 65, 80

and 100 m/min, Jawaid et al. [41]

Chipping Case, Turning, Ti6Al4V

Wurtzite boron nitride (wBN) tool at 80 min, V =75m/min, Bhaumik et al. [38]

Table 2.15: Chipping and Flanking observed for different cutting tool materials when machining titanium alloys

2.5 SURFACE INTEGRITY Surface integrity is based on a group of properties of a machined surface that influence the performance of this surface during post processing and service life [66]. The properties directly linked with surface integrity are surface roughness, texture, microhardness, residual stresses, microstructure, and surface cracking etc. These properties affect the service life by governing wear and frictional behaviour of contacting bodies, controlling lubrication efficiency, and growth of surface crack etc [67]. Surface condition is very critical if part has to be used under fatigue loading [68].

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Carbide cracking/ Tearing, most of the coating materials and workpiece materials consist of small carbide particles. It has been observed that at lower values of depth of cut in micro-machining, 50μm approximately, carbide particles were smeared against the workpiece surface. The reported size of carbide particle is 20 μm approximately [66]. Tearing can be seen as shown in Figure 2.16a. White layer formation, One of the most commonly observed microstructural defects in titanium and nickel alloys is called white layer formation. It is a condition where upper most surface of machined workpiece contains high hardness as compared to the bulk material inside [69]. This upper hard layer appears white under microscope due to which it is named as white layer [70]. White layer usually consists of very fine grains. White layer is very hard and brittle in nature that can support cracks to grow easily in to the material and it has direct influence on fatigue life of the product [71, 72]. White layer observed in the machining of Ti6Al4V is presented in Figure 2.16b.

(a)

(b)

Figure 2.16: (a) Surface tearing (Ti6Al4V), V=100 m/min, f=0.15 mm/ tooth, DoCa = 2.0 mm, DoCr = 8.8 mm [73], (b) White layer in Ti6Al4V machined at V=95 m/ min, f = 0.35 mm/rev, and DoC = 0.10mm [74] Work hardening layer formation, Materials with high work hardening behaviour form a work hardening layer in response to the machining operation. Titanium alloys have high tendency to form this work hardening layer under machining phase [75]. Presence of this extremely hard layer on top of bulk material makes further processing very difficult. Pawade et al. [73] studied influence of machining parameters and cutting edge geometry on surface integrity of Inconel 718. In the study machining affected zones

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were observed and degree of work hardening was measured for cutting experiments. The study revealed high values of microhardness in the vicinity of 200 μm below the machined surface. The study revealed that degree of work hardening increased with depth of cut and cutting edge geometry. The study showed that chamfered plus honed cutting edge geometry produced highest degree of work hardening. Figure 2.17 shows the behaviour of hardness under machined surface.

(a)

(b)

Figure 2.17: Microhardness behaviour observed in machining FGH 95 (a) Microhardness region (b) Microhardness measurement [69]

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Surface roughness, it is most widely treated as main indicator to estimate surface integrity. Titanium and nickel alloys are prone to produce high cutting temperature in cutting zone due to which high tool wears are reported in several studies [5, 76]. Factors like cutting speed, feed rate and depth of cut are found to be effective parameters influencing surface finish [40, 53]. Che-Haron et al. [77] investigated effect of cutting speed in machining of Ti6Al4V through turning experiments. The study was performed using straight tungsten carbide tools. It revealed that at lower speed of 45 m/min surface roughness increases in the start and then at the end of tool life surface tends to become smoother. At higher speeds of 60, 75 and 100 m/ min surface roughness increased rapidly towards the end of tool life as shown in Figure 2.18.

(a)

(b)

Figure 2.18: Surface roughness behaviour with cutting speed for turning Ti6Al4V (a) ISO-883-MR4 Tool at feed = 0.35 mm/ rev, (b) ISO-890-MR3 Tool at feed = 0.25 mm/ rev [77]

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2.6 CUTTING FLUIDS The accurate usage of lubricants/ coolants during machining processes significantly prolongs cutting tool life. Lubricants/ coolants are used to lubricate the cutting zone and dissipate the heat efficiently from the cutting zone. The effect of lubrication and cooling results in lower cutting forces during machining. In order to dissipate heat rapidly water based fluids are more efficient than the oil based, but oil based lubricants provides better lubrication. As available in literature [78], a weak solution of rust inhibitor and/or water-soluble oil (5 to 10%) is the most commonly used fluid for high-speed titanium machining processes. However for slow speed machining of titanium alloys chlorinated or sulphurized oils are recommended for better lubrication. Chlorine containing fluids, if chlorine based cutting fluids are used in the machining of alloys that may be subject to stress corrosion cracking, careful post-machining and cleaning operations must be performed afterwards. In order to investigate the applicability of chlorinated cutting fluids, US Air Force Materials Laboratory [78] concluded some interesting findings. The findings are mentioned below [78];

“Sulfurized and chlorinated soluble-oil emulsions used in low-stress grinding and end milling/end cutting did not degrade the high-cycle fatigue strength of annealed Ti-6Al-4V (34 HRC) at 25 °C (75 °F) and 315 °C (600 °F) relative to results from a neutral soluble-oil emulsion. Sulfurized and chlorinated soluble-oil emulsions used in abusive grinding did not degrade the 25 °C (75 °F) high-cycle fatigue strength of Ti-6Al-4V relative to results from a neutral soluble-oil emulsion. Sulfurized and chlorinated oils and soluble-oil emulsions as crack tip environments did not accelerate 25 °C (75 °F) fatigue crack propagation rates in Ti-6Al-4V at 1 cpm and 1800 cpm relative to results in laboratory air environment. A 100 h exposure under stress to sulfurized and chlorinated soluble oil emulsions did not affect 25 °C (75 °F) bend test results from low-stress ground and end milled end cut Ti-6Al-4V relative to results from a neutral soluble oil emulsion”.

Simon et al. [129] performed experiments and utilized auger analysis to study the influence of cutting fluids containing chlorine. It was found that films were formed on the surface with thickness equal to or less than 150 nm with chlorine content of 3% at the most. Similar behaviour with 100-150 nm thick film and 1.5% content was observed when machining

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titanium with demineralized water. It was concluded that prohibition of machining titanium with chlorinated cutting fluids cannot be continued. 2.7 ENVIRONMENTAL FRIENDLY COOLING STRATEGIES

Metal working fluids (MWF) are essential in the machining of titanium alloys to increase tool life, improve surface finish and chip removal from the cutting zone. Metal working fluids acts as lubricant or coolant during machining. These days metal working fluids (MWF) are being questioned extensively for their economics and environmental related issues. These lubricants and coolants impose danger to environment due to their toxicity and non-biodegradability. In order to make machining process sustainable in nature, toxicity has to be reduced whereas biodegradability has to be enhanced. This section provides a brief over view of different cooling strategies utilized to improve the machinability of titanium alloys.

2.7.1 Dry cutting

Dry cutting of titanium alloys is consider as the ideal desired approach but most of the literature strongly recommends that generous amount of coolant should be used while cutting titanium alloys. Cutting fluids offer an advantage of clearing chips easily, however in dry cutting dust and difficult chip removal is experienced. Dry cutting also results in higher friction and higher cutting temperature that initiates higher and rapid wear rates.

Ginting and Nouari [5] examined the machinability of Titanium alloy Ti6242S under dry condition using uncoated carbide inserts. The study analysed surface roughness, cutting temperature, flank wear and chip formation to evaluate machinability. Adhesion was found responsible for flank wear, whereas abrasion and diffusion produced crater wear. Nabhani [49] investigated the machinability of Ti 48 titanium alloy using PCD (SYNDITE)1, PCBN (AMBORITE)2 and CVD-TiN /TiCN /TiC multi layered carbide insert under dry conditions. The study showed that PCD tool outperformed other tools. Better performance of PCD (SYNDITE) was attributed to the reason that carbon substrate of the tool reacts with titanium to form TiC layer. This layer provides protection again abrasion and diffusion [49, 130]. 1 Syndite is a composite material that combines diamond with the toughness of tungsten carbide. 2 PCBN tool with low CBN content is called an AMBORITE

http://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Diamondhttp://en.wikipedia.org/wiki/Tungsten_carbidehttp://en.wikipedia.org/wiki/Tungsten_carbide

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2.7.2 Minimum Quantity Lubrication (MQL)

Due to the negative influence of lubricants on environment, many researchers have focused their work to utilize small amount of cutting fluid during machining. These procedures are termed under minimum quantity lubrication (MQL) methods. Rahim and Sasahara [43] conducted an experimental comparative study using palm oil based and synthetic ester based MQL systems. The study was performed to investigate the effectiveness of palm oil as lubricant in MQL system. The study revealed that palm oil based MQL arrangement out performed synthetic ester based MQL system. Zeilmann and Weingaertner [79] performed drilling experiments on Ti6Al4V using uncoated and coated drills (TiALN, CrCN and TiCN) under MQL environment. The study measured cutting temperature during drilling operation to evaluate the performance of MQL technique. The study revealed that internal MQL arrangement performed better than external MQL arrangement.

Figure 2.19: Schematic illustration of MQL [80]

Wang et al. [81] executed orthogonal turning experiments on Ti6Al4V using dry, flood and MQL cutting environments. The study was conducted under continuous and interrupted cutting cases. The study pointed out that MQL performed better than flood cooling at higher cutting speeds due to better lubrication capacity. The study also revealed that MQL was more effective in interrupted cutting scenario. Cia et al. [82] performed end milling experiments to investigate the controlling parameters for MQL system. The study used oil flow rates of 2 ml/h – 14 ml/h for optimized value. The study revealed that diffusion wear was present for low oil supply rates 2ml/h – 10ml/h, however at 14ml/h relatively less diffusion wear was found. Yasir et al. [83] utilized physical vapour deposition (PVD) coated cemented carbide tools to machine Ti6Al4V using MQL system. The study

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utilized coolant flow rates of 50 – 100 ml/h at three cutting speed levels of 120, 135 and 150 m/ min. Improved tool life was observed at 135 m/min with high flow rates. Mist 1 was found more effective for worn tool. A possible explanation of getting relatively better performance at 135 m/ min speed can be attributed to the better lubrication capacity at higher flow rate of 100 ml/h.

2.7.3 High pressurized cooling (HPC)

Klocke et al. [84] performed machining experiments on Titanium alloys to investigate the effect of high pressurized coolant supply. The study analysed cutting tool temperature, tool wear, chip formation and cutting forces. The study pressurized the cutting fluid up to 300 bars (55l/min) and compared the effects with conventional flood cooling. The study revealed that 25% cutting tool temperature reduction and 50% tool wear improvement, in best case, were achieved using high pressure coolant. Nandy et al. [85] performed machining experiments on Ti6Al4V using uncoated K20 cemented carbide inserts using conventional wet, high pressure neat oil and high pressure water soluble oil. Machining tests were conducted using cutting speeds of 90, 100 and 111 m/min and supply pressures of 70, 100 and 140 bar. The studies revealed 250% improved tool life when compared to conventional wet cutting. Sandvik coromant has also revealed sample testing studies to show better tool life and higher material removal rates (MRR) for Ti6Al4V and Inconel 718. The coolant delivery arrangement is shown in Figure 2.20.

Figure 2.20: High pressure coolant delivery systems by Sandvik Coromant

[86] 1 Small cutting oil droplets suspended in air is called as mist.

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2.7.4 Cryogenic cooling

When cutting operation is conducted at very low temperature, generally lower than – 153 °C, it is termed as cryogenic machining [87]. Su et al. [88] performed end milling machining experiments on titanium alloys to evaluate the performance of different cooling strategies by analysing oil mist, compressed cold nitrogen gas (CCNG) at 0, and−10 ◦C, and compressed cold nitrogen gas and oil mist (CCNGOM) as the cooling strategies. The study revealed that compressed cold nitrogen gas and oil mist (CCNGOM) cooling strategy outperformed other strategies by resulting longer tool life. A possible hypothesis to explain this better performance can be attributed to the arrangement of combining oil mist with liquid nitrogen setup. The combination provides lubricant and coolant both at same time. Yildiz et al. [89] reviewed the application methods of cryogenic coolants. The study revealed that cryogenic coolants effectively controlled the cutting temperature at cutting zone, and provided good tool life with reasonable surface finish. Sun et al. [90] evaluated the machining performance of titanium alloys by utilizing cryogenic compressed air. The study showed great potential of cryogenic compressed air cooling strategy as it reduced tool wear significantly. Bermingham et al. [91] performed machining experiments using cryogenic cooling technique. Cutting speed and material removal rate were kept constant during the study, however feed rate and depth of cut were varied to analyse cutting force. The study revealed that less heat was generated for low feed rate and high depth of cut. Table 2.3 shows a brief comparison of different cooling strategies used for the machining of titanium alloys. 2.8 SUSTAINABLILITY CONCEPTS IN MACHINING Currently the manufacturing sector is under enormous burden to improve environmental and ecological performance due to the development of strict international environmental protocols. By implementing the sustainable practices in the metal cutting sector, environmental performance and economics can be improved considerably.

Sustainability concepts can be incorporated in manufacturing sector by implementing the following practices [92 - 94];

• Reducing amount of input resources like energy, material and water.

• Improving environmental quality of resources, reducing the use of toxic and non-biodegradable chemicals.

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• Efficient designing of life cycles. • Adopting eco-friendly manufacturing technologies. • Improving ergonomic, health and safety requirements, equity and

fairness, and employee personal development. Table 2.3: Advantages and disadvantages of different environmental friendly cooling strategies employed in the machining of titanium alloys under continuous cutting processes

Dry

Minimum Quantity

Lubrication (MQL)

High Pressurized Cooling (HPC) Cryogenic Cooling

No lubricant/

coolant required, no

need for coolant disposal

Poor

temperature co