IMPROVED MACHINABILITY OF TITANIUM ALLOY TI-6AL-4V THROUGH WORKPIECE
PREHEATING
BY
TURNAD LENGGO GINTA
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy (Engineering)
Kulliyyah of Engineering
International Islamic University Malaysia
MARCH2010
ABSTRACT
Titanium alloys are generally regarded as difficult-to-cut materials and plentiful research works have been conducted on their machinability in the past few decades with several diverse objectives, such as improvement on tool life, surface finish and surface integrity, investigation on chip and tool wear morphology, cutting force and vibration/chatter as well as reducing cost of machining. This study introduces induction preheating as one of the new variable parameters to improve the machinability of titanium Ti-6Al-4V alloy. In the current work the influence of heating parameter i.e. preheating temperature, and the cutting variable, i.e. cutting speed, feed and axial depth of cut were investigated during end milling operation on a vertical machining center (VMC). The inserts used were uncoated tungsten-cobalt (WC-Co) carbide and polycrystalline diamond (PCD) attached to a 20 mm diameter end mill tool. The study comprehensively investigated the effect of preheating temperature on machinability parameters i.e. tool life, surface finish and cutting force.
The effect of preheating on vibration/chatter, surface integrity, chip-tool contact length and chip serration morphology were also investigated. Central composite design (CCD) of response surface methodology (RSM) coupled with Design-Expert 6.0.8 software was used to develop empirical models of tool life, cutting force and surface roughness both for room temperature and preheated machining. The software was further used to maximize the tool life and concurrently minimize the surface roughness by optimising the cutting parameters and preheating temperature. As a result, in preheated machining, the performance of uncoated WC-Co was tremendously improved (almost three times compared to room temperature machining), even the values of tool life are much higher than those cutting with PCD under room temperature machining. Preheated machining substantially contributes to the reductions of vibration/ chatter and resultant cutting force, facilitating increased tool life. Furthermore, preheated machining facilitates increased chip-tool contact length, stable chip serrations resulting in reduction of tool wear rate.
11
~ 1 .. IA'JI ~}J F 1 obi J4>' ~ ~ 0_;>-1 J>lf''JJ ~ ~ \ j Aj.,,.J. ..:..,y-..)1
4./y-P u-4 ~ )I
J>-. ~\ JJz
u'}Jl:;! JI ,u~\ uljlp'JIJ F ' 0 j ) ~)I o....,l_;.:i J~ ~ \ j JL::J~) JljJ'J\ ~ ~ \ j ~\..j i f ~WJ ~\.b,. \Au ~WI oh ~ ....).J_~
j>-
~ .,.,,,,,JJJI oh .~\.:JI a.,. J\..i... .j ._;L:.::: ... ,.'~JI ~Lil ,ul.,ol 4WI o))-1 ul.>.-_;.:iJ>-
~WI oh ~ \ j ~ ) y ._\!..\.:>.-J-W-- ~ \
~\..
~ I i \ ~ I )~L...,'j\ F l u l ~ JI .;j~\ JJ'J\ ~ I ~Li .i...lJ.:i
J>-
~ JLJ.-1J~., •.
11.~I.i..s;.pll F l
_;LS:.
i i ~ ~ J4JI _;:_;.:ll ~ JyG:. F l ~ ) ~Lll J...G..- F l 4>-.r"~ j5') o....,\JJJ\ .J\+JI fl_;:)\ ob\ .j ~ ~ ..::,_:;; -./' <i,.,4~\ F ' obi ~.:i...WI 0} , ~ I ,.\A:}1,FI obi J4>' , ~ I ~ \ j
J>-
F l ~ \.. JJ:11 ~ l ~ \ j F ' obi i.P.:/\.. JL,a;'jl djL...,__.,~I .. IA:11J-t5:;
~ Jl...l::,'}I ~ \ _,Jw.FIi ~ I ~
e
i i ~ ~ ..h.WI~?
)1 ~ I .J:-:)1 ,.4-:il i.,i"'}}Jy) ~,J:-:)\J '"%' .:J J4-JI ~ 1 ~~ Ji Aj~:J~ F 1 0}, F 1 01.:i1 __,..,..,J ~c~.r' _;;!µ
~~~JI Aj~:J~ i ~ \ ~ e:-41..;_r.-ll JI .Jl...l::,:}I ~ I J Aj_;.l\ o).r ~J.:i J,;1\.)...1
~ \ J F l ul_0 ~ J J_r,=l-~ ~'~~~~}I~ .jJ F l ob\ J4>' . ...2. ~ • \,, ;\\ obi bl 0u J ;J\ .. -· ·'I I c.. ~ \ I .:. 0 -1\ . ·~ .w\.i. ~ . '\..l:., 'j\
,,_r--
c-- .. .)
~ ~v---
y - - (,I .~\;;,,;\JI Aj~:J~ AjjJ\ o).r ~J.:i .j ~ \ ~ ~ y \... jl.,_.p\ ..::..>~ ~_rlo oJ.r'2-!
~ <,,s.UIJ F ' ul).:i\ i f t_.,JI lh ~
t.
Ll:i .J 'j _;.bj~ ,,u _;JI o ).r 4>.-J.:i .j i i ~ \ i f~ ~L... \'"""'~ ~ Jl...l::: '}\ ~ I . ~.:il,.cil y.j- -./' Ju .y, ~
? '
GI J'J>"~ ' J\ · F ' ob\ J4>' o.:i~j) ~ J1 ,u~'j~ 0~1
~ ?
~£) ~ .:J~ j.o-,JI lh 0\ . jsl.:JI o.:i~j J...G..- uW
e
J:-:)\e
~ I ol.:il v\i ;; fl i f --4p Jl...l::: 'jl·i~l:?I ~ W ~ \ ~ \ j i f ~ Jl...l;;:11 ~ \ 0~ ~
111
APPROVAL PAGE
The thesis of Tumad Lenggo Ginta has been approved by the following:
Supervisor
~km~
Co-Supervisor
M. Y eakub Ali Internal Examiner
Internal Examiner
Che Hassan Che Haron External Examiner
Nasr Elaffi Ibrahi; Ahmed
Chairman
IV
DECLARATION
I hereby declare that this thesis is the result of my own investigations, except where otherwise stated. I also declare that it has not been previously or concurrently submitted as a whole for any other degrees at IIUM or other institutions.
Tumad Lenggo Ginta
Signature ... .
3o -03 -20(0
Date ... .
V
INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA
DECLARATION OF COPYRIGHT AND AFFIRMATION OF FAIR USE OF UNPUBLISHED RESEARCH
Copyright © 2010 by Tumad Lenggo Ginta. All Rights Reserved.
IMPROVED MACHINABILITY OF TITANIUM ALLOY TI-6AL-4V THROUGH WORKPIECE PREHEATING
No part of this unpublished research may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without prior written permission of the copyright holder except as provided below.
I. Any material contained in or derived from this unpublished research may only be used by others in their writing with due acknowledgement.
2. IIUM or its library will have the right to make transmit copies (print of electronic) for institutional and academic purposes.
3. The IIUM library will have the right to make, store in a retrieval system and supply copies of this unpublished research if requested by other universities and research libraries.
Affirmed by Tumad Lenggo Ginta
vi
~ - C
3 -
2..ol o
Date
ACKNOWLEDGEMENTS
In the Name of Allah, the Most Compassionate, the Most Merciful
I would like to express my utmost gratitude and thankfulness to Allah subhanahu wa ta'ala for the shower of His blessings and mercies that I have successfully completed my research work. In particular, sincere appreciation and thanks to my dissertation supervisor, Prof. Dr. A.K.M Nurul Amin and co-supervisor, Assoc. Prof. Dr. A.N.
Mustafizul Karim for their unrelenting guidance, patience and support throughout the study. I would like also to thank Assoc. Prof. Dr. M. Yeakub Ali for his supporting guidance as post supervisor after the viva voce presentation.
Furthermore, I would also like to thank all the staff and technicians from different laboratories of Kulliyyah of Engineering, particularly Br. Zakaria (Tool and Die laboratory), Br. Ibrahim (Metalography Laboratory) and Br. Kahar (Measurement and Instrumentation) for guiding me in use the equipments.
I would like to pay my gratitude to all members of machining research group, Br. Anayet U Patwari, Br. Mohammed Ishtiyaq Hossein, Br. Mohd Amri Lajis, Br.
Khalid Hafiz, Br. Shah Alam, and Br. Suleiman from Department of Manufacturing and Materials Engineering, for their friendly cooperation and inspirations.
This study was conducted under the Science Project (03-01-08-SFOOOl) titled as "Implementation of workpiece preheating to control chatter and improve machinability during end-milling of titanium alloys Ti-6Al-4V", funded by the Ministry of Science, Technology and Innovation (MOSTI), Malaysia. I would take the opportunity to acknowledge the financial support of the ministry. I also wish to thank the Research Management Center of IIUM for their inspiration and overall management of the project.
At last, heartfelt appreciation goes to my family, especially to my lovely wife, Sri Fadilah, and my handsome sons, Raynor Fathur Ghafary Lenggo Ginta, and Rivald Fathur Ghaffary Lenggo Ginta, who are always behind the inspiration of my works.
Vll
TABLE OF CONTENTS
Abstract ... .
Abstract in Arabic . . . 11
Approval Page ... iv
Declaration Page ... v
Copyright Page ... v1
Acknowledgements... vu Table of Contents... v111
List of Tables ... xi
List of Figures ... xiii
List of Abbreviations ... xvii
CHAPTER ONE: INTRODUCTION ... 1
1.1. Background . . . 1
1.1.1. Titanium Alloys ... 1
1.1.2. Machinability of Titanium Alloys ... 2
1.1.3. Hot Machining of Titanium alloys ... 3
1.2. Problem Statements ... 4
1.3. Research Objectives ... 5
1.4. Thesis Organization ... 6
CHAPTER TWO: LITERATURE REVIEW ... 7
2.1. Machinability ... 7
2.2. Titanium Alloys and Their Properties ... 8
2.3. Metallurgy of Titanium Alloys ... 11
2.4. a alloys ... 12
2.3.1. Near a alloys ... 12
2.3.2.
a-P
alloys ... 122.3.3.
p
alloys ... 132.4 Machinability of Titanium Alloys ... 14
2.4.1. High Cutting Temperature ... 15
2.4.2. High Cutting Stress ... 17
2.4.3. Chatter ... 17
2.4.4. Surface Roughness and Surface Integrity ... 18
2.4.5. Chip Morphology and Segmentation ... 19
2.5 Tool Material For Machining Titanium Alloys ... 20
2.5.1. WC Tools ... 22
2.5.2. CBN and PCD Tools ... 24
2.6 Tool Failure in Machining of Titanium Alloys ... 29
2. 7 Hot Machining ... 31
2. 7.1. Induction Heating Assisted Machining ... 32
2.7.2. Plasma Assisted Machining ... 35
2.7.3. Gas Heating Assisted Machining ... 38
Vlll
2.7.4. Laser Assisted Machining ... 40
2.8 Response Surface Methodology ... 43
2.8.1. The Response Function and the Response Surface ... 46
2.8.2. Experimental Design ... 47
2.8.3. Test for Significance of the Regression Model ... 49
2.8.4. Test for Significance on Individual Model Coefficients ... 49
2.8.5. Test for Lack-of-Fit ... 50
2.8.6. Central Composite Design of RSM ... 51
2.8.7. Optimization Technique by Desirability Function ... 52
CHAPTER THREE: PROCEDURE AND EXPERIMENTAL SET-UP ... 55
3 .1 Experimental Procedure . . . 5 5 3.2 Equipment for Machining ... 56
3.2.1. Vertical Machining Centre ... 56
3 .2.2. Data Acquisition System ... 57
3.3 Workpiece Materials and Cutting Tools ... 58
3.3.1. Workpiece Materials ... 58
3.3.2 Cutting Tools ... 60
3.4 Preheating Equipment and Temperature Control ... 60
3 .4.1. Induction Heating Equipment ... 60
3 .4.2. Temperature Controller ... 61
3.5 Equipments for Machinability Assessment ... 62
3.5.1. Tool Maker Microscope ... 62
3.5.2. Cutting Force Dynamometer ... 62
3.5.3. Surface Roughness Tester ... 63
3.5.4. Scanning Electron and Optical Microscope ... 63
3.6 Predictive Models ... 63
CHAPTER FOUR: DEVELOPMENT OF MODELS FOR TOOL LIFE, SURFACE ROUGHNESS AND CUTTING FORCE. ... 65
4.1 Machining under Room Temperature Conditions ... 65
4.1.1. Tool Life Model for Machining with Uncoated WC-Co ... 68
Under Room Temperature Conditions 4.1.2. Surface Roughness Model for Machining with ... 69
Uncoated WC-Co Under Room Temperature Conditions 4.1.3. Cutting Force Models for Machining with ... 70
Uncoated WC-Co Under Room Temperature Conditions 4.1.4. Tool Life Model for Machining with PCD ... 71
Under Room Temperature Conditions 4.1.5. Surface Roughness Model for Machining with PCD ... 72
Under Room Temperature Conditions 4.1.6. Cutting Force Model for Machining with PCD ... 73
Under Room Temperature Conditions 4.2 Machining under Preheated Conditions ... 74
4.2.1. Tool Life Model for Machining with Uncoated WC-Co ... 77 Under Preheated Conditions
lX
4.2.2. Surface Roughness Model for Machining with ... 78
Uncoated WC-Co Under Preheated Conditions 4.2.3. Cutting Force Model for Machining with ... 79
Uncoated WC-Co Under Preheated Conditions 4.2.4. Tool Life Model for Machining with PCD ... 80
Under Preheated Conditions 4.2.5. Surface Roughness Model for Machining with PCD ... 81
Under Preheated Conditions 4.2.6. Cutting Force Model for Machining with PCD... .. 82
Under Preheated Conditions 4.3 Optimization for Tool Life and Surface Roughness ... 83
CHAPTER FIVE: RES UL TS AND DISCUSSIONS ... 86
5.1 Tool Life Assessment ... 86
5 .2 Surface Roughness Assessment ... 100
5.3 Cutting Force Assessment ... 104
5.4 Discussion ... 108
5.4.1. Discussion on Tool Life Models ... 108
5.4.2. Discussions on Surface Roughness Models ... 110
5.4.3. Discussions on Cutting Force Models ... 111
5.4.4. Discussion on Optimum Cutting Parameters ... 112
CHAPTER SIX: CONCLUSIONS AND RECOMMENDATIONS ... 114
6.1 Conclusions ... 114
6.2 Major Contributions ... 117
6.3 Recommendation for Further Studies ... 118
BIBLIOGRAPHY ... 119
RELATED PUBLICATIONS ... 126
APPENDIX A ... 128
APPENDIX B ... 134
APPENDIX C .. . . .. .. . .. . .. . . .. .. . . .. .. . .. . .. . . .. . .. . . .. . .. .. . .. .. . .. .. . .. . .. .. . . . .. . .. 137
APPENDIX D . . . 145
APPENDIX E ... 152
X
LIST OF TABLES
Table No. Page No.
2.1 Physical and chemical properties of Ti-6Al-4V 10 compared to Steel CK 45.
2.2 Softening points of several tool materials. 21
2.3 The comparison of WC-Co, CBN, and PCD cutting tools. 28
3.1 Specification of vertical machining center 57
(MCFV 1060 LR, ZPS, Czech Republic).
3.2 The chemical composition, mechanical and physical properties 59 of titanium alloy Ti-6Al-4V.
3.3 Cutting tool material and geometry data. 60
3.4 Technical specifications of the induction heating system. 61 4.1 Coding identification and level of independent variables
in end milling with uncoated WC-Co under room temperature
conditions. 66
4.2 Coding identification and level of independent variables in
end milling with PCD under room temperature conditions. 66 4.3 Data of tool life, surface roughness and resultant cutting force
in end milling of Ti-6Al-4V using uncoated WC-Co inserts
under room temperature conditions. 67
4.4 Data of tool life, surface roughness and resultant cutting force in end milling of Ti-6Al-4V using PCD inserts
under room temperature conditions. 67
4.5 Coding identification and level of independent variables 75 in end milling with uncoated WC-Co under preheated conditions.
4.6 Coding identification and level of independent variables in
end milling with PCD under preheated conditions. 75 4.7 Data of tool life, surface roughness and resultant cutting force
in end milling of Ti-6Al-4V using uncoated WC-Co inserts
under preheated conditions. 76
XI
4.8 Data of tool life, surface roughness and resultant cutting force in end milling of Ti-6Al-4V using PCD inserts
under preheated conditions. 76
4.9 Possible optimal solutions in end milling with
uncoated WC-Co under room temperature conditions. 84 4.10 Possible optimal solutions in end milling with
uncoated WC-Co under preheated conditions. 85
4.11 Possible optimal solution in end milling with PCD
under room temperature conditions. 85
4.12 Possible optimal solution in end milling with PCD
under preheated conditions. 85
5.1 Tool life models for end milling of titanium alloy Ti-6Al-4V with uncoated WC-Co and PCD under room temperature
and preheated conditions 87
5.2 Acceleration amplitudes of vibration and the percentage of reduction
in end milling with uncoated WC-Co. 95
5.3 Surface roughness models in end milling of titanium alloy Ti-6Al-4V with uncoated WC-Co and PCD under room temperature
and preheated conditions 100
5.4 Cutting force models in end milling of titanium alloy Ti-6Al-4V with uncoated WC-Co and PCD under room temperature and
preheated conditions 105
XU
LIST OF FIGURES
Figure No. Page No.
2.1 Evaluation of titanium alloy and other materials use
in aerogas turbines. 8
2.2. General characteristics and typical applications of titanium alloys. 9 2.3 Distribution of thermal load when machining titanium and steel. 11 2.4 Schematic psedo-binary phase diagram for Ti-6Al alloys with 13
additions of Vanadium.
2.5 Microstructure ofTi-6Al-4V (etchant: 10% HF, 5% HN03, 14 85% H20, alpha grains (light) and inter-granular beta phases (dark).
2.6 Measured temperature in turning of Ti-6Al-4V. 16
2.7 Elevated temperature strength of several aeroengine alloys). 17 2.8 Typical hot hardness characteristics of several tool materials. 21
2.9 WC-Co tool inserts and geometry. 24
2.10 SEM picture of the flank of BCBN tools at two different conditions 25 where the non-uniform flank wear is the dominant wear
for these two cases: (a) depth of cut = 0.125 mm, (b) depth of cut= 0.075 mm (feed= 0.075 mm/tooth, cutting speed= 400 m/min).
2.11 The variations of surface roughness with cutting speed during 27 machining titanium alloys.
2.12 Typical wear on a flank face during end milling of titanium alloy 30 Ti-6242S with cutting conditions: cutting speed = 100 m/min,
feed = 0 .15 mm/tooth, flank wear (VB) = 0. 3 mm.
2.13 Brittle fracture ( cracking, chipping, flaking) when cutting speed 30 of 60 m/min and feed of 0.15 mm/tooth during milling of
titanium alloy Ti-6242S.
2.14 Block diagram of experimental setup of induction 33
heating assisted end milling.
Xlll
2.15 FFT output during preheated end milling of carbon steel 34 ( cutting speed = 100 m/min, depth of cut = 1 mm and
feed= 0.2 mm/tooth) at: (a) 29
°c
and (b) 304°c.
2.16 Details of a plasma arc generator. 35
2.17 The relationship between the tool life and cutting speed 36 during machining of manganese steel.
2.18 Resultant cutting force vs surface temperature for 37 various cutting speeds during machining of Inconel 718 with
cutting conditions: feed= 0.124 mm/tooth, cutoff length= 0.8 mm, cutting time= 0.5 min.
2.19 Schematic representation of the gas heating assisted machining. 39 2.20 The relationship between cutting temperature and the tool life 40
during gas heated assisted machining of manganese steel,
with cutting conditions: feed= 0.2 mm/tooth, depth of cut= 1.5 mm.
2.21 The system of laser assisted machining. 40
2.22 Cost comparison when machining 1 m length of Inconel 718 42 by conventional machining and LAM.
2.23 Comparison of tool flank wear in conventional and laser assisted 42 longitudinal turning of AISI D2 steels.
2.24 Generation of a central composite design for two factors. 52 2.25 Individual desirability functions for simultaneous optimization 54
3.1 Research activities. 56
3.2 Vertical machining centre (MCFV 1060 LR, ZPS, Czech Republic). 58 3.3 Microstructure of Ti-6Al-4V with equiaxed and columnar 59
alpha grains (light) with intergranular beta phase ( dark);
Etchant: 10% HF, 5% HN03, 85% H20.
3.4 High frequency induction heating machine (GP-30AB, 61 Airocoat, Malaysia): (a) matching box (transformer and condenser),
and (b) cooling unit (pure water unit).
3.5 Experimental set-up for determining the current-temperature- 62 feed rates relationships.
4.1 Perturbation plot for tool life in end milling with 68 uncoated WC-Co under room temperature condition.
XIV
4.2 Perturbation plot for surface roughness in end milling with 69 uncoated WC-Co under room temperature conditions.
4.3 Perturbation plot for resultant cutting force in end milling with 70 uncoated WC-Co under room temperature conditions.
4.4 Perturbation plot for tool life in end milling with 71 PCD under room temperature conditions.
4.5 Perturbation plot for surface roughness in end milling with 73 PCD under room temperature conditions.
4.6 Perturbation plot for cutting force in end milling with 74 PCD under room room temperature conditions.
4.7 Perturbation plot for tool life in end milling with 77 uncoated WC-Co under preheated conditions.
4.8 Perturbation plot for surface roughness in end milling 79 with uncoated WC-Co and preheated conditions.
4.9 Perturbation plot for cutting force in end milling with 80 uncoated WC-Co under preheated conditions.
4.10 Perturbation plot for tool life in end milling with 81 PCD under preheated conditions.
4.11 Perturbation plot for surface roughness in end milling 82 with PCD under preheated conditions.
4.12 Perturbation plot for cutting force in end milling with 83 PCD under preheated conditions.
5.1 The effects of cutting speed and preheating temperature 88 on tool life in end milling with (a) uncoated WC-Co,
and (b) PCD (feed= 0.088 mm/tooth, axial DOC= 1 mm).
5.2 The effect of preheating temperature on tool life improvement 90 in end milling with (a)uncoated WC-Co, and (b) PCD.
5.3 The effect of feed on tool life in end milling with 91 (a) uncoated WC-Co, and (b) PCD.
5.4 SEM pictures of worn tools in end milling with 92
(a-b) uncoated WC-Co, and (c-d) PCD.
xv
5.5 FFT output of end milling with uncoated WC-Co at 94 (a) room temperature, and (b-d) three preheating temperatures
(Cutting speed= 70 m/min, axial DOC= 1 mm, feed= 0.088 mm/tooth).
5.6 The effects of preheating temperature on acceleration amplitude 96 of vibration in end milling with (a) uncoated WC-Co, and (b) PCD.
5.7 Type of chips produced at different temperature for the same 97 cutting conditions in end milling with uncoated WC-Co at:
(a) room temperature, (b) 450
°c,
and (d) 650°c
(Cutting speed=70 m/min, fz=0.088 mm/tooth, axial DOC=l mm).
5.8 Peak to valley ratio of chips produced vs. preheating temperature 98 in end milling with uncoated WC-Co (Cutting speed= 70 m/min,
feed = 0.088 mm/tooth and axial DOC = 1 mm).
5.9 Comparison of the performance of uncoated WC-Co 99
and PCD inserts in end milling of Ti-6Al-4V (axial DOC=l mmm, feed=0.088 mm/tooth).
5.10 The effects of preheating temperature on surface roughness 101 in end milling with (a) uncoated WC-Co, and (b) PCD.
5.11 SEM picture of built-up edge in end milling with 102 uncoated WC-Co under preheated conditions.
5.12 SEM pictures of machined surface in end milling with 103 uncoated WC-Co at: (a). Room temperature,
(b). Preheating at 650
°c
(Cutting speed=70 m/min, axial DOC=l mm, feed=0.088 mm/tooth).5.13 The effects of (a) cutting speed and 105
(b) feed on the resultant cutting force in end milling with uncoated WC-Co.
5.14 Effects of (a) cutting speed (b) feed on cutting force in 106 end milling with PCD.
5.15 SEM pictures of chip-tool contact length in end milling with 107 uncoated WC-Co (Cutting speed= 70 m/min, axial DOC= 1 mm,
feed =0.088 mm/tooth).
5.16 SEM pictures of chip-tool contact length in end milling 108 with PCD (Cutting speed=127 m/min, axial DOC=l mm,
feed=0.088 mm/tooth).
XVI
LIST OF ABBREVIATIONS
2FI Two factor interaction
3D Three dimensional
d Axial depth of cut ( axial DOC) ANOVA Analysis of variance approach bee Body center cubic
CCD Central composite design CNC Computer numerical control cph Closed-packed hexagonal DAQ Data acquisitions
DF Degrees of freedom
fz Feed
FR
Resultant cutting force F1 Tangential cutting forceFz Thrust force
fee Face centre cubic FEM Finite element model FFT Fast fourier transformation
Hz Hertz
hep hexagonal closed pack
Mz Torque along z-axis
MS Mean square
MRR Metal removal rate
mm millimeter
µm micrometer
N Newton
Ra Average surface roughness RSM Response surface methodology SEM Scanning electron microscope
ss
Sum of squareV Cutting speed
Vs Flank wear
Ysmax Maximum flank wear VMC Vertical machining center VMR Volume of metal removal 8 Preheating temperature
xvn
1.1 BACKGROUND
CHAPTER ONE INTRODUCTION
Titanium alloys and its machinability are presented in this section. Some factors affecting the machinability of titanium alloys and hot machining are discussed in this section.
1.1.1 Titanium Alloys
Titanium alloys are developed in order to satisfy the need for a class of strong and lightweight materials for aircraft engine and airframe manufacture. Because of their outstanding strength-to-weight ratio, fracture and corrosion resistance, titanium alloys are also used in advanced industrial equipment, generation of energy and transportation. It represents 25% of its market and 75% of all the world production are consumed by the aeronautical industry (Ribeiro et al., 2003).
Titanium alloys can be classified into corrosion resistant and structural alloys (Lopez et al., 2000). Corrosion resistant alloys are used in chemical, paper and food industries, as pipes, heat exchangers, valves and containers, etc. Structural alloys are used for application requiring resistance to fatigue at high temperatures, such as engines with internal combustion turbines at working temperature more than 600
°c.
Titanium alloy Ti-6Al-4V is the most widely known which lies within this group. In an aged state, this alloy has a good combination of mechanical properties at working temperatures over 315
°c.
Highly resistant to ambient temperature beta alloys are used in applications which demand a high resistance to relatively low temperatures. When considering the percentage share of use of titanium alloys in the aerospace industry,1
25% are used as corrosion resistance alloys, 60% of Ti-6Al-4V alloy, and 15% of other structural alloys.
1.1.2 Machinability of Titanium Alloys
Machinability of a material refers to an indication of the ease or difficulty with which it can be machined. Machinability assessment can be done mainly through the tool life, cutting force, and quality of the surface finish (Trent, 1991 ). Progress in the machining of titanium alloys has been hindered basically due to following reasons (Ribeiro et al., 2003):
1. High chemical reactivity and its low thermal conductivity which generates high temperature during machining. About 80% of the generated heat is retained in the tool and 20% is carried away with the chip.
2. The low modulus of elasticity of titanium alloys is considered to be a principal cause of the chatter during machining. When subjected to cutting pressure, titanium deflects nearly twice as much as the carbon steel resulting in a bouncing action as the cutting edge enters the cut (Ribiero, et al., 2003). Thus the greater spring-back behind the cutting edge results in premature flank wear, higher vibration and higher cutting temperature.
3. Low chip-tool contact length and chip segmentation.
4. Higher mechanical stresses occur in the immediate vicinity of the cutting edge attributing to the unusually small chip-tool contact length on the rake face.
2
1.1.3 Hot Machining of Titanium Alloys
To machine titanium alloys, tool manufacturers recommend hard metal tools, moderate cutting parameters and abundant cutting fluid for cooling, in order to control temperature near the cutting edge (Ezugwu, 2005). On the other hand dry machining and cooling with ecological cutting fluids have been receiving increasing attention because of ecological impact of conventional cutting fluids. Dry machining of titanium alloys is extremely difficult because of the temperature near the cutting edge, which should be controlled in order to guarantee the surface integrity of work-piece and to avoid tool wear (Cantero et al., 2005).
Hot machining found widespread application in the manufacture of engineering components in the late 20th century, a century after it was first introduced. The principle behind hot machining is the increase of the difference in hardness of the cutting tool and workpiece, leading also to the reduction in the component forces, improved surface finish and longer tool life (Ezugwu, 2005).
The manufacturing industry has explored various heating techniques such as electric current, arc, high frequency induction, laser beam, electron beam and plasma jet heating (Ezugwu, 2005). However, these techniques have limited applicability and are not suitable for all applications. Electron beam heating operated in a vacuum and was found to be expensive. Laser assisted machining (LAM) was found to decrease tool wear by 40%, cutting force by 18% and increase the metal removal rate by 33%.
However, the high costs and high power consumption slowed down the implementation of LAM (Chang and Kuo, 2007).
An economical alternative to LAM was plasma assisted machining (PAM) (Konig et al., 1990). It was observed from turning of ceramics using PCBN tool inserts that relief face tool wear was reduced by 40%. PAM was also used in turning
3
hard metals, such as hardened steel with tungsten carbide inserts. Although PAM was found to improve the machining performance, there was no protection to avoid the heat effect on the cutting edge. Furthermore, the notch wear of the tool was another problem associated with PAM (Konig et al., 1990).
Induction preheating method to improve machinability of medium carbon steel during end milling was conducted (Amin and Abdelgadir, 2003). It was found that induction preheating led to drastic reduction in the vibration/chatter in a wide frequency range. It also led to lower chip thickness and correspondingly lower value of shear angle, which was an indication of lower cutting forces in preheated end milling. The performance of circular carbide inserts in end milling of carbon steel under induction assisted machining was also conducted (Amin et al., 2007a). It was observed that tool wear was reduced up to 71 % by workpiece preheating. The uniform wear in preheated machining indicated that the main mechanism of wear was diffusion.
1.2 PROBLEM STATEMENT
Titanium alloy Ti-6Al-4V is extremely difficult-to-machine material (Canterro, 2005). The machinability of this alloy is poor owing to its inherent properties, such as high chemical reactivity, high specific strength maintained at elevated temperature and low chip-tool contact length. High cutting pressure and chip segmentation also lead to vibration/chatter, high tool wear/fracture rates, and poor surface finish.
Furthermore, the nature of intermittent cutting in end milling contributes to the difficulties of machining this alloy. Induction heating method has been found to offer promising results in machining of mild steel and hardened steel (Amin et al., 2007a;
Amin et al., 2008). Hence, investigations of the effectiveness of induction heating
4
method on the improvement of machinability of titanium alloy Ti-6Al-4V in a wide range of cutting speeds are also needed. The cemented carbide is a recommended tool in the industry for machining titanium alloy Ti-6Al-4V in the lower cutting speed range of 80-90 m/min, whereas PCD is an efficient emerging tool recommended also for machining of this alloy in the higher cutting speed range of 80-200 m/min (Amin et al., 2007b). However, due to the high price, PCD tool is not recommended for application in the lower cutting speed range. Hence, the combination of these two tools would ensure the wide cutting speed range.
1.3 RESEARCH OBJECTIVES
As the machinability of titanium alloy Ti-6Al-4V during end milling is a critical issue, the main objectives of this research are as follows:
1. To investigate the influence of cutting parameters on tool life, surface roughness and cutting force in end milling of titanium alloys Ti-6Al-4V using uncoated WC-Co and PCD inserts under room temperature and preheated conditions.
2. To develop empirical models of tool life, surface roughness and cutting force with machining parameters under room temperature and preheated conditions.
3. To identify the optimum preheating temperature and cutting parameters for end milling of Ti-6Al-4V using uncoated WC-Co and PCD inserts.
4. To compare the performances of uncoated WC-Co and PCD inserts in end milling.
5
1.4 THESIS ORGANIZATION
The thesis is presented in six chapters, starting from the introduction in chapter one.
Literature review on titanium machining, hot machining and response surface methodology are presented in chapter two. The experimental set up and procedure, equipment, and the design of experiments for both room temperature and preheated conditions are presented in the subsequent chapter. The models development and results and discussions are then presented in chapter four and five respectively.
Conclusions and recommendations then complete the thesis.
6
CHAPTER TWO LITERATURE REVIEW
This chapter is divided into several sections. The term machinability starts the first section of this chapter. After words, discussion on titanium alloys and their properties are presented. Furthermore, research progresses on titanium machining are reviewed.
Moreover, hot machining, and its applications in metal cutting are presented. In the final section, the concept of response surface methodology (RSM) and its application in metal cutting research are also presented.
2.1 MACHINABILITY
The term machinability of a material can be defined as an indication of the ease or difficulty with which it can be machined (Trent, 1991 ). In general, machinability of a work material can often be measured in terms of the numbers of components produced per hour, the cost of the machining the component, or the quality of the surface finish.
To deal with the complex situation, the approach is to discuss the behaviour of the main classes of metals and alloys during machining, and to explain this behavior in terms of their composition, structure, heat treatment and other properties. The machinability of a material may be assessed by one or more of the following criteria (Trent, 1991 ):
1. Tool life: The amount of material removed by a tool, under standardized cutting conditions, before the tool performance becomes unacceptable or the tool is worn by a standard amount.
2. Limiting Metal Removal Rate (MRR): The maximum rate at which the material can be machined for a standard tool life.
7