SURFACE INTEGRITY OF INCONEL 718 DURING
DRILLING OPERATION
ALI AKHAVAN FARID
A thesis submitted in fulfillment of the requirement for the award of the degree of
Master of Engineering
(Advanced Manufacturing Technology)
Faculty of Mechanical Engineering
Universiti Teknologi Malaysia
MAY, 2008
To my parents, Hossein Akhavan Farid and Parvin Sami Zadeh, my brother and my
sweet sister, Amin and Elham.
ACKOWLEDGEMET
My foremost thank goes to my thesis supervisor Prof. Dr. Safian Sharif.
Without him, this dissertation would not have been possible. I thank him for his
patience and encouragement that carried me on through difficult times, and for his
insights and suggestions that helped to shape my research skills. His valuable
feedback contributed greatly to this dissertation.
I thank all the students and staffs in department of Manufacturing and
Industrial Engineering, whose presences and fun-loving spirits made the otherwise
gruelling experience tolerable. They are: Mr. Rival, Mr. Denny, Mr. Chia, Mr. Ayub
and Mr. Ali. Not forgetting my friends especially Hamid Reza, Amir and Hesam for
their ideas and involvement during discussion and ideas of the project.
Lastly, I would like to thank my family for all their love and encouragement.
For my parents who raised me with a love of science and supported me in all my
pursuits.
ABSTRAK
Aloi-aloi super seperti Inconel 718 memiliki kekuatan yang tinggi pada suhu
tinggi. Dan ini menjadikan mereka menarik digunakan untuk aplikasi industri
angkasa. Walau bagaimanapun, bahan-bahan ini merupakan bahan yang sukar untuk
dimesin. Keadaan permukaan yang digerudi pastilah dipengaruhi oleh parameter
pemotongan, seperti halaju pemotougan, kadar uluran, jenis dan geometri mata
gerudi. Ujian penggerudian pada berbagai halaju, jenis dan sudut mata gerudi
dilakukan untuk menilai kesan parameter diatas pada kualiti lubang-lubang termesin
dan integriti permukaan Inconel 718. Kualiti lubang-lubang yang dimesin dinilai dari
segi ketepatan geometri dan pembentukan gerigis. Integriti permukaan yang dinilai
melibatkan aspek-aspek kekasaran permukaan, perubahan metalurgi, dan kekerasan
mikro substrat permukaan lubang. Dari kajian yang dilakukan, lubang-lubang yang
dihasilkan memiliki kualiti yang tinggi meskipun digerudi menggunakan mata alat
yang telah haus, jika dinilai dari sudut ukuran, kekasaran permukaan, dan tinggi
gerigis. Walau bagaimanapun, nilai kekerasan mikro dan analisis struktur mikro
menunjukkan perubahan-perubahan struktur mikro yang jelas yang berkait dengan
kemerosotan sifat-sifat mekanikal. Secara umumnya, parameter pemotongan didapati
memberikan kesan-kesan yang signifikan pada kualiti dan integriti permukaan pada
penggerudian Inconel 718 menggunakan mata gerudi karbida tak bersalut.
ABSTRACT
Superalloys such as Inconel 718 have high strength at elevated temperatures,
which make them attractive towards various applications in aerospace industry.
However, these materials are considered difficult to machine materials. The state of a
workpiece surface after machining is definitely affected by cutting parameters, such
as cutting speed, feed rates, drill types and drill geometries. Drilling tests, at different
spindle-speed, feed rates, drills and point angles of drill, were conducted in order to
investigate the effect of the above parameters on the quality of machined holes and
surface integrity of Inconel 718. The quality of machined holes was evaluated in
terms of the geometrical accuracy and burr formation. Surface integrity involved the
aspect of surface roughness, metallurgical alterations and microhardness of the
substrate of the hole surface. High hole quality was observed even at holes produced
using worn tools, in relation to dimensions, surface roughness and burr height.
However, microhardness measurements and microstructural analysis of work-piece
showed significant microstructural changes related with a loss of mechanical
properties. In general the cutting parameters have significant effects on the surface
quality and surface integrity when drilling Inconel 718 using uncoated carbide drill.
TABLE OF COTETS
CHAPTER
TITLE
PAGE
STATUS OF THESIS
SUPERVISOR DECLARATIO
TITLE PAGE
i
DECLARATIO OF ORGIALITY
ii
DEDICATIO
iii
ACKOWLEDGEMETS
iv
ABSTRAK
v
ABSTRACT
vi
TABLE OF COTETS
vii
LIST OF TABLES
x
LIST OF FIGURES
xii
LIST OF SYMBOLS
xv
CHAPTER1 ITRODUCTIO
1.1
Introduction
1
1.2
Project scope
3
1.3
Project objective
3
1.4
Problem statement
3
CHAPTER2 LITREATURE REVIEW
2.1
Nickel
4
2.1.1
Production of nickel
4
2.2
Nickel- copper alloys
5
2.3
Nickel-chromium alloys
6
2.4
Nickel-iron-base supperalloys
6
2.4.1
Chemical composition and typical application
6
2.4.2
Microstructure
7
2.4.3
Solid-solution strengtheners
7
2.4.4
Precipitation strengtheners
8
2.5
Inconel 718
8
2.6
High-temperature stress-rupture properties
9
2.7
Machinability
10
2.8
Drilling
13
2.9
Twist drill parts
13
2.9.1
Shank
14
2.9.2
Body
15
2.9.3
Point
16
2.9.4
Drill point characteristics
17
2.9.5
Drill point angle and clearance
18
2.10
Drilling facts and problems
19
2.11
Cause of drill failure
22
2.12
Cutting tool
23
2.12.1
24
Cutting Tool Materials
2.13
Surface Integrity
28
2.14
Surface roughness
28
2.14.1 Quantification of surface roughness
29
2.14.2 Effective parameters on surface roughness
in drilling
2.15
Microhardness
32
36
2.15.1 Effective parameters on microhardness changes
in drilling
38
2.16
Microstructural changes
39
2.17
Burr formation
41
CHAPTER3 METHODLOGY
3.1
Introduction
46
3.2
Research Design Variables
46
3.3
Workpiece Material
47
3.3.1
48
Analysis the workpiece material
3.4
Cutting Tools
51
3.5
Machining Procedure
52
3.6
Selection of independent variables
53
3.7
Investigation of Surface Finish
53
3.8
Dimensional accuracy
53
3.9
Burr height measuring
54
3.10
Preparing the samples
56
3.11
Microstructural analysis
57
3.12
Microhardness measurement
57
CHAPTER4 RESULTS AD DISCUSSIO
4.1
Introduction
59
4.2
Tool wear
59
4.3
Surface roughness
60
4.4
Dimensional accuracy
61
4.5
Burr height
62
4.6
Microstructure
64
4.7
Microhardness
68
CHAPTER5 COCLUSIOS
5.1
Conclusions
70
5.2
Future study
71
REFERECES
72
LIST OF TABLE
TABLE
TITLE
2.1
Chemical composition and typical application of
PAGE
7
nickel-iron-base superalloys
2.2
Geometry and coating details
33
3.1
Machining parameters
47
3.2
Mechanical properties of Inconel 718
47
3.3
Chemical composition of Inconel 718
47
3.4
Average hardness of Inconel 718
51
3.5
Drill information
51
3.6
Experimental planning at all levels
53
4.1
Number of holes drilled under different conditions
60
LIST OF FIGURES
FIGURE
TITLE
2.1
Electron micrographs of Inconel 718
10
2.2
The main parts of twist drill
14
2.3
Types of drill shanks
15
2.4
The web of a twist drill
16
2.5
The point of a twist drill
16
2.6
The lip clearance angle of the cutting edge
17
2.7
The drill angle of a twist drill
18
2.8
Wear at outer corners of drill
20
2.9
Breakdown of chisel point
21
2.10
Excessive and insufficient clearance of the cutting edge
21
2.11
Improper web thinning of twist drill
21
2.12
Cutting lips with unequal angles
22
2.13
Cutting lips with unequal length
22
2.14
Cross-section of a surface
29
2.15
Sampling length
30
2.16
Several different elements of a normal finish
31
2.17
Different profile in different directions
32
2.18
Surface roughness measurements
34
2.19
Surface roughness values when different cutting fluids
34
was applied
PAGE
2.20
Surface roughness of AISI 1045 steel finished by different
35
coated drills
2.21
SEM micrograph showing three microindentation marks
38
2.22
Typical microhardness profile from drilling
39
2.23
(a) Grain boundary deformation and white layer from drilling
40
(b) Microstructure resulting from Mill Boring
2.24
Burr types formed in dry cutting
43
2.25
Formation of a burr with drill cap
44
2.26
Burrs produced in wet cutting
44
2.27
Correlation between the burr formation and the cutting conditions 45
2.28
Correlation between the burr formation and the point angle
45
and the lip relief angle
3.1
Workpiece material
48
3.2
Mounted specimen
49
3.3
Buehler electromet
49
3.4
Grain structure of Inconel 718 at 100X magnification
50
3.5
Grain structure of Inconel 718 at 200X magnification
51
3.6
Image of uncoated tool drill
52
3.7
MAHO MH 700S CNC machining center
52
3.8
Coordinate measuring machine
54
3.9
Samples are separated from the workpiece plate
55
3.10
Optical microscope used to measure burr height
55
3.11
Linear precision saw
56
3.12
Preparations of samples to metallographic studies
56
3.13
Toolmakers’ light optical microscope
57
3.14
Vickers pyramid microhardness tester
58
4.1
Surface roughness measurement at different
60
experiment condition
4.2
Surface roughness measurement comparison in
61
different experimental trials
4.3
Variation of machined hole dimension and tool diameter
62
4.4
Burr heights obtained using an optical microscope
63
4.5
Comparison of burr height at different cutting condition
63
4.6
Burr height versus point angle
64
4.7
Comparison between first and last hole produced in exprement1
65
4.8
Subsurface microstructure in last holes produced using worn tool 67
4.9
Microhardness changes versus distance from machined surface
68
LIST OF SYMBOLS & ABBREVIATIOS
%
percent
°C
Degree celsius
mm
Millimetre
µm
Micrometer
Å
Angstrom
MPa
Mega Pascal
W
Watt
N
Newton
wt
Weight
HV
Hardness Vickers
HK
Hardness Knoop
K
Degree Kelvin
Ni
Nickel
Cu
Copper
Fe
Iron
Mo
Molybdenum
Al
Aluminium
Ti
Titanium
Mn
Manganese
Si
Silicon
C
Carbon
Cr
Chromium
Nb
Niobium
N
Nitrogen
CW
Tungsten carbide
γ
Gamma phase
γ'
Gamma prime phase
η
Eta phase
δ
Delta phase
µ
Mu phase
FCC
Face-centered cubic
HCL Hydrochloric acid
V
Cutting speed
PA
Point angle
f
Feed rate
BUE
Built-up edge
MRR Material removal rate
H.S.S High speed steel
ANSI American National Standards Institute
CBN Cubic boron nitride
PCD
Polycrystalline diamond
PVD
Physical vapor deposition
CVD Chemical vapor deposition
CMM Coordinate measuring machine
CHAPTER 1
ITRODUCTIO
1.1
Introduction
Nickel-based alloys account for 80% of the superalloy usage within the
aerospace industry, with the remainder being iron and cobalt based. Approximately
45–50% of the total material requirements for a gas turbine engine are met using
nickel alloys [1]. Other areas of application are within space exploration (main space
shuttle engine, nickel–hydrogen batteries (international space station)), power
generation (industrial gas turbines), chemical industry (cryogenic tanks), etc. [1–3].
The properties that make nickel-based superalloys attractive to industry are: high
yield strength (retained to approximately 750° C), high ultimate tensile strength, high
fatigue strength, retention of corrosion and oxidation resistance up to elevated
temperatures and good creep resistance [1,4,5].
Numerous publications have shown that nickel based superalloys are difficult
to machine regardless of the process being used [6-11]. The properties that make
Inconel 718 an important engineering material are also responsible for its generally
poor machinability.
Low thermal conductivity (11.4 W/mK) leads to high cutting temperature
being developed in the cutting zone. In turning, temperatures of around 900°C have
been reported at the relatively low cutting speed of 30 m/min with over 1300°C found
at 300 m/min [12]. In addition, temperature gradients in the tool are much steeper than
for steels with the maximum temperature being generated in the tool nose region [13].
The materials ability to retain its mechanical properties at elevated temperature results
in high cutting forces being generated, around double that found when cutting
medium carbon alloy steels. This in combination with the relatively short chip tool
contact length means that stress is concentrated on the area of maximum tool
temperature leading to chipping and/or plastic deformation of the cutting edge [10,
13]. Nickel based superalloys have a high chemical affinity for many tool materials
and as such form an adhering layer leading to diffusion and attrition wear[14]. They
are also highly sensitive to strain rate and rapidly work harden causing abrasive wear,
particularly at the depth of cut and leading edge positions. The presence of hard
phases in the microstructure, such as carbides, nitrides, oxides, etc, further
exacerbates tool abrasion.
In contrast to other machining processes drilling has received relatively little
attention and most literature available for nickel base superalloys are related only to
tool wear and productivity [15]. Drilling is one of the most important processes in
aerospace manufacture and being the last operation performed, particular emphasis on
the reliability of the process due to the costs already entailed. In addition a hole
amplifies the stress around it by a factor of two, placing considerable restraints on
dimensional tolerance and hole quality.
1.2
Problem statement
-
The metallurgical and mechanical characteristics that give nickel alloys highly
valued properties also make them one of the most difficult-to-machine
aerospace materials.
-
The tendency of nickel alloys to accrue surface damage during machining.
-
Burr formation during drilling can increase the cost of manufacturing due to
extra time give in removing the burrs.
1.3
Project objective
The objectives of the project are as follows:
To evaluate the machined hole quality and surface integrity of a
Inconel 718 when drilling using carbide drill with respect to surface
roughness, microhardness, microstructure defects.
To study the influence of the cutting conditions on the surface
roughness, microstructure defects and burr formation when drilling of
Inconel 718.
1.4
Project scope
This study will be focused on drilling of Inconel 718 using uncoated carbide
tools. This process is conducted under various independent variables which include
cutting speed, feed rate and tool geometries. The surface roughness, microhardness
and microstructural changes of subsurface will be evaluated.
CHAPTER 2
LITERATURE REVIEW
2.1
ickel
Nickel is an excellent structural metal for many engineering application. It has
the desirable FCC crystal structure, so it is tough and ductile. It also has good highand low- temperature strength as well as high oxidation resistance and good corrosion
resistance for most environments. Few metals can match the attractive engineering
properties of nickel. Unfortunately, its greatest disadvantage is its relatively high cost,
and thus its use as a base metal for alloy is greatly limited. Nickel-base alloys are
therefore used when no cheaper material can provide the necessary corrosion- or heat
– resisting properties required for special engineering application [16].
2.1.1
Production of nickel
In general, there are three major types of nickel deposits: nickel-copper
sulfides, nickel silicates, and nickel laterites and serpentines. The sulfide deposits,
which are located mainly in Canada, provide most of the western world’s supply of
the metal. The second most important source is the nickel silicate ores of New
Caledonia. Laterite ores, which have relatively low nickel contents, are located mainly
in tropical and subtropical regions of the world. These deposits have not been
extensively developed because of the high cost of the recovering the nickel. There are
several established processes for the extraction of nickel from its ores with the process
used depending mainly on the type of ore being treated. The Canadian Sudbury,
Ontario, deposits which are controlled by the Inco metals company are processed in
the following manner. After the nickel-copper-iron sulfide ore is crushed and ground,
an iron sulfide (pyrrhotite) concentrate is separated magnetically and processed in an
iron-ore recovery plant. The remaining ore product is subjected to froth flotation
treatment which produces separates nickel and copper concentrates [17].
The copper concentrate is sent to the copper product. The nickel concentrate is
processing to produce copper products. The nickel concentrate is processed separately
and is roasted, smelted in a reverberatory furnace, and converted to a Bessemer matte
which consist mainly of nickel and copper sulfide and a Nickel copper metallic alloy
are formed. After the cooled matte is crushed and ground, the metallic alloy is
separated by forth flotation. The copper sulfide is returned to the copper smelter for
further processing while the nickel sulfide is roasted to produce various grades of
nickel oxides. The purest nickel oxide products are marketed directly and the less pure
oxides are processed further at Inco’s port colborne, Ontario, and clydach, wales,
nickel refineries to produce commercially pure nickel and other nickel-alloy products
[17].
2.2
ickel- copper alloys
Nickel and copper are completely soluble in each other in all properties.
However, the most important nickel-copper alloys are those containing about 67% Ni
and 33% Cu, which are called Monels [16].
2.3
ickel-chromium alloys
Chromium is an important alloying element for many corrosion-resistant and
high-temperature-resistance nickel-base alloys. It has a high solid solubility
(approximately 30 wt% at room temperature) in nickel [16].
2.4
ickel-iron-base supperalloys
Nickel-base superalloys containing substantial amounts of both nickel and iron
form a second important class of supperalloys. In these alloys, lower-cost iron is
substituted in part for nickel. However, because of their lower nickel content, they are
not able to be utilized at as high temperatures as the nickel-base superalloys [18].
2.4.1
Chemical composition and typical application
Knowledge of the stainless steel and the nickel-base supperalloy led to the
development of the nickel-iron-base superalloys. Most of them contain from 25 to
45% Ni and from 15 to 60% Fe. Chromium from 15 to 38 percent is added for
oxidation resistance at elevated temperatures, while 1 to 6% Mo is also added to most
of them for solid-solution strengthening. Titanium, aluminum, and niobium are added
to combine with nickel for strengthening precipitates. Carbon, boron, zirconium,
cobalt, and some other elements are added for various complex effects. Table 2.1
shows the lists of the chemical compositions and typical application for selected
nickel-iron-base superalloys [19].
Table 2.1 Chemical composition and typical application of nickel-iron-base
superalloys
Alloy
%Ni %Fe %Cr %Mo %Al %Ti %Mn %Si %C
%other
Typical
applications
Inconel
41.5
40
16
0.5
0.2
1.75
0.2
0.2
.03
706
Inconel
53
18.5
18.6
3.1
0.4
0.9
0.2
0.3
0.04
2.9 Nb,
Gas
0.5 Co
components
5.0 Nb
Jet
718
Inconel
turbine
engines,
rocket motores
32.5
44.5
21
0.4
0.4
0.8
0.5
0.05
0.4 Cu
800
Furnace, heat
exchanger
parts
Inconel
32
46
20.5
-
-
1.1
0.8
0.5
0.5
0.2Cu
Heat exchange
42.5
36.0
12.5
5.7
0.2
2.8
0.1
0.1
0.05
0.015 B
Gas
801
Inconel
901
turbine
rotors, blades,
bolts
2.4.2
Microstructure
Most nickel-iron-base superalloys are desired so they have an austenitics FCC
matrix. Since they contain less than 0.1% C and relatively large amounts of ferrite
stabilizers such as chromium and molybdenum, the minimum level of nickel required
to maintain an austenite stabilizers can slightly lower this nickel level. High-nickel
contents are associated with higher useful temperatures and improve malleability, but
also considerably lower the oxidation resistance of these alloys [19].
2.4.3
Solid-solution strengtheners
The solid-solution strengthening elements added to nickel-iron supperalloy are
10 to 25% Cr,0 to 9% Mo, 0 to 5% Ti, 0 to 2% Al, and 0 to 7% Nb. Of these,
molybdenum is the most useful. Chromium is also solid-solution strengthener of the
γ matrix and also enters carbides and γ ' . however, its chief function is to provide
oxidation resistance. Niobium, titanium, and aluminum also provide some solidsolution strengthening of the austenite matrix, but this is not their primary function in
nickel-iron base alloys. Small amounts of carbon and boron are also potent solidsolution strengtheners [19].
2.4.4
Precipitation strengtheners
The most important precipitation strength-enters in nickel-iron-base alloys are
titanium, aluminum, and niobium since they combine with nickel to from
intermetallic phases. An important different in the structure of γ ' and γ ′′ strengthened nickel-iron-base superalloys from the nickel-base alloys is that the Ni-Fe
alloys are all susceptible to the precipitation of one or more secondary phases such as
η , δ , µ , or laves. These phases can be detrimental or beneficial to rupture
properties, depending on their morphology and distribution. Titanium is major γ '
forming element in γ ' strengthened nickel-iron superalloy, which in contrast most
nickel-base superalloys are strengthened principally by aluminum-rich γ ' . aluminum
however, does provide some oxidation resistance to nickel-iron alloys. Niobium is the
principal γ ′′ forming element in γ ′′ strengthened nickel-iron-base superalloys [19].
2.5
Inconel 718
Inconel 718 is an example of a nickel-iron-base superalloy that is strengthened
by niobium-rich γ ' ( NI3 NB , FCC) precipitates. Some aluminum and titanium atoms
may substitute for the niobium. This type of precipitate is in contrast to that found in
other nickel-iron-base superalloys in which the γ ' precipitate is NI3 (Al, Ti).
According to barker et al [20]. FCC γ ' is the main phase which is initially present in
the matrix of alloy 718 heat-treated in the standard precipitation-strengthened
naodition. The γ ' particles were found to be 7.5 to 30 nm in size and were both
spherical and dislike in morphology. When the samples of alloy 718 were exposed for
long period of time at elevated temperatures, the γ ' phase transformed into a BCT
phase of uncertain composition designated NIx NB . Upon even longer exposure
times, part of the NIx NB phase transformed into orthorhombic NI3 NB , which is
lamellar (needle like). After prolonged exposure in the 650 to 700 C range, three
distinct structural shapes were identified the spherical precipitates as FCC γ ' . X-ray
diffraction analysis identified the spherical precipitates as FCC X, the BCT NIx NB
as the small plates, and orthorhombic NI3 NB as the large plates [17].
2.6
High-temperature stress-rupture properties
In general, the nickel-iron-base superalloys cannot be used at as high
temperatures as the nickel-base alloys. Nickel-iron-base alloys that are strengthened
by ordered FCC γ ' (such as A-286 and V-57, which contain about 25 to 26 wt% Ni)
can be used to about 650° C, while alloys which have higher nickel contents (such as
860 and 901, with 42 to 43 wt%) can be used to about 815° C. Inconel 706 and 718,
which are strengthened by a niobium-containing γ ' , can be used to about 650° C.
Figure 2.1 shows the electron micrographs of Inconel 718 sample exposed 705 at 37
Ksi for 6,048 hours [17].
Figure 2.1 Electron micrographs of Inconel 718. (a) immersion etched in 20% HCLmethanol. (b) Electrolytically etched at 2 V in a chormic-phosphoric sulfuric solution.
2.7
Machinability
The properties that make Inconel 718 an important engineering material are
also responsible for its generally poor machinability. Low thermal conductivity (11.4
W/m/K) leads to high cutting temperatures being developed in the cutting zone. These
have been shown to rise from around 900° C at a relatively low cutting speed of 30
m/min up to 1300° C at 300 m/min [21]. The cutting forces generated are also very
high, around double that found when cutting medium carbon alloy steels. Literature
detailing the effects of operating parameters on tool life when machining nickel based
superalloys is comprehensive, however, relatively little of this data refers to the
effects of machining on workpiece surface integrity. The main problems reported are
surface tearing, cavities, cracking, metallurgical recrystalisation, plastic deformation,
microhardness increases and the formation of residual stresses [22–27]. Residual
stress is defined as the stress that persists in the absence of external force [28].
The properties responsible for the poor machinability of the nickel-based superalloys,
especially of Inconel 718, are [29–34]:
-
A major part of their strength is maintained during machining due to their
high-temperature properties.
-
They are very strain rate sensitive and readily work harden, causing further
tool wear.
-
The highly abrasive carbide particles contained in the microstructure cause
abrasive wear.
-
The poor thermal conductivity leads to high cutting temperatures up to 1200C’
at the rake face [21].
-
Nickel-based superalloys have high chemical affinity for many tool materials
leading to diffusion wear.
-
welding and adhesion of nickel alloys onto the cutting tool frequently occur
during machining causing severe notching as well as alteration of the tool rake
face due to the consequent pull-out of the tool materials.
-
Due to their high strength, the cutting forces attain high values, excite the
machine tool system and may generate vibrations which compromise the
surface quality.
The microstructure of Inconel 718 is comprised of an austenitic face centred cubic
(FCC) matrix phase, which is a solid solution of Fe, Cr and Mo in nickel together with
other secondary phases. The main strengthening phase is the precipitate gamma
double prime (denoted γ''). This phase consists of uniformly distributed body centred
tetragonal (BCT) disc shaped particles (of composition NI3 NB ) that are coherent with
the parent matrix. The diameter of these particles is approximately 600 Å by around
50-100 Å thick. Inconel 718 is often used in a solution treated and aged condition, this
involves a solution treatment at 970-1175° C, followed by a precipitation treatment at
600- 815° C [35]. This results in a microstructure of large grains containing the
NI3 NB precipitated phase and a heavy concentration of carbides at the grain
boundaries. The difficulty of dislocation motion through the γ''/ γ' microstructure is
responsible for high tensile and yield strength of Inconle 718 (approximately 1300
and 1100 MPa, respectively, at temperature up to 600 °C) [35].
In machining Inconel 718 alloy, it is well known that the tool temperature rises
easily due to its poor thermal properties. Micro-welding at tool-tip and chip interface
takes place leading to the formation of built-up edge (BUE). The excellent material
toughness results in difficulty in chip breaking during the process. In addition,
precipitate hardening γ ′′ secondary phase ( NI3 NB ) together with work-hardening
during machining makes the cutting condition even worse. All these difficulties lead
to serious tool wear and less material removal rate (MRR) [32, 36].
The difficulty of machining resolves itself into two basic problems: short tool life
and severe surface abuse of machined workpiece [31, 22]. The heat generation and the
plastic deformation induced during machining affect the machined surface. The heat
generated usually alters the microstructure of the alloy and induces residual stresses.
Residual stresses are also produced by plastic deformation without heat. Heat and
deformation generate cracks and microstructural changes, as well as large
microhardness variations [37]. Residual stresses have consequences on the
mechanical behaviour, especially on the fatigue life of the workpieces [38]. Residual
stresses are also responsible for the dimensional instability phenomenon of the parts
which can lead to important difficulties during assembly [39, 40]. Extreme care must
be taken therefore to ensure the surface integrity of the component during machining.
Most of the major parameters including the choice of tool and coating materials, tool
geometry, machining method, cutting speed, feed rate, depth of cut, lubrication, must
be controlled in order to achieve adequate tool lives and surface integrity of the
machined surface [37, 38].
Field and Kahles [41] summarized the metallurgical alterations that occur in
the surface layer as a function of machining parameters in conventional and
nonconventional machining operations of several alloy systems, including Inconel
718. They concluded that it is highly desirable to develop surface integrity data for
specific situations and only in the absence of specific data should general guidelines
be employed or considered for the manufacture of critical components. Bellows [42]
inferred that the mechanical properties of components made from Inconel 718 are
more sensitive to residual stresses than to surface finish, consequently sharp tools
must be maintained at all times.
2.8
Drilling
Twist drills are end-cutting tools used to produce holes in most types of
material. On standard drills, two helical grooves, or flutes, are cut lengthwise around
the body of the drill. They provide cutting edges and space for cutting to escape
during the drilling process. Since drills are among the most efficient cutting tools, it is
necessary to know the main parts, how to sharpen the cutting edges, and how to
calculate the correct speeds and feeds for drilling various metals to use them most
efficiently and prolong their life.
2.9
Twist drill parts
Most twist drills used in machine shop work today are made of high-speed
steel. High-speed steel drills have replaced carbon-steel drills, since they can be
operated at double the cutting speed and the cutting edge lasts longer. High-speed
steel drills are always stamped with the letters “H.S.” or “H.S.S.” since the
introduction of carbides-tripped drills, speeds for production drilling have increased
up to 300% over high-speed steel drills. Carbide drills have made it possible with
high-speed steel drills. Carbide drills have made it possible to drill certain materials
that would not be possible with high-speed steels.
A drill may be divided into three main parts: shank, body, and point (Figure 2.2).
Figure 2.2 The main parts of twist dill
2.9.1
Shank
Generally drills up to ½ in. or 13 mm in diameter have straight shanks, while
those over this diameter usually have tapered shanks. Straight-shank drills are held in
a drill chuck; tapered-shank drills fit into the internal taper of the drill press spindle. A
tang is provided on the end of tapered-shank drills to prevent the drills from slipping
while is cutting and to allow the drill to be removed from the spindle or socket
without the shank being damaged. Figure 2.3 shows two types of drill shanks.
Figure 2.3 Types of drill shanks: (a) straight; (b) tapered
2.9.2
Body
The body is the portion of the drill between the shank and the point. It consists
of a number of parts important to the efficiency of the cutting action.
1- The flutes are two or more helical grooves cut around the body of the drill.
They form the cutting edges, admit cutting fluid, and allow the chips to escape
from the hole.
2- The margin is the narrow, raised section on the body of the drill. It is
immediately next to the flutes and extends along the entire length of the flutes.
Its purpose is to provide a full size to the drill body and cutting edges.
3- The body clearance is the undercut portion of the body between the margin
and the flutes. It is made smaller to reduce friction between the drill and the
hole during the drilling operation.
4- The web is the thin portion in the center of the drill that extends the full length
of the flutes (Figure 2.4). This part forms the chisel edge at the cutting end of
the drill. The web gradually increases in thickness toward the shank to give the
drill strength.
Figure 2.4. The web is a tapered metal column that separates the flutes.
2.9.3
Point
The point of a twist drill consists of the chisel edges, lips, lip clearance,
and heel (Figure 2.5). The chisel edge is the chisel-shaped portion of the drill
point. The lips (cutting edges) are formed by the intersection of the flutes. The lips
must be of equal length and have the same angle so that the drill will run true and
will not cut a hole larger than the size of the drill.
Figure 2.5 The point of a twist drill
The lip clearance is the relief ground on the point of the drill extending from
the cutting lips back to the heel. The average lip clearance is from 8° to 12°
depending on the hardness or softness of the material to be drilled (Figure 2.6).
Figure 2.6 The lip clearance angle of the cutting edges should be 8° to 12° degree.
2.9.4
Drill point characteristics
Efficient drilling of the wide variety of materials used by industry requires a
great variety of drill points. The most important factors determining the size of the
drilled hole are the characteristics of the drill point [43].
A drill is generally considered a roughing tool capable of removing metal quickly. It
is not expected to finish a hole to accuracy possible with a reamer. However, a drill
can often be made to cut more accurately and efficiently by proper drill point
grinding. The use of various point angles and lip clearance, in conjunction with the
thinning of the drill web, will:
1- Control the size, quality, and straightness of the drilled hole
2- Control the size, shape, and formation of the chip
3- Control the chip flow up the flutes
4- Increase the strength of the drill’s cutting edges.
5- Reduce the rate of wear at the cutting edges
6- Reduce the amount of drilling pressure required
7- Control the amount of burr produced during drilling
8- Reduce the amount of heat generated
9- Permit the use of various speeds and for more drilling
2.9.5
Drill point angle and clearance
Drill point angles and clearance are varied to suit the wide variety of material
that must be drilled. The general drill points are commonly used to drill various
materials; however, there may be variation of these to suit various drilling conditions.
The conventional point (118°) is the most commonly used drill point and gives
satisfactory result for most general-purpose drilling (Figure 2.7). The 118° point angle
should be ground with 8° to 12° lip clearance for best results. Too much lip clearance
weakens the cutting edge and causes the drill to chip and break easily. Too little lip
clearance results in the use of heavy drilling pressure; this pressure causes the cutting
edge to wear quickly because of the excessive heat generated and places undue strain
on the drill and equipment.
Figure 2.7. The drill angle of 118 degree is suitable for most general work; (b) a
drill point angle of 60 to 90 degree is used for soft material; (c) a drill point angle
of 135 to 150 degree is best for hard material.
The long angle point (60° to 90°) is commonly used on low helix drills for the
drills for the drilling of nonferrous metals, soft cast iron, plastics, fibers, and wood.
The lip clearance on long angle point drills is generally from 12° to 15°. on standard
drill, a flat may be ground on the face of the lips to prevent the drill from drawing
itself into the soft material.
The flat angle point (135° to 150°) is generally used to drill hard and tough
materials. The lip clearance on flat angle point drills is generally only 6° to 8° to
provide as much support as possible for the cutting edges. The shorter cutting edge
tends to reduce the friction and heat generated during drilling.
2.10
Drilling facts and problems
The cutting efficiency of a drill is determined by the characteristics and
condition of the point of the drill. Most new drills are provided with a general-purpose
point (118° point angle and an 8° to 12° lip clearance). As a drill is used, the cutting
edges my wear and become chipped, or the drill my break. Drills are generally
resharpened by hand. A properly ground drill should have thee flowing
characteristics:
-
The length of both cutting lips should be the same. Lips of unequal length will
force the drill point off center, causing one lip to do more cutting than the
other and producing an oversize hole.
-
The angle of both lips should be the same. If the angles are unequal, the drill
will cut an oversize hole because one lip will do more cutting than the other.
-
The lips should be free from nicks or wear.
-
There should be no sign of wear on the margin.
If the drill does not meet all of these requirements, it should be resharpened. If
the drill is not resharpened, it will give poor service, will produce inaccurate holes,
and may break because of excessive drilling strain.
While a drill is being used, there will be signs to indicate that the drill is not cutting
properly and should be resharpened. If the drill is not sharpened at the first sign of
dullness, it will require extra power to force the slightly dulled drill into the work.
This causes more heat to be generated at the cutting lips and results in a faster rate of
wear.
When any of the following conditions arise while a drill is in use, it should be
examined and reground:
-
The color and shape of the chips change.
-
More drilling pressure is required to force the drill into the work.
-
The drill turns blue because of the excessive heat generated while drilling.
-
The top of the hole is out of round.
-
A poor finish is produced in the hole.
-
The drill chatters when it contacts the metal.
-
The drill squeal and may jam in the hole.
-
An excessive burr is left around the drilled hole.
Excessive speed will cause wear at outer corners of drill (Figure 2.8); this
permits fewer regrinds of drill due to amount of stock to be removed in
reconditioning. Discoloration is warning sign of excess speed [43].
Figure 2.8 Wear at outer corners of drill
Excessive feed sets up abnormal end thrust, which causes breakdown
of chisel point and cutting lips (Figure 2.9). Failure induced by this cause will
be broken or split drill [43].
Figure 2.9 Breakdown of chisel point
Excessive clearance results in lack of support behind cutting edge with quick
dulling and poor tool life (Figure 2.10-a), despite initial free cutting action. Clearance
angle behind cutting lip for general purposes is 8° to 12° degree. Insufficient
clearance causes the drill to rub behind the cutting edge, it will make the drill work
hard, generate heat, and increase end thrust (Figure 2-10-b). This results in poor holes
and drill breakage.
(a) Excessive clearance
(b) Insufficient clearance
Figure 2.10 Cutting angle
Improper web thinning is the result of taking more stock from one cutting
edge than from the other, thereby destroying the concentricity of the web and outside
diameter (Figure 2.11).
Figure 2.11 The web is the tapered central position of the body that joints the
lands
Cutting lips with unequal angles will cause one cutting edge to work harder
than the other this cause torsion strain, Bellmouth holes, rapid dulling, and poor tool
life (Figure 2.12).
Figure 2.12 Cutting lips with unequal angles
Cutting lips unequal in length cause chisel point to be off center with axis and
will drill holes oversize by approximate twice the amount of eccentricity (Figure 213).
Figure 2-13 Cutting lips with unequal length
2.11
Cause of drill failure
Drills should not be allowed to become so dull that they cannot cut. Over
dulling of any metal-cutting tool generally results in poor production rates, inaccurate
work, and the shortening of the tool life [43]. Premature dulling of a drill may be
caused by any one of a number of factors:
-
The drill speed may be too high for the hardness of the material being cut.
-
The feed may be too heavy and overload the cutting lips.
-
The feed may be too light and cause the lips to scrape rather than cut.
-
There may be hard spots or scale on the work surface.
-
The work or drill may not be supported properly, resulting in springing and
chatter.
-
The drill point may be incorrect for the material being drilled.
-
The finish on the lips may be poor.
2.12
Cutting tool
Principal categories of cutting tools include single point lathe tools, multipoint
milling tools, drills, reamers, and taps. All of these tools may be standard catalogue
items or tooling designed and custom-built for a specific manufacturing need.
Different machining applications require different cutting tool materials. The ideal
cutting tool material should have all of the following characteristics:
• Harder than the work it is cutting
• High temperature stability
• resists wear and thermal shock
• Impact resistant
• Chemically inert to the work material and cutting fluid
No single cutting tool material incorporates all these qualities. Instead, tradeoffs occur among the various tool materials. For example, ceramic cutting tool
material has high heat resistance, but has a low resistance to shock and impact. Every
new and evolving tool development has an application where it will provide superior
performance over others. Many newer cutting tool materials tend to reduce, but not
eliminate the applications of older cutting tool materials.
2.12.1 Cutting Tool Materials
As rates of metal removal have increased, so has the need for heat resistant
cutting tools. The result has been a progression from high-speed steels to carbide, and
on to ceramics and other superhard materials.
a) High Speed Steel (HSS): Developed around 1900, high-speed steels cut four
times faster than the carbon steels they replaced. There are over 30 grades of
high-speed steel, in three main categories: tungsten, molybdenum, and
molybdenum-cobalt based grades. Since the 1960s the development of
powdered metal high-speed steel has allowed the production of near-net
shaped cutting tools, such as drills, milling cutters and form tools. The use of
coatings, particularly titanium nitride, allows highspeed steel tools to cut faster
and last longer. Titanium nitride provides a high surface hardness, resists
corrosion, and it minimizes friction.
b) Cemented Tungsten Carbides: In industry today, carbide tools have replaced
high speed steels in most applications. These carbide and coated carbide tools
cut about 3 to 5 times faster than high-speed steels. Cemented carbide is a
powder metal product consisting of fine carbide particles cemented together
with a binder of cobalt. The major categories of hard carbide include tungsten
carbide, titanium carbide, tantalum carbide, and niobium carbide. Each type of
carbide affects the cutting tool’s characteristics differently. For example, a
higher tungsten content increases wear resistance, but reduces tool strength. A
higher percentage of cobalt binder increases strength, but lowers the wear
resistance. Carbide is used in solid round tools or in the form of replaceable
inserts. Every manufacturer of carbide tools offers a variety for specific
applications. The proper choice can double tool life or double the cutting
speed of the same tool. Shock-resistant types are used for interrupted cutting.
Harder, chemically-stable types are required for high speed finishing of steel.
More heat resistant tools are needed for machining the superalloys, like
Inconel and Hastelloy. There are no effective standards for choosing carbide
grade specifications so it is necessary to rely on the carbide suppliers to
recommend grades for given applications. Manufacturers do use an ANSI
code to identify their proprietary carbide product line. Two-thirds of all
carbide tools are coated. Coated tools should be considered for most
applications because of their longer life and faster machining. Coating
broadens the applications of a specific carbide tool. These coatings are applied
in multiple layers of under 0.001 of an inch thickness. The main carbide insert
and cutting tool coating materials are titanium carbide, titanium nitride,
aluminum oxide, and titanium carbonitride.
c) Ceramic: Ceramic cutting tools are harder and more heat-resistant than
carbides, but more brittle. They are well suited for machining cast iron, hard
steels, and the superalloys. Two types of ceramic cutting tools are available:
the alumina-based and the silicon nitride-based ceramics. The alumina-based
ceramics are used for high speed semi- and final-finishing of ferrous and some
non-ferrous materials. The silicon nitride-based ceramics are generally used
for rougher and heavier machining of cast iron and the superalloys.
d) Cermet: Cermet tools are produced from the materials used to coat the carbide
varieties: titanium carbides and nitrides. They are especially useful in
chemically reactive machining environments, for final finishing and some
turning and milling operations.
e) Superhard Materials: Superhard tool materials are divided into two categories:
cubic boron nitride, or "CBN", and polycrystalline diamond, or "PCD". Their
cost can be 30 times that of a carbide insert, so their use is limited to wellchosen, cost effective applications. Cubic boron nitride is used for machining
very hard ferrous materials such as steel dies, alloy steels and hard-facing
materials. Polycrystalline diamond is used for non-ferrous machining and for
machining abrasive materials such as glass and some plastics. In some high
volume applications, polycrystalline diamond inserts have outlasted carbide
inserts by up to 100 times.
For the improvement of tool lives, surface and coating technologies have
developed rapidly to produce several types of coated tools for machining of difficultto-machine materials. The cemented carbide tools are still largely used for machining
the nickel-based superalloys, especially for the Inconel 718 [39,40,31]. However, In
order to achieve higher cutting speeds, coated cemented carbides have been developed
[44,45].
In the following, typical results from the literature using coated and uncoated
carbide tools in turning, milling and drilling operations of Inconel 718 will be
presented. . Itakura et al. [46], conducted dry turning experiments to identify clearly
the tool wear mechanisms when a commonly used coated cemented carbide tool cuts
Inconel 718. Jindal et al. [47] studied the relative merits of PVD TiN, TiCN and
TiAlN coatings on cemented carbide substrate (WC—6wt.% Co alloy) in the turning
of Inconel 718. The tested cutting speeds were 46 and 76 m/min, the feed rate and the
depth-of-cut were maintained constant and respectively equal to 0.15 mm/rev and
1.5mm. At both speeds, TiAlN and TiCN coated tools performed significantly better
than tools with TiN coatings. The maximum flank wear was about 0.15mm after a
cutting time of 5 min. In addition the TiAlN tools exhibit lower nose and crater wear
than the TiCN and TiN coated tools. . Panjan et al. [48] studied TiN/AlTiN and
CrN/TiN nanolayer coatings deposited on a K20 cemented carbide and its machining
performance was tested by turning Inconel 718 alloy. The performance of the
nanolayer coated tools was compared with those of classical mono and multilayer
coated and uncoated inserts. Abrasive nose wear and chipping at the cutting edge
were the main failure modes observed. The depth-of-cut notch is considered as
determinant for tool life during machining Inconel 718. The notching is influenced by
burr formation on the uncut diameter; this failure mode is mainly due to the hardening
of the material during machining. This phenomenon appeared for uncoated or
CrN/TiN coated tool and was attenuated with TiN/AlTiN nanolayer coated insert.
According to the authors, this was probably due to better chip sliding and a reduced
cutting temperature with this coating. Abrasive wear is mainly due to carbide particles
in Inconel 718. The high hardness of the TiN/AlTiN nanolayer coating (Hardness
HV0.05 = 3900) provides better abrasion resistance than classical multilayer and
monolayer structures. In addition, TiN/AlTiN nanolayer coating presents a better
resistance to welding. High temperature resistance of AlTiN included in this coating
allows better resistance to the built-up-edge phenomenon than CrN/TiN nanolayer
coating.
Derrien and Vigneau [49] found that TiN coated tools resulted in higher tool
life and lower surface roughness (Ra) than uncoated tools when contour milling
Inconel 718. Gatto and Iuliano [3] suggested that CrN and TiAlN coatings improved
tool performance by acting as a thermal barrier and therefore preventing the high
temperature generated in the cutting process from softening the substrate. Sharman et
al. [29] also examined TiAlN and CrN coated carbide tools in end milling of Inconel
718. They found that TiAlN gave the overall better performance compared to CrN,
due to the lower hardness (lower abrasive wear resistance) and higher chemical
affinity of CrN to Inconel 718. This resulted, based on the wear mechanism proposed
by Liao and Shiue [44], in faster exposure of the carbide substrate and therefore
higher wear.
Sharman et al. [50] studied TiAlN multilayer PVD, TiN/TiAlN multilayer
PVD coated and uncoated cemented carbide tool and its machining performance
during drilling Inconel 718. They have reported that drills, TiAlN multilayer PVD and
TiN/TiAlN multilayer PVD tested failed due to localised wear exceeding 0.5mm at
the drill periphery. Chen [15] used multi-layer TiAlN PVD coated tungsten carbide
twist drill when drilling Inconel 718. He stated that Friction force is found to be the
most important factor governing tool failure. Wear mechanisms can be classified into
four stages. The coated layer is abraded-off first. It is followed by flank wear, and
chipping at the outer cutting edge.
2.13
Surface Integrity
Numerous investigations confirm that the quality and especially the lifetime of
the dynamically loaded parts are very much dependent on the properties of the
material in the surface [51]. Severe failures produced by fatigue, creep and stress
corrosion cracking invariably start at the surface of components and their origins
depend to a great extent on the quality of the surface [52]. Therefore, in machining
any component it is first necessary to satisfy the surface integrity requirements.
Surface
integrity
refers
to
residual
stress
analysis,
microhardness
measurements, surface roughness and degree of work hardening in the machined subsurfaces and they were used as criteria to obtain the optimum machining conditions
that give machined surfaces with high integrity. Field and Kahles [41] have defined
surface integrity as the relationship between the physical properties and the functional
behaviour of a surface. The surface integrity is built up by the geometrical values of
the surface such as surface roughness (for example, Ra and Rt), and the physical
properties such as residual stresses, hardness and structure of the surface layers.
2.14
Surface roughness
In addition to the more straightforward dimensional characteristics of an
engineering product, its performance, appearance and cost are likely to be strongly
influenced by the quality of the finish on the various surfaces. This may be important
for a variety of reasons. The most oblivious is that the surface has a function which
involves contact with another surface. This can be moving contact, as in the case of a
bearing diameter, or static, as in the case of surfaces required to provide an oil tight
joint. Finish might also be important in the interests of reducing stress on the part and,
particularly if on an external surface, it might be important merely for aesthetic
appearance.
2.14.1 Quantification of surface roughness
Surface finish can be accurately quantified, and several different principles
have been used to achieve the desirable objective of expressing the requirement and
the measuring in terms of only one number. The four main methods are indicated in
the diagrammatic representation of the cross-section of a surface shown in Figure
2.14. These are Center Line Average, referred to as Ra, Root Mean Square, refereed
to as RMS, maximum peak to valley, refereed to as Ra, and maximum peak to mean,
referred to as Rp.
Figure 2.14 Cross-section of a surface
Whilst all these parameters have some relevance, depending on the role the
surface has to play, the most common method is the Center Line Average, and it is
worth describing in some detail the way in witch this figure is derived.
Figure 2.15 Sampling length
In the figure diagram at Figure 2.15 a straight line x-x is drawn by eye
following the general direction of the profile and covering the sampling length L. The
areas of the profile p above, and q below this line are then measured and a distance z
is obtained by dividing the difference between these areas by the sampling length, i.e.
z=
areas p - areas q
L
If a new line y-y is now drawn parallel to x-x at a distance z from it, this new
line will be a mean centerline, and the CLA value (Ra) will be the sum of the area
above and below this line divided by the sampling length, i.e.
Ra =
areas r + area s
L
The length over which the sample is taken is obviously very important, and in
the example shown in Figure 2.15 the sample length L is adequate to give a measure
of the total surface because it covers a significant number of complete surface finish
cycles. If, however, the combination of sampling length and finish characteristics is
such that the sample contains say, only one or even less total cycles, the result will not
include all the characteristics of the surface. The sampling length is used extensively
in surface measurement to segregate the various characteristic of the surface.
As shown in Figure 2.16 a normal finish consists of several different elements.
These are referred to in this drawing as primary texture, secondary texture and form
errors, but they might also be described as roughness, waviness and flatness
respectively. The term “surface finish” is normally used to describe the first, and
perhaps the second of these elements. Errors of flatness are usually considered, and
measured, separately.
Figure 2.16 Several different elements of a normal finish
The parameter which is necessary to achieve this is the cut-off value. This is
the length of the surface sample to be considered, and all features within this length
will be included to arrive at, foe example, the CLA value. In figure 2.16 the effect of
three cut-off values, L1, L2 and L3, is indicated. It will be seen that if a cut-off value
of L1 were selected, the reading H1 obtained would cover only the primary texture.
The values H2 and H3 obtained from L2 and L3, however, would, in addition, include
the secondary texture and the form respectively. Whilst it is possible to obtain a finish
reading for any cut-off value, there are again preferred values. The imperial units
range, for example, cover six option between 0.003 in and 1.0 in. Since surface for
which measurement of the finish in quantitative terms is required are likely to be fine,
small sampling lengths are appropriate and the normal standard is 0.030 in. such a
sample is not likely to cover all the characteristics of a surface, but experience has
shown that, in practice, it is the most useful. In metric units the figure used is 0.8 mm.
Surface produced by normal engineering methods would, if looked at in cross-section,
generally show a different profile in different directions. Cutting processes, such as
turning and boring, produce a surface which is evenly spaced and unidirectional, as
indicated in Figure 2.17.
Figure 2.17 Different profile in different directions
Grinding generally produces a surface which is unidirectional, but does not
have regular cycles. Operation such as lapping and polishing produce very fine
surface but they are both multidirectional and irregular. The direction in which the
cutting tool moves is known as the “lay”. Normally, surface finish would be measured
in the direction which gives maximum roughness, and this is likely to be in a direction
at right angles to the lay.
2.14.2 Effective parameters on surface roughness in drilling
Sharman et al [50] did some drilling experiments on Inconel 718 using five
different 8mm diameter drills. Each of these drills had slightly different geometries,
substrate grades and coatings although a number of similarities can be seen (Table 2.2).
Table 2.2 Geometry and coating details
Tool
Coating
Substrate
Point angle
Helix angle
Web width
SS
TiAlN
multilayer
90% WC 10% Co
<1µm grain
140
35
0.15
130
30
0.2
130
30
0.15
130
30
0.15
130
30
0.15
PVD
size
DS
TiAlN
multilayer
PVD
90% WC 10% Co
<1µm grain
size
CS
TiN/TiAlN
multilayer
PVD
90% WC 10% Co
<1µm grain
size
SD
TiAlN
multilayer
PVD
90% WC 10% Co
<1µm grain
size
SD2
Uncoated
90% WC 10% Co
<1µm grain
size
They reported that the majority of surface roughness results fall between 1 and
1.5 µm Ra with a wide range of scatter around these values and there is little difference
between the values obtained with different drills, only for drill CS was a significant
difference (Figure 2.18). For both new and worn conditions this drill produced the lowest
surface roughness and the lowest scatter in surface roughness measurements. For drills
CS and SS it appears that surface finish has improved with increasing drill wear however
only for drill CS did an independent t-test show this to be statistically significant at the
5% level.
Figure 2.18: Surface roughness measurements
Chen and Lio [15] investigated wear mechanism of the TiAlN coated carbide
tool in drilling Inconel 718. They reported that deterioration of surface roughness is
greatly improved with the application of nano-modifier fluid and the use of uncoated
carbide drill and nano-modifier fluid results in even better drill life than the use of
coated carbide drill and traditional cutting fluid (Figure 2.19).
Figure 2-19. Surface roughness values when different cutting fluids was applied
Nouari and List [53] stated that drilling with the coated and uncoated
carbide drills produced similar surface finishes while higher surface roughness
values, hence poor surface finish, were recorded when drilling with HSS drills
during dry drilling of AA2024 aluminium alloy.
Kao and Yao [54] have studied on thrust forces, torque, flank wear and hole
surface roughness during the drilling of AISI 1045 steel workpieces using Tibased (Ti–C:H, Ti–C:H/TiC/TiCN/TiN and TiC/TiCN/TiN) and Cr-based (Cr–
C:H/CrC/CrCN/CrN, Cr–C:H and CrC/CrCN/CrN) Me–C:H coated high-speed
steel drills. They reported that the surface roughness of the hole is affected by
two factors, namely the cutting speed and the coating properties. As the cutting
speed increases, the workpiece material readily adheres to the cutting edge.
This causes the formation of a built-up edge, which increases the surface
roughness (as shown in Figure 2.20). They showed that the hole roughness
generated by the Ti-based coated drills with a top Ti–C:H coating (Ti0025,
Ti0050 and Ti2525) is lower than that produced by the Ti-based coated drill
with no overcoat, or by any of the Cr-based coated drills. Furthermore, the
machined surfaces of the holes produced by the Ti2500, Ti5000, Cr1010 and
Cr4400 coated drills at 310 rpm, or by the Ti2500, Cr0025 and Cr1010 coated
drills at 480 rpm, are of an inferior quality to those produced by an uncoated
drill.
Figure 2.20. Surface roughness of AISI 1045 steel finished by different coated drills
after drilling 12 holes at spindle speed of 310 rpm and 480 rpm, respectively.
Tsann and Lin [55] have investigated the tool life and surface roughness,
tool wear and burr formation during drilling of stainless steel using a Ti-N
coated carbide tool. They stated that the surface roughness increases with feed
rate for different cutting speeds. They found that the optimum cutting speed is
V=75 m/min from the standpoint of the minimum surface roughness. This is
because at high cutting speed (V =85 m/min) there was high vibration, whilst
outer corner wear occurred easily at low cutting speed (V =65 m/min). The
surface roughness produced was less than 1 mm with a cutting speed of V =75
m/min and a feed rate of f =0.1 mm/rev, and the surface produced was
generally very smooth.
2.15
Microhardness
The term "microhardness" has been widely employed in the literature to
describe the hardness testing of materials with low applied loads; however,
microhardness implies that the hardness is very small rather than the load. A more
precise term is "microindentation hardness testing." In microindentation hardness
testing, a diamond indenter of specific geometry is impressed into the surface of the
test specimen using a known applied force (commonly called a “load” or “test load”)
of 1 to 1000 gf. Microindentation tests typically have forces of 2 N and produce
indentations of about 50 µm. Due to their specificity, microhardness testing can be
used to observe changes in hardness on the microscopic scale. Unfortunately, it is
difficult to standardize microhardness measurements; it has been found that the
microhardness of almost any material is higher than its macrohardness. Additionally,
microhardness values vary with load and work-hardening effects of materials.
Regardless, the two most commonly used microhardness tess are the Knoop and
Vickers tests.
In microindentation testing, the hardness number is based on measurements
made of the indent formed in the surface of the test specimen. The hardness number is
based on the surface area of the indent itself divided by the applied force, giving
hardness units in kgf/mm². Microindentation hardness testing can be done using
Vickers as well as Knoop indenters. For the Vickers test, both the diagonals are
measured and the average value is used to compute the Vickers pyramid number. In
the Knoop test, only the longer diagonal is measured, and the Knoop hardness is
calculated based on the projected area of the indent divided by the applied force, also
giving test units in kgf/mm².
The Vickers microindentation test is carried out in a similar manner to the
Vickers macroindentation tests, using the same pyramid. The Knoop test uses an
elongated pyramid to indent material samples. This elongated pyramid creates a
shallow impression, which is beneficial for measuring the hardness of brittle materials
or thin components. Both the Knoop and Vickers indenters require prepolishing of the
surface to achieve accurate results. Scratch tests at low loads, such as the Bierbaum
microcharacter test, performed with either 3 gf or 9 gf loads, preceded the
development of microhardness testers using traditional indenters. In 1925, Smith and
Sandland of the UK developed an indentation test that employed a square-based
pyramidal indenter made from diamond. They chose the pyramidal shape with an
angle of 136° between opposite faces in order to obtain hardness numbers that would
be as close as possible to Brinell hardness numbers for the specimen. The Vickers test
has a great advantage of using one hardness scale to test all materials. ASTM
Specification E384 states that the load range for microhardness testing is 1 to 1000 gf.
For loads of 1 kgf and below, the Vickers hardness (HV) is calculated with an
equation (Equation 2.1), wherein load (L) is in grams force and the diagonal (d) is in
micrometers:
Equation 2.1: Vickers hardness HV = 1854.4 ×
L
d2
For any given load, the hardness increases rapidly at low diagonal lengths,
with the effect becoming more pronounced as the load decreases. Thus at low loads,
small measurement errors will produce large hardness deviations. Thus one should
always use the highest possible load in any test. Also, in the vertical portion of the
curves, small measurement errors will produce large hardness deviations.
2.15.1 Effective parameters on microhardness changes in drilling
Canteroa and Tardi [56] evaluated the tool wear, quality of machined holes
and surface integrity of work-piece, in the dry drilling of alloy Ti–6Al–4V using TiNcoated fine-grain carbide drill. They measured Microhardness (Vickers) in five points
(located at a distance from machined surface ranging from 50 to 600 mm) in two lines
perpendicular to the drill displacement direction (Figure 2-21). They also reported in
the zone close to machined surface (distance 75 mm) value of 420 HV was obtained,
approximately 30% greater to hardness obtained in material before machining. These
phenomena, but less pronounced, were observed at shorter cutting time, because
prolonged machining with nearly worn tools, produced severe plastic deformation and
thicker disturbed layer on the machined surface and the hardness of the disturbed
layer of the machined surface increased significantly.
Figure 2-21 SEM micrograph showing three microindentation marks on a region
approximately 2 mm away from the drill exit, for the 128th hole machined
In other research Sharman [50] stated that in drilling Inconel 718 For all the
drills examined the workpiece surface hardness was increased compared to bulk
(~500 HK0.05) with a return to bulk values within the first 50µm depth below the
surface (Figure 2.22). There was also no difference between the hardness profiles seen
when cutting with a worn or unworn drill. This result was most likely caused by the
fact that it is the flute margins that are responsible for forming the hole surface and in
comparison to the wear encountered at the cutting edges the flutes are relatively
unworn.
Figure 2.22. Typical microhardness profile from drilling (range bars show standard
deviation of results)
2.16 Microstructural changes
Sharman [50] showed that in drilling Inconel 718 the subsurface microstructural
damage seen in all the holes produced consisted of deformed grain boundaries and
white layer in the direction of cutting (Figure 2.23).
Figure 2.23 (a) Grain boundary deformation and white layer from drilling. (b)
Microstructure resulting from Mill Boring.
White layer is a resulte of microstructural alteranation. It is called “white”
layer because it resists standard etchants and appears white under an optical
microscope (or featureless in a scanning electron microscope). In addition, the white
layer has high hardness, often higher than the bulk. White layers are found in many
material removal processes such as grinding [57-59], electrical discharge machining
[60] and drilling [61]. Large plastic deformation and or rapid heating cooling are
possible formation mechanisms. White layers seem to be detrimental to product
performance, and therefore require a post-finishing process. White layers seem to be
detrimental to product performance, and therefore require a post-finishing process.
Most noted that white layer occurs when cutting tools wear out to a certain level, but
did not provide an in-depth explanation.
Tonshoff et al [62] studied the influence of hard turning on workpiece
properties and properties and reported that retained austenite is the major composition
of white layer structures. A higher thrust force component seems to accompany white
layer occurrence, as does tensile residual stress. They further showed that the white
layer decreases bending fatigue strength probably due to associated tensile residual
stress. Tool wear was suggested at the most influential parameter on white layer
formation, though frequently it was the only variable studied. However, the
explanation of white layer formation was rather qualitative and, thus, there was no
important that optimization of surface structure or minimization of white layer is
possible. Several factors may limit tool life and therefore affect machining cost. In a
finishing process, surface integrity is often of great concern because of its impact on
product performing; indeed, it may be used as a tool-changing criterion. Thus,
understanding tool wear and cutting parameter effect on surface integrity is of
practical significant.
2.17 Burr formation
Nickel and its alloys are used widely in aerospace, pressure vessels, aircraft
turbine and compressor blades and disks, surgical implants, etc. The alloys are
difficult to machine and, in particular, burr formation due to drilling is troublesome in
aerospace applications due to the difficulty of completely removing the burrs.
Estimates are that up to 30% of the cost of some components is due to deburring
operations. Most drilling processes create a burr on both entrance and exit surfaces.
The exit burr is much larger in size and is the main concern. In multi-layer materials
(for example multi-layer metal-composite laminates) the burr at the exit surface
between layers is a major problem requiring disassembly of the laminate, deburring
and re-assembly.
Preventing, or at least minimizing (or controlling) the formation of drilling
burrs is therefore very important. Burr formation in drilling has not been as
extensively studied as drilling itself. Most studies are concerned with tool wear or
hole quality and don’t consider burr formation. And, the studies usually focus on
conventional materials. Gillespie [63] was one of the first researchers to study burr
formation at an academic level in several machining operations including drilling. For
drilling, he studied the effects of process conditions, tool geometry and material
properties on burr formation over a wide range of test conditions and proposed a basic
model of burr formation in drilling. Gillespie’s studies on titanium alloys covered
hole quality issues and evaluated the influence of drill wear land size on burr size. No
influence was found in that study. Importantly, most of Gillespie’s tests were done
with hand fed drills (hence feed rate is unknown and not controlled) so the influence
of feed rate is confounded with other parameters studied. Sakurai et al [64] noted the
reduction in burr height with the vibratory step feed drilling of Ti-6AI-4V but offered
no explanation. Sofronas [65] made a fundamental study of burr formation in drilling
but for carbon steel.
Stein and Domfeld [66] studied the burr formation of miniature hole drilling in
stainless steel. Increases in feed, cutting speed and drill wear were found to increase
the burr height and thickness. Drill pecking stabilized the burr formation leading to a
process with less variation and uncertainty regarding the exit burrs. Link [67]
concluded that it is necessary to take the temperature dependency of material
properties into account when explaining burr formation phenomena. The nonlinearity
of these properties is the main reason there exists no general model of burr formation.
Dornfeldl and Kim [68] have done some studies on burr deformation during
drilling the Ti-6Al-4V using carbide drills for dry cutting and Cobalt high speed steel
drills for wet cutting. They have reported that four type of burr will be formed during
dry cutting. Figure 2.24 shows four burr types categorized by cross-section shape
created under different machining conditions. Type I is a uniform burr which has
uniform height and thickness. Type II is similar to Type I but has a "leaned-back"
cross section. Type Ill has a severe rolledback shape and Type IV is also rolled-back
but has a relatively small burr height with widened exit. For each cutting condition
and burr type, a drill cap was formed. However, the drill caps were separated from the
workpiece during the process. The shape of the drill caps were different depending on
the drill type due to the difference in point angle. Drill caps can be a problem in
intersecting holes or interior cavities where removal is difficult.
(a) Type I
(b) Type II
(c) Type 111
(d) Type IV
Figure 2.24: Burr types formed in dry cutting.
The roll back phenomena can be explained through the burr formation
mechanism illustrated in Figure 2.25. As the drill approaches the exit surface, the
material under the chisel edge begins to deform, (b). The distance from the exit
surface to the point where the deformation starts depends on the thrust force of the
drill. As the drill advances, the plastic deformation zone expands from the center to
the edge of the drill, (c). One of the cutting edges advancing will cause separation of
the cap from the hole perimeter, (d). Depending on the point angle and drill point
geometry, initial rupture may occur near the center of the remaining material. At the
final stage, the remaining material at the hole perimeter is pushed out to form a burr
with a drill cap, (e). While the remaining material at hole perimeter is being formed
into a burr no more cutting occurs (no chip formation) which means there is no way to
dissipate the generated heat. The low thermal conductivity of the material inhibits
heat dissipation. Thus, there should be a localized temperature increase at the inner
surface of the burr. This temperature increase and resulting thermal expansion is
believed to be the main cause of the lean back and roll back phenomena observed.
The amount of heat generation and temperature rise is proportional to cutting speed
and feed rate. Higher feed rate and cutting speed will generate more heat, and result in
more rolling-back as observed here.
Figure 2.25: Formation of a burr with drill cap
They have also reported three type burr will be formed during wet cutting.
Figure 2.26 shows three representative types of burrs observed in wet cutting
experiments. They are a standard uniform burr without any attachment, a burr with a
drill cap and a burr with "ring formation". The most common type is the burr without
attachment. A burr with a drill cap occurred at the lowest feed rate of 0.04 mm/rev.
The uniform portion of all the burrs was the Type I burr seen in dry cutting. This is
the burr at the hole perimeter. No rolled-back burr was observed and this supports the
proposed explanation of roll back formation in dry cutting.
Burr with ring formation
Burr without attachment
burr with a drill cap formation
Figure 2.26: Burrs produced in wet cutting.
The burr with a ring is believed to be an intermediate type between the plain
uniform burr and drill cap formation. The drill continues to remove material from the
workpiece even after breaking through the exit surface. However before the normal
bending process occurs which is responsible for the formation of uniform burr, the
remaining ring-shaped material in front of the drill is not able to sustain the thrust
force of the drilling operation and detaches from the workpiece. This detached
material has the shape of a ring and leaves a small "secondary burr" on the hole
perimeter as often seen in peripheral milling.
Both feed rate and cutting speed seem to have influence on the burr formation. Figure
2.27 shows the correlation between the burr formation and the cutting conditions.
Figure 2.27: Correlation between the burr formation and the cutting conditions
The correlation between burr formation and tool geometry is seen in Figure
2.28 (a) and (b). Lip relief angle seems to have little influence on burr formation. Lip
relief angles used were large enough compared to the feed rate so that contact
between the flank of the drill and the workpiece was minimal. Increasing point angle
produced burrs of decreased height and thickness and increasing helix angle increased
burr size. Concerning the style of the drills, the helical point drill proved to be very
suited to minimize the exit burr formation. The burr height and thickness were
reduced with a helical point drill. This reduction in burr size is due to the smaller
thrust force that is required for the helical drill compared to the split point drill.
(a): Lip relief angle (degree)
(b): Point angle (degree)
Figure 2.28: Correlation between the burr formation and the point angle and the lip
relief angle.
CHAPTER 3
RESEARCH METHODOLOGY
3.1
Introduction
Proper experimental plan is necessary to achieve good results in conducting
research. In this chapter all the equipments used in this study are described
accordingly.
3.2
Research Design Variables
The design variables are described into two main groups, which are dependent
variables response variables and independent variables (machining parameters).
The response variables include:
1- Surface integrity which include surface roughness, microhardness changes and
microstructure of drilled surface.
2- geometrical accuracy
3- burr height
The parameters involved in this study are shown in Table 3.1.
Table 3.1 Machining parameters
Machine Tools/Equipment
MAHO
MH
700S
CNC
milling
machine
3.3
Workpiece Material
Inconel 718
Cutting Tool, 2 flute drill
1) Uncoated carbide (WC/Co)
Cutting speed (m/min)
10-20
Feed rate (mm/rev)
0.03-0.012
Depth of cut (mm)
18
Tool diameter (mm)
6
Type of Cutting
Through Hole
Coolant
6% concentration
Workpiece Material
Inconel 718 (nickel alloys) was chosen as the workpiece material for the test
specimens. The mechanical properties and chemical compositions of the Inconel 718
is shown in Tables 3.2 and 3.3 respectively. Figure 3.1 shows the workpiece material
of 105 mm × 105 mm× 18 mm that was prepared for experiments.
Table 3.2 Mechanical properties of Inconel 718
Tensile strength (ksi)
199.9
Yield strength (ksi)
161.1
Elongation (%)
20
Reduction in Area (%)
50
Density (lb/in3)
0.296
Hardness (HRC)
43
Grain size (µm)
6
Table 3.3 Chemical composition of Inconel 718
i
Cr
Mo
Fe
b
Ti
≥54
18
3.0
18.5
5.0
1.0
Figure 3.1 Workpiece material
3.3.1
Analysis the workpiece material
In order to specify the microstructure study of workpiece material, a sample
specimen was prepared using standard metallography techniques. In the first step
metallographic specimen was cold mounted (Figure 3.2) using BUEHLER low
viscosity epoxy that requared 18-20 hour curing time in the temperature of 27° C.
After mounting, the specimen was wet ground to reveal the surface of the metal. The
specimen was successively ground with fine and finer grades of silicon carbide paper
from 100 to 4000 mesh number to remove damage from sectioning and then from
each grinding step. After grinding the specimen, polishing was performed. Typically,
specimen was polished with slurry of alumina on a napless cloth to produce a scratchfree mirror finish, free from smear, drag, or pull-outs and with minimal deformation
remaining from the preparation process.
Figure 3.2 Mounted specimen
After polishing the specimen was etched using electrolytic etchant by Buehler
electromet (Figure 3.3). The speciment was etched in sulfuric acid (3%) at the
electrical condition of 6 volts and temperature of 24° C for 15 sec. The material of the
cathode used was stainless steel. This etchant method is suitable for showing the
carbides and grain boundaries of Inconel and nickel alloys.
Figure 3.3 Buehler electromet
Prepared specimen should be examined after etching with the unaided eye to
detect any visible areas that respond differently to the etchant as a guide to where the
microscopical examination should be employed. Specimen was examined under an
Olympus toolmakers’ light optical microscope which is connected to Sony Digital
hyper head color video camera. Figure 3.4 and 3.5 show samples grain structure of
specimen at magnification of 100X and 200X.
Figure 3.4 Grain structure of Inconel 718 at 100X magnification
Figure 3.5 Grain structure of Inconel 718 at 200X magnification
The average hardness measurement of the workpiece was performed at three
different places on the specimen using Matsuzawa Seiki microhardness tester with
Vickers pyramid indenter of 10 kg load. The obtained hardness values have been
shown in Table 3.4.
Table 3.4 Average hardness of Inconel 718
Vickers hardness
3.4
1
247.5
2
249.2
3
248.4
Cutting Tools
In this experiment, uncoated of solid carbide (WC/Co) twist drill with
different geometry were used to drill Inconel 718. Tool geometry information is
presented in Table 3.5. Sample of the tool is shows in Figure 3.6.
Table 3.5 Drill information
Description
No of flute
Shank diameter tolerance
Shank diameter (mm)
Tool diameter (mm)
0
Helix angle ( )
LOC (mm)
OAL (mm)
0
Point angle ( )
Fluting web (mm)
Margin width (mm)
0
Chisel edge angle ( )
0
Lip relief angle ( )
Bevel width (mm)
Web Thickness (mm)
0
Secondary relief angle ( )
2
h6
6
6
25
35
65
120,125,130
1.65
0.6
135
10
1.1
0.4-0.6
20
Figure 3.6 Image of uncoated tool drill (PA: 1250)
3.5
Machining Procedure
The drilling experiments were carried out on a MAHO MH 700S CNC
machining center shown in Figure 3.7. The drills were clamped to the tool holder with
an overhang of 35mm.
Figure 3.7 MAHO MH 700S CNC machining center.
3.6
Selection of independent variables
The independent variables considered in this investigation are of two types:
(1) variables related to machining conditions, and (2) variables related to geometry of
cutting tool. The machining parameters were selected on the basis of the information
available in the literature. The value of independent variables and the values were
selected in different runs are shown in Table 3.6.
Table 3.6 Experimental planning at all levels
Experiment
o.
1
2
3
4
3.7
Cutting Speed
(m/min)
13
8
8
18
Feed rate
(mm/rev)
0.12
0.1
0.1
0.05
Point Angle
(0)
125
130
120
130
Investigation of Surface Finish
In evaluating the roughness of the drilled hole a Handysurf model E-35A was
used. Two surface roughness readings were taken at four positions spaced at 90 deg
intervals around the hole circumference and approximately mid-way down the depth
of the hole.
3.8
Dimensional accuracy
The hole diameter was been measured at 6 points located at different height
and orientation using of CMM- KN810 Mitutoyo (Figure 3.8).
Figure 3.8 Coordinate measuring machine
3.9
Burr height measuring
The first and last hole of each run are separated from the main plate by
electro-discharge wire cut, AQ537L Sodick. To measure the burr height optical
microscope is used (ZEISS, Figure 3.10), the separated sample are located under the
microscope and the burr was focused and captured digitally. The height of burr was
analyzed using an image analyser. Figure 3.9 show sample preparation for measuring
of burr height
Figure 3.9 Samples are separated from the workpiece plate
Figure 3.10 Optical microscope used to measure burr height
3.10
Preparing the samples
Samples created by wire-cut are sectioned along the holes axis and
perpendicular of hole axis using precision cutter (Figure 3.11) to prepare
metallographic samples for investigating the microstructure of machined surface and
sub-surface and for measuring microhardness.
Figure 3.11 Linear precision saw
Cross-sections of each surface will be prepared using standard metallography
techniques of sample mounting and polishing as like as described previously. These
steps are shown in Figure 3.12.
Figure 3.12 Preparations of samples to metallographic studies
3.11
Microstructural analysis
Subsurface microstructural analysis is conducted with Olympus toolmakers’
light optical microscope (Figure 3.13) which is connected to Sony Digital hyper head
color video camera.
Figure 3.13 Toolmakers’ light optical microscope
3.12
Microhardness measurement
Microhardness (Vickers) was measured at five points (located at a certain
distance from machined surface ranging from 40 to 480 µm) in four line positions
spaced at 90 degree intervals around the hole perpendicular to the drill direction and
2mm away from the drill entrance. Measurement was conducted using SHIMADZU
(Figure 3.14) micro hardness tester under 4.903 N force with Vickers pyramid
indenter.
Figure 3.14 Vickers pyramid microhardness tester
CHAPTER 4
RESULTS AD DISCUSSIO
4.1
Introduction
This chapter present the experimental results and discussion. The results from
the surface roughness measurement, geometrical accuracy, burr height,
microstructural changes and microhardness are shown graphically. The effect of
various parameters on the machining response such as tool wear, cutting speed, feed
rate and point angles are investigated. The collected data are analyzed graphically
using Microsoft Excel 2004.
4.2
Tool wear
Tool wear was measured with a toolmakers microscope fitted with a digital
camera and image analysis software. When flank wear reached VB= 0.25mm or
VBmax=0.5mm the trials were stopped. Number of holes created in each run is shown
in Table 4.1.
Table 4.1. Number of holes drilled under different conditions
Experiment
Cutting
Feed rate
Point
Number
No.
speed
(mm/rev)
angle
of holes
(m/min)
4.3
(deg)
1
13
0.12
125
10
2
8
0.1
130
29
3
8
0.1
120
24
4
18
0.05
130
27
Surface roughness
Figure 4.1 shows results of surface roughness values of machined hole of
different conditions. All the drills at various conditions produced irregular results in
the values of surface roughness profile under various positions measured, however no
trend could be noted. This is probably due to presence and absence of built up edge
during drilling trials. Majority of the results fell between 0.6 and 1.1 µm Ra with a
wide range of scatter around these values and there was little difference between the
values obtained at different runs. It can be seen that smoother surface finish has been
obtained at higher cutting speed and lower feed rate.
1.4
0.77
1.2
1
1.04
Ra ( µm )
1
V=13m/mim, f=0.12mm/rev, A=125deg
0.8
V=8m/min f=0.1mm/rev, A=130deg
V=8m/min, f=0.1mm/rev, A=120deg
0.6
0.57
0.4
0.76
0.7
0.81
V=18m/min f=0.05mm/rev A=130deg
0.66
0.2
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
machinig time (min)
Figure 4.1 Surface roughness measurement at different experiment condition
Similar results are reported by Sharman and Ridgway [50] when drilling
Inconel 718. They have reported that surface finish has improved with increasing drill
wear. Figure 4.2 shows the comparison of average surface roughness values produced
with new tools and worn tools at different conditions. Surface roughness values of
worn tool were higher than new tools while these values were lower in experiment
number 3 and 4. This may be described due to presence of built up edge during
drilling of Inconel 718.
Figure 4.2 Surface roughness measurement comparison in different experimental trials
4.4
Dimensional accuracy
All diameter measurements ranged from 6.009 to 6.088 mm, and mean values
were between 6.013 to 6.074 mm. These values corresponded to the dimensional
tolerance reasonable in drilling operation. Recorded values of first and last hole
diameters were subtracted form the actual diameter of drill at each run and results are
shown in Figure 4.3. It can be seen that this variation in the hole diameter produced
using worn tool are higher than those produced using new tools. The heat generated
by the drilling process can lead to thermal expansion of the drill and work-piece
which may affect the size and quality of the holes leading to oversized holes [69].
0.1
0.086 0.088
0.09
variation (mm)
0.08
0.073
0.066
0.07
0.06
0.054 0.057
new tool
worn tool
0.05
0.04
0.036
0.03
0.03
0.02
0.01
0
1
2
3
4
Experiment number
Figure 4.3 Variation of machined hole dimension and tool diameter
4.5
Burr height
The burr height of first and last holes of each run was measured (Figure 4.4)
and the maximum values of each specimen are shown in Figure 4.5. It can be seen
that burr height in last holes using worn tool were higher than those obtained using
new tool. This phenomenon was observed in all experiments except experiment
number 1. It could be due to the experimental errors as a result of clamping the plate
during the separation of samples by wire-cut machine.
It can be concluded that burr height increased with increase in tool wear which
may be due to the ploughing of the worn tool. Cantero et al. [56] have reported similar
results in dry drilling of Ti-6Al-4V. They concluded burr formation presented more
sensibility to heat accumulation than the resultant diameter.
Figure 4-4 Burr heights obtained using an optical microscope
0.4
0.35
burr height (mm)
0.35
0.3
0.25
0.2
new tool
worn tool
0.16
0.15
0.1
0.1
0.09
0.07
0.07
0.08
0.03
0.05
0
1
2
3
4
Experiment number
Figure 4.5 Comparison of burr height at different cutting condition
The burrs created during drilling are burrs with a drill cap. The shape of the
drill caps were different depending on the drill type due to the difference in point
angle. Drill caps can be a problem in intersecting holes or interior cavities where
removal is difficult. Dornfeld et al. [68] has stated that both feed rate and cutting
speed seemed to have very little influence on the burr formation. The correlation
between burr formation and point angle is seen in Figure 4.6. It is observed that
increasing point angle produced smaller burr, as reported by Dornfeld [68] increasing
the point angle may produced burrs of decreased height and thickness and similarly
increasing the helix angle may increase burr size. Larger helix angle and increasing
point angle reduced the burr height and thickness.
0.11
0.1
burr heigth (mm)
0.1
0.09
0.09
Series1
0.08
0.07
0.07
0.06
115
120
125
130
135
point angle (deg)
Figure 4.6 Burr height versus point angle
4.6
Microstructure
The microstructrul changes of the first hole and last hole of run1 and other
runs subsurface are investigated using the optical microscope. Figure 4.7 shows the
comparison of microstructure of the first hole and last hole of run 1 in two 200 and
500 magnification.
No specific changes are observed in the first hole using a new tool while the
subsurface microstructural damage was seen in all holes produced using worn tools at
different runs. The changes involved deformed grain boundaries in the direction of
drilling and formation of white layer as shown in Figure. 4.7 (c,d).
A discontinuous white layer (up to 7µm depth) was present on all drilled surfaces but
not present at the first hole of each condition using new tools (Figure 4.7, a, b). It is
obvious that the holes produced by the drilling alone would not meet the aerospace
standards due to the high levels of white layer presence.
(a)
(b)
(c)
(d)
Figure 4.7 Comparison between first and last hole produced in exprement1
(V=8m/min, f=0.1 mm/rev, point angle= 130 deg). (a, b) first hole produced using
new tool, (c, d) last hole produced worn tool
Prior researches had stated that white layers are produced due to grain
refinement induced by severe plastic deformation and/or thermally induced phase
transformation [70, 71]. Li et al.[72] showed that when drilling plain carbon steel a
white layer was produced by both thermal and deformation driven phase
transformations acting in combination, with the dominant mechanism defined by the
relative workpiece material properties and cutting conditions used. Osterle and Li [71]
found that the white layer produced in ground IN738LC nickel-based superalloy
contained equiaxed grains of 50–100nm diameter produced by melting and rapid
quenching. They suggested that in cases where the wheel dressing rate was not
sufficient, wheel loading would occur causing chips to be plastically deformed
between the wheel and the workpiece. These chips become pressure welded to the
workpiece surface and are spread across it as the wheel rotates, forming a white layer
due to incipient melting and severe plastic deformation.
It was well understood from the analysis of the hole surface that during
drilling chips were become entrap between the flute margins and the hole wall. These
chips are extruded between the flute margins and the hole wall as the drill rotates and
become pressure welded to the workpiece surface and forming a white layer due to
incipient melting and severe plastic deformation. This process continues as the drill is
fed down the hole and as it is retracted. Prior work on turning has shown that greater
levels of grain boundary deformation are produced when cutting with worn tools (at
least two to three times higher). This may be due to the higher cutting and frictional
forces that would be developed when cutting with a worn tool, due to the increase in
tool/workpiece contact area [27]. Wear on the tool flank reduces the tools clearance
angle leading to greater rubbing of the workpiece surface.
Figure 4.8 shows the microstructural changes during drilling Inconel 718 in
other experiments with different conditions. Results obtained in this study are
compatible with prior studies. It can be observed that white layer depth progressively
increases with flank wear as well increases with cutting speed. Though whit layer
depth increases with increasing speed, it clearly becomes saturated at high speed.
Flank wear land rubbing may be the primary heat source for white layer formation.
Thus it can be suggested tool wear is the most influential parameter on white layer
formation, though it was the only variable under studied.
(a)
(b)
(c)
Figure 4.8 Subsurface microstructure in last holes produced using worn tool. (a)
V=8m/min, f= 0.1 mm/rev, point angle=130 deg, (b) V= 8m/min, f= 0.1 mm/rev,
point angle=120 deg, (c) V= 18m/min, f= 0.05 mm/rev, point angle=130 deg
4.7
Microhardness
Data of microhardness are recorded and the average values versus distance
from machined surface are shown graphically in Figure 4-9.
360
hardness (HV)
340
320
run1 new tool, V=13m/min,
f=0.12mm/rev, A=125 deg
300
run 1 worn tool,V=
13m/min,f=0.12mm/rev, A=125deg
280
run2 worn tool,V=8m/min,f=0.1
mm/rev,A=130deg
260
run 3 worn tool,V=8/min,f=0.1 mm/rev,
A=120 deg
240
run 4 worn tool,V=8m/min,
f=0.1mm/rev, A=120 deg
220
200
0
40
80
120 160 200 240 280 320 360 400 440 480 520
depth (microns)
figure 4.9 Microhardness changes versus distance from machined surface
It was found that the surface hardness values increase as compared to bulk
material (240-250 HK. Figure 4.7 illustrates the average microhardness at the depth
below 120 µm for the first hole of run1 using new tool was lower than that
microhardness produced using worn tools. Therefore, it may be suggested that tool
wear has great influence on the degree of work hardening of material during
machining. On the other word the hardening of work material increases with drilling
force and accelerated tool wear.
In drilling of titanium alloy, Cantero [56] has found that as distance to
machined surface increases microhardness decreases until values to those obtained on
the bulk material before machining. This phenomenon, but less pronounced, were
observed at shorter cutting time, because prolonged machining with nearly worn tools
produced severe plastic deformation and thicker disturbed layer on the machined
surface and the hardness of the disturbed layer of the machined surface increased
significantly. These microstructural changes originated during machining were mainly
because of the elevated temperatures which influenced mechanical properties of
material, decreasing fatigue and stress corrosion resistance [73-75].
CHAPTER 5
COCLUSIO
5.1
Conclusion
This project is focused on the drilling of Inconel 718 in evaluating quality of
machined holes, burr formation and surface integrity after machining at various
conditions. Four different drilling conditions were analysed in order to observe the
effect of tool wear of the drill and work-piece. The most number of holes produced
was in run 2 with 29 holes under condition of 8m/min cutting speed, 0.1 mm/rev feed
rate and 130 degree point angle.
From the obtained results, the following conclusions can be drawn:
-
Surface roughness values were between 0.6 and 1.1 µm Ra at all condition
investigated. Smoother surface finish was obtained at higher cutting speed and
lower feed rate (V=18 m/min, f= 0.05 mm/rev, point angle= 130 deg).
-
The size of the hole varies between 6.009 to 6.088 mm, and values ranged
from 6.013 to 6.074 mm. Holes with higher accuracy were obtained in first
hole of each experiment using new tools as compared to the last holes
produced using worn tools.
-
The burrs created during drilling are burrs with drill caps. The burr height in
last holes were higher than those obtained during the initial cut.
-
Increasing point angle tends to reduce the burr height.
-
Subsurface microstructural damage was very obvious in the holes produced
using worn tools, consist of deformed grain boundaries in the direction of
drilling and a formation white layer. The white layer progressively increases
with flank wear and cutting speed.
-
Subsurface hardness was observed to increase compared as to bulk material
(240-250 HK) within the first 40 to 120µm depth below the surface.
5.2
Future study
It is suggested that further investigation should be focused on finding ways to
improve the surface quality and reducing the whit layer during drilling. The effect
other processes such as reaming are be analyzed on the quality of surface finish and
microstructural changes of the workpiece. In addition studies can be performed on
coated carbide tool when drilling Inconel 718 and also such as TiALN and AlTiN
coatings.
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