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Ezugwu-Ti.pdf
]awa~(ff
Materials
Processing
Technology
ELSEVIER
Journal of Materials Processing Technology 68 (1997) 262-274
Titanium alloys and their machinability
a review
E.O. Ezugwu *, Z.M. Wang
School of Engineering Systems and Design, South Bank University, London SEI OAA, UK
Received 20 October 1995
Abstract
Although there have been great advances in the development of cutting tool materials which have significantly improved the
machinability of a large number of metallic materials, including cast irons, steels and some high temperature alloys such as
nickel-based alloys, no equivalent development has been made for cutting titanium alloys due primarily to their peculiar
characteristics. This paper reviews the main problems associated with the machining of titanium as well as tool wear and the
mechanisms responsible for tool failure. It was found that the straight tungsten carbide (WC/Co) cutting tools continue to
maintain their superiority in almost all machining processes of titanium alloys, whilst CVD coated carbides and ceramics have not
replaced cemented carbides due to their reactivity with titanium and their relatively low fracture toughness as well as the poor
thermal conductivity of most ceramics. This paper also discusses special machining methods, such as rotary cutting and the use
of ledge tools, which have shown some success in the machining of titanium alloys. © 1997 Elsevier Science S.A.
Keywords: Titanium alloys; Machinability; Notching; Flank wear; Cratering; Chipping; AttrRion wear; Dissolution-diffusiom Plastic deformation;
Rotary cutting; Ledge tools
1. Introduction
Titanium and its alloys are used extensively in
aerospace because of their excellent combination of
h i ~ specific strength (strength-to-weight ratio) which is
maintained at elevated temperature, their fracture resistant characteristics , and their exceptional resistance to
corrosion. They are also being used increasingly (or
being considered for use) in other industrial and commercial applications, such as petroleum refining, chemical pro~ssing, surgical implantation, pulp and paper,
pollution control, nuclear waste storage, food processing, electrochemical (including cathodic protection and
extractive metallurgy) and marine applications [1]. They
have become established engineering materials available
in a range of alloys and in all the wrought forms, such
as billet, bar, plate, sheet, strip, hollows, extrusions,
wire, etc.
Despite the increased usage and production of titanium and its alloys, they are expensive when compared
to many other metals because of the complexity of the
extraction process, difficulty of melting, and problems
* Corresponding author. Fax: +44 171 8157699.
0924-0136/97/$1Z00 © 1997 Elsevier Science S.A. All rights reserved.
Pll S0924-0136(96)00030-1
during fabrication and machining [2,3]. Near net-shape
methods such as castings, isothermal forging, and powder metallurgy have been introduced to reduce the cost
of titanium components [4-9]. However, most titanium
parts are still manufactured by conventional machining
methods. Virtually all types of machining operations,
such as turning, milling, drilling, reaming, tapping,
sawing, and grinding, are employed in producing
aerospace components [10]. For the manufacture of gas
turbine engines, turning and drilling are the major
machining operations, whilst in airframe production,
end milling and drilling are amongst the most important machining operations.
The machinability of titanium and its alloys is generally considered to be poor owing to several inherent
properties of the materials. Titanium is very chemically
reactive and, therefore, has a tendency to weld to the
cutting tool during machining, thus leading to chipping
and premature tool failure. Its low thermal conductivity
increases the temperature at the tool/workpiece interface, which affects the tool life adversely. Additionally,
its high strength maintained at elevated temperature
and its low modulus of elasticity further impairs its
machinability [11]. In 1955, Siekmann [12] pointed out
E.O. Ezugwu, Z.M. Wang/Journal of Materials Processing Technology 68 (1997) 262-274
that '"machining of titanium and its alloys would always be a problem, no matter what techniques are
employed to transform this metal into chips". The
poor machinability of titanium and its alloys have led
many large companies (for example Rolls-Royce and
General Electrics) to invest large sums of money in
developing techniques to minimise machining cost.
Reasonable production rates and excellent surface
quality can be achieved with conventional machining
methods if the unique characteristics of the metal and
its alloys are taken into account [13|.
2. Metallurgy of titanium alloys
2.1. Alloying additions of titanium alloys
Pure titanium undergoes an allotropic transformation at 882°C, changing from the low-temperature
close-packed hexagonal • phase to the higher-temperature body-centred cubic fl-phase. Alloying elements
in titanium alloying tend to stabilise either the
phase, or the allotrope fl phase that alters the transformation temperature and changes the shape and extent of the e-/3 field [14,15]. Elements that raise the
transformation temperature are e-stabilisers, these being aluminium (AI), oxygen (O), nitrogen (N) and
carbon (C), of which AI is a very effective ~-strengthening element at ambient and elevated temperatures
up to 550°C. The low density of AI is an important
additional advantage. O, N and C are regarded as
impurities in commercial alloys. However, O is used
as a strengthening agent to provide several grades of
commercially-pure titanium offering various combinations of strength and fabricability [16].
Although the addition of tin (Sn) or zirconium (Zr)
also strengthen the 0~ phase, these elements have little
influence on the transformation temperature because
they exhibit extensive solubility in e- and fl-titanium
and are known as 'neutral elements'.
Elements that produce a decrease in the transformation temperature are fl-stabilisers, involving twc
types, fl-isomorphous and fl-eutectoid [16]. The most
important fl-isomorphous alloying additions are
molybdenum (Mo), vanadium (V), niobium (Nb).
These elements are mutually soluble with fl-titanium,
increasing addition of the solute element progressively
depressing the fl to e transformation up to ambient
temperature, fl-eutectoid elements have restricted solubility in fl-titanium and form intermetaUic compounds by eutectoid decomposition of the fl-phase.
The two most important examples of such elements
used in commercial alloys are copper (Cu) and silicon
(Si).
263
2.2. Ch~.ss(fication o/' titanium alloys
Titanium alloys may be divided into four main
groups, according to their basic metallurgical characteristics: e alloys, near e alloys, e-fl alloys and
alloys [10,14,17-19].
e alloys:
These contain ~-stabilisers, sometimes in combination with neutral elements, and hence have an ephase microstructure. One such single phase
a-alloy, Ti 5-2½ (Ti-SAI-2½Sn), is still available
commercially and is the only one of its type to
survive besides commercially-pure titanium. The alloy has excellent tensile properties and creep stability at room and elevated temperatures up to 300°C.
a-alloys are used chiefly for corrosion resistance
and cryogenic applications.
Near ~ alloys:
These alloys are highly e-stabilised and contain
only limited quantities of fl-stabilising elements.
They are characterised by a microstructure consisting of c~ phase containing only small quantities of
fl phase. Ti 8-1-1 ( T i - 8 A I - I M o - I V ) and IM! 685
(Ti-6AI-5Zr-0.5Mo-0.25Si) are examples of near
e alloys. They behave more like ~-alloys and are
capable of operating at greater temperatures of between 400 and 520°C.
e-fl alloys:
This group of alloys contains addition of .e- and
fl-stabilisers and they possess microstruetures consisting of mixtures of e- and /3-phases. Ti 6-4 (Ti6AI-4V, designated IMI 318) and IMI 550
(Ti-4AI-2Sn-4Mo-0.5Si) are its most common alloys. They can be heat-treated to high strength levels and hence are used chiefly for high-strength
applications at elevated temperatures of between
350 and 400°C.
fl alloys:
These alloys contain significant quantities of/3-stabilisers and are characterised by high hardenability,
improved forgeability and cold formability, as well
as high density. Basically, these alloys offer an ambient temperature strength equivalent to that of e-B
alloys, but their elevated temperature properties are
inferior to those of the e-fl alloys.
As far as the gas turbine engine is concerned,
the most important alloys are those in the near
and e-/3 groups, the ct-/3 alloy Ti--6AI-4V being
the most commonly used titanium alloy, accounting
for over 45% of the total titanium production [20].
Table 1 gives important properties of Ti-6AI-4V,
and of AISI 1045 steel, as a basis for comparison
[181.
264
E.O. Ezugwu, Z.M. Wang/Journal of Materials Processing Technology 68 (1997) 262-274
3. Machining of titanium alloys
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Progress in the machining of titanium alloys has not
kept pace with advances in the machining of other
materials due to their high temperature strength, very
low thermal conductivity, relatively low modulus of
elasticity and high chemical reactivity. Therefore, success in the machining of titanium alloys depends largely
on the overcoming of the principal problems associated
with the inherent properties of these materials, as discussed below:
High cutting temperature:
It is well known that high cutting temperatures are
generated when machining titanium alloys and the
fact that the high temperatures act close to the
cutting edge of the tool are the principal reasons for
the rapid tool wear commonly observed. As illustrated in Fig. l, a large proportion (about 80%) of
the heat generated when machining titanium alloy
Ti-6AI-4V is conducted into the tool because it
cannot be removed with the fast flowing chip or bed
into the workpiece due to the low thermal conductivity of titanium alloys, which is about 1/6 that of
steels [21,22]. About 50% of the" heat generated is
absorbed into the tool when machining steel. Investigation of the distribution of the cutting temperature
has shown that the temperature gradients are much
steeper and the heat-affected zone much smaller and
much closer to the cutting edge when machining
titanium alloys because of the thinner chips produced
(hence short chip-tool contact length) and the presence of a very thin flow zone between the chip and
the tool (approximately 8 ~tm compared with 50 lam
when cutting iron under the same cutting conditions)
which causes high tool-tip temperatures of up to
about 1100°C [23-27].
High cutting pressures:
The cutting forces recorded when machining titanium
alloys are reported to be similar to those obtained
when machining steels [28], thus the power consumption during machining is approximately the same or
lower (Table 2) [10]. Much higher mechanical stresses
do, however, occur in the immediate vicinity of the
cutting edge when machining titanium alloy. Konig
[21] has reported higher stresses on the tool when
machining Ti-6AI-4V (titanium alloy) than when
machining Nimonic 105 (nickel-based alloy) and
three to four times those observed when machining
steel Ck 53N (Fig. 2). This may be attributed to the
unusually small chip-tool contact area on the rake
face, (which is about one-third that of the contact
area for steel at the same feed rate and depth of cut)
[29] and partly to the high resistance of Ti-aUoy to
deformation at elevated temperatures, which only
reduces considerably at temperatures in excess of
800°C [21,30].
E O. Ezugwu, Z.M. H"atlg Journal o/Materials Process'ing Technology 68 (1997)262 274
0
42
84
125
265
~68
100
8o
A
Ti.~Ab4V
l
Steel Ck 45
0
.Q
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20
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0
'
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;
i ¢ , I ' ~~ 1/ / ~/
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./
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Fig. I. D i s t r i b u t i o n of thermal load w h e n m a c h i n i n g t i t a n i u m a n d steel (after K o n i g [2t]).
Chatter:
Chatter is another main problem to be overcome
when machining titanium alloys, especially for finish
machining, the low modulus of elasticity of titanium
alloys being a principal cause of the chatter during
machining. When subjected to cutting pressure, titanium deflects nearly twice as much as carbon steel
the greater spring-back behind the cutting edge resulting in premature flank wear. vibration and higher
cutting temperature [30]. In effect, there is a bouncing action as the cutting edge enters the cut. The
appearance of chatter may also be partly ascribed to
the high dynamic cutting forces in the machining of
titanium. This can be up to 30% of the value of the
static forces [21] due to the 'adiabatic or catastrophic
thermoplastic shear' process by which titanium chips
are formed [31-36].
Additional criteria for tool materials:
Besides high cutting temperatures~ high mechanical
pressure and high dynamic loads in the machining of
titanium alloys, which result in plastic deformation
and/or rapid tool wear, cutting tools also suffer from
strong the chemical reactivity of titanium. Titanium
and its alloys react chemically with almost all tool
materials available at cutting temperature in excess of
500°C due to their strong chemical reactivity. The
tendency for chips to pressure weld to cutting tools,
severe dissolution-diffusion wear, which rises with
increasing temperature, and other peculiar characteristics already mentioned, demand additional criteria
in the choice of the cutting tool materials.
These problems may be minimised by employing very
rigid machines, using proper cutting tools and set-ups,
minimising cutting pressures, providing copious coolant
flow and designing special tools or non-conventional
cutting methods.
3.1. Tool materials for machin#lg tit,~ziunl alloys
Major improvements in the rate at which workpieces
are machined usually result from the development and
application of new tool materials. Over the last few
decades, there have been great advancements in the
development of cutting tools, including coated carbides,
ceramics, cubic boron nitride and polycrystalline diamond. These have found useful applications in the
machining of cast irons, steels and high temperature
alloys such as nickel-based alloys. However, none of
these newer developments in cutting tool materials have
had successful application in improving the machinability of titanium alloys because of the paramount qualities required of tool materials, which are: (i) high hot
hardness to resist the high stresses involved; (ii) good
thermal conductivity to minimise thermal gradients and
thermal shock; (iii) good chemical inertness to depress
the tendency to react with titanium; (iv) toughness and
fatigue resistance to withstand the chip segmentaron
process; and (v) high compressive, tensile and shear
strength.
Straight tungsten carbide (WC/Co) cutting tools have
proven their superiority in almost all machining processes of titanium alloys and interrupted cutting (end
266
E.O. E'.,ugwu, Z.M. Wang/Journal of Materials Process#tg Technology 68 (1997) 262-274
Table 2
Average unit power requirements for turning, drilling and milling (horsepower per cubic inch/minute) (after Kahles et al. [10])
Material
Hardness Rc or Bhn
(3000 kg)
Turning with HSS and carbide
tools
Drilling with HSS
d rills
Milling with HSS and carbide tools
Steels
Titanium alloys
Nickel based alloys
35-40 Rc
250- 375
200-360
1.4
1.2
2.5
1.4
1. I
2.0
1.5
I. 1
2.0
milling, tapping, broaching and planing), drilling and
reaming being performed best by high-speed steel tools.
Freeman [25] established the better performance of the
WC/Co grades, no matter which wear mechanism is
taking place. It has been found that the best grades in
cutting applications are the C-2, represented by ISO
K20 [37-39]. Dearnley, Grearson and Aucote [23,40],
carrying out many trials involving various tool materials in the continuous turning of Ti-6AI-4V, also confirmed the K grade carbides as the best choice. They
suggested that those WC/Co alloys with Co contents of
6 wt% and a medium WC grain size (about 0.8 and 1.4
~tm) gave the optimum performance. A recent study
[41] advises that straight cobalt-base tungsten carbide
cutting tools implanted with either chlorine or indium
are very effective in the machining of titanium and their
alloys.
It has been proven that steel cutting grades (P grades
of ISO codes) of cemented carbides are not suitable for
machining titanium alloys because of the greater wear
rate of the mixed carbide grains than that of the WC
grains and because of their thermal properties
[21,23,25,42]. All coated carbide tools tested (cemented
carbides coated by TiC, TiCN, TiN-TiC, AI20~-TiC,
TiN-Ti(C,N)-TiC, A1203, HfN, and TiB_,) also show
greater wear rates than those of straight grade cemented carbides [15,42,43]. Ezugwu and Pashby have,
however, reported that a very fine grain TiN/steel compound coated with a layer of TiN (using the PVD
technique) shows outstanding performance when end
milling titanium alloy (IMI 318) at high cutting conditions beyond those possible with carbide end mills [44].
General-purpose high-speed steel tools (such as M 1,
M2, M7, and M10) are often suitable in the machining
of titanium. However, the best results have been
achieved with highly alloyed grades, such as M33, M40,
and M42 [10,13].
Even though ceramics have improved in quality and
found: increased application in the machining of
difficult-to-cut materials, especially high-temperature
alloys (such as nickel-based alloys), they have not replaced cemented carbides and high-speed steels due to
the poor thermal conductivity of most ceramics, their
relatively low fracture toughness and their reactivity
with titanium [45].
The superhard cutting tool materials (cubic boron
nitride and polycrystalline diamond) have also shown a
good performance in terms of wear rate in the machining of titanium [23,46]. However, their application are
limited due to their high price.
3.2. Tool failure modes and wear mechanisms
Some specific studies on tool failure modes and wear
mechanisms when machining titanium alloys have been
conducted [23,25,40,42,43,47-49]. Cutting tool materials encounter severe thermal and mechanical shocks
when machining titanium alloys, the high cutting
stresses and high temperatures generated at and/or
close to the cutting edge greatly influencing the wear
rate and hence the tool life. Notching, flank wear,
crater wear, chipping and catastrophic failure are the
prominent failure modes when machining titanium alloys, these being caused by a combination of high
temperature, high cutting stresses, the strong chemical
reactivity of titanium, the formation process of
catastrophic shear (lamellar) chips, etc.
Different tool materials tend to have different responses to different wear mechanisms when machining
titanium alloys. Because of the rapid loss of their
hardness at elevated temperatures above 600°C, highspeed steel tools suffer severe plastic deformation which
accelerates the rate of wear [21]. Plastic deformation
can also be a major contributor to wear mechanisms of
other tool materials when machining titanium alloys,
especially in the case of high-speed machining, due to
the presence of high compressive stresses and the development of high temperatures close to the cutting edge
[18,23,46].
Freeman [25] carried out a tool life study of steel
cutting grades (containing carbides other than tungsten)
and straight WC/Co grades of cemented carbides when
machining two commercially-available titanium alloys
(an alpha-beta alloy and a beta alloy) and reported that
plastic deformation occurred, especially at higher cutting speeds, and that a crater can also be formed by
shearing on the rake face, both of these effects accelerating other wear mechanisms considerably. The tools
tested also suffered diffusion during machining. The
steel-cutting grades of cemented carbide are inferior to
Jq
E.O. Ezugwu. Z.M. Wang Journal of Mawrials Pr.ce.~sing Techmdogy 68 (1997) _6_
"~ ~ ."~"
267
2000
7"
O
1500
"~A"
~. 1ooo
E
500
2o
,<
Work material:
Tool material:
Cutting speed:
"Foolgeometry:
Nose radius:
Steel Ck53N
Carbide P 10
v=lO0 rn/min
Nhnonic 105
Carbide K 10
v=30 m/rain
Ti-6AI-4V
Carbide K20
v=40 rn/min
6 °18°10°190 °185 o
15~18Ol0Ol70
o
0ol8ob4
r = 0.5 ram, Chip cross-section: a × s = 1.5 × 0.25 mm 2
Fig. 2. Normal and tangential stresses in machining (alter Konig [21]).
straight grades because of the presence of the mixed
carbide grains (such as TiC and TaC). The mechanism
of attrition acts preferentially on the mixed carbide
grains, and tools containing mixed carbides also wear
by diffusion quicker than WC/Co tools because these
mixed carbides dissolve preferentially in titanium.
According to Dearnley et al. [23,40], the rake and
flank wear of all of the tool materials tested resulted
from dissolution-diffusion and attrition when turning
titanium alloys. Dissolution-diffusion wear predominated on the 'rake face' of all the uncoated cemented
carbides and ceramics, except for sialon, where attrition
is the competitive wear mechanism. On the 'flank face',
attrition wear controls the wear rates of ceramics and
steel-cutting grades of cemented carbides, whilst it is
less predominant on the flank faces of straight grades of
cemented carbides, which can probably be attributed to
the increased toughness of WC/Co alloys compared to
that of other grades. For these materials, dissolutiondiffusion wear controls the wear rates of flank wear.
Coatings of TiN, TiC, A1203 and HfN on both the rake
and flank faces are worn more rapidly than uncoated
WC/Co by either dissolution-diffusion or attrition wear
mechanisms. Coatings of TiB2 are relatively more resistant than others, as are CBN tools [23]. Notch wear,
which severely affects ceramic tools, is caused mainly
by a fracture process, which agrees with the fracture
mechanism proposed by Katayama and lmai [43].
However, a smoother notch wear surface (perhaps
caused by reaction with the atmosphere) has also been
reported with Sialon tools [23].
Hartung and Kramer [42] have suggested that the
presence of a 'flow zone" at the chip-tool interface will
eliminate the sliding between them, thus maximising the
wear resistance. If a flow zone is formed the wear will
be limited by the diffusion rate of the tool constituents
through this layer. This process of weal" is believed to
occur at a lower rate compared to that caused by
physical motion of the chip under sliding conditions
(i.e. attrition); however, attrition has been found in
other machining operations when a flow zone is
present, It was found that WC/Co grades of cemented
carbide and polycrystailine diamond are the best tool
materials to machine titanium because a stable reaction
layer is formed between the tool and the chip. The
carbon from either WC/Co-based composites or polycrystalline diamond reacts with the workpiece to form
TiC. This reaction layer has high deformation resistance at the cutting temperature and adheres strongly
to both the tool and the chip. This layer quickly
becomes saturated, limiting the mass transport of tool
constituents from the tool surface and reducing the
wear rate. This, however, seems to conflict with the fact
that titanium carbide formed by chemical vapour depo-
268
E.O. Ezugwu, Z.M. Wang/Journal of" Materials Processing Tectmology 68 (1997) 262-274
Table 3
Typical parameters for machining Ti-6AI-4V jet engine components (after Kahles et al. [10])
Operation
Tool materials
Cutting speed (in./min)
Feed rate
Depth of cut (in3
Turning (rough)
Turning (finish)
Turning (finish)
End mill (~-1' dia.)
End mill (~-1' dia.)
Drill ~i- ~1, dta.)
•
Drill (~-~
i i, dia.)
Ream
Ream
Tap
Broach
Spline shape
C-2
C-2
C-2
M42 HSS
C-!0
M42 HSS
C-2
M42 HSS
C-2
M7 HSS
M3 HSS
M42 HSS
150
200
300
60
200
30
40
20
35
15
12
12
001,~ in./rev.
0.006-0.008 in./rev.
0.006-0.008 in./rev.
0.003 in./tooth
0.005 in./tooth
0.005 in./rev.
0.004 in./rev.
0.010 in./rev.
0.010 in./rev.
-0.003 in.,'tooth max.
0.012 in.-',,stroke
0.250
0.010 0.030
0.0t0 0.030
0.125"
0.150-0.200"
~Axial depth. Radial depth is up to two-thirds the cutter diameter.
sition on commercial tool tips is not effective in suppressing wear.
It has been reported that plastic deformation and the
development of cracks by a thermal shock process will
dominate the wear mechanisms when machining titanium at high cutting speeds with cemented carbide and
ceramic tools and that the crater wear is closely related
to the chemical composition of the tools [43,50].
In the milling of titanium with WC/Co grades and
coated tools, chipping is a major failure mode of the
tools [39]. This type of wear is a result of the combination of high temperature, and high thermal, mechanical
and cyclical stresses, as well as adhesion of the work
material onto the tool faces [50,51]. Ezugwu and
Machado [39] found that prior to chipping, an initial
normal flank wear takes place and that this contributes
to enhancing the critical conditions for the first appearance of chipping. Min and Youzhen [47,52] suggested
that a carbide-rich layer in the tool surface region and
a carbide deficient layer in the tool subsurface region
are formed by diffusion between the too! and the
workpiece. The carbon redistribution results in surface
weakening and embrittlement of the tool, which encourages chipping and increased tool wear rate. Bhattacharyya et al. [51] have found that at high cutting
speeds the high temperature developed enables chemical
interactions between the work material and the coating
layers to take place and the layers are thus rapidly
removed resulting in the substrate acting as the cutting
edge over most of the tool life.
Cutting speed has the most considerable influence on
tool life. The latter can be plotted against cutting speed
for a given cutting tool material at a constant feed rate
and depth of cut [10], Fig. 3 being a typical example of
a large number of tool-life charts available elsewhere
[53-58]. It can be seen that tool life is extremely short
at high cutting speeds but improves dramatically as the
speed is reduced.
Another important variable affecting the tool life is
the feed rate. Often the tool life is not changed dramatically with a change in feed, but titanium alloys, however, are very sensitive to changes in feed (Fig. 3).
Chandler [l 3] has suggested that operation at high feeds
is more desirable to increase productivity.
When machining titanium, the effect of the depth of
cut must be considered also. As indicated in Fig. 4 [13],
increasing the depth of cut from 0.75 to 3 mm decreases
the tool life from 46 to 14 min at a cutting speed of 60
m/min.
When machining titanium alloys, the tool geometry
has a considerable influence on the tool life. Komanduri and Reed [59] suggested that a new tool geometry,
consisting of a high clearance angle (from l0 to 15°)
together with a high negative rake angle (from - l0 to
-150), increases the tool life of straight~ cemented
tungsten carbide (WC/Co) significantly compared with
the standard tool geometry ( - 5° rake angle and 5°
clearance angle).
3.3. Cutting parameters and tool geometry
The high temperature and the high stresses developed
at the cutting edge of the tool are the principal problems when machining titanium alloys. To minimise the
problem, a cutting fluid must be applied, as a basic
rule. The cutting fluid not only acts as a coolant but
also functions as a lubricant, reducing the tool temperatures and lessening the cutting forces and chip welding
that are commonly experienced with titanium alloys,
Data on cutting parameters have been developed
experimentally on a wide variety of titanium alloys. An
example of typical machining parameters currently used
for machining Ti-6AI-4V jet engine components (such
as fan disks, spacers, shafts, and rotating seals) are
shown in Table 3 [10].
3.4. Cutting fluid
E.O. Ezugwu, Z,M. ~ 'ang Jmtrmd ~! Materiat~ Pr~ces.~in~ Techm,h,¢y 68 (1997J 262 274
260
35
3O
25
i
\
.005in/rev
20
a
15
10
,
i
50
L
150
I
I
.
.
.
.
.
.
250
350
Cutting speed - feet/minute
Work Material:
Tool material:
Ti-6AI-4V (Solution treated and aged 388 BHN)
C-2 (883) Carbide
Fig. 3. Effect of cutting speed and feed on tool life in t,:,ning T i - 6 A I -4V (after Kahles et al. [10]L
thus improving the tool life. The correct choice of
cutting fluid has a significant effect on tool life. Copious, uninterrupted flow of coolant will also provide a
good flushing action to remove chips, minimise thermal
shock of milling tools and prevent chips from igniting,
especially when grinding titanium [10,13,18,30,60,61].
Additionally, a high pressure coolant supply can result
in small, discontinuous and easily disposable chips,
unlike the long continuous chips produced when machining with a conventional coolant supply [62].
Catt and Milwain r601 fbund that an extreme-pressure emulsion oil gives reasonable results, whilst those
containing phosphates give the best results due to their
good cooling properties and great anti-welding properties with a suitable lubricant. Difficulties were, however,
experienced due to the activity of the fluid, which
caused corrosion of the machine tool. Chlorine compounds are used partly because of their undoubted
superiority for particular operations, such as grinding,
broaching, and tapping. It was found that sulphur
compounds led to sulphur attack on turbine blades
made in titanium alloys, which led to an embargo on
their use. Many ~f the early chlorinated cutting fluids
containing chlorinated hydrocarbons also were effective, but these were banned because of their toxicity, it
have been found that chlorokerosenes are equally effective without the attendant risks [60].
Konig and Schroder [61] suggested that the application of coolants could suppress the built-up edge that
L
J
was observed generally during the face milling of titanium with HSS- and carbide-tools. Tests did however
show, that the application of coolants as concentrates,
emulsions, or solutions on a mineral oil-, mineral oil
free-, or synthetic, basis in liquid jet or in spray cooling
causes more wear than does dry cutting. Work carried
out at the Air Force Materials Laboratory [53-58],
concluded that chlorine-containing cutting fluids do not
always provide a better tool life. For particular alloys
and operatious, dry machining is preferred, which
agrees with the observations of Konig and Schroder.
Usually the heavy chlorine-bearing fluids excel in operations such as drilling, tapping, and broaching.
According to Chandler [13], water-base fluids are
more efficient than oils. He found that a weak solution
of rust inhibitor and/or water-oil (5-10%) solution is
the most practical fluid for high-speed cutting operations. Slow speed and complex operations may require
chlorinated or sulfurized oils to minimise frictional
forces and the galling and seizing tendency of titanium.
Chandler [13] pointed our that chlorinated cutting
fluids should be used with great caution because of
their potential to cause stress corrosion cracking.
A series of tests on the chlorine problem was run in
Germany [63]. It was found that the machining of
titanium with a lubricant containing a chlorine additive
developed surface films of a thickness equal to or less
than 150 lam (1500 ~,) and a chlorine content of at most
3 at.%. Similar films with i.5 at.% and 100--150 ~tm
E.O. Ezugwu. Z.M. Wang/Journal of Materials ProcessOJg Technology 68 (1997) 262-274
270
5O
4o
............................................
.........................................................................................
T .............................................
! ...........................
0
~5o
~Ts
2oo
225
2so
275
Cutting speed, feet/rain
W o r k Material:
Tool material:
Ti-5AI-2Sn (Annealed 321 B H N )
C-2 (883) Carbide
Fig. 4. Effect of cutting speed and depth of cut on tool life in turning Ti-5AI-2Sn (after Chandler [13]).
(1000-1500 ,~,) thickness were obtained by machining
titanium with demineralized water. The work concluded
that the prohibition of machining titanium with lubricants containing chlorine additives can no longer be
maintained.
3.4.1. Special machining techniques
The inability to improve cutting-tool performance by
developing new cutting-tool materials has been very
frustrating, Likewise, very little improvement in productivity has been experienced by exploring new combinations of speeds, feeds, and depths of cut. However,
increased productivity and long tool life have been
achieved by special machining techniques, including
specially designed ledge tools and rotary tools.
3.4.2. Ledge tools
Ledge tools are characterised by a thin cutting edge
that overhangs a small distance equal to the desired
depth of cut (Fig. 5) [64,65]. The advantage of these
tools, developed by the General Electric Company, lies
in the limited maximum flank wear of the tools during
machining. As cutting proceeds, they first achieve maximum flank wear and then the length of the overhang
wears back without further development in flank wear
due to a restricted clearance face. Thus the tools can
perform for a long time, as the tool life is not limited by
the amount of flank wear but by the size of the edge
[65]. Because of its restricted geometry these tools are
applicable only to straight cuts in turning, facing, bor-
ing, face milling, and some peripheral milling operations.
3.4.3. Rotary tools
Rotary cutting tools are in the form of circular discs
that rotate about their central axis in addition to the
main cutting and feed motion (Fig. 6). It has been
shown conclusively that rotary tools give rise to several
hundred degrees centigrade lower cutting temperatures
when machining titanium alloys (Fig. 7) [66]. The toollife improvements are considerable when machining
difficult-to-cut materials with rotary tools due to their
superior wear-resistivity (Fig. 8), which may be attributed to their peculiar characteristics such as continuous shifting of the cutting edge during machining and
lower cutting temperature [67,68]. Komanduri et al.
concluded that the tool lives are approximately seven
times those of conventional tools when machining Ti6A1-4V using rotary tools at very high feed rates (up to
1 mm/rev), achieved without sacrificing the surface
finish or the stability of the cutting process [64]. Due to
the cutting edge being circular the rotary tool can also
lead to a very fine machined surface, provided that the
tool spindle assembly is adequately rigid.
Although the improvement in tool life achieved by
rotary cutting is very significant, very few industrial
applications, have been reported. The reasons for this
may be their reduced effectiveness for machining complex surfaces and the requirement for either rigid machine-work systems or light cuts.
E.O. Ezt¢gwu, Z.AI. Wang Jottrna! qf Materials Proce.ssin.~' TeJm¢~logy 68 11997) 262 274
271
Ledg~
~Tool
holder
(o) Ledgetoot r~oun~ec~
on
o conven't,onol ¢oo~holder
q ~-Feec~
rote
-
GO) T~rnin() o p e r o t l o n
w;'th o Ledge "too~
~or~
Lea~
l
I
ed
-tool
Ledge
[
Fig. 5. Ledge tool (after Komanduri and Lee [64.05]).
3.5. Surface hltegrity
Titanium is generally used for a material for parts
requiring the greatest reliability, and therefore the surface integrity must be maintained. However, the surFace
of titanium alloys is easily damaged during machining
and grinding operations due to their poor machinability, damage appearing in the form of microcracks,
built-up edge, plastic deformation, heat-affccted zones,
and tensile residual stresses. Specific studies on surface
integrity parameters (microstructures hardness, surface
roughness and residual stress) have been carried out
[26,69-76]. When machining titanium in an abusive
manner (such as using a dull tool) an overheated white
layer can be produced which may be harder or softer
than the base materials [70]. Under both gentle and
abusive machining conditions, however, the surface
residual stresses appear compressive and their values
differ according to the cutting conditions (such as the
cutting speed). In grinding, abusive grinding practices
(o)
~
(u)
Fig. 6. Rotary cutting tools (after Ping Chen [66]).
produce high-residual tensile surface stresses, whilst
gentle grinding produces beneficial shallow compressive
stresses [10]. The surfaces produced under abusive conditions are also damaged by deformation and microcracks, which contribute to the loss of fatigue strength
and stress corrosion resistance in combination with the
resi,:lual stress pattern discussed above.
4. Conclusions
I. Titanium and its alloys are considered as difficultto-cut materials due to the high cutting temperature
and the high stresses at and/or close to the cutting edge
during machining. The high cutting temperature is due
to the heat generated during machining (catastrophic
thermoplastic shear process), the thin chips, a thin
seco~.dary zone, a short chip-tool contact length and
the poor heat-conductivity of the metal, whilst the high
stresses are due to the small contact area and the
strength of titanium even at elevated temperature.
2. Straight grade (WC/Co) cemented carbides are
regarded as the most suitable tool material available
commercially for the machining of titanium alloys as a
continuous operation. The C-2, identical to ISO K20, is
the best carbide grade. High-speed steel tools are also
very useful for some interrupted cuts, but the development of new tool materials is still required.
3. Cutting tool materials undergo severe thermal and
mechanical loads when machining titanium alloys due
272
E.O. Ezugwu, Z.M. Wang/Journal of Materials Processing Technology 68 (1997)262-274
1500
Fixed round tool
1300
1100
g~
r~
~
900
700
~
~
'
1
0.5
~
gutting tool
'
1.5
'
2
2.5
Cutting speed (m/s)
Feed rate: 0.4 mm/rev, Depth o f cut: 0.25 mm
Fig. 7. Measured temperature in turning Ti-6AI-4V (after Ping Chen [66]).
notch wear is caused mainly by a fracture process
and/or chemical reaction.
4. As a basic rule, a cutting fluid must be applied
when machining titanium alloys. The correct use of
coolants during machining operations greatly extends
the life of the cutting tool. Chemically active cutting
fluids transfer heat efficiently and reduce the cutting
forces between the tool and the workpiece.
to the high cutting stresses and temperatures near the
cutting edge, which greatly influence the wear rate and
hence the tool life. Flank wear, crater wear, notch wear,
chipping and catastrophic failure are the prominent
failure modes when machining titanium alloys. Flank
and crater wear may be attributed to dissolution-diffusion, attrition and plastic deformation, depending on
the cutting conditions and the Iool material, whilst
0.45
0.4
0,35
0.3
o.2s
~
0.2
-~
0.IS
0.1
0.05
I
I
I
I
5
10
15
20
Cutting time (min)
© o . . Cutting speed 60 m/min,
• • . . Cutting speed 120 m/min
Fig. 8. Tool wear curves in machining Ti-6AI-4V (after Ping Chen [67,68]).
25
E.O. Ezugwu, Z.M. Wang/Journal o/ Materkds Processklg Tectmology 6g (19971 262 274
5. The machining methods used for titanium are
essentially those that have been used since titanium
became used widely in the early 1960s. However, some
special machining techniques (such as the use of [edge
tools and rotary tools and other non-conventional machining methods~ may be thought of as alternative
methods to increase the metal removal rate in the
production of titanium components, provided that the
component geometry integrity permits this.
6. Great care must be exercised to avoid loss of
surface integrity in the machining of titanium, especially grinding, or a dramatic loss in mechanical behaviour such as fatigue can result. Generally, the
crack-free, compressive residual stress produced during
machining gives excellent fatigue properties, whilst surface damage and a tensile residual-stress pattern will
result in a dramatic loss in performance.
273
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Fly UP