Int J Adv Manuf Technol (2011) 52:929–935
DOI 10.1007/s00170-010-2804-2
ORIGINAL ARTICLE
The performance of PCD tools in high-speed milling
of Ti6Al4V
Gert Adriaan Oosthuizen & Guven Akdogan &
Nico Treurnicht
Received: 16 March 2010 / Accepted: 17 June 2010 / Published online: 10 July 2010
# The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract Tool performance of conventional tools is poor
and a major constraint when used in milling titanium alloys
at elevated cutting speeds. At these high cutting speeds, the
chemical and mechanical properties of Ti6Al4V cause
complex wear mechanisms. In this paper, a fine-grain
polycrystalline diamond (PCD) end mill tool was tested,
and its wear behavior was studied. The performance of the
PCD tool has been investigated in terms of tool life, cutting
forces, and surface roughness. The PCD tool yielded longer
tool life than a coated carbide tool at cutting speeds above
100 m/min. A slower wear progression was found with an
increase in cutting speeds, whereas the norm is an
exponential increase in tool wear at elevated speeds.
Observations based on scanning electron microscope
(SEM) and energy dispersive spectroscopy (EDAX) analysis suggest that adhesion of the workpiece is the wear
main type, after which degradation of the tools accelerates
probable due to the combined effect of high temperature
degradation coupled with abrasion.
Keywords Milling . Tool wear . High speed .
Thermal softening . Ti6Al4V . PCD tool
G. A. Oosthuizen (*) : N. Treurnicht
Department of Industrial Engineering, University of Stellenbosch,
Private Bag X1,
Matieland 7602, South Africa
e-mail: tiaan@sun.ac.za
G. Akdogan
Department of Process Engineering, University of Stellenbosch,
Private Bag X1,
Matieland 7602, South Africa
1 Introduction
Titanium alloys have found wide application in the
aerospace and biomedical industries due to their good
strength-to-weight ratio and superior corrosion resistance
[1]. High-speed machining is defined whereby conventional
cutting speeds are exceeded by a factor of 5 to 10. Ti6Al4V
is generally difficult to machine at cutting speeds of over
30 m/min with high-speed steel tools and over 60 m/min
with cemented tungsten carbide tools [2]. The thermal
conductivity [3] of Ti6Al4V (7 W/m K) is around 1/25 of
that of aluminum (6061, 177 W/m K). The result is a
concentration of heat in the cutting zone (thermal stress),
causing the characteristically high tool wear for which
titanium alloys are known [1, 3]. Temperatures [4] of
900°C have been measured at a cutting speed of 75 m/min.
Titanium's chemical reactivity becomes problematic at
temperatures above 500°C. Apart from diffusion wear, it
has a strong affinity to adhere, which leads to chips seizing
onto the tool cutting surface. Once a built-up edge develops,
tool failure follows rapidly [5]. The combination of a low [6]
Young's modulus (114 GPa) coupled with a high yield stress
ratio allows only small plastic deformations [1–3] and
encourages deflections, chatter, and movement of the
workpiece away from the tool. The thermal conductivity
for PCD (λ≈400 W/m K) is roughly four times greater than
that of tungsten carbide (λ≈100 W/m K) [7], and PCD is
significantly harder (approximately 6,000 HV) than carbides
(approximately 2,500 HV). The softening temperature (hot
hardness) [1] of PCD is higher than that of other
commercially available cutting materials, indicating better
performance at elevated temperatures [8]. Research [9]
showed that PCD represents a substitute tool material for
turning titanium alloys and that it produced a better
930
Int J Adv Manuf Technol (2011) 52:929–935
Table 1 Mechanical and physical properties of the workpiece at room
temperature
Test specimen
Hardness
Ultimate tensile strength
Ti6Al4V
360 HV
1,080 MPa
workpiece surface integrity in finish turning operations [10].
Regardless of all these positive findings in turning, very little
data exists on finish milling Ti6Al4V and even less on the
rough milling using PCD. Studies [7] reported a tool life of
215 min (Vc =457 m/min) with PCD, concluding that highspeed milling of Ti6AlV4 is possible. Nurul Amin et al. [11]
studied the effectiveness of PCD and compared it to
uncoated tungsten carbide–cobalt inserts machining
Ti6Al4V. The authors concluded that PCD inserts can be
used effectively up to cutting speeds of 160 m/min, as the
wear rate is relative low and the amount of metal removal
per unit of tool life is acceptable. Similarly, research [3]
reported a rather satisfactory long tool life (T=381 min) with
good surface finish and geometrical accuracy, finish milling
titanium compressor blades with PCD at a relatively slower
cutting speed (Vc =110 m/min). In addition to this, research
[2, 12] indicated that the performance of binderless cubic
boron nitride (BCBN) is a manifestation of the phenomenon
envisaged in a study [7], namely that a tool material with
sufficiently hot hardness will be able to yield better
machining productivity through the utilization of the thermal
softening of Ti6Al4V by means of an α-β phase change
[13]. This counterintuitive phenomenon has been discussed
in the literature [7, 13], and it is believed [14] that higher
cutting speeds lead to decreasing cutting forces [8] if the
cutting material is able to maintain its hot strength, whereas
the workpiece material softens at the cutting edge, and chips
can be removed more easily. In addition, Eckstein et al. [15]
describe a “slower wear progression” when working at high
cutting speeds as compared to conventional milling, whereas
the norm is an exponential increase in tool wear at elevated
speeds. According to research, the wear mechanisms and
mechanical issues of Ti6Al4V still require thorough study
[16]. Furthermore, it is indicated [17] that additional research
Fig. 1 Schematic view of shoulder milling process
is required to determine the dominant tool wear mechanisms
of PCD tool material. The objective of this work is to
analyze the tool wear progression and the cutting forces
variation relative to cutting speed. Similarly, the main wear
types for PCD milling Ti6Al4V are studied.
2 Nomenclature
Vb
Vc
ƒz
hex
Fx, Fy,
Fz
Fc, FcN,
Fa
kc
ae , a p
θ
ω
8
8 st
8 ex
Flank wear land (μm)
Cutting speed (m/min)
Feed per tooth (mm/tooth)
Maximum chip thickness (mm)
Force components in x-, y-, and z-directions (N)
Cutting, cutting perpendicular, and axial force
components (N)
Cutting coefficient (N/mm2)
Radial- and axial depth of cut (mm)
Engagement angle (degree)
Spindle rotational speed (rev/min)
Instantaneous angle position of the shoulder mill
with respect to the negative y-direction and
measured clockwise (degree)
Entry angle of tool (degree)
Exit angle of tool (degree)
Table 2 Milling conditions and parameters of experiments
Parameter
Condition
Cutting speed (Vc)
Feed/tooth (fz)
Depth of cut (ap)
Working engagement (ae)
Coolant
100–500 m/min
0.025, 0.0375, and 0.05 mm/z
2 mm
0.5 mm
Flood
3 Experimental procedure
3.1 Work material
A Ti6Al4V (ASTM B265, grade 5) sample was used, with a
hardness measured to be 360 HV and a high ultimate tensile
strength (1,080 MPa) as indicated in Table 1. This grade of
Int J Adv Manuf Technol (2011) 52:929–935
931
Fig. 2 Wear progression of
PCD at different cutting speeds
(fz =0.05 mm/z)
material was chosen to produce the most demanding
conditions achievable when machining this alloy.
3.2 Milling experiment
Research [18] suggested a cutting speed in the range of 185
to 220 m/min for titanium alloys and the chip load (fz)
below 0.05 mm [7] to reduce the possibility of catastrophic
tool failure in climb milling. Table 2 tabulates the cutting
conditions for the experiments.
Experiments were done at cutting speeds ranging from
100 to 500 m/min and feed rate of 0.025–0.05 mm/z. A
Hermle C40U milling machine was used with a lubrication emulsion designed specifically for the cutting of
titanium alloys. Due to the low modulus of elasticity of
the workpiece, the length was limited to 280 mm, in order
to ensure rigid clamping of the material to minimize
vibration. The PCD (CMX850) tools were examined after
set cutting intervals, and the wear was continuously
observed. The performance of the PCD was compared to
that of coated carbide. The flank wear land (Vb) was
analyzed with an optical microscope for dimensional
measurements. The SEM was used for wear characterization, and the surface roughness was measured with a digital
Fig. 3 Tool life of PCD compared to the coated carbide at
elevated cutting speeds
surface roughness meter. The failure criterion due to wear
on the flank [11] was set for a maximum when Vb =0.3 mm,
and Ra =1.6 μm [8] was used for surface roughness. Cutting
forces are determined by the machined cross-section width
and thickness, the cutting speed, and the tool wear. Figure 1
is a schematic view illustrating the experimental setup.
Once the force components in the x-, y-, and z-directions
were measured with a Kistler 9255B dynamometer, Fc, FR,
and Fa could be expressed by the following transformation
[19]:
2
3 2
sin ϕ
Fc
4 FcN 5 ¼ 4 cos ϕ
0
Fa
32 3
cos ϕ 0
Fx
sin ϕ 0 54 Fy 5
0
1
Fz
ð1Þ
The entry angle of the tooth in the down milling
operation, starting from the negative y-axis should be
ϕst ¼ 90
cos 1 ½ðR
ae Þ=R
ð2Þ
The tooth exits the cutting segment at an angle
of approximately 8 ex =90°, and the chip thickness is
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Int J Adv Manuf Technol (2011) 52:929–935
Fig. 4 Tool life of PCD and
coated carbide (fz =0.05 mm/z)
zero at this point. Thereby, Eq. 1 can be modified as
follows:
32 3
2
3 2
Fx
sin ϕ cos ϕ 0
Fc
4 FcN 5 ¼ 4 cos ϕ sin ϕ 0 54 Fy 5 when ϕst < ϕ < ϕex
Fz
Fa
0
0
1
ð3Þ
The cutting coefficients (kc) may be considered as a
constant with respect to cutting speed for conventional
cutting speeds, but under high-speed conditions, the value
may decrease or increase due to thermal softening or strain
rate hardening, respectively, as per Eq. 4 [20]:
Fc
kc ¼
ð4Þ
hex ap
4 Results and discussion
4.1 Tool life
The tools were examined after set cutting intervals, and the
wear was continuously observed. The Vb was analyzed with
Fig. 5 Measured cutting forces
at different cutting speeds (fz =
0.05 mm/z)
an optical microscope for dimensional measurements, and
the SEM was used for wear characterization. PCD was
compared with a commercially available coated carbide
material. The PCD performed satisfactorily for cutting
speeds in the range of 100–200 m/min with fz =0.05 mm/z.
As shown in Fig. 2, an increase in cutting speed resulted
in sudden tool failure, because of the significant reduction
in the strength and hardness of the tool material at elevated
temperatures.
Lowering the feed rate to 0.01 mm/z and increasing the
cutting speed to 500 m/min enabled the PCD to cut for
similar extended tool life as when cutting at 100 m/min
(fz =0.05 mm/z), and a distinctly better surface finish was
attained. The surface roughness measurements for the
different cutting speeds all resulted in Ra <1. At elevated
cutting speeds, the PCD outperformed the benchmark
coated carbide, which is illustrated in Fig. 3.
The failure of the coated carbide in the high temperature environment could be due to the lower hot hardness
of the tool as depicted in the literature [1, 2]. Figure 3
indicates the generally accepted decreasing trend in tool
life, associated with an increase in cutting speed. For tool
Int J Adv Manuf Technol (2011) 52:929–935
933
Fig. 6 SEM EDAX analysis of
the material build-up on the
PCD tool material; as supporting
evidence, the build-up is
Ti6Al4V
materials with lower transverse rupture strength, such as
PCD compared to carbide, there may be a minimum cutting
speed below which mechanical overload may occur. In the
same way, a local maximum tool life may exist if there is
a phase change in the workpiece material, and the insert
can withstand the harsh operating conditions associated
with the elevated temperature. This PCD proved that there
is a definite increase in tool life, at a cutting speed of
200 m/min (fz =0.05 mm/z) compared to 150 m/min (Fig. 4).
In this study, the tool life increased when the cutting
speed was increased from 150 to 200 m/min. The
literature shows that increasing the cutting speed beyond
200 m/min will increase the cutting temperature significantly beyond 1,000°C [21]. The α phase (hexagonal close
packed) of Ti6Al4V is hard and brittle, with a strong
hardening tendency. The β phase (body-centered cubic) is
ductile and can be cut more easily, but also has a strong
tendency to adhere [3]. The body-centered cubic structure
has more slip systems, thereby reducing the yield stress for
failure in shear [13]. Also, considering that the strength of
Ti6Al4V is reduced from 1,000 MPa at ambient temperature to below 550 MPa above 500°C [6], the explanation
for this increase in tool life could be the phase transformation from α to β phase, which occurs at about 950°C and
above [22]. At a cutting speed of 250 m/min, the tool life
was significantly reduced. Although the strength of the
workpiece material is low at this cutting speed [22], the
chemical attack on the tool is more aggressive. Thermal
softening and chemical reactions weakened the intergranular bonds of the cobalt and diamonds in the PCD
tool, which caused the grains to tear apart under the action
of the machining shear stress.
Table 3 SEM EDAX analysis of the material build-up on the PCD
tool material; as supporting evidence, the build-up is Ti6Al4V
In stats
C
O
Al
Ti
V
Total
Weight %
3.95
–
5.34
86.9
3.81
100
4.2 Cutting forces
Cutting is a process of extensive stresses and plastic
deformations. The high compressive and frictional contact
stresses on the tool face result in substantial cutting force.
Figure 5 illustrates the Fc for the different cutting speeds.
Similar to literature [1, 2, 8, 12], the cutting force
decreased with an increase in Vc for CMX850. There is a
noticeable reduction in cutting force when increasing the
cutting speed from Vc =100 to Vc =200 m/min. This is at the
same cutting speed that resulted in slower wear progression
with an increase in cutting speed. This step decrease in kc
from 2,400 N/mm2 (Vc =100 m/min) to 2,085 N/mm2
(Vc =200 m/min) might be due to the new slip system
after the phase change. A clear minimum of all cutting
force components is discovered when finishing with
Vc =300 m/min.
4.3 Observations of tool wear
SEM analysis was also employed in order to identify the
build-up on the flank wear scar. The build-up on the PCD
material was found to be Ti6Al4V. Figure 6 illustrates the
results of this analysis. Compared with the nominal 6%
aluminum and 4% vanadium in Table 3, it becomes clear
that the deposit is work material.
This was supported using image mapping with the SEM
and its software as shown in Fig. 7.
An EDM sectioned sample gives an indication of the
formed wear scar on the flank. Figure 8 indicates where the
sample was cut and the form of the wear scar. The sample
was ×350 enlarged under the SEM.
As illustrated, the PCD tool was cut through the wear
scar (section A-A) by means of electric discharge machining (EDM) and polished. The SEM on the sectioned sample
yielded supportive information to establish that the depth of
the wear on the flank to be around 20–25-μm deep. Acid
cleaning (hydrochloric acid) enabled that the true wear scar
was exposed below the deposited Ti6Al4V layer on the
flank. Post-acid cleaning SEM analysis revealed that a
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Int J Adv Manuf Technol (2011) 52:929–935
Fig. 7 Analysis of material
build-up on the PCD using
image mapping with the SEM,
showing that the build-up is
Ti6Al4V on the PCD tool
material
Fig. 8 EDM sectioned wear
scar showing the depth of the
wear on the flank
typical flank wear scar had different wear regions as
illustrated in Fig. 9.
These different regions from the edge inward are as
follows:
Region A: The highest load is exerted on the edge of the
insert. This zone is where the initial chipping
occurs, due to mechanical impact, and is
measured to be around 45-μm deep with the
SEM.
Region B: This region is also characterized with impactrelated wear. The workpiece material tends to
stick and break away under the cutting forces.
This causes small particles to be gradually
dislodged out of the diamond matrix.
Region C: This region was covered with a Ti6Al4V layer,
and the region is characterized by a relatively
Fig. 9 Etched PCD wear scar on the flank illustrating the different
wear regions
Int J Adv Manuf Technol (2011) 52:929–935
smoother (polished) surface. The removed and
adhered layers caused more adhesive rubbing,
which is believed to increase the cutting
temperature. This causes the workpiece to
weld onto the wear scar. The critical temperature for adhesion to occur for machining with
PCD is reported [23] to be roughly 760°C,
around which, the high temperature strength of
PCD is believed [1] to be degrading as well.
Also above 800°C, the probability of chemical
interaction of PCD with the workpiece material
[6] increases.
5 Conclusion
The performance of PCD tools have been investigated in
terms of tool life, surface roughness, cutting forces, and the
wear mechanisms under different cutting conditions.
Results from this study showed that for PCD, a decrease
in feed and an increase in cutting speed (Vc) produced a
better surface finish, while achieving a similar tool life
when compared to lower speeds and higher feeds. PCD has
outperformed the carbide tool at elevated cutting speeds.
Complementary to the slower wear progression that was
found at a cutting speed 200 m/min, the cutting force was
also reduced by a significant amount. Observations based
on SEM analysis suggested that adhesion of the workpiece
took place during milling of Ti6Al4V, after which,
degradation of the tool accelerated, probably due to the
combined effect of thermal and mechanical wear.
Acknowledgements This research is funded by the Advanced
Manufacturing Technology Strategy (AMTS) program, an initiative
of the South African Department of Science and Technology (DST).
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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