CIRP Annals - Manufacturing Technology 59 (2010) 77–82
s on turning Ti–6Al–4V alloy with multi-layer coated inserts. Turning of Ti–6Al–4V using
lN + cBN coated single and multi-layer coated tungsten carbide inserts is conducted, forces and
ARTICLEINFO
element modelling is utilized to predict chip formation, forces, temperatures and tool wear on
models with strain softening effect are developed to simulate chip formation with finite element
ture fields for coated inserts. Predicted forces and tool wear contours are compared with
Keywords: Machining Finite
ributions and tool wear contours demonstrate some advantages of coated insert designs.
element method Titanium Tool
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology
journal homepage: http://ees.elsevier.com/cirp/default.asp
Investigations on the effects of multi-layered coated inserts in machining Ti–6Al–4V alloy
with experiments and finite element simulations
̈
T. O zel (2)a,*, M. Sima a , A.K. Srivastava (3)b, B. Kaftanoglu (1)c
a
Manufacturing Automation Research Laboratory, Department of Industrial and Systems Engineering, Rutgers University, NJ, USA b TechSolve Inc.,
Cincinnati, OH, USA c Department of Manufacturing Engineering, Atilim University, Ankara, Turkey
© 2010 CIRP.
7-8506/$ – see front matter © 2010 CIRP.
10.1016/j.cirp.2010.03.055
coated and multi-layer cBN + TiAlN coated tungsten carbide inserts is compared in
1. Introduction
terms of cutting forces and tool wear.
In addition, finite element (FE) simulations are utilized in investigating the
Titanium alloys such as Ti–6Al–4V offer high strength-to- weight
tool temperatures and wear development. Two dimensional and three dimensional
ratio, high toughness, superb corrosion resistance, and bio- compatibility and are
FE simulations have been designed and conducted to predict forces, temperatures
increasingly used in aerospace and biomedical applications. However, titanium
and tool wear to investigate the advantages of coatings in machining of Ti–
alloys are difficult to machine due to their low thermal conductivity and diffusivity,
6Al–4V. Finite element modelling and simulations of chip formation process in
high rigidity and low elasticity modulus and high chemical reactivity at elevated
titanium alloy machining present significant challenges due to the nature of
temperatures [1]. These alloys exhibit serrated and cyclical chip formation resulting
complicated dynamic material behaviour of these alloys at elevated temperatures,
in detrimental tool vibrations [2]. Titanium alloy machining performance can be
strain and strain rates. In general, adiabatic shearing is considered as responsible
increased by improving cutting tool materials and coatings [3,4]. Cubic boron
for serrated chip formation. Increasing temperatures in the primary shear zone due
nitride (cBN) material offers outstanding properties such as high hardness and wear
to shear deformation weaken the material by thermal softening; therefore, the
resistance. However, cBN material has lower toughness and is not suitable for
deformation is concentrated in shear bands, leading to serrated chip formation [5].
forming inserts into complex shapes. Recently, cBN coatings have been explored
Although it is also possible to simulate serrated chip formation by using damage
by applying several deposition techniques. Among these, physical vapor deposition
models [6], in this paper it is assumed that serration is caused by adiabatic shearing.
(PVD) has been preferred since thinner coatings can be deposited and sharp edges
and complex shapes can be easily coated at lower temperatures. On the other hand,
coating applied affects the edge radius of the inserts and must be taken into 2. Material constitutive model for Ti–6Al–4V and validation
consideration during tool performance analysis [4].
In this research, single and multi-layer TiAlN and cBN coatings are In FE models, a constitutive material model is required to relate the flow
experimented on tungsten carbide inserts (WC/Co) for possible improvements in stress to strain, strain rate and temperature, which often obtained from
machining of Ti–6Al–4V alloy. Positive tool geometry WC/Co inserts are coated Split-Hopkinson pressure bar (SHPB) tests per- formed under various strain rates
by cBN using a PVD system. Machining performance of multi-layered coated and temperatures. Dynamic material behaviour for Ti–6Al–4V titanium alloy has
inserts is exam- ined in longitudinal turning of titanium alloy (Ti–6Al–4V) without been widely published in literature [7,8]. Nemat-Nasser [7] reported that a
using coolant. The performance of uncoated, TiAlN coated, cBN phenomenon known as strain (flow) softening is observed which is responsible for
adiabatic shearing in titanium alloys. Localized softening is described as offering
less resistance to local deforma- tions due to rearrangement of dislocations caused
by subsequent cycling in hard materials. This phenomenon is usually seen during
* Corresponding author.
The an increase in strain beyond a critical strain value. Specifically, Lee
experimental flow stress data by Lee and Lin [8] has been and Lin [8] investigated temperature and strain-rate sensitivity of
taken as the base for this modified material model. The most Ti–6Al–4V and presented flow stress data at temperatures from 20
optimum set of model parameters that was identified with inverse to 1100 8C and strain rates ranging from 800 up to 3300 sÀ1. They
analysis are; A = 782.7 MPa, B = 498.4 MPa, n = 0.28, C = 0.028, used The Johnson-Cook (JC) material model to represent their flow
m = 1.0, a = 2, s = 0.05, r = 2, d = 1, b =5. The details of this stress data. However, their model did not include temperature-
̈
methodology are outlined in the work by O zel et al. [10]. dependent strain softening effect.
2.2. Orthogonal cutting tests 2.1. Modified material model
Orthogonal turning of Ti–6Al–4V titanium alloy tubes (50.8 mm A modification to the JC model is offered to include strain (flow)
in diameter and 3.175mm in thickness) have been performed softening effects at elevated temperatures as proposed by Calamaz
using uncoated and TiAlN coated tungsten carbide (WC/Co) cutting et al. [9]. In this study, further modifications to the strain hardening
tools in a rigid CNC turning centre at TechSolve Inc. The cutting part of the JC model by including strain softening at higher strain
forces were measured with a force dynamometer and high-speed values and thermal softening part are proposed and the model is
data acquisition devices. The experiments have been conducted given in Eq. (1). In this model, the influence of strain, strain rate,
using tool holders with 08 and 58 rake angle (g) at a cutting speed of temperature and temperature-dependent strain softening on the
vc = 120 m/min and three different feeds (f = 0.075, 0.1, 0.125 mm/ flow stress is defined by four multiplicative terms.
rev). Images of micro-chip geometries were captured with optical digital microscopy.
s 2.3. 2D finite element simulations and validation
Finite element model is developed using updated Lagrangian (DEFORM-2D) software in which chip separation from workpiece is achieved with
continuous remeshing. A plane-strain coupled thermo-mechanical analysis was performed. In these simulations, serrated chip formation process is
simulated from the incipient to the steady-state by using adiabatic shearing based on strain (flow) softening elasto-viscoplastic work material
assumption. The
Table 1 Mechanical and thermo-physical properties of work and tool materials and friction values used in FE simulations.
Ti–6Al–4V WC/Co (Ti,Al)N cBN
E(T) [MPa] 0.7412*T + 113375 5.6e5 6.0e5 6.52e5 a(T) [mmmmÀ18CÀ 1] 3.10À9*T + 7.10À6 4.7eÀ6 9.4eÀ6 5.2eÀ6 l(T) [WmÀ1 8CÀ 1] 7.039e0.0011*T 55 0.0081*T+ 11.95 100 cp(T) [NmmÀ2
8CÀ 1] 2.24e0.0007*T 0.0005*T + 2.07 0.0003*T + 0.57 3.26
1
1⁄4 A þ Bεn
expðεa Þ
ε
̇
ln ̇ε
1 þ C 0
T À T
1 À 0 Tm
À T0
m
1
D þ ð1 À DÞ tanh
pÞr
ðε þ
s (1)
is 1⁄4 true 1 À strain ð T/TmÞrate, d, p 1⁄4 ðT/T ̇ε0 is mÞb, s i s flow stress, ε is true reference true strain, and T, Tm, T0 are
where strain, ̇ε D
work, material melting and ambient temperatures respec- tively.
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T. O zel et al. / CIRP Annals - Manufacturing Technology 59 (2010) 77–82 78
Fig. 1. Comparison of measured and simulated serrated chip geometry.
simulations included a workpiece as elasto-viscoplastic with a mesh containing 10,000 quadrilateral elements. Tool is modelled as rigid with a mesh containing
5 mm for uncoated WC/Co, and rb =
2500 elements. Rake angles of 08 and 58 are employed in the tool geometry. Tool edge radius was estimated to be rb = 10
mm for TiAlN coated WC/Co respectively. Thermal boundary conditions are defined accordingly in order to allow heat transfer from workpiece to the cutting tool.
The heat conduction coefficient (h) is taken as 1.0e5 kW mÀ2 KÀ1 to allow rapid temperature rise in the tool. Mechanical and thermo-physical properties of titanium
Ti– 6Al–4V alloy are defined as temperature (T) dependent. Tempera- ture-dependent (T in 8C) modulus of elasticity (E), thermal expansion (a), thermal
conductivity (l), and heat capacity (cp) are given in Table 1.
In this paper, three contact regions are considered at the tool– chip interface: (i) a sticking region from the tool tip point to the end of the round edge
k where t i s frictional shear stress and k is the work material shear flow stress), (ii) a shear friction region (m = t/k) from the end of the curvature to
curvature (t =
0.7 for WC/Co, m = 0.5 for
the uncut chip thickness boundary (m = 0.9 for WC/Co, m = 0.85 for TiAlN), (iii) a sliding region along the rest of the rake face (m =
TiAlN as the friction coefficient).
Simulations are run for 0.1s cutting time. Comparison of simulated chips with experiments that are shown in Fig. 1 and Table 2 indicate close agreements.
In Table 2, h1 and
h2 indicate
minimum and maximum serrated chip thickness respectively.
Predicted forces from simulations are compared with measured forces in orthogonal cutting tests of Ti–6Al–4V alloy tubes as
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T. O zel et al. / CIRP Annals - Manufacturing Technology 59 (2010) 77–82 79
Table 2 Comparison of measured and simulated chip geometry.
Cutting condition Experimental Simulated
Tool g [8 ] f [mm/rev] h1 [mm] h2 [mm] h1 [mm] h2 [mm]
WC/Co 0 0.075 0.095 0.133 0.081 0.101 WC/Co 0 0.1 0.130 0.192 0.108 0.135 WC/Co 0 0.125 0.104 0.177 0.120 0.163 WC/Co 5 0.125 0.156 0.230 0.135 0.171 WC/Co + TiAlN 0 0.1 0.137 0.182
0.107 0.135 WC/Co + TiAlN 0 0.125 0.140 0.216 0.133 0.172 WC/Co + TiAlN 5 0.1 0.102 0.158 0.107 0.134 WC/Co + TiAlN 5 0.125 0.120 0.208 0.135 0.172
Fig. 2. Comparison of measured and simulated forces in orthogonal cutting tests.
and shown in Fig. 2. Especially, cutting forces are in close agreements
multi-layer coatings at National Boron Research Institute with 5% prediction error. Thrust force predictions which show 10–
(BOREN) in Turkey at a deposition pressure of 3 Â 10À3 Torr and 15% prediction error can be further improved with finer adjust-
heater temperature of 100 8C. Applied magnetron power is fixed at ments of friction regions and their values.
900 W and argon to nitrogen gas ratio is adjusted to 5/1 and run at the lowest possible bias voltage to obtain uniform cBN coating. 3.
Experimental work
Longitudinal turning of annealed Ti–6Al–4V titanium alloy bars (90 mm in diameter, 100 mm in length) was performed by using In this study,
four different inserts at the same cutting conditions have been tested; uncoated/unalloyed tungsten carbide
118) in (insert nose radius of rε = 0.8 mm and a rigid CNC turning centre under dry (WC/Co),
TPG432 relief type insert geometry angle of a =
tungsten carbide (WC/Co) PVD coated with TiAlN,
machining conditions at Rutgers University Manufacturing Auto- tungsten carbide (WC/Co) PVD coated with cBN, tungsten carbide
mation Research Laboratory. The inserts were used with a tool multi-layer PVD coated with cBN over TiAlN coating. Tungsten
holder that provided 08 lead, À58 side rake, and À58 back rake carbide (WC/Co) and PVD coated WC/Co with TiAlN inserts are
angles. The cutting forces were measured with a force dynam- coated with cBN by magnetron sputtering PVD system as mono
ometer mounted on the turret disk of the CNC turning centre. A
Table 3 Summary of FE simulation predictions.
Tool type Fc [N] Ft [N] Fz [N] Ttool [8 C] Tchip [8 C] dW/dt [mm/s]
WC/C0 590
(485–615) 93 (69–106) 229 (137–244) 785 791 0.0038 WC/C0+ cBN 602 (490–612) 99 (86–100) 236 (145–280) 762 778 0.0019 WC/C
0+ TiAlN 571 (490–593) 98
(77–103) 228 (135–250) 811 810 0.0024 WC/C0+ TiAlN + cBN 575 (481–606) 97 (81–115) 243 (166–286) 773 774 0.0025
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T. O zel et al. / CIRP Annals - Manufacturing Technology 59 (2010) 77–82 80
Fig. 3. Configuration of longitudinal bar turning experiments and force results.
Fig. 4. Predicted temperature distributions in 8 C.
ected as cutting conditions. Each test was replicated at least twice. The averages
the measured forces for each insert are shown in Fig. 3.
In order to observe the performance of coatings at different cutting speeds,
two sets of tests are done at cutting speeds of vc = 50 and 100 m/min respectively.
Thrust force was the lowest since inserts use 118 relief angle; hence flank contact
area is very
constant depth of cut (ap =
2 mm) and a constant feed (f = 0.1 mm/ rev) were
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T. O zel et al. / CIRP Annals - Manufacturing Technology 59 (2010) 77–82 81
Fig. 5. Simulated chip formation with effective strain distributions.
Fig. 6. Experimental and predicted wear rate distributions in [mm/s].
small. According to force measurements, cBN and cBN+TiAlN coated inserts The advantage of cBN coatings on forces is apparent for the lower
exhibit lowest cutting forces at 50 m/min cutting speed but the highest at 100 g speed. Adding cBN coating over TiAlN coating decreases forces. As cutting
m/min cutting speed. Moreover, the highest thrust force is seen in cBN coated
increases, the effect of larger edge radius (rb) due to added layer of coatings
WC/Co inserts at high cutting speed.
and TiAlN) becomes the dominant mechanism on forces. This larger edge
radius in multi-layer coated tools hinders the potential benefits of coatings, hence 2
results in higher forces especially when cutting speed is doubled. Hence, it may be (2)
beneficial to modify edge preparation of the coated tools (TiAlN and cBN) to lower (2)
the cutting edge radius. (2)
4. 3D finite element simulations
. Conclusions
Several FE studies on 3D turning are presented in the past such as
the work by Aurich and Bil [11] for segmented chip formation. In this study, In this study, a modified material model for Ti–6Al–4V titanium alloy is
updated Lagrangian FE modelling software (DEFORM- 3D) was used. The developed where strain (flow) softening, strain hardening and thermal softening
workpiece is modelled as elastic–viscoplastic material where the material ffects are coupled. This model is validated with elasto-viscoplastic FE simulation
constitutive model of this deformable body is represented with modified J-C f adiabatic shearing based serrated chip formation in machining Ti–6Al–4V
material model. The workpiece is represented by a curved model with 87 mm itanium alloy. The simulation predictions are compared with orthogonal cutting
diameter which is consistent with the experimental conditions. Only a segment (38) est results by using measured forces and chip morphology. Turning experiments
of the workpiece was modelled in order to keep the size of mesh elements small. have been conducted with uncoated, mono and multi-layer coated WC/Co carbide
Workpiece model includes 90,000 elements. The bottom surface of the workpiece ools and cutting perfor- mance of these coatings are evaluated. Although cBN and
0.8 mm with 118 relief angle) is TiAlN + cBN coated WC/CO inserts exhibit largest cutting forces at higher cutting
is fixed in all directions. The cutting tool (rε =
modelled as a rigid body which moves at the specified cutting speed by using peeds, they reveal favourable wear develop- ment. Tool wear zone measurements
180,000 elements. A very fine mesh density is defined at the tip of the tool and at nd predictions show that cBN coated WC/Co inserts depict smallest wear zone.
the cutting zone to obtain fine process output distributions. The minimum element Conse- quently, cBN coatings may lead to reduction in tool wear dry machining of
size for the workpiece and tool mesh was set to 0.008 and 0.024mm respectively. A itanium alloyed Ti–6Al–4V material.
tool edge radius of 5, 10 and 15 mm are designed for uncoated, single layer and
multi-layer coated tools respectively for each simulation, since added layers in Acknowledgements
multi-coating design is increasing the edge radius of the inserts [4].
All simulations were run at the same experimental cutting condition The authors gratefully acknowledge the support by the National Science
Foundation (CMMI-0758820 and CMMI-0757954), and BOREN Institute of
(vc = 100 m/min, f = 0.1 mm/rev, ap = 2 mm). In 3D FE modelling,
constant shear
Turkey.
friction factor (m = 0.9–0.95) was used to represent friction between tool and
workpiece. The averages, minimum and maximum of the simulated forces (Fc,
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