TEMPERATURE FIELDS IN ALUMINUM DURING ORTHOGONAL
CUTTING UNDER DIFFERENT RAKE ANGLES
1
1,2
2
3
K.M. Vernaza-Peña , J.J. Mason and M. Li
Department of Aerospace and Mechanical Engineering, University of Notre Dame
Notre Dame, Indiana 46556
3
Alcoa Technical Center, 100 Technical Drive, Alcoa Center, PA 15069
ABSTRACT-- A modified split Hopkinson bar apparatus is
employed to perform orthogonal machining of 6061-T6
aluminum alloy, and an array of HgCdTe high-speed infrared
detectors is used to experimentally measure the temperature
field distribution at the surface of the workpiece during this
process. The effect of rake angle on the temperature field
generated during machining is examined. Three different
rake angles are employed: 5, 10 and 15 degrees at a
constant speed of 30 m/s and a constant depth of cut of 0.5
mm. It is seen that the rake angle affects the maximum
temperature as well as the distribution of the temperature
field in the chip. For a cutting speed of 30 m/s, the
maximum temperature decreased with increasing rake angle
from 251 °C for 5° rake angle to 237 °C for 10° rake angle
and 196 °C for 15° rake angle. As the rake angle increases,
the primary shear zone in the workpiece contributes less to
the temperature distribution and the friction at the tool/chip
interface dominates.
and heat generation. Increasing the rake angle decreases
both the cutting force and the feed force.
INTRODUCTION
Metal cutting is a highly non-linear and coupled
thermomechanical process where mechanical work is
converted into heat through plastic deformation during chip
formation and through the frictional work between the tool
and chip. Temperatures attained can be quite high, thus
having a considerable effect on the residual stresses and the
overall distortion in the workpiece. One of the most
important parameters in the orthogonal cutting process is the
rake angle between the face of the cutting tool and the plane
perpendicular to the cutting direction [1-3] (See Figure 1).
The correct selection of the rake angle allows the
achievement of optimum cutting performance for particular
tool materials, work materials and cutting conditions. The
finished surface integrity as well as the performance of the
cutting tool are dependent on the magnitude of the rake
angle.
It is clear from the analytical [4-7] and numerical models [812] that the rake angle significantly effects the tool forces
Figure 1: Schematic of the 2-D orthogonal metal cutting
process
The amount of plastic deformation, expressed as plastic
shear strain γ on the shear plane, is directly related to both
the shear angle, φ , and the rake angle,
strain varies as [4]
γ =
cos α
.
sin φ cos (φ − α )
α.
The shear
(1)
From Equation (1), it is seen that as the rake angle is
increased, the shear strain becomes smaller. Hence, less
heating due to plastic deformation is expected.
Finite elements have been used extensively to generate data
regarding the metal cutting mechanics. They provide useful
insights for understanding and improving the cutting process,
but at the same time they raise many issues that demand
experimental data to verify the models proposed. In this
study, an experimental technique based on the Hopkinson
pressure bar and measurement of infrared radiation is used
to investigate the effects of the rake angle on the
temperature field distribution in the workpiece when cutting
aluminum at 30 m/s.
pre-cut was made by impacting the bar at low velocity,
approximately 9-10 m/s. After the pre-cut, a full cut was
made at the cutting speed.
EXPERIMENTAL METHOD
Temperature Field Measurements
Orthogonal Machining Hopkinson Bar Apparatus
A stationary, focused, linear array of 16 photoconductive
Mercury-Cadmium-Tellerium (HgCdTe) detectors was used
in these experiments to measure the infrared radiation
emitted by the workpiece’s surfaces as the cutting process
took place. The array consists of sixteen 80 mm x 80 mm
detectors with a center-to-center spacing of 100 mm and a
total length covered of 1.58 mm. The array is mounted
behind a sapphire window in a liquid nitrogen Dewar. In
order to measure the temperature distribution, the sixteen
elements of the detector array are focused on the surface of
the workpiece along a line perpendicular to the path of the
cutting tool. This non-invasive radiometric technique has
been employed by several researchers [15-17].
The
detectors posses a fast response and can measure in real
time the dynamic temperature fields. The Newtonian optical
system employed here consists of a concave mirror and two
flat mirrors (see Figure 3). The two flat mirrors are used to
o
rotate the image 90 so that proper orientation of the
detector array with respect to the cutting direction is
achieved. The magnification of the optical system has been
chosen to be one.
The modified Hopkinson bar used to perform orthogonal
machining in this study is shown schematically in Figure 2.
An elastic incident bar and projectile are made of maraging
steel with yield strength of 2.4 GPa. The bar is 2 m long and
a 0.3 m long projectile was employed. Both are 19 mm in
diameter. The bar is mounted over an I-beam and is
supported through its entire length using brass bearings that
allow it to move freely. One end of the bar is aligned with
the barrel of the air gun to ensure normal impact by the
projectile while the other end, which has the cutting tools
attached to it, is aligned to the workpieces to again have a
normal impact. The cutting tools are made of D2 tool steel
and are bolted to the tool holder, which in turn is held in
position on the elastic incident bar by means of setscrews.
For all the tests performed, the clearance angle was 8
degrees and three rake angles were employed: 5, 10 and 15
degrees. The width of the tools is 14.29 mm, making them
slightly wider than the specimens. The maximum allowable
velocity of the tool is equal to the maximum allowable impact
velocity of the projectile.
Figure 2: Schematic of the orthogonal machining Hopkinson
bar apparatus
The specimens of 6061-T6 aluminum alloy used for all the
cutting tests were machined into rectangles with dimensions
of 10.1 mm x 10.1 mm x 34.7 mm. Polishing was always
necessary to obtain a tight fit in the specimen holder as well
as a simultaneous contact between the tools and the
specimens. The side of the specimen on which the
temperature measurement was taken was polished to a 600grit finish [13]. The specimens were held in position by
setscrews located on the specimen holders.
When the properly aligned projectile impacts the bar, an
approximately square stress pulse is generated that
propagates along the bar (from left to right in Figure 1).
When the pulse reaches the opposite end, the cutting tool
impacts the two specimens and the stress pulse is reflected.
The cutting tool can be modeled as a point mass at the end
of the bar. It has been proven in the analysis by VernazaPeña et al. [14] that the simplification of the tool geometry is
acceptable and that the system launches the tool at the
projectile velocity. In all the experiments reported here, a
Figure 3: Schematic of Newtonian optical system
The sixteen detectors are calibrated by heating plates of the
same material as the specimen to a known temperature
while recording the temperature and the detectors output
voltages simultaneously. This calibration method provides a
direct relation between the surface temperature of the
specimen and the detector output voltage.
Typical
calibration curves are shown in Figure 4.
be estimated at this point. As one continuous to move up in
the direction parallel to the rake face, the material flows up
allowing for the chip formation. As the material passes over
the contact area on the tool, it is primarily heated by friction.
Figure 4: Typical calibration curve for 6061-T6, multiple
detectors are shown.
RESULTS AND DISCUSSION
The goal of this series of experiments is to experimentally
determine the effect of the rake angle on the temperature
distribution during orthogonal machining of aluminum. For
all the experiments discussed here, the cutting velocity was
30 m/s and the depth of cut, 0.5 mm. The three selected
rake angles were: 5, 10 and 15 degrees, with a common
clearance angle of 8 degrees.
The voltage vs. time readings provided by the infrared
detectors are converted into temperature readings with the
calibration curve. Then, a Galilean transformation is
employed to convert the time axis into distance parallel to
the cutting tool path and a temperature field is constructed
as the one shown in Figure 5.
Figure 5: Temperature field for aluminum 6061-T6, 5° rake
angle, 0.48 mm cut and 30.1 m/s. A maximum temperature
of 251 °C is recorded.
In all the experimental temperature fields constructed in this
study, it is observed that the undeformed material ahead of
the cutting tool is not heated. As one travels from right to left
toward the cutting tool in Figure 5, for example, a
concentration of isotherms is encountered. These isotherms
are an indication of the presence of a primary shear zone
and of the beginning of the flow zone. The shear angle can
Figure 6: Temperature field for aluminum 6061-T6, 10° rake
angle, 0.48 mm cut and 30.3 m/s. A maximum temperature
of 237 °C is recorded.
Figure 5 presents the temperature field distribution for the
orthogonal cutting of 6061-T6 aluminum alloy for a 5° rake
angle. For this particular test, the maximum temperature
was recorded as 251 °C. The y-axis is the position of the
infrared detectors while the x-axis is the position along the
cutting path. At the bottom of the temperature field, where
the concentration of isotherms is observed, the shear angle
was estimated to be 36°. In addition, the beginning of a
thermal trail can be observed below the cutting tool tip. In
Figure 6, the results for the orthogonal cutting of 6061-T6
alloy with a rake angle of 10° are reported. The maximum
temperature reported in this test was 237 °C. The shear
angle was estimated to be 51°. The results for the third rake
angle, 15°, are presented in Figure 7. For this test, the
maximum temperature was reported to be 196 °C and the
shear angle was estimated to be 50°. As the rake angle
increases, the maximum temperature decreases.
As the temperature fields are examined, a distinctive
primary shear zone is observed, being the isotherms the
evidence of its presence. This heating along the shear plane
contributes to the deformation, i.e. the formation of the chip.
As the rake angle increases from 5 to 15 degrees, the
temperature becomes localized near the tool face.
The
higher rake angle creates a longer heated contact zone
along the length of the rake face of the tool and decreases
the plastic zone ahead of the tool. The friction work
appears to have increased while the plastic work at the
primary shear zone seems to have decreased.
The amount of plastic deformation can be expressed as a
measurement of plastic shear strain [3]. Employing Equation
(1) and the shear angles measured from the temperature
field, estimates of the plastic shear strain can be obtained:
for a 5° rake angle, the plastic strain is calculated to be 1.98;
for a 10° rake angle, it is 1.67; and for a 15° rake angle, it is
1.54. These results validate previous statements. At lower
rake angles (for these particular cutting conditions) the
plastic deformation, i.e. plastic work, is larger. As the rake
angle increases, the magnitude of the plastic shear strain
decreases.
Figure 7: Temperature field for aluminum 6061-T6, 15° rake
angle, 0.50 mm cut and 29.0 m/s. A maximum temperature
of 196 °C is recorded.
CONCLUSIONS
The rake angle effect study presented here shows that the
maximum temperature is higher for lower rake angles and
that the maximum temperature shows an obvious trend to
decrease as the tool rake angle increases. This decrease in
the temperature magnitude is likely related to a decrease in
the cutting forces and power consumption during the cut. In
addition, for the aluminum alloy studied here, 6061-T6,
evidence of a primary shear zone is not as noticeable at
higher rake angles. At higher rake angles, plastic shear
strain decreases. A high temperature zone appears along
the length of the cutting tool rake face indicating dominance
of the friction component of heating.
The experimental technique developed can be employed to
test a broad range of cutting parameters; a large range of
speeds can be investigated, from conventional machining of
aluminum alloys to very high speed machining.
Acknowledgements:
The authors greatly acknowledge support from the Alcoa,
Inc. and the Helen Kellogg Institute for International Studies
at the University of Notre Dame.
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