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https://doi.org/10.1007/s00170-020-05392-w
ORIGINAL ARTICLE
Experimental investigations on longitudinal-torsional
vibration-assisted milling of Ti-6Al-4V
Philipp M. Rinck1·Alpcan Gueray1·Robin Kleinwort1·Michael F. Zaeh1
Received: 3 January 2020 / Accepted: 1 May 2020
©The Author(s) 2020
Abstract
In vibration-assisted milling (VAM), an additional high-frequency oscillation is superimposed on the kinematics of a
conventional machining process. This generates oscillations of the cutting edge in the range of a few micrometers, thereby
causing a high-frequency change in the cutting speed and/or the feed. Consequently, a reduction of cutting forces, an
increase of the tool life, and an improvement of the workpiece quality can be achieved. This paper shows and compares
the effects of longitudinal and longitudinal-torsional (L-T) vibrations on the cutting force, the tool life, and the surface
quality when milling Ti-6Al-4V. In comparison with the conventional milling process, the cutting forces are significantly
reduced and the surface finish of the workpiece can be improved by introducing ultrasonic vibrations to the milling process.
Longitudinal-torsional vibration assistance showed better overall process performance than the pure longitudinal vibration
assistance.
Keywords Vibration-assisted milling ·Longitudinal-torsional vibration ·Ti-6Al-4V ·Cutting forces ·Kinematics ·
Surface roughness ·Burr formation
1 Introduction
The continuing trend toward lightweight construction is
increasingly leading to the use of high-strength construction
materials. For example, high-temperature titanium alloys
are used in gas and steam turbines to increase the
maximum possible temperatures in the turbine—and thus its
efficiency. In medical technology, biocompatible and highly
wear-resistant materials are required for the manufacture of
implants and endoprotheses (e.g., teeth, hip, or knee joints)
Philipp M. Rinck
philipp.rinck@iwb.mw.tum.de
Alpcan Gueray
alpcan.gueray@tum.de
Robin Kleinwort
robin.kleinwort@iwb.mw.tum.de
Michael F. Zaeh
michael.zaeh@iwb.mw.tum.de
1Institute for Machine Tools and Industrial Management
(iwb), Technical University Munich, Boltzmannstr. 15, 85748
Garching, Germany
[23]. In aerospace technology, the titanium alloy Ti-6Al-4V
is used to produce structural components due to its high
strength and low density [18]. For structural components
of Ti-6Al-4V, the material removal by machining can be
up to 95% of the total workpiece mass [8]. Considering
conventional machining processes, these high-performance
materials can only be machined with high cutting forces,
high tool wear, and a low material removal rate. It has been
proven that these problems can be successfully mitigated
using vibration-assisted machining [4,7,24].
In vibration-assisted machining, high-frequency oscil-
lations are added to the conventional kinematics of
the machining process, thereby causing a high-frequency
change in the cutting speed and/or the feed. The tool usu-
ally oscillates at a frequency over 16 kHz, with an amplitude
of 1 to 20 μm. When the tool oscillates with the fre-
quency fand a vibration amplitude A, the vibration velocity
vos,crit =2·π·fos ·Acan be calculated [30]. In
the case of a vibration superimposition in the cutting direc-
tion, the tool separates from the workpiece in each vibration
cycle if the vibration velocity exceeds the cutting speed.
This periodically interrupted process leads to several advan-
tages compared with the conventional cutting process [25,
29]:
/ Published online: 22 June 2020
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•Reduced cutting forces
•Increased machining stability
•Reduced tool wear
•Improved workpiece quality
Figure 1shows different types of vibration-assisted
machining, which can be distinguished by the character-
istics of the tooltip trajectory. In 1D vibration-assisted
machining, the cutting tool vibrates either in the cutting
direction or in direction perpendicular thereto. If the tooltip
vibrates simultaneously in both the cutting and normal
direction at different frequencies or with a phase differ-
ence, the tooltip will perform an elliptical motion [19].
This process is called 2D vibration-assisted machining. In
vibration-assisted turning, the influence of the tooltip trajec-
tory on the process performance has already been studied
extensively [10,20,30]. Most of the publications state that
a vibration superimposition in cutting direction yields more
benefits regarding a reduction in cutting forces and tool
wear than a superimposition perpendicular to the workpiece
surface [30].
Vibration-assisted machining has already been studied
for over 50 years [2]. It has been applied to machining
processes like turning, drilling, milling, and grinding. The
vibration assistance can be achieved by either vibrating the
tool or the workpiece. Most of the researches were focused
on vibration assisted turning (VAT), as it is easier to apply
ultrasonic vibrations to a stationary turning tool rather than
to a rotating, drilling, milling, or grinding tool.
Nath et al. [11] studied the effects of vibration-assisted
turning of Inconel 718 with vibrations in the cutting
direction regarding cutting forces, tool wear, surface quality,
and chip formation. During the experiments, the cutting
speed and the feed rate were varied and the ultrasonic
vibration frequency and the amplitude were kept constant
at 19 kHz and 15 μm respectively. The results showed that
the surface quality depends mostly on the tool-to-workpiece
contact ratio and the relative speed between the tool and
epyT
Vibration in
cutting direction
Vibration normal
to the surface
Vibration normal
to cutting direction
Combined
vibration
y
r
otcejarTtnemecalpsiD
=
y=A
ysin 2
z=0
=
y=0
z=A
zsin 2
=+A
xcos(2 )
y=A
ysin(2 +1)
z=A
zsin 2 + 2
Workpiece
Tool
=+A
xcos(2 )
y=0
z=0
Workpiece Workpiece Workpiece
ToolToolTool
Cutting
direction
Fig. 1 Types of vibration-assisted milling
the workpiece when machining with vibration assistance. It
was found that to achieve high cutting qualities, the tool-
workpiece contact ratio must be kept low, which can be
achieved by increasing the amplitude and the frequency
of the vibration or decreasing the cutting speed. The
resulting cutting forces during vibration-assisted turning
were reduced by 75 to 88% compared with conventional
turning. The tool life of the cubic boron nitride cutters used
was increased 4 to 8 times during turning with vibration
assistance with a cutting speed of 10 m/min. However,
beyond 10 m/min cutting speed, tools catastrophically failed
under ultrasonic vibration assistance. Ultrasonic vibration
assistance enabled surface finishes of Ra≤1μm, whereas
the best surface roughness achieved with conventional
turning was Ra=2.4μm. It was mentioned that vibration
assistance caused longer, thinner chips, which is favorable
for high-quality machining. Patil et al. [16] studied the
effects of feed direction ultrasonic vibration assistance
during dry turning of Ti-6Al-4V. The cutting speed and
the feed rate were varied and the ultrasonic vibration
frequency and the amplitude were kept constant at 20 kHz
and 20 μm, respectively. A cutting force reduction of
up to 50% was observed due to the vibration assistance,
and an increasing tool–workpiece contact ratio resulted
in a decrease in force reduction. Better surface quality
and thinner, longer chips were observed with vibration
assistance. Another important effect of vibration-assisted
machining is the influence on the residual stress state near
the surface of the machined workpiece. Nestler et al. [12]
showed that higher compressive residual stresses can be
achieved by applying the vibration assistance perpendicular
to the machined surface. Similar results are shown in [29]
regarding residual stresses. For vibration-assisted turning
of Ti-6Al-4V, several approaches for numerical modelling
of the cutting mechanism exist [16,22]. Cakir et al. [3]
modelled the chip formation mechanism and showed that
the average cutting force significantly drops with vibration
assistance due to the intermittent cutting effect, which in
turn may improve the tool life. Although the cutting edge
has the possibility to cool down when the tool separates
from the workpiece, the peak temperature during each
vibration cycle was found to be higher compared with
conventional turning.
Compared with VAT, VAM has not yet been thoroughly
investigated. In VAM, two different methods for superim-
posing an ultrasonic vibration exist. The first one involves
using an ultrasonic worktable, which excites the workpiece
with the ultrasonic vibration [1]. Although the installation
of an ultrasonic worktable is relatively easy, this method has
some disadvantages. When different workpieces are milled,
the ultrasonic system needs to be adjusted because of a
change of the systems’ resonance frequency. A more flex-
ible method is achieved by superimposing the vibration on
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the milling tool. Usually, the tool is superimposed with a
longitudinal vibration along its axis (longitudinal vibration-
assisted milling (LVAM)). Many researchers reported posi-
tive effects regarding surface roughness, cutting forces, and
tool life [9,13,24,25,27,31]. Razfar et al. [17] stud-
ied the effects of longitudinal vibration-assisted milling of
AISI 1020 steel by superimposing a vibration on the work-
piece. Experiments were carried out at a constant vibration
frequency of 20 kHz. During the experiments, the depth of
cut, the cutting speed, and the feed rate were varied. While
the positive effect of vibration superimposition tended to
decrease with increasing cutting speed and feed rates, an
increase in depth of cut did not result in a significant dete-
rioration of the surface quality. A decrease in the cutting
force was observed during the intermittent cutting process,
which was explained both by an aerodynamic lubrication
effect and by the improved cooling behavior of the tool.
A positive correlation between the tool-workpiece separa-
tion time and the reduction in cutting forces was found. Niu
et al. [15] studied longitudinal-torsional vibration-assisted
milling (LTVAM) of Ti-6Al-4V by superimposing the vibra-
tions on the milling tool. The torsional vibrations were
produced by degenerating longitudinal vibrations into tor-
sional vibrations by helical grooves in the sonotrode. The
research included the influence of the ultrasonic vibration
on residual stresses, surface hardness, and surface quality
of the milled workpiece. The ultrasonic vibration frequency
was kept constant at 35 kHz and the amplitude was var-
ied between 2 and 6 μm. The experiments showed that the
vibration amplitude has the least significant effect on resid-
ual stresses. However, sensitivity analysis revealed that a
higher vibration amplitude increases the surface hardness of
the bottom surface, which is explained by the increase of
the impact energy with the increasing vibration amplitude.
Zhou et al. [33] studied the cutting mechanisms in the hole-
making process of longitudinal-torsional ultrasonic-assisted
helical milling. The results showed a reduction in the axial
cutting force of up to 12%, whereas the other cutting force
components did not change significantly compared with
conventional helical milling. An overall reduction in the
burr height at the hole entry was also achieved. Similar
experiments were performed by Wu et al. [28], who stud-
ied a longitudinal-torsional vibration-assisted side milling
process. The measured reduction in the cutting forces was
45.8% in the x-direction, 27.6% in the y-direction, and 48%
in the z-direction. Wu et al. also stated that the cutting
force reduction is in correlation with the total contact time
between the tool and the workpiece. With a higher contact
time, a decrease of the force reduction was observed. Thus, a
higher cutting speed also resulted in a lower force reduction.
In the present work, the influence of LVAM and LTVAM
on machining Ti-6Al-4V is demonstrated and compared
with each other for the first time. After analyzing the
process kinematics of LVAM and LTVAM in chapter
4, an ultrasonic actuator is presented that is capable
of superimposing longitudinal and longitudinal-torsional
vibrations onto an endmill. Experiments were carried out
for slot and peripheral milling. In Section 4.1, the effects
of LVAM and LTVAM on the cutting forces are discussed.
The resulting workpiece properties in the form of surface
roughness, residual stresses, hardness, and burr formation
are described in Section 4.2. The effect on the tool life is
showninSection4.3.
2 Kinematic analysis of LTVAM with helical
end mills
In vibration-assisted milling, the conventional kinematic of
the cutting tool is superimposed by an additional vibration.
The resulting process kinematics of a L-T vibration will be
discussed in the following. The movement of the cutting
tool in conventional milling consists of the cutting motion
and the feed motion. The two-dimensional trajectory of a
point at the cutting edge is shown in Fig. 2(top), where
the current position of the point is a function of time t,
spindle speed n, tool diameter D, and the feed rate vf.Due
to the longitudinal vibration assistance, each point of the
tool vibrates along the z-axis. This superimposed vibration
movement is a function of time t, vibration frequency
fos , and vibration amplitude Al. The longitudinal vibration
assistance causes a high-frequency, reciprocating movement
along the tool’s axis. In Fig. 2(middle), the trajectory of
a cutting edge for one tool rotation under the influence
of a longitudinal vibration is shown. Torsional vibration
assistance causes a reciprocating movement that acts in the
x-andthey-directions. The reciprocating motion of a point
on the cutting edge caused by the L-T vibration can be
calculated using the following equation where Atis the
z- no
i
tce
rid z- noitc
er
id
z-noit
c
e
r
i
d
1
2
3
4
5
6
7
8
9
Fig. 2 Schematic trajecory of the cutting edge for one tool rotation.
Conventional milling (top), LVAM (middle) and LTVAM (bottom)
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torsional vibration amplitude:
x(t) =d
2·n(2·π·n·t+At·n(2·π·fos ·t)) +vf·t
y(t) =d
2·cos(2·π·n·t+At·n(2·π·fos ·t))
z(t) =Al·n(2·π·fos ·t+φ)
(1)
In order to determine whether the cutting tool loses
contact with the workpiece due to the vibration assistance,
the vibration velocity in the cutting direction can be
calculated for LVAM and LTVAM by
vos,crit =2·π·fos ·At+2·π·fos ·Al·tan(β)(2)
where βis the helix angle of the milling tool. When the
vibration velocity vos,cri t exceeds the cutting speed vc,
the cutting edges will lose contact with the workpiece in
each vibration cycle, resulting in an intermittent cutting
process. If the vibration velocity vos,cri t does not exceed
the cutting speed vc, the tool will not lose contact with the
workpiece, resulting in a continuous cutting process with
an ultrasonically modulated cutting speed. The principle of
the intermittent cutting process can be seen in Fig. 3.The
figure shows the position of a point on the cutting edge in
cutting direction for conventional machining (pcon) and for
vibration-assisted machining (pos ), which can be derived
from Eq. 2.
When the cutting edge moves opposite to the direction
of cutting, the rake face of the tool loses contact with the
chip (exit point te). From that point on, the cutting forces
drop significantly. It should be noted that although the
cutting process stopped, the flank face of the tool is still
in contact with the workpiece, creating frictional forces.
Between the points teand tt(turning point), the friction
force acts opposite to the cutting direction. When the cutting
edge passes the turning point tt, the friction force acts in
the cutting direction. The cutting process starts again when
the cutting edge reaches the start point (ts). The result is an
intermittent cutting process, which is repeated periodically
Fig. 3 Position of a point on the cutting edge in cutting direction under
the influence of a longitudinal-torsional vibration
Spindle
Tool
Piezoelectric
transducer
Rotor coil
Stator coil
Workpiece Dynamometer
Power supply
L-T sonotrode
Fig. 4 Experimental setup for peripheral milling and slot milling with
longitudinal-torsional vibration assistance
with each vibration cycle. The ratio of relative cutting time
Tccan be calculated by:
Tc=te2−ts1
ts2−ts1
(3)
Many researchers already stated that the relative cutting
time is directly proportional to the cutting force in vibration-
assisted machining. In order to achieve a lower cutting
time Tc, the vibration amplitude and the frequency must be
increased or the cutting speed must be decreased.
3 Experiments
3.1 Experimental setup
The experiments were carried out on a CNC milling machine
(Hermle UWF 900), as seen in Fig. 4. The cutting forces were
measured in x-, y-, and z-directions with a dynanometer
(Kistler 9257A). To determine the surface roughness in
accordance with DIN EN ISO 4287, a 3D laser scanning
microscope (Keyence VK-X1000) was used. The residual
stresses were analyzed with an X-ray diffractometer
(Stresstech XSTRESS G2R with Ti-X-ray tube). Hardness
measurements were carried out with a Vickers hardness
tester (LECO LV-Series). The workpiece made from Ti-6Al-
4V had the dimensions 100 ×100 ×5 mm. The chemical
properties of the titanium alloy are shown in Table 1.
The solid carbide milling tool with 3 flutes has a diameter
of 8 mm, a helix angle of 55◦, a positive rake angle,
Tab le 1 Chemical composition of the workpiece material
%Al %V%Fe %O2%N2%H2%Ti
6.18 3.96 0.18 0.15 <0.01 <0.001 rest
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Tab le 2 Overview of the parameter range that was used for full
factorial DoE; for peripheral milling, the experiments were performed
with a constant axial depth of cut apof 5 mm and a radial depth of cut
aeof 0.5 mm
Level Cutting speed Feed per tooth Amplitude Frequency
vcin m/min fzin mm Alin μmfin Hz
130 0.04 0 0
280 0.05 ≈4≈32,000
3≈6
4≈8
5≈10
6≈12
Slot milling was performed with a depth of cut apof 0,5 mm. Each
experiment was carried out twice
and a free angle >9◦. The tool is PVD coated with ZrN
(zirconium nitride) because of its low chemical affinity
to Ti-6Al-4V, which reduces adhesive tool wear during
milling.
The parameter range used for the experiments is given in
Table 2.
3.2 Ultrasonic actuator
To superimpose an ultrasonic vibration onto the milling tool,
an ultrasonic actuator was designed in ANSYS. The actuator
consists of a Langevin–Bolt transducer that converts a high -
frequency electrical current into a mechanical vibration. The
Milling tool
for LTVAM
Screw
connector
Slotted
sonotrode
Tool holder
Stator coil
Rotor coil
Ultrasonic converter
with piezoelectirc discs
Milling tool
for LVAM
Fig. 5 Actuator for ultrasonically assisted milling with the milling tool
for LVAM and sonotrode with the milling tool for LTVAM
Vibrati on-
antinode
Vibrati on-
node
Too l
Screw
connector
Bolt for
pretension
Piezoelectric
discs
Back mass
Vibrati on-
amplitude
Flange
Fig. 6 Ultrasonic converter with milling tool; the vibration amplitude
is schematically shown along the actuator with its amplitude maximum
at the tooltip
transducer is powered by an ultrasonic generator (Weber
Typ ULC MD/MFD). The electrical energy is transferred
via induction through a stator and rotor coil to the rotating
transducer. In order to generate a pure longitudinal vibration
at the tool center point, a customized milling tool that
resonates at its first longitudinal eigenmode at 34 kHz was
designed and fabricated. The milling tool is attached to the
transducer using a screw connection with an axial centering.
This method ensures minimal damping of the vibration
amplitude when compared with conventional collet chucks.
The length of the cutting edges was kept short to prevent
the longitudinal vibration mode from being converted into
a torsional vibration mode through the helix form of the
cutting edges.
In order to generate a longitudinal-torsional vibration
output at the tool center point, a slitted horn was designed.
By means of geometrical changes of the cross section of
the horn, a part of the longitudinal vibration is converted
into a torsional vibration. The helical form of the cutting
Fig. 7 Longitudinal and torsional vibration amplitude (peak to peak)
at the cutting edges in LTVAM (left) and LVAM (right); the amplitude
can be adjusted by changing the generator power output
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edges further amplifies the torsional vibration output. The
actuator is shown in Fig. 5, and the converter in Fig. 6,
and the longitudinal and torsional vibration output on the
cutting edge is depicted in Fig. 7. The achieved ratio
between torsional and longitudinal vibration outputs in
LTVAM ( At/Al) is about 1.3. In the following descriptions,
only the longitudinal vibration amplitude Alwill be given.
For LTVAM, the corresponding torsional amplitude can be
calculated by:
At=1.35 ·Al(4)
The amplitude at the cutting edges can be adjusted to a
certain extent (Albetween approximately 4 and 12 μm) by
adjusting the output power of the ultrasonic generator.
4 Results and discussion
In the following, the effects of ultrasonic vibration
assistance on the milling forces, workpiece properties, and
tool wear will be discussed.
4.1 Effect on the cutting force
Figure 8shows the cutting force in relation to the longi-
tudinal vibration amplitude Alf o r LVA M a n d LT VA M f o r
different cutting speeds for peripheral milling. Ultrasonic
assistance reduces the cutting forces significantly due to the
intermittent cutting process and the reduced macroscopic
friction coefficient between the tool and the workpiece. The
highest cutting force reduction for LVAM in comparison
with conventional milling (Al= 0) is 44% for Fx, 47% for
Fy, and 44% for Fzat a vibration amplitude Alof 11.8
μm. In LTVAM, the highest force reduction is 53% for Fx,
59% for Fy, and 61% for Fzat a vibration amplitude Al
of 11.8 μm. The gray parts in Fig. 8indicate the regions
in which the cutting tool was in constant contact with the
workpiece according to Eq. 2. For a cutting speed of vc=
30 m/min, the tool loses contact with the workpiece at a
vibration amplitude of Al=4.6μm for LVAM, whereas in
LTVAM, only a longitudinal vibration amplitude of 1.7 μm
is needed. For a cutting speed of vc= 80 m/min, no tool-
workpiece separation occurs for LVAM. In the region, where
vos,crit <v
c, the reduction in cutting forces is moderate.
Here, the force reduction can be explained by an ultra-
sonic modulation of the cutting speed, which decreases the
macroscopic friction coefficient between the rake face and
the flowing chip, and between the flank face and the work-
piece, thus creating a higher shear angle when compared
with conventional machining. When the vibration ampli-
tude is increased until the vibration velocity vos,cri t equals
the cutting speed vc, the cutting edge loses contact. In this
Fig. 8 Cutting forces Fx,Fy,andFzfor peripheral milling at different
vibration amplitudes for vc= 30 m/min (left) and vc= 80 m/min
(right). Top: LVAM. Bottom: LTVAM
region, as described in Section 4, the friction force tem-
porarily acts opposite to the cutting direction which lowers
the average cutting forces considerably. LTVAM shows a
higher reduction than LVAM because the torsional compo-
nent of the vibration leads to a lower contact time. The
effects described are equal for LVAM and LTVAM, whereas
LTVAM shows lower cutting forces in all regions.
4.2 Effect on surface roughness, residual stresses,
and burr formation
The effects of vibration-assisted milling on the workpiece
properties are discussed in the following. The results are
described for the side wall of the workpiece for peripheral
milling and for the bottom surface for slot milling.
Surface roughness of the bottom surface Figure 9(top)
shows the resulting average roughness value Rafor dif-
ferent vibration amplitudes. At a vibration amplitude of
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Fig. 9 Resulting surface roughness for slot milling (top) and
peripheral milling (bottom) with different vibration amplitudes for
LVAM and LTVAM
Al=4μm, the surface roughness decreased for both
LVAM and LTVAM from Ra= 0.9 to 0.78 μmand
0.75 μm, respectively. Microscopic images (Fig. 10)show
that the feed marks are reduced compared with conven-
tional milling, which reduces the corresponding surface
roughness. When the vibration amplitude is increased fur-
ther, the surface roughness also increases slightly for both
LVAM and LTVAM. The surface shows vibration marks
that increase with rising vibration amplitude. In all cases,
LTVAM shows a better surface roughness when compared
with LVAM.
Surface roughness of the side wall Different results occur
when the surface roughness of the side wall for peripheral
milling is analyzed. With increasing amplitude, the surface
Fig. 10 Images of the bottom surface for slot milling for LVAM
and LTVAM at a cutting speed of vc= 30 m/min with different
magnifications
Fig. 11 Images of the side wall surface for peripheral milling for
LVAM and LTVAM at a cutting speed of vc= 30 m/min with different
magnifications
roughness is reduced for LVAM and LTVAM, whereas the
latter shows lower values overall. Images of the resulting
surfaces are shown in Fig. 11. The effect of the ultrasonic
vibration smoothens the surface as micropeaks are flattened
by the oscillating cutting edge. Other researchers stated that
the resulting surface has beneficial properties such as a
higher oil-locking property, which can improve the friction
coefficient [21,25].
Residual stresses and surface hardness The machining of
Ti-6Al-4V inevitably leads to changes in the near-surface
residual stresses, due to the high mechanical and thermal
loads between the tool and the workpiece during the
milling process [5]. The thermal load usually causes tensile
stresses near the surface. The friction between the tool
and the workpiece (especially the friction between the
flank face and the workpiece) causes the surface layer
to heat up locally and expand, which initially induces
compressive stresses. If the yield point of the material
is exceeded by these loads, plastic compression occurs.
When the previously compressed region cools down and
stretches, tensile residual stresses occur. The mechanical
load usually generates compressive residual stresses in the
Fig. 12 Residual stresses in feed direction and normal to feed direction
for different vibration amplitudes in LVAM; left: bottom surface for
slot milling; right: side surface for peripheral milling
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Fig. 13 Vickers hardness at the bottom surface and at the side wall for
LVAM
cutting direction by preventing elastic deformation through
areas where plastic deformation has already occurred. The
residual stress state influences the workpiece properties
regarding fatigue life and corrosion resistance. Compressive
stresses are to be preferred since tensile stresses applied
must first exceed the level of the residual compressive
stresses before the material would be loaded at all [29].
The influence of the vibration amplitude in LVAM and
LTVAM on the surface residual stresses in the feed, as well
as in its corresponding normal direction, is shown in Fig. 12
for peripheral and slot milling. In slot milling (Fig. 12
left), the compressive residual stresses on the surface rise
with higher amplitudes for both LVAM and LTVAM. The
longitudinal vibration component causes repeated impacts
between the tool and the bottom surface. These impacts
have an effect similar to shot peening, thus creating higher
compressive residual stresses. LVAM shows a rise in
compressive stresses in feed direction from 240 MPa for
conventional milling to 508 MPa with an amplitude Alof
12 μm. In LTVAM, on the other hand, the compressive
stresses in feed direction increase from 240 to 465 MPa at a
vibration amplitude Alof 11.4 μm. The lower compressive
stresses in LTVAM may be explained by the reduced
contact time between the tool and the workpiece due to
the torsional vibration component and its correspondingly
higher reduction of cutting forces. The reduced contact
time in vibration-assisted milling also results in a lower
temperature in the process zone, as has already been
described by [26]. Figure 14 shows temperatures measured
with thermoelements that were embedded in the workpiece
for LVAM and LTVAM. The thermoelements were placed in
drilled holes at a distance of 0.1 mm from the milled surface.
Although the measured temperatures do not represent the
temperatures in the cutting zone (as these are much higher),
it is possible to conclude that the heat transfer into the
workpiece is lower for both LVAM and LTVAM than for
conventional milling. LTVAM shows a greater drop in
temperature, mainly due to a lower tool–workpiece contact
time and reduced friction. The lower heat transfer into
the workpiece and the corresponding lower temperatures
also contribute to higher compressive residual stresses
because high temperatures lead to tensile residual stresses.
In peripheral milling, the effect of the ultrasonic vibration
on the residual stresses of the side wall showed a different
effect than in slot milling. The resulting residual stresses for
different vibration amplitudes are depicted in Fig. 12 (right)
for LVAM. With higher amplitudes, the residual stresses
tend to become less compressive. This can be explained by
reduced cutting forces, and thus lower mechanical loads on
the surface. In comparison with slot milling, in peripheral
milling, there is no impact mechanism through the vibration
that causes higher compressive residual stresses.
The surface hardness correlates with the measured
residual stresses. Figure 13 provides the Vickers hardness
for LVAM for the side wall and the bottom surface. On the
bottom surface, the hardness increases from HV 322 to HV
359 with rising amplitude because of the hardening effect of
the tool–workpiece impacts. On the side wall, the hardness
decreases from HV 328 to HV 322 with rising amplitude.
The lower cutting forces with rising amplitude lead to a
lower hardness (Fig. 14).
Burr formation A burr is a part of the material that remains
attached to the edge of a workpiece after the machining
process [6]. As the deburring process of machined parts is
time consuming and expensive, burr formation should be
minimized [32]. In this study, the burr height at the down-
milling side is analyzed. The burr height was measured at
5 different defined points at the edge of a workpiece and
the average burr height was calculated. Figure 15 presents
the result in relation to different cutting speeds as well as
Fig. 14 Temperature measurement for LVAM (top) and LTVAM
(bottom) with a thermocouple embedded in the workpiece for
peripheral milling at different vibration amplitudes
3614 Int J Adv Manuf Technol (2020) 108:3607–3618
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Fig. 15 Top burr height at the down-milling side for different cutting
speeds and exemplary measurements of the burr height with vc=30
m/min, fz= 0.05 mm, ap= 5 mm, ae= 0.5 mm, and Al= 11.9 μm
sample measurements of the burr height at given process
parameters. Both LVAM and LTVAM show very similar
results in comparison with conventional milling. A slight
reduction from 0.029 to 0.020 mm (LTVAM) and 0.023 mm
(LVAM) can be noticed at a cutting speed of vc= 30 m/min.
At higher cutting speeds, no significant differences in burr
height between vibration-assisted milling and conventional
milling were observed. The mechanisms of burr formation
in milling are very complex and are not yet fully understood
[14], but it has already been stated that the top burr height
increases with a higher friction coefficient. Thus, vibration-
assisted machining can reduce burr formation as it reduces
the friction between the tool and the workpiece.
4.3 Effect on tool wear
Due to the poor thermal conductivity of titanium, a con-
siderable amount of the heat generated during machining is
dissipated via the tool, which leads to high thermal loads on
the cutting edge and promotes chemical reactions between
the workpiece and the cutting edge. In addition, the low
modulus of elasticity combined with high tensile strength
causes a high load on the cutting edges, resulting in high tool
wear. As shown in Fig. 16, vibration-assisted machining has
a significant influence on the tool wear.
For peripheral milling (Fig. 16 top), the primary cutting
edge was analyzed in regard to abrasive flank wear,
adhesion wear, and chipping. Both LVAM and LTVAM
showed less tool wear compared with conventional dry
milling. With LTVAM, the milling distance was increased
from 12 to 15 m (+ 20%) until a wear mark of 0.20 mm
was reached. LVAM increased the tool life to 14.5 m
traverse in feed direction (+ 17.2%). The predominant tool
wear mechanism was abrasive flank wear. Occasionally,
chipping was observed along the cutting edge. Abrasion
wear mostly occurs due to the friction between the tool
and the workpiece. Because of the lower cutting forces and
temperatures, as well as the lower friction between the tool
and the workpiece in vibration-assisted milling, abrasive
wear is reduced. LTVAM demonstrates better performance
than LVAM because of even lower forces and temperatures.
Furthermore, adhesive wear in the form of built-up edge
was reduced in LTVAM and LVAM. Adhesive wear occurs
when a part of the workpiece material adheres to the tool
because of high pressures, high temperatures, and a high
chemical affinity between the tool and the workpiece. The
lower temperatures and cutting forces and the reduced
Fig. 16 Tool wear for conventional milling, LVAM, and LTVAM on
the primary (top) and minor (bottom) cutting edges
3615Int J Adv Manuf Technol (2020) 108:3607–3618
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
friction in vibration-assisted milling significantly reduced
the occurrence of built-up edges, whereas conventional
milling showed small amounts of built-up layers. Another
factor that could have caused the reduced tool wear in
VAM is the lower contact time between the tool and
the workpiece, as diffusion mechanisms are suppressed
if there is no contact. For slot milling, different wear
mechanisms were observed (Fig. 16 bottom). In comparison
with conventional milling, both LVAM and LTVAM showed
higher flank wear. The tool life decreased in LTVAM
from 11 to 9 m traverse (−18.2%) and in LVAM to
8m(−33.3%) until a wear mark of 0.20 mm was
reached. The predominant factor in increasing the flank
wear seems to be the longitudinal part of the vibration,
which creates a ploughing force on the flank face of
the milling tool. LTVAM shows slightly less tool wear
which is because of the lower friction and temperature in
comparison with LVAM. At cutting lengths above 4 m,
occasional chipping on the tool nose was observed in LVAM
and LTVAM. Chipping occurs due to high mechanical
stresses and high temperatures at the cutting edge. These
stresses lead to microscale cracks, which will eventually
cause chipping through crack propagation. Although the
temperature is lower in LVAM and LTVAM than that in
Fig. 17 Images of the tool wear for conventional milling, LVAM, and
LTVAM on the primary (left) and minor (right) cutting edges
conventional milling, the probability of chipping is higher.
This may be due to the mechanical loads that are caused
by the longitudinal vibration component. The longitudinal
vibration causes high-frequency impacts between the minor
cutting edge and the workpiece that enhance the formation
and propagation of microcracks, which eventually will lead
to chipping of the outer cutting edge (Fig. 17).
5 Conclusion and outlook
This paper presents an approach to enhance the machining
performance during milling of materials that are difficult to
cut, such as Ti-6Al-4V. An ultrasonic actuator for superim-
posing longitudinal and longitudinal-torsional vibrations on
an end mill is shown. The resulting kinematics of the milling
tool is described. Experimental results for peripheral milling
and slot milling allow the following conclusions:
•Cutting forces decrease significantly in vibration-assisted
milling of Ti-6Al-4V. In LVAM, an average maximum
decrease in the cutting force of 44.3% can be observed
for peripheral milling. A positive correlation between the
cutting forces and the contact time between the tool
and the workpiece was observed. A higher contact time
leads to less force reduction. LTVAM leads to a further
decrease in cutting forces (57%), as the torsional-
vibration component leads to a lower contact ratio.
•The bottom surface roughness of the slot-milled
specimens decreases from Ra= 0.9 to 0.78 μm(LVAM)
and 0.75 μm (LTVAM) with a vibration amplitude Al
of 4 μm. With a further increase in vibration amplitude,
the surface roughness increases. The surface roughness
of the side wall improves for both LVAM and LTVAM
with increasing vibration amplitude.
•LVAM and LTVAM can significantly increase the
compressive residual stresses and the surface hardness
in slot milling. For peripheral milling, the ultrasonic
vibration only has a small influence on residual stresses.
•Burr formation can be slightly reduced by vibration
assistance.
•Vibration assistance increases the tool wear of the
minor cutting edges, whereas the tool wear of the
primary cutting edges is reduced by up to 20%. LTVAM
consistently shows lower tool wear than LVAM.
The reductions in cutting forces in LVAM and LTVAM, even
in the region without an interruption of the cutting process,
need further investigation and modeling in order to get a
better understanding of the friction forces between the tool
and the workpiece and its influence on the chip formation. In
the region of an interrupted cutting process, different cutting
fluids and coolants have a significant potential to further
3616 Int J Adv Manuf Technol (2020) 108:3607–3618
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improve process performance, as the vibration may make
the cutting zone more accessible for the coolant. Future
research will investigate the influence of cryogenic cooling
or minimum quantity lubrication on the process.
Funding information Open Access funding provided by Projekt
DEAL. This work was funded by the Deutsche Forschungsge-
meinschaft (DFG) within the research project “Machining of high-
performance materials with ultrasonically modulated cutting speed”
(Project number 406283248).
Open Access This article is licensed under a Creative Commons
Attribution 4.0 International License, which permits use, sharing,
adaptation, distribution and reproduction in any medium or format, as
long as you give appropriate credit to the original author(s) and the
source, provide a link to the Creative Commons licence, and indicate
if changes were made. The images or other third party material in
this article are included in the article’s Creative Commons licence,
unless indicated otherwise in a credit line to the material. If material
is not included in the article’s Creative Commons licence and your
intended use is not permitted by statutory regulation or exceeds
the permitted use, you will need to obtain permission directly from
the copyright holder. To view a copy of this licence, visit http://
creativecommonshorg/licenses/by/4.0/.
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