179
Wear, 69 (1981) 179 - 188
0 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
NEW OBSERVATIONS
ON THE MECHANISM
WHEN MACHINING TITANIUM ALLOYS
OF CHIP FORMATION
R. KOMANDURI
General Electric Corporate Research and Development,
Schenectady,
NY 12301
(U.S.A.)
B. F. VON TURKOVICH
Mechanical Engineering
(U.S.A.)
Department,
The University of Vermont, Burlington,
VT 05401
(Received August 12,198O)
Summary
Titanium and other aerospace structural super-alloys are extremely difficult to machine except at low cutting speeds because of rapid tool wear.
To increase productivity
it is necessary to understand the mechanics of chip
formation when machining these alloys. In this paper we report some new
findings towards that goal.
1. Introduction
Titanium alloys, although very much sought after for aerospace structural applications, are notorious for their response to machining. Most tool
materials wear rapidly even at moderate cutting speeds. Current machining
practice limits the cutting speed to less than 1 m s-l in order to
minimize tool wear. In addition, tool vibrations are induced by the gross
inhomogeneous
plastic deformation
in the primary zone, producing distinct
chip segmentation,
and are accentuated
by inadequate
stiffness of
component parts of the machine tool structure. These factors together limit
the machining productivity
of these materials. Unfortunately,
very little is
known about the detailed mechanism of chip formation when machining
these alloys although certain aspects of it have been discussed in the pioneering work of Shaw et al. [l] .
Figures l(a) and l(b) are micrographs
of polished and etched
longitudinal midsections of a typical Ti-6Al-4V
catastrophic shear-failed
chip [ 21 and a steel continuous chip respectively, showing the differences in
the nature of deformation.
On comparison, the following contrast can clearly be distinguished. The titanium alloy chip is serrated and the strain in it is
not uniformly distributed but is confined mainly to narrow bands between
(a)
(b)
Fig. 1. Micrographs
of polished and etched longitudinal
midsections
of (a) a typical
Ti-6Al-4V
catastrophically
shear-failed
chip and (b) a continuous
chip.
the segments, as indicated by the arrows in Fig. l(a). In a continuous chip,
however, the deformation
is largely uniform.
Shaw et al. [l] suggested that chip serration of titanium alloys is
due to the onset of instability in the cutting process which results from
competing thermal softening and strain-hardening
mechanisms in the
primary shear zone. Shaw [3] also suggested that the formation
of
concentrated
shear (also called adiabatic shear) bands was due to the poor
thermal properties (the low thermal conductivity
and the low specific heat)
of these alloys and the consequent concentration
of thermal energy in these
bands. On the basis of motion picture analysis of titanium alloy chip formation, Cook [4] observed no relative motion between the chip and the tool
for a finite time and this, he suggested, should facilitate welding between the
chip and the tool. However, no mention was made as to its influence on tool
temperature
and tool wear, factors important in machining titanium alloys at
higher speeds. Cook [4] also attributed vibration excitation to a varying
shear stress on the shear plane. There are, however, many other aspects of
chip formation which have not been explained, e.g. how the rest of the chip
segmentation is formed, the probable mechanism for the rapid tool wear at
almost any cutting speed, and the metallurgical nature of the shear bands.
With an increasing need to use titanium alloys for aerospace structural
applications and the need to increase productivity
(the cutting speed and/or
the metal removal rate) so as to reduce U.S. Government
Department
of
Defense procurement
costs, an advanced machining research project (Defense
Advanced Research Project Agency (DARPA)) was initiated at General
Electric Corporate Research and Development
(GE-CRD). One of the
objectives of this program was to investigate the fundamental
mechanism of
chip formation when machining these alloys; the present paper is a part of
that work.
2. Experimental equipment and test conditions
Chip formation studies were conducted at various machining speeds
from an extremely low speed of 2.12 X 10s5 m s-l to a moderately high
speed of 5.1 m s-l under orthogonal cutting conditions. The low speed
experiment was conducted inside a scanning electron microscope and the
cutting process was recorded on a video tape using the technique originally
conceived by Bell et al. [ 51, Doyle [ 61 and Iwata and Euda [ 71. Chip formation studies at higher cutting speeds were conducted on a lathe with the aid
of a high speed (up to 8000 frames s-l ) movie camera.
3. Observations
On the basis of detailed studies of the videotapes and the films, the
mechanism of chip formation when machining titanium alloys was found to
be different from the continuous chip formation. In addition this study has
confirmed many of the findings of the pioneering work of Shaw, Cook and
associates and permitted further clarification of several important aspects of
the process. The following specific observations were made.
(1) The sequence of events leading to the catastrophic shear-failed chip
can be divided basically into two stages. The first stage involves plastic
instability, leading to strain localization along a shear surface, This surface
originates from the tool tip almost parallel to the cutting velocity vector and
gradually curves with the concave surface upwards until it meets the free surface. The shear failure of the chip on the outside appears as a crack and on
the inside remains as a heavily deformed band which, when metallurgically
etched, appears as a white band (Fig. l(a)). The second stage involves gradual
flattening of a softer half-wedge (the inclined shear-failed surface on the work
material side from the previous segment), with very little deformation, by
the compression on a flat die (advancing tool). The net result is that the chip
thickness will be about the same as the depth of cut for orthogonal machining. This observation led some researchers to believe, falsely, that the shear
angle is very high (about 45”) when machining titanium alloys. The use of
the term “shear angle” on the above basis is inappropriate for chip formation
of this type and it would be a good practice to discontinue its use.
Figures 2(a) - 2(f) are schematic diagrams (based on the analysis of the
videotapes of the chip formation process inside the scanning electron microscope) of a sequence of events, showing various stages involved in the
catastrophic shear failed chip formation when machining titanium alloys,
Build-up of the segments starts with the gradual flattening of the wedgeshaped work material ahead of the tool. This phase of the process is similar
to the discontinuous chip formation process which has been described with
some variation by Field and Merchant [8], Cook et al. [9] and Palmer [lo]
and the chip se~entation
process described by Rice [ ll] and Sullivan et al.
1121. The initial contact on the tool face with the segment being formed is
182
--_----__
(a)
(b)
/
crack and/or adiabatic
shear initiation
(d)
crack and/o:adtabalic
shear propagation
(el
(f)
Fig. 2. Schematic diagrams of a sequence of events, showing various stages involved in
chip formation when machining titanium alloys.
extremely short and the contact length increases as the flattening progresses
(i.e. as the cutting tool proceeds towards the work material). There is almost
no relative motion between the bottom surface of the chip segment which is
being formed and the tool face, almost until the end of the flattening stage
of the chip segmentation process. This forces rapid transfer of heat into the
tool tip and enables rapid chemical interaction between the chip and the tool
that can lead to accelerated tool wear. The gradual bulging of the chip seg-
183
ment slowly pushes the chip segment previously formed upwards. The
contact between the previous segment and the segment being formed shifts
gradually, starting close to the work surface and moving towards the tool
face, as flattening progresses. The velocity of the chip along the rake face
will be the same as the velocity of bulging of the chip segment but, once
shear is initiated and progresses rapidly, it will push the segment being
formed faster parallel to the shear surface. This will then push the previously
formed segment rather rapidly. This part of the cycle involving motion of
the chip segment on the tool face is similar to the slip portion of a stick-slip
oscillatory process, except that very little force is required to push the segment.
The process as described also has a great influence on the dynamics of
cutting, as originally pointed out by Cook [13]. First, the principal cutting
force rises gradually during the upsetting stage of the segment formation
(this deflects the tool away from the workpiece gradually) and drops precipitously, once shear failure occurs (this relieves the load on the tool and
allows it to spring back rapidly). The process is repeated, leading to severe
vibrations and possible fatigue of the tool [14]. Secondly, depending on the
compliance
of the tool-work
material-machine
tool system, complex
vibratory events (such as hammering of the tool) may take place during
cutting.
(2) The deformation
in the chip segment in the present case can be
elucidated by the methods of classical plasticity [15, 161 and can be
compared with that of continuous chip formation. Let us consider Fig. 3
which shows schematically the alternative deformation
modes in machining.
As the tool advances from position 1 to 2 in Fig. 3, the original layer 1234
will be transformed
by deformation
into either (a) 2365 by a process of
simple shear, leading to continuous chip formation, as can be described by
the classical Merchant-Piispanen
model (see ref. 17 for details) or (b) a
segment 237 by upsetting only with the line 15 equal to the line 27 or into
a segment 2398 by upsetting combined with a small amount of shear. However, in order to complete the formation of the segment into the form 2398
an intense deformation
(or adiabatic band) develops, starting from 1 and
propagating along the path 12 and then into 23. The shear failed surface will
1
2
Fig. 3. Schematic diagram showing alternate deformation modes in machining.
184
undergo further extensive shear by rolling during upsetting of the next segment and transforms into a shape similar to 2398. Thus, part of the deformation seen in Fig. l(a) is to be interpreted in the above context and not as a
result of conventional secondary deformation along the tool face, which
occurs for continuous chip formation.
(3) A distinct feature of the process is the periodic development
of a
concentrated
shear band of very large strain accompanied
by rapid shear
failure due to plastic instability (Fig. l(a)). The process of deformation
in
this narrow shear band is very probably one of adiabatic shear [ 31.
(4) During flattening of the chip segment the shear failed surface of the
segment rolls onto the rake surface of the tool with almost no sliding along
it. The surface of the chip which is coming in contact with the tool is
actually a portion of the high shear layer (on the work material side)
stemming from the shear failure in the previous segment, i.e. the line 15 is
equivalent to 28 and the line 14 equivalent to 289. The line 93 is actually
the original surface with very little deformation
(line 34 becomes line 39 in
Fig. 3).
(5) In the median plane of the deforming material (i.e. the longitudinal
midsection for orthogonal machining) the process of deformation
is very
probably one of adiabatic shear [ 31, leading to a large strain concentration
in a narrow shear band. Rupture is prevented or postponed by the restraint
imposed by the adjoining material (the plane strain condition). On the sides
of the chip, however, material is free or unconstr~ned
(plane stress), and this
allows the segment to crack near the surfaces and to form fissures as can be
seen in Fig. 4(a). Shaw et al. [ 181 have discussed the plasticity problem
which involves simultaneous plane stress and plane strain conditions in
orthogonal machining. In the present investigation, evidence showing
fissures due to cracking on the sides and adiabatic shear without cracking in
the interior (Figs. 4(a) and 4(b)) was obtained.
(6) Hill [ 161 has shown that the wedge-flattening
problem can be
solved when the semiwedge angle is greater than 26.6” (corresponding
to a
shear angle of about 26.6”), but the mode of deformation
for narrower
wedges is unknown. In the present case the theory implies that flattening
occurs until the upper surface of the chip segment (Fig. 2(a)) becomes
approximately
normal to the rake face of the tool. Our experimental
results
support this theory. In Hill’s solution of the wedge-flattening
problem, the
existence of a dead zone which adheres to the die is clearly indicated. Such a
dead zone corresponds to a true built-up edge in machining, as evidenced in
this investigation (Fig. 5).
(7) The free boundary surfaces of a chip segment are thus composed of
a portion of the freshly formed shear failed surface on the one side (this surface is the one close to the work surface) and the other side by a portion of a
relatively undeformed
work surface. The remaining two boundaries of the
chip segment are composed of (a) the continuation
of the remaining fraction
of the adiabatic shear failed surface and (b) the adiabatic shear failed surface
(on the work material side) from the previous cycle of chip segmentation.
(a)
(b)
Fig. 4. (a) End view of a frozen chip root when machining a Ti-GAL-4V alloy inside the
scanning electron microscope at low speed, showing fracture between the segments and
formation of fissures due to plane stress conditions at the sides of the chip formation
process; (b) mirror image of the longitudinal midsection of the same chip root, showing
thermoplastic shear bands without fracture due to plane strain conditions in the interior
of the chip formation process.
Fig. 5. Shear failure of the built-up.edge at the apex of the tool in machining a Ti-GAl4V alloy at low speed inside the scanning electron microscope.
The adiabatic shear failed surface is further deformed and rolled by the
wedge-flattening process during the current cycle of the chip segmentation.
(8) During the process of wedge (chip segment) flattening the upper
boundary pushes the previous chip segment along the rake face at a velocity
V, . Once shear failure commences, the forming chip segment begins to slide
along the tool face and pushes the previously formed segment at a velocity
186
much higher than V,. Thus chip velocity along the rake face fluctuates
cyclically and within a given cycle asymmetrically.
(9) During the formation of the chip segment but prior to the formation of the shear failure, material below the shear layer flows towards the
clearance face in a fashion similar to a drawing operation.
(10) Almost no secondary deformation
zone along the rake face was
observed when machining the Ti-GAl-4V alloy unlike that in conventional
machi~ng of a ductile material which yields a continuous chip. It is also
found that it is not necessary to have any pressure on the chip segment
already formed for the process to continue. From this point of view the chip
formation process is very similar to discontinuous
chip formation.
(11) By an energy balance of the plastic deformation
energy and the
resulting thermal energy in the primary cutting zone, it can be shown that
the ~mperature
rise in a nascent chip segment will be only moderate during
the formation of most of the segment, because of the low strains involved
during compression of the segment. However, the temperature
rise in the
shear failure layer will be substantially higher as the shear strain in this layer
will be larger at least by an order of magnitude or more. In materials such as
the titanium alloys, the heat generated tends to remain in the shear failed
surface and the rolling of one of these surfaces on the tool face, during the
subsequent flattening process of the chip segmentation,
transfers the heat to
the tool tip. As there is almost no relative motion between the tool face and
the segment being formed, substantial chemical and thermal damage of the
tool can result in the immediate vicinity of the cutting edge. Also, by
flowing in a manner analogous to drawing, the material below the shear layer
rubs the clearance face severely, producing a high temperature
in the flank
face and substantial deformation
of the machined surface (Fig, 4). It is
unfortunate
that the highest temperature
in machining of such materials
should occur at the most critical region of the tool, i.e. the apex. This is
probably the principal reason for the rapid wear of cutting tools when machining ti~ium
alloys at all speeds.
Having acquired some knowledge of the mechanism of the chip formation when machining titanium alloys, the question immediately arises as to
what can be done to reduce the tool wear so that these alloys can be
machined economically
at higher speeds. The modification
of the thermal
and mechanical properties of these materials (perhaps the part that ends up
as the chip) to obtain a continuous
chip instead of the widely spaced
adiabatic shear bands by techniques such as laser-assisted machining appears
to be one promising approach, This would probably reduce vibration, toolchip interface temperature
and tool wear. The other approach is to provide
external relative motion between the segment being formed and the tool,
e.g. by rotary tool machining [ 191, Work is under way on these topics and
will be reported elsewhere [ 201 together with details of the present work.
187
4. Conclusions
On the basis of the present study and the high chemical reactivity of
titanium alloys with most materials, the circumstances leading to rapid tool
wear when machining these alloys at any speed can be summarised as
follows: (1) rolling of the virgin shear failed surface of the chip (in the
primary shear zone from the previous segmentation cycle) onto the tool
face, thereby es~bli~~g int~a~ contact between the tool and the freshly
sheared titanium during upsetting of the following segment under formation;
(2) high temperatures generated in a very narrow (adiabatic) shear band as a
result of (a) intense shear concentration in this band and (b) poor heat
dissipation from this band due to poor thermal properties of this material;
(3) continuous contact at or near the apex of the tool with the segment
being formed due to no relative motion between the segment and the tool
face for a considerable portion of the chip segmentation cycle; (4) high reactivity of titanium with most tool materials including the cemented carbides,
nitrides, oxides, borides, diamond and cubic boron nitride etc.; (5) rapid
flank wear due to the flow of material towards the clearance face in a
fashion similar to a drawing operation during the upsetting stage of the chip
segmentation.
Acknowledgments
The work reported in this paper was performed under Contract
F33615-79-C-5119 on advanced machining research sponsored by DARPA.
The authors thank Dr. M. Buckley of DARPA and Mr. W. A. Harris and
Mr. R. Stach of the Wright-Patterson Air Force Base for their encouragement
and support, Drs. D. G. Flom, A. W. Urquhart, M. Lee and M. Aven of GECRD, for their interest, Mr. S. R. Hayashi of GE-CRD for conducting the
scanning electron microscopy experiments and Mr. D. Montross of the Photography Section, GE-CRD, for assisting one of us (R.K.) with the high speed
photography.
References
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Department
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Further work on chip formation
characteristics
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Development,
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