Note: Descriptions are shown in the official language in which they were submitted.
CA 02237074 1998-07-03
BACKGROUND OF THE INVENTION
This is a divisional of application Serial Number
2,146,665 filed March 31, 1994 for Group IVB Boride Based
Cutting Tools.
The present invention relates to Group IVB
(titanium, hafnium, zirconium) boride based articles, cutting
tools and their densification techniques. It is especially
related to titanium diboride based cutting tools and their use
to machine Group IVB metals and alloys, especially titanium
and its alloys.
It was recognized as early as 1955 that "machining
of titanium and its alloys would always be a problem, no
matter what techniques are employed to transform this metal
into chips," (Siekmann, H. J. Tool Engng, Jan. 1955, Vol. 34,
Pages 78-82).
Over approximately the past forty years, commercial
machining technology for most workpiece materials has advanced
significantly. Ceramic, cermet and ceramic coated cutting
tools have been developed and commercialized which have
significantly improved productivity in machining of steels,
cast irons and superalloys. However, during that same time
period, progress in the field of machining titanium alloys has
been minor. The commercial cutting tool materials of choice
for most titanium machining applications remain high speed
tool steels and an uncoated, approximately 6 weight percent
cobalt cemented tungsten carbide, such as Kennametal K313
cemented carbide grade. Where coated cemented carbide tools
(e.g., Kennametal, KC720 and KC730 grades) have been applied
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to titanium alloy machining, they have met with only limited
success. The use of uncoated cemented carbides to machine
titanium based metallic materials has greatly limited
productivity advances in the machining of these materials,
since uncoated carbides are limited in most commercial
applications to speeds of 250 surface feet/minute or less when
machining titanium alloys (see Dearnley et al., "Evaluation of
Principal Wear Mechanisms of Cemented Carbides and Ceramics
used for Machining Titanium Alloy IMI318," Materials Science
and Technology, January 1986, Vol. 2, Pages 47-58; Dearnley et
al., "Wear Mechanisms of Cemented Carbides and Ceramics used
for Machining Titanium," High Tech Ceramics, ed. by P.
Vincenzini, Elsevier Sci. Publ. (1987) Pages 2699-2712; Metals
Handbook, Ninth Edition, Vol. 16, "Machining," (1989), Pages
844-857; Marchado et al., "Machining of Titanium and Its
Alloys - A Review," Proc. Instn. Mech. Engrs., Vol. 204 (1990)
Pages 53-60; and "Kennametal Tools, Tooling Systems and
Services for the Global Metalworking Industry," Catalogue No.
A90-41(150)E1, (1991) Page 274.
Kennametal, KC, K313, KC720 and KC730 are trademarks
of Kennametal Inc., of Latrobe, Pennsylvania, for its cutting
tool grades.
The machining speed used when machining titanium
alloys with uncoated cemented carbide tools may be increased
to 500 to 1000 surface feet/minute, through the use of a high
pressure coolant machining system (e.g., U. S. Patent No.
4,621,547). These systems are expensive, difficult to
integrate into existing machine tools, and require a
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significant amount of maintenance. Their application in
titanium alloy machining has, therefore, been limited.
Clearly, there has thus been an unfulfilled
long-felt need for improved cutting tool materials, and
improved methods for machining titanium based metallic
materials.
SUMMARY OF THE INVENTION
The present inventors have now surprisingly
discovered a new cutting tool material for machining titanium
based metallic materials, which significantly advances
titanium machining productivity and fulfils the long-felt need
identified above. Applicants have found that the present
invention may be utilized in the machining of a titanium alloy
at a metal removal rate of about two to three times that
obtained with uncoated carbide cutting tools using flood
cooling while maintaining about the same amount of metal
removed per cutting edge. This results in a significant
reduction in the labor time required to machine a given
titanium alloy workpiece while significantly increasing
machine availability. These results are achieved using
standard flood cooling techniques. The present invention,
therefore, has the further advantage that it does not require
the use of a high pressure coolant system to achieve high
machining speeds.
In a broad aspect, this invention provides a
densified ceramic composition comprising the following phases:
a first TixlMyl boride phase, where xl > Yl, Yl 2 0, and
M includes tungsten;
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a second TiX2My2Z phase, where x2 ~ Y2l Y2 > 0 and M
includes tungsten;
wherein Y2/X2 > Y1/Xli
wherein said phases are distributed throughout the
densified ceramic composition; and
the densified ceramic composition has an average grain
size of 8~m or less.
In a second broad aspect, this invention provides a
metal boride ceramic composition comprising:
a first metal diboride phase having a first metal
selected from the group of titanium, hafnium, zirconium, alone
or in combination with each other and a second diboride based
phase having a first metal selected from the group of Ti, Hf
and Zr, alone or in combination with each other, and a second
metal selected from the group of W, Mo, Ta, Nb, Fe, Ni, Co,
Al, Cr, alone or in combination wlth each other;
and wherein said ceramic has an average grain size of 6~m
or less.
In a third broad aspect, this invention provides a
titanium diboride based densified ceramic composition
comprising:
a microstructure having a TiXlMylB2 first phase with a
second phase of a TiX2My2Z about said first phase,
wherein xl > yl;
wherein yl 2 0;
whereln x2 > Y2
wherein Y2 ' ~;
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wherein Y2/X2 ~ Yl/Xl; and
wherein M includes tungsten.
In a fourth broad aspect, this invention provides a
densified titanium diboride based ceramic article of
manufacture comprising:
a microstructure composed of phases consisting
essentially of a TiB2 crystal structure; and
wherein said phases include phases containing tungsten at
differing concentration levels;
a density of at least 97~ of theoretical density;
a hardness of 94.3 to 96.5 Rockwell A at room
temperature; and
wherein said microstructure has an average grain size of
8~m or less.
These and other aspects of the present invention
will become more apparent upon review of the figures briefly
described below in conjunction with the detailed description
of the invention which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows an embodiment of a cutting tool in
accordance with the present invention.
Figure 2 shows an embodiment of the microstructure
of the present invention as obtained by a scanning electron
microscopy back scattered imaging technique.
Figure 3 shows an embodiment of the microstructure
of the present invention at five times the magnification used
in Figure 2.
Figure 4 is a graph of nose wear against cutting
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time for the present invention and a prior art uncoated
cemented carbide tool during the turning of a Ti-6Al-4V alloy.
Figure 5 is a graph of maximum flank wear as a
function of cutting time in the turning of a Ti-6Al-4V alloy
for the present invention and a prior art uncoated cemented
carbide.
Figure 6 is a graph of maximum flank wear as a
function of cutting time in the machining of Ti-6Al-4V alloy
for the present invention and the prior art uncoated cemented
carbide at 152 and 213 surface meters/minute (500 and 700
surface feet/minute).
DETAILED DESCRIPTION OF THE INVENTION
A preferred embodiment of an article of manufacture
in accordance with the present invention is shown in Figure 1.
While the present invention may find use in many applications,
the present inventors have found it to be particularly useful
as a cutting tool.
Figure 1 shows an embodiment of an indexable
metalcutting insert 10 composed of the ceramic material
discovered by the present inventors. The present invention is
preferably used in the high speed (>400 surface feet/minute)
chip forming machining (e.g., turning, milling, grooving,
threading, drilling, boring, sawing) of Group IVB metallic
materials (i.e., zirconium and its alloys, titanium and its
alloys, and hafnium and its alloys). The inventors have found
the present invention to be particularly useful in the high
speed machining of titanium alloys. Preferably, the
speed should be at least 500 sfm, and preferably, 1,000 sfm or
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less, to obtain the most advantageous use of the present
invention when machining these materials. Preferred feed
rates contemplated for machining titanium alloys are .002 to
.015 inch/revolution, and more preferably, .002 to .010
inch/revolution. Preferred depths of cut contemplated for
machining titanium alloys are about 0.01 to about 0.2 inch,
and more preferably, about 0.01 to about 0.15 inch.
The cutting tool 10 has a rake face 30 over which
chips formed during said high speed machining of a Group IVB
metallic material flow. Joined to the rake face 30 is at
least one flank face 50. At at least one juncture of the rake
face 30 and flank faces 50, a cutting edge 70 is formed, for
cutting into the Group IVB metallic material.
While the cutting edge 70 may be in a sharp, honed,
chamfered, or chamfered and honed condition, it is preferred
that it be in a chamfered condition, an embodiment of which is
illustrated in Figure 1.
Preferably, the cutting insert 10 has a cutting edge
lifetime of at least 3 minutes, and more preferably, at least
5 minutes during the high speed machining (e.g., turning) of a
titanium alloy. In addition, the tool in accordance with the
present invention has a maximum flank wear rate preferably no
greater than one-half, and more preferably, no greater than
one-third that of an uncoated cemented carbide tool when
machining (e.g., turning) a titanium alloy under the same high
speed cutting conditions, including flood cooling.
The cutting tool shown in Figure 1 is, preferably,
composed of the TiB2 based ceramic material in accordance with
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the present invention. Figures 2 and 3 show typical
microstructures of a preferred embodiment (see Example No. 1,
Table I) of the present invention at two different
magnifications. From Figure 2, it can be seen that the grain
structure is substantially fine and uniform, with an average
grain size estimated to be about 4~m. It can be most clearly
seen in Figure 3 that the grain structure is typically
characterized by a dark central phase, or central portion,
which may, preferably, be TiB2 or TiXlMylB2l where M may
preferably include W and/or Co and y 2 0. This first phase
appears to be embedded in a matrix composed of a second, and
possibly, a third phase. In many instances, adjacent to and
substantially surrounding the central grain is a light grey
second phase which is believed to be composed of a TiX2My2Z
phase, where x2 > Y2 and Y2 ' ~~ Y2/X2 ' Y1/X1 and M
preferably includes W and/or Co. Around many of these phases
is a third phase which is of a shade of grey intermediate that
of the central portion and the second phase. This third phase
is believed to be composed of a TiX3My3Z phase, where My3 is
preferably W and/or Co and where X3 > y3, y3 > 0 and y2/x2 >
y3/x3 > yl/x1 (e.g., the second phase has a greater
concentration of tungsten in it than the third phase matrix).
The concentration of titanium, however, is preferably greatest
in the central portion of the grain. X-ray diffraction
analysis has also shown that the major phase(s) present is of
the TiB2 type crystal structure; however, because of the lack
of sensitivity of x-ray diffraction to minor levels of phases
and minor levels of solid solutioning, it is unclear, based on
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x-ray diffraction alone, as to what minor phases or solid
solutions may be present.
It is also unclear from the x-ray diffraction work
alone what phases form the second and third phases mentioned
above. However, since from the photomicrographs there appears
to be substantial amounts of the second and third phases
present, their absence from the x-ray diffraction studies done
is believed to be explainable if these phases are also
TiXnMynB2 (i.e., Z=B2) phases containing minor amounts of W
and/or Co in solid solution (e.g., TiX2Wy2B2 and TiX3Wy3B2).
In this case, their absence from the x-ray diffraction trace
would be explained by the almost identical lattice constants
they would have with TiB2. That is, the TiB2 peaks are
substantially identical to, and therefore mask, the peaks of
the second and third phases.
While it is believed that the phases forming the
halos about the first phase (see Figure 3) are diborides, they
may also possibly contain minor amounts of, boroncarbide,
boronitride, boronoxide, borocarbonitride, boroxycarbide,
boroxynitride or a boroxycarbonitride; however, this has not
been confirmed. What appears to be definite, however, is that
the inner halo, or second phase, has a greater concentration
of tungsten than the outer halo, or third phase, and all three
phases contain titanium as the major metallic element present.
In addition to the TiB2 phase observed by x-ray
diffraction, other phases that have been at times observed in
minor amounts by x-ray diffraction include CoW2B2, CoWB5, WB,
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W2B, W3CoB, TiB and Ti3B4. The white phase visible in Figure
3 is believed to be one of the tungsten rich phases mentioned
above. The black spots shown in Figure 3 are believed to be
porosity.
In the alternative, similar compositions may be made
based on ZrB2 or HfB2, or their mixtures and solid solutions
with each other or TiB2. These compositions are less
preferred than the TiB2 based composition described above
because of their higher cost. In general, therefore, it can
be stated that the present invention includes a densified
composition, including a first metal diboride phase having a
first metal selected from the group of titanium, hafnium and
zirconium alone, or in combination with each other, and
optionally in combination with W and/or Co, and preferably a
second metal diboride phase having a metal which includes W
and/or Co, in combination with Ti, Hf and/or Zr, Mo, Nb, Ta
may be partially or wholly substituted for the W in the
material, while iron and/or nickel may be partially or wholly
substituted for the Co in the material. In addition, W, Mo,
Al and/or Cr may be partially substituted for the cobalt in
the material.
The densification of the present invention may be
achieved either by hot pressing a blend of the appropriate
powders or by cold pressing the blended powders to form a
compact which is then sintered and hot isostatically pressed.
These processes will be illustrated by the following
discussion directed to TiB2 based compositions, but it should
be understood that the techniques described are also
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applicable to ZrB2 and HfB2 based compositions and their
mixtures and solid solutions with each other and/or TiB2 in
accordance with the present invention.
In accordance with the present invention, a blend of
powders is prepared, composed of at least 60 w/o; preferably,
at least 75 w/o; more preferably, at least 85 w/o; and most
preferably, at least 90 w/o TiB2.
It is preferred that the level of TiB2 utilized
should be as high as possible commensurate with the ability of
the composition to be densified by either the hot pressing or
the cold pressing-sintering-hot isostatic pressing route in
order to achieve the high wear resistance while machining
titanium alloys. Applicants have found that TiB2 has
excellent resistance to reactivity with titanium during
titanium alloy machining and has good thermal conductivity
compared with other ceramics; however, it is very difficult to
densify, while maintaining a fine grain size.
Applicants have surprisingly discovered that TiB2
based ceramics can be readily densified if WC and Co are added
to the TiB2 powder blend. The WC and Co may be added: (1)
directly as individual WC (or W and C) and Co powders; or (2)
as a result of the attrition of the cemented WC-Co milling
media during milling of the TiB2 powder; (3) as a cemented
WC-Co powder; or (4) by a combination of (1), (2) and/or (3).
At least 2.5 w/o total of WC + Co should be added to the TiB2
powder to assure densification at 2000~C or less in hot
pressing. Where densification is to be achieved by cold
compaction-sintering and hot isostatic pressing, it is
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preferred that there be at least 3.0 w/o total of WC + Co.
While not optimized, the inventors have found that
the ratio of W/Co on a weight percent basis may be about 9:1
to about 20:1. It has been found that the addition of the
combination of Co and WC in the minimum amounts indicated
significantly improves the ease with which densification is
achieved, without an adverse effect on the grain size of the
resulting material. It is believed that this effect is due to
a low melting point eutectic alloy formed by the WC and Co
during the sintering process. It is, therefore, believed that
W/Co ratios as low as 1:20 may also be useful and may result
in a further lowering of the sintering or hot pressing
temperatures required to achieve substantial densification.
The total WC + Co addition preferably should be less than
about 12 w/o, and more preferably, less than 10 w/o, since
increasing WC + Co content increases the observed wear rate
during the high speed machining of titanium alloys.
The inventors have also found that the grain size of
the densified article may be further controlled by the
addition of an effective amount of a grain growth inhibitor to
the powder blend. The inventors, therefore, prefer to add BN
powder to the blend at a preferred level of about 0.25 to 1.0
v/o of the powder blend.
Limited amounts (not exceeding about 35 v/o total)
of other elements and/or compounds may be added to the powder
blend to improve various properties of the material for
specific applications. Such additions that are now
contemplated may include: (1) TiC, ZrC, B4C, TaC and MO2C to
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improve wear resistance; (2) TiN, TiC to assist in
densification. Hafnium diboride and/or zirconium diboride may
also be substituted for TiB2 to improve wear resistance,
preferably, the total content of HfB2 and ZrB2 in the
composition is also held below 35 v/o. It is also
contemplated that a portion of the Co addition may be
partially replaced by, or supplemented by, small amounts of W,
Fe, Mo, Ni, Al and Cr, and totally replaced by Fe and/or Ni.
Fracture toughness may be further improved through
the use of starting powders having an elongated or whisker
morphology. For example, some of the TiB2 starting powder may
be replaced by TiB2 whiskers, or SiC, B4C, TiC, ZrC, TaC or
MO2C may be added as elongated particles or whiskers.
The foregoing powders are preferably blended for a
time appropriate to provide the desired pick up in WC and Co
from the WC-Co cemented carbide milling media. Preferably, at
least about 2.5 w/o of WC + Co is added to the blend in this
manner.
The blended powder is then densified. If it is
densified by uniaxial hot pressing, then the hot pressing
temperatures and pressures used are preferably about
1800-2000~C and about 1 to 5 Ksi, and more preferably, 1 to 2
Ksi. It is desirable that the hot pressing temperature be
minimized to minimize grain growth. In order to achieve
maximum densification during hot pressing, the pressure should
be maintained sufficiently low during temperature elevation to
allow gases generated during heat-up to escape. After these
gases have escaped, the full hot pressing pressure may then be
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applied.
Alternatively, the powder blend may also be
densified by cold compaction to form a green compact,
followed by sintering, preferably at 1800 to 2200~C,
preferably followed by hot isostatic pressing, preferably at
1700 to 2100~C, and up to 30,000 psi using argon or helium or
other inert gas, but not nitrogen. This manufacturing route
is preferable over the hot pressing route if equivalent levels
of densification and a fine grain size can be achieved for a
given composition, since the cutting and grinding of a hot
pressed ceramic billets is avoided, thereby reducing the
manufacturing cost.
- The inventors believe that the grain size in the
densified article is very important to achieving the best
metalcutting properties and, therefore, prefer that the
average grain size be 8~m, or less, more preferably, 6~m, or
less, and most preferably, 4~m or less. The inventors believe
that a fine grain size is important because TiB2 has a very
high modulus of elasticity, E, and an anisotropic thermal
expansion coefficient, ~, which would tend to reduce the
thermal shock resistance of a ceramic containing large TiB2
grains. The inventors, however, believe that they have
minimized any adverse consequences of these properties by
maintaining the fineness of the grains, as described above,
which are believed to be substantially randomly oriented.
The resulting articles made in accordance with the
present invention preferably have a Rockwell A room
temperature hardness of about 94.3 to 96.5, more preferably,
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about 94.7 to 96.0, and most preferably, 95.0 to 96Ø Their
density is, preferably, at least 97~, and more preferably, at
least 98~ of the theoretical calculated density. The KIC
(Evans & Charles) fracture toughness of these articles is
difficult to measure, but is estimated to be (using 300 to 500
gm loads), by the Palmqvist indentation method of fracture
toughness measurement, about 3.5 to about 4.5 MPam~. Despite
this low mechanical fracture toughness, the articles in
accordance with the present invention have been surprisingly
found to have excellent toughness during the turning of a
titanium alloy as described in the examples which follow.
These examples are provided to further illustrate the
significant benefit provided by the present invention in the
high speed machining of titanium alloys.
In accordance with the present invention, articles
were made of the compositions shown in Table I.
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N ~ E ~
~n P~~
t
q
,-
I O L
U oP O U
a u, ~ a
" ~O CD a~
U O
a bl
~, _
-rl
U2
U~ ~ X r~ r~ ~ r~ r~ r~ r~
a) O o o o o o o o
_ . o In O O ~ O O
Q~ Ul ~ -~ r~ ~I r~ r~ r1
O
tl r~
~~ r~ O ~ ~ ~ ~
r- J~ + ~ I 111
~ -, o 3 3
E~
u
~ a, ~ ,~~NO ~1'1 ~ ~
E
.r -rl
U ~UL
O -,
O ~ o L
O O
0 ~0 0 ~V
r~l U N O ~ r
~D ~ O O
~ O ~ O N~
+~ +' + ~ + ~a
m~ a:) 3
b- r-~ In O
O o O oo ,~ oo ~ o
~~ ~ ~~ O ~ N
m
a + + + -rl
rcN r~ N NN U NN C~~D
n ~. ~2 . . 3 3 (L
r
~L
Izr; N(~1 ~ Il~D ~ L
a
lC rl
- 16 -
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The titanium diboride starting powder used was Grade
F obtained from Hermann C. Starck Berlin GmbH & Co. KG, P.O.B.
1229, D-7887 Laufenburg/Baden, Germany. This powder is
composed of crushed and milled irregular shape particles
having an hexagonal crystal structure. The specification,
along with an example of the actual properties for this grade
of TiB2 powder, are shown in Table II.
TABLE II
SPecification Measured Property
BET Specific Surface Area > 4 m2/g 4. m2/g
Scott TAP Density/Apparent Density-- 9.2 g/in3
FSSS Particle Size max. 0.9~m 0.9 ~m
Max. Particle Size 98~ < 6~m
Ti 2 66.5 wt. ~ remainder
B 2 28.5 wt. ~ 29.8
Nonmetallic Impurities
C s 0.25 wt. ~ 0.21
O s 2.0 wt. ~ 1.92
N s 0.25 wt. ~ 0.11
Metallic Impurities
Fe s 0.25 wt. ~ 0.18
Other - Total s 0.2 wt. ~ c 0.2
The boron nitride starting powder was obtained from
Union Carbide as grade HCP.
The WC powder had the following properties:
Total Carbon 6.11 w/o Cr .01 w/o
Free Carbon .01 w/o Ta .12 w/o
~2 .17 w/o Ca .21 w/o
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Ni .01 w/o Fe .02 w/o
BET 1.36 m2/g
The cobalt powder was an extra fine grade cobalt.
These powders were milled together in the ratios
shown in Table I to form 100 gm lots. Wet milling was
performed in a polyurethane lined ball mill with isopropanol
and about 3900 gm of WC-Co cemented carbide cycloids for the
times shown in Table I. These cemented carbide cycloids have
a nominal composition containing about 5.7 w/o Co, 1.9 w/o Ta
and a nominal Rockwell A hardness and nominal magnetic
saturation value of about 92.7 and about 92 percent,
respectively.
From our experience in milling these powders under
the conditions described, it is estimated that, for a milling
time of 45 to 50 minutes, about 2.4 to about 2.7 w/o of WC +
Co, and for a milling time of 120 minutes, about 4.1 to about
5.8 w/o WC + Co, is added to the blend due to the attrition of
the WC-Co cemented carbide cycloids during milling.
After milling, powder blends were dried, screened
and then uniaxially hot pressed according to the conditions
shown in Table I in an argon atmosphere. During heating,
pressure was not applied. The pressing pressure was first
applied at the hot pressing temperature and held for typically
one hour. The resulting articles produced were essentially
fully dense and have the densities, hardnesses and grain sizes
shown in Table I. Billets made in accordance with Example 1
were cut and ground to produce SNGN-453T (.002-.004 inch x 20~
chamfer) style indexable metalcutting inserts (see Figure 1).
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These inserts were tested in the metalcutting tests
described below in Table III against prior art sharp edged
K313 grade cemented carbide SNGN-433 style cutting inserts.
These tests were run under flood coolant at 600, 800 and 1,000
surface feet/minute, 0.005 ipr and 0.050 inch depth of cut as
described in Table III. The cutting tools composed of
material in accordance with the present invention had more
than twice the life of the prior art cemented carbide tools.
It was observed that chemical reaction between the titanium
alloy workpiece and the cutting tool according to the present
invention was the dominant wear mechanism on the rake and
flank faces. The present invention, however, had a
significantly lower wear rate than the prior art tool, as
illustrated by the graphs of nose wear and maximum flank wear
shown in Figures 4 and 5, respectively, which are based on the
results of the 600 sfm test shown in Table III.
A water soluble coolant, which may be used in these
applications, is Cimtech 500. Cimtech 500 is a synthetic
fluid concentrate for machining and stamping ferrous metals.
It is supplied by Cincinnati Milicron Marketing Co., of
Cincinnati, Ohio. It is typically diluted in water at a water
to coolant ratio of 30:1 to 20:1 for machining applications.
TABLE III
METALCUTTING TEST RESULTS
Tool Life in Minutes
600 sfm 800 sfm 1,000 sfm
Invention > 5.0 > 1.5 1.5 BK
Prior Art 2.5 FW c 0.5 BK Not Run
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Conditions
Operation: Turning
Workpiece Material: Ti-6Al-4V titanium alloy (Annealed at
1300~F for 2 hours and air cooled)
Cutting Speed: as noted above
Feed: .005 inch/revolution
Depth of Cut: .050 inch
Lead Angle: 45 degrees
Rake Angle: -5 degrees back rake and -5 degrees side rake
Cutting Fluid: Flood water soluble coolant diluted 20:1 with
water
End of Life Criteria
Breakage (BK)
Flank Wear (FW) 2 . 030 inch
Maximum Flank Wear (MW) 2 . 040 inch
Nose Wear (NW) 2 . 040 inch
Depth of Cut Notching (DN) 2 . 80 inch
It was further surprisingly found that the low
fracture toughness, mentioned above, of the invention did not
adversely affect the ability of the material to turn the above
titanium alloy. It was further surprisingly found that the
use of flood coolant did not cause the invention to break from
excessive thermal shock. These results demonstrate that the
present invention has at least twice the cutting edge lifetime
of the prior art cemented carbide tools at machining speeds
far beyond those recommended (i.e., ~250 sfm) for uncoated
carbide.
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While uncoated cemented carbides can achieve similar
lifetimes to those found for the present invention at lower
speeds (c250 sfm), these lower speeds greatly reduce the metal
removal rate, which is important in determining machining
costs and machine availability.
In another example, the composition used in Example
1 was blended in accordance with Example 1. After milling,
the powders were dried, pelletized with a lubricant/fugitive
binder (e.g., rosin/polyethylene glycol) and then uniaxially
cold pressed to form green cutting inserts. The green inserts
were heated in a vacuum up to about 460~C to volatilize the
lubricant and fugitive binder. Heating was then continued in
one atmosphere of argon to a sintering temperature of about
2000~C, which was held for 60 minutes and then cooled to room
temperature. The sintered inserts were then hot isostatically
pressed at 1850~C for 60 minutes under 15 Ksi argon.
Sintering and hot isostatic pressing were performed by placing
the inserts on a bed of boron nitride setting powder. The
inserts were then ground to final size. In this manner,
RNGN-45T (.002-.004 inch x 20~ chamfer) style cutting inserts
were fabricated. These cutting inserts were tested against
prior art K313 grade cemented carbide cutting inserts in the
RNGN-45 style with a sharp cutting edge in the turning of
Ti-6Al-4V titanium alloy. The test conditions, as well as the
results of these tests, are shown in Table IV and Figure 6,
and are summarized below.
A single test trial was run at 152 m/minute (500
sfm), comparing the pill pressed-sinter-Hipped inserts to the
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prior art K313 grade of cemented carbide. Figure 6 is a plot
of maximum flank wear. The important observation is that the
wear rate for the present invention is relatively uniform
through the end of life, at 10 minutes (based on .040 inch
maximum flank wear). The 152 m/minute cutting speed is too
high for the prior art, which had less than three minutes of
tool life due to maximum flank wear exceeding 0.040 inch.
T~3LE IV
CONDITION Time MAXIMUM FLANK WEAR (inch)
500 sfm/ (minutes) RUN 1 RUN 2
.0072 IPR/ INVENTION PRIOR ART INVENTION PRIOR ART
.050"DOC/
Flood Coolant
1 .0151 .0081
2 .0169 .0203
3 .0182 .0428
4 .0214
0.223
6 .0255
7 .0286
8 .0301
9 .0345
.0401
700 sfm/
.0072 IPR/
.050" DOC/
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CA 02237074 1998-07-03
Flood Coolant
1 .0163.0263 .0170 .0354
2 .0228.0984 .0230 .0510
3 .0285 .0296
4 .0367 .0359
.0445 .0434
1000 sfm/
.005 IPR/
.050" DOC/
Flood Coolant
1 .0202 .1436
2 broken
Two test trials were run at 212 m/minute (700 sfm)
(see Table IV and Figure 6). It was found that the pill
pressed-sintered and Hipped inserts (~) had equal or better
wear rate compared to the hot pressed inserts (~) under these
conditions. (The hot pressed insert (RNGN-43T) failed
prematurely by cracking because it was too thin for this
application.) At 700 sfm, the present invention maintains a
uniform wear rate that is significantly superior to that
produced in the prior art cutting tool. The 700 sfm cutting
speed is also clearly out of the useful range of the uncoated
carbide tested, which experienced extreme localized wear in
less than two minutes.
In an attempt to determine the upper limit of the
cutting speed for the present invention, a test was run at
1000 sfm (see Table IV). The invention failed at two minutes
68188-71D
CA 02237074 1998-07-03
due to breakage. The prior art cemented carbide tool
experienced extreme localized wear and resulting rake and
flank face chipping in less than one minute.
Based on the foregoing examples, it is clear that
the pill pressed, sintered and Hipped cutting tools according
to the present invention have the same capabilities for
machining titanium based materials as the hot pressed cutting
tools according to the present invention. The present
invention is capable of withstanding cutting speeds which are
significantly beyond the useful operating range of uncoated
cemented carbide. It was further found that the present
invention can withstand larger wear scars without experiencing
the acceleration in wear rate that is typical of cemented
carbide cutting tools.
It is further believed that the metalcutting
performance of the present invention may be further improved,
allowing longer cutting edge lifetime and/or higher machining
speed capabilities, through the application of a refractory
coating to the rake face, flank face and cutting edge. The
coating may be applied by known PVD or CVD techniques now used
to coat cutting tools. A refractory coating having one or more
layers is preferably composed of one or more of the following
refractory materials: alumina, and the borides, carbides,
nitrides and carbonitrides of
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CA 02237074 1998-07-03
zirconlum, hafnium and tltanlum, thelr solld solutlons with
each other and thelr alloys. It ls further proposed that use
of such a refractory coatlng may allow the use of hlgher
levels of toughenlng agents or WC + Co to further lmprove the
slnterabillty of the present lnventlon, while minimizing the
adverse lmpact of such increases on the wear rate when
machining tltanium alloys.
It ls also contemplated that cuttlng lnserts ln
accordance with the present lnventlon may be fabrlcated wlth
elther a ground ln, or molded, chlpbreaker structure.
Examples of chlpbreaker structures whlch may be used hereln
are descrlbed ln Unlted States Patent No. 5,141,367. Titanium
alloy chlps are notorlously hard to break. Thls may be
partlally due to the slow speeds used when uncoated cemented
carbldes are utlllzed to turn tltanlum alloys. It ls our
bellef that the hlgher machlnlng speeds now posslble wlth the
present lnvention, ln comblnatlon wlth a chipbreaker
structure, may lead to lmproved chlp control durlng the
turnlng of tltanlum alloys.
Other embodlments of the lnventlon wlll be apparent
to those skllled ln the art from a conslderatlon of thls
speclflcation or practlce of the lnventlon disclosed hereln.
It ls lntended that the speclflcatlon and examples be
consldered as exemplary only, wlth the true scope and splrlt
of the lnventlon belng lndlcated by the followlng clalms.
68188-71
