U.S. patent application number 15/560533 was filed with the patent office on 2018-02-15 for alpha- titanium alloy.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Koichi AKAZAWA, Yoshio ITSUMI, Hideto OYAMA, Keitaro TAMURA.
Application Number | 20180044763 15/560533 |
Document ID | / |
Family ID | 57242933 |
Filed Date | 2018-02-15 |
United States Patent
Application |
20180044763 |
Kind Code |
A1 |
TAMURA; Keitaro ; et
al. |
February 15, 2018 |
Alpha- TITANIUM ALLOY
Abstract
To provide an .alpha.-.beta. titanium alloy that has high
strength and excellent hot workability of the level of the
.alpha.-.beta. titanium alloy, typified by the Ti-6Al-4V, while
exhibiting more excellent machinability than the Ti-6Al-4V. The
.alpha.-.beta. titanium alloy includes, in percent by mass: at
least one element of 0.1 to 2.0% of Cu and 0.1 to 2.0% of Ni; 2.0
to 8.5% of Al; 0.08 to 0.25% of C; and 1.0 to 7.0% in total of at
least one element of 0 to 4.5% of Cr and 0 to 2.5% of Fe, with the
balance being Ti and inevitable impurities.
Inventors: |
TAMURA; Keitaro;
(Takasago-shi, JP) ; AKAZAWA; Koichi; (Kobe-shi,
JP) ; ITSUMI; Yoshio; (Takasago-shi, JP) ;
OYAMA; Hideto; (Takasago-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) |
Kobe-shi, Hyogo |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
57242933 |
Appl. No.: |
15/560533 |
Filed: |
March 16, 2016 |
PCT Filed: |
March 16, 2016 |
PCT NO: |
PCT/JP2016/058247 |
371 Date: |
September 22, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22F 1/18 20130101; C22F
1/183 20130101; C22F 1/00 20130101; C22C 14/00 20130101 |
International
Class: |
C22C 14/00 20060101
C22C014/00; C22F 1/18 20060101 C22F001/18 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2015 |
JP |
2015-064275 |
Jan 21, 2016 |
JP |
2016-009417 |
Claims
1. An .alpha.-.beta. titanium alloy comprising, in percent by mass:
Ti; at least one element of 0.1 to 2.0% of Cu and 0.1 to 2.0% of
Ni; 2.0 to 8.5% of Al; 0.08 to 0.25% of C; and 1.0 to 7.0% in total
of at least one element of 0 to 4.5% of Cr and 0 to 2.5% of Fe.
2. The .alpha.-.beta. titanium alloy according to claim 1, further
comprising, in percent by mass: more than 0% and 10% or less in
total of one or more elements selected from the group consisting of
more than 0% and 5.0% or less of V; more than 0% and 5.0% or less
of Mo; more than 0% and 5.0% or less of Nb; and more than 0% and
5.0% or less of Ta.
3. The .alpha.-.beta. titanium alloy according to claim 1, further
comprising, in percent by mass, more than 0% and 0.8% or less of
Si.
4. The .alpha.-.beta. titanium alloy according to claim 2, further
comprising, in percent by mass, more than 0% and 0.8% or less of
Si.
Description
TECHNICAL FIELD
[0001] The present invention relates to an .alpha.-.beta. titanium
alloy. More particularly, the present invention relates to an
.alpha.-.beta. titanium alloy with excellent machinability.
BACKGROUND ART
[0002] A high-strength .alpha.-.beta. titanium alloy, typified by
Ti-6Al-4V, can have its strength level changed easily by a heat
treatment, in addition to being lightweight and having high
strength and high corrosion resistance. For this reason, this type
of .alpha.-.beta. titanium alloy has been hitherto used very often,
especially in the aircraft industry. To further make use of these
characteristics, in recent years, applications of the
.alpha.-.beta. titanium alloy have been increasingly expanded into
the fields of consumer products, including vehicle parts, such as
engine members of automobiles or motorcycles, sporting goods such
as golf goods, materials for civil engineering and construction,
various working tools, and spectacle frames, the development fields
of deep sea and energy, and the like.
[0003] For example, as such an .alpha.-.beta. titanium alloy,
Patent Document 1 mentions an .alpha.-.beta. titanium alloy
extruded material with excellent fatigue strength and a
manufacturing method for the .alpha.-.beta. titanium alloy extruded
material. Specifically, the .alpha.-.beta. titanium alloy extruded
material includes specified contents of C and Al, and also includes
2.0 to 10.0% in total of one or more of V, Cr, Fe, Mo, Ni, Nb, and
Ta, in which an area ratio of a primary .alpha.-phase is within a
certain range, a direction of a major axis of each of 80% or more
of primary a grains in the primary .alpha.-phase is positioned
within a specified angle range, and an average minor axis of a
grains in a secondary .alpha.-phase is 0.1 .mu.m or more.
[0004] As the .alpha.-.beta. titanium alloy with enhanced
forgeability, Patent Document 2 mentions an .alpha.-.beta. titanium
alloy for casting that has higher strength and more excellent
castability than a Ti-6Al-4V alloy. Specifically, this
.alpha.-.beta. titanium alloy mentioned includes specified contents
of Al, Fe +Cr +Ni, and C +N +0, and further a specified content of
V if needed, with the balance being Ti and inevitable
impurities.
[0005] However, the .alpha.-.beta. titanium alloy has extremely
high manufacturing cost, and in addition, especially bad
machinability, which interferes with the expansion of the
applications of the .alpha.-.beta. titanium alloy. The usage range
is limited in practice. In view of such circumstances, various
titanium alloys with improved machinability have been recently
proposed.
[0006] For example, Patent Document 3 mentions a titanium alloy for
a connecting rod that has improved the machinability while
suppressing the reduction in toughness and ductility by containing
rare earth elements (REM) and Ca, S, Se, Te, Pb, and Bi as
appropriate to form granular compounds. Patent Document 4 mentioned
a free-cutting titanium alloy that has improved the machinability
by containing a rare earth element and improved the hot workability
by containing B.
[0007] Patent Document 5 mentions a free-cutting titanium alloy
that achieves the reduction in ductility of a matrix and the
refinement of inclusions to improve the free cutting properties,
while suppressing the reduction in the fatigue strength and
ensuring hot workability, by adding P and S, P and Ni, or P, S and
Ni, or further REM in addition to these elements as free-cutting
component.
[0008] Further, Patent Document 6 mentions an .alpha.-.beta.
titanium alloy with excellent machinability and hot working. The
.alpha.-.beta. titanium alloy includes specified contents of C and
Al and 2.0 to 10% in total of one or more elements selected from
the group of .beta.-stabilizing elements consisting of respective
specified contents of V, Cr, Fe, Mo, Ni, Nb, and Ta, with the
balance being Ti and impurities. In the titanium alloy, an average
area ratio of TiC precipitates in a microstructure is 1% or less,
and an average value of the average circle equivalent diameter of
the TiC precipitates is 5 .mu.m or less.
PRIOR ART DOCUMENT
Patent Document
[0009] Patent Document 1: JP 2012-52219 A
[0010] Patent Document 2: JP 2010-7166 A
[0011] Patent Document 3: JP 06-99764 B
[0012] Patent Document 4: JP 06-53902 B
[0013] Patent Document 5: JP 2626344 B1
[0014] Patent Document 6: JP 2007-84865 B
Disclosure of the Invention
Problems to be Solved by the Invention
[0015] In the methods like Patent Documents 3 and 4 mentioned
above, metallic inclusions are precipitated by using REM. In the
method like Patent Document 5 mentioned above, P is positively
contained to form a P inclusion. In the method like Patent Document
6, the size of a TiC precipitate is controlled. However, in these
methods, it is considered that precipitation of these precipitates
and inclusions are more likely to be affected by the temperatures
and cooling rates of melting to forging steps, thus making it
difficult to control the size of the precipitate or the like.
Furthermore, the shape or size of a raw material tends to cause
variations in the size or the like of the precipitate or inclusion.
Thus, to achieve the excellent machinability by precipitating
inclusions of interest, there is a problem that strict control of a
manufacturing process is necessary.
[0016] The present invention has been made in view of the foregoing
circumstance, and it is an object of the present invention to
achieve an .alpha.-.beta. titanium alloy that has high strength and
excellent hot workability of the level of the .alpha.-.beta.
titanium alloy, typified by the Ti-6Al-4V, while exhibiting more
excellent machinability than the Ti-6Al-4V, without the necessity
for the strict control or the like of the manufacturing
process.
Means for Solving the Problems
[0017] An .alpha.-.beta. titanium alloy according to the present
invention, which can solve the above-mentioned problem, is
characterized by including, in percent by mass: at least one
element of 0.1 to 2.0% of Cu and 0.1 to 2.0% of Ni; 2.0 to 8.5% of
Al; 0.08 to 0.25% of C; and 1.0 to 7.0% in total of at least one
element of 0 to 4.5% of Cr and 0 to 2.5% of Fe, with the balance
being Ti and inevitable impurities.
[0018] The .alpha.-.beta. titanium alloy may further include, in
percent by mass: more than 0% and 10% or less in total of one or
more elements selected from the group consisting of more than 0%
and 5.0% or less of V; more than 0% and 5.0% or less of Mo; more
than 0% and 5.0% or less of Nb; and more than 0% and 5.0% or less
of Ta.
[0019] The .alpha.-.beta. titanium alloy may further include, in
percent by mass, more than 0% and 0.8% or less of Si.
Effects of the Invention
[0020] Accordingly, the present invention can provide the a-8
titanium alloy that has high strength and excellent hot
workability, such as forgeability, of the level of an
.alpha.-.beta. titanium alloy, typified by the Ti-6Al-4V, and also
exhibits more excellent machinability than the Ti-6Al-4V, making it
possible to ensure satisfactory lifetime of working tools.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a photomicrograph of a titanium alloy according to
the present invention.
MODE FOR CARRYING OUT THE INVENTION
[0022] The inventors have intensively studied to solve the
foregoing problems. As a result, it has been found that especially,
a specified content of at least one of Cu and Ni is contained in a
titanium alloy, thereby significantly improving the ductility of
the titanium alloy at high temperatures. In particular, thin chips
are formed on the titanium alloy during a cutting process due to
the reduction in deformation resistance, leading to a reduced
cutting resistance, i.e., improving the machinability thereof. The
composition of the .alpha.-.beta. titanium alloy according to the
present invention will be described in sequence below, starting
from Cu and Ni, which are the features of the present
invention.
At least one element of Cu: 0.1 to 2.0%, and Ni: 0.1 to 2.0%
[0023] These elements are solid-soluted into the .alpha.-phase and
the .beta.-phase in the alloy, thereby increasing its ductility at
a high temperature and improving the hot workability. Thus,
especially, the cutting resistance of the titanium alloy becomes
lower, and the machinability thereof is improved. These elements
may be used alone or in combination. If the content of each of
these elements is less than 0.1%, the effect of improving the
ductility is lessened. Thus, the content of each of these elements
is set at 0.1% or more. The content of each of these elements is
preferably 0.3% or more, and more preferably 0.5% or more. In
contrast, if the content of each of these elements exceeds 2.0% by
mass, the hardness of the titanium alloy is increased, thereby
making it more likely to reduce the machinability and the hot
workability, such as forgeability. Thus, the content of each of
these elements is set at 2.0% or less. The content of each of these
elements is preferably 1.5% or less, and more preferably 1.0% or
less. [0024] Al: 2.0 to 8.5%
[0025] Al is an a-stabilizing element and thus is contained in the
titanium alloy to form the .alpha.-phase. If the Al content is less
than 2.0%, the formation of the .alpha.-phase is lessened, failing
to obtain sufficient strength. Thus, the Al content is set at 2.0%
or more. The Al content is preferably 2.2% or more, and more
preferably 3.0% or more. Meanwhile, if the Al content exceeds 8.5%
to become excessive, the ductility of the titanium alloy is
degraded. Thus, the Al content is set at 8.5% or less. The Al
content is preferably 8.0% or less, more preferably 7.0% or less,
and still more preferably 6.0% or less. [0026] C: 0.08 to 0.25%
[0027] C is an element that exhibits the effect of improving the
strength of the titanium alloy. To exhibit such an effect, the C
content needs to be 0.08% or more. The C content is preferably
0.10% or more. Meanwhile, if the C content exceeds 0.25%, coarse
TiC particles not solid-soluted in the .alpha.-phase will remain,
thus degrading the mechanical properties of the titanium alloy.
Therefore, the C content is set at 0.25% or less. The C content is
preferably 0.20% or less. [0028] 1.0 to 7.0% in total of at least
one element of Cr: 0 to 4.5% and Fe: 0 to 2.5%
[0029] These elements are .beta.-stabilizing elements. These
elements may be used alone or in combination. To exhibit the
above-mentioned effects, the total content of these elements needs
to be 1.0% or more. The total content of these elements is
preferably 2.0% or more and more preferably 3.0% or more. The lower
limit of the total content of these elements only needs to be 1.0%
or more as mentioned above, and the lower limit of the content of
each of these elements is not limited specifically. Regarding the
lower limit of the content of the individual element, for example,
when Cr is contained in the titanium alloy, the lower limit of Cr
content can be set at 0.5% or more, and further 1.0% or more. When
Fe is contained in the titanium alloy, the lower limit of Fe
content can be set at 0.5% or more, and further 1.0% or more.
[0030] In contrast, when the total content of these elements is
excessive, the ductility of the titanium alloy is degraded. Thus,
the total content of these elements is set at 7.0% or less. The
total content of these elements is preferably 5.0% or less, and
more preferably 4.0% or less. Even when the total content of these
elements is within the above-mentioned total content range, if the
Fe content is excessive, the degradation in the ductility becomes
significant. Thus, the Fe content should be restrained to 2.5% or
less. The Fe content is preferably 2.0% or less. Meanwhile, if the
Cr content is excessive, the machinability of the titanium alloy is
degraded. Thus, the Cr content is set at 4.5% or less. The Cr
content is preferably 4.0% or less, and more preferably 3.0% or
less.
[0031] The .alpha.-.beta. titanium alloy according to the present
invention contains the above-mentioned components, with the balance
being Ti and inevitable impurities. The inevitable impurities may
include P, N, S, O, and the like. In the .alpha.-.beta. titanium
alloy according to the present invention, the P content is
restrained to 0.005% or less; the N content is restrained to 0.05%
or less; the S content is restrained to 0.05% or less; and the O
content is restrained to 0.25% or less. The .alpha.-.beta. titanium
alloy according to the present invention may further contain the
following elements.
[0032] More than 0% and 10% or less in total of one or more
elements selected from the group consisting of V: more than 0% and
5.0% or less, Mo: more than 0% and 5.0% or less, Nb: more than 0%
and 5.0% or less, and Ta: more than 0% and 5.0% or less
[0033] These elements are p-stabilizing elements. These elements
may be used alone or in combination. To form a .beta.-phase, the
total content of these elements is preferably 2.0% or more and more
preferably 3.0% or more. As long as the total content of these
elements is more than 0%, the lower limit of the content of the
individual element is not limited specifically. Regarding the lower
limit of the content of the individual element, for example, when V
is contained in the titanium alloy, the lower limit of V content
can be set at 0.5% or more, and further 2.0% or more. When Mo is
contained in the titanium alloy, the lower limit of Mo content can
be set at 0.1% or more, and further 1.0% or more. When Nb is
contained in the titanium alloy, the lower limit of Nb content can
be set at 0.1% or more, and further 1.0% or more. When Ta is
contained in the titanium alloy, the lower limit of Ta content can
be set at 0.1% or more, and further 1.0% or more.
[0034] In contrast, if the total content of these elements is
excessive, the ductility of the titanium alloy is degraded. Thus,
the total content of these elements is preferably 10% or less and
more preferably 5.0% or less. Even when the total content of these
elements is within the above-mentioned range, if the content of at
least one element of them is excessive, the ductility of the
titanium alloy is degraded. Thus, the upper limit of the content of
any of these elements is preferably 5.0% or less. The content of
any of these elements is more preferably 4.0% or less. [0035] Si:
more than 0% and 0.8% or less
[0036] Si acts to precipitate Ti.sub.5Si.sub.3 in the titanium
alloy. During cutting, stress is concentrated on the
Ti.sub.5Si.sub.3, causing voids from Ti.sub.5Si.sub.3 as a starting
point, which makes it easy to separate chips. Consequently, the
cutting resistance is supposed to be reduced. To efficiently
exhibit this effect, the Si content is preferably 0.1% or more, and
more preferably 0.3% or more.
[0037] Meanwhile, if the Si content is excessive, the strength of
the titanium alloy becomes extremely high, whereby a working tool
might be worn significantly or broken, which makes it difficult to
cut the titanium alloy. Accordingly, the Si content is set at 0.8%
or less. The Si content is more preferably 0.7% or less, and still
more preferably 0.6% or less.
[0038] The titanium alloy according to the present invention has
the microstructure at room temperature that is composed of the
.alpha.-phase and the .beta.-phase, or the .alpha.-phase, the
.beta.-phase, and a third-phase, such as Ti.sub.2Cu or Ti.sub.2Ni.
When Si is contained in the titanium alloy, Ti.sub.5Si.sub.3 is
precipitated in the titanium alloy as mentioned above.
[0039] A manufacturing method for the .alpha.-.beta. titanium alloy
is not limited specifically. However, the .alpha.-.beta. titanium
alloy can be manufactured, for example, by the following method.
That is, the .alpha.-.beta. titanium alloy is manufactured by
smelting titanium alloy material with the above-mentioned
components, casting to produce an ingot, and then performing hot
working, i.e., hot forging or hot-rolling on the ingot, followed by
annealing as needed. The above-mentioned hot working involves:
heating the ingot in a temperature range of a .beta.-transformation
temperature T.sub..beta. to approximately
(T.sub..beta.+250).degree. C., followed by rough forging or rough
rolling at a processing ratio of approximately 1.2 to 4.0, which is
represented by "original cross-sectional area/cross-sectional area
after the hot working"; and then performing finish processing at a
processing ratio of 1.7 or more in a temperature range of
approximately (T.sub..beta.-50) to 800.degree. C. After the
above-mentioned finish processing, annealing may be performed at a
temperature of 700 to 800.degree. C. as needed. The annealing is
performed, for example, for two to 24 hours. Then, an aging
treatment may be performed as needed.
[0040] Note that the above-mentioned T.sub..beta. is determined
from the formula (1) below. The formula (1) below corresponds to
formulas (1) to (3) mentioned in Morinaga et al., "Titanium alloy
design using d electron theory", Light metal, Vol. 42, No. 11
(1992), p. 614-621.
Boave=0.326Mdave-1.95.times.10.sup.-4T.sub..beta.+2.217 (1)
In the formula (1), the respective symbols mean the following:
Boave=.SIGMA.Xi(Bo)I (2)
Mdave=.SIGMA.Xi(Md)I (3)
where T.sub..beta. is the .beta.-transformation temperature
(K).
[0041] When each element is represented as an element i in the
formula (2), Boave is an average value of a bond order Bo of the
element i, Xi is an atomic ratio of the element i, and (Bo) i is a
value of the bond order Bo of the element i.
[0042] When each element is represented as an element i in the
formula (3), Mdave is an average value of a d-orbital energy
parameter Md of the element i, Xi is an atomic ratio of the element
i, and (Md)i is a value of the d-orbital energy parameter Md of the
element i.
[0043] The bond order Bo and the d-orbital energy parameter Md of
each element are mentioned in Table 1 at p.616 of the
above-mentioned reference. Xi is determined from the composition.
From these data, Boave and Mdave of each element including Ti are
determined and substituted into the above-mentioned formula (1),
thereby making it possible to calculate a T.sub..beta.. Note that
this reference does not have data on Bo and Md of C. However, since
the C content in the present invention is small, C is neglected to
calculate the T.sub.a.
[0044] This application claims priority based on Japanese Patent
Application No. 2015-064275 filed on Mar. 26, 2015, and Japanese
Patent Application No. 2016-009417 filed on Jan. 21, 2016, the
disclosure of which is incorporated by reference herein.
EXAMPLES
[0045] The present invention will be more specifically described
below by way of Examples, but is not limited to the following
Examples. It is obvious that various modifications can be made to
these examples as long as they are adaptable to the above-mentioned
and below-mentioned concepts and are included within the scope of
the present invention.
First Example
[0046] Test materials were fabricated in the following way. The
titanium alloy with each composition shown in Table 1 below was
processed by button arc melting to manufacture an ingot with a size
of about 40 mm in diameter.times.20 mm in height. In any example,
the P content was restrained to 0.005% or less; the N content was
restrained to 0.05% or less; the S content was restrained to 0.05%
or less; and the 0 content was restrained to 0.25% or less. In
Table 1, the mark "-" means that the corresponding element was not
contained. The ingot was heated to 1,200.degree. C. and subjected
to the rough forging at a processing ratio of 2.4, represented by
the "original cross-sectional area/cross-sectional area after the
hot working", followed by forging at a processing ratio of 4.4 at
870.degree. C. to perform finish processing. Thereafter, annealing
was performed on the forged material by holding it at 750.degree.
C. for 12 hours, thereby producing a test material. Note that as
shown in Comparative Example 7 of Table 1 below, a test material in
which a crack occurred by the rough forging was not subjected to
the finish forging.
Evaluation on Forgeability
[0047] In this example, the hot workability was evaluated by the
hot forgeability. In detail, the presence or absence of a crack in
each of forging steps, namely, the rough forging and the finish
forging mentioned above, was evaluated. That is, the surface of the
above-mentioned test material after each forging step was visually
observed. The test materials having any crack were rated as NG,
while the test materials having no cracks were rated as OK. Then,
the test materials rated as OK in terms of both the rough forging
and the finish forging were evaluated to have excellent
forgeability.
Evaluation on Machinability
[0048] The test materials having good forgeability were evaluated
for the machinability as follows. That is, a test specimen with the
size below was taken out of the above-mentioned test material, and
a cutting test was performed on the test specimen on the cutting
conditions below. The machinability was evaluated as an average
cutting resistance by measuring a cutting resistance in the cutting
direction with a Kessler' s cutting dynamometer, Model: 9257 B,
from the start to the end of cutting and then determining an
average value of the cutting resistance from the start to the end
of the cutting. When performing the cutting test on Ti-6Al-4V as a
general .alpha.-.beta. titanium alloy on the same conditions, an
average cutting resistance was 180 N. Because of this, in the first
example, the test materials having an average cutting resistance of
lower than 180 N were evaluated to be superior in the
machinability, while the test materials having an average cutting
resistance of 180 N or higher were evaluated to be inferior in the
machinability.
Cutting Conditions
[0049] Test Specimen: 10 mm in height.times.10 mm in
width.times.150 mm in length [0050] Tool: Carbide tip S30T (nose
0.4 mm) manufactured by Sandvik Corporation
[0051] End mill R390 manufactured by Sandvik Corporation (20 mm in
diameter, one blade) [0052] Cutting speed Vc: 100 m/min [0053]
Cutting amount in the axial direction: 1.2 mm [0054] Cutting amount
in the radial direction: 1 mm [0055] Feeding speed: 0.08 mm/blade
[0056] Cutting length: 150 mm [0057] Cutting oil: None
Measurement of Tensile Strength
[0058] The tensile strength of the .alpha.-.beta. titanium alloy
according to the present invention was also measured for reference.
In detail, the titanium alloys of Examples 1 and 3, and Comparative
Example 1 were used and subjected to the tensile test on the
following conditions of the shape and testing speed of the test
specimen. As a result, the test materials had a strength of 948 MPa
in Example 1, 1, 125 MPa in Example 3, and 948 MPa in Comparative
Example 1, all of these strengths being relatively high.
Specifically, the strengths of these test materials exhibited
higher strength than the strength of 896 MPa of an annealed
material of Ti-6Al-4V as a general .alpha.-.beta. titanium alloy.
[0059] Shape of Test Specimen: ASTM E8/E8M FIG. 8 Specimen 3 [0060]
Test Speed: 4.5 mm/min
[0061] The evaluation result of the above-mentioned forgeability
and an average cutting resistance are also shown in Table 1.
TABLE-US-00001 TABLE 1 Composition (% by mass) Balance Forgeability
Average cutting being Ti and inevitable impurities T.sub..beta.
Rough Finish resistance Cu Ni Si Al C Cr Fe (.degree. C.) forging
forging (N) Example 1 0.5 0.5 -- 4.5 0.10 2.5 1.2 976 OK OK 148
Example 2 1.0 1.0 -- 4.5 0.10 2.5 1.2 974 OK OK 170 Example 3 2.0
2.0 -- 4.5 0.10 2.5 1.2 969 OK OK 167 Example 4 -- 0.5 -- 4.5 0.10
-- 1.2 1,010 OK OK 129 Example 5 0.5 0.5 -- 4.5 0.10 -- 1.2 1,011
OK OK 148 Example 6 -- 1.0 -- 4.5 0.10 -- 1.2 1,006 OK OK 140
Example 7 1.0 1.0 -- 4.5 0.10 -- 1.2 1,009 OK OK 146 Example 8 2.0
2.0 -- 4.5 0.10 -- 1.2 1,004 OK OK 155 Comparative -- -- -- 4.5
0.10 2.5 1.2 979 OK OK 199 Example 1 Comparative -- 3.0 -- 4.5 0.10
2.5 1.2 957 OK OK 229 Example 2 Comparative 3.0 3.0 -- 4.5 0.10 2.5
1.2 965 OK OK 241 Example 3 Comparative 4.0 -- -- 4.5 0.10 2.5 1.2
989 OK NG -- Example 4 Comparative 6.0 -- -- 4.5 0.10 2.5 1.2 994
OK OK 234 Example 5 Comparative 4.0 4.0 -- 4.5 0.10 2.5 1.2 960 OK
OK 261 Example 6 Comparative 6.0 6.0 -- 4.5 0.10 2.5 1.2 950 NG --
-- Example 7
[0062] Table 1 shows the following. All Examples 1 to 8 satisfied
the composition specified by the present invention and were found
to enable good forging and to have excellent forgeability.
Furthermore, these examples were found to have a lower average
cutting resistance than that of Ti-6Al-4V as a general
.alpha.-.beta. titanium alloy and also to have good
machinability.
[0063] In contrast, all Comparative Examples 1 to 7 did not satisfy
the composition specified by the present invention and thereby were
consequently inferior in forgeability or machinability. In detail,
in Comparative Example 1, neither Cu nor Ni was contained,
resulting in a high average cutting resistance. Comparative Example
1 had the same composition as that mentioned in Patent Document 6.
The comparison of the above-mentioned Examples 1 to 3 with
Comparative Example 1 in which the constituent elements, other than
Cu and Ni, and their contents are the same as those in Examples 1
to 3 shows that in order to surely obtain good machinability by
sufficiently decreasing the average cutting resistance, it is
necessary to contain a specified content of at least one of Cu and
Ni, as mentioned in the present invention.
[0064] In Comparative Example 2, which contained Ni, the Ni content
was excessive. In Comparative Example 5, which contained Cu, the Cu
content was excessive. In both examples, the average cutting
resistance was higher than 180 N, resulting in bad
machinability.
[0065] In Comparative Examples 3 and 6, the respective contents of
Cu and Ni were excessive. In both comparative examples, the average
cutting resistance was higher than 180 N, resulting in bad
machinability.
[0066] In Comparative Example 4, since the Cu content was
excessive, the forgeabililty was degraded. In Comparative Example
7, since the respective contents of Cu and Ni were drastically
excessive, cracking occurred at the stage of the rough forging,
resulting in degradation in the forgeability.
Second Example
[0067] In the second example, the influence of the Si content,
especially, on the machinability were studied. As shown in Table 2,
various ingots with different Si contents were manufactured to
produce test materials in the same way as that in the first
example. In any example, the P content was restrained to 0.005% or
less; the N content was restrained to 0.05% or less; the S content
was restrained to 0.05% or less; and the 0 content was restrained
to 0.25% or less. In Table 2, the mark "-" means that the
corresponding element was not contained.
[0068] Each of the above-mentioned test materials was used to
confirm the presence or absence of a precipitation phase, as
mentioned below, and the Vickers hardness of the test material was
measured as an index of strength in the second example.
Furthermore, the forgeability of the test material was evaluated in
the same way as that in the first example, and the machinability
thereof was evaluated as mentioned below. For reference, the
tensile strength of test material No. 3 in Table 2 was measured in
the same way as that in the first example. This test material No. 3
had a tensile strength of 968 MPa, which was higher than a
strength, i.e., 896 MPa of an annealed material of Ti-6Al-4V as the
general .alpha.-.beta. titanium alloy.
Evaluation on Presence or Absence of Precipitation Phase
[0069] The cross section of the test material was polished to a
mirror-smooth state, followed by acid treatment using hydrofluoric
acid to an extent that crystal grain boundaries could be seen, and
then visually observed at ten field of views, each field of view
having a size of 40 .mu.m.times.40 .mu.m, with a field
emission-scanning electron microscope (FE-SEM) at a magnification
of 4,000 times. The test materials in which the precipitation phase
with a circle equivalent diameter of 2 .mu.m or more was recognized
at five or more of the above-mentioned ten field of views in total
were evaluated to be in the "presence" of the precipitation phase.
The test materials in which the precipitation phase was recognized
at four or less of the above-mentioned ten field of views in total
were evaluated to be in the "absence" of the precipitation phase.
Note that the above-mentioned precipitation phase was separately
recognized as Ti.sub.5Si.sub.3 by an X-ray diffraction (XRD).
[0070] FIG. 1 shows one example of a photomicrograph observed with
the above-mentioned microscope. FIG. 1 is one obtained by
measurement of the test material No. 3 shown in Table 2, with an
arrow indicating one precipitation phase.
Measurement of Vickers Hardness HV
[0071] A Vickers hardness HV was measured at five sites of each
test material on the condition of a load 10 kgf, and the measured
values were averaged. In this way, an average value of the Vickers
hardness was determined.
Evaluation on Machinability
[0072] The test materials evaluated to have good forgeability in
the same way as that in the first example, that is, all examples
shown in Table 2 were evaluated for the machinability as follows.
That is, a test specimen with the size mentioned below was taken
out of the above-mentioned test material, and a cutting test was
performed on the test specimen on the cutting conditions below. The
machinability was evaluated as an average cutting resistance by
measuring a cutting resistance in the cutting direction by the
Kessler' s cutting dynamometer, Model: 9257 B, from the start to
the end of cutting and then determining an average value of the
cutting resistance from the start to the end of the cutting. When
performing the cutting test on Ti-6Al-4V as the general
.alpha.-.beta. titanium alloy on the same conditions, an average
cutting resistance was 122 N. Because of this, in the second
example, the test materials having an average cutting resistance of
lower than 122 N were evaluated to be superior in the
machinability, while the test materials having an average cutting
resistance of 122 N or higher were evaluated to be inferior in the
machinability.
Cutting Conditions
[0073] Test Specimen: 10 mm in height.times.10 mm in width.times.60
mm in length [0074] Tool: Carbide tip S30T (nose 0.4 mm)
manufactured by Sandvik Corporation
[0075] End mill R390 manufactured by Sandvik Corporation (20 mm in
diameter, one blade) [0076] Cutting speed Vc: 100 m/min [0077]
Cutting amount in the axial direction: 1.2 mm [0078] Cutting amount
in the radial direction: 1 mm [0079] Feeding speed: 0.08 mm/blade
[0080] Cutting length: 15 mm [0081] Cutting oil: None
[0082] These results are also shown in Table 2.
TABLE-US-00002 TABLE 2 Composition (% by mass) Balance being
Forgeability Average cutting Ti and inevitable impurities
T.sub..beta. Precipitation Rough Finish resistance No. Cu Ni Si Al
C Cr Fe (.degree. C.) phase HV forging forging (N) 1 0.5 0.5 -- 4.5
0.10 2.5 1.2 976 Absent 298 OK OK 111 2 0.5 0.5 0.1 4.5 0.10 2.5
1.2 993 Present 316 OK OK 99 3 0.5 0.5 0.3 4.5 0.10 2.5 1.2 1,027
Present 320 OK OK 105 4 0.5 0.5 0.8 4.5 0.10 2.5 1.2 1,110 Present
335 OK OK 112 5 0.3 0.3 0.3 4.5 0.10 2.5 1.2 1,028 Present 316 OK
OK 106 6 2.0 2.0 0.5 4.5 0.10 2.5 1.2 1,054 Present 365 OK OK 120 7
2.0 2.0 1.0 4.5 0.10 2.5 1.2 1,137 Present 360 OK OK 134 8 2.0 2.0
2.0 4.5 0.10 2.5 1.2 1,303 Present 397 OK OK Measurement was
impossible due to damage to a working tool
[0083] Table 2 shows the following. That is, as clearly shown, the
test material No. 1 having the same composition as that in Example
1 of Table 1 were compared with test materials No. 2 to 6,
particularly, test materials No. 2 to 4 in which the contents of
elements other than Si were the same as those in Example 1 of Table
1. Based on the comparison, the arrangement that contains Si in the
titanium alloy made it possible to further reduce the average
cutting resistance and to ensure the sufficiently high
machinability, compared to a case in which Si was not contained. In
contrast, when the Si content was excessive, like the test
materials No. 7 and No. 8, the hardness of the titanium alloy
becomes extremely high, increasing the average cutting resistance
and also causing inconveniences, such as a damage of a working
tool.
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