U.S. patent application number 10/303731 was filed with the patent office on 2003-05-08 for titanium alloy having high ductility, fatigue strength and rigidity and method of manufacturing same.
Invention is credited to Ariyasu, Nozomu, Matsumoto, Satoshi.
Application Number | 20030084970 10/303731 |
Document ID | / |
Family ID | 11736086 |
Filed Date | 2003-05-08 |
United States Patent
Application |
20030084970 |
Kind Code |
A1 |
Ariyasu, Nozomu ; et
al. |
May 8, 2003 |
Titanium alloy having high ductility, fatigue strength and rigidity
and method of manufacturing same
Abstract
A titanium alloy is provided wherein metal boride is uniformly
crystallized and/or precipitated in the matrix. The heating
temperature in the finishing hot working is set smaller than the
.beta. transus temperature by not less than 10.degree. C., thereby
causing the matrix to include an equiaxial .alpha. structure in a
rate of not less than 40 vol %. This titanium alloy has excellent
properties, i.e., high rigidity, ductility and fatigue strength,
which are all required for structural components, and therefore can
be widely applied to a mechanical component such as an engine of an
automobile, a structural component in an aircraft as well as a
component for a high speed rail vehicle.
Inventors: |
Ariyasu, Nozomu; (Osaka,
JP) ; Matsumoto, Satoshi; (Joetsu-shi, JP) |
Correspondence
Address: |
CLARK & BRODY
Suite 600
1750 K. Street N.W.
Washington
DC
20006
US
|
Family ID: |
11736086 |
Appl. No.: |
10/303731 |
Filed: |
November 26, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10303731 |
Nov 26, 2002 |
|
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PCT/JP00/03461 |
May 29, 2000 |
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Current U.S.
Class: |
148/421 ;
420/417 |
Current CPC
Class: |
C22F 1/183 20130101;
C22C 14/00 20130101 |
Class at
Publication: |
148/421 ;
420/417 |
International
Class: |
C22C 014/00 |
Claims
1. A titanium alloy having a high ductility, fatigue strength and
rigidity, wherein said titanium alloy includes B: 0.5-3.0% in mass
%, and metal boride is uniformly crystallized and/or precipitated
in the matrix, and wherein the matrix includes an equiaxial .alpha.
structure in a rate of not less than 40 vol %.
2. A titanium alloy having a high ductility, fatigue strength and
rigidity according to claim 1, wherein said titanium alloy is
either of .alpha. type or of .alpha.+.beta. type.
3. A titanium alloy having a high ductility, fatigue strength and
rigidity according to claim 1, wherein said titanium alloy further
includes Al: 5.5-10%, oxygen (O): 0.07-0.25%, C: not more than
0.1%, H: not more than 0.05% and N: not more than 0.1% in mss
%.
4. A titanium alloy having a high ductility, fatigue strength and
rigidity according to claim 3, wherein said titanium alloy further
includes one or more than two of Sn, Zr and Hf in not more than 20%
in mass % in amount and/or one or more than two of .beta. phase
stabilizing elements in not more than 10% of V equivalent given by
the below equation (a): 3 V equivalent = V + 15 10 Mo + 15 6.3 Cr +
15 4.0 Fe + 15 36 Nb + 15 9 Ni + 15 25 W ( a )
5. A method for manufacturing a titanium alloy having a high
ductility, fatigue strength and rigidity, wherein said titanium
alloy includes B: 0.5-3.0% in mass %, and metal boride is uniformly
crystallized and/or precipitated in the matrix, and wherein the
heating temperature in the finishing hot working is set smaller
than the .beta. transus temperature by not less than 10.degree.
C.
6. A method for manufacturing a titanium alloy having a high
ductility, fatigue strength and rigidity according to claim 5,
wherein the solution treatment is carried out within a temperature
range between (the .beta. transus temperature-350.degree. C.) and
(the .beta. transus temperature-10.degree. C.).
7. A method for manufacturing a titanium alloy having a high
ductility, fatigue strength and rigidity according to claim 6,
wherein the aging treatment is further carried out.
8. A method for manufacturing titanium alloy having a high
ductility, fatigue strength and rigidity, wherein said titanium
alloy includes B: 0.5-3.0%, Al: 5.5-10%, oxygen (O): 0.07-0.25%, C:
not more than 0.1%, H: not more than 0.05% and N: not more than
0.1% in mass %, and metal boride is uniformly crystallized and/or
precipitated in the matrix, and wherein the heating temperature in
the finishing hot working is set smaller than the .beta. transus
temperature by not less than 10.degree. C.
9. A method for manufacturing a titanium alloy having a high
ductility, fatigue strength and rigidity according to claim 8,
wherein the solution treatment is carried out within a temperature
range between (the .beta. transus temperature-350.degree. C.) and
(the .beta. transus temperature-10.degree. C.).
10. A method for manufacturing a titanium alloy having a high
ductility, fatigue strength and rigidity according to claim 9,
wherein the aging treatment is further carried out.
11. A method for manufacturing a titanium alloy having a high
ductility, fatigue strength and rigidity, wherein said titanium
alloy includes B: 0.5-3.0%, Al: 5.5-10%, oxygen (O): 0.07-0.25%, C:
not more than 0.1%, H: not more than 0.05% and N: not more than
0.1% in mass %, and further includes one or more than two of Sn, Zr
and Hf in not more than 20% in mass % in amount and/or one or more
than two of .beta. phase stabilizing elements in not more than 10%
of V equivalent given by the below equation (a), and wherein the
heating temperature in the finishing hot working is set smaller
than the .beta. transus temperature by not less than 10.degree. C.:
4 V equivalent = V + 15 10 Mo + 15 6.3 Cr + 15 4.0 Fe + 15 36 Nb +
15 9 Ni + 15 25 W ( a )
12. A method for manufacturing a titanium alloy having a high
ductility, fatigue strength and rigidity according to claim 11,
wherein the solution treatment is carried out within a temperature
range between (the .beta. transus temperature-350.degree. C.) and
(the .beta. transus temperature-10.degree. C.).
13. A method of manufacturing a titanium alloy having a high
ductility, fatigue strength and rigidity according to claim 12,
wherein the aging treatment is further carried out.
Description
TECHNICAL FIELD
[0001] The present invention relates to a titanium alloy having a
high ductility, fatigue strength and rigidity, which alloy is used
in a mechanical component requiring excellent mechanical properties
and a light weight as well, for instance, a connecting rod, valve,
camshaft, crankshaft and push rod in an engine of an automobile or
a structural component in an aircraft, a high-speed rail vehicle or
the like. The present invention also relates to a method of
manufacturing such a titanium alloy.
BACKGROUND ART
[0002] A titanium alloy has excellent properties for the corrosion
resistance and the heat resistance, along with a high mechanical
strength and a lightweight property, so that an application of the
alloy to various mechanical components in an automobile, an
aircraft and a high-speed rail vehicle is now widely extending.
However, titanium alloy has a relatively small Young's modulus,
i.e., about half of that in iron or steel materials. Accordingly,
buckling and bending must be taken into account when the alloy is
used in such a mechanical structure. For instance, when the
titanium alloy is used to a mechanical component having a long
axial length, such as a camshaft, a connecting rod or the like, the
cross section of the component must be increased in a design work
in order to obtain a required mechanical strength. However, such
design work makes it impossible to effectively utilize the specific
properties of the titanium alloy, i.e., the lightweight and the
high mechanical strength.
[0003] In view of these facts, several investigations have been
made so far to enhance the Young's modulus of the titanium alloy by
providing a composite material into which fibers or particles
having a high Young's modulus are dispersed in titanium. For
instance, in Japanese Patent Application Laid-open No. 5-5142, a
method of producing a titanium-based composite material has been
proposed, in which a TiB solid solution is dispersed into the
matrix of the titanium alloy in a predetermined volume percentage.
In this specification, it has been demonstrated that the production
method is capable of providing a high mechanical strength, a high
rigidity, and a high wearing resistance over a wide range from room
temperature to a high temperature.
[0004] However, the composite material has a less plastic
workability in the production method proposed therein, and
therefore the application of a melting/casting method or a powder
metallurgy method is prerequisite for this material, thereby making
it impossible to employ the composite material to a large sized
structural component. Moreover, the finding regarding the matrix
structure in the composite material has not been disclosed, and
therefore it is not clear whether or not the ductility and the
fatigue strength required for such a structural element can
securely be obtained with the method proposed therein.
[0005] Furthermore, in Japanese Patent Application Laid-open No.
10-1760, a particle-strengthened type titanium-based composite
material has been proposed, in which material the matrix is formed
by a .alpha.-.beta. type titanium alloy including TiB or TiC
particles, and the structure is controlled so as to obtain a
needle-shaped .alpha. phase structure. In the composite material
proposed therein, however, TiB or TiC particles are used as
strengthened ceramic particles and therefore the powder metallurgy
method is prerequisite for the production method, thereby making it
difficult to apply the composite material to a large-scale
structural elements. In addition, the needle-shaped structure in
the matrix provides a high Young's modulus. Nevertheless, a
sufficiently high ductility can hardly be obtained.
DISCLOSURE OF INVENTION
[0006] As described above, there is a problem that titanium alloy
has a relatively higher mechanical strength, but a smaller Young's
modulus, compared with the iron or steel materials. Various
composite materials have been produced to overcome this problem.
However, no improvement has been succeeded yet to obtain a high hot
workability and a high ductility.
[0007] On the other hand, it is required that the structural
elements may be used in a much severer environment and the
manufacturing cost may also be reduced, along with an excellent hot
workability and mechanical strength. For instance, a high hot
workability, a high rigidity, an excellent ductility and fatigue
strength are all required for a connecting rod of an automobile,
although it can be used in such a sever environment and the
manufacturing cost is further reduced. Nevertheless, any titanium
alloy having such properties has not developed yet.
[0008] In view of these requirements on the development of titanium
alloys for such a mechanical part, it is an object of the present
invention to provide titanium alloy having an excellent properties
with regard to the hot workability, the ductility, the fatigue
strength and the rigidity, and it is further another object of the
present invention to provide a method of manufacturing such a
titanium alloy. More specifically, an object of the invention is to
develop a titanium alloy which is capable of hot forging or hot
rolling, and which has a tensile strength not less than 1100 MPa
and a Young's modulus not less than 130 GPa, together with a
provision of the ductility and fatigue strength in a predetermined
magnitude.
[0009] The present inventors studied on the composition of
elements, the fine particles to be dispersed and the structure in
the matrix in order to develop titanium alloys having the
above-mentioned properties, and obtained the following findings (a)
to (c):
[0010] (a) The Young's modulus of a titanium alloy may be
effectively enhanced by dispersing particles having a high Young's
modulus into a matrix. The dispersed particles are titanium carbide
or titanium boride particles, which are produced by the
crystallization and/or precipitation in the matrix. In this case,
titanium boride is more effective in usage, since it has 1.3 times
greater Young's modulus than titanium carbide.
[0011] (b) In a titanium alloy, various matrix structures appear
even if it includes the same alloy composition. Fundamentally,
these structures can be classified into the equiaxial .alpha.
structure and the needle-shaped .alpha. structure. In order to
obtain an excellent ductility and fatigue strength, the matrix
structure must have a certain rate of equiaxial .alpha.
structure.
[0012] In the formation of the equiaxial .alpha. structure in the
matrix, it is necessary to carry out a thermal treatment after a
working stress is applied thereto. The temperature in the hot
working should be smaller than the .beta. transus temperature.
Moreover, it is preferable that the subsequent solution treatment
should also be carried out at a temperature smaller than the .beta.
transus temperature.
[0013] (c) Elements Al, oxygen (O), C, H and N, which serve to
stabilize the .alpha. phase, enhance the Young's modulus of the
matrix, when they are included therein at an appropriate content.
Moreover, neutral type elements Sn, Zr and Hf provide a very weak
effect on the enhancement of the Young's modulus, but an appreciate
effect on the enhancement of the mechanical strength at a high
temperature and the creep resistance.
[0014] When an aging treatment is applied to the titanium alloy
including the above-mentioned elements, Al, oxygen, or Sn, Zr, Hf,
these elements provide an aged hardening property of promoting to
generate an intermetallic compound (Ti.sub.3Al), thereby enabling
the fatigue strength to be greatly increased.
[0015] Complete solid solution or isomorphous type elements V and
Mo among the .beta. phase stabilizing elements greatly reduce the
Young's modulus, whereas eutectoid type elements Fe and Cr reduces
not so greatly, compared with the isomorphous type elements. At any
rate, the .beta. phase stabilizing elements reduce the Young's
modulus to greater or less extent, but enhance the hot workability.
Accordingly, it is desirable to add these elements to the alloy in
an appropriate manner.
[0016] The present invention is realized on the basis of the
above-mention finding, and the gist is that the following titanium
alloys (1), (3) and (4), and the following methods of producing the
titanium alloys (2), (3) and (4) are provided:
[0017] (1) A titanium alloy having a high ductility, fatigue
strength and rigidity, wherein said titanium alloy includes B:
0.5-3.0% in mass %, and metal boride is uniformly crystallized
and/or precipitated in the matrix, and wherein the matrix includes
an equiaxial .alpha. structure in a rate of not less than 40 vol %.
The titanium alloy is either of .alpha. type or of .alpha.+.beta.
type.
[0018] (2) A method for manufacturing a titanium alloy having a
high ductility, fatigue strength and rigidity, wherein the titanium
alloy includes B: 0.5-3.0% in mass %, and metal boride is uniformly
crystallized and/or precipitated in the matrix, and wherein the
heating temperature in the finishing hot working should be set
smaller than the .beta. transus temperature by not less than
10.degree. C.
[0019] In the above manufacturing method, it is preferable that the
solution treatment should be applied within a temperature range
between (the .beta. transus temperature-350.degree. C.) and (the
.beta. transus temperature-10.degree. C.), and, if necessary, the
aging treatment should be further applied.
[0020] (3) It is preferable that the above-mentioned titanium alloy
(1) or (2) further includes Al: 5.5-10%, oxygen (O): 0.07-0.25%, C:
not more than 0.1%, H: not more than 0.05% and N: not more than
0.1% in weight %.
[0021] (4) Similarly, it is preferable that the above-mentioned
titanium alloy (3) further includes one or more than two of Sn, Zr
and Hf in not more than 20% in mass % in amount and/or one or more
than two of .beta. phase stabilizing elements in not more than 10%
of V equivalent given by the below equation (a): 1 V equivalent = V
+ 15 10 Mo + 15 6.3 Cr + 15 4.0 Fe + 15 36 Nb + 15 9 Ni + 15 25 W (
a )
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a table representing properties after various
solid solution treatments are applied to titanium alloys in Example
1; and
[0023] FIG. 2 is a table representing properties after various
solid solution or aging treatments are applied to titanium alloys
in Example 1.
BEST MODE FOR CARRYING OUT THE INVENTION
[0024] A titanium alloy according to the invention is characterized
by an excellent ductility and fatigue strength of the matrix
structure, in which the rate of the equiaxial .alpha. structure
(hereinafter denoted by "the isometric rate") is controlled into an
area rate (the same as the volume rate) more than 40% by finely and
uniformly crystallizing and/or precipitating metal boride in a
matrix, and, if necessary, by including one or more of the .alpha.
phase stabilizing elements Al, oxygen and the like thereto.
[0025] Moreover, in the titanium alloy according to the invention,
one or more of Sn, Zr and HF is included therein to enhance the
mechanical strength at high temperature and the creep resistance.
Otherwise, the amount of .beta. stabilizing elements to be added is
restricted in an appropriate v equivalent so as not to form a
.beta. phase monolayer, and thus the hot workability is enhanced by
decreasing the .beta. transus temperature. In the following, the
reason for the above specification will be described as for the
microstructure, the element composition and the manufacturing
method.
[0026] 1. Microstructure
[0027] Titanium alloy can be classified into three types in
accordance with the microstructure at normal temperature: .alpha.
type; .alpha.+.beta. type; and .beta. type. The subject matter of
the present invention extends to the .alpha. type and the
.alpha.+.beta. type.
[0028] Generally, either in the .alpha. type alloy or in the
.alpha.+.beta. type alloy, the equiaxial .alpha. structure is
favorable for the ductility and the fatigue strength, compared with
the needle-shaped .alpha. structure. Furthermore, in accordance
with the author's investigation, it is found that the matrix of the
alloy does not always need to be entirely constituted by the
equiaxial .alpha. structure, and the mixture of the needle-shaped
structure transformed from the .beta. phase therewith is allowed.
However, in order to obtain a high ductility and fatigue strength
in the mixed structure, it is necessary to set the rate of the
equiaxial .alpha. structure, i.e., the equiaxial rate to be not
less than 40% in the area rate. Furthermore, a more stable
ductility and fatigue strength require an equiaxial rate of not
less than 50% preferably.
[0029] The microstructure was inspected in the following steps: A
specimen was collected from the matrix of the alloy and then
observed after polishing and etching. The area rate of the
equiaxial .alpha. structure, i.e., the equiaxial rate which is
defined in the present invention, is determined by the area ratio
of the equiaxial .alpha. structure to the needle-shaped structure,
these structures being color-classified in the image analysis of a
micrograph of the matrix. The reason of the equiaxial rate used in
the present invention is due to the fact that the ductility and
fatigue strength strongly depend on the area rate of the equiaxial
.alpha. structure.
[0030] 2. Element Composition
[0031] B Composition:
[0032] In order to uniformly disperse metal boride (TiB) into the
matrix of titanium alloy, B is added thereto and then crystallized
and/or precipitated in the course of solidification and cooling.
Thereby, the Young's modulus of the titanium alloy can be enhanced
in accordance with the composite rule in proportion to the
magnitude of volume in TiB particles having a greater Young's
modulus than the titanium alloy.
[0033] A B content of less than 0.5% provides a reduced amount of
TiB crystallized and/or precipitated, thereby making it impossible
to sufficiently enhance the Young's modulus of the titanium alloy.
On the other hand, a B content of greater than 3.0% provides an
excess amount of dispersed TiB and an enhanced Young's modulus of
the matrix. Nevertheless, the hot ductility and the cold ductility
are markedly reduced. Accordingly, it is preferable that the
content of B to be added should be 0.5-3.0%.
[0034] .alpha. Phase Stabilizing Elements:
[0035] Either Al or oxygen is a .alpha. phase stabilizing element,
and has a prominent effect of solid solution hardening, thereby
causing the Young's modulus to be greatly enhanced. Either an Al
content of less than 5.5% or oxygen content of less than 0.07%
provides no such sufficient effect. On the other hand, either an Al
content of greater than 10% or an oxygen content of greater than
0.25% reduces the workability and the ductility. As a result, it
can be stated that the content of the two elements to be included
should be set preferably, Al: 5.5 to 10%; O: 0.07 to 0.25%, and
more preferably Al: 7 to 9%; O: 0.07 to 0.15%.
[0036] As another .alpha. phase stabilizing element, C, H or N can
be used. All of these elements reduce the ductility at normal
temperature. Therefore, the upper limit of the content should be
set such that C: 0.1%; H: 0.05% and N: 0.1%.
[0037] Neutral Type Elements
[0038] In the present invention, neutral type elements and/or
.beta. phase stabilizing elements may be added to the titanium
alloy. In this case, any of these elements is solved in the matrix.
Regarding neutral type elements Zr and Hf, most amounts of these
elements can be solved in the matrix, and a very small amount of
zirconium boride and hafnium boride is crystallized and/or
precipitated in the matrix. However, such a very small amount of
the borides provides no prominent enhancement of the Young's
modulus.
[0039] One or more than two of the neutral type elements Sn, Zr and
Hf can be solved in the alloy. Sn, Zr or Hf provides no enhancement
of the Young's modulus, but enhances the effect of the solid
solution strengthening to increase the mechanical strength at high
temperature. More than 20% content of these elements reduces both
the hot workability and the cold workability, and further increases
the cost of manufacturing the alloy. Accordingly, the upper limit
of the content should be 20% in amount, and preferably not more
than 5%.
[0040] .beta. Phase Stabilizing Elements
[0041] Elements V, Mo, Cr, Fe, Nb, Ni or W may be used as a .beta.
phase stabilizing element. The .beta. phase stabilizing element
included in the alloy decreases the .beta. transus temperature and
improves the hot workability. These elements are solved in the
matrix and suppress an excessive generation of metallic compound
(Ti.sub.3Al), thereby enabling a greater content of Al to be
solved. However, an excessive content of these elements causes the
Young's modulus to be markedly reduced. Accordingly, one or more
than two of these elements should be added to the alloy within a
range not more than 10% in the v equivalent given by the below
equation (a), and more preferably not more than 5% in the V
equivalent: 2 V equivalent = V + 15 10 Mo + 15 6.3 Cr + 15 4.0 Fe +
15 36 Nb + 15 9 Ni + 15 25 W ( a )
[0042] 3. Manufacturing Process
[0043] The titanium alloy ingot is produced in the form of a
compact shape of a raw material by appropriately selecting some of
pure Al, electrolyzed Sn, Zr sponge, pure Hf, Al--V alloy, Al--Mo
alloy and Mo, Cr, V and the like and by adding them to a titanium
sponge in predetermined contents. In order to crystallize or
precipitate TiB in the matrix of the titanium alloy in a dispersed
state, Al boride, Fe boride or the like is used as a boron source
in the raw material. Moreover, the oxygen amount in the ingot can
be adjusted to some extent by appropriately selecting the type of
titanium sponge. When, however, a much greater amount of oxygen is
required, TiO.sub.2 can be used as an adjusting material. The raw
material thus adjusted is arc-melted either by the consumable
electrode melting in a vacuum melting furnace or by the
non-consumable electrode melting in a plasma arc melting to form an
alloy ingot.
[0044] The titanium alloy ingot thus produced is hot worked by
forging or rolling to obtain a desired microstructure, and then is
appropriately heat-treated to adjust the mechanical properties. As
described above, in order to generate the equiaxial .alpha.
structure in the matrix, the material must undergo a proper thermal
history after applying a working stress thereto.
[0045] The structure in the matrix is widely changed by the heating
condition at a temperature close to the .beta. transus temperature.
The hot working at a temperature greater than the .beta. transus
temperature frequently generates the needle-shaped .alpha.
structure, whereas the hot working at a temperature smaller than
the .beta. transus temperature frequently generates the equiaxial
.alpha. structure. Accordingly, in the manufacturing method
according to the invention, the heating temperature in the
finishing hot working must be set smaller than the .beta. transus
temperature.
[0046] Since there exist the .alpha. and .beta. phases in a mixed
state within a temperature range just below the .beta. transus
temperature, the process of cooling down to room temperature
provides a mixed state of the needle-shaped structure and the
equiaxial structure. As described above, in order to obtain the
ductility and fatigue strength in a predetermined magnitude by
adjusting the equiaxial .alpha. structure at an area rate of not
less than 40%, the heating temperature in the finishing hot working
must be set smaller than the .beta. transus temperature by not less
than 10.degree. C. There is no special limitation regarding the
lower limit of the heating temperature. However, the temperature
can be set greater than the lower limit temperature in the hot
working. In the manufacturing method according to the invention,
the heating temperature in the finishing hot working is specified
such that a temperature greater than the .beta. transus temperature
can be used as for the heating temperature in the state of the
rough work prior to the finishing work.
[0047] In other words, the hot working of the titanium alloy ingot
is employed not only to produce a predetermined profile of a
structural component, but also to obtain a predetermined
microstructure of the matrix. As described above, the heat
treatment after undergoing the working stress must be applied to
generate the equiaxial .alpha. structure in the matrix. Once, for
example, the needle-shaped microstructure is formed, any heat
treatment applied to the alloy no longer provides the equiaxial
microstructure. In order to transform the needle structure of the
matrix to the equiaxial structure, the hot working must again be
applied after the alloy is heated at a temperature smaller than the
.beta. transus temperature.
[0048] In order to securely transform the needle structure of the
matrix to the equiaxial structure, it is effective to provide a
sufficient working stress and it is referable that the hot working
is carried out at a working rate not less than 50%. The
crystallization and/or precipitation of coarse TiB particles causes
the ductility and the fatigue strength to be reduced. To avoid such
reduction, it is necessary to destroy the coarse particles by the
hot working. In this case, the working rate should be preferably
not less than 70%.
[0049] In the titanium alloy, a decreased temperature for working
provides a reduction in the hot workability as well as the
generation of working fractures. To obtain a proper working
temperature, either a heat insulation material is coated onto the
ingot, or the temperature in the circumference is appropriately
increased within a temperature range for the warm working or the
hot working, or the ingot is re-heated at a temperature smaller
than the .beta. transus temperature after the temperature is
decreased.
[0050] The titanium alloy thus hot worked undergoes such a heat
treatment as a solution treatment and/or an aging treatment to
adjust the mechanical properties. When the temperature in the
solution treatment is set smaller than the .beta. transus
temperature by not less than 10.degree. C., the equiaxial .alpha.
structure, which is formed in the hot working, remains unchanged.
On the other hand, a decreased temperature of the treatment
provides no effect of the solution treatment, so that the
temperature should be set not less than (the .beta. transus
temperature-350.degree. C.). In accordance with the invention, the
solution treatment should be made preferably within a temperature
range between (the .beta. transus temperature-350.degree. C.) and
(the .beta. transus temperature-10.degree. C.), more preferably
within a temperature range between (the .beta. transus
temperature-200.degree. C.) and (the .beta. transus
temperature-100.degree. C.).
[0051] Moreover, the aging treatment promotes to generate the
intermetallic compound (Ti.sub.3Al), thereby enabling the fatigue
strength of the titanium alloy to be further enhanced. The
conditions of the aging treatment vary from composition to
composition of the alloy. It is preferable that the temperature of
treatment should be 500-600.degree. C. and the duration of
treatment should be more than 5 hours.
EXAMPLES
[0052] The effect resulting from the invention will be described in
detail, as for the case (Example 1), in which the solution
treatment is carried out after the hot forging, and the case
(Example 2), in which the aging treatment is further applied to the
above treatment.
Example 1
[0053] A titanium alloy having the composition shown in Table 1 was
arc-melted in a vacuum melting furnace to form an ingot having a
140 mm diameter. The .beta. transus temperature of the titanium
alloy used in the test was 1070.degree. C.
1TABLE 1 Composition of elements (mass %) Al V Mo B O H Ti 7.72
0.41 0.50 0.90 0.094 0.014 Bal.
[0054] By applying twice the hot forging and the solution treatment
to the alloy ingot obtained under the following conditions, test
pieces were produced:
[0055] 1. Rough-Forging
[0056] Size after forging: outside diameter 80 mm (working rate
68%, forging rate 3)
[0057] Heating temperature: 1170.degree. C. (the .beta. transus
temperature+100.degree. C.)
[0058] 2. Finish Forging
[0059] Size after forging: outside diameter 25 mm (working rate
90%, forging rate 10)
[0060] Heating temperature: 1040.degree. C. to 1170.degree. C. (the
respective heating temperatures being indicated in FIG. 1)
[0061] 3. Solution Treatment
[0062] Heating temperature: 700.degree. C. to 1100.degree. C. (the
respective heating temperatures being indicated in FIG. 1)
[0063] Heating duration: 2 hours
[0064] The tensile property at normal temperature, the fatigue
strength at normal temperature and the Yong's modulus were
determined as the properties of the titanium alloy used to test
after the solution treatment. Furthermore, the microstructure of
each test piece was observed to determine the isometric rate (vol.
%) of the matrix. The obtained results are given in FIG. 1.
[0065] From the results in FIG. 1, it is found that all the test
pieces have a tensile strength of 1100 MPa or more and a Young's
modulus of 130 Gpa or more, thereby exhibiting a high rigidity. In
particular, inventive examples No. 3 to 6 provide an isometric rate
of not less than 40 vol % and further exhibit excellent properties
regarding the fatigue strength and the ductility, along with high
rigidity.
[0066] In other words, a high ductility and high fatigue strength
can be obtained without any reduction of high rigidity so long as
the rate of the equiaxial .alpha. structure according to the
invention is attained.
Example 2
[0067] Utilizing the alloy ingot obtained in Example 1, the effect
of the aging treatment after the solution treatment was studied by
varying the conditions of hot forging. The titanium alloys used to
test were treated according to the following processes A to D.
[0068] 1. Process A (Comparative Example)
[0069] 1-1. Finishing Forging
[0070] Size after forging: outside diameter 25 mm (working rate
97%, forging rate 30)
[0071] Heating temperature: 1170.degree. C. (the .beta. transus
temperature+100.degree. C.)
[0072] 1-2. Solution Treatment
[0073] Condition of treatment: 900.degree. C..times.2 hours
[0074] 2. Process B (Comparative Example)
[0075] 2-1. Finishing Forging
[0076] Size after forging: outside diameter 25 mm (working rate
97%, forging rate 30)
[0077] Heating temperature: 1170.degree. C. (the .beta. transus
temperature+100.degree. C.)
[0078] 2-2. Solution Treatment
[0079] Treatment condition: 900.degree. C..times.2 hours
[0080] 2-3. Aging Treatment
[0081] Treatment condition: 580.degree. C..times.8 hours
[0082] 3. Process C (Inventive Example)
[0083] 3-1. Rough-Forging
[0084] Size after forging: outside diameter 80 mm (working rate
68%, forging rate 3)
[0085] Heating temperature: 1170.degree. C. (the .beta. transus
temperature+100.degree. C.)
[0086] 3-2. Finishing Forging
[0087] Size after forging: outside diameter 25 mm (working rate
90%, forging rate 10)
[0088] Heating temperature: 1040.degree. C. (the .beta. transus
temperature-30.degree. C.)
[0089] 3-3. Solution Treatment
[0090] Treatment condition: 900.degree. C..times.2 hours
[0091] 4. Process D (Inventive Example)
[0092] 4-1. Rough-Forging
[0093] Size after forging: outside diameter 80 mm (working rate
68%, forging rate 3)
[0094] Heating temperature: 1170.degree. C. (the .beta. transus
temperature+100.degree. C.)
[0095] 4-2. Finishing Forging
[0096] Size after forging: outside diameter 25 mm (working rate
90%, forging rate 10)
[0097] Heating temperature: 1040.degree. C. (the .beta. transus
temperature-30.degree. C.)
[0098] 4-3. Solution Treatment
[0099] Treatment condition: 900.degree. C..times.2 hours
[0100] 4-4. Aging Treatment
[0101] Treatment condition: 580.degree. C..times.8 hours
[0102] The tensile property at normal temperature, the fatigue
strength at normal temperature, the Yong's modulus and further the
equiaxial rate (vol %) of the matrix were determined as the
properties of the titanium alloy used to test after the solution
treatment or the aging treatment. Furthermore, the microstructure
of each test piece was observed to determine the equiaxial rate
(vol. %) of the matrix. The obtained results are given in FIG.
2.
[0103] In the processes A and B of the comparative examples, a
tensile strength of 1100 Mpa or more and a Young's modulus of 130
Gpa or more were attained and a high rigidity was also obtained.
However, an improper setting of the heating temperature in the
finishing forging provided no sufficiently high ductility and
fatigue strength. On the contrary, in the processes C and D of the
inventive examples, an excellent ductility and fatigue strength
were attained, along with a high rigidity. In the process D,
moreover, an application of the aging treatment enhances the proof
stress and tensile stress and, at the same time, greatly enhances
the fatigue strength.
INDUSTRIAL APPLICABILITY
[0104] In accordance with the titanium alloy and the manufacturing
method proposed in the present invention, excellent properties,
i.e., the rigidity, the ductility and the fatigue strength, which
are all required for a structural component can be obtained,
thereby making it possible to provide mechanical components having
excellent mechanical properties and a light weight as well.
Accordingly, the titanium alloy according to the present invention
can be widely applied to a mechanical component such as a
connection rod, camshaft, crankshaft and push rod in an engine of
an automobile as well as a structural element for an aircraft and
parts for a high-speed rail vehicle.
* * * * *