Note: Descriptions are shown in the official language in which they were submitted.
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Specification
Light-alloy Wheel for Vehicles
Technical Field
5The present invention relates to a light-alloy wheel
for vehicles.
Background Art
As vehicles are increased in number, recently, the
proportion of vehicles which are provided with light-
alloy wheels is increased. Because of a reduced unspring
weight and beauty in the view point of the design, many
light-alloy wheels which are mainly made of a light metal
such as aluminum or magnesium are used. As the method of
producing such wheels, a casting process and a forging
process are dominantly employed. Other useful methods
include a high-density casting process, and a so-called
casting forging process in which a hot forging process is
conducted after preform casting. In a working step of the
forging process, employed is a spinning process, a
pressing process, or the like. Wheels for vehicles are
classified into a l-piece wheel, a 2-piece wheel, a
3-piece wheel, etc. according to the structural feature.
Particularly, a l-piece wheel is excellent in rigidity,
functionality, weight, etc.
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The technique of producing a wheel by using a
forging process has already been established by the
techniques of producing a forged 1-piece wheel which were
proposed by the applicant in Japanese Patent Publications
Nos. 3-2573 and 3-2574.
The structure of a wheel consists of two components,
a rim portion (outer and inner rims), and a disk portion.
The disk portion has a disk-like shape and is designed so
as to have a complex pattern. The rim portion has a
drum-like shape and is required to have rigidity against
deflection. In order to adequately harmonize the two
components with each other, it is preferable to separate-
ly produce the rim portion and the disk portion, and
thereafter join them together. When an inner rim portion
of an integral forged wheel is to be produced, the
spinning process is usually employed. However, the
portion is produced by conducting plastic deformation in
a reduced thickness as small as about 4 to 5 mm. This
causes the production process to require a prolonged
process period and the yield to be lowered.
When two metal members are to be joined together, a
so-called welding process is usually employed in which
end faces of the portions to be joined are melted by
using an energy source such as electricity or a gas and
the end faces are joined to each other after cooling.
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However, this process has drawbacks that the high
temperature heating easily produces strains in the
structure and that harmful oxides are likely sealed in
the structure. There is another joining process which is
called a solid-phase joining. Typical examples of the
solid-phase joining include the diffusion joining, the
friction pressure joining, the cold joining, the explo-
sion joining, the gas pressure joining, the ultrasonic
joining, the hot pressure joining, and the crush welding.
According to the spirit of the invention, the friction
pressure joining is most suitable in view of the required
working period, the easiness of the work, and the sizes
of articles to be joined.
A known technique of producing a wheel by using a
friction pressure joining process is described in Japa-
nese Patent Publication Laying-Open No. 55-109586. In the
known technique, a process of joining a disk portion and
a rim portion to each other is conducted in parallel with
the rotation axis (line X-X) of a wheel, and the joint
portion is located at a position immediately adjacent to
an edge of a conical curved surface which is contiguous
to a side face of the rim portion. Japanese Patent
Publication Laying-Open No. 1-168501 discloses a techni-
que which is characterized in that the joint portions
between outer and inner rim portions and a disk portion
-
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elongate in a radial direction with respect to the
rotation axis of a wheel. Japanese Patent Publication
Laying-Open No. 4-334601 asserts a novelty in a joining
structure in which an inner rim portion made of a steel
product is joined to an outer rim portion and a disk
portion which are made of a light alloy. Generally,
points which are common to these techniques are mainly
directed to the structure of a wheel, easiness of working
of a material to be used, and reduction of the production
cost.
In the prior art, however, nothing is described
about a combination of joined members in which the
reduction of the unspring weight and toughness of a
material are considered with respect to the controlla-
bility of a vehicle to which wheels for vehicles areattached. Furthermore, the prior art shows no concern for
the maintenance of the quality and the required produc-
tion period.
In view of the above, a simulation test in which a
bending moment of 337 Kgf-m is applied was conducted on
light-alloy wheels for vehicles which has a disk portion
bearing a design pattern, to see how stress distributes
during a running operation. Fig. 6 shows an example of
results, and Fig. 7 shows positions where strain gauges
were attached. From the results, it will be seen that a
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larger stress is generated in a disk portion. In the
selection of a material of a member constituting disk and
rim portions, therefore, in order to attain properties
comparable to those of a conventional forged wheel, it is
preferable to employ a process such as a casting forging
process, or a high-pressure casting process represented
by a squeeze casting process and a non-pored die-casting
process, and selectively use a quality of a light-alloy
cast member which is superior in strength than a usual
casting light alloy material. However, it will be seen
that at least a disk portion is preferably produced by a
forging process, or a casting forging process.
Disclosure of the Invention
It is an object of the invention to provide a
light-alloy wheel for vehicles having a novel structure
in which a member produced by a forging process, or a
casting forging process is used at least for a disk
portion, outer and inner rim portions, or an inner rim
portion are separately worked and these members are then
joined together.
According to the invention, in a light-alloy wheel
for vehicles, (1) a light-alloy cast member in which an
outer rim portion and a disk portion are integrally
molded, and an inner rim portion which is made by casting
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a light alloy are joined to each other by a friction
pressure joining process. According to the invention, in
a light-alloy wheel for vehicles, (2) a member in which
an outer rim portion and a disk portion are integrally
molded is produced by a casting forging process according
to a pressurization, such as a squeeze casting process,
a non-pored die-casting process, or a forging cast
process, and the members are joined to a cast inner rim
member in the same manner as the (1) above.
According to the invention, furthermore, (3) an
outer rim portion, and a disk portion which is formed by
casting a light alloy are joined so as to be integrated
with each other, by a friction pressure joining process.
According to the invention, furthermore, (4) a raw
member which will be formed as an outer rim portion and
an inner rim portion or an inner rim portion is formed by
casting a light alloy, and an outer and inner rim member
or an inner rim member in which the outer rim portion and
the inner rim portion, or the inner rim portion are
integrally molded after forging by a roll molding process
such as a spinning process is joined to a forged disk
portion by a friction pressure joining process in the
same manner as the (3) above.
According to the invention, furthermore, (5) an
integrated light-alloy wheel is obtained in which the
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joint portion is made thick in order to compensate for
reduction of strength of a member due to heat generated
in the friction pressure joining process.
The forged disk portion has a metallographic
structure having grain flows directed from the center
portion to the peripheral portion, the grain flows
elongating to an interface extending to the joining face.
In the joining, among solid-phase joining processes,
a friction pressure joining process is most advantageous
because of the following reasons that the articles to be
joined are rotation bodies, that, since the heat genera-
tion occurs at the same time in the whole of the joint
portion and the temperature is lower by about 20 % than
a usual welding process, thermal deformation is prevented
from occurring, and that the process period is very short
or several tens of seconds.
In the thus configured light-alloy wheel for
vehicles, a forged member having a tough metallographic
structure is used in the disk portion which must bear a
larger stress, and hence the strength of the wheel can be
maintained. Since an integrated wheel can be obtained by
a friction pressure joining process, the disk portion and
the inner rim portion, or an outer rim portion can
separately be produced so that a difficult step such as
a forging step is eliminated. The employment of a casting
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forging process reduces a cost of designing various
patterns of the disk portion, and improves the material
yield, whereby the production cost of the light-alloy
wheel for vehicles can be suppressed.
Brief Description of the Drawings
A of Fig. 1 is a half section view showing an
embodiment of the light-alloy wheel for vehicles accor-
ding to the invention, and B is an enlarged view of a
part of the embodiment.
Fig. 2 is a half section view showing another
embodiment of the light-alloy wheel for vehicles accor-
ding to the invention.
Fig. 3 is a section view showing a state where an
outer rim portion and a raw member which will be formed
as an inner rim portion are subjected to a spinning
process.
Fig. 4 is a half section view showing a preforming
casting process in which a disk portion and an outer rim
portion are produced by a casting forging process.
A and B of Fig. 5 are section views of test pieces
which were used in a test for demonstrating the coupling
state of a friction pressure joining process.
Fig. 6 is a diagram showing a graph of stress
distribution in a running simulation of a light-alloy
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wheel for vehicles which has a disk portion bearing a
design pattern.
Fig. 7 is a plan view showing a part of a wheel disk
portion and indicating measuring positions shown in the
graph of Fig. 6.
Fig. 8 is a diagram showing a graph in which the
hardness of the test pieces obtained by subjecting the
test pieces of Fig. 5 to a friction pressure joining
process is plotted against the distance from the joint
interface.
Fig. 9 is a table showing test conditions of test
specimens produced in the dimensions shown in Fig. 5.
Fig. 10 is a table showing results of measurements
in which tensile strength of test pieces produced from
test specimens undergone the friction pressure joining
process were measured.
Fig. 11 is a table showing measured values of
experiments which checked the distance range from the
pressure joining center where hardness change was ob-
served.
Best Modes for Carrying Out the Invention
Embodiment 1
First, a proof test of the joint state of a friction
pressure joining process was conducted. As a material,
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test pieces which have a shape such as those shown in A
and B of Fig. 5 were used. In A, 21 indicates a cast
piece subjected to a heat treatment and made of AC4C-T6.
The conditions of the heat treatment were as follows: As
a solution treatment, a heating was conducted at 530 C
for 8 hours, and an aging treatment was conducted at 170
C for 4 hours after water cooling. In B, 22 indicates a
forged piece subjected to a heat treatment and made of
A6061-T6. As a solution treatment, a heating was con-
ducted at 525 C for 1 hours, and an aging treatment wasconducted at 180 C for 8 hours after water cooling. 23
and 24 indicate joining faces of the test pieces. The
joining faces 23 and 24 are rotated about the center
lines (one-dot chain lines).
Four sets of the friction pressure joining test
pieces in the dimensions shown in Fig. 5 were produced to
~ be used as test specimens. Fig. 9 shows test conditions
of each test specimen set. Then test pieces of a width of
15 mm and a thickness of 5 mm were produced from the test
specimens subjected to a friction pressure joining
process, and the tensile strength of the joint portions
were measured. The results are shown in Fig. 10. In a
light-alloy wheel, the elongation which is indicated in
the table of Fig. 10 is desired to have a value of about
5 %. With respect to this point, first, the test pieces
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are judged to be "no problem". The test pieces shown in
Fig. 10 were previously subjected to a heat treatment,
and, after the friction pressure joining process, they
were not subjected to a further heat treatment.
The range of the heat-affected zone was judged by
checking the range which is centered at the pressure
joint portion and in which hardness change is observed.
The influence of heat generated in the friction pressure
joining process on the periphery of the pressure joint
portion was investigated by measuring the hardness of the
periphery. The measured distribution state is shown in
the form of a graph in Fig. 8. In Fig. 8, the measured
values listed in the table of Fig. 11 are plotted. The
unit of hardness is of micro Vickers.
As shown from Fig. 8, judging from abrupt changes in
hardness in both the cast and forged pieces, it is seen
that a range extending from the joint interface and a
position separated therefrom by about 15 mm was affected
by heat. When an outer rim portion, a disk portion, etc.
of a light-alloy wheel are to be joined to an inner rim
portion, therefore, it is preferable to conduct joining
at a position which is separated from the outer rim
portion or the disk portion by at least 15 mm.
Embodiment 2
Fig. lA is a section view of the light-alloy wheel
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for vehicles according to the invention in a friction
pressure joining process. Fig. lB shows an enlarged
portion of the wheel. In Fig. lA, a section of a com-
pleted wheel obtained by machining is shown by broken
lines. In the embodiment, a disk portion 1 and an outer
rim portion 2 were integrally molded by a forging
process, and made of A6061-T6. An inner rim portion 3 was
produced by a casting process, and made of AC4C-T6. The
friction pressure joining process was conducted under the
conditions which are indicated as specification of test
specimen set No. 1 shown in the table of Fig. 9. The
joining face was formed at a position which is separated
by 16 mm from a position of a dot line C in Fig. 1,
whereby the outer rim portion 2 and the disk portion 1
are prevented from being affected by heat. In order to
reinforce the joint portion, as shown in Fig. lB, a thick
portion D was formed in the range of 30 mm so as to be
centered at the joint portion. While a conventional
forged integral wheel has a thickness of 6 mm, a thick-
ness T of the embodiment was increased by about 20 % orset to be 7.2 mm. 4-1 and 4-2 are burrs which are removed
away in the machining process.
Fig. 2 shows an embodiment in which a rim portion 5
was produced by integrally molding outer and inner rim
portions by a casting process, and the rim portion and a
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forged disk portion 6 were joined together by a friction
pressure joining process. In the figure, since tl is
affected by heat generated in the friction pressure
joining process, tl is thickened by 10 % as compared with
a conventional product, thereby reinforcing the wheel.
The friction pressure joining process was conducted under
the conditions conforming to test specimen set No. 4
shown in the table of Fig. 9. However, the number of
rotation was changed so as to attain the same speed of
the press joining face. The press joining face has an
L-shaped section which is directed in the direction of
the gathering margin of the friction pressure joining
(which is parallel with the rotation axis).
Embodiment 3
In Fig. 3, an outer rim portion 7 and a raw member
8 of an inner rim portion were integrally cast. The cast
portions were heated to about 400 C, and clamped by
mandrels 9 and 10 of a spinning machine. While rotating
the portions, the raw member 8 of the inner rim portion
was spread by a roller 11 to mold the inner rim portion
indicated by a one-dot chain line so that the outer and
inner rim portions are integrally formed. The portions
were then friction-joined to a forged disk in the same
manner as described above.
Embodiment 4
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The embodiment is an embodiment based on a casting
forging process. As shown in Fig. 4, first, an outer rim
portion 12 and a disk portion 13 were integrally cast to
form a preform. As the material, used was an Al-Si
eutectic alloy to which Mg and Cu were added. After
conducting a fluorescent penetration inspection, the
preform was heated and then subjected to a hot forging
process. The embodiment has an advantage that a crude
mold is not required and the forging cost can remarkably
be reduced as compared with a forging process starting
from billets. In the case of employing a casting forging
process, furthermore, the disk portion may be designed so
as to attain a mesh structure which has a high degree of
freedom and is fine. After the hot forging process, a
trimming process was conducted. After a heat treatment of
T6, an inner rim portion was joined by a friction
pressure joining process in the same manner as Embodiment
2.
Industrial Applicability
In the light-alloy wheel for vehicles according to
the invention, a forged member having a tough metal-
lographic structure is used in the disk portion which
must bear a larger stress, whereby the strength of the
wheel can be maintained. The light-alloy wheel for
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vehicles according to the invention can be obtained in
the form of an integrated wheel produced by a friction
pressure joining process. Therefore, the disk portion and
the inner rim portion, or the outer rim portion can
separately be produced so that a difficult step such as
a forging step is eliminated. According to the invention,
furthermore, the employment of a casting forging process
can reduce a cost of designing various patterns of the
disk portion, and improve the material yield, whereby the
production cost of the light-alloy wheel for vehicles can
be suppressed.
