U.S. patent application number 10/353050 was filed with the patent office on 2003-08-07 for method for manufacturing plastic worked article.
This patent application is currently assigned to Mazda Motor Corporation. Invention is credited to Sakamoto, Kazuo, Sakate, Nobuo, Uosaki, Yasuo.
Application Number | 20030145918 10/353050 |
Document ID | / |
Family ID | 18946734 |
Filed Date | 2003-08-07 |
United States Patent
Application |
20030145918 |
Kind Code |
A1 |
Sakamoto, Kazuo ; et
al. |
August 7, 2003 |
Method for manufacturing plastic worked article
Abstract
The method for manufacturing a plastic worked article includes
the steps of shaping an article for plastic working by a semisolid
molding in which molten metal including both a solid phase and a
liquid phase is introduced into a die; performing a pre-plastic
working heat treatment on the article for plastic working for
causing blisters by expanding gas cavities included in the article
for plastic working; and forming a plastic worked article by
plastic working the article for plastic working after the
pre-plastic working heat treatment. The plastic working of the
article for plastic working is performed in such a manner that a
tensile stress applied direction at a tensile stress applied
portion to which tensile stress is principally applied in actual
use of the plastic worked article substantially accords with a
plastic flow direction at a portion of the article for plastic
working corresponding to the tensile stress applied portion.
Inventors: |
Sakamoto, Kazuo; (Hiroshima,
JP) ; Uosaki, Yasuo; (Hiroshima, JP) ; Sakate,
Nobuo; (Hiroshima, JP) |
Correspondence
Address: |
NIXON PEABODY, LLP
8180 GREENSBORO DRIVE
SUITE 800
MCLEAN
VA
22102
US
|
Assignee: |
Mazda Motor Corporation
Hiroshima
JP
|
Family ID: |
18946734 |
Appl. No.: |
10/353050 |
Filed: |
January 29, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10353050 |
Jan 29, 2003 |
|
|
|
PCT/JP02/02532 |
Mar 18, 2002 |
|
|
|
Current U.S.
Class: |
148/557 |
Current CPC
Class: |
B22D 17/007 20130101;
B21J 5/004 20130101; B21J 5/00 20130101; B21K 1/28 20130101; B21J
5/02 20130101; C22C 1/005 20130101; C22F 1/06 20130101 |
Class at
Publication: |
148/557 |
International
Class: |
C22F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 28, 2001 |
JP |
2001-092234 |
Claims
What is claimed is:
1. A method for manufacturing a plastic worked article by plastic
working an article for plastic working, comprising the steps of:
shaping an article for plastic working by a semisolid molding in
which molten metal including both a solid phase and a liquid phase
is introduced into a die; performing a pre-plastic working heat
treatment on the article for plastic working for causing blisters
by expanding gas cavities included in the article for plastic
working; and forming a plastic worked article by plastic working
the article for plastic working after the pre-plastic working heat
treatment, wherein the plastic working of the article for plastic
working is performed in such a manner that a tensile stress applied
direction at a tensile stress applied portion to which tensile
stress is principally applied in actual use of the plastic worked
article substantially accords with a plastic flow direction at a
portion of the article for plastic working corresponding to the
tensile stress applied portion.
2. The method for manufacturing a plastic worked article of claim
1, wherein the pre-plastic working heat treatment is performed with
a processing temperature in a range of 300.degree. C. to
550.degree. C.
3. The method for manufacturing a plastic worked article of claim
2, wherein the pre-plastic working heat treatment is performed with
a processing time of 1 hour or more.
4. The method for manufacturing a plastic worked article of claim
3, wherein the pre-plastic working heat treatment is performed with
a processing temperature in a range of 350.degree. C. to
450.degree. C. and a processing time of 20 hours or less.
5. The method for manufacturing a plastic worked article of claim
4, wherein the pre-plastic working heat treatment is performed with
a processing time in a range of 10 hours to 20 hours.
6. The method for manufacturing a plastic worked article of claim
1, further comprising a step of performing a post-plastic working
heat treatment on the plastic worked article obtained by the
plastic working with a processing temperature in a range of
250.degree. C. to 400.degree. C. and a processing time in a range
of 20 minutes to 10 hours.
7. The method for manufacturing a plastic worked article of claim
1, wherein the article for plastic working is shaped by the
semisolid molding using an injection molding apparatus equipped
with a cylinder and a screw.
8. The method for manufacturing a plastic worked article of claim
1, wherein a thick part is provided on the portion of the article
for plastic working corresponding to the tensile stress applied
portion, and the article for plastic working is shaped into a
desired shape by the plastic working by allowing the thick part to
flow in a direction substantially according with the tensile stress
applied direction.
9. The method for manufacturing a plastic worked article of claim
1, wherein the plastic worked article is a road wheel, and the
plastic working of the article for plastic working is performed in
such a manner that a direction substantially along an extending
direction of a spoke of a wheel disk of the road wheel
substantially accords with a plastic flow direction at a portion of
the article for plastic working corresponding to the wheel
disk.
10. The method for manufacturing a plastic worked article of claim
1, wherein the plastic worked article is a road wheel, and the
plastic working of the article for plastic working is performed in
such a manner that a direction substantially along a width of a
wheel rim of the road wheel substantially accords with a plastic
flow direction at a portion of the article for plastic working
corresponding to the wheel rim.
Description
[0001] This is a continuation of Application PCT/JP02/02532, filed
Mar. 18, 2002, now abandoned.
FIELD OF THE INVENTION
[0002] The present invention relates to a manufacturing method for
a plastic worked article in which an article for plastic working
shaped by a semisolid molding is plastic worked after causing
blisters by expanding gas cavities included in the article for
plastic working through a pre-plastic working heat treatment.
BACKGROUND OF THE INVENTION
[0003] Cast-forging in which an article for forging having been
shaped into a near-net shape by casting is forged into a forged
article, that is, a final product, is a process capable of
providing a forged article with mechanical properties equivalent to
those obtained by casting at comparatively low cost. On the other
hand, the mechanical properties of a forged article are generally
improved by a T6 heat treatment (a solution treatment followed by
an ageing precipitation hardening treatment). In the case where the
T6 heat treatment is applied to a forged article obtained by the
cast-forging, however, blisters derived from gas cavities, that is,
internal defects introduced during the forging, are caused on the
surface thereof, which degrades the mechanical properties and the
appearance quality of the forged article.
[0004] As a countermeasure, Japanese Laid-Open Patent Publication
No. 2000-197956 discloses the following method: An article for
forging is shaped by filling a cavity in a die with molten light
metal, the obtained article for forging is subjected to a heat
treatment for causing blisters derived from expansion of an
internal gas, and the resultant article for forging having been
heat treated is forged. According to this publication, when
blisters are thus caused at an article stage, the blisters can be
ruptured by forging, so that blisters can be definitely prevented
from being caused in an ultimately obtained forged article.
[0005] When the heat treatment is carried out before the forging as
disclosed in Japanese Laid-Open Patent Publication No. 2000-197956,
however, anisotropy in the mechanical properties is caused in the
ultimately obtained forged article, and hence, the tensile strength
is higher in a plastic flow direction than in a forging direction.
This is probably for the following reason: An article for forging
produced by the semisolid molding has a microstructure in which a
matrix of a liquid phase part formed by hardening a liquid phase is
interspersed with solid phase parts that were solid in a semisolid
state, and there remains an interface between the liquid phase part
and the solid phase parts after the heat treatment performed before
the forging. When this article is forged, the solid phase parts are
compressed in the forging direction as well as elongated in the
plastic flow direction substantially perpendicular to the forging
direction. As a result, interfacial flaking between the liquid
phase part and the solid phase parts can be easily caused in the
forging direction than in the plastic flow direction.
SUMMARY OF THE INVENTION
[0006] An object of the invention is providing a method for
manufacturing a plastic worked article in consideration of
anisotropy in the mechanical properties caused in a plastic worked
article manufactured by plastic working an article for plastic
working shaped by the semisolid molding and having been heat
treated before the plastic working.
[0007] In order to achieve the object, an article for plastic
working is plastic worked, in consideration of the anisotropy in
the mechanical property caused in a plastic worked article, in such
a manner that a tensile stress applied direction at a tensile
stress applied portion to which tensile stress is principally
applied in actual use of the plastic worked article substantially
accords with a plastic flow direction at a portion of the article
for plastic working corresponding to the tensile stress applied
portion.
[0008] Specifically, the present invention is a method for
manufacturing a plastic worked article by plastic working an
article for plastic working, including the steps of shaping an
article for plastic working by a semisolid molding in which molten
metal including both a solid phase and a liquid phase is introduced
into a die; performing a pre-plastic working heat treatment on the
article for plastic working for causing blisters by expanding gas
cavities included in the article for plastic working; and forming a
plastic worked article by plastic working the article for plastic
working after the pre-plastic working heat treatment, and the
plastic working of the article for plastic working is performed in
such a manner that a tensile stress applied direction at a tensile
stress applied portion to which tensile stress is principally
applied in actual use of the plastic worked article substantially
accords with a plastic flow direction at a portion of the article
for plastic working corresponding to the tensile stress applied
portion.
[0009] According to this manufacturing method, the plastic working
of the article for plastic working is performed in such a manner
that the tensile stress applied direction at the tensile stress
applied portion to which tensile stress is principally applied in
actual use of the plastic worked article substantially accords with
the plastic flow direction at the portion of the article for
plastic working corresponding to the tensile stress applied
portion. Therefore, even when the anisotropy in the mechanical
properties occurs in the plastic worked article obtained by plastic
working the article for plastic working having been subjected to
the pre-plastic working heat treatment, the reinforcing effect of
the plastic working can be effective exhibited in the resultant
plastic worked article. Also, even when the plastic worked article
has a smaller thickness, necessary strength can be attained, so as
to lower the cost.
[0010] The plastic working is not particularly specified and may
be, for example, forging, spinning or the like.
[0011] The tensile stress applied portion is the portion to which
the tensile stress is principally applied in actual use of the
plastic worked article, and specifically corresponds to a portion
where the integrated value of the tensile stress is relatively
large.
[0012] Also, the semisolid molding is a shaping method in which
molten metal in a semisolid state including both a solid phase and
a liquid phase of a raw material metal is introduced into a die,
and the semisolid state is generally obtained by beating the raw
material metal below the melting point. In the resultant article
for plastic working, a microstructure in which a matrix of a liquid
phase part formed by hardening the liquid phase is interspersed
with solid phase parts that were solid in the semisolid state is
formed.
[0013] The pre-plastic working heat treatment is performed in order
to cause blisters, which are derived from expansion of gas
cavities, that is, internal defects such as gas defects included in
the vicinity of the surface of the article for plastic working, on
and/or in the vicinity of the surface thereof, and the blisters are
ruptured by the plastic working. As a result, the number of gas
cavities included in the plastic worked article is reduced, whereby
improving the mechanical properties of the plastic worked article.
Accordingly, in using an article for plastic working shaped by a
shaping method where gas cavities are more easily formed, such as
the semisolid molding using an injection molding apparatus equipped
with a cylinder and a screw, the effect to improve the mechanical
properties of the plastic worked article can be particularly
remarkably exhibited.
[0014] The pre-plastic working heat treatment is preferably carried
out with a processing temperature in a range of 300.degree. C. to
550.degree. C. Thus, the blisters can be definitely caused on
and/or in the vicinity of the surface of the article for plastic
working while suppressing oxidation caused by the heat treatment.
Specifically, when the processing temperature of the pre-plastic
working heat treatment is lower than 300.degree. C., the blisters
may be insufficiently caused on and/or in the vicinity of the
surface of the article for plastic working. Alternatively, when the
heat treatment is carried out at a temperature exceeding
550.degree. C., the surface may be largely oxidized or partially
eutectic fused. In consideration of these, the processing
temperature is more preferably in a range of 350.degree. C. to
450.degree. C.
[0015] Furthermore, the pre-plastic working heat treatment is
preferably carried out with a processing time of 1 hour or more.
Thus, the blisters can be effectively caused on the surface of the
article for plastic working. In addition, the microstructure
composed of the liquid phase part and the solid phase parts is
homogenized as in the solution treatment of the T6 heat treatment,
so that the anisotropy in the mechanical properties caused by the
plastic working can be reduced. In the case where the pre-plastic
working heat treatment is carried out at a processing temperature
in a range of 350.degree. C. to 450.degree. C., the processing time
is preferably 20 hours or less and more preferably in a range of 10
hours to 20 hours.
[0016] When the pre-plastic working heat treatment is performed at
a comparatively high temperature for a comparatively long time so
as to cause the blisters by expanding the gas cavities included in
the article for plastic working, a post-plastic working heat
treatment, if performed, may be performed at a comparatively low
temperature for a comparatively short time. Thus, occurrence of
blisters can be suppressed in the post-plastic working heat
treatment.
[0017] The post-plastic working heat treatment is preferably
performed with a processing temperature in a range of 250.degree.
C. to 400.degree. C. and a processing time in a range of 20 minutes
to 10 hours. Thus, the post-plastic working heat treatment is
performed after the plastic working at a higher temperature for a
shorter time than in the ageing precipitation hardening treatment
of the T6 heat treatment, and hence, ductility can be effectively
improved while retaining the tensile strength and the yield
strength of the plastic worked article as obvious from an
experiment described below. The processing temperature is in a
range of 250.degree. C. to 400.degree. C. because when the
processing temperature is lower than 250.degree. C., the ductility
cannot be sufficiently improved and when it is higher than
400.degree. C., the yield strength is largely lowered. Furthermore,
the processing time is in a range of 20 minutes to 10 hours because
when the processing time is shorter than 20 minutes, the ductility
cannot be sufficiently improved and when it is longer than 10
hours, the ductility may be lower than that attained without the
heat treatment. Accordingly, the processing time is preferably 5
hours or less and most preferably 1 hour.
[0018] The plastic worked article manufactured in the
aforementioned manner is not particularly specified but may be, for
example, a road wheel, an engine bracket or the like.
[0019] When the plastic worked article is a road wheel, the plastic
working of the article for plastic working is performed in such a
manner that a direction substantially along an extending direction
of a spoke, namely, the tensile stress applied direction, of a
wheel disk corresponding to the tensile stress applied portion of
the road wheel substantially accords with a plastic flow direction
at a portion of the article for plastic working corresponding to
the wheel disk. Thus, the strength along the extending directions
of the spokes of the wheel disk can be effectively improved.
Alternatively, the plastic working of the article for plastic
working is performed in such a manner that a direction
substantially along a width of a wheel rim of the road wheel,
namely, the tensile stress applied direction at the tensile stress
applied portion, substantially accords with a plastic flow
direction at a portion of the article for plastic working
corresponding to the wheel rim. Thus, the strength along the width
of the wheel rim can be effectively improved.
[0020] In a specific aspect of the plastic working, a thick part is
provided on the portion of the article for plastic working
corresponding to the tensile stress applied portion, and the
article for plastic working is shaped into a desired shape by the
plastic working by allowing the thick part to flow in a direction
substantially according with the tensile stress applied
direction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a partial cross-sectional view of an injection
molding apparatus according to an embodiment of the invention;
[0022] FIG. 2A is a front view of a wheel disk of a road wheel
exemplified as a forged article of the embodiment and FIG. 2B is a
cross-sectional view taken on line IIB-IIB of FIG. 2A;
[0023] FIG. 3A is a vertical cross-sectional view of a spoke
corresponding portion of an article for forging used for shaping
the road wheel exemplified as the forged article of the embodiment
and FIG. 3B is a vertical cross-sectional view of a spoke of the
road wheel obtained by forging the article for forging;
[0024] FIG. 4A is a vertical cross-sectional view of a spoke
corresponding portion of an article for forging of a first example
and a resultant spoke obtained by forging, FIG. 4B is a vertical
cross-sectional view of a spoke corresponding portion of an article
for forging of a second example and a resultant spoke obtained by
forging, and FIG. 4C is a vertical cross-sectional view of a spoke
corresponding portion of an article for forging of a third example
and a resultant spoke obtained by forging;
[0025] FIG. 5 is a vertical cross-sectional view of a road wheel
exemplified as a forged article of another embodiment of the
invention;
[0026] FIG. 6A is a vertical cross-sectional view of an article for
forging used for shaping a road wheel exemplified as a forged
article of another embodiment of the invention and FIG. 6B is a
vertical cross-sectional view of the road wheel obtained by forging
the article for forging;
[0027] FIGS. 7A and 7B are explanatory diagrams of states attained
before and after forging in Experiment 1;
[0028] FIG. 8 is a graph for showing the 0.2% yield strength, the
tensile strength and the elongation of tensile test samples of a
forged article produced under conditions A in Experiment 1;
[0029] FIG. 9 is a graph for showing the 0.2% yield strength, the
tensile strength and the elongation of tensile test samples of a
forged article produced under conditions B in Experiment 1;
[0030] FIGS. 10A and 10B are drawings of microstructures on
surfaces of the forged articles produced under the conditions A and
B in Experiment 1;
[0031] FIG. 11 is a graph for showing the relationship, obtained in
a forged article made from an alloy B, between the processing
temperature of a post-forging heat treatment carried out in
Experiment 2 and the 0.2% yield strength, the tensile strength and
the elongation after fracture;
[0032] FIG. 12 is a graph for showing the relationship, obtained in
a forged article made from an alloy C, between the processing
temperature of a post-forging heat treatment carried out in
Experiment 2 and the 0.2% yield strength, the tensile strength and
the elongation after fracture;
[0033] FIGS. 13A, 13B, 13C and 13D are drawings of microstructures
on surfaces of forged articles made from the alloy B having been
subjected to the post-forging heat treatment under various
conditions in Experiment 2;
[0034] FIGS. 14A, 14B, 14C and 14D are drawings of microstructures
on surfaces of forged articles made from the alloy C having been
subjected to the post-forging heat treatment under various
conditions in Experiment 2;
[0035] FIG. 15 is a graph for showing the relationship, obtained in
a forged article made from the alloy B, between the processing time
of a post-forging heat treatment carried out in Experiment 3 and
the 0.2% yield strength, the tensile strength and the elongation
after fracture; and
[0036] FIG. 16 is a graph for showing the relationship, obtained in
a forged article made from the alloy C, between the processing time
of a post-forging heat treatment carried out in Experiment 3 and
the 0.2% yield strength, the tensile strength and the elongation
after fracture.
DETAILED DESCRIPTION OF THE INVENTION
[0037] A method for manufacturing a forged article according to a
preferred embodiment of the invention will now be described.
[0038] (Forging Process--Injection Molding Process)
[0039] <Injection Molding Apparatus>
[0040] FIG. 1 shows an injection molding apparatus 1 for shaping an
article for forging (an article for plastic working) of light metal
alloy used in this embodiment.
[0041] The injection molding apparatus 1 includes a main body 2, a
screw 3 rotatably supported by the main body 2, a rotation driving
unit 4 disposed behind the main body 2 for rotationally driving the
screw 3, a cylinder 5 fixed on the main body 2 so as to surround
the screw 3, heaters 6 arranged around the circumference of the
cylinder 5 at a predetermined pitch along the lengthwise direction
of the cylinder 5, a hopper 7 for storing a light metal alloy raw
material introduced therein, a feeder 8 for measuring the material
stored in the hopper 7 and supplying the material into the
injection molding apparatus 1, and a die 9 attached on the end of
the cylinder 5.
[0042] An injection mechanism for propelling the screw 3 along the
lengthwise direction within the cylinder 5 is provided on the main
body 2. When the injection mechanism detects that the screw 3 has
retracted a preset distance due to the force of molten light metal
alloy transported forward, it stops the rotation and retraction of
the screw 3, and propels the screw 3 forward with a predetermined
timing to inject molten metal. The speed at which the screw 3 is
propelled forward is controllable, so as to control the speed at
which the molten metal is introduced into a cavity 12 in the die
9.
[0043] A nozzle 10 is provided at the end of the cylinder 5, so
that molten metal having been stirred and kneaded within the
cylinder 5 can be injected into the cavity 12 through the nozzle
10. The molten metal is injected into the cavity 12 when a
predetermined amount of molten metal is gathered in a front portion
of the cylinder 5, and hence, it is necessary to prevent the molten
metal from flowing out through the nozzle 10 until the sufficient
amount for the injection is gathered. For this purpose, the
temperature of the nozzle 10 is controlled as follows: While the
molten metal is gathering in the front portion of the cylinder 5,
the nozzle 10 is clogged with a cold plug made from solidified
molten metal, and when the molten metal is to be injected, the cold
plug is easily removed and pressed out toward the die 9 together
with the injected molten metal.
[0044] The heaters 6 provided around the circumference of the
cylinder 5 are divided into a plurality of zones in each of which
the temperature is controlled, so that the temperature increases
along the lengthwise direction of the cylinder 5 toward the front
portion thereof Thus, the temperature of the light metal alloy raw
material is increased while it is being transported within the
cylinder 5 by the screw 3, so that the light metal alloy raw
material can become molten metal in a semisolid state below the
melting point when it reaches the front portion of the cylinder
5.
[0045] The hopper 7, the feeder 8, the cylinder 5 and passages
joining them are filled with an inert gas (such as an Ar gas) for
preventing oxidation of the light metal alloy.
[0046] The die 9 has a runner 11 for guiding the molten metal
injected from the nozzle 10. The runner 11 extends straight from
the nozzle 10 of the cylinder 5 and then rises vertically to form
an L-shape. A plug receptacle 11a is provided at the corner of the
L-shape for receiving the cold plug having been removed from the
nozzle 10. Also, the die 9 includes a cavity 12 connected to the
runner 11, a gate 13 serving as the boundary between the cavity 12
and the runner 11, and an overflow 14 positioned away from the gate
13 of the cavity 12 for accepting a gas having been contained in
the cavity 12 and displaced by the molten metal.
[0047] <Injection Molding Method>
[0048] The injection molding method employed for an article for
forging of light metal alloy will now be described.
[0049] First, chips of light metal alloy (such as Mg--Al alloy) are
placed in the hopper 7 of the injection molding apparatus 1 as a
raw material. A predetermined weight of the light metal alloy chips
is measured and supplied to the injection molding apparatus 1 by
the feeder 8.
[0050] Thereafter, the light metal alloy chips are transported by
the rotation of the screw 3 into the cylinder 5 that is heated, and
are sufficiently stirred and kneaded by the rotation of the screw 3
and heated to a predetermined temperature within the cylinder 5. As
a result, the light metal alloy chips are changed into semisolid
light metal alloy below the melting point.
[0051] The thus obtained molten metal alloy is pushed forward by
the screw 3 to be gathered in the front portion of the cylinder 5,
and the screw 3 retracts due to the pressure of the gathered molten
metal alloy. At this point, the temperature of the nozzle 10
provided on the cylinder 5 is lowered so as to clog the nozzle 10
with a cold plug formed by solidifying some of the molten metal.
Thus, the molten metal is prevented from flowing through the nozzle
10 out of the cylinder 5.
[0052] When the screw 3 retracts by a predetermined distance, the
injecting mechanism of the main body 2 detects the retraction so as
to stop the rotation and the retraction of the screw 3. At this
point, the molten metal in an amount for a single injection is
gathered in the front portion of the cylinder 5.
[0053] Then, the screw 3 is pushed forward by the injection
mechanism to apply a pressure to the molten metal. As a result, the
molten metal pushes out the cold plug toward the die 9 and is
injected through the nozzle 10 into the cavity 12. At this point,
the semisolid light metal alloy enters the cavity as a laminar flow
or near-laminar flow, and hence, air is minimally trapped,
resulting in shaping an article for forging with few gas defects
such as gas cavities and shrinkage cavities. The removed cold plug
is caught by the plug receptacle 11a of the runner 11.
[0054] Finally, after the molten metal is solidified, the die 9 is
opened and the obtained injection molded material to be used as the
article for forging is taken out.
[0055] The thus obtained article for forging has a microstructure
in which a matrix of a liquid phase part formed by hardening a
liquid phase is interspersed with solid phase parts that were solid
in the semisolid state.
[0056] (Pre-Forging Heat Treatment)
[0057] The article for forging injection molded in the
aforementioned manner is subjected to a pre-forging heat treatment
(a pre-plastic working heat treatment) carried out with a
processing temperature in a range of 300.degree. C. to 550.degree.
C. and a processing time of 1 hour or more (preferably with a
processing temperature in a range of 350.degree. C. to 450.degree.
C. and a processing time in a range of 1 to 20 hours, and more
preferably with a processing temperature in a range of 350.degree.
C. to 450.degree. C. and a processing time in a range of 10 to 20
hours). Through this pre-forging heat treatment, the microstructure
of the article for forging is homogenized (not completely but) to
an extent that the boundaries between the liquid phase part and the
solid phase parts become difficult to identify. Also, the gas
cavities, that is, the internal defects in the vicinity of the
surface of the article for forging, are expanded to cause blisters
on and in the vicinity of the surface of the article for
forging.
[0058] (Forging Process)
[0059] The article for forging having been subjected to the
pre-forging heat treatment is then subjected to either fully
enclosed die forging (fully enclosed die plastic working) or
non-fully enclosed die forging (non-fully enclosed die plastic
working). The fully enclosed die forging is performed in a forging
die whose forging space is completely closed, while the non-fully
enclosed die forging is performed in a forging die where at least
part of the article for forging is not inhibited and so is free to
deform plastically. At this point, the article for forging is
forged in such a manner that a tensile stress applied direction at
a tensile stress applied portion to which tensile stress is
principally applied in actual use of the resultant forged article
substantially accords with a plastic flow direction at a portion of
the article for forging corresponding to the tensile stress applied
portion.
[0060] This will be specifically described by exemplifying a road
wheel. In actual use of a wheel disk 16 of a road wheel 15, as
shown in FIGS. 2A and 2B, tensile stress is principally applied to
spokes 16a along directions shown with arrows in the drawings
(namely, directions substantially along extension of the spokes).
In other words, the spokes 16a correspond to the tensile stress
applied portion. Therefore, an article for forging 17 having one or
more thick parts on the back face of a spoke corresponding portion
17a as shown in FIG. 3A is shaped by casting, and as shown in FIG.
3B, the thick part of the spoke corresponding portion 17a is made
to plastic flow along directions shown with arrows in the drawing
so that the plastic flow directions can substantially accord with
the tensile stress applied direction in actual use of the spoke
16a, namely, the direction substantially along the extension of the
spoke. Strictly speaking, tensile stress is applied onto the front
face of the spoke 16a with compression stress applied onto the back
face thereof, and hence, it is desired to provide the forging
strongly on the front face of the spoke 16a. When the forging is
provided strongly on the front face of the spoke 16a, however, the
surface property of the front face attained after the forging is
degraded. Therefore, the material for the article, the dimension of
the article 17 for forging, the finished dimension of the road
wheel 15 are properly set so that the reinforcing effect of the
forging can be exhibited on the front face even when the forging is
performed from the back face. Specific examples are shown in FIGS.
4A through 4C. In a first example shown in FIG. 4A, a spoke 16a
having a uniform thickness is shaped by forging an article 17 for
forging having a thick part rising at the center in the radial
direction on the back face thereof similarly to that shown in FIGS.
3A and 3B. In a second example shown in FIG. 4B, a spoke 16a having
a uniform thickness is shaped by forging an article 17 for forging
having wavy irregularities formed in the radial direction on the
back face thereof with projected portions serving as thick parts.
In a third example shown in FIG. 4C, a spoke 16a with a smaller
thickness at the center and a larger thickness at the ends thereof
is shaped by forging an article 17 for forging having a thick part
rising at the center in the radial direction on the back face
thereof similarly to that shown in FIG. 4A. In any of these
examples, when the dimensions and shapes of the article 17 for
forging and the spoke 16a are properly set, forging enters a
portion closer to the front face beyond the center line along the
thickness direction of the spoke 16a to be shaped as shown with
arrows in each drawing even when the forging is performed from the
back face, so that the reinforcing effect can be exhibited on the
front face.
[0061] During the forging, some of the blisters caused in the
pre-forging heat treatment on and in the vicinity of the surface of
the article for forging are ruptured. As a result, the number of
internal defects can be reduced as compared with that in the
article for forging obtained after the injection molding.
[0062] (Post-Forging Heat Treatment)
[0063] The forged article having been shaped by the forging process
is then subjected to a post-forging heat treatment (a post-plastic
working heat treatment) carried out with a processing temperature
in a range of 250.degree. C. to 400.degree. C. and a processing
time in a range of 20 minutes to 10 hours, and the resultant is
cooled to obtain a final forged article. Since the pre-forging heat
treatment is carried out at a comparatively high temperature for a
comparatively long time, the post-forging heat treatment can be
carried out at a comparatively low temperature for a comparatively
short time. In addition, since the number of internal defects is
reduced through the forging process, occurrence of blisters can be
suppressed in the post-forging heat treatment.
[0064] According to the aforementioned method for manufacturing a
forged article, an article for forging is forged in such a manner
that a tensile stress applied direction at a tensile stress applied
portion to which tensile stress is principally applied in actual
use of the resultant forged article substantially accords with a
plastic flow direction at a portion of the article for forging
corresponding to the tensile stress applied portion. Therefore,
even when the anisotropy in the mechanical properties is caused in
the forged article to be manufactured by forging the article for
forging after a pre-forging heat treatment, the reinforcing effect
attained by the forging can be effectively exhibited in the final
forged article to be manufactured. Also, even when the forged
article has a smaller thickness, the necessary strength can be
attained, and hence, the cost can be lowered.
[0065] Furthermore, since the pre-forging heat treatment is carried
out with a processing temperature in a range of 300.degree. C. to
500.degree. C. (preferably 350.degree. C. to 450.degree. C.) and a
processing time of 1 hour or more (preferably in a range of 1 to 20
hours and more preferably 10 to 20 hours), blisters can be
definitely caused due to expansion of gas cavities formed on and in
the vicinity of the surface of the article for forging. Since these
blisters are ruptured through the forging, the number of gas
cavities included in the resultant forged article can be reduced
and the mechanical properties of the forged article can be
improved. In addition, the microstructure composed of the liquid
phase part and the solid phase parts is homogenized, resulting in
reducing the anisotropy in the mechanical properties of the article
for forging.
[0066] Moreover, the post-forging heat treatment is carried out
after the forging process at a higher temperature for a shorter
time than in the ageing precipitation hardening treatment of the T6
heat treatment. Therefore, while retaining the tensile strength and
the yield strength of the forged article, the ductility can be
effectively improved.
[0067] Alternative Embodiments
[0068] Although the article for forging is shaped by the injection
molding in the aforementioned embodiment, the method for shaping
the article for forging is not limited to this but may be any of
other shaping methods such as a die-cast method.
[0069] Although the article for forging is made from a light metal
(such as Mg--Al alloy) in the embodiment, the material for the
article for forging is not limited to this but may be another metal
material.
[0070] Although the plastic working is performed by forging in the
embodiment, the plastic working is not limited to this but may be
spinning or the like.
[0071] Furthermore, although the road wheel 16 is described as a
specific example of the forged article in the embodiment, the
forged article is not limited to this but may be an engine bracket
or the like.
[0072] Although the wheel disk 16 is plastic worked by forging in
the embodiment, a wheel rim 18 may be plastic worked by spinning as
follows: As shown in FIG. 5, in actual use of the wheel rim 18 of
the road wheel 15, tensile stress is principally applied in the
wheel rim 18 along directions shown with arrows in the drawing
(namely, directions substantially along the width of the wheel rim
18). In other words, the wheel rim 18 corresponds to the tensile
stress applied portion. Therefore, through the spinning of a rim
corresponding portion 19a of an article 19 for forging as shown in
FIG. 6A, the plastic flows are caused in directions shown with
arrows in FIG. 6B so that the plastic flow direction can
substantially accord with the tensile stress applied direction in
actual use of the wheel rim 18, namely, the direction substantially
along the width of the wheel rim 18. As a result, even when the
wheel rim 18 has a smaller thickness, the necessary strength can be
attained.
[0073] (Experiment 1)
[0074] <Experiment Method>
[0075] Two articles for forging each in the form of a metal block
were shaped by using the injection molding apparatus from an alloy
A having a composition listed in Table 1.
[0076] One of the article for forging was subjected to a forging
process and a heat treatment under conditions A listed in Table 2.
Specifically, the article for forging was subjected to a
pre-forging heat treatment at 400.degree. C. for 10 hours, cooled
and then subjected to non-fully enclosed die forging to give a
forged article in the shape of a plate. Thereafter, the forged
article was subjected to a post-forging beat treatment at
350.degree. C. for 1 hour and cooled to give a final forged
article. In this case, the non-fully enclosed die forging was
performed to attain a forging working rate of 50%. Specifically, as
shown in FIG. 7, the non-fully enclosed die forging was performed
so as to attain (h.sub.0-h.sub.1)/h.sub.0.times.100=50 (the forging
working rate), wherein h.sub.0 is an initial thickness along the
forging direction and h.sub.1 is a thickness obtained after the
forging.
[0077] The other article for forging was subjected to a forging
process and a heat treatment under conditions B listed in Table 2.
Specifically, the article for forging was subjected to non-fully
enclosed die forging to give a forged article. The resultant was
subjected to a solution treatment at 400.degree. C. for 10 hours
and cooled, and then subjected to an ageing precipitation hardening
treatment at 175.degree. C. for 16 hours and cooled (namely,
subjected to the T6 heat treatment) to give a final forged article
in the form of a plate. In this case, the non-fully enclosed die
forging was performed to attain a forging working rate of 50%.
[0078] From each of the thus obtained forged articles, a tensile
test sample 21 in which the tensile direction accords with the
forging direction and a tensile test sample 22 in which the tensile
direction accords with the plastic flow direction perpendicular to
the forging direction were cut out as shown in FIG. 7B.
[0079] A tensile test was carried out on the tensile test sample 21
having the tensile direction according with the forging direction
and the tensile test sample 22 having the tensile direction
according with the plastic flow direction cut out from each of the
forged articles.
[0080] Also, a microstructure on the surface of each forged article
was observed with a microscope.
1 TABLE 1 Al Zn Mn Si Cu Ni Fe Mg Alloy A 7.1 0.66 0.24 -- 0.002
<0.001 0.002 remainder Alloy B 9.1 0.78 0.24 -- 0.001 0.0009
0.003 remainder Alloy C 6.9 0.60 0.23 0.01 0.002 0.0005 0.002
remainder unit: mass %
[0081]
2TABLE 2 Condition A Heat treatment at 400.degree. C. for 10 hrs.
.fwdarw. Air cooling .fwdarw. Hot forging .fwdarw. Heat treatment
at 350.degree. C. for 1 hr. .fwdarw. Air cooling Condition B Hot
forging .fwdarw. Heat treatment at 400.degree. C. for 10 hrs.
.fwdarw. Air cooling .fwdarw. Heat treatment at 175.degree. C. for
16 hrs. .fwdarw. Air cooling
[0082] <Experiment Results>
[0083] FIG. 8 shows the results of the tensile test (0.2% yield
strength, the tensile strength and the elongation after fracture)
obtained in each tensile test sample of the forged article
manufactured under the conditions A. FIG. 9 shows the relationship
corresponding to that shown in FIG. 8 obtained in each tensile test
sample of the forged article manufactured under the conditions B.
When these graphs are compared, it is understood with respect to
the former forged article that all the mechanical properties of the
0.2% yield strength, the tensile strength and the elongation after
fracture are higher in the tensile test sample having the tensile
direction according with the plastic flow direction than in the
tensile test sample having the tensile direction according with the
forging direction. In contrast, with respect to the latter forged
article, all the mechanical properties of the 0.2% yield strength,
the tensile strength and the elongation after fracture are
equivalent in the tensile test sample having the tensile direction
according with the forging direction and the tensile test sample
having the tensile direction according with the plastic flow
direction.
[0084] FIGS. 10A and 10B respectively show the microstructures on
the surfaces of the forged article manufactured under the
conditions A and the forged article manufactured under the
conditions B. It is understood from these drawings that the forged
article manufactured under the conditions A has a microstructure in
which the matrix of the liquid phase part is interspersed with the
solid phase parts and the solid phase parts are aligned along the
plastic flow direction while the forged article manufactured under
the conditions B has a homogeneous microstructure. The anisotropy
in the mechanical properties of the forged articles is probably
derived from this difference in the microstructure. Also, the
forged article manufactured under the conditions A has higher
mechanical properties along the plastic flow direction than along
the forging direction probably because interfacial flaking between
the liquid phase part and the solid phase parts can be easily
caused in the latter direction than in the former direction.
[0085] (Experiment 2)
[0086] <Experiment Method>
[0087] An injection molded material in the form of a metal plate
was shaped by using the injection molding apparatus from an alloy B
having a composition listed in Table 1. In this case, the
temperature of the molten metal was controlled so as to attain a
solid phase proportion of 5% in the resultant injection molded
material, and the solid phase proportion was confirmed through
image analysis of the surface of the resultant injection molded
material. The alloy B used herein is AZ91D according to ASTM
standards. Similarly, an injection molded material in the form of a
metal plate is shaped from an alloy C having a composition listed
in Table 1. In this case, the temperature of the molten metal was
controlled so as to attain a solid phase proportion of 10% in the
resultant injection molded material.
[0088] From each of the injection molded materials in the metal
plate shape made from the alloys B and C, several articles for
forging each in the form of a block with a width of 10 mm, a length
of 35 mm and a thickness of 21 mm were cut out. The articles for
forging cut out from the material of the alloy B were subjected to
a pre-forging heat treatment at 410.degree. C. for 16 hours and
cooled, and those cut out from the material of the alloy C were
subjected to a pre-forging heat treatment at 400.degree. C. for 10
hours and cooled.
[0089] Then, each of the articles for forging having been subjected
to the pre-forging heat treatment was forged, with its width
direction constricted, until the thickness was reduced by half from
21 mm to 10.5 mm (namely, with a forging working rate of 50%), to
give a forged article.
[0090] These forged articles made from the alloys B and C were then
subjected to a post-forging heat treatment for 4 hours at a
temperature of 170.degree. C., 250.degree. C., 300.degree. C.,
350.degree. C. or 400.degree. C. and cooled to give final forged
articles. For comparison, some articles for forging were not
subjected to the post-forging heat treatment to be directly used as
final forged articles.
[0091] Then, a tensile test was carried out on the forged articles
having been subjected to the post-forging heat treatment and the
forged articles not having been subjected to the post-forging heat
treatment.
[0092] With respect to each of the forged articles made from the
alloys B and C and having been subjected to the post-forging heat
treatment at 300.degree. C., 350.degree. C. or 400.degree. C., the
microstructure on the surface obtained after the tensile test was
observed with a microscope. At this point, for comparison, the
surface observation was performed on forged articles having been
subjected to not pre- and post-forging heat treatments but the T6
heat treatment (in which the solution treatment was carried out at
410.degree. C. for 16 hours and the ageing precipitation hardening
treatment was carried out at 170.degree. C. for 16 hours on the
forged article made from the alloy B, and the solution treatment
was carried out at 400.degree. C. for 10 hours and the ageing
precipitation hardening treatment was carried out at 175.degree. C.
for 16 hours on the forged article made from the alloy C).
[0093] <Experiment Results>
[0094] FIG. 11 shows the relationship between the temperature of
the post-forging heat treatment and the 0.2% yield strength, the
tensile strength and the elongation after fracture obtained in the
forged articles made from the alloy B, and FIG. 12 shows the
relationship corresponding to that of FIG. 11 obtained in the
forged articles made from the alloy C. It is understood from FIGS.
11 and 12 that as the processing temperature is increased, the 0.2%
yield strength is lowered, the tensile strength is slowly lowered
and the elongation after fracture is increased in the forged
articles made from both the alloys B and C. Also, when an article
for forging is subjected to a heat treatment equivalent to the
ageing precipitation hardening treatment (at 170.degree. C. through
230.degree. C.) of the T6 heat treatment, the elongation after
fracture is smaller than in the case where an article for forging
is not subjected to the heat treatment, but if the processing
temperature is 250.degree. C. or more, the elongation after
fracture can be largely improved without largely lowering the 0.2%
yield strength and the tensile strength.
[0095] FIGS. 13A through 13D show the microstructures of the forged
articles made from the alloy B, and specifically, FIG. 13A shows
that of a forged article having been subjected to the T6 heat
treatment, FIG. 13B shows that of a forged article having been
subjected to a heat treatment at 300.degree. C., FIG. 13C shows
that of a forged article having been subjected to a heat treatment
at 350.degree. C. and FIG. 13D shows that of a forged article
having been subjected to a heat treatment at 400.degree. C. FIGS.
14A through 14D show the microstructures of forged articles made
from the alloy C respectively corresponding to FIGS. 13A through
13D. According to FIGS. 13A through 13D and 14A through 14D, it can
be confirmed that crystal grains are coarse in those having been
subjected to the T6 heat treatment (namely, those shown in FIGS.
13A and 14A) and also segregation (shown as black areas in the
drawing) of a compound (Mg.sub.17Al.sub.12) is observed in the
forged article made from the alloy B. In contrast, in those having
been subjected to the post-forging heat treatments at a higher
temperature for a shorter time than in the T6 heat treatment, when
the processing temperature is 300.degree. C. (namely, in those
shown in FIGS. 13B and 14B), no clear grain boundary is observed
and a compound is homogeneously precipitated. When the processing
temperature is 350.degree. C. (namely, in those shown in FIGS. 13C
and 14C), fine grain boundaries are observed and the compound is
homogenously precipitated. When the processing temperature is
400.degree. C. (namely, in those shown in FIGS. 13D and 14D),
crystal grains are coarse and the compound is homogeneously
precipitated.
[0096] Judging from these results of the tensile test and the
microstructure observation, the microstructure of a forged article
obtained after a post-forging heat treatment is probably concerned
in the ductility. Specifically, a structure that is not
recrystallized is difficult to deform and hence has high tensile
strength but low ductility. As crystal grains are more deformed
through recrystallization, the ductility becomes higher, but when
the crystal grains are too large, they are difficult to deform and
hence are stiffened to degrade in the tensile strength and the
ductility.
[0097] Accordingly, the post-forging heat treatment should be
performed with a processing temperature properly selected so as to
attain a microstructure where no crystal grains are observed in
order to obtain a forged article with high tensile strength, and
with a processing temperature properly selected so as to attain a
microstructure where fine crystal grains are observed in order to
obtain a forged article with high ductility.
[0098] (Experiment 3)
[0099] <Experiment Method>
[0100] In the same manner as in Experiment 2, several articles for
forging each in the form of a block with a width of 10 mm, a length
of 35 mm and a thickness of 21 mm were prepared from each of the
alloys B and C. The articles made from the alloy B were subjected
to a pre-forging heat treatment at 410.degree. C. for 16 hours, and
the articles made from the alloy C were subjected to a pre-forging
heat treatment at 400.degree. C. for 10 hours.
[0101] Each of the articles for forging having been subjected to
the pre-forging heat treatment was forged, with its width direction
constricted, until its thickness was reduced by half from 21 mm to
10.5 mm (namely, with the forging working rate of 50%), to give a
forged article.
[0102] A post-forging heat treatment was carried out for 1 hour, 4
hours, 10 hours or 15 hours at a temperature of 300.degree. C. on
the forged articles made from the alloy B and at a temperature of
350.degree. C. on the forged articles made from the alloy C, to
give final forged articles.
[0103] A tensile test was carried out on each of the thus obtained
forged articles.
[0104] <Experiment Results>
[0105] FIG. 15 shows the relationship between the processing time
of the post-forging heat treatment and the 0.2% yield strength, the
tensile strength and the elongation after fracture obtained in the
forged articles made from the alloy B, and FIG. 16 shows the
relationship corresponding to that of FIG. 15 obtained in the
forged articles made from the alloy C. Data with the processing
time of 0 (zero) correspond to data obtained in forged articles not
subjected to the post-forging heat treatment in Experiment 2.
According to FIGS. 15 and 16, it is understood that the 0.2% yield
strength is comparatively largely lowered when the processing time
is shorter than 1 hour but is slowly lowered when the processing
time is longer in the forged articles made from both the alloys B
and C. Also, the tensile strength is slightly improved when the
processing time is shorter than 1 hour but is slowly lowered when
the processing time is longer. On the other hand, the elongation
after fracture is largely improved when the processing time is
shorter than 1 hour but is substantially constant when the
processing time is longer in the forged article made from the alloy
B while it is largest when the processing time is 1 hour but is
lowered when the processing time is longer in the forged article
made from the alloy C. Accordingly, in the forged articles made
from both the alloys B and C, the elongation after fracture can be
largely improved during first 1 hour after the start of the heat
treatment, and in the forged article made from the alloy C, the
elongation after fracture can be largely improved when the
processing time is 10 hours or less (preferably 5 hours or
less).
INDUSTRIAL APPLICABILITY
[0106] As described so far, the present invention is useful for a
manufacturing method for a plastic worked article in which an
article for plastic working shaped by the semisolid molding is
plastic worked after causing blisters by expanding gas cavities
included in the article for plastic working through a pre-plastic
working heat treatment.
* * * * *