U.S. patent application number 10/888447 was filed with the patent office on 2005-02-17 for pressure casting method of magnesium alloy and metal products thereof.
Invention is credited to Motegi, Tetsuichi, Takayama, Kazutoshi, Takizawa, Kiyoto.
Application Number | 20050034837 10/888447 |
Document ID | / |
Family ID | 34131352 |
Filed Date | 2005-02-17 |
United States Patent
Application |
20050034837 |
Kind Code |
A1 |
Motegi, Tetsuichi ; et
al. |
February 17, 2005 |
Pressure casting method of magnesium alloy and metal products
thereof
Abstract
The present invention provides a pressure casting method of a
magnesium alloy. In the method, a molten magnesium alloy is cooled
to form a partially molten state containing a solid-phase, and the
partially molten state is further cooled to form a solid-phase
granularly crystallized solid material. The solid material is
partially-melted and pressure cast into a mold by a molding
machine. A ratio of primary crystals in said solid material is set
to 55 to 65%. The solid material is partially-melted in a
solid-phase and liquid-phase coexisting state at a selected heating
temperature so that a semi-solid having a thixotropic properties
and having the size of a main solid phase of 50 to 250 .mu.m and a
solid-phase ratio of 30 to 70% is formed. The semi-solid is
pressure cast into a mold through a nozzle while maintaining the
semi-solid state to form metal products having a ratio of primary
crystals of 20 to 50%.
Inventors: |
Motegi, Tetsuichi;
(Chiba-ken, JP) ; Takayama, Kazutoshi;
(Nagano-ken, JP) ; Takizawa, Kiyoto; (Nagano-ken,
JP) |
Correspondence
Address: |
WEINGARTEN, SCHURGIN, GAGNEBIN & LEBOVICI LLP
TEN POST OFFICE SQUARE
BOSTON
MA
02109
US
|
Family ID: |
34131352 |
Appl. No.: |
10/888447 |
Filed: |
July 9, 2004 |
Current U.S.
Class: |
164/113 ;
164/900 |
Current CPC
Class: |
Y10S 164/90 20130101;
B22D 17/007 20130101 |
Class at
Publication: |
164/113 ;
164/900 |
International
Class: |
B22D 017/08; B22D
023/00; B22D 025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 2003 |
JP |
2003-195948 |
Claims
1. A pressure casting method of a magnesium alloy comprising the
steps of cooling a molten magnesium alloy to form a partially
molten state containing a solid-phase, further cooling the
partially molten state to form a solid-phase granularly
crystallized solid material and partially-melting the solid
material to pressure cast it into a mold by a molding machine,
wherein a ratio of primary crystals of said solid material is set
to 55 to 65%, the solid material is melted to be a semi-solid body
having a thixotropic properties in a solid-phase and liquid-phase
coexisting state so that the semi-solid having the size of a main
solid phase of 50 to 250 .mu.m and a solid-phase ratio of 30 to
70%, and pressure casting the semi-solid into a mold through a
nozzle while maintaining the semi-solid state to mold metal
products having a ratio of primary crystals of 20 to 50%.
2. The pressure casting method of a magnesium alloy according to
claim 1, wherein the temperature of a heating means in a molding
machine for holding the semi-solid in said semi-solid state is set
at a temperature 5 to 15.degree. C. higher than the temperature of
the semi-solid in accordance with time from the starting of melting
of the solid material to the pressure casting of the
semi-solid.
3. The pressure casting method of a magnesium alloy according to
claim 1, wherein said semi-solid is pressure cast from a nozzle
having a diameter of 8 to 15 mm into a mold through a gate having a
thickness of 1 mm or less.
4. A metal product of a magnesium alloy molded by the pressure
casting method of a magnesium alloy according to claim 1,
comprising a metal structure whose main primary crystal is
spherical and has a diameter of 10 .mu.pm or more.
5. The metal product of a magnesium alloy according to claim 4,
wherein said metal product has a wall thickness of 0.4 to 1.5 mm,
preferably 0.6 to 1.0 mm.
6. A metal product of a magnesium alloy molded by the pressure
casting method of a magnesium alloy according to claim 2,
comprising a metal structure whose main primary crystal is
spherical and has a diameter of 10 .mu.m or more.
7. A metal product of a magnesium alloy molded by the pressure
casting method of a magnesium alloy according to claim 3,
comprising a metal structure whose main primary crystal is
spherical and has a diameter of 10 .mu.m or more.
8. The metal product of a magnesium alloy according to claim 6,
wherein said metal product has a wall thickness of 0.4 to 1.5 mm,
preferably 0.6 to 1.0 mm.
9. The metal product of a magnesium alloy according to claim 7,
wherein said metal product has a wall thickness of 0.4 to 1.5 mm,
preferably 0.6 to 1.0 mm.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a casting method of using a
granularly crystallized magnesium alloy solid as a casting
materials, melting the solid material to a solid- phase and
liquid-phase coexisting state to form metal products by a pressure
casting means and relates to their metal products.
[0003] 2. Description of the Related Art
[0004] In a conventional manufacturing method for a product from a
partially molten metal, a molten alloy is held in a solid-phase and
liquid-phase coexisting state in an insulating vessel for a desired
time, generating many fine spherical primary crystals and pressure
casting them in a mold of a die cast machine at a desired liquid
phase ratio to form cast products (see for example Patent Reference
1).
[0005] Further, in molding of metal products using a magnesium
alloy, a solid material potentially holding thixotropy is heated in
a partially molten state and the obtained material is charged into
a mold by an injection apparatus (see for example Patent Reference
2).
[0006] [Patent Reference 1]
[0007] Japanese A-HEI 9-10893 (on pages 3 to 5, FIG. 9)
[0008] [Patent Reference 2]
[0009] JP-A-2001-252759 (on pages 6 and 7, FIG. 1)
[0010] The above-mentioned patent reference 1 describes a pressure
casting method of cast products by a die cast machine including the
steps of pouring a molten alloy into a tilt cooling jig held at a
temperature lower than the melting point to flow down and holding
the alloy in an insulating vessel in a state at a temperature equal
to or lower than a liquidus line temperature and higher than a
eutectic temperature or a solidus line temperature for five seconds
to 60 minutes to form the cast products in a liquid phase ratio of
20 to 90%, preferably 30 to 70%.
[0011] Further, the Patent Reference 2 describes a casting method
including the steps of allowing a molten magnesium alloy to flow on
a cooling tilt plate to cool in a partially molten, reserving the
alloy in a reservoir until it becomes a metal slurry having fine
spherical crystals, then solidifying the slurry by rapid cooling to
form a metal material potentially holding a thixotropy performance,
and melting the metal material into a partially molten state
magnesium alloy exhibiting a thixotropy performance to charge into
a mold by an injection apparatus.
[0012] In the prior art described in Patent Reference 1, after a
molten alloy is cooled in a partially molten state the obtained
alloy must be held in an insulating vessel until it has a desired
liquid phase ratio. Thus it takes much time from the melting of the
material to pressure casting of products. In order to shorten the
time, many insulating vessels and their transfer means are
required. Further, since the material is cooled close to lower
temperature close to the casting temperature and is transferred to
a molding machine to conduct molding immediately, some molding
machines have a problem, which cannot be adopted.
[0013] Even in the prior art described in Patent Reference 2, since
the solid phase ratio of a partially molten slurry is high, so much
time is also needed until holding the thixotropy performance
potentially. However, the molding steps of remelting a rapid
cooling solidified metal material in a partially molten state by a
molding machine and pressure casting the obtained material into a
mold in a state having a thixotropic properties can be completed
for short time. Further, the supply of the metal material into the
molding machine is also easy and continuous casting is also
possible, whereby the prior art has adaptability to the casting
machine.
[0014] However, in Patent Reference 2, it is difficult to set
temperature conditions and holding time for metal slurry crystals
crystallized out at a solid-phase and liquid-phase coexisting
temperature region to be uniform spherical crystals, and there is a
problem in maintaining a solid-phase ratio, which is preferable in
molding. The present inventors have studied these problems. As a
result they have found that even if crystals do not become uniform
spherical ones, if a primary crystal ratio in which a solid
material is granularly crystallized is within a certain range, the
primary crystal becomes a sphered solid phase in a solid-phase and
a liquid-phase coexisting state, and at the same time a main solid
phase has a solid-phase ratio preferable for casting in a grain
size of 50 .mu.m or more, and that if the holding time is within 30
minutes, the material is pressure cast into molds without changing
set conditions so that a number of metal products of a magnesium
alloy extremely excellent in a distribution state of primary
crystals can be formed.
SUMMARY OF THE INVENTION
[0015] In view of the above problems, the object of the present
invention is to provide a new pressure casting method of a
magnesium alloy comprising steps of melting previously granularly
crystallized solid material to form a partially molten state
(hereinafter referred to as a "semi-solid") having a thixotropic
properties in a solid-phase and liquid-phase coexisting state
(hereinafter referred to as a "semi-solid") and pressure casting
the semi-solid into a mold, wherein the grain diameter and
solid-phase ratio in a solid phase in the semi-solid are set to a
preferable state for molding so that metal products having an
excellent metal structure can be formed stably.
[0016] This invention comprises the steps of cooling a molten
magnesium alloy to form a partially molten state containing a
solid-phase, further cooling the partially molten state to form a
solid-phase granularly crystallized solid material and
partially-melting the solid material to pressure cast it into a
mold by a molding machine, wherein a ratio of primary crystals of
said solid material is set to 55 to 65%, the solid material is
melted to be a semi-solid body in a solid-phase and liquid-phase
coexisting state so that the semi-solid having a main solid phase
of 50 to 250 .mu.m and a solid-phase ratio of 30 to 70%, and
pressure casting the semi-solid into a mold through a nozzle while
maintaining the semi-solid state to form metal products having a
ratio of primary crystals of 20 to 50%.
[0017] Further, in this invention, the temperature of a heating
means in a casting machine for holding the semi-solid in the
semi-solid state is set at a temperature 5 to 15.degree. C. higher
than the temperature of the semi-solid in accordance with the time
from the starting of melting of the solid material to the pressure
casting of the semi-solid. In addition, the semi-solid is pressure
cast from a nozzle having a diameter of 8 to 15 mm into a mold
through a gate having a thickness of 1 mm or less.
[0018] The metal product of this invention is a metal product
molded by the above-mentioned pressure cast molding method of a
magnesium alloy which comprises a metal structure whose main
primary crystal is spherical and has a diameter of 10 .mu.m or
more, and has a wall thickness of 0.4 to 1.5 mm, preferably 0.6 to
1.0 mm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1(A) is a view of a structure by a metallurgical
micrograph of a magnesium alloy (AZ91D) used in a pressure casting
molding method according to this invention, and FIG. 1(B) is a view
in which the micrograph is two-valued in black and white by an
image processing;
[0020] FIG. 2(A) is a view of a structure by a metallurgical
micrograph of a solid obtained by rapidly cooling a semi-solid in a
solid-phase and liquid-phase coexisting state (570.degree. C.) and
FIG. 2(B) is a view in which the micrograph is two-valued in black
and white by an image processing;
[0021] FIG. 3(A) is a view of a structure by a metallurgical
micrograph of a solid obtained by rapidly cooling the semi-solid
after holding it for 30 minutes in a solid-phase and liquid-phase
coexisting state (570.degree. C.), and FIG. 3(B) is a view in which
the micrograph was two-valued in black and white by an image
processing;
[0022] FIG. 4(A) is a view of a structure by a metallurgical
micrograph of a solid obtained by rapidly cooling a semi-solid in a
solid-phase and liquid-phase coexisting state (590.degree. C.) and
FIG. 4(B) is a view which the micrograph is two-valued in black and
white by an image processing;
[0023] FIG. 5(A) is a view of a structure by a metallurgical
micrograph of a solid obtained by rapidly cooling the semi-solid
after holding the semi-solid for 30 minutes in a solid-phase and
liquid-phase coexisting state (590.degree. C.), and FIG. 5(B) is a
view in which the micrograph is two-valued in black and white by an
image processing;
[0024] FIG. 6(A) is a view of a structure by a metallurgical
micrograph of a metal product molded of a semi-solid in a
solid-phase and liquid-phase coexisting state (580.degree. C.,
holding time of 25 minutes), and FIG. 6(B) is a view in which the
micrograph is two-valued in black and white by an image
processing;
[0025] FIG. 7(A) is a view of a structure by a metallurgical
micrograph of a metal product molded of a semi-solid in a
solid-phase and liquid-phase coexisting state (585.degree. C.,
holding time of 25 minutes), and FIG. 7(B) is a view in which the
micrograph is two-valued in black and white by an image
processing;
[0026] FIG. 8(A) is a view of a structure by a metallurgical
micrograph of a metal product molded of a semi-solid in a
solid-phase and liquid-phase coexisting state (590.degree. C.,
holding time of 25 minutes), and FIG. 8(B) is a view in which the
micrograph is two-valued in black and white by an image processing;
and
[0027] FIG. 9(A) is a view of a structure by a metallurgical
micrograph of a metal product molded of a semi-solid in a
solid-phase and liquid-phase coexisting state (595.degree. C.,
holding time of 25 minutes), and FIG. 9(B) is a view in which the
micrograph is two-valued in black and white by an image
processing.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] In each Figure, (A) is a photograph taken by a metallurgical
microscope and (B) is a view in which a part of the photograph is
two-valued into black and white by image processing and the grain
diameter of a solid phase and the solid phase ratio or a primary
crystal ratio are calculated from the dot number of the white and
the black dots.
[0029] FIG. 1 shows a metal structure of a magnesium alloy (AZ91D)
used as a solid material in this invention. The solid material of
the granular crystal structure is manufactured by the steps of
heating and melting a magnesium alloy at a temperature higher than
the liquidus line temperature, allowing the molten alloy to flow
down on a surface of a cooled (for example 60.degree. C.) tilt
plate, cooling the alloy to a temperature at which a solid-phase
and liquid-phase below the liquidus line temperature coexist
(hereinafter referred to as a solid-phase and liquid-phase
coexisting temperature) to produce a solid phase, then holding the
solid phase and liquid phase coexisting temperature for desired
time until the solid phase ratio reaches 55 to 65% a primary
crystal a of a solid material, and cooling the alloy below the
solidus line temperature. As one example, the solid material is
manufactured by cooling a magnesium molten alloy of 605.degree. C.
to a temperature 5.degree. C. to 25.degree. C. lower than
595.degree. C. (the liquidus line temperature), holding the
temperature in its range for one minute and then rapidly cooling
the alloy below the solidus line temperature.
[0030] As the use form of the solid materials any type of the solid
material such as a round bar, ingot or the like, and of a granular
solid material such as a chip, pellet or the like may be selected.
The material form can be optionally selected by a structure of the
metal casting machine used. Further, as the metal casting machine,
a metal casting machine having the same structure as an inline
screw injection molding machine, a plunger injection molding
machine, a (pre-plasticizing system) injection molding machine or
the like, or a molding machine, which can pressure cast a
semi-solid charged into a cylinder into a mold from a nozzle
through a gate, such as a die cast machine or the like, may be all
adopted.
[0031] The molding of metal products of the above-mentioned solid
material is performed as follows. First, a solid material is made a
semi-solid in a solid-phase and liquid-phase state at a selected
melting temperature. Then the temperature of the semi-solid is held
at the liquidus line temperature or less and the solidus line
temperature or more to maintain a solid-phase and liquid-phase
coexisting state. After that the semi-solid is pressure cast into a
mold from a nozzle through a gate.
[0032] In the above-mentioned melting step, when a solid material
reaches the solidus line temperature or more, eutectic crystals b
in the metal structure are melted to be semi-solid liquid phases
b'. And a primary crystal a is dispersed into the liquid-phase as a
solid-phase a'. Alternatively, corners of the primary crystal a,
which are liable to be influenced by heating, are melted to become
sphered solid phases.
[0033] The size (grain diameter) of this main solid phase a' and
the solid-phase ratio of the semi-solid are changed in ranges of 50
to 250 .mu.m and of 25 to 75%, respectively, by the melting
temperature of the solid material and the holding temperature and
time for the semi-solid. If the solid material has a solid phase a'
of a size in the range (most preferably 50 to 100 .mu.m and, the
average grain diameter of 80 .mu.m) and a solid phase ratio of the
range (preferably 30 to 70%), pressure casting into a mold can be
carried out without any trouble while maintaining thixotropy
performance (viscous fluid performance).
[0034] FIG. 2 is a metal structure of a solid obtained by rapid
cooling a semi-solid produced when a solid material having a ratio
of a primary crystal a of 61% and a grain diameter of 50 to 100
.mu.m shown in FIG. 1 is melted at 570.degree. C., without holding
time. The primary crystal a of this solid material before melting
has a solid phase a' of 100 to 200 .mu.m by melting so that the
solid phase ratio is increased to 64%. Further, even a metal
structure of a solid rapidly cooled after holding a semi-solid at a
melting temperature of 570.degree. C. for 30 minutes has no extreme
enlargement of the solid phase a' due to a lapse of time and grows
generally, as shown in FIG. 3. However, the main solid phase a'
reaches a size of 150 to 250 .mu.m and the solid phase ratio is
increased to 69% at most.
[0035] FIG. 4 is a metal structure of a solid obtained by rapid
cooling a semi-solid produced when a solid material is melted at
590.degree. C., and the sizes of solid phases a' are various, but
in a range of 100 to 200 .mu.m and the solid phase ratio is 48%.
Further in a metal structure of a solid rapidly cooled after the
semi-solid is held at a temperature of 590.degree. C. for 30
minutes, as shown in FIG. 5, a solid-phase ratio is remarkably
increased to 65%, but some sizes of solid phases a' reach 50 to 250
.mu.m in the grain diameter, which is smaller than before melting.
This reason is guessed to be that since the holding temperature is
close to the liquidus line temperature (595.degree. C.), a small
solid phase, which is liable to be thermally influenced, is
partially melted so that the size of the diameter is decreased.
[0036] In the above-mentioned semi-solids at the temperatures of
570.degree. C. and 590.degree. C., the almost solid phases a' are
sphered and the size of the solid phase and solid-phase ratio are
further increased as compared with a solid material. Further, the
solid-phase ratios during melting are 48% at 590.degree. C., and
64% at 570.degree. C. That is when the temperature is high, melted
parts are increased, resulting in a decreased solid-phase ratio.
However, all solid-phase ratios of the solid materials held for 30
minutes do not exceed 70% and the sizes of the solid phases a' are
in a range of 50 to 200 .mu.m. This means that if the semi-solid
maintains a solid-phase and liquid-phase coexisting state, it can
be pressure cast into a mold at the same set conditions in a state
having thixotropy performance until at least 30 minutes have
passed.
[0037] Further, a temperature of a heating means in a molding
machine for maintaining the material at a semi-solid state can be
set at about 5 to 15.degree. C. higher than the temperature of a
semi-solid in accordance with time from the starting of melting of
the solid material to the pressure casting of the semi-solid. It is
only that a solid phase and liquid phase coexisting state is
maintained until the pressure casting whereby a state where the
thixotropy performance is produced, exists.
[0038] In a semi-solid having a low solid phase ratio of 25% or
less, even if the semi-solid is in a solid phase and liquid phase
coexisting state, the liquid phase ratio is too much and the
fluidity is increased. Thus, the semi-solid does not have
appropriate thixotropy performance or a shortage of material
resistance necessary for pressure casting, whereby the molding of
the semi-solid becomes unstable. As a result the molding of metal
products cannot be carried out. On the other hand, in a semi-solid
having a solid-phase ratio higher than 75%, the thixotropy
performance due to the intervening of liquid phases therein is lost
and it becomes extremely difficult to pressure cast the semi-solid
into a mold through a nozzle. However, when the solid phase ratio
of the semi-solid is in a range of 30 to 70%, pressure casting can
be easily carried out at thixotropy performance although having a
difference in itself between the upper limit and the lower
limit.
[0039] It is preferred that the pressure casting of the semi-solid
into a mold is carried out by use of a nozzle having a diameter of
8 to 15 mm and a gate having a thickness of 1 mm or less. If this
nozzle diameter and the gate thickness are used, the semi-solid is
liable to receive shearing force at the time when it passes through
the limited nozzle and gate. Thus the solid-phases a' are
subdivided and a metal structure having a small bias in the
distribution of primary crystals a" in a metal product can be
formed.
[0040] FIGS. 6 to 9 are views of structures of metal products
obtained by the steps of temperature-holding (for 25 minutes)
semi-solids while providing a difference in temperature of
5.degree. C. from 580.degree. C. to 595.degree. C. after setting
the temperature of a heating means in a molding machine at
5.degree. C. higher than the temperature of the semi-solids, and
pressure casting each of the obtained semi-solids into a mold from
a nozzle having a diameter of 8 mm through a gate having a
thickness of 0.5 mm to form the metal product. In the respective
structures of the semi-solids used in the molding of the metal
products of FIGS. 6 to 9, the size (grain diameter) of the main
solid phase is 50 .mu.m or more although omitted in the
Figures.
[0041] In the metal product molded of said semi-solid, as apparent
from the view of the structure, the main primary crystal a" is
sphered in the size of 10 .mu.m or more, and the distribution state
of the crystals are uniformly dispersed in eutectic crystals b" as
a whole. The ratio of the primary crystals a" in the metal product
is increased from 46% to 50% as the temperature of the semi-solid
is increased from 580.degree. C. to 590.degree. C. However, the
primary crystal a" of a metal product obtained from a semi-solid of
590.degree. C. which is thought to be a liquidus line temperature
is fined by melting and shearing so that the entire grain diameter
of the crystal is decreased. However the size of the main primary
crystal a" is 10 um or more and the ratio of the crystal maintains
28%.
[0042] As described above, in a magnesium metal product having the
primary crystal a" ratio of 20 to 50% and the size of the main
primary crystal a" of 10 .mu.m or more, a test piece of the product
thickness of 0.8 mm can obtain effects of increases in 60 % in
elongation, 20% in hardness, 30% in tensile strength and the like
in comparison with a magnesium metal product molded of a molten
magnesium alloy perfectly melted at a temperature of 620.degree. C.
or more, and the magnesium metal product facilitates mechanical
working such as pressing, cutting and the like. Further, according
to this invention, the distribution of primary crystals in a metal
product are more uniform than that of a metal product obtained by a
conventional thixo-molding method in which a molten material is
cooled to a solid-phase and liquid-phase coexisting temperature,
and is agitation sheared by screw rotation, and then the obtained
material is pressure cast into a mold. Thus strength of the
magnesium metal product of this invention becomes more
excellent.
* * * * *