U.S. patent application number 10/829953 was filed with the patent office on 2004-12-30 for method for grain refinement of magnesium alloy castings.
This patent application is currently assigned to Tetsuichi Motegi. Invention is credited to Kikuchi, Tetsuo, Motegi, Tetsuichi, Yoshihara, Kiyotaka.
Application Number | 20040261576 10/829953 |
Document ID | / |
Family ID | 32959722 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040261576 |
Kind Code |
A1 |
Motegi, Tetsuichi ; et
al. |
December 30, 2004 |
Method for grain refinement of magnesium alloy castings
Abstract
A method for grain refinement of magnesium alloy castings
includes adding graphite (C) powder and manganese dioxide
(MnO.sub.2) to a melt of magnesium alloy containing aluminum (Al)
and manganese (Mn) to finely divide crystal grains of a cast
structure.
Inventors: |
Motegi, Tetsuichi;
(Funabashi-shi, JP) ; Yoshihara, Kiyotaka;
(Fukushima-shi, JP) ; Kikuchi, Tetsuo; (Yuki-shi,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Tetsuichi Motegi
Funabashi-shi
JP
JP
Kiyotaka Yoshiwara
Fukushima-shi
JP
Seiko Idea Center Co., Ltd.
Tokyo
JP
|
Family ID: |
32959722 |
Appl. No.: |
10/829953 |
Filed: |
April 23, 2004 |
Current U.S.
Class: |
75/604 |
Current CPC
Class: |
C22C 1/03 20130101; C22C
1/02 20130101; C22B 26/22 20130101; C22C 23/02 20130101; C22B 9/103
20130101 |
Class at
Publication: |
075/604 |
International
Class: |
C22B 026/22 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2003 |
JP |
2003-122730 |
Claims
What is claimed is:
1. A method for grain refinement of magnesium alloy casting,
comprising adding graphite (C) powder and manganese dioxide
(MnO.sub.2) to a melt of magnesium alloy containing aluminum (Al)
and manganese (Mn) to refine crystal grains of a cast
structure.
2. The method according to claim 1, wherein said graphite (C)
powder and said manganese dioxide (MnO.sub.2) are accommodated in a
metallic capsule.
3. The method according to claim 2, wherein said metallic capsule
is formed of pure aluminum or aluminum alloy.
4. The method according to claim 2, wherein said metallic capsule
is formed of pure magnesium or magnesium alloy.
5. The method according to claim 1, wherein said manganese dioxide
(MnO.sub.2) has an amount in the range of 0.10 mass % to 0.22 mass
% based on a mass of the melt of said magnesium alloy.
6. The method according to claim 2, wherein said manganese dioxide
(MnO.sub.2) has an amount in the range of 0.10 mass % to 0.22 mass
% based on a mass of the melt of said magnesium alloy.
7. The method according to claim 3, wherein said manganese dioxide
(MnO.sub.2) has an amount in the range of 0.10 mass % to 0.22 mass
% based on a mass of the melt of said magnesium alloy.
8. The method according to claim 4, wherein said manganese dioxide
(MnO.sub.2) has an amount in the range of 0.10 mass % to 0.22 mass
% based on a mass of the melt of said magnesium alloy.
9. The method according to claim 1, wherein said manganese dioxide
(MnO.sub.2) has an amount in the range of 0.20 mass % to 0.22 mass
% based on a mass of the melt of said magnesium alloy.
10. The method according to claim 2, wherein said manganese dioxide
(MnO.sub.2) has an amount in the range of 0.20 mass % to 0.22 mass
% based on a mass of the melt of said magnesium alloy.
11. The method according to claim 3, wherein said manganese dioxide
(MnO.sub.2) has an amount in the range of 0.20 mass % to 0.22 mass
% based on a mass of the melt of said magnesium alloy.
12. The method according to claim 4, wherein said manganese dioxide
(MnO.sub.2) has an amount in the range of 0.20 mass % to 0.22 mass
% based on a mass of the melt of said magnesium alloy.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a method for grain refinement of
magnesium alloy castings, which method is capable of grain
refinement of solidified structure of the cast product and
improving the mechanical properties thereof without generating
dioxin.
[0003] 2. Description of the Prior Art
[0004] The method for the grain refinement of aluminum
(Al)-containing magnesium alloy, such as the AZ type, is known in
two kinds, namely a method that shuns addition of a grain refiner
and a method that necessitates addition of the grain refiner.
[0005] The former method includes a method of superheat-treatment
that comprises superheating he whole of molten alloy to about
850.degree. C. to 900.degree. C. (1123K to 1173K), holding it for
about 5 minutes to 15 minutes (0.3 ks to 0.9 ks), cooling it at a
rate of 150.degree. C./min or more, and thereafter quenching it at
the casting temperature. The mechanism of grain refinement of the
superheat-treatment is considered to nucleate an Al--MN--(Fe)
compound. Since this method of superheat-treatment requires an
inevitably high treating temperature, however, it incurs undue cost
of energy, boost the expense for preventing the molten alloy from
oxidation and performing works of maintenance and inspection on the
crucible, and entails the problem of imperiling economy and
safety.
[0006] The latter method includes carbon addition that comprises
adding a carbon (C)-containing compound at a temperature in the
neighborhood of 750.degree. C. to the molten alloy. The mechanism
of grain refinement by carbon addition is considered to nucleate
aluminum carbide (Al.sub.4C.sub.3) resulting from the reaction of
carbon in a compound with aluminum in the molten alloy.
Commercially, the practice of adding hexachloroethane
(C.sub.2Cl.sub.6) as the grain refiner has prevailed. Since the
hexachloroethane induces generation of dioxins
{2,3,7,8-tetrachloro-dibenzo-p-dioxin:
Cl.sub.2(C.sub.6H.sub.2)O.sub.2(C.- sub.6H.sub.2)Cl.sub.2}, the use
thereof is now prohibited.
[0007] The latter method further includes a method of Carbon
Addition by Blowing that comprises directly blowing a graphite (C)
powder together with argon gas (Ar) into the molten alloy (refer,
for example, to JP-A 2003-41331). This method of Carbon Addition by
Blowing is capable of grain refinement of the solidified structure
without inducing generation of dioxins.
[0008] The carbon that is added by blowing, however, can promote
the grain refinement of the solidified structure proportionately to
the rise of the temperature of the molten alloy. Though the method
of Carbon Addition by Blowing indeed defies generation of dioxins,
it requires heightening the temperature of treatment for the
purpose of promoting the grain refinement of the solidified
structure and entails the problem of incurring the cost of energy.
This invention aims to provide a method for fine grain refinement
of magnesium alloy castings while encouraging decrease of the cost
of energy, promoting the grain refinement of the solidified
structure and improving the mechanical properties of the magnesium
alloy castings without inducing generation of dioxins.
SUMMARY OF THE INVENTION
[0009] For the sake of accomplishing the object described above,
the method contemplated by this invention for fine gain refinement
of the magnesium alloy castings comprises adding a graphite (C)
powder and manganese dioxide (MnO.sub.2) into a melt of magnesium
alloy containing aluminum (Al) and manganese (Mn), thereby enabling
crystal grains of a cast structure to be refined finely.
[0010] In the method for fine grain refinement of the magnesium
alloy castings, the addition of a graphite (C) powder and manganese
dioxide (MnO.sub.2) embraces the case of them being added as
contained in a metallic capsule formed of pure aluminum, aluminum
alloys, pure magnesium or magnesium alloys.
[0011] The amount of the manganese dioxide ((MnO.sub.2) to be added
is in the range of 0.10 mass % to 0.22 mass %, preferably in the
range of 0.20 mass % to 0.22 mass %, based on the mass of the
molten magnesium alloy mentioned above.
[0012] According to this invention, since a graphite powder and
manganese dioxide are added to the molten magnesium ally, the
temperature of the molten alloy can be elevated by the reaction of
oxidation-reduction with the manganese dioxide. Since both the
addition of hexachloroethane and the generation of dioxins are
absent, the cost of energy can be decreased, the grain refinement
of a crystal can be promoted, and the mechanical properties of the
cast article of magnesium alloy can be improved. Then, since the
graphite powder and the manganese dioxide are allowed to be
contained in a metallic capsule, the graphite powder and the
manganese dioxide can be prevented from being scattered and can be
utilized efficiently in small amounts for the grain refinement
Further, since the metallic capsule is made of aluminum alloy or
magnesium alloy, they can participate in effecting the grain
refinement without adversely affecting the components of the
composition of the alloy. Since manganese dioxide is added in an
amount in the range of 0.10 mass % to 0.22 mass % based on the mass
of the molten magnesium alloys, it enables the crystal gains to be
finely divided to an average particle diameter of 170 .mu.m or
less. Since manganese dioxide is added in an amount in the range of
0.20 mass % to 0.22 mass % based on the mass of the molten
magnesium alloys, it enables the grain refinement to be effected to
the same degree as the addition of hexachloroethane.
BRIEF EXPLANATION OF THE DRAWING
[0013] FIG. 1 shows a schematic illustration of an apparatus used
in the method for fine grain refinement according to this
invention.
[0014] FIG. 2 shows a press diagram illustrating a procedure of
this invention, starting with melting alloy and finishing with
casting the molten alloy in a metallic mold.
[0015] FIG. 3 is a diagram illustrating changes of the temperature
in a phosphorizer and the temperature of the melt caused by the
addition of manganese dioxide.
[0016] FIG. 4 is a copy of optical microphotogaphs showing the
solidified structure of an untreated material and the solidified
structure of a manganese dioxide-added treated material resulting
from adding manganese dioxide in an amount of 0.20 mass % based on
the mass of molten alloy.
[0017] FIG. 5 shows a copy of optical microphotographs showing the
solidified structure of a hexachloroethane-added treated material,
that of an untreated material and that of a capsule-added treated
material, respectively.
[0018] FIG. 6 shows a copy of optical microphotographs showing the
solidified structures of graphite-added treated materials having
the amount of added graphite sequentially increased
[0019] FIG. 7 shows a characteristic diagram showing the relation
between the amount of graphite added and the average particle
diameter of crystal grains.
[0020] FIG. 8 shows a copy of optical microphotographs showing the
solidified structures of various complex-added treated materials
having the amount of added graphite varied.
[0021] FIG. 9 shows a characteristic diagram showing the relation
between various complex-added materials and the average particle
diameters of crystal grains.
[0022] FIG. 10 shows a copy of optical microphotographs showing the
solidified structures of various complex-added treated materials
having the amount of added MnO.sub.2 varied.
[0023] FIG. 11 shows a characteristic diagram showing the relation
be various complex-added materials and average particle diameters
of crystal grains.
[0024] FIG. 12 shows a copy of an optical microphotograph showing
the solidified structure of a complex-added treated material using
a graphite powder having a diameter on the order of nm.
[0025] FIG. 13 shows a characteristic diagram showing tensile
strength, 0.2% proof strength (MPa) and elongation (%) in the F
specimen of an untreated material, a manganese dioxide-added
treated material, a hexachloroethane-added treated material and a
complex-added treated material.
[0026] FIG. 14 shows a characteristic diagram showing tensile
strength, 0.2% proof strength (MPa) and elongation (%) in the T4
specimen of an untreated material, a manganese dioxide-added
treated material, a hexachloroethane-added treated material and a
complex-added treated material.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0027] The method according to this invention for fine grain
refinement of the magnesium alloy castings comprises adding a
graphite (C) powder and manganese dioxide (MnO.sub.2) to a melt of
magnesium alloy containing aluminum (Al) and manganese (Mn) to
enable crystals of a cast structure to be finely refined The
magnesium alloy containing aluminum and manganese does not need to
be particularly restricted with respect to its composition. The
AZ9E alloy that contains aluminum in the composition and manganese
as an impurity is used also for sand casting as will be described
specifically herein below may be used as such magnesium alloy.
[0028] The graphite powder to be used in this invention does not
need to be particularly restricted to the graphite 5 .mu.m in
particle diameter mentioned in an example cited herein below.
Commendably, it has the smallest possible diameter. Good results
are obtained by using a graphite powder having a diameter of the
order of nanometer (nm), for example.
[0029] The manganese dioxide to be used in this invention is
depicted as a powder 100 .mu.m in particle diameter in an example
to be cited herein below, it may be formed massively (such as in
the shape of tablets and pellets). The amount (mass %) of the
manganese dioxide to be added falls preferably in the range of 0.10
mass % to 0.22 mass % and more preferably in the range of 0.20 mass
% to 0.22 mass % eased on the amount (mass) of the melt of
magnesium alloy. If the amount of manganese dioxide added falls
short of 0.10 mass %, the shortage will result in preventing the
effect of grain refinement from being manifested fully
satisfactory. If the amount of manganese dioxide added exceeds 0.22
mass %, the excess will not bring a proportionate addition to the
effect of grain refinement. Incidentally, the particle diameter
level of the magnesium alloy castings in an untreated state is
about 30 .mu.m The effect of grain refinement becomes discernible
when the particle diameter falls short of this level. Since the
solidified structure of alloy is preferred to possess the finest
possible particle diameter, the conditions for obtaining as high an
effect of grain refinement as possible are aimed at a particle
diameter which equals the particle diameter attained by the method
resorting to the addition of hexachloroethane.
[0030] In this invention, it is commendable to add a graphite
powder and manganese dioxide as contained in a metallic capsule. By
having the graphite powder and manganese dioxide contained in a
metallic capsule, it is made possible to prevent the graphite
powder and manganese dioxide from being scattered and utilize them
efficiently in small amounts for the grain refinement Further,
since the metallic capsule is made of pure aluminum or aluminum
alloy or of pure magnesium or magnesium alloy, they are enabled to
effect the grain refinement without adversely affecting the
composition of the alloy.
[0031] FIG. 1 shows a schematic view illustration of the apparatus
that is used in the examples of this invention. Referring to FIG.
1, reference numeral 1 denotes a crucible that is formed of iron
(Fe)-chromium (Cr)-based SUS 430 stainless steel (Fe-18% Cr)
containing no nickel (Ni) and is manufactured by bending this
stainless steel plate in a cylindrical shape and gas welding the
seam. Further, for the purpose of enhancing the ability to resist
high-temperature oxidation, the crucible is subjected to immersion
plating in a pure aluminum bath and the layer of pure aluminum
formed thereon is subjected to thermal diffusion so as to coat the
surface of the crucible 1 with a FeAl.sub.3 layer sparingly liable
to be wetted by magnesium. All the casting implements including the
crucible 1 are coated with magnesium oxide of the reagent chemical
grade so as to prevent inclusion of impurities during the melting
of alloy.
[0032] Denoted by reference numeral 2 is an electric furnace and by
numeral 3 is a phosphorizer. The phosphorizer 3 which is destined
to be passed through a hole in the lid of the electric furnace 2
and immersed in molten alloy 11 held in the crucible 1 is made to
hold therein a metallic capsule 4 accommodating a graphite (C)
powder or manganese dioxide (MnO.sub.2), or accommodating a
graphite powder and manganese dioxide. Incidentally, the
phosphorizer 3 is manufactured in the same manner as the crucible
1. The metallic capsule is formed of pure aluminum or aluminum
alloy, or pure magnesium or magnesium alloy. As the graphite
powder, graphite (purity of 99.9and particle size of about 5 .mu.m)
made by Kojundo Kagaku Kenkyusho Ltd. and sold under the product
designation of "Pure Carbon." was used. As the manganese dioxide, a
reagent chemical grade manganese dioxide (purity of 99.9% and
particle size of about 100 .mu.m) made by Takaraitesuku K. K. was
used.
[0033] Denoted by reference numerals 5 and 6 are thermocouples. The
thermocouple 5 is set inside the electric furnace 2 for the purpose
of measuring the temperature of the interior of the electric
furnace 2, and the thermocouple 6 is set in the phosphorizer 3 next
to the metallic capsule 4 to measure the temperature of the molten
alloy 11. A temperature-controlling device is labeled with 7. It is
intended to control the temperature of the interior of the electric
furnace 2, namely the temperature of the molten alloy 11 at a
prescribed temperature, based on the output of the thermocouple 5.
A pen recorder is labeled with 8. It serves to record the
temperature of the molten alloy 11 based on the output of the
thermocouple 6.
[0034] Now, the method for testing the effect of the addition of
manganese dioxide on the grain refinement of crystals will be
described below.
[0035] For the test, commercially available AZ91E magnesium alloy
having a composition shown in Table 1 below was used. The test was
performed through the procedure shown in FIG. 2. The amounts, in
mass %, of graphite powder and/or manganese dioxide added in the
individual runs of this test were as shown in Table 2 below.
1TABLE 1 Al Zn Mn Si Cu Ni Fe Mg 9.01 0.82 0.22 0.01 0.001 0.002
0.0017 Balance
[0036]
2TABLE 2 Complex-added Variation in amount of Graphite-added
treated material MnO.sub.2 added treated material Amount of
graphite powder Amount of MnO.sub.2 added Amount of added (%), with
added (%), with added amount graphite power amount of MnO.sub.2
fixed of graphite powder fixed added (mass %) at 0.20 mass % at
0.02 mass % 0.005 0.04 0.30 0.01 0.02 0.20 0.02 0.01 0.10 0.04
0.005 0 0.06 0.003 -- 0.08 0.001 --
[0037] First, the question whether or not the addition of manganese
dioxide induces a rise of temperature was studied. For a start, 700
g of AZ91E magnesium alloy pickled with nitric acid for the purpose
of depriving the surface thereof of impurities was melted in the
electric furnace 2, and the resultant molten alloy 11 was heated to
800.degree. C. Then, the metallic capsule 4 accommodating therein
manganese dioxide in an amount of 0.20 mass % based on the mass of
the molten alloy 11 was set in place in the phosphorizer 3, and the
phosphorizer 3 was inserted together with the thermocouple 6 into
the molten alloy 11. Manganese dioxide was added to the molten
alloy 11 while the measurement of temperature was continued
meantime. Then, the molten alloy 11 was left cooling outside the
electric furnace 2 to 700.degree. C. and cast in a metallic mold
held at room temperature. An untreated sample was also made without
adding manganese dioxide, with the object of investigating the
effect of addition of manganese dioxide on a solidified
structure.
[0038] The amount of manganese dioxide to be added was decided,
with due respect to the fact that the manganese content in the
AZ91E magnesium alloy conforming to the ASTM standard shown in
Table 3 below is in the range of 0.17 mass % to 0.35 mass %, at the
maximum such that the total amount of the manganese content in the
commercially available AZ91E magnesium alloy and the amount of
manganese separated as reduced in the molten alloy 11 in
consequence of the addition of manganese dioxide may not surpass
the upper limit of the range of the manganese content in the AZ91E
magnesium alloy conforming to the ASTM standard. The reason for
using this maximum is that the actual manganese content conforms
the ASTM standard and shuns exertion of adverse effect on the
components of the composition of the alloy.
3TABLE 3 Al Zn Mn Si Cu Ni Fe Impurity Mg 8.1.about.9.3
0.40.about.1.0 0.17.about.0.35 0.20 0.015 0.0010 0.005 0.01
Balance
[0039] When manganese dioxide was added as described above, it
induced an oxidation-reduction reaction and consequently varied the
temperature of the interior of the phosphorizer 3 and the temperate
of the whole of the molten alloy 11, as shown in FIG. 3. It is
clear from FIG. 3 that the interior of the phosphorizer 3 showed a
discernible sign of temperature elevation, though instantaneously,
to the neighborhood of about 1370.degree. C. (1643 K). The
temperature of the whole of the molten alloy 11, however, showed
virtually no discernible change. The oxidation-reduction reaction
that was caused by the addition of manganese dioxide, therefore,
was found to be a reaction that elevated the temperature inside the
phosphorizer 3 or in a narrow region encompassing the phosphorizer
3.
[0040] The solidified structure, magnified by an optical
microscope, of an untreated material of the molten alloy 11 having
nothing added thereto and the solidified structure, magnified by an
optical microscope, of a manganese dioxide-added treated material
having manganese dioxide added in an amount of 0.20 mass % based on
the mass of the molten alloy 11 are shown in FIG. 4. From FIG. 4,
it may be safely inferred that manganese dioxide has no effect of
grain refinement of the solidified structure.
[0041] Now, the effect manifested by the sole addition of a
graphite powder on the grain refinement of the solidified structure
was studied. First, 700 g of AZ91E magnesium alloy pickled with
nitric acid for the purpose of depriving the surface thereof of
impurities was melted in the electric furnace 2, and the
temperature of the molten alloy 11 was elevated to 800.degree. C.
by following the procedure shown in FIG. 2. Then the graphite
powder of a gradually increased amount, namely 0.005 mass %, 0.01
mass %, 0.02 mass %, 0.04 mass %, 0.06 mass % and 0.08 mass % each,
based on the mass of the molten alloy 11 as shown in Table 2 was
added as held in the metallic capsule 4 to the interior of the
phosphorizer 3. The sample which bad the graphite powder added
thereto and had undergone the treatment will be referred to as
"graphite-added treated material." For the purpose of comparing and
studying the degree of grain refinement of a solidified structure
caused by the addition of graphite, a sample having
hexachloroethane (C.sub.2Cl.sub.6) added thereto (conditions of
addition: temperature of addition of 750.degree. C., amount of
addition: 1 mass % based on the mass of the molten alloy 11)
(hexachloroethane-added tread material) was also manufactured.
Then, for the purpose of studying the effect of the metallic
capsule 4 formed of magnesium alloy on the solidified structure, a
sample having the metallic capsule 4 alone added thereto (a
capsule-added treated material) was manufactured. The metallic
capsule 4 was a product obtained by preparing a blind container
measuring 19 mm in outside diameter 15 mm in inside diameter, 20 mm
in height and 2 mm in wall thickness and sealing the container with
a lid 2 mm in wall thickness.
[0042] The solidified structure, magnified by an optical
microscope, of the hexachloroethane-added treated material, the
solidified structure, magnified by an optical microscope, of the
untreated material and the solidified structure, magnified by an
optical microscope, of the capsule-added treated material are shown
in FIG. 5. The solidified structure, magnified by an optical
microscope, of the graphite-added treated material having the
graphite powder content gradually increased as described above is
shown in FIG. 6. The relation (characteristic) between the amount
of the graphite powder added and the average crystal grain diameter
is shown in FIG. 7. For the purpose of allowing visual observation
of the structure, the sample obtained by performing a solution
treatment under the conditions of 673 K and 14.4 ks with the object
of transforming a eutectic crystal generated by quenching into a
solid solution was tested for average crystal grain diameter by the
slice method using an optical microscope. The average crystal grain
diameters reported hereinafter were determined in the same
manner.
[0043] From FIG., 5 which shows that the untreated material and the
capsule-added treated material formed equal solidified structures,
it may be safely inferred that the metallic capsule 4 had no effect
of fine grain refinement. Hexachloroethane, however, had an effect
of fine grain refinement as clearly noted from FIG. 5. It is
clearly noted from FIG. 6 and FIG. 7 that the treated materials
having the graphite powder added in the amounts of 0.005 mass %,
0.001 mass % and 0.02 mass % each had an average crystal grain
diameter of 300 .mu.m equal to that of the untreated materials.
From the data, it may be safely inferred that the graphite powder
had no effect of grain refinement When the amount of the graphite
powder added reached 0.04 mass %, the average crystal grain
particle became 180 .mu.m. Thus, the effect of grain refinement was
attained to a certain extant. When the amount of the graphite
powder added further rose to 0.06 mass % and 0.08 mass %, the
average crystal grain diameter became substantially constant at
about 70 .mu.m. Thus, the effect of grain refinement in his case
equaled that obtained by the addition of hexachloroethane. The
smallest amount of the graphite powder required to be added for the
sale of grain refinement of the solidified structure, therefore, is
0.04 mass %. For the purpose of obtaining the same effect of grain
refinement as the hexachloroethane-added treated material, the
amount of addition is required to exceed 0.06 mass %.
[0044] Next, the effect brought by the addition of a graphite power
and manganese dioxide on the grain refinement of the solidified
structure was investigated. First, 700 g of AZ91E magnesium alloy
pickled with nitric acid for the purpose of depriving the surface
thereof of impurities was melted by the electric furnace 2 and the
temperature of the molten alloy 11 was elevated to 800.degree. C.
by following the procedure shown in FIG. 2. Then, the manganese
dioxide in a fixed amount of 0.20 mass % and the graphite powder of
a gradually decreased amount, namely 0.04 mass %, 0.02 mass %, 0.01
mass %, 0.005 mass %, 0.003 mass % and 0.001 mass % each, based on
the mass of the molten alloy 11 as shown in Table 2 were added as
held in the metallic capsule 4 to the interior of the phosphorizer
3. The material resulting from the addition of a complex additive,
namely graphite powder plus manganese dioxide, and the subsequent
treatment will be referred to as a "complex-added treated
material."
[0045] The solidified structure, magnified by an optical
microscope, of various complex-added treated materials mentioned
above are shown in FIG. 8 and the relation (characteristic) between
the various complex additives and the average crystal grain
diameters is shown in FIG. 9. It is clearly noted from FIG. 5 and
FIG. 9 that the amounts of the graphite powder added were 0.04 mass
% and 0.02 mass %, the average crystal grain diameters became
substantially constant at about 70 .mu.m. Thus, the effect of grain
refinement of the solidified structure obtained here was equal to
that obtained by the addition of hexachloroethane. When the amount
of the graphite powder added was 0.01 mass % or less, however, the
trend of the crystal grains toward coarsening proportionately to
the decease of the amount of addition of the graphite powder became
visible. A discussion based on FIG. 6 to FIG. 9, therefore, leads
to an inference that in the case of the addition of the graphite
powder in a fixed amount, the complex-added treated material had a
higher effect of grain refinement of the solidified structure than
the treated material which had no graphite powder added thereto and
that the complex-added treated material having the graphite powder
added in an amount of 0.02 mass % or more obtained the same effect
of gain refinement of the solidified structure as the addition of
hexachloroethane. This phenomenon may be logically explained by a
supposition that in consequence of the addition of both manganese
dioxide and the graphite powder, the oxidation-reduction reaction
occurred locally at the position act joining graphite and the
elevation of t r e due to the generation of heat by the reaction
promoted the formation of Al.sub.4C.sub.3, a nucleus-forming
substance. Though the optimum amount of addition of graphite in the
complex-added treated material is 0.02 mass %, it is commendable
for the purpose of securing the effect of grain refinement of the
solidified structure to fix the amount of addition of graphite at
0.01 mass % or more and 0.1 mass % or less.
[0046] Next, the effect brought by the change in the amount of
addition of manganese dioxide on the refinement of crystal grains
was investigated. First, 700 g of AZ91E magnesium alloy pickled
with nitric acid for the purpose of depriving the surface thereof
of impurities was melted by the electric furnace 2 and the
temperature of the molten alloy 11 was elevated to 800.degree. C.
by following the procedure shown in FIG. 2. Then, the graphite
powder in a fixed amount of 0.20 mass % and the manganese dioxide
of a gradually decreased amount, namely 0.30 mass %, 0.20 mass %,
0.10 mass % and 0 mass % each, based on the mass of the molten
alloy 11 as shown in Table 2 were added as held in the metallic
capsule 4 to the interior of the phosphorizer 3.
[0047] The solidified structures, magnified by an optical
microscope, of the various complex-added treated materials are
shown in FIG. 10, and the relation (characteristic) between the
various complex additives and the average crystal grain diameters,
is shown in FIG. 11. It is clearly noted from FIG. 10 and FIG. 11
that when manganese dioxide was added in amounts of 0.30 mass % and
0.20 mass %, the average crystal grain diameters became
substantially constant at about 70 .mu.m. Thus, the effect of grain
refinenent of the solidified structure obtained here equaled that
obtained by the addition of hexachloroethane. When the amount of
addition of manganese dioxide fell short of 0.10 mass %, however,
the trend of crystal grains toward coarsening proportionately to
the decrease of the amount of addition of manganese dioxide became
visible. In the complex-added treated material having the graphite
powder added in a fixed amount, therefore, it is inferred in view
of the data of FIG. 11 that the addition of manganese dioxide in an
amount of 0.10 mass % or more resulted in bringing the effect of
grain refinement having an average crystal grain diameter of about
170 .mu.m or less and the addition of manganese dioxide in an
amount of 0.20 mass % or more resulted in binging the same effect
of refined crystal grain as in the case of adding hexachloroethane.
Though the optimum amount of addition of manganese dioxide in the
complex-added treated material is 0.2 mass %, it is commendable for
the purpose of securing the effect of grain refinement of the
solidified structure to fix the amount of addition of manganese
dioxide at 0.10 mass % or more and 0.4 mass % or less.
[0048] Next the effect brought by the particle diameter of graphite
powder on the grain refinement of the solidified structure was
investigated. First, 700 g of an AZ91E magnesium alloy pickled with
nitric acid for the purpose of depriving the surface thereof of
impurities was melted by the electric furnace 2 and the temperature
of the molten alloy 11 was elevated to 800.degree. C. by following
the procedure shown in FIG. 2. Then, carbon nanotubes in an amount
of the nm order of 0.2 mass %, such as 100 nm in diameter and 5
.mu.m in length, based on the mass of the molten alloy 11 and
manganese dioxide in an amount of 0.20 mass % based on the mass of
the molten alloy 11 were added as held in the metallic capsule 4
into the phosplhorizer 3.
[0049] The solidified structure, magnified by an optical
microscopes, of the complex-added treated material mentioned above
is shown in FIG. 12. It is clearly noted from FIG. 12 that the
average crystal grain diameters became substantially constant at
about 75 .mu.m and the effect of grain refinement of the solidified
structure obtained here equaled that obtained by the addition of
hexachloroethane. Thus, the graphite obtained the effect of grain
refinement of the solidified structure equaling that obtained by
the addition of hexachloroethane even when the particle diameter of
the graphite was changed to the nm order.
[0050] Next, the F specimens (specimens as cast) and the T4
specimens (specimens having undergone a treatment for
transformation into a solid solution, specifically heat-treated at
400.degree. C. for 16 hours) of the various treated materials were
tested for tensile strength, 0.2% proof strength (MPa) and
elongation (%). First, FIG. 13 is a characteristic diagram showing
the data of tensile strength, 0.2% proof strength (MPa) and
elongation (%) obtained of the F specimens of the untreated
material, manganese dioxide-added treated material,
hexachloroethane-added treated material and complex-added treated
material, Then, FIG. 14 is a characteristic diagram showing the
data of tensile strength, 0.2% proof strength (MPa) and elongation
(%) obtained of the T4 specimens of the untreated material,
manganese dioxide-added treated material, hexachloroethane-added
treated material and complex-added treated material.
[0051] It is clearly noted from FIG. 13 and FIG. 14 that the F
specimens and the T4 specimens of the complex-added treated
materials equaled the F specimens and the T4 specimens of the
hexachloroethane-added treated material in terms of tensile
strength, 0.2% proof strength and elongation.
[0052] The preceding example depicted a case of using an AZ91E
magnesium alloy as a magnesium alloy. The castings shown in Table 4
below, for example, may be used instead so long as they are
magnesium alloys containing aluminum (Al) and manganese (Mb). As
the graphite powder, a carbon nanotube measuring 100 nm in diameter
and 5 .mu.m in length has been cited above. Carbon nanotubes
measuring 50 nm to 200 nm in diameter and 1 .mu.m to 20 .mu.m in
length are capable of producing similar effects.
4TABLE 4 Kind of article of Chemical compositions (mass %) cast
metal Al Zn Mn Si Cu Ni Fe Impurity Mg 1.sup.st grade 5.3.about.6.7
2.5.about.3.5 0.15.about.0.35 .ltoreq.0.30 .ltoreq.0.25
.ltoreq.0.01 -- .ltoreq.0.30 Balance 2.sup.nd grade C 8.1.about.9.3
0.40.about.1.0 0.13.about.0.35 .ltoreq.0.30 .ltoreq.0.10
.ltoreq.0.01 -- .ltoreq.0.30 Balance 2.sup.nd grade E 8.1.about.9.3
0.40.about.1.0 0.17.about.0.35 .ltoreq.0.20 .ltoreq.0.015
.ltoreq.0.0010 .ltoreq.0.005 .ltoreq.0.30 Balance 3.sup.rd grade
8.0.about.10.0 1.5.about.2.5 0.10.about.0.5 .ltoreq.0.3
.ltoreq.0.20 .ltoreq.0.01 .ltoreq.0.05 .ltoreq.0.30 Balance
5.sup.th grade 9.3.about.10.7 .ltoreq.0.3 0.10.about.0.35
.ltoreq.0.30 .ltoreq.0.10 .ltoreq.0.01 -- .ltoreq.0.30 Balance ISO
5.00.about.7.0 2.0.about.3.5 0.10.about.0.5 .ltoreq.0.3 .ltoreq.0.2
.ltoreq.0.01 .ltoreq.0.05 -- Balance 1.sup.st grade ISO 2.sup.nd
7.0.about.9.5 0.3.about.2.0 .ltoreq.0.15 .ltoreq.0.5 .ltoreq.0.35
.ltoreq.0.02 .ltoreq.0.05 -- Balance grade A ISO 2.sup.nd
7.50.about.9.0 0.2.about.1.0 0.15.about.0.6 .ltoreq.0.3 .ltoreq.0.2
.ltoreq.0.01 .ltoreq.0.05 -- Balance grade B ISO 3.sup.rd
8.3.about.10.3 0.2.about.1.0 0.15.about.0.6 .ltoreq.0.3 .ltoreq.0.2
.ltoreq.0.01 .ltoreq.0.05 -- Balance grade
[0053] According to this invention, owing to the addition of
manganese dioxide besides a graphite powder to the melt of
magnesium alloy, it is made possible to utilize the reaction of
oxidation-reduction caused by manganese dioxide for elevating the
temperature of the molten alloy, promoting the grain refinement of
the solidified structure by carbon, and materializing production of
a cast article of the magnesium alloy which enjoys improved
mechanical properties. This invention is enabled by the avoidance
of addition of hexachloroethane to shun the generation of dioxins
and by the utilization of the reaction of oxidation-reduction of
manganese dioxide to elevate the temperature of the melt of
magnesium alloy and promote the decrease of the cost of energy
consequently and enjoy the same effect as in the case of adding
hexachloroethane.
* * * * *