JP2002309322A - Method for manufacturing magnesium alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a magnesium alloy in which the occurrence of environmental pollutants can be prevented and cast texture can be fine-grained while maintaining the ductility of the magnesium alloy and improving simultaneously high-temperature strength. SOLUTION: SiC powder having SiO2 film on the surface of powder particles is directly added in an additive quantity of 0.1-10 mass % to the molten magnesium alloy. It is preferable to form the SiO2 film having a thickness of 1-10% of powder particle diameter by heating the SiC powder in the air.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a magnesium alloy, and more particularly, to a method for producing a magnesium alloy capable of improving the high-temperature strength while reducing the size of a cast structure.

[0002]

2. Description of the Related Art Refining of a cast structure of a magnesium alloy is extremely important in improving mechanical properties of the alloy. Conventionally, a method of adding hexachloroethane (C ₂ Cl ₆ ) to a molten magnesium alloy has been known as a general method of refining the cast structure of a magnesium alloy.
However, when hexachloroethane comes into contact with a high-temperature magnesium alloy melt, this method produces a chlorinated hydrocarbon (CH).
C) is generated and becomes a source of environmental pollution.

Japanese Patent Application Laid-Open No. 2000-104136 discloses a method for refining a magnesium alloy that does not generate environmental pollutants.
A technique of adding B and Mn to a magnesium alloy melt has been proposed. However, in this method, Mn can be added to the molten magnesium alloy only in the form of an Al—Mn alloy, and there is a problem that the ductility of the magnesium alloy is reduced due to the presence of an unnecessary Al component.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to produce a magnesium alloy capable of reducing the size of the cast structure and improving the high-temperature strength while maintaining good ductility of the magnesium alloy without generating environmental pollutants. The aim is to provide a method.

[0005]

In order to achieve the above object, a method of manufacturing a magnesium alloy according to the present invention comprises the steps of: preparing a SiC powder having a SiO ₂ coating on the surface of powder particles;
It is characterized in that it is added directly to the magnesium alloy melt at an addition amount of 10 to 10 mass%. By heating the SiC powder in the atmosphere, the SiO 2 having a thickness of 1 to 10% of the particle size of the powder is heated.
₂ It is desirable to form a coating.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION By adding SiC powder in which the surface of powder particles is coated with SiO ₂ (hereinafter abbreviated as “SiO ₂ coated SiC powder”) to a molten magnesium alloy, the cast structure of the magnesium alloy becomes finer. The mechanism by which the high-temperature strength is simultaneously improved is considered as follows.

When SiO ₂ -coated SiC powder is added to a molten Mg alloy, first, SiO ₂ and Mg on the surface of the powder particles become
The exothermic reaction of (1) occurs. As a result, the powder particle surface becomes 85
Since the temperature becomes 0 ° C. or higher, Mg and SiC can react as shown in the following (2), and Mg ₂ Si intermetallic compound and free C are generated. Mg ₂ Si generated here
High temperature strength is improved by the intermetallic compound.

[0008] On the other hand, free C is, as shown in the following (3), Mg
It reacts with carbide forming elements such as Al and Mn generally contained in the alloy to form fine metal carbide particles such as Al ₄ C ₃ and Mn ₂ C. These metal carbides are stable in the molten Mg alloy and act as solidified shells, so that the cast structure of the Mg alloy is refined.

(1) 2Mg + SiO ₂ → 2MgO + Si (exothermic reaction) (2) 2Mg + SiC → Mg ₂ Si + C (3) 3C + 4Al (or Mn, etc.) → Al ₄ C ₃ (or Mn ₂ C, etc.)

In the present invention, the SiO ₂ -coated SiC powder is added to the molten Mg alloy in an amount of 0.1 to 10 mass%. If it is less than 0.1 mass%, the effect of grain refinement cannot be obtained. If it exceeds 10 mass%, the viscosity of the molten Mg alloy becomes too high to make casting impossible.

[0011] In a preferred embodiment of the present invention, SiO 2
₂ The thickness of the coating is 1-10% of the SiC particle size. In less than 1%, Mg and SiO ₂ Reaction of Mg and SiC is difficult to occur because a small amount of heat generated by the, for exceeds 10% Mg and the SiO ₂ becomes susceptible to thermite reaction, Mg alloy melt Danger of explosion.

[0012]

EXAMPLE A magnesium alloy was produced by directly adding a SiO ₂ -coated SiC powder to a molten AZ91D magnesium alloy and casting it according to the following procedure and conditions.

[Formation of SiO ₂ film] Powder particle size 10 μm
Was heated in the air to form a SiO ₂ coating on the surface of the powder particles. First, the change in the coating thickness with respect to the heating temperature and the heating time was examined. FIGS. 1 and 2 show the change in the coating thickness with respect to the heating temperature and the heating time in the form of a graph and a table, respectively. Various similar preliminary experiments were conducted, and based on the results, the range of the temperature and the time at which the coating thickness was 1 to 10% of the particle size was obtained.

In this embodiment, heating is performed at 1500 ° C. × 1 hour, and a 411 nm thick Si
An O ₂ coating was formed. The thickness of this coating is 10%.
It is 4.1% of 000 nm (= 10 μm). X-ray diffraction was performed on the powder before and after heating. As a result, only the diffraction peak of SiC was observed before heating, whereas the diffraction peak of SiO ₂ was clearly observed in addition to the SiC peak after heating. Admitted. As a result, the coating composition becomes Si
O ₂ was confirmed.

[Addition to molten magnesium alloy] SF ₆
The AZ91D magnesium alloy melt maintained at 700 ° C. in a gas atmosphere is coated with the SiO ₂ coating S prepared above.
The iC powder was added in various addition amounts, stirred and uniformly dispersed in the molten metal, and then cast into a mold (JIS No. 4 boat type).

[Measurement of Crystal Grain Size] The microstructure of the above cast magnesium alloy was observed under a microscope to determine the crystal grain size. FIG. 3 shows a typical observation example of SiO ₂ coated Si
The casting structure when 1 mass% of C powder is added is shown. FIG.
The casting structure in the case of no addition is shown for comparison. FIG.
The graph and table show the relationship between the amount of added SiO ₂ -coated SiC powder and the crystal grain size.

As shown in FIG. 5, SiO ₂ coated SiC
When the addition amount of the powder is in the range of 0 to 0.05 mass%, the crystal grain size is as coarse as 200 to 180 μm.
When the content is more than mass%, the particles are remarkably reduced to 80 to 50 μm. The figure shows the results up to the addition amount of 0.5 mass%. However, even if the addition amount is further increased, the degree of grain refinement gradually progresses, but does not remarkably progress.

[Measurement of High-Temperature Strength] (1) Measurement of High-Temperature Hardness The high-temperature hardness of the cast magnesium alloy at 150 ° C. was measured. FIG. 6 is a graph and a table showing the relationship between the added amount of the SiO ₂ -coated SiC powder and the high-temperature hardness. FIG.
As shown in the above, the 150 ° C. high temperature hardness is SiO ₂ coated Si
It increases monotonically as the amount of C powder added increases. In particular, in the range of up to 4 mass% shown in the figure, SiO ₂ -coated SiC which is a hard dispersed phase in a magnesium alloy matrix is used.
It is observed that the hardness has a substantially linear relationship with the amount of powder added.

The increase in the high-temperature hardness by the addition of the SiO ₂ -coated SiC powder is basically due to the formation of the Mg ₂ Si intermetallic compound as described above. FIG. 7 shows a micrograph of a magnesium alloy casting structure to which 1 mass% of SiO ₂ -coated SiC powder was added. In the figure, the massive particles that look gray, which is the same as the ground color, are Mg
Mg-S presumably formed with Si intermetallic compound as nucleus
It is an i-Al-O-based quaternary compound and contributes to improvement in high-temperature strength similarly to the Mg ₂ Si intermetallic compound.

(2) Measurement of high-temperature axial force retention For a magnesium alloy to which 1 mass% of SiO ₂ -coated SiC powder was added, an initial load of 64 MPa and a test temperature of 150 were used.
At ℃, the axial force retention was measured with respect to the elapsed time from the start of loading. For comparison, the same measurement was performed for an unadded magnesium alloy. FIG. 8 shows the measurement results as a graph and a table.

As shown in FIG. 8, the axial force retention decreased with the passage of time, and in the case of the non-added comparative example, the axial force retention became zero after 100 hours. On the other hand, when 1 mass% of the SiO ₂ -coated SiC powder was added according to the present invention, the retention of the axial force was still maintained at 20% at the time of 100 hours, and the axial force retention decreased to 0. It took 300 hours. As described above, the addition of the SiO ₂ -coated SiC powder significantly improves the axial force retention characteristics.

[0022]

As described above, according to the present invention, it is possible to reduce the size of the cast structure and improve the high-temperature strength while maintaining good ductility of the magnesium alloy without generating environmental pollutants. Is provided.

[Brief description of the drawings]

FIG. 1 is a graph and a table showing the relationship between the heating temperature of SiC powder and the thickness of a SiO ₂ coating formed on the surface of powder particles.

FIG. 2 is a graph and a table showing the relationship between the heating time of a SiC powder and the thickness of a SiO ₂ coating formed on the surface of a powder particle.

FIG. 3 is a micrograph showing a casting structure of a magnesium alloy manufactured by adding a SiO ₂ -coated SiC powder according to the present invention.

FIG. 4 is a micrograph showing a casting structure of a magnesium alloy manufactured without addition as a comparative example.

FIG. 5 is a graph and a table showing the relationship between the amount of added SiO ₂ -coated SiC powder and the crystal grain size of a magnesium alloy.

FIG. 6 is a graph and a table showing the relationship between the amount of added SiO ₂ -coated SiC powder and the high-temperature hardness.

FIG. 7 is a micrograph showing precipitated compounds in a cast structure of a magnesium alloy manufactured by adding a SiO ₂ -coated SiC powder according to the present invention.

FIG. 8 shows the retention of axial force at high temperature over time for a magnesium alloy manufactured by adding a SiO ₂ -coated SiC powder according to the present invention and a magnesium alloy manufactured without addition as a comparative example. 3 is a graph and a table shown for FIG.

Claims (2)

[Claims] 1. A method for producing a magnesium alloy, wherein SiC powder having a SiO ₂ coating on the surface of powder particles is directly added to a molten magnesium alloy in an amount of 0.1 to 10 mass%. 2. The method for producing a magnesium alloy according to claim 1, wherein said SiO ₂ coating having a thickness of 1 to 10% of the powder particle size is formed by heating SiC powder in the air.