JPH0734172A - Magnesium alloy for die casting - Google Patents

Abstract

PURPOSE:To provide a magnesium alloy for die casting excellent in creep resistance. CONSTITUTION:This magnesium alloy is a one contg., instead of the conventional Al and Zn, by weight, 0.5 to 2.0% Ag, 0.5 to 3.0% Ca and 0.2 to 0.7% Mn, and the balance Mg with impurity elements. Ag increases its proof stress and strength by solid solution strengthening and contributes to the improvement of the force retaining rate of a bolt shank, Ca crystallized out Mg-Ca series compounds on the grain boundaries and improves its creep resistance and Mn increases its proof stress and strength by solid solution strengthening or precipitation strengthening and improves the force retaining rate of a bolt shank. In this way, in this magnesium alloy for die casting, creep resistance can remarkably be improved compared with that of the conventional magnesium alloy for die casting and proof stress and strength can furthermore be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnesium alloy for die casting having excellent creep resistance.

[0002]

2. Description of the Related Art Among conventional magnesium alloys, Mg-A
l-based alloy (ASTM standard-AM60B, AM50A, A
M20A, etc.) contains 2 to 12% of Al, to which a small amount of Mn is added. On the Mg side, an eutectic system of α-Mg solid solution and β-Mg ₁₇ Al ₁₂ compound is used. As a result, the toughness is improved, and age hardening occurs due to precipitation of the intermediate phase of Mg ₁₇ Al ₁₂ by the aging heat treatment.

Further, Al is 5 to 10% and Zn is 1 to 3%.
Mg-Al-Zn system contained (ASTM standard-AZ91
D) has a large α-solid solution region on the Mg side,
The -Al-Zn compound crystallizes out. As-cast, it is tough and has excellent corrosion resistance, but its aging heat treatment further improves its mechanical properties.

In the Mg-Zn alloy, the Mg content is 4%.
When Zn is added, the highest strength can be obtained as it is in the casting, but it is not used in die casting because it causes casting cracks when die casting.

[0005]

Among the above-mentioned magnesium alloys, Mg-Al-based or Mg-Al-Zn-based alloys have low cost and can be die-casted, so that they are members used at relatively low temperatures. Is being adopted by M
Since the g-Al-based compound has a low melting point and is unstable at high temperatures, the strength and creep resistance at high temperatures are greatly reduced.

For example, AZ91D is excellent in castability, corrosion resistance and strength from room temperature to 150 ° C, but is poor in creep resistance at 100 ° C or higher. If the high temperature creep property is low, for example, when the temperature of the bolt fastening portion rises during use, the tightening force (axial force) decreases. This tendency is particularly remarkable in the case of die casting materials.

FIG. 6 shows AZ91D and AM60M at 150 ° C.
Shows the axial force retention rate after being retained for 100 hours.
Both are below a few percent. The axial force retention rate test method is shown in FIG. The Mg test piece is fastened with a bolt at a predetermined surface pressure, held in a predetermined furnace for a predetermined time, and calculated from the change in the bolt fastening length before and after that by the following formula. Axial force retention rate = (l ₂ −l ₀ ) / (l ₁ −l ₀ ) × 100
(%) Where, l ₀ ; length before bolt fastening (20 ° C) l ₁ ; length immediately after bolt fastening (20 ° C) l ₂ ; length after holding in furnace (20 ° C)

As described above, the conventional Mg-based alloy for die casting uses Al as an additional element. Al addition is M
Although the strength of g is improved and good castability is exhibited, the creep resistance deteriorates with the addition amount of 1% as a peak. Also,
Although Zn is an additional element other than Al, the creep resistance deteriorates as the added amount of Zn increases, and if it exceeds 2%, die cracking easily occurs in casting cracks.

The present invention has been made to solve the above-mentioned problem that the conventional magnesium alloy for die casting is poor in creep resistance, and maintains the strength and corrosion resistance of the conventional magnesium alloy for die casting. At the same time, it is an object of the present invention to provide a magnesium alloy for die casting which has excellent creep resistance and a good axial force retention.

[0010]

In order to solve the above problems, the inventors of the present invention have made Al, Zn which deteriorates the creep resistance.
Various studies were conducted on the addition of other alloying elements without adding. As a result, they have found that good creep resistance can be obtained while maintaining strength and yield strength by adding Ag, Ca, and Mn, and completed the present invention.

The magnesium alloy for die casting having excellent creep resistance according to the present invention has a weight ratio of Ag: 0.5-2.
0%, Ca; 0.5 to 3.0%, Mn; 0.2 to 0.7
%, With the balance being Mg and impurity elements.

[0012]

The alloy of the present invention has a deterioration in creep resistance A
Creep resistance could be improved by adding Ag, Ca, and Mn without adding 1 and Zn. Also,
Fe that deteriorates the corrosion resistance by the addition of Mn even in the presence of a trace amount
It is also possible to reduce the influence of minute and improve the corrosion resistance.

Ag forms a solid solution in the Mgα phase, and plays a role of enhancing the yield strength and strength by solid solution strengthening. For the improvement of the bolt axial force retention, it contributes to the improvement of the yield strength because it withstands the increase in compression to the Mg side caused by the difference in thermal expansion between the bolt and Mg when the temperature rises. In order to obtain the above effect, it is necessary to contain at least 0.5% or more. But 2.0%
If the content is exceeded, the corrosion resistance will deteriorate, so the upper limit is 2.
It was set to 0%.

Ca crystallizes in the grain boundaries as a Mg-Ca compound and suppresses diffusion at the grain boundaries to improve creep resistance. With respect to the improvement of the bolt axial force retention rate, it has the effect of suppressing compression creep even when exposed to high temperatures for a long time. Also,
Although it is a small amount, it forms a solid solution in the Mgα phase and also has the effect of improving the yield strength. In order to obtain the above effect, it is necessary to contain at least 0.5% or more. However, if the content exceeds 3.0%, the strength decreases, so the upper limit was made 3.0%.

Mn forms a solid solution in the Mgα phase and strengthens the yield strength and strength by solid solution strengthening like Ag or by precipitation strengthening as a fine Mg-Mn compound. The improvement of the bolt axial force retention rate is similar to Ag.
In order to obtain the above effect, it is necessary to contain at least 0.2% or more. However, the amount of Mn that can be melted at 700 ° C, which is the upper limit of the molten metal temperature used in the die casting machine, is 0.72%.
Therefore, the upper limit was set to 0.7%.

[0016]

EXAMPLES Examples of the present invention will be described together with comparative examples and conventional examples to clarify the effects of the present invention. (Example 1) Examination of addition of Ag The tensile strength characteristics when Ag was sequentially added to Mg-2.2% Ca-0.30% Mn were examined, and the results are shown in FIG. As shown in FIG. 1, it was found that Ag was improved in both strength and 0.2% proof stress by adding 0.5%, and was gradually improved by adding more. However, as is clear from FIG. 2 showing the relationship between the addition of Ag and the corrosion rate, Ag
If the addition of Al exceeds 2%, the corrosion resistance deteriorates significantly. Therefore, it was found that it is appropriate to limit the Ag addition amount to 0.5 to 2.0% in consideration of practicality.

(Example 2) Examination of Ca addition The bolt axial force retention rate when Ca was added to Mg-1.9% Ag-0.30% Mn was measured, and the obtained results are shown in FIG.
It was shown to. As is clear from Fig. 3, the addition of 0.5% of Ca drastically increases the axial force retention rate, and when 1% or more, the axial force retention rate is
The above-mentioned good axial force retention rate was shown. On the other hand, FIG. 4 is a diagram showing the relationship between the added amount of Ca and the tensile strength. It was found that when the added amount of Ca exceeds 3%, the strength tends to decrease. Therefore, it was found that it is appropriate to limit the amount of Ca added to 0.5 to 3.0%.

Example 3 Examination of Mn Addition Amount of bolt axial force when Mn was added to Mg-1.5% Ag-2.0% Ca was measured, and the obtained results are shown in FIG. Indicated. As shown in FIG. 5, the addition of 0.2% of Mn had a remarkable effect on the axial force retention rate, but the effect was saturated even if Mn was added more. Also, the melting limit in the molten metal is 700 ° C.
Was 0.72%. Die casting temperature is 70
If the temperature exceeds 0 ° C, the injection system of the die casting machine is severely damaged by heat and is not suitable for actual production. Therefore, it was proved appropriate to limit the amount of Mn added to 0.2 to 0.7%.

(Example 4) Mg-within the composition range of the present invention
A 1.5% Ag-1.1% Ca-0.32% Mn alloy is melted, the melt temperature is 690 ± 10 ° C, and the mold temperature is 75 ° C to 11 ° C.
A test piece was prepared using a cold chamber die casting machine at 0 ° C. and a casting pressure of 830 kgf / cm ² . With respect to the obtained test piece, an axial force retention test and a tensile test of a bolt fastening portion were performed. The results are shown in Table 1.
For comparison, a conventional material, AZ91D material, was die cast under the same conditions as those of the examples of the present invention to prepare a test piece, and the same test was conducted. The results are also shown in Table 1.

[0020]

[Table 1]

As shown in Table 1, the conventional alloy AZ
In 91D, the axial force retention rate of the bolt fastening portion was only 3%, whereas the axial force retention rate of the bolt fastening portion of the embodiment of the present invention was 82%, which is remarkable in creep resistance. It was confirmed that it was excellent. It was also found that the tensile strength of AZ91D was 249 MPa, whereas the tensile strength of the inventive example was 260 MPa, which was also excellent.

[0022]

INDUSTRIAL APPLICABILITY The magnesium alloy for die casting according to the present invention has a weight ratio of Ag in place of conventional Al or Zn;
0.5-2.0%, Ca; 0.5-3.0%, Mn;
It is characterized by containing 0.2 to 0.7%, and the balance being Mg and impurity elements. Ag contributes to the improvement of proof strength and strength by solid solution strengthening and at the same time, improvement of bolt axial force retention rate. However, Ca crystallizes the Mg-Ca-based compound in the grain boundary to improve the creep resistance, and Mn improves the yield strength and strength by solid solution strengthening or precipitation strengthening and improves the bolt axial force retention rate. The present magnesium alloy for die casting has significantly improved creep resistance as compared with conventional magnesium alloys for die casting, and can also have improved yield strength and strength.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between Ag content, tensile strength and 0.2% proof stress in a Mg—Ca—Mn alloy.

FIG. 2 is a diagram showing the relationship between Ag content and corrosion rate in a Mg—Ca—Mn alloy.

FIG. 3 is a diagram showing a relationship between a Ca content and a fastening bolt axial force retention rate in a Mg-Ag-Mn alloy.

FIG. 4 is a diagram showing a relationship between Ca content and tensile strength in a Mg—Ag—Mn alloy.

FIG. 5 is a graph showing the relationship between the Mn content in a Mg-Ag-Ca alloy and the fastening bolt axial force retention rate.

FIG. 6 Conventional magnesium alloys AZ91D and AM6
It is a diagram showing a fastening bolt axial force retention rate of 0B die-cast material.

FIG. 7 is a side view illustrating an axial force retention rate test method.

Claims (1)

[Claims] 1. A weight ratio of Ag: 0.5 to 2.0%, C
A magnesium alloy for die casting having excellent creep resistance, characterized by containing a: 0.5 to 3.0%, Mn: 0.2 to 0.7%, and the balance being Mg and an impurity element.