JPH06145872A - Mg alloy excellent in creep strength and its production - Google Patents

Abstract

PURPOSE:To obtain an Mg alloy excellent in creep strength and which is widely applicable at a low cost by improving its proof stress. CONSTITUTION:6-10% Al or <=4.0% Zn or both are compositely added to an Mg alloy. The cooling rate in the solidification section is controlled to <=10 deg.C/sec when the Mg alloy is cast, and the cast alloy is used as it is. The cast Mg alloy is further homogenized at 350-420 deg.C and cooled to room temp., and the size of the grain of the compd. deposited in the succeeding ageing treatment is controlled to >=1mum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to, for example, automobile parts,
M used for computer parts, precision machine parts, etc.
The present invention relates to a g-alloy and a method for producing the same, and more particularly to an Al-Zn-based Mg alloy that can be used for a wide range of purposes by reducing weight and improving heat resistance.

[0002]

2. Description of the Related Art Mg alloys have the lowest density of practically used metals, can contribute to weight reduction, and have a large damping capacity of materials, so that automobile parts and computer parts are apt to generate vibration and noise. , Its use is drawing attention as a precision machine part.

By the way, the biggest drawback of the above Mg alloy is that it is inferior in heat resistance and has a low yield strength at high temperatures. For example, in the case of Al-Zn-based Mg alloy, which is the mainstream of Mg alloys, it is 8
It has been pointed out that when used at a high temperature of 0 ° C. or higher, the bolts that fasten and fix the Mg alloy parts are likely to loosen, and there is a problem that applications are limited. In order to prevent such loosening of the bolt, it has been conventionally proposed to add rare earth to the Mg alloy to improve the yield strength at high temperature.

[0004]

However, the Mg alloy to which the rare earth is added has a problem that the material cost increases. When such an alloy is used as the above-mentioned various parts, it is not possible to meet the demand for low cost because of the large amount used, and it is also disadvantageous in terms of recycling. Therefore, Al-Z, which is currently used in large quantities,
It is expected that the heat resistance of the n-based Mg alloy will be improved.

The present invention has been made in view of the above conventional circumstances, and an object of the present invention is to provide a Mg alloy which can improve the heat resistance without increasing the cost and can be expanded in its use, and a manufacturing method thereof.

[0006]

DISCLOSURE OF THE INVENTION The inventors of the present invention have examined the manufacturing conditions of conventional Al—Zn based Mg alloys in order to achieve the above object. According to it, in order to improve the slack of the bolt, in the conventional method, from the viewpoint of obtaining a material having high yield strength at high temperature, as heat treatment, homogenization treatment + aging treatment,
A method is used in which the aging treatment is performed at a temperature as low as possible (190 ° C. or less) to achieve high yield strength. That is, it has been considered that the cause of the slack of the bolt is the deformation of the Mg alloy due to the decrease in proof stress at high temperature.
This viewpoint is not wrong, and there are alloys that can improve the sag resistance by such a method.

Based on the above point of view, a more detailed analysis of the slack of the bolt due to the Mg alloy was carried out. As a result, the improvement of the yield strength at high temperature was a secondary factor, and the major factor was the creep strength. It was also found that high yield strength is more effective for sagging resistance if the creep strengths are substantially the same. Furthermore, the biggest finding is that, in the case of Al-Zn-based Mg alloy, the above-mentioned method of increasing the yield strength is a disadvantageous method from the viewpoint of creep strength, and this is the largest cause of increasing the slack of the bolt. Has been found.

[0008] In view of these points, the inventors of the present invention attach great importance to creep strength and have conducted earnest studies. As a result, they have found that in order to improve the creep strength, it is necessary to first increase the precipitation rate of the compound and secondly to control the particle size of the compound when precipitating the compound by aging treatment. For that purpose, it is important to regulate the added amounts of Al and Zn within a predetermined range, and the Mg alloy produced in this way can be improved by about 50 ° C. from the conventional service temperature, and by extension, the yield strength at high temperature. The present invention has been made with the idea that it can be improved.

Therefore, the invention of claim 1 is: Al: 6 to 10
%, Zn: 4.0% or less, either or both of them are added in combination, and Mg having excellent creep strength is characterized.
It is an alloy. The invention of claim 2 is characterized in that the Mg alloy is used as cast.

A third aspect of the present invention is a method for producing the Mg alloy, wherein Al: 6-10% and Zn: 4.0%.
Mg obtained by adding one or both of the following
When casting the alloy, the cooling rate in the solidification zone is 10
It is characterized in that it is cast into a mold so that the temperature is not higher than ° C / sec and that it is used as it is after this casting. Furthermore, the invention of claim 4 is characterized in that the cast Mg alloy is 350-420.
After homogenizing at a temperature of ℃, cool to room temperature, and then perform aging treatment to reduce the size of the precipitated compound particles to 1 μm.
The feature is that it is m or more.

The structure of the present invention will be described in detail below. I. First, the reason why the amount of Al is set to 6 to 10% The amount of Al is set to 6% or more in order to improve the creep strength by utilizing the segregation that occurs during casting as a means to accelerate the precipitation rate. This is because the minimum amount of addition required to cause sufficient segregation is 6%. Further, the amount of Al is 10
% Or less is because if it exceeds this value, it is advantageous in terms of segregation, but the ductility extremely decreases and the creep strength decreases conversely.

II. Reason for Zn content of 4% or less Zn is effective in improving the ductility and accelerating the precipitation rate of the compound. Therefore, addition of Zn is effective. However, when the addition amount of Zn exceeds 4%, the eutectic temperature is remarkably lowered, and it takes a long time to form a solid solution of the eutectic compound. Therefore, the optimum addition amount is 4% or less.

III. Reasons for using the Mg alloy as it is cast The above alloys are used in the as-cast state (As-cast), that is, without heat treatment, for the following reasons. As described above, in order to improve the creep strength, it is effective to accelerate the precipitation of the compound by utilizing the segregation of the solid solution element. This is because such compound protrusion is most effectively obtained in the As-cast state.

IV. Reason for cooling rate of 10 ° C./sec or less When casting the above alloy, if the cooling rate exceeds 10 ° C./s, a dendritic structure is formed, and as a result, the creep strength decreases.

V. The size of the dust particles produced by aging treatment is 1
Reasons for setting μm or more Since Mg alloys are prone to dimensional changes when precipitation occurs, it is necessary to use them after sufficient precipitation for applications where dimensional changes are unfavorable. It is effective to do. Conventionally, a method of precipitating a fine compound to improve the yield strength has been adopted, but this method is not effective for improving the slack resistance of the bolt. As a result of studying this point, loosening of the bolt is caused by a stress relaxation phenomenon. Therefore, it is necessary to increase the creep strength on the low stress side to suppress this, and the larger the precipitate, the more effective it is. I found that.

That is, since the creep mechanism of the Mg alloy of the present invention, which is on the low stress side, is governed by the dislocation rising motion, the dislocation must overcome the precipitate in order to pass through the grain, and the creep rate is the grain speed. This is because it is inversely proportional to the size of. From this, it is necessary to make the particle shape larger for the slack of the bolt,
This is because, as a result of experiments, it was found that it is desirable to set the length of the long piece to 1 μm or more.

[0017]

According to the Mg alloy excellent in creep strength and the method for producing the same according to the present invention, as described in the above-mentioned constitution, the Mg alloy added with Al: 6 to 10% and Zn: 4% or less can be cast. Since it has been used up to now, it is possible to improve the creep strength and eventually the yield strength at high temperatures. Therefore, when the Al-Zn-based Mg alloy of the present invention is adopted as parts for automobiles, parts for computers, etc., it is possible to prevent slackening of bolts at high temperature, and it becomes possible to use it in a high temperature range which was difficult in the past. The applications can be expanded accordingly. Further, since it can be realized only by controlling the addition amount of the Al-Zn-based Mg alloy which is widely used at present, the cost can be reduced as compared with the case of the alloy to which rare earth is added, and the application can be expanded also from this point.

[0018]

EXAMPLES Examples of the present invention will be described below. In this example, a test conducted to confirm the effect of the Mg alloy of the present invention will be described. First, the samples used in this test will be described. An AZ91 material prepared by adding Al: 8.5% -Zn: 0.7% in combination is prepared, and is cast at a cooling rate of 10 ° C./sec or less. It was prepared (hereinafter, referred to as alloy A of the present invention). Moreover, the cast AZ91 material is 420 ° C. × 24
After homogenizing treatment for a period of time, aging treatment was performed at 250 ° C. for 10 hours to prepare a sample in which the size of precipitated particles was 1.2 μm or more (hereinafter, referred to as alloy B of the present invention).

For comparison, a homogenizing treatment and an aging treatment were performed, whereby the size of the precipitated particles was 0.2 μm.
The conventional AZ91-T6 material was used. further,
ZE41 alloy (Zn4% -R added with rare earth
E1.25% -Zr0.6%) was adopted as a comparative material.

Next, the test method will be described. This test was performed by a bolt slack test as shown in FIG. This has a thickness of 20 m consisting of the alloys A and B of the present invention.
12φ holes 1a are formed in the plate-shaped block 1 of m.
A steel bolt 2 having a shaft diameter of 8φ is inserted into a, and a nut 5 is screwed into the bolt 2 with washers 3 and 3 and a spring washer 4 interposed therebetween and tightened and fixed with a predetermined torque. The strain was measured with a strain gauge 6 attached to the.
Then, the changes in the surface pressure (MPa) during the temperature rise and the heating were examined, and the decrease amount of the surface pressure after heating for 170 hours was also examined. The same test was conducted on the conventional material and the comparative material.

2 and 4 are characteristic diagrams for explaining the above test results, respectively. Figure 2 shows about 15
FIG. 9 is a characteristic diagram showing a change in surface pressure during the temperature increase to 0 ° C. (tightening torque was 3.5 Kgm).
As is clear from the figure, in the case of the conventional material, the surface pressure sharply decreases when the temperature exceeds 80 ° C., and slack easily occurs. On the other hand, in the case of the alloys A and B of the present invention, 11
No decrease in surface pressure occurred up to about 0 ℃, 130 ℃
It can be seen that a sufficient surface pressure has been obtained to a certain extent, and a proof stress that is substantially the same as that of the comparative material has been obtained.

FIG. 3 shows that when heated to 130.degree.
FIG. 10 is a characteristic diagram showing changes in surface pressure during heating for 10 ⁵ min (tightening torques of 2.5 Kgm and 4 Kgm). As is clear from the figure, in the case of the conventional material, the decrease rate of the surface pressure increases with the passage of time, whereas in the case of the alloys A and B of the present invention, the decrease rate is small. Further, it can be seen that in the alloys A and B of the present invention in which the tightening torque is 2.5 kgm, the surface pressure hardly changes.

FIG. 4 is a characteristic diagram showing the test temperature and the decrease amount% from the peak surface pressure after heating for 170 hours. As is clear from the figure, the alloys A and B of the present invention have a temperature of 130 ° C.
The amount of decrease is about 20%, which shows that the bolt has slack resistance comparable to that of the comparative material.

Thus, according to the alloys A and B of the present invention, A
l, Zn amount is limited, and it is used in the as-cast state. Furthermore, the size of precipitated particles in the aging treatment is 1 μm.
Since it is set to m or more, as shown by the above-mentioned test results, it is possible to obtain the creep strength and the slack resistance of the bolt that are substantially the same as those of the ZE41 alloy to which rare earth is added. Therefore, when it is used as an automobile part, a computer part, or a precision machine part, the heat resistance at high temperatures can be improved, and the application can be expanded accordingly.

In the alloys A and B of the present invention, Al--Zn
Since the amount of Al and Zn added to the Mg-based alloy can be regulated and the production conditions can be set, the cost can be reduced and the number of parts can be increased as compared with the alloy containing rare earth. Can be handled at cost.

[0026]

As described above, according to the Mg alloy excellent in creep strength and the method for producing the same according to the present invention, Al: 6
-10%, Zn: 4% or less are added in combination, and the cooling rate, the size of precipitated particles, etc. are specified, so that the yield strength can be improved at low cost and at high temperature, and the application can be expanded. .

[Brief description of drawings]

FIG. 1 is a diagram illustrating a method for testing a Mg alloy according to an example of the present invention.

FIG. 2 is a characteristic diagram showing a change in surface pressure during temperature increase by the above test.

FIG. 3 is a characteristic diagram showing changes in surface pressure during heating by the above test.

FIG. 4 is a characteristic diagram showing the relationship between the residual surface pressure and temperature in the above test.

Claims (4)

[Claims] 1. An Mg alloy having excellent creep strength, which is characterized in that one or both of Al: 6 to 10% and Zn: 4.0% or less is added in combination. 2. The Mg alloy having excellent creep strength according to claim 1, which is used as it is cast. 3. When casting a Mg alloy formed by adding one or both of Al: 6 to 10% and Zn: 4.0% or less, the cooling rate in the solidification zone is 10 ° C. /
A method for producing a Mg alloy having excellent creep strength, which comprises casting into a mold so as to be less than or equal to sec and using the casting as it is. 4. The size of compound particles precipitated according to claim 3, wherein the cast Mg alloy is homogenized at a temperature of 350 to 420 ° C., cooled to room temperature, and then aged. Of Mg alloy having excellent creep strength, characterized in that