JPH0853722A - Production of magnesium-base alloy excellent in high temperature creep strength - Google Patents

Abstract

PURPOSE:To develop an Mg-base alloy having high proof stress and excellent in high temp. creep properties by adding specified amounts of rare earth elements, Al or the like to Mg. CONSTITUTION:At the time of casting the molten metal of an Mg-base alloy having a compsn. contg., by weight, 0.2 to 7% lanthanoid-series rare earth elements such as La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu as well as Sc and Y and 1 to 10% Al or furthermore contg. <2% Mn, the casting is rapidly cooled at >=7 deg.C/sec cooling rate. Al is added to the Mg alloy to improve its proof stress, furthermore deterioration in the solid solution rate of rare earth elements as the elements for improving its high temp. creep caused by the addition of Al is prevented by the rapid cooling of the casting to enter >=0.2% rare earth elements into solid solution, and moreover, Mn is incorporated by <=2% as the element for improving its corrosion resistance. The Mg alloy casting having the minimum creep rate of less than 1X10<-4>%/hr at 5kgf/mm<2> (at 150 deg.C) and excellent in strength, creep properties and corrosion resistance can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a Mg-based alloy having high yield strength and excellent high temperature creep characteristics.

[0002]

2. Description of the Related Art Mg-based alloys are one of the lightest alloys currently in practical use, and are used to reduce transportation costs in various industrial fields including the automobile industry, such as transportation equipment. As one of the measures to reduce the weight of
The fields of application of g-based alloys are also gradually increasing.

However, when the Mg-based alloy is applied to, for example, a portion of an automobile part exposed to a high temperature, creep deformation occurs and not only the deformation of the component becomes a problem, but also loosening of the component mounting bolt during use is promoted. The problem of causing it will arise. Therefore, it has been attempted to contain a rare earth element as a means for enhancing the high temperature creep property of the Mg-based alloy, but it is not possible to sufficiently enhance the mechanical properties such as tensile strength and proof stress alone, so that the structural material Lacks aptitude as.

From such a point, for example, British Patent No. 8
75929, Japanese Patent Publications 54-11765, 59-1
In Japanese Patent No. 8457, etc., a method has been developed in which an Mg-based alloy contains an effective amount of a rare earth element and an expensive alloying element such as Ag is added to enhance the high temperature creep property. However, these methods have a problem in versatility for economical reasons as a general-purpose structural material for automobiles because the cost is increased by adding expensive alloy elements.
Further, in JP-A-46-6202, a Mg-based alloy contains Al and Mn together with a rare earth element.
Methods for improving strength and creep properties are also disclosed,
With this method, the alloy elements used are also inexpensive,
It is considered that it can be put to practical use without causing any difficulty in cost.

However, since this method is limited to die-cast casting, it can be effectively used for castings with a special shape, but it is difficult to apply it to fields where the die-cast method cannot be applied. There is a problem with the versatility of. Moreover, the above-mentioned publication does not make a sufficient theoretical investigation into the existence forms of Al and rare earth elements added to the Mg-based alloy, but merely shows the influence of the addition amount of these alloy elements. However, the solid solution state of each alloying element, which is thought to change significantly depending on the cooling rate and the like, has not been clarified at all.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its purpose is to obtain Mg
In particular, by clarifying the existence form of rare earth elements in the Al-based alloy, it is possible to suppress Al obstruction due to the addition of rare earth elements, which has a detrimental effect on the high temperature creep characteristics, as much as possible.
It is intended to establish a manufacturing method of an Mg-based alloy that can effectively exert the strength improving effect by the addition of 1 and satisfy both strength characteristics and high temperature creep characteristics.

[0007]

The structure of the manufacturing method according to the present invention, which has been able to solve the above-mentioned problems, is a rare earth element: 0.
2 to 7 wt% and Al: 1 to 10 wt%, the balance of the Mg-based alloy melt consisting essentially of Mg is rapidly cooled at a rate of 7.0 ° C./sec or more to reduce the rare earth element content to 0.2. With 5% by weight or more solid solution, a stress of 5 kgf / mm ² (at150
The main point is to obtain a minimum creep rate of 1 × 10 ⁻⁴ % / hr or less at (° C.). In carrying out the above method, when a Mg-based alloy containing 2% by weight or less of Mn as another element is used,
It is preferable because the corrosion resistance of the system alloy can be enhanced.
In addition, as the rare earth element (RE) used in the present invention,
In addition to Sc, Y, La series elements La, Ce, Pr, N
d, Pm, Sm, Eu, Gd, Tb, Dy, Ho, E
r, Tm, Yb, Lu, etc. are included, and these can be used alone or as a mixture of two or more kinds. Therefore, a misch metal which is a mixture containing a rare earth element as a main component is also used. be able to.

[0008]

OPERATION AND EXAMPLES Hereinafter, the structure and operation and effect of the present invention will be described in detail with the background of the experiment. First of all,
Creep curve of Mg-2% RE alloy (stress: 10 kg /
mm ^2, a temperature: 0.99 ° C.) and shows the, this alloy has a low yield strength, the amount of change the moment of adding the load, i.e. the moment although distortion is large, immediately thereafter creep strain most gone later Does not cause deformation,
It exhibits very good creep properties. Therefore, in order to effectively utilize such creep characteristics in practical use, without impairing such excellent creep characteristics of the Mg-RE alloy,
It is necessary to add a third alloying element that can increase the yield strength to a practical level.

Here, when the reason why the high temperature creep property is enhanced by adding the RE element to the Mg alloy was investigated, it was found that among the added RE elements, the Mg alloy had a favorable effect on the improvement of the creep property. When the Mg-based alloy, which is a solid solution of RE element, that is, in the state where the RE element is in solid solution, is subjected to high-temperature creep deformation, very fine Mg-RE-based compound of 0.1 μm or less is deposited, which is high-temperature creep. It was confirmed that it had a remarkable influence on the improvement of the characteristics.

However, when Al is added as a proof stress improving element to this Mg-RE type alloy, for example, as shown in FIG. 2, the solid solution of the RE element in the alloy necessary for the precipitation of the Mg-RE type compound is obtained. It was revealed that the amount decreased sharply, and by extension, the effect of improving the high temperature creep characteristics by adding the RE element was hardly exhibited. The influence of the added amount of Al on the reduction of the solid solution amount of the RE element appears remarkably when a small amount of about 1.0 wt% is added, and even if the added amount of Al is further increased, the solid solution amount of the Re element is increased. Is almost unchanged. Therefore, in an Mg-RE-Al based alloy containing 1 wt% or more of Al as a proof stress improving element in the Mg-RE based alloy, in order to effectively exert the high temperature creep property improving effect by the RE element inclusion, Nevertheless, it is necessary to clarify other means capable of increasing the solid solution amount of RE element in the alloy.

From this point of view, studies have been conducted from various angles. Even in the Mg-RE-Al alloy, the molten metal cooling rate when casting the molten alloy is increased to 7.0 ° C./sec or more. It was revealed that the solid solution amount of the RE element due to the coexistence of Al can be remarkably increased, and the high temperature creep property of the alloy cast product can be remarkably enhanced.

By the way, FIG. 3 shows Mg-2% RE-4% A.
Table 1 and FIG. 4 show the relationship between the cooling rate at the time of casting a molten alloy and the amount of RE solid solution in the cast alloy.
Shows the results of examining the relationship between the cooling rate of the obtained cast product and the minimum creep rate. From FIG. 3, it is seen that the RE solid solution starts when the cooling rate during casting exceeds 7 ° C / sec. The amount of RE solid solution increases sharply at 10 ° C./sec or more, and from Table 1 and FIG. 4, the cooling rate is 7 ° C./sec or more, more clearly 9 ° C. / Sec or more, the minimum creep speed is 1 ×
It can be seen that a very small value of 10 ⁻⁴ % / hr or less is exhibited.

[0013]

[Table 1]

That is, as is clear from FIGS. 3 and 4,
A clear correlation was found between the cooling rate, the amount of RE elemental solid solution, and the minimum creep rate. These results are summarized as follows: By setting the cooling rate during casting to 7 ° C / sec or more, If a solid solution amount of RE element of not less than wt% is secured, the high temperature creep property of the obtained Mg-RE-Al alloy cast product is 1 x 10 ^-4 ° C / h at the minimum creep rate.
It can be confirmed that it can be made very excellent with r or less.

As specific means for obtaining the above cooling rate, a method of increasing the cooling rate by reducing the heat capacity of the molten alloy by reducing the wall thickness of the cast product as much as possible, and cooling the mold with water It is possible to adopt a method of increasing the cooling rate by using a structure or using a mold material having high heat conductivity, etc. Furthermore, it is also possible to perform rapid cooling by a roll casting method or a water atomizing method. There are no restrictions on the means.

Considering the above results, the effect of improving the high temperature creep characteristics by increasing the amount of RE element solid solution is as follows:
Cooling rate is 7 ° C / sec or more, more preferably 10 ° C / sec
By setting it to be not less than sec, the solid solution amount of RE element is 0.2.
When the cooling rate is 10 ° C./se, which is remarkably exhibited by setting the content by weight% or more, more preferably 0.5 weight% or more.
Even if the RE elemental solid solution amount is increased to about 1.0% or more by increasing it to c or more, almost no further decrease in the minimum creep rate is observed. From such a tendency, the effect of improving the high temperature creep property by increasing the solid solution amount of the RE element can be sufficiently exerted if the solid solution amount of the RE element is 0.2 to 1.0% by weight. It can be seen that the object of the present invention can be achieved by setting the conditions.

Therefore, in order to achieve the object of the present invention, R
The content of the E element should be at least 0.2% by weight or more, but the solid solution amount of the RE element is naturally limited, and as described above, the solid solution amount of the RE element exceeds 1.0%. However, the high-temperature creep property is not improved more than that, and if it is too much, the elongation rate of the cast product tends to decrease obviously as shown in Table 2, so that it is preferably about 5% or less, more preferably It is desirable to suppress it to about 2.5% or less.

[0018]

[Table 2]

As for Al, as shown in FIG. 2, even if the content of Al exceeds 1% by weight, the effect of inhibiting the solid solution of RE element hardly changes, and the adverse effects thereof are as described above. Since it can be suppressed by setting such a cooling rate, the content can be up to about 1 to 10% by weight so that the effect of improving the strength by adding Al can be effectively exhibited. But,
As is clear from Table 3 below, if the Al content is too high, the elongation of the alloy cast product will be adversely affected, so it is preferably 10% by weight or less, more preferably 6% by weight or less. Is desired.

[0020]

[Table 3]

Further, according to the present invention, in addition to the improvement of proof stress and high temperature creep characteristics by the inclusion of RE element and Al, an appropriate amount of M is added.
Increasing the corrosion resistance by incorporating n is also very effective in practical use. By the way, Table 4 shows Mg-2%
Corrosion loss of RE-5% Al alloy with appropriate amount of Mn added (sodium chloride solution was sprayed on the sample,
This is a test for evaluating the corrosion resistance of the material by accelerating the corrosion, and the results of examining the effect on the test conditions: salt concentration 5%, test temperature 35 ° C. are shown. If Mn of about 2.0 wt% or less is added in combination, the corrosion resistance of the alloy cast product can be significantly improved. However, if the Mn content becomes too large, the elongation of the alloy tends to decrease (the alloy becomes brittle).
The Mn content should be kept below about 2.0% by weight.

[0022]

[Table 4]

[0023]

EFFECTS OF THE INVENTION The present invention is constituted as described above, and by defining the cooling rate of the molten alloy, it is possible to prevent the RE element solid solution amount from being drastically reduced by the addition of Al and to reduce the content of 0.2 wt%
By securing the above RE element solid solution amount, both the yield strength and the high temperature creep property can be enhanced, and further, it is possible to enhance the corrosion resistance by adding an appropriate amount of Mn in combination, the strength, the creep property, It has become possible to provide a high-performance Mg-based alloy cast product that satisfies all the required properties of corrosion resistance.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the strain amount of Mg-2% Re alloy and time.

FIG. 2 is a graph showing the influence of the amount of Al in the Mg alloy on the amount of RE solid solution.

FIG. 3 is a graph showing the influence of the cooling rate on the RE solid solution amount.

FIG. 4 is a graph showing the relationship between the cooling rate during casting and the minimum creep rate.

Claims (2)

[Claims] 1. A rare earth element: 0.2 to 7% by weight, and A
1: 1 to 10% by weight, the balance of the Mg-based alloy melt consisting essentially of Mg is rapidly cooled at a rate of 7.0 ° C./sec or more to form a solid solution of 0.2% by weight or more of a rare earth element. ,
1 × 10 ^-4 % at a stress of 5 kgf / mm ² (at 150 ° C)
A method for producing a Mg-based alloy having excellent high-temperature creep characteristics, which is characterized by obtaining a minimum creep rate of / hr or less. 2. The method for producing a Mg-based alloy according to claim 1, wherein the Mg-based alloy further contains 2% by weight or less of Mn as another element.