JPH0578775A - Magnesium alloy excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain an Mg alloy excellent in corrosion resistance by regulating the grain size of a beta crystallized substance, in an equilibrium state, to be dispersed to a specific value or below in an Mg alloy containing specific amounts of Al. CONSTITUTION:This is an Mg alloy containing 2-12wt.% Al. In this alloy, a beta crystallized substance (Mg17Al12) of <=10mum grain size in an equilibrium state is dispersed or a pearlite-state precipitate of alpha solid solution and beta crystallized substance is formed by >=60% by area ratio by means of heat treatment. The beta crystallized substance is dispersed in an alpha-Mg solid solution. The beta crystallized substance can be finely dispersed by means of rapid cooling, addition of a third element, etc., and corrosion resistance can be remarkably improved when the grain size of this beta crystallized substance is regulated to <=10mum. There are cases where the beta crystallized substance is dispersed in the grains and also where it is grown into pearlite state. Because corrosion resistance cane rapidly be improved when the amount of the beta crystallized substance in pearlite state reaches the prescribed value, the area ratio is regulated to >=60%.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a magnesium alloy having excellent corrosion resistance.

[0002]

2. Description of the Related Art Magnesium has a specific gravity of 1.74, which is the lightest in the weight of industrial metal materials, and its mechanical properties are not inferior to those of aluminum alloys. It has been attracting attention as a material that responds to fuel consumption.

Among conventional magnesium alloys, Mg-Al
Alloys (ASTM standard-AM60B, AM50A, AM
20A) contains 2 to 12% of Al, to which a small amount of Mn is added, and the Mg side is α-Mg solid solution and β.
-Mg ₁₇ Al ₁₂ compound eutectic system
Age hardening occurs due to the precipitation of the intermediate phase of ₁₇ Al ₁₂ . Further, solution treatment improves strength and toughness.

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,
Mg ₁₇ Al ₁₂ or Mg ₃₂ (Al, Z
n) Crystallization of either or both of the ₄₉ compounds. Although as-cast, it is tough and has excellent corrosion resistance, but mechanical properties are improved by aging heat treatment, and a compound phase precipitates in pearlite form at grain boundaries by quenching and tempering.

[0005]

However, these magnesium alloys containing aluminum have poor corrosion resistance, which is a major obstacle to the application of parts. Therefore, appropriate corrosion protection is applied regardless of cast or wrought material. There is a need.

The corrosion resistance of magnesium alloys is particularly inferior to gravity cast materials compared to die cast materials.
The results of the salt spray test of the material are shown below. The corrosion weight loss at 400 hours is 0.12 to 0.18 mg / mm for the die cast material.
² , the weight loss of gravity cast material is 0.6m
It can be as ^high as g / mm ² .

The present invention has been made to solve the problem that conventional magnesium alloys containing aluminum have poor corrosion resistance, and an object thereof is to provide a magnesium alloy having excellent corrosion resistance.

[0008]

The inventor has shown that A shown in FIG.
From the results of a salt spray test on the casting surface of Z91D alloy die cast material, the machined surface, and the machined surface of gravity cast material, we found that there is a close relationship between β crystallized grain size and corrosion resistance. It was Therefore, based on the salt spray test results of FIG. 3, as shown in FIG. 4, the relationship between the β crystallized product particle size and the corrosion resistance was summarized. As shown in FIG. 4, it became clear that the grain size of the β crystallized substance was reduced and the corrosion resistance was improved.

Based on this knowledge, as a result of further studies to clarify the relationship between the β crystallized substance particle size and the corrosion resistance, as a result, the β crystallized substance particle size was smaller than that of the die-cast material at a certain level. The present invention has been completed by newly finding that the corrosion resistance is remarkably improved when the content is within the range.

The magnesium alloy excellent in corrosion resistance according to the present invention is a magnesium alloy containing 2 to 12% by weight of aluminum, and β crystallized products (Mg ₁₇ Al ₁₂ ) in an equilibrium state having a grain size of 10 μm or less are dispersed. That is the summary.

As shown in the micrograph of the metal structure of the die-cast material (AZ91D) shown in FIG. 10, the Mg-Al alloy in the as-cast state has β-crystallized products of α-Mg.
Dispersed in solid solution. Further, when heat treatment after gravity casting (for example, T-6 treatment) is performed, β precipitates are formed as shown in the micrograph showing the metal structure of the gravity cast heat treated material (AZ91C-T6) in FIG. 11. It is known that all of these β-phase compounds are Al-rich Mg ₁₇ Al ₁₂ metal compounds.

The β crystallized product can be finely dispersed by quenching, addition of a third element, or the like. In addition, the β precipitate may be finely dispersed in the grain, or may be grown in a pearlite state at a layer interval of several μm from the grain boundary.

Therefore, the inventor examined the relationship between the morphology of β precipitates and corrosion resistance. Figure 6 shows F of AZ91D alloy
The salt spray test results of the (as cast) material, the T4 treated (solution treated) material, and the T6 treated (solution treated-aging) material are shown. From the results shown in FIG. 6, the corrosion resistance was highest in the T6 treated material in which β precipitates were microscopically dispersed.

Based on this knowledge, the inventor further studied the relationship with the corrosion resistance by changing the morphology of β precipitates. As a result, the inventors have newly found that when the amount of pearlite-like β precipitates generated from grain boundaries reaches a predetermined amount, the corrosion resistance is rapidly improved, and the present invention has been completed.

The magnesium alloy excellent in corrosion resistance of the second invention is a magnesium alloy containing 2 to 12% by weight of aluminum, wherein the α solid solution and the β phase compound (Mg ₁₇ A
The area ratio of the pearlite-like precipitates of ( ₁₂ ) is 6 by heat treatment.
The main point is to form 0% or more.

The aluminum to which the present invention is applied is 2-1.
A magnesium alloy containing 2% by weight has an A content of 2 to 12%.
alloy containing 1 and a small amount of Mn added thereto, for example, Mg-Al alloy (ASTM standard-AM60B, AM
50A, AM20A, etc.) or 5-10% of Al,
An alloy containing 1 to 3% Zn, for example Mg-Al-Zn
System (ASTM standard-AZ91D, etc.).

Β crystallized or precipitated in α-Mg solid solution
The phase compound is mainly Mg ₁₇ Al ₁₂ ,
_{A part of 32} (Al, Zn) ₄₉ compound may be mixed.
Further, the grain size of the β crystallized product can be controlled by changing the cooling rate after casting. Further, in order to precipitate the β-phase compound in a pearlite state at a desired area ratio, T6 is used.
This can be achieved by appropriately selecting the aging temperature and time in the treatment. In order to precipitate the α solid solution and the β phase compound in a pearlite state with an area ratio of 60% or more, Al
It is preferable that the amount is 5% or more because it is easy.

[0018]

[Function] In a magnesium alloy containing aluminum, when α-solid solution and β compound coexist, corrosion is α
Preferentially progresses from solid solution. Corrosion that proceeds in the α phase stops at the β phase interface. This is because the oxide film formed on the Al-rich β-phase compound is strong.

The above corrosion mechanism is schematically shown in the case where the β-phase compound in FIG. 7 is finely crystallized, in the case of pearlite-like precipitation in FIG. 8 and in the case of coarse grains in FIG. It will be described with reference to the drawings. 7 to 9 show the case where the magnification is the same and there are three corrosion nuclei per unit area. When the β crystallized substance particle size in FIG. 9 is large, when the corrosion progresses to the β crystallized substance, the area per α solid solution is large, so that the corrosion spreads over a wide range.

However, when the β-phase compound of FIG. 7 is finely crystallized or when it is precipitated in the pearlite form of FIG. 8, the area of the α solid solution surrounded by the β-phase compound is very small. Even if the corrosion progresses to reach the β-phase compound, the area to be corroded remains in a small range.

The alloy of the present invention has a particle size of 10 μm in α solid solution.
Since the following fine β crystallized substances were dispersed, as shown in FIG. 7, even if the corrosion progressed to β crystallized substances, the corroded area remained in a small range and the corrosion resistance was excellent. It becomes an alloy. Further, in the second invention alloy, since the pearlite precipitates were formed by the heat treatment in an area ratio of 60% or more, the area of the α solid solution surrounded by the β phase compound was very narrow and small as shown in FIG. Similarly, the area that is corroded remains in a small range, and the alloy has excellent corrosion resistance.

In the present invention, the β crystallized product (M
The grain size of g ₁₇ Al ₁₂ ) is set to 10 μm or less because if the grain size exceeds 10 μm, the area of the α solid solution becomes large and the effect of improving the corrosion resistance cannot be obtained. In addition, the area ratio of pearlite precipitates formed by heat treatment is 60%.
The reason for the above is that when the area ratio is 60% or less, the area of the α solid solution increases and the effect of improving the corrosion resistance cannot be obtained.

[0023]

EXAMPLES Examples of the present invention will be described to clarify the effects of the present invention. (Example 1) The amount of molten metal was 20 g to 15 in a water-cooled copper hearth.
Change the cooling rate in 6 steps between 0g and AZ9
A 1D molten metal was poured into it to prepare a rod-shaped test piece. The results of measuring the particle size of β crystallized substances on the surface of each test piece are shown in FIG. 5 as a relationship diagram with the cooling rate.

Further, using this test piece, 400
A salt spray test was performed for an hour to measure the corrosion weight loss. The obtained results are shown in FIG. 1 as a relationship diagram with the particle size of β crystallized substances. As shown in Fig. 1, the corrosion weight loss is due to the crystal grain size of 10μ.
When it becomes less than m, it sharply decreases, and when it reaches 7 μm, the decrease amount of corrosion weight loss becomes saturated. As a result, it was confirmed that a magnesium alloy having excellent corrosion resistance can be obtained by making the grain size of the β crystallized product smaller than that of the die-cast material and 10 μm or less.

From the relationship with FIG. 5, 10 ⁴ ° C / se
It was found that at a cooling rate of c or higher, there is no further improvement in corrosion resistance. Rather, premature cooling forms a supersaturated solid solution and reduces the amount of β-phase crystallized substances, which is rather thought to be the opposite effect.

(Example 2) After subjecting the AZ91D alloy to solution treatment, the aging temperature and time are controlled to form a pearlite precipitate of an α solid solution and a β phase compound in an area ratio of 30 to 100.
It was made to precipitate by changing between%. The obtained sample was subjected to a salt spray test for 400 hours to measure the corrosion weight loss. The obtained results are shown in FIG. 2 as a relational diagram with the area ratio of the pearlite precipitates.

As is clear from FIG. 2, it was found that the corrosion weight loss drastically decreases from the area ratio of 30% to 60% and gradually decreases from 60% to 100%. From the above results, it was confirmed that a magnesium alloy excellent in corrosion resistance can be obtained by forming a pearlite precipitate of an α solid solution and a β phase compound by heat treatment so as to have an area ratio of 60% or more.

[0028]

As described above, the magnesium alloy excellent in corrosion resistance of the present invention is a magnesium alloy containing 2 to 12% by weight of aluminum, and is a β-phase compound (Mg ₁₇ Al ₁₂ ) in equilibrium with a grain size of 10 μm or less. ) Is dispersed, or α solid solution and β crystallized product (Mg ₁₇ Al ₁₂ ).
The pearlite-like precipitate of No. 2 was formed by heat treatment in an area ratio of 60% or more. Since the area of the α solid solution surrounded by the β phase compound is very small, the α solid solution is corroded. Even if the corrosion progresses up to the β-phase compound, the corroded area remains in a small range, so that the magnesium alloy has excellent corrosion resistance.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the crystal grain size of β crystallized substances and the corrosion weight loss in a salt spray test.

FIG. 2 is a graph showing the relationship between the area ratio of pearlite precipitates of β compound and the corrosion weight loss in a salt spray test.

FIG. 3 is a diagram showing the relationship between the corrosion weight loss and the spraying time of a salt spray test of AZ91 material.

[Fig. 4] Corrosion weight loss and β by salt spray test of AZ91 material
It is a diagram which shows the relationship with a crystallized substance particle size.

5 is a diagram showing the relationship between the cooling rate and the crystal grain size of β crystallized substances in Example 1. FIG.

FIG. 6 is a diagram showing the relationship between the corrosion weight loss and the spray time of a heat-treated AZ91 material by a salt spray test.

FIG. 7 is a diagram schematically showing a situation of corrosion when a β-phase compound is finely crystallized.

FIG. 8 is a diagram schematically showing a situation of corrosion when a β-phase compound is precipitated in a pearlite form.

FIG. 9 is a diagram schematically showing a situation of corrosion when β crystallized substances are coarsely crystallized.

FIG. 10 is a 100 × micrograph showing the metal structure of a die cast material (AZ91D).

FIG. 11 is a 100 × micrograph showing the metal structure of a gravity cast heat-treated material (AZ91C-T6).

Claims (2)

[Claims] 1. A magnesium alloy containing 2 to 12% by weight of aluminum, wherein β crystallized substances (Mg ₁₇ Al ₁₂ ) in an equilibrium state having a grain size of 10 μm or less are dispersed, and the corrosion resistance is improved. Excellent magnesium alloy. 2. A magnesium alloy containing 2 to 12% by weight of aluminum, wherein α solid solution and β phase compound (M
A magnesium alloy excellent in corrosion resistance, characterized in that pearlite precipitates of g ₁₇ Al ₁₂ ) are formed by heat treatment in an area ratio of 60% or more.