JPH05171333A - Magnesium alloy excellent in heat resistance, corrosion resistance and castability - Google Patents

Abstract

PURPOSE:To provide a magnesium alloy excellent in heat resistance, corrosion resistance and castability. CONSTITUTION:The objective magnesium alloy contains, by weight, 1.0 to 6.0% Zn, 0.1 to 2.0% R.E., 0.1 to 2.0% Zr, 0.1 to 3.0% Si and 0.1 to 6.0% Al, and the balance Mg with inevitable impurities. By Al and Zn, its castability can be improved as well as its room temp. strength can be improved. Moreover, by adding R.E. besides Al and Zn, Mg-Al-Zn-R.E crystallized products are crystallized out on the grain boundaries to improve its high temp. strength. In addition, the content of R.E. is reduced in a range in which its high temp. strength can be maintained, by that, its castability is made excellent, its tensile strength at a room temp. is made high and its high temp. properties and creep properties are made improved. Furthermore, R.E. forms a R.E. rich protective film in the initial period of corrosion to improve its corrosion resistance. By the incorporation of Zr, its room temp. strength and high temp. strength improve without deteriorating its castability, and, by the incorporation of Si, its creep resistance improves.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnesium alloy having excellent heat resistance, corrosion resistance and castability.

[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, Al is 5 to 5
In a Mg-Al-Zn system containing 10% and 1 to 3% Zn (ASTM standard-AZ91C, etc.), a wide α on the Mg side.
-There is a solid solution region, and the Mg-Al-Zn compound crystallizes out. 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.

On the other hand, a magnesium alloy having excellent heat resistance and suitable for use at high temperature has been sought, and an alloy containing a rare earth element has a mechanical property somewhat inferior to that of an aluminum alloy at room temperature, but at a high temperature of 250 to 300 ° C. It has been found that properties comparable to alloys are obtained. For example, R. E. As a practical alloy containing
n-free EK30A alloy (2.5-4% RE-
0.2% Zr), ZE41A alloy (1% RE, -2.0% Zn-0.6% Zr) and the like have been put to practical use as those containing Zn.

[0005]

However, Mg-
Al-Zn-based AZ91C alloy (Mg-9% Al-1%
Zn) has a high Al content and a good molten metal flow and is excellent in castability, but since the α solid solution crystallizes in a resinous state in the solidification process, there is a problem that cast holes are likely to occur.
Such a porosity often becomes a starting point of fracture, and FIG. 2 is a micrograph showing a metal structure of a fracture example starting from the porosity, and FIG. 3 shows the positions of the porosity in the photograph of FIG. It is a mimicking figure shown.

[0006] Further, Mg-Al system or Mg-Al-Z
In the n-based alloy, the Mg ₁₇ Al ₁₂ compound crystallizes at the grain boundaries, and the crystallized product is unstable at high temperatures, so that the strength and creep resistance at high temperatures are greatly reduced. Figure 4 shows stress 6 at 373K, 393K and 423K of AZ91C alloy.
The creep curve in the case of 3 MPa is shown, but the creep strain is remarkably increased at 423K.

Further, as shown in FIG. 5, the axial force retention rate in the bolt loosening test is as follows.
Aluminum alloy 98% at 5 kg / mm ² , 100 hours, heat resistant alloy with rare earth element added (EQ21A)
Is 80%, whereas that of AZ91C which is a Mg-Al-Zn alloy is 40%. In addition, the axial force retention rate is a value obtained by tightening a cylindrical test piece with a bolt nut and leaving it in a furnace at 150 ° C., then measuring the elongation of the bolt and directly measuring the axial force. It is a simple measure.

On the other hand, an alloy doped with a rare earth element, for example ZE41A alloy (Mg-4% Zn-1 % R.E.) In, Mg ₂₀ Zn ₅ R. in grain boundaries E. Due to the presence of the _two crystallized substances, properties comparable to those of aluminum alloys can be obtained at high temperatures up to 250 to 300 ° C. FIG. 6 shows the tensile creep curves of AZ91C and ZE41A at a test temperature of 423K and a stress of 63 MPa. The ZE41A alloy has AZ9.
The creep resistance is remarkably superior to that of the 1C alloy.

In a magnesium alloy to which a rare earth element is added, micro-shrinkage occurs and causes a defect. Therefore, this micro-shrinkage is filled with a eutectic structure,
In order to form a complete ingot, Mg-R. E. Always Z for alloys
r is added. However, these alloys are not suitable for casting because the addition of Zr causes casting cracks. Further, the crystallized substance deteriorates the flowability of the molten metal and causes cracking in casting.

The present invention is a Mg-Al-Zn-based AZ91C.
The alloy is excellent in castability but inferior in high temperature strength and creep resistance, and the ZE41A alloy added with a rare earth element is excellent in heat resistance but inferior in castability. Therefore, it is an object of the present invention to provide a magnesium alloy having improved castability while ensuring high-temperature strength and creep resistance comparable to those of the ZE41A alloy and also improved corrosion resistance.

[0011]

Means for Solving the Problems The present inventor has conducted extensive studies on improving castability while securing high temperature strength and creep resistance, and conceived to add Al to an alloy based on the ZE41A alloy. The optimum composition range has been found that can improve castability while securing high temperature strength. Further, R. E. And reduce the contents of Zr within the range where high temperature strength can be maintained,
The present invention was completed by improving the creep resistance by adding Si.

The magnesium alloy excellent in heat resistance, corrosion resistance and castability of the present invention has a weight ratio of Zn: 1.0-6.
0%, R. E. 0.1-2.0%, Zr; 0.1
2.0%, Si; 0.1 to 3.0%, Al; 0.1
The gist is that it contains 6.0% and the balance consists of Mg and unavoidable impurities.

[0013]

The function of the magnesium alloy of the present invention is Al;
6.0%, Zn; since 1.0 to 6.0% was contained,
In magnesium alloys with improved castability and addition of rare earth elements, Mg-Al-Zn-based crystallized substances whose brittleness was improved compared with the crystallized substances of conventional materials were uniformly dispersed in the crystal grains. , Room temperature strength can be improved.

In addition to Al and Zn, R. E. By adding 0.1 to 2.0% of Mg-Al-Zn-R.
E. Since the system crystallization product was crystallized at the grain boundaries, the high temperature strength was improved. In addition, R. E. Since the content has been reduced within the range where high temperature strength can be maintained, the heat resistant magnesium alloy has excellent castability, high tensile strength at room temperature, and excellent high temperature characteristics and creep characteristics. In addition, R. E. R.I. E. It forms a rich protective film and improves corrosion resistance.

Further, the Zr content of 0.1 to 2.0% improves room temperature strength and high temperature strength without deteriorating castability, and the Si content of 0.1 to 3.0% improves creep resistance. Is improved.

Next, the reason why the component range of the magnesium alloy of the present invention is limited will be described. Zn; 1.0 to 6.0% Zn improves the room temperature strength of the magnesium alloy and improves the castability. In order to obtain the above effect, it is necessary to contain at least 1.0% or more. However, if contained in a large amount, the high temperature characteristics deteriorate and casting cracks easily occur, so the upper limit was made 6.0%.

R. E. 0.1-2.0% R.I. E. Is an element that improves the high temperature strength and creep resistance of magnesium alloys. In order to obtain the above effect, it is necessary to contain at least 0.1% or more. However, if it is contained in a large amount, the castability is lowered and it causes casting cracks, so the upper limit was made 2.0%.

Al: 0.1-6.0% Addition of Al improves the room temperature strength and the castability of the magnesium alloy. In order to obtain the above effect, it is necessary to contain at least 0.1% or more.
However, if contained in a large amount, the high temperature characteristics deteriorate, so
The upper limit was 6.0%. The upper limit of the preferable content of Al is 5.0%.

Zr: 0.1 to 2.0% Zr improves the room temperature strength and high temperature strength of the magnesium alloy. In order to obtain the above effect, it is necessary to add at least 0.1% or more. Further, if contained in a large amount, the castability deteriorates, which causes casting cracking, so the upper limit is 2.0%.
And

Si: 0.1 to 3.0% Si improves the creep resistance of the magnesium alloy.
It is considered that this is because fine Mg ₂ Si is precipitated by the T ₄ heat treatment, which becomes an obstacle to dislocation movement. However, if it is contained in a large amount, the castability deteriorates and it causes casting cracks, so the upper limit was made 3.0%.

[0021]

EXAMPLES Examples of the present invention will be described together with comparative examples to clarify the effects of the present invention. (Example 1) Magnesium alloys having the chemical components shown in Tables 1 and 2 were melted, a casting temperature of 690 ° C, and a mold temperature of 80 to
The test piece was gravity cast at 120 ° C.

[0022]

[Table 1]

[0023]

[Table 2]

In Tables 1 and 2, the numbers 1 to 1
5 has a Zn content of 4% and R. E. The content is fixed to 2% and the Al content is changed, and the numbers 1 to 3 are A
1 is an invention example in which the content is within the composition range of the present invention, No. 4 is a comparative example containing no Al, and No. 5 is a comparative example containing Al in the composition range of the present invention or more.

Nos. 6 to 10 have an Al content of 4% and R.
E. The content was fixed to 2% and the Zn content was changed, and numbers 6 to 8 are invention examples in which the Zn content is within the composition range of the present invention, and number 9 is a comparative example not containing Zn. No. 10 is a comparative example containing Zn in the composition range of the present invention or more.

Nos. 11 to 14 have an Al content of 4% and Zn
By fixing the content to 4%, R. E. The contents are changed, and the numbers 11 to 12 are R.I. E. Inventive example in which the content is within the composition range of the present invention, No. 13 is R. E. No. 14 is a comparative example containing no R. E. Is a comparative example containing at least the composition range of the present invention.

The tensile strengths of the obtained test pieces were measured at room temperature and 150 ° C., and the results are shown in FIG. 7 for the numbers 1 to 5 in which the Al content was changed, and in the numbers in which the Zn content was changed. 6 to 10 are shown in FIG.
E. FIG. 9 shows the numbers 11 to 14 in which the contents were changed.
Respectively shown.

As shown in FIG. 7, the Zn content is 4%,
R. E. When the content is fixed at 2%, the tensile strength at room temperature increases with increasing Al content and exceeds 240 MPa at 1% Al. Further, the tensile strength at 150 ° C exceeds 200 MPa at an Al content of 1.0%,
The maximum content is 4%, and the tensile strength decreases with increasing Al content.
It becomes below MPa. As a result, the Al content is 1.0 to 6.
At 0%, the tensile strength at room temperature was 240 MPa or higher, and the tensile strength at 150 ° C. was 200 MPa or higher, confirming the effect of the present invention.

As shown in FIG. 8, the Al content is 4%,
R. E. When the content is fixed at 2%, the tensile strength at room temperature increases with increasing Zn content and exceeds 240 MPa at 1% Zn. Moreover, the tensile strength at 150 ° C. exceeds 200 MPa at a Zn content of 1.0%,
The maximum content was 4%, and the tensile strength decreased with increasing Zn content.
It becomes below MPa. As a result, the Zn content is 1.0 to 6.
At 0%, the tensile strength at room temperature was 240 MPa or higher, and the tensile strength at 150 ° C. was 200 MPa or higher, confirming the effect of the present invention.

As shown in FIG. 9, the Al content is 4%,
When the Zn content is fixed at 4%, the tensile strength at room temperature is R.V. E. It decreases as the content increases, and R.I. E. When the content exceeds 2%, it becomes 240 MPa or less. Also, 1
The tensile strength at 50 ° C. is R. E. The content rapidly increases up to a content of 1%. E. The tensile strength gradually decreases as the content increases, and R. E. 20 if the content exceeds 2%
It becomes 0 MPa or less. As a result, R. E. Content 0.1
~ 2.0%, the tensile strength at room temperature is 240M
It was found that the tensile strength at 200 Pa or higher at 150 ° C was 200 MPa or higher, and the effect of the present invention was confirmed.

FIG. 10 shows a metallographic structure of a test piece, which was heat-treated at 330 ° C. for 2 hours in accordance with the invention example No. 12, 100.
Double microscope image, and FIG. 11 is a 250 × microscope image. As is clear from the photographs of FIGS. 10 and 11, Mg-Al-Zn-R. E.
It can be clearly seen that the crystallized substances are crystallized at the grain boundaries. Further, FIG. 12 shows the test piece No. 15 of the present invention as T ₄
It is a microscope photograph of 250 times showing the metal structure at the time of processing. It can be confirmed from the photograph of FIG. 12 that fine acicular Mg ₂ Si is deposited.

(Example 2) Mg-4% as the alloy of the present invention
Zn-3% Al-1% R.I. E. -0.4% Zr-0.4
% Si (all weight%, the same applies below) was melted, and a test piece was gravity cast at a casting temperature of 690 ° C. and a mold temperature of 80 to 120 ° C. This test piece is tested at the test temperature 423
A tensile creep test was performed at K and a stress of 63 MPa to obtain a creep curve. For comparison, test pieces of the AZ91C alloy and the ZE41A alloy were also cast under the same casting conditions, and the tensile creep curve was determined under the same test conditions. The obtained results are also shown in FIG.

As shown in FIG. 13, the material of the present invention is 300
The creep strain is about 1.5% smaller than that of the AZ91C alloy in time, and the creep strain is almost the same as that of the ZE41A alloy. Was confirmed.

(Example 3) Mg-4% Zn-1% R.V.
E. A magnesium alloy of -0.4% Zr-0.4% Si was melted, and 0 to 8% by weight of Al was added thereto.
The cast crack test was carried out by casting on a square-shaped test piece provided with an R portion having a predetermined shape as shown in (1) under the casting conditions of a casting temperature of 690 ° C. and a mold temperature of 80 to 120 ° C.

The casting crack test piece of FIG. 14 will be described. The test piece 10 has a wall thickness of 3 to 4 mm and a side length of 200.
mm-shaped tubular body, the center of the side 16 facing the side 14 to which the sprue 12 is attached is covered with a heat insulating material 18, and one of the sides 16 facing the sprue has a corner R of R = 1.0 mm. The other part is a corner R part 22 having R = 0.5 mm. The cast crack test piece 10 utilizes the difference in the solidification time between the portion covered with the heat insulating material 18 and the other portion to cause the casting crack in the corner R portion 20 or 22 due to the stress due to the solidification shrinkage. In the casting crack test, the crack occurrence rate at the corner R portion 22 with R = 0.5 mm was measured, and the results are shown in FIG.

As shown in FIG. 15, the casting crack occurrence rate was 90% or more when Al was not contained at all.
When the Al content is 1%, it sharply decreases to 40% and the Al content is
% To about 10%. based on the above results,
It was confirmed that the alloy of the present invention has excellent castability.

Example 4 Mg-4% as the alloy of the present invention
Zn-3% Al-1% R.I. E. -0.4% Zr-0.4
% Si is melted, casting temperature is 690 ° C., mold temperature is 80 to 1
The casting test piece shown in FIG. 14 was cast under the casting condition of 20 ° C.,
A casting crack test was conducted. For comparison, casting test pieces were cast under the same casting conditions for the AZ91C alloy and the ZE41A alloy, and a casting crack test was performed. R = 1.0m
The rate of occurrence of casting cracks in the corner R portion 20 of m and the corner R portion 22 of R = 0.5 mm was measured and shown in FIG.

As known from FIG. 1, R = 0.5 mm
The rate of occurrence of casting cracks in the corner R portion 22 was 60% for the ZE41A alloy and 5% for the AZ91C alloy, whereas it was 10% for the alloy of the present invention. The rate of occurrence of casting cracks in the corner R portion 20 with R = 1.0 mm is 3 for the ZE41A alloy.
2% and 3% for the AZ91C alloy, and 7% for the alloy of the present invention. As a result, the alloy of the present invention is AZ91.
It was found that the castability was close to that of C.

Example 5 Alloy of the present invention (Mg-4% Zn
-3% Al-1% R. E. -0.4% Zr-0.4% S
i), AZ91C alloy (Mg-9% Al-1% Zn) and Al alloy (Al-6% Si-3% Cu-0.3% M)
g-0.3% Mn) was subjected to a corrosion test in which it was immersed in a salt aqueous solution containing H ₂ SO _{4 at} 85 ° C. for 192 hours. The corrosion resistance was evaluated by measuring the weight increase due to the oxide adhesion and calculating the ratio when the original weight was 1.0. The obtained results are shown in FIG.

As shown in FIG. 16, the conventional Mg material, AZ91C, had a weight change ratio of 1.2 due to corrosion, whereas the material of the present invention showed almost no weight change due to corrosion. The change ratio is 1.0, which is 1.0 for Al alloys.
It was confirmed that it showed the same corrosion resistance as

FIG. 17 is a copy of the metallographic structure of the cross section of the corroded surface of the material of the present invention, and FIG. 18 is a copy of the metallographic structure of the cross section of the corroded surface of the AZ91C alloy. In the invention material of FIG. 17,
Mg-R. E. -Al layer is formed, and R. E. Is concentrated, which prevents the corrosion pit from propagating inside. On the other hand, in the AZ91C alloy shown in FIG. 18, a MgAl oxide layer is formed on the corroded surface, and Al is deficient in the vicinity of the Mg ₁₇ Al ₁₂ crystallized substance at the grain boundary, which is the starting point of corrosion pit generation. ..

The entire surface of the AZ91C alloy after the corrosion test was covered with white rust, and many corrosion pits were observed. Moreover, when one of the corrosion pits was enlarged and seen, the corrosion pit extended to a deep portion. On the other hand, the surface condition after the corrosion test of the material of the present invention is only scattered white rust, the number of pits generated is also very small,
It was comparable to the Al alloy. Also,
An enlarged view of one of the corrosion pits revealed that the corrosion pit was extremely shallow.

[0043]

The magnesium alloy excellent in heat resistance, corrosion resistance and castability according to the present invention has a weight ratio of Zn;
6.0%, R.I. E. 0.1-2.0%, Zr; 0.1
~ 2.0%, Si; 0.1-3.0%, Al; 0.1
The content of Mg is 6.0% with the balance being Mg and inevitable impurities, and Mg having improved castability and brittleness due to Al and Zn.
The -Al-Zn-based crystallized substance is uniformly dispersed in the crystal grains, and the room temperature strength can be improved. Further, R.O. E. With the addition of Mg-Al-Zn-R.
E. Since the system crystallization product was crystallized at the grain boundaries, the high temperature strength was improved. In addition, R. E. Since the content has been reduced within the range where high temperature strength can be maintained, the heat resistant magnesium alloy has excellent castability, high tensile strength at room temperature, and excellent high temperature characteristics and creep characteristics. In addition, R. E. R.I. E. It forms a rich protective film and improves corrosion resistance. The Zr content improves the room temperature strength and the high temperature strength without deteriorating the castability, and the Si content improves the creep resistance. As a result, the alloy of the present invention is ZE41.
It is a magnesium alloy that has improved castability while maintaining high-temperature strength and creep resistance comparable to those of alloy A, and also improved corrosion resistance. Since the alloy of the present invention has excellent heat resistance and corrosion resistance, engine parts that require these characteristics, particularly E
It can be applied to an intake manifold in which corrosion due to condensation of GR gas poses a problem, and the weight of an automobile can be significantly reduced. Further, since it is superior in castability as compared with the conventional heat-resistant magnesium alloy, it is possible to perform casting using a die and mass-produce engine parts such as an intake manifold having a complicated shape.

[Brief description of drawings]

FIG. 1 is a diagram showing the rate of occurrence of casting cracks of the alloy of the present invention and a conventional alloy.

FIG. 2 is a micrograph showing a metal structure of a fracture example starting from a porosity.

FIG. 3 is a copy diagram showing the position of the porosity in the photograph of FIG.

FIG. 4: 373K, 393K and 4 of AZ91C alloy
It is a creep curve in case of stress 63MPa in 23K.

FIG. 5 is a diagram showing an axial force retention rate in a bolt loosening test.

FIG. 6: Test temperature 423K for AZ91C and ZE41A,
It is a tensile creep curve in stress 63MPa.

FIG. 7: Room temperature and 15 when Al content was changed.
It is a diagram which shows the tensile strength in 0 degreeC.

FIG. 8: Room temperature and 15 when Zn content was changed.
It is a diagram which shows the tensile strength in 0 degreeC.

9: R. E. It is a diagram showing the tensile strength at room temperature and 150 ° C when the content is changed.

FIG. 10 is a 100 × micrograph showing the metal structure of the alloy of the present invention when heat-treated at 330 ° C. for 2 hours.

FIG. 11 is a 250 × micrograph showing the metal structure of the alloy of the present invention when heat-treated at 330 ° C. for 2 hours.

FIG. 12 is a 250 × micrograph showing the metal structure of a test piece of the alloy of the present invention, which has been treated with T ₄ .

FIG. 13 is an alloy of the present invention, an AZ91C alloy and ZE41.
It is a creep curve at a stress of the alloy A of 63 MPa and a test temperature of 423K.

FIG. 14 is a perspective view of a casting crack test piece.

FIG. 15 is a diagram showing the relationship between changes in Al content and incidence of casting cracking.

FIG. 16 is a view showing a weight change rate in a corrosion test of an alloy of the present invention, an AZ41C alloy and an Al alloy.

FIG. 17 is a copy of the metallographic structure of the cross section of the corroded surface of the alloy of the present invention after the corrosion test.

FIG. 18 is a copy of the metallographic structure of the cross section of the corroded surface of the AZ91C alloy after the corrosion test.

[Explanation of symbols]

10 test piece 12 sprue 18 heat insulating material 20 and 22
Corner R section

Claims (1)

[Claims] 1. Zn: 1.0 to 6.0% by weight, Ce
A main component of misch metal (hereinafter referred to as RE); 0.1 to 2.0%, Zr; 0.1 to 2.0%,
Magnesium alloy excellent in heat resistance, corrosion resistance and castability, characterized by containing Si; 0.1 to 3.0%, Al; 0.1 to 6.0%, and the balance being Mg and inevitable impurities. ..