JP4154480B2 - Heat-resistant magnesium alloy and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a heat-resistant magnesium alloy having excellent creep resistance and high-temperature strength. More specifically, the present invention relates to a structure composed of elongated crystal grains having an aspect ratio of 5 or more. The present invention relates to a heat-resistant magnesium alloy imparted with strength and a method for producing the same. INDUSTRIAL APPLICABILITY The present invention is useful for manufacturing and providing a heat-resistant magnesium alloy that can be used in a wide range of fields such as transportation equipment parts including automobiles, space / aircraft parts, and electrical / electronic equipment parts.
[0002]
[Prior art]
In general, magnesium has the lowest density (= 1.7 g / cm ³ ) among structural metal materials in practical use, has excellent recyclability unique to metals, and has abundant resources. Therefore, it is a material attracting attention as a next-generation structural lightweight material. At present, many magnesium products in Japan are manufactured by casting. Examples of main magnesium products include automobile parts such as steering wheels and cylinder heads, and parts of household electrical appliances such as personal computers and mobile phone casings. However, since the heat resistance of magnesium alloys rapidly decreases at high temperatures, most magnesium products can only be used in an environment of 100 ° C. or lower. In order to manufacture parts used in high-temperature environments such as engine blocks with magnesium alloy, it is essential to improve heat resistance.
[0003]
Magnesium alloys used in current magnesium products are mainly Mg—Al—Zn (—Mn) and Mg—Zn—Zr. The addition amount of each element of these magnesium alloys is generally Al: 15 mass% or less, Zn: 10 mass% or less, Mn: 3 mass% or less, and Zr: 3 mass% or less. The strength of these magnesium alloys in practical use rapidly decreases when the temperature reaches 150 ° C. or higher. Therefore, heat resistance has been improved by adding special elements or the like.
[0004]
Specifically, as a technique for improving the heat resistance of a magnesium alloy, (1) composite reinforced magnesium reinforced with ceramic fibers and particles having excellent high-temperature strength (for example, H. Ferkel and B. L. Mordike, “Magnesium strongened by” SiC nanoparticulates ", Mater.Sci.Eng.A, Vol.A298 (2001), pp.193-199), (2) Additive elements such as yttrium and rare earth metals with small diffusion coefficients in magnesium are dissolved in magnesium. Solid solution strengthened magnesium (for example, T. Mohri et al., “Microstructure and mechanical properties of a Mg-4Y-3RE alloy processed”). by thermomechanical treatment ", Mater.Sci.Eng.A, Vol.A257 (1998), pp.287-294).
[0005]
Further, (3) precipitation-strengthened magnesium that precipitates and strengthens high-temperature precipitates containing silicon, calcium, rare earth metals and the like as constituent elements (for example, M. Mabuchi and K. Higashi, “Strengthening machinism of Mg-Si alloy”). Acta. Mater., Vol. 44 (1996), pp. 4611-4618.), (4) Magnesium combined with the above (1) to (3) (for example, JG Wang et al., “ "Creep of a heat treated Mg-4Y-3RE alloy", Mater. Sci. Eng., Vol. A315 (2001) pp. 81-88).
[0006]
However, the conventional techniques for improving the heat resistance of magnesium alloys have focused on the improvement of heat resistance by adding special elements or the like to magnesium. And addition of such special elements etc. inhibits the recyclability which magnesium originally has, and becomes a big problem practically.
[0007]
[Problems to be solved by the invention]
Under such circumstances, the present inventors have conducted intensive research with the goal of developing a new method capable of drastically solving the problems of the prior art in view of the prior art. As a result of this, we focused on the fact that the decrease in heat resistance of magnesium is caused by grain boundary sliding, and that the intended purpose can be achieved by actively utilizing the suppression of grain boundary sliding by controlling the grain shape. The headline and the present invention were completed.
In the present invention, the molten magnesium is solidified from one direction to control the crystal grain to have an elongated crystal grain structure with an aspect ratio of 5 or more, thereby realizing superior heat resistance than making grain boundary sliding difficult to occur. An object of the present invention is to provide a method for producing a heat-resistant magnesium alloy and a material thereof. In the method of the present invention, since heat resistance can be improved without adding a special element or the like, recyclability is not impaired for improving heat resistance.
[0008]
[Means for Solving the Problems]
The present invention for solving the above-described problems comprises the following technical means.
(1) A molten magnesium alloy containing aluminum in an amount of more than 0 to 15 mass%, zinc in an amount of more than 0 to 10 mass% and manganese in an amount of more than 0 to 3 mass%, and the balance magnesium and unavoidable impurities is subjected to a temperature gradient (K / mm) and the solidification rate (mm / s) quotient (temperature gradient / solidification rate) is 1.0 × 10 ⁰ (K · s) to 1.0 × 10 ⁴ (K · s). An alloy produced by directional solidification , having an elongated crystal grain group with an aspect ratio (= long grain length / short grain length) of 5 or more, and an area ratio in heat resistant magnesium alloy at least 50 percent of the alloy, characterized in that it is constituted by the extension of grain group.
( 2 ) A molten magnesium alloy containing more than 0 to 15 mass% of aluminum, more than 0 to 10 mass% of zinc, more than 0 to 3 mass% of manganese, and the balance magnesium and unavoidable impurities , temperature gradient and solidification rate in terms of the quotient value of the (temperature gradient / solidification rate) of ^{1.0 × 10 0 (K · s} ) from ^{1.0 × 10 4 (K · s} ), alloy you prepared by unidirectional solidification The aspect ratio (= length of major axis of crystal grain / length of minor axis of crystal grain) is an elongated crystal grain group having an aspect ratio of 5 or more, and the area ratio is 50% of the alloy. A method for producing a heat-resistant magnesium alloy, comprising the above-described elongated crystal grain group.
( 3 ) A magnesium alloy structural member comprising the heat-resistant magnesium alloy according to (1 ) as a constituent element.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Next, the present invention will be described in more detail.
The present inventors paid attention to the form control of crystal grains elongated in a certain direction for the purpose of improving heat resistance of the magnesium alloy by suppressing grain boundary sliding. In the present invention, specifically, the molten magnesium alloy is cooled from one direction at an appropriate cooling rate, and a magnesium alloy having a structure composed of elongated crystal grains having an aspect ratio of crystal grains of 5 or more is produced. I tried to do that. In the present invention, the magnesium alloy material has a quotient of temperature gradient and solidification rate (temperature gradient / solidification rate) of 1.0 × 10 ⁰ (K · s) to 1.0 × 10 ⁴ (K · s). Heat-resistant magnesium having excellent heat resistance by producing a grain structure having a group of grains elongated in one direction by solidifying in one direction under conditions, thereby making it difficult for grain boundary sliding An alloy was produced.
[0010]
In the method of the present invention, as the magnesium alloy material, preferably, aluminum 0 super ～15Mass%, zinc 0 super ～10Mass%, manganese containing 0 super ～3Mass%, balance magnesium and unavoidable impurities A magnesium alloy is used. The method of the present invention includes, for example, AZ91 (Mg-9 mass% Al-1 mass% Zn-0.3 mass% Mn) and AM60 (Mg-6 mass% Al-0.3 mass% Mn) alloy, which are in great demand as practical magnesium alloys. Can be adapted to.
[0011]
In the method of the present invention, the magnesium alloy is melted and then solidified in one direction under a predetermined condition. In a specific example, the side surface is a heat insulating wall, and the inside of the crucible in which a cooling metal is installed on the bottom surface. The magnesium alloy is melted and the chilled gold is exposed to a coolant to solidify the magnesium alloy. Unidirectional solidification starts from the chill metal part, and the form of the formed crystal grains extends from the chill metal in the molten metal direction. A state in which elongated crystal grains are obtained by the above process is schematically shown in FIG. According to this process, it is possible to control the cooling condition of the molten metal relatively freely by selecting the material of the metal used as the cooling metal and the refrigerant for cooling the cooling metal.
[0012]
Factors that must be controlled when solidifying a crystal of a metal material in one direction are temperature gradient (K / mm) near the solidification interface of the metal (interface between liquid phase and solid phase), and progress of the solidification interface. Speed (coagulation speed (mm / s)). The quotient of temperature gradient and coagulation rate (temperature gradient / coagulation rate) is widely known as a parameter that controls the shape of dendritic tissue (eg: W. Kurz and DJ Fisher, Fundamentals of Solidification, Trans Tech Publications). Ltd., Switzerland, 1989, pp. 63-92.).
[0013]
For example, when the value of the quotient of temperature gradient and solidification rate (temperature gradient / solidification rate) is less than about 1, the metal structure becomes a structure having equiaxed crystals, and in the case of a value greater than about 1 and less than about 10,000. It is known that the metallographic structure exhibits finger-like dendrite growth. In addition, the solidification interface does not proceed apparently at a value of about 10,000 or more.
[0014]
In the present invention, the unidirectional solidification apparatus for carrying out the unidirectional solidification of the magnesium alloy described above produces a structure composed of elongated crystal grains by cooling the molten magnesium alloy from one direction at a predetermined cooling rate. Appropriate apparatuses can be used as long as they can be used, and specific configurations of these apparatuses can be appropriately designed according to the purpose of use, types of products, and the like.
[0015]
According to the method of the present invention, an aspect ratio (= length of major axis of crystal grain / length of minor axis of crystal grain) has an elongated crystal grain group having 5 or more, and 50% or more of the structure of the material is A heat-resistant magnesium alloy characterized by being composed of the stretched structure and exhibiting a tensile strength of 150 MPa or more at a temperature of 200 ° C. is obtained. By making the grain structure of a high aspect ratio by the above-mentioned unidirectional solidification, the grain boundary slip is suppressed, thereby obtaining a heat resistant aluminum alloy having high high temperature strength. When a crystal grain group having an aspect ratio of 5 or more constitutes 50% or more of the structure of the material, it is possible to prevent a rapid decrease in high-temperature strength. Further, in the present invention, a heat-resistant magnesium alloy can be produced without using special elements such as rare earth metals and ceramics that have been added to improve the heat resistance of magnesium alloys. It becomes possible to produce and provide recyclable products. The heat-resistant aluminum alloy according to the present invention can be used as any kind of structural member including a magnesium alloy as a constituent element.
[0016]
Improving the heat resistance of magnesium alloys without impairing recyclability is extremely important for increasing the demand for magnesium products. The present invention relates to a magnesium alloy whose heat resistance is improved by controlling the form of crystal grains and a method for producing the same. Specifically, the present invention is characterized in that a magnesium alloy exhibiting a tensile strength of 150 MPa or more at a temperature of 200 ° C. is obtained by setting the aspect ratio of the crystal grains to 5 or more. According to the present invention, it is possible to obtain an excellent heat-resistant magnesium alloy while maintaining easy recyclability without adding a special element or the like.
[0017]
【Example】
EXAMPLES Next, although this invention is demonstrated concretely based on an Example, this invention is not limited at all by the following Examples.
Example 1
By melting the AZ91 magnesium alloy and then cooling the cooling metal with water in a unidirectional solidification device having a diameter of 40 mm, a height of 100 mm, a side surface being a heat insulating wall (magnesia wall), and a bottom surface made of copper chiller Unidirectional solidification of the magnesium alloy was performed. As a result of installing a plurality of thermocouples inside the crucible and measuring the temperature distribution in the height direction, the quotient of the temperature gradient during solidification and the solidification rate was about 100 (K · s). A micrograph of the obtained magnesium alloy is shown in FIG. (A) is a microscopic structure | tissue photograph after making it solidify unidirectionally in the crucible in which the heat insulation wall and the cooling metal were installed. The aspect ratio of the crystal grains after solidification was about 5. As a result of image analysis, it was confirmed that crystal grains having an aspect ratio of 5 or more existed in at least 60% (area ratio) in the material. On the other hand, (b) shows a typical structure of an AZ91 magnesium alloy that is air-cooled in an alumina crucible having a diameter of 40 mm and a height of 100 mm that does not have a cooling metal at the bottom. The aspect ratio of the crystal grains after solidification was about 1.
[0018]
Example 2
The material produced in Example 1 was processed into a round bar test piece having a parallel part length of 10 mm and a parallel part diameter of 4 mm, and a tensile test was performed in a temperature range from room temperature to 400 ° C. FIG. 3 shows the relationship between the tensile strength and the temperature of an AZ91 magnesium alloy (invention material) having an aspect ratio of 5 and an AZ91 magnesium alloy (comparative material) having an aspect ratio of 1. The invented material of FIG. 3 refers to an AZ91 magnesium alloy having a crystal group having an aspect ratio of 5 or more produced by using the present invention. The proportion of the crystal group having an aspect ratio of 5 or more was 60 %. The comparative material in FIG. 3 refers to a sample having an aspect ratio of 1 produced by air-cooling the alloy in a normal alumina crucible in Example 1. The tensile strength of the inventive material was larger than that of the comparative material at any temperature. In addition, the strength of the inventive material showed a high strength of 200 MPa or more even at 200 ° C. The improvement in tensile strength shown in FIG. 3 was noticeable when a crystal group having an aspect ratio of 5 or more had 50% (area ratio) or more in the material.
[0019]
Example 3
The photograph which observed the side surface of the test piece after carrying out the tension test at 200 degreeC is shown in FIG. (A) shows the side of the test piece of the inventive material in FIG. 3, and (b) shows the side of the test piece of the comparative material. In the inventive material, no trace of grain boundary slip was observed, whereas in the comparative material, a trace of grain boundary slip (distortion of the sample surface having almost the same shape as the crystal grain) was confirmed. From the above results, it can be seen that the grain boundary slip was suppressed by the high aspect ratio crystal grain structure, and high high-temperature strength was obtained.
[0020]
The grain boundary sliding refers to a phenomenon in which the shape of the metal crystal itself does not basically change, and the deformation is achieved by the crystals sliding between the interfaces. The mechanism by which grain boundary sliding is suppressed due to the elongated crystal grain structure is schematically shown in FIG. From the results of Example 2 and Example 3, it can be confirmed that the aspect ratio of the crystal grains must be 5 or more in order to prevent a rapid decrease in high temperature strength.
[0021]
【The invention's effect】
As described above, the present invention relates to a heat-resistant magnesium alloy and a method for producing the same, and according to the present invention, 1) the high-temperature strength of the magnesium alloy can be obtained even with a composition of a practical alloy. 2) The heat-resistant magnesium alloy of the present invention is lightweight and excellent in recyclability, and is useful as a material with low environmental impact. 3) The present invention adds special elements such as rare earth metals and ceramics. It is possible to provide a magnesium alloy that exhibits excellent heat resistance while maintaining easy recyclability, and the industrial significance of the heat-resistant magnesium alloy of the present invention can be said to be extremely large. This is an exceptional effect.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing a process in which solidification starts from one direction by chilling metal and crystal grains extending from chilling metal in a molten metal direction are obtained.
FIG. 2 shows a structure photograph of AZ91 magnesium alloy.
FIG. 3 is a diagram showing the relationship between tensile strength and temperature.
FIG. 4 is a view of a side surface of an AZ91 magnesium alloy test piece after a tensile test at 200 ° C. is observed.
FIG. 5 is a diagram schematically showing that grain boundary slip is less likely to occur because crystal grains have an elongated shape.

Claims (3)

A molten magnesium alloy containing aluminum in excess of 0 to 15 mass%, zinc in excess of 0 to 10 mass%, manganese in excess of 0 to 3 mass%, the balance magnesium and unavoidable impurities, and a temperature gradient (K / mm) Solidification in one direction occurs under the condition that the quotient (temperature gradient / solidification rate) of the solidification rate (mm / s) is 1.0 × 10 ⁰ (K · s) to 1.0 × 10 ⁴ (K · s). a fabricated alloy by an aspect ratio (= length of the minor axis length / grain grain long axis) has a 5 or more elongated grain group, the alloy with an area ratio 50% or more of the above is constituted by the elongated crystal grain group . A molten magnesium alloy containing aluminum in excess of 0 to 15 mass%, zinc in excess of 0 to 10 mass%, manganese in excess of 0 to 3 mass%, and the balance magnesium and unavoidable impurities is used as a quotient of temperature gradient and solidification rate ( under conditions of a temperature gradient / value solidification rate) of ^{1.0 × 10 0 (K · s} ) from ^{1.0 × 10 4 (K · s} ), the manufacturing method of the alloy you prepared by unidirectional solidification And an aspect ratio (= long axis length of crystal grains / long axis length of crystal grains) has an elongated crystal grain group of 5 or more, and 50% or more of the alloy in the area ratio is the above A method for producing a heat-resistant magnesium alloy, characterized by comprising elongated crystal grain groups. A magnesium alloy structural member comprising the heat-resistant magnesium alloy according to claim 1 as a constituent element.

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