Manufacturing method of heat-resistant magnesium alloy and heat-resistant magnesium alloy

Abstract

To provide a method for producing a heat-resistant magnesium alloy having excellent heat resistance without using rare elements such as rare earth elements.SOLUTION: A method for producing a heat-resistant magnesium alloy includes the steps of: solution-treating a magnesium alloy extrusion material that comprises aluminum of 1-7 mass% and zinc of 0.5-6 mass% with the balance being substantially magnesium; compressing the solution-treated magnesium alloy extrusion material in parallel with an extrusion direction at a compressibility of 0.1-5%; and annealing the compressed magnesium alloy extrusion material.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a heat-resistant magnesium alloy and a heat-resistant magnesium alloy. [Background technology]

[0002] Magnesium alloys are the lightest structural metal materials in practical use, and have been widely studied as a material for reducing the weight of transportation equipment such as automobiles and aircraft. However, their heat resistance is inferior to that of other lightweight metal materials such as aluminum alloys.

[0003] Therefore, a method for improving the heat resistance of magnesium alloys by adding rare earth elements is known.Specifically, magnesium alloys are known that contain, for example, aluminum (Al), lanthanum (La), cerium (Ce), manganese (Mn), beryllium (Be), and optionally zinc (Zn), tin (Sn), neodymium (Nd), and praseodymium (Pr) in predetermined amounts, with the remainder being magnesium and unavoidable impurities (e.g., Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2018-521213 Summary of the Invention [Problem to be solved by the invention]

[0005] However, rare earth elements and misch metals are expensive and it is difficult for Japan to achieve self-sufficiency, so there is a risk that they will no longer be available in stable supply due to changes in the international situation or economic conditions.

[0006] The present invention has been made in view of the above circumstances, and has an object to provide a method for producing a heat-resistant magnesium alloy having excellent heat resistance even without using rare elements such as rare earth elements. Another object of the present invention is to provide a heat-resistant magnesium alloy having excellent heat resistance even without using rare elements such as rare earth elements. [Means for solving the problem]

[0007] One aspect of the present invention provides a method for producing a heat-resistant magnesium alloy, comprising the steps of: subjecting a magnesium alloy extrusion material containing 1-7 mass% aluminum, 0.5-6 mass% zinc, and the remainder being substantially magnesium; compressing the solution-treated magnesium alloy extrusion material parallel to the extrusion direction at a compression ratio of 0.1-5%; and annealing the compressed magnesium alloy extrusion material. According to this production method, by controlling the internal structure of the magnesium alloy, a magnesium alloy having excellent heat resistance can be obtained without using scarce elements such as rare earth elements. It is presumed that this is because the production method can introduce very stable twin boundaries into the internal structure of the magnesium alloy.

[0008] In one embodiment, the strain rate in the compressing step is 1.0×10 -4 ~1.0s -1 It may be.

[0009] In one embodiment, the annealing step may be a step of heating the magnesium alloy extruded material at 300 to 500° C. for 30 minutes or more, and then quenching the same.

[0010] Another aspect of the present invention provides a heat-resistant magnesium alloy having a crystal grain size of 5 μm or more, a twin volume fraction of 5 to 50%, and containing 1 to 7 mass % aluminum, 0.5 to 6 mass % zinc, and the remainder being substantially magnesium. Such a magnesium alloy can have excellent heat resistance without using scarce elements such as rare earth elements. Effect of the Invention

[0011] According to the present invention, it is possible to provide a method for producing a heat-resistant magnesium alloy having excellent heat resistance without using rare elements such as rare earth elements. Also, according to the present invention, it is possible to provide a heat-resistant magnesium alloy having excellent heat resistance without using rare elements such as rare earth elements. [Brief description of the drawings]

[0012] [Figure 1] Figure 1 shows a visualization of twin domains in a magnesium alloy. [Diagram 2] FIG. 2 shows the results of creep tests on magnesium alloys. [Diagram 3] FIG. 3 shows the results of creep tests on magnesium alloys. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Preferred embodiments of the present disclosure will be described in detail below, however, the present disclosure is not limited to the following embodiments.

[0014] <Method of manufacturing heat-resistant magnesium alloy> The manufacturing method of the heat-resistant magnesium alloy includes a step of solution treating the magnesium alloy extrusion (solution treating step), a step of compressing the solution-treated magnesium alloy extrusion parallel to the extrusion direction (compressing step), and a step of annealing the compressed magnesium alloy extrusion (annealing step). The manufacturing method can be said to have a process of removing processed structures from a magnesium alloy having texture by heat treatment, introducing twins by pre-processing considering the crystal orientation, and stabilizing the twins by another heat treatment.

[0015] (Solution treatment process) The magnesium alloy extrusion material is prepared by any known method, an example of which is shown below.

[0016] For example, a raw metal is melted (dissolved) in a high-frequency induction furnace or the like to obtain a molten metal containing a desired component, which is then cast to obtain a billet of a desired composition.

[0017] The resulting billet is heated to 200 to 450° C., preferably 250 to 400° C., and extruded to obtain an extruded material. A heating temperature of 200° C. or higher makes it easier to suppress breakage during extrusion, while a heating temperature of 450° C. or lower makes it easier to form an aggregate structure of crystal orientation.

[0018] The extrusion ratio during extrusion can be 5 to 100, preferably 5 to 50. The extrusion ratio is the cross-sectional area of ​​the billet material relative to the cross-sectional area of ​​the bar material after extrusion (cross-sectional area of ​​the billet material / cross-sectional area of ​​the bar material). When the extrusion ratio is 5 or more, the crystal orientation is easily textured, while when the extrusion ratio is 100 or less, the crystal grain size is not too fine and breakage during extrusion is easily suppressed.

[0019] Although the above description has been given of an extruded material, it is presumed that a heat-resistant magnesium alloy can be obtained by the manufacturing method of this embodiment as long as the material is formed to have a texture. Examples of materials formed to have a texture include rolled materials in addition to extruded materials.

[0020] The magnesium alloy extrusion material contains aluminum (Al), zinc (Zn), and substantially the remainder magnesium (Mg). The remainder may be magnesium and unavoidable impurity elements. It can be said that the magnesium alloy extrusion material contains magnesium alloy components that contain aluminum (Al), zinc (Zn), and substantially the remainder magnesium (Mg). The magnesium alloy extrusion material preferably contains the magnesium alloy components in a single phase, and preferably consists of the magnesium alloy components. The amount of each element in the magnesium alloy can be measured by inductively coupled plasma (ICP) mass spectrometry.

[0021] The aluminum content is 1 to 7 mass%, preferably 1.5 to 6 mass%, more preferably 2 to 5 mass%, and further preferably 2 to 4 mass%. When the aluminum content is 1 mass% or more, the strength can be improved compared to pure magnesium, while when it is 7 mass% or less, the precipitation of intermetallic compounds can be suppressed.

[0022] The zinc content is 0.5 to 6 mass%, preferably 0.7 to 4 mass%, more preferably 0.9 to 2 mass%, and further preferably 1 to 1.5 mass%. A zinc content of 0.5 mass% or more can improve the strength compared to pure magnesium, while a zinc content of 6 mass% or less can suppress the precipitation of intermetallic compounds.

[0023] The magnesium content is adjusted by the aluminum and zinc contents, but is 90 to 97 mass%, preferably 93 to 97 mass%. When the magnesium content is 90 mass% or more, the precipitation of intermetallic compounds is suppressed and the magnesium solid solution phase is easily stabilized, while when the magnesium content is 97 mass% or less, the strength is easily improved.

[0024] The magnesium alloy may contain other components other than those described above, to the extent that the introduction of a desired crystal structure is not hindered. The other components may be unavoidable impurities. Examples of the other components include iron, manganese, tin, lead, nickel, silicon, copper, and rare earth elements (misch metals). According to the manufacturing method of this embodiment, a magnesium alloy having excellent heat resistance can be obtained without using rare elements such as rare earth elements, but this does not exclude the magnesium alloy from containing such rare elements. The content of the other components may be less than 0.01 mass%.

[0025] Although the above describes an Mg-Al-Zn alloy (e.g., an AZ31-based alloy), a heat-resistant magnesium alloy can be obtained by the manufacturing method of this embodiment as long as it is a hexagonal single-phase magnesium alloy derived from pure magnesium.

[0026] The magnesium alloy extrusion material prepared as described above is subjected to solution treatment. Solution treatment is a process in which the alloy is heated to a temperature within the range of a uniform solid solution, held for a sufficient period of time, and then rapidly cooled to bring the solid solution state to room temperature.

[0027] In the solution treatment, the magnesium alloy extrusion material is preferably heated at 300 to 500°C, more preferably 400 to 500°C. By setting the heating temperature at 300°C or higher, a uniform solid solution phase can be easily obtained even with a short holding time, while by setting the temperature at 500°C or lower, melting and ignition can be easily suppressed. The holding time (heating time) at the above heating temperature is preferably 1 hour or more. By setting the heating time to 1 hour or more, a sufficiently uniform solid solution phase can be easily obtained. The cooling rate during rapid cooling after heating is not particularly limited, but can be, for example, 100°C / sec or more, preferably 150°C / sec or more. The atmosphere in the solution treatment process can be an inert gas atmosphere from the viewpoint of suppressing oxidation of the magnesium alloy, and specifically, an argon gas (Ar) atmosphere, a nitrogen gas (N2) atmosphere, etc. can be mentioned.

[0028] (Compression process) The compression process is a process for forming a coherent interface called a twin interface inside the magnesium alloy and introducing a twin structure. In this process, the solution-treated magnesium alloy extrusion is compressed parallel to the extrusion direction. Compressing parallel to the extrusion direction means, for example, compressing a roughly cylindrical extrusion in the height direction of the cylinder (the direction in which the bottom surfaces face each other). The compression process is carried out at room temperature (25 to 30°C) in an air atmosphere.

[0029] The magnesium alloy extrusion is compressed parallel to the extrusion direction at a compression ratio of 0.1 to 5.0%, preferably 1.0 to 5.0%, and more preferably 2.0 to 4.0%. A compression ratio of 0.1% or more allows twins to be sufficiently introduced, while a compression ratio of 5% or less allows the disappearance of twin boundaries due to the enlargement of twins to be suppressed. The compression ratio is the ratio of the compressed length to the length of the extrusion before compression in the compression direction of the extrusion, and is expressed by the following formula. Compression ratio (%) = (compressed length / length of extruded material before compression) x 100

[0030] The strain rate is preferably 1.0×10 -4 ~1.0s -1 , more preferably 1.0×10 -3 ~1.0s -1 The strain rate is 1.0×10 -4 s -1 By setting the temperature at 1.0s or more, it is easy to introduce twins efficiently. -1 By keeping it at or below this, the amount of compression is easy to control.

[0031] (Annealing process) The annealing process is a process for reducing lattice defects, such as dislocations, in the metal structure of the magnesium alloy that are caused by compressive deformation, promoting recovery, and homogenizing the structure, thereby stabilizing the induced twin crystals.

[0032] In this step, the magnesium alloy extrusion material after the compression step is heated at preferably 300 to 500°C, more preferably 400 to 500°C. When the heating temperature is 300°C or higher, the processed structure is easily restored, while when the heating temperature is 500°C or lower, melting and ignition are easily suppressed. The holding time (heating time) at the heating temperature is preferably 30 minutes or more, more preferably 1 hour or more. When the heating time is 30 minutes or more, the processed structure is easily restored. After heating, it is preferable to rapidly cool the material, and the cooling rate is not particularly limited, but can be, for example, 100°C / sec or more, preferably 150°C / sec or more. The atmosphere in the annealing step can be an inert gas atmosphere from the viewpoint of suppressing oxidation of the magnesium alloy, and specifically, an argon gas (Ar) atmosphere, a nitrogen gas (N2) atmosphere, etc. can be mentioned.

[0033] <Heat-resistant magnesium alloy> The heat-resistant magnesium alloy has a crystal grain size of 5 μm or more, a twin volume fraction of 5 to 50%, and contains 1 to 7 mass % aluminum, 0.5 to 6 mass % zinc, and the remainder is substantially magnesium. Such a heat-resistant magnesium alloy can be obtained by the above-mentioned manufacturing method.

[0034] (crystal grain size) The crystal grain size is 5 μm or more, preferably 10 μm or more, and more preferably 15 μm or more. By having a crystal grain size of 5 μm or more, twins can be introduced into the crystal grains. The upper limit of the crystal grain size is not particularly limited, but from the viewpoint of suppressing crystal grain growth due to the presence of alloying elements, it can be 1000 μm or less, and may be 500 μm or less, 250 μm or less, 100 μm or less, or 50 μm or less. The crystal grain size can be controlled by processing for forming texture, such as extrusion or rolling, and by adjusting the heat treatment temperature and heat treatment time. The crystal grain size is measured by a section method using an inverse pole figure map obtained by the electron backscatter diffraction (EBSD) method or a microscopic observation image of an etched sample.

[0035] (Twin volume fraction) The twin volume fraction is 5 to 50%, preferably 8 to 50%, more preferably 10 to 40%, and further preferably 15 to 40%. A twin volume fraction of 5% or more can sufficiently improve heat resistance, while a twin volume fraction of 50% or less can suppress disappearance of twin boundaries due to enlargement of twins. The twin volume fraction can be adjusted by the compression amount in the compression step before annealing. The twin volume fraction is calculated from image analysis using an inverse pole figure map obtained by the EBSD method. EXAMPLES

[0036] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

[0037] <Magnesium alloy production> (Preparation of magnesium alloy extrusions) As a magnesium alloy extrusion material, an AZ31 alloy extrusion round bar (AZ31B (V14001), 89 mmΦ×1625 mm, manufactured by Soltec Co., Ltd.) was prepared. This was a cylindrical material obtained by heating and extruding a billet containing 3 mass% aluminum, 1 mass% zinc, and the remainder essentially containing magnesium. Processing conditions: Heating temperature 350℃, extrusion ratio 13.4 (330mmφ→89mmφ)

[0038] (Solution treatment process) The magnesium alloy extrusion material prepared as described above was heat-treated in an argon gas (Ar) atmosphere at 400°C for 1 hour. A muffle furnace (HPM-0G, manufactured by AS ONE Corporation) was used for the heat treatment. After that, it was quenched in ice water (cooling rate was approximately 200°C / sec). The magnesium alloy extrusion material that had undergone the solution treatment process was used as a control sample (ST). The control sample can be said to be the so-called conventional AZ31 alloy.

[0039] (Compression process) The solution-treated magnesium alloy extrusions were compressed parallel to the extrusion direction at a predetermined compression ratio (1.0%, 1.6%, 2.1%, 2.6%, or 3.6%) using a compression device (Instron 5584, manufactured by Instron). The strain rate was 1.0×10 -3 s -1 This process was carried out at room temperature (25-30°C) and in an air atmosphere. The magnesium alloy compressed at a compression rate of X% is referred to as PCAX.

[0040] (Annealing process) The compressed magnesium alloy extrusion material was heat-treated at 400°C for 1 hour in an argon gas (Ar) atmosphere. A muffle furnace (HPM-0G, manufactured by AS ONE Corporation) was used for the heat treatment. After that, it was quenched in ice water (cooling rate was approximately 200°C / sec). Thus, a magnesium alloy was obtained.

[0041] <Evaluation of heat-resistant magnesium alloys> (Preparing the sample for EBSD evaluation) The bottom surface of the obtained magnesium alloy was polished with emery paper (#2000), buffed, and electrolytically polished in that order to prepare an evaluation sample for EBSD.

[0042] (Textural Observation) The twin structure introduced into the obtained magnesium alloy was observed by EBSD. A field emission scanning electron microscope (JEOL, JSM-7000F) equipped with an EBSD camera (TSL, MSC-2200) was used for the observation. TSL OIM Analysis 7 was used for the analysis of EBSD data. Specifically, the twin structure of the hexagonal magnesium phase was observed for the inverse pole figure map obtained by EBSD.

number

number

[0043] (Crystal grain size measurement) The grain size was measured by the intercept method using the inverse pole figure map obtained by the EBSD method. The step size during EBSD analysis was set to 0.5 μm. The results are shown in Table 1.

[0044] (Twin volume ratio calculation) The twin volume fraction of the obtained magnesium alloy was calculated from the microstructure observation images. ImageJ 1.53 was used as the calculation software. The results are shown in Table 1.

[0045] [Table 1]

[0046] (Creep test) Cylindrical tensile test pieces for creep tests were prepared from the obtained magnesium alloys by cutting. The magnesium alloys used were ST, PCA1.0, PCA2.1, and PCA3.6. The gauge length of the test pieces was 20 mm, and the gauge diameter was 4 mm. The creep test was carried out using a creep test apparatus equipped with a tubular electric furnace. The test specimen was placed in the electric furnace, and the temperature in the electric furnace was raised to 200°C and held for 1.5 hours until the temperature was sufficiently stabilized. Then, a load (50 MPa) was applied to the test piece to start the creep test. The load was adjusted so as to obtain a predetermined stress based on the load measured by the load cell and the cross-sectional area of ​​the test piece. The test was then continued until the test piece broke. The test results are shown in Table 2 and Figures 2 and 3. Figures 2 and 3 are diagrams showing the creep test results of the magnesium alloy.

[0047] [Table 2]

[0048] The magnesium alloy obtained in this example had a longer creep life and a lower creep rate than the conventional AZ31 alloy. [Industrial Applicability]

[0049] The heat-resistant magnesium alloy of the present invention is expected to be in demand in industrial fields where both light weight and heat resistance are required, and specifically, the heat-resistant magnesium alloy of the present invention can be suitably used in the field of transportation equipment such as automobiles and aircraft.

Claims

1. A step of subjecting a magnesium alloy extrusion material containing 1 to 7 mass % aluminum, 0.5 to 6 mass % zinc, and the remainder magnesium, to a solution treatment; compressing the solution-treated magnesium alloy extrusion material parallel to the extrusion direction at a compression ratio of 0.1 to 5%; annealing the compressed magnesium alloy extrusion; The present invention relates to a method for producing a heat-resistant magnesium alloy, the method comprising the steps of:

2. The strain rate in the compressing step is 1.0×10 －４ ~1.0s －１ The method according to claim 1,

3. 3. The method according to claim 1, wherein the annealing step comprises heating the magnesium alloy extrusion material at 300 to 500° C. for 30 minutes or more, and then quenching the material at a cooling rate of 100° C. / second or more.

4. The crystal grain size is 5 μm or more, The twin volume fraction is 5 to 50%, A heat-resistant magnesium alloy containing 1 to 7 mass % aluminum, 0.5 to 6 mass % zinc, and the remainder being magnesium.

Images