A near-zero thermal expansion magnesium alloy and preparation method thereof

Abstract

The present invention discloses a near-zero thermal expansion magnesium alloy and a preparation method thereof: 1) negative thermal expansion material MnCoGe is mixed with magnesium alloy powder and aluminum powder in proportion, fully ground until the powder is evenly mixed, and a negative thermal expansion material-aluminum-magnesium alloy mixed powder is obtained, wherein the mass ratio of MnCoGe, aluminum powder and magnesium alloy is 3-15:0.5-7:78-96.5; 2) the mixed powder obtained in step 1) is loaded into a mold, and a negative thermal expansion material-aluminum-magnesium-based alloy material is obtained by solid phase vacuum hot pressing and sintering, wherein the sintering temperature is 480-550°C, the sintering pressure is 18-30MPa, and the heat preservation and pressure holding time is 50-70 minutes. The magnesium-based alloy obtained by the present invention can maintain a near-zero thermal expansion coefficient in the temperature range of 25°C to 150°C, and has both high strength and toughness and a wide range of applications.

Description

Technical Field

[0001] The present invention relates to the technical field of metal composite materials, and in particular to a near-zero thermal expansion magnesium alloy and a preparation method thereof. Background Art

[0002] Magnesium is a light metal material with a density of 1.74cm 3 , far lower than the density of aluminum alloys, titanium alloys, zinc alloys and steel, and has the advantages of high specific strength, high specific modulus, high thermal conductivity, good electromagnetic shielding and easy recycling. It is known as the "21st century green engineering material". In recent years, a large number of magnesium alloy parts have been produced to replace plastic and aluminum alloy parts, and are used in automobiles, 3C (i.e. computers, communications, consumer electronics) and civil industries, especially in the aerospace field. However, when magnesium alloys are used in conjunction with other dissimilar alloys, components are often formed by welding or bolting. The huge difference in thermal expansion coefficients will cause large thermal stress concentrations at the joints when the components undergo temperature changes. In severe cases, cracks will form at the joints, leading to component failure. In addition, studies have shown that due to the mismatch in the thermal expansion coefficients of the materials, thermal fatigue damage will occur under temperature loads, causing 55% of electronic equipment to fail. This limits the application of magnesium alloys in high-temperature resistant systems, heat dissipation systems, and precision electronic instruments.

[0003] Low thermal expansion magnesium-based alloys can be obtained by combining low or negative thermal expansion materials with magnesium alloys. However, the addition of large amounts of hard and brittle secondary phases can severely degrade the mechanical properties of magnesium-based alloys, particularly their plasticity, rendering them impractical. Therefore, while ensuring a low thermal expansion coefficient, the mechanical properties of the resulting magnesium-based alloy must also be considered. Summary of the Invention

[0004] The object of the present invention is to provide a near-zero thermal expansion magnesium alloy and a preparation method thereof in view of the above problems.

[0005] In order to achieve its purpose, the present invention adopts the following technical solutions:

[0006] A method for preparing a near-zero thermal expansion magnesium alloy comprises the following steps:

[0007] 1) Mixing a negative thermal expansion material MnCoGe with magnesium alloy powder and aluminum powder in proportion, and grinding thoroughly until the powders are evenly mixed to obtain a negative thermal expansion material-aluminum-magnesium alloy mixed powder, wherein the mass ratio of MnCoGe, aluminum powder, and magnesium alloy is 3-15:0.5-7:78-96.5;

[0008] 2) The mixed powder obtained in step 1) is loaded into a mold, and a negative thermal expansion material - aluminum-magnesium based alloy material is obtained by solid phase vacuum hot pressing sintering, the sintering temperature is 480-550° C., the sintering pressure is 18-30 MPa, and the heat and pressure holding time is 50-70 minutes.

[0009] Preferably, the magnesium alloy in step 1) is WE43.

[0010] Preferably, in step 1), the grinding is to put the negative thermal expansion material powder, aluminum powder and magnesium alloy powder into a planetary ball mill, with a ball-to-material ratio of 10 to 20:1, a rotation speed of 60 to 100 r / min, and ball milling for 3 to 5 hours.

[0011] Preferably, in step 1), the mass ratio of MnCoGe, aluminum powder and magnesium alloy is 3-7:1-7:86-96 or 3-7:3-7:86-94 or 4-6:4-6:88-92 or 4.5-5.5:4.5-5.5:89-91 or 5:5:90.

[0012] Preferably, in step 2), after the heat preservation and pressure holding time reaches the set time, the pressure on the mold is released and the mold is cooled to obtain a negative thermal expansion material - aluminum-magnesium based alloy material.

[0013] Preferably, step 2) is specifically as follows: the mixed powder obtained in step 1) is placed in a mold, vacuumed in a hot pressing sintering furnace, and heated from room temperature to 480-550°C at a heating rate of 15-25°C / min, while simultaneously pressurizing to 18-30 MPa, holding the temperature for 50-70 minutes, and cooling with the furnace to obtain a negative thermal expansion material - an aluminum-magnesium-based alloy material.

[0014] In step 2), the mixed powder obtained in step 1) is loaded into a mold. Since the raw material is a magnesium alloy, when the sintering temperature is too high, the matrix magnesium will evaporate, resulting in a decrease in the density of the final product. A heating rate of 15 to 25°C / min is selected to heat the temperature from room temperature to a preset temperature value. The holding time should not be too long, as too long will cause the secondary phase to grow and affect the mechanical properties of the product. Therefore, the holding time is selected to be 50 to 70 minutes.

[0015] More preferably, a heating rate of 15-25°C / min is selected from room temperature to 480-520°C, while the pressure is increased to 18-25 MPa or 18-23 MPa, and the heat and pressure holding time is 50-70 minutes or 55-65 minutes.

[0016] Preferably, in step 2), the mold is a graphite mold, which is first vacuumed to 1×10 -5The temperature was raised from room temperature to 500°C at a rate of 20°C / min while simultaneously applying pressure to 20 MPa. The temperature was then maintained at this temperature for 1 hour. The mold was then released from pressure and cooled to room temperature to produce an aluminum-magnesium alloy material with negative thermal expansion. Since magnesium is chemically active and can react with a variety of substances, a chemically stable graphite mold that does not react with magnesium was preferred as the reaction mold.

[0017] A near-zero thermal expansion magnesium alloy is prepared by mixing negative thermal expansion material MnCoGe, aluminum powder and magnesium alloy powder, grinding until the powders are uniformly mixed, and then sintering by solid phase vacuum hot pressing. The alloy is prepared by any of the preparation methods described above.

[0018] The near-zero thermal expansion magnesium alloy has near-zero thermal expansion in a temperature range of 25 to 150°C.

[0019] The beneficial effects of the present invention are:

[0020] The present invention uses metal or its alloy as the matrix material and negative thermal expansion material as the reinforcing material, which can reduce the thermal expansion coefficient of the metal matrix. The introduction of aluminum powder and the regulation of pressure during cooling cause a short-range disordered structure to appear in the interface area of ​​the negative thermal expansion material, thereby improving the interface bonding of the composite material and enhancing the load transfer at the interface. The obtained magnesium-based alloy has high strength, good plasticity, extremely low thermal expansion coefficient, and is lightweight. The preparation method is simple, the price is low, and the application range is wide. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Thermal expansion curve (a) and compressive stress-strain curve (b) of x wt.% MnCoGe / WE43 (x=3, 5, 7, 15).

[0022] Figure 2 Thermal expansion curve (a) and compressive stress-strain curve (b) of (5wt.%MnCoGe+ywt.%Al) / WE43 (y=0.5, 1.0, 3.0, 5.0, 7.0).

[0023] Figure 3 Thermal expansion curve (a) and compressive stress-strain curve (b) of (5wt.% MnCoGe+5wt.% Al) / WE43 (the pressures during cooling are 20MPa, 10MPa, and 0MPa, respectively).

[0024] Figure 4 X-ray diffraction pattern of magnesium-based alloy (a), and the change of MnCoGe different phase contents with Al mass fraction after Rietveld refinement (b).

[0025] Figure 5 The SEM micromorphology test results of magnesium-based alloys, among which,

[0026] a: WE43, b: 5wt.%MnCoGe / WE43, c: (5wt.%MnCoGe+5wt.%Al) / WE43(20MPa), d: (5wt.%MnCoGe+5wt.%Al) / WE43(0MPa).

[0027] Figure 6 This is the energy spectrum scan of (5wt.%MnCoGe+5wt.%Al) / WE43(0MPa). DETAILED DESCRIPTION

[0028] The present invention will be described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those skilled in the art, several variations and improvements can be made without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0029] The experimental methods in the following examples are conventional methods unless otherwise specified; the reagents and materials used are conventional reagents and materials in the art unless otherwise specified and can be obtained commercially.

[0030] WE43 magnesium alloy is a Mg-Y-RE alloy with the chemical composition of Mg-4%Y-3%RE (Nd, Gd).

[0031] The negative thermal expansion material MnCoGe used in the embodiment can be prepared by vacuum arc melting. The preparation method can be referred to the following literature:

[0032] GiantNegative Thermal Expansion in BondedMnCoGe-Based CompoundswithNi2In-Type Hexagonal Structure (ZhaoYY, HuF

[0033] Example 1

[0034] 1. Preparation of magnesium-based alloys

[0035] (1) Preparation of xwt.% MnCoGe / WE43 alloy

[0036] MnCoGe powders with negative thermal expansion materials of 3%, 5%, 7%, and 15% mass fraction were mixed with metal matrix WE43 powder (purity 99.99%). The mixed powders were placed in a planetary ball mill for 4 hours at a speed of 80 r / min and a ball-to-material ratio of 15:1 to obtain mixed powders. The mixed powders were placed in a hot pressing sintering furnace and vacuumed to 1×10 -5 Pa, heating from room temperature to 500°C at a rate of 20°C / min, pressurizing to 20 MPa, keeping at this temperature and pressure for 1 hour, and finally cooling to room temperature with the furnace to obtain magnesium-based alloys - x wt.% MnCoGe / WE43 alloys (x = 3, 5, 7, 15).

[0037] (2) Preparation of (5wt.%MnCoGe+ywt.%Al) / WE43 alloy

[0038] Mix 5% MnCoGe powder, a negative thermal expansion material, 0.5%, 1.0%, 3.0%, 5.0%, and 7.0% Al powder (purity 99.99%) with a metal matrix WE43 powder (purity 99.99%). The mixed powder is placed in a planetary ball mill for 4 hours at a speed of 80 r / min and a ball-to-material ratio of 15:1 to obtain a mixed powder. The mixed powder is placed in a hot pressing sintering furnace and vacuumed to 1×10 -5 Pa, heated from room temperature to 500°C at a rate of 20°C / min, and pressurized to 20 MPa, kept at this temperature and pressure for 1 hour, and finally cooled to room temperature with the furnace to obtain a magnesium-based alloy - (5wt.%MnCoGe+ywt.%Al) / WE43 alloy (y=0.5, 1.0, 3.0, 5.0, 7.0).

[0039] (3) Preparation of (5wt.%MnCoGe+ywt.%Al) / WE43 alloy and changing the pressure during cooling

[0040] Mix 5% MnCoGe powder (negative thermal expansion material), 5.0% Al powder (purity 99.99%) and WE43 powder (purity 99.99%), and mill the mixed powder in a planetary ball mill for 4 hours at a speed of 80 r / min and a ball-to-material ratio of 15:1 to obtain a mixed powder. The mixed powder is placed in a hot pressing sintering furnace and vacuumed to 1×10 -5Pa, heated from room temperature to 500℃ at a rate of 20℃ / min, and pressurized to 20MPa, and kept warm for 1h. After the end of the heat preservation and pressure holding, the pressure on the mold was adjusted during cooling. Two groups of experiments were done. One group was to cancel the pressure on the mold so that the pressure of the equipment on the mold was 0MPa, and the other group was to adjust the pressure of the equipment on the mold to 10MPa during cooling, and cooled to room temperature to obtain a magnesium-based alloy.

[0041] 2. Product Testing

[0042] (1) Analysis of thermal expansion properties and mechanical properties

[0043] All the samples were tested for thermal expansion performance. The test results are as follows: Figure 1 、 Figure 2 、 Figure 3 As shown by Figure 1 (a) It can be seen that the thermal expansion coefficient (CTE) of the magnesium-based alloy obtained by adding MnCoGe is lower than that of pure WE43. When the mass fraction of MnCoGe is 15%, the CTE of the magnesium-based alloy (15%MnCoGe / WE43) is 16.9×10 - 6 K -1 However, the elongation of this sample is significantly lower than that of pure WE43 alloy, reaching only 6.5% (pure WE43 has an elongation of 13.2%). The compressive strength of the alloy is 432 MPa. Table 1 shows that increasing the mass fraction of MnCoGe particles decreases the CTE of the resulting sample, but the elongation drops sharply. The elongation of the 5wt.% MnCoGe / WE43 sample is essentially the same as that of pure WE43, so we chose to continue optimizing the composition based on this.

[0044] Table 1

[0045]

[0046] like Figure 2 As shown in the figure, when pure aluminum powder is added to 5 wt.% MnCoGe, the CTE value of the magnesium-based alloy (5 wt.% MnCoGe + y wt.% Al / WE43) obtained by adding 5 wt.% MnCoGe shows a trend of first decreasing and then increasing with the increase of Al content. When the Al mass fraction is 5 wt.%, the CTE value of the sample obtained is the lowest, which is 14.2×10 -6 K -1 At this time, the compressive strength of the alloy is 498MPa and the elongation is 12.5%.

[0047] By changing the pressure in the furnace during cooling, the CTE value of the sample can be further adjusted. Figure 3As shown in Figure 2, the CTE value of the (5wt.%MnCoGe+5wt.%Al) / WE43 sample decreases with the decrease of the furnace pressure (i.e., the pressure on the mold) during cooling. When the furnace pressure is 0 MPa, the (5wt.%MnCoGe+5wt.%Al) / WE43 sample exhibits near-zero expansion at 25-158°C, and the CTE value is 2×10 -8 K -1 At this time, the room temperature compressive strength is 423MPa, and the elongation is 11.6%. The compressive strength is 38MPa higher than that of pure WE43 (385MPa), and the elongation is only 1.7% lower than that of pure WE43. When the furnace pressure is 10MPa, the room temperature compressive strength of the (5wt.%MnCoGe+5wt.%Al) / WE43 sample is 478MPa, and the elongation is 11.1%. However, the CTE does not change significantly, being 13.8×10 -6 K -1 .

[0048] Table 2

[0049]

[0050]

[0051] (2) X-ray diffraction analysis

[0052] The prepared magnesium-based alloy was subjected to X-ray diffraction analysis, and the results were as follows: Figure 4 As shown, no tetragonal MnCoGe phase was detected in the sample without Al powder. However, after Al powder was added, the tetragonal MnCoGe phase appeared in the sample, and its content decreased slightly with increasing Al powder content. When the furnace pressure was 0 MPa during cooling, the tetragonal MnCoGe phase was more abundant, and no hexagonal MnCoGe phase was detected. This suggests that the addition of Al effectively retains the MnCoGe particles in the sample, preventing them from reacting with the magnesium matrix. Similarly, the furnace pressure during cooling also affects the content of different MnCoGe phases.

[0053] (3) Micromorphology and composition analysis

[0054] The prepared magnesium-based alloy was subjected to EBSD and EDS analysis, and the results are as follows: Figure 5 、 Figure 6 As shown. Figure 5It can be seen that the introduction of MnCoGe particles significantly refines the grain size of the magnesium matrix, indicating that the addition of MnCoGe particles not only brings precipitation strengthening but also grain refinement. However, when MnCoGe particles and Al powder are added simultaneously, if the pressure on the mold is maintained at 20 MPa during cooling, the grain size distribution of the magnesium matrix in the sample is uneven, with large variations in grain size. When the pressure on the mold is removed during cooling, reducing the pressure to 0 MPa, the grain size becomes relatively uniform, with no noticeable coarse grains present.

[0055] The (5wt.%MnCoGe+5wt.%Al) / WE43(0MPa) sample was subjected to energy spectrum scanning analysis. Figure 6 As shown in the figure, it can be seen that MnCoGe particles are present in the sample, and Al elements are clearly distributed around the MnCoGe particles, with some Al elements diffusing into the interior of the MnCoGe particles. This not only ensures that the MnCoGe particles do not react with the magnesium matrix, but also creates a transition interface between the MnCoGe particles and the magnesium matrix, improving the mechanical properties of the composite.

[0056] Based on the above test results, adding MnCoGe to WE43 magnesium alloy can obtain a magnesium-based alloy with low thermal expansion. However, since MnCoGe itself is a brittle material with high strength but low toughness, adding a large amount of it will lead to a serious loss of mechanical properties of the resulting magnesium-based alloy. In addition, the negative thermal expansion effect caused by the structural phase transition only occurs when MnCoGe has a tetragonal phase structure. The phase transition temperature of MnCoGe is sensitive to pressure and doping elements. Therefore, the addition of Al powder or the change of external pressure can regulate the phase transition temperature zone of MnCoGe particles, making the negative expansion temperature range of MnCoGe wider, and a wider low expansion range can be obtained after compounding with magnesium alloy. At the same time, MnCoGe has a large negative thermal expansion coefficient and its volume changes dramatically during heating. When the interface bonding is unstable, the low CTE temperature range of the magnesium-based alloy obtained is very narrow, the CTE value is unstable, and it has no application value. After adding Al powder, due to the interaction between MnCoGe and Al, the interface layer between MnCoGe and the magnesium matrix becomes a reaction product containing Mn, Co, Ge, and Al elements. The reaction layer thickness is about 5μm. This is more stable than an unreactive interface. When the sample is heated, it can resist the drastic volume change of MnCoGe. The volume contraction of MnCoGe particles can continuously compensate for the positive thermal expansion of the magnesium matrix. The resulting magnesium-based alloy has a wider low CTE temperature range and a more stable CTE value. At the same time, the stable interface will improve the interface's load transfer capacity and enhance the material's strength.

Claims

1. A method for preparing a near-zero thermal expansion magnesium alloy, characterized in that: The steps include: 1) Mixing a negative thermal expansion material MnCoGe with magnesium alloy powder and aluminum powder in appropriate proportions and grinding thoroughly until the powders are uniformly mixed to obtain a negative thermal expansion material-aluminum-magnesium alloy mixed powder, wherein the mass ratio of MnCoGe, aluminum powder, and magnesium alloy is 3-7:0.5-5:78-96.5; the magnesium alloy is WE43; 2) The mixed powder obtained in step 1) is charged into a mold and subjected to solid phase vacuum hot pressing to obtain a negative thermal expansion material - aluminum-magnesium-based alloy material. The sintering temperature is 480-550°C, the sintering pressure is 18-30 MPa, and the holding time is 50-70 minutes. After the holding time reaches the set time, the pressure on the mold is released and the mold is cooled to obtain the negative thermal expansion material - aluminum-magnesium-based alloy material.

2. The preparation method according to claim 1, wherein: In step 1), the grinding is to put the negative thermal expansion material powder, aluminum powder and magnesium alloy powder into a planetary ball mill, with a ball-to-material ratio of 10-20:1, a rotation speed of 60-100 r / min, and ball milling for 3-5 hours.

3. The preparation method according to claim 1, wherein: In step 1), the mass ratio of MnCoGe, aluminum powder, and magnesium alloy is 5:5:

90.

4. The preparation method according to claim 1, wherein: Step 2) is specifically as follows: the mixed powder obtained in step 1) is placed in a mold, vacuumed in a hot pressing sintering furnace, and heated from room temperature to 480-550°C at a heating rate of 15-25°C / min, while simultaneously applying pressure to 18-30 MPa, maintaining the temperature for 50-70 minutes, and cooling in the furnace to obtain a negative thermal expansion material - an aluminum-magnesium-based alloy material.

5. The preparation method according to claim 4, wherein: The temperature was raised from room temperature to 480-520°C at a heating rate of 15-25°C / min, while the pressure was increased to 18-25 MPa, and the holding time was 50-70 minutes.

6. The preparation method according to claim 5, wherein: In step 2), the mold is a graphite mold, which is first vacuumed to 1×10 -5 Pa, and heated from room temperature to 500 ° C at a rate of 20 ° C / min, while pressurizing to 20 MPa, holding time of 1 h, removing the pressure on the mold and cooling to room temperature to obtain a negative thermal expansion material - aluminum-magnesium based alloy material.

7. A near-zero thermal expansion magnesium alloy, characterized in that: The negative thermal expansion material MnCoGe, aluminum powder and magnesium alloy powder are mixed and ground until the powders are evenly mixed and then sintered by solid phase vacuum hot pressing. The preparation method according to any one of claims 1 to 6 is used to prepare the material.

8. The near-zero thermal expansion magnesium alloy according to claim 7, characterized in that: The near-zero thermal expansion magnesium alloy has near-zero thermal expansion in a temperature range of 25° C. to 150° C.

Images