Nano boron carbide / aluminum neutron absorption composite material and preparation method thereof

Abstract

The invention discloses a nano boron carbide / aluminum neutron absorption composite material and a preparation method thereof. The method comprises the following steps: selecting high-purity aluminum powder and micron boron carbide powder, and uniformly mixing; performing vacuum drying on the mixed powder; and the dried mixed powder is subjected to additive manufacturing forming through a selective laser melting technology, and the nano boron carbide / aluminum neutron absorption composite material is obtained. And carrying out hot isostatic pressing densification treatment on the nano boron carbide / aluminum neutron absorption composite material. The nano boron carbide / aluminum neutron absorption composite material provided by the invention has excellent mechanical properties (the tensile strength is 380-480 MPa, and the ductility is 9.3%-12.5%); meanwhile, the material provided by the invention has good neutron absorption rate (80-95%), and the mechanical-functional comprehensive performance of the material is superior to that of most of existing boron carbide / aluminum composite materials.

Description

Technical Field

[0001] The invention belongs to the technical field of metal composite materials and relates to a nano boron carbide / aluminum neutron absorption composite material and a preparation method thereof. Background Art

[0002] With the development of society and the growth of population, humanity's demand for energy is increasing. Traditional fossil energy reserves are limited and can easily cause serious environmental pollution. Due to its advantages of cleanliness and low resource consumption, nuclear energy has become one of the most promising energy alternatives to bioenergy. Nuclear energy utilization has also become a key energy development strategy for countries around the world. With the restart of my country's nuclear power projects and the advancement and development of nuclear industry technology and other industries, high-energy radiation-related equipment or rays have undoubtedly been widely used in many fields such as scientific research, nuclear power, military, medical care, and aerospace. The resulting radiation shielding and protection issues are increasingly attracting close attention and high attention, making the development of stable, high-performance neutron absorbing materials a necessity.

[0003] Neutrons are neutral particles with extremely strong penetrating power. When colliding with atomic nuclei, neutrons transfer energy to the nuclei, stimulating charged particles and thus causing indirect radiation. Neutron radiation protection requires neutron moderation followed by absorption of the moderated neutrons using nuclides with large neutron absorption cross sections. Among nuclides with large neutron absorption cross sections, boron is abundant and inexpensive. Boron-10, with a thermal neutron absorption cross section as high as 3844b, is an ideal neutron-absorbing nuclide. Boron carbide is the most commonly used boron compound. It has a stable structure, strong hardness, high melting point, and corrosion resistance. It is often combined with other materials to form composite materials for neutron shielding applications. Boron carbide / aluminum composites have been used in the nuclear industry and other radiation protection fields due to their high neutron absorption capacity, high thermal neutron capture cross section, lack of radioactive isotopes, low secondary radiation energy, and excellent thermal conductivity, strength, ductility, and corrosion resistance. They possess promising properties.

[0004] For boron carbide / aluminum neutron-absorbing composites, neutron absorption performance is directly related to the boron carbide content. Increasing the boron carbide content enhances the material's neutron absorption capacity, but its toughness and ductility decrease dramatically. The key to achieving both excellent neutron shielding and mechanical properties lies in the small size and uniform distribution of the boron carbide reinforcement phase particles. Nano-boron carbide particles offer advantages over micron-sized boron carbide particles in slow neutron absorption. Dispersed nano-sized boron carbide particles effectively avoid the self-shielding effect of neutron-absorbing particles, fully utilizing the material's neutron absorption capacity. Furthermore, the smaller the dispersed boron carbide particles, the better the strengthening effect on the aluminum matrix. Nano-sized second-phase particles are ideal for reinforcing metals, increasing their strength while maintaining their toughness and ductility. However, the process of adding nano-sized particles to a metal matrix is ​​complex, requires high control requirements, and achieving uniform distribution of the nanoparticles is difficult.

[0005] Currently, the main methods for preparing boron carbide reinforced aluminum-based composites are metal melt stirring casting, infiltration process, and powder metallurgy. Existing methods for preparing boron carbide / aluminum neutron absorption composites also have problems such as difficult to control particle size, uneven particle distribution (see Chinese Patent Publication No. CN108251673A, which discloses a preparation process for aluminum-based boron carbide neutron absorption materials), low overall material performance (see Chinese Patent Publication No. CN102392148A, which discloses a preparation method for aluminum-based boron carbide neutron absorption composites), and long preparation processes and high costs (see Chinese Patent Publication No. CN106978563A, which discloses a new Al-B4C-B neutron absorption material and its preparation method). Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the above-mentioned prior art and to provide a nano-boron carbide / aluminum neutron absorption composite material and a preparation method thereof. The present invention utilizes laser excitation to melt micron boron carbide. The melted droplets are further refined and reduced in size by the Marangoni convection of the aluminum matrix molten pool. At the same time, the boron carbide droplets are evenly distributed in the molten pool under the action of strong melt convection in the molten pool, ultimately achieving uniform dispersion of nano-scale boron carbide particles in the aluminum matrix. The obtained nano-boron carbide / aluminum neutron absorption composite material has good composite effect, high density, high modulus, high strength and toughness, and has a neutron absorption capacity close to the theoretical limit, and has broad application prospects.

[0007] It should be noted that the application area of ​​the nano-B4C / Al composite material of the present invention is primarily to meet the demand for neutron shielding in the nuclear industry, which places high demands on the material's specific strength and cost. Furthermore, the B4C and Al in the system readily react, resulting in products that are detrimental to the material's mechanical properties. The present invention reduces the degree of reaction and controls the size of the reaction products by regulating the laser scanning speed and substrate temperature during the preparation process, thereby alleviating the low mechanical properties commonly associated with traditional B4C / Al materials.

[0008] The present invention provides an in-situ nano-processing method for boron carbide / aluminum materials. The B4C and Al particles in the initial powder are melted under a high-energy laser. The micron-sized B4C particles are shattered and rapidly melted under laser irradiation. They are dispersed throughout the molten pool under the action of macroscopic eddies within the molten pool, and are finally dispersed uniformly in the matrix. The crushed B4C particles and the surrounding Al matrix are heated to a high temperature of over 2000°C in a short period of time. At this time, B4C and Al have a tendency to react spontaneously. Under normal conditions, they easily react to form products Al4C3 and AlB2. Al4C3 is a coarse, needle-shaped brittle phase that is prone to fracture or cleavage when the material is deformed by stress, leading to premature failure of the material and causing significant damage to the material's mechanical properties and neutron absorption capacity. The present invention regulates the printing parameters in the laser additive manufacturing process of the material, increases the scanning speed, shortens the laser scanning time per unit area of ​​the material, increases the rate of molten pool formation and cooling, and shortens the effective reaction time of B4C and Al in the material. This reduces the probability and extent of the reaction between B4C and Al. Furthermore, the faster cooling rate inhibits elemental diffusion, preventing some of the resulting Al4C3 phase from growing further into coarse, needle-like phases, maintaining nanoscale dimensions. This significantly minimizes damage to the material and preserves its high mechanical properties. Furthermore, the B4C-Al reaction products and B4C particles undergo rapid cooling and heating after rapid laser scanning, ultimately forming an amorphous particle phase within the material. This amorphous phase regulates internal dislocations under stress, enhancing the material's strength while maintaining its plasticity. By controlling the raw materials and additive manufacturing process parameters, the in-situ nanocrystallization of micronized boron carbide particles within the aluminum matrix was innovatively achieved. This significantly increases the content of nanoreinforced phases in conventionally prepared boron carbide / aluminum materials, significantly contributing to the material's improved mechanical properties. Furthermore, the nanocrystallized B-containing phase significantly enhances the material's neutron absorption capacity, resulting in improved mechanical properties and neutron absorption compared to conventionally prepared boron carbide / aluminum materials, promising broad application prospects.

[0009] The purpose of the present invention is achieved through the following technical solutions:

[0010] The present invention provides a method for preparing a nano-boron carbide / aluminum neutron absorption composite material, the method comprising the following steps:

[0011] S1. Mix high-purity aluminum powder and micron boron carbide powder evenly; and vacuum dry the mixed powder;

[0012] S2. Additively manufacturing the dried mixed powder using a selective laser melting process to obtain a nano-boron carbide / aluminum neutron absorbing composite material; the selective laser melting process uses a laser scanning speed of 600-1100 mm / s;

[0013] S3. Performing a hot isostatic pressing densification treatment on the nano-boron carbide / aluminum neutron absorption composite material.

[0014] As an embodiment of the present invention, in step S1, the high-purity aluminum powder is spherical with a particle size of 30-50 μm; the micron boron carbide powder has a particle size of 7-15 μm; and the purity of the raw materials is above 99.9%.

[0015] As an embodiment of the present invention, in step S1, the volume fraction of the micron boron carbide powder in the mixed powder is 10-15 vol.%, and the volume fraction of the spherical high-purity aluminum powder is 85-90 vol.%.

[0016] As an embodiment of the present invention, in step S1, the mixing is mechanical mixing under the protection of inert gas, the stirring speed of the mechanical mixing is 800-1200 rpm, the stirring time is 1-3 min, and it is repeated 10-15 times.

[0017] As an embodiment of the present invention, in step S1, the vacuum degree of vacuum drying is 10 -4 Pa, the drying temperature is 70-100℃, the drying time is 5-6h, and after drying, the powder is allowed to cool to room temperature in the furnace.

[0018] As an embodiment of the present invention, in step S2, the additive manufacturing equipment used is a selective laser melting equipment, model BLT-S210 metal 3D printer; during operation, the oxygen content in the molding chamber of the additive manufacturing equipment needs to be reduced to below 0.026%.

[0019] As an embodiment of the present invention, in step S2, the process parameters of the selective laser melting technology are: laser spot diameter is 60-100μm, laser power is 130-220W, laser scanning speed is 600-1100mm / s, laser scanning spacing is 20-80μm, laser scanning mode is orthogonal strip scanning, and powder laying thickness is set to 10-30μm.

[0020] The preferred process parameters of the selective laser melting technology are as follows: for uniform mixed powder (10 vol.% micron boron carbide + 90 vol.% high-purity aluminum powder), the laser spot diameter is 60 μm, the laser power is 210 W, the laser scanning speed is 600 mm / s, the laser scanning spacing is 60 μm, and the single layer powder thickness is 30 μm.

[0021] The preferred process parameters of the selective laser melting technology are as follows: for uniform mixed powder (15 vol.% micron boron carbide + 85 vol.% high-purity aluminum powder), the laser spot diameter is 60 μm, the laser power is 160 W, the laser scanning speed is 900 mm / s, the laser scanning spacing is 60 μm, and the single-layer powder thickness is 30 μm.

[0022] As an embodiment of the present invention, in step S2, the additive manufacturing process is carried out under an argon protective atmosphere with an argon purity of 99.999%; during the molding chamber gas washing stage to reduce the oxygen content, the argon flow rate is 40-50 L / min; during the laser forming stage, the argon flow rate is 4-5 L / min; and the outlet pressure is 3-5 KPa.

[0023] As an embodiment of the present invention, in step S3, the hot isostatic pressing treatment is performed at a pressure of 50-150 MPa, a temperature of 350-550° C., a holding time of 1-3 h, and a pressurizing medium of high-purity argon.

[0024] As an embodiment of the present invention, in step S3, after the hot isostatic pressing treatment is completed, the obtained material is furnace-cooled to room temperature under an argon atmosphere.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The present invention has simple requirements for raw materials (boron carbide particles, aluminum powder), and the required boron carbide particle size and aluminum powder particle size are both micron-level. On the one hand, compared with the use of nano-level boron carbide particles, the present invention can significantly reduce production costs and achieve more economical material preparation. On the other hand, the use of micron-level boron carbide to in-situ generate nano-boron carbide can effectively avoid the inherent strong electrostatic effect of nanoparticles, easy agglomeration, and difficulty in powder mixing; the low fluidity and poor formability of nano-boron carbide mixed powder; the difficulty in increasing the content of nano-boron carbide, and many other shortcomings. Boron carbide / aluminum neutron absorption composite materials with high nanoparticle volume fraction, strong interface bonding, and uniform particle dispersion can be prepared.

[0027] (2) The present invention adopts a mechanical mixing powder making method. Compared with the traditional powder making process for preparing boron carbide / aluminum composite materials, such as ball milling and co-precipitation, mechanical mixing is simpler and more effective; the powder making cost is lower, the powder making cycle is shorter, and it is suitable for large-scale precursor powder material preparation;

[0028] (3) The present invention adopts the same additive manufacturing process to produce nano boron carbide / aluminum neutron absorption composite materials. The additive manufacturing technology has a short preparation cycle and high efficiency. At the same time, the process characteristics of near-net forming make the additive manufacturing process more suitable for producing workpieces with complex structures compared with traditional preparation processes, and provide possibilities for complex structures and topology optimization. When the laser scans the micron boron carbide in the precursor powder, the boron carbide has a high absorption efficiency for the laser, so that the micron boron carbide melts quickly. At the same time, the laser refines and reduces the boron carbide during the melting process of the micron boron carbide. The melted boron carbide droplets are subjected to the strong effect of Marangoni convection generated by the large temperature gradient in the molten pool, and are further refined and reduced. At the same time, the droplets are evenly distributed in the molten pool, and finally the nano-scale boron carbide particles are uniformly dispersed in the aluminum matrix. The nano-boron carbide particles evenly dispersed in the matrix effectively improve the strength of the material and have little damage to the toughness and plasticity of the material. At the same time, the dispersed distribution of nano-boron carbide particles reduces the loss of the neutron absorption performance of the B element due to agglomeration, fully exerting the neutron absorption performance of the material, and the material has excellent neutron absorption performance.

[0029] (4) The present invention achieves high strength and plasticity that traditional neutron absorption materials do not have, and achieves higher strength and plasticity under the premise of ensuring neutron absorption capacity, so that the obtained nano-boron carbide / aluminum neutron absorption composite material has greater application potential in the field of neutron absorption. The nano-boron carbide / aluminum neutron absorption composite material provided by the present invention has excellent mechanical properties (tensile strength 380-480MPa, elongation 9.3%-12.5%); at the same time, the material provided by the present invention has a good neutron absorption rate (80-95%), and the "mechanical-functional" comprehensive performance is better than most existing boron carbide / aluminum composite materials (the neutron absorption rate of neutrons by neutron absorption materials is affected by many factors such as B content, test sample thickness, material density, particle size, etc.; currently, under the same conditions, the neutron absorption rate of other materials is in the range of 50%-60%). BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0031] Figure 1 This is a surface morphology of the uniformly mixed boron carbide / aluminum composite powder used in Example 2 of the present invention;

[0032] Figure 2 The microstructure morphology of the densified nano-boron carbide / aluminum composite material in Example 2 of the present invention; wherein, (a) is a SEM image of a 15 vol% B4C-Al material (microstructure), and (b) is a SEM image of a 15 vol% B4C-Al material (particle size). DETAILED DESCRIPTION

[0033] The present invention will be described in detail below with reference to the examples. The following examples 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 those skilled in the art may make several adjustments and improvements without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0034] Example 1

[0035] This example prepares a 10 vol.% nano-boron carbide / aluminum neutron absorption composite material. The preparation method is as follows:

[0036] 31.1g of 10μm boron carbide powder and 300g of 30-50μm spherical aluminum powder were placed in a mixing tank and mechanically mixed under an argon atmosphere at a stirring speed of 1000rpm for 2 minutes. This process was repeated 12 times to obtain a uniformly mixed boron carbide / aluminum composite powder. The composite powder was then additively manufactured using a selective laser melting process. The selective laser melting process parameters were as follows: laser spot diameter of 60μm, laser power of 210W, laser scan rate of 600mm / s, laser scan spacing of 60μm, orthogonal strip scanning, and a powder layer thickness of 30μm. The additive manufacturing process was carried out under an argon atmosphere at an outlet pressure of 5kPa. The nano-boron carbide / aluminum composite material obtained by additive manufacturing was placed in a hot isostatic pressing furnace, maintained at 70 MPa in an argon medium at 450°C for 1 hour, and cooled to room temperature in an argon environment to obtain a further densified 10 vol.% nano-boron carbide / aluminum composite material. The nano-boron carbide was uniformly dispersed in the aluminum matrix, and the particle size was about 200 nm.

[0037] In this embodiment, the obtained nano-boron carbide / aluminum neutron absorption composite material has a tensile strength of 380 MPa, an elongation of 12.5%, and a neutron absorption rate of 80%.

[0038] Example 2

[0039] This example prepares a 15 vol.% nano-boron carbide / aluminum neutron absorption composite material. The preparation method is as follows:

[0040] 49.4 g of boron carbide powder with a particle size of 10 μm and 300 g of spherical aluminum powder with a particle size of 30-50 μm were placed in a stirring tank and mechanically mixed under an argon atmosphere at a stirring speed of 1000 rpm for 2 min. This was repeated 12 times to obtain a uniformly mixed boron carbide / aluminum composite powder (e.g. Figure 1As shown). Using composite powder as raw material, the selective laser melting process is adopted to carry out additive manufacturing of nano-boron carbide / aluminum composite materials. The selective laser melting process parameters are as follows: the laser spot diameter is 60μm, the laser power is 160W, the laser scanning speed is 900mm / s, the laser scanning spacing is 60μm, the laser scanning mode is orthogonal strip scanning, and the powder thickness is set to 30μm. The additive manufacturing process is carried out in an argon atmosphere with an outlet pressure of 5KPa. The nano-boron carbide / aluminum composite material obtained by additive manufacturing is placed in a hot isostatic pressing furnace, and maintained at 70MPa under 450℃ argon medium for 1 hour. It is cooled to room temperature with the furnace under an argon environment to obtain a further densified 15vol.% nano-boron carbide / aluminum composite material, as shown Figure 2 As shown in (a), nano-boron carbide is uniformly dispersed in the aluminum matrix, and the particle size is about 200nm (as shown in Figure 2 (as shown in (b)).

[0041] In this embodiment, the obtained nano-boron carbide / aluminum neutron absorption composite material has a tensile strength of 480 MPa, an elongation of 9.3%, and a neutron absorption rate of 95%.

[0042] Comparative Example 1

[0043] In this comparative example, a 15 vol.% nano-boron carbide / aluminum neutron absorption composite material was prepared. The preparation method is as follows:

[0044] 49.4g of 10μm boron carbide powder and 300g of 30-50μm spherical aluminum powder were placed in a mixing tank and mechanically mixed under an argon atmosphere at a stirring speed of 1000rpm for 2 minutes. This process was repeated 12 times to obtain a uniformly mixed boron carbide / aluminum composite powder. Using the composite powder as the raw material, the nano-boron carbide / aluminum composite was additively manufactured using a selective laser melting process. The selective laser melting process parameters were as follows: laser spot diameter of 60μm, laser power of 140W, laser scan rate of 300mm / s, laser scan spacing of 60μm, orthogonal strip scanning, and a powder layer thickness of 30μm. The additive manufacturing process was carried out under an argon atmosphere at an outlet pressure of 5kPa. The nano boron carbide / aluminum composite material obtained by additive manufacturing was placed in a hot isostatic pressing furnace, maintained at 70 MPa under argon medium at 450°C for 1 hour, and cooled to room temperature in an argon environment to obtain a further densified 15 vol.% boron carbide / aluminum composite material.

[0045] In this comparative example, the obtained nano-boron carbide / aluminum neutron absorption composite material has a tensile strength of 410 MPa, an elongation of 2.7, and a neutron absorption rate of 80%.

[0046] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various variations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a nano-boron carbide / aluminum neutron absorption composite material, characterized in that: The method comprises the following steps: S1. Mix high-purity aluminum powder and micron boron carbide powder evenly; and vacuum dry the mixed powder; S2. Additively manufacturing the dried mixed powder using a selective laser melting process to obtain a nano-boron carbide / aluminum neutron absorbing composite material; the selective laser melting process uses a laser scanning speed of 600-1100 mm / s; S3, performing hot isostatic pressing densification treatment on the nano boron carbide / aluminum neutron absorption composite material.

2. The method for preparing the nano-boron carbide / aluminum neutron absorption composite material according to claim 1, characterized in that: In step S1, the high-purity aluminum powder is spherical and has a particle size of 30-50 μm; the micron boron carbide powder has a particle size of 7-15 μm; and the purity of the raw materials is above 99.9%.

3. The method for preparing the nano-boron carbide / aluminum neutron absorption composite material according to claim 1, characterized in that: In step S1, the volume fraction of the micron boron carbide powder in the mixed powder is 10-15 vol.%, and the volume fraction of the spherical high-purity aluminum powder is 85-90 vol.%.

4. The method for preparing the nano-boron carbide / aluminum neutron absorption composite material according to claim 1, characterized in that: In step S1, the mixing is performed mechanically under the protection of an inert gas, the stirring speed of the mechanical mixing is 800-1200 rpm, the stirring time is 1-3 min, and the mixing is repeated 10-15 times.

5. The method for preparing the nano-boron carbide / aluminum neutron absorption composite material according to claim 1, characterized in that: In step S1, the vacuum degree of vacuum drying is 10 -4 Pa, the drying temperature is 70-100℃, the drying time is 5-6h, and after drying, the powder is allowed to cool to room temperature in the furnace.

6. The method for preparing the nano-boron carbide / aluminum neutron absorption composite material according to claim 1, characterized in that: In step S2, the additive manufacturing equipment used is a selective laser melting equipment, model BLT-S210 metal 3D printer; when working, the oxygen content in the molding chamber of the additive manufacturing equipment needs to be reduced to below 0.026%.

7. The method for preparing the nano-boron carbide / aluminum neutron absorption composite material according to claim 1, characterized in that: In step S2, the process parameters of the selective laser melting technology are: laser spot diameter is 60-100μm, laser power is 130-220W, laser scanning speed is 600-1100mm / s, laser scanning spacing is 20-80μm, laser scanning mode is orthogonal strip scanning, and powder thickness is set to 10-30μm.

8. The method for preparing the nano-boron carbide / aluminum neutron absorption composite material according to claim 1, characterized in that: In step S2, the additive manufacturing process is carried out under an argon protective atmosphere with an argon purity of 99.999%; during the stage of gas washing in the molding chamber to reduce the oxygen content, the argon flow rate is 40-50 L / min; during the laser forming stage, the argon flow rate is 4-5 L / min; and the outlet pressure is 3-5 KPa.

9. The method for preparing the nano-boron carbide / aluminum neutron absorption composite material according to claim 1, characterized in that: In step S3, the hot isostatic pressing treatment is performed under a pressure of 50-150 MPa, a temperature of 350-550° C., a holding time of 1-3 hours, and a pressurizing medium of high-purity argon.

10. The method for preparing the nano-boron carbide / aluminum neutron absorption composite material according to claim 1, characterized in that: In step S3, after the hot isostatic pressing treatment is completed, the obtained material is furnace-cooled to room temperature under an argon atmosphere.

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