CN113798493B - Method for improving mechanical property of CuCrZr alloy prepared by additive manufacturing - Google Patents

Abstract

The invention provides a method for improving the mechanical properties of CuCrZr alloy prepared by additive manufacturing. The soluble rare earth oxide salt and CuCrZr powder are added into absolute ethanol or deionized water, so that the soluble rare earth oxide salt is in the solution, and the CuCrZr spherical powder is completely Wetting to obtain a solid-liquid mixture; drying and evaporating the solid-liquid mixture, and then calcining and reducing to obtain a preliminary composite spherical powder; ball milling the mixed powder of the preliminary composite spherical powder and the rare earth element powder to make it fully mixed to obtain a composite Spherical powder; using composite spherical powder as raw material, the composite spherical powder layer is printed layer by layer through additive manufacturing technology to melt and solidify, and at the same time, laser rapid remelting is performed on each solidified layer to prepare rare earth oxides Doped CuCrZr composites. In the present invention, the microstructure of the composite material is regulated by adding rare earth oxide to improve the mechanical properties of the material.

Description

A method for improving the mechanical properties of CuCrZr alloys prepared by additive manufacturing

technical field

The invention belongs to the technical field of additive manufacturing of metal composite materials, in particular to a method for improving the mechanical properties of CuCrZr alloy prepared by additive manufacturing

Background technique

Selective Laser Melting (SLM) is a typical powder bed additive manufacturing (AM) technique that manufactures parts layer by layer by fusing powders together according to slices of a 3D digital model. First, a layer of powder is spread over the substrate. Second, the powder is melted and solidified in selected areas using a computer-controlled laser beam. Third, the plate moves down one layer thickness, and then another layer of powder continues on top of the solidified powder. Repeat these steps until the part is manufactured. Therefore, the advantages of additive manufacturing technology for preparing samples with complex structures have received extensive attention.

CuCrZr alloys are considered candidates for many applications due to their excellent thermal conductivity, electrical conductivity, and high strength, such as lead frame materials, high-speed railway contact wires, heat transfer elements, and nuclear reactor assemblies. Up to now, the preparation method of CuCrZr alloy is mainly based on traditional methods, such as casting. In addition, the CuCrZr alloy prepared by casting needs to further improve the mechanical properties of the CuCrZr alloy through subsequent processing, such as hot rolling, heat treatment and cold working. However, with the industry's demand for functional components with complex structures, the choice of additive manufacturing technology to prepare CuCrZr alloys has received more and more attention. Such as the original powder preparation, additive manufacturing, post-hot isostatic pressing, annealing and other steps proposed in the Chinese Invention Patent Announcement text of application publication number CN111676386A to improve the properties of CuCrZr alloy. However, the steps are cumbersome, the preparation cost is too high, and it is difficult to achieve large-scale production, and hot isostatic pressing is only suitable for small-sized samples and cannot prepare large-sized samples. Preparation of zirconia dispersion-strengthened Cu or CuCrZr alloy based on smelting method proposed in the Chinese Invention Patent Announcement with Application Publication No. CN110129609A. The invention overcomes the floating problem of ZrO2 powder caused by the large difference in specific gravity between oxide and matrix when the alloy is prepared by traditional smelting. The main method is to use a Cu-ZrO2 combination with a specific gravity close to that of the matrix Cu. However, this invention is only applicable to ZrO2 as the second phase to strengthen CuCrZr alloy, and other second phases cannot be used, and the invention adopts traditional melting technology, which has long preparation cycle, high energy consumption, low processing efficiency, and is difficult to manufacture complex components of the structure. In addition, the main alloying elements such as Nb, Sc, Er, Y and Mg are added to the CuCrZr alloy based on the smelting method proposed in the Chinese Invention Patent Announcement text of the application publication number CN110747365A, and the subsequent solid solution treatment, drawing deformation, aging treatment , stress relief annealing and other steps to prepare high-performance CuCrZr alloy. However, this method has too many steps, complicated operations, and low processing efficiency. The cold drawing deformation has limitations on the size and shape of the material, and the preparation method is still a traditional smelting technology, making it difficult to manufacture components with complex structures. The application publication number CN105925922A proposed in the Chinese invention patent announcement text adopts the continuous equal-diameter angular extrusion technology for casting CuCrZr alloy to prepare high-performance materials. This method can only prepare wires such as alloy wires, but cannot prepare bulk materials.

As can be seen from the above-mentioned patents, there is a need to develop a way to prepare high-performance CuCrZr alloys using additive manufacturing and few steps. This patent uses selective laser melting technology with high forming precision to additively manufacture high-performance nano-rare earth oxide doped CuCrZr alloys. The proposal of this patent will provide a practical method for additive manufacturing of high-performance oxide dispersion-strengthened CuCrZr alloys.

SUMMARY OF THE INVENTION

In order to solve the above technical problems, the present invention provides a method for improving the mechanical properties of CuCrZr alloy prepared by additive manufacturing, using additive manufacturing technology and a nanosecond-phase dispersion strengthening mechanism to prepare rare-earth oxide-doped CuCrZr without obvious defects alloy, its mechanical properties are significantly enhanced.

In order to achieve the above purpose, the technical solutions adopted in the embodiments of the present invention are as follows:

The invention provides a method for improving the mechanical properties of CuCrZr alloy prepared by additive manufacturing, the method comprises the following steps:

Step S1, adding the soluble rare earth oxide salt and CuCrZr spherical powder into absolute ethanol or deionized water, and making the soluble rare earth oxide salt dissolve in the solution and the CuCrZr spherical powder completely wet by ultrasonic vibration or mechanical stirring to obtain a solid-liquid mixture ;

In step S2, the solid-liquid mixture obtained in step S1 is dried and evaporated, so that the rare earth oxide salt is deposited on the CuCrZr spherical particles, and then calcined at 400 to 700° C. for 2 to 8 hours in an atmosphere containing hydrogen for reduction to obtain a uniform rare earth oxide. Preliminary composite spherical powder dispersed on the surface of CuCrZr spherical particles;

Step S3, adding rare earth elemental powder to the preliminary composite spherical powder obtained in step S2, and performing ball milling on the mixed powder of the preliminary compound spherical powder and the rare earth elemental powder to make it fully mixed to obtain a compound spherical powder; the rare earth elemental powder The rare earth element is the same as the rare earth element in the soluble rare earth oxide salt in step S1;

In step S4, using the composite spherical powder prepared in step S3 as a raw material, the composite spherical powder layer is printed layer by layer through additive manufacturing technology to melt and solidify, and at the same time, laser rapid remelting is performed on each solidified layer to prepare a composite spherical powder layer. Rare earth oxide doped CuCrZr composites.

Preferably, the additive manufacturing technology in step S4 is a laser selective melting technology, and the process parameters are: the energy density of the laser body ranges from 200 to 1000 J/mm ³ .

Preferably, the process parameters of the laser rapid remelting in step S4 are: the energy density of the laser body is in the range of 200-1200 J/mm ³ , and the scanning speed is in the range of 800-1600 mm/s.

Preferably, the soluble rare earth oxide salt in step S1 is yttrium nitrate or lanthanum nitrate; if the soluble rare earth oxide salt is yttrium nitrate, the rare earth element added in step S3 is pure yttrium; if the soluble rare earth oxide salt is nitric acid lanthanum, the rare earth element added in step S3 is pure lanthanum.

Preferably, the rare earth oxide content in the composite spherical powder obtained in step S3 is 0.25 wt.% to 2.0 wt.%.

Preferably, in step S2, a preliminary composite spherical powder in which nano-sized rare earth oxides are uniformly dispersed and wrapped on the surface of the CuCrZr spherical particles is obtained.

Preferably, the atmosphere containing hydrogen in step S2 is pure hydrogen or a mixture of hydrogen and argon.

The present invention has the following beneficial effects:

The invention provides a method for improving the mechanical properties of CuCrZr alloy prepared by additive manufacturing. The additive manufacturing technology and the nano-second phase dispersion strengthening mechanism are used to prepare rare-earth oxide-doped CuCrZr alloy without obvious defects. Performance is significantly enhanced.

1. The patent proposes to add rare earth oxide and rare earth element in turn by using two methods. The first addition of rare earth oxides is based on physical deposition, reduction and nucleation mechanisms, so that rare earth oxides can tightly coat the CuCrZr spherical powder on the basis of maintaining nanometer size, and can maintain sphericity. The second method is to uniformly mix the rare earth element with the initial composite powder by ball milling. The rare earth element can not only adsorb the oxygen atoms in the process of adding rare earth oxides, but also adsorb the oxygen atoms in the printing process. The added rare earth element forms rare earth oxides during the adsorption process, which avoids printing defects such as holes, cracks, etc. caused by the oxidation of CuCrZr powder and the preparation of rare earth oxide-doped CuCrZr composite materials, thereby ensuring the mechanical properties of the printed parts. performance. The above-mentioned two-step method of adding rare earth oxides and rare earth compounds can not only reduce the material defects caused by the presence of oxygen atoms in the method of adding rare earth oxides, but also save costs compared with only adding rare earth elements.

2. Compared with not using the fast laser remelting technology, this patent innovatively proposes a fast laser remelting method. This fast laser remelting can use the Marangoni convection effect to break up and redistribute the agglomerated rare earth oxides to eliminate the agglomeration of rare earth oxides, so that the rare earth oxides can be dispersed in the CuCrZr alloy to play a dispersion strengthening effect. Enhance the mechanical properties of your prints.

3. The present invention has low cost, can prepare defect-free alloys, and is beneficial to mass industrial production. In addition, based on the advantages of additive manufacturing technology, the invention has a high degree of freedom for sample size and structure, and can efficiently prepare dispersion-strengthened CuCrZr alloys with different contents and types of rare earth oxides.

Description of drawings

In order to illustrate the technical solutions of the embodiments of the present invention more clearly, the following briefly introduces the accompanying drawings used in the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the embodiments of the present invention. .

Figure 1 is a picture of pure CuCrZr alloy powder;

Figure 2 is a picture of 0.5 wt.% Y ₂ O ₃ -CuCrZr composite powder;

Figure 3 shows (a) the transmission electron microscope picture of the pure CuCrZr sample produced by additive manufacturing and (b) the mechanical property diagram in Comparative Example 1;

FIG. 4 is a transmission electron microscope picture of (a) additively manufactured 0.5 wt.% Y ₂ O ₃ -CuCrZr sample and (b) a mechanical property diagram in Example 1;

5 is a transmission electron microscope picture of the additively manufactured 0.25 wt.% Y ₂ O ₃ -CuCrZr sample in Example 2;

FIG. 6 is a transmission electron microscope picture of the additively manufactured 2.0 wt.% La ₂ O ₃ -CuCrZr sample in Example 3. FIG.

Detailed ways

In order to make those skilled in the art better understand the technical solutions of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

The present invention also provides a method for improving the mechanical properties of CuCrZr alloy prepared by additive manufacturing, the method comprising the following steps:

Step S1, adding the soluble rare earth oxide salt and CuCrZr spherical powder into absolute ethanol or deionized water, and making the soluble rare earth oxide salt dissolve in the solution and the CuCrZr spherical powder completely wet by ultrasonic vibration or mechanical stirring to obtain a solid-liquid mixture ; The soluble rare earth oxide salt is yttrium nitrate or lanthanum nitrate.

In step S2, the solid-liquid mixture obtained in step S1 is dried and evaporated, so that the rare earth oxide salt is deposited on the CuCrZr spherical particles, and then calcined at 400-700° C. for 2-8 hours in pure hydrogen or hydrogen-argon mixture for reduction to obtain nanometers. Preliminary composite spherical powders of rare earth oxides of different sizes are uniformly dispersed and wrapped on the surface of CuCrZr spherical particles.

Step S3, adding rare earth elemental powder to the preliminary composite spherical powder obtained in step S2, and performing ball milling on the mixed powder of the preliminary compound spherical powder and the rare earth elemental powder to make it fully mixed to obtain a compound spherical powder, a compound spherical powder The content of rare earth oxides is 0.25 wt.% to 2.0 wt.%; the rare earth element of the rare earth element is the same as the rare earth element in the soluble rare earth oxide salt in step S1; if the soluble rare earth oxide salt is yttrium nitrate, The rare earth element is pure yttrium; if the soluble rare earth oxide salt is lanthanum nitrate, the rare earth element is pure lanthanum.

In step S4, using the composite spherical powder prepared in step S3 as a raw material, the composite spherical powder layer is printed layer by layer through additive manufacturing technology to melt and solidify, and at the same time, laser rapid remelting is performed on each solidified layer to prepare a composite spherical powder layer. Rare earth oxide-doped CuCrZr composite material, the oxide-doped CuCrZr composite material is free of cracks and defects, and its density is as high as 99.5% or more. The rare earth oxides in the prepared rare earth oxide doped CuCrZr composite samples are dispersed and distributed, and the size of the second phase formed in the samples is nanoscale, ranging from 20 to 500 nm. The additive manufacturing technology is a laser selective melting technology, and the process parameters are as follows: the energy density of the laser body ranges from 200 to 1000 J/mm ³ . The process parameters of the laser rapid remelting are as follows: the energy density of the laser body is in the range of 200-1200 J/mm ³ , and the scanning speed is in the range of 800-1600 mm/s.

Comparative Example 1

Selective laser melting (SLM) was used to prepare pure CuCrZr samples. The volume energy density used was 200J/mm ³ . The size of the printed samples met the requirements of standard tensile samples, and the density was over 99.5%. Figure 1 shows the spherical powder of pure CuCrZr with a particle size in the range of 15-150 μm. The microstructure of the CuCrZr sample was observed by transmission electron microscope (TEM), as shown in Fig. 3(a), it can be seen that there is a dispersed nanosecond phase with a size of about 100 nm and an area density of 5.3/μm ² . The tensile fracture mechanical properties of the samples were tested, and the ultimate tensile strength was 285 MPa and the ductility was 20.5%, as shown in Fig. 3(b).

Example 1

51.28 g of yttrium nitrate hexahydrate and 5000 g of CuCrZr spherical powder were added to an appropriate amount of absolute ethanol, the yttrium nitrate hexahydrate was completely dissolved in absolute ethanol by mechanical stirring, and the CuCrZr spherical powder was completely wetted in absolute ethanol. , obtain a solid-liquid mixture, dry the solid-liquid mixture, and then reduce it under a hydrogen atmosphere of 400 ° C for 8 hours to obtain a preliminary composite spherical powder, add 10 g of pure yttrium powder to the preliminary composite spherical powder, and compare the pure yttrium powder with The preliminary composite spherical powder is ball-milled to be thoroughly mixed to prepare composite spherical powder for additive manufacturing. Using the prepared composite spherical powder as the printing precursor powder, a 0.5 wt.% Y ₂ O ₃ doped CuCrZr composite was prepared by selective laser melting (SLM) with a bulk energy density of 1000 J/mm ³ , the laser rapid remelting parameters are the laser volume energy density range of 200J/mm ³ and the scanning speed of 800 mm/s. The size of the printed sample meets the size requirements of the standard tensile sample, and the density is above 99.5%. The sample is free of defects such as cracks and holes. Figure 2 shows the spherical powder of CuCrZr coated with 0.5 wt.% Y ₂ O ₃ , and its particle size is in the range of 15-150 μm. It can be seen that yttrium oxide is completely coated on the surface of the CuCrZr spherical powder. The microstructure of the 0.5 wt.% Y ₂ O ₃ doped CuCrZr sample was observed by transmission electron microscopy (TEM), as shown in Fig. 4(a), and it can be seen that there is a dispersed nanosecond phase. Its size is about 30 nm, and the area density is 250/μm ² . Its number is significantly higher than that of the second phase in pure CuCrZr samples and its size is much lower than that of pure CuCrZr samples. The tensile fracture mechanical properties of the samples were tested, and the ultimate tensile strength was 347 MPa and the ductility was 38 %, as shown in Fig. 4(b). By comparing with the ultimate tensile strength and ductility of the pure CuCrZr sample, it can be found that the ultimate tensile strength and ductility of the doped sample are higher than those of the pure CuCrZr sample of Comparative Example 1. This shows that the addition of nano-second phase and additive manufacturing technology can simultaneously improve the performance of CuCrZr samples.

Example 2

17.04 g of yttrium nitrate hexahydrate and 4000 g of CuCrZr spherical powder were added to an appropriate amount of deionized water, and the yttrium nitrate hexahydrate was completely dissolved in deionized water by ultrasonic vibration, and the spherical powder of CuCrZr was completely wetted in deionized water to obtain a solid-liquid The solid-liquid mixture is dried, and then reduced in a hydrogen atmosphere at 700 ° C for 2 hours to obtain a preliminary composite spherical powder. 5g of pure yttrium powder is added to the preliminary composite spherical powder. The pure yttrium powder and the preliminary composite spherical powder The powder is ball-milled and thoroughly mixed to produce composite spherical powders for additive manufacturing. Using the prepared composite spherical powder as the printing precursor powder, a 0.25 wt.% Y ₂ O ₃ doped CuCrZr composite was prepared by selective laser melting (SLM) with a bulk energy density of 800 J/mm ³ , and the parameters of laser rapid remelting are that the laser volume energy density range is 1200J/mm ³ , and the scanning speed is 1600 mm/s. The size of the printed sample meets the size requirements of the standard tensile sample, and the density is above 99.5%. The microstructure of the 0.25 wt.% Y ₂ O ₃ doped CuCrZr sample was observed by transmission electron microscopy (TEM), as shown in Fig. 5, and it can be seen that there is a dispersed nanosecond phase. Its size is about 40 nm, and the area density is 200/μm ² . Its number is significantly higher than that of the second phase in pure CuCrZr samples and its size is much lower than that of pure CuCrZr samples. It can be concluded that its mechanical properties are higher than those of the pure CuCrZr sample in Comparative Example 1. This shows that the addition of nano-second phase and additive manufacturing technology can simultaneously improve the performance of CuCrZr samples.

Example 3

661.2 g of lanthanum nitrate hexahydrate and 5000 g of CuCrZr spherical powder were added to an appropriate amount of deionized water, and the lanthanum nitrate hexahydrate was completely dissolved in deionized water by mechanical stirring, and the CuCrZr spherical powder was completely wetted in deionized water to obtain a solid-liquid The solid-liquid mixture was dried, and then reduced in a hydrogen atmosphere at 600 ° C for 4 hours to obtain a preliminary composite spherical powder. 15g of pure lanthanum powder was added to the preliminary composite spherical powder. The powder is ball-milled and thoroughly mixed to produce composite spherical powders for additive manufacturing. Using the prepared composite spherical powder as the printing precursor powder, a 2.0 wt.% La ₂ O ₃ doped CuCrZr composite was prepared by selective laser melting (SLM), and the volume energy density used was 1000 J/mm ³ , the laser rapid remelting parameters are the laser volume energy density range of 1000J/mm ³ and the scanning speed of 1200 mm/s. The size of the printed sample meets the size requirements of the standard tensile sample, and the density is above 99.5%. The microstructure of 2.0 wt.% La ₂ O ₃ doped CuCrZr was observed by transmission electron microscopy (TEM), as shown in Fig. 6, and it can be seen that there is a dispersed nanosecond phase. Its size is about 30 nm and the area density is 300/μm ² . Its number is significantly higher than that of the second phase in pure CuCrZr samples and its size is much lower than that of pure CuCrZr samples. It can be concluded that its mechanical properties are higher than those of the pure CuCrZr sample in Comparative Example 1. This shows that the addition of nano-second phase and additive manufacturing technology can simultaneously improve the performance of CuCrZr samples.

It can be seen from the above technical solutions that this embodiment provides a method for improving the mechanical properties of CuCrZr alloys prepared by additive manufacturing, using additive manufacturing technology and a nanosecond-phase dispersion strengthening mechanism to prepare rare-earth oxide-doped rare earth oxides without obvious defects. Hetero CuCrZr alloy, its mechanical properties are significantly enhanced.

1. The two methods proposed in this embodiment are used to sequentially add rare earth oxides and rare earth elements. The first addition of rare earth oxides is based on physical deposition, reduction and nucleation mechanisms, so that rare earth oxides can tightly coat the CuCrZr spherical powder on the basis of maintaining nanometer size, and can maintain sphericity. The second method is to uniformly mix the rare earth element with the initial composite powder by ball milling. The rare earth element can not only adsorb the oxygen atoms in the process of adding rare earth oxides, but also adsorb the oxygen atoms in the printing process. The added rare earth element forms rare earth oxides during the adsorption process, which avoids printing defects such as holes, cracks, etc. caused by the oxidation of CuCrZr powder and the preparation of rare earth oxide-doped CuCrZr composite materials, thereby ensuring the mechanical properties of the printed parts. performance. The above-mentioned two-step method of adding rare earth oxides and rare earth compounds can not only reduce the material defects caused by the presence of oxygen atoms in the method of adding rare earth oxides, but also save costs compared with only adding rare earth elements.

2. Compared with not using the fast laser remelting technology, this embodiment innovatively proposes a fast laser remelting method. This fast laser remelting can use the Marangoni convection effect to break up and redistribute the agglomerated rare earth oxides to eliminate the agglomeration of rare earth oxides, so that the rare earth oxides can be dispersed in the CuCrZr alloy to play a dispersion strengthening effect. Enhance the mechanical properties of your prints.

3. The cost of this embodiment is low, and defect-free alloys can be prepared, which is beneficial to mass industrial production. In addition, based on the advantages of additive manufacturing technology, this example has a high degree of freedom for sample size and structure, and can efficiently prepare dispersion-strengthened CuCrZr alloys with different contents and types of rare earth oxides.

The embodiments of the present invention have been described in detail above through the embodiments, but the contents are only exemplary embodiments of the embodiments of the present invention, and cannot be considered to limit the implementation scope of the embodiments of the present invention. The protection scope of the embodiments of the present invention is defined by the claims. Any technical solutions described in the embodiments of the present invention, or those skilled in the art, under the inspiration of the technical solutions of the embodiments of the present invention, within the essence and protection scope of the embodiments of the present invention, design similar technical solutions to achieve the above-mentioned Technical effects, or equivalent changes and improvements made to the scope of the application, shall still fall within the scope of protection covered by the patent of the embodiments of the present invention.

Claims (3)

1. a method for improving the mechanical properties of CuCrZr alloy prepared by additive manufacturing, is characterized in that, described method comprises the steps: Step S1, adding the soluble rare earth oxide salt and CuCrZr spherical powder into absolute ethanol or deionized water, and making the soluble rare earth oxide salt dissolve in the solution and the CuCrZr spherical powder completely wet by ultrasonic vibration or mechanical stirring to obtain a solid-liquid mixture ; The soluble rare earth oxide salt is yttrium nitrate or lanthanum nitrate; In step S2, the solid-liquid mixture obtained in step S1 is dried and evaporated, so that the rare earth oxide salt is deposited on the CuCrZr spherical particles, and then calcined at 400 to 700° C. for 2 to 8 hours in an atmosphere containing hydrogen for reduction to obtain a uniform rare earth oxide. Preliminary composite spherical powder dispersed on the surface of CuCrZr spherical particles; Step S3, adding rare earth elemental powder to the preliminary composite spherical powder obtained in step S2, and performing ball milling on the mixed powder of the preliminary compound spherical powder and the rare earth elemental powder to make it fully mixed to obtain a compound spherical powder. The rare earth oxide content in the powder is 0.25 wt.% to 2.0 wt.%; the rare earth element of the rare earth element is the same as the rare earth element in the soluble rare earth oxide salt in step S1; if the soluble rare earth oxide salt is nitric acid Yttrium, the rare earth element added in step S3 is pure yttrium; if the soluble rare earth oxide salt is lanthanum nitrate, the rare earth element added in step S3 is pure lanthanum; In step S4, using the composite spherical powder prepared in step S3 as a raw material, the composite spherical powder layer is printed layer by layer through additive manufacturing technology to melt and solidify, and at the same time, laser rapid remelting is performed on each solidified layer to prepare a composite spherical powder layer. Rare earth oxide doped CuCrZr composite material; the additive manufacturing technology is laser selective melting technology, and its process parameters are: the laser volume energy density range is 200-1000J/mm ³ ; the process parameters of the laser rapid remelting are: The laser volume energy density range is 200-1200 J/mm ³ , and the scanning speed is 800-1600 mm/s. 2 . The method for improving the mechanical properties of CuCrZr alloys prepared by additive manufacturing according to claim 1 , wherein, in step S2 , a preliminary composite spherical powder in which nano-sized rare earth oxides are uniformly dispersed and wrapped on the surface of CuCrZr spherical particles is obtained. 3 . 3 . The method for improving the mechanical properties of CuCrZr alloy prepared by additive manufacturing according to claim 1 , wherein the atmosphere containing hydrogen in step S2 is pure hydrogen or a hydrogen-argon mixture. 4 .

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