A method for improving sintering density of a unit system powder

Abstract

The application relates to a method for improving sintering density of unit system powder, belonging to the technical field of powder metallurgy; the method comprises the following steps: mixing unit system powder and a binder to obtain a mixture; preparing a green body from the mixture; performing a degreasing treatment on the green body to remove the binder in the green body and form connected pores, thereby obtaining an intermediate body; performing pre-sintering on the intermediate body to generate reactant particles at the boundaries of the unit system powder in the intermediate body; performing densification sintering on the intermediate body after pre-sintering to eliminate pores in the intermediate body; performing sintering on the intermediate body after densification sintering to reduce the reactant particles in the intermediate body, thereby completing sintering; and then through a continuous three-step sintering process, a small amount of reaction product is formed between the powders, the removal of the crystal boundary and the pores in the sintering process is inhibited, the pores are eliminated by means of atomic crystal boundary diffusion, and therefore the sintering density of the final sintering sample is improved.

Description

Technical Field

[0001] This application relates to the field of powder metallurgy technology, and in particular to a method for improving the sintering density of unit system powders. Background Technology

[0002] Powder metallurgy is a green manufacturing technology with high processing efficiency and one-time net forming, widely used in the large-scale production of complex parts. The presence of porosity significantly affects the performance of powder metallurgy parts, and porosity elimination mainly occurs during the sintering process. Therefore, controlling the sintering process to form high-density powder metallurgy parts is of great significance.

[0003] During the sintering of unitary powder, porosity is eliminated through atomic diffusion, which can be categorized into surface diffusion, grain boundary diffusion, and volume diffusion. Grain boundary diffusion has the highest coefficient, and maintaining atomic grain boundary diffusion is beneficial for achieving high densification. However, in the later stages of sintering, rapid grain growth and the detachment of pores from grain boundaries weaken grain boundary diffusion, hindering pore elimination. This results in lower density of unitary powder sintered parts, failing to meet the performance requirements of some special applications. Therefore, the density of unitary powder sintering can be improved by controlling the sintering regime to suppress the detachment of grain boundaries and pores, thereby utilizing atomic grain boundary diffusion. In the prior art, Chinese invention patent CN103769589B describes a method for preparing high-strength, high-toughness, and high-conductivity pure copper sintered bulk materials. This method uses hot isostatic pressing (HIP) to obtain high-density copper sintered parts by maintaining a medium pressure of 205–210 MPa during sintering. However, HIP equipment is expensive and has limited applicability to complex structural parts. Summary of the Invention

[0004] This application provides a method for improving the sintering density of unit system powders, thereby addressing the problems of high difficulty and high cost in the preparation of high-density metal parts by powder metallurgy.

[0005] In a first aspect, this application provides a method for improving the sintering density of unitary powder, characterized in that the method comprises:

[0006] The unit system powder and the binder are mixed to obtain a mixture;

[0007] The mixture is then prepared into a green body;

[0008] The green blank is degreased to remove the binder and form interconnected pores, thus obtaining an intermediate blank;

[0009] The intermediate blank is pre-sintered to generate reactant particles at the boundaries of the unit system powder in the intermediate blank;

[0010] The pre-sintered intermediate billet is densified by sintering to eliminate porosity in the intermediate billet;

[0011] The densified sintered intermediate blank is sintered to reduce the reactant particles in the intermediate blank, thus completing the sintering process.

[0012] As an optional implementation, the pre-sintering temperature is 60% to 80% of the melting point of the unit system powder.

[0013] As an optional implementation, the pre-sintering atmosphere includes an oxidizing gas and an inert gas; and / or

[0014] The oxidizing gases include, but are not limited to, oxygen, water vapor, and nitric oxide.

[0015] As an optional implementation, the densification sintering temperature is 80% to 95% of the melting point of the unit system powder.

[0016] As an optional implementation, the densification sintering atmosphere includes an inert gas.

[0017] As an optional implementation, the sintering temperature is 80% to 95% of the melting point of the unit system powder.

[0018] As an optional implementation, the sintering atmosphere includes a reducing gas and an inert gas; and / or

[0019] The reducing gases include, but are not limited to, carbon monoxide, formic acid, and hydrogen.

[0020] As an optional implementation, in the green body, the mass ratio of the unit system powder to the binder is (70-85):(15-30); and / or

[0021] The method of forming the mixture into a green body includes compression molding or indirect additive manufacturing; and / or

[0022] The compression molding pressure is 5–10 MPa; and / or

[0023] The indirect additive manufacturing includes material extrusion additive manufacturing, binder spraying additive manufacturing, or photopolymerization additive manufacturing.

[0024] As an optional implementation, the peak temperature of the degreasing treatment is 200–500°C.

[0025] The technical solutions provided in this application have the following advantages compared with the prior art:

[0026] The method provided in this application provides a green body composed of unit system powder and binder. The binder is removed by degreasing treatment to form uniform interconnected pores inside the green body. Then, a small amount of reaction products are formed between the powders through a continuous three-step sintering process to suppress the removal of grain boundaries and pores during sintering. The pores are eliminated by diffusion of atoms through grain boundaries, thereby improving the density of the final sintered sample. Attached Figure Description

[0027] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0028] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 A flowchart illustrating the method provided in the embodiments of this application;

[0030] Figure 2 This application provides an internal schematic diagram of the intermediate billet after pre-sintering for the purposes of this embodiment;

[0031] Figure 3 This application provides an internal schematic diagram of the intermediate billet after densification sintering for the purposes of this embodiment;

[0032] Figure 4 Internal schematic diagram of sintering unit system powder provided by existing technology;

[0033] Figure 5 This is a microstructure image (100x magnification) of the pure copper sample prepared in Example 1 of this application.

[0034] Figure labels: 201-pores, 202-grain boundaries, 203-reactant particles. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0036] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0037] Figure 1 A flowchart of the method provided in the embodiments of this application is shown below. Figure 1 As shown in the embodiment of this application, a method for improving the sintering density of unit system powder is provided, characterized in that the method includes:

[0038] S1. Mix the unit system powder and the binder to obtain a mixture;

[0039] In some embodiments, the unit system powder may be selected from copper powder, steel powder, etc.

[0040] Specifically, in this embodiment, the unit system powder and binder are mixed evenly and ball-milled to prepare a raw material with room temperature solidification characteristics, namely, a mixture.

[0041] S2. The mixture is prepared into a green body;

[0042] In some embodiments, in the green body, the mass ratio of the unit system powder to the binder is (70-85):(15-30); the method of forming the mixture into a green body includes compression molding or indirect additive manufacturing; wherein the pressure of compression molding is 5-10 MPa; the indirect additive manufacturing includes material extrusion additive manufacturing, binder spraying additive manufacturing or photopolymerization additive manufacturing.

[0043] Specifically, in this embodiment, the raw material is placed in a mold and pressed into a green sample, or a three-dimensional green sample is printed using an indirect additive manufacturing method. The pressure used in placing the raw material in the mold and pressing into a green sample is 5-10 MPa. The indirect additive manufacturing method can be: material extrusion additive manufacturing, binder spraying additive manufacturing, or photopolymerization additive manufacturing.

[0044] S3. The green blank is degreased to remove the binder in the green blank to form interconnected pores, thereby obtaining an intermediate blank;

[0045] In some embodiments, the peak temperature of the degreasing treatment is 200–500°C.

[0046] S4. The intermediate blank is pre-sintered to generate reactant particles at the boundaries of the unit system powder in the intermediate blank;

[0047] In some embodiments, the pre-sintering temperature is 60% to 80% of the melting point of the unit system powder. The pre-sintering atmosphere includes oxidizing gases and inert gases; further, the oxidizing gases include, but are not limited to, oxygen, water vapor, and nitric oxide.

[0048] Specifically, in this embodiment, pre-sintering is carried out at a temperature of 60% to 80% of the powder's melting point, and the sintering atmosphere is an oxidizing gas (0.1 vol% to 0.5 vol%) + an inert gas (balance).

[0049] S5. The pre-sintered intermediate billet is densified by sintering to eliminate porosity in the intermediate billet;

[0050] In some embodiments, the densification sintering temperature is 80% to 95% of the melting point of the unit system powder. The densification sintering atmosphere includes an inert gas.

[0051] Specifically, in this embodiment, densification is carried out at a temperature of 80% to 95% of the powder's melting point, and the sintering atmosphere is an inert gas.

[0052] S6. The densified sintered intermediate blank is sintered to reduce the reactant particles in the intermediate blank, thereby completing the sintering process.

[0053] In some embodiments, the sintering temperature is 80% to 95% of the melting point of the unit system powder. The sintering atmosphere includes reducing gases and inert gases; further, the reducing gases include, but are not limited to, carbon monoxide, formic acid, and hydrogen.

[0054] Specifically, in this embodiment, the powder is reduced at a high temperature of 80% to 95% of its melting point, and the sintering atmosphere is a reducing gas (5 vol% to 10 vol%) + an inert gas (balance).

[0055] This method prepares a green body composed of unitary powder and a binder. The binder is removed through a debinding process to form uniform, interconnected pores within the green body. Then, a three-step sintering process is employed, controlling the sintering atmosphere and peak temperature at different stages. This process generates small amounts of reaction products between the powder particles, inhibiting the separation of grain boundaries from pores during sintering. The pores are eliminated through atomic diffusion across grain boundaries, thereby increasing the density of the final sintered sample. The specific principle of the continuous three-step sintering method is as follows: In the first step of the sintering process (i.e., pre-sintering), pre-sintering is carried out at a temperature of 60% to 80% of the powder's melting point. The sintering atmosphere is an oxidizing gas + inert gas. The metal atoms on the powder surface react with a small amount of oxidizing gas, generating reactant particles at the powder boundaries. In the second step of the sintering process (i.e., densification sintering), densification is carried out at a temperature of 80% to 95% of the powder's melting point. The sintering atmosphere is an inert gas. Atomic diffusion is intensified, and porosity is rapidly eliminated. The presence of reactant particles can effectively inhibit the detachment of pores from grain boundaries. Higher density is achieved at high temperatures through atomic grain boundary diffusion. In the third step of the sintering process (i.e., sintering), reduction is carried out at a high temperature of 80% to 95% of the powder's melting point. The sintering atmosphere is a reducing gas + inert gas. The high temperature reduces the reactant particles in the sintered sample. In addition, this method has a simple process, is applicable to powder metallurgy and indirect additive manufacturing of unit system powders, and has low cost and high production efficiency, making it suitable for large-scale production.

[0056] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0057] Example 1

[0058] A method for improving the density of sintered pure copper specifically includes the following steps:

[0059] Raw materials for preparation: The matrix material is pure copper powder, and the binder system consists of paraffin wax and low-density polyethylene; the average particle size of the pure copper powder is 10 μm, and its volume percentage is 85%, while the volume percentage of the binder is 15%; the pure copper powder and the binder are ball-milled at high speed at 60°C for 2 hours to obtain raw materials with uniform composition and stable properties.

[0060] Prototype manufacturing: The raw material is placed in a mold and pressed into a green blank under a pressure of 8MPa.

[0061] Degreasing: The green blank is placed in a tube furnace and calcined at a heating rate of 1℃ / min to 520℃. After holding at this temperature for 2 hours, the binder is completely removed. After degreasing, the sample has uniform interconnected pores inside.

[0062] Sintering: A three-step continuous sintering process was used to sinter the samples. The first step involved holding the sample at a peak temperature of 800℃ for 1 hour at a heating rate of 5℃ / min, under a sintering atmosphere of 0.1 vol% oxygen and 99.9 vol% nitrogen. During this process, atoms reacted with oxygen, generating copper oxide particles at the powder boundaries. The second step involved holding the sample at a peak temperature of 1000℃ for 2 hours at a heating rate of 5℃ / min, under a nitrogen atmosphere. During this process, atomic diffusion intensified, porosity was rapidly eliminated, and the presence of oxide particles effectively inhibited grain growth. The detachment of pores from grain boundaries was reduced, and higher density was achieved through atomic diffusion at high temperatures. The third step involved holding the sample at a peak temperature of 1000℃ for 1 hour under a sintering atmosphere of 5 vol% formic acid and 95 vol% nitrogen. During this process, some internal oxide particles were reduced to copper. The final formed copper parts were dense, without obvious defects, and had a porosity of less than 0.5%. Their microstructure was as follows: Figure 5 As shown.

[0063] Example 2

[0064] A method for improving the density of sintered steel specifically includes the following steps:

[0065] Raw materials: The matrix material unit is steel powder, and the binder system consists of paraffin wax, low-density polyethylene, and stearic acid. The steel powder has an average particle size of 20 μm and accounts for 70% of the volume, while the binder accounts for 30% of the volume. The steel powder and binder are ball-milled at 60℃ for 2 hours to obtain a raw material with uniform composition and stable properties.

[0066] Prototype manufacturing: The slice file of the 3D model is imported into the material extrusion printing equipment. The printing nozzle moves in a two-dimensional plane according to the slice file. At the same time, the semi-solid slurry is extruded from the nozzle under the action of hot pressure to form a single-layer cross section. Layers are stacked to form a three-dimensional green body.

[0067] Degreasing: The green blank is placed in a tube furnace and calcined at a heating rate of 0.8℃ / min to 520℃. After holding at this temperature for 2 hours, the binder is completely removed. After degreasing, the sample has uniform interconnected pores inside.

[0068] Sintering: The samples were sintered using a three-step continuous sintering process. The first step involved holding at a peak temperature of 950℃ for 1 hour at a heating rate of 5℃ / min, under a sintering atmosphere of 0.2 vol% oxygen and 99.8 vol% argon. During this process, atoms reacted with oxygen, generating oxide particles at the powder boundaries. The second step involved holding at a peak temperature of 1350℃ for 3 hours at a heating rate of 5℃ / min, under an argon atmosphere. During this process, atomic diffusion intensified, porosity was rapidly eliminated, and the presence of oxide particles effectively inhibited grain growth. The detachment of pores from grain boundaries was reduced, and higher density was achieved through atomic diffusion at high temperatures. The third step involved holding at a peak temperature of 1400℃ for 0.5 hours under a sintering atmosphere of 5 vol% hydrogen and 95 vol% nitrogen. During this process, some oxide particles were reduced to iron. The final steel parts were dense, free of significant defects, and had a porosity of less than 1%.

[0069] Comparative Example 1

[0070] The difference between this comparative example and Example 1 is that pure nitrogen is used as the protective gas during the sintering process, and no oxidizing or reducing gases are added to obtain the sintered part.

[0071] Comparative Example 2

[0072] The difference between this comparative example and Example 1 is that in the first step of the process, sintering, 2 vol% oxygen (excess oxidizing gas) and 98 vol% nitrogen are added to obtain the sintered part.

[0073] Comparative Example 3

[0074] The difference between this comparative example and Example 2 is that the peak temperature during the second and third sintering steps is 1200℃ (lower than 80% of the powder melting point), resulting in a sintered part.

[0075] The porosity measurement results of the final sintered parts of Examples 1-2 and Comparative Examples 1-3 of the present invention are shown in Table 1.

[0076] Table 1

[0077] porosity of sintered parts Example 1 0.29％ Example 2 0.58％ Comparative Example 1 2.63％ Comparative Example 2 1.76％ Comparative Example 3 7.34％

[0078] As shown in the table above, the sintered part obtained by the method provided in this invention has a smaller porosity. In Comparative Example 1, since only a nitrogen atmosphere was used during the sintering process, no oxide particles were generated inside to inhibit the movement of grain boundaries, and the porosity could not be eliminated by grain boundary diffusion. In Comparative Example 2, due to the excessive addition of oxidizing gas, more oxide particles were generated during the pre-sintering process, which could not be completely reduced to copper during the reduction process, resulting in a decrease in density. In Comparative Example 3, due to the low sintering temperature in the second and third steps, the diffusion movement of powder was restricted, and a large number of pores existed in the final sintered part.

[0079] Detailed description of the attached diagram:

[0080] Figure 2 This application provides an internal schematic diagram of the intermediate blank after pre-sintering, where 201 represents pores, 202 represents grain boundaries, and 203 represents reactant particles; during the pre-sintering process, reactant particles appeared at the powder boundaries.

[0081] Figure 3 This application provides an internal schematic diagram of the intermediate billet after densification sintering, where 201 represents pores, 202 represents grain boundaries, and 203 represents reactant particles. During this process, the reactant particles inhibit grain boundary movement, reducing the detachment of grain boundaries from pores. Pore elimination is achieved through grain boundary diffusion, resulting in a significant reduction in the number of pores inside the sintered sample.

[0082] Figure 4 The diagram shows the internal structure of the sintered unit system powder provided by the prior art, where 201 represents pores and 202 represents grain boundaries. During the high-temperature sintering process, the internal grains grow rapidly, the grain boundaries separate from the pores, the diffusion of atoms at the grain boundaries weakens, the pores are difficult to eliminate, and a large number of pores exist in the sintered sample.

[0083] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0084] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the orientation shown in the accompanying drawings. Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to."

[0085] In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, without necessarily requiring or implying any actual relationship or order between these entities or operations. In this document, "and / or" describes the association between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c," or "at least one of a, b, and c," can both represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0086] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for improving the sintering density of unitary powder systems, characterized in that, The method includes: The unit system powder and the binder are mixed to obtain a mixture; The mixture is prepared into a green body, wherein the mass ratio of the unit system powder to the binder in the green body is (70~85):(15~30). The green blank is degreased to remove the binder and form interconnected pores, thus obtaining an intermediate blank; The intermediate billet is pre-sintered to generate reactant particles at the boundaries of the unit system powder in the intermediate billet. The pre-sintering temperature is 60% to 80% of the melting point of the unit system powder, and the pre-sintering atmosphere is 0.1 vol% to 0.5 vol% oxidizing gas and the balance inert gas. The intermediate billet after pre-sintering is subjected to densification sintering at a temperature of 80% to 95% of the melting point of the unit system powder. The densification sintering atmosphere is an inert gas to eliminate the porosity of the intermediate billet under the condition that the reactant particles inhibit the separation of grain boundaries and pores. The intermediate blank after densification sintering is sintered to reduce the reactant particles in the intermediate blank and complete the sintering. The sintering temperature is 80% to 95% of the melting point of the unit system powder, and the sintering atmosphere is 5 vol% to 10 vol% of reducing gas and the balance is inert gas.

2. The method for improving the sintering density of unit system powder according to claim 1, characterized in that, The oxidizing gas includes at least one of oxygen, water vapor, and nitric oxide.

3. The method for improving the sintering density of unit system powder according to claim 1, characterized in that, The reducing gases are carbon monoxide, formic acid, and hydrogen.

4. The method for improving the sintering density of unit system powder according to claim 1, characterized in that, The method of forming the mixture into a green body includes compression molding or indirect additive manufacturing; The pressure for the pressing process is 5~10MPa; The indirect additive manufacturing includes material extrusion additive manufacturing, binder spraying additive manufacturing, or photopolymerization additive manufacturing.

5. The method for improving the sintering density of unit system powder according to claim 1, characterized in that, The peak temperature of the degreasing treatment is 200~500℃.

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