Sintering method for alternately strengthening oxygen reduction in vacuum and hydrogen environments

Abstract

The invention relates to a sintering method for alternately strengthening oxygen reduction in a vacuum and hydrogen environment, which comprises the following steps: uniformly doping organic carbon into a metal powder raw material, utilizing the characteristics of high-temperature volatilization and decomposition of the organic carbon, heating the organic carbon to volatilize or decompose the organic carbon into CO, CO2, H2O and other gases in a presintering stage, and forming through open pores in a metal blank after overflowing. The pore structure provides a channel penetrating into the green body for hydrogen, so that the hydrogen can be directly subjected to reduction reaction with metal oxide in the green body, and the problem that the oxide in the green body is difficult to fully reduce due to limited mass transfer in the traditional process is effectively solved. In the reduction process, vacuum hydrogen circulation deoxidation can be carried out for multiple times according to the requirement of a sintered product on oxygen element; in the high-temperature sintering process, a vacuum dehydrogenation method can be carried out according to the requirement of the sintered product on the hydrogen element, the oxygen and hydrogen element content of the final product is reduced, and the comprehensive performance of the sintered product is improved.

Description

Technical Field

[0001] The invention relates to a sintering method for metal powder metallurgy, in particular to a sintering method for alternately strengthening oxygen reduction in vacuum and hydrogen environments. Background Art

[0002] Powder metallurgy is widely used in numerous fields due to its significant advantages, including high material utilization, the ability to produce materials with specialized properties, the ability to manufacture complex parts with high precision, suitability for large-scale and efficient production, and its environmental and energy-saving characteristics. It is commonly used to produce products composed of pure metals or metal alloys. Metal powder or a blend of multiple metal powders is pressed into an intermediate form of the desired shape. The intermediate form is then heated and sintered until the metal powder is densified.

[0003] However, in this process, the oxygen present in the metal powder usually impairs sintering and the quality of the final product. First, oxygen reacts with the metal powder to form oxide impurities, which reduce the purity of the material, destroy the microstructure, and cause fluctuations in product performance. Second, the surface oxide film hinders sintering densification, affecting material properties. In addition, during sintering, the oxide decomposes to produce gas and form pores, and cracks due to thermal expansion differences, reducing fatigue life and reliability. In addition, the oxide will also destroy the surface state of the metal, accelerate corrosion, and shorten the life. Therefore, strict control of oxygen content is extremely critical to ensuring product quality and performance.

[0004] For this reason, deoxidation is a crucial consideration at every stage of the powder metallurgy process. Oxygen in metal products originates both from the metal's inherent oxygen content and from oxides formed on the surface of metal particles during atomization or ball milling. This oxygen cannot be completely removed during the powdering and pressing processes, directly impacting the subsequent sintering stage.

[0005] In the powder metallurgy process, the deoxidation problem during the sintering process is a key link that must be addressed and urgently needs to be improved. Since the last century, many researchers have continued to explore effective methods to reduce the oxygen content of metals during the sintering process. Among them, pressureless sintering is the most basic and widely used sintering method. It is the sintering of powder compacts under pressureless conditions, and has the significant advantages of simple equipment and convenient operation. However, this method has obvious shortcomings. It is difficult to remove oxygen from the voids of the metal blank during the sintering process, which directly leads to the difficulty in increasing the density of the sintered body and relatively poor performance. In view of this, in order to reduce the oxygen content in the voids of the metal blank, researchers have made a lot of efforts and developed a series of effective methods, such as adding deoxidizers, using inert gas protection, vacuum degassing treatment, and implementing electrochemical deoxidation. However, the above methods still have problems that need to be improved and solved.

[0006] Elemental metal deoxidizers and carbon and carbide deoxidizers are the most commonly used types of deoxidizers. However, these deoxidizers often carry impurities. After reacting with oxygen, these impurities can enter the metal, affecting its purity. More seriously, some impurities may form coarse inclusions, reducing the density of the metal blank and ultimately negatively impacting product quality. Deoxidation using inert gas primarily isolates the metal from oxygen, preventing further oxygen from entering the metal blank. However, this method has significant shortcomings. It only prevents the intrusion of external oxygen and cannot remove oxides already present in the blank, making it difficult to fundamentally reduce the oxygen content of the metal blank. Vacuum degassing also presents certain difficulties in removing oxygen from the metal blank. The difficulty in removing oxygen from the metal blank leads to prolonged degassing time and reduced production efficiency. Furthermore, in a vacuum environment, metal powder may agglomerate or change particle size, negatively impacting the powder's molding and sintering properties, thereby compromising the quality of the final product. Electrochemical deoxidation and sintering involve complex electrolytic reactions. In practical application, precise control of multiple parameters, including voltage, current, and temperature, is required. Furthermore, this method consumes significant amounts of electricity, significantly increasing production costs. Given its high cost and complex operational requirements, electrochemical deoxidation and sintering has been difficult to implement widely as an industrial production method.

[0007] In summary, the defects of the existing sintering deoxidation methods are as follows:

[0008] (1) Poor deoxidation effect: The method of deoxidation by isolating oxygen from contact with metal materials can only prevent external oxygen from entering, but cannot remove the original oxides in the blank. The deoxidation effect needs to be improved.

[0009] (2) Other impurities are easily introduced: The purpose of sintering deoxidation is to improve the density of the material, eliminate pores and promote atomic diffusion, thereby improving the physical, chemical and mechanical properties of the product. If other impurities are introduced during the deoxidation process, it will also affect the sintering effect and damage the product performance.

[0010] (3) Highly targeted, but poorly universal: Each sintering method has its own corresponding applicable materials. Sintering using inert gas protection and vacuum degassing is more universal. However, the method of adding deoxidizers during sintering requires different deoxidizers due to the different chemical properties of different materials. Electrochemical deoxidation sintering is not suitable for non-conductive materials or materials that react chemically with electrolyte solutions.

[0011] Currently, sintering deoxidation technology faces many challenges in practical application, primarily due to the difficulty in achieving a balance between deoxidation effectiveness, impurity introduction, and universal applicability. Specifically, some sintering deoxidation methods with excellent deoxidation effectiveness are prone to introducing impurities and have high energy consumption in practical applications; while some sintering deoxidation methods designed to save energy have less than satisfactory deoxidation results. Given this, developing a sintering deoxidation method that simultaneously meets the requirements of excellent deoxidation effectiveness, no introduction of other impurities, and applicability to a wide range of materials has become a pressing challenge that needs to be overcome. Summary of the Invention

[0012] The purpose of the present invention is to provide a sintering method for enhancing oxygen reduction by alternating vacuum and hydrogen environments, which can effectively reduce the oxygen content of metal powder, save energy consumption and achieve the goal of industrial production.

[0013] In order to achieve the above object, the technical solution adopted by the present invention is:

[0014] 1) Mixing: Metal powders are mixed according to the target metal product composition, and the organic carbon solution and metal powder are wet-mixed at a mass ratio of 1:8-9 to uniformly disperse the organic carbon solution and then dried to obtain metal powder with uniformly distributed organic carbon molecules, which is then pressed into a metal blank;

[0015] 2) Through-hole drilling + pre-reduction: The metal body with uniformly distributed organic carbon molecules is placed in a vacuum hydrogen sintering furnace (ZL200520129463.4). After the sintering furnace is evacuated to a vacuum, argon gas is introduced, and the furnace temperature is raised to the temperature at which the organic matter volatilizes or decomposes. This allows the CO and H2 generated by the organic carbon molecules to react with the metal oxides in the metal body to complete the pre-reduction of the body.

[0016] 3) Low-temperature reduction with hydrogen: After the pre-reduction is completed, the closed pores inside the metal body are opened to form open pores. At this time, the vacuum system is restarted to evacuate the furnace to a vacuum state. The decomposition products and reduction products of organic carbon are pumped out of the furnace, achieving a negative pressure state in the furnace cavity and the body. Hydrogen is then introduced and the sintering furnace is heated to the reduction temperature of the metal oxide with the lowest reduction temperature among the components of the target metal product, thereby deoxidizing the reduced metal.

[0017] 4) Hydrogen medium-temperature reduction + pre-sintering: Argon is again filled into the furnace to replace the hydrogen, and the water vapor produced by the hydrogen reduction is discharged until the outlet cannot ignite. The vacuum is then drawn again to achieve a negative pressure in the furnace cavity and the embryo body. Hydrogen is again injected into the sintering furnace. Under the combined effect of negative pressure and its permeability, the hydrogen penetrates deeply into the embryo body. The sintering furnace temperature is raised to the reduction temperature of the metal oxides in the target metal product composition that require a higher reduction temperature for the metal embryo body. The metal oxides that are easy to reduce at medium temperature are reduced. This process is repeated according to the target metal product composition. Through "vacuum negative pressure-hydrogen positive pressure intake and reduction-vacuum negative pressure discharge", the reduced metal is deoxidized;

[0018] 5) Vacuum dehydrogenation + high-temperature sintering: Argon is slowly introduced to replace the hydrogen in the furnace until the gas at the outlet cannot be ignited. The sintering furnace is then evacuated to a vacuum, and argon is introduced again while maintaining a slight negative pressure in the furnace. Under the argon atmosphere, the temperature is raised according to the sintering temperature of the metal with the highest sintering temperature among the target metal product components to complete sintering densification;

[0019] 6) Cooling: Cool the sintering furnace to room temperature under argon atmosphere to obtain the target metal.

[0020] The organic carbon solution is 10-15% oxalic acid (C2H2O4) anhydrous ethanol solution, formic acid (CH2O2) anhydrous ethanol solution, citric acid (C6H8O7) anhydrous ethanol solution or glucose (C6H 12 O6) anhydrous ethanol solution.

[0021] The wet mixing is carried out using a planetary ball mill.

[0022] The present invention employs a ratio of 1:8 to 9 of organic solvent to metal powder, which allows for the creation of open pores in the metal blank without compromising sintering. However, a high concentration of the organic solution can easily lead to agglomeration, hindering uniform distribution within the metal powder. Therefore, a concentration of 10% to 15% is appropriate. However, due to the varying chemical properties of metals and organic solutions, a low ratio of organic solution to metal powder results in a low number of open pores, preventing hydrogen from entering the blank. A high ratio of organic solution to metal powder can lead to excessive porosity in the blank, compromising sintering.

[0023] The present invention carries out deoxidation work by means of a sintering method that alternately strengthens oxygen reduction by volatilization or decomposition of organic carbon to open pores and vacuum hydrogen environment, thereby achieving the goal of effectively reducing oxygen content, saving energy consumption and realizing industrial production during the sintering process.

[0024] The present invention achieves directional control of material properties and efficient manufacturing of complex structures through precise proportioning and sintering of powder metallurgy raw materials. This method breaks through the limitations of traditional smelting processes and is particularly suitable for the preparation of high-melting-point metals (such as tungsten and molybdenum), composite materials, and difficult-to-process alloys, significantly improving the density, strength, and wear resistance of the materials. Excessive oxygen content in any metal material will significantly reduce the material's density, strength, and corrosion resistance. Therefore, oxygen reduction has become a key issue in powder metallurgy processes.

[0025] Compared with most metals, hydrogen is more likely to react with oxygen under certain conditions. Based on this characteristic, hydrogen has gradually been widely used in industrial scenarios where metals need to be deoxygenated. The oxygen reduction in the sintering process has significant core advantages over other links such as raw material pretreatment and mixed pressing. It relies on the active reduction and deep purification achieved in a high-temperature environment: during the sintering stage, by heating the material to above 1000°C (even above 2000°C), the reduction reaction rate of metal oxides and reducing gases (such as H2) increases exponentially. At the same time, the vacuum system reduces the oxygen partial pressure, and synergistically with the high temperature, it prompts oxygen atoms to be discharged in the form of water vapor, forming a dual deoxygenation mechanism of "high-temperature activation + vacuum degassing". This deep purification not only eliminates the oxides remaining in the raw material mixing stage, but also eliminates the micropores at the sintering neck through hydrogen diffusion, so that the material density is increased to more than 98% of the theoretical value.

[0026] Based on this synergistic effect, Gu Zhuoming invented a vacuum hydrogen sintering furnace (ZL200520129463.4) in 2006. This equipment is primarily used in the field of synthetic diamond materials. Its core principle is to extract air from the furnace through a vacuum system to create a negative pressure environment. Graphite rod heating elements provide high-temperature sintering conditions. Combined with hydrogen gas filled into the bottom as a reducing atmosphere, the iron-based catalyst is reduced and sintered within the insulation-wrapped furnace. This device addresses the problems of low material purity, poor process stability, high cost, and harsh working conditions associated with the use of nickel-based flake catalysts in traditional synthetic diamond production. By combining a single-chamber structure, a double-layer water-cooled furnace, graphite rod heating, and layered insulation, along with vacuum degassing and hydrogen reduction sintering, it effectively improves material purity and synthesis process stability, resulting in a more concentrated diamond particle size and a significantly increased proportion of high-grade material. It also reduces energy consumption and production costs, improves the production environment, and achieves automated operation and pollution-free production, providing a reliable solution for the industrial production of high-quality diamonds using iron-based powder catalysts.

[0027] The principle of vacuum hydrogen sintering furnaces is to leverage the reducing properties of hydrogen and the oxygen-isolating effect of the vacuum environment to ensure the purity and low oxygen content of the finished product during sintering. However, the above analysis reveals that these vacuum hydrogen sintering furnaces have the drawback of poor oxygen removal: within the sintering furnace, hydrogen primarily enters the synthetic diamond composite block via vapor diffusion. Diffusion is driven by the hydrogen concentration difference between the interior and exterior of the composite block. This hydrogen concentration difference is greatest after the vacuum is removed and hydrogen is introduced. As the water vapor generated by the reduction reaction of iron oxide equilibrates, the concentration difference decreases, and the diffusion rate slows. In particular, a number of closed pores form in the composite block during the pressing process, preventing the formation of vapor diffusion channels and thus restricting hydrogen entry and reduction. Furthermore, due to the increased diffusion resistance within the composite block, hydrogen diffusion forms a high-to-low concentration gradient from the surface to the interior, severely impacting the reduction of oxides deep within the metal. Finally, for hydrogen-sensitive materials, residual hydrogen in the metal composite can pose a risk of hydrogen embrittlement. Therefore, controlling the hydrogen content within the sintered body is crucial for material processing performance.

[0028] The present invention aims to solve the problem of high oxygen and hydrogen content in vacuum and hydrogen environment sintering, and realizes the control of hydrogen and oxygen elements in the sintered body by strengthening deep hydrogen diffusion, reduction and overflow.

[0029] The main reason for poor deoxidation is that the metal body's internal structure is dense, with small and irregular pores. This obstructs the diffusion path of hydrogen molecules, limiting their diffusion speed and depth. Furthermore, the internal atomic mobility is weak, making it difficult to meet the activation energy required for the reduction reaction, resulting in low reactivity. Furthermore, the actual sintering time is limited, making it difficult for hydrogen to fully diffuse into the interior to complete the reduction within the specified timeframe. Furthermore, sintering metals in a hydrogen atmosphere increases the hydrogen content, which in turn causes hydrogen embrittlement. The main reason is that hydrogen decomposes into hydrogen atoms at high temperatures and penetrates into the metal, existing in the form of interstitial atoms in the lattice or defects, resulting in lattice distortion and internal stress accumulation. Hydrogen is easily segregated in grain boundaries, precipitated phases and other areas to reduce grain boundary bonding strength. It may also react with some metals to form brittle hydrides, or "pin" dislocations to hinder their slip and reduce the plasticity of the material. At the same time, if the cooling is too fast after sintering, supersaturated hydrogen will remain and accumulate in the micropores to form internal pressure. In addition, the reducing effect of hydrogen makes the metal surface directly contact with hydrogen atoms, accelerating adsorption. Ultimately, hydrogen embrittlement is caused by the combined effects of lattice damage, stress concentration and brittle phase formation caused by hydrogen.

[0030] It can be seen from this that in order to improve the reduction effect during the sintering process and reduce the oxygen content, the breakthrough point lies in unblocking the hydrogen diffusion path, increasing the hydrogen diffusion power, and realizing the deep diffusion of hydrogen in the blank, thereby creating material and thermodynamic conditions for full reduction.

[0031] The present invention addresses the problems of incomplete deoxidation inside the green body and the risk of hydrogen embrittlement in traditional vacuum hydrogen sintering furnaces, and proposes an improved process based on a molecular-level reaction mechanism. By uniformly doping organic carbon into the metal powder raw material and utilizing its high-temperature volatilization and decomposition characteristics, during the pre-sintering stage, the organic carbon is volatilized or decomposed into gases such as CO, CO2 and H2O by heat, which overflow and form through-holes inside the metal green body. This pore structure provides a channel for hydrogen to penetrate deep into the green body, allowing hydrogen to directly react with the metal oxides inside the green body to undergo a reduction reaction, effectively solving the problem in traditional processes where the oxides inside the green body are difficult to fully reduce due to limited mass transfer. During the reduction process, multiple vacuum hydrogen cycle deoxidation can be carried out according to the requirements of the sintered product for the oxygen element; during the high-temperature sintering process, according to the requirements of the sintered product for the hydrogen element, a vacuum dehydrogenation method can be carried out to reduce the oxygen and hydrogen content of the final product and improve the comprehensive performance of the sintered product.

[0032] The present invention addresses the problems of low deoxidation efficiency inside the green body, high risk of hydrogen embrittlement, cumbersome operation, and high energy consumption in traditional sintering processes. Based on the advantages of easy operation of vacuum hydrogen sintering furnaces, the present invention achieves efficient deoxidation inside the metal green body and timely discharges residual hydrogen and other impurity gases through innovative processes, thereby suppressing the occurrence of hydrogen embrittlement. Specifically, functional components are introduced into the metal powder raw materials to construct a through-pore structure inside the green body to enhance mass transfer efficiency. Combined with segmented atmosphere control technology, reducing gas is used to achieve deep deoxidation in the pre-sintering stage. In the main sintering stage, inert gas is used for replacement and a slightly negative pressure environment is maintained to remove residual gas. Ultimately, an industrialized sintering deoxidation process is formed that combines efficient deoxidation, low risk of hydrogen embrittlement, convenient operation, and economical and energy-saving characteristics, effectively balancing the deoxidation effect, product performance, and production economy during the sintering process.

[0033] The organic matter described in this invention volatilizes and decomposes upon heating to produce CO and CO₂ gases, which act as pore-opening agents to open closed pores. Simultaneously, alternating between a vacuum and hydrogen atmosphere creates a positive and negative pressure, enhancing the discharge of the reducing agent (hydrogen) and the product (water vapor). Finally, controlling the vacuum or hydrogen atmosphere at the sintering terminal allows for control of the hydrogen and oxygen content. DETAILED DESCRIPTION

[0034] Taking oxalic acid, which can be decomposed into reducing gas at 200°C, as an example, the specific operation steps are as follows:

[0035] 1. Mixing: Metal powders are mixed according to the target metal product composition. A 10-15% oxalic acid anhydrous ethanol solution is wet-mixed with the metal powders in a mass ratio of 1:8-9 using a planetary ball mill to achieve uniform dispersion. The mixture is then dried to obtain a metal powder mixed with oxalic acid molecules, which is then pressed into a green body.

[0036] 2. Opening through holes + pre-reduction: Place the metal blank with evenly distributed oxalic acid in a vacuum hydrogen sintering furnace. Pump the furnace to a vacuum and then introduce argon. Raise the furnace temperature to a temperature where organic matter will volatilize or decompose. At this point, oxalic acid will decompose to form H2C2O4=CO(g)+CO2(g)+H2(g); the generated CO and hydrogen will react with the metal oxides in the blank to form MO n +nH2=M+nH2O、2MO n +nCO = 2M +nCO2; complete the pre-reduction of the embryo body.

[0037] 3. Low-temperature reduction with hydrogen: After pre-reduction is complete, the closed pores within the metal body are opened, forming open pores. At this point, the vacuum system is reactivated, evacuating the furnace to a vacuum state. The decomposition and reduction products of oxalic acid are then pumped out of the furnace, creating a negative pressure state within the furnace cavity and the body. Hydrogen is then introduced, and the sintering furnace is simultaneously heated to the reduction temperature of the metal oxide with the lowest reduction temperature. This temperature is then maintained for a specified period to deoxidize the easily reducible metal. As the reduction process continues, the reaction rate steadily decreases, the gas diffusion rate driven by the concentration gradient decreases, and the gas exchange between the body and the outside of the body weakens and reaches equilibrium.

[0038] 4. Hydrogen Medium-Temperature Reduction + Pre-Sintering: The furnace is again filled with argon to displace the hydrogen and expel water vapor produced by hydrogen reduction until the outlet becomes unignitable. The vacuum is then re-established, creating a negative pressure within the furnace and the embryo. Hydrogen is again injected into the sintering furnace. Due to the combined effects of negative pressure and its permeability, hydrogen penetrates deeply into the embryo. The sintering furnace is then heated to medium temperature (the reduction temperature for metal oxides requiring higher reduction temperatures). At this point, sintering is in the sintering neck formation stage, with porosity decreasing but not yet sealed. Metal oxides that are easily reduced at medium temperatures are reduced during this stage. The medium-temperature stage is when the voids are about to close and is the core stage for oxygen reduction. For easily reducible metal oxides, this process can be repeated as needed, using a combination of "vacuum negative pressure - hydrogen positive pressure inlet and reduction - vacuum negative pressure exhaust" to enhance the deoxidation process. Volatile metal oxides can be removed during this stage by increasing the vacuum level and extending the sintering time.

[0039] 5. Vacuum dehydrogenation + high-temperature sintering: The high-temperature sintering stage is the key to forming. It is necessary to reduce the temperature step, slow down the heating rate, extend the gap sealing time, prevent the surface of the blank from closing prematurely, and create conditions for the discharge of hydrogen and products. Before heating, slowly introduce argon to replace the hydrogen in the furnace to avoid large temperature fluctuations in the furnace due to excessive air flow rate. The gas at the outlet cannot be ignited, indicating that the replacement is complete. The sintering furnace is then evacuated to a vacuum. At this time, the sintering furnace and the inside of the blank are in a negative pressure state, which is conducive to the discharge of hydrogen and other products. Argon is introduced again and a slight negative pressure is maintained in the furnace. In an argon atmosphere, the temperature is raised according to the sintering temperature of the metal with the highest sintering temperature in the target metal product composition to complete sintering densification.

[0040] 6. Cooling: Cool the sintering furnace to room temperature under argon atmosphere.

[0041] The present invention has at least the following effects:

[0042] 1. This invention utilizes the property of organic carbon to volatilize or decompose into reducing gases at high temperatures. In an oxygen-free environment, as the sintering furnace gradually heats up, the organic carbon molecules uniformly mixed within the green body decompose into CO, CO₂, and H₂, which escape and leave uniform open pores in the metal green body. For example, oxalic acid decomposes into reducing gases at high temperatures and exhibits stable chemical properties. Other substances with similar properties can also be used in this field.

[0043] 2. In traditional vacuum hydrogen sintering furnaces, hydrogen can only remove oxides inside the furnace and on the surface of the metal body, resulting in poor deoxidation. From a molecular perspective, organic carbon creates pathways for hydrogen in the metal body, allowing it to enter the body and reduce metal oxides, fundamentally solving the problem of incomplete deoxidation.

[0044] 3. The medium-temperature reduction + pre-sintering temperature is between the low-temperature reduction temperature and the high-temperature sintering temperature. At this temperature, sintering necks form between the powder particles, promoting initial bonding and partial densification of the powder particles, stabilizing the green body size, reducing shrinkage and deformation during subsequent high-temperature sintering, and improving product dimensional accuracy. At the same time, the powder particle surface is activated, enhancing atomic diffusion capacity and the reduction effect, creating conditions for further bonding and densification of the particles during subsequent sintering, thereby helping to improve material performance.

[0045] 4. During the high-temperature sintering stage, slowly replace hydrogen with argon and maintain negative pressure in the sintering furnace to facilitate the discharge of hydrogen from the green body, thereby preventing hydrogen atoms from remaining inside the green body and causing hydrogen embrittlement, which would damage product quality.

[0046] 5. Low cost: Vacuum hydrogen sintering has a lower cost among many sintering methods, and the cost of the organic carbon oxalic acid added in the sintering method described in this patent is also low. Under the condition of reducing the oxygen content of the metal product, the cost is not increased.

[0047] 6. Excellent oxygen reduction effect: In traditional vacuum hydrogen sintering, hydrogen can only react with oxygen molecules in the sintering furnace and on the surface of the metal body, reducing the corresponding oxides, but cannot reduce the oxides inside the metal. This patented method utilizes the volatilization or decomposition of organic carbon at high temperatures to form uniform, through-hole open pores in the metal body. Hydrogen molecules enter the metal body through the open pores, contact and react with the internal oxide molecules, reducing the oxides inside the metal body and improving the overall reduction effect.

[0048] 7. Strong universality: Existing sintering deoxidation methods, such as adding deoxidizers and electrochemical deoxidation, are only applicable to a very limited range of materials. It is difficult to flexibly adjust the process for different materials, resulting in low equipment utilization. Furthermore, production efficiency is significantly reduced due to limited green body processing and complex processes. The sintering method described in the present invention is applicable to all metals that do not react with pore-opening agents (organic carbon) and whose oxides are reduced by hydrogen at a temperature lower than the sintering temperature. Parameters can be easily optimized based on different material properties and product requirements, greatly improving equipment utilization.

[0049] 8. Prevent hydrogen embrittlement: In the sintering method described in the present invention, hydrogen is used as a reducing agent to enter the interior of the metal blank and react with the oxide to produce a reduction reaction. However, if the sintering process is continued under a hydrogen atmosphere, hydrogen is likely to remain inside the blank. Taking into account that residual hydrogen may cause hydrogen embrittlement in the product, seriously affecting product quality, the present invention implements argon replacement and vacuuming operations before reaching the target sintering temperature in the process design: first, argon is introduced to replace the hydrogen in the furnace, and the inert properties of argon are used to gradually replace hydrogen, thereby reducing the hydrogen content in the system; then a vacuuming treatment is performed to reduce the pressure in the furnace, thereby causing the residual hydrogen inside the blank to be discharged under the drive of the pressure difference. This process effectively avoids hydrogen embrittlement problems caused by residual hydrogen in the product after sintering by precisely controlling the atmosphere conversion and pressure regulation, thereby ensuring the stability and reliability of product quality.

[0050] 9. Clean reaction process: The decomposition reaction of organic carbon oxalic acid and the reduction reaction of hydrogen and metal oxides produce only a small amount of by-products such as CO, CO2, and H2O. No subsequent waste gas treatment is required, which is an environmentally friendly production.

[0051] 10. Industrial production is possible: After the green body is formed by mixing the materials using this sintering method, the subsequent sintering operations are intelligently controllable, thus facilitating industrial production.

Claims

1. A sintering method for enhancing oxygen reduction by alternating vacuum and hydrogen environments, characterized in that The following steps are involved: 1) Mixing: Metal powders are mixed according to the target metal product composition, and the organic carbon solution and metal powder are wet-mixed at a mass ratio of 1:8-9 to uniformly disperse the organic carbon solution and then dried to obtain metal powder with uniformly distributed organic carbon molecules, which is then pressed into a metal blank; 2) Through-hole drilling + pre-reduction: The metal body with uniformly distributed organic carbon molecules is placed in a vacuum hydrogen sintering furnace. After the sintering furnace is evacuated to a vacuum, argon gas is introduced, and the furnace temperature is raised to the temperature at which the organic matter volatilizes or decomposes. This allows the CO and H2 generated by the organic carbon molecules to react with the metal oxides in the metal body to complete the pre-reduction of the body. 3) Hydrogen low-temperature reduction: After the pre-reduction is completed, the closed pores inside the metal embryo are opened to form open pores; At this time, the vacuum system is restarted to evacuate the furnace to a vacuum state. The decomposition products and reduction products of organic carbon are pumped out of the furnace, achieving a negative pressure state in the furnace cavity and the embryo body. Then, hydrogen is introduced and the sintering furnace is heated to the reduction temperature of the metal oxide with the lowest reduction temperature among the target metal product components to deoxidize the reduced metal. 4) Hydrogen medium-temperature reduction + pre-sintering: Argon is again filled into the furnace to replace the hydrogen, and water vapor produced by hydrogen reduction is discharged until the outlet cannot ignite. The vacuum is then drawn again to achieve a negative pressure in the furnace cavity and the embryo body. Hydrogen is again injected into the sintering furnace. Under the combined effects of negative pressure and permeability, hydrogen penetrates deeply into the embryo body. The sintering furnace temperature is raised to the reduction temperature of the metal oxides in the target metal product composition that require a higher reduction temperature for the metal embryo body. The metal oxides that are easily reduced at medium temperatures are reduced. This process is repeated according to the target metal product composition. Through "vacuum negative pressure - hydrogen positive pressure intake and reduction - vacuum negative pressure discharge", the reduced metal is deoxidized; 5) Vacuum dehydrogenation + high-temperature sintering: Argon is slowly introduced to replace the hydrogen in the furnace until the gas at the outlet cannot be ignited. The sintering furnace is then evacuated to a vacuum, and argon is introduced again while maintaining a slight negative pressure in the furnace. Under the argon atmosphere, the temperature is raised according to the sintering temperature of the metal with the highest sintering temperature among the target metal product components to complete sintering densification; 6) Cooling: Cool the sintering furnace to room temperature under argon atmosphere to obtain the target metal.

2. The sintering method for enhancing oxygen reduction by alternating vacuum and hydrogen environments according to claim 1, characterized in that: The organic carbon solution is 10-15% oxalic acid (C2H2O4) anhydrous ethanol solution, formic acid (CH2O2) anhydrous ethanol solution, citric acid (C6H8O7) anhydrous ethanol solution or glucose (C6H 12 O6) anhydrous ethanol solution.

3. The sintering method for enhancing oxygen reduction by alternating vacuum and hydrogen environments according to claim 1, characterized in that: The wet mixing is carried out using a planetary ball mill.