Preparation process of tungsten-nickel alloy composite material based on MIM technology

Abstract

The present application relates to the technical field of metal powder injection molding, and discloses a tungsten-nickel alloy composite material preparation process based on MIM technology, which comprises the following steps: determining the Hall flow rate, loose bulk density and average sphericity of different batches of spherical tungsten powder, and selecting a batch with measured values meeting preset threshold values as a substrate; synchronously coating nickel and iron on the surface of the spherical tungsten powder by means of gas phase deposition to form tungsten-nickel-iron powder with a core-shell structure in which the components are uniformly pre-combined on a microscale; mixing the tungsten-nickel-iron powder with a core-shell structure as a metal raw material with an organic binder to prepare injection molding feedstock; injection molding the feedstock to obtain a green body, and sequentially performing debinding, sintering and heat treatment on the green body to obtain a tungsten-nickel alloy composite material part. The present application realizes microscale uniform pre-combination of alloy components by using core-shell structure composite powder, thereby ensuring the uniformity of product organization and the stability of performance.

Description

A preparation process of tungsten-nickel alloy composite materials based on MIM technology Technical Field

[0001] This invention relates to the field of metal powder injection molding technology, and in particular to a process for preparing tungsten-nickel alloy composite materials based on MIM technology. Background Technology

[0002] The standard process for manufacturing tungsten-nickel-iron alloy parts using traditional metal injection molding technology is as follows: First, tungsten powder, nickel powder, iron powder and other elemental powders are mechanically mixed and ball-milled, then mixed with a binder to obtain feedstock, followed by injection, debinding and sintering.

[0003] This process has an inherent, fundamental flaw caused by the physical properties of the raw materials: due to the significant density differences between tungsten powder and nickel / iron powder, the powders of different components are easily separated by gravity or centrifugal force during feeding, mixing, conveying, and injection, resulting in macroscopic segregation. This segregation directly leads to uneven distribution of the nickel-iron binder phase in the microstructure of the sintered parts, causing large fluctuations in the mechanical properties of the products within and between batches, making it difficult to guarantee reliability and consistency.

[0004] To address this issue, existing technologies primarily optimize the mixing process, such as extending ball milling time or improving mixing equipment. However, these methods only improve mixing uniformity to a limited extent and cannot fundamentally eliminate the segregation tendency caused by density differences. Instead, they increase process complexity, energy consumption, and production costs. Therefore, fundamentally solving the compositional segregation problem of MIM tungsten-nickel alloys at the raw material level has become a long-standing technical challenge in this field.

[0005] Therefore, this invention proposes a process for preparing tungsten-nickel alloy composite materials based on MIM technology. Summary of the Invention

[0006] The purpose of this invention is to solve the problems of uneven mixing, component segregation and unstable performance in the prior art by proposing a tungsten-nickel alloy composite material preparation process based on MIM technology.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a process for preparing tungsten-nickel alloy composite materials based on MIM technology, comprising the following steps:

[0008] Step S1: Measure the Hall flow rate, loose packing density and average sphericity of different batches of spherical tungsten powder, and select the batches whose measured values ​​meet the preset threshold as the matrix;

[0009] Furthermore, step S1 also includes the following sub-steps:

[0010] S1-1, for different batches of spherical tungsten powder, the Hall flow rate was determined according to GB / T1482 standard. The measuring device was a Hall flow meter with a feeding funnel aperture of 2.5 mm and a test powder mass of 50 g. The loose density was determined according to GB / T1479 standard using a standard loose density meter with a capacity of 25 mL and the test was completed by natural sedimentation. Representative images of each batch of spherical tungsten powder were collected using a scanning electron microscope at a magnification of 2000x. At least 5 fields of view were collected for each batch. The aspect ratio of the powder particles was statistically analyzed using image analysis software, and the average sphericity of each batch of powder was calculated.

[0011] S1-2, the Hall flow rate, loose packing density, and average sphericity values ​​measured for each batch are compared one by one with the preset joint performance thresholds according to the requirements of powder bed fusion additive manufacturing process. The joint performance thresholds are specifically set as follows: Hall flow rate not higher than 16s / 50g, loose packing density not lower than 4.1g / cm³, and average sphericity not lower than 0.88. The loose packing density and tapped density are set with reference to the general requirements of metal additive powder and MIM process to ensure that the selected spherical tungsten powder has good flowability and spreadability, and can be adapted to the feeding preparation of MIM process and the powder spreading requirements of metal additive manufacturing.

[0012] S1-3, batches with a Hall flow rate not exceeding 16 s / 50 g, a loose packing density not less than 4.1 g / cm³, and an average sphericity not less than 0.88 were selected as the matrix. Through multi-dimensional performance parameter screening, the quality of the raw material powder is ensured from the source, laying the foundation for subsequent core-shell structure preparation and the entire MIM process, while also ensuring the powder's potential for cross-process applications.

[0013] Step S2: Nickel and iron are simultaneously coated on the surface of spherical tungsten powder by vapor deposition to form a core-shell structured tungsten-nickel-iron powder with uniform pre-composite composition at the microscale.

[0014] Furthermore, step S2 also includes the following sub-steps:

[0015] In step S2-1, spherical tungsten powder, serving as the matrix, is placed into a fluidized bed reactor. Argon gas, with a purity of not less than 99.99%, is introduced as the fluidizing gas, and the gas flow rate is adjusted to 0.5 to 1.0 L / (min·kg) to ensure the tungsten powder is in a stable fluidized state. The fluidized state is determined by the powder being uniformly suspended within the reactor, without localized accumulation or sedimentation. The inert fluidizing gas effectively prevents oxidation of the spherical tungsten powder during fluidization and ensures uniform suspension of the powder particles, providing a stable reaction environment for subsequent deposition reactions.

[0016] S2-2, the fluidized bed reactor is heated to a deposition temperature range of 170 to 190 degrees Celsius. This temperature range is higher than the thermal decomposition initiation temperature of nickel carbonyl gas (150 degrees Celsius) and iron carbonyl gas (160 degrees Celsius), but lower than their rapid homogeneous nucleation temperature (200 degrees Celsius). This temperature range ensures that nickel carbonyl and iron carbonyl gases undergo thermal decomposition within the reactor, releasing nickel and iron atoms, while preventing rapid homogeneous nucleation of nickel and iron atoms in the gas phase, thus ensuring that nickel and iron can be directionally deposited on the surface of spherical tungsten powder.

[0017] In step S2-3, a reaction gas consisting of a mixture of nickel carbonyl gas and iron carbonyl gas at a mass ratio of 2:1 to 4:1 is introduced into the fluidized bed reactor located within the deposition temperature range. The purity of both the nickel carbonyl gas and the iron carbonyl gas is not less than 99.9%. The ratio of the total flow rate of the reaction gas to the mass of the spherical tungsten powder is controlled to be 0.7 to 0.9 L / (min·kg). By adjusting the mixing ratio of nickel carbonyl gas and iron carbonyl gas, the nickel and iron composition ratio in the core-shell structured tungsten nickel-iron powder can be precisely controlled. The total nickel-iron content is designed with reference to the classic proportions of tungsten nickel-iron alloys to meet the alloy preparation requirements of different mechanical properties. Controlling the ratio of the total flow rate of the reaction gas to the mass of the spherical tungsten powder ensures sufficient contact between the reaction gas and the powder particles, improving deposition efficiency and coating uniformity.

[0018] S2-4, the deposition reaction is carried out within the deposition temperature range to form a core-shell structured tungsten-nickel-iron powder with a uniform coating thickness, which is controlled to be 1.5 to 2.5 μm.

[0019] The deposition reaction process specifically includes: real-time monitoring of the concentrations of undecomposed nickel carbonyl and iron carbonyl in the tail gas of the deposition reaction using a gas chromatograph, with a monitoring frequency of once per minute; based on the change data of characteristic gas component concentrations and combined with the correlation formula between gas decomposition efficiency and deposition rate, the cumulative deposition amount of nickel and iron on the surface of spherical tungsten powder is calculated in real time; the real-time cumulative deposition amount is compared with the theoretical total deposition amount calculated based on the preset target thickness of the coating layer, which is calculated based on the specific surface area of ​​tungsten powder and the thickness of the coating layer; when the real-time cumulative deposition amount reaches the theoretical total deposition amount, the deposition reaction is considered complete, and the introduction of reaction gas and heating of the reactor are terminated, with the cooling rate controlled at 5 degrees Celsius / min. Through tail gas monitoring and closed-loop control of deposition amount, the thickness and compositional uniformity of the coating layer can be precisely controlled, avoiding problems such as local over-thickness, under-thickness, or voids in the coating layer, ensuring the quality stability of the core-shell structured tungsten nickel-iron powder.

[0020] Step S3: The core-shell structured tungsten-nickel-iron powder is used as a metal raw material and mixed with an organic binder to form an injection molding feed. The core-shell structured tungsten-nickel-iron powder is used as the only metal raw material for the alloy components, thus eliminating the need for the mechanical mixing of tungsten, nickel and iron elemental powders.

[0021] Furthermore, step S3 also includes the following sub-steps:

[0022] In step S3-1, core-shell structured tungsten-nickel-iron powder and polyoxymethylene (POM)-based binder are added to an internal mixer at a mass ratio of 6.8:3.2 to 7:3. The POM-based binder is composed of polyoxymethylene, polyethylene wax, and zinc stearate at a mass ratio of 8:1.5:0.5. This binder system is suitable for the characteristics of tungsten-nickel-iron powder and meets the general requirements for feed preparation in the MIM process. The first mixing stage is carried out at a first mixing temperature of 125 to 135 degrees Celsius and a rotation speed of 45 to 55 r / min for 25 to 35 minutes, allowing the binder to melt and initially wet and coat the powder particles. The first mixing stage uses a relatively low temperature and rotation speed to allow the binder to melt slowly and evenly cover the surface of the core-shell structured tungsten-nickel-iron powder, avoiding the collapse of the core-shell structure due to excessively high temperature or rotation speed, and reducing the oxidation of the powder particles.

[0023] S3-2, the internal mixer is heated to a second mixing temperature of 155-165 degrees Celsius, which is 30 degrees Celsius higher than the first mixing temperature. A second stage of refining and mixing is then performed at a speed of 95-105 rpm for 55-65 minutes, continuing until the feed reaches a highly uniform paste. The paste is judged by the absence of obvious particles and a uniform color. This second stage of refining and mixing, by increasing the temperature and speed, enhances the shear force of the material, promotes the full integration of the binder and powder particles, eliminates voids and agglomerates within the material, and improves the uniformity and rheological properties of the feed. The mixing temperature, speed, and time parameters are optimized based on the MIM feed preparation process standard.

[0024] S3-3, During the refining and mixing stage, the melt index of the feed is measured at intervals. When the measured value of the melt index falls stably within the preset qualified range for several consecutive times, the mixing is deemed complete.

[0025] The specific steps for determining the completion of mixing include: during the refining and mixing stage, samples are taken from the internal mixer at fixed time intervals of 10 to 15 minutes, with each sample weighing 5g; the melt index of each sample is measured using a melt indexer under standard test conditions of 190 degrees Celsius and a load of 2.16 kg, and the corresponding mixing time is recorded; a process curve of melt index versus mixing time is plotted, and the instantaneous slope of the process curve near the most recent sampling time point is calculated using a linear fitting method; when the absolute value of the instantaneous slope is consistently lower than 0.4 to 0.6 g / (10 min·min), and the melt index values ​​measured in 3 to 4 consecutive samples all fall within the preset qualified range of 18 to 32 g / 10 min, the rheological properties of the feed are considered stable, and mixing is considered complete. By using both melt index and slope as indicators, the mixing endpoint can be accurately controlled, avoiding problems such as poor feed uniformity due to insufficient mixing, or binder decomposition and powder oxidation due to over-mixing.

[0026] S3-4, the mixed feed is extruded, cooled and pelletized to produce feed pellets for injection molding.

[0027] The specific steps include: transferring the completed compounded paste feed to the feed inlet of a single-screw extruder with a screw diameter of 30 mm and a length-to-diameter ratio of 25:1; controlling the temperature of each temperature control zone of the single-screw extruder to form a decreasing temperature gradient from 145 to 150 degrees Celsius at the feed inlet to 115 to 120 degrees Celsius at the die, and ensuring that the temperature of each temperature control zone is lower than the second compounding temperature; starting the single-screw extruder with a screw speed of 30 to 40 r / min, and feeding the material forward. After feeding, compression, and melting homogenization, the material is extruded through a die to form continuous, slender strips with uniform cross-sectional dimensions, each 2 to 3 mm in diameter. The extruded strips are then cooled by cooling water or oil at 25 to 30 degrees Celsius for 5 to 10 minutes to ensure complete solidification. The solidified strips are then fed into a pelletizer, where rotating blades cut them into cylindrical particles 2.5 to 3 mm in length, with a length deviation not exceeding ±0.2 mm. By controlling the decreasing temperature gradient, decomposition of the feed material due to excessive temperature during extrusion is prevented, while ensuring thorough melting and homogenization, thus improving the quality stability of the feed particles and providing a good raw material foundation for subsequent injection molding.

[0028] Step S4: The feedstock is injection molded to obtain a green blank, and the green blank is then degreased, sintered and heat-treated in sequence to obtain a tungsten-nickel alloy composite material part.

[0029] Furthermore, step S4 also includes the following sub-steps:

[0030] S4-1: Feed the material into the screw-type injection molding machine and inject it into the preset mold. Control the injection pressure to 75-80 MPa, the holding pressure to 60-64 MPa (holding pressure is 80% of the injection force), the holding time to 10-11 seconds, the mold temperature to 50-60 degrees Celsius, and the cooling time to 8-9 seconds. After cooling, demold to obtain the green preform. During the injection molding process, by setting the process parameters reasonably, ensure that the material fully fills the mold cavity and avoid defects such as material shortage and shrinkage cavities. By setting the cooling time reasonably, ensure that the green preform is fully solidified, and control the Shore hardness of the green preform to 60-70D, improving the strength and dimensional accuracy of the green preform and facilitating the subsequent debinding process. The injected green preform must flow into the next process within 24 hours. The material box must be cleaned and lined with foam at the bottom. The quantity in each box should be controlled within 2000 pieces to avoid damage.

[0031] S4-2, the green body is placed in a nitric acid vapor environment and catalytically degreased at a constant temperature. Nitric acid vapor acts as a catalyst, accelerating the decomposition and removal of organic binders while preventing deformation or collapse of the green body during degreasing, thus ensuring the shape integrity of the degreased green body.

[0032] The specific steps include: placing the green body in the working area of ​​a degreasing furnace filled with nitric acid vapor, with the nitric acid vapor concentration controlled at 50% to 60%, and using 98% concentrated nitric acid as the catalyst; controlling the temperature of the working area within a constant degreasing temperature range of 110 to 120 degrees Celsius, with the specific process as follows: pre-rinsing 45 min (110 degrees Celsius, no acid injection), degreasing stage 1 135 min (110 degrees Celsius, 4g acid injection), degreasing stage 2 65 min (115 degrees Celsius, 4.5g acid injection), degreasing stage 3 65 min (120 degrees Celsius, 5g acid injection), post-rinsing stage 1 40 min (120 degrees Celsius, no acid injection), and post-rinsing stage 2 5 min (natural cooling, no acid injection); calculating and setting the total degreasing time based on the maximum wall thickness of the green body and the preset degreasing time per unit thickness of 1.0 to 1.2 h / mm; maintaining the nitric acid vapor concentration within the constant degreasing temperature range and continuing the degreasing reaction until the set total time is reached. After degreasing, the binder removal rate should be no less than 98%. The degreasing effect is verified by a crushing test; the degreased blank is considered qualified if it can be completely crushed and has no binder core inside. By controlling the constant temperature and concentration, it is ensured that the binder is removed evenly, avoiding defects caused by incomplete degreasing or localized over-degreasing. At the same time, the degreasing time can be adjusted according to the wall thickness of the green blank to improve the adaptability and reliability of the degreasing process.

[0033] In step S4-3, the degreased green body is heated under vacuum to a first temperature range of 580 to 600 degrees Celsius, with the vacuum level controlled at 0.0008 to 0.001 Pa, and held for 2 to 2.5 hours to complete deep decarburization and deoxidation. The vacuum environment effectively prevents oxidation of the green body during heating. By raising the temperature to the first temperature range and holding it there, residual small amounts of binder decomposition products, carbon impurities, and oxide phases can be removed from the green body, reducing the carbon impurity content to below 0.05%, further improving the purity of the green body and laying the foundation for subsequent sintering and densification. The parameters for this step are set according to the general standards for vacuum decarburization and deoxidation in MIM processes to ensure effective decarburization while avoiding green body deformation.

[0034] In step S4-4, argon gas with a purity of not less than 99.99% is introduced into the sintering furnace as an inert protective gas, and the furnace pressure is increased to a set partial pressure of 75 to 80 kPa. The temperature is then raised to a second temperature range of 1480 to 1500 degrees Celsius, which is higher than the melting point of the nickel-iron binder phase (1450 degrees Celsius). The holding time is 3 to 3.5 hours to achieve liquid-phase sintering and densification, resulting in sintered parts. The inert protective gas prevents oxidation of the parts during sintering. By controlling the partial pressure of the protective gas and the sintering temperature, the uniform melting of the nickel-iron binder phase and its spreading at the tungsten grain boundaries can be promoted, achieving dense bonding of the tungsten grains and improving the density and interfacial bonding strength of the alloy. The sintering process is divided into a negative pressure degreasing section (350 to 600 degrees Celsius, parameters are adjusted according to carbon content), a vacuum decarbonization section (1050 degrees Celsius, parameters are adjusted according to carbon and oxygen content), and a partial pressure sintering section (1480 to 1500 degrees Celsius, parameters are adjusted according to product color and density). The argon flow rate is controlled at 30 to 40 L / min.

[0035] The specific steps for achieving liquid-phase sintering densification include: during the holding period in the second temperature range, real-time pressure data is monitored using an in-furnace pressure sensor with a monitoring accuracy of ±1 kPa and a monitoring frequency of once per minute; the apparent color change of the sintered parts is monitored using an optical observation device; pressure stability and color uniformity indices are established. The pressure stability index is defined as pressure data fluctuation within a preset time period of less than 4 to 5 kPa, and the color uniformity index is defined as the overall surface color of the parts transforming into a uniform silvery-white metallic luster; when the pressure data fluctuation within a preset time period of 30 to 40 minutes is less than 4 to 5 kPa, and the overall apparent color transforms into a uniform silvery-white metallic luster and lasts for 20 to 25 minutes (a second threshold time), liquid-phase sintering densification is considered complete. After sintering, the density of the parts is not less than 99.0%, and the surface is bright without graying or darkening. By monitoring both pressure and color indices, the sintering endpoint can be accurately controlled, avoiding problems such as low density due to insufficient sintering or grain growth and performance degradation due to over-sintering, ensuring the quality stability of the sintered parts.

[0036] S4-5 involves solution treatment and aging treatment of sintered parts to obtain tungsten-nickel alloy composite parts. Solution treatment is performed at 1055-1065°C for 1.5-2 hours, followed by rapid water cooling at a rate of 100°C / min, with a vacuum level controlled to ≤0.02 Pa. Aging treatment is performed at 536-539°C for 4.5-5.5 hours, followed by furnace cooling to room temperature, with a vacuum level controlled to ≤0.001 Pa. Solution treatment eliminates the unbalanced microstructure formed during sintering, homogenizing the alloy composition. Aging treatment optimizes the strength-toughness balance by controlling the precipitation morphology and distribution of second-phase particles, improving the hardness, strength, and toughness of the parts to meet the stringent mechanical performance requirements of high-end applications. After heat treatment, the parts have a hardness of HV580 to 610, a tensile strength of 2000 to 2080 MPa, and an elongation of 4.2% to 4.5%, which meets the performance requirements for high-end applications of tungsten-nickel alloy composite materials.

[0037] The beneficial effects of the technical solution provided by this invention include at least the following:

[0038] This invention uses a "core-shell structure composite powder" formed by simultaneously coating nickel and iron on the surface of spherical tungsten powder as the sole metal raw material. This completely eliminates the macroscopic segregation of components caused by the density differences of tungsten, nickel, and iron elemental powders from a physical structure perspective. Since each tungsten powder particle has been pre-composite with the target proportion of nickel and iron components, there is no physical basis for density separation during subsequent mixing and injection molding. This ensures the extreme uniformity of the microstructure of the sintered parts and significantly improves the consistency and reliability of the product's mechanical properties.

[0039] This invention eliminates the cumbersome mechanical mixing and ball milling steps of tungsten, nickel, and iron elemental powders that are indispensable in traditional processes by directly using core-shell structured composite powders for feeding and mixing. This not only simplifies the process flow, shortens the production cycle, and reduces energy consumption and equipment wear, but also eliminates quality fluctuations caused by improper control of the mixing process from the source, thereby improving production efficiency and overall yield.

[0040] This invention uses core-shell structure powder as the sole raw material for the already prepared alloy components, which enables the standardization and simplification of raw materials in the MIM process. It moves the alloying process forward to the powder preparation stage, allowing subsequent MIM processes to focus on shape forming and densification. This simplifies process control and provides a solid foundation for process stability and product quality traceability. Attached Figure Description

[0041] To more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 is a schematic diagram of the process flow provided in an embodiment of the present invention;

[0043] Figure 2 is an original test image of the average sphericity value provided in an embodiment of the present invention;

[0044] Figure 3 is a SEM image of the MIM tungsten nickel iron powder provided in the embodiment of the present invention;

[0045] Figure 4 is a TEM image of the MIM tungsten nickel iron powder provided in the embodiment of the present invention;

[0046] Figure 5 is a STEM image of the MIM tungsten nickel iron powder provided in the embodiment of the present invention;

[0047] Figure 6 is a schematic diagram of the process curve of the feed melt index changing with mixing time provided in the embodiment of the present invention. Detailed Implementation

[0048] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the following, in conjunction with the accompanying drawings and preferred embodiments, details the specific implementation, structure, features, and effects of a tungsten-nickel alloy composite material preparation process based on MIM technology proposed in accordance with the present invention. In the following description, different "one embodiment" or "another embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0049] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0050] The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0051] The following describes in detail, with reference to the accompanying drawings, a specific scheme for the preparation process of tungsten-nickel alloy composite materials based on MIM technology provided by the present invention.

[0052] Please refer to Figure 1, which shows a schematic process flow diagram of a tungsten-nickel alloy composite material preparation process based on MIM technology according to an embodiment of the present invention, including the following steps:

[0053] Step S1: Measure the Hall flow rate, loose packing density and average sphericity of different batches of spherical tungsten powder, and select the batches whose measured values ​​meet the preset threshold as the matrix;

[0054] Step S2: Nickel and iron are simultaneously coated on the surface of spherical tungsten powder by vapor deposition to form a core-shell structured tungsten-nickel-iron powder with uniform pre-composite composition at the microscale.

[0055] Step S3: The core-shell structured tungsten-nickel-iron powder is used as a metal raw material and mixed with an organic binder to form an injection molding feedstock.

[0056] Step S4: The feedstock is injection molded to obtain a green blank, and the green blank is degreased, sintered and heat-treated in sequence to obtain a tungsten-nickel alloy composite material part.

[0057] In step S3, the core-shell structured tungsten-nickel-iron powder is used as the only metal raw material for the alloy components that have been prepared, thus eliminating the need for mechanical mixing of tungsten, nickel, and iron powders.

[0058] Please refer to Figure 2, which shows the original test image of the average sphericity value provided by an embodiment of the present invention.

[0059] It should be noted that the determination of Hall flow rate of different batches of spherical tungsten powder refers to the use of a Hall flow meter in accordance with the GB / T1482 standard, under specific conditions of feed funnel aperture and test powder mass, to detect the flow rate of the powder. Its main purpose is to evaluate the flow performance of the powder in subsequent processes and provide key indicators for raw material screening.

[0060] The determination of bulk density refers to obtaining the bulk density data of powder by using a standard bulk density meter with fixed capacity according to GB / T1479 standard and natural sedimentation method. It mainly reflects the packing characteristics of powder particles and, together with flowability, serves as the basis for judging the qualification of raw materials.

[0061] The determination of average sphericity refers to the use of scanning electron microscope to collect representative images of each batch of powder, and to calculate the average sphericity by statistically analyzing the aspect ratio of the particles using image analysis software. This is mainly to ensure that the morphology of the powder particles meets the requirements of subsequent coating and molding processes, and to improve process adaptability.

[0062] Selecting batches whose measured values ​​meet the preset thresholds as the matrix means comparing the Hall flow rate, loose packing density, and average sphericity values ​​of each batch of powder with the combined performance thresholds set according to the dual requirements of powder bed fused additive manufacturing and MIM processes, and screening out qualified powder batches. This is mainly to ensure the quality of raw materials from the source and lay the foundation for subsequent core-shell structure preparation and molding processes.

[0063] Simultaneous coating of nickel and iron on the surface of spherical tungsten powder by vapor deposition refers to placing spherical tungsten powder in a specific reaction device, introducing reaction gases containing nickel and iron sources, and depositing nickel and iron atoms on the surface of the tungsten powder under preset temperature and atmosphere conditions to form a coating layer. Its main purpose is to achieve uniform distribution of nickel and iron on the surface of tungsten powder and avoid the problem of uneven mixing of elemental powders.

[0064] The formation of core-shell structured tungsten-nickel-iron powder with uniform composition at the microscale refers to the process of precisely controlling deposition process parameters to ensure uniform coating thickness and consistent composition, forming a composite structure with tungsten powder as the core and nickel-iron alloy as the shell. This is mainly to promote the uniform melting and spreading of the binder phase during subsequent sintering and improve the uniformity of the alloy structure.

[0065] Mixing core-shell structured tungsten nickel-iron powder with an organic binder as a metal raw material refers to adding the core-shell structured powder and the organic binder to a mixing equipment in a specific ratio and mixing them under set temperature, speed and time conditions. The main purpose is to make the binder uniformly coat the powder particles and form a mixture with good molding properties.

[0066] Preparing injection molding feedstock refers to the process of mixing materials to achieve a uniform paste state, followed by extrusion, cooling, pelletizing, and other treatments to obtain feedstock with uniform particles and stable rheological properties. Its main purpose is to meet the requirements of injection molding process for the form and properties of raw materials and to ensure the quality of green body molding.

[0067] Using core-shell structured tungsten-nickel-iron powder as the sole metallic raw material in the preparation of already completed alloy components means that there is no need to add additional tungsten, nickel, or iron elemental powders. The core-shell structured powder is used directly as the source of metallic components. This is mainly to eliminate the mechanical mixing steps in traditional processes and fundamentally solve the problem of component segregation.

[0068] Injection molding to obtain a green body refers to feeding material into a screw-type injection molding machine and injecting it into a preset mold. By controlling process conditions such as injection pressure, holding pressure parameters, mold temperature, and cooling time, the material fills the mold cavity and solidifies to form a shape. Its main purpose is to obtain a preliminary green body that is consistent with the shape of the target part.

[0069] The process of sequentially degreasing, sintering, and heat treatment of green blanks refers to first removing the organic binder from the green blank through a specific process, then achieving densification through high-temperature sintering in a protective atmosphere or vacuum environment, and finally optimizing the mechanical properties through treatments such as solution aging. The main purpose is to gradually improve the density, strength, and toughness of the blank, and ultimately obtain alloy parts that meet the requirements for use.

[0070] This application overcomes the limitations of traditional MIM (Metal Injection Molding) processes, such as uneven mixing of elemental powders, component segregation, and poor process compatibility, through innovative raw material structure and optimized process flow. Specifically, firstly, spherical tungsten powder adapted for powder bed melt additive manufacturing is used as the matrix. High-quality raw materials are screened through multi-dimensional performance testing to ensure that the raw materials have good flow properties and morphological characteristics, providing a foundation for cross-process applications. Next, nickel and iron are simultaneously coated on the surface of the tungsten powder using vapor deposition to form a core-shell structured composite powder, achieving uniform pre-composite bonding of alloy components at the microscale, thus avoiding the risk of component segregation caused by mechanical mixing of elemental powders from the source. Then, the core-shell structured powder is used as the sole metal raw material and mixed with an organic binder to prepare the feedstock, eliminating the traditional mechanical mixing step, simplifying the process flow, and improving the uniformity of the feedstock. Finally, through precise control of the entire process of injection molding, debinding, sintering, and heat treatment, the densification and performance optimization of the billet are achieved, ultimately obtaining tungsten-nickel alloy composite parts with high density, excellent mechanical properties, and no component segregation. The entire process forms a complete closed loop of "raw material screening - structure preparation - molding treatment", which not only solves the core pain points of traditional processes, but also realizes the compatible application of the same batch of powder in MIM process and additive manufacturing process, improving production flexibility and cost-effectiveness.

[0071] In one specific implementation, step S1 involves the following steps: For five different batches of spherical tungsten powder, the Hall flow rate of 50g of powder is tested using a Hall flow meter with a 2.5mm aperture funnel, according to GB / T1482 standard; the loose density is determined by natural sedimentation using a 25mL standard loose density meter, according to GB / T1479 standard; and images of at least five fields of view for each batch are acquired using a scanning electron microscope at 2000x magnification. Image analysis software is used to statistically analyze the particle aspect ratio and calculate the average sphericity. The preset joint performance thresholds are: Hall flow rate not exceeding 16s / 50g, loose density not less than 4.1g / cm³, and average sphericity not less than 0.88. Batches with a Hall flow rate of 14s / 50g, loose density of 4.3g / cm³, and average sphericity of 0.90 are selected as the matrix.

[0072] The specific implementation of step S2 is as follows: The selected spherical tungsten powder is placed into a fluidized bed reactor. Argon gas with a purity of not less than 99.99% is introduced as the fluidizing gas, and the gas flow rate is adjusted to 0.7 L / (min·kg) to ensure the tungsten powder is in a stable fluidized state. The reactor is heated to a deposition temperature range of 170 degrees Celsius, which is higher than the thermal decomposition initiation temperature of nickel carbonyl and iron carbonyl but lower than their rapid homogeneous nucleation temperature. A reaction gas composed of a 4:1 mass ratio of nickel carbonyl gas and iron carbonyl gas is introduced into the reactor, and the ratio of the total flow rate of the reaction gas to the mass of the tungsten powder is controlled at 0.7 L / (min·kg). The concentration of undecomposed nickel carbonyl and iron carbonyl in the exhaust gas is monitored in real time using a gas chromatograph. The cumulative deposition amount is calculated based on the concentration changes. When the theoretical total deposition amount corresponding to a 1.5 μm coating thickness is reached, the reaction is terminated, and the cooling rate is controlled at 5 degrees Celsius / min to obtain core-shell structured tungsten-nickel-iron powder.

[0073] The specific implementation of step S3 is as follows: Core-shell structured tungsten-nickel-iron powder and polyoxymethylene-based binder are added to a mixer at a mass ratio of 6.8:3.2. The polyoxymethylene-based binder is composed of polyoxymethylene, polyethylene wax, and zinc stearate at a mass ratio of 8:1.5:0.5. At a first mixing temperature of 125 degrees Celsius, the mixture is first-stage mixed at a speed of 45 r / min for 35 minutes to allow the binder to initially melt and wet the powder particles. Subsequently, the mixer is heated to a second mixing temperature of 155 degrees Celsius, and a second-stage refining and mixing is performed at a speed of 95 r / min for 65 minutes. During the refining and mixing stage, samples were taken every 15 minutes, with each sample weighing 5g. The melt index was measured under standard test conditions of 190°C and 2.16kg load. A curve showing the change of melt index with mixing time was plotted. When the absolute value of the instantaneous slope of the curve was continuously lower than 0.4g / (10min·min), and the melt index of four consecutive samples fell within the range of 18 to 28g / 10min, the mixing was considered complete. The mixed material was transferred to a single-screw extruder, and a decreasing temperature gradient was formed in each temperature control zone from 145°C at the feed inlet to 115°C at the die orifice. The screw speed was 30r / min, and strips with a diameter of 2.5mm were extruded. After cooling with 30°C cooling water for 10 minutes, the strips were cut into feed pellets with a length of 2.5mm by a pelletizer.

[0074] The specific implementation of step S4 is as follows: feed material into the screw injection molding machine, inject it into the preset mold, control the injection pressure to 75MPa, the holding pressure to 60MPa, the holding time to 11s, the mold temperature to 55 degrees Celsius, and the cooling time to 8s. After cooling, demold to obtain the green blank. The green blanks are transferred to the degreasing process within 24 hours. The green blanks are placed in a degreasing furnace filled with nitric acid vapor, with the nitric acid vapor concentration controlled at 50%. A multi-stage degreasing process is adopted: pre-rinse 45min (110 degrees Celsius, no acid injection), degreasing stage 1 135min (110 degrees Celsius, 4g acid injection), degreasing stage 2 65min (115 degrees Celsius, 4.5g acid injection), degreasing stage 3 65min (120 degrees Celsius, 5g acid injection), post-rinse stage 1 40min (120 degrees Celsius, no acid injection), and post-rinse stage 2 5min (natural cooling, no acid injection). The total degreasing time is calculated based on the maximum wall thickness of the green blank (4mm) and the degreasing time per unit thickness (1.2h / mm), and is determined to be 4.8h. After degreasing, a crushing test is conducted to verify that the degreased blank can be completely crushed without any core. The degreased blank was placed in a sintering furnace, evacuated to 0.0008 Pa, and heated to 580°C for 2.5 hours to complete decarburization and deoxidation. Argon gas was then introduced to bring the furnace pressure to 75 kPa, and the temperature was further increased to 1480°C for 3.5 hours for liquid-phase sintering. During the holding period, the furnace pressure and the surface color of the parts were monitored in real time. Sintering was considered complete when the pressure fluctuation was less than 4 kPa within 40 minutes and the surface of the parts exhibited a uniform silvery-white metallic luster for 25 minutes. The sintered parts were then subjected to heat treatment: solution treatment at 1055°C for 2.5 hours, followed by water cooling at a rate of 100°C / min with a vacuum degree ≤0.02 Pa; aging treatment at 536°C for 5.5 hours, followed by furnace cooling to room temperature with a vacuum degree ≤0.005 Pa, ultimately yielding tungsten-nickel alloy composite parts. The part was tested and found that the nickel-iron binder phase was uniformly distributed in a continuous network structure at the tungsten grain boundaries, with no macroscopic component segregation bands, a density of 99.0%, a hardness of HV580, a tensile strength of 2000MPa, an elongation of 4.2%, and dimensional accuracy conforming to the general tolerance standards for MIM products.

[0075] Step S1 further includes the following sub-steps:

[0076] S1-1, For different batches of spherical tungsten powder, the Hall flow rate was determined according to GB / T1482 standard, the loose density was determined according to GB / T1479 standard, and the average sphericity was statistically calculated based on scanning electron microscope images;

[0077] S1-2, compare the Hall flow rate, loose packing density and average sphericity values ​​measured for each batch with the joint performance threshold preset according to the requirements of powder bed melt additive manufacturing process;

[0078] S1-3, select batches with Hall flow velocity not higher than the flow velocity threshold, loose packing density not lower than the density threshold, and average sphericity not lower than the sphericity threshold as the matrix.

[0079] It should be noted that the GB / T1482 standard is a national recommended standard for standardizing the method of measuring Hall flow rate of metal powder. It clarifies the equipment required for the test, the operating procedures and the calculation method of the results, and serves as the basis for ensuring the accuracy and consistency of flow rate measurement.

[0080] GB / T1479 is a national recommended standard for the determination of the loose density of metal powders. It specifies test methods such as the natural sedimentation method and related equipment parameters, providing support for the reliability of loose density data.

[0081] Hall flow rate refers to the flow time of metal powder through a funnel with a specific aperture. It reflects the smoothness of powder flow under gravity and is one of the key indicators for evaluating the adaptability of powder to processes.

[0082] Loose packing density refers to the mass per unit volume of powder in its natural packing state. It reflects the packing characteristics of powder particles and is closely related to flow properties.

[0083] Scanning electron microscope (SEM) images are images of the microscopic morphology of powder particles taken using a scanning electron microscope. They can clearly show the shape, size, and surface condition of the particles.

[0084] Mean sphericity refers to the average degree to which the shape of powder particles approximates a sphere, calculated by statistical analysis of the aspect ratio of powder particles in scanning electron microscope images. It is an important factor affecting the flowability and spreadability of powder.

[0085] The combined performance threshold refers to a comprehensive performance judgment standard that includes Hall flow rate, loose packing density and average sphericity, based on the characteristics and process requirements of the powder bed additive manufacturing system. It clarifies the minimum performance limit that the powder can meet the powder bed requirements of additive manufacturing.

[0086] One-by-one comparison refers to comparing and analyzing the measured performance data of each batch of powder with the corresponding indicators in the joint performance threshold to ensure that each performance indicator meets the preset requirements.

[0087] The flow rate threshold is the upper limit set for Hall flow rate in the combined performance threshold. It is used to limit the maximum allowable range of powder flow rate and ensure that the powder has good flow efficiency.

[0088] The density threshold is the lower limit set for loose density in the combined performance threshold. It is used to regulate the minimum compaction of powder packing and ensure the uniformity of the powder layer.

[0089] The sphericity threshold is the lower limit set for the average sphericity in the joint performance threshold. It is used to ensure the regularity of the powder particle shape and improve the stability of the powder spreading process.

[0090] The matrix refers to the core raw material for the subsequent preparation of the core-shell structure. Its properties directly affect the final quality of the composite powder and the compatibility with subsequent molding processes.

[0091] Step S2 further includes the following sub-steps:

[0092] S2-1, spherical tungsten powder, which serves as the matrix, is placed into a fluidized bed reactor, and fluidizing gas is introduced to keep it in a stable fluidized state;

[0093] S2-2, the fluidized bed reactor is heated to a deposition temperature range that is higher than the initial thermal decomposition temperature of carbonyl nickel and carbonyl iron gases, but lower than their rapid homogeneous nucleation temperature;

[0094] S2-3, In the fluidized bed reactor at the deposition temperature range, a reaction gas consisting of a mixture of nickel carbonyl gas and iron carbonyl gas in a preset ratio is introduced, and the ratio of the total flow rate of the reaction gas to the mass of the spherical tungsten powder is controlled within a preset range.

[0095] S2-4 undergoes a deposition reaction within the deposition temperature range to form a core-shell structured tungsten-nickel-iron powder with a uniform coating thickness.

[0096] Please refer to Figures 3, 4 and 5, which show SEM images of core-shell structured tungsten nickel-iron powder, TEM images of MIM tungsten nickel-iron powder and STEM images of MIM tungsten nickel-iron powder provided in the embodiments of the present invention.

[0097] Furthermore, in sub-steps S2-4, the steps for carrying out the deposition reaction include:

[0098] Real-time monitoring of the concentration of characteristic gas components in the tail gas of the deposition reaction;

[0099] Based on the change data of characteristic gas component concentration, the cumulative deposition amount of nickel and iron on the surface of spherical tungsten powder is calculated in real time;

[0100] The real-time cumulative deposition amount is compared with the theoretical total deposition amount calculated based on the preset target thickness of the coating layer;

[0101] When the real-time cumulative deposition reaches the theoretical total deposition amount, the deposition reaction is determined to be complete, and the introduction of reaction gas and heating of the reactor are terminated.

[0102] It should be noted that a fluidized bed reactor is a gas-phase deposition reactor with fluidization function. It can keep solid powder particles in a suspended state through gas flow. Its core function is to ensure that the powder and the reaction gas are in full contact, thereby improving the uniformity of deposition.

[0103] Fluidizing gas refers to the gaseous medium used to fluidize spherical tungsten powder. Inert gases are usually selected, and their main function is to prevent powder oxidation while providing stable fluidization power.

[0104] A stable fluidized state refers to the state in which powder particles are uniformly suspended in the reactor without local accumulation or sedimentation. It reflects the dynamic balance between powder and gas and is the basis for ensuring the uniformity of the coating layer.

[0105] The deposition temperature range refers to the temperature range set to achieve uniform deposition of nickel-iron. This range needs to be higher than the thermal decomposition initiation temperature of carbonyl nickel and carbonyl iron to ensure that the reaction occurs, while being lower than their rapid homogeneous nucleation temperature to avoid the formation of independent particles in the gas phase. It directly affects the deposition efficiency and coating quality.

[0106] Nickel carbonyl gas and iron carbonyl gas refer to gaseous precursors containing nickel and iron elements. They can decompose and release nickel and iron atoms at the deposition temperature, and are the material source for forming the core-shell structure shell.

[0107] The preset ratio refers to the mixing ratio of carbonyl nickel gas and carbonyl iron gas set according to the target alloy composition requirements, which determines the composition ratio of nickel and iron in the core-shell structure.

[0108] The ratio of the total flow rate of the reactant gas to the mass of the spherical tungsten powder refers to the rate of reaction gas introduction per unit mass of tungsten powder. It controls the amount of contact between the gas and the powder to adjust the deposition rate and the uniformity of the coating thickness.

[0109] Core-shell structured tungsten-nickel-iron powder refers to a composite powder with spherical tungsten powder as the core and nickel-iron alloy as the shell. Its characteristics are that the shell has uniform composition and consistent thickness, realizing the pre-composite of alloy components at the microscale.

[0110] The concentration of characteristic gas components refers to the content of undecomposed carbonyl nickel and carbonyl iron in the exhaust gas, and its changes directly reflect the progress and efficiency of the deposition reaction.

[0111] Cumulative deposition refers to the total mass of nickel and iron atoms deposited on the surface of tungsten powder during the deposition process, which reflects the actual growth of the coating layer.

[0112] The preset target thickness of the coating layer refers to the standard for shell thickness set according to the requirements of subsequent sintering and forming, and it is the core basis for calculating the theoretical total deposition.

[0113] The theoretical total deposition refers to the total mass of nickel-iron deposition calculated based on the target thickness and the specific surface area of ​​tungsten powder, which provides a clear criterion for terminating the deposition reaction.

[0114] The deposition reaction is complete when the coating thickness reaches the preset requirement. Terminating the reaction at this point ensures the consistency and stability of the core-shell structure.

[0115] In one specific embodiment, taking the preparation of core-shell structured tungsten-nickel-iron powder with a coating thickness of 2 μm as an example, the implementation process of the deposition reaction is specifically described as follows:

[0116] 1. Characteristic gas monitoring settings

[0117] A gas chromatograph was used as the monitoring device, with a monitoring frequency of 1 time / minute. The target monitored components were undecomposed nickel carbonyl and iron carbonyl gases, and the detection accuracy was 0.01 vol%, ensuring that subtle changes in the exhaust gas components could be accurately captured.

[0118] 2. Calculation of cumulative sediment volume

[0119] A correlation model between characteristic gas concentration and deposition rate was established through preliminary experiments. The model was derived based on the ideal gas law and deposition reaction kinetics. When the concentration of nickel carbonyl in the exhaust gas decreased from the initial 5 vol% to 1 vol% and the concentration of iron carbonyl decreased from the initial 3 vol% to 0.8 vol%, the real-time deposition rate of nickel was calculated to be 0.035 g / cm², the real-time deposition rate of iron was 0.015 g / cm², and the cumulative deposition rate reached 0.05 g / cm².

[0120] 3. Determination of theoretical total sediment volume

[0121] Based on the specific surface area of ​​spherical tungsten powder (measured to be 0.8 m² / g) and the preset target coating thickness of 2 μm, combined with the density of nickel-iron alloy (8.2 g / cm³), the theoretical total deposition amount is calculated to be 0.052 g / cm².

[0122] 4. Determination and Verification of Reaction Termination

[0123] When the cumulative deposition reached 0.052 g / cm², the reaction gas supply and reactor heating were immediately stopped, and the cooling rate was controlled at 5 degrees Celsius / min. Scanning electron microscopy revealed that the uniformity of the coating thickness was less than ±0.1 μm. Energy dispersive spectroscopy analysis showed that the nickel-iron content ratio deviated from the preset value by less than ±0.5%, verifying the effectiveness of this implementation method.

[0124] Step S3 further includes the following sub-steps:

[0125] S3-1, core-shell structured tungsten nickel iron powder and binder are added to an internal mixer and the first stage of mixing is carried out at the first mixing temperature, so that the binder melts and initially wets and coats the powder particles.

[0126] S3-2, heat the internal mixer to a second mixing temperature higher than the first mixing temperature, and perform the second refining mixing at a speed higher than the first mixing speed, continuing until the feed reaches a highly uniform paste.

[0127] S3-3, During the refining and mixing stage, the melt index of the feed is measured at intervals. When the measured value of the melt index falls stably within the preset qualified range for multiple consecutive times, the mixing is deemed complete.

[0128] S3-4, the mixed feed is extruded, cooled and pelletized to produce feed pellets for injection molding.

[0129] Please refer to Figure 6, which is a schematic diagram of the process curve showing the change of the feed melt index with mixing time according to an embodiment of the present invention.

[0130] Furthermore, in sub-step S3-3, the steps for determining whether mixing is complete include:

[0131] During the refining and mixing stage, samples are taken from the internal mixer at fixed time intervals;

[0132] The melt flow index of each sample was determined under standard test conditions, and the corresponding mixing time was recorded.

[0133] Plot the process curve of melt index as a function of mixing time, and calculate the instantaneous slope of the process curve near the most recent sampling time point;

[0134] When the absolute value of the instantaneous slope is continuously lower than the preset slope threshold, and the melt index values ​​measured by sampling for a preset number of consecutive times all fall within the preset qualified range, it is determined that the rheology of the feed has stabilized and the mixing is completed.

[0135] Furthermore, in sub-steps S3-4, the steps of extruding, cooling, and pelletizing the compounded feed include:

[0136] The mixed paste feed is transferred to the feed inlet of the single-screw extruder;

[0137] Control the temperature of each temperature control zone of the single screw extruder to form a decreasing temperature gradient from the feed inlet to the die orifice, and ensure that the temperature of each temperature control zone is lower than the second mixing temperature;

[0138] Start the single screw extruder, feed the material forward, compress it, melt and homogenize it, and then extrude it through the die to form a continuous and slender strip with uniform cross-sectional dimensions;

[0139] The extruded strip feed is cooled by a cooling medium to completely solidify it;

[0140] The solidified strip feed is fed into a pelletizer, where it is cut into uniform cylindrical particles by rotating blades.

[0141] It should be noted that the binder refers to the organic bonding system used to mix with core-shell structured tungsten nickel iron powder. It is usually a polyoxymethylene-based composite binder, which is used to bind the powder particles together and give the feed good flowability and formability.

[0142] A self-mixing machine is a specialized piece of equipment used for mixing powders and binders. It achieves uniform mixing of materials through heating, pressurization, and shearing.

[0143] The first mixing temperature refers to the initial temperature set for the first stage of mixing. This temperature is lower than the temperature at which the binder completely decomposes. The main purpose is to allow the binder to melt slowly, initially wet the powder particles, and avoid compositional changes caused by local overheating.

[0144] The first stage of mixing refers to the initial mixing process carried out at a lower temperature and speed. Its core function is to achieve the initial bonding between the binder and the powder, laying the foundation for subsequent refining and mixing.

[0145] The second mixing temperature refers to a refining temperature that is higher than the first mixing temperature. This temperature needs to be adapted to the melting characteristics of the binder and the thermal stability of the powder. Its main purpose is to enhance the shear mixing effect of the material and promote uniform coating of the binder.

[0146] The second stage of refining and mixing refers to a deep mixing process carried out at higher temperatures and speeds. Its core function is to eliminate voids and agglomerates within the material and improve the uniformity and rheological properties of the feed.

[0147] A highly uniform paste-like consistency refers to a state in which the material, after mixing, has no obvious particle texture, consistent color, and uniform texture. This state is a key characteristic for the feedstock to have good molding performance.

[0148] Melt flow index refers to the mass of feed material passing through a specific aperture within a specified time under standard test conditions. It is a core indicator characterizing feed material flowability, and its value directly reflects the molding adaptability of the feed material.

[0149] Standard test conditions refer to the uniformly set temperature, load, and time parameters when determining the melt flow index, ensuring the accuracy and comparability of test results.

[0150] A process curve is a curve plotted with mixing time on the horizontal axis and melt flow index on the vertical axis. Its shape reflects the evolution of feed flowability during the mixing process.

[0151] Instantaneous slope refers to the slope of the tangent line of the process curve near a specific sampling time point. Its absolute value characterizes the rate of change of the melt index and reflects the stability of the feed rheology.

[0152] The slope threshold is a preset critical value used to determine the rate of change of the melt flow index. When the absolute value of the instantaneous slope is lower than this threshold, it indicates that the feed flowability tends to be stable.

[0153] The acceptable range refers to the allowable range of melt flow index set according to the injection molding process requirements, ensuring that the feedstock has suitable flow properties.

[0154] A single-screw extruder is a device used to extrude paste-like feed into shapes. It achieves material conveying, compression, and melting homogenization through the rotation and propulsion of the screw.

[0155] A decreasing temperature gradient refers to a gradually decreasing temperature distribution set from the feed inlet to the die in a single-screw extruder. Its function is to prevent the feed from decomposing due to excessive temperature during extrusion, while ensuring that the material is fully melted and homogenized.

[0156] Slender strips refer to continuous strips of material with uniform cross-sectional dimensions formed through the die of an extruder, which are the basic form for subsequent pelleting.

[0157] Cooling medium refers to the substance used to cool the strip-shaped feed, usually cooling water or cooling oil. Its function is to quickly reduce the temperature of the strip-shaped material, causing it to solidify from a paste-like state to a solid state, making it easier to cut later.

[0158] A pelletizer is a device used to cut solidified strip feed into pellets. Through the shearing action of rotating blades, it obtains cylindrical feed pellets of uniform length, which meets the requirements of injection molding process for raw material morphology.

[0159] In one specific embodiment, taking the mixing of core-shell structured tungsten-nickel-iron powder with polyoxymethylene-based binder as an example, the implementation process for determining the completion of mixing is specifically explained:

[0160] 1. Sampling and Testing Setup

[0161] During the refining and mixing stage, a fixed sampling interval of 10 minutes was set, with each sample weighing 5g. The standard test conditions were set at a temperature of 190 degrees Celsius and a load of 2.16kg. A melt flow indexer was used for testing, and each sample was tested 3 times and the average value was taken.

[0162] 2. Process curve plotting and slope calculation

[0163] The melt flow index was recorded for different mixing times: 22 g / 10 min at 30 minutes, 26 g / 10 min at 40 minutes, 28 g / 10 min at 50 minutes, 29 g / 10 min at 60 minutes, and 29.2 g / 10 min at 70 minutes. After plotting the process curve, linear fitting was used to calculate the instantaneous slope of the two most recent sampling points (60 minutes and 70 minutes), with an absolute value of 0.02 g / (10 min·min).

[0164] 3. Threshold setting and mixing determination

[0165] The preset slope threshold is 0.5 g / (10 min·min), the preset acceptable range is 25 to 35 g / 10 min, and the preset number of consecutive acceptable times is 3. When mixing reaches 50 minutes, 60 minutes, and 70 minutes, the melt index is 28 g / 10 min, 29 g / 10 min, and 29.2 g / 10 min, respectively, all falling within the acceptable range, and the instantaneous slope absolute value of 0.02 g / (10 min·min) is lower than the threshold, indicating that mixing is complete.

[0166] 4. Rheological verification

[0167] The compounded feed was tested using a rotational rheometer. The ratio of its storage modulus to loss modulus was between 1 and 1.2, and the viscosity fluctuation was less than ±5%, indicating that the feed rheological properties were stable and met the requirements for injection molding.

[0168] In one specific embodiment, taking the preparation of feed pellets with a diameter of 2.5 mm and a length of 3 mm as an example, the process of extruding, cooling, and pelletizing the mixed feed is specifically described:

[0169] 1. Extruder temperature control

[0170] The single-screw extruder is equipped with three temperature control zones: the feed inlet temperature control zone is set at 150 degrees Celsius, the middle temperature control zone at 135 degrees Celsius, and the die temperature control zone at 120 degrees Celsius, forming a decreasing temperature gradient. The second mixing temperature is 160 degrees Celsius. The temperature of each temperature control zone is lower than this temperature to avoid feed decomposition.

[0171] 2. Extrusion parameter settings

[0172] The single-screw extruder has a screw diameter of 30mm, a length-to-diameter ratio of 25:1, a screw speed set at 35r / min, and a die orifice diameter of 2.5mm. After the equipment is started, the feed is conveyed, compressed, melted, and homogenized before being extruded through the die to form a continuous, slender strip with a cross-sectional diameter deviation of less than ±0.1mm.

[0173] 3. Cooling and pelletizing operations

[0174] Cooling water is used as the cooling medium, and the temperature is controlled at 28 degrees Celsius. The strip feed is passed through the cooling water for 8 minutes to ensure complete curing. After curing, the Shore hardness reaches 65D. The cured strip feed is then fed into a pelletizer. The pelletizer's rotating blade speed is set to 250 r / min, the cutting length is set to 3 mm, and the particle length deviation is less than ±0.2 mm.

[0175] 4. Particle quality verification

[0176] 100 feed pellets were randomly selected for testing. The roundness of the pellets was greater than 0.9, and there was no sticking or burrs. Through sieving test, more than 95% of the pellets could pass through the 2.36mm standard sieve and did not pass through the 1.7mm standard sieve, which met the feeding requirements of the injection molding equipment.

[0177] Step S4 further includes the following sub-steps:

[0178] S4-1, The feed material is injected into the mold, and after holding pressure and cooling, it is demolded to obtain the green body;

[0179] S4-2, the green body is placed in a nitric acid vapor environment and catalytically degreased at a constant temperature;

[0180] S4-3, the degreased green body is heated to the first temperature range under vacuum and held at that temperature to complete deep decarburization and deoxidation;

[0181] S4-4, Inert protective gas is introduced into the sintering furnace to the set partial pressure, and the temperature is continued to rise to the second temperature range above the melting point of the binder phase for holding, so as to achieve liquid phase sintering densification and obtain sintered parts;

[0182] S4-5 involves solution treatment and aging treatment of sintered parts to obtain tungsten-nickel alloy composite parts.

[0183] Furthermore, in sub-step S4-2, the step of catalytic degreasing at a constant temperature includes:

[0184] The green blanks are placed in the working area of ​​a degreasing furnace filled with nitric acid vapor;

[0185] The temperature of the working area is controlled within a constant degreasing temperature range of 110 degrees Celsius to 120 degrees Celsius;

[0186] Based on the maximum wall thickness of the green body and the preset degreasing time per unit thickness, calculate and set the total degreasing time for this operation;

[0187] The nitric acid vapor concentration is maintained within a constant degreasing temperature range, and the degreasing reaction continues until the set total time is reached.

[0188] Furthermore, in sub-step S4-4, the steps for achieving liquid-phase sintering densification include:

[0189] During the heat preservation process in the second temperature range, pressure data is monitored in real time by an in-furnace pressure sensor, and the surface color change of the sintered parts is monitored by an optical observation device.

[0190] Establish pressure stability index and color uniformity index;

[0191] When the fluctuation range of the pressure data within the preset time is less than the first threshold, and the overall appearance color changes to a uniform silvery-white metallic luster and continues for the second threshold time, the liquid phase sintering densification is determined to be complete.

[0192] It should be noted that a mold is a special molding tool used for injection molding. Its cavity shape is consistent with the target part and it is the core component that gives the green part its shape.

[0193] Holding pressure refers to the process of continuously applying a certain pressure after injection, the purpose of which is to compensate for the volume shrinkage of the material in the mold cavity and avoid defects such as shrinkage cavities and depressions in the green blank.

[0194] Cooling refers to the process of reducing the temperature of the green blank through a mold cooling system, causing the feed material to solidify from a molten state to a solid state. Its speed directly affects the molding quality and dimensional stability of the green blank.

[0195] Catalytic degreasing refers to a process that uses nitric acid vapor as a catalyst to accelerate the decomposition and removal of organic binders in green bodies. Its core function is to efficiently remove binders while avoiding deformation of the green body.

[0196] The degreasing furnace working area refers to the specific area inside the degreasing furnace used to place green billets, and it is necessary to ensure uniform temperature and atmosphere.

[0197] The constant degreasing temperature range refers to the temperature range of 110 degrees Celsius to 120 degrees Celsius. This range is suitable for the decomposition characteristics of the binder, which can ensure degreasing efficiency and prevent the green body from deforming or oxidizing due to excessive temperature.

[0198] Degreasing time per unit thickness refers to the degreasing time required per unit thickness based on the green body wall thickness. Its value is determined based on the binder content and degreasing efficiency, and it is a key parameter for calculating the total degreasing time.

[0199] Total degreasing time refers to the total time required to complete the degreasing of the green body. It is calculated by multiplying the maximum wall thickness of the green body by the degreasing time per unit thickness, ensuring that the binder in thick-walled areas is completely removed.

[0200] Nitric acid vapor concentration refers to the content of nitric acid vapor in the degreasing furnace, which needs to be kept stable to ensure uniform degreasing effect.

[0201] The first temperature range refers to the temperature range used for deep decarburization and deoxidation. This temperature can remove carbon impurities and oxide phases from the green body without causing excessive sintering of the green body.

[0202] Deep decarburization and deoxidation refers to removing residual binder decomposition products, carbon elements, and oxygen compounds from the green body through a high-temperature vacuum environment, thereby improving the purity of the green body and laying the foundation for subsequent sintering and densification.

[0203] Inert protective gas refers to a chemically stable gas that does not react with the blank. Argon is usually selected. Its function is to isolate the air during the liquid phase sintering stage and prevent the parts from oxidizing.

[0204] The set partial pressure refers to the protective gas pressure value set to promote the spread of the binder phase. By adjusting the gas pressure, the sintering environment is optimized, and the density of the alloy is improved.

[0205] The second temperature range refers to the temperature range above the melting point of the nickel-iron binder phase. This temperature allows the binder phase to completely melt, achieving a dense bond of tungsten particles.

[0206] Liquid phase sintering densification refers to the process of filling the gaps between tungsten particles with a molten nickel-iron binder phase, causing the billet to shrink in volume and increase in density. It is a core step in improving the density and mechanical properties of parts.

[0207] A pressure sensor is a device used to monitor pressure changes inside a sintering furnace in real time, and its data reflects the stability of the atmosphere inside the furnace.

[0208] Optical observation devices are equipment used to observe the surface color of sintered parts. Color changes directly reflect the sintering state and oxidation of the parts.

[0209] Pressure stability index refers to a parameter that measures the degree of pressure fluctuation inside the furnace. Atmosphere stability is determined by the pressure fluctuation amplitude within a preset time period.

[0210] Color uniformity index refers to a parameter that measures the consistency of the appearance color of parts. A uniform silvery-white metallic luster indicates that the parts are free from localized oxidation or uneven sintering.

[0211] The first threshold refers to the critical fluctuation range for determining pressure stability; below this threshold, it indicates that the atmosphere inside the furnace is stable.

[0212] The second threshold duration refers to the duration required to determine color stability, ensuring that the overall sintering state of the parts is consistent.

[0213] Solution treatment refers to the process of heating sintered parts to a high temperature and holding them at that temperature to homogenize the alloy composition and eliminate internal stress. Its purpose is to improve the microstructure of the parts and lay the foundation for aging treatment.

[0214] Aging treatment refers to heating solution-treated parts to a specific temperature and holding them at that temperature, thereby optimizing the alloy's strength and toughness through the precipitation of second-phase particles, ultimately improving the parts' hardness, strength, and toughness.

[0215] Tungsten-nickel alloy composite parts refer to the final products prepared through the above-mentioned full-process technology, which have high density, uniform microstructure and excellent mechanical properties.

[0216] In one specific implementation, taking a green body with a maximum wall thickness of 5 mm as an example, the process of catalytic degreasing at a constant temperature is specifically described:

[0217] 1. Degreasing oven parameter settings

[0218] Fifteen firing plates are placed in the working area of ​​the degreasing furnace. The green billets are evenly spread on the firing plates. Each furnace has 35 acid stripping trays, arranged in a straight line, front to back and top to bottom. The concentration of nitric acid vapor is controlled at 55%, and the concentration is maintained by adjusting the temperature of the nitric acid evaporator and the nitrogen flow rate (6 to 8 m³ / h).

[0219] 2. Degreasing temperature and time settings

[0220] The constant degreasing temperature range is set at 115 degrees Celsius, and the temperature fluctuation is controlled within ±2 degrees Celsius through real-time monitoring by an in-furnace temperature sensor. The preset degreasing time per unit thickness is 1 hour / mm. Based on the maximum wall thickness of 5mm for the green body, the total degreasing time is calculated to be 5 hours. The specific process is as follows: pre-rinsing 45 minutes, degreasing stage 1 135 minutes, degreasing stage 2 65 minutes, degreasing stage 3 65 minutes, post-rinsing stage 1 40 minutes, and post-rinsing stage 2 5 minutes.

[0221] 3. Control of the defatting process

[0222] During production, a process check is performed every 2 hours, recording parameters such as temperature, acid inlet volume, and nitrogen flow rate. During degreasing, the acid degreasing tank, acid degreasing tray, and transfer cart are kept clean to avoid contaminating the green blanks.

[0223] 4. Verification of degreasing effect

[0224] After degreasing, a crushing test was conducted on the degreased blank, which could be completely crushed and had no binder core inside; the degreasing rate was calculated by gravimetric method and reached 98.5%, which is higher than the target value of 98%; differential scanning calorimetry was used to detect that there were no characteristic peaks of binder remaining, indicating that the degreasing was thorough.

[0225] In one specific implementation, taking the liquid-phase sintering of tungsten-nickel-iron core-shell powder as an example, the implementation process for achieving densification through liquid-phase sintering is described in detail:

[0226] 1. Monitoring equipment and indicator settings

[0227] The furnace pressure sensor has a monitoring accuracy of ±1 kPa and a monitoring frequency of 1 time / minute. The optical observation device uses a high-temperature resistant industrial camera to capture an image of the part's surface every 5 minutes. The pressure stability index is defined as the pressure fluctuation amplitude within a preset time period, and the color uniformity index is defined as the proportion of silver-white luster covering the part's surface.

[0228] 2. Threshold setting

[0229] The preset duration is set to 30 minutes, the first threshold is set to 5 kPa, the second threshold duration is set to 20 minutes, and the color uniformity qualification standard is that the silver-white gloss coverage ratio is ≥98%.

[0230] 3. Monitoring and Judgment of Sintering Process

[0231] The second temperature range was set at 1500 degrees Celsius. During the holding period, the furnace pressure fluctuated from 75 kPa to 78 kPa in the first 10 minutes, with a fluctuation range of 3 kPa. From 10 to 30 minutes, the pressure stabilized between 77 and 78 kPa, with a fluctuation range of 1 kPa, which was less than the first threshold. After holding for 15 minutes, the surface color of the parts began to turn silvery-white. After holding for 25 minutes, the silvery-white gloss coverage reached 99%, and this continued until holding for 45 minutes, meeting the second threshold duration requirement, and the liquid phase sintering densification was determined to be complete.

[0232] 4. Verification of densification effect

[0233] The density of the sintered parts was tested using the Archimedes water displacement method and reached 99.3%. Metallographic microscopy showed that the tungsten particles were uniformly distributed, and the nickel-iron binder phase was continuously spread at the tungsten grain boundaries without obvious porosity or compositional segregation. The surface roughness Ra ≤ 0.8 μm, which meets the product quality requirements.

[0234] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A process for preparing tungsten-nickel alloy composite materials based on MIM technology, characterized in that, Includes the following steps: Step S1: Measure the Hall flow rate, bulk density, and average sphericity of different batches of spherical tungsten powder, and select batches whose measured values ​​meet a preset threshold as the matrix; Step S2: Simultaneously coat the surface of the spherical tungsten powder with nickel and iron using vapor deposition to form a core-shell structured tungsten-nickel-iron powder with uniform pre-composite composition at the microscale; Step S3: Mix the core-shell structured tungsten-nickel-iron powder as a metal raw material with an organic binder to prepare an injection molding feedstock; Step S4: Inject the feedstock into a green body to obtain a green body. The green blank is then subjected to degreasing, sintering, and heat treatment sequentially to obtain tungsten-nickel alloy composite parts. In step S3, during the mixing process, the core-shell structured tungsten-nickel-iron powder is used as the sole metallic raw material for the completed alloy composition, thus eliminating the need for mechanical mixing of tungsten, nickel, and iron elemental powders. Step S2 further includes the following sub-steps: S2-1, placing spherical tungsten powder, serving as the matrix, into a fluidized bed reactor and introducing fluidizing gas to maintain a stable fluidized state; S2-2, ... The fluidized bed reactor is heated to a deposition temperature range higher than the thermal decomposition initiation temperature of nickel carbonyl and iron carbonyl gases, but lower than their rapid homogeneous nucleation temperature; S2-3, a reaction gas composed of nickel carbonyl and iron carbonyl gases mixed in a preset ratio is introduced into the fluidized bed reactor within the deposition temperature range, and the ratio of the total flow rate of the reaction gas to the mass of the spherical tungsten powder is controlled within a preset range; S2-4, a deposition reaction is carried out within the deposition temperature range to form a core-shell structured tungsten-nickel-iron powder with a uniform coating thickness; in the sub-step S2-4, the deposition reaction includes: real-time monitoring of the concentration of characteristic gas components in the deposition reaction tail gas; real-time calculation of the cumulative deposition amount of nickel and iron on the surface of the spherical tungsten powder based on the change data of the characteristic gas component concentration; comparison of the real-time cumulative deposition amount with the theoretical total deposition amount calculated based on the preset target coating thickness; when the real-time cumulative deposition amount reaches the theoretical total deposition amount, the deposition reaction is determined to be completed, and the introduction of the reaction gas and the heating of the reactor are terminated.

2. The preparation process of tungsten-nickel alloy composite material based on MIM technology according to claim 1, characterized in that: Step S1 further includes the following sub-steps: S1-1, for different batches of spherical tungsten powder, the Hall flow rate is measured according to GB / T1482 standard, the loose packing density is measured according to GB / T1479 standard, and the average sphericity is statistically calculated based on scanning electron microscope images; S1-2, the Hall flow rate, loose packing density, and average sphericity values ​​measured for each batch are compared one by one with the joint performance thresholds preset according to the powder bed fused additive manufacturing process requirements; S1-3, batches with Hall flow rates not higher than the flow rate threshold, loose packing density not lower than the density threshold, and average sphericity not lower than the sphericity threshold are selected as the matrix.

3. The preparation process of tungsten-nickel alloy composite material based on MIM technology according to claim 1, characterized in that: Step S3 further includes the following sub-steps: S3-1, adding core-shell structured tungsten nickel-iron powder and binder to a mixer, and performing a first stage of mixing at a first mixing temperature to melt the binder and initially wet and coat the powder particles; S3-2, heating the mixer to a second mixing temperature higher than the first mixing temperature, and performing a second stage of refining mixing at a speed higher than the first stage mixing, continuing until the feed reaches a highly uniform paste; S3-3, during the refining mixing stage, taking samples at intervals to measure the melt index of the feed, and determining that mixing is complete when the measured melt index value falls stably within the preset qualified range for multiple consecutive times; S3-4, extruding, cooling, and pelletizing the mixed feed to produce feed pellets for injection molding.

4. The preparation process of tungsten-nickel alloy composite material based on MIM technology according to claim 3, characterized in that, In sub-step S3-3, the step of determining that mixing is complete includes: taking samples from the internal mixer at fixed time intervals during the refining and mixing stage; measuring the melt index of each sample under standard test conditions and recording the corresponding mixing time; plotting a process curve of melt index versus mixing time and calculating the instantaneous slope of the process curve near the most recent sampling time point; when the absolute value of the instantaneous slope is continuously lower than a preset slope threshold, and the melt index values ​​measured by a preset number of consecutive samplings all fall within a preset qualified range, it is determined that the rheology of the feed has stabilized and mixing is complete.

5. The preparation process of tungsten-nickel alloy composite material based on MIM technology according to claim 3, characterized in that, In sub-steps S3-4, the step of extruding, cooling, and pelletizing the mixed feed includes: transferring the mixed paste feed to the feed inlet of a single-screw extruder; controlling the temperature of each temperature control zone of the single-screw extruder to form a decreasing temperature gradient from the feed inlet to the die, and ensuring that the temperature of each temperature control zone is lower than the second mixing temperature; starting the single-screw extruder to convey, compress, melt, and homogenize the feed forward, and then extruding it through the die to form a continuous and uniformly sized slender strip; cooling the extruded strip feed through a cooling medium to completely solidify it; and feeding the solidified strip feed into a pelletizer, where it is cut into uniformly long cylindrical particles by rotating blades.

6. The preparation process of tungsten-nickel alloy composite material based on MIM technology according to claim 1, characterized in that: Step S4 further includes the following sub-steps: S4-1, the feed material is injected into the mold, and after pressure holding and cooling, it is demolded to obtain a green blank; S4-2, the green blank is placed in a nitric acid vapor environment and catalytically degreased at a constant temperature; S4-3, the degreased blank is heated to the first temperature range under vacuum and held at that temperature to complete deep decarburization and deoxidation; S4-4, an inert protective gas is introduced into the sintering furnace to the set partial pressure, and the temperature is further increased to the second temperature range higher than the melting point of the binder phase and held at that temperature to achieve liquid phase sintering densification and obtain a sintered part; S4-5, the sintered part is subjected to solution treatment and aging treatment to obtain a tungsten-nickel alloy composite material part.

7. The preparation process of tungsten-nickel alloy composite material based on MIM technology according to claim 6, characterized in that, In sub-step S4-2, the step of catalytic degreasing at a constant temperature includes: placing the green blank in the working area of ​​a degreasing furnace filled with nitric acid vapor; controlling the temperature of the working area within a constant degreasing temperature range of 110 degrees Celsius to 120 degrees Celsius; calculating and setting the total degreasing time based on the maximum wall thickness of the green blank and a preset degreasing time per unit thickness; maintaining the nitric acid vapor concentration within the constant degreasing temperature range and continuing the degreasing reaction until the set total time is reached.

8. The preparation process of tungsten-nickel alloy composite material based on MIM technology according to claim 6, characterized in that, In sub-step S4-4, the step of achieving liquid phase sintering densification includes: during the heat preservation process in the second temperature range, monitoring pressure data in real time through an in-furnace pressure sensor and monitoring the change in the apparent color of the sintered parts through an optical observation device; establishing pressure stability index and color uniformity index; when the fluctuation range of pressure data within a preset time is less than a first threshold, and the overall apparent color changes to a uniform silvery-white metallic luster and continues for a second threshold time, it is determined that liquid phase sintering densification is complete.

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