Injection molding feeding method for producing sintered high-hardness 316 material through in-situ method

Abstract

The invention discloses an injection molding feeding method for producing a sintered high-hardness 316 material by an in-situ method, which comprises the following steps: S1, dissolving ammonium molybdate in a solvent to form an ammonium molybdate solution; s2, 316L stainless steel powder is added into the ammonium molybdate solution and stirred, and ammonium molybdate is evenly attached to the surfaces of 316L powder particles; s3, the mixture obtained in the step S2 is subjected to vacuum drying, and ammonium molybdate attached to the surfaces of the powder particles is subjected to in-situ thermal decomposition in the drying process and converted into a uniformly-distributed molybdenum oxide coating; and S4, the 316L powder with the surface coated with the molybdenum oxide and a binder are subjected to banburying and mixing, and the injection molding feed is prepared. Through liquid phase adsorption and in-situ thermal decomposition technologies, molecular-level initial dispersion and nano-level uniform coating of molybdenum oxide on the surface of the 316L powder are realized, and the problem of poor uniformity of a traditional mechanical mixing method is fundamentally solved.

Description

Technical Field

[0001] This invention belongs to the field of metal injection molding technology, specifically relating to a method for preparing metal injection molding feedstock, and more particularly to a method for preparing injection molding feedstock capable of obtaining high sintering hardness 316 stainless steel parts by introducing a molybdenum oxide coating layer on the surface of 316L stainless steel powder through an in-situ chemical method. Background Technology

[0002] Metal injection molding is a novel near-net-shape forming technology that combines modern plastic injection molding technology with powder metallurgy technology. Its basic process involves: first, uniformly mixing metal powder with an organic binder to create a feedstock with good flowability; then, injection molding to obtain a preform on an injection molding machine; followed by debinding to remove the binder; and finally, sintering to densify and obtain the final metal part. This technology boasts significant advantages such as high production efficiency, high material utilization, and the ability to form complex three-dimensional shapes, and is widely used in fields such as electronics, medical devices, automotive, and aerospace.

[0003] 316L stainless steel is a low-carbon austenitic stainless steel widely used in the MIM (Metal Injection Molding) field due to its excellent corrosion resistance, good biocompatibility, and moderate mechanical properties. However, the hardness of traditional 316L MIM parts after sintering is generally low, typically with a Vickers hardness (HV) between 120 and 160. While this hardness level is sufficient for many conventional applications, it is insufficient for precision structural components requiring higher wear resistance, higher load-bearing capacity, or higher strength (such as joint components in certain medical devices, precision gears, wear-resistant bushings, and high-stress fasteners). The hardness of sintered 316L is mainly affected by its austenitic matrix, grain size, and density. Conventional 316L MIM processes primarily improve density by increasing sintering temperature or extending sintering time; however, excessively high temperatures or times can lead to coarse grains, potentially reducing hardness and strength, and are detrimental to dimensional accuracy control.

[0004] To improve the performance of stainless steel, alloying or composite methods are commonly used. For example, adding molybdenum (Mo) to steel can significantly improve its strength, hardness, and pitting resistance. However, in the MIM process, how to introduce trace amounts of uniform Mo into the 316L matrix is ​​a technical challenge. The traditional mechanical mixing method directly mixes molybdenum powder or molybdenum compound powder with 316L powder, which has the following problems: (1) It is difficult to ensure the uniformity of mixing, especially when adding trace amounts; (2) Density differences may lead to component segregation; (3) The diffusion of molybdenum powder during sintering may be uneven, affecting the uniformity of the final properties; (4) Directly added molybdenum compounds (such as molybdenum oxide) may react incompletely or adversely with the matrix.

[0005] Therefore, developing a feed preparation method that enables the reinforcing phase (such as a molybdenum-containing phase) to be uniformly distributed on the surface of 316L powder particles at the nanoscale, thereby achieving uniform alloying or composite formation in the subsequent sintering process and ultimately effectively improving the hardness of sintered parts, has significant industrial application value and technological innovation significance. Summary of the Invention

[0006] The purpose of this invention is to address the deficiency of low sintering hardness in existing MIM 316L materials by providing an in-situ injection molding feeding method for producing high-hardness sintered 316L material. This method utilizes chemical means to generate a uniformly distributed molybdenum oxide nano-coating on the surface of 316L powder in situ. This effectively inhibits grain growth, promotes densification, and achieves fine-grain strengthening and solid solution / second-phase strengthening during sintering, ultimately yielding high-hardness 316L material parts.

[0007] The technical solution of this invention to solve the above-mentioned technical problems is as follows: A method for in-situ production of sintered high-hardness 316 material via injection molding feeding, characterized by comprising the following steps:

[0008] S1. Dissolve ammonium molybdate in a solvent to form an ammonium molybdate solution;

[0009] S2. Add 316L stainless steel powder to the ammonium molybdate solution and stir to make the ammonium molybdate uniformly adhere to the surface of the 316L powder particles;

[0010] S3. Vacuum dry the mixture obtained in step S2, so that the ammonium molybdate adhering to the surface of the powder particles is thermally decomposed in situ during the drying process and transformed into a uniformly distributed molybdenum oxide coating.

[0011] S4. The 316L powder coated with molybdenum oxide is mixed with a binder by intensive kneading to prepare the injection molding feed.

[0012] The beneficial effects of this invention are:

[0013] By in-situ generating a uniformly distributed molybdenum oxide nanocoating on the surface of 316L stainless steel powder, this method effectively solves the problem of low sintering hardness in traditional MIM 316L parts. During sintering, the molybdenum oxide coating inhibits abnormal grain growth, promotes densification, and simultaneously achieves fine-grain strengthening and solid solution / second-phase strengthening mechanisms, resulting in 316L parts with a hardness of HV 180 or higher. Compared to traditional mechanical mixing methods, this technology achieves nanoscale uniform coating of the reinforcing phase on the powder particle surface through chemical means, avoiding problems such as component segregation and uneven diffusion, and significantly improving the uniformity of material properties. Furthermore, this method eliminates the need for additional molybdenum powder or molybdenum compound powder, simplifying the process, reducing production costs, and possessing significant industrial application value.

[0014] Based on the above technical solution, the present invention can be further improved as follows.

[0015] Furthermore, in step S1, the concentration of the ammonium molybdate solution is 5 wt% to 30 wt%.

[0016] The beneficial effect of adopting the above-mentioned further approach is that, by precisely controlling the concentration range of the ammonium molybdate solution, the thickness and uniformity of the subsequently formed molybdenum oxide coating can be effectively adjusted. Too low a concentration may result in incomplete coating coverage, while too high a concentration may cause powder particle agglomeration; both will affect the hardness uniformity of the final sintered part. Experiments show that when the concentration is controlled between 5wt% and 30wt%, it ensures both uniform coating of molybdenum oxide on the powder surface and maintains the dispersion stability of the powder, laying the foundation for obtaining a high-hardness sintered body.

[0017] Furthermore, in step S2, the mass ratio of the 316L stainless steel powder to ammonium molybdate is 100:0.5-5.

[0018] The beneficial effect of adopting the above-mentioned further solution is that, by precisely controlling the mass ratio of 316L stainless steel powder to ammonium molybdate, the formation process of the molybdenum oxide coating can be further optimized. If the proportion of ammonium molybdate in the mass ratio is too low, the molybdenum oxide coating may not form sufficiently, making it difficult to achieve the expected strengthening effect; while if the proportion of ammonium molybdate is too high, it will not only waste raw materials, but may also affect the subsequent processing performance of the powder due to an excessively thick coating. When the mass ratio is controlled within the range of 100:0.5-5, it can be ensured that the molybdenum oxide coating uniformly covers the surface of the 316L stainless steel powder with an appropriate thickness, thereby effectively improving the hardness and performance uniformity of the sintered parts.

[0019] Furthermore, the stirring process in step S2 is carried out under ultrasonic assistance or high-speed shearing conditions, and the stirring time is 30 minutes to 2 hours to ensure uniform dispersion and adhesion of the ammonium molybdate solution.

[0020] The beneficial effects of adopting the above-mentioned further scheme are that, during the stirring process, ultrasonic assistance can generate a cavitation effect, effectively breaking up the agglomeration in the ammonium molybdate solution, making it finer and more uniformly dispersed around the 316L stainless steel powder; high-speed shearing, through the powerful mechanical force, promotes full contact and adhesion between the ammonium molybdate solution and the powder. Controlling the stirring time within the range of 30 minutes to 2 hours ensures that the ammonium molybdate solution is fully dispersed and adhered, without wasting energy and reducing efficiency due to excessive time, thus providing a strong guarantee for the subsequent formation of a uniform and high-quality molybdenum oxide coating.

[0021] Furthermore, in step S3, the vacuum drying temperature is 80°C to 150°C, the vacuum degree is less than 10 kPa, and the drying time is 4 to 12 hours.

[0022] The beneficial effect of adopting the above-mentioned further scheme is that precise control of the temperature, vacuum level, and drying time of vacuum drying is crucial for the formation of molybdenum oxide coating. If the temperature is too low, ammonium molybdate may not be completely thermally decomposed into molybdenum oxide, affecting the coating quality; if the temperature is too high, it may cause oxidation or other adverse reactions in the powder particles. A vacuum level below 10 kPa effectively avoids the introduction of impurities during the drying process, ensuring the purity of the coating. If the drying time is too short, the ammonium molybdate will not decompose completely; if the drying time is too long, it will not only increase production costs but may also adversely affect the performance of the powder. When the drying temperature is controlled between 80℃ and 150℃, the vacuum level is below 10 kPa, and the drying time is between 4 and 12 hours, it can ensure that ammonium molybdate is fully and uniformly converted into molybdenum oxide coating, laying a good foundation for the subsequent preparation of high-hardness 316 material parts.

[0023] Furthermore, the average particle size of the 316L stainless steel powder is 5μm to 25μm, and the particle size distribution D90 ≤ 40μm.

[0024] The beneficial effects of adopting the above-mentioned further solution are that 316L stainless steel powder within this particle size range is selected because it has good flowability and filling properties, enabling it to fill the mold cavity more evenly during injection molding, reducing the generation of defects such as porosity, and helping to improve the density and performance of the final sintered high-hardness 316 material parts. Simultaneously, a particle size distribution D90 ≤ 40μm ensures the relative concentration of powder particle size, further optimizing the molding effect and resulting in more stable and reliable quality of the produced parts.

[0025] Further, in step S4, the binder comprises a mixture of at least two components selected from polyoxymethylene, polyethylene, polypropylene, paraffin wax, and stearic acid; the volume mixing ratio of the 316L powder coated with molybdenum oxide to the binder is 50:50 to 65:35; the mixing process in step S4 is carried out in a mixer or extruder at a mixing temperature of 150°C to 200°C for 1 to 3 hours.

[0026] The beneficial effects of adopting the above-mentioned further scheme are that using a mixture containing at least two components as a binder can fully utilize the advantages of each component, giving the binder good comprehensive performance and better integration with the 316L powder coated with molybdenum oxide, thus improving the uniformity and stability of the feed. Controlling the volume mixing ratio of powder to binder within the range of 50:50 to 65:35 ensures sufficient binder filling between the powder particles, giving the feed a certain degree of fluidity and plasticity, while preventing excessive binder from affecting the performance of the final part. The mixing process is carried out in a mixer or extruder, with the mixing temperature controlled between 150℃ and 200℃ for 1 to 3 hours. This temperature and time range allows the binder to fully melt and mix uniformly with the powder, forming a stable and high-performance injection molding feed.

[0027] Furthermore, the injection molding feedstock prepared by the method, after injection molding, degreasing and sintering, results in 316 material parts with a hardness of HV 180 and above.

[0028] The beneficial effect of adopting the above-mentioned further solution is that the injection molding feedstock prepared by this method, after undergoing a complete injection molding, debinding, and sintering process, can produce 316 material parts with a hardness of HV 180 or higher. This hardness level is significantly improved compared to traditional MIM 316L parts, and can meet the application needs of more precision structural parts with higher requirements for wear resistance, load-bearing capacity, and strength.

[0029] Furthermore, the molybdenum oxide coating has a continuous or discontinuous nano- to submicron-scale coating morphology on the surface of 316L powder particles, and can inhibit abnormal grain growth and promote densification during subsequent sintering, thereby improving the hardness of the sintered body.

[0030] The beneficial effects of adopting the above-mentioned further solution are that the molybdenum oxide coating, uniformly coated on the surface of 316L powder particles at the nanometer to submicron scale, plays an important role in the sintering process regardless of its continuous or discontinuous morphology. It effectively inhibits abnormal grain growth, avoiding the decrease in hardness and strength caused by coarse grains; simultaneously, it promotes the densification process during sintering, reduces the generation of defects such as porosity, and further improves the density of the sintered body. Through the combined effect of these two mechanisms, the hardness of the sintered body is significantly improved, resulting in 316L parts with superior performance.

[0031] Compared with the prior art, the present invention has the following significant advantages and beneficial effects:

[0032] 1. By using liquid-phase adsorption and in-situ thermal decomposition technology, the initial molecular-level dispersion and nanoscale uniform coating of molybdenum oxide on the surface of 316L powder were achieved, fundamentally solving the problem of poor uniformity in traditional mechanical mixing methods.

[0033] 2. The feedstock prepared by this method, after being processed by standard MIM process, can produce 316 material parts with a hardness that can stably reach HV 180 or higher, and can even reach HV 220-250. This is more than 50% higher than the hardness of traditional MIM 316L, which can meet the application requirements of higher strength.

[0034] 3. Uniformly distributed nano-molybdenum oxide particles can effectively pin grain boundaries during sintering, inhibiting abnormal growth of austenite grains and obtaining a fine sintered structure, thereby improving hardness and strength. At the same time, in the reducing atmosphere of high-temperature sintering, some molybdenum oxide can be reduced to molybdenum atoms and diffuse into the austenite matrix of 316L, forming solid solution strengthening of molybdenum. The incompletely reduced fine molybdenum oxide particles are dispersed as a second phase, playing a dispersion strengthening role. Furthermore, uniformly distributed nanoparticles may promote the growth of sintering necks by affecting material migration mechanisms such as surface diffusion, which helps to obtain sintered bodies with higher density. Attached Figure Description

[0035] Figure 1 This is a process flow diagram of the method described in this invention. Detailed Implementation

[0036] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0037] like Figure 1 The present embodiment provides an in-situ injection molding feeding method for producing sintered high-hardness 316 material, including the following steps:

[0038] S1. Solution preparation: Dissolve ammonium molybdate in deionized water or a low-boiling-point organic solvent to prepare an ammonium molybdate solution of a certain concentration.

[0039] S2. Powder surface treatment: 316L stainless steel powder with an average particle size of 5-25μm is added to the above ammonium molybdate solution. Under stirring, the ammonium molybdate molecules or ions are uniformly adsorbed and dispersed on the surface of the 316L powder particles.

[0040] S3. Vacuum Drying and In-situ Conversion: The above slurry is subjected to vacuum drying. Under vacuum and heating conditions, the solvent evaporates, and at the same time, the ammonium molybdate adhering to the powder surface undergoes thermal decomposition, converting in-situ into molybdenum oxide (mainly MoO3), thereby forming a uniform, dense or discontinuous nano- to submicron-scale molybdenum oxide coating layer on the surface of each 316L powder particle.

[0041] S4. Feed preparation: 316L composite powder coated with molybdenum oxide and multi-component organic binder are mixed at a certain volume ratio in a mixer or twin-screw extruder at high temperature, then granulated to obtain a uniform feed for injection molding with good flowability.

[0042] The above-mentioned injection molding feeding method utilizes the good solubility of ammonium molybdate in water or ethanol, allowing it to be uniformly dispersed in the solvent in a molecular or ionic state. When 316L metal powder is added, through sufficient mechanical and physical action, molybdenum-containing species are uniformly adhered to the surface of each powder particle due to physical adsorption, electrostatic adsorption, and other forces; the subsequent vacuum drying process is crucial. The vacuum environment lowers the boiling point of the solvent, allowing for rapid solvent removal at lower temperatures while preventing powder oxidation. At a certain heating temperature (80-150℃), the ammonium molybdate adhering to the powder surface undergoes a thermal decomposition reaction. The process is roughly as follows:

[0043] (NH4)6Mo7O 24 ·4H₂O→7MoO₃+6NH₃↑+ 7H₂O↑

[0044] The gaseous NH3 and H2O produced during decomposition are removed by a vacuum system, while the solid MoO3 is directly deposited in situ onto the surface of the 316L powder particles. Because the precursor is uniformly adhered, the resulting MoO3 coating is also uniform.

[0045] The MoO3-coated composite powder exhibits good contact between the organic phase and the powder surface during mixing with the binder. The following key changes occur during subsequent sintering:

[0046] 1. Degreasing stage: The adhesive is completely removed.

[0047] 2. Early stage of sintering: As the temperature rises, sintering necks begin to form at the contact points between particles. The MoO3 nanolayer on the surface begins to undergo a reduction reaction with hydrogen: MoO3 + 3 H2 → Mo + 3 H2O. This reaction occurs on the powder surface and near the particle contact points.

[0048] 3. Mid-to-late stage of sintering: The newly formed molybdenum atoms are highly reactive. On the one hand, they rapidly dissolve into the 316L matrix through surface diffusion and grain boundary diffusion, achieving microalloying of molybdenum. On the other hand, molybdenum atoms that do not diffuse in time may aggregate to form finer particles, or some incompletely reduced MoO3 / MoO2 particles may remain. These nanoscale heterogeneous phases are very effectively pinned to the moving grain boundaries, strongly inhibiting the growth of austenite grains.

[0049] 4. Densification and property formation: Under the combined effect of fine grain strengthening, solid solution strengthening and possible dispersion strengthening, the final sintered body maintains good corrosion resistance while significantly improving hardness.

[0050] To demonstrate the in-situ injection molding feeding method for producing sintered high-hardness 316 stainless steel and to improve the hardness of 316, the chemical composition of the 316L stainless steel powder used in the following examples is as follows: C≤0.03%, Si≤1.0%, Mn≤2.0%, P≤0.025%, S≤0.01%, Cr:16.0-18.0%, Ni:10.0-14.0%, Mo:2.0-3.0%, Fe balance. The powder morphology is nearly spherical, with an average particle size D50 of approximately 15 μm and a particle size distribution D90 ≤ 38 μm. The binder uses a common wax-based multi-component system: 60wt% paraffin (PW), 35wt% polypropylene (PP), and 5wt% stearic acid (SA).

[0051] Example 1

[0052] S1. Preparation of ammonium molybdate solution: Weigh 10g of analytical grade ammonium molybdate tetrahydrate, dissolve it in 90g of deionized water, and stir until completely transparent to obtain a 10wt% ammonium molybdate aqueous solution.

[0053] S2. Powder Surface Treatment: Weigh 200g of 316L powder and pour it into the above 100g solution. Transfer the mixed slurry to a container equipped with a mechanical stirrer and an ultrasonic probe. First, mechanically stir at 200rpm while simultaneously turning on the ultrasonic probe at a power of 300W and a frequency of 40kHz for 60 minutes to ensure the slurry is uniform and free of sediment.

[0054] S3. Vacuum Drying and Conversion: Transfer the slurry into the tray of the vacuum drying oven and level it. Set the drying temperature to 120℃, turn on the vacuum system, and maintain the pressure in the drying chamber below 5 kPa. After drying for 8 hours, remove the slurry. At this point, the powder is grayish-black, loose, and without lumps. X-ray diffraction (XRD) analysis confirmed that the powder surface is mainly composed of the MoO3 phase.

[0055] S4. Feed Preparation: Weigh 150g of the treated powder (approximately equivalent to 200g of untreated powder) at a volume ratio of 60:40 (powder loading approximately 62 vol%) and pre-mix it with 100g of binder (PW:PP:SA = 60:35:5). Add the mixture to the feed inlet of a twin-screw internal mixer (screw diameter 30mm, L / D = 40). Set the mixing temperature as follows: Zone 1 160℃, Zone 2 170℃, Zone 3 175℃, and die head 170℃. The screw speed is 80 rpm. After mixing for 90 minutes, extrude strips from the die head, cool in a water bath, and then pelletize to obtain cylindrical feed pellets with a diameter of approximately 3mm and a length of approximately 4mm. The feed pellets have a uniform appearance and a glossy finish.

[0056] S5. Injection, defatting, and sintering:

[0057] First, injection molding: using a regular plastic injection molding machine, the barrel temperature is 170-180℃, the mold temperature is 40℃, and the injection pressure is 80MPa, the feed material is injected into standard tensile test specimens and hardness test block blanks.

[0058] Secondly, solvent degreasing: the blank is immersed in n-heptane and soaked at 40°C for 4 hours to remove most of the paraffin and stearic acid.

[0059] Finally, hot degreasing and sintering: The degreased blanks are placed in a pusher-type sintering furnace. Under the protection of flowing high-purity nitrogen, the temperature is increased to 450°C at 2°C / min and held for 60 minutes to completely pyrolyze and remove polypropylene. Then, the atmosphere is switched to hydrogen (dew point < -40°C), and the temperature is increased to 1380°C at 5°C / min and held for 120 minutes, followed by furnace cooling.

[0060] The sintered samples prepared in the above manner were ground and polished, and their density (Archimedes displacement method) and Vickers hardness (HV1, load 1 kgf, holding pressure 15 s) were measured. The results showed that the sintered density was 98.5% of the theoretical density, and the hardness value was HV 215.

[0061] Example 2

[0062] This example mainly examines the effect of different ammonium molybdate concentrations on 316L powder:

[0063] S1. Preparation of solutions: Prepare 5wt%, 20wt%, and 30wt% ammonium molybdate aqueous solutions respectively.

[0064] S2. Powder Processing: Weigh 100g of 316L powder and add 100g of each of the three concentrations mentioned above (i.e., powder to solution mass ratio 1:1). As a high-proportion control, add 80g of an aqueous solution containing 20g of ammonium molybdate (powder:ammonium molybdate = 100:20). Disperse all powders using a high-speed shear disperser at 5000rpm for 30 minutes.

[0065] S3. Vacuum drying: The conditions are the same as in Example 1, with a temperature of 100°C, a vacuum degree of <8kPa, and a drying time of 10 hours.

[0066] S4. Feeding and Sintering: The powder loading was 60 vol%, and the binder system was the same as in Example 1. Mixing conditions: temperature 155-165℃, time 80 minutes. Injection molding, debinding, and sintering processes were the same as in Example 1, with a sintering temperature of 1360℃.

[0067] Analysis of the above three ammonium molybdate aqueous solutions shows that...

[0068] Treatment with a 5wt% solution resulted in a thinner MoO3 coating on the powder surface and a sintering hardness of HV 185.

[0069] Treatment with 20wt% solution: The coating layer is moderate, and the sintering hardness is HV 225;

[0070] Treatment with a 30wt% solution resulted in a thicker coating, but the powder was slightly clump-bound. After dispersion, the sintering hardness was HV235, but the dimensional shrinkage fluctuated slightly.

[0071] High ratio (100:20) treatment: powder agglomeration is severe, feeding and mixing uniformity is poor, sintered body has uneven micro-composition, hardness HV 205, but large fluctuation.

[0072] Therefore, an ammonium molybdate solution concentration in the range of 10-30 wt% yields good results, preferably 10-20 wt%. The mass ratio of powder to ammonium molybdate should be controlled at 100:(0.5-5). In this example, a 20 wt% solution (i.e., 100:2) provides a balanced treatment effect.

[0073] Example 3

[0074] This embodiment mainly analyzes the influence of vacuum drying process parameters: taking the slurry treated with 10wt% ammonium molybdate solution in Example 1 as the object, different drying conditions are compared.

[0075] Condition A: 120℃, vacuum degree <5kPa, drying for 8 hours;

[0076] Condition B: 80℃, vacuum degree <3kPa, drying for 15 hours;

[0077] Condition C: 150℃, vacuum degree <10kPa, drying for 4 hours;

[0078] Condition D: Drying in air at 120℃ and normal pressure for 12 hours;

[0079] Condition E: 180℃, vacuum degree <10kPa, drying for 2 hours;

[0080] The results showed that the powders obtained under conditions A, B, and C were all loose and uniform in color. XRD analysis showed that the degree of MoO3 crystallinity was A>C>B. The corresponding sintering hardnesses were HV 215, HV 205, and HV 210, respectively. This indicates that effective conversion can be achieved under vacuum conditions of 80-150℃. Under condition D (normal pressure drying): the powder surface was uneven in color, with some parts turning yellow (possibly due to the formation of some undesirable molybdates or more complex oxidation), and there was slight agglomeration. The sintering hardness was only HV 175, and the properties were highly dispersible, indicating that the vacuum environment is crucial for preventing the formation of impurity phases and ensuring the purity and uniformity of the reaction. Under condition E (excessively high temperature): the powder was severely sintered and agglomerated into hard lumps that were difficult to break, indicating that the excessively high temperature caused local melting of MoO3 or intermediate products or premature reaction with the matrix, destroying the powder's dispersibility. This powder could not be used for subsequent feed preparation. Therefore, the vacuum drying temperature is preferably 80-150℃, more preferably 100-130℃; the vacuum degree is preferably below 10kPa, more preferably below 5kPa; the time is adjusted according to the temperature and the amount of material to ensure that the solvent evaporates completely and the reaction is complete, usually 4-12 hours.

[0081] Example 4

[0082] This embodiment analyzes the compatibility of different binder systems and mixing processes: powder treated with the method of Example 1 (10wt% solution treatment).

[0083] 1) Wax-based binder system 1: Same as in Example 1, such as PW, PP, SA, etc.

[0084] 2) Polyoxymethylene-based adhesive system 2: 73wt% polyoxymethylene (POM), 25wt% high-density polyethylene (HDPE), 2wt% adipic acid, as stabilizer and lubricant.

[0085] 3) Comparison of Refining Conditions:

[0086] For the internal mixer: a Hacker torque rheometer (mixing chamber volume 60cm³) was used, and the rotor speed was set to 60rpm;

[0087] For System 1: Mixing temperature 175℃, time 60 minutes;

[0088] For System 2: Mixing temperature 185℃ (POM melting point is about 165℃), time 50 minutes;

[0089] For twin-screw extruders: Same setup as in Example 1.

[0090] 4) Feed evaluation and sintering: Evaluate the mixing uniformity and melt flow rate of the feed. After injection molding, the appropriate debinding process is adopted (system 1 is the same as before; system 2 adopts nitric acid catalytic debinding + thermal debinding), and finally sintered in hydrogen at 1360℃ for 120 minutes.

[0091] The results analysis shows that both binder systems are well compatible with the surface-modified powder of this invention, producing uniform feedstocks. The wax-based system (1) has a fast degreasing rate and a simple process; the POM-based system (2) has high injection molding strength and potentially better dimensional accuracy. The hardness of the sintered parts obtained from the two systems is not significantly different, both falling within the HV range of 210-220. Therefore, the method of this invention is universally applicable to common MIM binder systems. The mixing temperature needs to be set according to the melting point of the binder, generally 10-30°C higher than the melting point of the main binder components, for 1-3 hours to ensure that the powder and binder are fully wetted and mixed.

[0092] Example 5

[0093] This example analyzes the effect of different sintering processes on the final hardness:

[0094] Using the feed prepared in Example 1, with a fixed degreasing process, the sintering regime was varied:

[0095] Procedure A: Hydrogen, 1350℃ × 180min;

[0096] Procedure B: Hydrogen, 1380℃ × 120min;

[0097] Procedure C: Hydrogen, 1420℃ × 60min;

[0098] System D: Vacuum (10) - (2 Pa) → Argon partial pressure sintering, 1380℃ × 120min;

[0099] System E: Nitrogen-hydrogen mixture (95 / 5), 1380℃ × 120min;

[0100] The results analysis shows that: Regulation A yielded a density of 98.0%, a hardness of HV 230, and the finest grains; Regulation B yielded a density of 98.5%, a hardness of HV 215, and the best overall performance; Regulation C yielded a density of 99.0%, but the grains were significantly coarser, and the hardness decreased to HV 195; Regulation D yielded a density of 98.2%, a hardness of HV 210, close to Regulation B, indicating that a reducing atmosphere is not absolutely necessary, but a hydrogen atmosphere is more economical and conducive to the reduction of MoO3; Regulation E yielded a density of 97.5%, with slight surface oxidation and a hardness of HV 185, indicating that a weakly reducing or neutral atmosphere is not conducive to the full reduction of surface molybdenum oxide and the diffusion of molybdenum. Therefore, using a hydrogen or strongly reducing atmosphere, sintering at 1360-1400℃ for 1-3 hours yields the optimal combination for high hardness. While excessively high temperatures can increase density, they lead to grain coarsening, negating the strengthening effect of the second phase and ultimately hindering hardness improvement.

[0101] In addition, the following comparative analysis was also conducted in this implementation:

[0102] 1) Comparative Example 1: Using the conventional MIM 316L method, untreated raw 316L powder was directly used with the same binder as in Example 1 (volume ratio 60:40). Sintered samples were prepared under the same process conditions (mixing, injection molding, debinding, and hydrogen sintering at 1380°C for 120 minutes). The sintered density of the 316 material was 98.2%, and the hardness was HV 155.

[0103] 2) Comparative Example 2: Using a mechanical mixing method, 200g of original 316L powder was weighed and mechanically dry-mixed with approximately 1.5g of molybdenum trioxide (MoO3) nanoparticles (equivalent to the molybdenum content of ammonium molybdate in Example 1) using a three-dimensional mixer (mixing for 4 hours). Then, it was mixed with a binder at a volume ratio of 60:40, granulated, and the subsequent processes were the same as in Example 1. The resulting sintered sample of 316 material had a density of 98.0% and a hardness of HV 170, showing an improvement in hardness, but still significantly lower than that of Example 1 (HV 215).

[0104] 3) Comparative Example 3: A solution was prepared and powder was processed according to the steps of Example 1 without vacuum drying and thermal decomposition after mixing. However, after stirring, vacuum heating and drying were not performed; instead, centrifugation and natural air drying were used. At this time, ammonium molybdate adhered to the powder surface in an undecomposed crystalline form. This powder was then mixed with a binder. In the early stage of debinding the injection molding blank (low temperature stage), ammonium molybdate decomposes, but the decomposition product gas is generated inside the blank, which may cause defects. The resulting 316 material blank after debinding showed visible cracks, low density (96.5%), a hardness of HV 160, and poor surface quality after sintering. This indicates that directly introducing the precursor without a controlled vacuum drying in-situ conversion step will have a destructive impact on the entire MIM process chain.

[0105] The data above clearly show that the sintering hardness of the feed prepared by the in-situ method of the present invention (Examples 1 and 2) is significantly higher than that of the traditional process (Comparative Example 1); and the hardness improvement effect is significantly better than that of the simple mechanical mixing method (Comparative Example 2), proving the technical advantages of in-situ uniform coating; the key process parameters in the present invention, such as ammonium molybdate concentration and vacuum drying, have an important impact on the final performance; inappropriate processes (such as atmospheric pressure drying, Example 3-condition D) will lead to performance degradation or even deterioration.

[0106] In summary, this invention constructs a uniform nano-molybdenum oxide coating layer on the surface of 316L powder through a process of uniform adsorption with ammonium molybdate solution followed by in-situ thermal decomposition and conversion via vacuum drying. Through the synergistic effects of grain refinement, solid solution strengthening, and dispersion strengthening of the nano-molybdenum oxide coating layer, the hardness of the final MIM 316L parts achieves a qualitative leap, increasing from the traditional HV120-160 to over HV180, and even reaching HV220-250. This significantly expands the application prospects of MIM 316L materials in the field of high-performance components.

[0107] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An in-situ injection molding feeding method for producing sintered high-hardness 316 material, characterized in that, Includes the following steps: S1. Dissolve ammonium molybdate in a solvent to form an ammonium molybdate solution; S2. Add 316L stainless steel powder to the ammonium molybdate solution and stir to make the ammonium molybdate uniformly adhere to the surface of the 316L powder particles. S3. Vacuum dry the mixture obtained in step S2 so that the ammonium molybdate adhering to the surface of the powder particles is thermally decomposed in situ during the drying process and transformed into a uniformly distributed molybdenum oxide coating. S4. The 316L powder coated with molybdenum oxide is mixed with a binder to prepare the injection molding feed.

2. The in-situ injection molding feeding method for producing sintered high-hardness 316 material according to claim 1, characterized in that, In step S1, the concentration of the ammonium molybdate solution is 5 wt% to 30 wt%.

3. The in-situ injection molding feeding method for producing sintered high-hardness 316 material according to claim 2, characterized in that, In step S2, the mass ratio of the 316L stainless steel powder to ammonium molybdate is 100:0.5-5.

4. The in-situ injection molding feeding method for producing sintered high-hardness 316 material according to claim 3, characterized in that, The stirring process in step S2 is carried out under ultrasonic assistance or high-speed shearing conditions, and the stirring time is 30 minutes to 2 hours to ensure uniform dispersion and adhesion of the ammonium molybdate solution.

5. The in-situ injection molding feeding method for producing sintered high-hardness 316 material according to claim 1, characterized in that, In step S3, the vacuum drying temperature is 80°C to 150°C, the vacuum degree is less than 10 kPa, and the drying time is 4 to 12 hours.

6. The in-situ injection molding feeding method for producing sintered high-hardness 316 material according to claim 1, characterized in that, The average particle size of the 316L stainless steel powder is 5 μm to 25 μm, and the particle size distribution D90 ≤ 40 μm.

7. The in-situ injection molding feeding method for producing sintered high-hardness 316 material according to claim 1, characterized in that, In step S4, the adhesive comprises a mixture of at least two components selected from polyoxymethylene, polyethylene, polypropylene, paraffin wax, and stearic acid; the volume mixing ratio of the 316L powder coated with molybdenum oxide to the adhesive is 50:50 to 65:

35.

8. The in-situ injection molding feeding method for producing sintered high-hardness 316 material according to claim 7, characterized in that, The mixing process in step S4 is carried out in a mixer or extruder at a mixing temperature of 150°C to 200°C for 1 to 3 hours.

9. The in-situ injection molding feeding method for producing sintered high-hardness 316 material according to claim 1, characterized in that, The injection molding feedstock prepared by the method, after injection molding, degreasing and sintering, produces 316 material parts with a hardness of HV 180 and above.

10. The in-situ injection molding feeding method for producing sintered high-hardness 316 material according to claim 1, characterized in that, The molybdenum oxide coating is a continuous or discontinuous nano- to submicron-scale coating layer on the surface of 316L powder particles. In the subsequent sintering process, it can inhibit abnormal grain growth and promote densification to improve the hardness of the sintered body.

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