High-fluidity MIM metal feed and forming process thereof

Abstract

The invention belongs to the technical field of powder metallurgy, and particularly relates to a high-fluidity MIM metal feed and a forming process thereof.The high-fluidity MIM metal feed is composed of the following materials: metal powder: spherical metal powder (a titanium alloy TC4 material), polycaprolactone and graphene nanosheets; wherein the volume ratios are 68-72 vol%, 0.5-2 wt% (based on the mass of the metal powder) and 0.1-0.5 wt% (based on the mass of the metal powder) respectively; a powder oxide layer is removed through vacuum plasma cleaning (300-500 W, argon atmosphere), a metal core-PCL / graphene shell structure is prepared in combination with ultrasonic-assisted in-situ polymerization, lubrication and adhesion enhancement are synchronously achieved, the powder oxygen content is controlled to be smaller than 500 ppm, and sintering oxidation defects are reduced.

Description

Technical Field

[0001] This invention belongs to the field of powder metallurgy technology, specifically a high-flowability MIM metal feedstock and its forming process. Background Technology

[0002] Metal injection molding (MIM) technology, as a key branch of powder metallurgy, has been widely used in aerospace (such as titanium alloy TC4 engine structural parts), medical (such as artificial joints), and electronic (such as micro connectors) fields due to its near-net-shape forming advantage. It can efficiently produce complex, precise and high-performance metal parts.

[0003] A patent application with publication number CN107127348A discloses a method for preparing metal powder for MIM (Metal Injection Molding). The method includes high-pressure water atomization, reduction, and electrolysis to obtain fine metal powder, followed by a shaping process to obtain near-spherical powder. Further processing, including powder classification and sieving, produces a fine, uniform, near-spherical powder with high bulk density and good flowability. This method offers high feed loading, low viscosity, high production efficiency, low cost, easy process control, and good product consistency; it is suitable for the technical requirements of MIM molding and near-net-shape forming processes.

[0004] In the current environment, existing technologies mostly use simple surface modification such as silane coupling agents, which only solve the compatibility between powder and binder, and cannot simultaneously achieve "reducing particle friction (improving flowability)" and "enhancing interfacial bonding (improving green strength)". Moreover, the powder oxygen content control accuracy is low (often >800ppm), which easily leads to oxidation inclusions in the sintered product, affecting the stability of mechanical properties.

[0005] Therefore, the present invention provides a high-flowability MIM metal feedstock and its forming process. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, at least one technical problem raised in the background art is solved.

[0007] The technical solution adopted by this invention to solve its technical problem is: a high-flowability MIM metal feedstock, which is composed of the following materials: Metal powder: spherical metal powder (titanium alloy TC4 material), polycaprolactone, and graphene nanosheets; wherein, the volume percentages are 68-72 vol%, 0.5-2 wt% (based on metal powder mass), and 0.1-0.5 wt% (based on metal powder mass), respectively; the mass percentages are 88-92 wt%, 0.04-0.18 wt% (based on the total mass of the metal feed), and 0.01-0.05 wt% (based on the total mass of the metal feed), respectively; the metal powder system ultimately exists in the core-shell structure of (metal core-PCL graphene shell), wherein the proportions of PCL and graphene are calculated based on the original metal powder mass. Since the amount added is extremely small, the impact on the overall volume percentage of the feed is negligible, and the overall volume of the feed is still mainly composed of the metal core (original spherical metal powder); Composite binder: polybutylene succinate, tributyl citrate, nano-hydroxyapatite, trace additives (trace antioxidants, dispersants); volume percentages are 40-50 wt%, 25-35 wt%, 1-3 wt%, and 12-34 wt% (to be made up to 100%); mass percentages are 4.2-8.3 wt%, 2.6-5.8 wt%, 0.1-0.5 wt%, and 1.3-5.7 wt%. The volume percentages of metal powder and composite binder were 68–72 vol% and 28–32 vol%, respectively; their mass percentages were 88–92 wt% and 8–20 wt%, respectively.

[0008] Preferably, a molding process for a high-flowability MIM metal feedstock is characterized in that: the molding process is applicable to the high-flowability MIM metal feedstock described above, and the process includes the following steps: 1. Core-shell structure modification of metal powders; constructing a "lubricating-adhesive bifunctional layer" on the powder surface to simultaneously improve flowability and interfacial bonding. Methods include: Select spherical metal powder (titanium alloy TC4) with D50=3-8μm, and first clean it with vacuum plasma (power 300-500W, argon atmosphere) to remove the surface oxide layer and impurities, and expose the fresh metal surface. Using ultrasound-assisted in-situ polymerization technology, 0.5-2 wt% of polycaprolactone (PCL, a biodegradable resin) is mixed with 0.1-0.5 wt% of graphene nanosheets (to enhance conductivity and lubricity). In an ultrasonic field (frequency 20-40 kHz) at 60-80℃, PCL is polymerized on the powder surface to form a coating layer 50-100 nm thick, with the graphene sheets uniformly embedded in the shell layer. Keep the temperature at 80-100℃ in a vacuum oven for 1-2 hours to completely solidify the shell layer and form a core-shell structure powder of "metal core-PCL / graphene shell". The oxygen content is controlled below 500ppm. Design of a composite binder system (biodegradable elastomer-biobased plasticizer); improving the binder's elastic recovery (preventing green deformation) and environmental friendliness (degreasing and VOC-free) on the basis of low viscosity (high fluidity), achieved through the following methods: Replacing traditional PP / PE with 40-50 wt% polybutylene succinate (PBS, a biodegradable elastomer with a melting point of 110-120℃) results in a higher elastic modulus (1.5-2 GPa) than PP and a lower melt viscosity (500-800 mPa·s at 180℃). Adding 25-35 wt% tributyl citrate (TBC, bio-based) to replace traditional DOP improves its compatibility with PBS by 30% and prevents the release of benzene VOCs during defatting. Adding 1-3 wt% nano-hydroxyapatite (n-HA) as a "powder-binder interface enhancer" allows its hydroxyl groups to form hydrogen bonds with the ester groups of the PCL shell, further improving the uniformity of feed dispersion. The volume ratio of metal powder to composite binder is increased to 68-72 vol% (the traditional maximum is 65 vol%), while maintaining low viscosity (<1000 mPa·s at 190℃). (Variable pitch twin-screw + ultrasonic-assisted) feeding and mixing process; reduces "powder breakage" and "local agglomeration" in traditional mixing, achieving 100% uniform dispersion of core-shell powder in the binder. Method: A customized "variable pitch twin-screw extruder" is adopted, with the screw divided into three sections: Feed section (40mm pitch): Low shear conveying reduces damage to the core-shell powder shell layer; Melting section (pitch 25mm): medium shear mixing, combined with an ultrasonic vibration device on the outer wall of the barrel (power 100-200W, frequency 20kHz), using the ultrasonic cavitation effect to break up local agglomeration; Homogenization section (15mm pitch): High shear extrusion ensures uniform embedding of n-HA into the interface; Screw speed 20-30 rpm (lower than the traditional 30-50 rpm), feeding rate 3-8 kg / h, barrel temperature gradient 110-130-150℃ (matching the low melting point characteristics of PBS); Using "low-temperature air pelletizing" (outlet temperature 10-15℃) instead of underwater pelletizing reduces water absorption by the binder on the pellet surface, resulting in a more efficient pellet production process. 1.5-2mm "dumbbell-shaped particles" (rounded ends and slightly thinner in the middle) improve the barrel conveying efficiency during injection molding; (Gradient holding pressure + dynamic mold temperature) injection molding process; solves the pain points of "incomplete filling" and "green warpage" in high-load feeding injection molding, improves the molding accuracy of complex parts, and achieves this through the following methods: The "low-temperature melting" strategy is adopted, with the front section of the barrel at 140-150℃, the middle section at 150-160℃, the rear section at 160-170℃, and the nozzle at 170-180℃ (20-30℃ lower than the traditional method), to reduce excessive degradation of PBS. Based on the mold cavity structure, pressure holding is divided into 3 stages: Initial filling phase (0-2s): High pressure 80-100MPa, rapidly filling thin-walled areas (<1mm). Mid-filling stage (2-5s): Medium pressure 50-70MPa, to reduce flash in thick-walled areas (>5mm); End of pressure holding period (5-10s): Low pressure 30-40MPa to compensate for the elastic contraction of PBS; Through the built-in partitioned heating tubes of the mold, dynamic cooling is achieved from 60-70℃ during filling (to improve fluidity) to 30-40℃ during pressure holding (to accelerate binder solidification), with a cooling rate of 5-8℃ / s and green body warpage controlled within 0.1mm / m. A combined process of pulsed atmosphere degreasing and low-temperature sintering solves the problems of long degreasing time (>10h) and high residual carbon (>0.1%) in traditional degreasing methods, while reducing degreasing deformation of green bodies with high loading. The method is as follows: Pulsed fat removal design: Pre-degreasing stage (30-200℃): Pulsed nitrogen gas is introduced (pressure 0.1-0.3MPa, pulse frequency 10-20 times / min) to accelerate the volatilization of TBC and PBS by using airflow disturbance, shortening the degreasing time to 3-4h (traditional 6-8h). High-temperature degreasing stage (200-450℃): switch to a hydrogen-argon mixed atmosphere (volume ratio 1:9), pulse frequency 5-10 times / min, use hydrogen to reduce residual carbon, and reduce the residual carbon content to below 0.05%; Low-temperature sintering optimization: Heating rate control: 5-8℃ / min (lower than the traditional 10-15℃ / min), reducing cracking of green billets due to thermal stress; Sintering temperature: 50-80℃ lower than traditional (e.g., TC4 titanium alloy sintering temperature 1150-1200℃). By utilizing the "pore-forming effect" of PCL in the core-shell structure, the diffusion of metal particles is promoted, and the sintering density still reaches 96-98% (traditional above 95%). After degreasing, the green billets are directly fed into the sintering furnace (without cooling), reducing the risk of oxidation exposure and lowering energy consumption by 20%.

[0009] Preferred, preferred, the The adaptation formula for the structural modification of metal powder (core-shell) is as follows: in, PCL / graphene composite shell thickness (nm), target range 50-100nm; Polycaprolactone (PCL) quality ( Dosage: 0.5–2 wt% (based on metal powder mass); The graphene dispersion coefficient, due to the improved uniformity resulting from graphene embedding in the shell, ranges from 1.05 to 1.15 (graphene dosage 0.1–0.5 wt%, higher dosage increases this value). The larger); PCL density ( / At room temperature, it is 1.14. / ; Specific surface area of ​​metal powder ( / The value is calculated based on spherical powder with a D50 of 3–8 μm, and is taken as 0.3–1.0. / / ; Original spherical metal powder mass ( ); This formula controls the shell thickness, which is determined by the "volume coverage" of PCL—the volume of PCL is calculated based on its mass and density, then divided by the total specific surface area of ​​the metal powder (i.e., the total coverage area), while also incorporating the graphene dispersion coefficient. The correction (graphene embedding makes the shell more uniform and slightly increases the thickness) brings the result to the 50-100nm range; in, : Final oxygen content of core-shell structured powder ( ) Initial oxygen content of metal powder ( The original titanium alloy TC4 powder is 800-1000 ; Plasma cleaning efficiency coefficient ( / (W・h)), with values ​​ranging from 0.8 to 1.2 under a vacuum argon atmosphere; Vacuum plasma cleaning power (W), range 300-500W; Plasma cleaning time (h); This formula controls the oxygen content of the powder. The oxygen content is reduced by removing the surface oxide layer through vacuum plasma cleaning. Higher cleaning power and longer cleaning time result in more thorough oxide layer removal. Therefore, the oxygen content is negatively correlated with power and time. <500 .

[0010] Preferably, the The adaptation formula for the design of the medium-composite adhesive system is as follows: in, : Feed melt viscosity (mPa·s), <1000 mPa·s at 190℃; The melt viscosity (mPa·s) of pure PBS matrix resin is 1500-1800 mPa·s at 190°C. Plasticizing synergy factor (unitless), PBS and TBC synergistically reduce viscosity, value is 1.2-1.5; The mass percentage of PBS in the adhesive (%) is 40-50%. The percentage of TBC in the adhesive (by mass) ranges from 5% to 35%. n—HA interface enhancement coefficient, n—HA improves compatibility, with a value of 0.05–0.1; n—The mass percentage of HA in the adhesive (%), ranging from 1% to 3%; The influence coefficient of the powder-to-binder ratio: an excessively high powder-to-binder ratio will increase stickiness; the value is 0.03-0.05. The volume ratio of metal powder to binder is 68-72 vol% (i.e., 68:32 to 72:28). Critical powder-to-binder ratio, taken as 0.65 (the highest powder-to-binder ratio in traditional processes, used as a benchmark); Molecular components: PBS (matrix) and TBC (plasticizer) work synergistically to reduce viscosity, while n-HA improves interfacial compatibility and further assists in viscosity reduction; Denominator: When the powder-to-binder ratio is higher than the traditional critical value (0.65), the powder bulk density will increase slightly, resulting in increased viscosity. Correction; Thus making <1000mPa・s, while achieving a high powder-to-binder ratio (68-72 vol%).

[0011] Preferably, the The appropriate formula for the medium (variable pitch twin-screw + ultrasonic-assisted) feeding and mixing process is: in, Uniformity of core-shell powder dispersion in the binder (%), target 100%; Dispersion correction factor (%·kg / (W·)) •h)), takes values ​​from 0.08 to 0.12; Feeding rate (kg / h), range 3-8 kg / h; Ultrasonic assisted power (W), range 100-200W; Twin screw speed ( ), range 20-30 ; This formula is for calculating the uniformity of feed dispersion. in, Particle diameter (mm), ranging from 1.5 to 2 mm; Pellet size factor (mm・m / s・K), with a value of 0.02–0.03; Low-temperature air pelletizing outlet temperature (°C), range 10-15°C; Linear velocity (m / s) corresponding to the rotational speed of the pelletizer, ranging from 0.8 to 1.2 m / s; Temperature unit conversion factor (°C → K); This formula is for calculating particle size.

[0012] Preferably, the The appropriate formula for the (gradient holding pressure + dynamic mold temperature) injection molding process is: in, Green body warpage (mm / m), value <0.1mm / m: Pressure holding fluctuation influence coefficient (mm / (m·MPa)), with a value of 0.002–0.003; Gradient pressure holding pressure fluctuation value (MPa), i.e. ; The influence coefficient of mold cooling rate (mm / (m・℃ / s)) ranges from 0.01 to 0.015. Dynamic cooling rate of the mold (°C / s), range 5-8°C / s; This formula is for calculating the warpage of green blanks.

[0013] Preferably, the The suitable formula for the (pulse-atmosphere degreasing-low-temperature sintering) synergistic process is: in, Total defatting time (h), target 3-4h; Degreasing efficiency coefficient (h・MPa・times / min), with a value of 0.4–0.6; Pulse-type nitrogen pressure (MPa), range 0.1–0.3 MPa; Pulse frequency (times / min), range 10-20 times / min; This formula is for calculating defatting time; in, Density (%) of the sintered product: 96-98%; Density coefficient (%・℃ / min・vol%), ranging from 0.025 to 0.03; Low-temperature sintering temperature (°C): 1150-1200°C for TC4 titanium alloy; Sintering heating rate (°C / min), document range 5-8°C / min; The volume ratio of metal powder to binder is 68-72 vol%. This formula is for calculating sintered density.

[0014] Preferably, the In the (ultrasound-assisted in situ polymerization) step, when the ultrasonic power is 20-25kHz, the polymerization time needs to be matched to 40-60min. When the ultrasonic power is increased to 30-40kHz, the polymerization time can be shortened to 25-35min.

[0015] Preferably, in the composite adhesive system, the molecular weight of polybutylene succinate (PBS) needs to be compatible with the plasticizing efficiency of tributyl citrate (TBC); when the number-average molecular weight of PBS is 80,000 to 100,000, the mass percentage of TBC in the adhesive needs to be controlled at 30 to 35 wt%; when the number-average molecular weight of PBS is increased to 120,000 to 150,000, the mass percentage of TBC needs to be increased to 32 to 35 wt% simultaneously.

[0016] Preferably, in the metal powder (core-shell) structure modification, the pretreatment of graphene nanosheets needs to be compatible with the PCL polymerization process; the graphene nanosheets need to be ultrasonically dispersed in a 0.5-1wt% polyethylene glycol (PEG-6000) solution for 30-40 minutes (ultrasonic power 150-200W) to remove the van der Waals forces between the graphene layers, so that the dispersed graphene particle size is controlled at 50-100nm; when the pretreated graphene nanosheets are mixed with PCL, the "graphene dispersion coefficient (…)" in the "PCL / graphene composite shell thickness formula" can be reduced. The P-value remains stable within the 1.10–1.12 range, reducing the impact of graphene agglomeration. Fluctuations (during reunion) It may drop to 1.02-1.05.

[0017] The beneficial effects of this invention are as follows: 1. The present invention discloses a high-flowability MIM metal feedstock and its molding process, which removes the oxide layer of powder by vacuum plasma cleaning (300-500W, argon atmosphere) and prepares a "metal core-PCL / graphene shell" structure by ultrasonic-assisted in-situ polymerization, thereby simultaneously achieving lubrication and bonding enhancement, and controlling the oxygen content of powder to <500ppm to reduce sintering oxidation defects.

[0018] 2. The high-flowability MIM metal feedstock and its molding process described in this invention use biodegradable PBS as the matrix resin, bio-based TBC as the plasticizer, and add n-HA to enhance interfacial bonding, replacing traditional fossil-based binders to achieve VOC-free degreasing; through the close packing of core-shell powder and the synergistic viscosity reduction of the binder, the powder-to-binder ratio is increased to 68-72 vol% while maintaining low viscosity, solving the problem of "poor flowability with high powder-to-binder ratio". Attached Figure Description

[0019] The invention will now be further described with reference to the accompanying drawings.

[0020] Figure 1 This is a flowchart of the metal powder "core-shell" structure modification system in this invention; Figure 2 This is a flowchart of the composite binder preparation + feeding mixing and granulation system in this invention; Figure 3 This is a flowchart of the injection molding + debinding-sintering synergistic system in this invention. Detailed Implementation

[0021] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0022] like Figures 1 to 3 As shown in the embodiment of the present invention, a high-flowability MIM metal feed is provided, which is composed of the following materials: Metal powder: Spherical metal powder (Titanium alloy TC4 material), polycaprolactone Graphene nanosheets Among them, the volume percentages are respectively =68-72 vol% =0.5-2wt% (based on metal powder mass) =0.1-0.5wt% (based on metal powder mass); the mass percentages are respectively =88-92wt%, =0.04—0.18wt% (based on the total mass of the metal feed) =0.01—0.05wt% (based on the total mass of the metal feed); the final metal powder system is composed of (metal core—PCL) The core-shell structure of the graphene shell exists, and the proportions of PCL and graphene are calculated based on the mass of the original metal powder. Since the amount added is very small, the impact on the overall volume of the feed can be ignored. The overall volume of the feed is still mainly composed of the metal core (original spherical metal powder). Composite adhesive: Polybutylene succinate Tributyl citrate Nano-hydroxyapatite Micro-adjuvants (Trace amounts of antioxidants and dispersants); the volume percentages are respectively =40-50wt% =25-35wt% =1-3wt% =12-34wt% (make up to 100%); the mass percentages are respectively =4.2-8.3wt%, =2.6-5.8 wt%, =0.1-0.5wt%, =1.3–5.7 wt%; The volume percentages of metal powder and composite binder were 68–72 vol% and 28–32 vol%, respectively; their mass percentages were 88–92 wt% and 8–20 wt%, respectively.

[0023] like Figures 1 to 3 As shown, a molding process for a high-flowability MIM metal feedstock is described above. This molding process is applicable to the high-flowability MIM metal feedstock described above, and the process includes the following steps: 1. Core-shell structure modification of metal powders; constructing a "lubricating-adhesive bifunctional layer" on the powder surface to simultaneously improve flowability and interfacial bonding. Methods include: Select spherical metal powder (titanium alloy TC4) with D50=3-8μm, and first clean it with vacuum plasma (power 300-500W, argon atmosphere) to remove the surface oxide layer and impurities, and expose the fresh metal surface. Using ultrasound-assisted in-situ polymerization technology, 0.5-2 wt% of polycaprolactone (PCL, a biodegradable resin) is mixed with 0.1-0.5 wt% of graphene nanosheets (to enhance conductivity and lubricity). In an ultrasonic field (frequency 20-40 kHz) at 60-80℃, PCL is polymerized on the powder surface to form a coating layer 50-100 nm thick, with the graphene sheets uniformly embedded in the shell layer. Keep the temperature at 80-100℃ in a vacuum oven for 1-2 hours to completely solidify the shell layer and form a core-shell structure powder of "metal core-PCL / graphene shell". The oxygen content is controlled below 500ppm. Traditional surface modification only addresses compatibility. This step, through a core-shell structure, simultaneously achieves "reduced particle friction (graphene lubrication)" and "enhanced binder bonding (PCL shell)," laying the foundation for balancing subsequent feed flowability and green strength.

[0024] Design of a composite binder system (biodegradable elastomer-biobased plasticizer); improving the binder's elastic recovery (preventing green deformation) and environmental friendliness (degreasing and VOC-free) on the basis of low viscosity (high fluidity), achieved through the following methods: Replacing traditional PP / PE with 40-50 wt% polybutylene succinate (PBS, a biodegradable elastomer with a melting point of 110-120℃) results in a higher elastic modulus (1.5-2 GPa) than PP and a lower melt viscosity (500-800 mPa·s at 180℃). Adding 25-35 wt% tributyl citrate (TBC, bio-based) to replace traditional DOP improves its compatibility with PBS by 30% and prevents the release of benzene VOCs during defatting. Adding 1-3 wt% nano-hydroxyapatite (n-HA) as a "powder-binder interface enhancer" allows its hydroxyl groups to form hydrogen bonds with the ester groups of the PCL shell, further improving the uniformity of feed dispersion. The volume ratio of metal powder to composite binder is increased to 68-72 vol% (the traditional maximum is 65 vol%), while maintaining low viscosity (<1000 mPa·s at 190℃). For the first time, biodegradable elastomers are combined with bio-based plasticizers to solve the industry problems of "overly thick feed under high powder loading" and "degreasing and environmental protection". At the same time, the introduction of n-HA breaks through the bottleneck of interfacial bonding force. (Variable pitch twin-screw + ultrasonic-assisted) feeding and mixing process; reduces "powder breakage" and "local agglomeration" in traditional mixing, achieving 100% uniform dispersion of core-shell powder in the binder. Method: A customized "variable pitch twin-screw extruder" is adopted, with the screw divided into three sections: Feed section (40mm pitch): Low shear conveying reduces damage to the core-shell powder shell layer; Melting section (pitch 25mm): medium shear mixing, combined with an ultrasonic vibration device on the outer wall of the barrel (power 100-200W, frequency 20kHz), using the ultrasonic cavitation effect to break up local agglomeration; Homogenization section (15mm pitch): High shear extrusion ensures uniform embedding of n-HA into the interface; Screw speed 20-30 rpm (lower than the traditional 30-50 rpm), feeding rate 3-8 kg / h, barrel temperature gradient 110-130-150℃ (matching the low melting point characteristics of PBS); Using "low-temperature air pelletizing" (outlet temperature 10-15℃) instead of underwater pelletizing reduces water absorption by the binder on the pellet surface, resulting in a more efficient pellet production process. 1.5-2mm "dumbbell-shaped particles" (rounded ends and slightly thinner in the middle) improve the barrel conveying efficiency during injection molding; By combining variable pitch screws with ultrasonic assistance, high dispersibility is achieved under low shear. Meanwhile, the dumbbell-shaped particle design is an industry first, solving the "bridging" problem in the traditional cylindrical particle conveying process.

[0025] (Gradient holding pressure + dynamic mold temperature) injection molding process; solves the pain points of "incomplete filling" and "green warpage" in high-load feeding injection molding, improves the molding accuracy of complex parts, and achieves this through the following methods: The "low-temperature melting" strategy is adopted, with the front section of the barrel at 140-150℃, the middle section at 150-160℃, the rear section at 160-170℃, and the nozzle at 170-180℃ (20-30℃ lower than the traditional method), to reduce excessive degradation of PBS. Based on the mold cavity structure, pressure holding is divided into 3 stages: Initial filling phase (0-2s): High pressure 80-100MPa, rapidly filling thin-walled areas (<1mm). Mid-filling stage (2-5s): Medium pressure 50-70MPa, to reduce flash in thick-walled areas (>5mm); End of pressure holding period (5-10s): Low pressure 30-40MPa to compensate for the elastic contraction of PBS; Through the built-in partitioned heating tubes of the mold, dynamic cooling is achieved from 60-70℃ during filling (to improve fluidity) to 30-40℃ during pressure holding (to accelerate binder solidification), with a cooling rate of 5-8℃ / s and green body warpage controlled within 0.1mm / m. By combining gradient pressure holding with dynamic mold temperature, and targeting the rheological characteristics of high-load biodegradable feedstock, the system achieves synergy between "rapid filling" and "low warpage," breaking through the molding bottleneck of complex thin-walled parts (wall thickness 0.8-1mm).

[0026] A combined process of pulsed atmosphere degreasing and low-temperature sintering solves the problems of long degreasing time (>10h) and high residual carbon (>0.1%) in traditional degreasing methods, while reducing degreasing deformation of green bodies with high loading. The method is as follows: Pulsed fat removal design: Pre-degreasing stage (30-200℃): Pulsed nitrogen gas is introduced (pressure 0.1-0.3MPa, pulse frequency 10-20 times / min) to accelerate the volatilization of TBC and PBS by using airflow disturbance, shortening the degreasing time to 3-4h (traditional 6-8h). High-temperature degreasing stage (200-450℃): switch to a hydrogen-argon mixed atmosphere (volume ratio 1:9), pulse frequency 5-10 times / min, use hydrogen to reduce residual carbon, and reduce the residual carbon content to below 0.05%; Low-temperature sintering optimization: Heating rate control: 5-8℃ / min (lower than the traditional 10-15℃ / min), reducing cracking of green billets due to thermal stress; Sintering temperature: 50-80℃ lower than traditional (e.g., TC4 titanium alloy sintering temperature 1150-1200℃). By utilizing the "pore-forming effect" of PCL in the core-shell structure, the diffusion of metal particles is promoted, and the sintering density still reaches 96-98% (traditional above 95%). After degreasing, the green body is directly fed into the sintering furnace (without cooling), reducing the risk of oxidation exposure and lowering energy consumption by 20%. The pulsed atmosphere degreasing method is an industry first, which greatly improves degreasing efficiency; the low-temperature sintering combined with the pore-forming effect of the core-shell structure reduces energy consumption while ensuring density, solving the core problem of "degreasing of high-load green billets - sintering deformation".

[0027] like Figures 1 to 3 As shown, the The adaptation formula for the structural modification of metal powder (core-shell) is as follows: in, PCL / graphene composite shell thickness (nm), target range 50-100nm; Polycaprolactone (PCL) quality ( Dosage: 0.5–2 wt% (based on metal powder mass); The graphene dispersion coefficient, due to the improved uniformity resulting from graphene embedding in the shell, ranges from 1.05 to 1.15 (graphene dosage 0.1–0.5 wt%, higher dosage increases this value). The larger); PCL density ( / At room temperature, it is 1.14. / ; Specific surface area of ​​metal powder ( / The value is calculated based on spherical powder with a D50 of 3–8 μm, and is taken as 0.3–1.0. / / ; Original spherical metal powder mass ( ); This formula controls the shell thickness, which is determined by the "volume coverage" of PCL—the volume of PCL is calculated based on its mass and density, then divided by the total specific surface area of ​​the metal powder (i.e., the total coverage area), while also incorporating the graphene dispersion coefficient. The correction (graphene embedding makes the shell more uniform and slightly increases the thickness) brings the result to the 50-100nm range; in, : Final oxygen content of core-shell structured powder ( ) Initial oxygen content of metal powder ( The original titanium alloy TC4 powder is 800-1000 ; Plasma cleaning efficiency coefficient ( / (W・h)), with values ​​ranging from 0.8 to 1.2 under a vacuum argon atmosphere; Vacuum plasma cleaning power (W), range 300-500W; Plasma cleaning time (h); This formula controls the oxygen content of the powder. The oxygen content is reduced by removing the surface oxide layer through vacuum plasma cleaning. Higher cleaning power and longer cleaning time result in more thorough oxide layer removal. Therefore, the oxygen content is negatively correlated with power and time. <500 .

[0028] like Figures 1 to 3 As shown, the The adaptation formula for the design of the medium-composite adhesive system is as follows: in, : Feed melt viscosity (mPa·s), <1000 mPa·s at 190℃; The melt viscosity (mPa·s) of pure PBS matrix resin is 1500-1800 mPa·s at 190°C. Plasticizing synergy factor (unitless), PBS and TBC synergistically reduce viscosity, value is 1.2-1.5; The mass percentage of PBS in the adhesive (%) is 40-50%. The percentage of TBC in the adhesive (by mass) ranges from 5% to 35%. n—HA interface enhancement coefficient, n—HA improves compatibility, with a value of 0.05–0.1; n—The mass percentage of HA in the adhesive (%), ranging from 1% to 3%; The influence coefficient of the powder-to-binder ratio: an excessively high powder-to-binder ratio will increase stickiness; the value is 0.03-0.05. The volume ratio of metal powder to binder is 68-72 vol% (i.e., 68:32 to 72:28). Critical powder-to-binder ratio, taken as 0.65 (the highest powder-to-binder ratio in traditional processes, used as a benchmark); Molecular components: PBS (matrix) and TBC (plasticizer) work synergistically to reduce viscosity, while n-HA improves interfacial compatibility and further assists in viscosity reduction; Denominator: When the powder-to-binder ratio is higher than the traditional critical value (0.65), the powder bulk density will increase slightly, resulting in increased viscosity. Correction; Thus making <1000mPa・s, while achieving a high powder-to-binder ratio (68-72 vol%).

[0029] like Figures 1 to 3 As shown, the The appropriate formula for the medium (variable pitch twin-screw + ultrasonic-assisted) feeding and mixing process is: in, Uniformity of core-shell powder dispersion in the binder (%), target 100%; Dispersion correction factor (%·kg / (W·)) •h)), takes values ​​from 0.08 to 0.12; Feeding rate (kg / h), range 3-8 kg / h; Ultrasonic assisted power (W), range 100-200W; Twin screw speed ( ), range 20-30 ; This formula calculates the uniformity of feed dispersion. Dispersion uniformity is positively correlated with ultrasonic power (breaking up agglomerates) and screw speed (shear mixing), and negatively correlated with feed rate (too fast a rate leads to insufficient dispersion). The formula allows us to... Approaching 100%, while reducing the risk of core-shell powder shell damage due to excessive screw speed. ); in, Particle diameter (mm), ranging from 1.5 to 2 mm; Pellet size factor (mm・m / s・K), with a value of 0.02–0.03; Low-temperature air pelletizing outlet temperature (°C), range 10-15°C; Linear velocity (m / s) corresponding to the rotational speed of the pelletizer, ranging from 0.8 to 1.2 m / s; Temperature unit conversion factor (°C → K); This formula is for calculating particle size. The higher the temperature of the low-temperature air, the slower the particle cooling and solidification rate, resulting in a slightly larger particle size. The faster the pelletizing speed, the finer the particles are cut. Therefore, the diameter is positively correlated with temperature and negatively correlated with pelletizing speed. Reduce water absorption by particles in the 1.5-2mm range.

[0030] like Figures 1 to 3 As shown, the The appropriate formula for the (gradient holding pressure + dynamic mold temperature) injection molding process is: in, Green body warpage (mm / m), value <0.1mm / m: Pressure holding fluctuation influence coefficient (mm / (m·MPa)), with a value of 0.002–0.003; Gradient pressure holding pressure fluctuation value (MPa), i.e. ; The influence coefficient of mold cooling rate (mm / (m・℃ / s)) ranges from 0.01 to 0.015. Dynamic cooling rate of the mold (°C / s), range 5-8°C / s; This formula is for calculating the warpage of green billets. Green billet warpage originates from "uneven pressure holding" and "internal stress caused by excessively rapid cooling"—the greater the pressure fluctuation and the faster the cooling rate, the higher the warpage. Therefore, the formula shows a positive correlation. By controlling... (<50MPa) and (5—8℃ / s), let <0.1mm / m.

[0031] 7. The molding process for a high-flowability MIM metal feedstock according to claim 2, characterized in that: preferably, the... The suitable formula for the (pulse-atmosphere degreasing-low-temperature sintering) synergistic process is: in, Total defatting time (h), target 3-4h; Degreasing efficiency coefficient (h・MPa・times / min), with a value of 0.4–0.6; Pulse-type nitrogen pressure (MPa), range 0.1–0.3 MPa; Pulse frequency (times / min), range 10-20 times / min; This formula calculates degreasing time. Higher pulse pressure and faster frequency result in stronger airflow turbulence, leading to higher binder (TBC / PBS) evaporation efficiency and shorter degreasing time. The formula states... It falls within the 3-4 hour range; in, Density (%) of the sintered product: 96-98%; Density coefficient (%・℃ / min・vol%), ranging from 0.025 to 0.03; Low-temperature sintering temperature (°C): 1150-1200°C for TC4 titanium alloy; Sintering heating rate (°C / min), document range 5-8°C / min; The volume ratio of metal powder to binder is 68-72 vol%. This formula is for calculating sintering density. Density is positively correlated with sintering temperature (higher temperature leads to more complete particle diffusion) and negatively correlated with heating rate (slower rate leads to lower internal stress). Furthermore, a high powder-to-binder ratio (higher powder content) can improve the final density. The formula ensures... The density falls within the 96-98% range, and the "pore-forming effect" of the PCL shell is used to compensate for the density loss during low-temperature sintering.

[0032] like Figures 1 to 3 As shown, the In the (ultrasound-assisted in situ polymerization) step, when the ultrasonic power is 20-25kHz, the polymerization time needs to be matched to 40-60min. When the ultrasonic power is increased to 30-40kHz, the polymerization time can be shortened to 25-35min.

[0033] Through this synergistic regulation, the thickness deviation of the PCL graphene shell is controlled within ±5nm (i.e., the shell thickness range is stable at 50-100nm), reducing local viscosity fluctuations in the feed caused by uneven shell thickness. At the same time, this core-shell structure can reduce the angle of repose of the metal powder from 35-40° of the original TC4 powder to 28-32°, further improving the fluidity of the metal powder itself and providing a basis for subsequent high powder-to-binder ratio (68-72 vol%) mixing.

[0034] like Figures 1 to 3 As shown, in the composite adhesive system, the molecular weight of polybutylene succinate (PBS) needs to be compatible with the plasticizing efficiency of tributyl citrate (TBC); when the number-average molecular weight of PBS is 80,000-100,000, the mass percentage of TBC in the adhesive needs to be controlled at 30-35 wt%; when the number-average molecular weight of PBS is increased to 120,000-150,000, the mass percentage of TBC needs to be increased to 32-35 wt% simultaneously. This compatibility ensures that the compatibility between PBS and TBC remains above 90% (compared to approximately 60% for traditional DOP and PP), and that the melt viscosity (190°C) of the composite binder remains stable at 300-400 mPa·s. Simultaneously, this compatibility guarantees the "plasticizing synergistic coefficient" in the feed melt viscosity formula described in claim 4. The viscosity remains stable in the range of 1.3-1.4, further ensuring that the melt viscosity of the feed is <1000mPa·s and that no binder is released at a powder-to-binder ratio of 68-72 vol%.

[0035] like Figures 1 to 3 As shown, in the metal powder (core-shell) structure modification, the pretreatment of graphene nanosheets needs to be compatible with the PCL polymerization process; the graphene nanosheets need to be ultrasonically dispersed in a 0.5-1wt% polyethylene glycol (PEG-6000) solution for 30-40 minutes (ultrasonic power 150-200W) to remove the van der Waals forces between the graphene layers, so that the dispersed graphene particle size is controlled at 50-100nm; when the pretreated graphene nanosheets are mixed with PCL, the "graphene dispersion coefficient (…)" in the "PCL / graphene composite shell thickness formula" can be reduced. The P-value remains stable within the 1.10–1.12 range, reducing the impact of graphene agglomeration. Fluctuations (during reunion) It may drop to 1.02-1.05). Meanwhile, this pretreatment can increase the in-plane orientation of graphene in the shell to over 80%, further enhancing the lubrication effect of the shell and reducing the friction coefficient of the core-shell powder from 0.6-0.7 for the original TC4 powder to 0.3-0.4.

[0036] Example 1 Raw material preparation: Select D50=3 Spherical TC4 titanium alloy powder (initial oxygen content) =800 Take 1000g for later use; Core layer pretreatment: The above powder is fed into a vacuum plasma cleaning equipment, and the power is set. =300W, argon gas (99.999% purity) is introduced, cleaning time =2h, according to the formula: ( =0.8 / (W) h))Calculation, theoretical oxygen content = 800 - 0.8 × 300 × 2 = 320 ; Shell coating: Weigh 5 Polycaprolactone (PCL, 0.5 wt% based on metal powder mass), 0.5 Graphene nanosheets (0.1 wt% based on metal powder mass) were mixed and added to an ultrasonic reactor. The mixture was heated to 60°C, and the ultrasonic frequency was set to 20 kHz. The reaction was carried out for 1.5 hours. The reaction proceeded according to the formula: ( The theoretical shell thickness is calculated as (5 × 1.05) / (1.14 × 1.0 × 1000) × nm≈46nm (actually measured at 50nm, which is within the 50-100nm range). Drying and curing: Transfer the coated powder into a vacuum oven and keep it at 80℃ for 2 hours to fully cure; Finished product testing: The oxygen content was measured to be 310 using an oxygen-nitrogen analyzer. (<500) Scanning electron microscopy revealed that the shell was uniformly covered and there were no exposed metal particles.

[0037] Example 2 Raw material preparation: Select D50=5 Spherical TC4 titanium alloy powder (initial oxygen content) =900 Take 1000g for later use; Core layer pretreatment: Setting the vacuum plasma cleaning power =400W, argon atmosphere, cleaning time =1.5h, calculated according to the formula ( =1.0 / (W・h)), theoretical oxygen content = 900 - 1.0 × 400 × 1.5 = 300 ; Shell coating: Weigh 12g PCL (1.2wt%) and 3g graphene nanosheets (0.3wt%), and react in an ultrasonic reactor at 70℃ and 30kHz for 1 hour; calculate according to the formula ( =1.1, =0.6m² / g), theoretical shell thickness = (12×1.1) / (1.14×0.6×1000)×10 7 nm≈191nm (actually measured as 95nm; due to the increased specific surface area of ​​the powder, the actual thickness is lower than the theoretical value, but still within the range). Drying and curing: Vacuum insulation at 90℃ for 1.5 hours; Finished product testing: oxygen content 290ppm, shell thickness 95nm, graphene no agglomeration.

[0038] Example 3 Raw material preparation: Select D50=8 Spherical TC4 titanium alloy powder (initial oxygen content) =1000 Take 1000g for later use; Core layer pretreatment: Setting the vacuum plasma cleaning power =500W, argon atmosphere, cleaning time =1.5h, calculated according to the formula ( =1.0 / (W・h)), theoretical oxygen content = 900 - 1.0 × 400 × 1.5 = 300 ; Shell coating: Weigh 12g PCL (1.2wt%) and 3g graphene nanosheets (0.3wt%), and react in an ultrasonic reactor at 70℃ and 30kHz for 1 hour; calculate according to the formula ( =1.1, =0.6m² / g), theoretical shell thickness = (12×1.1) / (1.14×0.6×1000)×10 7 nm≈191nm (actually measured as 95nm; due to the increased specific surface area of ​​the powder, the actual thickness is lower than the theoretical value, but still within the range). Drying and curing: Vacuum insulation at 90℃ for 1.5 hours; Finished product testing: oxygen content 290ppm, shell thickness 95nm, graphene no agglomeration.

[0039] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A high flowability MIM metal feedstock, which is composed of: Metal powder: includes spherical metal powder Polycaprolactone Graphene nanoplatelets ; wherein, Volume ratio is respectively 68-72vol%, 0.5-2wt%, 0.1-0.5wt%; The mass ratio is respectively 88-92wt%, 0.04-0.18wt%, 0.01-0.05wt%; the metal powder system finally exists in the form of core-shell structure with any one of metal core-PCL or graphene shell, wherein the mass ratio of PCL and graphene is calculated based on the original metal powder. Composite binder: including polybutylene succinate , tributyl citrate , nano-hydroxyapatite , trace adjuvant ; the volume ratio is respectively 40-50wt%, 25-35wt%, 1-3wt%, 12-34wt%, Supplement 100%; the mass ratio is respectively 4.2-8.3wt%, 2.6-5.8wt%, 0.1-0.5wt%, 1.3-5.7wt% wherein the volume ratio of metal powder and composite binder is 68-72 vol% and 28-32 vol% respectively, and the mass ratio is 88-92 wt% and 8-20 wt% respectively.

2. A forming process for high flow MIM metal feedstock, characterized by: The molding process is suitable for the high flowability MIM metal feedstock of claim 1, and the process comprises the following steps: "core-shell" structure modification of metal powder; building functional layer on the surface of powder, while improving fluidity and interface bonding force, the implementation method: , spherical metal powder with D50 = 3-8 μm is selected, the spherical metal powder comprises titanium alloy TC4, and the fresh metal surface is exposed by vacuum plasma cleaning with a power of 300-500 W in an argon atmosphere to remove the surface oxide layer and impurities; , 0.5-2wt% polycaprolactone (PCL) and 0.1-0.5wt% graphene nanosheet are mixed to enhance the conductivity and lubricity, PCL is polymerized on the surface of the powder in the ultrasonic field at 60-80℃ to form a 50-100nm thick coating layer, and the graphene sheet is uniformly embedded in the shell layer; , in a vacuum oven 80-100 ℃ heat preservation 1-2 h, the shell layer is completely cured, forming a "metal core - PCL / graphene shell" core-shell structure powder, the oxygen content is controlled below 500 ppm; , degradable elastomers including bio-based plasticizers; on a low viscosity base, improving the elastic recovery and environmental friendliness of the binder: , variable pitch twin screw feeding mixing process, for realizing uniform dispersion of core-shell powder in binder, the method is: , a customized "variable pitch twin-screw extruder" is used, and the screw is divided into three sections: The feeding section has a pitch of 40 mm: low shear conveying, reducing the shell damage of core-shell powder; The melting section has a pitch of 25 mm: medium shear mixing, cooperating with the ultrasonic vibration device on the outer wall of the barrel, and breaking local agglomeration by using ultrasonic cavitation effect; The homogenization section has a pitch of 15 mm: high shear extrusion, ensuring that n-HA is uniformly embedded in the interface; Screw rotation speed 20-30 rpm, feeding rate 3-8 kg / h, temperature gradient 110-130-150 °C to match the low melting point of PBS; Low temperature air cutting: 10-15℃ outlet temperature to reduce water absorption of the binder on the surface of the granules, to produce 1.5-2mm "dumbbell" shaped granules, to improve the efficiency of the barrel conveying during injection molding; , gradient pressure and dynamic mold temperature injection molding process; avoid incomplete filling and green warping when high load feeding injection molding, improve the molding precision of complex parts; , pulsed atmosphere debinding and low temperature sintering synergistic process, while reducing high loading green body debinding distortion: , pulsed defatting design: The pre-debinding stage: pulse nitrogen is introduced, and the volatilization of TBC and PBS is accelerated by using airflow disturbance, and the debinding time is shortened to 3-4 h; The high-temperature debinding stage: switch to hydrogen-argon mixed gas atmosphere, volume ratio 1:9, pulse frequency 5-10 times / min, use hydrogen reduction to reduce residual carbon, and the residual carbon content is reduced to below 0.05%; Low temperature sintering optimization: The heating rate control is 5-8 ℃ / min, which is used to reduce the cracking of green body due to thermal stress; , directly into the sintering furnace after debinding, without the need for cooling, for reducing the risk of exposure of the green body to oxidation.

3. A process for forming a high flow MIM metal feedstock according to claim 2, wherein: The steps The thickness of the PCL / graphene composite shell was set as follows: wherein, : PCL / graphene composite shell thickness (nm); : Polycaprolactone (PCL) mass ( ) ; : graphene dispersion factor; : PCL density ( / ); : metal powder specific surface area (m2 / g) / ); : mass of the original spherical metal powder ); The oxygen content control formula of the powder is as follows: wherein, : final oxygen content of the core-shell structured powder ); : initial oxygen content of the metal powder ); : plasma cleaning efficiency coefficient (EPC) (W-h); : vacuum plasma cleaning power (W); : Plasma cleaning time (h).

4. A process for forming a high flow MIM metal feedstock according to claim 2, wherein: The steps In the formula, the feeding melt viscosity is calculated as follows: wherein, : feed melt viscosity (mPa-s); : Pure PBS base resin melt viscosity (mPa-s); : plasticization synergy coefficient, PBS and TBC synergistically reduce viscosity; : Mass fraction of PBS in the adhesive (%); : mass fraction of TBC in the binder (%) : n-HA interface enhancement coefficient, n-HA enhances compatibility; : mass fraction of n-HA in the adhesive (%); : powder glue ratio influence coefficient, powder glue ratio is too high to increase viscosity; : volume ratio of metal powder to binder; : critical bitumen content; Molecular term: PBS (matrix) and TBC (plasticizer) cooperate to reduce viscosity, and n-HA improves interface compatibility to further assist in reducing viscosity; Denominator: When the powder / gum ratio is higher than the traditional critical value (0.65), the viscosity will increase slightly due to the increase of the powder bulk density, which is compensated by correction; Thus, the <1000 mPa s, while achieving a high powder-to-binder ratio of 68-72 vol %.

5. A process for forming a high flow MIM metal feedstock according to claim 2, wherein: The S2 step comprises: , 40-50wt% of polybutylene succinate (PBS), PBS is a degradable elastomer, the melting point is 110-120℃, the elastic modulus is 1.5-2GPa, and the melt viscosity is 500-800mPa・s at 180℃; , adding 25-35 wt% of tributyl citrate (TBC), which has 30% higher compatibility with PBS and no benzene VOCs release during defatting; , 1-3 wt% of nano-hydroxyapatite (n-HA) is added, the hydroxyl groups of which can form hydrogen bonds with the ester groups of the PCL shell layer, further improving the uniformity of the feed dispersion; , the volume ratio of metal powder to composite binder is raised to 68-72 vol%.

6. A process for forming a high flow MIM metal feedstock according to claim 2, wherein: The S4 step comprises: , barrel front section 140-150°C, middle section 150-160°C, rear section 160-170°C, nozzle 170-180°C to reduce excessive degradation of PBS; , 3 segments of pressure maintaining according to the structure of the mold cavity The filling initial time is 0-2 s: high pressure 80-100 MPa, quickly filling the thin-walled area; The filling middle time is 2-5 s: medium pressure 50-70 MPa, reducing the burr generated in the thick-walled area; The pressure holding final time is 5-10 s: low pressure 30-40 MPa, compensating for the elastic shrinkage of PBS; By the partition heating pipe built-in the mould, the dynamic cooling is realized, the cooling rate is 5-8℃ / s, and the green body warping degree is controlled within 0.1mm / m.

7. A process for forming a high flow MIM metal feedstock according to claim 2, wherein: The steps In this case, the sintering density calculation formula is as follows: The debinding time calculation formula is: wherein, : total delipidation time (h); : defatting efficiency coefficient (h MPa min; : Pulse nitrogen pressure (MPa); : pulse frequency (Hz); The sintering density calculation formula is: wherein, : sintered product density (%) : density factor (%・℃ / min・vol%); : Low temperature sintering temperature (°C); : sintering heating rate (°C / min); : Metal powder to binder volume ratio.

8. A process for forming a high flow MIM metal feedstock according to claim 2, wherein: The steps When the ultrasonic power is 20-25 kHz, the polymerization time needs to be matched to 40-60 min, and when the ultrasonic power is raised to 30-40 kHz, the polymerization time can be shortened to 25-35 min.

9. A process for forming a high flow MIM metal feedstock according to claim 4, wherein: In the composite binder system, the molecular weight of polybutylene succinate (PBS) needs to be adapted to the plasticizing efficiency of tributyl citrate (TBC); when the number average molecular weight of PBS is 80-100 thousand, the mass ratio of TBC in the binder needs to be controlled to 30-35 wt%, and when the number average molecular weight of PBS is increased to 12-15 thousand, the mass ratio of TBC needs to be increased to 32-35 wt%.

10. A process for forming a high flow MIM metal feedstock according to claim 3, wherein: When the pretreated graphene nanosheets are mixed with PCL, the "graphene dispersion coefficient (K) in the "PCL / graphene composite shell thickness formula" is stably in the range of 1.10-1.12, avoiding fluctuations caused by graphene agglomeration. )” is stably in the range of 1.10-1.12, avoiding fluctuations caused by graphene agglomeration. ​

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