Low temperature catalytic debinding and rapid densification process for MIM shaped parts

Abstract

This invention discloses a low-temperature catalytic degreasing and rapid densification process for MIM molded parts, comprising the following steps: (1) Low-temperature catalytic degreasing: under a nitrogen atmosphere, catalytic decomposition of POM binder is carried out at 80-120℃ using a nitric acid / organic acid composite catalytic system; (2) Surface activation treatment of the degreased blank: vacuum plasma cleaning is used to remove surface catalyst residues, and the process is switched to H2 / N2 mixed gas to reduce residual impurities; (3) Gradient rapid sintering densification: gradient rapid sintering densification is adopted, with three-stage gradient temperature and pressure control, and a total sintering time ≤3 hours. This invention uses low-temperature catalytic degreasing, shortens the degreasing time, and performs surface activation treatment on the degreased blank, avoiding traditional acid washing or secondary degreasing steps. Gradient rapid sintering densification improves material density and performance.

Description

Technical Field

[0001] This invention relates to the field of metal materials technology, specifically to a low-temperature catalytic degreasing and rapid densification process for MIM molded parts. Background Technology

[0002] Metal Powder Injection Molding (MIM) is a novel powder metallurgy forming technology that introduces modern plastic injection molding technology into the field of powder metallurgy. MIM technology utilizes molds to inject and mold blanks, and then rapidly manufactures high-precision parts with complex three-dimensional shapes through sintering. It is particularly suitable for mass production and is widely used in aerospace, medical devices, and automotive industries.

[0003] The basic process steps of metal injection molding are as follows: First, select metal powder and binder that meet the requirements of MIM. Then, mix the powder and binder into a uniform feed at a certain temperature using an appropriate method. After granulation, the feed is injected and molded. The resulting preform is degreased and then sintered to become the final product. Chinese invention patent application number 202010154524.1 discloses an injection molding method for high-strength and high-toughness metal parts. The material composition is Ni 17.0-18.0%, Co 8.5-9.5%, Mo 4.5-5%, with the balance being Fe. After sintering and heat treatment, the yield strength only reaches 1350 MPa, and the elongation is 6%. Chinese invention patent application number 202010852638.3 discloses an injection-molded ultra-high-strength steel. The material composition is C<0.1%, Ni 15.5-19.5%, Co 7.0-10.0%, Mo 4.0-6.0%, Ti<1.5%, with the balance being Fe. Although the yield strength can reach the 1500 MPa level, the elongation is only up to 6%, and the elongation decreases as the yield strength increases.

[0004] In traditional MIM processes, the degreasing stage often employs thermal or solvent degreasing, which suffers from high energy consumption (>400℃), long degreasing time (10-48 hours), and a tendency to generate cracks, defects, and deformation of the green body. The subsequent sintering stage typically uses single-pressure or vacuum sintering, resulting in a long densification cycle (8-15 hours) and a high risk of grain coarsening. While existing catalytic degreasing technology can lower the temperature, it suffers from low nitric acid vapor utilization (<60%), residual catalyst after degreasing easily contaminates the sintering furnace and the green body, and lacks a design for seamless integration with rapid densification processes.

[0005] Compared with existing technologies, this invention uses a nitric acid / formic acid composite system mixed with nitrogen to improve catalytic effect, decompose polyoxymethylene (POM) based binder, and shorten the degreasing time to 2-4 hours, compared with 10-48 hours in traditional processes. During degreasing, alternating positive and negative pressure cycles are implemented to compensate for degreasing shrinkage through plastic deformation, increasing the density of the green body and reducing subsequent sintering shrinkage. Ar / O2 mixed gas plasma cleaning is used to remove surface catalyst, reducing catalyst contamination of the sintering furnace and green body. H2 / N2 mixed gas is introduced to reduce residual impurities, avoiding traditional acid washing or secondary degreasing steps, improving sintering activity, and lowering the subsequent densification temperature by 50-100°C. In the densification process, the first stage eliminates micropores under Ar atmosphere; the second stage applies isostatic pressure in a vacuum environment to achieve grain boundary diffusion densification; and the third stage completes final densification in N2 / H2 mixed gas. The resulting product has a higher relative density, lower grain size, and shorter sintering time than traditional processes. Summary of the Invention

[0006] Purpose of the invention: The purpose of this invention is to provide a low-temperature catalytic degreasing and rapid densification process for MIM molded parts, supporting the manufacturing of high-precision parts with complex three-dimensional shapes using MIM technology, which can be widely used in aerospace, medical devices, automobiles and other fields.

[0007] Technical solution: A low-temperature catalytic degreasing and rapid densification process for MIM molded parts, comprising the following steps:

[0008] (1) Low-temperature catalytic degreasing: POM binder is catalytically decomposed in a nitrogen atmosphere at 80-120℃ using a nitric acid / organic acid composite catalytic system;

[0009] (2) Surface activation treatment of degreased blanks: Vacuum plasma cleaning removes surface catalyst residues, and H2 / N2 mixed gas is switched to reduce residual impurities;

[0010] (3) Gradient rapid sintering densification: Gradient rapid sintering densification is adopted, with three-stage gradient temperature and pressure control, and the total sintering time is ≤3 hours.

[0011] Preferably, the nitric acid / organic acid composite catalytic system comprises 2-6 vol% nitric acid and 0.5-1.5 vol% formic acid.

[0012] Preferably, low-temperature catalytic degreasing, achieved through multi-stage pressure gradient penetration to decompose the adhesive, includes the following steps:

[0013] (1) The adhesive is decomposed by multi-stage pressure gradient permeation at 80-120℃, and the degreasing time is 2-4 hours.

[0014] (2) Alternately implement positive pressure 0.5MPa / negative pressure -0.08MPa cycle to accelerate the diffusion of acidic vapor into the interior of the billet.

[0015] Preferably, the surface activation treatment of the degreased preform includes the following steps:

[0016] (1) Remove surface catalyst residue by Ar / O2 mixed gas plasma cleaning for 10-20 min under a vacuum of 10-3 Pa;

[0017] (2) Introduce H2 / N2 mixed gas and reduce the catalyst and carbon impurities at 200-300℃.

[0018] Preferably, the proportion of O2 in the Ar / O2 mixture is 2-5%, and the proportion of H2 in the H2 / N2 mixture is 5-10%.

[0019] Preferably, gradient rapid sintering densification is achieved by using a 2.45 GHz microwave source in synergy with 10–20 MPa Ar gas pressure.

[0020] Preferably, gradient rapid sintering densification is implemented by gradient sintering, which includes the following steps:

[0021] (1) First stage: 500℃→800℃, heating rate 10~15℃ / min, eliminate micropores under Ar atmosphere;

[0022] (2) Second stage: 800℃→1200℃, heating rate 5~8℃ / min, applying isostatic pressure of 10~30MPa in vacuum environment to achieve grain boundary diffusion densification;

[0023] (3) Third stage: maintain a constant temperature of 1200~1350℃ for 10~30 minutes, and complete the final densification in N2 / H2 mixed gas.

[0024] Beneficial effects:

[0025] (1) The present invention adopts low-temperature catalytic degreasing, and uses a nitric acid / formic acid composite system mixed with nitrogen to improve the catalytic effect, decompose the polyoxymethylene (POM) based binder, and shorten the degreasing time to 2-4 hours, while the traditional process requires 10-48 hours; during the degreasing process, positive pressure / negative pressure cycles are alternately implemented to compensate for degreasing shrinkage through plastic deformation, increase the density of the green body, and reduce the subsequent sintering shrinkage rate.

[0026] (2) The present invention adopts a degreased green body surface activation treatment, uses Ar / O2 mixed gas plasma cleaning to remove surface catalyst, reduces catalyst contamination of sintering furnace and green body, introduces H2 / N2 mixed gas to reduce residual impurities, avoids traditional acid washing or secondary degreasing steps, improves sintering activity, reduces subsequent densification temperature by 50-100℃, and increases green body density.

[0027] (3) The present invention adopts gradient rapid sintering densification. In the first stage, micropores are eliminated under Ar atmosphere with gradient heating. In the second stage, isostatic pressure is applied in vacuum environment to achieve grain boundary diffusion densification. In the third stage, final densification is completed in N2 / H2 mixed gas. The resulting product has a higher relative density than the traditional process, a lower grain size than the traditional process, and a shorter sintering time than the traditional process. Detailed Implementation

[0028] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments.

[0029] Example 1

[0030] A low-temperature catalytic debinding and rapid densification process for MIM molded parts is obtained by the following preparation method:

[0031] (1) 316L powder (D50 = 10μm) was mixed with POM-based binder (8%), injection pressure 80MPa, mold temperature 60℃, under nitrogen atmosphere, POM binder was catalytically decomposed in a composite catalytic system of 2vol% nitric acid / 0.5vol% formic acid at 80℃, and alternating positive pressure 0.5MPa / negative pressure -0.08MPa cycle was performed for 2h degreasing;

[0032] (2) Under a vacuum of 10-3 Pa, a mixed gas of Ar / O2 with 2% O2 was introduced for plasma cleaning for 10 min; a mixed gas of H2 / N2 with 5% H2 was introduced for reduction reaction at 200 °C for 1 h.

[0033] (3) Gradient sintering was carried out using a 2.45 GHz microwave source and 10 MPa Ar: ① 500℃→800℃, heating rate 10℃ / min, micropores were eliminated under Ar atmosphere; ② 800℃→1200℃, heating rate 5℃ / min, isostatic pressure of 10 MPa was applied in vacuum environment to achieve grain boundary diffusion densification; ③ 1200℃ was kept constant for 10 minutes, and final densification was completed in N2 / H2 mixed gas.

[0034] Example 2

[0035] A low-temperature catalytic debinding and rapid densification process for MIM molded parts is obtained by the following preparation method:

[0036] (1) 316L powder (D50 = 10μm) was mixed with POM-based binder (8%), and the injection pressure was 80MPa, the mold temperature was 60℃, and the POM binder was catalytically decomposed in a nitrogen atmosphere at 100℃ using a composite catalytic system of 4vol% nitric acid / 1.0vol% formic acid. The positive pressure was alternately 0.5MPa / negative pressure -0.08MPa for 3h degreasing.

[0037] (2) Under a vacuum of 10⁻³ Pa, a mixture of Ar / O₂ gas with an O₂ content of 3.5% was introduced for plasma cleaning for 15 min; a mixture of H₂ / N₂ gas with an H₂ content of 7.5% was introduced for reduction reaction at 250 °C for 1 h.

[0038] (3) Gradient sintering was carried out using a 2.45 GHz microwave source and 15 MPa Ar: ① 500℃→800℃, heating rate 12.5℃ / min, micropores were eliminated under Ar atmosphere; ② 800℃→1200℃, heating rate 6.5℃ / min, isostatic pressure of 20 MPa was applied in vacuum environment to achieve grain boundary diffusion densification; ③ 1290℃ was kept constant for 20 minutes, and final densification was completed in N2 / H2 mixed gas.

[0039] Example 3

[0040] A low-temperature catalytic debinding and rapid densification process for MIM molded parts is obtained by the following preparation method:

[0041] (1) 316L powder (D50 = 10μm) was mixed with POM-based binder (8%), injection pressure 80MPa, mold temperature 60℃, under nitrogen atmosphere, POM binder was catalytically decomposed at 120℃ in a composite catalytic system of 6vol% nitric acid / 1.5vol% formic acid, and alternating positive pressure 0.5MPa / negative pressure -0.08MPa cycles were performed for degreasing for 4h;

[0042] (2) Under a vacuum of 10⁻³ Pa, a mixed gas of Ar / O₂ with 5% O₂ was introduced for plasma cleaning for 20 min; a mixed gas of H₂ / N₂ with 10% H₂ was introduced for reduction reaction at 300 °C for 1 h.

[0043] (3) Gradient sintering was carried out using a 2.45 GHz microwave source and 10-20 MPa Ar: ① 500℃→800℃, heating rate 15℃ / min, micropores were eliminated under Ar atmosphere; ② 800℃→1200℃, heating rate 8℃ / min, isostatic pressure of 30 MPa was applied in vacuum environment to achieve grain boundary diffusion densification; ③ 1350℃ was kept constant for 30 minutes, and final densification was completed in N2 / H2 mixed gas.

[0044] Comparative Example 1

[0045] A low-temperature catalytic debinding and rapid densification process for MIM molded parts is obtained by the following preparation method:

[0046] (1) Mix 316L powder (D50 = 10μm) with POM-based binder (8%), injection pressure 80MPa, mold temperature 60℃; soak in n-hexane at 37℃ for 24-48h, and dry in hot air oven at 60-80℃ for 2h;

[0047] (2) The degreasing rate (A) is calculated as follows:

[0048]

[0049]

[0050] In the formula: W A W represents the weight (g) of the workpiece before solvent immersion. B The weight (g) of the workpiece after solvent immersion; if the degreasing is not up to standard, continue the second degreasing according to step (1);

[0051] (3) Gradient sintering was carried out using a 2.45 GHz microwave source and 15 MPa Ar: ① 500℃→800℃, heating rate 12.5℃ / min, micropores were eliminated under Ar atmosphere; ② 800℃→1200℃, heating rate 6.5℃ / min, isostatic pressure of 20 MPa was applied in vacuum environment to achieve grain boundary diffusion densification; ③ 1290℃ was kept constant for 20 minutes, and final densification was completed in N2 / H2 mixed gas.

[0052] Comparative Example 2

[0053] A low-temperature catalytic debinding and rapid densification process for MIM molded parts is obtained by the following preparation method:

[0054] (1) 316L powder (D50 = 10μm) was mixed with POM-based binder (8%), injection pressure 80MPa, mold temperature 60℃, under nitrogen atmosphere, POM binder was catalytically decomposed in a composite catalytic system of 4vol% nitric acid at 100℃, and alternating positive pressure 0.5MPa / negative pressure -0.08MPa cycle was performed for degreasing for 3h;

[0055] (2) Gradient sintering was carried out using a 2.45 GHz microwave source and 15 MPa Ar: ① 500℃→800℃, heating rate 12.5℃ / min, micropores were eliminated under Ar atmosphere; ② 800℃→1200℃, heating rate 6.5℃ / min, isostatic pressure of 20 MPa was applied in vacuum environment to achieve grain boundary diffusion densification; ③ 1290℃ was kept constant for 20 minutes, and final densification was completed in N2 / H2 mixed gas.

[0056] Comparative Example 3

[0057] A low-temperature catalytic debinding and rapid densification process for MIM molded parts is obtained by the following preparation method:

[0058] (1) 316L powder (D50 = 10μm) was mixed with POM-based binder (8%), and the injection pressure was 80MPa, the mold temperature was 60℃, and the POM binder was catalytically decomposed in a nitrogen atmosphere at 100℃ using a composite catalytic system of 4vol% nitric acid / 1.0vol% formic acid. The positive pressure was alternately 0.5MPa / negative pressure -0.08MPa for 3h degreasing.

[0059] (2) Under a vacuum of 10⁻³ Pa, a mixture of Ar / O₂ gas with an O₂ content of 3.5% was introduced for plasma cleaning for 15 min; a mixture of H₂ / N₂ gas with an H₂ content of 7.5% was introduced for reduction reaction at 250 °C for 1 h.

[0060] (3) Gradient rapid sintering densification: Gradient sintering is carried out as follows: ① Nitrogen pressure 0.2MPa, 500℃→800℃, heating rate 2℃ / min; ② Vacuum degree above -0.095MPa, 800℃→1200℃, heating rate 2℃ / min; ③ Constant temperature of 1300℃ for 3h, nitrogen protection, to complete the final densification.

[0061] Comparative Example 4

[0062] (1) Mix 316L powder (D50 = 10μm) with POM-based binder (8%), injection pressure 80MPa, mold temperature 60℃; soak in n-hexane at 37℃ for 24-48h, and dry in hot air oven at 60-80℃ for 2h;

[0063] (2) The degreasing rate (A) is calculated as follows:

[0064]

[0065] In the formula: W A W represents the weight (g) of the workpiece before solvent immersion. B The weight (g) of the workpiece after solvent immersion; if the degreasing is not up to standard, continue the second degreasing according to step (1);

[0066] (3) Gradient rapid sintering densification: Gradient sintering is carried out as follows: ① Nitrogen pressure 0.2MPa, 500℃→800℃, heating rate 2℃ / min; ② Vacuum degree above -0.095MPa, 800℃→1200℃, heating rate 2℃ / min; ③ Constant temperature of 1300℃ for 3h, nitrogen protection, to complete the final densification.

[0067] Comparison Data Table of Examples and Comparative Examples

[0068]

[0069] The results show that the low-temperature catalytic degreasing and rapid densification of MIM molded parts provided by the present invention results in a product with a higher relative density, lower grain size, and shorter sintering time than the traditional process. Through Examples 1-3 and Comparative Examples 1-4, it can be seen that: (1) Comparative Example 1 uses a traditional degreasing process, which has a long degreasing time and involves secondary degreasing; (2) In the process of Comparative Example 2, after nitric acid catalysis, the catalyst cleaning and reduction to remove residual impurities are cancelled, which leads to easy cracking of the workpiece and a high scrap rate; (3) In the traditional sintering process of Comparative Example 3, the sintering time is long, the product density is relatively low, and the grain size is large; (4) Comparative Example 4 uses traditional degreasing and traditional sintering processes, resulting in a product with a large grain size and low relative density.

[0070] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A low-temperature catalytic degreasing and rapid densification process for MIM molded parts, characterized in that, Includes the following steps: (1) Low-temperature catalytic degreasing: POM binder is catalytically decomposed in a nitrogen atmosphere at 80-120℃ using a nitric acid / organic acid composite catalytic system; (2) Surface activation treatment of degreased blanks: Vacuum plasma cleaning removes surface catalyst residues, and H2 / N2 mixed gas is switched to reduce residual impurities; (3) Gradient rapid sintering densification: Gradient rapid sintering densification is adopted, with three-stage gradient temperature and pressure control, and the total sintering time is ≤3 hours; Low-temperature catalytic degreasing, achieved through multi-stage pressure gradient penetration to decompose the adhesive, includes the following steps: (1) The adhesive is decomposed by multi-stage pressure gradient permeation at 80-120℃, and the degreasing time is 2-4h; (2) Alternately implement positive pressure 0.5MPa / negative pressure -0.08MPa cycle to accelerate the diffusion of acidic vapor into the interior of the billet; The surface activation treatment of the degreased preform includes the following steps: (1) Remove surface catalyst residue by plasma cleaning with Ar / O2 mixed gas for 10-20 min under a vacuum of 10⁻³Pa; (2) Introduce H2 / N2 mixed gas and reduce the catalyst and carbon impurities at 200-300℃; In an Ar / O2 mixture, O2 accounts for 2-5% of the total gas volume, while in an H2 / N2 mixture, H2 accounts for 5-10%.

2. The low-temperature catalytic degreasing and rapid densification process for MIM molded parts according to claim 1, characterized in that, The nitric acid / organic acid composite catalytic system includes nitric acid at a concentration of 2-6 vol and formic acid at a concentration of 0.5-1.5 vol.

3. The low-temperature catalytic degreasing and rapid densification process for MIM molded parts according to claim 1, characterized in that, Gradient rapid sintering densification is achieved by using a 2.45 GHz microwave source in synergy with 10~20 MPa Ar gas pressure.

4. The low-temperature catalytic degreasing and rapid densification process for MIM molded parts according to claim 1, characterized in that, Gradient rapid sintering densification is achieved by implementing gradient sintering, which includes the following steps: (1) First stage: 500℃ to 800℃, heating rate 10~15℃ / min, eliminate micropores under Ar atmosphere; (2) Second stage: 800℃ to 1200℃, heating rate 5~8℃ / min, applying isostatic pressure of 10~30MPa in vacuum environment to achieve grain boundary diffusion densification; (3) Third stage: maintain a constant temperature of 1200~1350℃ for 10~30 minutes, and complete the final densification in N2 / H2 mixed gas.

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