Low-temperature catalytic degreasing and rapid densification process of MIM (metal injection molding) forming part

Abstract

The invention discloses a low-temperature catalytic degreasing and rapid densifying process of an MIM (metal injection molding) forming part, which comprises the following steps: (1) low-temperature catalytic degreasing: in a nitrogen atmosphere, catalytically decomposing a POM (polyoxymethylene) binder by a nitric acid / organic acid composite catalytic system at 80-120 DEG C; (2) surface activation treatment of the degreased green body: performing vacuum plasma cleaning to remove surface catalyst residues, and switching to H2 / N2 mixed gas to reduce residual impurities; and (3) gradient rapid sintering densification: gradient rapid sintering densification is adopted, three-stage gradient temperature control-pressure control is adopted, and the total sintering time is less than or equal to 3 hours. According to the method, low-temperature catalytic degreasing is adopted, the degreasing time is shortened, the surface of a degreased green body is activated, a traditional acid pickling or secondary degreasing step is avoided, gradient rapid sintering densification is achieved, and the density and performance of the material are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and in particular to a low-temperature catalytic degreasing and rapid densification process for MIM molded parts. Background Art

[0002] Metal Powder Injection Molding (MIM) is a new powder metallurgy technology that integrates modern plastic injection technology into the field of powder metallurgy. MIM technology uses molds to inject blanks and then sinters them to rapidly produce high-precision, complex three-dimensional parts. It is particularly well-suited for mass production and is widely used in aerospace, medical equipment, automotive, and other fields.

[0003] The basic process steps of metal injection molding are: first, select metal powder and binder that meet the requirements of MIM, then use appropriate methods to mix the powder and binder into a uniform feed at a certain temperature, granulate it and then inject it into the molded blank. After degreasing, it is sintered and densified to become the final product. The Chinese invention patent with 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%, Mo4.5-5%, and the balance is Fe. After sintering and heat treatment, the yield strength only reaches 1350MPa and the elongation is 6%; the Chinese invention patent with application number 202010852638.3 discloses an injection-molded ultra-high-strength steel with a material composition of C <0.1%, Ni 15.5-19.5%, Co 7.0-10.0%, Mo4.0-6.0%, Ti <1.5%, and the balance is Fe. Although the yield strength can reach 1500MPa, the elongation is only 6% at most, and the elongation decreases with the increase of yield strength.

[0004] In traditional MIM processes, thermal or solvent debinding is often used in the debinding stage, which presents problems such as high energy consumption (>400°C), long debinding times (10-48 hours), prone to crack defects, and cracking and deformation of the green body. The subsequent sintering stage typically uses single-pressure or vacuum sintering, with a long densification cycle (8-15 hours) and a high risk of grain coarsening. While existing catalytic debinding technology can reduce temperatures, the utilization rate of nitric acid vapor is low (<60%), residual catalyst after debinding easily contaminates the sintering furnace and green body, and there is a lack of design integration with rapid densification processes.

[0005] In contrast to the existing technology, the present invention uses a nitric acid / formic acid composite system mixed with nitrogen to improve the catalytic effect and decompose the polyoxymethylene (POM)-based binder, shortening the degreasing time to 2-4 hours, while the traditional process requires 10 to 48 hours. During the degreasing process, positive and 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. Ar / O2 mixed gas plasma cleaning is used to remove surface catalysts, reducing catalyst contamination of the sintering furnace and green body. H2 / N2 mixed gas is introduced to reduce residual impurities, avoiding traditional pickling or secondary degreasing steps, improving sintering activity, and reducing the subsequent densification temperature by 50 to 100°C. In the first stage of the densification process, micropores are eliminated under an Ar atmosphere. In the second stage, isostatic pressing is applied in a vacuum environment to achieve grain boundary diffusion densification. In the third stage, final densification is completed in an N2 / H2 mixed gas mixture. 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. Summary of the Invention

[0006] Purpose of the invention: The purpose of the present invention is to provide a low-temperature catalytic degreasing and rapid densification process for MIM molded parts, supporting MIM technology to manufacture high-precision, three-dimensional complex parts, which can be widely used in aerospace, medical equipment, automobile 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: In a nitrogen atmosphere, the POM binder is catalytically decomposed at 80-120°C using a nitric acid / organic acid composite catalytic system;

[0009] (2) Surface activation treatment of degreased blank: vacuum plasma cleaning is used to remove surface catalyst residues, and then H2 / N2 mixed gas is used 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 includes nitric acid at a concentration of 2-6 vol% and formic acid at a concentration of 0.5-1.5 vol%.

[0012] Preferably, low-temperature catalytic degreasing, which achieves adhesive decomposition through multi-stage pressure gradient infiltration, comprises the following steps:

[0013] (1) The binder is decomposed by a multi-stage pressure gradient infiltration method at 80-120°C, with a degreasing time of 2 to 4 hours;

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

[0015] Preferably, the degreased body surface activation treatment comprises the following steps:

[0016] (1) Using Ar / O2 mixed gas plasma cleaning at a vacuum of 10-3Pa for 10-20min to remove surface catalyst residues;

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

[0018] Preferably, O2 accounts for 2-5% in the Ar / O2 mixed gas, and H2 accounts for 5-10% in the H2 / N2 mixed gas.

[0019] Preferably, the gradient rapid sintering densification is carried out by using a 2.45 GHz microwave source in conjunction with a 10-20 MPaAr gas pressure.

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

[0021] (1) The first stage: 500°C → 800°C, heating rate 10-15°C / min, eliminating micropores under Ar atmosphere;

[0022] (2) The second stage: 800°C → 1200°C, heating rate 5-8°C / min, isostatic pressure 10-30 MPa in a vacuum environment to achieve grain boundary diffusion densification;

[0023] (3) The third stage: constant temperature at 1200-1350 °C for 10-30 minutes, and final densification is completed in N2 / H2 mixed gas.

[0024] Beneficial effects:

[0025] (1) The present invention adopts low-temperature catalytic degreasing, using 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 to 48 hours; during the degreasing process, positive pressure / negative pressure cycles are alternately implemented to compensate for degreasing shrinkage through plastic deformation, thereby increasing the density of the green body and reducing 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 catalysts, reduces catalyst contamination of the sintering furnace and green body, introduces H2 / N2 mixed gas to reduce residual impurities, avoids traditional pickling or secondary degreasing steps, improves sintering activity, reduces the subsequent densification temperature by 50 to 100°C, and increases green body density.

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

[0028] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to specific embodiments.

[0029] Example 1

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

[0031] (1) 316L powder (D50 = 10 μm) was mixed with a POM-based binder (8%) at an injection pressure of 80 MPa and a mold temperature of 60°C. The POM binder was catalytically decomposed at 80°C using a 2 vol% nitric acid / 0.5 vol% formic acid composite catalyst system under a nitrogen atmosphere. The positive pressure was 0.5 MPa and the negative pressure was -0.08 MPa, and the degreasing process lasted for 2 h.

[0032] (2) At a vacuum of 10-3 Pa, introduce an Ar / O2 mixed gas with an O2 ratio of 2% and perform plasma cleaning for 10 minutes; introduce an H2 / N2 mixed gas with an H2 ratio of 5% and perform reduction reaction at 200°C for 1 hour;

[0033] (3) Using a 2.45 GHz microwave source and 10 MPa Ar, gradient rapid sintering and densification were carried out: ① 500°C → 800°C, heating rate 10°C / min, eliminating micropores in Ar atmosphere; ② 800°C → 1200°C, heating rate 5°C / min, applying isostatic pressure of 10 MPa in a vacuum environment to achieve grain boundary diffusion densification; ③ Maintaining temperature at 1200°C for 10 minutes, and completing final densification in a N2 / H2 mixture.

[0034] Example 2

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

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

[0037] (2) At a vacuum of 10-3 Pa, introduce an Ar / O2 mixed gas with an O2 ratio of 3.5% and perform plasma cleaning for 15 minutes; introduce an H2 / N2 mixed gas with an H2 ratio of 7.5% and perform reduction reaction at 250°C for 1 hour;

[0038] (3) Using a 2.45 GHz microwave source and 15 MPa Ar, gradient rapid sintering and densification were carried out: ① 500°C → 800°C, heating rate 12.5°C / min, eliminating micropores in Ar atmosphere; ② 800°C → 1200°C, heating rate 6.5°C / min, applying isostatic pressure 20 MPa in a vacuum environment to achieve grain boundary diffusion densification; ③ maintaining a constant temperature at 1290°C for 20 minutes, and completing final densification in a N2 / H2 mixture.

[0039] Example 3

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

[0041] (1) 316L powder (D50 = 10 μm) was mixed with a POM-based binder (8%) at an injection pressure of 80 MPa and a mold temperature of 60°C. The POM binder was catalytically decomposed at 120°C using a 6 vol% nitric acid / 1.5 vol% formic acid composite catalyst system under a nitrogen atmosphere. The mixture was alternately subjected to a positive pressure of 0.5 MPa and a negative pressure of -0.08 MPa cycle, and degreasing was performed for 4 h.

[0042] (2) Under a vacuum of 10-3 Pa, introduce an Ar / O2 mixed gas with an O2 ratio of 5% and perform plasma cleaning for 20 minutes; introduce an H2 / N2 mixed gas with an H2 ratio of 10% and perform reduction reaction at 300°C for 1 hour;

[0043] (3) Using a 2.45 GHz microwave source and 10-20 MPaAr, gradient rapid sintering and densification were carried out: ① 500℃→800℃, heating rate 15℃ / min, eliminating micropores in Ar atmosphere; ② 800℃→1200℃, heating rate 8℃ / min, applying isostatic pressure 30 MPa in vacuum environment to achieve grain boundary diffusion densification; ③ maintaining constant temperature at 1350℃ for 30 minutes, and completing final densification in N2 / H2 mixed gas.

[0044] Comparative Example 1

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

[0046] (1) 316L powder (D50 = 10 μm) was mixed with a POM-based binder (8%) at an injection pressure of 80 MPa and a mold temperature of 60°C. The mixture was then immersed in 37°C n-hexane for 24 to 48 hours and dried in a hot air oven at 60 to 80°C for 2 hours.

[0047] (2) The calculation method of degreasing rate (A) is:

[0048]

[0049] Where: W A is the weight of the workpiece before immersion in solvent (g), W B is the weight of the workpiece after immersion in solvent (g); if degreasing does not meet the standard, continue secondary degreasing according to step (1);

[0050] (3) Using a 2.45 GHz microwave source and 15 MPa Ar, gradient rapid sintering and densification were carried out: ① 500°C → 800°C, heating rate 12.5°C / min, eliminating micropores in Ar atmosphere; ② 800°C → 1200°C, heating rate 6.5°C / min, applying isostatic pressure 20 MPa in a vacuum environment to achieve grain boundary diffusion densification; ③ maintaining a constant temperature at 1290°C for 20 minutes, and completing final densification in a N2 / H2 mixture.

[0051] Comparative Example 2

[0052] A low-temperature catalytic degreasing and rapid densification process for MIM molded parts is obtained by the following preparation method:

[0053] (1) 316L powder (D50 = 10 μm) was mixed with a POM-based binder (8%) at an injection pressure of 80 MPa and a mold temperature of 60°C. The POM binder was catalytically decomposed at 100°C using a 4 vol% nitric acid composite catalyst system under a nitrogen atmosphere. The positive pressure was 0.5 MPa and the negative pressure was -0.08 MPa, and degreasing was performed for 3 h.

[0054] (2) Using a 2.45 GHz microwave source and 15 MPa Ar, gradient rapid sintering and densification were carried out: ① 500℃→800℃, heating rate 12.5℃ / min, eliminating micropores in Ar atmosphere; ② 800℃→1200℃, heating rate 6.5℃ / min, applying isostatic pressure 20 MPa in vacuum environment to achieve grain boundary diffusion densification; ③ maintaining constant temperature at 1290℃ for 20 minutes, and completing final densification in N2 / H2 mixed gas.

[0055] Comparative Example 3

[0056] A low-temperature catalytic degreasing and rapid densification process for MIM molded parts is obtained by the following preparation method:

[0057] (1) 316L powder (D50 = 10 μm) was mixed with a POM-based binder (8%) at an injection pressure of 80 MPa and a mold temperature of 60°C. The POM binder was catalytically decomposed at 100°C using a 4 vol% nitric acid / 1.0 vol% formic acid composite catalyst system under a nitrogen atmosphere. The positive pressure was 0.5 MPa and the negative pressure was -0.08 MPa, and the degreasing process lasted for 3 h.

[0058] (2) At a vacuum of 10-3 Pa, introduce an Ar / O2 mixed gas with an O2 ratio of 3.5% and perform plasma cleaning for 15 minutes; introduce an H2 / N2 mixed gas with an H2 ratio of 7.5% and perform reduction reaction at 250°C for 1 hour;

[0059] (3) Gradient rapid sintering densification Gradient sintering was implemented as follows: ① nitrogen pressure 0.2 MPa, 500℃→800℃, heating rate 2℃ / min; ② vacuum degree above -0.095 MPa, 800℃→1200℃, heating rate 2℃ / min; ③ constant temperature at 1300℃ for 3h, nitrogen protection, to complete the final densification.

[0060] Comparative Example 4

[0061] (1) 316L powder (D50 = 10 μm) was mixed with a POM-based binder (8%) at an injection pressure of 80 MPa and a mold temperature of 60°C. The mixture was then immersed in 37°C n-hexane for 24 to 48 hours and dried in a hot air oven at 60 to 80°C for 2 hours.

[0062] (2) The calculation method of degreasing rate (A) is:

[0063]

[0064] Where: W A is the weight of the workpiece before immersion in solvent (g), W B is the weight of the workpiece after immersion in solvent (g); if degreasing does not meet the standard, continue secondary degreasing according to step (1);

[0065] (3) Gradient rapid sintering densification Gradient sintering was implemented as follows: ① nitrogen pressure 0.2 MPa, 500℃→800℃, heating rate 2℃ / min; ② vacuum degree above -0.095 MPa, 800℃→1200℃, heating rate 2℃ / min; ③ constant temperature at 1300℃ for 3h, nitrogen protection, to complete the final densification.

[0066] Comparative data table of examples and comparative examples

[0067]

[0068] The results show that the low-temperature catalytic degreasing and rapid densification of a MIM molded part provided by the present invention has a higher relative density, a lower grain size, and a shorter sintering time than that of the traditional process. It can be seen from Examples 1 to 3 and Comparative Examples 1 to 4 that: (1) Comparative Example 1 uses a traditional degreasing process, which takes a long time to degreasing 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 eliminated, resulting in easy cracking of the workpiece and a high scrap rate; (3) In Comparative Example 3, the traditional sintering process takes a long time, the product density is relatively low, and the grain size is relatively large; (4) Comparative Example 4 uses traditional degreasing and traditional sintering processes, and the product grain size is relatively large and the relative density is relatively low.

[0069] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A low-temperature catalytic degreasing and rapid densification process for MIM molded parts, characterized in that: The following steps are involved: (1) Low-temperature catalytic degreasing: In a nitrogen atmosphere, the POM binder is catalytically decomposed at 80-120°C using a nitric acid / organic acid composite catalytic system; (2) Surface activation treatment of degreased blank: vacuum plasma cleaning is used to remove surface catalyst residues, and then H2 / N2 mixed gas is used 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.

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 concentration of 2-6 vol% and formic acid 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: Low-temperature catalytic degreasing, which achieves adhesive decomposition through multi-stage pressure gradient infiltration, includes the following steps: (1) The binder is decomposed by a multi-stage pressure gradient infiltration method at 80-120°C, with a degreasing time of 2 to 4 hours; (2) Alternately implement positive pressure 0.5MPa / negative pressure -0.08MPa cycles to accelerate the diffusion of acidic vapor into the interior of the green body.

4. The low-temperature catalytic degreasing and rapid densification process for MIM molded parts according to claim 1, characterized in that: The surface activation treatment of the degreased green body comprises the following steps: (1) In 10 -3 Use Ar / O2 mixed gas plasma cleaning for 10-20 minutes under Pa vacuum to remove surface catalyst residues; (2) Introduce H2 / N2 mixed gas to reduce the catalyst and carbon impurities at 200-300℃.

5. The low-temperature catalytic degreasing and rapid densification process for MIM molded parts according to claim 4, characterized in that: O2 accounts for 2-5% of the Ar / O2 mixed gas, and H2 accounts for 5-10% of the H2 / N2 mixed gas.

6. The low-temperature catalytic degreasing and rapid densification process for MIM molded parts according to claim 1, characterized in that: Gradient rapid sintering densification uses a 2.45 GHz microwave source and 10-20 MPaAr gas pressure to work together.

7. The low-temperature catalytic degreasing and rapid densification process for MIM molded parts according to claim 1, characterized in that: Gradient rapid sintering densification Gradient sintering is carried out, including the following steps: (1) The first stage: 500°C to 800°C, heating rate 10-15°C / min, eliminating micropores under Ar atmosphere; (2) The second stage: 800°C to 1200°C, heating rate 5-8°C / min, isostatic pressure 10-30 MPa in a vacuum environment to achieve grain boundary diffusion densification; (3) The third stage: constant temperature at 1200-1350 °C for 10-30 minutes, and final densification is completed in N2 / H2 mixed gas.

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