A nanocrystalline cemented carbide additive manufacturing method

Abstract

The application relates to a nanocrystalline cemented carbide additive manufacturing method and belongs to the technical field of cemented carbide and additive manufacturing. The obtained nanocrystalline cemented carbide has high strength and high hardness, and the average size of WC grains is less than 0.2 microns. The application takes superfine polycrystalline WC powder, Co powder and grain growth inhibitor as raw materials, uniformly dispersed mixed powder is prepared through ball milling, then the mixed powder is mixed with a binder in a vacuum compounding-granulating machine to prepare printing feed, and then a fused deposition forming is used to print a cemented carbide green body in an inert gas environment, finally, the green body is subjected to degreasing and two-step sintering to prepare nanocrystalline cemented carbide. The application combines additive manufacturing and degreasing and sintering processes, and the prepared nanocrystalline cemented carbide has high relative density, excellent mechanical properties and no metallurgical defects, and is suitable for large-scale batch production.

Description

Technical Field

[0001] This invention relates to a method for additive manufacturing of nanocrystalline cemented carbide, belonging to the fields of cemented carbide and additive manufacturing. Background Technology

[0002] Cemented carbide is a metal-ceramic composite material made of refractory metal hard compounds and binder metals, possessing high hardness, high strength, and high wear resistance. Nanocrystalline cemented carbide refers to cemented carbide with an average grain size of less than 0.2 μm in the hard phase. It exhibits even higher hardness and strength than traditional cemented carbide, effectively solving the problem of efficient machining of difficult-to-machine materials such as PCBs (printed circuit boards), composite materials, and hardened steel, and is widely used in electronics, aerospace, and automotive fields. It is mainly prepared using powder metallurgy methods such as compression molding (CM) and extrusion molding (EM). Current powder metallurgy technology is insufficient to meet the needs of fabricating nanocrystalline cemented carbide parts with complex geometries; additive manufacturing (AM) technology provides a new approach for this purpose.

[0003] Among the existing technologies available for cemented carbide additive manufacturing, powder bed fusion (PBF) requires the raw material powder to have excellent properties, including sphericity, particle size distribution, bulk density, and flowability. The prepared samples are prone to a large number of metallurgical defects such as pores, cracks, and brittle η phase that are difficult to eliminate.

[0004] To address the aforementioned issues, Chinese patent (CN201510107078.8) discloses a method for manufacturing cemented carbide using 3D printing. This invention relates to a method for manufacturing cemented carbide using 3D printing, including wet milling and mixing cemented carbide raw material powder with an organic binder, spray drying and granulation, extrusion into filaments, 3D printing, and dewaxing and sintering. However, this patent method does not disclose information such as the relative density, grain size, and mechanical properties of the prepared cemented carbide. Chinese patent (CN201680084484.X) discloses a 3D printing method for cermet or cemented carbide, involving a powder for 3D printing cermet or cemented carbide tool bodies, the powder containing 30% to 70% by volume particles with a diameter <10 μm; and a method for manufacturing cermet or cemented carbide tool bodies using the BJAM method, comprising: forming the powder, using the powder together with a printing binder to 3D print the tool body, thereby forming a 3D printed cermet or cemented carbide green body, and subsequently sintering the green body to form the cermet or cemented carbide tool body. The porosity of the cermet or cemented carbide tool bodies prepared by this patent is 3-13%. Zhang Xinyue [Zhang Xinyue. Research on 3D Cold Printing of Cemented Carbide [D]. Beijing: Beijing University of Science and Technology, 2018.] prepared a WC-20%Co cemented carbide angle end mill using a 3DGP-debinding sintering process, the density of the sintered sample being 13.55 g / cm³. 3 The hardness is 87.7 HRC and the bending strength is 2612.8 MPa. Lee et al. [Lee SW, et al., Phase control of WC-Cohardmetal using additive manufacturing technologies[J]. Powder Metallurgy, 65(2021)13-21.] increased the bulk density of the FDM (Fused deposition modeling) wire, thereby increasing the green density. After degreasing and sintering, they obtained cemented carbide parts with a relative density of 96.3% and a hardness of 89.06 HRC.

[0005] Additive manufacturing methods such as photopolymerization and BJAM produce cemented carbide with relatively high porosity and poor mechanical properties, requiring HIP densification treatment, which increases the difficulty of the process and the preparation cost, and reduces the production efficiency. Cemented carbide parts prepared by FDM have problems such as low relative density, low strength and hardness, and poor toughness.

[0006] This invention presents a method for additive manufacturing of nanocrystalline cemented carbide with high relative density, high hardness, high strength, and complex shapes. First, using ultrafine polycrystalline WC powder and ordinary Co powder as main raw materials, and grain growth inhibitors (GGI) as additives, the polycrystalline WC powder is thoroughly crushed using a ball milling process to prepare a uniformly dispersed mixed powder of GGI. Second, a printing feedstock is prepared using vacuum mixing and granulation. Third, a high-relative-density cemented carbide green body is prepared by printing. By optimizing the FDM printing environment to an inert gas environment and optimizing the printing parameters, oxidation of the cemented carbide green body during printing is avoided, thus preventing decarburization of the sintered sample. Finally, the green body is debound and sintered to obtain a nanocrystalline cemented carbide product without metallurgical defects and with excellent mechanical properties. This invention, through the synergistic effect of alloy composition design, binder composition design, powder characteristic control, and process parameter optimization, eliminates microstructural defects and refines the grain size of the cemented carbide, thereby improving the relative density and comprehensive mechanical properties of cemented carbide parts.

[0007] Currently, there are no reports on the preparation of nanocrystalline cemented carbide using the FDM-debinding sintering process. Summary of the Invention

[0008] This invention provides a method for additive manufacturing of nanocrystalline cemented carbide. First, using ultrafine polycrystalline WC powder and ordinary Co powder as the main components, and grain growth inhibitors (GGI) as the second phase, the polycrystalline WC powder is fully crushed using a ball milling process to prepare a uniformly dispersed mixed powder of GGI. Second, vacuum mixing and granulation are used to prepare the additive manufacturing feedstock. Third, a high-density green compact is printed using an FDM device in an inert gas environment. Finally, the green compact is degreased and sintered in two steps to obtain a high-density, high-hardness, and high-strength nanocrystalline cemented carbide product. The specific steps are as follows:

[0009] (1) Ball milling to prepare mixed powder:

[0010] Ultrafine polycrystalline WC powder A, ordinary Co powder B, and nanocrystal growth inhibitor C were ball-milled in a designed ratio to prepare mixed powder D.

[0011] (2) Vacuum mixing-granulation preparation of printing feed:

[0012] Mix powder D and binder E according to the design ratio, and then put them into a vacuum mixer for internal mixing to prepare a uniformly mixed material F of mixed powder D and binder E. Then, put the prepared internally mixed material F into a vacuum granulator to prepare granular printing feed G with a particle size of 1-4 mm.

[0013] (3) FDM printing for green body preparation:

[0014] Using the granular printing feed G obtained in step (2) as raw material, cemented carbide green blank H is prepared by printing in an inert gas environment using a fused deposition modeling (FDM) device.

[0015] The inert gas should be nitrogen, helium, argon, or a mixture of argon and helium, with a purity of 99.99 wt%, of which the oxygen content is less than 0.0001 wt%.

[0016] (4) Degreasing of green blank: Solvent degreasing and thermal degreasing are performed on the cemented carbide green blank H prepared in step (3) to obtain degreased green blank I.

[0017] (5) Two-step sintering: The degreased green blank I treated in step (4) is sintered in two steps to obtain nanocrystalline WC-Co cemented carbide solid part J.

[0018] Among them, the particle size D of powder A in step (1) 50 Powder B particle size D <1.0μm 50 Powder particle size C <10μm D 50 The micrometer size is less than 0.2 μm, preferably less than 0.1 μm.

[0019] When ball milling is used, the ball-to-material ratio is between 3:1 and 18:1, preferably between 5:1 and 10:1; the rotation speed is 60 to 300 rpm, preferably 100 to 220 rpm; the ball milling time is 10 to 120 hours, preferably 60 to 100 hours; and the content of each powder in the ball-milled powder D is as follows: powder A mass fraction is 80 to 97%; powder B mass fraction is 3 to 19%; and powder C mass fraction is 0.1 to 5%.

[0020] In step (1), the powder ball mill uses alcohol as the medium for rolling ball milling.

[0021] Of course, other ball milling processes are also applicable to this invention.

[0022] Mixed powder design scheme:

[0023] Preferably, the mixed powder D contains WC-Co, V8C7, and Cr3C2, and the mass ratio of WC-Co:V8C7:Cr3C2 is 96-99:0.5-1:0.5-1.

[0024] In step (2), the volume percentage of the mixed powder D in the internally mixed mixture F is 40-75%, and the volume percentage of the binder E is 25-60%. Here, the volume percentage of D in F is defined as the powder loading. The present invention optimizes the composition of the binder E, which includes a skeleton component, a plasticizer component, and a dispersant component. The skeleton component is at least one of polyethylene, polypropylene, polyethylene glycol, ethylene-vinyl acetate copolymer, polymethyl methacrylate, polystyrene, polyvinyl butyral, polyacetal, etc. The plasticizer component includes at least one of paraffin wax, microcrystalline wax, tung oil, quartz, beeswax, palm wax, etc. The dispersant component is at least one of fatty acid, polyacrylamide, 2,6-di-tert-butyl-4-methylphenol (BHT), stearic acid, stearate. In this adhesive E, the volume percentage of the skeleton component is 35-65%, preferably 50-60%; the volume percentage of the plasticizer component is 30-60%, preferably 35-45%; and the volume percentage of the dispersant component is 1-15%, preferably 5%-10%.

[0025] As a preferred embodiment, adhesive E is composed of paraffin wax, palm wax, polypropylene, high-density polyethylene, low-density polyethylene, polymethyl methacrylate, stearic acid, dioctyl phthalate, and BHT (2,6-di-tert-butyl-4-methylphenol); by volume ratio, paraffin wax: palm wax: polypropylene: high-density polyethylene: low-density polyethylene: polymethyl methacrylate: stearic acid: dioctyl phthalate: BHT = 30-40: 18-25: 10-15: 10-15: 8-15: 3-10: 5-8: 4-6: 0.3-0.6.

[0026] In step (2), the binder E and the ball-milled mixed powder D are added sequentially to the vacuum mixing chamber, and then the mixing chamber is evacuated to a vacuum value of <5×10. -2 MPa, preferably vacuum value <1×10 -2 MPa, and finally heated to the working temperature of 100-200℃, preferably 110-180℃, and the mixing time is greater than 30min, preferably greater than 40min, to obtain the mixed material F.

[0027] In step (2), the intensively mixed material F is added to the vacuum granulator, and then the material hopper of the granulator is evacuated to a vacuum value of <5×10. -2 MPa, preferably vacuum value <1×10 -2 MPa, and finally heated to the working temperature of 100-200℃, preferably 110-180℃, using a screw pressure of 3-10kg, preferably 5-8kg; extruded to obtain FDM printing feed G with a particle diameter of 1-4mm, preferably 2-4mm.

[0028] In step (3), the cemented carbide green blank H is produced by printing in an inert gas environment using granular printing feed G prepared in step (2) as raw material and an FDM device. The 3D printing process parameters are designed and optimized to reduce defects in the cemented carbide green blank H (such as wedge-shaped or rhomboid pores, interlayer cracks, etc.). The selectable printing process parameters of this invention are: nozzle orifice diameter of 0.2-0.8 mm, preferably 0.4-0.7 mm; printing temperature of 120-200℃, preferably 130-170℃; layer thickness of 0.05-0.8 mm, preferably 0.1-0.6 mm; printing speed of 15-75 mm / s, preferably 20-50 mm / s; and flow rate of 50-130%, preferably 60-100%.

[0029] The inert gas should be nitrogen, helium, argon, or a mixture of argon and helium, with a purity of 99.99 wt%, of which the oxygen content is less than 0.0001 wt%.

[0030] In step (4), the green body is first soaked in n-heptane for degreasing and then dried; then the green body is placed in a sintering furnace and subjected to medium-low temperature hot degreasing treatment, with a temperature range of 400-750℃, a heating rate of 0.1-8℃, preferably 0.1-6℃, and more preferably 0.3-5℃ / min, and a holding time of more than 30min.

[0031] In step (5) and step (4), the hot degreasing process uses the same sintering furnace. The degreased green blank I is placed in a vacuum or inert gas environment for a two-step sintering treatment: First, using a hot degreasing-sintering integrated furnace, the hot degreased green blank is rapidly heated from the hot degreasing temperature to 1300-1380℃ in a vacuum at a heating rate of 5-20℃ / min, and held for 0.2-1h. The preferred sintering temperature is 1320-1375℃, and the preferred holding time is 0.2-0.5h. Second, inert gas is introduced and pressurized to 3-9MPa, and held for 0.2-1h, with the preferred pressure being 3-6MPa and the preferred holding time being 0.2-0.5h. Then, it is cooled to room temperature at a cooling rate of 10-20℃ / min to obtain the nano-WC-Co cemented carbide solid part J.

[0032] This invention solves the technical problem of easy oxidation and carbon loss in nano-hard alloys by controlling powder properties, using vacuum mixing and granulation process, optimizing the printing environment of FDM equipment to an inert gas environment, and reducing sintering temperature and sintering time. It can also eliminate the η phase in the microstructure, thereby achieving the goal of simultaneously improving strength and fracture toughness.

[0033] Advantages and positive effects of the present invention:

[0034] The difficulty in preparing nanocrystalline cemented carbide lies in the fact that the small size of the raw material powder makes it extremely susceptible to oxygen absorption and oxidation, leading to uncontrollable decarburization; nanocrystalline grains are also prone to abnormal growth, reducing mechanical properties. The combined challenges of cemented carbide additive manufacturing and nanocrystalline cemented carbide preparation make the additive manufacturing of nanocrystalline cemented carbide even more difficult. This invention proposes for the first time a method for additive manufacturing of nanocrystalline cemented carbide, which can efficiently prepare high-performance nanocrystalline cemented carbide products with complex structures.

[0035] (1) This invention proposes for the first time a method for additive manufacturing of nanocrystalline cemented carbide. Through the design of binder composition, the control of raw material powder characteristics, and the synergistic effect of process parameters in each step, complex-shaped nanocrystalline cemented carbide products with high relative density, fine and uniform grains, and excellent mechanical properties can be prepared. It effectively solves the long-standing problems in cemented carbide additive manufacturing, such as the difficulty in eliminating porosity, cracks, brittle η phase, and abnormal grain growth in existing additive manufacturing processes. In particular, the large specific surface area and high surface activity of the powder during the preparation process lead to difficulties in oxidation control, easy formation of brittle η phase, and abnormal grain growth in the preparation of nanocrystalline cemented carbide.

[0036] (2) The present invention designs a set of binders that combine plasticizers, skeletons and dispersants. The binder has good thermoplasticity and high temperature stability. It is suitable for powders of various particle sizes and can be used to prepare printing feeds with high powder loading and shear thinning characteristics of pseudoplastic fluids. It can be used for FDM to prepare cemented carbide green blanks with high relative density and good dimensional accuracy. In addition, the binder contains a large number of plasticizers and has good dispersion uniformity. It can significantly reduce the adhesion of nanopowders and the frictional resistance between powders. It can effectively coat the ball-milled nanopowders, so that the printing feed has excellent rheological properties.

[0037] (3) This invention is the first to propose a vacuum mixing-granulation process, which avoids oxidation of the raw material powder due to high temperature during mixing and effectively eliminates metallurgical defects such as the η phase in the microstructure. Optimization of process parameters improves the efficiency of vacuum mixing, ensuring uniform dispersion of the binder in the mixed powder and solving the problem of easy oxidation of nanocrystalline printing feedstock. Simultaneously, the vacuum mixing-granulation process provides the necessary conditions for the subsequent preparation of high-quality nano-hard alloys within a wider range of preparation processes.

[0038] (4) This invention optimizes the FDM working environment to an inert gas environment for the first time and optimizes the FDM printing process parameters, eliminating wedge-shaped or rhomboid pores in the green blank, avoiding the oxidation of the printing feed at a higher FDM printing temperature, which leads to the decarburization of the final sintered sample, and can prepare green blanks with complex shapes of high relative density and no defects in cemented carbide.

[0039] (5) This invention, by adding a suitable nanocrystal growth inhibitor and distributing it uniformly in the raw material powder through ball milling and vacuum mixing, facilitates uniform distribution of WC grains at the WC grain boundaries in the sintered sample, thereby reducing the temperature at which the liquid phase appears in the sintering system. Simultaneously, the two-step sintering process, combining vacuum and pressure sintering of the cemented carbide green blank at a temperature lower than the traditional liquid phase sintering temperature for cemented carbide, enables rapid densification of the sintered body while preventing the coalescence and growth of WC grains. The resulting nano-cemented carbide has finer grain size, with WC grains smaller than 0.2 μm; high relative density (greater than 99%); and excellent mechanical properties, with a hardness of 2000-2100 HV. 30 Flexural strength 3500-5000MPa, fracture toughness 9-11MPa·m 1 / 2 It has no decarburization and no brittle phase. Attached Figure Description

[0040] Figure 1 This is a metallographic microscope image of a cemented carbide sample after degreasing and sintering according to Embodiment 1 of the present invention.

[0041] Figure 2 This is a SEM image of a cemented carbide sample after degreasing and sintering according to Embodiment 1 of the present invention.

[0042] Figure 3 This is an XRD image of a cemented carbide sample after degreasing and sintering, which is a comparative example of this invention.

[0043] Figure 4 This is a metallographic microscope image of a cemented carbide sample after degreasing and sintering, which is a comparative example of this invention. Detailed Implementation

[0044] Example 1:

[0045] (1) The powder raw material composition consists of 89 wt.% of particles with a diameter of D 50 It is an ultrafine polycrystalline WC powder with a particle size of 1.0 μm and a particle size of 10 wt.%. 50 Co powder with a particle size of 5 μm and a particle size D of 0.5 wt.%. 50 V8C7 with a particle size of 0.06 μm and a particle size D of 0.5 wt.% 50 The nanocrystalline cemented carbide mixed powder, composed of 0.2 μm Cr3C2, was prepared by ball milling for 76 h. 1.45 kg was measured for later use.

[0046] (2) Measure 10cm at room temperature. 3 Stearic acid, 10cm 3 High-density polyethylene, 10cm 3 Low-density polyethylene, 10cm 3 Polypropylene, 25cm 3 Paraffin wax, 30cm 3Prepare palm wax. After the vacuum mixer has preheated to 50°C for 15 minutes, add 10cm of palm wax sequentially. 3 Stearic acid, 10cm 3 High-density polyethylene, 10cm 3 Low-density polyethylene, 10cm 3 Polypropylene, 25cm 3 Paraffin wax, 30cm 3 Microcrystalline wax and 1.45 kg of mixed powder D from step (1) were added to the mixing chamber of the internal mixer. The pressure hammer was turned off, and the internal air pressure was evacuated to 5 × 10⁻⁶ using a vacuum pump. -2 The pressure was increased to MPa, and then the internal mixer was heated to 160℃. The specific parameters were designed as follows: mixing temperature 160℃, mixing time 90min, to prepare a mixed material with a powder loading of 54%. After cooling, the mixed material was placed in a vacuum granulator, and the granulator was evacuated to a vacuum of 5×10 MPa. -2 After MPa, the temperature of the sealed chamber is raised to 120℃, the screw speed is 60rpm, and the pressure is 10kg for piston propulsion, producing granular printing feed with a particle size of about 2-3mm.

[0047] (3) Using the granular printing feed obtained in step (2) as raw material, the printing chamber of the fused deposition modeling (FDM) equipment is filled with 0.2m... 3 Inert gas was introduced at a flow rate of / min for 15min, and a cemented carbide blank was formed by 3D printing technology. The parameters were set as follows: nozzle diameter of 0.5mm, nozzle temperature of 150℃, layer thickness of 0.2mm, printing speed of 25mm / s, and fill rate of 100%. The resulting part blank was formed.

[0048] (4) The cemented carbide green blank printed in step (3) is immersed in n-heptane liquid at room temperature and sealed for 20 hours. After drying in a drying oven at 40°C for 6 hours, the blank is placed in a sintering furnace and subjected to medium-low temperature hot degreasing treatment. The temperature is raised from room temperature to 550°C at a heating rate of 2.5°C / min and held for 60 minutes to complete the hot degreasing.

[0049] (5) Using a hot degreasing-sintering integrated furnace, the cemented carbide green billet treated in step (4) is heated rapidly from the hot degreasing temperature to 1330℃ in a vacuum at a heating rate of 5-20℃ / min, held for 0.3h, then inert gas is introduced and pressurized to 6MPa, held for 0.2h; cooled to room temperature at a cooling rate of 10℃ / min, and depressurized to atmospheric pressure at a depressurization rate of 1MPa / min to obtain nano WC-Co cemented carbide solid parts.

[0050] The printing feedstock prepared using these parameters exhibits shear-thinning characteristics of a pseudoplastic fluid, demonstrating good printability. Through optimization of the nanocrystalline cemented carbide composition, improvement of the vacuum mixing-granulation process to protect the printing feedstock, and refinement of the sintering process, the relative density of the sintered sample after degreasing and sintering of the nanocrystalline cemented carbide green body reached 99.7%. Micro-CT analysis revealed no significant porosity in the sample. Metallographic microscopy showed that the porosity of the sample was A02B00, the non-combined carbon was C00 (100x metallographic analysis after polishing), and the η phase was E00 (slight corrosion from NaOH and K3Fe(CN)6 solution). Microstructure and mechanical property analysis showed that the average grain size of WC was 0.138 μm, and the hardness was 2093.8 HV. 30 The flexural strength is 5220 MPa, and the fracture toughness is 9.98 MPa·m. 1 / 2 .

[0051] Example 2:

[0052] (1) The powder raw material composition consists of 89 wt.% of particles with a diameter of D 50 Ultrafine polycrystalline WC powder with a particle size less than 1.0 μm and a particle size density of 10 wt.% 50 Co powder with a particle size of less than 5 μm and a particle size of 0.5 wt.% D 50 A nanocrystalline cemented carbide mixed powder was prepared by ball milling, consisting of V8C7 with a particle size of 0.1 μm and Cr3C2 with a particle size D5 of 0.5 μm, for 76 h. 1.45 kg was measured and prepared for use.

[0053] (2) Measure 10cm at room temperature. 3 Stearic acid, 10cm 3 High-density polyethylene, 10cm 3 Low-density polyethylene, 10cm 3 Polypropylene, 25cm 3 Paraffin wax, 30cm 3 Prepare palm wax. After the vacuum mixer has preheated to 50°C for 15 minutes, add 10cm of palm wax sequentially. 3 Stearic acid, 10cm 3 High-density polyethylene, 10cm 3 Low-density polyethylene, 10cm 3 Polypropylene, 25cm 3 Paraffin wax, 30cm 3 Microcrystalline wax and 1.45 kg of mixed powder D from step (1) were added to the mixing chamber of the internal mixer. The pressure hammer was turned off, and the internal air pressure was evacuated to 5 × 10⁻⁶ using a vacuum pump. -2The pressure was increased to MPa, and then the internal mixer was heated to 160℃. The specific parameters were designed as follows: mixing temperature 160℃, mixing time 90min, to prepare a mixed material with a powder loading of 50%. After cooling, the mixed material was placed in a vacuum granulator, and the granulator was evacuated to a vacuum of 5×10 MPa. -2 After MPa, the temperature of the sealed chamber is raised to 120℃, the screw speed is 60rpm, and the pressure is 10kg for piston propulsion, producing granular printing feed with a particle size of about 2-3mm.

[0054] (3) Using the granular printing feed obtained in step (2) as raw material, the printing chamber of the fused deposition modeling (FDM) equipment is filled with 0.2m... 3 Inert gas was introduced at a flow rate of / min for 15min. A cemented carbide blank was formed using fused deposition modeling equipment with 3D printing technology. The parameters were set as follows: nozzle diameter of 0.5mm, nozzle temperature of 150℃, layer thickness of 0.25mm, printing speed of 35mm / s, and fill rate of 90%. The resulting part blank was formed.

[0055] (4) The cemented carbide green blank printed in step (3) is immersed in n-heptane liquid and sealed. Then the n-heptane solution is heated to 40°C in a water bath and kept at that temperature for 14 hours. After drying at 60°C for 6 hours in a drying furnace, the blank is placed in a sintering furnace and subjected to medium-low temperature hot degreasing treatment. The temperature is raised from room temperature to 550°C at a heating rate of 2.5°C / min and held for 60 minutes to complete the hot degreasing.

[0056] (5) Using a hot degreasing-sintering integrated furnace, the cemented carbide green billet treated in step (4) is heated rapidly from the hot degreasing temperature to 1330℃ in a vacuum at a heating rate of 5-20℃ / min, held for 0.3h, then inert gas is introduced and pressurized to 6MPa, held for 0.2h; cooled to room temperature at a cooling rate of 10℃ / min, and depressurized to atmospheric pressure at a depressurization rate of 1MPa / min to obtain nano WC-Co cemented carbide solid parts.

[0057] The printing feedstock prepared using these parameters exhibits shear-thinning characteristics of a pseudoplastic fluid, demonstrating good printability. Through optimization of the nanocrystalline cemented carbide composition, improvement of the vacuum mixing-granulation process to protect the printing feedstock, and refinement of the sintering process, the relative density of the sintered sample after degreasing and sintering of the nanocrystalline cemented carbide green body reached 99.6%. Metallographic microscopy revealed that the porosity of the sample was A02B00, the non-combined carbon was C00 (100x metallographic analysis after polishing), and the η phase was E00 (slight corrosion from NaOH and K3Fe(CN)6) solution. Microstructure and mechanical property testing showed that the average grain size of WC was 0.188 μm, and the hardness was 2016.5 HV. 30The flexural strength is 4815 MPa, and the fracture toughness is 10.4 MPa·m. 1 / 2 .

[0058] Example 3:

[0059] (1) The powder raw material composition consists of 89 wt.% of particles with a diameter of D 50 Ultrafine polycrystalline WC powder with a particle size less than 1.0 μm and a particle size density of 10 wt.% 50 Co powder with a particle size of less than 5 μm and a particle size of 0.5 wt.% D 50 Nanocrystalline cemented carbide mixed powder was prepared by ball milling, consisting of V8C7 with a particle size of 0.1 μm and Cr3C2 with a particle size D5 of 0.5 wt.% for 76 h. 1.45 kg was measured for later use.

[0060] (2) Measure 10cm at room temperature. 3 Stearic acid, 10cm 3 High-density polyethylene, 10cm 3 Low-density polyethylene, 10cm 3 Polypropylene, 25cm 3 Paraffin wax, 30cm 3 Prepare palm wax. After the vacuum mixer has preheated to 50°C for 15 minutes, add 10cm of palm wax sequentially. 3 Stearic acid, 10cm 3 High-density polyethylene, 10cm 3 Low-density polyethylene, 10cm 3 Polypropylene, 25cm 3 Paraffin wax, 30cm 3 Microcrystalline wax and 1.45 kg of mixed powder D from step (1) were added to the mixing chamber of the internal mixer. The pressure hammer was turned off, and the internal air pressure was evacuated to 5 × 10⁻⁶ using a vacuum pump. -2 The pressure was increased to MPa, and then the internal mixer was heated to 160℃. The specific parameters were designed as follows: mixing temperature 160℃, mixing time 90min, to prepare a mixed material with a powder loading of 54%. After cooling, the mixed material was placed in a vacuum granulator, and the granulator was evacuated to a vacuum of 5×10 MPa. -2 After MPa, the temperature of the sealed chamber is raised to 120℃, the screw speed is 60rpm, and the pressure is 10kg for piston propulsion, producing granular printing feed with a particle size of about 2-3mm.

[0061] (3) Using the granular printing feed obtained in step (2) as raw material, the printing chamber of the fused deposition modeling (FDM) equipment is filled with 0.2m... 3Inert gas was introduced at a flow rate of / min for 15min. A cemented carbide blank was formed by 3D printing using a fused deposition modeling (FDM) machine. The parameters were set as follows: nozzle diameter of 0.4mm, nozzle temperature of 150℃, layer thickness of 0.2mm, printing speed of 45mm / s, and fill rate of 75%. The resulting part blank was formed.

[0062] (4) The cemented carbide green blank printed in step (3) is immersed in n-heptane liquid and sealed. Then the n-heptane solution is heated to 40°C in a water bath and kept at that temperature for 14 hours. After drying at 60°C for 6 hours in a drying furnace, the blank is placed in a sintering furnace and subjected to medium-low temperature hot degreasing treatment. The temperature is raised from room temperature to 550°C at a heating rate of 3°C / min and kept at that temperature for 60 minutes to complete the hot degreasing.

[0063] Step (5) is the same as in Example 1.

[0064] The printing feedstock prepared using these parameters exhibits shear-thinning characteristics of a pseudoplastic fluid, demonstrating good printability. Through optimization of the nanocrystalline cemented carbide composition, improvement of the vacuum mixing-granulation protection printing feedstock, and refinement of the sintering process, the relative density of the sintered sample after degreasing and sintering of the nanocrystalline cemented carbide green body reached 99.3%. Metallographic microscopy results showed that the porosity of the sample was A02B00, the non-combined carbon was C00 (100x metallographic analysis after polishing), and the η phase was E00 (slight corrosion from NaOH and K3Fe(CN)6 solution). Microstructure and mechanical property testing results showed that the average grain size of WC was 0.170 μm, and the hardness was 2026 HV. 30 The flexural strength is 4837 MPa, and the fracture toughness is 10.1 MPa·m. 1 / 2 .

[0065] Example 4:

[0066] (1) The powder raw material composition consists of 89 wt.% of particles with a diameter of D 50 Ultrafine polycrystalline WC powder with a particle size less than 1.0 μm and a particle size density of 10 wt.% 50 Co powder with a particle size of less than 5 μm and a particle size of 0.5 wt.% D 50 Nanocrystalline cemented carbide mixed powder was prepared by ball milling, consisting of V8C7 with a particle size of 0.1 μm and Cr3C2 with a particle size D5 of 0.5 wt.% for 76 h. 1.45 kg was measured for later use.

[0067] (2) Consistent with Example 1

[0068] (3) The printing chamber of the fused deposition modeling (FDM) equipment is positioned at 0.2m... 3Inert gas was introduced at a flow rate of / min for 15min. A cemented carbide green blank was formed by 3D printing using a fused deposition modeling (FDM) machine. The parameters were set as follows: nozzle diameter of 0.4mm, nozzle temperature of 148℃, layer thickness of 0.2mm, printing speed of 28mm / s, and fill rate of 100%. The green blank of the formed part was produced.

[0069] (4) Consistent with Example 1.

[0070] (5) Using a hot degreasing-sintering integrated furnace, the cemented carbide green billet treated in step (4) is heated rapidly from the hot degreasing temperature to 1375℃ in a vacuum at a heating rate of 5-20℃ / min, held for 0.4h, then inert gas is introduced and pressurized to 5MPa, held for 0.2h; cooled to room temperature at a cooling rate of 10℃ / min, and depressurized to atmospheric pressure at a depressurization rate of 1MPa / min to obtain nano WC-Co cemented carbide solid parts.

[0071] The relative density of the sintered sample after degreasing and sintering of the nanocrystalline cemented carbide green billet reached 99.5%. Metallographic microscopy results showed that the porosity of the sample was A02B00, the non-combined carbon was C00 (100x metallographic analysis after polishing), and the η phase was E00 (slight corrosion from NaOH and K3Fe(CN)6 solution). Microstructure and mechanical property tests showed that the average grain size of WC was 0.182 μm, and the hardness was 2038 HV. 30 The flexural strength is 4888 MPa, and the fracture toughness is 10.2 MPa·m. 1 / 2 .

[0072] Example 5:

[0073] (1) The powder raw material composition consists of 89 wt.% of particles with a diameter of D 50 Ultrafine polycrystalline WC powder with a particle size less than 1.0 μm and a particle size density of 10 wt.% 50 Co powder with a particle size of less than 5 μm and a particle size of 0.5 wt.% D 50 Nanocrystalline cemented carbide mixed powder was prepared by ball milling, consisting of V8C7 with a particle size of 0.1 μm and Cr3C2 with a particle size D5 of 0.5 wt.% for 100 h. 1.45 kg was measured for later use.

[0074] (2) Consistent with Example 1.

[0075] (3) A cemented carbide blank was formed by 3D printing using a fused deposition modeling (FDM) equipment. The parameters were set as follows: nozzle diameter of 0.4 mm, nozzle temperature of 148 °C, layer thickness of 0.2 mm, printing speed of 60 mm / s, and fill rate of 75%.

[0076] Other operating procedures are the same as in the embodiment.

[0077] The relative density of the sintered sample after degreasing and sintering of the nanocrystalline cemented carbide green billet reached 99.0%. Metallographic microscopy results showed that the porosity of the sample was A02B00, the non-combined carbon was C00 (100x metallographic analysis after polishing), and the η phase was E00 (slight corrosion from NaOH and K3Fe(CN)6 solution). Microstructure and mechanical property tests showed a coercivity of 32.9 kA / m, an average WC grain size of 0.198 μm, and a hardness of 2003.5 HV. 30 The flexural strength is 4789 MPa, and the fracture toughness is 10.4 MPa·m. 1 / 2 .

[0078] Comparative Example 1:

[0079] The difference from step (2) in Example 1 is that in Comparative Example 4, the vacuum environment in step (2) of Example 1 is eliminated, that is, the mixed powder and binder prepared in step (1) of Example 1 are mixed and granulated in an air environment to prepare granular printing feed.

[0080] The other operating steps are the same as in Example 1.

[0081] Microscopic observation revealed the presence of a distinct η phase in the sintered nanocrystalline cemented carbide, with a relative density of 99.8%. Furthermore, due to the formation of the brittle phase, the mechanical properties of the sample significantly decreased, with hardness, bending strength, and fracture toughness all reduced to 1445 HV. 30 1435MPa and 7.3MPa·m 1 / 2 .

[0082] Compared with Example 1, Comparative Example 4 did not use vacuum mixing-granulation. Due to the fine size and high surface activity of the powder raw material, it is very easy to oxidize in the air environment, resulting in the appearance of carbon-deficient η phase in the microstructure of the nanocrystalline cemented carbide, which reduces its mechanical properties.

[0083] Comparative Example 2:

[0084] The difference from step (3) in Example 1 is that, in Comparative Example 2, inert gas was not introduced into the printing chamber of the FDM device as in step (3) in Example 1, that is, the cemented carbide green blank prepared in step (3) of Example 1 was processed in an air environment.

[0085] Microscopic observation revealed the presence of a distinct η phase in the sintered nanocrystalline cemented carbide, with a relative density of 99.7%. Furthermore, due to the formation of the brittle phase, the mechanical properties of the sample significantly decreased, with hardness, bending strength, and fracture toughness all reduced to 1585 HV. 301595MPa and 8.8MPa·m 1 / 2 .

[0086] Compared with Example 1, in Comparative Example 2, no inert gas was introduced into the printing chamber of the FDM equipment. Due to the fine size and high surface activity of the powder raw material, it is very easy to oxidize in the air environment, resulting in the appearance of carbon-deficient η phase in the microstructure of the nanocrystalline cemented carbide, which reduces its mechanical properties.

[0087] Comparative Example 3:

[0088] The difference from step (1) described in Example 1 is that the powder raw material composition consists of 89 wt.% of particles with a diameter D 50 Ultrafine polycrystalline WC powder with a particle size less than 1.0 μm and a particle size density of 10 wt.% 50 Co powder with a particle size of less than 5 μm and a particle size of 0.5 wt.% D 50 Nanocrystalline cemented carbide mixed powder was prepared by ball milling, consisting of V8C7 with a particle size of 2-4 μm and 0.5 wt.% Cr3C2 with a particle size D5 of 2-4 μm. The ball milling time was 120 h. 1.45 kg was measured and prepared for use.

[0089] The other operating steps are the same as in Example 1.

[0090] After debinding and sintering, microscopic observation and mechanical property testing showed that the prepared nanocrystalline cemented carbide had no η phase formation, a relative density of 99.6%, and a slight reduction in the average grain size of WC to 0.32 μm. The hardness, bending strength, and fracture toughness of the parts were 1931 HV. 30 3816 MPa and 13.1 MPa·m 1 / 2 .

[0091] Compared to Example 1, when the grain growth inhibitor itself has excessively coarse grains, it promotes the aggregation of WC grains during sintering, leading to WC grain aggregation and recrystallization. This results in uneven distribution of the grain growth inhibitor, causing uneven growth of WC grains. Furthermore, while increased ball milling time reduces the size of WC grains in the mixed powder, it also increases lattice distortion, negatively impacting the control of alloy grain size during sintering. This results in coarser WC grains in the sintered sample, decreased sample hardness and flexural strength, and a slight increase in fracture toughness.

[0092] Comparative Example 4:

[0093] The difference from step (1) in Example 1 is that in Comparative Example 4, the printing parameters are modified as follows: nozzle diameter is 0.6 mm, nozzle temperature is 150 °C, layer thickness is 0.5 mm, printing speed is 60 mm / s, extrusion flow rate is 100%, and filling rate is 100%.

[0094] The other operating steps are the same as in Example 1.

[0095] After debinding and sintering, microscopic observation and mechanical property testing showed that the prepared nanocrystalline cemented carbide had no η phase formation, a relative density of up to 96.4%, and an average WC grain size of 0.146 μm. The hardness, bending strength, and fracture toughness of the parts were 1917.2 HV. 30 3610MPa and 9.1MPa·m 1 / 2 .

[0096] After 3D printing using these parameters, due to the excessive printing speed and high layer thickness, the feed extruded from the nozzle could not fill the matrix well. Microscopic observation showed that obvious stacked wedge-shaped holes or rhomboid pores appeared in the prepared cemented carbide green blank. The cemented carbide sintered sample after degreasing and sintering had a lower relative density. Compared with Example 1, the mechanical properties of the sintered sample were reduced.

[0097] Comparative Example 5:

[0098] The difference from step (5) in Example 1 is that in Comparative Example 5, the temperature is adjusted to 1400℃, the heating rate is adjusted to 8℃ / min, and the holding time for both vacuum sintering and pressure sintering is adjusted to 1h.

[0099] The other operating steps are the same as in Example 1.

[0100] Microscopic observation showed that the nanocrystalline cemented carbide sample prepared after sintering had a relative density of 99.7%, an average grain size of 0.462 μm, and hardness, bending strength, and fracture toughness of 1887 HV. 30 3899MPa and 13.1MPa·m 1 / 2 .

[0101] Compared with Example 1, Comparative Example 5 had a higher sintering temperature. While the relative density of the prepared nanocrystalline cemented carbide did not change significantly, the higher sintering temperature and longer holding time resulted in a significant increase in the WC grain size. Compared with Example 1, the hardness and flexural strength of the sintered sample decreased, while the fracture toughness increased slightly.

[0102] Comparative Example 6:

[0103] (1) Unlike step (1) in Example 1, the powder raw material composition consists of 89 wt.% of particles with a diameter D 50 Ultrafine polycrystalline WC powder with a particle size less than 1.0 μm and a particle size density of 10 wt.% 50 Co powder with a particle size of less than 5 μm and a particle size of 0.5 wt.% D 50Nanocrystalline cemented carbide mixed powder was prepared by ball milling, consisting of V8C7 with a particle size of 0.1 μm and Cr3C2 with a particle size D5 of 0.5 wt.% for 90 h. 2.5 kg was measured for later use.

[0104] The other operating steps are the same as in Example 1.

[0105] Because the powder loading in the feed prepared by this method is too large (about 64%), the feed viscosity increases significantly. Furthermore, due to the limited pressure of the screw extruder in the fused deposition modeling equipment, the feeding and extrusion process is not smooth, the nozzle is blocked, and the feeding and deposition are not connected, which makes it impossible for the parts to be formed normally.

[0106] Obviously, the above embodiments and comparative examples are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for additive manufacturing of nanocrystalline cemented carbide, characterized in that: Includes the following steps: (1) Ball milling to prepare mixed powder: Ultrafine polycrystalline WC powder A, ordinary Co powder B, and nanocrystal growth inhibitor C were ball-milled in a designed ratio to fully break down the polycrystalline WC powder and prepare a uniformly dispersed mixed powder D. In step (1), the particle size D of powder A is... 50 Powder B particle size D <1.0μm 50 Powder particle size C <10μm D 50 The powder is less than 0.2 μm; powder A has a mass fraction of 80-97%; powder B has a mass fraction of 3-19%; powder C has a mass fraction of 0.1-5%, and the sum of the mass fractions of powders A, B, and C is 100%; powder D is a mixture obtained by ball milling powders A, B, and C; the mixture D contains WC-Co, V8C7, and Cr3C2, and the mass ratio of WC-Co:V8C7:Cr3C2 is 96-99:0.5-1:0.5-1; (2) Vacuum mixing-granulation preparation of printing feed: Mix powder D and binder E according to the design ratio, and put them into a vacuum mixing chamber for mixing to prepare a uniformly mixed mixture F of mixed powder D and binder E. Put the prepared mixed mixture F into a vacuum granulator to prepare granular printing feed G. The adhesive E comprises a skeleton component, a plasticizer component, and a dispersant component; the skeleton component is at least one selected from polyethylene, polypropylene, polyethylene glycol, ethylene-vinyl acetate copolymer, polymethyl methacrylate, polystyrene, polyvinyl butyral, and polyacetal; the plasticizer component is at least one selected from paraffin wax, microcrystalline wax, tung oil, quartz, beeswax, and carnauba wax; the dispersant component is at least one selected from fatty acids, polyacrylamide, 2,6-di-tert-butyl-4-methylphenol, stearic acid, and stearate; wherein the volume percentage of the skeleton component is 35-65%; the volume percentage of the plasticizer component is 30-60%; and the volume percentage of the dispersant component is 1-15%. In the intensively mixed material F mentioned in step (2), the volume percentage of mixed powder D is 40-75%, and the volume percentage of binder E is 25-60%. Here, the volume percentage of D in F is defined as the powder loading amount. (3) Fused deposition modeling (FDM) for green body preparation: Using the granular printing feed G obtained in step (2) as raw material, cemented carbide green blank H is prepared by printing in an inert gas environment using a fused deposition modeling (FDM) device. The printing parameters in step (3) are: nozzle diameter 0.2-0.8mm, printing temperature 120-200℃, layer thickness 0.05-0.8mm, printing speed 15-75mm / s, and flow rate 50-130%; The inert gas is nitrogen, helium, argon, or a mixture of argon and helium, with a purity of 99.99 wt%, wherein the oxygen content is less than 0.0001 wt%. (4) Degreasing of green blank: Solvent degreasing and thermal degreasing are performed on the cemented carbide green blank H prepared in step (3) to obtain degreased green blank I; (5) Two-step sintering: The degreased green blank I treated in step (4) is sintered in two steps to obtain nanocrystalline WC-Co cemented carbide solid part J; specifically, it includes: First step, using a hot degreasing-sintering integrated furnace, the hot degreased green blank is rapidly heated from the hot degreasing temperature to 1300-1380℃ in a vacuum atmosphere at a heating rate of 5-20℃ / min, and held for 0.2-1h; Second step, inert gas is introduced and pressurized to 6MPa, and held for 0.2-1h; Then, it is cooled to room temperature at a cooling rate of 10-20℃ / min to obtain nanocrystalline WC-Co cemented carbide solid part J.

2. The method for additive manufacturing of nanocrystalline cemented carbide according to claim 1, characterized in that: The ball milling time in step (1) is 10-120h, and the ball-to-material ratio is 3:1 to 18:

1.

3. The method for additive manufacturing of nanocrystalline cemented carbide according to claim 1, characterized in that: Step (2) Add the binder E and the ball-milled mixed powder D into a vacuum mixer in sequence, heat it to 100-200℃ in a vacuum environment, and mix for more than 30 minutes to obtain the mixed material F.

4. The method for additive manufacturing of nanocrystalline cemented carbide according to claim 1, characterized in that: Step (2) Place the intensively mixed material F into a vacuum granulator, heat it to 100-200℃ in a vacuum environment, and use a screw pressure of 3-10kg to extrude FDM printing feed material G with a particle diameter of 1-5 mm.

5. The method for additive manufacturing of nanocrystalline cemented carbide according to claim 1, characterized in that: In step (4), the green blank is first soaked in n-heptane for degreasing and then dried; then the green blank is placed in a sintering furnace and subjected to medium-low temperature hot degreasing treatment, with a temperature range of 400-750℃, a heating rate of 0.1-8℃ / min, and a holding time of more than 30min.

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