A method for additive manufacturing of ultrafine-grained cemented carbide

Abstract

This invention relates to an additive manufacturing method for ultrafine-grained cemented carbide. Using WC-Co as raw material, grain growth inhibitors and rare earth oxides as reinforcing phases are added. Paraffin wax and trace amounts of low-valent rare earth oxide powder are added, and the cemented carbide powder is ball-milled and oxygen is controlled. The ball-milled cemented carbide powder is then mixed with an organic binder in a vacuum mixer-granulator to prepare a printing feedstock. Fused deposition modeling (FDM) is then used to print a green compact, which is finally degreased and sintered in two steps to produce the ultrafine-grained cemented carbide. This invention, through the synergistic effect of composition and process optimization, controls powder characteristics, broadens the range of vacuum mixer-granulation and additive manufacturing processes, and solves problems such as porosity, interlayer cracks, oxygen absorption in printed green compacts, and abnormal grain growth, oxidation and decarburization, and formation of brittle η-phase in sintered samples. The prepared ultrafine-grained cemented carbide has high relative density, no metallurgical defects, and excellent mechanical properties, making it suitable for large-scale mass production.

Description

Technical Field

[0001] This invention relates to a method for additive manufacturing of ultrafine-grained cemented carbide, belonging to the field of cemented carbide and additive manufacturing technology. Background Technology

[0002] Cemented carbide is a metal-ceramic composite material made from refractory metal carbides and binder metals using powder metallurgy methods. It possesses high hardness, high strength, and high wear resistance, and is widely used in aerospace, mineral exploration, and machinery manufacturing. Ultrafine grain cemented carbide is a metal-ceramic composite material composed of refractory metal carbides with grain sizes less than 0.5 μm and binder metals. It is currently used to manufacture high-efficiency precision cutting tools, micro-drills, and micro-milling cutters, and is mainly prepared using powder metallurgy methods such as powder injection molding (PIM), powder compression molding (CM), and powder extrusion molding (EM).

[0003] In recent years, there has been an urgent need for cemented carbide parts with complex geometries, and additive manufacturing (AM) technology has provided a new approach to address this. Among existing AM technologies, powder bed fusion (PBF) has high requirements for the morphology, sphericity, particle size distribution, loose packing density, and tap density of the raw powder. The resulting samples are prone to exhibiting numerous metallurgical defects such as persistent porosity, cracks, and brittle η phase, leading to poor mechanical properties. In particular, ultrafine-grained cemented carbides are highly susceptible to powder oxidation, brittle η phase formation, and abnormal grain growth during PBF, resulting in severely cracked samples with low relative density, making it difficult to meet mechanical property requirements.

[0004] To address the aforementioned issues, Chinese patent (CN202310192309.4) discloses a cemented carbide and its additive manufacturing method. This method utilizes photopolymerization-debinding sintering followed by hot isostatic pressing (HIP) densification to obtain WC-Ni cemented carbide parts with a relative density of 99.8% and a hardness of 1400 HV3. Mariani et al. [Mariani M, et al., Mechanical and microstructural characterization of WC-Co consolidated by binder jetting additive manufacturing[J]. International Journal of Refractory Metals and Hard Materials 100(2021)105639.] prepared a WC-12Co cemented carbide with a relative density of 97.4% using BJAM (Binder jetting additive manufacturing). After HIP treatment, the relative density reached 99.3%, and its hardness was 1205 HV3. 10The bending strength was 2257 MPa, but the mechanical properties before HIP were not reported. Lee et al. [Lee SW, et al., Phase control of WC-Co hardmetal using additive manufacturing technologies[J]. Powder Metallurgy 65(2021)13-21.] increased the packing density of FDM (Fused deposition modeling) wire, thereby increasing the green density. After debinding and sintering, they obtained cemented carbide parts with a relative density of 96.3% and a hardness of 89 HRA. Lengauer et al. [Lengauer W, et al., Fabrication and properties of extrusion-based 3D-printed hardmetal and cermet components[J]. International Journal of Refractory Metals & Hard Materials 82(2019)141-149.] prepared WC-10%Co cemented carbide indexable inserts using FDM-debinding sintering process, but no data on relative density and mechanical properties were reported. Hard alloys prepared by additive manufacturing methods such as photopolymerization and BJAM have low relative density and require HIP densification treatment, which significantly increases post-processing costs and manufacturing process requirements. Hard alloy parts prepared by FDM method have problems such as low relative density, low strength and hardness, and poor toughness.

[0005] Currently, there are no reports of using AM technology to prepare high-quality ultrafine-grained cemented carbides. Summary of the Invention

[0006] To address the aforementioned problems, this invention presents a method for additive manufacturing of ultrafine-grained cemented carbide with high relative density, high hardness, and high strength. First, using WC-Co powder as the main raw material, and grain growth inhibitors (GGIs) and rare earth oxides as reinforcing phases, paraffin wax and / or trace amounts of low-valent rare earth elements are added, and a uniformly dispersed mixed powder is prepared using a ball milling process. Second, a printing feedstock is prepared using vacuum mixing and granulation. Third, a high-relative-density green blank is prepared using FDM printing. Finally, the green blank is degreased and sintered in two steps to obtain an ultrafine-grained cemented carbide product free of metallurgical defects and possessing excellent mechanical properties. This invention eliminates microstructural defects and refines the grain size of the cemented carbide through the synergistic effect of alloy composition control and process parameter optimization, thereby improving the relative density and comprehensive mechanical properties of cemented carbide parts.

[0007] This invention provides a method for additive manufacturing of ultrafine-grained cemented carbide:

[0008] First, paraffin wax is added to the powdered raw material, and a uniformly dispersed mixed powder is prepared by ball milling. The added paraffin wax can completely coat the raw material powder after ball milling.

[0009] Alternatively: First, paraffin wax and trace amounts of low-valent rare earth elements are added to the powdered raw material, and a uniformly dispersed mixed powder is prepared using a ball milling process; the raw material powder includes cemented carbide powder, grain growth inhibitors, and rare earth oxides; the added paraffin wax can completely coat the raw material powder after ball milling; the valence state of the rare earth elements in the low-valent rare earth elements is less than or equal to +2; the valence state of the rare earth oxides in the rare earth oxides is greater than +2.

[0010] Second, vacuum mixing-granulation is used to prepare additive manufacturing printing feedstock;

[0011] Third, fused deposition modeling (FDM) is used to prepare green blanks;

[0012] Fourth, the green blank is degreased and then sintered in two steps to obtain an ultrafine-grained cemented carbide product without metallurgical defects. The temperature of the first sintering step is 1350-1500℃, and the temperature of the second sintering step is 30-100℃ lower than that of the first sintering step.

[0013] In this invention, low-valent rare earth elements include zero-valent rare earth elements and rare earth oxides with a valence state less than or equal to +2. The zero-valent rare earth element can be an intermediate alloy of rare earth and Co, or elemental rare earth. However, due to the high reactivity of rare earth elements, they are easily oxidized during production and transfer. Therefore, it is generally recommended to use low-valent rare earth oxides. These low-valent rare earth oxides may have an outer layer of oxidized rare earth, and the resulting oxide layer prevents oxygen from entering, keeping the inner rare earth element at zero valence. This outer oxide layer will then detach during ball milling; subsequently, the inner zero-valent rare earth element will react rapidly with oxygen in other raw materials, thereby removing oxygen adsorbed by those materials.

[0014] This invention discloses an additive manufacturing method for ultrafine-grained cemented carbide, specifically comprising the following steps:

[0015] (1) Ball milling to prepare mixed powder:

[0016] WC-Co powder, grain growth inhibitor, rare earth oxide, and paraffin were ball-milled according to the designed ratio to prepare a uniformly dispersed mixed powder.

[0017] Alternatively: Ball mill WC-Co powder, grain growth inhibitor, rare earth oxide, paraffin, and low-cost rare earth according to the designed ratio to prepare a uniformly dispersed mixed powder.

[0018] Powder ball milling is carried out in a protective atmosphere. Through ball milling, the cemented carbide powder is fully coated and oxygen is controlled.

[0019] (2) Vacuum mixing-granulation preparation of printing feed:

[0020] Mix the powder and organic binder according to the design ratio, and put them into the vacuum mixing chamber of the internal mixer for internal mixing to prepare a uniformly mixed mixture of powder and organic binder. Then, put the prepared internal mixing mixture into a vacuum granulator to prepare granular printing feed.

[0021] (3) Fused Deposition Modeling (FDM) for Preparing Printed Green Parts:

[0022] Using the granular printing feed obtained in step (2) as raw material, cemented carbide printing green blanks were prepared using FDM equipment;

[0023] (4) Green blank degreasing: The cemented carbide green blank prepared by printing in step (3) is subjected to solvent degreasing and thermal degreasing to obtain a degreased green blank;

[0024] (5) Two-step sintering: The degreased green blank treated in step (4) is sintered in two steps to obtain ultrafine WC-Co cemented carbide products.

[0025] Preferably, in the additive manufacturing method of ultrafine-grained cemented carbide of the present invention, the raw materials WC-Co powder, grain growth inhibitors, and rare earth oxides with a particle size D are used. 50 The particle size can be less than 10 μm, preferably less than or equal to 5 μm. More importantly, the present invention does not have special requirements on the shape of the raw material powder.

[0026] In step (1), when using a drum ball mill, the ball-to-material ratio is 10:1 to 3:2, preferably 6:1 to 3:1; the rotation speed is 60 to 300 rpm, preferably 100 to 220 rpm; the milling time is 8 to 72 hours, preferably 24 to 60 hours; in the mixture, the mass fraction of WC-Co powder is 92 to 99%; the mass fraction of grain growth inhibitor is 0.5 to 4.5%; the mass fraction of raw material rare earth oxide is 0.1 to 1%; the mass fraction of paraffin is 0.5 to 5%; and the mass fraction of low-valent rare earth is 0 to 0.05%, preferably 0.01 to 0.05%.

[0027] Other ball milling processes are also applicable to this invention.

[0028] Both wet grinding and dry grinding are applicable to this invention; anhydrous ethanol is preferably used as the ball milling medium for wet grinding.

[0029] As a preferred embodiment, in the present invention, the paraffin wax added in step (1) accounts for 0.5 to 5% of the total mass of the mixture.

[0030] This invention discloses an additive manufacturing method for ultrafine-grained cemented carbide. When low-valent rare earth elements are added, the amount of low-valent rare earth elements added accounts for 0.01 to 0.05% of the total mass of the mixture. When the low-valent rare earth elements are added in the form of low-valent rare earth oxides, the amount added is calculated based on the amount of low-valent rare earth oxides. When the low-valent rare earth elements are added in the form of zero-valent rare earth elements, the amount added is calculated based on the amount of zero-valent rare earth elements added.

[0031] The present invention discloses an additive manufacturing method for ultrafine-grained cemented carbide. In step (1), powder ball milling is carried out in a protective atmosphere, wherein the protective atmosphere is one of nitrogen, argon, and helium, or a mixture of several gases, with a purity of 99.99 wt%, wherein the oxygen content is less than 0.0001 wt%.

[0032] This invention discloses an additive manufacturing method for ultrafine-grained cemented carbide. In step (1), paraffin wax or paraffin wax and trace amounts of low-valent rare earth elements are added. The powder is then fully coated and its residual oxygen is adsorbed through ball milling. This prevents the WC-Co powder, grain growth inhibitors, and the mixed powder prepared by ball milling from absorbing oxygen and oxidizing during the ball milling process, as well as from absorbing oxygen and oxidizing the mixed powder during subsequent processing. The prepared mixed powder is fully coated with paraffin wax, and the residual oxygen in the powder reacts in situ with the low-valent rare earth elements and their oxides to generate nano-rare earth oxides, thereby minimizing the absorption of oxygen and oxidation.

[0033] As a preferred embodiment, the present invention provides an additive manufacturing method for ultrafine-grained cemented carbide, wherein the mixed powder comprises paraffin wax, WC-Co, VC, Cr3C2, rare earth oxide powder, and low-valent rare earth oxide in a mass ratio of WC-Co:VC:Cr3C2:paraffin wax:rare earth oxide:low-valent rare earth oxide = 96-98.5:0.5-1:0.5-1:0.45-1:0.1-2:0.01-0.05; wherein the rare earth oxide is selected from at least one of CeO2, La2O3, Y2O3, etc., and the low-valent rare earth oxide is selected from Ce2O3, Ce7O 12 At least one of La5O7, YO, etc.

[0034] In the internal mixing mixture described in step (2), the volume percentage of the mixed powder is 40-75%, and the volume percentage of the organic binder is 25-60%. The volume percentage of the mixed powder in the internal mixing mixture is defined as the powder loading amount.

[0035] This invention optimizes the composition of an organic adhesive, which includes 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 includes at least one selected from paraffin wax (including the amount added during ball milling), microcrystalline wax, tung oil, quartz, beeswax, and palm wax. The dispersant component is at least one selected from fatty acids, polyacrylamide, 2,6-di-tert-butyl-4-methylphenol (BHT), stearic acid, and stearate. The volume percentage of the skeleton component in the organic adhesive 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%.

[0036] As a preferred embodiment, the organic adhesive is composed of paraffin wax, carnauba 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 (including the amount of paraffin wax added in the ball milling process): carnauba 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.

[0037] The amount of paraffin used in step (2) includes the amount of paraffin added in the ball milling process in step (1), that is, the amount of paraffin in the organic adhesive in step (2), which is composed of the amount of paraffin used in the ball milling process in step (1) and the amount of paraffin added later.

[0038] In step (2), the organic binder and the ball-milled mixed powder are added to the vacuum mixing chamber of the internal mixer in sequence, and then the mixing chamber is evacuated to a vacuum degree greater than -0.05MPa, preferably greater than -0.08MPa; protective gas is introduced, and then heated to a working temperature of 100-200℃, preferably 110-180℃, and the mixing time is greater than 30min, preferably greater than 40min, to ensure that the organic binder and the mixed powder are mixed evenly and fully coated, so as to obtain the mixed material.

[0039] In step (2), the intensively mixed material is added to a vacuum granulator, and the material hopper of the granulator is evacuated to a vacuum degree greater than -0.05MPa, preferably greater than -0.08MPa, and a protective gas is introduced; finally, it is heated to a working temperature of 100-200℃, preferably 110-180℃, and a screw pressure of 3-10kg is used, preferably 5-8kg; the material is extruded and sheared to obtain a printing feed with a particle diameter of 1-4mm, preferably 2-4mm.

[0040] In step (3), the cemented carbide green blank is obtained by printing using the granular printing feed 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 (such as wedge-shaped or rhomboid pores, interlayer cracks, etc.). The printing process parameters selectable in this invention are: nozzle orifice diameter of 0.2-0.8 mm, preferably 0.2-0.7 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%.

[0041] In step (4), the green body is first soaked in n-heptane for degreasing and then dried; then the green body degreased by n-heptane is placed in a hot degreasing-sintering integrated furnace for hot degreasing treatment. The degreasing temperature is 400-750℃, the heating rate is 0.1-8℃, preferably 0.1-6℃, and more preferably, the heating rate is 0.3-5℃ / min, and the degreasing time is greater than 30min.

[0042] In step (5), the degreased green billet is sintered in a vacuum or inert gas environment in an integrated hot degreasing-sintering furnace. The two-step sintering process can reduce the temperature of the liquid phase sintering system, avoid the coalescence and growth of WC grains, and eliminate abnormal grain growth: In the first stage, the temperature is rapidly increased from the hot degreasing temperature to the first stage sintering temperature T1 (1350-1500℃) at a heating rate of 3-20℃ / min in a vacuum atmosphere, and held for 0.5-4h; preferably, the heating rate is 5-10℃ / min, the preferred sintering temperature is T1 (1350-1450℃), and the preferred holding time is 0.5-4h. The first stage sintering temperature T2 is 30-100℃ lower than the first sintering temperature T1. The temperature is lowered from T1 to T2 (1200-1350℃) at a cooling rate of 5-20℃ / min, and held for 3-10 hours. The preferred cooling rate is 5-10℃ / min, the preferred sintering temperature is T2 (1250-1350℃), and the preferred holding time is 5-8 hours. This completes the two-step sintering process, yielding an ultrafine-grained WC-Co cemented carbide product. In the first stage (T1), the sintered sample rapidly densifies. In the second stage (T2), the temperature is 30-100℃ lower than T1, and the long holding time at this low temperature results in almost no growth driving force for WC grains. Residual pores in the sample are eliminated through grain boundary diffusion, which requires a relatively long diffusion time, ultimately resulting in a fine-grained ultrafine-grained WC-Co cemented carbide.

[0043] Advantages and positive effects of the present invention:

[0044] The challenge of manufacturing ultrafine-grained cemented carbide and its additive manufacturing lies in the difficulty of controlling oxidation and decarburization, which easily leads to numerous metallurgical defects such as persistent porosity, cracks, brittle η phase, and abnormal grain growth, resulting in poor mechanical properties. This invention proposes for the first time an additive manufacturing method for ultrafine-grained cemented carbide. Through the synergistic effect of composition and process design, it effectively eliminates metallurgical defects such as oxygen absorption, oxidation, decarburization, porosity, cracks, brittle η phase, and abnormal grain growth, thereby improving mechanical properties, broadening the process range, and enabling the efficient preparation of high-performance ultrafine-grained cemented carbide products with complex structures.

[0045] (1) This invention proposes for the first time an additive manufacturing method for ultrafine-grained cemented carbide. Through the synergistic effect of composition design, various processes and process parameters, it is possible to prepare complex-shaped ultrafine-grained cemented carbide products with high relative density, fine and uniform grains, and excellent mechanical properties. It effectively solves the metallurgical defects such as porosity, cracks, brittle η phase and abnormal grain growth that are difficult to eliminate in existing additive manufacturing processes, as well as the long-standing problems of low relative density and insufficient mechanical properties that have plagued cemented carbide additive manufacturing; in particular, it addresses the difficulties in preparing ultrafine-grained cemented carbide, such as the difficulty in controlling oxidation and decarburization, the easy formation of brittle η phase, and abnormal grain growth.

[0046] (2) The present invention provides an additive manufacturing method for ultrafine-grained cemented carbide. Through composition optimization design, grain growth inhibitors are added and rare earth oxides are further added as reinforcing phases, which can effectively inhibit abnormal grain growth and improve mechanical properties. Furthermore, a trace amount of low-valence rare earth (elemental rare earth or Co-rare earth intermediate alloy or partially oxidized rare earth) is added to adsorb oxygen in cemented carbide powder and generate nano-rare earth oxides through in-situ reaction. This prevents the cemented carbide from oxidizing, decarburizing, and forming a brittle η phase. At the same time, the rare earth oxides generated in-situ act as a dispersed reinforcing phase, which strengthens the cemented carbide and transforms harmful oxygen into beneficial reinforcing phase components.

[0047] (3) Through process design, this invention first uses an appropriate amount of paraffin wax to fully coat the powder, and then employs a vacuum mixing-granulation process. This avoids oxygen absorption and oxidation of the raw material powder during ball milling, mixing, and FDM, effectively eliminating metallurgical defects such as oxidation, decarburization, and brittle η phase. After optimizing the process parameters, the vacuum mixing efficiency is further improved, ensuring that the organic binder is uniformly dispersed in the mixed powder, thus solving the preparation problem of ultrafine crystal printing feedstock. At the same time, the vacuum mixing-granulation process provides the necessary conditions for the subsequent preparation of high-quality ultrafine crystal cemented carbide under a wider range of preparation process conditions.

[0048] (4) This invention broadens the FDM printing process parameters through component design (including the introduction of low-cost rare earth), powder coating and vacuum mixing granulation, eliminates wedge-shaped or rhomboid pores and interlayer cracks in the green blank, and can prepare high relative density, pore-free cemented carbide green blanks with complex shapes.

[0049] (5) This invention employs a two-step sintering process to reduce the temperature of the liquid phase sintering system, thereby avoiding the coalescence and growth of WC grains and eliminating abnormal grain growth. The prepared ultrafine-grained cemented carbide is free from decarburization and brittle phases, eliminating abnormal grain growth. The grain size is finer and more uniform, with a high relative density (greater than 99%) and excellent mechanical properties, achieving a simultaneous improvement in the hardness, strength, and fracture toughness of the ultrafine-grained cemented carbide.

[0050] (6) This invention fully utilizes the advantages of additive manufacturing and powder metallurgy debinding and sintering, effectively eliminating metallurgical defects such as porosity, interlayer cracks, oxygen absorption and oxidation in green blanks, as well as oxidation, decarburization, brittle η phase, and abnormal grain growth in sintered samples. It completely eliminates metallurgical defects such as oxidation, decarburization, porosity, cracks, brittle η phase, and abnormal grain growth in the PBF process. It has no special requirements for raw material powders and can use ordinary commercial cemented carbide powder raw materials, resulting in low production costs. Attached Figure Description

[0051] Figure 1 This is a SEM image of the WC-9Co cemented carbide raw material powder used in a specific embodiment of the present invention.

[0052] Figure 2 This is a metallographic microscope image of an FDM-formed ultrafine-grained cemented carbide green billet in Embodiment 3 of the present invention (eliminating green billet defects).

[0053] Figure 3 This is a SEM image of the cemented carbide sample after degreasing and sintering in Example 3 of the present invention.

[0054] Figure 4 This is a macroscopic structural photograph of the FDM-debinded sintered ultrafine-grained cemented carbide sample of the present invention.

[0055] Example 1:

[0056] (1) The powder raw material consists of 96.5 wt.% WC-9Co, 0.5 wt.% VC, 0.5 wt.% Cr3C2, 0.45 wt.% CeO2, 0.05 wt.% Ce2O3 and 2 wt.% paraffin wax, with a particle size of less than 2 μm. It is prepared as an ultrafine-grained cemented carbide mixed powder by ball milling in a nitrogen atmosphere with the addition of an appropriate amount of alcohol for 30 h. 1.45 kg is measured for later use.

[0057] (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 (including the amount added during the ball milling process), 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 a vacuum degree of -0.07 MPa using a vacuum pump. After argon was introduced, the internal mixer was heated to 160°C. The specific parameters were designed as follows: mixing temperature of 160°C and mixing time of 90 min, to prepare a mixed material with a powder loading of 55%. After cooling the mixed material, it was placed in a vacuum granulator. After the granulator was evacuated to a vacuum degree of -0.07 MPa and argon was introduced, the temperature of the sealed chamber was set to 120°C, the screw speed was 60 rpm, and the pressure was 10 kg for piston propulsion, to obtain a granular printing feed with a powder loading of 55% and a particle size of about 2-3 mm.

[0058] (3) Using the granular printing feed obtained in step (2) as raw material, a cemented carbide blank is formed by 3D printing technology using a fused deposition modeling (FDM) equipment. The parameters are set as follows: nozzle diameter is 0.5 mm, nozzle temperature is 150 °C, layer thickness is 0.2 mm, printing speed is 25 mm / s, and filling rate is 100%. The blank of the formed part is formed.

[0059] (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 for 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.

[0060] (5) The cemented carbide billet processed in step (4) is heated from 550°C to 1400°C at a heating rate of 8°C / min under vacuum conditions and held for 30 min; then it is cooled to 1350°C at a cooling rate of 10°C / min and held for 6 h; finally it is cooled to room temperature to obtain an ultrafine-grained cemented carbide solid part.

[0061] The printing feedstock prepared using these parameters exhibits shear-thinning characteristics of a pseudoplastic fluid, demonstrating good printability. Through optimization of the ultrafine-grained 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 ultrafine-grained 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 tests showed a coercivity of 31.5 kA / m, an average WC grain size of 0.395 μm, and a hardness of 2093.8 HV. 30 The flexural strength is 4920 MPa, and the fracture toughness is 12.5 MPa·m. 1 / 2 .

[0062] Example 2 (Expanding the range of vacuum mixing-granulation parameters):

[0063] (1) The powder raw material consists of 97.5 wt.% WC-9Co, 0.25 wt.% VC, 0.75 wt.% Cr3C2, 0.45 wt.% CeO2, 0.05 wt.% Ce2O3, and 1 wt.% paraffin wax, with a particle size of less than 2 μm. It is prepared as an ultrafine-grained cemented carbide mixed powder by ball milling in a nitrogen atmosphere with the addition of an appropriate amount of alcohol for 30 hours. 1.45 kg is measured for later use.

[0064] (2) Measure 10cm at room temperature. 3 Stearic acid, 10cm 3 High-density polyethylene, 5cm 3 Low-density polyethylene, 10cm 3 Polypropylene, 20cm 3 Paraffin wax (including the amount added during the ball milling process), 25cm 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, 5cm 3 Low-density polyethylene, 10cm 3 Polypropylene, 20cm 3 Paraffin wax, 25cm 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 a vacuum degree of -0.08 MPa using a vacuum pump. After argon was introduced, the internal mixer was heated to 160°C. The specific parameters were designed as follows: mixing temperature of 150°C and mixing time of 80 min, to prepare a mixed material with a powder loading of 55%. After cooling the mixed material, it was placed in a vacuum granulator. After the granulator was evacuated to a vacuum degree of -0.08 MPa and argon was introduced, the temperature of the sealed chamber was set to 120°C, the screw speed to 70 rpm, and the pressure to 8 kg for piston propulsion, to obtain a granular printing feed with a powder loading of 55% and a particle size of about 2-3 mm.

[0065] (3) Using the granular printing feed obtained in step (2) as raw material, a cemented carbide blank is formed by 3D printing technology using a fused deposition modeling (FDM) equipment. The parameters are set as follows: nozzle diameter is 0.5 mm, nozzle temperature is 150 °C, layer thickness is 0.25 mm, printing speed is 40 mm / s, and filling rate is 85%. The blank of the formed part is formed.

[0066] (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.

[0067] (5) The cemented carbide billet processed in step (4) is heated from 550°C to 1400°C at a heating rate of 10°C / min under vacuum conditions and held for 30 min; then it is cooled to 1350°C at a cooling rate of 10°C / min and held for 6 h; finally it is cooled to room temperature to obtain an ultrafine-grained cemented carbide solid part.

[0068] The printing feedstock prepared using these parameters exhibits shear-thinning characteristics of a pseudoplastic fluid, demonstrating good printability. Through optimization of the ultrafine-grained 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 ultrafine-grained cemented carbide green body 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 27.8 kA / m, an average WC grain size of 0.460 μm, and a hardness of 2019.5 HV. 30 The flexural strength is 4715 MPa, and the fracture toughness is 13.4 MPa·m. 1 / 2 .

[0069] Example 3 (Expanding the range of vacuum mixing-granulation parameters):

[0070] (1) The powder raw material consists of 97 wt.% WC-9Co, 0.75 wt.% VC, 0.25 wt.% Cr3C2, 0.45 wt.% CeO2, 0.05 wt.% Ce2O3, and 1.5 wt.% paraffin wax, with a particle size of less than 2 μm. It is prepared as an ultrafine-grained cemented carbide mixed powder by ball milling in a nitrogen atmosphere with the addition of an appropriate amount of alcohol for 45 h. 1.45 kg is measured for later use.

[0071] (2) Measure 10cm at room temperature. 3 Stearic acid, 15cm 3 High-density polyethylene, 15cm 3 Low-density polyethylene, 5cm 3 Polypropylene, 25cm 3 Paraffin wax (including the amount added during the ball milling process), 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, 15cm 3 High-density polyethylene, 15cm 3 Low-density polyethylene, 5cm 3 Polypropylene, 25cm 3 Paraffin wax, 30cm 3Microcrystalline 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 pressure was evacuated to a vacuum of -0.05 MPa using a vacuum pump. After argon was introduced, the internal mixer was heated to 150°C. The specific parameters were designed as follows: mixing temperature 150°C, mixing time 70 min, to prepare a powder-loaded 53% mixed mixture. After cooling the obtained mixed mixture, it was placed in a vacuum granulator. After the granulator was evacuated to a vacuum of -0.05 MPa and argon was introduced, the temperature of the sealed chamber was raised to 120°C, the screw speed was 80 rpm, and the pressure was 10 kg for piston propulsion, to obtain a powder-loaded 53% granular printing feed with a particle size of approximately 2-3 mm.

[0072] (3) Using the granular printing feed obtained in step (2) as raw material, a cemented carbide blank is formed by 3D printing technology using a fused deposition modeling (FDM) equipment. The parameters are set as follows: nozzle diameter is 0.4 mm, nozzle temperature is 150 °C, layer thickness is 0.2 mm, printing speed is 60 mm / s, and filling rate is 75%. The blank of the formed part is formed.

[0073] (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.

[0074] (5) The cemented carbide green blank treated in step (4) is heated from 550℃ to 1400℃ at a heating rate of 12℃ / min under vacuum conditions and held for 30min; then cooled to 1350℃ at a cooling rate of 10℃ / min and held for 6h; finally cooled to room temperature to obtain an ultrafine-grained cemented carbide solid part. The printing feed prepared using these parameters exhibits the shear-thinning characteristics of a pseudoplastic fluid and has good printability. After optimization of the ultrafine-grained cemented carbide composition design, improvement of vacuum mixing-granulation protection printing feed and sintering process, the relative density of the sintered sample after degreasing and sintering of the ultrafine-grained cemented carbide green blank can reach 99.5%. Metallographic microscopy results show that the porosity of the sample is A02B00, the non-combined carbon is C00 (100x metallographic detection after polishing), and the η phase is E00 (slight corrosion by NaOH and K3Fe(CN)6) solution). Microstructure and mechanical property testing results showed that the coercivity was 35.8 kA / m, the average grain size (WC) was 0.355 μm, and the hardness was 2252 HV. 30 The flexural strength is 5137 MPa, and the fracture toughness is 14.1 MPa·m. 1 / 2 .

[0075] Example 4 (Expanding the range of printing parameters):

[0076] (1) The powder raw material consists of 96.5 wt.% WC-9Co, 0.5 wt.% VC, 0.5 wt.% Cr3C2, 0.48 wt.% CeO2, 0.02 wt.% Ce2O3 and 2 wt.% paraffin, with a particle size of less than 2 μm. It is prepared by ball milling with an appropriate amount of alcohol in a nitrogen atmosphere using a roller ball mill for 30 h. 1.45 kg is then measured for later use.

[0077] (2) 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 0.5 mm, nozzle temperature of 150 °C, layer thickness of 0.25 mm, printing speed of 15 mm / s, and fill rate of 130%.

[0078] The other operating steps are the same as in Example 3.

[0079] The relative density of the sintered sample after degreasing and sintering of the ultrafine-grained cemented carbide green billet reached 99.1%. 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 34.8 kA / m, an average WC grain size of 0.362 μm, and a hardness of 2138 HV. 30 The flexural strength is 5088 MPa, and the fracture toughness is 14.3 MPa·m. 1 / 2 Example 5 (Expanding the range of printing parameters):

[0080] (1) The powder raw material consists of 97.5 wt.% WC-9Co, 0.5 wt.% VC, 0.5 wt.% Cr3C2, 0.46 wt.% CeO2, 0.04 wt.% Ce2O3 and 1.5 wt.% paraffin wax, with a particle size of less than 2 μm. It is prepared as an ultrafine-grained cemented carbide mixed powder by ball milling in a nitrogen atmosphere with the addition of an appropriate amount of alcohol for 30 h. 1.45 kg is measured for later use.

[0081] (2) 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 150 °C, layer thickness of 0.2 mm, printing speed of 75 mm / s, and filling rate of 60%.

[0082] The other operating steps are the same as in Example 3.

[0083] The relative density of the sintered sample after degreasing and sintering of the ultrafine-grained 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 0.373 μm, and a hardness of 2091 HV. 30 The flexural strength is 4989 MPa, and the fracture toughness is 13.8 MPa·m. 1 / 2 .

[0084] Comparative Example 1 (ball milling process without paraffin-coated powder):

[0085] The difference from step (1) of Example 3 is that in Comparative Example 1, 2wt% paraffin was not added to the drum ball mill as in step (1) of Example 3 to coat powders A, B, and C. Instead, powders A, B, and C and trace amounts of low-valence rare earth oxide Ce2O3 from step (1) of Example 3 were directly contacted and ball-milled with the grinding balls in a nitrogen atmosphere to prepare ultrafine-grained cemented carbide mixed powder. The ball milling time was 30 hours, and 1.45 kg was measured for later use.

[0086] The other operating steps are the same as in Example 3.

[0087] After debinding and sintering, microscopic observation and mechanical property testing showed that the prepared ultrafine-grained cemented carbide contained a small amount of η phase, with a relative density of 98.1%, a coercivity of 24.7 kA / m, and an average WC grain size of 0.561 μm. The hardness, bending strength, and fracture toughness of the parts were 1878.2 HV. 30 4186 MPa and 12.2 MPa·m 1 / 2 .

[0088] Compared with Example 3, in Comparative Example 1, without the addition of 2 wt% paraffin to coat powders A, B, and C, a brittle η phase was formed, resulting in a decrease in hardness, strength, and fracture toughness of 16.6%, 18.5%, and 13.5%, respectively. Adding paraffin coating to the powder during the ball milling process effectively eliminates adsorbed oxygen and oxidation, controlling the formation of the brittle η phase. Comparative Example 2 (ball milling process without a protective atmosphere):

[0089] The difference from step (1) of Example 3 is that in Comparative Example 2, nitrogen gas was not introduced into the ball mill as a protective atmosphere as in step (1) of Example 3. Instead, powders A, B, C, low-valent rare earth oxide Ce2O3 and paraffin were added in step (1) of Example 3, and ball milled in air to prepare ultrafine crystalline hard alloy mixed powder. The ball milling time was 30 hours, and 1.45 kg was measured for later use.

[0090] The other operating steps are the same as in Example 3.

[0091] After debinding and sintering, microscopic observation and mechanical property testing showed that the prepared ultrafine-grained cemented carbide contained a large amount of brittle η phase, with a relative density of up to 96.3%, a coercivity of 18.3 kA / m, and an average WC grain size of 0.861 μm. The hardness, bending strength, and fracture toughness of the parts were 1631.5 HV. 30 3616 MPa and 11.7 MPa·m 1 / 2 Compared with Example 3, in Comparative Example 2, no nitrogen gas was introduced as a protective atmosphere, resulting in the formation of a large amount of brittle η phase, and a decrease in hardness, strength, and fracture toughness of 27.6%, 29.6%, and 17.0%, respectively. Introducing a protective atmosphere during the ball milling process can eliminate the formation of brittle η phase and improve the mechanical properties of cemented carbide.

[0092] Comparative Example 3 (no trace amounts of low-valence rare earth oxides were added to the mixed powder):

[0093] The difference from step (1) in Example 3 is that, in Comparative Example 3, no trace amount of low-valence rare earth oxide Ce2O3 powder was added. The raw material composition consisted of 96.5 wt.% WC-9Co, 0.5 wt.% VC, 0.5 wt.% Cr3C2, 0.5 wt.% CeO2 and 2 wt.% paraffin wax, with a particle size of less than 2 μm. The powder was prepared by ball milling with 2 wt.% paraffin wax and an appropriate amount of alcohol in a nitrogen atmosphere for 30 h. 1.45 kg was then measured for later use.

[0094] The other operating steps are the same as in Example 3.

[0095] After debinding and sintering, microscopic observation and mechanical property testing showed that the prepared ultrafine-grained cemented carbide had a small amount of η phase formed, with a relative density of 98.8%, an increased coercivity of 23.3 kA / m, and a WC average grain size that grew to 0.513 μm. The part's hardness, bending strength, and fracture toughness were 1904.3 HV. 30 3962 MPa and 11.8 MPa·m 1 / 2 .

[0096] Compared with Example 3, Comparative Example 3 did not include trace amounts of low-valent rare earth oxide Ce2O3 powder. Hardness, strength, and fracture toughness decreased by 15.5%, 22.9%, and 16.3%, respectively. Appropriate addition of trace amounts of low-valent rare earth oxides can adsorb residual oxygen in the powder and undergo an in-situ reaction to generate nano-rare earth oxides, which act as a dispersed reinforcing phase, strengthening the cemented carbide and transforming harmful oxygen into a beneficial reinforcing phase component.

[0097] Comparative Example 4 (no rare earth oxides added to the mixed powder):

[0098] The difference from step (1) in Example 3 is that in Comparative Example 4, rare earth oxides are not added, and the parameters are modified as follows: the powder raw material consists of 96.5 wt.% WC-9Co, 1 wt.% Cr3C2, 0.5 wt.% Cr3C2 and 2 wt.% paraffin, with a particle size of less than 2 μm. The ultrafine crystalline cemented carbide mixed powder is prepared by ball milling for 30 h, and 1.45 kg is measured for later use.

[0099] The other operating steps are the same as in Example 3.

[0100] After debinding and sintering, microscopic observation and mechanical property testing showed that the prepared ultrafine-grained cemented carbide had no η phase formation, a relative density of 99.5%, a coercivity of 23.1 kA / m, and an average WC grain size of 0.502 μm. The hardness, bending strength, and fracture toughness of the parts were 1748.9 HV. 30 3758 MPa and 10.3 MPa·m 1 / 2 .

[0101] Compared with Example 3, in Comparative Example 4, where no rare earth oxides were added, the WC grain size increased significantly, and the mechanical properties of the cemented carbide decreased by 22.3%, 26.8%, and 27.0%, respectively. Rare earth oxides can refine the grain size and have a pinning effect on grain migration. As a second phase, their addition can significantly improve the overall mechanical properties of the cemented carbide.

[0102] Comparative Example 5 (without vacuum mixing-granulation process):

[0103] The difference from step (2) of Example 3 is that in Comparative Example 5, the vacuum environment in step (2) of Example 3 is eliminated, that is, the mixed powder and binder prepared in step (1) of Example 3 are mixed and granulated in an air environment to prepare granular printing feed.

[0104] The other operating steps are the same as in Example 3.

[0105] Microscopic observation revealed the presence of a distinct η phase in the sintered ultrafine-grained cemented carbide, resulting in a reduced relative density of only 96.4%. Furthermore, the formation of the brittle phase significantly degraded the mechanical properties of the sample, with hardness, bending strength, and fracture toughness all dropping to 1745 HV. 30 3435MPa and 9.3MPa·m 1 / 2 .

[0106] Compared with Example 3, Comparative Example 5 did not use vacuum mixing-granulation. Due to the fineness of the powder raw material and its high surface activity, it is very easy to oxidize in the air environment, which leads to a decrease in the relative density of the ultrafine-grained cemented carbide and the appearance of carbon-deficient η phase in the microstructure. The mechanical properties decreased by 22.5%, 33.1% and 34.0%, respectively.

[0107] Comparative Example 6 (without using a two-step sintering process)

[0108] The difference from step (5) in Example 3 is that in Comparative Example 6, the two-step sintering process is adjusted to a conventional one-step sintering process, and the temperature is held at the highest temperature of 1400℃ for 1 hour.

[0109] The other operating steps are the same as in Example 3.

[0110] Microscopic observation showed that the ultrafine-grained cemented carbide sample prepared after sintering had a relative density of 99.0%, an average grain size of 0.462 μm, and hardness, bending strength, and fracture toughness of 1887 HV. 30 4499MPa and 13.1MPa·m 1 / 2 .

[0111] Compared with Example 3, Comparative Example 6 did not use a two-step sintering process. The relative density of the ultrafine-grained cemented carbide prepared by conventional one-step sintering did not change significantly, but the WC grain size increased significantly, and the hardness, bending strength and fracture toughness decreased by 16.2%, 12.4% and 7.1%, respectively.

[0112] Comparative Example 7 (changing the two-step sintering process parameters):

[0113] The difference from step (5) in Example 3 is that in Comparative Example 7, the two-step sintering process is adjusted so that the first sintering temperature T1 is 50°C lower than the second sintering temperature T1. That is, the first sintering temperature is 1350°C and held for 30 minutes, then the temperature is increased to 1400°C at a rate of 10°C / min, held for 1 hour, and finally cooled to room temperature.

[0114] The other operating steps are the same as in Example 3.

[0115] Microscopic observation showed that the cemented carbide sample prepared by the adjusted two-step sintering had a relative density of 99.1%, an average WC grain size of 0.493 μm, and hardness, bending strength, and fracture toughness of 1783.6 HV. 30 4413 MPa and 13.2 MPa·m 1 / 2 .

[0116] Compared with Example 3, the sintering temperature of the two-step sintering process in Comparative Example 7 was lower at first and then higher. The relative density of the prepared ultrafine-grained cemented carbide did not change significantly, the WC grain size increased by 0.098 μm, and the hardness, bending strength and fracture toughness decreased by 20.8%, 14.1% and 5.6%, respectively.

Claims

1. A method for additive manufacturing of ultrafine-grained cemented carbide, characterized in that: Includes the following steps: (1) The powder raw material consists of 97wt.% WC-9Co, 0.75wt.% VC, 0.25wt.% Cr3C2, 0.45wt.% CeO2, 0.05wt.% Ce2O3 and 1.5wt.% paraffin, with a particle size of less than 2μm. It is prepared by ball milling with an appropriate amount of alcohol in a nitrogen atmosphere using a roller ball mill for 45h. 1.45kg is then measured for later use. (2) Measure 10cm at room temperature. 3 Stearic acid, 15cm 3 High-density polyethylene, 15cm 3 Low-density polyethylene, 5cm 3 Polypropylene, 25cm 3 Paraffin wax, including 25cm 3 Paraffin wax, including the amount added during the ball milling process, 30cm 3 Prepare palm wax; after the vacuum mixer has been preheated to 50°C for 15 minutes, add 10cm of palm wax sequentially. 3 Stearic acid, 15cm 3 High-density polyethylene, 15cm 3 Low-density polyethylene, 5cm 3 Polypropylene, 25cm 3 Paraffin wax, 30cm 3 Palm wax and 1.45 kg of mixed powder from step (1) are added to the mixing chamber of the internal mixer. The pressure hammer is turned off and the internal air pressure is evacuated to a vacuum degree of -0.05 MPa using a vacuum pump. After argon is introduced, the internal mixer is heated to 150°C. The specific parameters are designed as follows: mixing temperature is 150°C and mixing time is 70 min. A powder loading of 53% is obtained by internal mixing. After cooling the obtained internal mixing, it is placed in a vacuum granulator. After the granulator is evacuated to a vacuum degree of -0.05 MPa and argon is introduced, the temperature of the sealed chamber is set to 120°C, the screw speed is 80 rpm, and the pressure is 10 kg for piston propulsion. A powder loading of 53% and a particle size of 2-3 mm are obtained as a printing feed. (3) Using the granular printing feed obtained in step (2) as raw material, a cemented carbide blank is formed by 3D printing technology using a fused deposition modeling (FDM) equipment. The parameters are set as follows: nozzle diameter is 0.4 mm, nozzle temperature is 150 °C, layer thickness is 0.2 mm, printing speed is 60 mm / s, and filling rate is 75%. The blank of the formed part is formed. (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 40°C 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 60 minutes to complete the hot degreasing. (5) The cemented carbide blank processed in step (4) is heated from 550°C to 1400°C at a heating rate of 12°C / min under vacuum conditions and held for 30 min; then it is cooled to 1350°C at a cooling rate of 10°C / min and held for 6 h; finally it is cooled to room temperature to obtain an ultrafine-grained cemented carbide solid part.

Images