High-performance NANO cemented carbide product and preparation method therefor

Abstract

A high-performance nano cemented carbide product and a preparation method therefor. The method comprises: carrying out wet ball milling and powder mixing on 85-95% by mass of tungsten carbide powder and 5-15% by mass of cobalt powder, and drying same; loading WC-Co cemented carbide mixed powder into a mold; carrying out powder tap on the WC-Co cemented carbide mixed powder loaded into the mold; degassing the tapped WC-Co cemented carbide mixed powder by means of a vacuum environment; by means of a press, carrying out powder forming on the degassed WC-Co cemented carbide mixed powder in the vacuum environment, such that the powder is formed into a WC-Co cemented carbide primary product in the mold; separating the WC-Co cemented carbide primary product from the mold in the presence of a protective gas; and carrying out heat treatment on the demolded WC-Co cemented carbide primary product in a protective gas environment to obtain a high-performance nano cemented carbide material product. Thus, efficient batch production of the cemented carbide product is achieved, and it is ensured that all properties of the prepared product meet the requirements.

Description

High-performance nano-hard alloy product and preparation method thereof Technical Field

[0001] The present invention relates to the field of hard alloy material preparation, and in particular to a high-performance nano-hard alloy product and a preparation method thereof. Background Art

[0002] Cemented carbide is a multiphase composite material typically prepared using powder metallurgy, using one or more refractory metal compounds with high hardness and high elastic modulus as a matrix and a transition metal or its alloy as a binder. Refractory metal compounds, particularly refractory metal carbides (such as WC, TaC, TiC, and NbC), have the characteristics of high melting point, high hardness, good chemical and thermal stability, and minimal dissolution interaction with the bonding metal at room temperature. The transition metal used for bonding (such as Co, Ni, and Fe) is required to have good wettability with the refractory metal hard compound, transform into a liquid phase during the sintering process, effectively wet the surface of the refractory metal hard compound, and even partially dissolve the refractory metal hard compound. It also does not chemically react with the refractory metal hard compound at the sintering temperature, exhibits good mechanical properties, and is guaranteed to not form a liquid phase at the service temperature of the cemented carbide.

[0003] Tungsten-cobalt (WC-Co) alloys are a common type of cemented carbide, primarily composed of tungsten carbide and cobalt. Compared to other cemented carbides containing the same amount of cobalt, WC-Co alloys possess the highest transverse rupture strength, compressive strength, fracture toughness, and elastic modulus. Therefore, as an important cemented carbide material, WC-Co cemented carbide has found widespread application in metal cutting, metal forming tools, mining, oil drilling, national defense, and stone and wood cutting, earning it the nickname "the teeth of industry."

[0004] At present, the industrial production of WC-Co cemented carbide mainly adopts the technical process scheme of cold pressing + low temperature degreasing treatment + high temperature liquid phase sintering. In the cold pressing stage, the cemented carbide powder is pressed and formed; in the low temperature degreasing treatment stage, the organic forming agent inside the compact is degreased at a relatively low heating temperature; in the liquid phase sintering stage, the fully mixed WC-Co powder is heated to a temperature above the WC-Co eutectic point, so that the WC powder is surrounded by a Co-rich liquid phase containing W and C. The rotation and rearrangement of the WC powder and the filling of the remaining pores by the Co-rich liquid phase reduce the porosity of the material. In order to achieve complete densification of the material, it is necessary to sinter at a higher temperature to obtain a sufficient amount of liquid phase and to keep it warm for a long time (usually more than 24 hours). However, this method easily leads to abnormal growth of WC grains after sintering. At present, carbides of transition elements such as TaC, NbC, VC and Cr3C2 are commonly used as grain growth inhibitors to suppress the abnormal growth of WC grains during the sintering process.

[0005] my country currently produces over 40% of the global WC-Co cemented carbide market share. However, China still lags far behind leading international companies in processing high-end WC-Co cemented carbide products, particularly nano-WC-Co cemented carbide products. The country lacks mature and stable manufacturing capabilities for high-performance nano-WC-Co cemented carbide products. Key challenges include: 1. Abnormal growth of nano-WC powder during sintering. Due to the high surface activation energy of nano-WC powder, it is prone to coalescence with adjacent WC particles during prolonged, high-temperature sintering, leading to abnormal growth. To prevent this abnormal growth and achieve nano-WC-Co cemented carbide with stable microstructure and properties, complex sintering processes are required, placing high demands on production line equipment and presenting significant manufacturing challenges. Consequently, my country has yet to achieve independent industrialization of nano-WC-Co cemented carbide products. 2. The impact of Co content on the microstructure and properties of nano-WC-Co cemented carbide. Co acts as a metal binder to fill the gaps between WC particles during the sintering process, preventing them from coalescing and growing. However, some WC dissolves in the liquid Co during the sintering process and re-precipitates on the surfaces of other WC particles, which also causes the WC particles to grow. Furthermore, an increase in the Co content can reduce the hardness and wear resistance of the WC-Co cemented carbide. Therefore, minimizing the Co content while ensuring that the WC particles are encapsulated is an important research direction in this field. 3. Incomplete dewaxing. Currently, organic substances such as synthetic rubber, paraffin, and polyethylene glycol are commonly used as forming agents. During the cold pressing process, they enhance powder fluidity, maintain the shape of the compact, and prevent microcracks within the compact, as well as cracking during demolding and transportation. The forming agent must be removed during the sintering process, also known as dewaxing. Common dewaxing methods include vacuum dewaxing, nitrogen dewaxing, and hydrogen dewaxing, with hydrogen dewaxing being the most effective. Incomplete dewaxing can lead to excessive free carbon content in WC-Co cemented carbide and the formation of η phase, which reduces the material's hardness, fracture toughness, and wear resistance. Furthermore, because the sintering process of WC-Co cemented carbide powder metallurgy requires multiple steps, the equipment requirements and production costs are high.

[0006] Lin Yaojun, Xie Yongqiang and others published a method for rapid semi-solid hot pressing to manufacture cemented carbide in a Chinese invention patent (CN111455206B, 2021.07.06). In this method, the thickness of the cemented carbide powder when it is fully dense is first accurately calculated; then the cemented carbide powder is placed in the inner cavity of the hot pressing mold, the pressing head is inserted into the inner cavity of the hot pressing mold, the hot press working room is evacuated or evacuated and then filled with inert protective gas, and the cemented carbide powder in the hot pressing mold is rapidly heated to a high temperature corresponding to a certain liquid phase; finally, the pressing head is rapidly lowered to axially press the cemented carbide powder. When the pressing head descends to a thickness slightly less than the thickness of the cemented carbide powder when fully dense, the downward movement of the pressing head is immediately stopped and the pressure on the pressing head is stopped. At the same time, the heating of the cemented carbide powder is stopped, and the manufacture of cemented carbide is completed. This method has the following shortcomings: 1) For cemented carbide powders with more components and more complex composition, and considering the volume expansion of different components at high temperature, it is difficult to obtain the accurate thickness of the product when fully dense by calculation; 2) It is difficult to control the melting of a part of the cemented carbide powder in advance, and due to the high sintering temperature, excessively high heating temperature can easily cause liquid phase elements to re-precipitate on the surface of solid phase cemented carbide grains, resulting in coarse grains; 3) When the method of liquefaction followed by pressurization is adopted, since the liquid phase does not appear at the same temperature point at the same time, it may cause uneven distribution of the liquid phase, thereby affecting the uniformity of the final structure; 4) The time for the liquid phase in the cemented carbide compact to fill the pores is too short, and in the absence of subsequent heat treatment, it may result in a high porosity.

[0007] Zhu Liu, Wang Jinfang and others published a method for preparing WC-Y2O3 binderless cemented carbide by cold pressing and hot pressing in a Chinese invention patent (CN111778436B, 2021.08.31). The method comprises the following steps: ball milling WC powder and Y2O3 powder to obtain WC-Y2O3 powder; adding saturated oxalic acid solution to the WC-Y2O3 powder until it is completely wetted, and cold pressing and sintering the wetted WC-Y2O3 powder to obtain a sintered body; the pressure of the cold pressing and sintering is 200-400MPa; the heating program of the cold pressing and sintering is: first heating to 100-150℃ and keeping warm for 1 hour, then continuing to heat to 200-300℃ and keeping warm for 1-2 hours; hot pressing and sintering the sintered body to obtain WC-Y2O3 binderless cemented carbide. This method has the following shortcomings: 1) the oxalic acid solution cannot achieve trace dissolution of the Co element without affecting the main phase WC particles, and the precipitation of the Co element on the surface of the main phase particles at low temperature; 2) because this method uses WC-Y2O3 binderless cemented carbide powder, although a pre-sintered compact with a certain density is obtained by pre-sintering, a sintering temperature (1400-1650°C) far higher than the eutectic temperature (1280-1330°C) of WC-Co cemented carbide is still required for final powder forming, and due to the lack of liquid phase, the density of the final product (97.87%) is lower than that of commercial WC-Co cemented carbide products (>99%); 3) the entire process of this method requires many steps and workstations, resulting in low preparation efficiency.

[0008] He Qiushuang, Chen Haiyan, and others published a Chinese invention patent (CN104630529B, September 12, 2017) on fine-grained WC-Co cemented carbide with B4C as a dispersion-strengthening additive and a method for preparing the same. The method includes the following steps: weighing WC (0.6-0.8μm, 99.9% purity) and Co (0.6-0.8μm, 99.9% purity) powders in proportion, ball milling them in a mill at 400-450 rpm for 45-50 hours (using anhydrous ethanol as the milling medium and WC balls as the grinding media); adding B4C (0.6-0.8μm, >99.9% purity) at a certain volume fraction (5%-10%); 99.9%) powder, uniformly mixed at a speed of 100-150 rpm for 20-24 hours; the dried raw materials are placed in a graphite mold, placed in a hot pressing sintering furnace, heated to 1100-1200°C at a rate of 15-20°C / min, pressurized to 5-10 MPa, then heated to 1300-1350°C at a rate of 10-15°C / min and pressurized to 20-25 MPa, kept at this temperature and pressure for 1.5-2 hours, then released and cooled in the furnace. This method has the following disadvantages: 1) This method adds 5%-10% by volume of B4C as a dispersion strengthening agent, and at the same time reduces the volume fraction of the metal binder phase Co to ensure the hardness of the final product. However, the fracture toughness of the prepared alloy product (less than 6 MPa·m 1 / 2 ) is significantly lower than commercial WC-Co cemented carbide products (>8MPa·m 1 / 2 ); 2) The WC particle size used is 0.6-0.8μm, which is larger than the WC particle size in nano-cemented carbide specified in the internationally adopted Sandvik WC grain grading standard (<300nm), which will have a direct adverse effect on the final wear resistance, fracture toughness and bending strength of the product; 3) The single-furnace single-piece production method has low production efficiency and is not suitable for mass production.

[0009] Zhang Jiuxing, Huang Hao and others announced a SPS sintering method for large-sized binderless pure tungsten carbide cemented carbide in a Chinese invention patent (CN108165859B, 2019.08.30). The method includes the following steps: according to the size of the required tungsten carbide cemented carbide sample, a graphite mold of corresponding size is set; the binderless pure tungsten carbide powder is placed in the graphite mold for pre-pressing, and then placed in a spark plasma sintering system, and the temperature is increased while the gradient is pressurized until the displacement of the lower pressure head of the graphite mold no longer changes, and the temperature is stopped, sintered at a constant temperature, and then cooled with the furnace to obtain a large-sized binderless pure tungsten carbide cemented carbide sample. This method has the following shortcomings: 1) This method requires the addition of graphite sheets, carbon paper and other additives between the upper and lower pressure heads of the graphite mold and the powder to ensure that the powder and the graphite mold do not directly diffuse with each other during the sintering process, which makes demolding difficult. The process is relatively complicated. It also does not mention whether the subsequent graphite sheet or carbon paper will undergo diffusion reaction with the cemented carbide sample, thereby affecting the material structure and element distribution, and whether a special machining process is required to remove a surface diffusion layer of a certain depth; 2) This method requires coating the surface of the graphite mold with multiple layers of carbon felt to ensure uniform temperature of the entire graphite mold. The process is complicated and only one cemented carbide product can be prepared each time, resulting in low production efficiency; 3) In order to achieve a binderless phase, the sintering temperature is 1600-1700°C. At this temperature, only graphite molds can be used for forming, which cannot avoid carburizing reaction between the graphite mold and WC-Co cemented carbide at high temperature, resulting in excessive free carbon content in the surface layer of the product and easy generation of brittle phase η phase, which has an adverse effect on the material structure and properties, especially wear resistance during service; 4) The diameter of the cemented carbide product prepared by this method can reach about 100 mm, but the thickness can only reach about 5 mm. This size range limits its application in important machined parts such as cemented carbide drills and milling cutters.

[0010] Summary of the Invention

[0011] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a method for preparing high-performance nano-hard alloy materials, so as to realize the efficient batch production of nano-WC-Co hard alloy products and ensure that the various properties of the prepared nano-WC-Co hard alloy products meet the requirements.

[0012] The preparation method of the high-performance nano-hard alloy product of the present invention is characterized by comprising the following steps: wet ball milling 85-95% WC powder and 5-15% Co powder to obtain WC-Co hard alloy mixed powder after drying; loading the WC-Co hard alloy mixed powder into a mold; performing powder compaction on the WC-Co hard alloy mixed powder loaded into the mold; degassing the compacted WC-Co hard alloy mixed powder in a vacuum environment; powder forming the degassed WC-Co hard alloy mixed powder by means of a press to form a WC-Co hard alloy primary product in the mold; separating the WC-Co hard alloy primary product from the mold under a protective gas environment; and heat treating the cooled WC-Co hard alloy primary product under a protective gas environment to obtain a high-performance nano-hard alloy material product.

[0013] The preparation method of the present invention improves the tap density of nano-hard alloy powder through powder tapping treatment, reduces the air content in the powder, makes the arrangement of nano-hard alloy particles denser, and makes the powder filling surface in the mold smoother, which is beneficial to subsequent pressing and forming, and reduces the structural defects of the final product after powder forming.

[0014] The preparation method of the present invention purifies the surface of nano-cemented carbide powder through vacuum degassing, promoting wetting between hard phase particles and the metal binder phase at high temperatures, a key factor in ultimately achieving a dense and uniform material structure. Furthermore, vacuum degassing prior to powder forming increases the powder's bulk density, achieving improved filling properties. Continued vacuuming during the powder forming process ensures that no air gaps remain in the powder particles during the forming process. Furthermore, vacuum degassing prevents gases adsorbed on the nano-powder surface from chemically reacting with the powder during the high-temperature forming process, creating inclusions that could affect the material's final structure and properties.

[0015] The above-mentioned method of the present invention reduces the content of the metal binder phase in the nano-hard alloy through powder forming, thereby improving the hardness, corrosion resistance and high temperature resistance of the hard alloy. During the powder forming process, high forming pressure is applied to the nano-hard alloy powder. Since the hard phase hardly undergoes plastic deformation, the metal binder phase distributed between the hard phases undergoes plastic deformation with the forming pressure, which on the one hand improves the coating effect of the binder phase on the hard phase and on the other hand reduces the thickness of the metal binder phase between the hard phase particles. During the high-temperature forming process, the high forming pressure is used to reduce the pores between the hard phase particles, which makes it easier for the liquid binder phase to enter the pores to complete the filling and obtain a dense structure, thereby achieving the preparation of a nano-hard alloy product with a uniform and dense structure under the condition of reducing the metal binder phase.

[0016] The above-mentioned method of the present invention combines a reasonable WC-Co powder formulation, powder tapping, vacuum degassing and powder forming. 1) Without using any grain growth inhibitor, it effectively suppresses the abnormal growth of WC grains, so that the average grain size of the WC in the prepared alloy product is less than 300nm, while avoiding the adverse effects of the introduction of other undesirable elements such as additional carbon on the structure and properties of the cemented carbide; 2) with a very low content of the metal binder phase, it achieves a good coating effect of the binder phase on the hard phase, thereby ensuring that the various properties of the prepared nano WC-Co cemented carbide product meet the requirements or even exceed them; 3) the common sintering temperature of WC-Co cemented carbide of 1350°C is reduced to 1200°C, which, on the one hand, relaxes the restrictions on mold materials, thereby allowing for greater forming force to be applied during powder forming, resulting in various improved properties of the nano WC-Co cemented carbide product, and on the other hand, saves energy consumption and reduces manufacturing costs.

[0017] In one embodiment of the present invention, the powder forming comprises the following two sub-stages: a) holding temperature at 600-700°C and pressure at 20-30 MPa, and b) holding temperature at 1100-1200°C and pressure at 50-70 MPa.

[0018] Therefore, by applying pressure and sintering the nano-cemented carbide at a relatively low temperature and then maintaining the temperature and pressure for a period of time, the friction generated between the hard phase (WC) powder, the metal binder phase (Co) powder, and the mold wall can be effectively reduced, thereby improving the uniformity of the compact density during the forming process. Under high forming pressure, the metal binder phase undergoes plastic deformation, increasing the contact area with the hard phase, and achieving atomic interdiffusion at temperatures below the eutectic point. After a period of temperature and pressure maintenance, an interface structure with good metallurgical bonding is finally obtained, ensuring the material's density and high mechanical properties.

[0019] In one embodiment of the present invention, in sub-stage a), the pressure is increased to 20-30 MPa at a pressurization rate of 5 MPa / min, and the temperature is increased to 600-700°C at a heating rate of 100-130°C / min; in sub-stage b), the pressure is increased to 60-70 MPa at a pressurization rate of 10 MPa / min, and the temperature is increased to 1100-1200°C at a heating rate of 100°C / min.

[0020] Therefore, the staged heating and pressurization of WC-Co cemented carbide powder further helps improve the microstructure uniformity of the final product and reduce stress concentration between particles. At lower forming temperatures, the formability of the metallic binder phase Co in the powder material is poor. Using a lower strain rate can reduce the porosity between the powders and the stress concentration between the particles, thus achieving pre-forming of the powder compact. As the forming temperature increases, the formability of the metallic binder phase Co improves, and by appropriately increasing the strain rate, it can quickly fill the pores between the hard phase WC particles, resulting in a dense and uniform final microstructure.

[0021] In one embodiment of the present invention, the size of the tungsten carbide powder is in the range of 150-300 nm.

[0022] Therefore, tungsten carbide powder with a scale range of 150-300nm is selected to ensure that the original particle size is smaller than the nano-tungsten carbide grain size required by the Sandvik WC grain grading standard commonly used internationally. Combined with the forming method of the present invention, the abnormal growth of tungsten carbide particles is suppressed, further ensuring that WC-Co cemented carbide products with nano-grain size are obtained.

[0023] In one embodiment of the present invention, one or more of anhydrous ethanol, acetone, and ethane are used as grinding aids during the wet ball milling process. In one embodiment of the present invention, the ball-to-material ratio during the wet ball milling process is 6-10:1. In one embodiment of the present invention, the liquid-to-solid ratio during the wet ball milling process is 0.4-0.8:1.

[0024] Thus, preferably one or more of anhydrous ethanol, acetone, and ethane are used as grinding aids, which, on the one hand, help to suppress powder agglomeration and phenomena such as surface oxidation, and on the other hand, are easy to realize complete volatilization in a short time at a lower temperature, avoid introducing excessive carbon or other undesirable elements due to the residual grinding aid, causing the powder to produce harmful phases in the subsequent liquid phase forming process, thereby reducing the uniformity of the final product microstructure and the mechanical properties of the material. In addition, the above-mentioned ball-to-material ratio and liquid-to-solid ratio further improve the ball milling quality, help to realize the ideal particle size distribution of the WC-Co cemented carbide mixed powder, thereby further improving the final performance of the nano cemented carbide material product.

[0025] In one embodiment of the present invention, the vibration frequency of the powder compaction is 50-200 Hz, and the vibration duration is 1-3 minutes.

[0026] Therefore, considering the density of WC-Co cemented carbide powder, a vibration frequency of 50-200Hz and a vibration time of 1-3 minutes are selected to achieve more uniform powder filling. This method of powder compaction effectively reduces the porosity between particles. Compared with the loose packing density of traditional direct powder filling, the powder packing density can be increased by more than 30%, which is more conducive to the subsequent vacuum forming process.

[0027] In one embodiment of the present invention, the vacuum degree of the vacuum environment is 2.0-5.5×10 -3 Pa, degassing duration is 10-60 minutes.

[0028] As a result, the volatilization of the grinding aid during the low-temperature forming process is further promoted, and the air remaining between the pores of the powder particles is discharged in time, ensuring that the hard phase WC particles and liquid phase Co do not come into contact with the gas at the high temperature stage to generate harmful phases, or cause the generation of tissue defects such as pores, thereby reducing the uniformity of the microstructure of the final product and the mechanical properties of the material.

[0029] In one embodiment of the present invention, the heat treatment is to firstly cool the demoulded WC-Co cemented carbide primary product to room temperature by oil quenching, and then temper the cooled WC-Co cemented carbide primary product at a temperature of 400-500°C for 2-8 hours.

[0030] Therefore, through oil quenching-tempering treatment, more α-CO high-temperature phase with good plasticity is retained, and the residual thermal stress of WC-Co cemented carbide products is eliminated, so as to avoid the interaction between the preparation residual thermal stress generated in the cooling stage due to the different linear thermal expansion coefficients of WC phase and Co phase and the external load, thereby affecting the mechanical behavior and service performance of the material.

[0031] In one embodiment of the present invention, the protective gas environment is a high-purity nitrogen atmosphere or a high-purity argon atmosphere.

[0032] This prevents air, especially oxygen, from oxidizing the WC-Co cemented carbide product during high-temperature forming, demoulding, and subsequent heat treatment, thereby preventing the microstructure and mechanical properties of the material from being affected.

[0033] Another object of the present invention is to provide a high-performance nano-hard alloy product, which is made by the above preparation method.

[0034] The alloy product has a relatively low metal binder phase content, and the average grain size of WC is less than 300 nm, so the alloy product has excellent mechanical properties.

[0035] The above content of the present application will be more concise and understandable in the following description of multiple embodiments with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The following are the drawings of the present application. These drawings are only for presenting the present application in a more intuitive form. They are exemplary and are not intended to limit the scope of the present application.

[0037] FIG1 is a schematic flow diagram of the preparation method according to the present invention.

[0038] FIG2 is an alloy structure image of a high-performance nano-cemented carbide product prepared according to an embodiment of the preparation method of the present invention.

[0039] FIG3 is an alloy structure image of a high-performance nano-cemented carbide product prepared according to another embodiment of the preparation method of the present invention. DETAILED DESCRIPTION

[0040] In order to make this application easier to understand, the present application is further described below with reference to specific examples. The experimental methods described in this application are conventional methods unless otherwise specified; the materials described are commercially available unless otherwise specified. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by technicians in the technical field of this application. In the event of any inconsistency, the meaning described in this specification or the meaning derived from the contents recorded in this specification shall prevail. In addition, the terms used herein are only for the purpose of describing the embodiments of this application and are not intended to limit this application.

[0041] In order to accurately describe the technical content of this application and to accurately understand the present invention, the following explanations of the words and terms used in this specification are given before describing the specific implementation methods.

[0042] As used in this specification, the phrases "one embodiment" or "an embodiment" mean that a particular feature, step, or characteristic described in connection with that embodiment is included in at least one embodiment of the present invention. Thus, the phrases "in one embodiment" or "in an embodiment" appearing throughout this specification do not necessarily refer to the same embodiment, but may do so. Furthermore, in one or more embodiments, the particular features, steps, or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure.

[0043] The present invention provides a new high-performance nano-hard alloy product and a preparation method thereof, which realizes the low-temperature, high-pressure and efficient preparation of nano WC-Co hard alloy materials under vacuum conditions through vacuum powder forming technology. In the preparation method of the present invention, high-temperature and high-strength materials such as non-graphite or graphite-based composite materials, including but not limited to titanium alloys, molybdenum alloys, nickel-based high-temperature alloys, high-temperature ceramic materials and high-temperature composite materials, can be selected to prepare the forming mold. The powder compaction method is used to increase the bulk density of the powder during the powder filling stage, and vacuum degassing treatment is performed during the forming process. The powder forming temperature is lower than 1300°C. The preparation method of the present invention has a relatively simple process flow, has no special requirements for the mold, and can achieve high-efficiency and low-cost production of nano-hard alloy materials. In addition, without using any grain growth inhibitor, the average WC grain size in the prepared material or product is less than 300nm, and the prepared material or product has a relatively low metal binder phase content and excellent mechanical properties.

[0044] The term "powder forming" as used in the specification and claims refers to a process in which a composite powder containing two or more phases is heated under vacuum or protective atmosphere to a temperature near the eutectic point of the two or more phases, while simultaneously applying a certain forming pressure to the powder. This causes the low-melting-point auxiliary phase in the powder to liquefy and come into close contact with the surface of the main phase particles. After a certain period of atomic interdiffusion, a material with a dense and uniform structure is formed. The powder forming process of the present invention combines pressure forming and sintering processes and is suitable for the rapid prototyping of composite materials containing two or more phases.

[0045] The term "vacuum degassing" used in the specification and claims refers to the discharge of gases remaining on the surface of particles or encapsulated between particles during the powder making process, as well as gases adsorbed on the surface of the powder during the heating process, by continuous vacuuming before and during powder forming. The surface of the powder particles is a gas-solid interface. When the powder is heated to a certain temperature and is in a high vacuum state, the gas adsorbed on the surface of the particles will begin to desorb and detach from the powder surface. Since desorption is an endothermic reaction, the degassing rate will increase by increasing the degassing temperature and vacuum degree. For powders degassed under vacuum heating, at 200-500°C, the main gas desorbed is H2O gas, etc. When the temperature rises to above 700°C, H2, CO2 and other gases can be desorbed. The method of the present invention can effectively reduce the occurrence of pores and air holes in the powder metallurgy preparation process through vacuum degassing.

[0046] The term "powder compaction" as used in the specification and claims refers to the process of causing relative displacement and rearrangement between powders through vibration. When powder particles are freely stacked, an "arch bridge phenomenon" will occur, resulting in an increase in the pores between the powder particles. The method of the present invention can allow the powder particles to obtain a considerable alternating speed and acceleration through powder compaction. The originally stationary powder particles generate a considerable inertial force. Under the action of this inertial force, relative displacement occurs between the powder particles, destroying the arch bridge phenomenon and causing the particles to rearrange. The air in the loose powder is discharged due to the continuous movement of the particles, and the pores between the particles gradually decrease, ultimately improving the density and uniformity of the powder.

[0047] 1 , the preparation method of the high-performance nano-hard alloy product of the present invention mainly comprises the following steps: wet ball milling tungsten carbide (WC) powder with a mass fraction of 85-95% and cobalt (Co) powder with a mass fraction of 5-15% to obtain WC-Co hard alloy mixed powder after drying; loading the WC-Co hard alloy mixed powder into a mold; vibrating the WC-Co hard alloy mixed powder loaded into the mold; degassing the vibrated WC-Co hard alloy mixed powder in a vacuum environment; powder forming the degassed WC-Co hard alloy mixed powder in a mold in a vacuum environment by means of a press; separating the WC-Co hard alloy primary product from the mold in the presence of a protective gas; and heat treating the demolded WC-Co hard alloy primary product in a protective gas environment to obtain a high-performance nano-hard alloy material product.

[0048] To facilitate implementation, exemplary embodiments of the present invention are described below.

[0049] <Example 1>

[0050] The preparation method of this embodiment is to prepare (diameter × length) nano WC-Co cemented carbide products. The preparation method of this embodiment adopts the mold female mold size of (diameter × height), the mold wall thickness is 80mm. The preparation method of this embodiment adopts a two-way pressing process, and the sizes of the upper and lower pressing molds are respectively and Powder forming is carried out in a vacuum powder forming furnace. The specific steps are as follows:

[0051] Nano WC carbide powder with a particle size of 150-300 nm was used: 90 wt.% WC powder and 10 wt.% Co powder were wet-milled in anhydrous ethanol. The ball-to-material ratio during wet milling was 8:1, the liquid-to-solid ratio was 0.5:1, the milling time was 48 hours, and the mixture was dried after wet milling.

[0052] WC-Co cemented carbide powder was loaded into the mold and subjected to powder compaction treatment at 100 Hz for 1.5 minutes.

[0053] The mold is closed and the entire mold is transferred to a vacuum powder forming furnace for vacuum degassing treatment with a vacuum degree of 3.0×10 -3 Pa, the vacuum time is 20 minutes;

[0054] Continue to evacuate the furnace, first increase the temperature in the furnace to 650°C at a heating rate of 100°C / min, and at the same time increase the forming pressure to 25MPa, keep warm and hold the pressure for 10 minutes, then continue to increase the temperature in the furnace to 1200°C at a heating rate of 100°C / min, and at the same time increase the pressure to 50MPa, and then keep warm and hold the pressure for 10 minutes;

[0055] After the heat preservation and pressure maintenance is completed, high-purity argon is filled into the furnace as a protective gas, and the furnace is cooled to 1100°C, and the formed product is demoulded and taken out from the mold;

[0056] The product is transferred to an oil quenching chamber, which is sealed, evacuated, and filled with high-purity nitrogen as an inert protective gas for oil quenching until the product cools to room temperature;

[0057] The cooled product is transferred to a heat treatment chamber, which is sealed, evacuated and filled with high-purity nitrogen as an inert protective gas, and tempered at 500°C for 2 hours. The product is then rapidly cooled to room temperature and the final product is transported out of the furnace.

[0058] <Example 2>

[0059] The preparation method of this embodiment is to prepare (diameter × length) nano WC-Co cemented carbide products. The preparation method of this embodiment adopts the mold female mold size of (diameter × height), the mold wall thickness is 80mm. The preparation method of this embodiment adopts a two-way pressing process, and the sizes of the upper and lower pressing molds are respectively and Powder forming is carried out in a vacuum powder forming furnace. The specific steps are as follows:

[0060] Nano WC carbide powder with a particle size of 150-300 nm was used: 91 wt.% WC powder and 9 wt.% Co powder were wet-milled in acetone. The ball-to-material ratio during wet milling was 8:1, the liquid-to-solid ratio was 0.7:1, the milling time was 48 hours, and the mixture was dried after wet milling.

[0061] WC-Co cemented carbide powder was loaded into the mold and subjected to powder compaction treatment at 100 Hz for 2.5 minutes.

[0062] The mold is closed and the entire mold is transferred to a vacuum powder forming furnace for vacuum degassing treatment with a vacuum degree of 3.0×10 -3 Pa, the vacuuming time is 45 minutes;

[0063] Continue to evacuate the furnace, first increase the temperature in the furnace to 650°C at a heating rate of 110°C / min, and at the same time increase the forming pressure to 25MPa, keep warm and hold the pressure for 10 minutes, then continue to increase the temperature in the furnace to 1150°C at a heating rate of 100°C / min, and at the same time increase the pressure to 50MPa, and then keep warm and hold the pressure for 15 minutes;

[0064] After the heat preservation and pressure maintenance is completed, high-purity argon is filled into the furnace as a protective gas, and the furnace is cooled to 1150°C, and the formed product is demoulded and taken out from the mold;

[0065] The product is transferred to an oil quenching chamber, which is sealed, evacuated, and filled with high-purity nitrogen as an inert protective gas for oil quenching until the product cools to room temperature;

[0066] The cooled product is transferred to a heat treatment chamber, which is sealed, evacuated and filled with high-purity nitrogen as an inert protective gas, and tempered at 500°C for 3 hours. The product is then rapidly cooled to room temperature and the final product is transported out of the furnace.

[0067] <Example 3>

[0068] The preparation method of this embodiment is to prepare (diameter × length) nano WC-Co cemented carbide products. The preparation method of this embodiment adopts the mold female mold size of (diameter × height), the mold wall thickness is 80mm. The preparation method of this embodiment adopts a two-way pressing process, and the sizes of the upper and lower pressing molds are respectively and Powder forming is carried out in a vacuum powder forming furnace. The specific steps are as follows:

[0069] Nano WC carbide powder with a particle size of 150-300 nm was used: 91.5 wt.% WC powder and 8.5 wt.% Co powder were wet-milled in acetone. The ball-to-material ratio during wet milling was 10:1, the liquid-to-solid ratio was 0.6:1, the milling time was 48 hours, and the mixture was dried after wet milling.

[0070] The WC-Co cemented carbide powder was loaded into the mold of a vacuum powder forming furnace and subjected to a powder compaction treatment at 120 Hz for 1 minute.

[0071] The mold is closed and the entire mold is transferred to a vacuum powder forming furnace for vacuum degassing treatment with a vacuum degree of 3.5×10 -3 Pa, the vacuum time is 30 minutes;

[0072] Continue to evacuate the furnace, first increase the temperature in the furnace to 700°C at a heating rate of 120°C / min, and at the same time increase the forming pressure to 30MPa, keep warm and hold the pressure for 10 minutes, then continue to increase the temperature in the furnace to 1200°C at a heating rate of 100°C / min, and at the same time increase the pressure to 60MPa, and then keep warm and hold the pressure for 10 minutes;

[0073] After the heat preservation and pressure maintenance is completed, high-purity argon is filled into the furnace as a protective gas, and the furnace is cooled to 1180°C, and the formed product is demoulded and taken out from the mold;

[0074] The product is transferred to an oil quenching chamber, which is sealed, evacuated, and filled with high-purity argon as an inert protective gas for oil quenching until the product cools to room temperature;

[0075] The cooled product is transferred to a heat treatment chamber, which is sealed, evacuated and filled with high-purity argon as an inert protective gas, and tempered at 550°C for 4 hours. The product is then rapidly cooled to room temperature and the final product is transported out of the furnace.

[0076] <Example 4>

[0077] The preparation method of this embodiment is to prepare (diameter × length) nano WC-Co cemented carbide products. The preparation method of this embodiment adopts the mold female mold size of (diameter × height), the mold wall thickness is 80mm. The preparation method of this embodiment adopts a two-way pressing process, and the sizes of the upper and lower pressing molds are respectively and Powder forming is carried out in a vacuum powder forming furnace. The specific steps are as follows:

[0078] Nano-WC cemented carbide powder with a particle size of 150-300 nm was used: 92.5 wt.% WC powder and 7.5 wt.% Co powder were wet-milled in anhydrous ethanol. The ball-to-material ratio during wet milling was 10:1, the liquid-to-solid ratio was 0.6:1, the milling time was 36 hours, and the mixture was dried after wet milling.

[0079] The WC-Co cemented carbide powder was loaded into the mold and subjected to a powder compaction treatment at 150 Hz for 2 minutes.

[0080] The mold is closed and the entire mold is transferred to a vacuum powder forming furnace for vacuum degassing treatment with a vacuum degree of 4.0×10 -3 Pa, the vacuum time is 50 minutes;

[0081] Continue to evacuate the furnace, first increase the temperature in the furnace to 700°C at a heating rate of 130°C / min, and at the same time increase the forming pressure to 25MPa, keep warm and hold the pressure for 15 minutes, then continue to increase the temperature in the furnace to 1100°C at a heating rate of 100°C / min, and at the same time increase the pressure to 70MPa, and then keep warm and hold the pressure for 10 minutes;

[0082] After the heat preservation and pressure maintenance is completed, high-purity nitrogen is filled into the furnace as a protective gas, and the furnace is cooled to 1120°C, and the formed product is demoulded and taken out from the mold;

[0083] The product is transferred to an oil quenching chamber, which is sealed, evacuated, and filled with high-purity nitrogen as an inert protective gas for oil quenching until the product cools to room temperature;

[0084] The cooled product is transferred to a heat treatment chamber, which is sealed, evacuated and filled with high-purity nitrogen as an inert protective gas, and tempered at 450°C for 5 hours. The product is then rapidly cooled to room temperature and the final product is transported out of the furnace.

[0085] <Experimental Example 1>

[0086] Metallographic observations were performed on the nano WC-Co carbide products prepared in Example 1 and Example 2. A 5 mm thick specimen was cut from the middle of the rod-shaped sample in the longitudinal direction by wire cutting, and hot-mounted to obtain a mounted specimen. The metallographic specimen was then polished to obtain a metallographic specimen.

[0087] The microstructure was observed under a Zeiss Gemini SEM 500 field emission scanning electron microscope. The magnification of Figure 2 was 2000 times, and the magnification of Figure 3 was 5000 times.

[0088] Figures 2 and 3 show the alloy structure images of the high-performance nano-cemented carbide products prepared according to Examples 1 and 2, respectively. The light-colored grains in the image are the hard phase WC phase, and the dark area is the metal bonding phase Co-rich phase. It can be seen from the image that the high-performance nano-cemented carbide products prepared have a small particle size, dense particles, neat arrangement, and no obvious pores, and it can be seen that the Co-rich phase forms a relatively ideal coating effect on the WC phase. The WC grain size and the area fraction of each phase were statistically analyzed using image processing software. The results show that the area fraction of the Co-rich phase in Figure 2 is 11.21%, and the area fraction of the Co-rich phase in Figure 3 is 13.08%, which shows that the present invention achieves a good coating effect of the Co-rich phase on the WC phase at a lower Co content.

[0089] <Experimental Example 2>

[0090] Samples were prepared for the products in the above examples, and the hardness, compressive strength, flexural strength, and fracture toughness of each sample were tested. The test results of these properties of each sample are shown in Table 1.

[0091] <Experimental Example 3>

[0092] The density of the products in the above embodiments was measured by the “Archimedes drainage method”, and then the relative density of each product was calculated using the following formula (1-3). The results are shown in Table 1.

[0093] First, a 5×5×5 mm block sample was prepared by wire cutting and the surface was polished. Then, according to GB 3850-83, the mass of the sample in air and suspended in water was measured respectively, and the sample density was calculated according to formula (1):

[0094] Where: ρ—actual density of the sample, g / cm 3 ;

[0095] M1—mass of sample in air, g;

[0096] M2—mass of the sample in water, g;

[0097] ρ W —Density of water at room temperature is 1g / cm 3 .

[0098] Then, the theoretical density of the sample is calculated according to formula (2):

[0099] Where: ρ0—theoretical density of cemented carbide, g / cm 3 ;

[0100] X—mass percentage of WC in cemented carbide (%);

[0101] Y—mass percentage of Co in cemented carbide (%);

[0102] Z i —Mass percentage (%) of grain growth inhibitor (this item is omitted in the present invention);

[0103] d WC —WC density, 15.6 g / cm 3 ;

[0104] d Co —The density of Co is 8.9 g / cm 3 ;

[0105] d i—Density of grain growth inhibitor, g / cm 3 (This item is omitted in the present invention).

[0106] Finally, the relative density of the sample is calculated according to formula (3):

[0107] Where: f—relative density of cemented carbide (%);

[0108] ρ—actual density of the sample, g / cm 3

[0109] ρ0—theoretical density of cemented carbide, g / cm 3 .

[0110] Table 1 Test results of various properties of the experimental products of the present invention

[0111] As can be seen from Table 1, the high-performance nano-hard alloy products prepared by the preparation method of the present invention have relatively high hardness, compressive strength and flexural strength, and their fracture toughness and relative density are better than those of commercial WC-Co hard alloy products (fracture toughness 8MPa·m 1 / 2 , relative density 99%), and the WC grain size is smaller than the WC particle size (300nm) in nano-cemented carbide specified in the internationally commonly used Sandvik company WC grain grading standard.

[0112] The above are only preferred embodiments of the present application and the technical principles employed. Those skilled in the art will understand that the present application is not limited to the specific embodiments described herein, and that various obvious changes, readjustments, and substitutions can be made by those skilled in the art without departing from the scope of protection of the present application. Therefore, although the present application has been described in more detail through the above embodiments, the present application is not limited to the above embodiments and may include many other equivalent embodiments without departing from the scope of protection of the present application.

Claims

1. A method for preparing a high-performance nano-hard alloy product, characterized in that: The following steps are involved: Wet-milling 85-95% by mass tungsten carbide powder and 5-15% by mass cobalt powder to obtain WC-Co cemented carbide mixed powder after drying. The WC-Co cemented carbide mixed powder is loaded into a mold; Performing powder compaction on the WC-Co cemented carbide mixed powder loaded into the mold; Degassing the compacted WC-Co cemented carbide mixed powder in a vacuum environment; By means of a press, under a vacuum environment, the degassed WC-Co cemented carbide mixed powder is powder-formed to form a WC-Co cemented carbide primary product in the mold; Separating the WC-Co cemented carbide primary product from the mold under a protective gas environment; as well as The WC-Co hard alloy primary product after demoulding is heat treated in a protective gas environment to obtain a high-performance nano-hard alloy material product.

2. The preparation method according to claim 1, characterized in that: The size of the tungsten carbide powder is in the range of 150-300 nm.

3. The preparation method according to claim 1, characterized in that: In the wet ball milling, one or more of anhydrous ethanol, acetone and ethane are used as grinding aids, the preferred ball-to-material ratio is 6-10:1, and the preferred liquid-to-solid ratio is 0.4-0.8:

1.

4. The preparation method according to claim 1, characterized in that: The vibration frequency of the powder compaction is 50-200 Hz, and the preferred vibration duration is 1-3 minutes.

5. The preparation method according to claim 1, characterized in that: The vacuum degree of the vacuum environment is 2.0-5.5×10 -3 Pa, the preferred degassing duration is 10-60 minutes.

6. The preparation method according to claim 1, characterized in that: The powder forming process includes the following two sub-stages: a) maintaining the temperature at 600-700°C and the pressure at 20-30MPa, and b) Maintaining the temperature at 1100-1200°C and the pressure at 50-70MPa.

7. The preparation method according to claim 6, characterized in that: In sub-stage a), the pressure is increased to 20-30 MPa at a pressurization rate of 5 MPa / min. The temperature is raised to 600-700°C at a heating rate of 100-130°C / min; In sub-stage b), the pressure is increased to 60-70 MPa at a pressurization rate of 10 MPa / min, and the temperature is increased to 1100-1200° C. at a heating rate of 100° C. / min.

8. The preparation method according to claim 1, characterized in that: The heat treatment is to firstly cool the demoulded WC-Co cemented carbide primary product to room temperature by oil quenching, and then temper the cooled WC-Co cemented carbide primary product at a temperature of 400-500° C. for 2-8 hours.

9. The preparation method according to claim 1, characterized in that: The protective gas environment is a high-purity nitrogen atmosphere or a high-purity argon atmosphere.

10. A high-performance nano-hard alloy product, characterized in that: Prepared by the preparation method according to any one of claims 1 to 9.

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