Abstract : The two key factors for the preparation of nano / ultrafine WC – Co cemented carbides are the prep -Aration of high – quality nano/ ultrafine WC – Co composite powders and the control of grain growth during sintering. The research progress at home and abroad in recent years is comprehensively reviewed on nano/ultrafine WC – Co composite powder preparation methods and nano/ ultrafine WC – Co cemented carbide sinte-Ring technologies. Additional , the development prospects and the future research focus of nano/ ultrafine WC – Co cemented carbides are also discussed.Keywords : cemented carbide, nano / ultrafine crystal; WC – Co composite powder;Carbide is a refractory metal hard compound (mainly refers to WC, TiC, TaC, NbC, VC, Cr 3 C 2, Mo 2 C, etc.) as hard phase and bond metal (mainly refers to Fe, Co, Ni Etc.) For the binder phase, an alloy material prepared by powder metallurgy. Compared with high-speed steel, diamond, ceramics and other materials, cemented carbide not only has good strength, but also has excellent toughness. It is one of the most widely used tool materials and plays a role in promoting China’s industrial manufacturing and national economic development. A decisive role. Nano/ultrafine grained carbides (when the average WC grain size in the alloy is 0.1 to 0.6 μm) can effectively overcome the inconsistencies between hardness and toughness in conventional cemented carbides, as well as greater brittleness and process softening. The problem is that it has a double high characteristic of high hardness and toughness. Now it has developed a series of high-end carbide products, such as micro drills for processing integrated circuit boards, dot matrix printer printing needles, overall hole machining tools, and milling cutters. , dental drills and precision molds, etc., are widely used in aerospace, precision machining, electronics industry, precision manufacturing and other fields. Since the powder metallurgy method is adopted for the production of cemented carbide, the steps include powder preparation, pressing and sintering. Therefore, the two key factors for the preparation of nano/ultra fine grain WC-Co cemented carbides are the high quality nano/ultrafine crystal powders. Control of grain growth during preparation and sintering. In this paper, the synthesis of nano/ultrafine WC-Co composite powders and nano/ultrafine-grained carbides sintering techniques are reviewed and reviewed in recent years.Preparation method of 1 nano/ultrafine WC-Co composite powderThe traditional method for producing WC-Co composite powders is as follows: 1) WO 3 is obtained by hydrogen reduction in the temperature range of 700-900 °C to obtain W powder; 2) W powder and C powder are mixed in the temperature range of 1400 to 1 600 °C. Carbonized to obtain WC powder; 3) WC powder and Co powder were mixed to obtain WC-Co composite powder. The traditional process method is not an ideal method for preparing nano/ultrafine WC-Co composite powders, and there are many disadvantages. First of all, the high carbonization temperature of W and C powders can easily cause the grain growth of powders and affect the uniformity of the particle size distribution. Secondly, there are many factors that affect the quality of powders in the traditional process, and it is difficult to control the powder properties. Finally, the traditional methods Long process flow and production cycle, high production costs.After nearly 20 years of development, many new nano/ultrafine WC-Co composite powder preparation methods have been developed under the unremitting efforts of researchers around the world. They can be divided into two major categories: top-down and self- Bottom up approach. The bottom-up method refers to obtaining nano/ultrafine crystalline powders from the microscopic level of atomic or molecular level, which mainly includes the solution method (sol-gel method, co-precipitation method, spray drying conversion method) and gas-phase synthesis. Law and so on. The top-down method refers to obtaining nano/ultrafine crystal powders from macroscopic viewpoints such as large particles. The main methods include high-energy ball milling and the like.Fig.1 Grain size of nanocrystalline carbide WC-7Co and WC-10Co1. 1 High-energy ball millingTraditional high-energy ball milling involves charging raw material powders and grinding balls into a ball mill tank in a certain proportion and introducing an inert gas to force the powders to undergo extrusion through the impact of the grinding balls – cold welding – crushing processes for grain refinement Preparation of nano/ultrafine WC-Co composite powders. EL-ESKANDARANY M S uses W powder (d<196 μ m) and C powder (d <45 μ m) as raw materials, using steel balls as ball milling media, and obtaining full ball milling at a ball material ratio of 10:1 for 120 hours. Nano WC powder. However, the use of high energy ball milling to produce nano/ultrafine WC-Co composite powder has the disadvantages of long ball milling time, impure powder after milling, and low work efficiency. In order to overcome the shortcomings of traditional high-energy ball milling, carbide balls are generally used as grinding balls to reduce the contamination of powders. At the same time, some new high-energy ball milling processes have also been developed, such as High Energy Dual-Drive Planetary Mill, Mechano-chemical Synthesis, and Integrated Mechanical and Thermal Activation.The high-energy dual-drive planetary ball mill mainly combines the rotation and revolution of the mill barrel, and increases the efficiency through the gravity acceleration field generated during the ball milling process. BUTLER B G et al. used a high-energy dual-drive planetary ball mill to reduce the particle size of 0.8 μm WC and WC-Co powders to 10-20 nm in just 10 h.Mechanochemical synthesis refers to the introduction of chemical reactions during the ball milling process, thereby shortening the milling time and improving the milling efficiency. Mechanochemical synthesis is mainly divided into two steps: the first step is to use active metals such as Mg and Zn as reducing agents, and carbon black and some carbon-containing organics as carbonization agents are added to the ball mill tank together with WO 3 . Because the ball milling process generates a large amount of energy, WO3 first reacts with the active metal to form W, and then C reacts with W to produce nano-WC. The second step is to put the powder obtained after the ball milling is completed into an acidic solution such as HCl to remove the metal oxides to obtain pure nano WC powder. HO-SEINPUR A et al. placed WO3, Zn and C into a ball mill tank, and after ball milling for 36 hours, the resulting powder was soaked in diluted hydrochloric acid for 2 hours to obtain a WC powder of about 20 nm.The mechanical heat-activated synthesis method is a new method that combines the ball milling process with the reduction-carbonization process. Its main feature is to make full use of the highly active surface produced by high-energy ball milling to reduce the reduction-carbonization temperature, and to prepare nano/ Ultrafine WC-Co composite powder. SHAWLL and so on with 1:2.4:0. 7 (molar ratio) Tungsten oxide, graphite and cobalt oxide of 20 μm were put into a ball mill for 6 h high-energy ball milling, and then the obtained powder was subjected to reduction-carbonization reaction at 1 000 °C under argon gas protection to obtain crystals. WC-Co composite powder with a grain size of 80 to 200 nm. Song Xiaoyan’s team reinvented the traditional mechanical heat-activated synthesis method, and put the compound oxide obtained by ball milling into a vacuum furnace directly for in-situ reduction-carbonization synthesis of nano/ultrafine WC-Co composite powders. The particle size distribution and composition of the prepared powder were uniform, and the particle size ranged from 70 to 500 nm.Fig. 2 Surface abrasion SEM photographs of nano-carbide and ordinary cemented carbide1. 2 solution methodIn the solution method, soluble tungsten salt, cobalt salt and other raw materials are added to a solution to disperse it at the atom or molecule level, and a precursor powder is prepared by a specific method; and then the precursor powder is dried, reduced, carbonized, etc. to prepare a nanometer. / Ultra-fine grain WC-Co composite powder. In the precursor powder obtained by the solution method, each phase is uniformly distributed and exists at the molecular and atomic level, and has a high chemical activity, which can effectively reduce the reduction and carbonization temperature, shorten the preparation time, and favor the nano/ultrafine crystal. Preparation of WC-Co Composite Powders.The solution method can be divided into sol-gel method, co-precipitation method and spray-drying conversion method according to different methods for obtaining precursor powder. The sol-gel method is a method of gradually forming a viscous colloid precursor by the process of hydrolysis and polycondensation of soluble salts, and then drying and sintering to obtain a nano/ultrafine crystal composite powder. HOLGATE M W R uses tungsten salt, cobalt salt, and soluble organic carbon as raw materials to obtain a gel-like precursor by controlling the synthesis conditions such as the pH value of the solution, and then obtains nano-WC-Co composite powder through drying, reduction, and carbonization processes.The co-precipitation method is to prepare a good dispersion of tungsten-cobalt composite precursor by co-precipitation of tungsten salt and cobalt salt in the liquid phase, and then to prepare a nano/ultrafine WC-Co composite powder by reduction-carbonization. MAJH etc. contains 66% W (mass fraction, the same below) tungsten salt and contains 14. The cobalt salt of 42% Co was used as a raw material, and a tungsten/cobalt composite precursor powder was prepared by a chemical co-precipitation method, followed by reduction in H 2 and carbonization in a CO/CO 2 atmosphere to obtain a nanoparticle having a particle size of about 50 nm/ Ultrafine WC-Co composite powder.In the spray drying conversion method, soluble tungsten salt, cobalt salt, etc. are dissolved in a solution to be spray-dried to obtain a tungsten-cobalt composite precursor powder, and then a nano-scale WC-Co composite powder is obtained through reduction and carbonization steps. The spray conversion method was first proposed by Rutgers University, and its specific process includes three steps: 1) Dissolve soluble tungsten salt and cobalt salt in high-purity water to obtain a uniform aqueous solution; 2) Spray-dry the aqueous solution. The solute in the solvent is rapidly crystallized to form a precursor powder that is evenly distributed at the molecular level; 3) The precursor powder is reduced under H 2 atmosphere, followed by the carbonization reaction in a fluidized bed under a CO/CO 2 atmosphere. A nano/ultrafine WC-Co composite powder was obtained. As spray drying technology and fluidized bed heat treatment technology are industrial production technologies, it is a technology with industrial application prospects. The Yang Jiangao team integrated and reinvented the traditional spray drying conversion method, abandoning complex fluidized bed equipment and switching to a fixed bed, and developed a new preparation technology for composite powders with “ion-layer mixing, rapid precipitation, and low-temperature synthesis”. In addition, a one-step method of high activity in-situ carbon and carbon thermal reaction was introduced into the preparation process of nano/ultrafine WC-Co composite powders.Uniformly distributed high activity in-situ carbon effectively reduced the reaction temperature and shortened the reaction time to suppress the crystal grains. Grown up, a simple, fast, low-cost, industrially-producible powder preparation method was proposed to prepare a nano/ultrafine WC-Co composite powder with controlled structure and performance and a WC crystal grain size of less than 100 nm. From the traditional 8 steps to 3 steps, the carbonization temperature is reduced from the conventional 1300 °C to 1000 °C.1. 3 gas phase reaction synthesisThe gas-phase reaction synthesis method is a method for preparing a superfine powder in which a thermodynamically unstable supersaturated precursor gas undergoes a physical reaction or a chemical reaction in a gas state and agglomerates and grows in the cooling process to form microparticles. According to the thermodynamically unstable saturated precursor method, the chemical vapor synthesis method can be divided into a laser ablation method, a spark discharge conversion method, an ion sputtering method, a flame synthesis method, a chemical vapor method, and a thermal plasma conversion method. At present, the widely used methods for preparing nano-WC-Co composite powders include chemical vapor deposition and thermal plasma conversion.In the chemical vapor method, a nano-WC-Co composite powder is prepared by passing a gasified precursor and a reducing carbonized gas into a hot wall reactor. Metal chlorides are ideal precursor materials due to their lower volatilization temperature. RYUT et al. used WCl 6 and CoCl 2 as precursors, H 2 and CH 4 as reducing and carbonizing gases, and Ar gas as carrier gas to successfully obtain nano-WC-Co composite powders with a particle size of (24±1) nm. In the preparation process, in order to avoid the formation of carbon-deficient phases such as Co3W3C, WCl6 and CoCl2 were fed at reactor temperatures of 440 and 1400°C, respectively, and there was almost no carbon-deficient phase in the resulting composite powder.The hot plasma conversion method is a method in which a plasma is used as a heat source, and the gasified precursor and the reduced carbonized gas are converted into atomic levels to promote their mutual reduction and carbonization to obtain a composite powder. SOHN H Y et al. used WCl 6, AMT, and C 2 H 4 as raw materials to carry out thermal plasma conversion in an induction plasma apparatus to prepare a 30-nm WC1-x powder, followed by a H 2 /CH 4 atmosphere at a temperature of 900°C. Heat treatment was performed to obtain 100 nm pure WC powder.2 Nano/Ultrafine WC-Co Cemented Carbide Sintering TechnologySintering is the last step in the preparation of cemented carbide. Sintering has a direct effect on product performance, and this change is irreversible, and therefore plays a decisive role in the process of producing cemented carbide.For nano/ultrafine WC-Co cemented carbides, the sintering process not only ensures the densification of the cemented carbide, but also controls the growth behavior of the grains during the sintering process. Compared with conventional size powders, nano/ultrafine WC-Co composite powders exhibit special sintering behavior due to small size effects, surface and interface effects and other factors. The thermodynamic driving force of the sintering process is mainly the reduction of the surface energy, but the nano/ultrafine WC-Co composite powder has a large surface energy and a large driving force for sintering, and the densification process can be performed at a lower temperature. At the same time, nano/ultrafine WC-Co composite powders have high activity, and they are prone to agglomeration of crystal grains during the sintering process and dissolution-dissolution processes, making grains very easy to grow. MA-HESHWARIP et al. studied the densification behavior of nano/ultrafine WC-Co composite powders with different particle sizes during the sintering process. WANG X et al. used WC-10Co (mass fraction) with a particle size of 10 nm as a raw material and sintered it in a vacuum furnace to study the effect of temperature on grain growth. The results showed that the increase in temperature caused a significant increase in grain length. The larger the temperature is, the higher the increase is. When the sintering temperature is 1 300 °C, the grain size grows from 10 nm to about 380 nm, which is a 38-fold increase. FANGZG et al. found that during the first 5 minutes of sintering, the nanopowder rapidly developed. In recent years, in order to effectively control the growth behavior of nano/ultrafine WC-Co composite powders in the sintering process, some new sintering processes have been developed, such as gas pressure sintering, hot press sintering, microwave sintering and spark plasma sintering, etc.2. 1 Gas pressure sinteringAt the end of the degassing process, the gas pressure sintering is performed under the conditions that the pores on the compact surface have been closed and the cobalt phase exists in the liquid phase. Using inert gas as the pressure medium, hot isostatic pressing is applied to the alloy to promote densification of the alloy. Gas pressure sintering effectively combines vacuum sintering and hot isostatic pressing to promote the flow of cobalt phase and suppress the high temperature volatility of Co, which helps to eliminate the pores and cobalt pools of the product, so that the alloy has a fine and uniform structure and the performance is greatly improved. Compared with traditional hot isostatic pressing, the pressure of gas pressure sintering is only equivalent to 1/10 or less of hot isostatic pressure, which greatly reduces equipment manufacturing costs and maintenance costs. Du Wei et al used a nano/ultrafine WC powder with a particle size of 0.53 μm and a spherical Co powder as raw materials to compare the effects of vacuum sintering and gas pressure sintering on the performance of WC-2.5% Co cemented carbide. The experimental results show that gas pressure sintering can reduce the porosity of the alloy and suppress the abnormal grain growth. The bending strength of the alloy increases from 1800 MPa to 2250 MPa. Wei Chongbin and others used the in-situ reduction/carbonization method of nano/ultrafine WC-10Co composite powder as raw material to compare the effects of vacuum sintering and gas pressure sintering on the microstructure and properties of the alloy at 1420°C for 1 h. The sintering pressure is 2 MPa. The results show that gas pressure sintering can greatly improve the performance of the alloy and increase its fracture toughness from 10.2MPa ? m1 / 2 to 13. 6MPa ? m1 / 2 Shi Xiaoliang et al used WC-10Co composite powders prepared by spray conversion method as raw materials, and after ball milling for 48 hours, produced WC-10Co-0.4VC-0. 4Cr 3 C 2 composite powder; followed by gas pressure sintering, sintering process for 1h at 320 °C, the pressure is 5. At 5 MPa, the obtained alloy has high mechanical properties, and the HRA hardness is 92. 8, the intensity is 3 780 MPa. From the previous research results, it can be seen that the grain size of the nano/ultrafine-grained hard alloy obtained by gas pressure sintering is small, the structure is uniform, and the toughness is also very good. At present, it has become an industrially manufactured nano/ultra fine crystalline hard alloy. One of the main sintering methods.2. 2 hot press sinteringHot-press sintering is a method that effectively combines the pressing and sintering processes and rapidly densifies the alloy under the combined action of pressure and temperature. Compared with traditional pressing and sintering processes, hot-press sintering can eliminate the need of adding forming agents and reduce the introduction of impurities; the plasticity and fluidity of powders are greatly improved under heat-pressing conditions, and the densification of alloys is promoted, and the sintering temperature can be reduced at a relatively low temperature. A fully dense alloy is obtained within a short sintering time.Li Zhixi et al. used nano/ultrafine WC powder (0.81 μm) and Co powder (1.35 μm) as raw materials, and Cr 3 C 2 and VC as grain growth inhibitors through planetary high-energy ball milling. The prepared particle size is less than 0. The 3 μm WC-Co composite powder was subsequently hot-pressed and sintered to study the effect of hot-press sintering on the sample performance. The results showed that WC-10Co cemented carbide with uniform microstructure and average grain size of less than 0.8 μm was obtained by hot-press sintering at 1 400°C, 2h in temperature and 30 MPa pressure. The grain size was increased. Inhibitor Cr 3 C 2 +0. 4VC microhardness value 56GPa. Zhu Qikou et al. used WC – 6Co composite powders with a diameter of 300 nm prepared by in-situ reduction under high temperature as raw materials, and applied them by hot-press sintering at 1 200°C for 20 MPa and kept warm. 5h Preparation of Nano/Ultrafine WC-6Co Cemented Carbide. The results show that hot-press sintering can effectively reduce the alloy pores and inhibit grain growth. The average grain size of WC in the alloy is 600 nm and the distribution is even. The HRA hardness is 93 and the transverse fracture strength is 1530 MPa. Liu Xuemei and others used WO 3 powder, Co 3 O 4 powder and carbon black powder as raw materials, firstly pretreating in a vacuum heat treatment furnace, and then using nanocomposite at a temperature of 1 370 °C under a pressure of 20 MPa for 1.5h. Fine grain WC – Co type carbide. The results show that the prepared cemented carbide has high density and pure WC and Co phases with an average grain size of 0.813 μm, HRA hardness and fracture toughness of 92.5 and 8.44 MPa?m1/2, respectively. From the above research results, it can be seen that the toughness of the alloy after hot-press sintering is generally low, mainly because the axial pressure can only be applied during the hot-press sintering process, so that the structure of the various parts of the alloy in the sintering process due to uneven force generated The anisotropy leads to a lower toughness of the alloy and affects the service life of the alloy.2. 3 microwave sinteringMicrowave sintering is a new rapid sintering technology that utilizes the dielectric loss of the material in the microwave electromagnetic field to heat the entire sintered body to the sintering temperature to achieve sintering and densification. Since the microwave energy increases the kinetic energy of atoms, molecules or ions inside the sintered material, the sintering activation energy of the material is reduced, which is advantageous in reducing the sintering temperature and shortening the sintering time. At the same time, microwave heating has the characteristics of rapid heating and rapid temperature reduction, so that the materials prepared by microwave sintering have the characteristics of uniform microstructure and fineness, good toughness and so on.The WC- 10Co composite powder prepared by high-energy ball milling was used as the raw material for the whole peak, and the microwave sintering process was used to prepare the hard alloy. The experimental results show that the dewaxing time and sintering temperature have a significant effect on the properties of the alloy, while the holding time and heating rate have little effect on the properties of the alloy. The results are obtained at a dewaxing time of 20 min and a sintering temperature of 1 320°C. The alloy grains are fine and uniform, with a density of 14. 32g/cm3, hardness of HV30 16. 11GPa, fracture toughness up to 9. 78MPa ? m1 / 2 Lu et al. found that the holding time has little effect on the grain growth of microwave sintered WC-8Co cemented carbide. BAO R et al. used a planetary ball milling method to mix and compress WC and Co powder with a particle size of 0.15 μm, followed by microwave sintering. The results show that the microwave sintering has the characteristics of rapid densification. After the sintering, the decarburized phase forms on the surface of the alloy. Adding a certain amount of carbon black during the mixing can inhibit the decarburization of the alloy surface and effectively improve the performance of the alloy. The HRA hardness of the alloy reached 93.2 after microwave sintering using a composite powder with a total carbon content of 6.08%. Although microwave sintering has the advantages of short sintering time, rapid heating rate, fine and uniform grain size and excellent mechanical properties, microwave sintering has a strong selectivity to materials, and is prone to thermal runaway and uneven heating. Material properties. At the same time, the preparation of high-power microwave ovens is still an industrial problem. At present, the main research is still concentrated in schools and research institutes, and no large-scale industrial production has yet been formed.2. 4 Discharge plasma sinteringDischarge plasma sintering is the direct application of pressure and DC pulse current between the powder particles. Under the combined action of mechanical pressure, discharge pulse pressure and instantaneous high temperature field, the sintered body particles spontaneously generate heat and activate the surface of the particles to achieve rapid densification. A new type of sintering process. Spark plasma sintering has the advantages of fast heating rate, short sintering time, and low sintering temperature, which helps to shorten the preparation cycle and suppress the growth of crystal grains. The obtained sintered body has fine microstructure controllable, fine grain size and uniform distribution, and excellent overall performance. . GAO Y and other nano-WC-10Co composite powders prepared by an in-situ reduction-carbonization process were used as raw materials, VC was used as a grain growth inhibitor, and spark plasma sintering was used to study the carbon distribution at a sintering temperature of 1 130 °C and a pressure of 60 MPa. The effect of volume on the performance of plasma cemented carbide sintered. The results show that the amount of carbon has a great influence on the phase, structure and properties of the alloy. Under the optimal carbon allocation, the alloy has the characteristics of uniform structure and pure phase, with hardness and fracture toughness reaching 20.50GPa and 14. 5MPa ? m1 / 2 Hao Quan et al. used the WC-10Co composite powder with a grain size of 250 nm prepared by the spray conversion process as the raw material for discharge plasma sintering, and studied the effect of sintering temperature and atmosphere. The results show that the sintering temperature increases, the pressure in the furnace decreases, the cobalt phase evaporates, and the alloy deviates from the equilibrium phase. The Co content of the WC-10.10Co composite powder sintered at 1 250 °C for 5 min becomes 10.02%. LIU W B et al. fully studied the influence of discharge plasma process parameters on the microstructure and properties of the alloy. The results show that during the spark plasma sintering process, the densification starting temperature of the nano/ultrafine WC-Co composite powder is about 804 °C. The HRA hardness, fracture toughness, and transverse rupture strength of 92.6, 12 MPa ? m1 / 2 and 2 180 MPa high-performance hard materials can be obtained under the optimized conditions of sintering temperature of 1 325 °C, pressure of 50 MPa, and holding time of 6 to 8 minutes. alloy. Because the spark plasma sintering has a special DC pulse voltage, which is conducive to the plastic flow and surface diffusion of the particles in the sintering process, and the material is rapidly densified at a relatively low temperature and in a short time. It is a promising new technology. , has been widely studied around the world. However, spark plasma sintering is difficult for the sintering of complex structures, and large-scale industrial application is still in the stage of exploration.Fig. 3 Abrasive wear traces of nano WC-7Co rake faceFig. 4 Friction coefficient of nano-carbide and ordinary cemented carbide under different loads3 ConclusionNano/ultrafine crystal cemented carbide is a high-performance, high-value-added cemented carbide product. The development of nano-/ultrafine-grained carbide products that can be industrialized has become one of the problems to be solved in the cemented carbide industry in China. It is of great significance to promote the healthy development of China’s hard alloy industry. In recent years, under the strong support of the national policy, the preparation of nano/ultrafine WC-Co composite powders in China has made a breakthrough, and high performance nano/ultrafine WC-Co composite powders have gradually been industrialized. However, in order to produce high-performance nano/ultra fine crystalline cemented carbides with stable quality and reliable products, especially for large-scale production of nano/ultra fine crystalline cemented carbides with a particle size of less than 0.2 μm, it is still necessary to increase the Research and development of alloy-related preparation processes.
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