Hard fine crystalline alloy (an average WC grain size of the alloy is 0.1 to 0.6 m) having a high strength, high hardness, high wear resistance and other excellent properties to meet the development of modern industrial and specialty materials difficult to machine, and therefore, near Ultrafine grained carbide has been a hot topic in international hard alloy academic and industrial research for 10 years.
Since the advent of cemented carbide, its strength and hardness have always been a pair of "irreconcilable contradictions", and the rapid development of advanced manufacturing technology, strongly demanding the combination of the two. Studies have shown that when the WC grain size is reduced below submicron, the hardness and wear resistance, strength and toughness of the cemented carbide material are improved. This ultrafine grained WC-Co cemented carbide is aptly called "double high" cemented carbide because it has high hardness and high flexural strength, high wear resistance and high toughness. Performance of carbide tool materials is getting higher and higher requirements. The author comprehensively reviews the research results of ultrafine WC-Co cemented carbide at home and abroad in recent years from the preparation process of alloy powder, sintering process, ultra-fine cemented carbide molding technology and cemented carbide grain inhibitor. Figure 1 shows the performance comparison between ultrafine grained cemented carbide and conventional cemented carbide. When the grain size of the material drops below 0.6 microns, the strength of the alloy is significantly improved under the premise of increasing the hardness.
Figure 1 Hardness and flexural strength of cemented carbides with different grain sizes
1. Preparation technology of ultrafine WC-Co composite powder
The conventional technique for preparing WC-Co powder is firstly mixing W powder with C powder, and solid-phase reaction at 1400 ° C to 1600 ° C to form WC powder, which is then mixed with diamond powder and ground. Alloys made by this technique have a particle size that is not less than the particle size of the original powder, and typically has a diameter of from 1 to 10 microns and a high brittleness. In recent years, many technologies for preparing ultrafine WC-Co composite powders have been developed. Their common features are: compared with the conventional technology, the composite powder prepared is relatively uniform, the particle size is small, and the process flow is simple and less time consuming. The following is a brief description.
(1) Chemical precipitation method
The chemical precipitation method firstly prepares a tungsten- cobalt compound precursor with good dispersibility and high activity, and then reduces and carbonizes it into an ultrafine WC-Co composite powder in a fixed bed or a fluidized bed. Zhang et al. prepared a 90 nm tungsten-cobalt precursor using ammonium paratungstate and cobalt hydroxide as raw materials. Cao Lihong et al. first prepared CoWO 4 by coprecipitation with Na 2 WO 4 and Co(NO 3 ) 2 as raw materials, and then used high purity H 2 and carbon-containing gas in the carbonization furnace or rotary kiln at 550 ° C to 750 ° C and The WC-Co composite powder with less than 0.1% free carbon and an average particle diameter of about 0.1 μm was prepared by reduction carbonization at 850 ° C to 900 ° C. The method has the advantages of small powder particle size, uniform distribution, high reaction activity, simple equipment and easy control of the process, but there are problems such as easy introduction of impurities in the preparation process, formation of precipitates which are easy to be colloidal, difficult to filter and wash, and high cost. .
(2) High energy ball milling
Mao Changhui mechanically grinds WC with a particle size of 20 μm and Co powder of 16 to 18 μm on a Spe×8000 high-energy grinder. It is protected by Ar gas during ball milling to prepare WC-Co with an average particle size of less than 10 nm. Powder, but WC crystals have a lot of defects. Ban et al. carried out ball milling using WO 3 , carbon black and CoO as raw materials to obtain a composite powder of 0.3 to 0.5 μm. Ma et al. also prepared WC-Co powder having a particle size of about 10 nm by high energy ball milling. This type of technology is simple in process, but has a small amount of processing, a large abrasion, and is prone to produce contaminated products.
(3) In-situ carburizing reduction method
Zhou et al. dissolved tungstic acid and Co(NO 3 )·6H 2 O in a polyacrylonitrile solution, dried at low temperature, and then transferred to a 90% Ar+10% H 2 mixed gas at 800 ° C to 900 ° C in a furnace. It is reduced to carbonized into WC-Co powder, and the obtained powder has a grain size of 50 to 80 nm. The method uses polyacrylonitrile as the in-situ carbon source instead of CO∕CO 2 , which can shorten the diffusion path and make the finished product have better uniformity through the uniform distribution of carbon in the original material of the Polymer, but still observe in the WC-Co powder. The presence of a small amount of undecomposed polymer and free carbon, the phase purity of the product is related to the process parameters such as carbonization temperature, reaction time, and atmosphere ratio. The biggest problem with this technology is the control of carbon content.
(4) Plasma method
Fan et al. used W 2 +Ar as a plasma initiator and C 2 H 2 as a carbon source to prepare a WC-Co powder having an average particle diameter of 40 nm by directly reducing carbonized CoWO 4 by arc plasma at about 3727 °C. The method has the advantages of high operation speed and high production speed, and the obtained powder particles are uniform, but the cost is high, and the electrode is easy to be melted or evaporated at a high temperature, and the pollution product is easily generated.
(5) Spray heat conversion preparation technology
Rutgers University in the United States uses a water-soluble precursor to synthesize nano-WC-Co by the following steps: preparing and mixing an aqueous solution of a precursor compound to fix the components of the starting solution, usually using ammonium metatungstate [(NH 4 ) 6 (H 2 ) W 12 O 40 )·H 2 O] and CoC 12 , Co(NO 3 ) 2 or Co(CH 3 COO) 2 are used as an aqueous solution of the precursor compound; the starting solution is spray-dried to obtain an amorphous precursor powder; The precursor powder was converted to a nano WC-Co powder in a fluidized bed using H 2 reduction, CO ∕ CO 2 as a carbon source. The biggest problem encountered in the industrialization of this technology is the high cost and complicated process control.
Second, the ultra-fine composite WC-Co molding process
The high density and high uniformity of the shaped body have a great effect on improving the physical and mechanical properties of the cemented carbide. In the preparation of nano-WC-Co cemented carbide, a reasonable molding process is adopted, and appropriate molding process parameters are selected to ensure a uniform structure and high density of the green body, thereby obtaining a high-performance alloy. There are many molding processes for nanocomposite WC-Co. In addition to conventional compression molding, the following are representative ones.
(1) Extrusion molding
Powder extrusion molding (PEM) is a mixture of powder and a certain amount of binder, plasticizer, etc., extruded into a blank of the desired shape and size by extrusion nozzle, which is small in production cross section and radial A good way to make large products. The basic process of PEM is as follows: powder body + binder - mixing - granulation - extrusion - degreasing - sintering. The PEM process can be operated at low temperature and low pressure. The length of the product is not limited, the longitudinal density is relatively uniform, and it has the advantages of strong molding continuity, low cost and high efficiency. It has become the most important method for forming the hard alloy bar.
(2) Injection molding
Injection molding is similar to injection molding except that it is pressurized with a fluid, not mechanically. Injection molding is a method of molding a concentrated slurry into a cavity with compressed air. Therefore, in theory, it can make the pressure at any point in a closed container of any complicated shape the same, or the density of the powder. Therefore, it can produce various shapes and complicated products, and is simple in operation and high in production efficiency.
(3) Injection molding
Injection molding (PIM) is a combination of traditional plastic forming processes and powder metallurgy techniques. The powder body and the molding agent are kneaded, granulated, heated and melted in an injection molding machine, and then injected into a cavity through a nozzle under pressure, and coagulated to obtain a preform having a uniform structure and a complicated geometric shape. . The product produced by this molding method has a good surface finish and a shape close to the shape of the final product. This process allows the powder to maintain good rheology during the injection process, improve the interaction between the binder and the alloy powder, and improve the sintering performance. The process flow of PIM is almost the same as that of PEM. Compared with the traditional molding methods such as molding, it has the following advantages: the shape of the product is not limited; the density of the finished product is uniform; the applicability is wide; the shrinkage of each part of the product is consistent and can be better. Control the dimensional tolerances of the product.
(4) Explosive forming
Explosive forming is a special method of forming compacted bodies in recent years. The pressing method is to place a violently explosive substance around a shell containing super-hard powder, which is caused by the pressure of the explosion. 10 MPa), the body with a relatively high density can be extruded in a very short time. Experiments have shown that the density of the WC-8Co composite powder after compression molding can reach 99.2%.
Third, the sintering technology of ultra-fine cemented carbide
Sintering is a process that has a decisive influence on the microstructure and properties of cemented carbide. In general, the smaller the particle size of the WC-Co powder, the lower the temperature required for the sintering to be completely dense, and the sintering temperature of the conventional WC-Co powder is generally about 1400 °C. The surface activity of ultrafine/nano WC-Co powder is improved on the one hand due to the deagglomeration of large agglomerates after high energy ball milling, and the specific surface area increases after powder refinement; on the other hand, WC powder continuously forms new during ball milling. The surface, while surface polarization and rearrangement, causes severe distortion of the surface lattice, which tends to amorphize the WC grain surface, thereby imparting high surface activity to the WC grains. Due to the large surface energy and lattice distortion energy of the powder, these energies are fully released during the sintering process, which is manifested in the rapid growth and rapid densification of the grains. Therefore, many new sintering methods have been developed, in order to achieve low-temperature short-time sintering by pressure, electromagnetic and other activation, and further control grain growth.
(1) Vacuum-pressure sintering
In the method, the cemented carbide compact is placed in a vacuum-pressure sintering furnace and first sintered under vacuum. When the sintering temperature is reached, the shrinkage rate of the sample is greatly reduced as the holding time is extended, indicating that the sample is in the sample. The shrinkage in the vacuum sintered state has been substantially completed. Thereafter, a pressure of 3 to 6 MPa is applied by using argon or nitrogen as a gas medium, so that the sample can be significantly shrunk. It can be seen that the gas pressure sintering plays an important role in promoting the final densification of the sample, improving the microstructure of the material and eliminating residual porosity.
(2) Microwave sintering
Microwave sintering is a new rapid sintering technology that utilizes dielectric loss of materials in a microwave electromagnetic field to heat the sintered body as a whole to a sintering temperature to achieve densification. Conventional sintering relies on the heating element to transfer heat through convection, conduction, and radiation. The material is heated from the outside to the inside, the sintering time is relatively long, and the grains are easy to grow. Microwave sintering relies on the absorption of microwave energy by the material itself to convert the kinetic energy and potential energy of the internal molecules of the material. The internal and external materials are heated uniformly at the same time, so that the internal thermal stress of the material can be reduced to a minimum. Secondly, under the action of microwave electromagnetic energy, the kinetic energy of molecules or ions inside the material. The increase is such that the sintering activation energy is lowered, the diffusion coefficient is increased, and the low-temperature rapid sintering can be performed, so that the fine powder can be sintered before it grows. Microwave sintering is undoubtedly one of the effective methods for preparing fine-grained materials. However, the main problem at present is that it is still difficult to prepare high-power microwave ovens suitable for the production of cemented carbide. This sintering process has not been widely used in industrial production. .
(3) Discharge plasma sintering
Discharge Plasma Sintering (SPS) is a new rapid sintering process. It is directly heated by a pulse current between powder particles. It is sintered by the instantaneous high temperature field generated by pulse energy, discharge pulse pressure and Joule heat. Through the instantaneous generation of discharge plasma, each particle in the sintered body produces uniform self-heating and activates the surface of the particle. Due to the high temperature rise and temperature drop rate, the holding time is short, so that the sintering process quickly skips the surface diffusion stage and reduces the particle growth. At the same time, the electricity shortens the preparation cycle and saves energy. Discharge plasma sintering is a new type of hot pressing sintering method. The obtained sintered sample has uniform grain size, high density and good mechanical properties. It is a new modern sintering technology with great application value and broad application prospects.
(4) Other new sintering technologies
In addition to the sintering techniques described above, there are some new sintering technologies that are emerging. Such as field assisted sintering, laser sintering, two-stage sintering. Sinter forging combines forging and sintering to effectively eliminate pores and refine grains by plastic deformation of the powder. Similar methods include hot extrusion and shock wave sintering, which use the large compressive stress generated by the explosion to produce large plastic deformation in the powder compact to achieve high density. These methods can be applied to the sintering of nano-powders, reducing the growth of grain size and improving performance.
Fourth, the addition of grain inhibitors
In order to refine the crystal grains during the sintering process, grain growth inhibitors such as VC, Cr 3 C 2 and TaC may be added to the powder, and the grain growth inhibition effect is remarkable, and the WC crystal can be effectively suppressed. Continuous or discontinuous growth of the granules. The following summarizes some of the characteristics of commonly used inhibitors.
(a) transition element carbide inhibitor
The inhibitory effects of various carbides are related to their thermodynamic stability, with VC being the most effective and reliable inhibitor. When the inhibitor addition amount reaches the saturation concentration in the liquid phase at the sintering temperature, the order of inhibition is as follows: VC>Mo 2 C>Cr 3 C 2 >NbC>TaC>TiC>ZrC∕HfC; For the agent, studies have shown that the inhibitory effect of the TaC∕VC complex inhibitor is better than that of the same content of Cr 3 C 2 ∕VC. The effect of the inhibitor at a given temperature and time depends on the chemical nature of the inhibitor (the ability to diffuse the inhibitor), the amount, and the geometric properties of the original powder (particle size, particle size composition). For the transition group element carbide grain growth inhibitor, the addition amount should reach the maximum saturation concentration of the liquid phase, and the inhibitor is most effective in hindering the dissolution and precipitation process. When the addition amount of the carbide inhibitor is small, the effect of suppressing the growth of the grain is not obvious; when the amount of addition is excessive, although the effect of suppressing grain growth is better, a brittle third phase is formed in the alloy ( W, M) C, reduce the strength of the alloy. According to the research of most scholars, the optimum content of VC is 3% to 5% (relative to the binder phase Co); the optimum content of Cr 3 C 2 is 1% to 3% (relative to the binder phase Co). The optimum content of VC+Cr 3 C 2 composite inhibitor is 3% to 7% (relative to the binder phase Co). The method of adding the inhibitor determines the distribution state of the inhibitor, so it also affects the effect of the inhibitor.
Although the transition metal carbide inhibitor can effectively suppress the growth of WC grains, these substances often cause an increase in pores, thereby deteriorating the properties of the material. At the same time, in order to suppress the grain growth, the grain growth inhibitor such as VC should be dissolved into the binder phase first, and the dissolution occurs in the micron-scale powder sintering process at about 1242 ° C; but for the nano-scale powder For example, grain growth has occurred at 1150 ° C sintering, which is much lower than the dissolution temperature of the inhibitor, so it is difficult for the inhibitor to inhibit grain growth.
(2) Rare earth inhibitors
Rare earth additives have an important influence on cemented carbide. Many studies have shown that rare earth additives can refine grains, and the refining effect becomes more obvious as the amount of addition increases. Xiong et al studied the effect of rare earth (1%～3%) on the WC particle size in cemented carbide (WC-9%Co), and pointed out that the type of rare earth has an important influence on WC grain, and it is best to refine Sm. Second, the influence of morphology and temperature, the amount of addition has the least impact. Studies have shown that the addition of mixed rare earth oxides can not only refine the WC grains, eliminate the discontinuously grown coarse crystal WC and increase the cubic cobalt phase content, but also increase the macroscopic stress of the alloy products, so the toughness of the alloy is remarkable. Improvement. The addition of trace amounts of rare earth/rare earth oxides in cemented carbides can reduce the temperature of liquid phase, reduce porosity and refine grains, which have an important effect on the properties of cemented carbides. Therefore, the addition of rare earths to the preparation of nano-hard alloys provides A new way.
V. Conclusion
With the improvement of precision and processing speed of CNC machine tools, and the development of printed circuit boards in the direction of miniaturization and high integration, the research direction of ultra-fine grained cemented carbides is gradually developing towards better comprehensive performance and finer grain size. Hard alloys with an alloy grain size of less than 0.2 microns are still in the research and testing stage and are the future research and development direction of ultra-fine alloys.
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