Progress in nano-hard alloys
Electric hand drill is a drilling tool powered by AC power or DC battery. It is a kind of hand-held electric tools.
Electric Hand Drill,Customization Power Tools,Electric Tools Set,Power Tools Set Behappy Crafts (suzhou)Co.,Ltd , https://www.jshaoyue.com
Synthesis of 1 nanometer powder
Since the growth of crystal grains is inevitably present in the densification process, in order to manufacture a nanocrystalline cemented carbide, it is necessary to first synthesize a finer nanopowder. The main methods used now include the following:
1.1 Mechanical alloying method
Mechanical alloying is a method of synthesizing materials at low temperatures using high-energy mechanical driving forces. Commonly used high energy ball milling as a mechanical driving force. At present, mechanical alloying and synthesis of nano-hard alloy powder mainly includes two aspects: one is to use mechanical alloying method to synthesize nano-WC powder by W and C, and the other is to mix WC and Co powder with high energy. The ball mill makes it pulverized and refined to nanocomposite.
Ma Xueming et al. synthesized a powder of WC-Co of 11.3 nm by mixing ball milling of W, C and Co for 100 h. El-Eskandarany et al. and China's Tan Guolong have successively prepared nano-sized WC by chemical mechanical alloying method. The method uses the mixing of WO3 and Mg and C in a ball mill tank under N2 or H2-Ar protective atmosphere. The explosion reduction reaction generates W and MgO, and then W undergoes a diffusion reaction with C to form W2C and WC. Its grain size is about 4 to 20 nm.
Another method is to directly refine the WC-Co by high-energy ball milling. Mao Changhui of Beijing Research Institute of Nonferrous Metals used this method to mill WC-10%Co for 40h to obtain an average of 10nm WC grains, and WC particles were separated and covered by Co. Goren-Muginstein et al. ball milled at a speed of 55 r/min for 300 h in the same manner to obtain an average 7 nm WC grain size.
The method of mechanical alloying is simple and efficient in synthesizing nano-powder, and the size of the produced powder is small, but it is often caused by powder contamination caused by friction with the can body and the sphere.
1.2 Injection conversion method
This method can also be called thermochemical method, or fluid bed method. McCandish et al. of Kurger University in New Jersey, USA, developed a spray conversion method to synthesize nano WC-Co composite powder. The method utilizes ammonium metatungstate (CH4)6(H2W12O40)·4H2O and cobalt chloride CoCl2·nH2O aqueous solution or Co(en)3WO4 and H2WO4 aqueous solution, spray drying and fluidized bed reduction and carbonization reaction to form uniform 20-50 nm. Grain powder. Nanodyne Corporation of the United States has used this method to produce and sell nano-sized WC-Co composite powder.
1.3 In-situ carburizing reduction method
Zhu et al. in the United States reported the use of polyacrylonitrile as an in-situ carbon source, which does not require gas phase carbonization. The tungstic acid and cobalt salts are dissolved in a polyacrylonitrile solution, dried at low temperature, and moved to an oven at 800-900 ° C for use. The mixed gas of 90% Ar-10% H2 is directly reduced to WC-Co powder, and the obtained powder has a grain size of about 50 to 80 nm.
1.4 Coprecipitation method
The patent of Muhammed et al. uses a coprecipitation method of sodium tungstate or ammonium tungstate (CH4)6 (H2W12O40) and cobalt acetate to obtain a [H2Co2W11O40]8-solid salt as a WC-Co powder precursor. Then, a WC-Co powder of about 50 nm is formed by a H2 reduction reaction and a carbonization reaction. However, this method is only applicable to powders having a W/Co atomic ratio close to 5.5. If (NH4)10[H2W12O42] is coprecipitated with cobalt hydroxide, the atomic ratio of W/Co can be changed to obtain a wider range of composite powder.
1.5 Other synthetic methods
Other synthetic methods include gas phase synthesis, and Japanese scholars have earlier research in this area. The method uses WCl6 and methane to react at 1300-1400 ° C, and after cooling, can obtain WC powder of about 20-30 nm: there is also a high-frequency plasma synthesis method, which uses Ar as a carrier to obtain WC1-x in a high temperature region. Powder, particle size 5~20nm: high frequency induction heating synthesis method, arc discharge gasification, filling methane to obtain nanometer size WC: ion arc method, using W as cathode, graphite rod as anode, direct current 300A And 60V, arc discharge produces WC1-x. The average grain size of the powder was 12 nm. However, these synthetic methods are generally less efficient.
2 nano-carbide sintering
Due to the large proportion of the surface and interface of the nanoparticles, the sintering behavior of the nanoparticles is different from that of the ordinary grain WC-Co cemented carbide due to the small size effect, surface and interface effects.
2.1 Densification temperature
The sintering of a conventional cemented carbide is usually at a eutectic temperature of 120,000 ° C or higher at WC-Co, which is called liquid phase sintering. However, the densification starting temperature is lower than the eutectic temperature, usually around 1280 ° C, so it is also called the solid phase sintering stage. For ultrafine grained and nanostructured composite WC-Co powders, the densification temperature is greatly reduced. Gille et al found that 0.4μm WC, the addition of grain growth inhibitors Cr3C2, VC, MaC can significantly reduce the eutectic temperature of cemented carbide, and its densification start temperature is between 770 ~ 850 ° C, Arato et al found crystal grains The densification onset temperature of WC-15Co having a size of 30 nm was 600 ° C, and the maximum densification was completed at 1200 ° C. The alloy of the same composition of 1.8 μm was densified at 1100 ° C. The ball-milled WC-Co composite powder having an average grain size of 5 nm began to shrink at 100 °C. Significant grain shape changes and grain growth begin to occur at 800 ° C, and maximum densification is achieved below 1300 ° C. The shrinkage onset temperature of the 30 nm WC-Co powder synthesized by the jet conversion method was 580 ° C, and the other processes were similar. Porat et al. believe that the nanopowder synthesized by the ball milling method contains a higher defect density and smaller nanoparticles, and the solid phase sintering temperature is also significantly lower than that of the nanopowder synthesized by the jet conversion method.
2.2 Grain growth
The smaller the radius of curvature of the particles, the greater the driving force for sintering. The sintering driving force of nano-WC-Co powder is tens or even hundreds of times that of ordinary cemented carbide. Therefore, the grain growth of the WC-Co powder tends to be large. The first is the effect of sintering time. Pang et al. studied the densification of nano-WC-Co powders and found that the grains have grown in the first 5 min of sintering. The second is the effect of the sintering temperature. The higher the sintering temperature, the more serious the grain growth. In addition, the original size of the powder also strongly affects the sintered grain size. The result of the sintering test of Wang Shequan, in the range of particle size less than 0.2 μm, the smaller the original powder, the larger the grain size at the selected temperature and time.
Therefore, suppressing the growth of crystal grains during sintering is the most critical process for obtaining nanocrystalline cemented carbides, and it is also a hot issue. On the one hand, by adding grain growth inhibition, such as VC, Cr3C2, TaC, NbC and other carbides, by affecting the interfacial energy of WC/Co and reducing the solubility of WC in the liquid phase of Co, thereby suppressing the growth of WC grains. . On the other hand, the growth of the crystal grains is controlled by controlling the sintering process or studying a new sintering method. The hot isostatic pressing (HIP) process can be used to quickly densify and reduce the grain growth. In addition, microwave sintering, pulse discharge sintering, and discharge plasma sintering are very promising methods for sintering nano-hard alloys, which can be highly efficient. Rapid heating to achieve densification and reduce grain growth.
Microstructure and mechanical properties of 3 nanometer cemented carbide
3.1 Microstructure
The microstructure of nano-hard alloys is very small, which determines its excellent mechanical properties. However, due to the different preparation methods and sintering processes of nano-powders, their microstructures are also different.
Jia et al. sintered the nano-WC-Co powder prepared by SPC method at 1350 °C to obtain nano-hard alloy WC grain size of about 70 nm. The grain boundary is the same as that of ordinary cemented carbide, and it is also a straight boundary. However, its dislocation density is significantly less than that of ordinary cemented carbide. In addition, it is found that the content of WC in the binder phase is 20% (mass fraction), and that of ordinary cemented carbide is 3% (mass fraction), and in the binder phase. The ratio of the fcC phase to the hcp phase is increased. Ungar et al. sintered ball-milled 10 nm WC-Co powder at 1420 ° C to obtain WC grains of about 100 nm, and the grain boundaries were also flat, but the dislocation density was calculated to be as high as 2.0×10 15 . The microstructure of WC-Co powder synthesized by chemical synthesis and mechanical ball milling method is very different, especially mechanical ball milling causes large deformation of crystal grains and accumulation of a large number of dislocations. Although the dislocations are mostly eliminated during sintering, they still have a high density. Fang et al. added VC with Nanocarb 20-50 nm WC-Co powder and sintered to obtain WC grains of less than 200 nm. When Goren-Muginstein sintered the nano-WC prepared by unbindered phase and mechanical ball milling at 1800 °C, it was found that the WC grains were long finger-like grains. In another sintering experiment, he obtained equiaxed nanocrystals with preferred growth surfaces of (1010) and (1103) (0111).
3.2 Mechanical properties
As the free phase of the binder phase decreases, the Vickers hardness of the cemented carbide increases remarkably. When the cobalt binder phase has a mean free path of 30 nm, its Vickers hardness is as high as 2300 kg/mm ​​2 or more. Moreover, the crack propagation resistance also increases, and the toughness of the alloy is correspondingly increased. Comparison of hardness and flexural strength of WC-10%Co (mass fraction) nano-carbide and ordinary cemented carbide.
3.3 Tool cutting performance
Tool products made of nano-hard alloys have excellent performance. For example, RTW's printed circuit board nano-hard alloy drills have a much smaller amount of wear when drilling the same number of micropores than ordinary carbide drills. Wei Rixi and others found that when cutting chilled cast iron, CrWMn and other materials with carbide molds of G2, D30, EM10, and F of Japan Toshiba Corporation with a grain size of about 200 nm, it is found in comparison with ordinary cemented carbide. In comparison, the amount of wear on the flank is greatly reduced, and the durability is significantly improved, and the durability of F is increased by up to 18 times.
4 Problems and prospects
At present, the main problem of nano-hard alloys is the grain growth during sintering. Although there are reports on nano-hard alloy products, the grain size is rarely around 100 nm or even smaller. Therefore, the use of advanced sintering methods, such as microwave sintering, accurate control of sintering temperature, time, pressure and other process parameters to obtain higher performance of nano-hard alloy is a key factor to solve the problem.
Hand electric drill is the largest sales of power tools industry products, widely used in construction, decoration, furniture and other industries, used in the object on the opening or piercing objects, some industries are also known as electric hammer.
The main components of the hand electric drill: drill chuck, output shaft, gear, rotor, stator, housing, switch and cable.
Electric hand drill (pistol drill) - used for drilling metal materials, wood, plastic, etc. When equipped with a positive and negative switch and electronic speed regulating device, it can be used as an electric screwdriver. Some models are equipped with rechargeable batteries, which can work normally without external power supply for a certain period of time.