For powder sintering, the process is generally divided into three phases. The first phase is the initial sintering, called bonding stage. In this phase due to the temperature rise, the atomic amplitude of contact point between the particles is increase and there will have diffusion occurs, making particle contact change from point to the surface. And inner grains of particle are not change. Besides, the particle shape has little change and sintered blank is no shrinkage. What’s more sintered blank density changed little, but the strength and conductivity of it has increased significantly.

The second phase is sintering neck growth and produce sticky plastic flow stage. In this stage, the temperature is higher, so the migration rate of atoms phase junction surface is increase. And migration rate increase makes sintering neck to grow up and particle distance to reduce, forming the associated pore network. Besides, the grains in the particles grow up and grain boundaries moves over the pores. What’s more, the placed where grain boundary swept the pores disappear, so in this phase sintering blank has obvious shrinkage phenomenon and density and strength has significantly enhanced.

The third stage is closed pores spheroidization and shrinking stage. At this stage, most of the pores are completely divided, so closed-pore number has increased considerably and the shapes of pores tend to spherical and shrinking. Besides, at this stage, the entire sintered body continues to shrink. Closed-pore number has increased and shape tends to spherical achieved by pores disappearing and pores quantity reducing. What’s more, this stage can last for a long time.

The core process of sintering is the material migration. According to the migration patterns the powder sintering mechanism can be divide into there types which is diffusion mechanism, evaporation mechanism and condensation flow mechanism.

Analysis at the same sintering process, different rare earth content tungsten powder sintered, to understand the role of rare earth in tungsten powder sintering process and its influence on the tungsten electrode. The sintering curves and temperature change of different content rare earth tungsten billet shown in Figure 2-7. From the Figure 2-7 you can see at the same sintering current, rare earth content increase will lead to sintering temperature rises. And the reason is due to the rare earth and other impurities content increased, the electron scattering effect increased as well, causing the resistivity of the sintered tungsten bar increased. For constant current fusion sintering verification, the performance is the temperature also rises.

To test density of many tested sintered blank found the density is normal distribution, showing in Figure 2-8. From Figure 2-8 we can see two type tungsten powders which have different rare earth content in the same sintering process, the density has significantly different. Rare earth content with 1%, the tungsten powder after sintered has large density and has narrow distribution. The standard deviation is about 0.028. Tungsten powder with 2% rare earth content in the sintering process has lower density and has wide distribution. The standard deviation is about 0.303. Description in this process, tungsten electrode blank has strong sensitivity and production stability is difficult to control.

Nuclear radiation, also known as ionizing radiation, is the microscopic particle flow released from the transition process of atomic nucleus (from an energy state of a structure or a structure into another energy state or another structure) that can cause the ionization or excitation in materials. Nuclear radiation is mainly produced by nuclear reactions, or released from fission products. Nuclear explosions and nuclear accidents will also generate nuclear radiation, including α, β and γ three kinds of rays. Nuclear radiation can cause harm to human body, while the sensitivity of the somatic and germ cells of the body to nuclear radiation and the damage effects of them are different. Once somatic cells are damaged, can lead to diseases in the organ tissues and ultimately death, and the damaged cells will not be transferred to the next generation after the death of the body cells; while once reproductive cells are damaged, the next generation will inherit the parent‘s changed gene, so that leads to defects in the offspring.

Radioactive substances of the nuclear radiation include iodine 131 and cesium 137, which can enter the body through breathing, skin wounds and gastrointestinal absorption, causing internal radiation. Once iodine 131 enters the human body, it can cause thyroid disease; while Cs-137 will damage the body's blood system and the nervous system after it entering the human body, therefore nuclear radiation must be avoided by humans. γ radiation can also penetrate a certain distance to be absorbed by the body, and cause radiation damage. Symptoms of radiation sickness caused by internal and external radiation include hair loss, leukemia, bleeding, vomiting, fatigue, skin redness, dizziness, insomnia, ulcers, diarrhea. Sometimes internal and external radiation will increase the incidence of distortion, cancer, hereditary disease, and affect the health of future generations. In general, the more radiation energy received by the body, the more serious their symptoms of radiation sickness, and the greater the risk of carcinogenic, teratogenic.

Tungsten alloy nuclear radiation shielding is a tungsten alloy shielding with excellent radiation shielding performance, high temperature resistance and strong applicability. Lead is used as a shielding material early, but its shielding effect is not as good as the tungsten alloy shielding. Tungsten alloy shielding is the best choice. Due to the high density of tungsten alloy shielding, its shielding effect is twice that of lead. In addition, lead and its compounds are toxic and dangerous to human health，while tungsten alloy shielding is harmless non-toxic, will not cause harm to human health.

There are some common used nano tungsten carbide powder processes as follow:

1. MA (Metal Alloying): It is a by high energy ball milling the powder is subjected to repeated deformation, cold welding, broken, so as to achieve inter-atomic level alloying element complex physical and chemical process. It has simple operation, high efficiency and the granularity of grains can meet the requirements, but the powder pollution often caused by the friction between tank and balls;

2. SCP (Spraying Conversion Process): It is a new kind of process by American scholars. It uses ammonium paratungstate (CH4) 6 (H2W12040) and an aqueous solution of cobalt chloride CoCl2 • nH2O or Co (en) 3WO4 and H2WO4 tungstate solution by spray drying and fluidized-bed reduction and carbonization reaction homogeneous grain powder (20-50nm);

3. In-situ carburization: It combines in-situ synthesis and carburizing process. In situ synthesis is the use of different elements or compounds react under certain conditions, and in the metal matrix to generate one or more ceramic phase particles have reached the improvement of a single metal alloy. American scholars used polyacrylonitrile as carbon source in situ, without vapor carbide, tungsten and cobalt salts are dissolved in acid polyacrylonitrile solution, low-temperature drying after move 90% Ar-10% argon, hydrogen H2 reduction of direct reduction furnace was cemented carbide powder, the grain size of about 50-80nm;

4. Co-precipitation: Mix different chemical composition and add in solution in a mixture of appropriate preparation precipitating agent precursor precipitate and then the precipitate is dried or calcined, to thereby obtain the corresponding powder particles, which is an important method for preparing a composite oxide containing two or more kinds of ultra-fine powders of metal elements. The researchers use the sodium tungstate or tungsten and cobalt ammonium acetate co-precipitation WC-Co precursor powder through hydrogen reduction and carbonization reaction made cemented carbide powder (about 50nm). However, this method is only applicable to the W / Co atomic ratio close to 5.5. While the use of ammonium tungstate and cobalt hydroxide precipitate will be able to co-atomic ratio of the high-side W / Co, access to a broader range of composite powder.

In addition, there are some other synthesis methods, such as vapor evaporation, high-frequency plasma synthesis, high-frequency induction heating synthesis, ARC. But they all still in the research step, which did not put into practical production. Nano carbide fine and uniform microstructure, mechanical properties have been significantly enhanced. The mean free path of cobalt binder phase is shortened, crack propagation resistance will be improved, the better the toughness of cemented carbide. As tungsten carbide micro drills, nano tungsten carbide micro drill has higher efficiency, lower wear consumption, longer service life.

Generally, tungsten carbide is a kind of super-hard composite material consists of refractory metal carbide (e.g., WC, TiC), and the binder by transition metals or alloys (such as Fe, Co, Ni). It has high hardness, high strength, high melting point, excellent thermal hardness and wear resistance and corrosion resistance and other advantages. In the early 1920s, germen first adapted PM (Powder Metallurgy) to fabricate WC tungsten carbide, which put forward tungsten carbide widely used in aerospace, machinery processing, metallurgy, oil drilling, mining tools, electronic communications, construction, military and other fields.

Originally, tungsten carbide mainly used in drawing die and some wear parts manufacturing. And due to the technologies, process and performance improvements, it gradually plays an important role in metal cutting industries. With the further study, the researchers found that the smaller size of WC in granularity, the shorter mean free path of the binder Co, which has a dramatical increase in the hardness and strength of the alloys. Especially when the WC grain size reduces to about 100 nm, there will be a obvious breakthrough in the mechanical properties.

Today, in some emerging industries as well as some areas of difficult to machine materials fields, such as air super-alloy materials processing, electronics industry, using a wide range of printed circuit board (PCB) drilling, composite sheet metal processing, glass-fiber reinforced structural thermoplastic material, dot matrix printer head, the precision cutting of glass, etc., all need to be completed by nano tungsten carbide cutting tools with higher hardness and better strength. However, in the process of tungsten carbide densification, the growth of grains is an inevitable phenomenon. So in order to obtain nano tungsten carbide, we should synthesize finer powder in advance.

Uneven tungsten billet sizes would cause the nonuniform resistance and large different temperature of each part of wolfram bar during a sintering process. Therefore, due to the different degree of sintering, the degree of combustion of the large size part is too low, but the small part is over-firing, which will cause the fuse. It is best to use the automatic charging machine and tracking piamater retractor and reasonable pressing parameter. So that it can ensure that diameter fluctuations of each part of the round billet can be stabilized within ±0.4mm after sintering.

At the beginning of it, we can see the difference between the square billet and round billet. On the universal testing machine, respectively compare to the compression strength and bending strength of green molding, pre-sintered compact, and isopressed compact. Pressed at the same pressure, the density of isostatic green compact is 1.15 times that of the green molded billet, and compression strength is 6.5 times, its bending strength is 2.7 times that of green die blanks. After pre-sintering, the density of square billet and round billet are approximately the same, compressive strength of round billet is 1.4 times that of square billet, and the flexural strength is 2.1 times. We can finger it out from here that round billet is not easy to be damaged during transport.

When swaging for both materials, it will be found that the surface of the tungsten square bar will often appear subtle cracks, if the swaging temperature rises, then the crack phenomenon would become more serious. Under the same conditions, the round tungsten bar doesn’t have this problem. This is because when the square tungsten is swaging, its edges, faces, and the center pat cannot receive strength evenly, and the degree of deformation also have a greater difference. The tungsten bar is the refined form of tungsten ore, which acts like silver grade tools. But armor and tools made of it will be classed as tungsten equipment. With the processing of tungsten elongated, and the part which did not receive would easy crack with the rest part. Uniform density, round face of cross-section and not forming agent of tungsten round bar eliminate defects of worked structure in a certain extent.