Main uses of tungsten
Tungsten and its alloys are widely used in electronic and electric light source industries. It is used to manufacture all kinds of light bulbs. The filament of electron tube is made of doped tungsten filament with anti droop property.
Rhenium is added to the doped tungsten wire. The thermocouple made of tungsten rhenium alloy wire with low rhenium content and tungsten rhenium alloy wire with high rhenium content has a wide temperature measurement range (0-2500 ℃), good linear relationship between temperature and thermoelectric EMF, fast reaction speed (3 seconds) and relatively low price. It is an ideal thermocouple for measuring in hydrogen atmosphere.
Tungsten wire not only triggered a revolution in lighting industry, but also became a kind of thermionic emitter of electron, such as scanning electron microscope and transmission electron microscope, because of its high melting point and without losing its mechanical integrity. It is also used as filament of X-ray tube. In an X-ray tube, electrons generated by tungsten wires are accelerated to collide with tungsten and tungsten rhenium alloy anodes, and then X-rays are emitted from the anode. In order to produce X-ray, the energy of the electron beam generated by tungsten wire is very high, so the spots on the surface impacted by the electron beam are very hot. Therefore, the rotating anode is used in most X-ray tubes.
In addition, large size tungsten wire is also used as heating element of vacuum furnace.
The density of tungsten is 19.25g/cm3, about 2.5 times of that of iron (7.87g/cm3), which is one of the heaviest metal elements in the periodic system. Based on this characteristic of tungsten, high density tungsten alloy (i.e. high specific gravity tungsten alloy) has become an important application field of tungsten. High density tungsten alloy can be made by liquid phase sintering process, adding nickel, iron, copper and a small amount of other elements into tungsten powder at the same time. According to the different composition, high density tungsten alloys can be divided into two alloy systems: W-Ni-Fe and W-Ni-Cu. The density can reach 17-18.6 g / cm3 by liquid phase sintering. The so-called liquid phase sintering refers to the sintering process in which a certain amount of liquid phase exists at the sintering temperature. The advantage of this method is that the liquid phase wettes the solid particles and dissolves a small amount of solid materials, which greatly speeds up the process of densification and grain growth, and reaches a very high relative density. For example, for ferronickel powder, which is usually used in liquid phase sintering, the nickel iron powder melts as sintering proceeds. Although the solubility of liquid ferronickel is very small in solid tungsten (95% volume fraction), solid tungsten is easy to dissolve in liquid ferronickel. Once the liquid ferronickel wetted tungsten particles and dissolved part of tungsten powder, the shape of tungsten particles changed, and the internal pores disappeared immediately when the liquid flow entered. If the process continues, the tungsten particles will be coarsened and grown, and the final product with nearly 100% density and the best microstructure will be produced.
High density tungsten alloy made by liquid phase sintering has better impact performance than pure tungsten in addition to high density. Its main purpose is to manufacture high penetrating military armor piercing projectile.
Tungsten carbide can maintain good hardness at high temperature above 1000 ℃, so it is an ideal tool for cutting and grinding. In 1923, schrotter of Germany invented WC Co cemented carbide by using this characteristic of WC. Because WC Co cemented carbide as cutting tool and drawing and stamping die has brought huge business opportunities, the industrial production was quickly realized in 1926-1927. In short, the mixture of tungsten powder (or W03) and carbon black is carbonized in hydrogen or vacuum at a certain temperature to produce tungsten carbide (WC). Then, WC and metal binder cobalt are proportioned according to a certain proportion. After milling, molding and sintering, cemented carbide products such as cutting tools, moulds, rolls, impact rock drilling bits are made.
At present, the tungsten carbide based cemented carbide can be generally divided into four categories: tungsten carbide cobalt, tungsten carbide titanium carbide cobalt, tungsten carbide titanium carbide tantalum carbide (niobium) - cobalt and steel bonded cemented carbide. In the current global annual consumption of about 50000 tons of tungsten, the tungsten carbide based hard alloy accounts for about 63%. According to the latest news, the global production of cemented carbide is about 33000 tons / year, which consumes 50% - 55% of the total supply of tungsten.
Tungsten is the main alloy element of high speed tool steel, alloy structural steel, spring steel, heat-resistant steel and stainless steel.
Tungsten can be alloyed by solution strengthening, precipitation strengthening and dispersion strengthening, so as to improve the high temperature strength and plasticity of tungsten materials. Through alloying, tungsten has formed a variety of non-ferrous metal alloys which have a great impact on contemporary human civilization.
The addition of rhenium (3% - 26%) to tungsten can significantly increase the ductility (plasticity) and recrystallization temperature. The elongation of some W-Re alloys can reach 5% after proper high-temperature annealing treatment, which is much higher than that of pure tungsten or doped tungsten (1% - 3%).
The tungsten thorium alloy formed by adding 0.4% to 4.2% thorium oxide (tho 2) has high thermal electron emission capability and can be used as hot cathode of electron tube and argon arc welding electrode, but the radioactivity of ThO2 has not been solved for a long time.
The cerium tungsten (w-ceo2) alloys developed in China and lanthanum tungsten and yttrium tungsten alloys (oxide content is generally less than 2.2%) prepared by using La2O3 and Y203 as dispersants instead of w-th02 alloy, have been widely used in argon arc welding, plasma welding and cutting, non consumable arc furnace and other high-temperature electrodes.
W-Cu and W-Ag alloy is a kind of powder metallurgy composite material which has no reaction between the constituent elements, so no new phase is formed. Tungsten silver and tungsten copper alloys are not alloys in fact, so they are regarded as pseudoalloys. Tungsten silver alloy is often referred to as silver infiltrated tungsten. This kind of alloy contains 20% - 70% copper or silver. It has excellent electrical and thermal conductivity properties of copper and silver, high melting point and ablation resistance of tungsten. It is mainly used as rocket nozzle, electrical contact and semiconductor support. The nozzle of a foreign Polaris A-3 missile is made of tungsten tube with 10% - 15% silver. The nozzle of Apollo spacecraft, which weighs hundreds of kilograms, is also made of tungsten. Tungsten molybdenum alloy has higher resistivity and better toughness than pure tungsten. It has been used as hot wire of electron tube and glass sealing lead. As an alloying element, superalloy should be mentioned in nonferrous metal alloys. In the 1940s, superalloy was born in the roar of gunfire in order to meet the needs of aviation turbine engine for high temperature materials. The superalloy is composed of three kinds of special structural alloys: Ni base, Co base and Fe base. They can maintain high strength, creep resistance, oxidation resistance and corrosion resistance when working at high temperature (500 ~ 1050 ℃). In addition, they can be guaranteed not to fracture during the service life of several years, that is, they have the characteristics of high cycle fatigue resistance and low cycle fatigue resistance. This kind of performance is very important to the aerospace industry, which is vital to human life.
The lowest content of tungsten in these alloys is 0.6% and the highest is 15%. The proportion is not high. However, from the absolute demand of high-temperature engineering such as aviation industry and thermal power plant, the amount of tungsten will be considerable. It is estimated that more than 2 / 3 of superalloys are used in aerospace industry, 1 / 7 in nuclear power plant and gas turbine power plant, and 1 / 7 in marine operation and transportation industry.
The main problem of tungsten is the shortage of resources. For this reason, in some areas where tungsten resources are short, such as Europe, researches have been carried out to replace tungsten with molybdenum. In addition, from the point of view of saving materials, high efficiency coated cemented carbides have emerged abroad. It is estimated that the hardness, chemical stability, wear resistance, friction coefficient (low requirement), thermal conductivity and the effectiveness of preventing the diffusion of cobalt and carbon from the substrate (substrate) have been largely solved.
In the electronic industry, especially in the manufacture of integrated circuits, the technology of forming thin films on substrates by chemical vapor deposition (CVD) is a completely different process from that of powder metallurgy to produce bulk tungsten (bulk) products. Tungsten hexafluoride is the most common tungsten source for CVD deposition. Wf6 is liquid at room temperature, but WE6 is combined with hydrogen due to its very high vapor pressure when it passes through the parts to be coated. WE6 is selectively coated on the surface of the workpiece through the reaction of wf6 + 3h2 → W + 6HF at about 300 ℃. For example, the tungsten channel (vias) deposited on the integrated circuit can be connected to another horizontal wire of the circuit board as a small metal plug. The diameter of the small plug is 0.4 mm, and the ratio of length to diameter is 2.5. In the future, the diameter can be reduced to 0.1 mm, so that the length diameter ratio can reach 5. Because tungsten has excellent conductivity and does not react with surrounding materials, CVD is the only way to fill the channel and purify the surface without tungsten.
