What is tungsten alloy
Tungsten alloys are widely used in electronics and electric light source industries, and are also used in aerospace, casting, weapons and other departments to make rocket nozzles, die-casting molds, armor-piercing cores, contacts, heating elements and heat shields.
Classification
Molybdenum Tungsten Alloy
Alloys containing molybdenum and tungsten include molybdenum-based molybdenum-tungsten alloys and tungsten-based tungsten-molybdenum alloys. This alloy can be formed in any proportion and is a complete solid solution alloy at all temperatures.
Niobium Tungsten Alloy
A niobium alloy formed by adding a certain amount of tungsten and other elements based on niobium. Tungsten and niobium form an infinite solid solution. Tungsten is an effective strengthening element of niobium, but with the increase of tungsten addition, the ductility-brittle transition temperature of the alloy will rise, and the grains will grow significantly. Therefore, in order to obtain a high-strength niobium-tungsten alloy, the amount of tungsten added must be properly controlled, and an appropriate amount of elements such as zirconium and hafnium that can refine grains and reduce the ductility-brittle transition temperature must be added. In 1961, the United States successfully developed Nb-10W-2.5Zr alloy for the skin of the space shuttle, and later developed into Nb-10W-1Zr-0.1C alloy. In the early 1970s, China also successfully developed NbWl0Zr2.5 and NbWl0Zr1C0.1 alloys.
Carbide
Cemented carbide is the most common and most important form of tungsten alloy. Different from the previous tungsten alloys, it is tungsten, carbon and cobalt, so it is also often called tungsten-cobalt alloy. The most widely used tools in the industrial field are basically cemented carbide tools, so the tungsten alloy of cemented carbide is also called "industrial teeth".
use
Filament industry
Tungsten was first used to make incandescent filaments. In 1909, W.D. Coolidge made tungsten wire by pressing, remelting, swaging and wire drawing of tungsten powder. Since then, the production of tungsten wire has developed rapidly. In 1913, I. Langmuir and W. Rogers discovered that tungsten-thorium wire (also known as thorium-tungsten wire) has better electron emission performance than pure tungsten wire, and began to use tungsten-thorium wire, which is still widely used today. In 1922, a tungsten wire with excellent sag resistance (called doped tungsten wire or non-sag tungsten wire) was developed, which is a major progress in the research of tungsten wire. Non-sag tungsten filament is an excellent filament and cathode material that is widely used. In the 1950s and 1960s, extensive research was carried out on tungsten-based alloys, hoping to develop tungsten alloys that can work at 1930-2760 ° C for making high-temperature parts used in the aerospace industry. Among them, there are many studies on tungsten-rhenium alloys. The smelting and forming technology of tungsten has also been studied, and tungsten ingots are obtained by consumable arc and electron beam smelting, and some products are made by extrusion and plastic processing; however, the smelted ingots have coarse grains and poor plasticity. , The processing is difficult, and the yield is low, so the melting-plastic processing process has not become the main means of production. With the exception of chemical vapor deposition (CVD) and plasma spraying, which can produce very few products, powder metallurgy is still the main means of manufacturing tungsten products.
sheet industry
China has been able to produce tungsten wire in the 1950s. In the 1960s, the smelting, powder metallurgy and processing technology of tungsten was studied, and now it can produce plates, sheets, foils, bars, pipes, wires and other special-shaped parts.
high temperature material
Tungsten material is used at high temperature, and the solid solution strengthening method alone has little effect on improving the high temperature strength of tungsten. However, on the basis of solid solution strengthening, dispersion (or precipitation) strengthening can greatly improve the high temperature strength, and the strengthening effect of ThO2 and precipitated HfC dispersed particles is the best. Both W-Hf-C and W-ThO2 alloys have high high temperature strength and creep strength at around 1900℃. For tungsten alloys used below the recrystallization temperature, it is an effective way to strengthen by warm work hardening to produce strain strengthening. For example, the thin tungsten wire has a high tensile strength, the total processing deformation rate is 99.999%, and the diameter of the thin tungsten wire is 0.015 mm, and the tensile strength at room temperature can reach 438kg·N/mm2.
Among the refractory metals, tungsten and tungsten alloys have the highest ductility-brittle transition temperature. The plastic-brittle transition temperature of sintered and smelted polycrystalline tungsten is about 150 to 450 °C, which causes difficulties in processing and use, while single crystal tungsten is lower than room temperature. Interstitial impurities, microstructure and alloying elements in tungsten materials, as well as plastic working and surface state, have a great influence on the plastic-brittle transition temperature of tungsten materials. Except for rhenium, which can significantly reduce the ductility-brittle transition temperature of tungsten, other alloying elements have little effect on reducing the ductility-brittle transition temperature.
Tungsten has poor oxidation resistance, and its oxidation characteristics are similar to those of molybdenum. Tungsten trioxide volatilizes above 1000 °C, resulting in "catastrophic" oxidation. Therefore, tungsten materials must be protected by vacuum or inert atmosphere when used at high temperature. If used in high temperature oxidizing atmosphere, protective coating must be added.
Military weapons industry
With the advancement of science, tungsten alloy materials have become the raw materials for the production of military products, such as bullets, armor and shells, shrapnel heads, grenades, shotguns, bullet warheads, bulletproof vehicles, armored tanks, military aviation, artillery parts, guns, etc. Armor-piercing projectiles made of tungsten alloys can penetrate high-angle armor and composite armor, and are the main anti-tank weapons.
processing
Tungsten has a high melting point, is hard and brittle, and is difficult to process, but as long as there is a reasonable process, tungsten can be processed into materials by powder metallurgy, extrusion, forging, rolling, spinning and drawing.
Prepare
Qualified billet is one of the keys to the production of tungsten materials. To make a billet, qualified tungsten powder must be selected first. Powder characteristics (average particle size, particle size distribution, chemical composition), compounding, forming and sintering processes have a direct impact on the composition, density and microstructure of the billet and strongly influence product processing and service properties.
The silicon, aluminum and potassium elements added to the non-sagging tungsten wire are added in the form of oxides in tungsten trioxide or "blue tungsten" (a mixture of various low-valent tungsten oxides). The mixture is usually composed of hydrofluoric acid. The solution is washed to remove impurities from the powder. The blanks for the production of filaments and small sheets are mostly formed on a press, and isostatic pressing can also be used.
The size of the powder blank is generally 12×12×400mm, and there are also larger-sized round rods, square rods or rectangular rods. The powder blank is first pre-sintered at 1200 °C for 1 hour in a hydrogen atmosphere to make it have a certain strength and conductivity, and then energized and self-resistive sintering.
Electric self-resistance sintering, commonly known as "vertical melting", is a method developed in tungsten processing. The principle is to pass the current directly through the sintered blank, and Joule heat is generated due to the resistance of the blank itself. This heat is used to sinter the blank. The sintering current is usually 90% of the fusing current. The resulting blank is a self-resisting sintered bar (also known as a vertical melting bar). The general standard of the vertical melting bar that can be processed into wire is to control the number of grains in the cross section to be about 10,000 to 20,000 per square millimeter, and the density to be 17.8 to 18.6g/cm3. For pipes, sheets or other large-sized products, isostatic pressing (pressure above 2500kg·N/mm2) is often used to form, and sintered in vacuum or hydrogen protection at a high temperature of 2300-2700 °C.
swaging
Rotary forging is a common plastic processing method for the production of tungsten wire blanks and thin bars. Bars of different sizes are heated to 1400-1600 °C in a hydrogen atmosphere, and are subjected to rotary forging on different types of rotary forging machines. The deformation amount of the first pass should not be too large, and then the deformation amount can be appropriately increased. During the swaging deformation process, graphite is used to lubricate between the workpiece and the mold. The density of the processed tungsten rod can reach 18.8～19.2g/cm3. Since the square billet is forged into a round billet, the deformation of each part is different, which makes the structure uneven. At this time, recrystallization annealing should be carried out. The final diameter of the swaged bar is around 3 mm.
brushed
Drawing blanks can be produced by rotary forging or rolling. The billet produced by the rolling method has a large amount of deformation and a uniform structure, which is conducive to subsequent processing. The tungsten wire blank is drawn by the "warm wire drawing" method. First, it was drawn to a diameter of 1.3 mm on a chain stretching machine, and then the diameters reached 0.2, 0.06 and less than 0.06 mm by rough drawing, medium drawing and fine drawing, respectively. As the diameter decreases, the heating temperature should be lowered and the wire drawing speed should be increased. The amount of deformation of the pass is generally between 10 and 20%.
The wire drawing is heated by gas-air mixing, and the temperature is 900-400℃. Hard alloy die is used for thick wire drawing, and diamond die is used for thin wire drawing. The mold material, hole type, and grinding technology have a great influence on the quality of the silk. The quality, particle size, proportion and coating method of the graphite lubricant also affect the quality of the silk.
The non-uniformity of the wire diameter is one of the main reasons for wire breakage during use. A deviation of 0.2 to 0.4 microns will greatly reduce the life of the tungsten wire in the vacuum tube. The diameter of the filament can be measured gravimetrically or by a vacuum amperometric method. During the wire drawing process, as the diameter decreases, the deformation resistance increases (for example, the breaking strength of a tungsten wire with a diameter of 0.1-0.3 mm can be as high as 350kg·N/mm2), and its plasticity also decreases accordingly. In order to improve reworkability, stress relief intermediate annealing is generally required. In addition, electrolytic etching can be used to process the wire into filaments with a diameter of less than 0.01 mm.
