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Titanium Alloy Steel
Titanium alloys are mainly used to make aircraft engine compressor components, followed by rocket, missile and high-speed aircraft components.
Special points: high strength, good corrosion resistance, high heat resistance
Titanium alloys are alloys based on titanium that are composed of other elements. Titanium has two homogenous heterocrystals: α -Ti with dense hexagonal structure below 882 ℃ and β -Ti with body-centered cubic structure above 882 ℃.
Alloy elements can be divided into three categories according to their effect on phase transition temperature:
1.stabilizing α phase and increasing phase transition temperature are α stable elements, such as aluminum, carbon, oxygen and nitrogen. Aluminum is the main alloy element of titanium alloy. It can improve the strength of alloy at room temperature and high temperature, reduce the specific gravity and increase the modulus of elasticity.
2. the β phase is stabilized and the elements that decrease the phase transition temperature are β stable elements, which can be divided into two types: homocrystalline type and eutectoid type. The former has molybdenum, niobium, vanadium and so on; the latter has chromium, manganese, copper, iron, silicon and so on.
3 the elements with little effect on phase transition temperature are neutral elements, such as zirconium, tin and so on.
The density of titanium alloys is generally about 4.51g/ cubic centimeters,The density of pure titanium is close to that of ordinary steel, and some high strength titanium alloys exceed the strength of many alloy structural steels. Therefore, the specific strength (strength / density) of titanium alloy is much higher than that of other metal structural materials, see Table 7-1, which can produce parts with high unit strength, good rigidity and light weight. Aircraft engine components, skeleton, skin, fasteners, landing gear and so on are made of titanium alloy.
High thermal strength
If the temperature is several degrees higher than that of aluminum alloy, the required strength can still be maintained at medium temperature, and the two kinds of titanium alloys can still have high specific strength in the range of 150 ℃ ~ 500 ℃ at 450 ~ 500 ℃ for a long time. However, the specific strength of aluminum alloy decreased obviously at 150 ℃. The operating temperature of titanium alloy can reach 500 ℃, and that of aluminum alloy is below 200 ℃.
Good corrosion resistance
The corrosion resistance of titanium alloy in moist atmosphere and seawater medium is much better than that of stainless steel, and its resistance to pitting corrosion, acid corrosion and stress corrosion is especially strong. Alkali, chloride, chlorine organic products, nitric acid, sulfuric acid and so on have excellent corrosion resistance. However, the corrosion resistance of titanium to the medium with reducing oxygen and chromium salt is poor.
Good performance at low temperature
Titanium alloy can maintain its mechanical properties at low and very low temperatures. Titanium alloys with good low temperature properties and extremely low interstitial elements, such as TA7, can maintain plasticity at -253 ℃. Therefore, titanium alloy is also an important low temperature structural material.
High chemical activity
Titanium has high chemical activity, and it has strong chemical reaction with the atmosphere, such as O ~ (2 +), H _ (2) O _ (2), water vapor, ammonia and so on. When the carbon content is larger than 0.2, the hard TiC; layer will be formed in the titanium alloy when the temperature is high, and the hard surface layer of TiN will be formed when the carbon content is higher than 0.2 ℃, and when the titanium absorbs oxygen above 600 ℃, the hardened layer with high hardness will be formed. The increase in hydrogen content will also form embrittlement layers. The depth of hard and brittle surface produced by gas absorption can reach 0. 1? 0. 15 mm, hardening degree is 20? 30? The chemical affinity of titanium is also large, and it is easy to adhere to the friction surface.
Titanium alloy has high strength, low density, good mechanical properties, good toughness and corrosion resistance. In addition, titanium alloy has poor processing performance and difficult cutting, so it is easy to absorb impurities such as hydrogen, oxygen, nitrogen and carbon in hot working. There is also poor wear resistance, complex production process. The industrial production of titanium began in 1948. With the development of aviation industry, titanium industry is growing at an average rate of about 8% per year. The annual output of titanium alloy in the world has reached more than 40,000 tons, and nearly 30 kinds of titanium alloy grade. The most widely used titanium alloys are Ti-6Al-4V (TC4), Ti-5Al-2.5Sn (T) A7) and commercial pure titanium (TA1,TA2 and TA3).
Titanium alloys are mainly used in the manufacture of aircraft engine compressor components, followed by rocket, missile and high-speed aircraft structures. In the mid-1960s, titanium and its alloys were used in general industry to make electrodes in the electrolytic industry. Condensers for power stations, heaters for oil refining and desalination, and environmental pollution control devices. Titanium and its alloys have become a kind of corrosion resistant structural material. In addition, it is also used to produce hydrogen storage materials and shape memory alloys.
Mainly used in manufacturing compressor components, such as forging titanium fan, compressor disc and blade, cast titanium compressor casing, intermediate casing, bearing shell, etc. The spacecraft mainly uses titanium alloy’s high specific strength, corrosion resistance and low temperature resistance to manufacture various pressure vessels, fuel tanks, fasteners, instrument straps, frames and rocket shells. Artificial earth satellites, lunar modules, manned spaceships and space shuttles are also welded with titanium alloy plates.
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