Titanium is a new type of metal. The properties of titanium are related to the content of carbon, nitrogen, hydrogen and oxygen. The purity of titanium is not more than 0.1%, but its strength is low and its plasticity is high. 99.5% of industrial pure titanium performance is: the rho = 4.5 g/cm3 density, melting point is 1725 ℃, coefficient of thermal conductivity of lambda = 15.24 W/(m.K), tensile strength of sigma b = 539 mpa, elongation delta = 25%, reduction of area bits = 25%, elastic modulus E = 1.078 x 105 mpa, hardness HB195.
The density of titanium alloy is usually about 4.51 g/m3.
The density of pure titanium is only near the density of ordinary steel. Some high strength titanium alloy exceeds the strength of many alloy structural steel. Therefore, the ratio strength (strength/density) of titanium alloy is much larger than that of other metal structure materials, as shown in table 7-1, it can produce parts with high strength, rigidity and light weight. The aircraft's engine components, skeleton, skin, fasteners and landing gear all use titanium alloys.
High thermal strength
Temperature is higher than aluminum alloy several baidu, under moderate temperature can still maintain the required strength, can work under the temperature of 450 ~ 500 ℃ long these two types of titanium alloy at 150 ℃ ~ 500 ℃ range still has high specific strength, and aluminum alloy at 150 ℃ than strength decreased obviously. Working temperature can reach 500 ℃ of titanium alloy, aluminum alloy under 200 ℃.
Good resistance to corrosion
Titanium alloy works in humid atmosphere and seawater medium, and its corrosion resistance is far superior to that of stainless steel. Resistance to pitting, acid erosion and stress corrosion is especially strong. It has excellent corrosion resistance to alkali, chloride, chlorine, nitric acid and sulfuric acid. But titanium has the resistance to the reducing oxygen and chromite medium.
Low temperature performance
Titanium alloy can maintain its mechanical properties at low temperature and ultra-low temperature. Good low temperature performance, low gap element of titanium alloy, such as TA7, under 253 ℃ can keep certain plastic. Therefore, titanium alloy is also an important cryogenic structure material.
The chemical activity of titanium is large, and it produces a strong chemical reaction with O, N, H, CO, CO2, water vapor and ammonia in the atmosphere. When carbon content is greater than 0.2%, hard TiC is formed in titanium alloy. When the temperature is high, the function of N will also form TiN hard surface. At above 600 ℃, titanium absorbing oxygen to form a high hardness of hardening layer; As the content of hydrogen rises, it also forms a brittle layer. The hard surface depth of the absorbed gas can reach 0.1 ~ 0.15mm and the hardening degree is 20% ~ 30%. The chemical affinity of titanium is also large, and it can produce adhesion phenomenon with friction surface.
The thermal conductivity of titanium =15.24 W/(m. k) is about 1/4 of nickel, 1/5 of the iron, 1/14 of aluminum, and the thermal conductivity of titanium alloys decreases by about 50% compared with that of titanium. Titanium elastic modulus is about half of the steel, so its rigidity is poor, easy to deformation, should not be made thin rod and thin-walled parts, the springback amount of cutting machining surface is very big, about 2 ~ 3 times of stainless steel, the friction of cutter knife after, adhesion, adhesion wear
Titanium alloys have high strength and low density, good mechanical properties, good toughness and corrosion resistance. In addition, the process performance of titanium alloy is poor, cutting processing is difficult, and in hot processing, it is very easy to absorb impurities such as hydrogen and oxygen nitrogen. There is also the resistance to wear, the production process is complex. The industrial production of titanium began in 1948. The need for aviation industry development has enabled the titanium industry to grow at an average annual rate of about 8 per cent. The annual output of the world titanium alloy processing material is more than 40,000 tons, the titanium alloy plate number nearly 30 kinds. The most widely used titanium alloys are ti-6al-4v (TC4), ti-5al-2.5 Sn(TA7) and industrial pure titanium (TA1, TA2 and TA3).
Titanium alloys are mainly used to make aircraft engine compressor parts, followed by rocket, missile and high-speed aircraft structure. 60 s, titanium and its alloy has set up a file in the general industrial application, used for making electrode electrolysis industry, power plant condenser, oil refining and seawater desalination heater and environmental pollution control devices, etc. Titanium and its alloys have become an anticorrosive structure. It is also used in the production of hydrogen storage materials and shape memory alloys.
China began research on titanium and titanium alloys in 1956; In the mid-1960s, the industrial production of titanium was developed and TB2 alloy was developed.
Titanium alloy is used in the aerospace industry is a new important structural materials, proportion, the intensity and the use of temperature between aluminum and steel, but better than aluminum, steel, high strength and excellent corrosion resistance to seawater and ultra-low temperature performance. In 1950, the United States first used the f-84 fighter-bomber as a non-bearing component, such as the rear fuselage heat shield, wind shield and tail cap. In the 1960s, the use of titanium alloy was moved from the rear fuselage to the fuselage, and in part, the structural steel frame, beam, flaps and other important bearing components. The amount of titanium alloy used in military aircraft increases rapidly, reaching 20% ~ 25% of the weight of the aircraft structure. Since the 1970s, the use of titanium alloys has been widely used in civilian machines, such as Boeing 747s with more than 3,640 kilograms of titanium. Aircraft with Mach number greater than 2.5 are mainly used to replace steel to reduce structural weight. For example, the United States sr-71 high-altitude reconnaissance aircraft (flight Mach 3, flight height of 26212 meters), with titanium accounting for 93% of the weight of the aircraft, is known as the "all-titanium" aircraft. When aeroengine in esteem than increased from 4 to 6 to 8 ~ 10, compressor outlet temperature accordingly increased to 500 from 200 ~ 300 ° C to 600 ° C, originally made of aluminium low-pressure compressor disk and blade must use titanium alloy material, or use titanium alloy instead of stainless steel high pressure compressor disk and blade, in order to reduce the structure weight. In the 70 s, the dosage of titanium alloy in the aircraft engine generally accounts for 20% ~ 30% of total weight structure, mainly used in the manufacture of compressor parts, such as forging of titanium fan, compressor disk and blade, titanium compressor casing, intermediate casing, bearing housing, etc. The spacecraft mainly USES the high ratio strength of titanium alloy, corrosion resistance and low temperature resistance to manufacture various pressure vessels, fuel storage tanks, fasteners, instrument bandages, frames and rocket housing. Artificial earth satellites, lunar modules, manned spacecraft and space shuttles also use titanium alloy sheet welding pieces.
Titanium alloy is a alloy of other elements based on titanium. Titanium has two homogenous different crystal: under 882 ℃ to close the six-party alpha titanium structure, above 882 ℃ for body centered cubic beta titanium.
The effects of alloy elements on the phase transition temperature can be divided into three categories:
The elements that stabilize alpha phase and improve phase transition temperature are alpha stable elements, including aluminum, carbon, oxygen and nitrogen. Aluminum is the main alloy element of titanium alloy. It has obvious effect on improving the normal temperature and high temperature strength, reducing the proportion and increasing the elastic modulus.
The elements that stabilize beta phase and decrease phase change temperature are beta stable elements, and can be divided into two types: crystal type and eutectic type. The former has molybdenum, niobium, vanadium and so on; The latter has chromium, manganese, copper, iron, silicon and so on.
The elements that have little influence on phase transition temperature are neutral elements, such as zirconium, tin and so on.
Oxygen, nitrogen, carbon and hydrogen are the main impurities of titanium. The oxygen and nitrogen have a great solubility in the alpha phase, which has a significant effect on the titanium alloy, but the plasticity decreases. The content of oxygen and nitrogen in titanium is usually set at 0.15 ~ 0.2% and 0.04 ~ 0.05% respectively. Hydrogen is very small in the alpha phase, and the dissolved hydrogen in the titanium alloy produces hydrogen compounds that make the alloy brittle. The hydrogen content in titanium alloys is usually controlled below 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.