Titanium alloys overiew
Titanium is a new type of metal. The performance of titanium is related to the content of impurities such as carbon, nitrogen, hydrogen, and oxygen. The purest titanium iodide impurity content does not exceed 0.1%, but its strength is low and its plasticity is high. The properties of 99.5% industrial pure titanium are: density ρ = 4.5g / cm3, melting point 1725 ° C, thermal conductivity λ = 15.24W / (mK), tensile strength σb = 539MPa, elongation δ = 25%, section Shrinkage ψ = 25%, elastic modulus E = 1.078 × 105MPa, hardness HB195.
The density of titanium alloy is generally about 4.51g / cm3, which is only 60% of steel. The density of pure titanium is close to that of ordinary steel. Some high-strength titanium alloys exceed the strength of many alloy structural steels. Therefore, the specific strength (strength / density) of the titanium alloy is much greater than that of other metal structural materials. See Table 7-1, which can produce parts with high unit strength, good rigidity, and light weight. The aircraft engine components, skeleton, skin, fasteners and landing gear are all made of titanium alloy.
High heat intensity
The operating temperature is several hundred degrees higher than that of aluminum alloys, and the required strength can be maintained at moderate temperatures. The two types of titanium alloys can work for a long time at a temperature of 450 to 500 ° C. The two types of titanium alloys are still very high in the range of 150 to 500 ° C. Specific strength, while the specific strength of aluminum alloy significantly decreased at 150 ° C. The working temperature of titanium alloy can reach 500 ℃, while that of aluminum alloy is below 200 ℃.
Good corrosion resistance
Titanium alloy works in humid atmosphere and seawater medium, and its corrosion resistance is far better than that of stainless steel. It is particularly resistant to pitting, acid, and stress corrosion; organic objects such as alkali, chloride, and chlorine, nitric acid, and sulfuric acid Etc. Have excellent corrosion resistance. However, titanium has poor resistance to reducing oxygen and chromium salt media.
Good low temperature performance
Titanium alloys can maintain their mechanical properties at low and ultra-low temperatures. Low temperature properties, titanium alloys with very low interstitial elements, such as TA7, can still maintain a certain degree of plasticity at -253 ° C. Therefore, titanium alloy is also an important low temperature structural material.
High chemical activity
Titanium has a large chemical activity and produces a strong chemical reaction with atmospheric O, N, H, CO, CO2, water vapor, ammonia, etc. When the carbon content is greater than 0.2%, hard TiC will be formed in the titanium alloy; at higher temperatures, it will also form a hard TiN surface layer when interacting with N; at 600 ° C or higher, titanium absorbs oxygen to form a hardened layer with high hardness ; Increased hydrogen content will also form a brittle layer. The hard and brittle surface layer produced by absorbing gas can reach a depth of 0.1 to 0.15 mm and a hardening degree of 20% to 30%. Titanium also has a large chemical affinity and is prone to adhere to friction surfaces.
Low thermal conductivity
The thermal conductivity of titanium is 15.24W / (m.K) is about 1/4 of nickel, 1/5 of iron, 1/14 of aluminum, and the thermal conductivity of various titanium alloys is about 50% lower than that of titanium. The elastic modulus of titanium alloy is about 1/2 that of steel, so its rigidity is poor and it is easy to deform. It is not suitable to make slender rods and thin-walled parts. The springback of the machined surface during cutting is large, about 2 to 3 Times, causing severe friction, adhesion, and abrasion on the flank of the tool.
Titanium alloy has high strength and small density, good mechanical properties, good toughness and corrosion resistance. In addition, the titanium alloy has poor process performance and is difficult to cut. In hot processing, it is very easy to absorb impurities such as hydrogen, nitrogen, nitrogen and carbon. There is also poor abrasion resistance and complicated production processes. The industrial production of titanium began in 1948. The development of the aviation industry has made the titanium industry grow at an average annual growth rate of about 8%. The world's annual output of titanium alloy processed materials has reached more than 40,000 tons, with nearly 30 types of titanium alloy grades. The most widely used titanium alloys are Ti-6Al-4V (TC4), Ti-5Al-2.5Sn (TA7) and industrial pure titanium (TA1, TA2 and TA3).
Titanium alloys are mainly used to make aircraft engine compressor parts, followed by rockets, missiles and high-speed aircraft structural parts. In the mid-1960s, titanium and its alloys have been used in general industry for making electrodes for 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 corrosion resistant structural material. 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; industrial production of titanium materials began in the mid-1960s and development into TB2 alloys.
Titanium alloy is a new important structural material used in the aerospace industry. Its specific gravity, strength and operating temperature are between aluminum and steel, but it is stronger than aluminum and steel and has excellent resistance to seawater corrosion and ultra-low temperature performance. In 1950, the United States first used the F-84 fighter bomber as a non-load-bearing component such as the rear fuselage heat shield, air baffle, and tail cover. In the 1960s, the use of titanium alloys moved from the rear fuselage to the middle fuselage, and partly replaced structural steel to manufacture bulkheads, beams, flaps, and other important load-bearing components. The use of titanium alloys in military aircraft has increased rapidly, reaching 20% to 25% of the aircraft structure weight. Since the 1970s, civilian aircraft have begun to use a large amount of titanium alloys. For example, the Boeing 747 passenger aircraft uses more than 3640 kg of titanium. Titanium for aircraft with Mach numbers greater than 2.5 is mainly used to replace steel to reduce structural weight. For another example, the US SR-71 high-altitude high-speed reconnaissance aircraft (with a flying Mach number of 3 and a flying altitude of 26212 meters), titanium accounts for 93% of the aircraft structure weight, and is known as an "all-titanium" aircraft. When the thrust ratio of aero engines is increased from 4 to 6 to 8 to 10 and the compressor outlet temperature is correspondingly increased from 200 to 300 ° C to 500 to 600 ° C, the original low-pressure compressor discs and blades made of aluminum must be used. Switch to titanium alloys, or use titanium alloys instead of stainless steel to make high-pressure compressor disks and blades to reduce structural weight. In the 1970s, the amount of titanium alloy used in aero engines generally accounted for 20% to 30% of the total weight of the structure. It was mainly used to manufacture compressor components, such as forged titanium fans, compressor disks and blades, cast titanium compressor casings, and intermediaries Case, bearing housing, etc. The spacecraft mainly uses the high specific strength, corrosion resistance and low temperature resistance of titanium alloys to manufacture various pressure vessels, fuel storage tanks, fasteners, instrument bands, frames and rocket shells. Artificial earth satellites, lunar modules, manned spacecraft and space shuttles also use titanium alloy plate weldments.