We are often exposed to air transport, such as air cargo, such as flying. When you see an airplane, you may have a question: What is it made of? Why can you carry so many heavy cargo and fly so high? We looked down with doubts.
Introduction of Titanium
Only in 1948 did DuPont produce titanium sponge in tons by magnesium method, which marked the beginning of industrialized production of titanium sponge. Titanium alloys are widely used in various fields because of their high specific strength, good corrosion resistance and high heat resistance.
Titanium is abundant in the crust, ranking ninth, far higher than copper, zinc, tin and other common metals. Titanium is widely found in many rocks, especially in sandstone and clay.
CHARACTERISTICS OF TITANIUM
High strength: 1.3 times as much as aluminium alloy, 1.6 times as much as magnesium alloy, 3.5 times as much as stainless steel, champion of metal materials.
High thermal strength: the use temperature is several Baidu higher than that of aluminium alloy, and it can work for a long time at 450-500 C.
Good corrosion resistance: acid resistance, alkali resistance, atmospheric corrosion resistance, pitting corrosion, stress corrosion resistance is particularly strong.
The low temperature performance is good: the titanium alloy TA7 with very low interstitial elements can maintain a certain plasticity at – 253 C.
High chemical activity: high chemical activity at high temperature, easy to react with hydrogen, oxygen and other gaseous impurities in the air to form a hardened layer.
The thermal conductivity of titanium alloys is about 1/4 of nickel, 1/5 of iron and 1/14 of aluminium, while the thermal conductivity of titanium alloys is about 50% lower than that of titanium. The elastic modulus of titanium alloy is about 1/2 of that of steel.
III. CLASSIFICATION AND USE OF TITANIUM ALLOYS
Titanium alloys can be divided into heat resistant alloys, high strength alloys, corrosion resistant alloys (Ti-Mo, Ti-Pd alloys, etc.), low temperature alloys and special functional alloys (Ti-Fe hydrogen storage materials and Ti-Ni memory alloys).
Although titanium and its alloys have not been used for a long time, they have gained many honorable titles because of their excellent properties. The first title won was “Space Metal”. It has light weight, high strength and high temperature resistance, and is especially suitable for manufacturing aircraft and various spacecrafts. At present, about three-quarters of the world’s titanium and titanium alloys are used in the aerospace industry. Titanium alloys have been used in many parts originally made of aluminium alloys.
IV. Aviation Applications of Titanium Alloys
Titanium alloys are mainly used for manufacturing materials of aircraft and engines, such as forging parts of titanium fans, compressor discs and blades, engine hood, exhaust devices, and structural frame parts of aircraft girders and diaphragms. Spacecraft mainly utilizes the high specific strength, corrosion resistance and low temperature resistance of titanium alloy to manufacture various pressure vessels, fuel tanks, fasteners, instrument straps, frameworks and rocket shells. Titanium alloy plate weldments are also used in artificial earth satellites, lunar module, manned spacecraft and space shuttles.
In 1950, the U.S. first used F-84 fighter bomber as non-load-bearing components such as rear fuselage heat insulation plate, air guide hood and tail hood. Since the 1960s, the use of titanium alloys has shifted from the rear fuselage to the middle fuselage, partially replacing structural steel to manufacture important load-bearing components such as partitions, beams, flaps and slides. Titanium alloys have been widely used in civil aircraft since the 1970s. For example, the amount of Titanium used in Boeing 747 passenger aircraft is more than 3640 kg, accounting for 28% of the aircraft weight. With the development of processing technology, titanium alloys are also used in rockets, satellites and spacecraft. The more advanced the aircraft, the more titanium it uses. Titanium alloy is used in F-14A fighter aircraft, accounting for about 25% of the aircraft weight; F-15A fighter aircraft is 25.8%; Titanium used in the fourth generation fighter aircraft is 41%, and Titanium used in F119 engine is 39%, which is the highest Titanium used aircraft at present.
5. Reasons for the extensive application of titanium alloys in aviation
The highest speed of modern aircraft has reached more than 2.7 times the speed of sound. Such a fast supersonic flight would cause the aircraft to rub against the air and generate a lot of heat. When the flight speed reaches 2.2 times the speed of sound, the aluminium alloy can’t stand it. High temperature resistant titanium alloys must be used. When the thrust-weight ratio of aero-engine increases from 4 to 6 to 8 to 10 and the outlet temperature of compressor increases from 200 to 300 to 500 to 600, the original low-pressure compressor disk and blade made of aluminium must be replaced by titanium alloy.
In recent years, scientists have made new progress in the research of properties of titanium alloys. The original titanium alloy consisting of titanium, aluminium and vanadium has a maximum working temperature of 550 ~600 C, while the newly developed TiAl alloy has a maximum working temperature of 1040 C. Using titanium alloy instead of stainless steel to manufacture high pressure compressor disc and blade can reduce the structural weight. For every 10% reduction in weight, the aircraft can save 4% of its fuel. For a rocket, a range of 15 km can be increased for each weight reduction of 1 kg.
VI. Analysis of Machining Characteristics of Titanium Alloys
Firstly, the thermal conductivity of titanium alloy is only 1/4 of steel, 1/13 of aluminium and 1/25 of copper. Because of the slow heat dissipation in the cutting area, which is not conducive to heat balance, the heat dissipation and cooling effect is very poor in the cutting process, which is easy to form high temperature in the cutting area. After processing, the parts deform and rebound greatly, resulting in increased cutting tool torque, rapid edge wear, and reduced durability.
Secondly, the low thermal conductivity of titanium alloy makes the cutting heat accumulated in a small area near the cutting tool not easy to emit, the friction of the rake face increases, the chip removal is not easy, the cutting heat is not easy to emit, and the tool wear is accelerated.