Why Do We Have To Use Titanium Alloy As The Material Of Air Transport Aircraft?
1. Introduction of Titanium
In 1948, the American DuPont Company used the magnesium method to produce tons of titanium sponge-this marked the beginning of the industrial 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 earth's crust, ranking ninth in content, much higher than common metals such as copper, zinc, and tin. Titanium is widely present in many rocks, especially sand and clay.
2.The characteristics of titanium
High strength: 1.3 times of aluminum alloy, 1.6 times of magnesium alloy, 3.5 times of stainless steel, champion among metal materials.
High thermal strength: The operating temperature is several hundred degrees higher than that of aluminum alloy, and it can work for a long time at a temperature of 450 to 500 ℃.
Good corrosion resistance: acid resistance, alkali resistance, atmospheric corrosion resistance, particularly strong resistance to pitting corrosion and stress corrosion.
Good low temperature performance: Titanium alloy TA7 with extremely low interstitial elements can maintain a certain degree of plasticity at -253°C.
High chemical activity: The chemical activity is high at high temperature, and it easily reacts with gas impurities such as hydrogen and oxygen in the air to form a hardened layer.
The thermal conductivity is small and the modulus of elasticity is small: the thermal conductivity is about 1/4 of nickel, 1/5 of iron, and 1/14 of aluminum. 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 of that of steel.
3.The classification and use of titanium alloys
Titanium alloys can be divided into heat-resistant alloys, high-strength alloys, corrosion-resistant alloys (titanium-molybdenum, titanium-palladium alloys, etc.), low-temperature alloys, and special functional alloys (titanium-iron hydrogen storage materials and titanium-nickel memory alloys) Wait. Although the history of titanium and its alloys is not long, but because of its superior performance, it has won many honorable titles. The first title won is "Space Metal". It is light in weight, strong and resistant to high temperatures, and is particularly suitable for manufacturing aircraft and various spacecraft. Currently, about three-quarters of the titanium and titanium alloys produced in the world are used in the aerospace industry. Many parts that originally used aluminum alloys have changed to titanium alloys.
4.Aerospace applications of titanium alloys
Titanium alloys are mainly used in aircraft and engine manufacturing materials, such as forged titanium fans, compressor disks and blades, engine hoods, exhaust devices and other parts, as well as structural frame parts such as aircraft girder frames. The spacecraft mainly uses the high specific strength, corrosion resistance and low temperature resistance of titanium alloys to manufacture various pressure vessels, fuel tanks, fasteners, instrument straps, frames and rocket shells. Artificial earth satellites, lunar modules, manned spacecraft and space shuttles also use titanium alloy sheet welded parts. Why do we have to use titanium alloy for the material of air transport aircraft? In 1950, the United States used it for the first time on F-84 fighter-bombers as non-load-bearing components such as rear fuselage heat shields, wind deflectors, and tail covers. Since the 1960s, the use of titanium alloy has moved from the rear fuselage to the middle fuselage, partially replacing structural steel to make important load-bearing components such as bulkheads, beams, and flap slides. Since the 1970s, civilian aircraft began to use titanium alloys in large quantities. For example, the Boeing 747 passenger aircraft uses more than 3,640 kilograms of titanium, which accounts for 28% of the aircraft's weight. With the development of processing technology, a large amount of titanium alloy is also used in rockets, artificial satellites and spacecraft. The more advanced the aircraft, the more titanium is used. The titanium alloy used by the American F-14A fighter jets accounts for about 25% of the aircraft weight; the F-15A fighter jets account for 25.8%; the fourth-generation American fighter jets use 41% of titanium, and the F119 engine uses 39% of titanium. The aircraft with the highest amount of titanium.
5.The reason why titanium alloy is widely used in aviation
The top speed of modern aircraft has reached more than 2.7 times the speed of sound. Such fast supersonic flight will 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 aluminum alloy cannot withstand it. High temperature resistant titanium alloy must be used. When the thrust-to-weight ratio of the aero-engine increases from 4-6 to 8-10, and the compressor outlet temperature increases from 200-300℃ to 500-600℃, the original low-pressure compressor discs and blades made of aluminum must be changed to Titanium alloy.In recent years, scientists have continuously made new progress in the research on the properties of titanium alloys. The original titanium alloy composed of titanium, aluminum, and vanadium has a maximum operating temperature of 550°C to 600°C, while the newly developed titanium aluminum (TiAl) alloy has a maximum operating temperature of 1040°C. Using titanium alloy instead of stainless steel to manufacture high-pressure compressor discs and blades can reduce structural weight. Every 10% weight reduction of the aircraft can save 4% of fuel. For the rocket, every 1kg weight reduction can increase the range of 15km.
6.Analysis of machining characteristics of titanium alloy
First, the thermal conductivity of titanium alloys is low, only 1/4 of steel, 1/13 of aluminum, and 1/25 of copper. Due to the slow heat dissipation in the cutting zone, it is not conducive to the heat balance. During the cutting process, the heat dissipation and cooling effect is very poor, and it is easy to form high temperature in the cutting zone. After processing, the deformation and rebound of the parts are large, causing the cutting tool torque to increase and the cutting edge wear fast. The durability is reduced.Secondly, the low thermal conductivity of titanium alloy makes the cutting heat hard to dissipate in a small area near the cutting tool. The friction of the rake face is increased, chip removal is not easy, and cutting heat is hard to dissipate, which accelerates tool wear. Finally, titanium alloys have high chemical activity and are easy to react with tool materials when processed at high temperatures, forming dissolution and diffusion, causing sticking, burning, and breaking of the knife.
