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Why must titanium alloy be used for aviation materials?

Why must titanium alloy be used for aviation materials?

Issue Time:2021/12/23
Titanium has an indissoluble bond with aviation. In 1953, titanium was used for the first time in the engine nacelle and fire wall of the DC-T engine produced by Douglas Company in the United States, thus unveiling the history of titanium aviation application. Since then, titanium has been used in aviation for more than half a century. Titanium can be widely used in aviation because it has many valuable properties suitable for aircraft applications. Today we will talk about why titanium alloys must be used in aviation materials.

One,Introduction to Titanium
In 1948, DuPont of the United States 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.

Two,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 service temperature is several hundred degrees higher than that of aluminum alloy, and it can work for a long time at a temperature of 450~500℃.
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 very 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 modulus of elasticity of titanium alloy is about 1/2 of that of steel.

Three,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 titanium and its alloys have a short history of application, they have won many honorable titles due to its superior performance. The first title won is "Space Metal". It is light in weight, strong and resistant to high temperatures, and is particularly suitable for the manufacture of 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.

Four,the aviation application of titanium alloy
Titanium alloys are mainly used in aircraft and engine manufacturing materials, such as forged titanium fans, compressor discs and blades, engine hoods, exhaust devices and other parts, as well as structural frame parts such as girder frames of aircraft. 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, structures and rocket shells. Artificial earth satellites, lunar modules, manned spacecraft and space shuttles also use titanium alloy sheet welded parts.
In 1950, the United States used it for the first time on the F-84 fighter bomber as a rear fuselage heat shield, wind deflector, tail cover and other non-load-bearing components. Starting in the 1960s, the use of titanium alloy 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 weight of the aircraft; 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.

Five,Reasons why titanium alloys are 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 can't stand 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 use Titanium alloy.
In recent years, scientists have continuously made new progress in the research work 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 the structural weight. For every 10% weight reduction of the aircraft, 4% of fuel can be saved. For the rocket, every 1kg of weight reduction can increase the range of 15km.

Six,Analysis of machining characteristics of titanium alloy

First of all, the thermal conductivity of titanium alloy 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, resulting in increased cutting tool torque and fast cutting edge wear. Durability is reduced.

Secondly, the low thermal conductivity of titanium alloy makes it difficult for cutting heat to accumulate 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 not easy 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.
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