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Why do aviation materials and mobile phones all choose titanium alloy?

The characteristics of titanium
High specific strength: 1.3 times that of aluminum alloy, 1.6 times that of magnesium alloy, 3.5 times that of stainless steel, * in metallic materials.
High thermal strength: The usage 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: resistant to acid, alkali, and atmospheric corrosion, with particularly strong resistance to pitting and stress corrosion.
Good low-temperature performance: Titanium alloy TA7 with extremely low interstitial elements can maintain a certain degree of plasticity at -253 ℃.
High chemical activity: At high temperatures, the chemical activity is very high, easily reacting with gas impurities such as hydrogen and oxygen in the air to form a hardened layer.
Low thermal conductivity and elastic modulus: The thermal conductivity is about 1/4 of nickel, 1/5 of iron, and 1/14 of aluminum, while the thermal conductivity of various titanium alloys is about 50% lower than that of titanium. The elastic modulus of titanium alloy is about half of that of steel.
Classification and uses 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, titanium nickel memory alloys) according to their uses. Although titanium and its alloys have a short history of application, they have earned multiple honorable titles due to their outstanding performance. The first title awarded is "Space Metal", which is lightweight, has high specific strength, and is resistant to high temperatures, making it particularly suitable for manufacturing airplanes and various spacecraft. Currently, about three-quarters of the titanium and titanium alloys produced worldwide are used in the aerospace industry. Many components that were originally made of aluminum alloy have switched to titanium alloy.
Aerospace applications of titanium alloys
Titanium alloy is mainly used as a manufacturing material for aircraft and engines, such as forged titanium fans, compressor discs and blades, engine covers, exhaust devices and other parts, as well as structural framework components such as aircraft beams and frames. Spacecraft mainly utilize the high specific strength, corrosion resistance, and low temperature resistance of titanium alloy 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 plate welding components.
In 1950, the United States first used non load bearing components such as rear fuselage heat shields, wind shields, and tail shields on F-84 fighter bombers. Starting from the 1960s, the use of titanium alloy shifted from the rear fuselage to the middle fuselage, partially replacing structural steel to manufacture important load-bearing components such as frames, beams, and wing slides. Since the 1970s, civilian aircraft have started to use a large amount of titanium alloy, such as the Boeing 747 aircraft, which uses over 3640 kilograms of titanium, accounting for 28% of the aircraft 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 it uses. The titanium alloy used in the F-14A fighter jet in the United States accounts for approximately 25% of the aircraft weight; The F-15A fighter jet is 25.8%; The fourth generation fighter jet in the United States uses 41% titanium, and its F119 engine uses 39% titanium, making it the aircraft currently using only titanium.
The reason why titanium alloys are widely used in aviation
Why does titanium alloy have to be used for the materials of air transport aircraft?
The speed of modern aircraft has reached over 2.7 times the speed of sound. Such rapid supersonic flight will generate a large amount of heat due to friction between the aircraft and the air. When the flight speed reaches 2.2 times the speed of sound, aluminum alloy cannot withstand it and high-temperature resistant titanium alloy must be used. When the thrust to weight ratio of aircraft engines increases from 4-6 to 8-10, and the outlet temperature of the compressor correspondingly increases from 200-300 ℃ to 500-600 ℃, the low-pressure compressor discs and blades originally made of aluminum must be replaced with titanium alloy.
In recent years, scientists have made new progress in their research on the properties of titanium alloys. The titanium alloy originally composed of titanium, aluminum, and vanadium has a working temperature of 550 ℃ to 600 ℃, while the newly developed titanium aluminum (TiAl) alloy has a working temperature of 1040 ℃. Replacing stainless steel with titanium alloy to manufacture high-pressure compressor discs and blades can reduce structural weight. For every 10% reduction in aircraft weight, fuel can be saved by 4%. For rockets, for every 1kg reduction in weight, they can increase their range by 15km.
3C Application of Titanium Alloy
Currently, the consumer electronics industry, represented by mobile phones, is highly competitive, with top manufacturers hoping to increase their product premium capabilities through titanium alloys.
Huawei, Apple, Xiaomi, Honor, and other mobile phones have all imported this material. Apple started with the Ultra series of watches, which come standard with titanium alloy cases. The iPhone 15 released by Apple has a new titanium body in the Pro version, making it Apple's aviation grade titanium phone; Huawei's MateXs2 foldable screen phone, released in 2022, uses titanium alloy materials in its structural components, and uses titanium alloy frames in the Watch4Pro; Honor released its flagship smartphone, Honor MagicVs2, on October 12th with a slim and large internal fold, featuring innovative materials such as Luban titanium hinges; Among the new Xiaomi 14 models, the pricing * is for the 14Pro titanium version.
It is reported that Samsung will use a titanium alloy center frame on the Galaxy S24 Ultra, which is similar in color to the original titanium color of the iPhone 15 Pro.
Overall, the advantages of high specific strength and lightweight of titanium alloys have become an important reason for their widespread promotion. They can make consumer electronics products lighter and provide a more comfortable consumer experience.
Analysis of processing characteristics of titanium alloy
Firstly, titanium alloy has a low thermal conductivity, only 1/4 of steel, 1/13 of aluminum, and 1/25 of copper. Due to slow heat dissipation in the cutting area, it is not conducive to thermal balance. During the cutting process, the heat dissipation and cooling effects are poor, making it easy to form high temperatures in the cutting area. After processing, the deformation and rebound of the parts are large, resulting in increased cutting tool torque, fast blade wear, and reduced durability.
Secondly, the thermal conductivity of titanium alloy is low, making it difficult to dissipate cutting heat in a small area near the cutting tool. This increases the friction force on the front cutting surface, making it difficult to remove chips and reducing cutting heat, accelerating tool wear, Titanium alloy has high chemical activity and is prone to react with tool materials during processing at high temperatures, resulting in solution deposition and diffusion, causing phenomena such as sticking, burning, and breakage of the tool.
The characteristics of machining titanium alloys on machining centers
The machining center can process multiple parts simultaneously, improving production efficiency. Improve the machining accuracy of parts and achieve good product consistency. The machining center has a tool compensation function, which can obtain the machining accuracy of the machine tool itself. It has a wide range of adaptability and great flexibility, such as the arc machining, chamfering, and transition fillet of this part, which can achieve multiple functions in one machine. The machining center can perform a series of processing such as milling, drilling, boring, and tapping. Accurate cost calculation can be performed to control production progress. No need for specialized fixtures, saving a lot of cost and expenses, and shortening the production cycle. Greatly reduces the labor intensity of workers. It can perform multi axis machining with processing software such as UG.
Selection of cutting tools and coolant materials
1. The selection of tool materials should meet the following requirements
Sufficient hardness is required, and the hardness of the cutting tool must be much greater than that of titanium alloy.
Sufficient strength and toughness are necessary due to the high torque and cutting force that the tool bears when cutting titanium alloys.
Sufficient wear resistance. Due to the good toughness of titanium alloy, the cutting edge needs to be sharp during machining. Therefore, the tool material must have sufficient wear resistance to reduce work hardening. This is an important parameter for selecting titanium alloy cutting tools for machining.
The affinity between tool material and titanium alloy is poor. Due to the high chemical activity of titanium alloy, it is necessary to avoid the formation of alloy by solution deposition and diffusion between tool material and titanium alloy, which may cause tool sticking and burning. After conducting experiments on commonly used tool materials in China and foreign tool materials, it has been shown that using high cobalt tools has an ideal effect. The main function of cobalt is to strengthen the secondary hardening effect, improve the red hardness and hardness after heat treatment, and have high toughness, wear resistance, and good heat dissipation.
2. Geometric parameters of milling cutters
The processing characteristics of titanium alloy determine that there are significant differences in the geometric parameters of cutting tools compared to ordinary cutting tools. The spiral angle β selects a smaller spiral rise angle, which increases the chip removal groove, makes chip removal easier, heat dissipation faster, and also reduces the cutting resistance during the cutting process. When cutting at the front angle, the cutting edge is sharp and the cutting is light and fast, avoiding excessive cutting heat generated by titanium alloy and thus avoiding secondary hardening. The back angle α reduces the wear rate of the blade, which is beneficial for heat dissipation and greatly improves durability.
3. Selection of cutting parameters
Titanium alloy machining should choose a lower cutting speed, appropriate large feed rate, reasonable cutting depth and precision machining amount, and sufficient cooling. The cutting speed is VC=30-50m/min, and the feed rate f is taken as the larger feed rate during rough machining, while the appropriate feed rate is taken for precision machining and semi precision machining. The appropriate cutting depth is ap=1/3d. Titanium alloy has good affinity and is difficult to remove chips. If the cutting depth is too large, it can cause tool sticking, burning, and fracture. The precision machining allowance is moderate, and the surface hardening layer of titanium alloy is about 0.1-0.15mm. If the allowance is too small, the cutting edge will cut on the hardening layer, and the tool is prone to wear. It is recommended to avoid machining the hardening layer, but the cutting allowance should not be too large.
4. Cooling fluid
Titanium alloy processing does not require chlorine containing coolant to avoid the generation of toxic substances and hydrogen embrittlement, and can also prevent high-temperature stress corrosion cracking of titanium alloys. Choose a synthetic water-soluble emulsion, or you can use your own coolant. During cutting, the coolant should be sufficient, the coolant circulation speed should be fast, the cutting fluid flow rate and pressure should be large, and the machining center should be equipped with dedicated cooling nozzles. As long as attention is paid to adjustment, the expected effect can be achieved.

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