Why do aviation materials and mobile phones use titanium alloys? Titanium properties

High specific strength: 1.3 times that of aluminum alloy, 1.6 times that of magnesium alloy, and 3.5 times that of stainless steel. It is the champion among metal materials.

High thermal strength: The operating temperature is several hundred degrees higher than that of aluminum alloy, and it can work at temperatures of 450 to 500°C for a long time.

Good corrosion resistance: acid, alkali, atmospheric corrosion, particularly resistant to pitting corrosion and stress corrosion.

Good low temperature performance: Titanium alloy TA7 with extremely low interstitial elements can maintain a certain plasticity at -253°C.

High chemical activity: The chemical activity is very high at high temperatures, and it easily reacts chemically with gas impurities such as hydrogen and oxygen in the air to form a hardened layer.

Small thermal conductivity and small elastic modulus: 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 that of steel.

Classification and uses of titanium alloys

Titanium alloys can be divided according to their uses: heat-resistant alloys, high-strength alloys, corrosion-resistant alloys (titanium-molybdenum, titanium-palladium alloys, etc.), low-temperature alloys and special function alloys (titanium-iron hydrogen storage materials, titanium-nickel memory alloys) wait. Although the application history of titanium and its alloys is not long, it has won many glorious titles due to its superior performance. The first title it won was "space metal". It is light in weight, has high specific strength and is resistant to high temperatures. It is especially suitable for manufacturing aircraft and various spacecrafts. Currently, about 3/4 of the titanium and titanium alloys produced in the world are used in the aerospace industry. Many parts that were originally made of aluminum alloy have been replaced with titanium alloy.

Aerospace applications of titanium alloys

Titanium alloys are mainly used as manufacturing materials for aircraft and engines, 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 girders and partitions. Spacecraft mainly utilize 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 casings. Artificial earth satellites, lunar modules, manned spacecraft and space shuttles also use titanium alloy plate welding parts.

In 1950, the United States first used it on the F-84 fighter-bomber as non-load-bearing components such as rear fuselage heat shields, wind guides, and tail cowls. Since the 1960s, the use of titanium alloys has moved from the rear fuselage to the middle fuselage, partially replacing structural steel in the manufacture of important load-bearing components such as bulkheads, beams, and flap slide rails. Since the 1970s, civil aircraft have begun to use titanium alloys in large quantities. For example, the Boeing 747 passenger aircraft uses more than 3,640 kilograms of titanium, accounting for 28% of the aircraft weight. With the development of processing technology, a large amount of titanium alloys are also used in rockets, artificial satellites and spacecrafts. The more advanced the aircraft, the more titanium is used. The titanium alloy used in the U.S. F-14A fighter accounts for about 25% of the aircraft weight; the F-15A fighter uses 25.8%; the titanium used in the U.S. fourth-generation fighter is 41%, and its F119 engine uses 39% titanium, which is currently Aircraft with the highest amount of titanium.

Reasons why titanium alloys are widely used in aviation

Why do air transport aircraft have to use titanium alloys as materials?

The maximum speed of modern aircraft has reached more than 2.7 times the speed of sound. Such fast supersonic flight will cause friction between the aircraft and 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, and high-temperature-resistant titanium alloy must be used. When the thrust-to-weight ratio of aeroengines increases from 4 to 6 to 8 to 10, and the compressor outlet temperature increases from 200 to 300°C to 500 to 600°C, low-pressure compressor discs and blades originally made of aluminum must be replaced. Titanium alloy.

In recent years, scientists have continuously made new progress in research on the properties of titanium alloys. The original titanium alloy composed of titanium, aluminum, and vanadium had a maximum operating temperature of 550°C to 600°C. However, the newly developed titanium aluminum alloy (TiAl) alloy has a maximum operating temperature of 1040°C. Using titanium alloy instead of stainless steel to manufacture high-pressure compressor disks and blades can reduce structural weight. Every 10% reduction in aircraft weight can save 4% in fuel. For rockets, every 1kg weight reduction can increase the range by 15km.

3C application of titanium alloy

At present, the consumer electronics industry represented by mobile phones is very "involved", and leading manufacturers hope to use titanium alloys to increase the premium capabilities of their products.

Many mobile phones such as Huawei, Apple, Xiaomi, and Honor have imported this material. Apple has started to equip its Ultra series of watches with titanium alloy cases as standard. Its latest iPhone 15, the Pro version of which uses a new titanium body, has become Apple’s first mobile phone to use aviation-grade titanium; Huawei’s folding screen mobile phone will be released in 2022 Titanium alloy materials are used in the structural parts of Mate The highest priced version is the 14Pro titanium version.

It is reported that Samsung will use a titanium alloy middle frame on the Galaxy S24 Ultra. The middle frame part is similar to the original titanium color of the iPhone 15 Pro.

Generally speaking, the advantages of titanium alloys with both high specific strength and lightweight have become an important reason for its promotion. It can make consumer electronics products lighter and the consumer experience will be more comfortable.

Analysis of processing 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. Because the heat dissipation in the cutting area is slow, it is not conducive to thermal balance. During the cutting process, the heat dissipation and cooling effect is very poor, and it is easy to form high temperatures in the cutting area. After processing, the parts deform and rebound greatly, resulting in increased cutting tool torque and rapid edge wear. Durability reduced.

Secondly, the thermal conductivity of titanium alloy is low, which makes the cutting heat accumulate in a small area near the cutting tool and is not easy to dissipate. The friction on the rake face increases, making it difficult to remove chips. The cutting heat is not easy to dissipate, which accelerates tool wear. Finally, titanium alloys are highly chemically active and tend to react with tool materials when processed at high temperatures, forming coatings and diffusions, resulting in phenomena such as sticking, burning, and breakage.

Characteristics of machining centers for processing titanium alloys

The machining center can process multiple parts at the same time to improve production efficiency. Improve the processing accuracy of parts and achieve good product consistency. The machining center has a tool compensation function to obtain the processing accuracy of the machine tool itself. It has wide adaptability and great flexibility, such as arc processing, chamfering and transition filleting of this part, which can realize multiple functions in one machine. The machining center can perform a series of processing such as milling, drilling, boring, and tapping. Accurate cost calculations can be performed and production progress can be controlled. No special fixtures are required, saving a lot of costs and shortening the production cycle. Greatly reduces the labor intensity of workers. It can be used with UG and other processing software for multi-axis processing.

Selection of tool and coolant materials

1. The selection of tool materials should meet the following requirements

To achieve sufficient hardness, the hardness of the tool must be much greater than the hardness of titanium alloy.

Sufficient strength and toughness. Since the tool bears large torque and cutting force when cutting titanium alloy, it must have sufficient strength and toughness.

Sufficient wear resistance. Due to the good toughness of titanium alloy, the cutting edge must be sharp during processing, so the tool material must have sufficient wear resistance to reduce work hardening. This is the most important parameter when choosing cutting tools for processing titanium alloys.

The affinity between tool materials and titanium alloys is poor. Due to the high chemical activity of titanium alloys, it is necessary to avoid the formation of alloys between the tool materials and titanium alloys by dissolution and diffusion, causing sticking and burning of the tools. Tests on commonly used domestic tool materials and foreign tool materials have shown that the use of high-cobalt tools has ideal effects. The main role of cobalt can strengthen the secondary hardening effect, improve red hardness and hardness after heat treatment, and at the same time have high toughness and wear resistance. properties and good heat dissipation.

2. Geometric parameters of milling cutter

The processing characteristics of titanium alloy determine that the geometric parameters of the tool are quite different from those of ordinary tools. The helix angle β is chosen to be a smaller helix rise angle, so that the chip removal groove is enlarged, chip removal is easy, heat dissipation is fast, and the cutting resistance during the cutting process is also reduced. When cutting with rake angle γ, the cutting edge is sharp and the cutting is light and fast, which prevents the titanium alloy from generating excessive cutting heat and thus avoids secondary hardening. The relief angle α reduces the wear rate of the blade, which is beneficial to heat dissipation and greatly improves the durability.

3. Cutting parameter selection

Titanium alloy machining should choose a lower cutting speed, a moderately large feed amount, a reasonable depth of cut and finishing amount, and sufficient cooling. The cutting speed vc=30~50m/min, the feed f is larger for rough machining, and a moderate feed for finishing and semi-finishing. The cutting depth ap=1/3d is appropriate. Titanium alloy has good affinity and difficulty in chip removal. If the cutting depth is too large, it will cause the tool to stick, burn, and break. The finishing allowance is moderate. The hardened layer on the surface of titanium alloy is about 0.1~0.15mm. If the allowance is too small, the cutting edge cuts on the hardened layer, and the tool is prone to wear. Processing of the hardened layer should be avoided, but the cutting allowance should not be too large.

4. Coolant

It is best not to use chlorine-containing coolants when processing titanium alloys to avoid producing toxic substances and causing hydrogen embrittlement, and also to prevent high-temperature stress corrosion cracking of titanium alloys. Use synthetic water-soluble emulsion, or use your own coolant. During cutting, the coolant must be sufficient, the coolant circulation speed must be fast, and the cutting fluid flow and pressure must be large. The machining center is equipped with a special cooling nozzle. As long as you pay attention to the adjustment, you can achieve the desired effect.