Reasons for using titanium
reduce weight
The high strength and low density of titanium (about 40% lower than steel) provide many opportunities for weight reduction. The best examples are its use on the landing gears of Boeing 777 and 787 aircraft and Airbus A380. Figure 1 shows the landing gear on the 777 aircraft. 1 All marked parts are made of Ti-10V-2Fe-3Al. The minimum tensile strength of this alloy is 1,193 MPa; it is used to replace the high-strength low-alloy steel 4340M used at 1,930 MPa. This replacement resulted in a weight reduction of more than 580 kg. 1 Boeing 787 uses the next generation of high-strength titanium alloy Ti-5Al-5V-5Mo-3Cr, which is slightly higher in strength and has certain processing advantages. The use of titanium in the landing gear structure should also significantly reduce the maintenance cost of the landing gear to its corrosion resistance. The low density and high strength make it very attractive for reciprocating parts, such as connecting rods for automotive applications. Similarly, the price of family cars is too high, but the U.S. Department of Energy is investing heavily to make the price of titanium components for cars and trucks reasonable. (Titanium has been successfully used in high-end racing cars, and the cost is not such a big problem.)
Space constraints
This application does not appear often, but it is important. The best examples are the landing gear beams used on the 737, 747, and 757. This component runs between the wings and the fuselage, supporting the landing gear. Other Boeing airplanes use aluminum alloy in this application, but for the above airplanes, the load is higher and the aluminum structure is not suitable for the wing envelope. Aluminum alloy will be the first choice because its cost is much lower. Steel is another option, but the weight will be higher.
Operating temperature
The structure of the engine and the exhaust area work at high temperatures, so the main choice is titanium-based or nickel-based alloys; similarly, nickel alloys will significantly increase weight. The service temperature of titanium engine alloy is as high as about 600°C. Some applications, such as plugs and nozzles (Figure 2), can withstand temperatures above this temperature for a short period of time under certain operating conditions. Except for special engine alloys, the temperature limit of titanium alloys is approximately 540°C. Above this temperature, oxygen contamination becomes a problem, making the surface brittle. Titanium is also used in structures at low temperatures, such as the impellers of rocket engines.
Corrosion resistance
Titanium has a very tough nascent oxide that will form immediately when exposed to the air. This oxide is responsible for the excellent corrosion resistance. In the aerospace environment, corrosion is not a factor in titanium. Titanium is not pitted. In the author's opinion, this is the essence of high-quality service experience. In use, aluminum and steel alloys will eventually form corrosion pits, which act as stress risers and then cause stress corrosion or fatigue cracks. This does not happen with titanium. This corrosion resistance runs through the chemical, petrochemical, pulp, paper and construction industries. Titanium and its alloys have excellent resistance under most oxidizing, neutral and inhibited reduction conditions. It also has corrosion resistance in the human body. The biocompatibility is also very good; it is used in a prosthetic device, and the bone will grow into a reasonably designed titanium structure. Commercial pure titanium is also used in exterior construction applications, and this practice started in Japan. It is used on the outer surface because it never needs any maintenance. The most famous of these is its use on the exterior of the Guggenheim Museum in Bilbao, Spain.
Composite material compatibility
Titanium is compatible with graphite fibers in polymer composites. There is a high electric potential between aluminum and graphite. If aluminum comes into contact with graphite when wet, the aluminum will be corroded away. It can be isolated from composite materials by methods such as glass fiber layers, but in areas that are difficult to inspect and replace, titanium is used as a conservative method. In addition, although the coefficient of thermal expansion (CTE) of titanium is higher than that of graphite, it is much lower than that of aluminum. Even within the operating temperature range of the fuselage structure, from about –60°C during cruising to +55°C in hot weather, the difference in CTE of the aluminum structure attached to the composite material will cause a very high load. This is not a problem with the titanium structure. Obviously, the longer the component, the greater the problem of using aluminum.
Low modulus
The main area of importance is the replacement of steel springs. Since the modulus is about half that of steel, only half the number of coils is needed. Combining high strength and density (approximately 60% of steel), steel springs can ideally reduce the weight by approximately 70%. In addition, titanium provides excellent corrosion resistance, thereby reducing maintenance costs.
Armor
Titanium has excellent ballistic resistance. Compared with steel or aluminum armor, it has the same ballistic protection at the areal density of interest and can reduce the weight by 15-35%, thereby greatly reducing the weight of military ground combat vehicles. Lighter vehicles have better transportability and maneuverability. Excellent corrosion resistance, low ferromagnetism, and compatibility with composite materials also provide significant advantages. Two projects that use titanium in upgraded vehicles are the Bradley infantry fighting vehicle (Figure 3) and the Abrams main battle tank. 2 The relatively high cost of titanium has been successfully reduced by using plates made of electron beams, cold hearths, and single-melting ingots. 3
