Grades and applications of titanium and titanium alloys in aerospace industry

4.β alloy and its application

Beta alloy is the highest strength of titanium alloys, tensile strength up to 1240MPa. After rapid cooling, 100% of the metastable β phase can be retained at room temperature. By using different aging temperature and time, the size and proportion of α phase precipitation in the β phase matrix of an alloy can be controlled, so as to obtain higher strength than α+β alloy, and different properties can be obtained by selecting the appropriate aging temperature and time.

With a few exceptions, beta alloys are not used for high temperature applications because, in general, beta alloys decline in strength more rapidly than near-alpha and α+ beta alloys with increasing temperature, and do not have the same creep resistance as near-alpha alloys.

Beta alloy is mainly used for structural parts with high strength requirements, such as aircraft landing gear, starting with the Boeing 777, and has been applied in a number of new large commercial aircraft. Other components used in beta alloy on the 777 and 787 include flap rails, springs, APU struts, fire tanks, clamps and brackets, and exhaust pipes.

The beta alloys used in aviation include Ti-10V-2Fe-3Al, Ti-5Al-5Mo-5V-3Cr, Ti-15V-3Cr-3Al-3Sn, Ti-6Al-2Sn-4Zr- 6Mo, Ti-5Al-2Sn-2Zr-4Mo-4Cr, Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-35V-15Cr and Ti-15Mo-2.7Nb-3Al-0.2Si.

(1) Ti-10V-2Fe-3Al(Ti-10-2-3). Ti-10V-2Fe-3Al (Ti-10-2-3) alloy is a high strength near beta alloy jointly developed by the American Timet Company, Boeing Company and Wyman-Gordon in the 1970s. It has been successfully applied to key structural parts such as main bearing beam, wing and shaft of aircraft landing gear. After trials on the Boeing 757, the alloy was officially approved for use in the landing gear of the Boeing 777, as shown in Figure 3. Since then, Airbus has also used the Ti-10-2-3 as landing gear on its A380 aircraft.

(2) Ti-5Al-5Mo-5V-3Cr (Ti-5553). Ti-5553 alloy is a new high-strength and high-toughness near β titanium alloy jointly developed by Russia's Upper Sarda (VSMPO) and European Airbus company, its nominal composition is Ti-5Al-5Mo-5V-3Cr-1Zr, which is slightly higher than Ti-10-2-3 alloy strength (about 1240MPa). After heat treatment, the tensile strength can exceed 1500MPa, with certain processing performance advantages and better hardenability. It is especially suitable for the manufacture of large bearing components such as wing/hanger joints, landing gear/wing joints and landing gear parts. Ti-5553 alloy is used for most parts of the landing gear of Boeing's new 787 aircraft, and it is also used for the landing gear parts of Airbus A350-1000.

(3) Ti-15V-3Cr-3Sn-3Al(Ti-15-3-3-3). Ti-15-3-3-3 is a metastable β-type titanium alloy developed in the United States in the 1970s. After treatment at 800℃ 30minAC+540℃ 8hAC, the tensile strength at room temperature reached 1100MPa, and the elongation was still above 9%. The alloy has excellent compression ductility, cold formability and welding properties, and is an ideal material for aviation components. It is mainly used as fuselage structural parts and aviation fasteners, and can also be used to make springs, as shown in Figure 4. Using beta titanium instead of steel springs can achieve a 70% weight reduction.

(4) Ti-6Al-2Sn-4Zr-6Mo (Ti-6-2-4-6). Titanium 6246 is a high Mo content high temperature titanium alloy developed by Timet in the 1960s, with high temperature resistance (using temperature at 420 ° C), good strength, corrosion resistance, welding and processing properties. The low cycle fatigue strength of the alloy after solution aging or double annealing is obviously higher than that of the corresponding Ti-6Al-4V alloy, and has higher high temperature creep strength and instantaneous strength, Titanium Alloy Aerospace, which can be used to manufacture turbine engine compressor discs and blades.

(5) Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti-17). Ti-17 is the United States General Electric Company in the early 1970s began to research and development of β-type alloy, high strength, good toughness, at room temperature yield strength of 1137~1166MPa, tensile strength of 1196~1235MPa, elongation of more than 8%. At the same time, it has good crack growth/fatigue resistance and fracture toughness. It is mainly used for some newly developed large aircraft engine fan disk and air pressure engine disk with high strength requirements. The United States General Electric Company and Wyman Gordon Company used Ti-17 alloy to manufacture engine disc and helicopter rotor clamp shaft. Japan's Kobe Steel also began to develop the alloy and used in the manufacture of engine discs.

(6) Ti-3Al-8V-6Cr-4Mo-4Zr(β-C). β-C is a metastable β-titanium alloy developed by RMI in 1969. The alloy contains more solid solution, tensile strength can reach 1240 MPa, due to high strength, its plastic and loss tolerance properties (fracture toughness and fatigue crack growth rate) lower than α+β alloy, so it is not commonly used in critical load-bearing components, usually used as aircraft springs, fasteners, connectors and missile components.

The research shows that adding a small amount of C(0.1%) to β-C and conducting a certain cold deformation before aging can accelerate the precipitation of α phase during aging, reduce the formation of grain boundary α (GB-α), and promote grain refinement, and maintain good ductility while obtaining a strength of up to 1500MPa.

(7) Ti-35V-15Cr (Alloy C). There is only one true (stable) beta Alloy with limited commercial application, Alloy C, nominally Ti-35V-15Cr-0.05C, manufactured by Pratt and Whitney, Inc. One of the two largest aircraft engine manufacturing companies in the United States) developed. Due to the high content of the β-stabilized Alloy, Alloy C does not decompose the β phase into the α+β phase at service temperature, as normal β alloys do. The alloy has a tensile property of 1071MPa at room temperature, a yield strength of 1023 MPa, a elongation of 14.7, and a creep temperature of 540℃, and is used by Pratt & Whitney as an exhaust system for military engines due to its fire protection (non-combustion) properties. Other titanium alloys will burn at high mass flow rates (such as jet engine airflow), and the "fuel" for combustion is titanium and aluminum, which are essential in almost all titanium alloys.

(8) T i-1 5 M o-2.7 N b-3 Al-0.2 S i (β-21 1 S). β-21S(Ti-15Mo-2.7Nb-3Al-0.2Si) is a new ultra-high strength β-alloy developed by TIMET. The alloy has high strength and good plasticity, and through heat treatment, aging to a very high strength level (tensile strength >1450MPa), the plasticity is still maintained at the level of Ti-1023. β-21S has significant antioxidant properties during processing and use, making it more suitable for processing into thin sheets. β-21S can withstand higher temperatures than other β alloys, and the long-term working temperature can reach 540℃.

Due to its better resistance to high temperatures, this alloy can be used as an aero-engine tail vertebrae, as shown in Figure 5, where the nozzle is exposed to engine exhaust. Replacing the nickel-based alloy with β-21S can greatly reduce the weight of the nozzle and tail vertebrae.

Peroration

The analysis of the American and European Aerospace Titanium and titanium alloys shows that in recent years, the high temperature titanium alloy, low temperature titanium alloy, high strength and high toughness β titanium alloy, flame retardant titanium alloy and damage tolerance titanium alloy developed abroad have been widely used in the aerospace field, representing the development direction of aerospace high performance titanium alloy materials.

(1) High temperature titanium alloy. The high-temperature titanium alloys developed in the 1950s are represented by the Ti-6Al-4V alloy developed in the United States, which is adapted to a temperature of 300-350 ° C. The high-temperature titanium alloys developed later are mainly near α type, represented by Ti-6-2-4-2S and Ti-1100 developed in the United States, IMI834 developed in the United Kingdom and BT-36 developed in Russia, and the temperature is as high as 600℃. High temperature titanium alloys have been widely used in aero engines because of their excellent thermal strength and high specific strength. Another development trend of high temperature titanium alloys is titanium aluminum alloy, that is, Ti3Al (α2) and TiAl (ϒ) intermetallic compounds based on titanium and aluminum, of which ϒ alloy has a high temperature resistance of 725℃. Titanium aluminum alloy has become the most competitive material for future aircraft engines and aircraft structural parts.

(2) Low temperature titanium alloy. Some titanium and titanium alloys can maintain their original mechanical properties at low and ultra-low temperatures. Studies on low-temperature titanium alloys in the United States mainly focus on α type Ti-5Al-2.5Sn ELI and α+β type Ti-6Al-4V ELI. By reducing the content of intermittent elements, the two titanium alloys maintain good strength and toughness at extremely low temperature of 20K. Used in cryogenic vessels, cryogenic pipes and liquid rocket engine impellers.

(3) high strength titanium alloy. High-strength titanium alloys generally refer to titanium alloys with tensile strength above 1,000MPa, and foreign high-strength titanium alloys are mainly developed in the United States and Russia. Beta alloy is the highest strength titanium alloy, currently representing the international advanced level and in the field of aviation to obtain practical applications of high-strength titanium alloy mainly β type titanium alloy, such as the United States Ti-10-2-3, Ti-15-3-3-3 and β-21s, Russia Ti-5-5-5-3-1, etc. It is mainly used for structural parts with high strength requirements, such as aircraft landing gear and fuselage parts.

(4) flame retardant titanium alloy. In order to solve the "titanium combustion" problem of titanium alloy materials for aircraft engines to meet the needs of high thrust-to-weight ratio engines, the United States and Russia have carried out the development of flame retardant titanium alloys since the 1970s. Flame retardant titanium Alloy mainly includes two alloy systems: the United States Ti-V-Cr system Alloy C (T-35V-15Cr); The Russian Ti-Cu-Al series is BTT-1 and BTT-3. [3] Alloy C is a stable β type flame retardant titanium alloy with high room and high temperature strength, good creep strength, excellent fatigue strength and cold formability, which has been successfully applied to the high-pressure compressor casing, guide blade and vector tail nozzle of F119 engine.

Damage tolerance titanium alloy. In order to meet the requirements of new aircraft on material specific strength, fatigue resistance, crack growth, fracture toughness, life cycle cost and other comprehensive properties, damage tolerance titanium alloys with high fracture toughness and low crack growth rate have been developed abroad. It is represented by the α+β alloys Ti-6Al-4V ELI and Ti-6-2-2-2-2S developed in the United States. Ti-6Al-4V ELI is a medium-strength damage tolerance titanium alloy, and Ti-6-2-2-2-2S is a high-strength damage tolerance titanium alloy, which has been widely used in F-22 fighter jets in the United States.