Grades and applications of titanium and titanium alloys in aerospace industry

1.α alloys include Commercially Pure titanium (CP) and titanium alloys containing only α-stable elements and/or neutral elements.

1.1 Industrial pure titanium

Industrial pure titanium is mainly composed of close-packed hexagonal crystals (HCP)α phase, while due to residual impurities from sponge titanium raw materials or man-made addition of Fe elements, industrial pure titanium also contains a small amount of (<5%) β phase, According to the tensile strength according to 240-550MPa divided into 4 grades (G1, G2, G3 and G4 in ASTM standards), the higher the grade, the higher the oxygen concentration that can play intermittent solid solution strengthening, so the higher the strength. Aerospace Titanium

CP titanium is mainly used in areas that require good corrosion resistance and welding properties, but do not have high strength requirements. In the aviation field, CP titanium is mainly used in air heating tubes for wing leading edge deicing systems, cabin environmental control system pipes, hydraulic pipes, and various clamping and support devices.

1.2 Titanium alloy Ti-5Al-2.5SnELI

Another class of α-type titanium alloys contains the α-stable element Al and the neutral alloy element Sn in order to obtain a higher strength than CP. The most common α-type titanium alloys in the aviation field include Ti-5Al-2.5Sn ELI (Extra low interstitial, ultra-low interval), developed by Russia and the United States, and the Russian brand is BT5-1. Based on common titanium alloy Ti-5Al-2.5Sn, the alloy significantly improves its strength and toughness at extremely low temperatures by reducing the content of intermittent elements, and still has good toughness and low thermal conductivity at 20K (-250℃) low temperature conditions, mainly used in low-temperature vessels, low-temperature pipelines and liquid rocket engine turbine pump impeller.

2 Near-alpha titanium alloy and its application

These alloys mainly contain Al, Sn and Zr and small amounts (not more than 2% by weight) of low-diffused β stable elements such as Mo or Nb, V and Si(not more than 0.5%). The addition of Mo or Nb can stabilize a small amount of the retained β phase at room temperature to play a certain strengthening role.

Near-alpha titanium alloys are not as strong as α+β or β alloys at room temperature, but have superior resistance to high temperature creep, which is particularly important for high temperature applications because they can maintain sufficient strength at high temperatures.

The main grades of near-alpha titanium alloys most commonly used in the aerospace application and aviation industry include Ti-3-2.5, Ti-6-2-4-2S, Ti-1100, IMI834, and BT-36.

(1) Ti-3Al-2.5V(Ti-3-2.5). Ti-3Al-2.5V is a near-alpha type titanium alloy developed in the United States, which is 20% to 50% stronger than pure titanium at room temperature and high temperature, and is suitable for aircraft and engine hydraulic and fuel piping systems. At Boeing, the Ti-3-2.5 is used for all hydraulic piping in the aircraft, except for the landing gear bay hydraulic piping that drives the main landing gear. A large number of oil pressure pipelines on the space shuttle are equipped with Ti-3Al-2.5V seamless alloy pipes, which can reduce the weight of the pipelines by 40%.

(2) Ti-6Al-2Sn-4Zr-2Mo-0.08Si(Ti-6-2-4-2S). Due to the high temperature creep strength of nearly α type titanium alloy is better than α+β alloy, in modern engines, compressor blades use two titanium material used in aircraft, the front blade gas temperature is lower than 300℃, the material is Ti-6-4, the rest of the final stage material is high creep strength alloy Titanium 6242 and Titanium 6246, can be used at up to 540℃. In the 1970s, the US RMI (Reactive Metals Inc) developed Ti-6242S alloy with a temperature of more than 500℃ by adding Si elements. By refining β grains to control the acicular structure, both fatigue strength and creep strength of the alloy are achieved, so that it has high strength, high stiffness, creep resistance and good thermal stability at 565℃, and is widely used in turbine engine components. Figure 2 shows the 3-9 grade titanium compressor rotor.

(3) Ti-1100 (Ti-6Al-2.75 Sn-4Zr-0.4Mo-0.45Si-0.7O2-0.2Fe). Ti-1100 nearly α high temperature titanium alloy is Timet to meet the needs of new aero engines for high temperature titanium alloy high creep resistance and high fracture toughness, and developed in the 1980s, the alloy is actually the development of Ti-6242Si, its use temperature can reach 593℃, It is currently used in the LYCOMING T55-712 engine.

(4) Ti-5.8 Al-4Sn-3.5Zr-0.5Mo-0.7Nb-0.35Si-0.06C (IMI834). IMI834 is an engine superalloy developed by Rolls-Royce (the largest aero engine company in Europe), and the working temperature can reach 600 ° C. At present, it is generally considered to be the highest temperature near alpha titanium alloy that has been put into industrial production. 834 alloy is mainly used in aero engine rings, compressor discs and blades.

(5)Ti-6.2Al-2Sn-3.6Zr-0.7 Mo-0.1Y-5.0 W-0.15 Si(BT36). BT36 is a titanium alloy with a temperature of 600-650℃ successfully developed in Russia in 1992. The alloy replaces 1%Nb with 5% high melting point W on the basis of BT18Y. The addition of W significantly improves the strength, creep and durability of the alloy at room temperature, and improves the thermal stability of the alloy.

3α +β alloy and its application

α+β alloy is by far the most widely used titanium alloy. It has a higher content of (4-6%) beta elements, so it has a higher beta phase content than near-alpha titanium alloys and can obtain higher strength through heat treatment. The main strengthening mechanisms include the retention of metastable β phase at room temperature and the formation of martensite from the original β phase by quenching to room temperature. By aging an alloy containing a metastable β phase, a flaky α can be generated in this region, which can increase strength with as little plastic loss as possible.

The most commonly used α+β alloy is Ti-6Al-4V (Ti-6-4), other α+β alloys for aviation include Ti-6Al-6V-2Sn(Ti-662), Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.2Si (6-2-2-2-2S), Ti-6al-2SN-2Zr-2Mo-2CR-0.2Si (6-2-2-2s), Ti-6al-6V-2SN (Ti-662), Ti-6al-2SN-2Zr-2Mo-2CR-0.2Si (6-2-2-2S), Ti-6al-6V-2SN (Ti-6V-2SN), Ti-6al-2SN-2Zr-2Mo-2CR-0.2Si. IMI550(Ti-4Al-2Sn-4Mo-0.5Si).

(1) Ti-6Al-4V (Ti-6-4). Ti-6-4 is the most widely used titanium alloy material, with good comprehensive properties, often used in the annealed state, the lowest tensile strength of 896MPa(130ksi). Ti-6-4 is a heat-treatable strengthened titanium alloy with good weldability, formability and forging properties. It is the main titanium alloy used in fuselage structural parts, and is also used in the manufacture of jet engine compressor blades, impellers, landing gear and structural parts, fasteners, brackets, aircraft accessories, frames, stringer structures, pipelines.

(2) Ti-6Al-6V-2Sn(Ti-662). The tensile strength of Ti-662 is 1030MPa, the yield strength is 970MPa, the strength is higher than Ti-6-4, excellent corrosion resistance, welding and processing performance is medium, used in aircraft fuselage, rocket engine, nuclear reactor components, in recent years, the application of oil drilling increased.

(3) Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.2Si (6-2-2-2-2S). Developed by RMI in the 1970s, 6-2-2-2S has excellent strength, fracture toughness, high temperature properties, as well as good machinability and weldability for thick structural parts. Used for fuselage, wing, engine structure. The alloy has high strength, the strength of 1068MPa in the annealed state, after solution strengthening and aging, it can reach the maximum strength of 1241MPa, and has a large damage tolerance, and is widely used in fighter structural parts, such as the United States Air Force F-22 Raptor fighter.

(3) Ti-4Al-2Sn-4Mo-0.5Si (IMI550). The IMI550, developed by the British Imperial Metal Company (IMI), has a tensile strength of 1100MPa, a yield strength of 940MPa, and a operating temperature of 400℃ for fuselage and engine structural parts. First used as a creep resistant alloy in Rolls Royce Pegasus and Olympus engines, it was later used in European civil and military aircraft fuselages such as Jaguar, Tornado and Airbus.