Titanium is widely distributed, its content exceeds 0.4% of the mass of the earth's crust, and the global proven reserves are about 3.4 billion tons, ranking 10th among all elements (oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium, hydrogen, titanium).
American scientists first obtained metallic titanium in 1910 using the "sodium method" (sodium reduction of TiCl4), but the titanium industry did not develop immediately with the discovery of titanium.
It was not until 1948 after World War II that the "magnesium method" (magnesium reduction TiCl4) invented by Luxembourg scientists was used for production in the United States and the titanium industry began to take off.
Titanium is 40% less dense than steel, and its strength is comparable to that of steel, which can improve structural efficiency. At the same time, titanium has good heat resistance, corrosion resistance, elasticity, elasticity and formability. Due to the above characteristics of titanium, titanium alloys have been used in the aviation industry since the appearance of titanium alloys. In 1953, titanium was used for the first time on the DC-T engine fire wall and nacelle produced by the Douglas Company of the United States, and the history of titanium alloys used in aviation began.
The space shuttle is the most important and the most widely used aircraft. Titanium is the main structural material of aircraft, and it is also the material of choice for important components such as aero-engine fans, compressor discs and blades, and is known as "space metal". The more advanced the aircraft is, the more titanium is used. For example, the titanium content of the fourth-generation aircraft of the US F22 is 41% (mass fraction), and the titanium content of the F119 engine is 39%, which is currently the aircraft with the highest titanium content. Titanium alloy research originated in aviation, and the development of aviation industry has also promoted the development of titanium alloys. The research of titanium alloys for aviation has always been the most important and active branch in the field of titanium alloys, but its development is also extremely difficult.
In this paper, titanium alloys are classified from the perspective of alloy matrix phase composition. Taking the aircraft as the representative of the aircraft, this paper focuses on the application and research of titanium alloys in aero-engines, aircraft fuselage, and aviation fasteners. Finally, the problems existing in the development of titanium alloys for aviation are analyzed.
1 Classification of titanium alloys
The classification of titanium alloys in the United States, Britain, Russia, France, Japan and other countries is mostly determined by manufacturers, and there are many names. Some companies directly use the chemical symbols and numbers of the elements to replace the added alloying elements and their contents, such as Ti-6Al-4V (equivalent to TC4 in my country). According to the phase composition, titanium alloys can be divided into: α-type titanium alloys with hexagonal close-packed structure (HCP) (including near-α-type alloys)—that is, domestic grades TA, and two-phase mixed α+β-type titanium alloys—that is, domestic grades TC and Body-centered cubic (BCC) β-type titanium alloys (including near-β-type alloys)—that is, the domestic brand is TB.
1.1 α-type titanium alloy
The single-phase solid solution alloy with α-titanium as the matrix in the annealed state is an α-type titanium alloy, which mainly contains elements such as Al and Sn. Al can increase the tensile and creep strength of the alloy, reduce the density of the titanium alloy, and improve the specific strength, and is an important alloying element in the titanium alloy. In order to maximize the solid solution strengthening effect of aluminum and avoid alloy embrittlement caused by excessive Al, the alloying work of high-temperature titanium alloys should follow the equivalent empirical formula proposed by ROSENBERG. Good thermal stability. These elements in alpha titanium alloys serve to stabilize by inhibiting or increasing the transformation temperature at the transformation temperature. Compared with β-type titanium alloys, α-type alloys have good creep resistance, strength, weldability and toughness, and are the preferred alloys for use at high temperatures. At the same time, α-type alloy does not have cold brittleness, and it is also suitable for use in low temperature environment, which expands its application range. α-type alloys have poor forgeability and are prone to forging defects. Forging defects can be controlled by reducing the processing rate per pass and frequent heat treatment. The α matrix is a stable phase, and for a given composition alloy, the change in its properties is mainly the change in grain size, because both the yield strength and creep strength are related to the grain size and the energy stored during deformation. The strength of α-type titanium alloy cannot be improved by heat treatment, and the strength has basically no or little change after annealing. Some alloys contain more Al, Sn, Zr and a small amount of β-stabilizing elements (generally less than 2%). Although these alloys contain β-phase, the matrix is mainly composed of α-phase, which is very close to α-type alloys in terms of heat treatment sensitivity and processability, and is called near-α-type titanium alloys. Near-α-type alloys were developed on the basis of the recognition that high creep strength can be obtained by strengthening the α-matrix with solid solution alloying elements. Most near-α-type alloys have now become high-temperature titanium alloys due to their good thermal stability. important alloy types. Its strengthening mechanism is that atoms in the β phase diffuse quickly and are prone to creep.
Common α-type titanium alloys (including near-α-type alloys) include Ti811 (Ti-8Al-1Mo-1V), Ti-6Al-2Zr-1Mo-1V, Ti-679 (Ti-2.25Al-11Sn-5Zr-1Mo- 0.25Si), BT18 (Ti-7.7Al-11Zr-0.6Mo-1Nb-0.3Si) and Ti6242S (Ti-6Al-2Sn-4Zr-2Mo-0.1Si), etc. The compositions and properties are listed in Table 2.
1.2 α+β type titanium alloy
In order to improve the strength and toughness of titanium alloys, people have developed α+β titanium alloys. Compared with other titanium alloys, α+β alloys are added with α-stabilizing elements and β-stabilizing elements to strengthen the α and β phases. α+β alloy has excellent comprehensive properties. For example, its room temperature strength is higher than that of α alloy. It has good thermal processing performance and can be strengthened by heat treatment, so it is suitable for aerospace structural parts. The annealed structure of α+β type titanium alloy is α+β phase, and the content of β phase is generally 5%~40%. However, its structure is not stable enough, and the maximum operating temperature can only reach 500 ℃, and its welding performance and heat resistance are lower than that of α-type titanium alloy.
α+β type titanium alloys mainly include TC4 (Ti-6 Al - 4 V ), TC 6 (Ti - 6 Al - 1.5 C r -2.5Mo-0.5Fe-0.3Si), TC11 (Ti- 6.5Al-3.5Mo-1.5Zr-0.3Si), TC17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr), TC19 (Ti-6Al-2Sn-4Zr-6Mo) and TC21 (Ti-6.2Al-2.8Mo) -2Nb-2Sn-2.1Zr-1.3Cr) and so on. Among them, TC11 alloy is also known as near-beta alloy.
ZHOU proposed a processing technology of TC11 alloy. First, the alloy is heat treated at 15° below the β-transition temperature, followed by rapid water cooling, and then undergoes high temperature and low temperature toughening and strengthening heat treatment to obtain a new microstructure. This new microstructure matrix consists of 15% equiaxed α grains, 50% to 60% layered α grains and transformed β grains. The research results show that the alloy exhibits high fatigue resistance, long creep fatigue life, high toughness and excellent high temperature service performance without reducing plasticity and thermal stability.
And the experimental principle of the new process and toughening mechanism is discussed. The key problem in the practical application of this processing technology is the accurate control of temperature.
This TC11 titanium alloy machining process has been used to produce reliable aero-engine compressor discs, rotors and other components.
1.3 β-type titanium alloy
The content of β-stabilizing elements is high enough, and the alloy obtained by rapidly cooling the β-phase after solution treatment and retaining it to room temperature is called β-type titanium alloy. According to the classification of stable state microstructure, β titanium alloys can be divided into stable β titanium alloys and metastable β titanium alloys, as shown in Figure 1. In Figure 1, MS is the martensitic transformation temperature line, βC is the minimum content of β-stable elements in metastable alloys, and βS is the minimum content of β-stable elements in stable alloys.
Beta alloys have good cold formability in the solution state, and are also excellent in hardenability and heat treatment responsiveness.
The commonly used heat treatment method is first solution treatment, and then aging at 450~650 ℃, fine α phase will be precipitated on the original β matrix of the alloy, forming a second phase with dispersed distribution, which is the strengthening mechanism of β alloy. Compared with other types of titanium alloys, β-titanium alloy precipitates more α phase during aging, and contains more α-β phase interface to hinder the movement of dislocations, so the room temperature strength of β-titanium alloy is the highest.
The ability of a metal material to absorb energy during deformation and fracture is called toughness. The more energy a material absorbs, the better the toughness. Fracture toughness is an indicator of the toughness of a material, reflecting the resistance of the material to the propagation of cracks and other sharp defects. Generally speaking, the fracture toughness and strength of titanium alloys show an inverse trend, that is, the fracture toughness decreases as the strength increases. To study the application of β-titanium alloys in the aerospace industry, it is necessary to design microstructures with good strength and fracture toughness, as well as processing technology and heat treatment regimes. Alloy composition and microstructure are the two main factors that determine the fracture toughness of beta titanium alloys. The alloy composition determines the amount of beta phase in the alloy, as well as the type and fracture toughness of the alloy. The morphology, quantity and volume of the microstructure also affect the fracture toughness of the alloy. Fu Yanyan and others believed that the β-stabilizing element and the medium-sized element Zr of β-titanium alloy can improve the strength of the alloy and reduce the fracture toughness. The fine β grains cannot effectively improve the strength of aged β titanium alloys, and will reduce the fracture toughness of Ti-15-3 alloys, but have no significant effect on the fracture toughness of β-C and Ti-1023 alloys.
The strength of aging β-titanium alloy mainly depends on the content and size of the secondary α phase precipitated by aging. In the case of containing the same primary α phase, the fine secondary α phase can significantly improve the strength of the alloy.
The coarsening of the primary α phase and the transformation of the primary phase from spherical to flaky will lead to a decrease in ductility and an increase in fracture toughness of β-titanium alloys. The dual-mode structure of β-titanium alloy has a good match of strength, ductility and toughness.
The reason why β-titanium alloy is widely used is also because it has the advantages of high strength and high plasticity that other types of titanium alloys cannot match after aging. At the same time, the heat-treatable strengthening and deep hardening ability of β titanium alloy make it gradually replace the α+β two-phase titanium alloy as the preferred structural material for aircraft fuselage and wings, and it plays a more and more important role in the aerospace industry. increasingly important role.
2 Development and application of titanium alloys for aviation
In the 1950s, military aircraft entered the era of supersonic speed, and the original aluminum and steel structures could no longer meet the new demands. It was at this time that titanium alloys entered the stage of industrial development. Titanium alloys have excellent properties such as low density, high specific strength, corrosion resistance, high temperature resistance, non-magnetic, weldable, wide operating temperature range (269~600℃), and can be used for various parts forming, welding and machining. Aeronautics soon became widely used. In the early 1950s, military aircraft began to use industrial pure titanium to manufacture structural parts with less stress such as heat shields, tail cowls, and speedbrakes of the rear fuselage. In the 1960s, titanium alloys were further applied to major stress-bearing structural parts such as aircraft flap sliding, load-bearing bulkheads, mid-wing box beams, and landing gear beams. By the 1970s, the application of titanium alloys in aircraft structures had expanded from fighter jets to large military bombers and transport aircraft, and a large number of titanium alloy structures had also been used in civil aircraft.
After entering the 1980s, the titanium used in civil aircraft has gradually increased, and has surpassed the titanium used in military aircraft. The more advanced the aircraft, the more titanium is used. Tables 3 and 5 list the mass fraction of titanium materials used in third- and fourth-generation fighters and advanced bombers and transport aircraft in the United States, the types of titanium alloys used in general aircraft, and the amount of titanium alloys and composite materials used in Airbus aircraft. It can be seen from Table 5 that the usage of titanium on Airbus A380 aircraft has reached 10%, and titanium has become an indispensable structural material for modern aircraft. According to different uses, titanium alloys for aviation can be divided into titanium alloys for aircraft engines, titanium alloys for aircraft fuselage and titanium alloys for aviation fasteners. In recent years, people have carried out in-depth research on the application of aviation titanium alloys in the above three aspects.
To sum up, titanium alloy has large thrust-to-weight ratio, high toughness, good strength and weldability, and is an aviation material with excellent comprehensive properties. In the past few decades, the alloying theory, comprehensive strengthening and toughening technology and heat treatment process of titanium alloys for aviation have been greatly developed. At present, the research on titanium alloys mainly focuses on thermal stability at high temperature, creep resistance and low-cost titanium alloy design and manufacturing process. With the deepening of research, the technological progress of low-cost processing of titanium alloys will be driven by high-end aviation applications, thereby fundamentally breaking through the cost bottleneck restricting the improvement of the dosage and application level of titanium alloys for aviation. An all-titanium aircraft may become a reality in the not too distant future.
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