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Research Progress of Titanium Alloys for Aviation
Introduction: Titanium alloys are widely used in aerospace industry because of their high specific strength and good corrosion resistance. In this paper, the research progress of main titanium alloys in recent years is introduced, including high strength and high toughness titanium alloys (Ti-1023, Ti-15-3, beta 21S and BT-22), high temperature titanium alloys (IMI834, Ti-1100, BT36 and Ti-60), damage tolerant titanium alloys (TC21 and TC4-DT) and flame retardant titanium alloys (Alloy C, BTT-l and BTT-3), and the prospect of aviation is also presented. The development direction of titanium alloys for aerospace applications.
Titanium is an important structural metal widely used in aerospace field. It has high specific strength, good corrosion resistance and wide use temperature range (-269-600 C). The specific strength of titanium material ranks first among the commonly used metal materials within -253-600 C, so it is quickly used in aerospace industry. Aviation has always been the largest user in the world titanium market in the past 50 years. Fig. 1 shows the main parts and components of titanium for A350. At present, the weight of titanium material on advanced aircraft has reached 30%-35% of the total weight of aircraft structure. Titanium material has become an indispensable structural material for modern aircraft. In recent years, the need for new structural materials with high strength, high modulus of elasticity, high temperature resistance and low density has become increasingly urgent in the aviation industry, which has greatly promoted the rapid development of titanium manufacturing industry. Therefore, this paper first introduces the typical structure types and corresponding properties of titanium alloys, and then summarizes the research progress of new titanium alloys developed in recent years, which are mainly divided into the following four categories: (1) high strength and high toughness titanium alloys; (2) high temperature titanium alloys; (3) Damage Tolerance Titanium alloys; (4) flame retardant titanium alloys.
Fig. 1 Main parts and components of titanium for A350
I. Typical Microstructure Types and Corresponding Properties of Titanium Alloys
The microstructures of titanium and titanium alloy castings are usually transformed beta structures formed by cooling down from high temperature beta zone. The microstructures maintain the original beta grain boundary, which is composed of needle-like or flake-like and basket-like alpha phases. Different microstructures can be obtained by deformation processing and heat treatment. Fig. 2 shows the typical structure and morphology of titanium alloy. Usually, the structure of titanium alloy can be divided into Widmanstatten structure, bimodal structure, basket structure and equiaxed structure according to its morphological characteristics. The formation of the above-mentioned structures mainly depends on the hot deformation and heat treatment process.
(1) Widmanstatten structure: the original beta grain is coarse, clear and complete, there is a very obvious continuous alpha phase on the grain boundary, there is a large sheet of regular parallel alpha phase bundle domains in the grain, and beta phase between the sheets (Fig. 2a). When the initial and final temperatures of alloy deformation are both in the beta phase region and the deformation is not large, or when the alloy is heated to the beta phase region and cooled slowly, the structure will be formed.
(2) Bimorphic structure: It is characterized by the distribution of a certain number of discontinuous primary equiaxed alpha-phase grains on the matrix of transformed beta tissue, but the total content is less than 50%. Therefore, there are two forms of equiaxed primary alpha and flaky alpha in transformed beta tissue (Fig. 2b). This structure can be formed when the alloy is heated and deformed at the upper temperature of the alpha+beta two-phase region. This kind of structure has good comprehensive mechanical properties.
(3) Basket structure: It is characterized by the destruction of the grain boundary of the original beta grain to varying degrees, the shortening of the intragranular alpha sheet, the interlacing arrangement of the orientation of the alpha phase, and the formation of a braided basket structure (Fig. 2c). When the alloy undergoes large deformation near the beta transition point and the deformation is not large enough in the two-phase region of alpha+beta, the structure is formed. Its characteristics are: good plasticity, impact toughness, high temperature durability and creep resistance.
(4) Equiaxed structure: It is characterized by more primary alpha grains (over 50%) and a certain number of beta transformation structures distributed at the boundary of alpha phase, all of which are equiaxed or polygonal (Fig. 2d). The flake alpha can form equiaxed alpha when it is heated after deformation. The degree of equiaxed alpha is affected by the degree of deformation, heating temperature and holding time. The general trend is that with the increase of deformation degree, heating temperature increases, holding time prolongs and equiaxed degree increases. The structure is characterized by good comprehensive properties, especially high plasticity and impact toughness.
Fig. 2 Basic structure of titanium alloy: (a) Widmanstatten structure, (b) basket structure, (c) equiaxed structure, (d) bimodal structure
II. New Titanium Alloys
1. High Strength and Toughness beta Titanium Alloy
The Beta-type titanium alloy has good processability, easy forging, rolling and welding, and high strength and fracture toughness can be obtained by solution aging treatment. At present, the high strength and high toughness Beta-type titanium alloys which have been applied in aircraft are mainly as follows:
Ti-1023 (Ti-10V-2Fe-3Al) titanium alloy was developed by Timet Company in 1971. It is a forged titanium alloy with high structural benefit, high reliability and low manufacturing cost. The Al equivalent of the alloy is 4.0, the Mo equivalent is 11.1, (a+beta)/beta phase transition temperature is 790-805 C. It has great hardenability and remarkable heat treatment strengthening effect. It has excellent forging performance. The forging performance can be isothermal forging at 760 C, providing various near net shape forgings. After heat treatment, the_b of the alloy is 965-1310 MPa, and the KIC is 99-33 MPa.m1/2. It has been used in the landing gear main beam of Boeing 777 passenger aircraft and the main landing gear support of Airbus A380.
Ti-l5-3 (Ti-15V-3Cr3Al-3Sn) high-strength beta-titanium alloy is a kind of near-beta-type high-strength corrosion-resistant alloy developed by USAF in 1970s. The Al equivalent of the alloy is 5.0, Mo equivalent is 15.7, (a+beta)/beta phase transition temperature is 750-770 C. The alloy has excellent cold deformation, aging hardening and weldability, and its cold working performance is better than that of industrial pure titanium.
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