Research and Application of Ti6Al4V Titanium Alloy

Ti6Al4V alloy, first successfully developed in 1954, is an equiaxed martensitic-type dual-phase alloy that has now become a globally recognized titanium alloy. It is widely used in aerospace components, with its fatigue performance being a primary focus of research.

Based on the degree of refinement, Ti6Al4V alloy can be classified into regular Ti6Al4V and Ti6Al4V (ELI), with their chemical compositions shown in Table 1. The measured mechanical properties of the alloy are as follows: tensile strength (σb) of 896 MPa, yield strength (σs) of 869 MPa, elastic modulus (E) of 110 GPa, shear modulus (G) of 42.7 GPa, and density (ρ) of 4.43 g/cm³. Currently, Ti6Al4V accounts for 50% of the total titanium alloy production and 95% of all processed titanium alloy components. Since its introduction, research on this alloy has continued unabated, and through extensive and in-depth studies, its processing technology has reached a relatively mature stage.

However, in recent years, with the shift in design concepts from purely static strength design to damage-tolerance design concepts and fail-safe design principles, as well as the exploration of new application fields, research on Ti6Al4V has once again surged. Extensive studies have been conducted on the influence of factors such as microstructure, texture, heat treatment, cross-sectional size, loading direction, stress ratio, surface condition, and corrosion environment on the fatigue performance of the alloy. As a result, Ti6Al4V has once again emerged as a prominent material for new applications.

Table 1: Chemical Composition of Ti6Al4V Alloy and Ti6Al4V (ELI) Alloy (Mass Fraction, %)

1. Industrial Application of β Heat Treatment

With the increasing demand for improved aviation efficiency and cost reduction, reducing the density of aerospace materials while enhancing their performance has become increasingly important. Lowering the material density can improve the thrust-to-weight ratio of aircraft, extend flight range, and reduce fuel consumption. One of the primary methods to reduce the weight of aircraft structural components is to use α+β titanium alloys, which offer high specific strength and excellent comprehensive properties, achieving a mass reduction of 10% or more. At the same time, to ensure the design life and damage tolerance of components, the material must possess good fracture toughness and crack growth resistance. Compared to other high-strength engineering materials, titanium alloys have a lower elastic modulus, making stiffness-designed components bulkier and heavier than strength-designed parts. Therefore, how to further improve the comprehensive performance of α+β titanium alloys has always been a focus of research.

The conventional forging temperature of titanium alloys is typically 40-50°C below the phase transformation point, where heating and deformation result in an equiaxed grain structure. This structure provides excellent room-temperature strength, ductility, and thermal stability, but it exhibits inferior high-temperature performance, fracture toughness, and crack growth resistance. On the other hand, β forging above the phase transformation point produces a basketweave structure, which offers superior high-temperature properties (such as creep and stress rupture resistance), fracture toughness, and crack growth resistance, albeit with a significant reduction in ductility and thermal stability.

To address this issue, researchers have integrated phase transformation theory, deformation heat treatment, and strengthening-toughening theory, combined with computer numerical simulation technology applied to forging processes, proposing the Near-β Forging Theory. According to this theory, heating and deformation at temperatures 45-75°C below the phase transformation point result in a three-phase structure — equiaxed α + lamellar α + transformed β matrix. This microstructure enhances yield strength, high-temperature creep performance, low-cycle fatigue life, fracture toughness, and crack growth resistance while maintaining good ductility and thermal stability. Additionally, it increases the applicable service temperature of the material.

According to the literature, when the forging heating temperature is maintained at 45-75°C below the β-transformation temperature, the room-temperature tensile strength and elongation of the material fully meet the requirements of the GJB391-87 standard. This is because the significant temperature difference between the heating temperature and the β-transformation temperature ensures that even with the temperature rise caused by deformation during forging, it does not exceed the β-transformation temperature. Consequently, deformation occurs entirely within the dual-phase region, ensuring a sufficient processing rate (approximately 70%). After final forging and heat treatment, the microstructure of the product predominantly consists of a primary α + β equiaxed grain structure.

2. Study on the Influence of Microstructure and Texture on Performance

The directional nature of deformation leads to the formation of texture. When the microstructure of a titanium alloy sheet is uniformly distributed in all directions, it promotes isotropic deformation behavior, ensuring similar deformation rates across different directions and reducing the likelihood of weak spots. At room temperature, the α phase in Ti6Al4V alloy accounts for more than 85% of the structure. When the hexagonal close-packed (HCP) α-phase texture orientation is perpendicular or nearly perpendicular to the surface of the sheet, the thickness direction has the highest texture density and maximum strength, making it less prone to failure during deep drawing processes.

Ti6Al4V alloy contains 6% aluminum (an α stabilizer) and 4% vanadium (a β stabilizer), giving it excellent comprehensive performance, making it widely used in the aerospace industry. Its semi-finished forms include bars, forgings, sheets, profiles, and wires, among others.

Effect of Different Microstructures and Textures on Mechanical Properties

Different microstructures and textures lead to different mechanical properties:

• High-density textures usually correspond to higher strength,

• While low-density textures result in lower strength.

Through appropriate deformation processes, it is possible to generate favorable textures in the material that support further forming. For sheet materials, when the texture orientation is perpendicular or nearly perpendicular to the sheet surface, the thickness direction is strengthened, and mechanical properties become more uniform in the longitudinal and transverse directions. Although the overall strength may be slightly lower, the sheet exhibits excellent cold workability and stamping formability.

Literature studies have shown that different rolling processes can produce various textures and microstructures, each corresponding to different performance characteristics.

Influence of Microstructure on Mechanical Performance

Ti6Al4V alloy is primarily used in the annealed condition, but can also be used in the quenched and aged condition. The influence of microstructure on performance varies significantly depending on the heat treatment state.

• In the annealed condition:

For both smooth and notched fatigue, the lamellar (plate-like) microstructure generally exhibits a higher fatigue limit than the bimodal microstructure.

However, under high stress conditions, the bimodal microstructure performs better in fatigue than the lamellar structure.

• In the quenched and aged condition:

The fatigue performance follows this descending order:

Bimodal > Fine equiaxed > Fine lamellar > Coarse equiaxed > Coarse lamellar.

In addition, when there are significant differences in macroscopic grain size, even if the microstructural type remains the same, the fatigue performance of the alloy can vary greatly. Therefore, controlling the microstructure and grain size of forged components is critically important for optimizing performance.