Overview of Titanium Alloy Materials for Aerospace Fasteners

Fasteners, as an important category of universal basic components, play a vital role in industry and are often referred to as the “rice of industry.”

Fasteners can be classified into 13 major categories: bolts, screws, studs, nuts, wood screws, self-tapping screws, washers, rivets, pins, retaining rings, coupling sets, and fastener assemblies. According to their application fields, fasteners can be divided into general-purpose fasteners and aerospace fasteners. In the aviation industry, mechanical joining remains the primary method of aircraft assembly, relying heavily on a wide variety of fasteners. In the aerospace sector, fasteners are also essential for connecting different sections of spacecraft.

With the ongoing trend of lightweight design in equipment, titanium alloy materials are increasingly favored for aerospace fasteners. Internationally, the use of titanium alloy fasteners dates back to the 1950s when the United States first used Ti-6Al-4V alloy bolts on the B-52 bomber, achieving significant weight reduction. This marked the beginning of titanium alloy fastener applications in the aerospace field. Today, in developed countries such as the United States and France, over 95% of titanium alloy fasteners are made from the internationally recognized Ti-6Al-4V alloy. In some advanced aircraft models, titanium alloy fasteners have completely replaced 30CrMnSiA steel.

For example, after titanium alloy fasteners were applied to the U.S. C-5A military transport aircraft, the overall weight was reduced by about 4,500 kg. In the Boeing 747 commercial aircraft, replacing steel fasteners with titanium reduced weight by approximately 1,814 kg. In Russia, titanium alloy fasteners and alloy systems have been used in aircraft models such as the Il-76, Il-86, Il-96, Tu-204, An-72, and An-124, significantly reducing aircraft weight. The Il-76 alone uses around 142,000 titanium alloy fasteners, resulting in a weight reduction of 600 kg.

China’s development of titanium alloy fasteners dates back to 1965. In the 1970s, relevant organizations carried out research on titanium alloy rivets and their applications. In the 1980s, a small number of titanium alloy rivets and bolts began to be used on some second-generation military aircraft. By the late 1990s, with the introduction of production lines for international third-generation heavy fighter jets and the development of domestic third-generation fighters, China began using some titanium alloy fasteners. In recent years, driven by the rapid advancement of China’s aerospace industry, many organizations have launched research on titanium alloy materials for fasteners and fastener manufacturing technologies. Titanium alloy fasteners have seen widespread use in aerospace applications and are also extensively used in civil aircraft. According to available data, each domestically produced C919 aircraft requires about 200,000 titanium alloy fasteners. Based on the planned annual production of 150 large aircraft in 2018, this would require 30 million titanium alloy fasteners per year.

1. Advantages of Titanium Alloy Fasteners

The table below compares the properties of titanium alloys and steel materials used for fasteners. Titanium alloy fasteners offer the following advantages:

1. Low Density: Titanium alloys have significantly lower density than steel materials, making titanium alloy fasteners lighter in weight than their steel counterparts.

2. High Specific Strength: Titanium alloys have a high specific strength (strength-to-weight ratio) compared to other common metals. This allows titanium alloys to replace even lightweight materials like aluminum alloys. For the same applied load, titanium components can be designed with smaller geometric dimensions, saving space—an especially critical factor in aerospace applications.

3. High Melting Point: Titanium alloys have a much higher melting point than steel, providing better heat resistance for titanium alloy fasteners.

4. Low Coefficient of Thermal Expansion and Low Elastic Modulus: This contributes to better dimensional stability and performance under thermal stress.

5. Non-Magnetic: Titanium alloys have extremely low magnetic permeability, making titanium fasteners essentially non-magnetic. This effectively prevents electromagnetic interference. While austenitic stainless steels are also non-magnetic, their magnetism can increase after cold working. In contrast, titanium alloys maintain their non-magnetic properties regardless of hot or cold processing, making them suitable for avionics applications.

6. High Yield-to-Tensile Strength Ratio: For fasteners under tensile load, the critical design criterion is the yield strength, followed by tensile strength. Once a fastener yields, it loses its fastening function. Compared to steel, titanium alloys have a yield strength close to their tensile strength, resulting in a high yield ratio and increased safety in fastener applications.

7. Electrode Potential Compatibility with Carbon Fiber Composites: One major reason titanium alloys are widely used in fasteners is that their electrode potential is well matched with that of carbon fiber composites, which helps prevent galvanic corrosion.

8. Additional Advantages: Titanium alloys also exhibit excellent corrosion resistance and high creep resistance, making them suitable for demanding applications.

Comparison of Material Properties for Fastener Applications

2. Overview of Titanium Alloy Materials and Their Properties for Fastener Applications

Titanium alloys used in fasteners are closely related to both manufacturing processes and intended applications. On one hand, the manufacturing process of titanium alloy fasteners generally consists of three main stages:

1. Plastic deformation – such as upsetting, necking, and thread rolling;

2. Surface strengthening – including the reinforcement of load-bearing transition zones between the bolt head and shank;

3. Machining – including turning, milling, grinding, etc.

On the other hand, different applications demand different mechanical properties from the fastener materials, which leads to the selection of different titanium alloys. For example, rivets require good plasticity, as one or both ends are deformed during installation (heading). In contrast, bolts require high strength, with performance comparable to high-strength steels such as 30CrMnSiA, which is why high-strength titanium alloys are typically used.

Considering both manufacturing and application requirements, titanium alloys used in fasteners are mainly divided into three categories: Commercially Pure Titanium, (α+β) titanium alloys, and β titanium alloys, as shown in Table 2.

From Table 2, it can be seen that Commercially Pure Titanium includes Grade 1 and Grade 2.

(α+β) titanium alloys mainly include Ti-6Al-4V, Ti-6Al-1.5Cr-2.5Mo-0.5Fe-0.3Si, and Ti-662, among others.

β titanium alloys are primarily metastable β alloys, as they typically have a molybdenum equivalent (Mo_eq) around 10%. Alloys with Mo_eq lower than 10% (near-β alloys) exhibit insufficient strengthening through heat treatment, while alloys with Mo_eq above 10% (stable β alloys) retain excessive β phase stability, which makes decomposition during aging difficult. Therefore, metastable β titanium alloys provide the most effective strengthening.

In addition, metastable β titanium alloys offer excellent cold formability, enabling cold heading without the need for specialized heating equipment or gas shielding – resulting in high production efficiency, good material utilization, tight dimensional tolerance, and superior surface quality.

Conversely, (α+β) titanium alloy fasteners must be hot-headed, which requires dedicated heating systems and inert gas shielding, typically resulting in lower efficiency and utilization, and a higher likelihood of uneven heating.

Table 2: Titanium Alloy Grades Used for Fasteners

Table 3: Mechanical Properties of Titanium Alloys for Rivets

Note:

Rm = Tensile strength; A = Elongation; Ψ = Reduction of area; τ = Shear strength

Table 4: Mechanical Properties of Titanium Alloys for Bolts (Solution + Aging Condition)

Note:

Rm = Tensile strength; Rp0.2 = Yield strength; A = Elongation; Ψ = Reduction of area; τ = Shear strength

Here is the English translation of the section:

3. Key Titanium Alloys for Fasteners

Ti-6Al-4V Titanium Alloy

Ti-6Al-4V is a medium-strength, dual-phase (α+β) titanium alloy and is the most widely studied and applied titanium alloy. The majority of titanium alloy fasteners are made from Ti-6Al-4V. Fasteners made from Ti-6Al-4V can only be produced using hot heading and require specialized hot forging and heating equipment, which not only reduces production efficiency but also results in low material utilization. For high-strength fasteners, the strength of Ti-6Al-4V fasteners is insufficient. After solution treatment and aging, the alloy’s ultimate tensile strength reaches up to 1100 MPa and shear strength is around 650 MPa. Due to its poor hardenability, the cross-sectional size of Ti-6Al-4V fasteners during solution-aging is generally below 19 mm. Ti-6Al-4V fasteners include bolts, high-lock bolts, blind rivets, screws, and lock-bolt rivets, and have been widely used in domestic aircraft, engines, airborne equipment, spacecraft, and satellites.

Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si Titanium Alloy

This is a martensitic (α+β) titanium alloy with excellent comprehensive properties. It is typically used in the annealed condition, but can also be strengthened by heat treatment, and exhibits good oxidation resistance.

Ti-3Al-5Mo-4.5V Titanium Alloy

Ti-3Al-5Mo-4.5V is a typical solution-aging strengthened dual-phase titanium alloy. After solution treatment, it has high room temperature ductility, making it suitable for cold heading, with a heading ratio of up to 1:4. In fastener manufacturing, this alloy can be formed by both cold and hot heading. Fasteners currently made from this alloy include bolts, screws, and self-locking nuts.

Ti-3Al-8Cr-5Mo-5V Titanium Alloy

This is a metastable β titanium alloy. In the solution-treated condition, it offers excellent cold formability and weldability. It is mainly used in satellite corrugated shells, satellite-launch vehicle connection bands, cold-headed rivets, and bolts. In particular, rivets made from this alloy have seen wide application in critical aerospace components.

Ti-10Mo-8V-1Fe-3.5Al Titanium Alloy

This is a heat-treatable, metastable β titanium alloy. Its main advantage is its excellent cold formability in the solution-treated state, with a cold heading ratio of up to 2.8. After aging, it can achieve high strength and is primarily used to manufacture aerospace fasteners with tensile strengths around 1100 MPa.

Ti-15V-3Cr-3Sn-3Al Titanium Alloy

A metastable β titanium alloy with outstanding cold formability, comparable to commercially pure titanium. After solution treatment, it can be cold-formed into various fasteners. After aging, it reaches room temperature tensile strengths of up to 1000 MPa. Boeing has applied this alloy in its aircraft, and China has used it to manufacture cold-headed rivets for use with fighter jet canopy beams and satellite panels.

Ti-3Al-2.7Nb-15Mo Titanium Alloy

This is a metastable β21S titanium alloy. It offers excellent hot and cold workability, good hardenability, and outstanding creep and corrosion resistance. Thanks to its use of high-melting-point β-stabilizing elements Mo and Nb with low self-diffusion coefficients, this alloy exhibits superior oxidation resistance—up to 100 times better than Ti-15-3. Currently, Ti-3Al-2.7Nb-15Mo high-strength bolts are widely used in key Chinese aerospace programs.

Oxidation Resistance Comparison: Ti-3Al-2.7Nb-15Mo vs Ti-15V-3Al-3Cr-3Sn

Ti-45Nb Alloy

Ti-45Nb is a stable β titanium alloy specifically developed for rivets. Initially, commercially pure titanium was used for rivets, but its strength was too low to meet the requirements of high-load applications. A new titanium alloy with plasticity similar to pure titanium but higher strength was needed. However, typical metastable β alloys have high deformation resistance and low room-temperature ductility compared to pure titanium. Ti-45Nb was developed to address this—offering excellent room-temperature formability, with elongation up to 20% and reduction in area up to 60%. Compared to pure titanium, Ti-45Nb has higher tensile and shear strengths, reaching 450 MPa and 350 MPa respectively.

Here is the English translation of section 4:

4. Future Development Trends

Ultra-High Strength Titanium Alloy Fasteners

With the continuous advancement of China’s aerospace industry, the connection technologies used in new-generation aircraft and spacecraft have significantly improved, bringing higher requirements for new types of fasteners. One of the future development trends is the design and application of ultra-high strength titanium alloy fasteners with tensile strengths ranging from 1200 to 1500 MPa and shear strengths ≥750 MPa.

High-Temperature Resistant Titanium Alloy Fasteners

Currently, the service temperatures of titanium alloys used in fasteners are relatively low, as shown in Table 5. In the aerospace field, as aircraft and spacecraft operate at increasingly higher speeds, the materials used must withstand higher service temperatures. Therefore, the development of high-temperature resistant titanium alloy fasteners is also a key future trend—especially in aerospace applications, where new high-temperature titanium alloys are expected to withstand short-term service temperatures of 600–800 °C.

Ti₂AlNb alloy is often used as a replacement for heavier high-temperature alloys, although it tends to experience significant deformation. While Ti₂AlNb offers benefits compared to other titanium alloys, it remains relatively heavy and does not fully meet lightweight requirements. Additionally, Ti-Al intermetallics generally have poor workability and immature processing technologies. As a result, future high-temperature titanium alloys for fasteners will continue to focus on near-α and high-aluminum content (α+β) titanium alloys.

At high temperatures, the strength and creep resistance of titanium alloys primarily depend on solid solution strengthening from elements such as Al, Sn, and Zr. However, due to aluminum equivalence limitations, the content of these elements cannot be increased indefinitely. Therefore, within the controlled ranges of Al, Sn, and Zr, additional strengthening can be achieved through multi-element alloying.

Among β-stabilizing elements, Mo (molybdenum) enhances high-temperature strength and creep resistance through solid solution strengthening. Nb, Cr, and V provide similar effects. Adding small amounts of β-stabilizers also helps prevent alloy embrittlement.

Silicon (Si) content in titanium alloys is also critical. When approximately 0.2 wt% of Si is added, ellipsoidal silicides precipitate non-uniformly and discontinuously along α-phase boundaries. These silicides effectively impede dislocation movement, resulting in dispersion strengthening and significantly improved creep resistance. However, the presence of silicides can adversely affect thermal stability, reducing ductility and increasing the degree of ordering in the alloy, which promotes the formation of the Ti₃Al phase. Therefore, the Si content should be kept low—generally not exceeding 0.5 wt%.

In summary, multi-element composite strengthening remains a key direction for designing new high-temperature titanium alloys for fasteners.

Table 6: Typical Service Temperatures of Titanium Alloys for Fasteners