Why Titanium Alloys Are Popular in Aerospace and Smartphones

The Characteristics of Titanium

Titanium has several notable properties that make it a favored material in various high-tech applications, including aerospace and consumer electronics like smartphones. These properties include:

1. High Strength-to-Weight Ratio: Titanium alloys have a strength-to-weight ratio that is 1.3 times that of aluminum alloys, 1.6 times that of magnesium alloys, and 3.5 times that of stainless steel, making them the strongest among metal materials.
2. High Thermal Strength: Titanium alloys can operate at temperatures several hundred degrees higher than aluminum alloys, maintaining their strength between 450°C to 500°C.
3. Excellent Corrosion Resistance: Titanium alloys are highly resistant to corrosion from acids, alkalis, and atmospheric conditions, making them particularly effective against pitting and stress corrosion.
4. Good Low-Temperature Performance: Titanium alloys with very low interstitial elements, such as TA7, can maintain plasticity even at -253°C.
5. High Chemical Reactivity: At elevated temperatures, titanium alloys react easily with gases like hydrogen and oxygen, forming a hardened layer on the surface.
6. Low Thermal Conductivity and Elastic Modulus: Titanium's thermal conductivity is about 1/4 that of nickel, 1/5 that of iron, and 1/14 that of aluminum. The thermal conductivity of titanium alloys is reduced by approximately 50% compared to pure titanium. The elastic modulus of titanium alloys is about half that of steel.

Types and Uses of Titanium Alloys

Titanium alloys are classified based on their applications:

• Heat-Resistant Alloys
• High-Strength Alloys
• Corrosion-Resistant Alloys (e.g., titanium-molybdenum, titanium-palladium alloys)
• Low-Temperature Alloys
• Special Function Alloys (e.g., titanium-iron hydrogen storage materials, titanium-nickel memory alloys)

Despite being relatively new, titanium and its alloys have quickly gained several prestigious titles due to their exceptional performance. Known as "space metals," they are lightweight, strong, and heat-resistant, making them ideal for aircraft and spacecraft manufacturing. Approximately three-quarters of the world's titanium production is used in the aerospace industry, replacing many parts previously made from aluminum alloys.

Titanium Alloys in Aerospace Applications

Titanium alloys are primarily used in the manufacturing of aircraft and engine components, including forged titanium fans, compressor discs and blades, engine casings, exhaust systems, and structural framework like beams and frames. In spacecraft, titanium alloys are utilized for their high strength-to-weight ratio, corrosion resistance, and low-temperature performance, making them suitable for pressure vessels, fuel tanks, fasteners, instrument straps, structures, and rocket casings. These materials are also used in artificial satellites, lunar modules, manned spacecraft, and space shuttles.
The first significant use of titanium in aerospace was in the 1950s with the U.S. F-84 fighter-bomber. By the 1960s, titanium alloys were used more extensively, moving from non-structural components to structural ones, partially replacing structural steel. In the 1970s, commercial aircraft began incorporating large amounts of titanium, with the Boeing 747 using over 3,640 kilograms, accounting for 28% of the aircraft's weight. Advanced aircraft use even more titanium: the F-14A uses 25%, the F-15A uses 25.8%, and the fourth-generation U.S. fighter jets use up to 41%, with 39% used in the F119 engine.

Reasons for Extensive Use of Titanium in Aviation

Modern aircraft can fly at speeds over 2.7 times the speed of sound. Such supersonic speeds generate significant heat due to air friction, which aluminum alloys cannot withstand at speeds above 2.2 times the speed of sound. Thus, heat-resistant titanium alloys are essential. Additionally, as the thrust-to-weight ratio of jet engines increases, the temperature at the compressor outlet rises, necessitating the use of titanium alloys for components that were once made from aluminum.
Recent advancements in titanium alloy research have led to new alloys, such as titanium-aluminum (TiAl), with working temperatures up to 1040°C, significantly higher than earlier titanium-aluminum-vanadium alloys. Using titanium alloys instead of stainless steel for high-pressure compressor discs and blades reduces structural weight, improving fuel efficiency and range.

Titanium Alloys in Consumer Electronics

In the highly competitive consumer electronics market, leading brands aim to enhance their products' premium appeal by using titanium alloys. Companies like Huawei, Apple, Xiaomi, and Honor have incorporated titanium in various products:

• Apple: Introduced titanium alloy casings in the Ultra series watches and used titanium bodies in the iPhone 15 Pro, marking the first use of aerospace-grade titanium in an iPhone.
• Huawei: Used titanium in the structure of its foldable MateXs2 and the Watch4Pro's frame.
• Honor: Used titanium in the hinge of its MagicVs2 foldable phone.
• Xiaomi: The Xiaomi 14 Pro features a titanium version as its highest-priced model.
• Samsung: Is expected to use a titanium middle frame in the Galaxy S24 Ultra.

Titanium alloys provide high strength-to-weight ratios and lightweight benefits, enhancing consumer electronics' durability and user experience.

Machining Characteristics of Titanium Alloys

1. Thermal Conductivity: Titanium alloys have low thermal conductivity, about 1/4 that of steel, 1/13 that of aluminum, and 1/25 that of copper. This makes heat dissipation during cutting challenging, leading to high temperatures in the cutting zone and potential deformation of parts.
2. Chemical Reactivity: Titanium's high chemical reactivity at elevated temperatures causes it to react with cutting tool materials, leading to tool wear and damage.

Machining Centers for Titanium Alloys

Machining centers offer several advantages for processing titanium alloys, including simultaneous multi-part machining, improved precision, consistency, and flexibility. These centers can perform milling, drilling, boring, and tapping, reducing labor intensity and production costs while increasing efficiency and accuracy.

Tool and Coolant Selection

1. Tool Material: The tool material must be significantly harder than titanium alloys, with sufficient strength, toughness, and wear resistance to handle the high cutting forces and prevent chemical reactions at high temperatures. High-cobalt tools are ideal due to their secondary hardening effect, high red hardness, and toughness.

2. Geometric Parameters: Titanium's machining characteristics require specific tool geometry. Smaller helix angles and increased chip clearance improve chip removal and reduce cutting resistance. Sharp cutting edges and optimized rake and relief angles minimize cutting heat and tool wear.

3. Cutting Parameters: Lower cutting speeds, appropriate feed rates, and reasonable depth of cut are crucial. Adequate cooling is essential to prevent tool damage and ensure efficient machining.

4. Coolant: Non-chlorinated coolants are preferred to avoid toxic byproducts and hydrogen embrittlement. Sufficient coolant flow and pressure are necessary to maintain low temperatures and effective cooling during machining.

By understanding and utilizing these properties and techniques, titanium alloys can be effectively used in both aerospace and consumer electronics, providing significant performance benefits.