In 1948, the American company DuPont began mass production of spongy titanium using the magnesium process, signaling the beginning of industrialized titanium production.
Titanium alloys, with their high specific strength, excellent corrosion resistance and superior heat resistance, are now widely used in various industries.
Titanium alloys have been used in the aviation industry for more than half a century; In the consumer electronics sector, brands such as Huawei, Apple, Xiaomi and Honor have incorporated this material into many of their smartphone models, and an increasing number of electronics manufacturers are expected to adopt titanium alloys. But what makes titanium alloys so universally favored?
Characteristics of titanium
1. High specific resistance:
1.3 times greater than aluminum alloys, 1.6 times greater than magnesium alloys and 3.5 times greater than stainless steel, making it the champion among metallic materials.
2. High thermal resistance:
It can operate long-term at temperatures hundreds of degrees higher than aluminum alloys, specifically between 450-500°C.
3. Excellent corrosion resistance:
It resists acids, alkalis and atmospheric corrosion well, and has particularly strong resistance to pitting and stress corrosion cracking.
4. Good Low Temperature Performance:
Certain titanium alloys, such as interstitially low TA7, retain some plasticity even at -253°C.
5. High chemical reactivity:
At high temperatures, titanium is highly reactive and combines easily with gases such as hydrogen and oxygen in the air, creating a hardened layer.
6. Low thermal conductivity and modulus of elasticity:
Its thermal conductivity is about one-fourth that of nickel, one-fifth that of iron, and one-fourteenth that of aluminum. The thermal conductivity of various titanium alloys is about 50% lower than that of pure titanium. The modulus of elasticity of titanium alloys is about half the modulus of elasticity of steel.
Classifications and applications of titanium alloys
Titanium alloys can be categorized into: heat-resistant alloys, high-strength alloys, corrosion-resistant alloys (such as titanium-molybdenum, titanium-palladium), low-temperature alloys, and special-purpose alloys (such as titanium-iron hydrogen materials storage, titanium-nickel shape memory alloys).
Despite the relatively short history of its application, its excellent properties have earned titanium and its alloys several prestigious titles, the first of which is “the metal of space”.
Its light weight, high specific strength and high temperature resistance make it particularly suitable for manufacturing aircraft and various spacecraft.
Approximately three-quarters of the world's production of titanium and its alloys is used in the aerospace industry, with many components originally made from aluminum alloys now being replaced by titanium alloys.
Aerospace Applications of Titanium Alloys
Titanium alloys are primarily used in the manufacture of aircraft and engine components such as forged titanium fan blades, compressor discs and blades, engine covers, exhaust systems, and structural structures such as aircraft spar bulkheads.
Spacecraft take advantage of the high specific strength, corrosion resistance and low-temperature performance of titanium alloys to manufacture various pressure vessels, fuel tanks, fasteners, instrument straps, structures and rocket casings.
Artificial satellites, lunar modules, manned spacecraft and space shuttles also use welded components made from titanium alloy sheets.
In 1950, the United States first used titanium alloys in the F-84 fighter-bomber for non-structural components such as heat shields for the rear fuselage, wind deflectors and tail covers.
Starting in the 1960s, titanium alloy applications moved from the rear to the mid-fuselage, partially replacing structural steel in the manufacture of frames, girders, and flap rails as critical load-bearing components.
From the 1970s onwards, civil aircraft began to use titanium alloys extensively, with the Boeing 747 jet incorporating over 3,640 kg of titanium, representing 28% of the aircraft's weight.
With the advancement of processing techniques, a considerable amount of titanium alloy has also been used in rockets, satellites and space shuttles. The more advanced the aircraft, the greater the use of titanium.
The American F-14A fighter jet uses titanium alloys that make up about 25% of its weight; the F-15A has 25.8%; fourth generation fighters use up to 41% titanium, with the F119 engine alone accounting for 39% of titanium usage, the highest of any aircraft to date.
Titanium alloys are widely used in aviation for good reason.
Why should air transport aircraft use titanium alloys? Modern aircraft can reach speeds of up to 2.7 times the speed of sound. At such high supersonic speeds, air friction generates a significant amount of heat.
When the speed of flight exceeds twice the speed of sound, aluminum alloys can no longer withstand the conditions, necessitating the use of high temperature resistant titanium alloys.
As the thrust-to-weight ratio of aviation engines increased from 4-6 to 8-10, and the compressor outlet temperature increased from 200-300°C to 500-600°C, the low-pressure compressor discs and blades , previously made of aluminum, had to be replaced by titanium alloys.
Recent advances in the study of titanium alloy properties have led to significant progress.
Traditional titanium alloys composed of titanium, aluminum and vanadium, which had a maximum working temperature of 550°C to 600°C, have been replaced by newly developed titanium aluminide (TiAl) alloys, with maximum working temperatures reaching up to 1040°C.
Replacing stainless steel with titanium alloys to manufacture high-pressure compressor discs and blades can reduce structural weight. A 10% reduction in aircraft weight can result in fuel savings of 4%. For rockets, a 1kg weight reduction can increase range by 15km.
The 3C applications of titanium alloys
In the highly competitive consumer electronics industry represented by cell phones, major manufacturers are interested in using titanium alloys to increase product appreciation.
Brands such as Huawei, Apple, Xiaomi and Honor have already incorporated this material into several products. Apple has equipped its Ultra series watches with titanium cases as standard, and its latest iPhone 15 includes a Pro model with an all-new titanium body, marking Apple's first phone to adopt aviation-grade titanium.
In 2022, Huawei used titanium alloy in the structural components of its foldable screen phone, the MateXs2, and incorporated a titanium frame in the Watch4Pro.
On October 12, Honor launched its new foldable smartphone, the Honor MagicVs2, featuring innovative materials like the Luban titanium hinge. In Xiaomi's new line, the most expensive model is the 14 Pro titanium version.
It is reported that Samsung will use a titanium alloy frame for its upcoming Galaxy S24 Ultra, similar to the original titanium color scheme of the iPhone 15 Pro.
Overall, the combination of high specific strength and lightweight properties is one of the main reasons why titanium alloys are widely promoted, allowing consumer electronics to be more portable and offering a more comfortable user experience.
Analysis of machining characteristics of titanium alloy
Firstly, titanium alloys have low thermal conductivity, just a quarter of that of steel, a thirteenth of that of aluminum and a twenty-fifth of that of copper. Slow heat dissipation in the cutting area does not lead to thermal equilibrium.
During the machining process, poor heat dissipation and cooling effects can lead to high temperatures, significant deformation and springback in machined parts, resulting in increased cutting tool torque and rapid tool wear, which reduces the tool durability.
Secondly, the low thermal conductivity of titanium alloys causes cutting heat to accumulate in a small area near the cutting tool, which is difficult to dissipate. This increases friction on the rake face, making chip evacuation difficult and accelerating tool wear.
Finally, the high chemical reactivity of titanium alloys means that they tend to react with tool materials at high temperatures during machining, leading to welding and diffusion, which can cause sticking, burning and even tool breakage.
Machining centers in titanium alloy processing
Machining centers can process multiple parts simultaneously, increasing production efficiency. They improve machining precision, ensuring good consistency in products.
These centers feature tool compensation capabilities that can achieve the inherent accuracy of the machine itself. With wide adaptability and considerable flexibility, machining centers are capable of multifunctional operations.
Tasks such as arc machining, chamfering and rounding transitions on parts are possible. They allow milling, drilling, reaming and threading operations.
Accurate cost calculations and production schedule control are also facilitated. Eliminating the need for specialized accessories saves substantial costs and shortens the production cycle, while significantly reducing the labor intensity of workers. Multi-axis machining with software like UG is also possible.
Tool and coolant material selection
- Tool material requirements
The tool material must have a significantly higher hardness than titanium alloys.
It must possess sufficient strength and toughness to withstand the large torques and cutting forces experienced when machining titanium alloys.
High wear resistance is critical because titanium alloys are tough and require sharp cutting edges to minimize work hardening. This is the most important parameter when selecting tools for machining titanium alloys.
The tool material must have little affinity with titanium alloys to avoid alloy formation by dissolution and diffusion, which can lead to tool sticking and burnout. Tests on domestic and foreign tool materials show that tools with high cobalt content exhibit optimal performance.
Cobalt improves secondary hardening, improves red hardness and hardness after heat treatment, while providing high toughness, wear resistance and good heat dissipation.
- Milling cutter geometric parameters
The unique machining characteristics of titanium alloys mean that the geometric parameters of the tools differ significantly from those of standard tools. A smaller helix angle β is chosen to facilitate chip removal and dissipate heat faster, which also reduces cutting resistance during machining.
The positive rake angle γ ensures a sharp cutting edge for light, fast cutting, avoiding excessive cutting heat and subsequent work hardening. A smaller clearance angle α slows tool wear and improves heat dissipation and tool durability.
- Selection of cutting parameters
Machining titanium alloys requires lower cutting speeds, appropriately large feed rates, reasonable depths of cut and finishing tolerances, with ample cooling. The cutting speed vc=30–50m/min is ideal, with higher feed rates for rough machining and moderate feed rates for finishing and semi-finishing.
The cutting depth ap=1/3d is suitable; Large depths may cause tool sticking, burning or breakage due to the good affinity and difficult chip removal of titanium alloys.
An adequate finishing margin is required, since the surface hardening layer in titanium alloys is about 0.1–0.15 mm; too small a tolerance may result in tool wear due to cutting into the hardened layer, but the tolerance should not be excessively large to avoid this problem.
- Refrigerator
It is best to avoid chlorine-containing coolants when machining titanium alloys to avoid toxic substances and hydrogen embrittlement, as well as to protect against high-temperature stress corrosion cracking.
Water-soluble synthetic emulsions are preferred, or a custom soda blend can be used. During cutting operations, ensure that the coolant is ample, with rapid circulation, high flow rate and pressure.
Machining centers come equipped with dedicated cooling nozzles that, when properly adjusted, can achieve the desired effect.
