In 1948, the American company DuPont began mass production of sponge titanium using the magnesium process, signaling the dawn of industrialized titanium production.
Titanium alloys, with their high specific strength, excellent corrosion resistance, and superior heat resistance, are now widely used across various sectors.
Titanium alloys have been utilized in the aviation industry for over half a century; in the consumer electronics sector, brands like 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?
1. High specific strength:
1.3 times that of aluminum alloys, 1.6 times that of magnesium alloys, and 3.5 times that of stainless steel, making it the champion among metal materials.
2. High thermal strength:
It can operate long-term at temperatures several hundred degrees higher than aluminum alloys, specifically between 450-500°C.
3. Excellent corrosion resistance:
It stands up well to acids, alkalis, and atmospheric corrosion, and has particularly strong resistance to pitting and stress corrosion.
4. Good low-temperature performance:
Certain titanium alloys, like the interstitially low TA7, retain some plasticity even at -253°C.
5. High chemical reactivity:
At high temperatures, titanium is highly reactive and easily combines with gases such as hydrogen and oxygen in the air, creating a hardened layer.
6. Low thermal conductivity and elastic modulus:
Its thermal conductivity is about a quarter that of nickel, a fifth that of iron, and a fourteenth that of aluminum. The thermal conductivity of various titanium alloys is about 50% lower than that of pure titanium. The elastic modulus of titanium alloys is about half that 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 storage materials, titanium-nickel shape memory alloys).
Despite the relatively short history of their application, their outstanding 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 suited for the manufacture of aircraft and various spacecraft.
Approximately three-quarters of the world’s production of titanium and its alloys are used in the aerospace industry, with many components originally made of aluminum alloys now being replaced with 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 disks and blades, engine covers, exhaust systems, and structural frames like aircraft spar partitions.
Spacecraft take advantage of titanium alloys’ high specific strength, corrosion resistance, and low-temperature performance to manufacture various pressure vessels, fuel tanks, fasteners, instrument straps, frames, 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-load-bearing components such as heat shields for the rear fuselage, wind deflectors, and tail covers.
Starting in the 1960s, titanium alloy applications shifted from the rear to the middle fuselage, partially replacing structural steel for manufacturing frames, beams, and flap tracks as critical load-bearing components.
From the 1970s onward, civil aircraft began using titanium alloys extensively, with the Boeing 747 jetliner incorporating over 3,640 kilograms of titanium, accounting for 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 making up about 25% of its weight; the F-15A has 25.8%; the 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 extensively used in aviation for good reason.
Why must aviation 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, friction with the air generates a significant amount of heat.
When the flight speed 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 has increased from 4-6 to 8-10, and the compressor exit temperature has risen from 200-300°C to 500-600°C, the low-pressure compressor disks and blades, formerly made of aluminum, have had to be replaced with 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 superseded 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 disks and blades can reduce structural weight. A 10% reduction in aircraft weight can result in a 4% saving in fuel. For rockets, a reduction of 1kg in weight can extend the range by 15km.
The 3C Applications of Titanium Alloys
In the highly competitive consumer electronics industry, represented by mobile phones, leading manufacturers are keen on using titanium alloys to enhance product premiumization.
Brands like Huawei, Apple, Xiaomi, and Honor have already incorporated this material into various products. Apple has fitted 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 the first Apple phone to adopt aviation-grade titanium.
In 2022, Huawei utilized titanium alloy in the structural components of its foldable screen phone, the MateXs2, and incorporated a titanium frame in the Watch4Pro.
On October 12th, Honor released its new flagship foldable smartphone, the Honor MagicVs2, featuring innovative materials like the Luban titanium hinge. In Xiaomi’s new lineup, the highest-priced 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 a key reason why titanium alloys are widely promoted, allowing consumer electronics to be more portable and offering a more comfortable user experience.
Analysis of Titanium Alloy Machining Characteristics
Firstly, titanium alloys have a low thermal conductivity, only one-quarter that of steel, one-thirteenth that of aluminum, and one-twenty-fifth that of copper. The slow heat dissipation in the cutting area is not conducive 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 tool durability.
Secondly, the low thermal conductivity of titanium alloys causes cutting heat to build up in a small area near the cutting tool, which is difficult to dissipate. This increases the friction on the rake face, makes chip evacuation difficult, and accelerates tool wear.
Finally, the high chemical reactivity of titanium alloys means they tend to react with tool materials at high temperatures during machining, leading to soldering and diffusion, which can cause tool sticking, tool burning, and even tool breakage.
Machining Centers in Titanium Alloy Processing
Machining centers can process multiple parts simultaneously, enhancing production efficiency. They improve the machining precision, ensuring good consistency in the products.
These centers feature tool compensation capabilities that can achieve the inherent precision of the machine itself. With a broad adaptability and considerable flexibility, machining centers are capable of multifunctional operations.
Tasks such as arc machining, chamfering, and rounding transitions on parts are all possible. They allow for milling, drilling, reaming, and tapping operations.
Precise cost calculations and production schedule control are also facilitated. The elimination of the need for specialized fixtures saves substantial costs and shortens the production cycle while significantly reducing labor intensity for workers. Multi-axis machining with software like UG is also achievable.
Tool and Coolant Material Selection
- Tool Material Requirements
The tool material must have a hardness significantly exceeding that of titanium alloys.
It should possess sufficient strength and toughness to withstand the large torque and cutting forces experienced during machining of 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 should have poor affinity with titanium alloys to prevent alloying through dissolution and diffusion, which can lead to tool sticking and burning. Tests on domestic and foreign tool materials show that high-cobalt tools perform ideally.
Cobalt enhances secondary hardening, improves red hardness, and the hardness after heat treatment, while also offering 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 for easier chip removal and faster heat dissipation, which also reduces cutting resistance during machining.
The positive rake angle γ ensures a sharp cutting edge for light and swift cutting, preventing excessive cutting heat and subsequent work hardening. A smaller clearance angle α slows down tool wear and improves heat dissipation and tool durability.
- Cutting Parameter Selection
Machining titanium alloys require lower cutting speeds, appropriately large feed rates, reasonable cutting depths, and finishing allowances, with ample cooling. The cutting speed vc=30–50m/min is optimal, with larger feed rates for rough machining and moderate feed rates for finishing and semi-finishing.
The cutting depth ap=1/3d is suitable; large depths can cause tool sticking, burning, or breakage due to the good affinity and difficult chip removal of titanium alloys.
An appropriate finishing allowance is necessary since the surface hardening layer on titanium alloys is about 0.1–0.15mm; too small an allowance may result in tool wear due to cutting in the hardened layer, but the allowance should not be excessively large to avoid this issue.
It is best to avoid chlorine-containing coolants when machining titanium alloys to prevent toxic substances and hydrogen embrittlement, as well as to protect against high-temperature stress corrosion cracking.
Synthetic water-soluble emulsions are preferred, or a custom coolant mix can be used. During cutting operations, ensure 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.