Material
Titanium Ti
The highest strength-to-weight ratio of any structural metal, with outstanding corrosion resistance, bought at roughly five to ten times the price of aerospace aluminum and a supply chain that begins outside the United States.
Overview
Titanium is the structural metal of choice where weight, temperature and corrosion all matter at once: airframe bulkheads, engine fan and compressor sections, landing gear, submarine and seawater hardware, and armor. It is difficult and expensive to produce because it is highly reactive, which forces every melting, welding and heat treating operation into vacuum or inert atmosphere, and it work hardens and holds heat at the cutting edge, which makes it slow and tool-hungry to machine. The metal itself is abundant in the earth's crust; what is scarce is the industrial capacity to reduce it to metal and the qualified capacity to convert it into flight hardware.
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Defense applications
Airframe structure
Bulkheads, wing carry-through, engine mounts and fasteners where aluminum lacks the strength or temperature capability and steel is too heavy. Also used wherever structure contacts carbon fiber composite, because titanium is galvanically compatible with carbon and aluminum is not.
F-35 bulkheads, F-22 structure, Composite-to-metal joints, Landing gear back-up structure
Gas turbine engines
Fan blades, fan discs, compressor blades, discs and casings through the temperature range up to roughly 600 degrees Celsius, above which nickel superalloys take over.
Fan blades and discs, Compressor rotors, Intermediate casings, Blisks
Landing gear and high-strength forgings
Beta alloys such as Ti-10V-2Fe-3Al and Ti-5Al-5V-5Mo-3Cr reach strengths that let titanium displace high-strength steel in landing gear, saving weight and eliminating the cadmium plating and hydrogen embrittlement problems that come with 300M steel.
Landing gear beams, Trunnions, Actuator bodies, Flap tracks
Armor and ordnance
Titanium armor plate offers substantially better mass efficiency than rolled homogeneous armor steel against several threat classes, and is used where weight is the binding constraint on a vehicle or aircraft.
Aircraft crew protection, Vehicle applique armor, Hatch and structural armor
Naval and seawater service
Titanium is effectively immune to seawater corrosion, including crevice and pitting attack, which makes it the material of record for seawater piping, heat exchangers, pumps and submarine hardware where the alternative is a lifetime of copper-nickel maintenance.
Seawater piping and valves, Heat exchangers and condensers, Submarine ball valves, Sonar domes
Missiles and space
Pressure vessels, propellant tanks, motor cases and structure where the strength-to-weight ratio directly buys range or payload.
Composite overwrapped pressure vessel liners, Propellant tanks, Motor cases, Interstage structure
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| Grade 1 (CP Titanium) | 1 | Commercially pure (alpha) | The softest and most formable commercially pure grade. Chosen where deep drawing or severe forming matters more than strength. | Sheet, Plate, Strip and Foil, Tube and Pipe |
| Grade 2 (CP Titanium) | 2 | Commercially pure (alpha) | The workhorse commercially pure grade and the most widely used non-aerospace titanium. Balances moderate strength with good formability and weldability. | Plate, Sheet, Bar and Rod, Tube and Pipe, Wire |
| Grade 4 (CP Titanium) | 4 | Commercially pure (alpha) | The highest strength commercially pure grade, trading formability for strength while keeping the corrosion behavior of unalloyed titanium. | Plate, Sheet, Bar and Rod |
| Ti-6Al-4V (Grade 5) | 5 | Alpha-beta | The dominant titanium alloy by a wide margin, commonly cited as roughly half of all titanium consumed. If a drawing says titanium without qualification, this is usually what it means. | Plate, Sheet, Bar and Rod, Billet, Forgings, Castings, Powder, Wire, Extrusions |
| Ti-6Al-4V ELI (Grade 23) | 23 | Alpha-beta | Extra low interstitial version of Ti-6Al-4V. Reduced oxygen and iron buy fracture toughness and cryogenic ductility at the cost of some strength. | Plate, Sheet, Bar and Rod, Forgings, Powder |
| Ti-3Al-2.5V (Grade 9) | 9 | Alpha-beta | Often called half 6-4. Stronger than commercially pure titanium but still cold formable, which makes it the standard aerospace hydraulic tubing alloy. | Tube and Pipe, Sheet, Bar and Rod, Wire |
| Ti-0.15Pd (Grade 7) | 7 | Commercially pure with palladium | Commercially pure titanium with a small palladium addition that dramatically improves resistance to reducing acids and crevice corrosion. | Plate, Sheet, Tube and Pipe, Bar and Rod |
| Ti-6Al-2Sn-4Zr-2Mo | Near-alpha | Near-alpha alloy developed for elevated temperature service, holding useful creep strength above where Ti-6Al-4V falls off. | Forgings, Billet, Bar and Rod, Sheet | |
| Ti-10V-2Fe-3Al | Beta | High strength beta alloy developed for large forged structure, notably landing gear, where it competes directly with high-strength steel. | Forgings, Billet, Bar and Rod | |
| Ti-5Al-5V-5Mo-3Cr | Beta | High strength deep-hardenable beta alloy used for heavy landing gear and structural forgings, offering better section-size capability than Ti-10-2-3. | Forgings, Billet, Bar and Rod | |
| Ti-6Al-6V-2Sn | Alpha-beta | Higher strength than Ti-6Al-4V with reduced weldability and toughness. Long associated with ordnance and armor applications. | Plate, Forgings, Bar and Rod | |
| Ti-3Al-8V-6Cr-4Mo-4Zr (Beta C) | 19 | Beta | Highly cold formable in the solution treated condition and age hardenable to very high strength, which suits springs and fasteners. | Wire, Bar and Rod, Tube and Pipe |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 4.51 | g/cm3 | 0.163 lb/in3; roughly 60 percent the density of steel |
| Melting point | 1668 | degrees C | 3034 degrees F |
| Elastic modulus | 103 to 116 | GPa | About half that of steel, which matters for stiffness-driven design |
| Tensile strength, Grade 2 CP | 345 minimum | MPa | 50 ksi minimum per ASTM B265 |
| Tensile strength, Ti-6Al-4V annealed | 895 minimum | MPa | 130 ksi minimum; typical values run higher |
| Yield strength, Ti-6Al-4V annealed | 828 minimum | MPa | 120 ksi minimum |
| Beta transus, Ti-6Al-4V | approximately 995 | degrees C | 1820 degrees F; the boundary that separates alpha-beta from beta processing |
| Thermal conductivity | 6.7 to 22 | W/m-K | Alloys sit near the bottom of this range, which is why heat concentrates at the cutting edge during machining |
| Coefficient of thermal expansion | 8.6 | micrometre/m-K | Low; close to carbon fiber composite, which is part of why the two are paired |
| Maximum service temperature | approximately 600 | degrees C | Near-alpha alloys such as Ti-6242 reach the top of this range; above it, nickel superalloys take over |
| Galvanic behavior | Noble | Compatible with carbon fiber composite; drives aluminum away from composite joints |
Advantages
- Highest strength-to-weight ratio of any structural metal in production use
- Effectively immune to seawater corrosion, including crevice and pitting attack
- Galvanically compatible with carbon fiber composite, unlike aluminum
- Retains useful strength to roughly 600 degrees Celsius
- Non-magnetic, which matters for mine countermeasures and sensor structure
- Biocompatible, which sustains a medical market that shares the same mills
Limitations
- Roughly five to ten times the cost of aerospace aluminum per pound, before machining
- Reactive at temperature, so melting, welding and heat treating all require vacuum or inert atmosphere
- Poor thermal conductivity concentrates heat at the tool tip, making machining slow and consumable-intensive
- Elastic modulus is about half that of steel, so stiffness-driven parts do not benefit from substitution
- Galls and seizes against itself and against many other metals, requiring coatings or dissimilar mating materials
- Poor unlubricated wear resistance
- Buy-to-fly ratios on machined-from-solid parts are often severe, which is why forging and near-net processes dominate
- No primary sponge production in the United States, so virgin feedstock is entirely imported
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| AMS 4911 | SAE International | Ti-6Al-4V sheet, strip and plate, annealed |
| AMS 4928 | SAE International | Ti-6Al-4V bar, wire, forgings and rings, annealed |
| AMS 4907 | SAE International | Ti-6Al-4V ELI sheet, strip and plate |
| AMS 2801 | SAE International | Heat treatment of titanium alloy parts |
| AMS 2750 | SAE International | Pyrometry. Governs furnace calibration and instrumentation for all heat treatment; the document a heat treater must conform to |
| ASTM B265 | ASTM International | Titanium and titanium alloy strip, sheet and plate |
| ASTM B348 | ASTM International | Titanium and titanium alloy bars and billets |
| ASTM B338 | ASTM International | Seamless and welded titanium tube for condensers and heat exchangers |
| ASTM B381 | ASTM International | Titanium and titanium alloy forgings |
| ASTM B367 | ASTM International | Titanium and titanium alloy castings |
| ASTM B863 | ASTM International | Titanium and titanium alloy wire |
| ASTM F136 | ASTM International | Ti-6Al-4V ELI wrought material for surgical implants |
| ASTM F2924 | ASTM International | Additive manufactured Ti-6Al-4V by powder bed fusion |
| ASTM F3001 | ASTM International | Additive manufactured Ti-6Al-4V ELI by powder bed fusion |
| MIL-DTL-46077 | U.S. Department of Defense | Titanium alloy armor plate |
| DFARS 252.225-7009 | U.S. Department of Defense | Restriction on acquisition of certain articles containing specialty metals. The clause that makes melt source contractual |