Ti-6Al-4V (Titanium)
Ti-6Al-4V (Titanium) is a titanium alloy used in aerospace applications. Melting point: 1,604 °C. Tensile strength: 950 MPa.
If you wanted a metal as strong as steel but only about 60% of the weight, you would be looking for Ti-6Al-4V. It is the most widely used titanium alloy in the world, and it shows up almost everywhere on a rocket where strength and low weight both matter.
Quick facts
- Also called: Titanium Grade 5, or simply “6-4.”
- What’s in it: mostly titanium, with about 6% aluminum and about 4% vanadium (plus trace iron and oxygen).
- Density: about 4.43 grams per cubic centimeter, roughly 60% of steel’s 7.85.
- Strength: ultimate tensile strength (the pull it can take before breaking) of about 900 to 1,170 megapascals (MPa).
- Melting point: roughly 1,604 to 1,660 degrees C.
- Temperature range: useful continuously up to about 300 to 350 degrees C, and it keeps its strength down to cryogenic cold near -253 degrees C.
- History: invented in the early 1950s by Stanley Abkowitz at the US Army’s Watertown Arsenal; commercial production began around 1954.
What it is and how it works
Pure titanium is light and resists corrosion, but it is relatively soft. Mixing in two other metals fixes that. Aluminum is an “alpha stabilizer,” meaning it locks in a phase (a particular arrangement of the metal’s atoms) that adds strength and keeps weight low. Vanadium is a “beta stabilizer,” which allows a second, more bendable phase to form. Because the alloy holds both phases at once, it is called an “alpha-beta” alloy.
The mix of those two phases can be tuned with heat treatment (carefully heating and cooling the metal). Annealed, it offers around 895 MPa of strength; heat-treated, it can reach about 1,170 MPa. That tuning is why one alloy can be both strong and tough.
The payoff is an outstanding strength-to-weight ratio: a part can be as strong as steel while weighing far less. On a rocket, where every kilogram of structure is a kilogram you cannot give to payload, that is decisive. A thin, self-healing layer of titanium oxide forms naturally on the surface, giving the metal near-immunity to rust and oxidation across a very wide temperature band.
Why it matters
For spaceflight, mass is everything. Lighter structure means more payload, more range, and easier reuse. Ti-6Al-4V hits a rare sweet spot: high strength, low weight, corrosion resistance, and good behavior from the cold of liquid-oxygen and liquid-hydrogen propellants up to several hundred degrees C. That heat resistance is what lets SpaceX’s titanium grid fins survive re-entry without a protective ablative coating, so they can be reused over and over.
It is not a free lunch. Titanium ore is expensive to refine, and the alloy is hard to machine: its thermal conductivity is very low (around 6.7 W/m-K), so heat piles up at the cutting tool instead of carrying away. It can also suffer “cold dwell fatigue,” a loss of fatigue life when held under steady load in the cold. And like all titanium, thin or finely divided pieces can ignite in pure high-pressure oxygen. That oxygen hazard is exactly why titanium has to be used carefully near liquid oxygen and high-pressure gas systems, and why pressure vessels that store gas inside a rocket’s oxidizer tank are studied so closely for ignition risk.
Where it’s used
- Grid fins: SpaceX’s steerable re-entry fins, introduced in 2017, are single-piece cast-and-cut titanium, among the largest single pieces of titanium ever made. The metal’s heat resistance lets them fly uncoated and be reused flight after flight.
- Pressure vessels: in a composite-overwrapped pressure vessel (COPV), a thin metal inner liner holds the gas in while a carbon-fiber overwrap carries the load. Ti-6Al-4V is one of the metals used for those liners in aerospace, valued as a leak-proof, chemically inert barrier; NASA has studied the strength, fatigue, and fracture toughness of titanium COPV liners. (Not every COPV uses titanium, though – Falcon 9’s helium COPVs, for example, use aluminum liners.)
- Apollo spacecraft: Apollo carried more than 40 pressure vessels, roughly 85% of them titanium alloys including Ti-6Al-4V, for lightweight gas and propellant storage.
- Engine and turbopump parts: rotating components such as impellers, disks, and shafts, plus engine housings, valued for keeping their strength at liquid-oxygen cryogenic temperatures near -183 degrees C.
- Airframes: landing gear, compressor blades and casings, and structural fittings. Modern airliners lean on it heavily too, with about 15% titanium by weight on the Boeing 787 and about 14% on the Airbus A350.
Ti 90%, Al 6%, V 4%
| DENSITY | 4 kg/m³ |
| TENSILE STRENGTH | 950 MPa |
| YIELD STRENGTH | 880 MPa |
| STRENGTH-TO-WEIGHT | 214447 kN·m/kg |
| MELTING POINT | 1,604 °C |
| MAX SERVICE TEMPERATURE | 315 °C |
| THERMAL CONDUCTIVITY | 6.7 W/m·K |
| THERMAL EXPANSION | 8.6 µm/m·K |
| CATEGORY | Titanium Alloy |
| DESIGNATIONS | UNS R56400, AMS 4911, Grade 5 |
| MANUFACTURER | TIMET / VSMPO-AVISMA |
| DENSITY | 4 kg/m³ |
| TENSILE STRENGTH | 950 MPa |
| YIELD STRENGTH | 880 MPa |
| MELTING POINT | 1,604 °C |
| MAX SERVICE TEMP | 315 °C |
| THERMAL CONDUCTIVITY | 6.7 W/m·K |
| THERMAL EXPANSION | 8.6 µm/m·K |
| CORROSION RESISTANCE | Excellent |
| WELDABILITY | Good |
| MACHINABILITY | Poor |
| COST RATING | Very High |
