Titanium's two crystal structures
Titanium changes crystal structure at 882 °C: below that temperature it is hexagonal close-packed (alpha), above it body-centred cubic (beta). Alloying elements determine the character of the material by stabilising one of these two phases.
- Alpha stabilisers
- Aluminium, oxygen, nitrogen, carbon. They give high-temperature strength and weldability; the alloy cannot be hardened by heat treatment.
- Beta stabilisers
- Vanadium, molybdenum, niobium, iron, chromium. They give hardenability by heat treatment and formability.
- Neutral
- Zirconium and tin; they do not shift either phase appreciably.
Ti-6Al-4V (Grade 5)
It takes its name from its composition: roughly 6% aluminium and 4% vanadium by weight, the balance titanium. It is an alpha-beta alloy; the aluminium stabilises the alpha phase and the vanadium the beta phase. This single composition accounts for more than half of all titanium alloy produced worldwide.
| Property | Typical value |
|---|---|
| Tensile strength | 895–1100 MPa |
| Yield strength | 830–900 MPa |
| Density | 4.43 g/cm³ (about 57% that of steel) |
| Continuous service temperature | around 400 °C |
| Modulus of elasticity | 110–114 GPa |
That combination is what makes it attractive: strength close to or above stainless steel at nearly half the weight, resistance to seawater and most chemicals, and compatibility with body tissue.
The ELI grade
Grade 23 is known as Ti-6Al-4V ELI, where ELI stands for "Extra Low Interstitials". The limits for oxygen, nitrogen, carbon and iron are tighter than in standard Grade 5.
Interstitial elements raise strength but lower fracture toughness and the reserve of ductility. The ELI grade gives up a little strength in exchange for markedly higher fracture toughness and better low-temperature performance. That is why implants, critical aerospace parts and cryogenic applications call for ELI.
Oxygen is not an impurity in titanium but effectively an alloying element. A titanium order with no oxygen ceiling in the specification is incomplete.
The main grades
| Grade | Composition | Use |
|---|---|---|
| Grades 1–4 | Commercially pure titanium | Chemical equipment, heat exchangers, architecture, plate |
| Grade 5 (Ti-6Al-4V) | Al 6%, V 4% | Aerospace structural parts, engine components, sports equipment |
| Grade 23 (ELI) | Al 6%, V 4%, low interstitials | Medical implants, critical aerospace, cryogenic |
| Grade 7 | Ti + Pd | Strongly reducing environments, chemical industry |
| Grade 9 | Al 3%, V 2.5% | Tubing and hydraulic lines; better machinability |
How sponge becomes an ingot
- Blending. Titanium sponge, recycled scrap and alloying elements (usually an aluminium-vanadium master alloy) are weighed out and mixed.
- Pressing. The blend is compacted into electrode blocks and welded together into a single electrode.
- VAR melting. It is melted in a vacuum arc furnace. For aerospace grades this is repeated two or three times; every cycle improves homogeneity and reduces inclusions.
- Forming. Forging, rolling and heat treatment give the final semi-finished product.
The oxygen, nitrogen and iron content of the final product is largely set by the purity of the starting sponge and scrap. Melting cannot remove these elements, so the control has to happen in the raw material at the very beginning.
The difficulties of working with titanium
- Low thermal conductivity: in machining, heat builds up at the cutting edge; low cutting speeds and plenty of coolant are needed.
- Reactive at high temperature: above 600 °C it picks up oxygen and nitrogen; welding and heat treatment call for a protective atmosphere.
- Tendency to gall: it interacts chemically with the cutting tool and tool life suffers.
- Springback: because the modulus of elasticity is low, forming has to compensate for it.
When buying raw material
- Write separate ceilings for oxygen, iron, nitrogen, carbon and chlorine in the sponge.
- State the particle size range and its suitability for your pressing method.
- Add the purity of the aluminium and vanadium source to the specification; these carry straight through into the final composition.
- Stipulate air- and moisture-tight packaging.
- Require a certificate of analysis for every lot and, where needed, independent laboratory verification.
Frequently asked questions
What is Ti-6Al-4V?
It is the titanium alloy containing roughly 6% aluminium and 4% vanadium by weight, also known as Grade 5. It has an alpha-beta structure, a tensile strength in the 895–1100 MPa range and a density about 57% that of steel. It accounts for more than half of all titanium alloy in use worldwide.
What is the difference between Grade 5 and Grade 23 (ELI)?
The composition is the same; the difference is in the limits on interstitial elements. In the ELI grade, oxygen, nitrogen, carbon and iron are held to tighter limits. That lowers strength slightly while markedly increasing fracture toughness and low-temperature performance; medical implants and critical aerospace parts call for ELI.
Which raw materials are used in titanium alloy production?
The basic raw material is titanium sponge. Recycled scrap and alloying elements are added to it; for Ti-6Al-4V a high-purity aluminium and vanadium source is used. The blend is pressed into an electrode and melted in a vacuum arc furnace.
Why does oxygen matter so much in titanium?
Oxygen forms an interstitial solid solution in titanium and raises strength, but lowers ductility and fracture toughness. Because it cannot be removed by melting, the control has to be exercised in the starting raw material, which is why an oxygen ceiling is always stated in the specification.
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