Titanium

What defines the titanium family?

Titanium is a metal used for its strength-to-weight ratio or its corrosion resistance in chlorides. Its density is about 4.5 grams per cubic centimetre, slightly more than half that of steel. Its elastic modulus is roughly half that of steel. A passive titanium dioxide film protects the surface and re-forms almost immediately after damage when oxygen or water is present.

Titanium grades comprise commercially pure, strength-alloyed and corrosion-alloyed groups. Commercially pure Grades 1 through 4 are unalloyed, and their strength depends on oxygen and iron content. In strength-alloyed grades, aluminium stabilises the hexagonal alpha phase, while vanadium or molybdenum stabilises the cubic beta phase. The resulting two-phase structures can receive heat treatment and attain strengths unavailable in unalloyed grades. Corrosion-alloyed grades contain small additions of palladium or of nickel and molybdenum.

The Laxcon Steels grade reference lists 12 titanium grades with their full composition.

Commercially pure grades

Grades 1, 2, 3 and 4 contain no alloying additions. Their principal differences are their interstitial oxygen content and their iron content. Their maximum oxygen contents are 0.18, 0.25, 0.35 and 0.4 percent, respectively. Their corresponding maximum iron contents are 0.2, 0.3, 0.3 and 0.5 percent.

Oxygen dissolved in the alpha lattice provides substantial strengthening per unit weight compared with substitutional elements. It also reduces ductility and fracture toughness. Grade 1 is the softest and most formable grade. It is used for deep drawing and applications requiring extreme corrosion resistance. Grade 2 is the general-purpose grade and is the grade most commonly supplied as bar, plate and tube. Grade 3 provides moderately higher strength for pressure applications. Grade 4 is the strongest and least formable commercially pure grade.

The extra-low interstitial versions of alloyed grades use the same principle. Grade 23 is derived from Grade 5 by reducing the oxygen limit from 0.2 to 0.13 percent and the iron limit from 0.4 to 0.25 percent. It also has a slightly lower hydrogen limit. These reductions restore fracture toughness, making Grade 23 the grade used for implants and fracture-critical applications.

Strength-alloyed and corrosion-alloyed grades

Grade 5, designated Ti-6Al-4V, accounts for more titanium consumption than all other grades combined. It contains 5.5 to 6.75 percent aluminium and 3.5 to 4.5 percent vanadium. Its specified minimum tensile strength is 895 N/mm squared, compared with 345 N/mm squared for Grade 2. Solution treatment and ageing can further strengthen its two-phase structure. Grade 9, designated Ti-3Al-2.5V, is a leaner alpha-beta alloy containing 2.5 to 3.5 percent aluminium and 2.0 to 3.0 percent vanadium. It is easier to cold work and draw into tube than Grade 5. It is therefore used for hydraulic tubing and bicycle frames instead of the stronger alloy.

Corrosion-alloyed grades contain additions intended to improve passivity rather than strength. Grade 7 is Grade 2 with 0.12 to 0.25 percent palladium. Grade 11 is Grade 1 with the same addition. Grades 16 and 17 are low-palladium versions containing 0.04 to 0.08 percent palladium. They were developed to provide most of the corrosion benefit at a fraction of the palladium cost. Palladium raises the surface potential and maintains passivity under reducing conditions in which unalloyed titanium would corrode. Grade 12 uses 0.6 to 0.9 percent nickel and 0.2 to 0.4 percent molybdenum for a similar purpose. It is specified for resistance to crevice corrosion in hot brines.

Workshop processing

Welding requires inert-gas shielding over the arc, the root and the cooling weld. Shielding must continue until the metal cools below the temperature at which it absorbs oxygen, nitrogen and hydrogen. Absorbed gases permanently embrittle the joint, and heat treatment cannot remove them. Straw, blue, grey or powdery discolouration indicates contamination. Contaminated metal must be cut out rather than dressed.

Machining is affected by low thermal conductivity and high chemical activity at elevated temperature. Cutting heat remains at the tool edge. Suitable practice uses low surface speeds, generous feeds, sharp positive-rake tooling, rigid setups and flood coolant. Fine chips and grinding dust present a fire hazard.

Titanium also galls readily against itself and many other metals. Threads, sliding faces and press fits therefore require anti-seize compound, a dissimilar mating material, anodising or a suitable hard coating from the design stage.

Questions about titanium grades

Where the titanium family is not the right answer

Titanium is unsuitable for hydrofluoric acid and fluoride-bearing solutions at any concentration. These media dissolve the oxide film and attack the metal rapidly, and no reliable inhibitor is available. It is also unsuitable for dry chlorine because water is absent to maintain the film. Hot, concentrated reducing acids require a palladium-bearing grade.

Titanium is unsuitable for high-temperature service. It loses strength rapidly as temperature increases and begins to absorb oxygen from air at temperatures well below those at which nickel alloys remain serviceable. Nickel-chromium alloys are used for such hot service.

Titanium is unsuitable when stiffness governs the design. Its elastic modulus is about half that of steel, so a component sized for deflection retains little of titanium's weight advantage. It is also unsuitable when the value of reduced weight or corrosion resistance does not offset the costs of the material, slow machining, scrap from cutting components from solid stock and the required inspection.

Titanium grades in detail