Titanium Grade 5

What is Titanium Grade 5?

Titanium Grade 5 is the alpha-beta alloy Ti-6Al-4V, designated UNS R56400. It contains 5.5 to 6.75 percent aluminium and 3.5 to 4.5 percent vanadium. Its consumption exceeds that of all other titanium grades combined. Aluminium stabilises the hexagonal alpha phase. Vanadium stabilises the cubic beta phase. The resulting two-phase structure permits heat treatment, unlike commercially pure grades.

ASTM B348 specifies minimum properties for annealed bar and billet. These are a tensile strength of 895 N/mm squared, a 0.2 percent yield strength of 828 N/mm squared and elongation of 10 percent. The specified tensile minimum is roughly two and a half times that of commercially pure Grade 2. The density is about 4.43 grams per cubic centimetre, which is slightly more than half the density of steel.

The beta transus is the temperature above which the alloy has a fully beta structure. The published value is approximately 995 degrees Celsius. Processing temperatures for Grade 5 are generally defined in relation to this value.

Laxcon Steels lists Titanium Grade 5 in its grades reference as a titanium grade. The same steel is written Ti Grade 5, Ti-6Al-4V, Ti 6Al 4V and R56400.

UNS R56400. ASTM B348 / B265. Titanium is the balance, determined by difference. Other elements 0.1 maximum each, 0.4 maximum total.

What is the chemical composition of Titanium Grade 5?

Composition limits in weight percent are carbon 0.08 maximum, oxygen 0.2 maximum, nitrogen 0.05 maximum, hydrogen 0.015 maximum, iron 0.4 maximum, aluminium 5.5 to 6.75 and vanadium 3.5 to 4.5.

Mechanical properties of Titanium Grade 5

Grade 5 is available in several conditions, and the specified condition determines its properties. Mill annealing is the general-purpose condition described by the ASTM B348 minimums. It provides the highest ductility and toughness among the standard conditions. Annealed tensile strength commonly ranges from 895 to 1170 N/mm squared, depending on section size and processing.

Solution treated and aged material has higher strength and lower ductility. The increase in strength is greater in thin sections. Solution treatment requires sufficiently rapid quenching to retain the beta phase, which becomes more difficult as section thickness increases.

The elastic modulus is approximately half that of steel, as with other titanium grades. Parts governed by stiffness therefore require sizing according to deflection rather than strength. The alloy provides a high ratio of strength to density, which is relevant when reduced weight is a design requirement.

Heat treatment of Titanium Grade 5

The alpha-beta structure enables heat treatment. Solution treatment takes place below the beta transus. The aerospace bar specification typically requires a range approximately 28 to 83 degrees Celsius below the transus. The temperature is held within a narrow tolerance, and the holding time depends on section thickness. Quenching follows. Ageing at a substantially lower temperature precipitates fine alpha within the retained beta and increases strength.

Working above the beta transus allows easier deformation. It also produces a coarse transformed beta structure after cooling. This structure has fatigue properties that differ markedly from those of the fine equiaxed structure in mill-annealed bar. Specifications may therefore define the required microstructure rather than hardness alone.

Section size limits the effect of solution treatment. Heavy sections cannot cool rapidly enough through the critical temperature range to retain sufficient beta for ageing. The strength benefit of the solution treated and aged condition consequently decreases with increasing thickness. Heavy parts are usually specified in the annealed condition.

Comparison of Grade 5 and Grade 23 ELI

Grade 23 is the extra-low interstitial version of the Grade 5 alloy. Its principal differences are lower limits for interstitial elements and iron.

Composition limits in weight percent, from the Laxcon Steels grade reference. Interstitials, iron and carbon are maximums; titanium is the balance.

The aluminium and vanadium ranges remain effectively unchanged. Oxygen decreases by more than one-third. Oxygen strengthens titanium but reduces fracture toughness. Grade 23 therefore has lower specified strength and greater damage tolerance and low-temperature toughness. It is specified for orthopaedic implants and fracture-critical subsea components. Grade 5 is used for general structural applications.

Designations and product specifications

ASTM B348 covers bars and billets. ASTM B265 covers plate, sheet and strip. In the aerospace system, AMS 4928 covers bar and billet, while AMS 4911 covers sheet and plate. Together, these specifications qualify most aerospace structural applications of the alloy. ASTM F1472 covers Grade 5 for surgical implants. ASTM F136 covers the extra-low interstitial Grade 23.

Welding and machining Titanium Grade 5

Grade 5 is weldable, but welding is more difficult than with commercially pure titanium grades. Inert gas must shield the arc, root and cooling weld until the metal falls below the temperature at which titanium absorbs oxygen, nitrogen and hydrogen. Absorbed gas embrittles the joint, and post-weld treatment cannot remove it.

The weld metal and heat-affected zone solidify with a transformed beta structure. This structure has lower ductility than the parent annealed bar. Welded assemblies therefore often receive a post-weld anneal. Designs also place joints away from the highest-stress locations.

Machining requires controlled conditions. Low thermal conductivity keeps cutting heat at the tool edge rather than transferring it to the chip. The alloy remains strong at the temperature reached by the edge and work hardens when a tool rubs. Common practice uses rigid setups, minimum overhang, sharp positive-rake tools, low surface speeds, generous feed, climb milling and continuous flood coolant. Fine chips and grinding dust present a fire hazard.

Applications of Titanium Grade 5

Grade 5 is used in aerospace structures, airframe fittings, fasteners and hydraulic tubing. Engine applications include fan blades and discs. Medical uses include orthopaedic and dental implants, bone screws and plates. Other applications include connecting rods, valves and other high-performance automotive and motorsport components. The alloy is also used in marine and offshore fittings, sports equipment and pressure vessels where strength relative to weight is required.

Where Titanium Grade 5 is not the right choice

Titanium Grade 5 is unsuitable when corrosion resistance is required without its higher strength. It has lower resistance than commercially pure titanium in some reducing acids. It is also harder to form and weld. Heat exchangers and chemical vessels that do not require its strength are better suited to Grade 2, or to palladium-bearing corrosion grades in severe environments.

Grade 5 has a strong tendency to gall. Its galling behaviour is worse than that of steel and most alternatives, particularly when it contacts itself. A titanium threaded fastener can seize in a titanium part during its first assembly and may be destroyed during removal. Anti-seize compound, a dissimilar mating material, anodising or a hard coating is therefore required on the drawing. Galling also limits unlubricated sliding and bearing applications.

Grade 5 is unsuitable for high-temperature service. Its strength decreases progressively above a few hundred degrees Celsius, and it begins to absorb oxygen from air. Hot sections instead require nickel alloys such as Inconel 625 or titanium alloys developed for high-temperature service.

Grade 5 is also unsuitable when weight reduction does not justify its cost. Bar is expensive, machining is slow, and parts cut from solid produce substantial scrap. Inspection requirements are also demanding. When steel can carry the load and weight is not the design driver, alloy steel provides a substantially lower finished-part cost.

Other titanium grades