Ti-6Al-6V-2Sn

Ti-662 is a strength alloy and its corrosion behaviour follows Ti-6Al-4V rather than the commercially pure grades — good in marine and oxidising environments, limited in reducing acids, and not a substitute for the CP or lean corrosion grades on corrosion duty. The copper and iron additions, which are deliberate here rather than residual, slightly reduce corrosion performance relative to Grade 5.

The reason to choose it is roughly 15% more strength than Ti-6Al-4V with deeper hardenability, which matters for thick-section forgings and for highly loaded fasteners where Grade 5 is marginal. The trade-offs are real and should be weighed: lower ductility, poorer weldability, and a lower useful service temperature than the near-alpha alloys.

Titanium alloys divide into four classes, and the class tells you more than the grade number does. Alpha and near-alpha alloys are weldable and creep resistant and run hottest, but are not heat treatable to high strength. Alpha-beta alloys — of which Ti-6Al-4V is the archetype — are heat treatable, strong and the general structural choice. Near-beta and metastable beta alloys are deeply hardenable, can be cold formed in the solution-treated condition and then aged to very high strength, which makes them the natural choice for fasteners and springs. Commercially pure and lean corrosion grades trade all strength away for corrosion performance.

Three absolute exclusions apply to every titanium grade: it must not be used in dry chlorine, with which it reacts violently; not with red fuming nitric acid; and it is rapidly attacked by hydrofluoric acid at any concentration. Fine titanium swarf and grinding dust are a fire hazard and are handled wet.

Titanium galls, and for a threaded fastener this is the dominant practical issue. A titanium thread run against another titanium thread will cold-weld and seize, often badly enough that the fastener has to be cut off. Every titanium bolted joint needs one of three mitigations: an anti-seize compound or dry-film lubricant such as molybdenum disulphide; a plated or coated thread, commonly silver or solid-film lubricant; or a nut in a dissimilar material. Specify which at order stage.

Titanium is galvanically noble. It will not corrode itself, but a titanium fastener in an aluminium, carbon steel, zinc or magnesium assembly drives accelerated corrosion of the structure around it, and a bolted joint’s small-anode, large-cathode geometry is the worst case. Insulating washers and sleeves or a barrier coating should be specified in those assemblies.