Ti-6Al-2Sn-4Zr-6Mo

Ti-6246 occupies a specific niche: very high strength that holds up to around 450°C. That is hotter than Ti-6Al-4V or Ti-662 will usefully go and considerably stronger than the near-alpha alloys such as Ti-6242 that run hotter still. The combination suits jet engine compressor discs and blades in the warmer stages, and highly loaded fasteners that see elevated temperature.

Corrosion resistance follows the alpha-beta pattern — adequate in marine and oxidising environments, limited in reducing acids, and not a substitute for the commercially pure grades on corrosion duty. As with the other aluminium-bearing alloys, methanol-bearing and hot-salt service under sustained tensile stress should not be assumed safe without specific qualification.

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.