Ti-15Mo-3Nb-3Al-2Si

Beta 21S exists to solve two specific problems that other beta alloys have. The first is oxidation: the silicon and niobium additions give it materially better oxidation resistance than conventional metastable beta alloys, which extends its useful temperature range above theirs. The second is resistance to attack by hydraulic fluids — specifically the phosphate ester fluids used in aircraft systems, which can embrittle some titanium alloys. That combination is why the alloy found its application in aerospace nacelle and exhaust structure, where hot gas, hydraulic fluid and the need for sheet formability all meet.

Density is on the high side for titanium at around 4.94 g/cm³, a consequence of 15% molybdenum, and as with the other beta alloys the aged yield strengths are specified as ranges so over-ageing can put material out of specification.

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.