Ti-15V-3Cr-3Sn-3Al

Ti-15-3 was developed to be formed, and that is why it is specified. The metastable beta structure in the solution-treated condition gives cold formability comparable to an austenitic stainless steel, so sheet can be rolled thin, drawn and formed into shapes that would be impossible in Ti-6Al-4V, and fasteners can be cold headed and thread rolled before ageing. The subsequent age then develops 1000 to 1240 MPa typical strength in the finished shape.

For a fastener that route produces a better part than machining hard material: the rolled thread carries continuous grain flow and a cold-worked surface, with the fatigue performance that follows. The trade-offs are a density on the high side for titanium at around 4.76 g/cm³, a modest service ceiling near 315°C, and corrosion behaviour that follows the alloyed rather than the commercially pure grades.

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.