Ti-6Al-4V

Titanium with six per cent aluminium and four per cent vanadium, and it accounts for more than half of all titanium alloy used anywhere. It is strong as a good steel, weighs a little over half as much, is essentially immune to seawater and body fluid, and keeps useful strength to around 400 °C.

Its stiffness is half of steel's, which is a disadvantage in a structure and an advantage in a bone implant: a stiff implant carries load the bone should be carrying, the bone resorbs, and the implant loosens. Titanium's lower modulus is closer to bone's and the mismatch is smaller.

What it is not is cheap or easy. Titanium is expensive because reducing it from its ore requires the Kroll process, a batch operation using magnesium that has resisted replacement for seventy years. And it machines badly — it is a poor conductor so the heat stays in the cutting edge, and it is chemically reactive with most tool materials at temperature.

Forged, rolled, and increasingly built by powder-bed additive manufacturing, which is where the alloy has found a genuinely new market: an aerospace bracket or a patient-specific implant is a low-volume complex shape, which is exactly the case where printing beats machining a billet away.

Welded under inert gas or in vacuum, because hot titanium takes up oxygen, nitrogen and hydrogen from air and becomes brittle. Machined slowly, with sharp tools, plenty of coolant and rigid setups — the standing advice is that titanium punishes every process compromise.

Airframe structure and engine components in the cooler sections; landing gear; fasteners. Hip and knee replacements, bone plates, spinal cages and dental implants — the biomedical use rests on osseointegration, where bone bonds directly to the oxide surface rather than merely sitting against it.

Marine hardware and offshore risers, where seawater immunity is worth the price. Chemical plant. Sports equipment and high-end consumer goods, where it is bought as much for what it signifies as for what it does.

Developed in the United States in the 1950s, and it has remained the default titanium alloy for seventy years — a stability that is unusual and says more about the cost of qualifying a replacement than about the alloy being optimal.

Osseointegration was found by accident. Per-Ingvar Brånemark implanted titanium chambers in rabbit bone in the 1950s to study blood flow and could not remove them afterwards; the observation became the basis of modern dental implantology.

Extremely energy intensive to produce — the Kroll process is a batch reduction with magnesium, and the metal costs several times what its abundance would suggest. Titanium is the ninth most abundant element in the crust and is expensive entirely because of the chemistry of getting it out.

Against that, it is durable to the point of being effectively permanent in most service, and machining swarf and aerospace offcuts are recovered and remelted because the metal is worth it. Additive manufacturing improves the arithmetic substantially: machining an aerospace bracket from billet can discard nine tenths of the metal, and printing it discards very little.