Titanium Alloys

Titanium alloys are the strong, light, corrosion-resistant metals of aerospace, medical and marine work, and they are among the most respected materials in the machine shop — respected because they machine nothing like steel. A titanium part weighs about 60 percent of the same part in steel, yet the aerospace alloys match the strength of quenched and tempered alloy steels, and the metal keeps that strength and its resistance to corrosion and fatigue at temperatures where steel and aluminium alloys give up. The cost of those properties is paid at the cutter: titanium holds the heat of cutting at the tool edge, galling and welding to the tool, and springing away from it, so it is machined slowly, firmly and with a respect that the shop ignores at its peril. This entry sets out the titanium alloy family and how it is machined.

The titanium alloy family

Titanium is alloyed, like steel, to set the balance of strength, toughness and formability, and the family falls into three broad kinds. The commercially pure (CP) grades are essentially unalloyed titanium: soft, ductile and the most corrosion-resistant of the family, used where the metal’s resistance matters more than its strength — chemical plant, marine fittings, surgical implants. The alpha-beta alloys are the structural workhorses, and their head is Ti-6Al-4V (Grade 5), alloyed with about 6 percent aluminium and 4 percent vanadium, which is the titanium of aircraft structure, engine parts, fasteners and racing components. The beta alloys are the strongest and hardest of the family, heat-treatable to very high strength for the springs, fittings and landing-gear parts that must carry the greatest loads. In machining terms the family runs from the relatively forgiving CP grades to the resistant beta alloys, with Ti-6Al-4V between them and by far the most machined.

Why titanium fights the cutter

The difficulty of titanium is not its hardness but its behaviour under the cut, and three properties govern it. The first is that titanium conducts heat poorly — about a sixth to an eighth of steel’s conductivity — so the heat of cutting, instead of flowing into the chip and the work, stays concentrated at the tool edge, where it softens the tool and drives wear. The second is that titanium is chemically reactive at cutting temperature: it welds and gall to the tool, building an edge that tears rather than cuts, and it reacts with the air’s oxygen and nitrogen when heated, hardening and spoiling the surface it is cut on. The third is that its stiffness is low — about half of steel’s modulus — so the work springs away from the tool under cutting pressure, flexes and chatters, and thin features are distorted rather than cut. Add the titanium habit of work-hardening wherever a tool rubs instead of cutting cleanly, and the metal hardens in the cut and the heat is all in the wrong place.

Machining titanium alloys

The practice that machines titanium well follows from that character, and its first rule is speed: titanium is cut at a fraction of the speed used for steel — commonly a third or less with the same carbide — because the heat of the cut is the enemy of the edge. The second rule is keep the tool cutting: a tool that dwells, rubs or takes a spring cut does not shear the metal but burnishes it, work-hardening the surface and blunting itself, so cuts are made continuous, with a depth deep enough to cut beneath any hardened skin, and climb milling is used where the cutter can take its full chip on entry. The third rule is rigidity: the setup is made as stiff as the machine allows, tools are held short, and the thin, springy work is supported, because any flex under the tool is rubbing, heat and chatter. And the fourth is coolant: a plentiful, high-pressure flood directed at the cut carries the heat away, washes the chips clear and keeps the edge cool, and it is not optional with a metal whose chips can burn as readily as they do. The shop treats titanium as a metal to be coaxed, not forced: firm and continuous cutting, at modest speed, with the coolant on.

Cutting tools and their wear

Titanium is machined with carbide — sharp, positive-rake edges that shear the metal cleanly — and the exotic alternatives mostly fail on it. Ceramics are too brittle and too hot-failing for the interrupted, heat-loaded cut; diamond tooling is avoided because titanium reacts with carbon and dissolves the edge; and even carbide is used with edges kept keen, because a rounded edge rubs, heats and gall. The wear on the tool is characteristic: flank wear at the cutting edge from the concentrated heat, and a notch at the depth-of-cut line where the tool meets the work-hardened skin or scale of the metal. The tool is watched, and an insert is changed at the first sign of that notch or of edge breakdown — titanium punishes a worn edge with sudden failure and a spoiled surface rather than giving warning, and a fresh, sharp edge is the cheapest tool on the job. The speeds and feeds are set from the feeds-and-speeds practice of this wiki, tuned for the grade: the CP grades and Ti-6Al-4V in its annealed state take the shop’s normal titanium practice, while the harder heat-treated beta alloys cut slower still.

Titanium alloys in the shop

Titanium earns its difficulty by what it gives the part, and the shop that machines it well has a place in the work that needs it. The stainless steel comparison is the useful one: both metals gall, work-harden and hold heat, and the coolant and the sharp, rigid practice that tames stainless is the same family of practice that titanium demands — only more so. The metal’s rewards are on the other side of the cut: the titanium part that leaves the machine is light where steel would be heavy, strong where it must carry load, and resistant to the corrosion and fatigue of aircraft, implant and offshore service. It is machined at the speeds and with the respect that this entry has set out — slowly, firmly, cool and always cutting — and the shop that observes them finds titanium a metal of steady, reliable work rather than the ordeal of its reputation. It is not a difficult metal to machine well; it is an unforgiving one to machine badly, and the difference is the discipline.