Titanium Grade 5 (Ti-6Al-4V)

Strength is what separates them; corrosion resistance does not. Grade 2 is commercially pure titanium at roughly 345 MPa tensile: formable, readily weldable, easier to machine and cheaper, and it is the right answer when the part exists to survive a corrosive medium — chemical handling, heat exchangers, marine and subsea fittings. Grade 5 (Ti-6Al-4V) is the alloy at roughly 950 MPa tensile and the same 4.43 g/cm³ density, which is why it carries structural and implant work. Both are effectively immune in seawater, so do not pay for Grade 5 to get corrosion performance you already have. If the part is loaded, or a section has to be thinned to save weight, Grade 5. If it is formed or welded and simply must not corrode, Grade 2.

Same alloy, tighter chemistry. Grade 23 is Ti-6Al-4V ELI — extra-low interstitial — with lower limits on oxygen, nitrogen, carbon and iron. Those interstitials raise strength but cost ductility and fracture toughness, so ELI trades a little strength for damage tolerance, which is what implant and cryogenic work wants. Surgical implant material is normally specified to ASTM F136 (Ti-6Al-4V ELI) or ASTM F1472 (standard Grade 5); general industrial bar and billet go to ASTM B348 or AMS 4928. Put the standard on the drawing rather than the word titanium, and state separately whether you need mill certificates traceable to the heat lot — that traceability, not the alloy name, is what makes material usable in a regulated build.

Three physical properties, and none of them is the price of the metal. First, thermal conductivity is low, so the heat generated at the cut stays at the tool edge instead of leaving in the chip — that is what limits surface speed and burns tools. Second, titanium is chemically reactive at cutting temperature and will attack tool materials, which shortens tool life further and rules out running it like steel. Third, the elastic modulus is about 114 GPa, roughly half that of steel, so the part springs away from the cutter and thin sections chatter, which forces light engagement and rigid fixturing. The result is low speeds, flood or through-tool high-pressure coolant, frequent tool changes and more machine hours per part. Read the cost as machine time and tooling, not stock. If 7075-T6 aluminium or 17-4 PH stainless satisfies the load case, either will cost a fraction.

Five things. Call out the grade with its standard — Grade 5 to ASTM B348 or AMS 4928, Grade 23 to ASTM F136 — not the word titanium alone. Give one datum scheme the part can realistically be clamped on, and mark which tolerances are functional, because holding everything tight in titanium is paid for in machine hours. Size internal corner radii for a tool that can survive the cut: a small radius in a deep pocket means a slender tool in the worst possible material, and it is usually cheaper to open the radius than to pay for the tool life. Specify the finish explicitly — titanium is commonly supplied as-machined or bead blasted, anodised to AMS 2488 where galling or wear resistance matters, and passivated where the specification calls for it. Finally, state the certification you need (material certificate, inspection report, first article) on the drawing rather than in an email, so it is quoted rather than discovered.

Stiffness, not strength, sets the limit. At about 114 GPa the elastic modulus is roughly half that of steel, so a titanium wall of a given thickness deflects about twice as much under the same cutting pressure — it pushes away from the tool, springs back behind it, and chatters. The consequences are practical: leave more stock for finishing than you would in aluminium, rough symmetrically so the part does not bow toward one side, take the final wall in a single light pass with a sharp tool, and support the wall while it is cut — soft jaws, wax, a sacrificial rib machined away at the end. On the drawing, say which wall thicknesses and flatness callouts are functional and state the restraint condition for inspection, because a thin titanium part measured free-state and measured clamped are two different parts.