Titanium

Titanium is the only common engineering metal that combines the specific strength of steel with the corrosion resistance of platinum. Density is 4.43 g/cm3 for the workhorse alloy Ti-6Al-4V—about 60% of steel and 1.6x aluminum—while yield strength of 880 MPa exceeds annealed 304 stainless by more than 4x. Specific strength reaches 230 kN m/kg, the highest of any structural metal in series production. That single ratio is why it dominates aerospace primary structure, orthopedic implants, and any part where mass and chloride corrosion both matter.

Six grades cover commercial demand. Grades 1 through 4 are commercially pure (CP) titanium, ranked by oxygen content—Grade 1 is softest and most ductile, Grade 4 is hardest and used for dental abutments. Grade 5 is Ti-6Al-4V, an alpha-beta alloy that takes 90% of all titanium tonnage. Grade 23 is Ti-6Al-4V ELI (extra-low interstitial), the medical implant standard under ASTM F136. Grade 9 (Ti-3Al-2.5V) is the bicycle-tube alloy. Beta alloys like Ti-5553 hit 1,200 MPa yield for landing-gear forgings.

Manufacturing routes split between wrought (CNC from forged or rolled bar), DMLS / L-PBF additive (where Ti-6Al-4V is the dominant aerospace and medical powder), investment casting (less common because of the reactivity), and cold-formed wire for fasteners. CNC of titanium is not casual—surface speeds run 30–60 m/min versus 250 m/min for aluminum, the chip is springy and welds to the cutter, and rigid fixturing with flood coolant is mandatory. DMLS bypasses the machinability problem entirely and is why titanium adoption has tripled in aerospace brackets since 2018.

Cost is the gating constraint. Raw billet runs USD 35–60 per kilogram for industrial Grade 5 and USD 80–140 per kilogram for medical Grade 23, against USD 4–6 for 6061 aluminum and USD 8–12 for 316L stainless. A typical CNC bracket that costs USD 120 in 6061 lands between USD 600 and USD 2,000 in Ti-6Al-4V because cycle time stretches 3–4x on top of the material premium.

Titanium gets specified for one of three reasons. First: weight at structural strength. A 6061 bracket holding the same load needs roughly 1.7x the cross-section to match Ti-6Al-4V yield, which often nullifies the lower density of aluminum. For aerospace primary and motorsport unsprung mass, the absolute mass at equal stiffness is what matters and titanium wins consistently above 200 MPa working stress.

Second: chloride and acid corrosion in environments that eat stainless. Titanium is essentially inert in seawater, holds up against 10% HCl at room temperature, and is the de facto choice for chlor-alkali heat exchangers and offshore fasteners. The protective TiO2 layer reforms in milliseconds when scratched and is stable up to 535 C. There are no recoat cycles on a titanium subsea bolt the way there are on a coated alloy steel one.

Third: biocompatibility and osseointegration. Titanium oxide is the only metal surface that bone osteoblasts colonize directly without an intermediate fibrous capsule. ASTM F136 Grade 23 ELI hip stems, dental abutments, and spinal cages are the regulatory and clinical default precisely because no other metal system gets to bone bonding without a coating step. For class III medical devices, switching away from titanium triggers a new 510(k) cycle.

For a representative aerospace bracket (180 x 90 x 35 mm pocketed, four mounting holes, AS9100 traceable, quantity 20), the comparison below shows where titanium prices against the materials it usually displaces. The premium versus aluminum is roughly 5x; versus 17-4 PH stainless about 2.5x. DMLS Ti-6Al-4V approaches CNC pricing only when geometry forces 70%-plus stock removal.

Six grades cover roughly 95% of real specifications. Use this matrix to pick before quoting. ELI denotes Extra Low Interstitial—a tighter spec on oxygen, nitrogen, hydrogen, and iron required for fatigue-critical implants and cryogenic service.

Titanium is fussier than stainless or aluminum on process choice. Casting needs vacuum or argon shielding because molten titanium dissolves oxygen, nitrogen, and hydrogen aggressively. Welding requires a trailing argon shield over the cooling weld pool—any blue or purple tint indicates oxygen contamination and brittle joints. The table below maps the practical pairings.

Reverse the flow: start from environment and certification, then pick the grade and process. The pairings below cover most incoming aerospace, medical, and industrial requests we see.

The expensive thing about titanium is the cycle time, not the kilogram. Designs that machine fast pay back the material premium; designs that fight the cutter waste both stock and time. Keep CNC pocket corner radii at 35% of tool diameter or larger, keep wall thickness above 1.2 mm to avoid chatter, and chamfer all sharp internal edges by R0.5 or more so the cutter does not stall under work-hardened chips. Titanium work-hardens fast, so dwelling is fatal—program continuous toolpaths with no air cuts mid-feature.

For DMLS, design for support reduction. Self-supporting overhangs up to 45 degrees print without contact supports; below 45 degrees you need teeth supports that consume powder, time, and post-processing labor. Topology-optimize the load path so the build orientation puts critical surfaces upward—as-built down-skin surfaces are 2–3x rougher than up-skin and may not pass an Ra 6.3 um requirement. Add 0.5 mm of CNC stock on any sealing or bearing face that needs Ra under 1.6 um.

On any titanium implant, specify Grade 23 ELI per ASTM F136, anodize for color identification (Type II is voltage-controlled oxide thickness, no dye, runs from gold at 25 V to blue at 80 V), and call out passivation per ASTM F86 (nitric acid 30–40% by volume, 30 minutes minimum). Skipping the passivation step leaves machining-induced iron contamination on the surface, which fails biocompatibility cytotoxicity testing per ISO 10993-5.

Titanium accepts a tighter finish menu than stainless. The five common finishes: as-machined for non-cosmetic aerospace internals; bead-blasted matte (ceramic media, never aluminum oxide—that contaminates the surface) for general-purpose aerospace and consumer; anodized Type II for color identification on implants and consumer goods (purple at 30 V, blue at 80 V, no pigment involved); electropolished to Ra 0.2–0.4 um for implants needing micro-deburr; and DLC (diamond-like carbon) PVD for high-wear knife and watch surfaces.

Passivation is non-negotiable on medical parts. ASTM F86 specifies nitric acid 20–45% by volume at room temperature for 30 minutes minimum, followed by deionized water rinse and air dry. The bath dissolves any iron pick-up from cutters and grinding wheels—iron on a titanium implant surface fails cytotoxicity and corrodes preferentially in saline. For aerospace parts, citric passivation per AMS 2700 is the standard alternative.

Avoid grinding without dedicated wheels and dust extraction. Fine titanium dust is pyrophoric—it ignites at 250 C in air and burns at 3,000 C with no easy extinction (water and CO2 are both ineffective; class D dry powder is required). Reputable shops segregate titanium grinding from steel grinding by physical room, not just by procedure.

A small-sat OEM replaced a CNC-machined 7075-T651 reaction-wheel mount (mass 312 g, 14-hour cycle, USD 740 per part) with a DMLS Ti-6Al-4V topology-optimized version. New mass 168 g (46% reduction), build time 18 hours per plate of six parts, post-processing 6 hours per part (stress relief, support removal, HIP at 100 MPa / 920 C / 2 hours, CNC finish on three mounting interfaces, citric passivation). Unit cost USD 1,180 each at quantity 30—60% premium over the milled aluminum, but the program saved 144 g x 4 mounts x 18 satellites = 10.4 kg of dry mass, worth USD 56,000 in launch cost.

The build qualification took 14 weeks: coupons cut from witness blocks each build, fatigue tested per ASTM E466, ultrasonic and CT-scan inspection per ASTM E2104. Print parameters were locked under the program's NADCAP additive heat-treat approval. Subsequent flight builds run on the same parameter set with no re-qualification.

An orthopedic device firm shipped patient-specific lumbar interbody cages from CT-imported geometry, printed in Grade 23 ELI on a Renishaw RenAM 500Q with 60 um lattice porosity to encourage bony ingrowth. Build time per plate of 14 cages was 22 hours, post-processing per cage 4.5 hours including stress relief, support removal, HIP, citric passivation per ASTM F86, and laser UDI marking per FDA 21 CFR 830. Unit cost USD 2,100 against USD 980 for a stock PEEK cage, but reimbursement under CPT 22633 covers the premium when patient-matched geometry is documented in surgical planning.

Regulatory path: ISO 13485 quality system, FDA 510(k) De Novo for the patient-specific instrument family, ASTM F2924 conformance for the powder-bed Ti-6Al-4V process, ISO 10993-5 cytotoxicity, and ISO 10993-6 implantation per cage geometry. Time from approved CT to delivered sterile implant is 11 calendar days for routine cases.

A subsea instrumentation supplier replaced 316 stainless M8 bolts in a sensor pod (480 m depth, splash zone deployment cycles, 5-year service interval) with cold-rolled Ti-6Al-4V to eliminate chloride pitting that was causing fastener-driven leaks at year three. Bolt unit cost rose from USD 4.20 (316) to USD 28.50 (Ti-6Al-4V) at quantity 5,000, but in-service inspection intervals stretched from 6 to 24 months and recovered units showed no thread degradation across two complete cycles.

Specification: AMS 4928 Grade 5 bar, cold-rolled threads class 6g, anodized Type II purple for visual identification, packed dry with anti-seize compound (titanium-on-titanium galls catastrophically without lubrication). Total program cost saving across the fleet of 240 pods over 8 years: USD 1.4 million in avoided dive-recovery operations.

Titanium is rarely the right answer to a single requirement. It becomes obvious when at least two of the following are true: the part is structurally loaded above 200 MPa working stress, weight at that strength matters more than dollars per part, the environment includes seawater or chlorides that pit stainless, or the part is a long-term implant. If only one applies, an aluminum or stainless alternative usually wins on total cost. If two or more apply, titanium pays back across the service life even at 5x the per-part cost.

Specify the grade by application, the process by geometry and volume, the finish by environment, and the certification by destination market—then write all four on the drawing. Skipping any one of those decisions transfers risk to the shop and the price comes back wide and slow. With them on paper, you get a tight quote, a clean first article, and no surprises at incoming inspection.