Titanium Alloys for Aerospace Hardware

The titanium grades on aerospace drawings

Titanium families are defined by their phase structure. Commercially pure (CP) grades are all-alpha, with strength set mainly by oxygen and iron content. Ti-6Al-4V is an alpha-beta alloy that can be used annealed or solution treated and aged. Ti-6Al-2Sn-4Zr-2Mo is a near-alpha alloy built for strength retention and creep resistance at elevated temperature.

Within the CP grades, strength rises and cold formability falls from Grade 1 to Grade 4. ASTM also defines H variants such as Grade 2H, identical in chemistry to Grade 2 but with a higher guaranteed minimum ultimate of 58 ksi, intended mainly for pressure vessel work.

Ti-6Al-4V properties and the numbers behind them

Ti-6Al-4V is roughly half of all titanium used, and its numbers are the ones engineers remember. The AMS 4911 and AMS 4928 titles both name a 120,000 psi yield strength for the annealed condition. Other values below are from a producer data sheet and a supplier summary of Grade 23, and are not design allowables.

The modulus is the number that surprises people coming from steel: at about 16.5 million psi, a titanium part is a little over half as stiff as the same steel part. Designs converted from steel to titanium for weight often need extra section to hold deflection, which eats into the weight savings.

Annealed, STA and ELI: choosing the condition

Most machined 6-4 parts are bought and used mill annealed. Solution treating and aging raises strength by roughly 20 percent according to producer data, but it reduces ductility and fracture toughness, it is harder to do in thick sections because 6-4 has limited hardenability, and it means a heat treat step after rough machining with the alpha case and distortion that come with it.

The ELI grade restricts interstitial elements, chiefly oxygen, which lowers strength slightly and improves toughness and ductility. Engineers specify it for fracture-critical parts, damage tolerant designs and cryogenic hardware such as tank fittings, where normal interstitial levels reduce toughness at low temperature. If the drawing says Ti-6Al-4V without ELI, a supplier will reasonably quote standard Grade 5.

Ti-6-2-4-2 is chosen when parts see sustained elevated temperature and creep matters, for example near engines or in hot structure. It is less common and less available than 6-4, so check stock forms before committing a design to it.

Alpha case: the defect you cannot see

Alpha case is an oxygen-enriched, hard and brittle surface layer that forms when titanium is exposed to air or other oxygen sources at heat treatment and forging temperatures. It has almost no ductility, so a small crack in the case can start a fatigue crack into the part.

• Where it comes from: heat treatment in air or poor vacuum, forging, hot forming, welding without adequate shielding, and casting.
• How it is removed: the usual approach is to leave parts oversize, heat treat, then remove the case by chemical milling in acid baths containing hydrofluoric and nitric acid. Machining and abrasive methods are used for simple shapes, sometimes followed by a light etch.
• How removal is verified: metallographic examination of a sample or coupon processed with the parts. Aerospace process specifications require proof that the case is gone, not an assumption based on stock removal.
• What to call out: heat treatment to the governing spec (AMS 2801 is the common one for titanium parts), vacuum or inert atmosphere where required, and a requirement that finished surfaces be free of alpha case.

Acid processes add their own risk, because titanium can absorb hydrogen during pickling and etching. That is why the hydrogen limit on the cert matters and why many specs require a hydrogen check after chemical processing.

Machining titanium without scrapping parts

Titanium machines slowly because it conducts heat poorly. Heat stays at the cutting edge instead of leaving with the chip, and cutting speed drives temperature far more than feed does. The consistent guidance from tool makers and experienced shops:

• Run low surface speeds and the highest feed the setup tolerates, so heat goes into a thicker chip.
• Use sharp, positive rake tools and replace edges early. Wear accelerates fast once it starts.
• Never let the tool dwell or rub. Titanium smears, work hardens at the surface and welds to the edge.
• Flood coolant, preferably high pressure through the tool, both for heat and to clear chips.
• Use rigid machines and short tool stick-out. Titanium's low modulus makes thin walls deflect and chatter.
• Keep fine chips and grinding swarf wet and cleared. Fine titanium chips can ignite, and the fire is hard to put out.

Galling is the other titanium habit engineers design around. Titanium threads against titanium or stainless gall readily, so threaded titanium parts usually get a dry film lubricant, an anodize or a coating on one member, and drawings for titanium fasteners and inserts should name it.

Environment and service limits

Ti-6Al-4V, Ti-3Al-2.5V, Ti-5Al-2.5Sn and Ti-10V-2Fe-3Al appear in MSFC-STD-3029 Table I, high resistance to stress corrosion cracking in sodium chloride environments, in all conditions. That makes titanium a strong choice for coastal launch site hardware. Producer data notes one exception: stress corrosion can occur if chloride salts are left on stressed parts that are later heated, so keep parts clean of fingerprints and salt residue before heat treatment or hot service.

Oxygen is the other limit. Titanium's ignition and burn behavior in oxygen-enriched environments is the reason oxygen system designers usually steer away from it for wetted parts. If a titanium part will see oxygen, the program's oxygen compatibility assessment under NASA-STD-6001B needs to address it explicitly.

Callouts, certs and sourcing

A complete titanium callout states the alloy and grade, ELI if required, condition (annealed, duplex annealed, STA), the procurement spec by product form, any heat treatment spec and alpha case requirement, and finish or anti-galling treatment. Require a mill cert with heat number, chemistry including oxygen, nitrogen, carbon, iron and hydrogen, and tensile results by the spec.

All titanium and titanium alloys are specialty metals under DFARS 252.225-7009, so defense contracts carrying that clause need titanium melted in the United States or a qualifying country. Confirm the melt source before the PO is placed. The certs and traceability page explains how to check it, and controlled programs covers how we handle export-controlled drawings.

Get titanium parts quoted

Titanium jobs go wrong at heat treatment, alpha case removal and machining heat, so the supplier choice matters more than with aluminum. Send your drawing through a CNC quote request; we match it to shops with titanium experience, flow down the heat treat and alpha case requirements, and check the mill cert and processing certs before the part ships. See also the materials hub, nickel superalloys and heat treatment.

Questions

Related

• Aerospace Materials: Selection, Specs and Paperwork
• Aerospace Aluminum Alloys and Tempers
• Nickel Superalloys and Nickel-Copper Alloys
• Material Certs, Specialty Metals and Lot Traceability

Sources

• United Performance Metals, Ti-6Al-4V data sheet (AMS 4911, 4928)
• SAE AMS 4928, Titanium Alloy Bars, Forgings, and Rings, 6Al-4V Annealed, 120,000 psi Yield
• SAE AMS 4911, Titanium Alloy Sheet, Strip, and Plate, 6Al-4V Annealed
• SAE AMS 4919, Ti-6Al-2Sn-4Zr-2Mo sheet, strip and plate, duplex annealed
• Performance Titanium, ASTM B348 Grade 23 summary
• NASA MSFC-STD-3029A, Table I-E titanium alloys
• Gosiger, Tips for Machining Titanium
• DFARS 252.225-7009 specialty metals