Which titanium grade: grade 2, 9, 23 (ELI) or Ti-6Al-4V?

Weight, stiffness and heat conduction. Values keep their own test conditions; see the notes in the sections.

Which titanium grade is the strongest?

Ti-6Al-4V (Grade 5) has the highest minimum yield on the ASTM B265 table. Table 1 for annealed sheet and plate, as quoted in the Grade 9 profile, gives minimum tensile and yield of 345 and 275 MPa for Grade 2, 620 and 483 MPa for Grade 9, and 895 and 828 MPa for Grade 5, with minimum elongation of 20 %, 15 % and 10 %. Our Grade 9 profile quotes only those three rows. Grade 23 is also listed in ASTM B265 and B348 (Grade 23 profile), but its row is not in our data.

The Ti-6Al-4V profile adds other bases. AMS 4928 annealed bar under 2.00 in diameter has minimums of 931 MPa tensile and 862 MPa yield. A technical reference page (typical ranges, not guarantees) lists 830 to 950 MPa yield annealed and 1,000 to 1,100 MPa solution treated and aged, where elongation falls to 6 to 10 %.

Condition moves Grade 9 as well. ASTM B338 gives tube minimums of 620 and 483 MPa annealed (row 9A, 15 % elongation) and 860 and 725 MPa cold-worked and stress-relieved (row 9D, 10 %). A retailer's typical 760 to 860 MPa for cold-worked tube overlaps annealed Ti-6Al-4V, but the guaranteed 725 MPa sits below the 862 MPa of AMS 4928.

The AMS hydraulic tube numbers add a spread. Aircraft Materials lists annealed hydraulic seamless tube at a 517 MPa minimum yield, and Boze, which calls AMS 4944 welded tube, lists 520 MPa with 10 % elongation. Both are above the 483 MPa of ASTM B338. The condition behind each AMS number is not settled: Alleima (Grade 2 profile) lists AMS 4943 as annealed and 4944 and 4945 as cold worked and stress relieved, while the Grade 9 profile lists all three as Ti-3Al-2.5V hydraulic tube. Do not assume one condition for all three: a buyer ordering 4945 should not expect the annealed number.

Grade 2 and Grade 23 have other bases. Smiths prints a typical Grade 2 yield of 352 MPa, above the 275 MPa minimum. One Grade 23 datasheet prints 759 MPa yield and 828 MPa tensile for annealed bar to ASTM B348 up to 3.00 in, L direction (along the length of the bar), 20 °C. It does not say minimum or typical, but 759 MPa is a round 110 ksi, which reads like a specification minimum (our reading, not stated). If so, it sits below the 828 MPa yield minimum of Ti-6Al-4V in ASTM B265, in line with general engineering knowledge, not in our data, that ELI gives up some strength for toughness.

Which titanium grade gives the most strength for its weight?

Ti-6Al-4V, because the weights are almost equal and its B265 minimum yield is the highest. Densities are 4.42 g/cm³ for Grade 23 at 22 °C, 4.43 g/cm³ for Ti-6Al-4V, 4.48 g/cm³ for Grade 9 tube and 4.51 g/cm³ for Grade 2 tube. Taking the top of each listed yield range, strength per density is 248 MPa per g/cm³ for Ti-6Al-4V, 192 MPa per g/cm³ for Grade 9, 78 MPa per g/cm³ for Grade 2 and 102 MPa per g/cm³ for 6061. Those tops mix typicals and minimums: read them as an order of magnitude.

Against the references, 6061 is the light option at 2.7 g/cm³. Grade 2 is 1.7 times as dense, yet its 275 MPa minimum yield only matches the 276 MPa typical of 6061-T6, so Grade 2 buys corrosion resistance, not weight. Annealed Ti-6Al-4V is 1.6 times as dense as 6061 and carries a typical 830 to 950 MPa. 316 is 1.8 times as dense as Ti-6Al-4V (largest density printed for each), with a 205 MPa minimum yield as ASTM A240 plate, sheet and strip.

How stiff is each grade, and does heat change it?

The titanium grades are stiffer than 6061 and less stiff than 316, and too close to choose between. Ti-6Al-4V is listed at 114 GPa in tension, Grade 2 at 103 GPa and Grade 23 at 107 to 122 GPa at 20 °C. 316 is listed at 190 to 205 GPa. Heat lowers stiffness: Grade 23 falls from 107 to 122 GPa at 20 °C to 95 to 111 GPa at 230 °C. Our data holds no modulus for any titanium grade at cold temperatures.

What temperature limits and heat conduction does the data show?

The Ti-6Al-4V profile cites about 400 °C for holding strength, from a Wikipedia statement. The Grade 9 datasheet says the alloy resists oxidation up to 316 °C (600 °F) and prints nothing above that. Grade 2 and Grade 23 have no service temperature in our data, and no titanium yield strength at an elevated temperature is stored.

For cold service, the only titanium tensile data is the Grade 23 research ELI alloy: 953 MPa tensile strength at 293 K and 1,502 MPa at 77 K.

Thermal conductivity is 22 W/(m·K) for Grade 2 at room temperature, 6.6 W/(m·K) for Grade 23 and 6.7 W/(m·K) for Ti-6Al-4V at 20 °C. 316 is 13 to 17 W/(m·K), and 6061-T6 is 167 W/(m·K) at 77 °F. Grade 9 has no conductivity value.

Which titanium grade is the toughest?

Our data cannot rank the four grades on toughness: only Grade 23 has fracture toughness, and impact energy exists for the other three on different bases. Charpy V-notch minimums are 41 J for Grade 2 (product form and direction not stated) and 48 J for Grade 9 pipe in the L direction, against a range of 18 to 30 J for annealed Ti-6Al-4V. Test temperature is not stated, and minimums and typical ranges are not comparable.

Grade 23 fracture toughness comes from one 1991 research forging of an ELI alloy with nominal oxygen 0.10 mass %, heat treated 7.2 ks at 973 K and tested by compact tension. K_Ic(J) was 57.5 MPa√m at 293 K, 42.9 at 77 K and 44.9 at 4 K. Use toughness at the service temperature, not the room-temperature figure. Carpenter and Titanium Industries tie the ELI limits to better cryogenic behaviour than standard Ti-6Al-4V, but our data holds no fracture toughness for standard Ti-6Al-4V, so we cannot show the difference.

Ductility is the other side: Ti-6Al-4V is listed at 10 to 18 % elongation annealed and Grade 23 annealed bar shows 10 %, against the 20 % (Grade 2) and 15 % (Grade 9) minimums above.

Which titanium grade resists corrosion best?

Grade 2 has the only quantified seawater statements and the only comparative test in our data, which makes it the best documented pick for seawater below 80 °C, not a proven safest one. For its Grade 2 tube, Alleima reports excellent general corrosion resistance in seawater and brines, no crevice corrosion below 80 °C at any pH, immunity to stress corrosion cracking in seawater, and negligible erosion in flowing seawater up to 40 m/s. These are manufacturer statements without test conditions.

The limit is hot, near-anaerobic seawater. Pang and Blackwood (2016) tested Grades 2, 5 and 7 in seawater with under 1 ppm oxygen, up to 200 °C. Grade 5, which is Ti-6Al-4V, showed crevice corrosion at 200 °C against 80 °C for Grade 2. That is a laboratory test, not a service limit, and it does not say why. The usual step up for hot chloride crevices is a palladium-stabilised grade (general engineering knowledge, not in our data); the Ti-6Al-4V profile names Grades 24 and 25, said to raise resistance to above 200 °C, and we hold no profile or test for them.

The other grades rest on statements. Bibus calls Grade 9 seawater resistance excellent, Smiths High Performance lists excellent corrosion resistance, and Smiths Metal Centres calls it good in oxidising, neutral and mildly reducing media, including chlorides. Titanium Industries lists marine uses for Grade 23. A supplier says Ti-6Al-4V is rapidly attacked by hydrofluoric acid, and Alleima advises against strong reducing acids, fluoride solutions, pure oxygen and anhydrous chlorine, and notes that titanium can speed corrosion of a less noble metal it touches.

Which titanium grade is easiest to weld and form?

Supplier ratings, not a test: Smiths calls Grade 2 weldability excellent, Smiths calls Grade 9 good while Boze calls it excellent for its seamless tube, and Ti-6Al-4V is rated fair. Grade 9 also cold-forms, unlike Grade 5. Alleima describes TIG welding of Grade 2 tube with inert gas on both sides, low heat input and normally no post-weld heat treatment, with springback about twice that of stainless steel. Our data has no welding or forming information for Grade 23 and no shielding guidance for Grade 9 or Ti-6Al-4V. General engineering knowledge, not in our data: hot titanium takes up oxygen, nitrogen and hydrogen and turns brittle, so shield every weld and heat treatment as for Grade 2 and ask the fabricator.

The bend radii in our data are not interchangeable. Grade 2 is Smiths' specific bend radius, 2.0 times thickness below 0.070 in and 2.5 times above. Grade 9 is the ASTM B265 bend-test mandrel (the rod the sample is wrapped around), an acceptance test on sheet, not a design radius: 2.5T below 0.070 in and 3T from 0.070 to 0.187 in. For Grade 9 tube Nexmetal gives 2.5 to 3.0 times OD as the minimum centreline radius (measured to the middle of the tube) for reliable production bends. Ti-6Al-4V is a supplier fabrication value, 4.5 times thickness below 0.070 in and 5.0 times above.

What are the uses and handling risks?

Grade 2 goes into seawater heat exchangers and desalination pipe, Grade 9 into hydraulic tube and bicycle frames, Ti-6Al-4V into aerospace bar, forgings and sheet, and Grade 23 into airframes, cryogenic and high-pressure vessels, and implants (Carpenter cites ASTM F136).

Treat swarf as a fire hazard. PubChem reports titanium as a flammable solid (H228) in aggregated reports, which applies to powder and swarf, while the Grade 23 profile read a record that lists it as Not Classified. Selection risks differ by grade: Ti-6Al-4V has poor shear strength and surface wear in some loadings, cold-worked Grade 9 tube can wrinkle or crack in tight bends, and the Grade 23 toughness data come from one research forging, with a drop at 77 K and 4 K.

Terms used

Yield strength

the stress where a metal starts to deform for good. Design loads are normally set below it.

Minimum and typical values

a minimum is what the specification guarantees; a typical value is a datasheet average that is not guaranteed. The two are not comparable.

Annealed

heated and cooled slowly to soften the metal. It is the usual supply condition for titanium sheet, plate and bar.

Cold-worked and stress-relieved (CWSR)

tube drawn cold to raise strength, then given a low-temperature stress relief. It is stronger and less ductile than annealed tube. ASTM B338 labels annealed Grade 9 tube row 9A and CWSR row 9D.

Solution treated and aged (STA)

a heat treatment that raises strength further, in Ti-6Al-4V at a cost in elongation.

ELI

extra-low interstitial: tighter limits on oxygen, iron, carbon, nitrogen and hydrogen than the standard grade. Grade 23 is the ELI version of Ti-6Al-4V.

Grade and UNS numbers

Grade 2 is UNS R50400, Grade 9 (Ti-3Al-2.5V) is UNS R56320, Grade 5 (Ti-6Al-4V) is UNS R56400 and Grade 23 (Ti-6Al-4V ELI) is UNS R56407. Name the grade and the UNS number on the order.

Charpy V-notch impact

the energy a notched bar absorbs when a pendulum breaks it, in joules.

Fracture toughness

how well a metal resists a growing crack, in MPa√m. It is a different test from Charpy impact. K_Ic(J) is a value estimated from a J-integral test.

Crevice corrosion

local attack inside a tight gap, such as under a gasket, where the liquid stops moving.

Elastic modulus

stiffness: the stress carried per unit of elastic stretch. A lower modulus flexes more under the same load.

What to check before you order

1. Ask for grade, UNS number, product form, specification and condition together, for example Grade 9 (UNS R56320) tube to ASTM B338 row 9A (annealed) or row 9D (cold-worked and stress-relieved).
2. For Grade 23, name ASTM B265 or B348 Grade 23, AMS 4907 or ASTM F136: the hydrogen maximum is 0.0125 % in B265 and B348 and 0.012 % in F136.
3. For hydraulic tube, name the AMS number, the grade and the condition. Our data does not settle which grade or condition AMS 4943, 4944 and 4945 cover (see the strength section), so do not assume the same condition for all three, and read the grade on the mill certificate.
4. For cold service, ask for toughness at 77 K or 4 K for your product form.
5. For hot seawater, chloride or acid service, ask for corrosion data in your medium, temperature and oxygen content, and avoid crevices above 80 °C.

What do we not know yet?

• Fracture toughness exists only for Grade 23, from one 1991 research forging. The other five profiles hold none.
• Grade 9 has no melting point, thermal conductivity, expansion or specific heat. Grade 2 has no melting point or expansion number. No titanium profile here holds strength at an elevated temperature, and the 400 °C figure for Ti-6Al-4V is a Wikipedia statement, not a test.
• Corrosion for Grade 9, Grade 23 and Ti-6Al-4V is supplier statements with no test conditions; no crevice, pitting or stress corrosion test is stored for them. No corrosion rate is stored for any of the six profiles.
• Fatigue is stored only for Ti-6Al-4V (500 to 600 MPa annealed at 10^7 cycles) among the titanium grades. 316 and 6061 have fatigue strength on other bases, so no fatigue comparison is made.
• Sources conflict or are thin: the grade and condition behind AMS 4943, 4944 and 4945, Grade 23 nitrogen limits that differ between suppliers, and the ASTM B265 copy behind Grade 2 and Grade 9, which is the 2008b edition. No welding data is stored for Grade 23.
• Not covered: palladium grades for hot chloride crevices, the commercially pure Grades 1, 3 and 4, and cost, lead time and stock availability by form. Grade 9 is mostly tube, so sheet is thinly covered.

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