TA18 Titanium Alloy Tube (Ti-3Al-2.5V): Properties, Processing and Applications
Mar 12, 2024
What Is TA18 Titanium Alloy Tube?
TA18 is the Chinese designation for the near-alpha titanium alloy Ti-3Al-2.5V, developed from the Ti-6Al-4V (TC4) alloy system but with reduced aluminum and vanadium content to improve cold formability and weldability. In international specifications the same composition is known as Grade 9 (UNS R56320). TA18 tubing combines room-temperature and elevated-temperature strength about 20-50% higher than commercially pure titanium with significantly better cold forming and welding performance than TC4 alloy.
Chemical Composition and Alloy Design
The alloy contains nominally 3% aluminum and 2.5% vanadium, with the balance titanium. Aluminum stabilizes the alpha phase and provides solid-solution strengthening; vanadium stabilizes the beta phase and improves workability. Because the total alloying addition is lower than in Ti-6Al-4V, TA18 retains a fine near-alpha microstructure in the annealed condition, giving it an attractive combination of medium strength, ductility and resistance to stress corrosion.
Mechanical Properties and Comparison
| Property | TA18 / Grade 9 (Ti-3Al-2.5V) | CP Titanium Grade 2 | TC4 / Grade 5 (Ti-6Al-4V) |
|---|---|---|---|
| Tensile strength, min | 620 MPa | 345 MPa | 895 MPa |
| Yield strength, min | 483 MPa | 275 MPa | 828 MPa |
| Elongation, min | 15% | 20% | 10% |
| Cold formability | Good | Excellent | Limited |
| Weldability | Good | Good | Moderate |
Values for TA18 are based on ASTM B861 Grade 9 tube requirements. The comparison shows why TA18 is selected when higher strength than CP titanium is needed but the part still has to be formed or welded without difficulty.
Applications
TA18 seamless tube is widely used in aerospace hydraulic, fuel and pneumatic systems operating up to about 315 deg C, where pressure resistance, light weight and leak integrity matter. Typical components include aircraft hydraulic lines, engine piping and lightweight structural tubing. Outside aerospace, TA18 tube is used for bicycle frames, marine instrumentation, chemical processing lines and other applications requiring a weldable, corrosion-resistant, medium-strength titanium tube.
Processing and Fabrication
Cold Working and Forming
TA18 can be cold worked and cold formed. In cold pilgering and drawing, the feed rate, rolling speed and friction must be balanced: excessive feed increases forming load and can cause cracking or folding, while higher rolling speed reduces the strain components and vertical load. For thick-walled tubes prone to internal surface defects, a slightly higher friction condition is generally recommended to control wall thickness distribution.
Surface Finishing
Electrolytic-magnetic composite grinding is an effective finishing route for TA18 tube surfaces. Under controlled conditions, burrs and scratches are removed and surface roughness can be reduced from about Ra 1.6 um to Ra 0.13 um, improving fatigue performance and corrosion resistance. Finished tubes are typically supplied pickled, polished or passivated.
Welding
TA18 welds readily with gas tungsten arc welding using matching filler, with argon shielding on both sides to prevent oxygen and nitrogen contamination. Post-weld annealing is applied when maximum ductility is required.
Standards and Equivalents
China: TA18 per GB/T 3625 and GB/T 12969.
International: Grade 9 per ASTM B861 (seamless tube), ASTM B338 (welded tube), ASTM B862 (welded large-diameter tube).
Aerospace: AMS 4943 (hydraulic tubing), AMS 4944.
UNS: R56320.
FAQ
Is TA18 the same as Grade 9 titanium?
Yes. TA18 is the Chinese designation for Ti-3Al-2.5V, which corresponds to Grade 9 (UNS R56320) in ASTM and AMS specifications.
What is the maximum service temperature of TA18 tube?
TA18 components are commonly designed for service up to approximately 315 deg C, where the alloy retains useful strength and oxidation resistance.
Why choose TA18 instead of TC4 (Ti-6Al-4V)?
TA18 offers better cold formability and weldability than TC4 while providing about 20-50% higher strength than pure titanium; TC4 is preferred only where maximum strength is required and forming complexity is low.
Can TA18 tube be welded?
Yes. TA18 has good weldability using GTAW with argon shielding; post-weld annealing is recommended when full ductility is needed.
What surface finishes are available for TA18 tube?
Pickled, polished, passivated and specially ground finishes are available; electrolytic-magnetic grinding can achieve Ra values down to about 0.13 um.
What standards govern TA18 tube supply?
Internationally, ASTM B861 and ASTM B338 govern seamless and welded tube respectively, with AMS 4943 for aerospace hydraulic tubing; in China, GB/T 3625 applies.
