The Hot Finished Seamless Route
Hot finishing starts with a solid titanium billet that is heated into the working range and pierced on a rotary mill to form a hollow, then elongated and sized by plug or mandrel rolling. Working above the recrystallisation range breaks down the cast structure, closes porosity and produces a uniform alpha-beta microstructure without any welded seam. The finished tube is subsequently annealed in vacuum to remove residual stress and to keep the surface free of a hardened oxygen enriched layer. Compared with cold drawn tube, the hot finished route suits larger diameters and heavier walls and gives a slightly rougher, matt surface that is usually machined or pickled before use.
Grade 5 Chemistry and Property Requirements
Ti-6Al-4V is an alpha-beta alloy with nominally 6 percent aluminium and 4 percent vanadium, balanced to give a high strength to weight ratio and useful high temperature capability. Minimum properties for seamless tube are 895 MPa (130 ksi) tensile strength, 828 MPa (120 ksi) 0.2% offset yield strength and 10 percent elongation in 50 mm. The beta transus sits near 995 C, so hot working is performed above that temperature while annealing for delivery is carried out in the 700 to 790 C range. Nominal density is 4.43 g/cm3 and the elastic modulus is about 114 GPa, roughly half that of steel, which means the tube deflects more under load for the same section and must be designed accordingly.
| Item | Requirement or Typical Value |
|---|---|
| Tensile strength, min | 895 MPa (130 ksi) |
| Yield strength, 0.2% offset, min | 828 MPa (120 ksi) |
| Elongation in 50 mm, min | 10% |
| Oxygen, max | 0.20% |
| Beta transus | approx. 995 C |
| Density | 4.43 g/cm3 |
Automotive Applications and Why Weight Matters
In automotive work every kilogram removed from the exhaust, driveshaft or suspension linkage improves acceleration, braking and fuel consumption. Grade 5 tube is used for exhaust headers and downpipes, turbocharger inlet and outlet pipework, driveshafts on competition vehicles, roll cage inserts and suspension pushrods. The material also tolerates the thermal cycling of exhaust service better than aluminium and resists road salt and condensation attack without plating. For high volume production the usual compromise is a thin wall welded tube in Grade 2 for the cooler sections, with Grade 5 hot finished tube reserved for hot, highly stressed or safety related areas.
Machining, Welding and Heat Treatment
Hot finished tube is normally supplied in the annealed condition with machining allowance where the outside diameter will be turned. Titanium work hardens rapidly, so cutting speeds are held low, feeds are kept up to avoid rubbing, and sharp carbide tooling with copious coolant is used. Welding is by TIG with a full purge and a trailing shield, and the joint is protected until it has cooled below about 400 C. When higher strength is required, solution treatment at 900 to 950 C followed by aging between 480 and 595 C raises the yield strength, while stress relief in the 540 to 650 C band is used after forming to restore dimensional stability. All procedures are qualified before production, and the qualified range is recorded on the route card for each batch.
Purchasing, Testing and Certification
Seamless tubing is ordered to ASTM B338 for tube product and ASTM B861 for pipe product, with bar and forging stock to ASTM B348 and sheet or plate to ASTM B265. Automotive customers commonly add AMS 4928 for bar and billet traceability and AMS 4911 for sheet. Each lot is verified by spectrographic chemical analysis, tensile testing in the annealed condition, flattening tests and hardness checks, with ultrasonic or eddy current inspection available for hollows and finished tube. Deliveries include an EN 10204 3.1 mill test certificate, dimensional records and heat number traceability, and production quantities can be scheduled to a monthly call off so that a single heat number covers a whole programme run.
Frequently Asked Questions
Q: What does hot finished mean for a seamless tube?
A: The tube is pierced and rolled at elevated temperature rather than cold drawn. The result is a wrought, seam free wall with good density, suitable for larger diameters and heavier walls than cold drawing can handle economically.
Q: Does the hot finished surface need further treatment?
A: In most cases yes. The surface is pickled, blasted or machined to remove the oxygen enriched layer that forms during hot working, because that layer is hard and brittle.
Q: How does Grade 5 compare with aluminium for a driveshaft?
A: Grade 5 is roughly 60 percent denser than aluminium but about twice as strong, so a smaller, thinner tube carries the same torque and often weighs less once the required stiffness is met.
Q: Can the tubing be supplied in cut lengths?
A: Yes. Tubing is supplied in random lengths or cut to drawing, with ends plain, machined, bevelled or threaded, and the cut length tolerance agreed when the order is placed.
Q: What documentation comes with the material?
A: An EN 10204 3.1 mill test certificate giving chemistry and mechanical results, a dimensional inspection record and full heat number traceability. Additional third party inspection can be arranged for programme requirements.
