Titanium Deep Processing Core Processes: Bending, Stamping, Spinning and Tube Expanding

Deep processing turns titanium plate, sheet, tube and bar into finished components such as pipe elbows, ducting, pressure vessel heads, heat exchanger tubesheets and spun housings. Titanium is more difficult to form than steel or aluminum, so each process must be matched to the alloy condition, the wall thickness and the required final geometry. This article reviews the four core deep-processing processes - bending, stamping, spinning and tube expanding - with the parameters and controls that determine success.

1. Why Titanium Is Difficult to Form

Three material properties dominate titanium forming behavior. First, the elastic modulus is about 103-114 GPa, roughly half that of steel, which produces large elastic recovery (springback) after forming. Second, the yield-to-tensile ratio is high, so the plastic working range is narrow and small errors in tooling geometry translate into large shape errors. Third, titanium reacts rapidly with oxygen and nitrogen above about 600 C, forming a brittle alpha case that must be avoided or removed. Practical consequences: bending and stamping tools need overbend allowances, hot forming must be done under controlled atmosphere or with tight process windows, and lubrication must prevent galling between the part and tool surfaces.

2. Bending of Titanium Tubes and Profiles

Tube bending combines elastic and plastic deformation; the outer wall thins and the inner wall thickens, while the neutral axis shifts outward. For titanium tube, the minimum bend radius is typically 2 to 3 times the tube diameter for mandrel bending, and springback of 5-15 degrees must be compensated by tooling overbend. Cold bending is practical for tubes below about 50 mm outside diameter and wall thickness ratios that avoid buckling; after cold bending, stress-relief annealing at 480-650 C is recommended to restore ductility and stabilize dimensions. Hot bending, performed as rotary draw bending or push bending at 600-800 C, reduces the yield strength and springback significantly and allows tighter radii and thicker walls, but requires protection against oxidation. Tension (stretch) bending is preferred for large-radius profiles where cross-section distortion must be minimized.

3. Stamping and Forming of Titanium Sheet

Stamping of titanium sheet is harder than for carbon steel or aluminum because of the larger bend radius required (typically 2-4 times the sheet thickness for 90-degree bends in Grade 2, more for high-strength alloys) and the strong directionality of rolled sheet. Three routes are used in practice: cold forming for thin sheet (up to about 3 mm) with small deformation and moderate precision; hot forming at 600-800 C for thick plate and large parts, where lower flow stress allows deep draws; and pre-forming followed by hot straightening, which combines the accuracy of cold work with the formability of hot work for complex shapes. Deep drawing of titanium requires careful blankholder force control, generous corner radii and effective lubrication; intermediate annealing is often needed for multi-stage draws.

4. Spinning (Rotary Forming)

Spinning, or metal spinning, progressively deforms a rotating blank over a mandrel using rollers. The process combines elements of forging, extrusion, stretching and bending, and is highly efficient for axisymmetric parts such as cones, domes, cylinders and nozzles. Compared with press forming, spinning on titanium offers material savings of 20-50 % because the starting blank can be close to the final developed shape, high surface finish, and dimensional accuracy on diameters of +/-0.5 mm or better. Shear spinning (flow forming) can reduce wall thickness intentionally, producing seamless conical and cylindrical shells with refined grain structure and improved mechanical properties. Titanium parts produced by spinning include aerospace ducts, chemical vessel heads, rocket motor casings and centrifuges.

5. Tube Expanding and Tube-to-Tubesheet Joints

Tube expanding is a mechanical joining method that deforms the tube wall radially into grooves or against a tubesheet hole to create a leak-tight, load-bearing joint, widely used in shell-and-tube heat exchangers, condensers and coolers. Three techniques are common: mechanical rolling, where tapered rollers expand the tube under controlled torque; hydraulic or flexible expanding, which uses liquid pressure or a flexible bladder for uniform expansion over long lengths; and explosive expanding, where a detonating charge expands the tube almost instantly, suited to thick tubesheets and difficult materials. The degree of expansion must be controlled precisely - typically 3-8 % wall reduction depending on the joint design - because under-expansion leaks and over-expansion causes wall thinning, work hardening and stress corrosion susceptibility. After expanding, the tube end is often flared or welded to the tubesheet for a stronger seal, and the joint is pressure-tested per the applicable code.

6. Process Selection and Quality Control

Select the process by geometry: axisymmetric hollow parts favor spinning; prismatic or box-like parts favor stamping; tubes and profiles favor bending; tube-to-tubesheet connections favor expanding. Regardless of route, verify the alloy condition (annealed, cold-worked or solution-treated), control forming temperature within the specified window, apply appropriate lubricants, and inspect for surface cracking, orange peel, wall-thinning beyond tolerance, and alpha case after any hot working. Dimensional checks, dye-penetrant testing on suspect areas, and hardness or tensile verification on test coupons complete the acceptance package. Formed titanium components are normally supplied to the same material standards as the starting product, for example ASTM B338 or ASME SB338 for tube, with the forming route documented in the manufacturing record.

7. FAQ

Q1: What is the minimum bend radius for titanium tube?

For mandrel bending of small-diameter annealed tube, a centerline radius of 2-3 times the tube diameter is a common minimum; tighter radii require hot bending, special tooling or a different alloy condition.

Q2: Does titanium spring back more than steel?

Yes, significantly. Because the elastic modulus of titanium is about half that of steel, springback is roughly two to three times larger; tooling must include overbend compensation, verified by trial forming.

Q3: Can titanium sheet be deep drawn?

Yes, with limitations. Grade 1 and Grade 2 sheet draw well when corner radii are generous, blankholder force is controlled, and lubrication is effective; high-strength alloys such as Grade 5 usually require hot forming or intermediate annealing for deep draws.

Q4: What is the advantage of spinning over stamping for titanium?

Spinning uses lower forming forces, requires less expensive tooling, and saves 20-50 % material for axisymmetric parts, while achieving good surface finish and controlled wall thickness through shear spinning.

Q5: How is expansion degree controlled in tube expanding?

By controlling torque for mechanical rolling, pressure for hydraulic expanding, or charge size for explosive expanding, and verifying with tube wall measurements and pull-out or pressure tests on sample joints.

Q6: What defects should be inspected after hot forming of titanium?

Check for alpha case (oxygen-rich brittle surface layer), discoloration from inadequate shielding, cracking at corners, excessive wall thinning, and distortion. Remove alpha case by machining or pickling before service.