Tube Material Selection and Performance Data for Hydroforming

During hydroforming, the blank sees changing circumferential tension, axial compression or tension, contact pressure, bending, and friction. A uniaxial coupon does not reproduce that history. Nominal yield strength, tensile strength, and elongation are useful inputs, but they do not describe the complete forming response.

The blank already carries a manufacturing history when it reaches the die. Welding, extrusion or drawing, sizing, straightening, heat treatment, and pre-bending can change residual stress, anisotropy, hardness, dimensions, and local formability. Record those operations and the welded or seamless construction separately.

In the round-to-square process studied by Manabe and Amino, wall distribution changed with stress ratio, friction, strain hardening, and plastic anisotropy. The study does not rank those variables for every part. It does show why a purchase description such as “aluminum tube” or “stainless tube” leaves out data needed for forming analysis.

Frame the material decision around the supplied product and the proposed route:

Use the information available at quotation to define the starting condition, then add deeper evidence when simulation or validation requires it.

No item in the table proves hydroformability by itself. Together, the records define the starting condition that analysis and tryout must represent.

Yield and tensile strength describe part of the tube’s response after plastic flow begins. Elongation is a broad ductility indicator. Forming pressure and force also depend on geometry, wall, end loading, friction, die contact, restraint, and the load path, so none of these certificate values is sufficient on its own.

Strain hardening affects whether deformation spreads or localizes after yielding. Plastic anisotropy describes directional differences in material flow. Both may matter in tube made from rolled and welded strip, where the rolling direction and seam have a known orientation to the part.

Forming-limit data must also match the loading history. Die contact, axial feed, and corner filling can create a nonlinear strain path. In the broader context of sheet characterization, NIST’s multiaxial-material program notes that strain-based limits are path-dependent. Check the origin and applicability of a forming-limit curve; the NIST program is not a tube-qualification method.

Available tensile data and a conservative model may be enough for early screening. Large expansion, sharp transitions, structural service, high tooling cost, or limited trial opportunities can justify tube-specific data before the route is released.

Neither construction is automatically the safer choice.

Hydroforming research and production applications include both welded and seamless tube. Compare the specified products, their manufacturing records, and the intended route. A construction label does not establish local uniformity or formability.

For welded tube, keep the welding process, applicable tube standard, available weld and heat-affected-zone evidence, and the weld position relative to the proposed strain path in the material review. Definitions and measurement records for the seam profile, scarfed region, incoming wall, and lot geometry belong in the incoming-dimensions briefing linked below.

The SAE paper *Material Aspects of Tube-Hydroforming* discusses welded tube, biaxial data, and the link between material characterization and simulation. Its laser-welded examples show that a welded product can be developed for hydroforming under defined conditions; they do not qualify other welded tubes.

For seamless, drawn, or extruded tube, retain the tube-making route, surface condition, heat-treatment history, and representative material evidence. Removing the longitudinal seam removes one material variable; it does not remove the need for a separate incoming-dimensional review.

A mill test report establishes traceability and specification compliance. It may not provide the local response needed for simulation. Rolling, welding, sizing, drawing, and heat treatment can change values measured before tube making; a specimen cut from finished tube still represents only its test direction and stress state.

A hydraulic bulge test can provide biaxial data at strains relevant to tube expansion. Sokolowski et al. combined hydraulic bulging, analytical treatment, and simulation for roll-formed, laser-welded 304 stainless tube in their fixture. The study supports that test concept under its stated conditions, not a requirement to copy the fixture or material model for every project. The 2020 review gives broader context.

Other tube tests answer narrower questions. ISO 8492 covers flattening of circular metallic tube, ISO 8493 covers drift expansion, and ISO 8496 covers ring tensile testing within their stated scopes. They can expose particular defects or deformation behavior. Passing one is not a hydroformability certificate for a complex part.

Select the tube with the part geometry and process path in view. Tight corners demand more local stretch than generous radii. Large perimeter growth may need axial feed, while bends and die contact can block that feed. Starting diameter and wall also change sealing, bending, pressure demand, and contact.

A material change after tool design reopens:

1. the simulation material model, including stress–strain response, hardening, and anisotropy;
2. pressure and axial-feed paths;
3. pre-bending and preform response;
4. predicted thinning, thickening, and instability;
5. springback or dimensional recovery where relevant;
6. seam orientation and acceptance criteria.

Published work covers several alloy families under material-specific conditions. For a ShuiYiYuan project, the drawing review uses the actual grade, delivered condition, available data, and proposed route before the forming route is defined.

Put known inputs and open assumptions in one material record. Use the same baseline for simulation, tryout, and validation.

Record available information and mark every missing item:

• material and tube standards, grade, and delivery condition;
• tube construction: welded or seamless;
• welding process and available weld or heat-affected-zone evidence where applicable;
• tube manufacturing route such as extrusion, drawing, sizing, or straightening;
• heat treatment and any controlled hardness condition;
• the controlled incoming-dimensional record for the tube and blank;
• available certificate data and representative tube tensile or bulge data;
• prior bending, piercing, flattening, sizing, or preforming operations;
• critical final wall, geometry, interface, and functional validation requirements.

Geometry, quantity, and commercial scope may sit elsewhere in the project file. Keep missing material data as explicit assumptions until the supplier provides them or the parties agree on characterization.

A narrow-margin part may justify tube bulge testing, simulation, or third-party characterization. Before work starts, state the required work, method, and responsible party in the quotation or validation plan.

Before production-tool release, drawings and purchasing documents should define the incoming tube closely enough for later lots to represent the developed process. A purchasing-approved substitute may still form differently. Recheck it against the validated route and record which simulations, samples, measurements, or functional tests must be repeated.

Conforming incoming material does not guarantee a conforming formed part. The validation plan must link tube controls to the finished geometry, wall, and performance requirements.

Frequently asked questions

References

1. Koç, M., and Altan, T. “An overall review of the tube hydroforming (THF) technology.” Journal of Materials Processing Technology, 108(3), 384–393, 2001.
2. Bell, C., Corney, J., Zuelli, N., and Savings, D. “A state of the art review of hydroforming technology.” International Journal of Material Forming, 13, 789–828, 2020.
3. Manabe, K., and Amino, M. “Effects of process parameters and material properties on deformation process in tube hydroforming.” Journal of Materials Processing Technology, 123(2), 285–291, 2002.
4. Gerlach, J., Blümel, K. W., Kneiphoff, U., and Eyl, G. “Material Aspects of Tube-Hydroforming.” SAE Technical Paper 1999-01-3204, 1999.
5. Sonobe, O., Hashimoto, Y., Iguchi, T., and Abe, H. “Effect of Mechanical Properties on Formability in Hydroforming of ERW tubes.” SAE Technical Paper 2003-01-2737, 2003.
6. National Institute of Standards and Technology. “NCAL: Multiaxial Material Performance.”
7. ISO 8492:2013, Metallic materials — Tube — Flattening test.
8. ISO 8493:1998, Metallic materials — Tube — Drift-expanding test.
9. ISO 8496:2013, Metallic materials — Tube — Ring tensile test.
10. Sokolowski, T., Gerke, K., Ahmetoglu, M., and Altan, T. “Evaluation of tube formability and material characteristics: hydraulic bulge testing of tubes.” Journal of Materials Processing Technology 98(1), 34–40 (2000).