Summary
Define underfilling at a specified section, process stage, and measurement condition. A photograph or a general statement that the corner looks open is not enough.
Target radius, transition geometry, preform shape, contact sequence, pressure, axial feed, and friction act together. None supplies a universal correction by itself.
Use controlled comparisons to determine what changed. Simulation and local wall evidence can narrow the investigation, but project acceptance still depends on measured parts and the specified acceptance method.
On this page
Define underfilling against a measurable target
Underfilling means that the formed surface does not reach the required contour at a defined location. The comparison needs more than a nominal corner radius. It should identify the section location and orientation, target contour, process stage, measurement method, and applicable acceptance requirement.
A profile measured after preforming answers a different question from one measured after final calibration and unloading. A cut section, scan, coordinate measurement, or dedicated gage may also report the feature differently. Retain the method and datum used so that samples from separate trials remain comparable.
| Record | What to identify | Why it matters |
|---|---|---|
| Target | Drawing or model revision, section location, orientation, contour, and tolerance | Establishes what the sample is being compared with |
| Sample condition | Tube lot, preform condition, forming stage, and unloading condition | Prevents intermediate and finished profiles from being mixed |
| Measurement | Method, datum, section extraction, and reported deviation | Makes samples and trials comparable |
| Acceptance | Controlled feature, limit, and sampling requirement | Separates diagnosis from final disposition |
Separate geometry demand from process response
The target corner radius describes the required shape, not how material will reach it. A smaller radius can make the final filling stage more demanding, while an abrupt axial transition can change local restraint and the available material path. Neither observation provides a universal minimum radius, transition-length ratio, or pressure.
Kridli and coauthors examined corner filling and wall-thickness response for straight round tubes formed toward a square section under their stated model and test conditions. Their work supports evaluating die radius, starting wall, strain hardening, and pressure together. It does not define a transferable radius or pressure for an unrelated three-dimensional part.
The actual tube and manufactured preform set the initial gap to the die and the regions that can contact first. Koç and Altan likewise treat geometry, material response, pressure, and axial feed as coordinated inputs. Their analytical screening does not supply a process recipe for a complex part.
Cross-section perimeter screening can identify stations with high circumferential material demand, but this page does not repeat that calculation. Underfilling diagnosis starts with the target and actual contour, the real preform, and the path by which contact developed.
Reconstruct the die-contact sequence
Once part of the tube contacts the die, that region becomes more constrained and friction can restrict later movement. The contact sequence therefore affects which areas continue to deform during the final filling stage.
Wang, Song, and Yuan studied an annealed 5A02 aluminum tube through hydro-bending, preforming, and hydroforming of a specified polygonal-section part. In their configuration, deformation concentrated at a corner that contacted late, and bursting occurred there under the reported conditions. This supports reconstructing contact as part of diagnosis; it does not support a rule that the last-contact corner always underfills or fails.
- measure the preform in its actual tool orientation;
- identify the first stage at which the contour deviation becomes measurable;
- compare tool witness marks and surface-contact patterns where they are interpretable;
- use planned intermediate samples or simulation contact output only when they belong to the project scope;
- align contact observations with pressure and punch-position records on one time base.
Compare pressure, feed input, and material delivery
Pressure helps drive the wall toward the cavity, but pressure alone does not show whether enough material is available at the corner. Raising final calibration pressure can increase local deformation without removing the restriction that caused the shortage. The contour may remain underfilled, or the risk may shift toward local thinning or splitting.
Axial feed also requires two separate records. Punch travel describes a machine action. Material delivery describes how much tube material actually reaches the forming zone. Die contact, friction, bends, and the preform can prevent additional punch motion from reaching the corner.
| Signal | What it can show | What it cannot establish alone |
|---|---|---|
| Pressure history | Fluid-pressure timing and level during the cycle | Local die contact or sufficient material at the corner |
| Punch position or axial-feed command | End-tool movement and timing | Material delivered to a remote section |
| Contour measurement | Where and by how much the target was missed | The unique cause |
| Local wall measurement | How material responded at selected points | The complete material path or final root cause |
D’Amours and coauthors compared corner-fill radii and end-feed levels for specified seam-welded AA5754 tubes. The response depended on the tested combination, and added feed was not a simple monotonic improvement. Their values and outcomes do not define stroke, pressure, friction, or acceptance for another part.
Friction, Lubrication, and the Contact State
Friction matters after contact because it affects whether the wall can slide and whether material introduced at the tube ends can reach the intended section. The condition includes lubricant identity and application, tube surface, die surface, contact pressure, and the order in which contact develops.
Orban and Hu used an analytical round-to-square model to examine wall thinning during corner filling. Under their assumptions, contact friction restricted material flow and changed the predicted strain and wall response. The model supports including friction in the diagnosis; it does not establish a production coefficient, lubricant choice, or correction for a different geometry.
Lower friction is not a complete process objective. A useful comparison records lubricant, application coverage, surface condition, and resulting contact pattern while holding the other planned inputs as consistent as practicable. An undocumented surface or lubricant change can otherwise be mistaken for a pressure or feed effect.
Use local wall evidence as a diagnostic clue
Local wall measurements help show how the corner approached the target. An underfilled contour with substantial local thinning may indicate that the wall was being stretched without enough material delivery. A contour that remains open with limited local deformation may justify a closer look at the initial gap, contact state, or pressure stage. Neither pattern proves a single root cause without the cycle and contact records.
Use the actual incoming wall-thickness distribution rather than nominal wall alone, and tie each measurement to the same section and coordinate system used for contour assessment. Local thickening or a ripple upstream may justify checking whether material accumulated before reaching the target zone, but the shape still needs its own diagnosis.
This article uses wall thickness only as supporting information. General thinning mechanisms, measurement-method selection, and finished-wall acceptance remain separate topics; no universal thinning limit is introduced here.
Build a controlled tryout comparison
A useful comparison changes one planned input, preserves the other recorded conditions as closely as practicable, and repeats the same contour and wall measurements. The project safety limits, tool limits, and validation plan govern what may be changed.
| Question | Controlled comparison | Conclusion limit |
|---|---|---|
| Does the starting gap affect contact? | Compare verified preforms with a documented local geometry difference | Applies only to the compared preforms and cycle |
| Does the final pressure segment affect the remaining gap? | Compare an approved change to the final pressure segment while retaining the other recorded inputs | Does not establish a universal corner-fill pressure |
| Does punch motion reach the corner? | Compare punch records with contour and local material-response measurements | Punch travel still does not equal delivered material |
| Does contact friction contribute? | Compare a documented lubricant or surface condition under an approved plan | Does not define a universal friction value |
| When does the deviation begin? | Compare planned intermediate profiles or samples | Does not by itself identify the upstream cause |
Simulation, when it belongs to the project scope, can compare contact sequence, strain concentration, and response to input changes. It remains tied to its geometry, material data, contact assumptions, and boundary conditions. It cannot replace measured samples or the agreed acceptance procedure.
Acceptance Records and Project Handoff
A project review is more useful when it receives the condition that produced the part, not only the final sample. The handoff should identify:
- the drawing and model revision, target section, contour, and tolerance;
- actual incoming tube dimensions, wall data, material condition, relevant seam orientation, and tube lot;
- the measured preform and its die orientation;
- pressure and punch-position records on a common time base;
- lubricant, application method, and noted surface or contact changes;
- samples from the first observed deviation and relevant comparison cycles;
- contour measurements and selected local wall measurements with their methods;
- the acceptance stage, measurement method, and sampling requirement.
These records support a project-specific scope discussion; they do not guarantee that a different radius, pressure, feed path, lubricant, or preform will resolve the part. To submit the geometry and available records for an initial review of scope, use the Hydroforming capabilities page.
Frequently asked questions
Why does a hydroformed tube fail to fill the die corner?
Is higher calibration pressure the first correction for underfilling?
Is there a minimum corner radius that every hydroformed tube can fill?
Does more axial feed guarantee better corner filling?
Does the last corner to contact the die identify the root cause?
Can simulation prove that the finished corner is acceptable?
References
- Kridli, G. T., Bao, L., Mallick, P. K., and Tian, Y. “Investigation of thickness variation and corner filling in tube hydroforming.” Journal of Materials Processing Technology, 133(3), 287–296, 2003.
- Orban, H., and Hu, S. J. “Analytical modeling of wall thinning during corner filling in structural tube hydroforming.” Journal of Materials Processing Technology, 194, 7–14, 2007.
- Wang, X., Song, P., and Yuan, S. “Investigation on Corner Filling Process in Hydroforming of Thin-Walled Aluminum Alloy Tubular Part with Polygonal Sections.” Materials Transactions, 53(5), 796–800, 2012.
- D’Amours, G., Rahem, A., Williams, B., Worswick, M., and Mayer, R. “Crashworthiness of Aluminium Tubes; Part 1: Hydroforming at Different Corner-Fill Radii and End Feeding Levels.” AIP Conference Proceedings, 908, 787–792, 2007.
- Koç, M., and Altan, T. “Prediction of forming limits and parameters in the tube hydroforming process.” International Journal of Machine Tools and Manufacture, 42(1), 123–138, 2002.