3 Fundamentals of press design

418 Hydroforming
Straight component axis
The inflation limit is restricted by the risk of bursting and buckling. The
likelihood of buckling increases with the length of the expanded section.
With a buckling length � 2 · the diameter of the tubular blank
(starting diameter), and if a rotationally symmetrical cross section is
being processed, the achievable deformation may be assumed to lie well
beyond the ultimate elongation of the material.
Axial curvature
In the case of components with a curved axis in the inflation contour,
axial displacement of material is possible without problems, provided
the curvature radius is R � 5 · the diameter of the initial preform. Partial
inflation takes place here at the inner shell of the bend.
The free buckling length should not exceed the starting diameter,
extend over no more than half the circumference of the tubular blank
and at the same time it should be continuously decreasing.
In free forming without support, the component buckles if axial pressure
is applied. The inflation capacity is limited by the ultimate elongation,
as forming takes place purely as a result of the internal pressure.
Cross sections
During free expansion, the maximum deformation is reached when the
free buckling length amounts to a maximum of 2 · starting diameter, and
the cross section is rotationally symmetrical and located in the tube axis.
An asymmetrical cross section with a deep, narrow contour area can
only be formed in cases where material is able to flow axially into the
contour. Sharp contour transitions lead to partial and localized stretching
in the wall and thus to premature bursting. Asymmetrical cross sections
outside the tube axis can be formed in cases where at least half the
circumference of the tubular blank is supported by the die contour.
Longitudinal sections
Contour transitions are best designed with large radii. Suddenly changing
transitions in the direction of material flow lead to the formation of
cross-wrinkles at the component (buckling) and act as a brake to material
flow in the axial direction. Beyond these underlying guidelines, the
following points should be observed in the configuration of hydroformed
components:
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998