3 Fundamentals of press design

424 Hydroforming
In the case of ferritic steel, the type and practicality of heat treatment
depend heavily on the respective material and its forming history. On
principle, either of two possible processes can be used: recrystallization
and normal annealing.
Generally, Mn or Si alloys are used for aluminium.The most favorable
method of heat treatment must be determined in each individual case,
whereby particular attention must be paid to the long-term behavior of
the material, and in the case of strain hardening alloys, to rapid further
processing.
5.5.2 Preforms and preparation
For hydroforming, parts made of the following semi-finished products
are used (Fig. 5.2.8):
– drawn or welded tubes,
– double-walled tubes,
– extruded profiles,
– welded or pre-formed tubular blanks.
In general, tubular preforms with circular cross sections are used.
Welded and extruded non-round sections are used less frequently.
The production steps required for the preparation of preforms for
technically and economically optimum production are described in the
following with reference to tubular blanks.
The tube cutting process must be extremely precise in order to avoid
undesirable effects such as leaks at the beginning of the hydroforming
sequence. These would lead to failure of the component as a result of
buckling. Fluctuations in the wall thickness along the periphery result
in a high scrap level, and in extreme cases even to failure of the component.
The following tolerances are generally admissible:
– length: +/– 0.5 mm
– angle of the cross section relative to the longitudinal axis: +/– 0.5°
Concentricity at the tube ends is important in order to avoid unwanted
shearing when the die is closed. The tubes must be cleaned of any
chips, as these lead to tool damage, forming errors or marks on the sur-
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998