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Developed forming process and guidelines for tube applications

The presented guidelines concentrate on the EM- tube compression process, which is analysed assuming that it is applied as a preforming operation for hydroforming workpieces. An important criterion for the process is that the used energy should be as small as necessary, i.e. the efficiency coefficient should be as high as possible.

With regard to this aspect, the following conclusions for a suitable choice of material and process parameters can be drawn:
- The electric conductivity of the material should be as high as possible.
- The gap width between workpiece and tool coil should be as small as possible.
- The inductance of the tool coil has to be applied to the forming task.

The assessment of the resulting preform is based on geometric criteria and material properties. The most important aspects are the roundness of the preform, the resulting longitudinal contour of the preform, and the hardening of the material.
For the assessment of the achieved roundness it has to be taken into account that the compression process is a preforming operation and that the calibration shall be realized by the subsequent hydroforming step. It is not necessary to achieve the geometric tolerances requested for the final part because up to a certain limit geometric deflections can be corrected and wrinkles are reversible.
The material properties have an important influence on the workpiece behavior and the forming operation. Important aspects are the stress strain behavior of the material, especially the yield strength, and the stiffness of the tube which is mainly determined by the ratio of radius and wall thickness of the tube.

In general it can be said that the material should be as homogenous and as round as possible because inhomogeneities might cause material failure. In case of electromagnetic compression this means wrinkling.

In addition to the material properties also the process parameters of the electromagnetic compression process have an important influence on the roundness of the resulting preform. Concerning the investigation of the roundness, the pressure course, the tangential strain, and the strain rate are the most important process parameters. Regarding the free forming process, an increasing tangential strain is correlated to a worsening of the roundness. This wrinkling effect can be reduced by increasing the strain rate. If this potential is depleted a further improvement can be realized by using a mandrel because of the supporting effect and increasing the pressure impulse.

This means that
- The maximum radial deformation is limited by the allowable wrinkling effect.
- The strain rate should be as high as possible to reduce the wrinkling effect.
- The maximum pressure in a forming operation with a mandrel has to be chosen high enough to avoid irreversible wrinkling.

In addition to the pressure course also the distribution of the magnetic pressure over the axis of the tube significantly influences the forming result.

Here, the most important aspects are
- The transition region between the compressed and the undeformed area of the workpiece and
- The length of the compression zone.

Regarding the possibilities to adjust the pressure profile, it has to be differentiated between a setup with a direct acting tool coil and with an indirect acting tool coil respectively with a tool coil including a fieldshaper. In case of a direct acting tool coil the pressure distribution is mainly determined by the number of turns per unit of length of the coil. The application of a fieldshaper offers the possibility to adapt the axial pressure distribution and thereby influence the contour of the workpiece.
Concerning the length of the compression zone, it can be said that the maximum length that can be compressed during one single forming step is limited. A possible solution for the compression of even longer areas of the tube is offered by the sequential forming. Thereby, during each forming step a tube compression process is being performed and than the workpiece is moved in axial direction relative to the tool coil.

In addition to the described geometric properties of the preform the hardening behavior of the preform represents an important aspect. The hardening, resulting from the electromagnetic preforming operation, is of particular importance for the subsequent hydroforming step because the remaining forming capability in the preformed workpiece areas will be reduced. Therefore, the strain hardening of the used material should be as low as possible. As it is known from quasistatic forming operations the hardness of a material increases with a rising deformation due to the strain hardening effect. Within the range of the velocities which are reached by EMF the strain hardening seems to depend only on the tangential strain and not on the strain rate.

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