Rezultaty
An important precondition for the valuation and optimization of the electromagnetic preforming operation is the knowledge of significant process parameters and their influence on the forming process as well as on the forming result.
In the project, two different kinds of the EM-forming process are investigated: the compression of tubular workpieces and the forming of sheet metal.
The investigation of the EM-compression of tubular workpieces is carried out assuming that it is one production step in the process chain bending - electromagnetic compression hydroforming. This means that the final product of the electromagnetic compression is the semi-finished part for the hydroforming process.
This has to be considered for the choice of material and process parameters.
By considering the pressure course and its distribution, which affect the load on the workpiece, the influence of tool coil and characteristics of the forming machine are excluded. The pressure course and its distribution give rise to reactions on the tube in terms of radial deformation and deformation velocity, which affect the final geometric properties.
For the tube compression process the following process parameters have been defined and their correlations and interactions have been investigated:
- Current pulse
- Pressure course characterized by pressure maximum and pressure rise time
- Pressure distribution
- Radial deformation
- Forming velocity/ strain rate
- Geometric properties characterized by roundness and contour and for the sequential forming using sequential forming steps additionally
- Movement of the tube relative to the tool coil between two forming steps
The current pulse is one important aspect affecting the process. It is influenced by properties of the tool coil, the forming machine, and the charging energy of the capacitor battery. To achieve a good efficiency coefficient of the process, the frequency of the current pulse has to be optimized by adjusting the tool coil parameters to the forming machine.
The pressure course directly depends on the current pulse and directly influences the forming velocity. The pressure course can be characterized by its maximum and duration, both influencing the achievable tangential strain in a free forming operation, as well as the pressure rise time, which significantly influences the forming velocity.
The pressure distribution over the axis of the tube significantly influences the contour of the tube, this means the shape along the z- coordinate. It can be adjusted by the design of the tool coil and an additional tool element called fieldshaper.
As mentioned, in a free forming process the radial deformation of the workpiece is determined by the pressure maximum and its duration. With an increasing tangential strain the resulting roundness of the workpiece decreases. This effect can be reduced by increasing the forming velocity.
The maximum length which 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.
The electromagnetic sheet metal forming can beneficially be used as a calibrating operation for small areas in deep drawing parts. Before performing a forming process with a die it was necessary to investigate the free forming process to obtain process fundamentals and a clear understanding of the process.
As a result of the performed investigations and several discussions between the project partners the following process parameters have been defined:
- Current impulse
- Pressure impulse defined by pressure maximum and duration of the pressure impulse
- Pressure distribution
- Workpiece deformation
- Forming velocity
Analogous to the tube compression process, also for sheet metal forming the current pulse is influenced by properties of the tool coil, the forming machine, and the charging energy of the capacitor battery and has to be optimized by adjusting the tool coil parameters to the forming machine.
In contrast to the tube compression process the pressure course is significantly influenced by the workpiece deformation and cannot be calculated analytically. Furthermore, there is no direct influence of the pressure rise time on the forming velocity.
The pressure distribution can be adjusted by the design of the tool coil and it significantly influences the forming velocity and its distribution.
Electromagnetic forming requires the generation of a high and transient magnetic field using an appropriate electrical device bringing energy of about hundreds of kJ. This field is developed in the surrounding of the forming coil and the metallic object to be formed. Like any other forming process, a die might be necessary to give the workpiece the required final geometry. The high current pulse passing through the coil is producing eddy currents in the opposite direction to the process current that causes repulsive forces between the two components due to Lorentz forces. Modelling such a forming process requires to couple multiphysical phenomena.
The effect of temperature variations is generally considered negligible for the simulation of the stamping process. Indeed temperature effects are much localised. Eddy currents generate a high thermal power density in a very short time, so as heat transfer cannot affect the complete workpiece and is only softening a small region. The plastic work is distributed in the whole plastically deformed part of the component and does not need to be considered more than for classical stamping process.
ESI group s commercial finite element software solution is designed to model such a complex process. SYSMAGNA can especially solve the electromagnetic part. PAMSTAMP code, dedicated to the stamping process simulation, is chosen for the structural analysis. In order to realise 3D simulations, some developments where performed in these software.
Some examples of application like tube compression test and plates show that virtual testing is very efficient in order to optimize coil geometry or process parameters.
In spite of the low weight of the aluminium, the use of this material in automtive inductry is limitated by its low formability. Sharp corners or edges achievable on steel parts are not achievable on aluminium parts.
At higher forming speed (higher strain rate), the formability is higher, because necking occurs later. Electro Magnetic Forming is a high speed forming technology compatible with automotive process and can be coupled with deep-drawing.
The results presented here give basic guidelines required to couple this innovative process with deep-drawing in order to design and manufacture an automotive aluminium sheet stamped part.
Targeting potential areas
Geometry not feasible with aluminium
Part design
= Simulation = deep drawing can be used to know the strain available before EMF - no necking and 15% max thinning is allowed = Geometry achievable by EMF = small edge radius : in principle sharp radii (e.g. R2) achievable. Min. radius depends on formability and process setup. maximal formed edge length/embossing area 70-200J/cm(160J/cm² in case of the sheet demonstrator)
Material choice
High electrical conductivity s required AA6016: sÜ26 MS/m or AA5182:sÜ17 MS/m
Deep-drawing tool design no influence on deep drawing cycle time coil insert can easily be included in a stamping tool holes to let the air flow are required PU (soft material) in the insert requires smooth radii of the preformed part life time of coil insert regarding stamping restraints is unknown
EMF equipment and parameters
Typical values
Current: 100KA max 8KV - 7/10KHz - 15micro seconds
Electro magnetic force 70/130MPa
EMF tooling design: insert
- A design of coil, which resist to both stamping and EMF forces was found during this project
- Mix of steel - copper wires in epoxy raisin reinforced with kevlar fiber - exterior in PU to resist stamping
- Cooling system might be necessary
Trying out
Deep drawing: PU can be milled if necessary after casting
EMF: after 10 tests, parameters can be fixed.
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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