Electromagnetic forming

The magnet is an electro- dynamic conversion high energy conversion process for cold forming a flat (e.g. sheet metal), and a cylindrical semi-finished (for example, pipes, profiles ) made of electrically conductive materials using pulsed magnetic fields.

The workpiece is positioned within or in the vicinity of a coil and shaped by the contact force of a pulsed magnetic field of very high intensity, that is without mechanical contact with a tool.

Can magnetic forming partially conventional technologies of deep drawing, rolling, soldering, welding and gluing replace in terms of production costs and product quality. In addition, the magnet forming offers the possibility to develop manufacturing methods that are not possible using conventional means.

For use when joining see also Electromagnetic pulse welding.

History

The electrodynamic findings on which the method is based, were outlined in 1873 by James Clerk Maxwell in his famous essay "A Treatise of Electricity and Magnetism ". But only after it was plasma physicists succeeded in the 1960s hot, highly conductive plasma of several million Kelvin without contact with material walls magnetically compress plants were built for fast magnetic forming of metals and used.

As a result, the magnetic shaping lost compared to competing mechanical process, at least in Western Europe temporarily importance. Reasons for this are probably to be found in essentially that one hand always better mechanical methods have been developed for joining and forming the other hand, work coils and energy storage for Magnetumformanlagen were initially expensive and not durable enough.

In the 1980s, the technique of magnetic forming process has been further developed especially in the U.S. and the USSR and made ​​industry ripe. Since then, the method has been successfully used in wide range of industries such as automotive, aerospace, electrical, nuclear and kitchen appliances industry.

Pros and Cons

The magnetic forming the following benefits:

• The pulse is very accurate dosing. This allows the injection of metals on glass, plastics, composites or metals with high repeatability.
• Magnetic fields and magnetic forces act freely through materials such as glass, ceramic and plastic. Therefore, the magnetic forming can also be under vacuum, an inert gas atmosphere or apply under clean room conditions.
• Magnetumformanlagen do not require mechanical contact with the workpiece, therefore occur either surface contamination or tool marks.
• Process time of <0.1 s

Disadvantages are:

• Requirement is a good conductive material
• The high magnetic fields can disturb or damage electronic equipment
• The coil must be very firm and is therefore expensive - its destruction is a potential danger
• Suitability must be provided in technological preliminary investigations

Physical Basics

The Magnetumformverfahren based on the physical fact that a time-varying magnetic field in adjacent electrically conductive bodies induces eddy currents. On these currents, the magnetic field exerts force, whose strength depends on the spatial gradient of magnetic flux density and the size of the induced currents. The conductive body (sheet, pipe ) experiences a targeted towards lower flux densities force.

Appropriate for the magnetic forming magnetic fields are generated by charged capacitors are discharged in the course of tens of microseconds is adapted to the workpiece geometry, the coil, which in the area of the workpiece surface creates a very high magnetic field pulse. It flows in sequence in the workpiece, a current which is oppositely directed to the coil current. The magnetic forces attempting to expand the coil, and for moving the workpiece towards lower fields.

The strength of the induced currents and thus the force acting on the workpiece depends critically on the electrical conductivity of the material used. With good conductivity, such as copper, aluminum and brass, act on the surface of the work pressures of several thousand megapascals.

This pressure is only a few microseconds, ie the duration of the discharge of the capacitors. During this time, the workpiece assumes the forming energy required in the form of a pulse. After a short acceleration phase, the material moves very quickly especially at low mass. The velocities reach values ​​up to 300 m / s In consequence, the stresses occurring in the workpiece so high that flow occurs in the meaning of the Forming. Welding the material meets with such high speeds, on the parts to be joined and develops local mixing processes which make it possible to weld not schmelzschweißbare material combinations. Since both forces act on the coil, the mechanical strength requirements of the coil construction are very high.

Special

Driver

Even poor conductors such as stainless steel tubes are reshaped by this process by the steel tube is surrounded by a driver of good conducting material - often just a few turns of aluminum foil. The magnetic forces acting here is not directly on the steel but to the driver by means of which the tube is formed.

The pulse or the mechanical energy introduced can be set exactly on the level of the capacitor charge.

Field shaper

For the effective use of magnetic forces during forming of the distance from the coil must be as small as possible to the workpiece. To at various workpiece dimensions to be able to nevertheless use the same coil, the field shaper are used, which enable the electromagnetic force to focus on specific areas of the workpiece.

So that a current can flow to the field shaper from highly conductive material has to be slit longitudinally to the coil axis, at least once on the inner wall. It should be noted that a rush current generated at the Feldformeroberfläche by induction due to the skin effect in the short time to be considered can not penetrate into the metal inside.

Figure 2 shows an arrangement with a double-slotted box former and a representation of the directions of current flowing in the coil, the field shaper and the workpiece current. The current flowing on the inner wall of the field shaper current is concentrated to the inner wall portion, which is close to the workpiece. Accordingly, there is a particularly high magnetic pressure by which the workpiece is formed in this area.

Coil shapes and arrangements

There are three basic forms of the magnetic forming: compression, expansion and flat forming.

The magnetic transformation is the most widely used compression. This is used as a work coil, a solenoid coil comprising the workpiece. The forces on the workpiece are directed radially inwardly and press it together and press it on an inner core. This is shown in Figure 3

In Figure 4, the application of compression during the crimping of the tubular work piece is shown on a yoke of a Cardan shaft. The use of a field shaper ensures magnetic forces that are large enough to drive the wall material of the tube into the wells of the universal joint element.

Since in the magnetic compression, a high degree can be achieved at rotational symmetry of the forces that occur, it is superior in the mechanical process of metal pipes Aufpressung on ceramic, glass or brittle plastics in general.

With the expansion tubular workpieces are expanded or pressed into a tube enclosing form. The work coil for generating the magnetic field suitable for this conversion is in this case a cylindrical coil, which is inserted into the tubular work piece. This is shown in Figure 5. The forces acting on the tube forces are directed radially outwardly.

Figure 6 shows an arrangement for the flat forming. The magnetic field is generated in the vicinity of a sheet lying on a die. The electromagnetic forces driving the plate into the recess of the die. The magnetic field replaces the conventional mechanical stamp here. The flat coil used for generating the magnetic field in this example has the form of a clock spring ( Archimedes spiral ). It is mounted parallel to the workpiece above it.

Figure 4: Connecting the shaft and joint through compression

Figure 5: Expansion of a pipe

Figure 6: Flat deformation

Figure 7: separation by flat coil

The three basic types of magnetic forming process described can be used with the use of appropriate tools for cutting and for forming, fastening and joining, however. This is illustrated in Figure 7 using the example of a flat coil. At the positions provided for the workpiece material is driven into the recesses of the support and separated from the workpiece. Similarly, can be arbitrarily shaped tubes with holes provided or cut.