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= Electromagnetic forming =
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Introduction
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A pinched aluminium can, produced from a pulsed magnetic field created
by rapidly discharging 2 kilojoules from a high-voltage capacitor bank
into a 3-turn coil of heavy gauge wire.
Electromagnetic forming (EM forming or magneforming) is a type of
high-velocity, cold forming process for electrically conductive
metals, most commonly copper and aluminium. The workpiece is reshaped
by high-intensity pulsed magnetic fields that induce a current in the
workpiece and a corresponding repulsive magnetic field, rapidly
repelling portions of the workpiece. The workpiece can be reshaped
without any contact from a tool, although in some instances the piece
may be pressed against a die or former. The technique is sometimes
called 'high-velocity forming' or 'electromagnetic pulse technology'.
Explanation
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A special coil is placed near the metallic workpiece, replacing the
pusher in traditional forming. When the system releases its intense
magnetic pulse, the coil generates a magnetic field which in turn
accelerates the workpiece to hyper speed and onto the die.
The magnetic pulse and the extreme deformation speed transforms the
metal into a visco-plastic state - increasing formability without
affecting the native strength of the material. See the
[http://www.bmax.com/technology/magnetic-pulse-forming/ magnetic pulse
forming illustration] for a visualization.
A rapidly changing magnetic field induces a circulating electric
current within a nearby conductor through electromagnetic induction.
The induced current creates a corresponding magnetic field around the
conductor (see Pinch (plasma physics)). Because of Lenz's Law, the
magnetic fields created within the conductor and work coil strongly
repel each other.
When the switch is closed, electrical energy stored in the 'capacitor
bank' (left) is discharged through the 'forming coil' (orange)
producing a rapidly changing magnetic field which induces a current to
flow in the metallic 'workpiece' (pink). The current flowing the
workpiece produces a corresponding opposite magnetic field which
rapidly repels the workpiece from the forming coil, reshaping the
workpiece -- in this case, compressing the diameter of the cylindrical
tube. The reciprocal forces acting against the forming coil are
resisted by the supportive coil casing' (green).
In practice the metal workpiece to be fabricated is placed in
proximity to a heavily constructed coil of wire (called the 'work
coil'). A huge pulse of current is forced through the work coil by
rapidly discharging a high-voltage capacitor bank using an ignitron or
a spark gap as a switch. This creates a rapidly oscillating, ultra
strong electromagnetic field around the work coil.
The high work coil current (typically tens or hundreds of thousands of
amperes) creates ultra strong magnetic forces that easily overcome the
yield strength of the metal work piece, causing permanent deformation.
The metal forming process occurs extremely quickly (typically tens of
microseconds) and, because of the large forces, portions of the
workpiece undergo high acceleration reaching velocities of up to 300
m/s.
Applications
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The forming process is most often used to shrink or expand cylindrical
tubing, but it can also form sheet metal by repelling the work piece
onto a shaped die at a high velocity. High-quality joints can be
formed, either by electromagnetic pulse crimping with a mechanical
interlock or by electromagnetic pulse welding with a true
metallurgical weld. Since the forming operation involves high
acceleration and deceleration, mass of the work piece plays a critical
role during the forming process. The process works best with good
electrical conductors such as copper or aluminum, but it can be
adapted to work with poorer conductors such as steel.
Comparison with mechanical forming
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Electromagnetic forming has a number of advantages and disadvantages
compared to conventional mechanical forming techniques.
Some of the advantages are;
*Improved formability (the amount of stretch available without
tearing)
*Wrinkling can be greatly suppressed
*Forming can be combined with joining and assembling with dissimilar
components including glass, plastic, composites and other metals.
*Close tolerances are possible as springback can be significantly
reduced.
*Single-sided dies are sufficient, which can reduce tooling costs
*Lubricants are reduced or are unnecessary, so forming can be used in
clean-room conditions
*Mechanical contact with the workpiece is not required; this avoids
surface contamination and tooling marks. As a result, a surface finish
can be applied to the workpiece before forming.
The principle disadvantages are;
*Non-conductive materials cannot be formed directly, but can be formed
using a conductive drive plate
*The high voltages and currents involved require careful safety
considerations
License
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License URL: http://creativecommons.org/licenses/by-sa/3.0/
Original Article: http://en.wikipedia.org/wiki/Electromagnetic_forming
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