| Opportunities in electromagnetic forming of metal workpiece |
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Electromagnetic forming (EMF or Magneforming) is a type of high energy rate metal forming process that uses pulsed power techniques to create ultrastrong pulsed magnetic fields to rapidly reshape metal parts. The technique is sometimes called high velocity forming.
A rapidly changing magnetic field induces a circulating electrical current within a nearby conductor through electromagnetic induction. The induced current creates a corresponding magnetic field around the conductor. Because of Lenz's Law, the magnetic fields created within the conductor and work coil strongly repel each other.
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, ultrastrong electromagnetic field around the work coil.
The high work coil current, typically tens or hundreds of thousands of amperes, creates ultrastrong magnetic forces that easily overcome the yield strength of the metal workpiece, 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.
The forming process is most often used to shrink or expand cylindrical tubing, but it can also form sheet metal by repelling the workpiece onto a shaped die at a high velocity. Since the forming operation involves high acceleration and deceleration, mass of the workpiece 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.
EMF offers elegant approaches to forming complex components and can improve forming limits in metals. It is the only high velocity forming technique to gain significant acceptance in commercial metal working. The electromagnetic forming technique has been in use commercially for the last 30 years. Mostly, it has been used for joining and assembly of concentric parts. The minimal springback inherent in all high velocity forming processes provides high-quality joints.
One of the most common applications of electromagnetic forming is the compression crimp sealing and assembly of axi-symmetric components such as automotive oil filter canisters. As the name implies, in this technique, electromagnetic forces are used to form the material. A current pulse from a capacitor bank is passed through a coil that is placed in close proximity to a workpiece. The current pulse causes a high-magnetic field around the coil. This field induces an eddy current in the workpiece and an associated secondary magnetic field. The two fields are repulsive and the force of magnetic repulsion causes deformation of the workpiece.
The nature of the electromagnetic forming process makes it highly suitable for automation. Results obtained are very repeatable because energy discharge characteristics are controlled essentially by the non-changing electrical parameters of the system and precise control of capacitor bank charge voltage. The fundamental physical characteristic of this technique is that the deformation forces initially are only magnetic body forces generated within the material by eddy currents induced by the drive coils. Surface pressures only occur upon contact with the form tool. This can provide deformation capabilities that are difficult to obtain with other forming methods.
How EMF works
EMF works by the magnetic induction effect. When a coil or solenoid is placed near a metallic conductor and pulsed via an energy store like a capacitor bank, a magnetic field is generated between the coil and the workpiece. If done quickly enough, the magnetic field is excluded from penetrating into the workpiece for a short period of time. During this time, a pressure is generated on the workpiece that is proportional to the magnetic flux density squared. This "magnetic" pressure is what provides the forming energy.
The energy is usually supplied to the workpiece in the form of kinetic energy. The magnetic pressure pulse accelerates the workpiece up to a certain velocity (such as 200-300 m/s). This kinetic energy drives the material into the die, causing forming on impact.
Choosing EMF over other processes
For some applications, EMF has distinct advantages over conventional forming processes. Any time you need to form a round part radially inward or outward, the radial direction of the electromagnetic forces is ideal. Examples are forming a ring-shaped part onto a tube-shaped part, or a tube-shaped part onto a disk-shaped part. The forming causes the two parts to swage or mechanically lock together. IAP has formed tubes and rings with diameters ranging from about 1-inch to above 24-inches. If you use your imagination, this describes a lot of parts.
Another reason to choose EMF is to form sheet materials in a different way than conventional processes to improve surface quality. EMF can eliminate sheet surface problems present in conventional metal forming methods such as stretching stringers or marring from punches.
EMF has the highest cost savings in the following situations:
● eliminate one or more processing steps
● combine assembly or joining operations
● eliminate the need for welding (especially in aluminum alloys)
● eliminate quality problems by improving forming quality of sheet
● systems are smaller and more flexible than hydraulic presses of comparable capacity
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