Energy based High Velocity Forming – Electromagnetic forming
Electromagnetic forming (EMF) was invented by Harvey and Brower in 1961 [US 2976907]. This technology includes all production processes using the energy pulsed of magnetic fields in order to apply Lorentz’ forces and thus deform a workpiece made of an electrically highly conductive material.
Depending on the geometry and the setup the following major process variants can be distinguished.
- the electromagnetic compression of tubes which are positioned inside a typically cylindrical tool coil,
- the electromagnetic expansion of tubes in which a typically cylindrical tool coil is positioned,
- the electromagnetic forming of flat sheets with a suitable (often a spirally wound) flat coil, and
- the electromagnetic forming of three-dimensionally preformed sheets with an accordingly shaped tool coil.
More exotic process variants are
- electromagnetic forming with a discharged (and not an induced) current in the workpiece [Fur62] and
- the electromagnetic forming with attractive forces, by which convex forms can be produced in such positions which cannot be reached from the other side [Pfe64, Fur62, Bir61].
The principle of electromagnetic forming is based on the mutual induction concept. The typical setup consists of
- a pulsed power generator,
- a tool coil including a fieldshaper if applicable, and
- the workpiece. (If the electrical conductivity of the workpiece is insufficient, a so-called driver can be used to establish the required Lorentz’ forces.)
In order to start the process, the charged capacitor of the pulsed power generator is discharged by closing of the high current switch. Consequently, a damped sinusoidal current flows through the tool coil inducing a magnetic field and a secondary current in the workpiece, which is directed opposed to the coil current. Due to the skin effect, both currents are concentrated near the surfaces facing each other. The secondary current shields the magnetic field, which is consequently concentrated in the small gap between tool coil and workpiece. According to [Kad59], the efficiency of this shielding depends on
- the frequency of the discharging current,
- the workpiece geometry (radius and wall thickness),
- the electrical conductivity of the material, and
- the time.
The energy density stored in the magnetic field is correlated to a pressure p(z,t) acting orthogonal to the field and the current, which can be calculated from the magnetic field outside the workpiece Ha(t,z), the penetrated magnetic field Hi(t,z), and the permeability µ0 as
In many practical applications the setup for electromagnetic tube compression is supplemented by a so-called fieldshaper or fieldconcentrator, i.e. a usually axis symmetric part with one or several axial slots made of a material of high electrical conductivity, which is positioned between tool coil and workpiece. In these cases, the coil current induces a secondary current in the fieldshaper surface (skin effect), which in turn induces a current in the workpiece.
Advantages and disadvantages of a fielfshaper application
The application of a fieldshaper features several advantages as e.g.
- an increased process flexibility by adapting one and the same tool coil to forming tasks and workpieces of different diameters, lengths and cross section geometries [Row67, Bau80]
- a reduction of the coil load [Kim59, Lan93] and thus
- an increase of the coil lifetime
- the possibility for a combined application of different tool coils via one and the same fieldshaper e.g. in order to compress a workpiece directly after an inductive heating [Hah04, Uhl03a, Uhl03b]
The major disadvantages are
- a reduced process efficiency [Fur57, Bau80]
- an inhomogeneous pressure distribution along the circumference
In principle, the use of a fieldshaper is not limited to compression [Neu88], but the application in electromagnetic expansion or sheet metal forming processes is rare.