Design of Hybrid Conductors for Electromagnetic Forming Coils

Abstract

The use of hybrid coil turns made of steel (St) and copper (Cu) is originally motivated by the increased mechanical strength compared to monolithic copper conductors. Due to the differing electrical conductivities of the two materials, the hybrid design also changes the current density distribution in the conductor cross section. This affects crucial process parameters such as the magnetic pressure and the Joule heat losses. The effect of the hybrid conductor design on the process efficiency is investigated. An electromagnetic sheet metal forming operation using a one-turn coil with rectangular cross section is used as reference case. The copper layer (CuCr1Zr) was deposited on a tool steel substrate (X40CrMoV5-1) using a selective laser melting process. The copper layer thickness is varied ranging from a monolithic steel conductor to a monolithic copper conductor. The workpiece (EN AW-5083, t_w = 1 mm) is formed through a drawing ring so that the final forming height is a qualitative measure for the process efficiency. The experimental results prove that the efficiency in case of a properly designed hybrid conductor can exceed the efficiency of a monolithic copper coil. The current density distribution in the hybrid cross section is investigated by means of numerical simulations. This way a deeper insight into the physical effects of a varying copper layer thickness is gained. The results reveal that the optimum layer thickness is not just a function of the coil cross section and the current frequency. It is also affected by the coil length and the resistance of the pulse generator.