Design of Hybrid Conductors for Electromagnetic Forming Coils

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

Investigation of Tailored Pressure Distributions by Vaporizing Tailored Foils

The rapid vaporization of thin metallic conductors can be used for innovative high speed forming processes. Metal wires or foils are vaporized when a high current is applied. The generated metal gas or plasma expands very rapidly with high pressure and impacts on an intermediate polyurethane plate near the wires or foils. A shock wave is induced into the polyurethane plate and provides the pressure pulse to the sheet metal, leading to a deformation of the sheet. This process requires no expensive tool coils and no electrical conductivity of the workpiece, which makes it attractive to multiple fields of application such as forming and impact welding. In this study, the basic process parameters that influence the shock pressure were experimentally identified including the charging energy of capacitor bank, foil geometry (thickness and width) and thickness of polyurethane plate. Based on the experiments of the parameter investigations, different new foil designs were investigated in order to acquire a tailored pressure distribution. The results show that the shock pressures can be located at different positions in a discontinuous way. Besides, the pressure amplitudes and areas at different positions can also be varied, which depends on the vaporized foil geometries at those positions

Experimental Investigations on the Optimum Driver Configuration for Electromagnetic Sheet Metal Forming

Electromagnetic forming is a high speed forming process especially suitable for materials with high electrical conductivity such as copper or aluminum. In case of materials with comparatively low electrical conductivity (e.g. stainless steel or titanium) the use of so-called driver sheets is a common approach. Various publications proved that this way materials with low electrical conductivity and even non-conductive materials can be formed. Although the use of driver sheets is common practice, there are no or only contradicting recommendations regarding the optimum driver sheet configuration. Based on experimental investigations of the electromagnetic sheet metal forming process, this paper investigates the optimum material and thickness of the driver sheet. The results prove that aluminum should be favored over copper as driver material. The optimum driver thickness was found to be dependent on thickness and electrical conductivity of the workpiece. Even in case of a workpiece made of aluminum the use of a driver sheet could enhance the efficiency of the process

Simulation of Wrinkle Formation in Free Electromagnetic Tube Compression

A 3-dimensional (3D) finite element (FE) simulation of free electromagnetic (EM) tube compression was performed with the aim of predicting wrinkle formation. Staggered coupling was applied between the EM and mechanical parts of the problem. The full 360° portion of the problem was modelled since the wrinkle formation does not represent any symmetry in circumferential direction. The initial geometric imperfections of the tube were measured and included in the model to trigger buckling. The deformed geometry with the wrinkles could be predicted accurately

Influence of Axial Workpiece Positioning during Magnetic Pulse Welding of Aluminum-Steel Joints

Magnetic Pulse Welding (MPW) offers a method to economically join similar and dissimilar metals without the need for external physical or chemical binders, while avoiding the adverse heating effects seen in many welding techniques. MPW allows for the fabrication of joints via the harnessing of Lorentz forces, which result from discharging a current pulse through a coil. In the process an outer piece (flyer) is accelerated onto an inner piece (parent), and welding is achieved using propagating impact fronts. There are several geometrical factors to be considered including the flyer-coil distance, the parentflyer distance, as well as the axial relationship between flyer and coil (working length). Various shapes of the front are possible and each configuration has its own advantages and drawbacks. The goal of this work is to show not only how the aforementioned parameters are related, but also ways to optimize front propagations, which are vital to the welding result. This is done primarily by determining the influence of the working length of tubular MPW specimens. It is shown that for steel-aluminum joints in the given arrangements, three different front regimes exist, which are related to geometrical factors. These results are especially useful to avoid seemingly favorable but nevertheless suboptimal conditions for flyer movement that would reduce weld quality and energy efficiency of the process

ICHSF2014

Since the first ICHSF, which was held in 2004 at the Technische Universität Dortmund, Germany, this biannual conference has grown into one of the major events for high speed forming technologies and its applications. This meeting series is now being organized with the support of the International Impulse Forming Group (I2FG) that was formed in October 2008 through the vision of Professor Erman Tekkaya. His goal was to model this in many ways after the International Cold Forging Research Group which has been instrumental in applying cold forging to wide manufacturing practice. The public face of this site can be found at http://www.i2fg.org with useful information as well as the proceedings of all the ICHSF meetings. This 6th conference is organized as a joint event of the Department of Mechanical Engineering of KAIST (Korea Advanced Institute of Science and Technology) and the Institute of Forming Technology and Lightweight Construction of Technische Universität Dortmund

Avoiding Bending in Case of Uniaxial Tension with Electromagnetic Forming

During electromagnetic forming, excessive bending of the specimen takes place due to high velocities and inertia. We show that the excessive bending can be prevented by optimizing the coil geometry in case of uniaxial tension. The process is simulated with various coil geometries, and the resulting amount of bending is compared to the case of standard Nakajima Test. The comparison shows that the bending can be minimised to acceptable levels to be able to call the method a decent way of determining forming limits. The results should be verified experimentally

Plastic flow and failure in single point incremental forming of PVC sheets

This paper presents an innovative and effective methodology to characterize plastic flow and failure in single point incremental forming (SPIF) of polymers that allows determining the stresses and the accumulated values of ductile damage directly from the experimental values of strain at various positions over the deformed polymer sheets. The approach traces the deformation path of material elements in conical and pyramidal SPIF parts, undergoing linear strain loading paths from beginning until failure, and is built upon the generalization of the analytical framework conditions assumed by Glover et al. [1] to the pressure-sensitive yield surfaces of polymers under incompressible, non-associated, plastic flow. Experimentation in conventional and multi-stage SPIF of Polyvinylchloride (PVC) sheets confirms the effectiveness of the proposed methodology and demonstrates that standard non-coupled damage models currently utilized in sheet metal forming are inapplicable to describe failure in polymers. Instead fracture forming limit lines (FFL’s) should be employed

Novel Layers for Dies Used in Electromagnetic Sheet Metal Forming Processes

Due to the high forming velocities during electromagnetic sheet metal forming processes, a high impact force acts between workpiece and die. Here, the die surface sustains high damages shown by high wear and galling of the workpiece on the die surface. To enhance the die lifetime, a novel coating concept based on the PVD (physical vapour deposition) process was developed. In doing so, the hardness and the toughness of the designed layers were varied and adjusted to the demands of AlMg-sheet forming process