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

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

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

Agile Production of Sheet Metal Aviation Components Using Disposable Electromagnetic Actuators

Electromagnetic forming is a process used to produce high strain rates that improve the formability of sheet metal. The objective of this paper is to discuss the feasibility of the use of disposable actuators during electromagnetic forming of two aluminum components: an industry part whose main feature is a convex flange with two joggles, and a simple part with a one-dimensional curve throughout. The main forming complications after the parts were formed using conventional methods were the presence of wrinkles and excessive springback. The goal of this work is to use large, controlled electromagnetic impulses to minimize the springback of these components from a roughformed shape, with the end result being a dimensionally correct part. The optimum test protocols for electromagnetic calibration of the components were determined by optimizing parameters such as design of the actuator, tool material, and capacitor discharge energy. The use of disposable actuators for electromagnetic calibration of the parts showed significant reductions in springback compared to the parts which were only preformed using conventional techniques (hydroforming and rubber-pad forming). Springback was decreased in the curved component by up to 87%. For the flanged component, the wrinkles were eliminated, the joggles were formed properly, and the average bending angle of the part was improved from 95.3° to 90.3°, very near the target bending angle of 90°. This study demonstrates that these forming techniques can be used to improve current sheet metal production processes

Development of design principles for form-fit joints in lightweight frame structures

Based on fundamental technological investigations, alternative joining strategies using electromagnetic forming (EMF) for the flexible production of lightweight frame structures are developed in the collaborative research project SFB/TR10. The results of these investigations will also be used to create general design principles for the joining process itself as well as for the joining zone. The focus of this article will be on dominating form-fit joints of aluminum frame structures and the parameters which have a significant influence on the strength of those joints. For the development of design principles regarding the joining zone, the groove geometry of the connection elements was varied in terms of size and shape, and the influence of those variations was analyzed. In terms of the joining process itself the effect on the joint strength of different forming pressures for a given groove geometry was also investigated. In the first step these experiments were performed on solid mandrels. In order to reduce the weight of the structure, experiments were then performed with hollow connection elements and similar groove geometries to analyze how the reduced stiffness of those elements affected the strength of the joints

Influencing Factors on the Strength of Electromagnetically Produced Form-Fit Joints using Knurled Surfaces

Joining by electromagnetic forming is a non-contact assembling method that is especially suitable for connections in aluminum space frame structures. By reason of increased joint strength along with lower charging energies, form-fit connections are favored over interference-fit connections for this joining process. In contrast to conventional form-fit concepts, in which the inner joining partner has grooves or pockets, the use of knurled surfaces offers several advantages like easier machinability or the resistance against combined axial and torsional loadings. The objective of this paper is to identify the influencing geometry and process parameters on the joint strength of tubular joints using mandrels with knurled surfaces, with tube and mandrel being made of the same aluminum alloy AA6060-T6. For that reason, experimental studies were conducted: In addition to pull-out tests to determine the axial strength of joints, first computed tomographic images and, afterwards, micrographs of joined components were produced to analyze the contact zone between tube and mandrel and the deformation behavior of the inner joining partner by non-destructive and destructive means. Based on the detailed knowledge of the influencing variables, guidelines for joint and process design are derived

Time-temperature-transformation diagram within the bainitic temperature range in a medium carbon steel

The time-temperature-transformation (ITT) diagram within the medium temperature range of medium carbon steel has been determined. A single type of C-curve is found within the bainite temperature range for the studied steel. Distinct reaction C-curves were not observed for both types of microstructure, upper bainite and lower bainite in the TTT diagram. Experimental results on the kinetics of the isothermal formation of bainite at different temperature have demonstrated that both type of microstructure, upper and lower bainite, possesses similar overall transformation kinetics. Some applications of phase transformation theory towards the formation of bainitic microstructures are discussed, with particular emphasis on the bainite start temperature, transition temperature from upper to lower bainite, martensite start temperature and the thickness of bainitic plates.Peer Reviewe