Pressure heterogeneity in small displacement electrohydraulic forming processes

Electrohydraulic (submerged arc discharge) forming of sheet metal parts has been used as a specialized high speed forming method since the 1960 s. The parts formed generally had a major dimension in the 5 to 25 cm range and required gross metal expansion in the centimeter range. In the descriptions of this process found in the literature, the pressure front emanating from the initial plasma generated by the arc is considered to be uniformly spherical in nature. At least one commercial system used this model to design hardware for pressure front focusing to optimize the forming process[1] and it has been the subject of continued research [2]. Recently, there has been commercial interest in adopting the electro-hydraulic method for the production of much smaller parts requiring very high die contact pressures but little gross sheet expansion. The forming of these small shallow parts required only a few kilojoules but proved to be problematic in other terms. The process development clearly showed indications of random patterns of large pressure heterogeneity across distances in the millimeter range. The apparent pressure heterogeneity produced unacceptable small scale variation in the part geometry. A test program was designed to verify and quantify this effect using a target (die) consisting of a flat plate having small closely spaced holes. This 50 mm diameter target proved very effective in clearly showing the extent of the heterogeneity as well as the approximate local pressures. Various discharge energies were investigated along with different chamber shapes and pressure transfer mediums. The pressure heterogeneity across the target face was a common feature to all experiments. These test results indicate that a uniform pressure front model can be seriously in error for the electrohydraulic process as implemented to date. The results of a qualitative hydro-code model of the test system including the discharge event are presented. The model results are similar enough to the experimental to imply that the coaxial electrode s inherent off center discharge is a primary suspect among potential explanations for the observed heterogeneity in terms of asymmetric shock interaction. The absence of this phenomena in the earlier electrohydraulic forming literature is also discussed

Impact Welding in a Variety of Geometric Configurations

Magnetic pulse welding is an electromagnetically assisted high strain rate impact welding technology. The physical principle is similar to explosive welding and it also belongs to solid state impact welding. This high velocity oblique impact welding has been applied to various lap joint configurations. Three different geometric configurations on plate-to-plate welding were studied in this paper. They are direct lap joint, pre-flange lap joint, and lap joint with embedded wires. All of the three welding configurations have been used to provide metallurgical bonds between both similar and dissimilar metal pairs. The welded materials include copper alloy, aluminium alloy, and steels. The plates are centimeter or more thick and often centimeter in extent. The critical welding process parameters were instrumentally investigated by Rogowski Coil and Photon Doppler Velocimetry. Metallographic analysis of the welded interface showed refined grain structure. The mechanical properties of the welded plates were studied by lap shearing, peeling and nano-indentation tests. The test results showed that the impact welded interface has a much greater micro-hardness and fracture toughness than the base metals

Improved Formability by Control of Strain Distribution in Sheet Stamping Using Electromagnetic Impulses

Stamping failures consist of, broadly speaking, either tearing (excessive local strain energy) or wrinkling (insufficient or inappropriate local strain energy). Good parts are produced when the strain energy or plastic work is effectively distributed during the forming process such that tears and wrinkles are eliminated. The process window framed by tearing and wrinkling limits can be rather small for some materials, notably aluminum alloys. At present, there are no established methods of directly controlling the forming energy distribution within the tool during a stamping operation. All current commercial methods attempt plastic strain control at the sheet boundary by various binder geometries and pressure profiles. While improvements by active control of draw beads and binder pressure have led to improved stamping performance, these methods still broadly rely on tool geometry to set the energy distribution. We have recently developed and demonstrated a method for more directly controlling the distribution of forming energy in a stamping operation based on an extension of electromagnetic (EM) impulse forming. We now have techniques for embedding and operating EM pulse actuator coils in stamping tools. These coils can be operated in a single high power pulse or as a series of lower energy pulses occurring several times during the forming stroke. A single high power pulse can provide the advantage of increased material forming limits of high velocity forming. However, applying a series of lower power pulses can increase forming limits without exposing the tooling and coil to large shock loads. Multiple pulses reduce the maximum strain levels by engaging more of the part material in the forming process which mimics (eliminates) the use of lubricants. Conventional production stamping rates are technically obtainable with proper integration of the EM impulse circuit with the forming press and tooling. This paper focuses on the basic design approach of our multiple pulse technique and integrated process forming results. Comparisons to other augmented stamping processes as well as conventional stamping are presented in terms of both simple metrics, such as draw depth and strain distributions

Impact Welding Structural Aluminium Alloys to High Strength Steels Using Vaporizing Foil Actuator

Dissimilar Al/Fe joining was achieved using vaporizing foil actuator welding. Flyer velocities up to 727 m/s were reached using 10 kJ input energy. Four Al/Fe combinations involving AA5052, AA6111-T4, JAC980, and JSC1500 were examined. Weld samples were mechanically tested in lap-shear in three conditions: as-welded, corrosion-tested with ecoating, and corrosion-tested without coating. In all three conditions, the majority of the samples failed in the base aluminium instead of the weld. This shows that the weld was stronger than at least one of the base materials, both before and after corrosion testing. Galvanic corrosion was not significant since the differences in open cell potential, which represent the driving forces for galvanic corrosion, were small among these materials—no more than 60 mV in all cases. Nonetheless, through corrosion testing, the base materials suffered general corrosion, which accounted for the weakening of the base materials

Dimensional Control and Formability in Impact Forming

Electromagnetic forming (EMF) is a high speed forming technique that can be used for embossing fine surface features onto sheet metals. Here two coupled experimental and analytical studies show how interface conditions including rebound and friction affect the ability to create a component in impact forming. In the first part of this work high velocity is generated with the Uniform Pressure Actuator (UPA) and impact with a die emboss fine features in a nominally flat component. The primary objective of this work is to develop a modelling facility that guides experimental design nominally flat grooved components. Both shape fidelity and formability aspects are presently considered. In a second short study expansion of a round tube into a square hole is considered. Traditional modelling techniques solve a coupled system of equations with spatially varying electromagnetic fluxes controlling the dynamics of the plastic deformation. Because the magnetic pressure is spatially uniform, the flux equations are obviated from the coupled system rendering them computationally efficient. The calibration of contact mechanics that influence the rebound behaviour of the sheet metal remains as a difficult issue. The interfaces between various sheet metals and the metal die play a critical role in controlling the shape of the final product. The characterization of such an interface using appropriate calibrated friction coefficients is assessed. The role of magnetic pressure in reducing the sheet metal rebound is demonstrated via a comparison between results from mechanical and electromagnetic simulations. The influence of the channel geometry on final shape is illustrated through simulation and experiments

Control of Velocity, Driving Pressure, and Planarity During Flyer Launch with Vaporizing Foil Actuator

Electrically-driven rapid vaporization of thin conductors produces a high-pressure pulse which can be used to accelerate thin metal sheets to high velocities. Recently, vaporizing foil actuators (VFA) have been applied toward a variety of impulse-based metalworking operations such as collision welding, closed-die forming, embossing, and shearing. To better apply VFA to different purposes, it is necessary to develop an understanding of how variations on the characteristics of the foil actuator affect its mechanical impulse generation. In this work, actuators made out of 0.0508, 0.0762, and 0.127 mm thick full hard temper AA1145 foil were used to launch 0.508 mm thick AA2024-T3 sheets toward a photonic Doppler velocimeter (PDV) probe. Launch velocities ranging between 300 and 1000 m/s were observed over a distance of less than 3 mm, and repeated trials demonstrated repeatable results. Velocity, current and voltage traces were used to examine the effect of deposited energy on average pressure and resulting velocity for foil actuators of various thicknesses. The planarity of the flyer sheets’ launch and flight was demonstrated with 0.0762 mm foil actuators by experiments that employed multiple PDV probes simultaneously recording the velocity evolution at different locations across the surface of the flyer

Collision Welding of Tungsten Alloy 17D and Copper Using Vaporizing Foil Actuator Welding

The objective of this study was to implement collision welding of tungsten alloy 17D (6.5% Ni, 3.3% Fe) and copper at a laboratory scale and subsequently investigate the relationship between interfacial structure and mechanical properties. Vaporizing Foil Actuator (VFA) has recently been demonstrated as a versatile tool for metalworking applications, such as impact welding of dissimilar materials. Its implementation for welding is termed as VFA welding or VFAW. With 8 kJ input energy into an aluminum foil actuator, a 0.5 mm thick Cu110 alloy sheet was launched toward a tungsten alloy target resulting in a collision at a velocity of 580 m/s. The two sheets were found to be welded in the region where the collision velocity and angle were optimal. This range, termed as the welding window was found to be narrow for this combination of target and flyer materials. Scanning electron microscopy of sectioned samples showed regions of wavy interface with significant plastic deformation on both sides. Microhardness tests revealed significant increase in hardness near the interface. Instrumented peel tests showed that the welds were quite strong with peel strength of 60 N/mm

Commercialization of Fuel Cell Bipolar Plate Manufacturing by Electromagnetic Forming

The cost of manufacturing bipolar plates is a major component to the overall cost structure of a Proton Exchange Membrane (PEM) fuel cell stack. To achieve the commercialization of PEM fuel cells, a high volume and low cost manufacturing process for the bipolar plate must be developed. American Trim has identified high velocity electromagnetic forming as a suitable technology to manufacture metallic fuel cell bipolar plates, because of its low capital cost, flexible tooling and rapid prototyping capability. Through the support from the State of Ohio Third Frontier Fuel Cell Program, a group of collaborators consisting of American Trim, The Ohio State University and General Motors have developed a commercially viable prototype production process to manufacture metallic fuel cell bipolar plates in which electromagnetic coils and forming dies were integrated. To manufacture fuel cell bipolar plates, a metal sheet is accelerated by electromagnetic force to impact against, and take the shape of, the forming die surface. A novel approach which introduces a compliant layer eliminates the need for expendable driver plates in order to reduce the production cost. This process enables continuous manufacturing of fuel cell bipolar plates in short-time cycles at very low cost, which demonstrates strong potential for commercialization. This paper will introduce the electromagnetic forming process developed to manufacture metallic bipolar plates, and include a discussion of the preliminary results. The benefits of using this high velocity electromagnetic forming process over a traditional stamping press will also be discussed. To commercialize electromagnetic forming, coil life and die wear are being investigated. The results of some preliminary experiments involving coil durability and die wear will also be presented

Constitutive relation development through the FIRE test

The importance of well-developed constitutive models for predicting deformation behavior of materials at high strain rates cannot be overstated. The study and development of these constitutive models is pertinent to several fields, yet the test methods utilized to probe this high strain-rate realm are limited in both number and standardization. In an effort to augment current high rate tests, new technologies have been leveraged to revive an old, under-utilized test method - the axisymmetric expanding ring. The combination of Photon Doppler Velocimetry (PDV) and one of several ring launch techniques allows the successful testing and instrumentation of samples loaded in tension without wave effects at strain rates exceeding 10^4 s^-1. Design and construction of the embodiment of this test at OSU, dubbed the Fully Instrumented Ring Expansion (FIRE) system, will be discussed. The key difficulties to implementation of the test are examined, along with our efforts to overcome them and preliminary results

Characterization of High-Speed Flyer Evolution by Multi-Probe Photon Doppler Velocimetry

In this paper, the shape evolution of an aluminium flyer is characterized by a 16-probe Photonic Doppler Velocimeter while being impulsed by a Vaporizing Foil Actuator. For high-speed manufacturing, understanding the shape evolution of a flyer can advance the understanding of the characteristics of the applied pressure as well as the dynamics of the material; however, shock-driven process conditions often make it difficult to perform an in-situ study due to its rapidity and high non-equilibrium nature. Characterization of flyer evolution is also essential for comprehending the mechanism of impact welding, as it can enable measuring the process parameters at the time of collision, thus allowing for the prediction of the weld interface structure. An example is provided with an Al-Mg weld interface, showing the process-microstructure relationship of an impact welding process