ICHSF 2016

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http://hdl.handle.net/2003/34229

The 7th International Conference on High Speed Forming (ICHSF 2016) is held in Dortmund, Germany on April 27th – 28th, 2016. The conference is organized as a joint event of the Institute of Forming Technology and Lightweight Construction (IUL) of TU Dortmund University and the Department of Materials Science and Engineering of the Ohio State University.
In addition to the presentation of new research results regarding high speed forming, the conference intention is to provide a forum for the exchange of experiences and the discussion between industrial operators and researchers on an international stage.

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A Study on the Critical Thickness of the Inner Tube for Magnetic Pulse Welding Using FEM and BEM
(2016-04-27) Geng, H.; Cui, J.; Sun, G.; Li, G.
Due to high efficiency and quality in welding dissimilar metals, Magnetic Pulse Welding (MPW) has attracted much attention. In this study, 3A21 aluminium alloy used as outer tube was welded to 20Fe tube by MPW. In order to investigate the critical thickness of the inner tube (20Fe) which is subjected to huge impact pressure from the outer tube (3A21), both numerical simulations and experiments were carried out. For the purpose of investigating the critical thickness of the inner tube under various impact velocities, four discharge voltages (9 kV, 11 kV, 13 kV and 14 kV) were employed in the MPW experiment. The diameters of inner tube at different locations were measured to obtain its plastic deformation at various discharge voltages. The simulations considering the coupled effects of the mechanical, thermal and electromagnetic process were performed to research the impact velocity and deformation of tubular fittings in the electromagnetic module (EM) in LS-DYNA. An inverse method was proposed to find the dynamic yield stress of inner tube, and the predicted yield stress was then employed in models with critical thickness. Both of the impact velocity and deformation were verified experimentally.
The Influence of Thermal and Mechanical Effects on the Bond Formation During Impact Welding
(2016-04-27) Pabst, C.; Groche, P.
Impact welding, usually applied as explosion welding or electromagnetic pulse welding, is a highly transient joining process. Strain rates in orders of magnitude far above 104 1/s and resultant thermal effects occur and influence the formation of the joint significantly. Experimental and microscopic investigations as well as analytical estimations are carried out and presented in this paper in order to gain a more comprehensive understanding of the effective mechanisms and their relevance. In addition to electromagnetic pulse welding, a specially built test rig is used to identify the process window and its change due to modified parameters. The test rig allows to change both impact parameters, angle β and velocity v_c , independently. It will be shown that the actual formation of the joint and its characteristics are greatly affected by the surrounding gaseous media. Strength and size of the joint can be influenced as well as the location of the process window. Theories will be developed to explain these results and to make them usable for the practical application. Furthermore, experimental results indicate that the compression of the ambient atmosphere in the closing gap between the two specimens evokes highly elevated temperature, which is in good accordance with earlier findings.
Electromagnetic Pulse Welded Aluminium to Copper Sheet Joints: Morphological and Mechanical Characterization
(2016-04-27) Faes, K.; Kwee, I.
This study investigated joining of Al to Cu sheets by electromagnetic pulse welding, which is a solid-state welding process that uses electromagnetic forces to join materials. The interfacial morphology and mechanical properties of the Al/Cu joints were analysed and related to the welding process parameters. The centre section of the Al/Cu joints evolved from a non-welded to a welded zone. The welded zone started with a wavy interface, consisting of thick interfacial layers with defects and evolved to a relatively flat interface without an interfacial layer. Interfacial phases resulted from solid-state mechanical mixing and/or very localised interfacial heating. The interfacial layers had a thickness ranging from 2-39 μm, an interface waviness amplitude up to 11 μm and contained 31-75 wt% Cu. The interfacial layer thickness and the weld length are determined by both the discharge energy and the stand-off distance. A trade-off existed between a homogeneous interface and the maximum weld length when the stand-off distance is changed. The interfacial layer exhibited an increased hardness compared to Al and Cu. A higher tensile force, up to 4,9 kN, was achieved at a higher energy and a lower stand-off distance. One of the factors determining the tensile force was the width of the welded area.
Benchmarking and Refining the Vaporizing Foil Actuator Spot Welding Process
(2016-04-27) Vivek, A.; Wright, S. M.; Liu, B. C.; Hansen, S. R.; Brune, R. C.; Thurston, B. P.; Taber, G. A.; Lee, T.; Mao, Y.; Dittrich, T. J.; Daehn, G. S.
Impact spot welding implemented by the vaporizing foil actuator welding method has been studied. With significantly lower input energy levels as compared to resistance spot welding, similar and dissimilar lap welding of aluminium alloys (AA) of types 5052 and 7075 was implemented. The dissimilar welds between 2 mm thick AA5052 and 2.3 mm thick AA7075 were created with 4 kilojoules input energy, whereas the similar welds between 1 mm thick AA5052 sheets required only 0.6 kilojoules. Flyer sheet velocities of approximately 750 m/s were measured with a PDV system. Microhardness measurements, performed across the dissimilar weld interfaces, showed no softening of the base materials due to the welding process. A few distinct welding configurations were investigated for improving process feasibility and obtaining the highest possible weld strength. Lap shear tests and pry tests revealed that the configuration of the starting weld geometry greatly affected weld quality.
Effects of Surface Coatings on the Joint Formation During Magnetic Pulse Welding in Tube-to-Cylinder Configuration
(2016-04-27) Bellmann, J.; Lueg-Althoff, J.; Goebel, G.; Gies, S.; Beyer, E.; Tekkaya, A. E.
Magnetic Pulse Welding (MPW) is a joining technique favorable for the generation of strong atomic bonded areas between different metals, e.g. aluminum and steel. Brittle intermetallic phases can be avoided due to the high-speed collision and the absence of external heat. The demand for the use of this technique in industries like automotive and plant engineering rises. However, workpieces used in these fields are often coated, e.g. in order to improve the corrosion resistance. Since the weld quality depends on the material’s behavior at the collision zone, surface layers in that region have to be taken into account as well. This work investigates the influences of different coating types. Aluminum to steel welding is used as an example system. On the inner steel part (C45) coatings like zinc, nickel and chrome are applied, while the aluminum flyer tubes (EN AW-6060) are anodized, chromated and passivated. Welding tests are performed using two different welding systems with varying discharging frequencies and four geometrical part setups. For all combinations, the flyer velocity during the process is measured by Photon Doppler Velocimetry (PDV). By using the uncoated material combination as a reference, the removal of surface layers due to jetting is analyzed. Finally, the weld quality is characterized in peel tests, shear-push tests and by the help of metallographic analysis. It is found that certain coatings improve the joint formation, while others are obstructive for the performance of MPW. Some coatings have no influence on the joining process at all.
Impact Welding Structural Aluminium Alloys to High Strength Steels Using Vaporizing Foil Actuator
(2016-04-27) Liu, B.; Vivek, A.; Daehn, G. S.
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.
Influence of the Wall Thicknesses on the Joint Quality During Magnetic Pulse Welding in Tube-to-Tube Configuration
(2016-04-27) Lueg-Althoff, J.; Schilling, B.; Bellmann, J.; Gies, S.; Schulze, S.; Tekkaya, A. E.; Beyer, E.
The implementation of multi-material concepts, for example, in automotive engineering or aerospace technologies, requires adequate joining techniques. The Magnetic Pulse Welding (MPW) process allows for joining both similar and dissimilar materials without additional mechanical elements, chemical binders, or adverse influences of heat on the joining partners. In this process, an electro-conductive at (‘flyer’) part is accelerated by Lorentz forces and impacts the inner (‘parent’) part under high velocity and high pressure, leading to the formation of a metallurgical joint. Besides joining of sheets and tubes to solid cylinders, the connection of two tubes is of particular interest due to the increased lightweight potential. The present paper focuses on the MPW of aluminum (EN AW-6060) to steel (C45) tubes. An experimental study was performed, in which the wall thickness of the parent part was reduced successively. The deformation behavior of both the flyer and parent parts was recorded during the experiments by a two-probe Photon Doppler Velocimeter (PDV). The final shape of the joined specimens was analyzed by a 3D digitizer. An instrumented peel test was used for the determination of the weld quality. It was found that defect-free MPW of aluminum tubes on steel tubes without supporting mandrel is possible.
Influence of Different Strain Rates on the Flow Curve and the Formability of Thin Aluminium and Tinplate Sheets
(2016-04-27) Linnemann, M.; Lieber, T.; Scheffler, C.; Psyk, V.; Müller, R.; Landgrebe, D.
Due to this high number of produced units and the very thin sheet metals used for beverage cans, precise production processes with high production volumes are necessary. To save expenses, while optimising these processes, numerical simulation methods are exploited. Considering this, it is indispensable to identify the material behaviour as exactly as possible. In practise, often results of quasi static tensile tests are used, although these are insufficient for the precise modelling of the material behaviour during can production, since strain rates of up to 10³ s-1 can occur, here. Therefore, quasi static and high speed tensile test have been done on specimens featuring the typical materials and thicknesses of semi-finished parts used for beverage can production. The results were compared with similar materials at higher sheet metal thicknesses and authenticated by numerical simulation. It was shown that there is an influence of the strain rate on the material behaviour and it is necessary to determine material characteristics at strain rates, which are close to the process speed. Furthermore, the results were classified in their signification for beverage can production and forming technologies in general.
Experimental and Numerical Prediction of the Static and Dynamic Forming Properties of Ti6Al4V
(2016-04-27) Verleysen, P.; Galan-Lopez, J.
The strain rate dependence of the plastic yield and failure properties displayed by most metals affects energies, forces and forming limits involved in high speed forming processes. In this contribution a technique is presented to assess the influence of the strain rate on the forming properties of Ti6Al4V sheet. In a first step, static and dynamic tensile experiments are carried out using a classical tensile test device and a split Hopkinson tensile bar facility respectively. Next to uniaxial tensile, also purpose-developed plain strain and shear stress samples are tested. The experimental results clearly show that the mechanical behaviour of Ti6Al4V is strain rate dependent. Indeed, with increasing strain rate, plastic stress levels increase, however, this occurs at the expense of the deformation capacity. Subsequently, to allow simulation of forming processes, Johnson-Cook, Swift and Voce material model parameters are determined. Finally, the influence of the strain rate on the forming limits is assessed using the uni-axial tensile test results. Prediction of the initiation of necking in the Ti6Al4V sheets subjected to multi-axial strain states is based on the Marciniak-Kuczynski model. The thus obtained forming limit diagrams (FLDs) show a non-negligible effect of the strain rate. The reduced ductility at higher strain rates is reflected into an unfavourable downward shift of the FLD. Compared with the experimental data, the static FLD is clearly conservative.
Development of Vibration During the Electromagnetic Ring Expansion Test
(2016-04-27) Yang, K.; Taber, G.; Sapanathan, T.; Vivek, A.; Daehn, G. S.; Raoelison, R. N.; Buiron, N.; Rachik, M.
Magnetic pulse forming (MPF) techniques work on the principle of Lorentz force induced by eddy current which can cause plastic deformation in a metal workpiece. Lorentz force depends on parameters such as frequency and amplitude of input current, electromagnetic properties of materials and distance between the work piece and coil. The development of vibration as a consequence of elastic strain recovery in a ring expansion process using a MPF technique has been identified and presented in this paper. Coupled mechanicalelectromagnetic 3D simulations were carried out to investigate the effect of various magnetic pulse currents in the development of reversal of motion during the MPF process using LS-DYNA package. Ring expansion using a multi-turn helix coil with an applied pulse current, with the rings made of aluminum alloy AA6061 –T6 is investigated for the effect of vibration during the process. The numerical results show good agreement with the experimental work for various currents. The underlying principle of vibration and formability has respectively been studied using force analysis and stress analysis. The results also show that the 5.6kJ energy already increased the formability by ~66 percent in comparison with the quasi-static formability value from the literature.
Development of an Interrupted Pulse Expanding Ring Test
(2016-04-27) Imbert, J.; Worswick, M.
An interrupted pulse electromagnetic (EM) expanding ring test is being developed at the University of Waterloo to study the high rate behaviour of sheet metals. In a classic EM expanding ring test, a ring is expanded radially using the forces induced on the ring by a high frequency high intensity current flowing in a nearby coil. If the driving force and the acceleration of the ring are known, then the stress-strain history of the ring can be determined. Coil currents are typically generated by large capacitor banks that produce a current discharge in the shape of a damped sinusoid. To properly determine the stress of the ring, the forces induced on the ring by the current pulse must be known, which is difficult to do in practice. The approach taken in this work is to interrupt the current by means of an exploding wire switch to eliminate the Lorentz forces and achieve a free flight condition, where the stress can be determined using only the measured velocity and density of the ring. The velocity of the rings was measured using a photon Doppler velocimeter (PDV). With this technique significant periods of free-flight were obtained, with the corresponding stressstrain data. Results for 1.5 mm sheet of AA 5182-O are presented.
A New Experimental Technique for Applying Impulse Tension Loading
(2016-04-27) Fan, Z. S.; Yu, H. P.; Su, H.; Zhang, X.; Li, C. F.
This paper deals with a new experimental technique for applying impulse tension loads. Briefly, the technique is based on the use of pulsed-magnetic-driven tension loading. Electromagnetic forming (EMF) can be quite effective in increasing the forming limits of metal sheets, such as aluminium and magnesium alloys. Yet, why the forming limit is increased is still an open question. One reason for this is the difficulty to let forming proceed on a certain influence monotonically: the main phenomena causing this increase in formability are considered to due to “body force” effect, inertia effect, changes in strain rate sensitivity. In this study, an impulse tension loading setup is presented. “Body force” effect and strain rate, which are known to be the two key factors leading to higher formability, can now be separated freely by our designed device. Reproducible and adjustable loading rate (80s-1~3267s-1) can be achieved by adjusting the discharge voltage and capacitance. The relation between the discharge voltage and strain rate was obtained with the help of finite element calculations and high-camera measurement results. The results of an exploratory experiment carried out on the designed device are presented for aluminum alloy AA5052 sheet. It shows that this technique could be used to study the dynamic response of sheets.
A Study to Improve the Crash Performance of Plastic Materials Considering the Strain Rate and Fracture Characteristic
(2016-04-27) Kim, H. Y.; Lee, C. A.; Bamg, J. H.; Cho, B. C.; Kim, D. Y.; Ha, D. Y.
The numerical simulation of structural parts made from plastics is becoming increasingly important nowadays. The fact that almost any structural requirement can be combined in a lightweight, durable and cost effective structure is the driving force behind its widespread application. More and more structural relevant parts are being constructed and manufactured from plastics. It is difficult accurately to predict the reliability according to finite element analysis, because plastics materials show the complex material behaviour. Thus, it is demanded for reliable and obvious methods to design these parts and to predict their material behaviour. For the finite element simulations of polymeric materials mathematical models are needed which cover all the phenomena of the material. In this paper, it is possible to describe accurately the mechanical behaviour of thermoplastic materials using a new constitutive model termed as SAMP-1(Semi- Analytical Model for Polymers) in LS-dyna. We performed the high speed tension tests (strain rate: 0.001/s, 0.1/s, 1/s, 50/s, 100/s) for the characterisation of the plastics material. Also, the parameters of the SAMP-1 model were identified by using multidirectional mechanical tests such as uniaxial tension, simple shear, and compression tests. As validation purpose, the SMAP-1 model was compared to the existing models for predicting the stress-strain behaviour in the test specimens and the dynatup impact test.
Material Constitutive Behavior Identification at High Strain Rates Using a Direct-Impact Hopkinson Device
(2016-04-27) Guo, X.; Sow, C.; Khalil, C.; Heuzé, T.; Racineux, G.
Modern numerical simulation techniques allow nowadays obtaining accurate solutions of magnetic pulse and electrohydraulic forming/welding processes. However, one major difficulty persists: the identification of material constitutive equations behavior at levels of high strain rates reached by these processes, and which varies between 103 and 105 s-1. To address this challenge, a direct-impact Hopkinson system was developed at ECN. It permits to perform dynamic tests at very high strain rates exceeding the range of the traditional Split Hopkinson Pressure Bars and hence enable us to identify constitutive models for a wide range of strain rates. The alloy used to test this device was Ti-6Al-4V. Strain rates up to 2.5×103 s-1 were attained.
Qualification of CuCr1Zr for the SLM Process
(2016-04-27) Uhlmann, E.; Tekkaya, A. E.; Kashevko, V.; Gies, S.; Reimann, R.; John, P.
Working coils for electromagnetic forming processes need to comply with a wide list of requirements such as durability, efficiency and a tailored pressure distribution. Due to its unique combination of high strength and high electrical conductivity CuCr1Zr meets these requirements and is a common material for coil turns. In combination with conventional coil production processes like winding or waterjet cutting the use of this material is state of the art. A promising approach for coil production is the use of additive manufacturing (AM) processes. In comparison to conventional manufacturing processes, AM offers tremendous advantages such as feature-integration e.g. undercuts or lattice structures. However, this increased design freedom only leads to improved working coils if copper alloys with high strength and high electrical conductivity such as CuCr1Zr can be processed. Due to the high thermal conductivity and reflectivity the use of suchlike materials in additive manufacturing processes is challenging. Considering the effects of the required pre- and post-processing treatments for additive manufactured parts the need for research is further increased. The objective of this paper is to develop a method for the qualification of CuCr1Zr for the selective laser melting (SLM) process. This comprises the powder characterization, the process parameter identification and the microstructure investigation of the generated test geometries.
Electrodynamics of Magnetic Pulse Welding Machines: Global and Local Electrical Analogues
(2016-04-27) Bouzerar, R.; Bougrioua, F.; Tekaya, I.; Foy, N.; Hamzaoui, M.; Bourny, V.; Durand-Drouhin, O.; Jouaffre, D.; Haye, D.
In this paper, a theoretical, experimental and numerical study of MPW machines is carried out. While it is known that such machines are very complex by nature because of the coupling between different parts, we used simple electrical analogues to describe its dynamics. A RLC circuit modeling the whole machine is depicted and experimental results are shown. A further study including numerical simulations allows to compute the current distribution and estimate the magnetic field within the coil but also the magnetic pressure generated in the process, all using a 2D model and reasonable assumptions. A late theoretical study opens the way for innovative experimental measurements regarding the kinetics of the deformations of metallic tubes, but also their mechanical behavior before the welding process, making use of their capacitive properties.
Efficient Coil Design by Electromagnetic Topology Optimization for Electromagnetic Sharp Edge Forming of DP980 Steel Sheet
(2016-04-27) Choi, M. K.; Huh, H.; Seo, M. H.; Kang, Y.
This paper proposes a design method of the tool coil by topology optimization for the electromagnetic sharp edge forming process. Topology optimization is an approach that optimizes material configuration in a given domain to meet the design requirements. The design problem for the tool coil is defined as enhancing efficiency of the forming process and optimization problem is set to be maximization of the Lorentz force induced on the tool coil. A new topology optimization formulation based on the numerical methods for electromagnetism using FEM and BEM is developed for maximization of the Lorentz force. Optimum design of the tool coil is obtained by the topology optimization using the element density approach. The optimized result is compared with other coils which have different configurations to show the effectiveness of the proposed method. The idea of applying topology optimization to the design of the tool coil is successful and this formulation deals effectively for the optimization problems.
Effect of Conductivity of the Inner Rod on the Collision Conditions During a Magnetic Pulse Welding Process
(2016-04-27) Sapanathan, T.; Yang, K.; Raoelison, R. N.; Buiron, N.; Jouaffre, D.; Rachik, M.
The Magnetic Pulse Welding (MPW) process involves a high speed collision between the flyer and inner rod. Conductivity of the inner rod may play a significant role in the collision speed and collision angle. The collision conditions were investigated with varying conductivity of the inner rod in this study. Coupled mechanical-electromagnetic 3D simulations were carried out using LS-DYNA package to investigate the effect of conductivity of the inner rod on the collision patterns during the MPW process. The simulation involves a welding process with a tube and a rod using a one turn coil with a separate field shaper. The electrical conductivity was varied to a wide range to investigate the influence on the collision condition. Moreover, in order to verify the independency of the collision condition with the mechanical properties of the inner rod, two cases including aluminum alloy AA2024-T351 and copper with appropriate Johnson-Cook parameters were used for the rod. In the entire simulations aluminum alloy was used as the tube material. It was identified that the impact velocity is almost consistent for each case and the impact angles vary between negative and positive values according to the angular measurement convention used in this study. Although, influence of the conductivity of the inner rod is not significant for the investigated current flow while it may sometime delay the incidence of collision at lower frequencies than the critical frequency (FCrit). Optimizing the collision conditions in the MPW process can help to identify the suitable materials for prescribed welding conditions.
A Coupled 3D/2D Axisymmetric Method for Simulating Magnetic Metal Forming Processes in LS-DYNA
(2016-04-27) L‘Eplattenier, P.; Çaldichoury, I.
LS-DYNA is a general purpose explicit and implicit finite element program used to analyse the non-linear dynamic response of three-dimensional solids and fluids. It is developed by Livermore Software Technology Corporation (LSTC). An electromagnetism (EM) module has been added to LS-DYNA for coupled mechanical/thermal/electromagnetic simulations, which have been extensively performed and benchmarked against experimental results for Magnetic Metal Forming (MMF) and Welding (MMW) applications. These simulations are done using a Finite Element Method (FEM) for the conductors coupled with a Boundary Element Method (BEM) for the surrounding air, hence avoiding the need of an air mesh. More recently, a 2D axisymmetric version of the electromagnetic solver was introduced for much faster simulations when the rotational invariance can be assumed. In many MMF and MMW applications though, the rotational invariance exists only for part of the geometry (typically the coil), but other parts (typically the workpiece or the die) may not have this symmetry, or at least not for the whole simulation time. In order to take advantage of the partial symmetry without limiting the geometry to fully symmetric cases, a coupling between 2D and 3D was introduced in the EM. The user can define the parts that can be solved in 2D and the ones which need to be solved in 3D and the solver will assume the rotational invariance only on the 2D parts, thus keeping the results accurate while significantly reducing the computation time. In this paper, the coupling method will be presented along with benchmarks with fully 3D and fully 2D simulations, comparing the accuracy of the results and the simulation times.
Increase of the Reproducibility of Joints Welded with Magnetic Pulse Technology Using Graded Surface Topographies
(2016-04-27) Rebensdorf, A.; Boehm, S.
The reproducibility of individual welding methods depends to large extents on the material properties. This is especially the case for impact welding as tests have shown that the surface properties influence the joint formation. With the aim to influence the formation and position of the lower curve of the welding process window, this paper focuses on how the surface topography influences an asymmetrical impact. Additionally, relevant process parameters (e.g. collision speed, collision angle, jet formation) will be included and disturbance contours that are placed transversely to the collision vector will be examined. A high-speed camera was used to measure the collision speed as well as the collision angle. The specific surface topographies were created using belt grinding (cutting with geometrically undefined edges) and laser ablation (non-cutting process, local vaporization of materials through pulsed laser beams). The tests exemplarily show a strong correlation between the surface geometries and the joint. The disturbance contours that were introduced transversely to the collision vector shift the lower weld seam boundary, whereas a reduction of the discharge energy leads to a relative strength of the joint of 1.0. In sum, this paper offers fundamental insights into the mechanisms of the joint formation when using magnetic pulse welding and shows the influence of the surface topographies on the conflict between relevant procedural parameters and the possibility to shift the lower procedural window.

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