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Vaporizing Foil Actuator Welding (VFAW) technique was applied to join 4 mm thick aluminium alloy 5754 to 2.5 mm thick High Strength Low Alloy (HSLA) steel 340 with a 1 mm thick aluminium alloy 3003-H14 interlayer. Welds with consistent peak loads of 17.6 kN were achieved with input energy less than 5 kJ. Tension-tension fatigue testing yielded high cycle around 1000k cycles under the peak load of 40% tensile strength from static testing. Microscopic characterization revealed no continuous melting zones in the welding interfaces. Coupled with lightweight designs, the approach was utilized on prototype production of an aluminium-steel vehicle subframe. Issues related to arcing arose and were tackled during the development of prototype production.

This paper mainly demonstrates an advanced type of the vaporizing foil actuator welding (VFAW) process between GPa-grade steel (TRIP1180) and aluminum alloy (AA5052-H32) without applying standoff. To secure a flying distance during the VFAW process, the preformed target sheet shaped like a circular indentation has been utilized. It is necessary to optimize process parameters integrated with geometrical design of the preform since the welding strength can be decreased beyond the optimum input energy in the standoff-free VFAW process. The welded surface was evaluated by SEM-EDS, XRD, EBDS, and TEM to analyze the welding mechanism and composition at the welding interface. The diffusion zone including the AlFe3 phase was observed at the welded interface which has high grain density due to the high-speed impact by increasing the welding strength, which leads to the perfect welding between the dissimilar materials.

Collision welding is a joining technology that is based on the high-speed collision and the resulting plastic deformation of at least one joining partner. The ability to form a high-strength substance-to-substance bond between joining partners of dissimilar metals allows us to design a new generation of joints. However, the occurrence of process-specific phenomena during the high-speed collision, such as a so-called jet or wave formation in the interface, complicates the prediction of bond formation and the resulting bond properties. In this paper, the collision welding of aluminum and copper was investigated at the lower limits of the process. The experiments were performed on a model test rig and observed by high-speed imaging to determine the welding window, which was compared to the ones of similar material parings from former investigation. This allowed to deepen the understanding of the decisive mechanisms at the welding window boundaries. Furthermore, an optical and a scanning electron microscope with energy dispersive X-ray analysis were used to analyze the weld interface. The results showed the important and to date neglected role of the jet and/or the cloud of particles to extract energy from the collision zone, allowing bond formation without melting and intermetallic phases.

Dissertação de Mestrado Integrado em Engenharia Mecânica apresentada à Faculdade de Ciências e Tecnologia The present work aims to study, by numerical simulation, using the commercial hydrodynamic program Ansys Autodyn, the morphology and interfacial phenomena in welding aluminum plates to steel by impact. The effect of the impact velocity and the collision angle on the conditions developed at the plate interface, namely on the formation of the jet, on the formation and morphology of waves, the existence of fusion and the temperature and pressure reached, were considered. The numerical results were compared with experimental results obtained in previous studies. The study focused on the bonding of AA 6061-T6 aluminum to AISI 304 stainless steel and high hardness V250 carbon steel.In the Ansys Autodyn simulation software, the mesh method used to reproduce the formation of jet and wave formation more accurately was the Smoothed Particles Hydrodynamics method. The equation of state chosen for the metal plates was the Mie-Grüneisen shock equation, which was associated with the Steinberg-Guinan behavior model. A total of 50 numerical tests were performed.The Al-SS weldability window revealed that the simulations obtained are generally within the weldability limits, with good welding results and interfaces with well-defined waves. It was found that the ideal pressure to obtain the best results was between 2 GPa and 4 GPa. On the contrary, in the Al-CS weldability window, the results obtained in the simulations were not in line with the weldability limits. The interfaces obtained were, in most cases, flat, due to the high hardness of the steel, being quite difficult to connect the two metals. The pressure close to the interface estimated to provide the best welding results was from 4 GPa to 7 GPa.The numerical and experimental results, when compared, did not show total harmony with regard to the morphology of the interface, mainly due to the difference in hardness of welded metals in both approaches. O presente trabalho tem como objetivo estudar, por simulação numérica, com recurso ao programa comercial hidrodinâmico Ansys Autodyn, a morfologia e os fenómenos interfaciais em soldadura de chapas de alumínio a aço por impacto. Foi considerado o efeito da velocidade de impacto e do ângulo de colisão nas condições desenvolvidas na interface das placas, nomeadamente na formação do jato, na formação e morfologia de ondas, a existência de fusão e a temperatura e pressão atingidas. Os resultados numéricos foram comparados com resultados experimentais obtidos em trabalhos anteriores. O estudo incidiu na ligação do alumínio AA 6061-T6 ao aço inoxidável AISI 304 e ao aço carbono V250 de elevada dureza.No software de simulação Ansys Autodyn, o método de malha utilizado para reproduzir com maior exatidão a formação de jato e de ondas foi o método Smoothed Particles Hydrodynamics. A equação de estado escolhida para as placas metálicas foi a equação de Shock de Mie-Grüneisen, que foi associada ao modelo de comportamento de Steinberg-Guinan. Foram realizados, no total, cerca de 50 ensaios numéricos.A janela de soldabilidade Al-SS revelou que as simulações obtidas estão, de modo geral, dentro dos limites de soldabilidade, com bons resultados de soldadura e interfaces com ondas bem definidas. Verificou-se que a pressão ideal para obter os melhores resultados situava-se entre os 2 GPa e os 4 GPa. Pelo contrário, na janela de soldabilidade Al-CS, os resultados obtidos nas simulações não estavam em sintonia com os limites de soldabilidade. As interfaces obtidas foram, na maior parte dos casos, planas, devido à elevada dureza do aço, sendo bastante difícil ligar os dois metais. A pressão próxima da interface estimada para proporcionar os melhores resultados de soldadura foi de 4 GPa a 7 GPa.Os resultados numéricos e experimentais, quando comparados, não mostraram total sintonia no que respeita à morfologia da interface, devido essencialmente à diferença de dureza dos metais soldados em ambas as abordagens.

Collision welding is a high-speed joining technology based on the plastic deformation of at least one of the joining partners. During the process, several phenomena like the formation of a so-called jet and a cloud of particles occur and enable bond formation. However, the interaction of these phenomena and how they are influenced by the amount of kinetic energy is still unclear. In this paper, the results of three series of experiments with two different setups to determine the influence of the process parameters on the fundamental phenomena and relevant mechanisms of bond formation are presented. The welding processes are monitored by different methods, like high-speed imaging, photonic Doppler velocimetry and light emission measurements. The weld interfaces are analyzed by ultrasonic investigations, metallographic analyses by optical and scanning electron microscopy, and characterized by tensile shear tests. The results provide detailed information on the influence of the different process parameters on the classical welding window and allow a prediction of the different bond mechanisms. They show that during a single magnetic pulse welding process aluminum both fusion-like and solid-state welding can occur. Furthermore, the findings allow predicting the formation of the weld interface with respect to location and shape as well as its mechanical strength.

Collision welding processes are accompanied by the ejection of a metal jet, a cloud of particles (CoP), or both phenomena, respectively. The purpose of this study is to investigate the formation, the characteristics as well as the influence of the CoP on weld formation. Impact welding experiments on three different setups in normal ambient atmosphere and under vacuum-like conditions are performed and monitored using a high-speed camera, accompanied by long-term exposures, recordings of the emission spectrum, and an evaluation of the CoP interaction with witness pins made of different materials. It was found that the CoP formed during the collision of the joining partners is compressed by the closing joining gap and particularly at small collision angles it can reach temperatures sufficient to melt the surfaces to be joined. This effect was proved using a tracer material that is detectable on the witness pins after welding. The formation of the CoP is reduced with increasing yield strength of the material and the escape of the CoP is hindered with increasing surface roughness. Both effects make welding with low-impact velocities difficult, whereas weld formation is facilitated using smooth surfaces and a reduced ambient pressure under vacuum-like conditions. Furthermore, the absence of surrounding air eases the process observation since exothermic oxidation reactions and shock compression of the gas are avoided. This also enables an estimation of the temperature in the joining gap, which was found to be more than 5600 K under normal ambient pressure.

High-velocity impact welding is a kind of solid-state welding process that is one of the solutions for the joining of dissimilar materials that avoids intermetallics. Five main methods have been developed to date. These are gas gun welding (GGW), explosive welding (EXW), magnetic pulse welding (MPW), vaporizing foil actuator welding (VFAW), and laser impact welding (LIW). They all share a similar welding mechanism, but they also have different energy sources and different applications. This review mainly focuses on research related to the experimental setups of various welding methods, jet phenomenon, welding interface characteristics, and welding parameters. The introduction states the importance of high-velocity impact welding in the joining of dissimilar materials. The review of experimental setups provides the current situation and limitations of various welding processes. Jet phenomenon, welding interface characteristics, and welding parameters are all related to the welding mechanism. The conclusion and future work are summarized.

This work aims to investigate the formation and evolution of the interfacial features which occur during a high-speed impact welding using numerical simulation. A robust numerical modelling based on Eulerian technique is developed to predict the interface kinetics of magnetic pulse welding (MPW). The model has been originally validated for similar material welding and in this study we extend the investigation to an aluminum/copper welding. Results of the kinetics along the welding interface including metal jet emission, wake, vortex, and wave formation are successfully predicted, and the predictions are in good agreement with the experimental observations. Moreover, the metal jet was mainly composed of the lower density metal, i.e. aluminum. It was also observed that the combination of impact velocity and impact angle affects the interface instabilities. The Eulerian model demonstrates its ability to predict the complex interfacial behaviors and the corresponding thermomechanical interactions during the magnetic pulse welding process.

Explosive welding is a field with a wide variety of applications of great value, such as corrosion resistant cladding and bi-metallic joints. It occupies a special place in the available metal joining techniques. Dissimilar metal welding is possible in metal pairings that don’t support other conventional bonds, and it can produce superior area welds regardless of the metal parts to be joined. The objectives of this dissertation were to further the understanding of explosive welding in general, as well as the empirical understanding of welding of Aluminum 6061-O, and to investigate the use of LS-DYNA’s Multi-Material Arbitrary Lagrangian-Eulerian formulation as a potential tool for the design of explosive welds. In the course of the work, the theory on formation of bond interfacial waves was identified as an area where there was not an apparent consensus, and this was addressed in light of both recent works and information from this study.For this dissertation, an experimental program of explosive welding tests, mechanical weld verification, and metallurgical observation were undertaken in order to add to the data available for this type of welding. Nine different explosive welding tests were conducted covering four scenarios, which were combinations of different explosive thicknesses and flyer inclination angles. Tensile shear tests with digital image correlation were used to test the welds, and optical microscope, Scanning Electron Microscope, and Transmission Electron Microscope images were used to investigate the nature of the bond. The numerical investigation was conducted and compared to both experiment and initial modeling results.The results reinforce the need for well-developed and material specific welding windows, adding additional data for the joining of Aluminum 6061-O. The endorsement of the continuous Kelvin-Helmholtz jet wake as the source of instability was supported with modeling results. The Multi-Material Arbitrary Lagrangian Eulerian modeling with Euler-Lagrange Coupling was demonstrated to yield results comparable to research codes for welding parameters, to be able to capture jetting, and provide meaningful temperature results. Bond interfacial waves were characterized with some success as well, concluding that this modeling technique is a viable means to assist in the design of explosive welds.

Introduced is a comprehensive numerical modeling framework that includes microstructure when simulating the laser impact welding (LIW) of metals to study the transient phenomena that occur during weld formation. Such transient phenomena include evolution of shear stresses, plastic strains, thermal response, and material jetting. Inhomogeneous microstructures for two dissimilar foils (aluminum 1100 and stainless steel 304) are first predicted using the Dynamic Kinetic Monte Carlo (KMC) method to simulate laser-based powder bed fusion (PBF-LB) additive manufacturing (AM). These microstructures are subsequently incorporated into an Eulerian finite element (FE) simulation of the LIW process, enabling prediction of grain elongations that result from the varying yield surfaces, stacking fault energies, and grain-boundary sliding effects. Trends in the predicted microstructure deformation patterns show strong agreement with those from experimental images in the literature. Compared to existing homogeneous models, the new framework with inhomogeneous AM microstructure reveals higher collision velocities at the weld interface, resulting in increased plastic strain rates, greater plastic heat dissipation, and increased material jetting with higher jet temperatures. The framework allows for new opportunities to study correlations between grain topography (as well as polycrystalline metal texture) and the transient process phenomena occurring at the impact weld interface.

Impact welding in general facilitates the metallurgical bond even between dissimilar metals due to the lack of extensive heat input. It thus offers superior joint properties such as a good mechanical strength. However, in contrast to fusion welding, the fundamentals are not fully understood. As a direct consequence, not all influencing process parameters are known or quantified yet. However, this is crucial in order to further comprehend, develop and optimize the process. The objective of this study is to investigate the influence of the surrounding gaseous medium on the weld strength as additional parameter. This issue has been discussed in earlier publications of other researchers already, though without any consistent conclusion yet. The results of this study show that a lower gas density results in higher weld strength, whereas higher densities may even inhibit the joint completely at identical impact velocities. High speed images of the electromagnetic pulse welding process indicate that this might be caused by the jet which is obstructed by the ambient gas atmosphere on its way out of the impact area. The higher the density, the slower the visible jet is. Thus, superficial material of the workpieces and contaminants cannot leave the welding area and hinder its formation instead.

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.

MPW (magnetic pulse welding) is a solid state joining technology that allows for the generation of strong metallic bonds, even between dissimilar metals. Due to the absence of external heat, critical intermetallic phases can largely be avoided. In this process, Lorentz forces are utilized for the rapid acceleration of at least one of the two metallic joining partners leading to the controlled high velocity impact between them. The measurement of the collision conditions and their targeted manipulation are the key factors of a successful process development. Optical measuring techniques are preferred, since they are not influenced by the prevalent strong magnetic field in the vicinity of the working coil. In this paper, the characteristic high velocity impact flash during MPW was monitored and evaluated using phototransistors in order to measure the time of the impact. The results are in good accordance with the established PDV (photon Doppler velocimetry) and show a good repeatability. Furthermore, the collision front velocity was investigated using adapted part geometries within a series of tests. This velocity component is one of the key Parameters in MPW; its value decreases along the weld zone. With the help of this newly introduced measurement tool, the magnetic pressure distribution or the joining geometry can be adjusted more effectively.

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.

Magnetic pulse welding (MPW) enables 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. The working length of the experimental setup allows for various shapes of the deformation front, and each configuration has its own advantages and drawbacks. The objective of this work is to show how the working length of tubular MPW specimens affects the front propagation as well as to indicate ways to optimize the front propagations, which are vital to the welding result. It is shown that for steel-aluminum joints, three different front regimes exist, which are related to geometrical factors. These results may be used to avoid seemingly favorable but nevertheless suboptimal conditions for flyer movement, which reduce the weld quality and energy efficiency of the process. Additionally, the methodology presented here may allow for faster process optimization without the need for time-consuming metallographic analyses.

Against the manufacturing requirement for both lower lead time and reduced machining time for titanium components, a new concept was conceived assembling sheet material and other stock into semi finished parts by (explosive) impact welding. It is believed that this concept (which we named SLAM) could be especially beneficial for titanium alloy and nickel alloy. The present investigation centered on the feasibility of this technology for the widely used Ti6Al4V alloy. Impact process parameters were established and mechanical properties were investigated. In general, static properties were good. Fatigue strength reduced however, although much less so for notched specimens compared to un-notched material. Fatigue crack initiation could be linked to complex features within the (wavy) weld interface associated with the 'first generation' impact welding parameters. Next steps are foreseen to improve the fatigue performance. Two manufacturing demonstrators were manufactured showing the application to a T-bar manufactured from plate and sheet, as well as a tool-cladding application. Copyright © 2006 SAE International.

Against the manufacturing requirement for both lower lead time and reduced machining time for titanium components, a new concept was conceived assembling sheet material and other stock into semi finished parts by (explosive) impact welding. It is believed that this concept (which we named SLAM) could be especially beneficial for titanium alloy and nickel alloy. The present investigation centered on the feasibility of this technology for the widely used Ti6Al4V alloy. Impact process parameters were established and mechanical properties were investigated. In general, static properties were good. Fatigue strength reduced however, although much less so for notched specimens compared to un-notched material. Fatigue crack initiation could be linked to complex features within the (wavy) weld interface associated with the 'first generation' impact welding parameters. Next steps are foreseen to improve the fatigue performance. Two manufacturing demonstrators were manufactured showing the application to a T-bar manufactured from plate and sheet, as well as a tool-cladding application. Copyright © 2006 SAE International.

High energy-rate metalworking has significant advantages, in that short time scales change the fundamental nature of the forming process and short duration impulses can be used with much lighter and more agile equipment because large static forces do not need to be resisted. Here, we propose the use of thin aluminum foils that are intentionally vaporized by a pulsed electric current, in order to create an intense mechanical impulse. The driving pressures can be a few GPa's, and have been used to launch sheet metal workpieces to velocities in excess of 1 km/s. Applications including impact welding, forming, embossing, shearing, and powder compaction have been demonstrated and will be presented in this paper.