Soeren Gies

Influence of the Wall Thicknesses on the Joint Quality During Magnetic Pulse Welding in Tube-to-Tube Configuration

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.

Analytic Prediction of the Process Parameters for Form-Fit Joining by Die-Less Hydroforming

The Commission of the European Communities aims for a reduction of new car CO2 emissions of 120 grams per kilometer in 2012. As a result of the omnipresent efforts of the automotive industry to hit these tighter emission standards innovative lightweight strategies, e.g. the use of lightweight materials are developed. This entails new joining techniques that are appropriated to the new lightweight materials. The die-less hydroforming process is a joining method for tubular joints which meets the new demands of lightweight strategies. Since there is no need for any additional connection elements or filling material, it is an interesting alternative to conventional welding and riveting processes. The present paper describes the basic principle of the die-less hydroforming joining technology with a special focus on form-fit connections. An analytical model, based on the membrane theory with an additional local consideration of bending stresses is developed. This analytic approach can be used to calculate the working fluid pressure, required to bulge the tube material into the groove of the outer joining partner. Taking into account the material parameters as well as the groove and tube geometry, this model allows a reliable process design. Additionally, validation of the model by experimental investigations will be provided.

Analytical Model to Determine the Strength of Form-Fit Connection Joined by Die-Less Hydroforming

Modern lightweight concept structures are increasingly composed of several dissimilar materials. Due to the different material properties of the joining partners, conventional and widely used joining techniques often reach their technological limits when applied in the manufacturing of such multimaterial structures. This leads to an increasing demand for appropriate joining technologies, like joining by die-less hydroforming (DHF) for connecting tubular workpieces. The present work introduces an analytical model to determine the achievable strength of form-fit connections. This approach, taking into account the material parameters as well as the groove and tube geometry, is based on a membrane analysis assuming constant wall thicknesses. Besides a fundamental understanding of the load transfer mechanism, this analytic approach allows a reliable joining zone design. To validate the model, experimental investigations using aluminum specimens are performed. A mean deviation between the calculated and the measured joint strength of about 19% was found. This denotes a good suitability of the analytical approach for the design process of the joining zone.

Parameter Identification for Magnetic Pulse Welding Applications

Magnetic pulse welding (MPW) is a promising technology to join dissimilar metals and to produce multi-material structures, e.g. to fulfill lightweight requirements. During this impact welding process, proper collision conditions between both joining partners are essential for a sound weld formation. Controlling these conditions is difficult due to a huge number of influencing and interacting factors. Many of them are related to the pulse welding setup and the material properties of the moving part, the so-called flyer. In this paper, a new measurement system is applied that takes advantage of the high velocity impact flash. The flash is a side effect of the MPW process and its intensity depends on the impact velocity of the flyer. Thus, the intensity level can be used as a welding criterion. A procedure is described that enables the user to realize a fast parameter development with only a few experiments. The minimum energy level and the optimum distance between the parts to be joined can be identified. This is of importance since a low energy input decreases the thermal and mechanical shock loading on the tool coil and thus increases its lifetime. In a second step, the axial position of the flyer in the tool coil is adjusted to ensure a proper collision angle and a circumferential weld seam.

Joining by Die-Less Hydroforming of Profiles with Oval Cross Section

Joining by die-less hydroforming is used to produce overlap joints by means of hydraulic expansion. Due to a difference in the elastic recovery of the two joining partners an interference pressure p remains at their contact area. Due to the possibility to produce multi-material joints without relying on heat, the process has great potential for joining parts in lightweight applications. Therefore, the process limits were extended so that profiles with non-rotationally symmetric cross-sections can be joined. For this purpose a new tool for profiles with oval cross-section was developed. The inner and the outer joining partner ware made of aluminum 6060 and aluminum 6082 respectively. The influence of the overlap length and different wall thicknesses of the outer joining partner were investigated by numerical simulations and validated by experiments. An upper limit in interference pressure was observed which was also found previously for profiles with circular cross sections. The fluid pressure limit is compared with the analytically calculated value for a configuration with circular tubes under equivalent conditions. The analytical model underestimated the pressure limit. In contrast to circular tubes, the strain distribution of profiles with oval cross sectional shapes is not uniform, which results in superposed bending stresses. Also a difference in stiffness of the inner and outer joining partner leads to a pressure depended contact area which is assumed constant in the analytical model.

Analytical Methodology for the Process and Joint Design of Form-Fit Joining by Die-Less Hydroforming

In modern lightweight concepts, for example in automotive engineering, structures are increasingly composed of several dissimilar materials. Due to the different material properties of the joining partners, conventional and widely used joining techniques often reach their technological limits when applied in the manufacturing of such multi-material structures. This leads to an increasing demand for appropriate joining technologies, like joining by die-less hydroforming (DHF) for connecting tubular workpieces. The present work introduces an analytical model to determine the achievable joint strength of this connection type. This approach, taking into account the material parameters as well as the groove and tube geometry, is based on a membrane analysis with constant wall thickness. Additionally, bending stresses and friction are considered locally. Besides a fundamental understanding of the load transfer mechanism, this analytic approach allows a reliable joining zone design. To validate the model, experimental investigations using aluminum specimens were performed.Copyright

Groove Filling Characteristics and Strength of Form-Fit Joints Produced by Die-Less Hydroforming

The manufacturing of modern lightweight structures and the implementation of multi material concepts, for example in automotive engineering, entails appropriate joining technologies. The absence of additional connection elements or filling materials as well as the possibility to join dissimilar metals are basic requirements in this field of application to reach the aspired weight reduction. In case of tubular joints the die-less hydroforming process meets these demands and thus makes it an interesting alternative to conventional welding and riveting processes. The present work focuses on form fit joints produced by die-less hydroforming. It provides a verification of a previously presented analytical approach that allows the calculation of the working fluid pressure required to bulge the tube material into the groove of the outer joining partner. For that purpose, the groove filling characteristics of joined specimens with different groove geometries are analyzed. Here both joining partners were made of the aluminum alloy EN AW-6060. Additionally the connection strength of the joined specimens are determined using tensile tests. The results prove that the groove angle is the main influencing factor on the connection strength and that it can be used for an ordinal comparison of different groove geometries.