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Clinching is a joining process that is becoming more and more important in industry due to the increasing use of multi-material designs. Despite the already widespread use of the process, there is still a need for research to understand the mechanisms and design of clinched joints. In contrast to the tool parameters, process and material disturbances have not yet been investigated to a relatively large extent. However, these also have a great influence on the properties and applicability of clinching. The effect of process disturbances on the clinched joint are investigated with numerical and experimental methods. The investigated process variations are the history of the sheets using the pre-hardening of the material, different sheet thicknesses, sheet arrangements and punch strokes. For the consideration of the material history, a specimen geometry for pre-stretching specimens in uniaxial tension is used, from which the pre-stretched secondary specimens are taken. A finite element model is set up for the numerical investigations. Suitable clinching tools are selected. With the simulation, selected process influences can be examined. The effort of the numerical investigations is considerably reduced with the help of a statistical experimental design according to Taguchi. To confirm the simulation results, experimental investigations of the clinch point geometry by using micrographs and the shear strength of the clinched joint are performed. The analysis of the influence of difference disturbance factors on the clinching process demonstrate the importance of the holistic view of the clinching process.

ZusammenfassungFlexible Fertigungsprozesse in der Umformtechnik ermöglichen es, schnell und effizient auf veränderliche Rahmenbedingungen beispielsweise durch Kundenwünsche und rechtliche Anforderungen reagieren zu können. Besonders die inkrementellen Umformverfahren der Massivumformung zeichnen sich durch eine hohe Flexibilität aus, da die Prozesse gut anpassbar sind und die verwendeten Anlagen und Werkzeuge in einem geringeren Maße auf ein spezifisches Bauteil hin ausgelegt werden.Der hier vorgestellte Fügeprozess mittels Radial-Walzen ist geeignet, hybride Bauteile form- und kraftschlüssig miteinander zu verbinden. So lassen sich die spezifischen Vorteile verschiedener Werkstoffe kombinieren und Nutzungs- sowie Leichtbaupotenziale können gezielt genutzt werden. Die in diesem Prozess entstehende charakteristische umlaufende Kerbe in der Wellenkomponente bildet eine Schwachstelle der gefügten Bauteile, da es dort unter Last zu einer Spannungskonzentration kommt, welche die Bildung eines Risses begünstigt. Dieser Herausforderung wird mit einer Kombination der Ausnutzung der Kaltverfestigung des gewählten Werkstoffs (1.7225) und gezielt eingebrachter Druckeigenspannungen in der Kerbe begegnet. Hier liegt der Fokus auf den Druckeigenspannungen, die das Potenzial aufweisen, sich verlängernd auf die Lebensdauer auszuwirken. Sie lassen sich gezielt über eine günstige Wahl des Walzpfads zur Herstellung der Fügeverbindung beeinflussen, wie sowohl in numerischen Analysen als auch durch Eigenspannungsmessungen an Realbauteilen gezeigt wird. Es können sehr hohe Eigenspannungen in der kritischen Zone des Kerbgrunds eingebracht werden, die einen deutlichen Einfluss auf die Lebensdauer haben, wie auch die Lebensdauerberechnungen mittels des Örtlichen Konzepts deutlich zeigen.

The proposed innovative cross-rolling process can be used to form hybrid components like shaft hub connections and electric drive parts. The advantages as well as the challenges are discussed in detail, especially the resulting notch geometry and possible approaches for mitigating adverse effects. While the shaft ideally should be a high strength steel, generally any other metal, ceramic or composite material can be used as a joining partner. This paper will present a numerical analysis of the joining process regarding work roll path and geometry as well as an experimental evaluation of the achievable properties and the resulting transmissible loads.

A number of different rolling processes are summarized under the term skew rolling. The process variant investigated in this paper is an incremental flexible rolling process with kinematic forming of the geometry. Two sets of actively driven work rolls are used in a transverse rolling process to selectively reduce the diameter of a cylindrical workpiece to form circumferential grooves. Simultaneously, the excess material is pushed in axial direction, thereby forming a bulge that can be used to join e.g. a gear wheel onto an axle. Since this is done in a cold rolling process the positive influence of the induced residual compressive stress as well as strain hardening can be used to improve fatigue strength of the workpiece under cyclical loads. The authors discuss the advantages of this hybrid skew and cross rolling process and propose a simplified numerical model to quickly analyze process parameters like axial and radial work roll feed and the influence of initial stress profiles in the workpiece. The model uses a 2 ½ D axis-symmetric approach in LS-Dyna R10.1.0 with implicit solver. Additionally, preliminary results of first tests on rolled specimen from 42CrMo4 are briefly discussed.

To pursue better quality, there is a strong need for efficient and robust non-destructive testing methods for joining processes. A new approach has been proposed to detect loose bolted joints using acoustic waves. This method measures the dissipation of energy by recording the propagated waves on the surfaces of the joining partners next to the joint. The authors show a link between the torque moments applied to the bolt and the ability of the joint to transfer acoustic energy using both simulations and experiments. In this article, the method’s ability to detect variations of certain process parameters of the clinching process was investigated numerically. To this extent, results of FEM calculations of the joining process are used to simulate the non-destructive testing method. By varying parameters of the clinching model, their influence on the transmission of energy within the joint is studied.

Clinching is a cost-effective mechanical joining process used for metallic as well as nonmetallic materials. Destructive testing methods such as peel and tensile tests can be used for strength investigations of such joints. Additionally, by measuring the geometrical properties such as undercut, neck thickness, and final bottom thickness, the joint quality can be estimated. These methods are cumbersome and do not meet the time and cost efficiency requirements of industrial production. The harmonic analysis benefits from changes in vibration characteristics. In this paper, the structural response of a clinched joint, subjected to a dynamic displacement in order to introduce an acoustic wave testing method is studied. This is accomplished by using simulated joining processes. The approach of this paper is based on a transient dynamic finite element analysis, followed by a fast Fourier transform. The results are presented, together with an analysis of sensitivity to different process parameters.

In conventional forming processes, quasi-static conditions are a good approximation and numerical process optimization is the state of the art in industrial practice. Nevertheless, there is still a substantial need for research in the field of identification of material parameters. In production technologies with high forming velocities, it is no longer acceptable to neglect the dependency of the hardening on the forming speed. Therefore, a method for determining material characteristics in processes with high forming speeds was developed by designing and implementing a test setup and an inverse parameter identification. Two acceleration concepts were realized: a pneumatically driven one and an electromagnetically driven one. The method was verified for a mild steel and an aluminum alloy proving that the identified material parameters allow numerical modeling of high-speed processes with good accuracy. The determined material parameters for steel show significant differences for different stress states. For specimen geometries with predominantly uniaxial tensile strain at forming speeds in the order of 104&ndash, 105/s the determined yield stress was nearly twice as high compared to shear samples, an effect which does not occur under quasi-static loading. This trend suggests a triaxiality-dependent rate dependence, which might be attributed to shear band induced strain localization and adiabatic heating.

Forming processes generally lead to residual stresses. Compressive residual stresses can have a beneficial impact on the fatigue life of components, which is demonstrated here in terms of a skew-rolled hybrid shaft-hub connection. In order to set the basis for a systematic investigation of this effect, a strategy for the evaluation of the influence of residual stresses on the fatigue resistance of formed components is proposed. It involves the determination of the expectable residual stresses through a simulation of the forming process as well as a fatigue analysis. While state-of-the-art fatigue concepts assume unchanging cyclic material behaviour throughout the entire fatigue life of the component, this assumption is not adequate for residual stress-afflicted components. Hence, a generalization to transient cyclic material behaviour with an asymmetry in tension and compression is introduced.

This paper presents a concept for a hybrid deep drawing tool based on polymer concrete and first results from actual deep drawing tests using a prototype of the newly developed tool. The hybrid tool presented here has a defined macro-structure of the blankholder and drawing die as implemented before by Brosius et al. [1] in a steel tool for a lubricant free deep drawing process. The macro structure has a positive effect on the lubrication free deep drawing process, because it reduces the contact surface, increases the sheet’s stability against wrinkling in the flange, reduces the necessary blank holder force and thereby the contact pressure, and offers a means to control the material flow [1]. The main objective in transferring this concept to a new hybrid tool from polymer concrete is an enhanced possibility to easily integrate other functional elements like temperature and pressure sensors, graphite inlays or roller bearings into the drawing die or blankholder. Since polymer concrete is cast in a cold e...