Der Einsatz eines gesteuerten Gegenstempels als aktives Element kann die Qualität und die Beanspruchbarkeit kaltfließgepresster Produkte signifikant verbessern. Durch konventionelle Prozessführung mittels Voll‐Vorwärts‐Fließpressen hergestellte Bauteile weisen hohe Zugeigenspannungen an der Oberfläche sowie einen steilen Spannungsgradienten auf. Neben der Gefahr des Verzugs beeinträchtigt dieser ungünstige Eigenspannungszustand das Ermüdungsverhalten. Von den Autoren wurde ein System entwickelt, das während des Fließpressvorgangs mit einem aktiv eingesetzten Gegenstempel eine Druckkraft auf das zu umzuformende Bauteil ausübt und eine signifikante Verbesserung des Spannungszustands des Endprodukts bewirkt. Der positive Effekt wird durch eine Homogenisierung der Deformationen im Bauteil während des Fließpressens bewirkt. Zur Prozesssimulation wurde ein Finite‐Elemente‐Modell in Simufact Forming® entwickelt. Die Simulationsergebnisse zeigen, wie sich die durch den aktiven Gegenstempel aufgebrachte Kraft auf den endgültigen Spannungszustand auswirkt. Die Validierung der Simulationsergebnisse erfolgte anhand von Versuchen mit den beiden austenitischen Stählen 1.4307 und 1.4404. Ein weiterer Schwerpunkt bildet die Entwicklung geeigneter Messstrategien zur röntgenographischen Eigenspannungsanalyse mit dem sin2Ψ‐Verfahren, um trotz grobkörniger und anisotroper Werkstoffzustände die Qualität der Messergebnisse sicherzustellen. Die Ergebnisse aus Simulation und Experiment belegen das Potenzial der entwickelten Umformstrategie. Die Produktqualität lässt sich ohne Änderung der Prozesszeit erhöhen.
The resulting tensile residual stresses on the surface of cold full-forward extruded parts are unfavorable for the fatigue life of these parts. The final stress state is determined by the combination of two process stages: forming and ejection. This is due to the fact that the workpiece undergoes a second plastic deformation after forming during the ejection from the die. So far, literature is focusing mainly on the identification of the parameters affecting the residual stresses during the first stage. In the present paper, the attention is drawn to the ejection phase during cold extrusion of workpieces made out of the austenitic stainless steel AISI 316 L. First of all, a system consisting of an active die is presented. This technology allows the control of the applied pre-stress on the die during the process. It is experimentally and numerically demonstrated that a significant shift of the residual stress state in the near-surface region can be achieved. Even compressive axial and tangential residual stresses can be induced in this area. Also the limits of this system are numerically investigated. It is observed that a different deformation mechanism occurs above a certain pre-stress level. Finally, an analytical model is created and observations are presented relatively to the mechanisms that influence the plastic deformation during ejection.
The family of bulk forming technologies comprises processes characterised by a complex three-dimensional stress and strain state. Besides shape and material properties, also residual stresses are modified during a bulk metal forming process. The state of residual stresses affects important properties, like fatigue behaviour and corrosion resistance. An adjustment of the residual stresses is possible through subsequent process steps such as heat treatments or mechanical surface modification technologies, like shot peening and deep rolling. However, these additional manufacturing steps involve supplementary costs, longer manufacturing times and harmful effects on the product quality. Therefore, an optimized strategy consists in a targeted introduction of residual stresses during the forming processes. To enable this approach, a fundamental understanding of the underlying mechanisms of residual stress generation in dependence of the forming parameters is necessary. The current state of the art is reviewed in this paper. Strategies for the manipulation of the residual stresses in different bulk forming processes are classified according to the underlying principles of process modification.
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