After conventional forming processes, the residual stress distribution in wires is frequently unfavorable for subsequent processes, such as bending operations. High tensile residual stresses typically occur near the wire surface and normally limit further processability of the material. Additional heat treatment operations or shot peening are often used to influence the residual stress distribution in the material after conventional manufacturing, which is time- and energy-consuming. This paper presents an approach for influencing the residual stress distribution by modifying the forming process, especially regarding die geometry. The aim is to reduce the resulting tensile stress levels near the surface. Specific forming elements are integrated into the dies to achieve this residual stress reduction. These modifications in the forming zone have a significant influence on process properties, such as plastic strain and deformation direction, but typically do not influence product geometry. This paper describes the theoretical approach and model setup, the FE simulation, and the results of the experimental tests. The characterization of the residual stress states in the specimen was carried out through X-ray diffraction using the sin²Ψ method.

In this contribution, we study the microstructural evolution of an age-hardenable AA6082 aluminum alloy during severe plastic deformation by gradation extrusion. A novel die design allowing an interruption of processing and nondestructive billet removal was developed. A systematic study on the microstructure gradient at different points of a single billet could be performed with the help of this die. Distinct gradients were investigated using microhardness measurements and electron microscopy. Our results highlight that gradation extrusion is a powerful method to produce graded materials with partially ultrafine-grained microstructures. From the point of view of obtaining an ultrafine-grained surface layer with maximum hardness, only a small number of forming elements is needed. It was also found that large incremental deformation by too many forming elements may result in locally heterogeneous microstructures and failure near the billet surface caused by localization of deformation. Furthermore, considering economical aspects of processing, fewer forming elements are preferred since several processing parameter-related cost factors are then significantly lower.

As a result of conventional wire-forming processes, the residual stress distribution in wires is frequently unfavorable for subsequent forming processes such as bending operations. High tensile residual stresses typically occur in the near-surface region of the wires and can limit further application and processability of the semi-finished products. This paper presents an approach for tailoring the residual stress distribution by modifying the forming process, especially with regard to the die geometry and the influence of the drawing velocity as well as the wire temperature. The aim is to mitigate the near-surface tensile residual stresses induced by the drawing process. Preliminary studies have shown that modifications in the forming zone of the dies have a significant impact on the plastic strain and deformation direction, and the approach can be applied to effectively reduce the process-induced near-surface residual stress distributions without affecting the diameter of the product geometry. In this first approach, the process variant using three different drawing die geometries was established for the metastable austenitic stainless steel X5CrNi18-10 (1.4301) using slow (20 mm/s) and fast (2000 mm/s) drawing velocities. The residual stress depth distributions were determined by means of incremental hole drilling. Complementary X-ray stress analysis was carried out to analyze the phase-specific residual stresses since strain-induced martensitic transformations occurred close to the surface as a consequence of the shear deformation and the frictional loading. This paper describes the setup of the drawing tools as well as the results of the experimental tests.

The residual stress distribution in extruded components and wires after a conventional forming process is frequently unfavourable for subsequent processes, such as bending operations. High tensile residual stresses typically occur near the surface of the wire and thus limit further processability of the material. Additional heat treatment operations or shot peening are often inserted to influence the residual stress distribution in the material after conventional manufacturing. This is time and energy consuming. The research presented in this paper contains an approach to influence the residual stress distribution by modifying the forming process for wire-like applications. The aim of this process is to lower the resulting tensile stress levels near the surface or even to generate compressive stresses. To achieve these residual compressive stresses, special forming elements are integrated in the dies. These modifications in the forming zone have a significant influence on process properties, such as degree of deformation and deformation direction, but typically have no influence on the diameter of the product geometry. In the present paper, the theoretical approach is described, as well as the model set-up, the FE-simulation and the results of the experimental tests. The characterization of the residual stress states in the specimen was carried out by X-ray diffraction using the sin²Ψ method.

Zusammenfassung: Nach dem Drahtziehen treten hohe Zugeigenspannungen in den oberflächennahen Schichten des Drahtes auf. Dieser Eigenspannungszustand ist für die weitere umformtechnische Verarbeitung, z. B. für das Biegen zur Herstellung von Torsionstabfedern, ungeeignet. Nach derzeitigen Standards wird dem Drahtziehen eine Wärmebehandlung nachgeschaltet. In diesem Artikel wird ein neuer Ansatz zur Einstellung des Eigenspannungszustandes direkt im Umformprozess des Drahtes vorgestellt. Durch eine Modifikation der Ziehwerkzeuge mit integrierten Umformelementen sollen niedrigere Zugspannungsniveaus in den oberflächennahen Schichten des Drahtes erzeugt werden. Diese Elemente haben wesentlichen Einfluss auf den Grad der Kaltverfestigung, ohne den Drahtdurchmesser zu verändern. Zunächst werden mittels FE-Simulationen verschiedene Werkzeuggeometrien hinsichtlich ihrer Wirkung auf den Umformgrad sowie den Eigenspannungszustand des gezogenen Drahtes analysiert und bewertet. Durch den Einsatz eines speziell konzipierten Drahtziehwerkzeugs werden experimentelle Versuche durchgeführt und Drahtproben erzeugt. Weiterhin wird die Vorgehensweise für die Messung der Eigenspannungen durch den Einsatz von Röntgenbeugung erläutert. Für weiterführende experimentelle Untersuchungen zum Biegen erfolgte die Konzipierung eines modularen Werkzeuges, das eine Variation der Biegeradien und Drahtdurchmesser ermöglicht.

Scientific research results show that graded ultrafine grained aluminum-based materials offer technical advantages by possible material property improvements (e.g. yield strength, corrosion resistance) especially when applied to heavily stressed components like screws. The commercial screw production is currently conducted without gradation of the semi-finished workpiece. Until now there is no evaluation on whether and under which conditions graded materials (screws) can technically and economically compete with conventional materials. In this paper, an innovative process chain for the production of graded ultrafine grained components is presented and analyzed based on a methodology for an integrated technical-economic evaluation. The evaluation especially of the yield strength of graded materials, energy consumption of the forming steps, energy costs, and revenue effects of improved material properties will outline the technical and economic potential of the technology. Despite higher energy consumptions positive profit effects can be expected due to improved material properties.