13th International Conference on the Technology of Plasticity (ICTP 2021): Residual Stresses in Metal Forming I
Program Organizers: Glenn Daehn, Ohio State University; Libby Culley, The Ohio State University; Anupam Vivek, Ohio State University; Jian Cao, Northwestern University; Brad Kinsey, University of New Hampshire; Erman Tekkaya, TU Dortmund; Yoshinori Yoshida, Gifu University

Wednesday 10:20 AM
July 28, 2021
Room: Virtual: Room D
Location: Virtual

Session Chair: Wolfram Volk, Technical University of Munich


Influence of Material Delivery Condition on Residual Stresses and Part Properties during Forward Rod Extrusion: Andreas Jobst1; Marion Merklein1; 1Friedrich-Alexander-University, Institute of Manufacturing Technology
    During the manufacturing of semi-finished products the material is subjected to various forming steps. It is annealed before component manufacturing in order to reduce hardening and to ensure a good formability. Inhomogeneous pre-strengthening of the material influences the stress distribution during full forward extrusion. This affects the part’s residual stress state and therefore its operating and failure behaviour. For prediction of the residual stress state, influences of the material delivery condition must be known. The aim of this paper is to derive dependencies between material properties and resulting residual stresses in the component. For this purpose, stainless steel in different delivery conditions is used. Residual stresses, microstructure and microhardness distribution of the materials are compared regarding the rods and extruded parts. The influences of the delivery condition are evaluated by comparing process and component properties.

Analysis of Cylindrically and Spherically Embossed Flux Barriers in Non-oriented Electrical Steel: Ines Gilch1; Simon Vogt1; Tobias Neuwirth2; Benedikt Schauerte3; Kay Hameyer3; Michael Schulz2; Wolfram Volk1; Alex Gustschin1; Hannes Weiss1; 1Technical University of Munich; 2Technical University of Munich - Heinz Maier-Leibnitz Zentrum; 3RWTH Aachen University - Institute of Electrical Machines
    In reluctance and permanent magnet synchronous machines, flux barriers are crucial for magnetic flux guidance. Designed as cutouts, flux barriers reduce the mechanical strength of the rotor construction. To operate these electric drives at higher rotational speed, an alternative flux barrier design is required. Since residual stress influences the magnetic properties of soft magnetic materials, this paper deals with embossing induced residual stress as flux barriers in non-oriented electrical steel with 2.4 wt% silicon and a sheet thickness of 0.35 mm. The investigated flux barriers were fabricated with a cylindrical or spherical punch at two different penetration depths and were compared to a flux barrier fabricated as cutout. A residual stress analysis using Finite Element Analysis helps understanding the mechanism of embossed flux barriers. Additionally, the influence of induced residual stress on the magnetic material behavior is measured using standardized single sheet tests and neutron grating interferometry measurements. This investigation aimed at a better understanding of the flux barrier design by local induction of residual stress.

Residual Stresses in Hot Bulk Formed Parts – A Phenomenological Approach for the Austenite-to-Martensite Phase Transformation: Sonja Uebing1; Dominik Brands1; Lisa Scheunemann1; Christoph Kock2; Hendrik Wester2; Bernd-Arno Behrens2; Jörg Schröder1; 1Institute of Mechanics, Department of Civil Engineering, University Duisburg-Essen; 2Institute of Forming Technology and Machines
    In production engineering, current research focuses on the induction of targeted residual stress states in components in order to improve their properties rather than following the usual path of minimizing residual stresses to prevent failure. In this contribution, a focus is laid on the investigation of the subsequent cooling process of hot bulk formed parts. Such cooling of a component leads to a microscopic phase transformation, which has to be considered in order to compute residual stresses inside the material. A numerical approach based on a phenomenological macroscopic material model is presented to depict the related stress evolution.

Adjusting Product Properties by Deliberate Induction of Residual Stresses in Single Point Incremental Forming: Mateus Dobecki1; Fabian Maass2; Marlon Hahn2; A. Tekkaya2; Walter Reimers1; 1Institute for Materials Science and Technology - Metallic Materials, TU Berlin; 2Institute of Forming Technology and Lightweight Components (IUL), TU Dortmund
    Single point incremental forming (SPIF) is a flexible manufacturing process for the production of customized hollow parts. The mechanical properties of the manufactured components are highly affected by the prevailing residual stress state. The objective of this research is the improvement of the product properties by selective induction of residual stresses. The residual stress state is influenced by the forming mechanisms bending, shearing and membrane stretching. These mechanisms are triggered by adjusting the ratio of step-down increment and tool radius. Numerical and experimental analysis of groove and cone geometries shows that higher step-down increments increase bending and smaller step-down increments increase shearing. Shearing leads to higher residual stress amplitudes. The resulting targeted residual stress state can improve the fatigue strength of a component up to 42%. Through superposed tensile and compression stresses, the residual stress amplitudes can be further increased or may even be inverted.

Formation of Residual Stresses in Austenitic Stainless Steels by Infeed and Recess Rotary Swaging: Holger Hoche1; Fabian Jaeger1; Alessandro Franceschi2; Matthias Oechsner1; Peter Groche2; 1Center for Engineering Materials (MPA-IfW), TU-Darmstadt; 2Institute for Production Engineering and Forming Machines (PtU), TU-Darmstadt
    The austenitic steels 1.4307 and 1.4404 significantly benefit from cold forming, due to their high work hardening ability. Optimizing the forming process chain could improve the component's fatigue properties in order to induce specific residual stresses in critical component areas. In this work, an analysis of the effects of the process parameters during rotary swaging on the resulting residual stresses is carried out. Through this incremental forming process, high strain hardening and a complex material flow history are induced in the workpieces. Therefore, measuring strategies for the residual stress measurement by XRD were developed. Here, especially the 1.4307 is a challenging material due to cold forming induced martensite. Despite phase change, both cold formed materials exhibit anisotropic microstructures as well as grain coarsening. Moreover, particular notched geometries are produced on the workpieces by rotary swaging. Measuring techniques are further developed for these complex geometries and the residual stresses are investigated.

Complementary Methods for Assessment of Residual Stress Fields Induced by Rotary Swaging of Steel Bars: Dhia Charni1; Svetlana Ishkina2; Jeremy Epp1; Marius Herrmann2; Christian Schenck2; Bernd Kuhfuss2; 1Leibniz Institute for Materials Engineering - IWT; 2Bremen Institute for Mechanical Engineering - bime
    The process investigated in this paper consists of cold rotary swaging of steel bars. In recent work, surface residual stresses were found to be sensitive to process parameters and their fluctuations, leading to variations of residual stresses at different length scales. To properly evaluate the residual stress states in rotary swaged bars, several complementary measurement techniques were applied. Complementary to surface X-ray diffraction measurements, the applicability of micromagnetic methods was evaluated for fast mapping of local residual stress distribution at the surface with a spatial resolution down to 20 µm. Additionally, an evaluation of the full residual stress profile over the cross section was achieved by neutron diffraction. The combination of the different methods thus allowed a complete characterization of the generated residual stresses at different length scales. Furthermore, a 3D FE-model was developed and process simulation using Chaboche material model was carried out to investigate the residual stress generation and compare the results with the experimental data. The results show an overall good agreement of the experimental data with the simulation results.

Analysis of Influencing Factors on the Achievability of Bistable Fully Closed Shells by Semi-analytical Modelling: Pavlo Pavliuchenko1; Marco Teller1; Gerhard Hirt1; 1Institute of Metal Forming, RWTH Aachen University
    Bistable fully closed shells can serve as long supporting structures that can be folded into a compact transport geometry and unfolded at the construction place and can be produced by incremental die-bending. In order to find a suitable bending radii a semi-analytical model was developed and experimentally validated in previous research. Nevertheless, minor deviations have occurred in the prediction of the stable geometries curvatures and the degree to which other parameters influence the achievability of bistable shells is still unclear. Therefore, an enhancement of the existing model is described and used for an analysis of the influence of different parameters on bistability and final shell geometries. To verify the results, experiments with two different materials (AISI 1095 and AISI 301) and different bending radii are presented. Finally, the paper shows the influence on the residual stress distribution and based on this, production limits for bistable fully closed shells are derived.