Characterization of damage in forward rod extruded parts
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ORIGINAL RESEARCH
Characterization of damage in forward rod extruded parts Oliver Hering 1
&
Anthony Dunlap 2 & A. Erman Tekkaya 1 & Anke Aretz 2 & Alexander Schwedt 2
Received: 29 March 2019 / Accepted: 6 November 2019 # Springer-Verlag France SAS, part of Springer Nature 2019
Abstract In addition to strain hardening and residual stress, damage influences the product performance of forward rod extruded parts. Damage is usually neglected and difficult to quantify. The evolution of ductile damage in metal forming is closely correlated to the load path. An experimental approach using automated energy dispersive X-ray spectroscopy (EDX) particle analysis in scanning electron microscopy (SEM) is used to successfully quantify the void area fraction and obtain information on ductile damage. The method is performed on forward rod extruded 16MnCrS5 workpieces with varying extrusion strains and shoulder opening angles (and thus varying underlying load paths). The quantified damage is directly correlated to the load path, which can be described by the stress triaxiality evolution during forming. Density measurements were performed to further validate the results. By observing the change of strain-weighted stress triaxiality and maximum stress triaxiality, it is shown, that the maximum stress triaxiality is the decisive parameter enabling void nucleation. Keywords Forward rod extrusion . Damage . Characterisation . SEM
Introduction In forward rod extrusion a cylindrical semi-finished part is pushed through a die by which the diameter is reduced from d0 to d1 (Fig. 1). To avoid buckling and uncontrolled upsetting the semi-finished part is supported by a container. After reaching the final shaft length, the punch is moved back and the extrudate is pushed out of the die by an ejector [1]. The main process parameters in forward rod extrusion are the extrusion strain ε and the shoulder opening angle 2α. The extrusion strain describes the exact local effective strain on the central axis and can be calculated by ε ¼ 2 lnðd 0 =d 1 Þ:
ð1Þ
Types of failure that can occur during forward rod extrusion are chevron cracks in the component core or surface cracks [2]. The occurrence of chevron cracks due to different * Oliver Hering [email protected] 1
Institute of Forming Technology and Lightweight Components (IUL), TU Dortmund University, 44227 Dortmund, Germany
2
Central Facility for Electron Microscopy (GFE), RWTH Aachen University, 52074 Aachen, Germany
combinations of shoulder opening angle and extrusion strain was studied analytically utilizing the upper-bound approach by Avitzur [3] assuming perfect plastic behavior and by Zimerman and Avitzur [4] considering hardening. Results of these investigations are process limit diagrams. Numerical investigations with different modeling approaches, summarized by Soyarslan et al. [5], show that chevron cracks are caused due to positive hydrostatic stress states during extrusion. Small extrusion strains in combination with high shoulder opening angles affect these st
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