Browsing by Author "Wegener, T."

Now showing 1 - 6 of 6

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Cyclic deformation response of ultra-fine grained titanium at elevated temperatures
(Elsevier, 2019-05) Sajadifar, Seyed Vahid; Yapıcı, Güney Güven; Demler, E.; Krooss, P.; Wegener, T.; Maier, H. J.; Niendorf, T.; Mechanical Engineering; YAPICI, Güney Güven; Sajadifar, Seyed Vahid
This study focuses on the high-temperature cyclic deformation response (CDR) of ultra-fine grained (UFG) titanium of commercial purity (grade 4) processed via equal channel angular extrusion as a severe plastic deformation method. Low-cycle fatigue experiments were conducted at elevated temperatures up to 600 degrees C and at strain amplitudes ranging from 0.2% to 0.6%. Besides temperature and strain amplitude, the influence of two processing routes (8B(C) and 8E) on the fatigue characteristics of UFG Ti was examined. It is clearly revealed that the CDR of UFG Ti is not strongly affected by the alteration of strain path during ECAE processing, as long as highly efficient routes are employed. Both routes lead to high volume fraction of high angle grain boundaries and improved fatigue performance up to 400 degrees C is demonstrated. Electron backscatter diffraction assisted microstructural characterization was used to analyze elementary degradation mechanisms affecting cyclic mechanical behavior. Micrographs reveal the occurrence of severe recrystallization and grain growth only at temperatures above 400 degrees C and, thus, grade 4 UFG Ti is characterized by unprecedented cyclic stability in comparison to other UFG alloys.
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Effect of friction stir processing on the fatigue performance of AZ31 magnesium alloy
(Wiley, 2023-05) Yapıcı, Güney Güven; Sajadifar, S. V.; Ghobadlou, Ali Hosseinzadeh; Wegener, T.; Sobrero, C.; Engelhardt, A.; Niendorf, T.; Mechanical Engineering; YAPICI, Güney Güven; Ghobadlou, Ali Hosseinzadeh
Herein, the cyclic mechanical behavior of AZ31 magnesium alloy after multipass friction stir processing (FSP) is investigated up to the very high-cycle fatigue (VHCF) regime. The grain refinement and texture evolution after processing are evaluated to enhance the understanding of the fatigue response. Although ultimate tensile strength and ductility of the friction stir processed AZ31 increase up to about 320 MPa and 25%, respectively, the fatigue performance deteriorates in comparison with that of the as-received condition due to the low yield strength and texture evolution after processing. Furthermore, analysis of fracture surfaces of the samples after cyclic loading reveals that the as-received AZ31 is more prone to brittle fracture with multiple-origin fatigue failure even at low stress amplitudes. On the contrary, the dominant failure mechanisms of the friction stir processed samples are initiation and propagation of cracks originating from the surface, porosities, and grain size inhomogeneity. Nevertheless, the capability of FSP for providing superior crack initiation resistance in the VHCF regime is demonstrated as a significant contribution. Based on a detailed study of prevalent microstructural features, processing–property–damage relationships are established indicating the major effect of FSP on the final performance of the AZ31 magnesium alloy.
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Effect of grain size on the very high cycle fatigue behavior and notch sensitivity of titanium
(Elsevier, 2019-12) Sajadifar, Seyed Vahid; Wegener, T.; Yapıcı, Güney Güven; Niendorf, T.; Mechanical Engineering; YAPICI, Güney Güven; Sajadifar, Seyed Vahid
The very high-cycle fatigue performances of coarse-grained and ultrafine-grained titanium samples with different geometries at ambient temperature and various stress amplitudes were investigated. Severe plastic deformation improves monotonic strength of titanium at the cost of a loss in ductility. Ultrafine-grained titanium demonstrates a superior fatigue performance compared to that of coarse-grained counterparts in the high-cycle fatigue regime, however, suffers notch sensitivity. Furthermore, in the very high-cycle fatigue regime stress-life curves merge unexpectedly. Microstructural inhomogeneity in the ultrafine-grained titanium is expected to be the reason. Analysis of fracture surfaces reveals that the formation of fatigue slip marks is evident on the fatigued samples of both microstructural states. Ultrafine-grained titanium is more prone to the intergranular fracture.
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On the friction stir processing of additive-manufactured 316L stainless steel
(Wiley, 2022-10) Sajadifar, S. V.; Ghobadlou, Ali Hosseınzadeh; Richter, J.; Krochmal, M.; Wegener, T.; Bolender, A.; Heidarzadeh, A.; Niendorf, T.; Yapıcı, Güney Güven; Mechanical Engineering; YAPICI, Güney Güven; Ghobadlou, Ali Hosseınzadeh
The novel combination of friction stir processing (FSP) and additive manufacturing (AM) is studied herein. Laser-based powder bed fusion of metals (PBF-LB/M) is used to establish 316 L stainless steel with a bimodal microstructure. Upon FSP, the as-built bimodal microstructure with an average grain size of 179 μm is transformed into the unimodal microstructure containing ultrafine grains with an average grain size of 1.2 μm. Results obtained by mechanical testing reveal that after FSP; the hardness, the yield point, and the ultimate strength of additively manufactured 316 L are enhanced by 45%, 77%, and 62%, respectively. Microstructure assessment reveals that such a unique improvement in the mechanical properties is due to considerable structural refinement leading to grain boundary strengthening. Energy-dispersive X-Ray diffraction analysis reveals that phase transformation does not occur upon FSP. Fracture analysis further indicates that severe plastic deformation (SPD) during FSP can promote the transformation of coarse voids to fine voids and, hence, densification of as-built parts.
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On the low-cycle fatigue behavior of a multi-phase high entropy alloy with enhanced plasticity
(Elsevier, 2023-08) Radi, Amin; Sajadifar, S.; Seyedmohammadi, Seyedveghar; Krochmal, M.; Bolender, A.; Wegener, T.; Niendorf, T.; Yapıcı, Güney Güven; Mechanical Engineering; YAPICI, Güney Güven; Radi, Amin; Seyedmohammadi, Seyedveghar
A multi-phase non-equiatomic FeCrNiMnCo high entropy alloy (HEA) was fabricated using vacuum induction melting. Thermo-mechanical treatments consisting of cold rolling and annealing at 750 °C and 850 °C were employed to improve the mechanical properties of the HEA in focus. Tensile experiments revealed that yield strength and ultimate tensile strength levels can be enhanced significantly after thermo-mechanical processing (TMP). At the same time, ductility remains at an adequate level. Strain-controlled low-cycle fatigue (LCF) experiments were carried out in order to assess the mechanical properties of this HEA under cyclic loading conditions. At the same strain amplitude, the stress levels of the processed samples were considerably higher than that of the as-received counterpart. Similarly, fatigue lives of the former could surpass the base condition at the strain amplitudes of 0.2% and 0.4%; however, at the higher strain amplitudes, cyclic softening was observed. Electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) results revealed that phase transformation from face-centered cubic (FCC) to body-centered cubic (BCC/B2) took place at a higher occurrence with increasing strain amplitude (0.2% to 0.6%). Furthermore, transmission electron microscopy (TEM) studies confirm that upon tensile deformation additional plasticity mechanisms, i.e., deformation twinning and phase transformation, contribute to the overall mechanical behavior of the multi-phase HEA.
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Severe plastic deformation as a processing tool for strengthening of additive manufactured alloys
(Elsevier, 2021-08) Ghobadlou, Ali Hosseinzadeh; Radi, Amin; Richter, J.; Wegener, T.; Sajadifar, S. V.; Niendorf, T.; Yapıcı, Güney Güven; Mechanical Engineering; YAPICI, Güney Güven; Ghobadlou, Ali Hosseinzadeh; Radi, Amin
For the first time, the novel combination of multi-pass equal channel angular extrusion/pressing (ECAE/P) and selective laser melting (SLM) was investigated. Herein, four passes of ECAP via route Bc at 150 °C were applied as a severe plastic deformation (SPD) technique on the SLM as-built AlSi12 to promote superior mechanical properties. The microstructure and mechanical behavior of AlSi12 fabricated by SLM were studied before and after ECAP, applying several mechanical and microstructural characterization techniques. Results of the tensile experiments revealed that the yield point, the ultimate strength, and the ductility of the as-built sample were improved by 56%, 11%, and 55% after 4 passes of ECAP, respectively. This enhancement is attributed to the effective grain refinement and the persisting silicon phase network after SPD as evidenced by electron backscatter diffraction and elemental mapping results. Moreover, micro-computed tomography analysis disclosed that ECAP considerably reduces the remnant porosity of the post-treated SLM AlSi12 samples eventually further affecting the strength of the ultra-fine grained AlSi12 in a positive way. Findings presented herein indicate that it is viable to utilize ECAP as a post-AM processing tool for mechanical property improvement of laser powder bed fused microstructures with the virtue of enhanced densification. Even if geometrical restrictions exist in ECAP, results obtained herein are transferrable to other SPD techniques with suitable processing windows, which would pave the way to advanced properties of adequately post-treated conditions.