Browsing by Author "Sajadifar, S. V."

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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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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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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.