Fatigue Behaviors of Materials Processed by Planar Twist Extrusion
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TRODUCTION
FATIGUE failure is defined as weakening of a material during the application of an iterative cyclic load. As the fluctuating stress is considerably lower than that of its monotonic counterpart, the problem plays an important role in the dynamic structures of many elements. Therefore, keeping the fatigue limit under a threshold magnitude is important while designing movable components. Accordingly, several experimental and numerical studies have been conducted to increase the fatigue life, and it has been observed that the strength-ductility combination is a good way to achieve this.[1–3] Since the beginning of the twenty first century, materials researchers have moved from using traditional metalworking processes, such as forging, rolling, and extrusion, to new methods and approaches, such as electroforming, hydroforming, and severe plastic deformation (SPD) techniques.[4–8] In fact, SPD is one of the most prominent of the top-down approaches for refining grains of materials to improve both fatigue properties and other mechanical, superplastic, wear, creep, and corrosion behaviors.[9–13] Although a large body of research has been reported on the monotonic and dynamic properties of materials prepared by the equal-channel angular pressing (ECAP) technique,[14–16] MAHMOUD EBRAHIMI is with the Department of Mechanical Engineering, Faculty of Engineering, University of Maragheh, Maragheh, Iran. Contact email: [email protected] Manuscript submitted April 14, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
little research has addressed the effects of various SPD methods, such as high pressure torsion,[17] accumulative roll bonding,[18] parallel tubular channel angular pressing,[19] vortex extrusion,[20] planar twist channel angular extrusion,[21] twist extrusion,[22] elliptical cross-sectional spiral equal-channel extrusion,[23] and equal-channel forward extrusion (ECFE),[24] on the fatigue behaviors of materials. In brief, various ECAP parameters and conditions have been emphasized during the previous decades, without paying attention to other SPD methods. Some researchers have emphasized the fatigue behaviors of different materials during the ECAP process. Different cyclic responses of materials to the ECAP process were reported by Kim et al. (AZ31 magnesium alloy and commercial low-carbon steel),[25,26] Vinogradov et al. (Al-Mg-Sc alloys),[27] and Liu (8090 Al-Li alloy).[28] Kulyasova et al. proved an increase in fatigue endurance with a reduction in grain size by fabricating ultrafine grain (UFG) magnesium alloy via an ECAP method at various temperatures.[29] The results indicated that the ECAP process at lower temperatures led to higher magnitudes of ultimate tensile strength without considerable change in ductility, unlike the coarse grain (CG) condition. Cavaliere and Cabibbo[30] focused on the influence of the addition of Sc and Zr on the microstructure and fatigue properties of 6106 aluminum alloy during ECAP. The results indicated an increase in static tensile strength and high-cycle fatigue limit, as
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