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    Formability improvement in Ti-6Al-4 V sheet at room temperature by pulsating hydraulic bulging: experimental and numerical investigations
    (Springer London Ltd, 2023) Ozturk, Osman; Korkmaz, Habip G.; Atas, Gurkan; Aydin, Mevlut; Turkoz, Mevlut; Toros, Serkan; Dilmec, Murat
    Ti-6Al-4 V sheets possess limited formability at room temperature due to low ductility with almost no strain hardening. Pressure pulsation during hydroforming may bring significant improvement as an alternative to the widespread solution hot forming. However, much uncertainty exists on the deformation mechanism and effects of pulsating on difficult-to-form materials. In this study, the effect of pulsating pressure on the hydraulic bulge test was investigated to increase the limited formability of the Ti-6Al-4 V sheet at room temperature. Experimental results of thickness distribution and bulge height obtained from the bulge tests were compared with the finite element simulation results. The results show that the tests with pulsation allow a higher thickness reduction with a slightly more homogenous thickness distribution. Pulsation causes a delay in the material's failure resulting in a 15.4% increase in the dome height with a 17% increased burst pressure compared to monotonic loading. The underlying microstructural phenomena of increased formability were elaborated using dislocation estimations, fracture surface analysis, and hardness. Test results suggest that pulsation improves formability by 47% in terms of maximum elongation due to stress relaxation.
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    Numerical and experimental investigation of the effect of double-sided hydroforming process on wrinkling damage by optimizing loading curves with adaptive control
    (Springer London Ltd, 2022) Akay, Selahattin Burak; Halkaci, Huseyin Selcuk; Ozturk, Ekrem; Ozturk, Osman; Atas, Gurkan; Aydin, Mevlut; Turkoz, Mevlut
    In this study, by using the double-sided hydroforming process (DSHP), scientific studies have been conducted to reach the highest formability possible with today's technology in deep drawing at room temperature and to produce difficult-to-shape parts. For the first time, the type-2 fuzzy logic controller (T2FLC) working with adaptive finite element analysis (aFEA) was applied to the DSHP, with the expectation that it would be a satisfactory solution to problems such as wrinkling, especially in the production of parts with different cross sections in the direction of the axis. In the literature, no study was found in which adaptive finite element analysis (aFEA) integrated with fuzzy logic control algorithms and genetic algorithm was applied to the DSHP to obtain the optimum back pressure profile. T2FLC was developed for the conical workpiece, and the variables used in the cylindrical workpiece were used first. DSHP is modeled with finite element method, and fuzzy logic algorithms have been developed to provide adaptive control in analyses. Using the aFEA-FLC optimum loading profiles also internal pressures and blank holder force loading profiles were determined. DSHP experiments were performed by the optimum loading profiles, and manufactured part geometry was compared with the part manufactured by non-applied back pressure in the sheet hydroforming process. By using the loading profiles obtained sheet metal parts have been successfully shaped numerically and experimentally without wrinkling or any other damage. It has been observed that as soon as the back pressure is applied, the wrinkling disappears, and the parts can be formed completely. It was concluded that DSHP is more effective than other hydroforming methods in terms of preventing wrinkling in axisymmetric parts with variable cross section.