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    Design, Fabrication and Performance Tests of a Double-sided Sheet Hydroforming Test System
    (Natl Inst Science Communication-Niscair, 2023) Urmamen, Mustafa Kemal; Atas, Gurkan; Dilmec, Murat; Turkoz, Mevlut; Ozturk, Osman; Halkaci, Huseyin Selcuk
    In this study, a double-sided sheet hydroforming (DSH) test system, which contains dies, sealing, pressure intensifiers, and a control unit, has been designed, built, and tested. The hydraulic numerical control system, which is currently used, has been modified as afour-axis where parameters are forming pressure, back pressure, punch position, and blank holder force. A hydromechanical deep drawing press has been modified in terms of die and sealing. New sealing components have been used to prevent leakage during the forming process because one side of the sheet is exposed to the forming pressure, and the other side is exposed to both the back pressure and the moving punch. Performance tests have been carried out to determine the limitations and capacity of the system. After the performance tests, it has been concluded that; the higher forming rates all along the process curve, the higher the pressure and force differences. However, the resultant error of the corresponding points has been only higher at the beginning of the process curve. In addition, the higher slopes in process curves have increased the pressure and force differences. A conical industrial part has been deformed by using hydromechanical deep drawing and DSH processes to test the performance of the DSH press. The wrinkling defect that occurred in previous hydroformed parts has been reduced remarkably by using back pressure in the DSH process. As a result, a double-sided sheet hydroforming test press has been successfully designed and manufactured. Finally, this study provides technical knowledge and can be used as a guideline for the design and performance evaluation of similar manufacturing systems.
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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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    Microstructural Characterization of Improved Formability of Ti-6Al-4V Sheet by Pulsating Hydraulic Bulging at Room Temperature
    (Springer, 2023) Yapan, Yusuf Furkan; Ozturk, Osman; Turkoz, Mevlut; Dilmec, Murat; Livatyali, Haydar; Halkaci, Huseyin Selcuk; Kotan, Hasan
    This study aims to increase the limited formability of the Ti-6Al-4V sheet at room temperature using the pulsating hydraulic bulging and to investigate the microstructural reasons for the increased formability. Accordingly, monotonic and pulsating hydraulic bulge tests (HBTs) were applied to Ti-6Al-4V sheets with 0.55 mm thickness at room temperature, and the underlying microstructural reasons for the improved formability were investigated by conducting detailed microstructural characterizations. The experimental results showed that the pulsating HBT samples reached 15.4% higher bulge dome height than the monotonic HBT samples, and a more homogeneous thickness distribution was achieved by pulsating bulging. The thickness and microhardness distributions, dislocation density estimations, and fracture surface analysis were utilized to correlate the increased formability with the microstructure. It was determined that stress relaxation occurred during the pulsating HBT, resulting in the achieved improved formability. The decrease in the dislocation density in the early stage of forming prevented the locking of the dislocations and delayed the occurrence of the damage, i.e., leading to the increased bulge dome height. The higher plastic deformation resulting from the increased formability also increased the hardness along the cross-section. The increased formability of the Ti-6Al-4V sheet during the pulsating HBT and the underlying microstructural phenomena elaborated in this study are expected to make significant contributions to the current literature on the sheet metals with limited formability at room temperature.
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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.