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    Design, Fabrication, and Experimental Validation of a Warm Hydroforming Test System
    (Asme, 2016) Turkoz, Mevlut; Halkacr, Huseyin Selcuk; Halkaci, Mehmet; Dilmec, Murat; Avci, Semih; Koc, Muammer
    In this study, a hydroforming system was designed, built, and experimentally validated to perform lab-scale warm hydromechanical deep drawing (WHDD) tests and small-scale industrial production with all necessary heating, cooling, control and sealing systems. This manuscript describes the detailed design and fabrication stages of a warm hydroforming test and production system for the first time. In addition, performance of each subsystem is validated through repeated production and/or test runs as well as through part quality measurements. The sealing at high temperatures, the proper insulation and isolation of the press frame from the tooling and synchronized control had to be overcome. Furthermore, in the designed system, the flange area can be heated up to 400 degrees C using induction heaters in the die and blank holders (BH), whereas the punch can be cooled down to temperatures of around 10 degrees C. Validation and performance tests were performed to characterize the capacity and limits of the system. As a result of these tests, the fluid pressure, the blank holder force (BHF), the punch position and speed were fine-tuned independent of each other and the desired temperature distribution on the sheet metal was obtained by the heating and cooling systems. Thus, an expanded optimal process window was obtained to enable all or either of increased production/test speed, reduced energy usage and time. Consequently, this study is expected to provide other researchers and manufacturers with a set of design and process guidelines to develop similar systems.
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    Numerical optimization of warm hydromechanical deep drawing process parameters and its experimental verification
    (Elsevier Sci Ltd, 2020) Turkoz, Mevlut; Cora, Omer Necati; Gedikli, Hasan; Dilmec, Murat; Halkaci, Huseyin Selcuk; Koc, Muammer
    Warm Hydromechanical Deep Drawing (WHDD) is considered as an effective sheet metal forming process to overcome low formability problems of lightweight materials, such as aluminum and magnesium alloys, at room temperature. WHDD process combines the advantages of Hydromechanical Deep Drawing (HDD) and Warm Deep Drawing (WDD) processes. In this study, interactive and combined effects of Pressure (P) and Blank Holder Force (BHF) variation on the formability of the AA 5754 aluminum alloy sheets in the WHDD process were investigated experimentally and numerically. Different from available studies, the optimal fluid pressure (P) and blank holder force (BHF) profiles, which were determined numerically using adaptive FEA integrated with fuzzy logic control algorithm (aFEA-FLCA), were validated experimentally for the first time in literature. Consequently, limiting drawing ratios (LDR) of AA5754 material were recorded as 2.5, 2.625, and 3.125 for HDD, WDD, and WHDD processes, respectively. Thus, it was demonstrated that the formability of lightweight materials, such as AA5754, could be increased significantly using the WHDD process through the proposed optimization method. This method was also implemented into the WHDD of an industrial part with complex geometry, successfully forming sharp features with minimal thinning at reduced levels of force, pressure, and time. Consequently, it is reasonably to state that the method developed in this study can be adopted for the manufacturing of any other part using the WHDD process.
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    Uniaxial and biaxial deformation characteristics of AA7075-O friction stir welded joints
    (Springer Heidelberg, 2020) Acar, Dogan; Karali, Mehmet; Cora, Omer Necati; Burford, Dwight; Koc, Muammer
    The formability of friction stir welded (FSW) AA7075-O aluminum alloy sheet specimens was tested under a range of warm uniaxial and biaxial loading conditions. To study the effect of FSW process conditions on formability, four groups of specimens were prepared using a range of FSW parameters. Two rotational speeds (500 and 1000 rpm), four travel speeds (4.2, 5.1, 8.5, 12.7 mm/s), and four axial force levels (6895, 7006, 7117, 7346 N) were included in the test matrix. Each specimen had a butt joint centrally located and oriented parallel to the sheet rolling direction. Specimens from each of the four groups were then tested under different combinations of uniaxial (e.g., transverse tensile test) and biaxial loading conditions (bulge test), including at two different strain rates (0.0013 and 0.013 1/s) and three different temperature levels (25, 200, and 300 degrees C). From the flow curves obtained from each test combination, the relative effect that FSW parameters (rotational speed, travel speed, and axial force) and forming parameters (temperature and strain rate) had on formability was investigated in detail. Mechanical and structural variations of the weld zone were compared with those of the base material. Tool rotational speed was observed to have a major effect on the yield and tensile strength of FSW blanks, with strengths varying by as much as 20% over the range process and forming parameters tested. Stress-strain curves obtained by using hydraulic bulge tests yielded a 5-10% increase in strain values compared with uniaxial tensile test results. In the bulge tests, the fracture zone was only observed to occur at the apex of the dome along the joint line for specimens tested at 300 degrees C and a strain rate of 0.0013 1/s. Specimens tested under other FSW parameters and process combinations did not fail in this location. In tensile tests, the specimens were only fractured on the welding line under forming conditions of 300 degrees C and at either the low or the high strain rate. Otherwise, the specimens failed away from the joint line. The maximum dome height, as a measure of formability, was obtained under forming conditions of 200 degrees C temperature and at a 0.0013 1/s strain rate. This result correlated well with stress-strain curves obtained from uniaxial tension testing.