Huseyin Selcuk Halkaci

Design, Fabrication, and Experimental Validation of a Warm Hydroforming Test System

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 °C using induction heaters in the die and blank holders (BH), whereas the punch can be cooled down to temperatures of around 10 °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.

Investigation on Earing Behavior of AA 2024-T4 and AA 5754-O Aluminum Alloys

Deep drawn parts usually have different wall heights because of earing behavior. This behavior is due to the planar anisotropy (Δr) of sheet metals. A measure of the variation of normal anisotropy with the angel to the rolling direction in sheet plane is known as planar anisotropy. If the magnitude of the planar anisotropy is relatively large as absolute value, the earing behavior becomes more effective so larger ears occur. Furthermore, the orientation of the sheet with respect to the die or the part to be formed will be important. In addition, cutting of scraps in the parts which have ears leads to material waste. The scope of this study is to determine the planar anisotropy of AA 5754-O and AA 2024-T4 aluminum alloys and to investigate the earing behavior by the way of deep drawing of cylindrical cups.

Comparison of Flow Curves of AA 5457-O Sheet Material Determined by Hydraulic Bulge and Tensile Tests at Warm Forming Temperatures

The deformation behavior of sheet materials changes according to temperature. It is possible that the formability of a material for different temperatures is investigated and the flow curves are obtained by using a hydraulic bulge test. Generally, biaxial stress state occurs in real forming processes. Flow curves can be derived from the hydraulic bulge test for the biaxial stress state, and the higher strain values can be achieved in comparison to the tensile test without extrapolation. Hydraulic bulge tests are preferred instead of tensile tests on account of presuming the problems can occur during the formation process of sheet material, being informed about the formability of material at the current pressure and temperature states, and obtaining flow curves to perform more accurate process simulations. In this study, the flow curves for the material AA5754-O were obtained using the warm bulge test and by considering the strain rates. The sections of the curves that can be used in simulation were identified, and these curves were comparatively investigated with respect to the curves obtained from the tensile test. In addition, case studies were performed in order to conduct more realisitic simulations using the results of the flow curves obtained from the bulge and tensile tests.

A new method for determining limit strains of materials that show post-uniform elongation behavior:

In this study, a new method is proposed to determine limit strains at the onset of localized necking for ductile materials that show post-uniform elongations. The new method is first applied for AA 5754-O using the ISO 12004-2 forming limit diagram determination standard. The method is also applied for ductile materials of 7114 steel, 304 stainless steel, and CuZn37 brass and finally for AA 2024-T4 having brittle fracture behavior. The results indicate that the new proposed method is quite successful, easy, and accurate for ductile materials that show post-uniform elongations.

A Study on DOE Methods for Hydromechanical Deep Drawing Process Parameters

Formability of sheet metals can be increased by Hydromechanical Deep Drawing (HDD) process. Formability of the deep drawn cups is generally assessed by Limiting Drawing Ratio (LDR) which is the ratio of the blank diameter to punch diameter. In order to increase LDR by HDD, process parameters of the HDD should be arranged properly. Arranging of the process parameters requires a great knowledge about the effects of the process parameters to certain performance criteria of the process. Determining of the effects of the process parameters by full factorial experiments is a hard duty. Hence certain statistical methods that decrease the number and the cost of the experiments and reduce the time should be used to find effective parameters and their appropriate levels. In this study orthogonal experimental array was applied and effective process parameters were determined by analyzing predicted data with Taguchis robust parameter design method and ANOVA method. Then the results were compared with each other to evaluate differences between the methods. By using the appropriate levels of the parameters the LDR of AA 5754 aluminum alloy which uses in automotive industry intensely was determined.

Effects of Heat Treatment Conditions on the Mechanical Properties of AA 2024 Alloy

Aluminum alloys have good properties such as high strength-to-weight ratio, corrosion resistance and relatively low cost. Nowadays they are primarily used as wrought and cast in many industries such as automotive, aviation and aerospace because of these properties. Aluminum alloys are classified into two categories as non-heat-treatable and heat-treatable. The mechanical properties of the heat-treatable alloys are improved by solution heat treatment and controlled ageing. While mechanical properties of some heat-treatable alloys, especially 2XXX series, become stable with natural ageing at room temperature within a few days, some of them are unstable and exhibit significant changes in properties even after many years. Heat treatment process of AA 2024 is very sensible and critical and therefore should be carefully performed. In this research, effects of the solution temperature, soaking time, heating rate and quenching delay condition of AA 2024 on the mechanical properties were investigated.