Experimental and numerical investigation of material behaviour in pulsating forming

In this dissertation, experimental and numerical investigations were performed on the pulsating forming process to increase the formability of Ti-6Al-4V titanium alloy, which has limited formability at room temperature. Pulsating forming processes are applied by loading the materials so that the stress level of the materials is reduced and increased again without the need to change the system infrastructure. In this respect, studies in the literature investigate the increase in the forming ability of different materials with pulsating forming processes, which is a low-cost solution. In this study, unlike the literature, the plastic deformation behavior of Ti-6Al-4V titanium alloy, which is widely used in the aerospace industry, under different pulsating loading conditions was investigated by tensile tests. Pulsating tensile tests were carried out for two different methods, loading-unloading and stress relaxation, and for two different thicknesses of 0.5 mm and 2.65 mm. As a result, it was found that an increase of up to 50% in the maximum elongation of the material was possible. To determine the microstructural reasons behind the pulsating forming process increasing the material's formability, the dislocation densities were calculated by XRD analysis, and the fracture surfaces of the materials were examined by SEM analysis. To generalize the pulsating forming processes, Finite Element Analysis (FEA) was first performed with the Yoshida Uemori material model. However, since this model cannot predict stress relaxation and consequently formability increase, an FEA model that can predict the pulsating forming behavior and formability increase of the material, which constitutes another unique aspect of the thesis, has been developed. In this context, modifications were made to the dislocation-based Bergström material model and the Gurson Tvergaard Needleman damage criterion that predicts the damage point. The FEA results using the proposed model were compared with the results of pulsating hydroforming experiments and the prediction success of the simulations was determined to be approximately 92%. The changes in the microstructure of Ti-6Al-4V material during pulsating forming were also determined by finite element analysis of two-dimensional real microstructure and three-dimensional synthetic microstructure. Accordingly, the deformation ratios, stress distributions and dislocation density distributions for each phase and grain along with the deformation occurring in the microstructure could be determined.