Görtan, Mehmet Okan (2014)
Severe plastic deformation of metallic materials by equal channel angular swaging: Theory, experiment and numerical simulation.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Ultrafine-grained metals produced by severe plastic deformation processes are distinguished with many outstanding mechanical properties such as a combination of high strength and ductility or improved fatigue behavior and thus can be used to meet the high strength requirements of the metal forming industry. Although several severe plastic deformation processes have been developed over the last three decades, the industrial utilization of such materials is still in an early stage. Production of bulk UFG materials is still expensive. Therefore, the few applications are limited to the sports goods, biomedical parts or sputtering targets where the price of the parts mostly doesn’t play a major role in the buying decision. To overcome the efficiency problems of current SPD methods, a new process, called “Equal Channel Angular Swaging” has been proposed which is based upon the combination of the conventional ECAP and the incremental bulk metal forming method of infeed rotary swaging. In the current study, firstly, mechanics of the process is investigated by using slip line field approach. It is determined that unlike conventional ECAP, due to the kinematics of the process, the friction forces help the deformation by drawing the samples into the forming operation. Accordingly, the loads in the feeding direction are relatively low. Therefore, ECAS has a great potential as a continuous and so economical SPD process. Secondly, in order to validate the slip line solution, a tool system has been developed. By the development, a thermo-mechanical coupled finite element simulation approach is utilized to investigate the effect of the geometrical parameters on material flow and temperature formation as well as process loads. It is determined that both, channel length and outer corner radius has a significant effect on the process. Accordingly, a middle channel length of 15 mm and an outer corner radius of 5 mm have been selected for the tool system of the model experiments. First experiments with the developed tool system prove the feasibility of ECAS process. The deformation of round bars from two different materials, commercially pure copper and low carbon steel was possible. Moreover, the deformation on the samples follows predominantly a simple shear pattern. Developed thermo-mechanical coupled finite element models are capable of representing the deformation as well as the process loads required to form the materials. However, due to the highly idealized assumptions by the slip line field approach, defined analytical formulas overestimate the real process forces. Nonetheless, the analytical theory is capable of representing the load correlations. The materials deformed by ECAS process are distinguished by an increased tensile strength compared to their as-received state. Moreover, microstructural investigations reveal that an average grain size under 1 µm can be achieved even after a single pass with the developed SPD process. Although model experiments prove the feasibility of ECAS and the process loads, especially in the feeding direction are low which demonstrate the potential for a continuous and economical processing, the investigated feed rate of 1 mm/s isn’t appropriate for a wide commercial implementation. Therefore, in the last step of investigations, the effect of the most important process parameters of friction, temperature, feeding speed and feeding type is investigated by means of finite element simulations. These simulations reveal that a discontinuous feeding is inevitable for the speed-up of ECAS process. Moreover, an active cooling is definitely necessary to keep the temperatures at acceptable levels and to ensure an efficient grain refinement.

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