Torsion testing for characterizing the plastic behavior in metals: determination of material flow stress for FEM-based design and optimization of metal forming processes

The thesis investigates the use of torsion testing as a method for characterizing the plastic behavior of metals, with particular emphasis on determining material flow stress, a critical parameter for FEM simulations in metal forming processes. The study addresses limitations in traditional testing methods, such as tensile and compression tests, which fail to capture the full extent of material flow stress. In contrast, characterization via torsion test offers several advantages, including the ability to analyze materials for high values of strain, strain rate, and temperature. A detailed comparison is initially conducted among the methods for characterizing the cold plastic behavior of ETPcopper. The testing methods and the different analytical models for data processing are analyzed. The comparison reveals that the material exhibits different plastic behaviors under distinct stress states, due to the development of varying textures and differing hardening rates. Consequently, a new approach to processing cold torsion test data is proposed. Subsequently, the results from torsion copper characterization are applied to the study, modeling, and optimization of the wire drawing process for electrical cable production. A theoretical multi-pass drawing model is developed, considering the entire process and focusing on the relationship between drawing and back-stresses. The model is implemented by incorporating the material’s plastic behavior as process conditions varied, obtained via cold torsion tests conducted on copper wire rods from four different suppliers. A numerical model is then proposed for simulating the drawing process in individual passes. The characterization performed via hot torsion tests on various AA6082 aluminum alloys is then presented and compared. The influence of flow stress modeling on numerical simulation results is assessed by comparing predicted data with experimental results from an industrial extrusion process. In conclusion, torsion testing proves to be an effective method for material characterization, enhancing FEM simulation accuracy and manufacturing process control.