@phdthesis{Imran2021, author = {Imran, Muhammad}, title = {Investigating and modeling the effect of metal forming-related measures for damage-controlled hot forming}, doi = {10.26127/BTUOpen-5710}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-57105}, school = {BTU Cottbus - Senftenberg}, year = {2021}, abstract = {Within the process chains of the semi-finished production, hot forming is used to eliminate pores and voids from the casting process under compressive stresses and to adjust the microstructure for further processing. In the case of caliber rolling process, tensile stresses occur at certain roll gap ratios which promote void formation (damage) on non-metallic inclusion. Decohesion of matrix and inclusion in void formation is determined by local flow stress. Since the process is carried out above recrystallization temperature, damage cannot only be controlled through stress state (stress triaxiality and Lode parameter) but also the softening processes like dynamic recovery (DRV) and recrystallization (DRX) which can relieve local stresses. Such processes can be used to delay or prevent the damage initiation and growth and hence the mechanical properties of components can be improved. In spite of this, the influence of DRX on damage development has not been quantified so far. This study focuses on the understanding the interaction between softening processes and damage during hot forming and to derive a model for coupled microstructure and damage evolution. For precise damage modeling, in-depth damage characterization of the specimens deformed during hot tensile tests under different stress states and deformation conditions (temperature and strain rate) is conducted. The stress states are varied by designing the specimens with different notch radii. To model the influence of local stresses on damage, continuum-based damage modeling approach is adapted where a system of matrix-inclusion-interface is considered. The influence of different parameters such as DRX, stress state, temperature, strain rate, total strain, inclusion size and type (soft/hard) is analyzed. Based on the damage quantification results, an extension to Gurson-Tvergaard-Needleman (GTN) damage model is devised where a new nucleation criterion couples the stress state and DRX to the void formation at inclusions. The extended GTN model is utilized to predict the internal damage during multi-step hot caliber rolling process and validated with experimentally observed damage at different locations of stress state and recrystallized volume fraction. Based on the model, a damage-controlled strategy is proposed by finding the solution of optimal control problem (OCP) to obtain an optimal variable deformation speed that can induce low damage as compared to constant speed while keeping the process time constant. To implement the damage-controlled strategy, an isothermal forming simulator is designed and manufactured that can replicate the real load paths during hot forming. It is suggested that damage can be controlled if material is allowed to recrystallize at low deformation speed without inducing damage and then, speed up the deformation after onset of recrystallization.}, subject = {Damage modeling; Dynamic recrystallization; Damage; Hot forming; Hot caliber rolling; Sch{\"a}digungskontrolle; Sch{\"a}digungsmodellierung; Dynamische Rekristallisation; Warmumformung; Spannungszustand; Warmumformen; Werkstoffsch{\"a}digung; Schadensanalyse}, language = {en} }