Fabrication of a newly developed high entropy alloy is an essential step to enable it to be used as an industrial structure for its potential applications, such as billets, frames and tubes. Bulk forming processes at high temperatures are preferably used requiring the hot flow behavior of the HEA, which needs to be thoroughly investigated for accurate construction of a robust constitutive model and, hence, reliable process simulation and optimizations. In this study, to compensate for the lack of modelling microstructure using conventional phenomenological models, a novel physical mechanism-based model of CoCrFeMnNi high-entropy alloy was established. Particularly, the adiabatic heat effect was taken into account for modelling the HEA for the first time. The hot flow behavior, as well as grain evolution of this alloy under different forming conditions, are well modelled. The modelling predictions obtain great agreement with the experimental results, the calculated R-value (all higher than 0.95) and AARE (all smaller than 0.05) because the different conditions provide validity to the accuracy of the model prediction. In addition, the temperature increase due to deformation heat was well predicted to further evident to the accuracy of model. Furthermore, the hardening behavior during hot deformation was also compared, enabling the provision of useful guides for process designers of hot bulk forming HEAs.
Hot gas forming (HGF) is an advanced technique for fabricating complex-shaped hollow tubular parts. Practically, multi-step pre-forming involving pre-deformation is often necessary prior to HGF. This paper performs an experimental investigation to simulate the pre-forming operation evaluating the effect of pre-strain on the subsequent HGF. Firstly, the dislocation density was accumulated with uniaxial pre-stretching with different strains (5%, 10% and 15%) at room temperature, simulating the pre-forming operations. Secondly, the sub-sized specimen from the pre-stretched sample was characterized at different temperatures (350, 400 and 450 ℃) to evaluate the effect of pre-strain on successive hot deformation for simulating practical HGF. The experimental results proves the coupled influence of pre-strain and temperature on the flow stress and stress-strain variations. To thoroughly understand the micro-mechanisms, EBSD analysis of grains and grain boundary angles was carried out under different pre-deformation levels and temperatures, which shows the recrystallization phenomenon at 15% pre-strain and 450 ℃ temperature. Finally, a physical-mechanism constitutive model is established based on the determined macro and micro results where the pre-strain effect shows the accurate modeling of stress flow behavior of the material.
Hot gas forming (HGF) is an advanced technique for fabricating complex-shaped hollow tubular parts.Practically, multi-step pre-forming involving pre-deformation is often necessary prior to HGF. This paper performs an experimental investigation to simulate the pre-forming operation evaluating the effect of prestrain on the subsequent HGF. Firstly, the dislocation density was accumulated with uniaxial prestretching with different strains (5%, 10% and 15%) at room temperature, simulating the pre-forming operations. Secondly, the sub-sized specimen from the pre-stretched sample was characterized at different temperatures (350, 400 and 450 ℃) to evaluate the effect of pre-strain on successive hot deformation for simulating practical HGF. The experimental results proves the coupled in uence of prestrain and temperature on the ow stress and stress-strain variations. To thoroughly understand the micro-mechanisms, EBSD analysis of grains and grain boundary angles was carried out under different pre-deformation levels and temperatures, which shows the recrystallization phenomenon at 15% prestrain and 450 ℃ temperature. Finally, a physical-mechanism constitutive model is established based on the determined macro and micro results where the pre-strain effect shows the accurate modeling of stress ow behavior of the material.
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