Pulsed current assisted forming of ultrathin superalloy sheet: Experimentation and modelling

Abstract

To realize the collaborative control of formability and microstructure of difficult-to-deform alloys, the electropulsing assisted forming technology provides a potential alternative with reduced flow stress, increased ductility, accelerated aging and recrystallization of the material. To clarify the relationship between the constitutive behavior, microstructural evolution and mechanical property, the electrically-assisted (EA) tensile tests of ultrathin superalloy sheet were performed. The experimental results indicated that the flow stress is obviously decreased and the elongation is slightly changed with increasing current intensity. In addition, the Portevine Le Chatelier (PLC) phenomenon occurs and transforms from the normal to inverse pattern as the current density grows, which is caused by the coupled effects of the premature precipitation of the second phase and the accelerated diffusion of solute atoms. The result of microstructural observation proves that the pulsed electric current can promote the formation of the incipient precipitations from the oversaturated solute, which suggests that the pulsed current aided forming process differs from the traditional hot deformation. Based on the experimental findings, a modified dislocation density based constitutive model was developed for the characterization of the electroplastic deformation considering the electrons and dislocations interaction, precipitation strengthening, solute-dislocation interaction and their relationship with the mechanical behavior, which is able to accurately predict the experimental data. In addition, the proposed model interprets the complexities during the EA deformation of ultrathin superalloy including the effect of pulsed electric current on the dislocation interactions and evolution, the formation of incipient second phase precipitations as well as the abnormal hardening phenomenon.