Electroplasticity in electrically-assisted forming: Process phenomena, performances and modelling

Abstract

Electrically-assisted forming has been proven to be an efficient process in terms of reducing forming flow stress, improving formability and supressing the springback for different hard-to-deform alloys. However, there is a controversial debate about whether the flow stress reduction with the introduction of electric current into a metallic specimen undergoing plastic deformation is attributed to Joule heat, additional athermal effect, namely the electroplasticity, or their combination thereof. There is a lack of thorough understanding of different mechanisms inducing the flow stress reduction and formability improvement, which would hinder the development and application of innovative electrically-assisted forming processes. Most prior works have examined only one or two materials with similar electroplastic behaviours, making it difficult to resolve this debate. In this study, three typical materials with diverse electroplastic behaviours, i.e. copper, SS304 and Ti6Al5V, were investigated and compared via electrically-assisted tensile tests and thermally-assisted tensile tests. The flow stress of the copper specimens in the electrically-assisted tensile test was lower than that in the thermally-assisted tensile test, which verifies the existence of electroplasticity. However, the flow stresses of the SS304 specimens in the electrically-assisted tensile test and thermally-assisted tensile test were similar at the same temperature, indicating that the athermal effect on the flow stress is not evident. For the Ti6Al4V specimens, the athermal stress reduction was not significant at lower current densities. However, the electrically-assisted and thermally-assisted tensile tests results began to differ when the current density exceeded 37.6 A/mm2. According to the further discussion on the microstructure evolution of different materials, the introduced current improves the dislocation motions and recovery and consequently reduces the flow stress in simple face-centered cubic structure materials such as copper. However, these effects can be suppressed by stronger effects such as the solution hardening in stainless steels. The current also affects the phase transition, which in turn influences the hardening behaviours of Ti6Al4V. By clarifying the cause of conflicting electroplastic effects, a uniform analytical method calculating the short- and long-range stresses that considers the correlation of electrical and thermal effects on dislocation motion, solution hardening and phase transformation was developed for electrically-assisted forming processes. The model was also experimentally verified.