The anomalous negative electric current sensitivity of a precipitation hardened Al alloy during electrically-assisted forming

Abstract

It has been known for decades that the formability of metal sheets can be improved effectively by electrically-assisted (EA) forming. However, it remains controversial as to the detailed mechanisms and the manifestation of electrical effects can vary in different materials. In this paper, the EA flow behavior and mechanism of a precipitation hardened AA7075-T6 were studied. The term “electric current density sensitivity” (ECDS) was introduced to describe the athermal effect. The results show that Joule heating is the main influence factor, while the athermal effect varies with the electric–thermal parameters. Here, we report an anomalous negative electric current sensitivity in AA7075-T6, i.e. the flow stress increases with current density under the same temperature and strain rate when the temperature is ≤ 150 °C and strain rate is ≥ 0.005/s. The above phenomena are attributed to the local Joule heating and promoted atom diffusion under the action of the current, leading to the dissolution of GPZ, the formation of fine precipitate and Cr segregation-induced vacancies. Further, an extended dislocation density based constitutive model was proposed to cover the ECDS and softening effect, which could predict the flow behavior of 7075-T6 under EA forming, including the negative electric current sensitivity. The above work provides a viable method to simultaneously control both the shape and performance of the metal part. Meanwhile, the corresponding mechanism shed new light into the understanding of electroplasticity.