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Electroplasticity of the Invar 36 alloy is investigated based on comparisions between warm and electrically-assisted (EA) uniaxial tensions experiments at different temperatures (100–600 °C) and corresponding microstructural characterizations. Under warm deformation (WD), the flow stress decreases with the increase in temperature, which is directly attributed to dynamic recovery and dynamic recrystallization. With the introduction of high electric current during deformation, it shows a temperature-dependent electroplasticity phenomenon. No difference could be observed between WD and EA samples below a critical temperature of 500 °C, above which, an obvious softening behavior takes place in EA samples. This transition in softening respective to deformation temperature clearly demonstrated that it correlates with the intrinsic current-induced dislocation annihilation and recovery rather than the traditional theory of the Joule heating effect. Our findings shed more light on the utilization of EA softening effect on the development of novel forming routines, especially for materials with limited deformability.

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.

Marciniack–Kuczinski (M–K) model is widely used to predict material’s forming limit curve (FLC). The prediction of FLC traditionally neglected through-thickness normal stress. However, it cannot be neglected in some forming processes. Much work has been done to study the effect of through-thickness normal stress on FLC with constant through-thickness normal stress or constant ratio of through-thickness normal stress and maximum principal stress. In addition, based on Nakazima test process, the ratio of through-thickness normal stress and maximum principal stress has been derived, which was a function of instantaneous thickness and loading path. Here, initial groove angle in M–K model was not considered. In this paper, uniaxial tension tests and Nakazima tests were performed on 7B04 aluminum alloy. Based on Hill 48 yield criterion and M–K model, the prediction model of FLC was established. The increase of thickness can enhance FLC. Meanwhile, it is necessary to consider through-thickness normal stress and initial groove angle in prediction model. On the left side of FLC, the effect of initial groove angle on FLC is weakened by increasing sheet thickness. On the right side of FLC, the effect of initial groove angle on FLC is strengthened by increasing sheet thickness. On the right side of FLC, the relation between limit strain points with different thicknesses is linear under one certain loading path. Thickness has decisive effect on through-thickness normal stress level and the changing trendy of through-thickness normal stress during calculation is different under different stress condition.