Analysis and Observations of Current Density Sensitivity and Thermally Activated Mechanical Behavior in Electrically-Assisted Deformation

Abstract

The flow of electric current through a metal during deformation has been observed to reduce its flow stress and increase its ductility. This observation has motivated the development of advanced “electrically-assisted” metal forming processes that utilize electric current to assist in the forming of high-strength and difficult-to-form materials, such as titanium and magnesium alloys. This method of heating provides attractive benefits such as rapid heating times, increased energy efficiency due to its localized nature, as well as the ability to heat the workpiece in the forming machine thus eliminating the transfer process between oven heating and forming. In this paper, a generalized method is proposed to relate applied electric current density to thermally activated mechanical behavior to better understand and improve the processing of metals during electrically-assisted deformation. A comparison is made of engineering metals studied experimentally as well as in the literature, and it is shown that the method provides insight into what some researchers have observed as the occurrence or absence of a “current density threshold” in certain materials. A new material parameter, “current density sensitivity,” is introduced in order to provide a metric for the relative influence of current density on a material's thermally activated plastic flow stress. As a result, the electric current necessary to induce thermal softening in a material can be estimated in order to effectively parameterize a wide range of advanced electrically-assisted forming processes. Thermally induced changes in material microstructure are observed and discussed with respect to the underlying deformation mechanisms present during electrically-assisted deformation. Finally, a strong correlation between thermally activated mechanical behavior and elastic springback elimination during sheet bending is demonstrated.