Method for Characterizing Electric Current Effects on the Deformation of Metals

Traditional metal forming requires large forces and substantial tooling, high power, and large amounts of energy. Passing an electric current through the metal during forming has been shown to reduce the deformation energy and increase the materials’ formability, leading to the possibility of increased formed-shape complexity. The primary challenge in characterizing electric current effects on plastic deformation has been decoupling the current and intrinsic Joule heating effects. Many of the experiments reported in the literature have specimen geometry and fixturing features that can lead to concentrations of stress, deformation, current, and/or temperature. The presence of one or more localized field concentrations in the specimen gage length can bias the observed deformation behavior and lead to erroneous conclusions. In this work, we addressed this challenge by conducting independent electro- and thermo-mechanical experiments for the current- and temperature-controlled characterizations, respectively. For the former, a novel cylindrical tensile specimen and grip design were developed to enable larger current densities with uniform deformations, currents, and temperature conditions in the gage section. Forced-air convection with vortex tube cooling was used to maintain uniform temperature conditions within the gage section. Non-contact video strain and infrared temperature measurements were employed, and test control and data acquisition were computer automated. A laboratory materials test system with an environmental chamber was used for tensile testing the cylindrical specimens under near-identical temperature histories as the applied current tests. These techniques were used to characterize the electro-deformation behavior of several pure metals, i.e., copper, iron, and titanium. The results convincingly demonstrated an electric current effect on the plastic deformation behavior of the titanium but not the copper and iron. Details of the experiments and methodology are described and discussed along with candidate mechanisms for current effects on the plastic deformation behavior of the titanium.