Session: 02-07-01: Advanced Material Forming, Friction Stir Welding, and Deformation

Paper Number: 77132

Start Time: Thursday, 01:30 PM

77132 - Effects of Electric Current on the Plastic Deformation Behavior of Pure Copper, Iron, and Titanium 

Tradition metal forming requires high power and consumes large amounts of energy. Application of an electric current through a metal has been shown to reduce the deformation energy and increase a materials formability leading to the possibility of increases in achievable shape complexity. The primary challenge in characterizing electrical current effects on plastic deformation is decoupling the current from the intrinsic Joule heating. Many experiments reported in the literature have specimen geometry and fixturing features that produce localized stress/deformation, current, and/or temperature concentrations. The presence of one or more localized field concentrations in the specimen gage length can bias the deformation behavior and lead to erroneous conclusions. In this research, we address the primary challenge by utilizing independent electro- and thermo-mechanical testing systems for current- and temperature-controlled characterizations, respectively. For the former, a novel wire test specimen and grip design was developed that enables larger current densities while achieving greater electrical and thermal uniformity. Forced-air convection with vortex tube cooling is used to maintain uniform temperature conditions within the gage section. Non-contact video strain and infrared temperature measurements are employed, and test control and measurement data acquisitions are all computer automated. For temporal temperature control, a parallel testing approach is developed employing a laboratory materials test system with an environmental chamber to achieve near-identical current-induced temperature history. Uniaxial tension tests were performed on pure polycrystalline copper, iron, and titanium specimens with various applied constant (dc) current levels and at matching temperatures, i.e., zero current with temperature histories matched to the current tests. The experiments achieved uniform strain, current density, and temperature conditions along the specimen gage length for unambiguous interpretation of the test data. The results showed non-thermal current effects only with the titanium; 20% reduction in ultimate strength with respect to the strength from the matching temperature tests was observed as well as significant inhomogeneous grain growth. No discernable changes in microstructure were observed in specimens deformed at matching temperatures or with applied current but no deformation (matching temperatures). The electron-wind and local Joule heating mechanisms for electrically-assisted deformation (EAD) do not produce effects large enough to explain the observed titanium results. Dislocation scattering by thermal phonons and electrons associated with the radial and axial heat fluxes generated in the titanium tensile specimens with bulk Joule heating is suggested as a potential mechanism for the observed EAD effects. The experimental results and the possible link to thermal phonon/electron scattering suggests several new avenues of research for understanding EAD of metals.

Presenting Author: Christopher Rudolf US Naval Research Laboratory

Authors:

Christopher Rudolf US Naval Research Laboratory