Mechanics of Electrically-Assisted Deformation in Metals

• Main Author: Magargee, James K
• Format: Dissertation
• Language: English
• Subjects: engineering, mechanical
• ISBN: 9781321079104; 1321079109

The flow of electric current through a metal during deformation has been observed to reduce flow stress and increase ductility, presumably separate from thermal-mechanical behavior. This observation has motivated the development of 'electrically-assisted' metal forming processes which utilize electric current to assist in the forming of high-strength and difficult-to-form materials, such as titanium and magnesium alloys. However, the existence of a non-thermal electro-mechanical behavior in metals, deemed as 'electroplasticity,' is debated in materials research. This dissertation provides an in-depth study of the mechanics associated with electrically-assisted deformation in order to uncover the dominant mechanisms behind the observed behavior.
First, a new experimental mechanical testing system and measurement method is described which decouples thermal-mechanical from electroplastic behavior and characterizes the effects of electrical and mechanical loading. Next, a generalized model is developed to relate electric current density to thermally activated behavior and provides insight into an observed 'current density threshold' in certain materials. A new material parameter, 'current density sensitivity,' is introduced to provide a metric for the relative influence of current density on thermally activated mechanical behavior.
The unique mechanics in electrically-assisted deformation are studied through experiments, analytical models, and numerical simulations. First, constitutive models based on thermally activated deformation are evaluated and shown to effectively predict behavior during experiments using a constant applied DC current. With electric current pulsing, it is shown that stress drops can be predicted through modeling the effects of thermal expansion, high temperature softening, and strain rate. Moreover, the rapid heating and sudden change in strain rate due to a current pulse are shown to cause short durations of dynamic strain aging, leading to accumulated plastic strain and transient high temperature strain hardening, and may help explain the large deformation of materials in the literature. Changes in material microstructure are observed with respect to the deformation mechanisms present during electrically-assisted deformation. Finally, a strong correlation between thermally activated behavior and elastic springback elimination during sheet bending is demonstrated. This study concludes that effects of temperature caused by Joule heating, thermal expansion, thermal softening, and local microstructure effects are the dominant mechanisms in electrically-assisted deformation.