Civil Engineering Reference
In-Depth Information
Fig. 3.6
Experimental flow
stress results comparing RT
and EAF incremental forming
3.3.2 Electrical Current with Metal Deformation
A similar theory is presented for the application of an electrical current during the
deformation of a metal. As the current is applied during deformation, the local “hot
spots” created from greater electron scattering at defects significantly enhance the
vibrational energy in the surrounding area of the dislocation. This greater energy
surrounding the dislocation allows for enhanced mobility along the slip plane as
it can pass by lattice obstacles with less resistance. Thus, the dislocation has a
greater quantity of energy and can move under a lower required stress (i.e., exter-
nal required force for deformation is reduced). The lower required stress is what
is observed on a macroscale when forming using an applied electric current. Also,
for the other defects within the material (point and interfacial defects), they have
an increased vibrational energy surrounding them as a result of larger amount of
electron scattering. As a result, if dislocations interact or become piled up at these
defects, this additional energy from scattering may allow the dislocation to pass by
the obstacle, where it otherwise would have remained pinned.
Aside from the local “hot spots” at dislocations and defects, the surrounding
defect-free lattice and the overall material temperature is rising. This overall bulk
temperature rise translates to traditional elevated temperature effects on material
deformation (i.e., thermal softening).
The reduction in material strength was observed in the square wave EAF tests
(Appendix B, Figs. B.2-B.5) where the material stress was significantly reduced
during the application of the current. From the theory, it is proposed that the main
effect is a result of localized “hot spots” which significantly increase the mobility
of the dislocations. Additionally, some dislocation annihilation may occur during
the time the current is applied. From the microstructure analysis that is presented
in Chap.
8
, it was noted that the EAF tests had a reduced amount of twinning
