Civil Engineering Reference
In-Depth Information
Fig. 2.7
Electrical threshold
versus material resistivity
comparison for several
lightweight metals [
35
].
The figure depicts that, as
the material resistivity is
increased, the electrical
threshold decreases
70
60
Al 6061-T6511 (BCC)
MG AZ31B-O (HCP)
304 SS (BCC)
Ti-6AL-4V (HCP)
50
40
30
20
10
0
Resistivity (
Ω
m)
• The resistivity has a direct correlation with the electrical threshold current den-
sity, as seen in Fig.
2.7
. For the same die speed, metals with a higher resistivity
require a lower electrical threshold to produce significant formability improve-
ments. This could be related to the first part of the electrical theory, where the
flowing electrons scatter off of the lattice obstacles and cause localized atomic
heating. A material with a higher resistivity will have a greater number of lattice
obstacles and will result in a greater amount of localized heating around these
obstacles, which ultimately lowers the electrical thresholds of these metals.
The process-related variables to be presented are as follows: (1) initial dimensions
of the workpiece (
r
o
and
h
o
), (2) deformation speed (i.e., die speed), (3) current
density (current per normal area), (4) applied voltage (
V
), (5) workpiece/die con-
tact area, (6) electrical application method, and (7) initial percent cold work. These
additional effects are detailed below:
• The initial dimensions of the workpiece determine the magnitude of current
needed for EAF. It was determined that the electroplastic improvements are a
function of current density and not current magnitude, so the current density
will determine the appropriate current magnitude to use.
• The deformation speed is important because the EAF technique is strain rate-
dependent, and therefore, the electrical application parameters (starting current
density) must be adjusted if the die speed is increased or decreased.
• The current density and applied voltage make up the applied electrical power to
the process. In an EA-forging process where the electricity is applied continu-
ously, these variables must be adjusted to produce a desired amount of electrical
power.
• The effect of the actual contact area between the workpiece and die was not
previously explored experimentally; however, since the dies and workpiece are
separate parts, are composed of different materials, and must both transfer elec-
tricity during the EAF process, this variable is to be explored. Additionally, the
roughness between surfaces is a widely studied topic in the field of electrical
connectors.
