The Effect of Electric Current on Metals - Electrically Assisted Forming - page 49

Experimental work supports the theory of localized resistive heating due to the
applied electricity. From this previous work, a threshold current density value has
become apparent which is specific to each metal tested [ 22 ]. This threshold value
is the current density value (for a specific metal/deformation speed combination)
whereby significant formability improvements are witnessed, by way of decreased
flow stress or increased achievable elongation as compared to conventional testing.
Upon further investigation, as the resistivity of metal is increased, the threshold
current density, where increased formability effects occur, is decreased (Fig. 2.7 ) .
This supports the theory of resistive heating. The metals with a higher resistivity
have a greater number of lattice obstacles, and there are more sites for the flowing
electrons to scatter off. The increased number of locations of local lattice heating
allows for easier dislocation movement through the lattice.
Another work examined the thermal profiles of stationary-electrical tests
of specimens with different amounts of cold work in them (up to 50 %) [ 4 ]. In
these specimens, as the percent cold work was increased, the dislocation density
within the metal was increased. After a constant current was applied to each of
the specimens, the maximum temperature value increased by 85 % as the level
of cold work was increased from 10 to 40 % CW. This can be seen in Fig. 3.11 .
Additionally, the voltage potential across the top and bottom dies was larger when
comparing a worked specimen to an annealed specimen of the same dimensions
at different levels of % CW. This indicated that the resistance was higher in the
worked specimen.
An interesting note is that although the dislocation density (number of disloca-
tions) was the only variable that changed with the cold work, and the dislocations
are the only moveable lattice defect, the temperature of the specimens during a sta-
tionary-electrical test still increased significantly with cold work, and the part did
not deform just due to the forces from the electrons. When considering deforma-
tion without electrical assistance, the dislocations receive the motivation to move
due to a stress exerted on them by the external forming force. This shows that the
extra energy imparted into the dislocations from the flowing electrons is lower in

Fig. 3.11 Temperature
versus percent cold work
relationship for SS304
[ 4 ]. The temperatures of
stationary-electrical tests
at the same current density
increased as the initial
percent cold work was
increased

400

Temp. (%CW) = 0.0875(%CW) 2 + 1.275(%CW) + 176.25

350

300

Worked
Annealed
Poly. (Worked)

250

200

150

100

0 10 20 30 40 50

Percent Cold Work (%CW)