Tribological and Contact Area Effects - Electrically Assisted Forming - page 234

9.2.6 Experimental Results and Discussion
9.2.6.1 Lubricant Evaluation Based on Shear Friction Factor
The reduction of the inner diameter was determined for all the specimens, and the
values were superimposed on the friction calibration curves determined earlier
for the corresponding ring height reduction. The friction factor was quantified for
each lubricant for conventional forming and EAF. Figure 9.35 shows in (a-c) the
measurements used to interpolate m .
Figure 9.36 shows the evolution of an average current density-dependent friction
factor for each lubricant. Conventional forming was represented by a current density
of 0 A/mm 2 . The oil-based lubricant exhibited an average friction factor of m = 0.18
in conventional forming. However, when the electricity was applied, the friction fac-
tor went to 0.25 and 0.28, respectively. The synthetic lubricant initially exhibited a
slightly lower friction factor, but it reached the same values when the electricity was
applied. The poorest performance was exhibited by the water-based lubricant, which
in conventional forming had the highest average friction factor ( m = 0.24), and
increased up to m = 0.34 for a current density of 35 A/mm 2 . This evolution observed
with the current density suggests that the dielectric permittivity of the lubricant has
less effect on the lubricant performance under electricity, than the temperature rise.
The heat generated will result in more severe evaporation for a water-based lubricant.
Note that the error bars depicted in the figure indicate some scattering, which
is to be expected when forming microscale specimens. At this scale, small differ-
ences in material structure, specimen dimensions, or applied current may result in
variation of the results.
The effective increase in friction with current density can be observed in the flow
pattern of the material and increase in the barrel shape. Figure 9.37 shows magnified
images taken on a section of the ring, for a deformed specimen at 0, 25, and 35 A/
mm 2 . The lubricant used was TufDraw 1919. The specimens were carefully cut on
a CNC milling machine. Then, sectioned rings were mounted in resin for the metal-
lographic preparation. Grinding, polishing, and etching were performed on the speci-
men, in order to have a clear delimitation of the grains. Thus, the specific “ x ” shape
of the higher localized strain could be observed with an increase in current density.
To re-emphasize the results discussed in Fig. 9.6 , an increase in friction can
also be observed from the increased surface roughness of the ring specimens.
Preliminary measurements indicated that the roughness increased from 0.89 µm in
conventional forming to 1.24 µm at 25 A/mm 2 and 1.4 µm at 35 A/mm 2 .
9.2.7 Lubricant Evaluation (Reduction in Forming Load)
When electric current is superimposed on the deformation, the forming load is
reduced. Figure 9.38 shows the compressive force versus stroke displacement