Control of Electrically Assisted Forming - Electrically Assisted Forming - page 168

Fig. 7.14 Constant stress forming sample. The magnesium sample exhibits diffuse necking [ 1 ]
to the room temperature testing. This can be explained by certain regions have
areas of localized deformation causing the material to fail.
The filtered current supplied to the process is summarized in Fig. 7.13 for the
three test cases performed (100, 150, and 200 MPa). Although not shown clearly
by the filtered data, the current quickly increases to the maximum allowable cur-
rent (300 A) once the controller is activated and quickly decreases at the point
where the material reaches the desired stress state. Once the stress state is reached,
the current slowly decreases until the specimen fractures. Again, lower set points
require greater current levels. When considering the current response in general,
it is expected that a process using this control architecture in compression will
exhibit similar behavior; however, the current input would increase over time to
counter the cross-sectional area increase and material strain hardening.
For the constant stress forming results, uniform strain was assumed for the
entire test length. However, as a result of the testing setup, there is a thermal gra-
dient within the test samples which causes diffuse necking during the test (see
Fig. 7.14 ). Due to the diffuse necking, this modifies the actual local stresses within
the material due to the presence of an area gradient along the sample length.
Consequently, the presented response is an averaging of the true stress within the
sample, and it can be seen that the experimental response decreases slightly near
the end of the tests due to larger amounts of diffuse necking present just prior to
fracture.
7.2.1 Benefits and Opportunities of Constant Stress Forming
With the introduction of constant stress forming, this opens additional areas
of research for determining the desired or optimal material flow stress response
during forming for a given material/process combination. Additionally, this