Thixoforming : Semi-solid Metal Processing

266j 8 Tool Technologies for Forming of Semi-solid Metals
Figure 8.21 Experimental setup for the compression tests.
situated on the movable plunger. The top plunger with the coated tool plate was
heated by the tube furnace to a temperature of 400 C. The setup was comparable to
the tool temperature in the current semi-solid forging process. The steel billet was
heated up to the semi-solid range. The testing procedure is illustrated in Figure 8.21.
The steel billet was heated up to 1350 C within 202 s and compressed at 1260 C.
The billet was compressed at a velocity of 10 mm s 1 up until a displacement of
10 mm was reached. The contact duration of hot billets with the tool was 1 s per test
cycle. The procedure was repeated 50 times for the each coated plate. The stress on the
surface of the compression plates was 75 MPa.
The PVD-coated compression plates do not show any visible signs of coating
failure or rupture at the surface. X-ray grazing incidence measurements show that the
deposited phase structures g-Al2O3 and also the high-temperature stable t-ZrO2
phase are still present after forming operations [40]. No traces of phase transformation
from g-Al2O3 to a-Al2O3 or tetragonal to monoclinic ZrO2 are observed, despite
the high surfaces temperatures. With respect to adhesion towards forming materials,
t-ZrO2 displays the least built-up of iron, which can be gathered from the EDS data
shown in Figure 8.22 after 50 compression cycles.
Figure 8.22 Compression plate after 50 forming cycles.