Thixoforming : Semi-solid Metal Processing

182j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
plastic deformation and facture caused by the heat extension. Each of the characterized
systems had specific advantages and disadvantages. As a consequence, the
transmission system had to be changed or mechanically reworked after each
experiment leading to an extensive experimental effort.
6.1.5.5 Compression Test
The compression test is a conventional testing method to acquire flow stress (yield
stress) data for cold and warm forming operations [25]. Generally, cylindrical
specimens are compressed and the resulting values for the required force subjected
to the reduced height characterize the mechanical properties of the utilized
material [26].
The compression test is carried out with a servo-hydraulic press. Figure 6.13 shows
the experimental setup of the compression test with the three steps of the complete
procedure. In the first step (1), the testing specimen is heated to the required
temperature interval in the induction furnace. Thus it is inserted into the preheated
testing chamber (2), where it is placed on the lower compression plate. After
compression, the specimen can be quenched in a water basin (3). To achieve adequate
data for the thixoforming process of X210CrW12, the tests are carried out at different
temperatures (900–1320 C) and strain rates (0.1, 1.0 and 10 s 1 ).
Cooling of the test specimen during its transport to the upsetting chamber cannot
be avoided and is a drawback of this experimental setup. This drawback is being dealt
with by slightly overheating the test specimen during the initial heating up, so that the
temperature of the test specimen is slightly above the experimental temperature after
the transport. The upsetting is conducted in the moment when the temperature of the
test specimen has dropped to the desired experimental temperature. The overheating
required for this procedure is about 30–40 C.
The consistency of the semi-solid material requires special preparation of the
testing samples above 1220 C. To avoid crumbling of the specimen that occurs in the
molten phase of carbides at the grain boundaries, the specimen is encapsulated in
thin tube shells made of the higher melting low-carbon steel S235JR (1.0038)
(cylinder, 20 mm diameter 30 mm height; tube shell, thickness 1.5 mm) [26].
Figure 6.13 Experimental setup of compression test (numbers indicate the steps of the procedure).