Thixoforming : Semi-solid Metal Processing

6.1 Empirical Analysis of the Flow Behaviourj193
Ostwald ripening can be neglected in capillary experiments, because the experimental
time is orders of magnitude shorter that the time necessary for particle growth.
Wall slip causes the core flow to slip along the wall on a thin pure liquid layer and
falsify the measured relation of rotation speed and turning moment and the flow rate
and pressure drop considering rotational and capillary rheometers. Conventional
evaluation yields an apparent shear stress gradient which is less steep than it would be
in the case of true wall adhesion. In order to reduce or even avoid wall slip, the walls of
the rheometer can be grooved evenly (as mentioned in Section 6.1.4). Thereby microeddies
and thus near-wall mixing are induced and the slip layer is destroyed.
However, it is important to note that by that at least near the wall viscosimetric flow
can no longer be assumed, which also may influence the measurement results for
small capillary heights. It has been observed that wall adhesion increases with an
applied treatment to the capillary surface. Roughening of the surface can be achieved
either by a mechanical treatment such as grinding or milling of defined structures or
by coating the surface. However, the surface treatments reduce but do not completely
destroy the liquid layer. Second, wall slip can also be accounted for by subsequent
correction of smooth wall measurement results. Beforehand, a correlation of shear
stress and slip velocity has to be determined. applying well-established methods.
These are based on comparing measurement results from different geometric setups
for both capillary and rotational rheometers [23]. However, this procedure is
applicable only to steady-state measurements and not to transient measurements.
6.1.6.3 Compression Tests
The material is first pressed via a conventional experimental technique with a
standard cylinder specimen starting with hot forming temperatures up to temperatures
above the solidus. As shown in Figure 6.31, the flow stresses of the compressed
samples decrease drastically when the solidus is passed. Beginning with expected
flow behaviour, the material shows less formability when the solidus (Ts 1220 C) is
passed. Metallography confirms this behaviour with the resulting microstructure.
Below the solidus, the microstructure shows an austenitic matrix with small areas of
dropped out chromium carbides. Depending on the position, a forming structure
appears. Above the solidus, eutectics occur at the grain boundaries, which grow with
increasing temperature and cause debilitation of the microstructure against tensile
stresses. The test specimen disintegrates in this condition, even when only slightly
loaded mechanically, so that a precise flow curve cannot be calculated because of the
missing cross-sectional area. To avoid the disintegration of the test specimen, it is
covered with a thin layer of a higher melting material.
This method to determine flow curves of such materials is the so-called coat
test [29]. The early development of cracks is inhibited by the hydrostatic pressure
caused by the shell. During evaluation of flow curves, the effect of the shell on the
measured force is determined by calculation (inverse modelling). The prerequisite
for this is the knowledge of the flow curve of the shell material.
Figure 6.32 shows the samples and the shell in its initial geometry and after
forming. Even a compression with a high deformation can be carried out without
disintegration of the material.