Thixoforming : Semi-solid Metal Processing

328j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
from 0.2 to 11.5 m s 1 according to the wall thickness of the component. For this set
of parameters (mould temperature 250 C, piston velocity of 0.05 m s 1 ), a 1 mm step
could only be partially filled. The observed surface quality and the obtainable wall
thicknesses demonstrate an excellent mould filling behaviour of the semi-solid steel
suspension. Mechanical properties were evaluated by a Rockwell hardness test.
Excellent mould filling was achieved using slugs with an initial solid fraction of
60–70%, the best results. The large amount of solid phase reduces the shrinkage so
that even compacted parts can be produced with a relatively small ingate cross-section
and supports a more easily controllable flow tip due to its higher viscosity. Compared
with aluminium alloys, a higher forming pressure is required at the beginning of the
forming operation. After breaking up the meshed structure, the viscosity drops
immediately, so that the semi-solid steel suspension can be processed at a forming
pressure comparable to that of aluminium alloys. It has been shown that for steel
alloys with their higher density and their resulting mass inertia, the processing
parameters have to be adjusted in terms of reduced maximum metal velocities.
The evolution of the solid fraction with the flow length and the wall thickness is
shown in Figure 9.17 for the step specimen cast of X210CrW12. The micrographs
illustrated indicate a fine globular structure of the solid particles embedded in the
eutectic phase.
Based on the 25 mm step, a slight increase in the liquid fraction is observed, which
might have been caused by squeezing out of liquid phase from the incompletely
broken structure during the initial compression of the billet. The strong supercooling
of the partially solidified suspension permits a dendritic growth initiated from the
globular solid particles. The grown dendrites break due to permanent shearing
during the die filling and form globules. Depending on the flow length and the metal
velocity, a larger amount of fine dispersed globular solid particles occurs on the flow
front. According to the digital picture analysis, the microstructure of the semi-solid
steel parts exhibit an average grain size of 35 mm and a shape factor of 0.7 of the
primary globules.
9.3.3.2 Determination of Flow Length Capabilities (Meander Die)
The meander die was developed for the comparative characterization of the flow
length in tools. It does not represent any standardized experiment for the rheological
characterization of the flow properties, like the capillary experiment (cf. Chapter 3). It
is used to correlate the process conditions such as velocity of flow, thermal boundary
conditions and initial liquid content, with consideration of material-specific qualities
such as morphology. The test geometry consists of a flow channel with a trapezoidal
crosscut of area 87 mm 2 which is integrated in a meander tools in a length
of 2830 mm.
Experiments with the meander die evaluate the flow length of semi-solid materials
in correlation with process and material parameters obtained from prior investigations
on the mould filling behaviour. The optional vacuum system allows investigations
of the influence of a slight evacuation of the cavity on the flow length. If
one considers the relatively low mould temperatures that cause a high temperature
gradient between the mould and SSM, these investigations are necessary to evaluate