Thixoforming : Semi-solid Metal Processing

348j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
workability of the semi-solid slurry. But using a steel mould with a 1 mm wall
thickness, the effective heat capacity is reduced decisively so that no chilling layer or
dendritic growth appears, because the supercooling is not sufficiently high to form
dendrites and only nucleation occurs. There is also the advantage that no holding to
spheroidize the dendrites is necessary.
During inflow of the melt, the seed crystals will be dispensed homogeneously, just
as well as using the channel. Hence all necessary factors for building a globular
microstructure are fulfilled.
An additional advantage of using the steel sheet mould is the heat conductivity,
which is about 20 times higher than that of the ceramic used. Because of the thinwalled
steel mould (1 mm), the decisive factor for cooling to the processing temperature
is the heat transfer from the mould to the air. Hence additional cooling by
pressurized air, for example, is completely unproblematic.
However, a disadvantage of using a channel for feedstock material production is
the loss of metal due to remnants on the channel. Comparing Table 9.4, the wastage is
about 20% of the cast metal. This wastage can be minimized by shortening
the channel length, but this will result in changes to the inclination, the pouring
rate and/or the pouring temperature, because the key task of the channel in the
cooling channel process is cooling of the metal below the liquidus temperature. With
the current length of 480 mm and an inclination of 5 , the pouring rate (Table 9.4) and
the pouring temperature of 630 C are set quite well.
The metal loss on the channel can also be reduced by increasing the channel
temperature, but this would influence the cooling of the melt. Indeed, investigations
showed that a variation of the channel temperature from 20 to 230 C does
not affect the resulting microstructure significantly. A temperature of at least
120 C was chosen to produce back-cooling of the channel to a steady state in less
than 150 s.
To investigate if the tempering works well, a Flir ThermaCam P640 thermographic
camera was used. The surface temperature was found to be very homogeneous with a
median deviation of less than 1 C over the channel length.
Summarizing, Table 9.5 shows the found and set parameters of the cooling
channel process.
In the following, the cooling to the process temperature is examined.
Table 9.5 Parameters of the channel in the cooling channel process.
Parameter Advice Set parameter
Length Preferably short 48 cm
Inclination Balanced (effect subsidiary) 5
Pouring rate Balanced 250 mL s 1
Temperature Preferably low 100–150 C
Cycle time Preferably short >150 s