Thixoforming : Semi-solid Metal Processing

244j 8 Tool Technologies for Forming of Semi-solid Metals
example, 1400 C. During the dwell time after forming and part ejection, the die
temperature decreases to a value above the preheating temperature, until the next
cycle starts. In the course of continuous forming, the lower die temperature increases
until a steady state is reached, in which the die temperature is alternating between
stable upper and lower boundaries. Two phases are distinguished on the time axis: a
transient state characterized by severe thermal shocks in each forming step and a
steady state in which thermal shocks are reduced compared with the onset of
forming, while the average die temperature during operation is significantly higher
than in the transient state. A process conducted in this way would require die systems
able to operate at high temperatures while concurrently exhibiting outstanding
thermal shock resistance. Since the upper thermal boundary is directly related to
the work alloy, the only parameter to influence thermal process conditions is the die
temperature. In order to reduce the transient state to a minimum and to obtain stable
process conditions, the die temperature may be fixed by heating/cooling devices to a
predefined value. The resulting die temperatures versus cycle times are depicted in
Figure 8.3. Although thermal shocks on forming dies are high, the average die
temperature is reduced compared with the aforementioned process layout. Hence
die materials of excellent thermal shock resistance are required, whereas the
demands on high temperature stability are decreased.
In an alternative approach, the die preheating temperatures may be increased to
>1000 C. This results in a significant decrease in thermal shock loads on forming
dies but requires tool materials that can withstand these high operating temperatures.
Moreover, a heating core has to be provided to establish the targeted die
temperatures, in addition to a thermal insulation shell to protect the machinery
environment from the heat generated in the die (Figure 8.4).
However, altering the die preheating temperature not only influences the demands
on tool materials but also strongly affects the entire process layout. Low die
temperatures lead to rapid solidification of the semi-solid slurry in the cavity, thereby
decreasing the maximum flow length and increasing the minimum wall thickness.
Hence this process layout is to be applied for manufacturing work pieces of bulk,
Figure 8.3 Schematic temperature evolution of steel thixoforming
dies during sequenced processing in thixoforging and
thixocasting with active cooling of the dies.