Thixoforming : Semi-solid Metal Processing

3. The temperature at the outer shell of the die, in contact with the metallic tool
frame, must not exceed 500 C, desirably 300 C, to ensure a long service life of the
surrounding tool frame.
4. The material between the hot die and the outer tool frame must exhibit sufficient
strength to bear the process forces, but also has to sustain the severe temperature
gradient present after die preheating.
The first demand is met by a huge number of dense and porous ceramic and
refractory materials, whereas the second demand considerably narrows the range of
potential die materials. The third and fourth demands reveal the crucial point of selfheating
dies operating at high temperature, since the tool frame has to be protected
from the heat generated in the die. The low thermal conductivity of thermal
insulation materials required hereto is typically realized by porosity, thereby affecting
mechanical strength. Thus, in a first approach to this concept, a multi-shell tool
construction was envisaged, in which the demands are fulfilled separately by the
individual shells (Figure 8.47). The inner forming die (A) is surrounded by a heating
core (B) that is resistance heated to the operating temperature. The inner parts are
fixed into an insulation shell (C) that is covered by a metallic tool frame (D). In an
advanced layout the heating core may be replaced by an induction coil to enhance
heating velocity.
This die concept is predestined for, although not limited to, thixoextrusion, since
this forming method exhibits relatively stable thermal process parameters in
comparison with sequenced forming methods, that is, thixoforging and thixocasting.
The applicability for the latter processes was evaluated by additional prototypes in
order to demonstrate the transferability of this concept to alternative forming
methods.
Figure 8.47 Schematic tool construction for self-heating ceramic
dies for the semi-solid processing of high melting point alloys.
(A) shaping die; (B) heating core; (C) insulation body; (D) tool
frame.
8.6 Bulk Ceramic Forming Toolsj295