Thixoforming : Semi-solid Metal Processing

298j 8 Tool Technologies for Forming of Semi-solid Metals
Figure 8.50 Self-heating ceramic thixocasting mould (a) and IR
image of tool at operating temperature (b), showing temperatures
of 1000 C in the centre (yellow) and 400 C outside the insulation
layer (blue).
8.6.4.3 Forming Performance and Wear Resistance
Results of thixoforming experiments using the ceramic tools introduced above
confirmed the beneficial influence of highly preheated tools on the form filling
behaviour of the semi-solid slurry, requiring very low forming pressures. As observed
in forming experiments using conventionally preheated ceramic dies (cf. Chaper 10),
excellent surface quality of the as-formed parts is obtained. Details of process
parameters and work piece quality of the respective forming experiments are given
in Chapter 11.
With regard to tool performance, results indicate that thermal shock is effectively
reduced to a minimum, allowing for the application of thermal shock-sensitive but
highly corrosion resistant oxide ceramics. However, the applied prototypes revealed
the weak points of tool construction. Rupture of ceramic die parts occurred due to
insufficient mechanical strength of the insulation shells and unintended process
loads such as canted extrusion discs. Moreover, heating cores of the prototypes did
not provide a fully homogeneous temperature distribution throughout the parts of
the die in contact with the ceramic surface. Hence, despite the active heating of tools,
thermally induced stresses may also contribute to critical stress states evolving in the
ceramic parts. Owing to the complex tool construction and the high operating
temperatures, determination of the causes of fracture of prototypes is difficult.
Regarding wear and corrosion resistance of the alumina dies and moulds, the tool
surfaces after forming experiments macroscopically yielded no evidence of wear or
corrosive attack. This is ascribed to the high corrosion resistance and hardness of this