Thixoforming : Semi-solid Metal Processing

294j 8 Tool Technologies for Forming of Semi-solid Metals
dependence on the steel grades intended for thixoforming and the extensive effort for
the development of specificSi3N4 grades. Since the reaction layer thickness on Si3N4
dies after thixoforming experiments is small and due to the competing reaction
mechanisms, in conjunction with the difficulties in determining exact oxygen partial
pressures evolving in the contact zone, prediction of corrosion rates is not feasible.
Hence the corrosion resistance of Si3N4 dies has to be investigated in long-term
forming series under genuine thixoforming conditions.
8.6.4
Self-heating Dies
8.6.4.1 Die Materials and Tool Characteristics
The load profile acting on self-heating dies operating at temperatures >1000 C
differs significantly from that of conventionally heated dies. Thermal shock is
drastically reduced and may even be eliminated from the load category of shortterm
effects in near-isothermal forming. Consequently, thermally induced stresses
are decreased, whereas the demands on high-temperature stability are increased.
This in turn allows for the application of thermal shock-sensitive materials, thereby
considerably expanding the range of potential ceramic die materials. Mechanical
loads are of the same magnitude as in conventional process layouts, possibly slightly
reduced due to the high temperature facilitating material flow in the cavity. Regarding
the long-term effects, corrosive attack is intensified due to the increased temperature
in the contact zone.
Hence material selection is carried out first according to corrosion resistance
instead of thermal shock resistance. Oxide ceramics such as alumina (Al2O3) and
zirconium oxide (ZrO2) are known to exhibit excellent corrosion resistance against
ferrous melts and slags, which is confirmed by their application as primary phases in
numerous refractories for steelmaking [68, 69]. Both ceramics are also widely used for
wear protection in the textile industry and for lubricant-free antifriction bearings [68].
Although the strength levels of oxide ceramics typically are lower than those of silicon
nitride or silicon carbide, these materials are successfully used as structural parts, if
critical tensile stresses are prevented or reduced to uncritical values. Based on
experimental results of extensive screening of the corrosion resistance of ceramic
materials in contact with solid, semi-solid and molten steel, alumina was selected as
the tool material for self-heating dies due to its higher ratio of thermal shock resistance
to corrosion resistance in comparison with zirconium oxide. This allowed for a
broader thermal process window for die construction, since no experience is available
concerning high-temperature dies in steel thixoforming. Tool construction thus has to
meet the following demands derived from the process analysis given in Section 8.1:
1. The inner surface of the die shaping the steel must be heatable to a minimum
temperature that is less than 150 K lower than the solidus temperature of the steel
grade to be formed, in the case of steel X210CrW12 being 1250 C.
2. The die surface must be corrosion, infiltration and wear resistant in contact with
semi-solid steel at operating temperatures.