Thixoforming : Semi-solid Metal Processing

242j 8 Tool Technologies for Forming of Semi-solid Metals
Figure 8.1 Load profile acting on steel thixoforming dies and resulting demands on tool materials.
in the literature ranging from less than 50 MPa to more than 1500 MPa [1, 2]. This
diversity is due to the fact that no explicit study has yet been conducted to determine
experimentally the required minimum forming pressure to obtain fully dense steel
parts, which may be due in part to the lack of suitable tools. However, a minimum
densification pressure of 100 MPa seems to be a reasonable approach [1, 2]. Thermal
loads are exerted by the preheated primary material in contact with the mould and the
amount of heat that is transferred from the semi-solid slurry to the die during form
filling and solidification. The thermal process window ranges from the temperature
of the primary material as an upper boundary to the die preheating temperature
defining the lower boundary. The forming pressure is directly dependent on the work
alloy, typically being in the range 1200–1500 C. The latter may be varied in a certain
range within the operating temperatures of the die material applied. Maximum tool
temperatures in conventional metal forming processes are restricted to the annealing
temperature of the metallic tool frames, being typically 500–550 C. Hence severe
thermal shocks act on the dies in each forming cycle.
Chemical attack on the die surface is exerted mainly by the liquid fraction of the
semi-solid slurry during form filling. Scale residues on the preheated primary
material also have a strong influence on the chemical interaction between the die
and the metal during forming. In order to avoid oxide inclusions in the formed parts,
the entire process chain preferably is covered with a protective atmosphere. Although
in first experimental setups an encapsulated forming process was envisaged, this
concept was abandoned in view of the process requirements in industrial production.
Consequently, provisions were made with the target of reducing the contact of the
steel with ambient air to a minimum by continuous flushing with argon during
preheating, transport and insertion of the steel billets. However, this is ineffective