Thixoforming : Semi-solid Metal Processing

8.1 Introduction – Suitable Tool Concepts for the Thixoforming Processj243
during die closing and opening, leading to strongly varying oxygen partial pressures
on the die surface that are virtually impossible to determine experimentally. Tribological
loads consist of abrasive attack by the solid particles of the semi-solid slurry
during material flow and by the solidified part during ejection. The different loads
may be related to failure effects on a time scale: mechanical and thermal loads result
in stress states that evolve in the parts during the different process steps. If peak
stresses exceed the material strength, cracks are initiated, causing rupture of the
parts. Since this will happen after only a few forming cycles, these are classified as
short-term effects. In contrast, chemical and tribological attacks result in surface
degradation during steady-state operation, hence they are classified as long-term
effects.
The combination of these impacts yields a multifaceted load profile, the severity of
which is significantly increased compared with the already challenging load profiles
acting on typical forging and casting dies. Contrary to first expectations derived from
light metal thixoforming, the decrease in process temperatures compared with sand
casting and investment casting does not lead to reduced demands on tool materials
[2, 3]. The targeted combination of the beneficial aspects of forging and casting
consequently causes a superposition of the respective loads on forming dies:
mechanical impacts are superior to pressureless casting, while chemical attack is
drastically increased by the presence of a ferrous melt in comparison with hot
forging. Therefore, instead of a plain transfer of state-of-the-art tool solutions applied
in conventional metal forming processes, the development of adapted tool systems is
mandatory for the semi-solid processing of high-melting alloys. In comparison with
light metal thixoforming, the significantly increased forming temperature for steels
is the most critical aspect, affecting the entire process chain from preheating and
material manipulation in the semi-solid state to the impact on forming dies. In
Figure 8.2, the evolution of the die temperature during cyclic thixoforming without
any temperature control is depicted schematically.
Starting at a typical die preheating temperature of 350 C, the die surface
temperature rises to a value close to the temperature of the work material, for
Figure 8.2 Schematic temperature evolution in steel thixoforming
dies during sequenced processing in thixoforging and
thixocasting without active temperature control.