Thixoforming : Semi-solid Metal Processing

246j 8 Tool Technologies for Forming of Semi-solid Metals
With respect to the maximum operating temperatures of common metallic
substrate materials, this approach is confined to conventional process layouts of
moderately preheated dies. Alternatively, bulk ceramic dies and die segments of high
thermal shock resistance may be applied in this process layout. Apart from the
strengths and limitations of different coatings and bulk ceramics regarding thixoforming
process conditions, the characteristics of the respective manufacturing
technologies have to be taken into consideration when designing tools for the semisolid
processing of steel parts of predefined geometry. That is, depending on the
deposition technology, cavities and undercuts are difficult or impossible to coat,
whereas processing of bulk ceramic green bodies is limited in size and geometric
complexity. Moreover, the intensity of the separate loads acting on forming dies, as
discussed above, varies locally inside the cavity as a function of work piece geometry.
In order to account for these local load collectives and specific tool systems, a hybrid
tool approach is proposed, in which individual tool solutions are developed for the
targeted work piece geometry by combining the different die systems, where
appropriate. An example of this hybrid concept is given in Figure 8.5, showing a
thixoforging tool with a flat, physical vapour deposition (PVD)-coated upper swage, a
plasma-enhanced chemical vapour deposition (PECVD)-coated liner and a bulk
ceramic punch mounted on a steel plunger. Conversely, the demands on die
materials to be applied in the concept of hot tools operating at temperatures >1000 C,
regarding high temperature strength and chemical stability in oxidizing atmospheres,
are met exclusively by ceramic materials.
A test strategy (Figure 8.6) was established according to the load profile depicted
above in order to examine the qualification of the different die materials and coating
systems for the semi-solid processing of steels. Based on fundamental characterization
methods typically applied for coatings and bulk ceramics, the resistance against
thermomechanical, chemical and tribological loads was subsequently examined in
Figure 8.5 Hybrid tool concept.