Thixoforming : Semi-solid Metal Processing

model tests, which were custom-developed to reproduce the specific attack on dies
during steel thixoforming separately [5, 6].
Further evaluations were undertaken using simple tool geometries in forming
tests under defined experimental conditions. The tool systems tested in these
experiments successfully were applied in small-scale forming series using geometrically
complex dies under industrial conditions in forging presses and die-casting
machines.
Forming experiments reveal the performance of the respective tool systems and
die materials but also indicate the effect on form filling, shape accuracy and work
piece quality. Owing to the pronounced temperature sensitivity of the rheological
properties of the semi-solid slurry and the resulting effects on form filling behaviour
and liquid phase separation (cf. Chapter 6), the different thermal die concepts, as
proposed here, directly influence the work piece microstructure and the properties of
the manufactured parts. Detailed descriptions of the characterization methods and
model tests on PVD/PECVD-coated tools and bulk ceramic dies are given in the
following sections. Experimental results on the load profile and the work piece quality
obtained are presented and discussed.
8.2
Thin-Film PVD/PECVD Coating Concepts for Die Materials
8.2 Thin-Film PVD/PECVD Coating Concepts for Die Materialsj247
Figure 8.6 Testing strategy for evaluation and qualification of potential tool materials and systems.
Metallic alloys such as hot working steel and high-melting alloys, for example nickeland
molybdenum-based alloys, have been evaluated for application as die materials
during semi-solid forming of steel [7–10]. The nickel steels are characterized by their
low thermal conductivity and their high coefficient of thermal expansion (CTE)
values. Consequently, they will be easily affected by strong thermal cycles and