Thixoforming : Semi-solid Metal Processing

8
Tool Technologies for Forming of Semi-solid Metals
Kirsten Bobzin, Erich Lugscheider, Jochen M. Schneider, Rainer Telle, Philipp Immich, David
Hajas, and Simon M€unstermann
8.1
Introduction – Suitable Tool Concepts for the Thixoforming Process
Semi-solid processing (thixoforming) is an innovative metal forming technology
bearing a high potential for cost reduction by reducing forming steps, forming forces
and improving work piece quality. These beneficial effects are currently exploited
only in light metals to be shaped in a semi-solid state, being in direct competition with
highly sophisticated conventional forming technologies such as high-pressure die
casting (HPDC). Although the potential of thixoforming technologies is notably more
pronounced for ferrous metals, economically in terms of market numbers and
technically in view of load-bearing structures, a transfer to high-melting alloys has not
yet been accomplished.
One major drawback for industrial implementation is the lack of suitable tools and
diesthat meet the processdemands andexhibit an economically satisfactory service life
within therange oftolerated degradation. Tool systemsandmaterialsthatareapplied in
established metal forming processes, that is, surface-treated and/or conventionally
coated tool steels, hot-working steels and hard metals, suffer from severe deformation,
hot tearing and build-up of metal and scale layers on the surface when applied in steel
thixoforming. This consequently leads to a rapid decrease in shape accuracy of the
manufactured parts and, thus, to short tool changing cycles. The reason for this severe
attack on forming dies is to be found in the complex load profile acting on these dies
during semi-solid processing. Four main load categories may be distinguished:
(i) mechanical, (ii) thermal, (iii) chemical and (iv) tribological impacts (Figure 8.1).
Mechanical loads comprise the forming pressure required for form filling and also
the final densification pressure to eliminate porosity in the work pieces. Whereas the
forming pressure is comparatively low at approximately 25 MPa according to our own
experiments and literature data, the latter is discussed equivocally with values given
A List of Symbols and Abbreviations can be
found at the end of this chapter.
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