Thixoforming : Semi-solid Metal Processing

2
Metallurgical Aspects of SSM Processing
Peter J. Uggowitzer and Dirk I. Uhlenhaut
2.1
Introduction
With the discovery of shear thinning and the thixotropic behaviour of partially
solidified alloys under vigorous agitation, a new era in forming technology was
started, namely semi-solid metal (SSM) processing. The new technology promises
several important advantages: improved die filling, less air entrapment and less oxide
inclusions due to the higher viscosity compared with fully liquid melts, longer die life,
shorter solidification time, reduced cycle time and therefore higher productivity due
to lower heat content and lower process temperature and reduced shrinkage, and
thus near net shape or even net shape production due to the partially solidified slurry.
Two basic routes of SSM processing – termed rheocasting and thixocasting –
proved their feasibility in industrial trials. The rheo-route involves the preparation of
an SSM slurry from liquid alloys and transfer of the prepared slurry directly to a die or
mould for component shaping. The thixo-route is basically a two-step process,
involving the preparation of a feedstock material with thixotropic characteristics,
reheating the solid feedstock material to semi-solid temperature and shaping the
semi-solid slurry into components. Both routes aim at the formation of an ideal
slurry that exhibits an accurately specified volume fraction of fine and spherical solid
particles uniformly distributed in the liquid matrix [1]. Such microstructural tuning
requires specific properties of the alloys used, namely sufficient width of the freezing
range and adequate temperature sensitivity S [2]. For all possible alloying systems,
the liquid and solid phases in the freezing range will differ in their chemical
composition due to near-equilibrium element partitioning. During complete solidification
in the die, however, for specific alloying systems, that is, single-phase
systems, this element partitioning might not be outweighed by diffusion processes
and can cause severe deterioration of properties. Such systems are less capable at
SSM processing. A further aspect concerning the selection of appropriate alloying
A List of Abbreviations and Symbols can be
found at the end of this chapter.
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