Thixoforming : Semi-solid Metal Processing

58j 3 Material Aspects of Steel Thixoforming
Figure 3.8 Schematic influence of Ostwald ripening and
coalescence on the fraction of intra-globular liquid phase (a) [37].
Micrograph with intra- and inter-globular liquid phase (b) [36].
. With the existence of dendritic structures, enclosed liquid phase areas are formed
during heating in areas with locally higher alloying element contents.
For thixoforming, the problem with the intra-globular liquid phase is that it
cannot contribute to the sliding of the globulites during forming and therefore the
liquid-phase volume fraction is lower than the theoretical value. From this follows
on the one hand an undesirably raised viscosity and on the other the sponge effect is
promoted. The solid-phase particles adhere more to each other and higher forces
are necessary to obtain a flowable suspension. The development of the enclosed
liquid-phase fractions during partial melting can be described by the two coarsening
phenomena of the globulites, Ostwald ripening and coalescence (Figure 3.8).
They have a reverse influence on the intra-globular liquid-phase fraction. Ostwald
ripening leads to the aggregation of enclosed liquid-phase areas due to the dissolution
of smaller particles and the growth of larger particles. In contrast, the
aggregation and coalescence of different particles lead to an increase in the
enclosed liquid phase [37].
During reheating of dendritic structures, intra-globular liquid phases are often
formed in discrete globulites which do not have a connection to the liquid phase
network due to the coalescence of the secondary dendrite arms. As a result of the
tendency to save interfacial energy, the intra-globular liquid-phase fractions are
nearly globular. An unambiguous determination of its fraction is only possible by
means of 3D analysis, because with a 2D polished section areas can appear to be
isolated even when they have a connection to the liquid-phase network (inter-globular
or inter-dendritic liquid phase) in deeper levels [38].
Contiguity, Matrix Character and Contiguity Volume In addition to the liquid-phase
content, the grain size and the grain shape, the steric arrangement of the solid phase
is of crucial importance for processing in the partial liquid state. To warrant a
dimensionally stable handling of the partial liquid billet, the solid-phase particles