Thixoforming : Semi-solid Metal Processing

of the material. In general, longer holding times, higher temperatures and higher
shear rates in the partial liquid state result in rounder grains. With equal holding
times and temperatures, electromagnetically stirred material exhibits rounder particles
than conventionally cast material [29, 33]. The form factor is generally defined
by the equation
F ¼ 4pA
U2 ð3:5Þ
where A is the grain section and U is the grain circumference of the globulites. For
ideal round globulites, the form factor takes the value 1. A low value for this parameter
indicates dilation or a complex (e.g. dendritic) grain structure. Occasionally the
form factor is also defined as 1/F, so that in this case large values (>1) yield a complex
grain structure. In the analyses conducted, the form factor according to Equation 3.5
is exclusively used and this form factor is also denoted the object-specific form factor.
With two-dimensional sections of complex grains, especially of dendrites, the form
factor is strongly dependent on the particular magnification utilized so that a simple,
object-specific appraisal by means of image analysis is subject to a large error. The
complexity of the solid phase actually present is not reflected in this case. Therefore, a
dimensionless grain-specific form factor, FG, according to Equation 3.6 was introduced
in the literature [33]:
FG ¼ 1 S2 v
ð3:6Þ
6pf s
NA
3.2 Backgroundj57
where f s is the volume fraction of the solid phase, Sv is the solid–liquid boundary
surface per volume unit and NA is the amount of globulites per volume unit of the
sample. This form factor takes the value 1 for perfect, round globulites and values >1
for complex structures.
Volume Fraction of the Intra-Globular Liquid Phase In contrast to the inter-globular
liquid phase, which can contribute to the sliding of the solid-phase particles during
forming, the intra-globular liquid phase is unwanted. Three phenomena can essentially
be held responsible for the development of the intra-globular liquid phase:
. With the existence of a very homogeneous primary structure, an obstruction of the
solid–liquid phase boundary surface can be observed due to the self-blockingremelting
effect. In this process, the material melts locally at the grain boundaries
due to the marginally enriched alloying elements there and the associated lower
solidus temperature. If now further alloying elements (e.g. carbon) diffuse over
short distances into the already liquid zone, this leads in the depleted areas to
higher solidus temperatures and with constant temperature to an obstruction of
the phase boundaries. In the grain interior, however, the nominal composition
remains, so that areas within the solid grains melt and form intra-globular liquid
phases with an increase in temperature [35].
. During remelting of highly segregated, banded structured, hot-rolled material,
coarse, clouded melting behaviour is exhibited, as the carbon-rich carbide bands
possess lower solidus temperatures [36].