Thixoforming : Semi-solid Metal Processing

54j 3 Material Aspects of Steel Thixoforming
reheating can be strongly reduced through specific alloy modification (addition of
barium to the conventional aluminium alloy AA6082). This can be ascribed to the
decreasing effect of barium on the contiguity, because barium raises the contact
penetration of the cast due to its influence on surface energy and thereby confines the
cohesion mechanism of the solid phase [15].
For aluminium alloy A356, analyses concerning the influence of primary material
during conventional thixoforming show that dendritic grains with grain radii of more
than 800 mm during isothermal holding in the partial liquid state exhibit an irregular,
round grain form, whereas dendritic primary material with grain radii from 200 to
600 mm with a relatively long holding time show a homogeneous, coarse-grained
globular structure. Primary material with grain sizes of less than 200 mm exhibits a
globular structure already after short holding times. Cast material with grain sizes of
less than 110 mm requires no isothermal holding time to reach a globular structure
for semi-solid forming. Based on these results, Equation 3.4 concerning the connection
of the grain size in the primary material and the size of the globulites after
remelting was derived [29]:
Size of globulites after remeltingðmmÞ ¼ 1 þ 0:09
particle sizeas cast ðmmÞ 110
timeðminÞþ0:5 3.2.2
Structural Examination
ð3:4Þ
3.2.2.1 Structural Parameters
All structural components can be characterized by their crystal structure and their
chemical composition and also by their geometry. While the structure of the
structural components is generally relatively easy to specify, the description of its
geometry and its proximity relationships (except for simple quantification) is
considerably more complicated and is, therefore, usually carried out qualitatively
with the aid of simplifying parameters. For the characterization of the microstructure
of partial liquid samples, these are usually quenched and interpreted metallographically.
The production and analysis of level sections of the to-be-examined
material provide only indirect information about the steric structural parameters.
The stereology provides the necessary procedures to reconstruct the structure of the
three-dimensional space based on two-dimensional sections or projections on to a
plain. Procedures of geometric probability computation and differential geometry
can be utilized. By means of two-dimensional grindings, thixoforming processrelevant
parameters can be detected or rather estimated. These are described in
detail in the following sections.
Volume Fraction in the Solid and Liquid Phases In partially liquid metals, the most
important parameters are the volume fractions of the solid and liquid phases, because
they crucially influence the viscosity and therefore the flow and form-filling behaviour
of the material. The microstructure development in the partial liquid state is also