Thixoforming : Semi-solid Metal Processing

52j 3 Material Aspects of Steel Thixoforming
Figure 3.7 Simplified schematic solidification or concentration
development of steel with 1% carbon [21].
centre of the primary crystals changes only slightly during solidification, so that the
average concentration is below equilibrium. For aluminium alloys with an element
concentration below the maximum solubility, the formation of brittle secondary
phases even has to be expected. This is the case, for example, for the forming of the
magnesium alloys AZ91 or AM60 by means of thixo- or pressure die casting. Despite
a maximum solubility of 12.6% Al, these alloys show a significant fraction of brittle,
eutectic b-phase (Mg17Al12) [17].
For the development of the conventional thixoforming process of steel, especially
the carbon concentration and the local carbon distribution exert a crucial influence on
the fusion behaviour during reheating. In Figure 3.7, micro-segregations for a steel
with a carbon concentration of 1% are depicted schematically under the assumption
that the carbon does not diffuse in the solid phase. Hence micro-segregations also
results in a shift of the solidus line to lower temperatures and a broadening of the
solidification interval. The chemical homogeneity adjusted during the primary
material production therefore has a considerable influence on the fusion behaviour
and the suitability of a material for thixoforming.
For the development of rheo-processes, accurate knowledge of the nucleation and
nucleus growth process is necessary. Typically, a dendritic structure of the solid phase
is formed during the solidification process of metals due to the constitutional
undercooling of the liquid phase. In most cases, the temperature gradient in the
liquid state is not sufficient or the solidification rate is not high enough to avoid
dendritic solidification. To obtain a globular instead of a dendritic microstructure, the
solidifying alloy has to be specially treated. For all primary material routes, local
concentration differences have to be avoided because they can lead to undercooling
and superheating in different areas of the liquid, leading to an inhomogeneous
distribution of the solid and liquid phases.
In order to understand the nucleation and crystal growth processes, numerical
modelling techniques are applied [22]. Here local undercooling is seen to be responsible
for the formation of a dendritic structure. The growth rate of a dendrite barb