Thixoforming : Semi-solid Metal Processing

desired behaviour, an extruded or rolled single-phase feedstock material features
completely different remelting characteristics. Due to dynamic recrystallization
during hot deformation, an extruded or rolled feedstock material does not exhibit
microsegregation (coring), that is, there is no gradient in the alloying content within a
single grain. When during reheating the temperature reaches the solidus temperature,
remelting starts at grain boundaries. As the liquid dissolves more alloying
elements than the solid, the adjacent solid becomes depleted in alloying elements.
The solidus temperature within this depleted seam is increased, that is, further
remelting into the solid grain can only occur when the temperature is raised. This
phenomenon is called self-blocking remelting [5]. Within the grains, where the
original alloying content has been preserved, the solidus temperature stays unchanged
and remelting may start simultaneously everywhere inside the grains, that
is, many tiny liquid droplets develop. Once again, the vicinal area of every small liquid
inclusion becomes depleted in solute content, the local solidus temperature is raised
and remelting stops. This is the reason for the formation of many small liquid
droplets within each grain in the early stage of reheating into the semi-solid state.
These liquid droplets become fewer and larger by growth and coalescence with
increasing holding time in the semi-solid state seeking to reduce the interfacial
energy. Such an entrapped liquid is shown in Figure 2.3 for single-phase AZ80 and
100Cr6 feedstock material.
Self-blocking remelting is an exclusive feature of hot deformed or homogenized
feedstock material and does not occur in microsegregated feedstock materials that
are produced by casting techniques.
Extruded or rolled feedstock material may not be ideal in respect of entrapped
liquid, but it has its advantage in superior sphericity of the solid particles compared
with most other feedstock materials. As it does not contain any dendrites or rosettelike
structures, little time is needed to spheroidize the grains in the semi-solid state.
The excellent sphericity of the solid particles is helpful for good fluidity and may
counterbalance the detrimental effect of the entrapped liquid.
Figure 2.3 Entrapped liquid pockets in extruded AZ80 (a) and
rolled 100Cr6 (b) (circles) feedstock material, reheated to the
semi-solid state.
2.3 Slurry Formation in the Rheo- and Thixo-routesj35