Thixoforming : Semi-solid Metal Processing

3.2.1.4 Microstructure in the Partial Liquid State
Influences on Microstructural Development During Solidification The compositions
of the liquid and solid phases for different temperatures are given in equilibrium
by the course of the solidus and liquidus lines of an alloying system (Figure 3.6).
ForeutecticalloyswithchemicalcompositionsbetweenCmax and CE, the liquid
phase is constituted by eutectic transformations. For single-phase alloys with chemical
compositions below Cmax, no eutectic is formed under equilibrium conditions. The
aluminium alloy AlSi7Mg0.3 (A356), used predominantly for thixoforming, belongs
to the eutectic type and exhibits at 580 C a favourable liquid-phase fraction
of about 45%, convenient for thixotropic forming with a eutectic temperature of
577 C [17].
In technical casting processes, the equilibrium condition according to the phase
diagram is not achieved. The equilibrium condition is only reached if, during solidification,
enough time is available, so that by means of diffusion in the solid or liquid
phase concentration differences of the alloying elements can be compensated. The
dashed line in Figure 3.6b shows that solidification in non-equilibrium conditions
causes not only a broadening of the solidification interval but also the occurrence of
an unexpected eutectic . In this case, melts and crystals appear with compositions
which do not comply with those in the finite state diagram. The composition at the
Figure 3.6 Crystallization by means of a schematic phase diagram
of a binary alloy or rather a quasi-binary cut of a multi-component
system with eutectic and a high solubility in the solid state:
solidification of a homogeneous alloy in equilibrium (a) and
solidification of a heterogeneous alloy in non-equilibrium (b).
With more rapid cooling and/or low diffusion potential the solidus
line is shifted to lower temperatures.
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