Thixoforming : Semi-solid Metal Processing

4.5 Consideration of the Forming Pressure on Thixoalloy Developmentj131
(Figures 4.16 and 4.17, both on the right). As the high-purity argon (6.0) used still
contains 0.2 vol.-ppm oxygen and the fact that during compression to 2000 bar this
content increases to 400 vol.-ppm, the oxide phase formation in the highly reactive
Al–Li–Mg alloys is understandable. The solubilities of Li and Mg in the aluminium
mixed crystal that were increased under pressure and the formation of non-metallic
contamination are responsible for the reduced mechanical properties of the samples
solidified under pressure. By using oxygen-free argon and applying heat treatment
this negative effect could possibly be reduced.
4.5.4
Possible Impacts of High Pressure for the Thixoforming Process
The cooling curves for Al–Li–Mg alloys under 2000 bar indicate that the pressure has
a very great influence on the melting temperature of these alloys. Increases in the
liquidus temperature of up to 47 C were identified. This has a direct effect on the
distribution of the alloying elements and on the liquid–solid phase proportions. This
effect is shown exemplarily for an Al-4% Mg–2.1% Li–0–8% sample/alloy
(Figure 4.18).
Under normal pressure, this alloy has a freezing range of 83 C(Tliquidus 639 C,
T solidus 556 C). A 50% liquid-phase fraction is achieved at 619 C, corresponding to
an undercooling of the melt by 20 C. During forming of the suspension, the pressure
increases suddenly and a pressure of, for instance, 2000 bar increases the liquidus
temperature by 25.3 C. Merely due to the rise in pressure the undercooling changes
from 20 to 45.3 C. This high value of undercooling leads to the formation of an
additional solid-phase portion. Such a process requires a considerable displacement
in the process window. This phenomenon, which also takes place to a slight extent at
low pressures [37], can cause increases in alloying elements in the component and
also changes in viscosity.
Figure 4.18 Al–Li phase diagram showing the change of the fluid/solid phase distribution.