Thixoforming : Semi-solid Metal Processing

130j 4 Design of Al and Al–Li Alloys for Thixoforming
ternary system Al–Li–Mg (DT ¼ 7.2–23.2 C). A systematic correlation between the
pressure effect on the increase in the liquidus temperature and the structure of the
corresponding binary and ternary systems could not so far be identified.
4.5.3
Effect of the Solidification Pressure on the Microstructure
All samples that were solidified under a pressure of 2000 bar were very brittle, which
is due to a phase modification and/or dissolution of a brittle phase on the phase
boundaries. An analysis of the microstructure indicates that the morphology of the
phase changes and that non-metallic contamination has accumulated at the phase
boundaries of the sample that was solidified at 5–8Ks 1 under pressure (Figures 4.16
and 4.17).
In the samples solidified under normal conditions, the intermetallic inclusions are
uniformly finely distributed within the alloy matrix, whereas in the samples solidified
under a pressure of 20 bar, these intermetallics are separated irregularly in a coarser
form (Figure 4.16). In the samples solidified under a pressure of 2000 bar, the
separation of the intermetallic is somewhat less again and at the phase boundaries as
well as between the mix crystals the non-metallic contamination is eliminated
Figure 4.16 Microstructures of Al–10Mg–1Li solidified under different pressures.
Figure 4.17 Microstructures of Al–6Mg–4Li solidified under different pressures.