Thixoforming : Semi-solid Metal Processing

136j 4 Design of Al and Al–Li Alloys for Thixoforming
Figure 4.24 Li and Mg contents of the Al–Li–X step samples (for
specification of A1420, see the rectangle).
the AlLi2.1Mg5.5ZrSc alloy was determined as 65 mm, as mentioned above. A
significant tendency for mix-separation of the solid and liquid phases was not
observed. Only in the immediate border area of the component did the liquid phase
become concentrated. In statistically distributed samples for determining the alloy
composition, there were indications that the target composition could largely not be
set in relation to the main alloying elements Li and Mg (Figure 4.24). This is due to the
already demonstrated non-homogeneity in the raw material or to burning off of
the alloying elements during the forming process. This needs to be improved for the
extensive forming of real components.
The modification in the microstructure during heat treatment is shown in
Figure 4.25. Two different T6 heat-treatment strategies were tested. Already after
a short solution annealing for 3 h, dissolution of the eutectic and intermetallic phase
portions commences, enhancing the aluminium-mix crystal. This is accompanied by
slight grain growth. If the solution annealing time is extended to 24 h, the modifications
in the microstructure are drastic in contrast.
Figure 4.25 Microstructure of AlLi2.1Mg5.5ZrSc alloy processed
by means of the RCP in its as cast state and after two T6 heat
treatment states (5 mm step) [32].