Thixoforming : Semi-solid Metal Processing

4.4 Development of Aluminium–Lithium-Alloys for Semi-solid Processingj125
Figure 4.12 Effects of Sc and Zr and casting temperature on the
grain size of AlLi2.1Mg5.5ScZr (a) and microstructure of a billet (b).
Figure 4.13 Dependence of the average grain size in mm (white
boxes) on the Sc and Zr contents of AlLi2.1Mg5.5ZrSc at three
casting temperatures [ (a) 750, (b) 800 and (c) 850 C].
4.4.4
Assessment of the Raw Material Development Programme
The results of the optimization programme can be summarized as follows:
. With the aid of the overpressure melt process, Al–Li–X alloys with Li contents of
1.5–4 wt% can be reproducibly produced. In order to prevent contamination by Na
and K, it is necessary to employ master alloys and high-grade metals.
. In spite of the complex permanent mould system, chemical non-homogeneity of
the raw material billet cannot be entirely eliminated. In particular, the elements Cu
and Mg are concentrated at the edge of the billet and are below the specified desired
values in the centre of the billet.
. DTA testing confirmed the results of the thermochemical calculations, while for
the AlLi4Cu4, AlLi4Mg8 and AlLi2.1Mg5.5 alloy systems the largest process
window for processing in the semi-solid range can be expected.