Thixoforming : Semi-solid Metal Processing

4.4
Development of Aluminium–Lithium-Alloys for Semi-solid Processing
The concept of adaptation/optimization of familiar Al–Li alloys for processing in a
partially liquid state is essentially based on the following points:
. influencing the micro-grain structure and the texture properties by suitable
selection of the composition and the process sequence;
. optimization of the degree of recrystallization and the grain size in relation to the
applicable component geometry;
. adaptation of the casting grain structure by addition of elements such as Ce, Sc, Ti,
B and Ag;
. minimization of the proportions of balancing phases T2 and d, of Fe and Si phases,
of Na and K and of hydrogen in solution;
. variations in the main alloying elements (Li, Mg, Cu, Sc, etc.).
The semiempirical methodology and sub-operations employed are shown in
Figure 4.10. After selecting suitable Al–Li–X basis systems, first an evaluation was
made of the important key parameters for processing in a partially liquid state, with
the aid of thermochemical calculations. After tuning of the alloys by means of
microalloying element additives, melt-based metallurgical synthesis of trial alloys as
well the characterization was performed. Finally, thixocast and rheocast principle
parts and real components were jointly produced and evaluated. Where necessary, an
adaptation of the selected alloy composition was carried out in addition to repeating
the development operation for reproducibility check.
4.4.1
Thermochemical Modelling
4.4 Development of Aluminium–Lithium-Alloys for Semi-solid Processingj121
The thermochemical calculations in the selected Al–Li–X system models [27–29]
produced the following results, compared with experimental data [30]:
. In the Al–Li–Cu system, the alloys AlLi4Cu4 (A3) and AlLi2.5Cu4 (A2) display the
lowest temperature sensitivity in the liquid-phase portion. Both are below the
critical value of dfl/dT < 0.015 for a liquid-phase proportion of 50% and therefore
should be suitable for processing in the semi-solid range. The alloys AlLi2.5Cu2.5
(A1) and AlLi4Cu1.5 (A4) display a smaller processing window in relation to
temperature sensitivity of the liquid-phase portion, but should prove of interest for
processing with low liquid-phase proportions (