Thixoforming : Semi-solid Metal Processing

138j 4 Design of Al and Al–Li Alloys for Thixoforming
overpressure melt process, the Al–Li–X alloys were reproducibly produced on a 3 kg
scale with Li contents of 1.5–4 wt%. In order to prevent contamination by Na and K,
it is necessary to employ prealloys and metals of high purity. The maximum Li
losses compared with the initial weighing amounted to 5%. Using the permanent
mould system available it was not yet possible, however, to prevent chemical nonhomogeneity
of the raw material billet. In particular, the elements Cu and Mg are
concentrated at the surface of the billet and are below the target values in the centre of
the billet. DTA tests confirmed the results of the thermochemical prediction
calculations that for the AlLi4Cu4, AlLi4Mg8 and AlLi2.1Mg5.5 alloy systems the
largest process window for processing in the semi-solid range can be expected. The
evaluations for the microstructure in the semi-solid range for quenched samples
indicate that in particular the Ti and Zr grain-refined alloys AlLi4Cu4 and AlLi4Mg8
and also the Sc and Zr microalloyed alloys AlLi2.1Mg5.5 achieve very good results
with globular and fine-grained grain structures in the range 38–60 mm. Apart from
that, these alloys have a high proportion of premelting eutectic phase, which supports
thixotropic flow during forming.
The alloys AlLi4Cu4Ti and AlLi4Mg8Ti with billet formats of 160 76 mm were
able to be transformed into the semi-solid state by inductive reheating without
any problems. Excessive oxidation of the surface of the billet could be minimized
by wrapping it with aluminium foil. The primary grain sizes achieved in the
thixocast step samples were 65 mm, where this was very globular and homogeneously
distributed over all step thicknesses. The thixotropic flow characteristics
can in this respect be described as excellent . The Sc and Zr microalloyed A1420
(AlLi2.1Mg5.5) alloys were successfully processed using the RCP at the Foundry
Institute. Also in this case grain sizes of 60 mm were achieved in the as cast
state. As indicated by the chemical analyses, with the RCP a slight Li burn-off
must be taken into account due to the nature of the process. No tendency for mixseparation
of the solid–liquid phases was identified. Only in the immediate shell
was the liquid phase concentrated. After suitable heat treatment (solution annealed
at 450 C for 24 h, quenched in water, precipitation heat treated at 160 C
for 17 h) and the virtually complete dissolution of the residual intermetallic
phases, in the principle component (step sample) made from the alloy A1420
(AlLi2.1Mg5.5 þ ScZr) mechanical characteristic values were achieved that are
within the range of rolled sheets and profiles of the same alloy, and in the case of
elongation even exceed them. Due to the high-purity raw material used here, it
was possible to safeguard the mechanical properties by keeping the Na content
below 6 ppm. In direct comparison with the rolled alloy 1421 (Sc added), it
appears that for a semi-solid cast component, already in this early step of
development, 80% of the yield limit, 88% of the tensile strength and equivalent
elongations can be achieved. The anisotropy of mechanical properties occurring in
rolled sheets does not occur in SSM Al–Li components due to the homogeneous
microstructure achieved. By semi-solid forming of the reactive Al–Li alloys, which
are difficult to process in conventional casting processes, a completely new
production method and an innovative component quality for near-net-shaped
production is being achieved.