Thixoforming : Semi-solid Metal Processing

Figure 4.19 Hardness development of step samples derived
from an Al–Li–X principle component.
4.6 Preparation of Principle Components from Al–Li Thixoalloysj133
transferred into a ceramic carrier basin and placed in the preheated casting
chamber (Foundry Institute of RWTH Aachen). The effect of the solidification
rate (wall thickness 1–25 mm) on the casting grain structure was studied by casting
into a step form (Figure 4.19). The form was preheated to 250 Cusingthree
heater–cooler units. When the component was removed from the form too early,
ignition of the casting residue took place that was still in a partially liquid state.
During removal from the form, some of the thinner component walls cracked. In
general, the alloy AlLi4Mg8Ti displays better flow characteristics and visual surface
quality than the alloy Ali4Cu4Ti.
Figure 4.20 shows the grain structure of the 25 and 2.5 mm steps for both
materials. Both materials display a virtually perfect thixo-grain structure with a
pronounced globulitic primary phase. The magnesium-alloyed variant has a significantly
smaller particle size of 35 mm than the Al–Li–Cu alloy with 48 mm. The form
factor of the globulitic primary phase for both alloys is 0.74. The image analysis
evaluation of the samples could not detect any notable signs of mix separation across
the step thickness. The solid-phase proportion for the copper-alloyed type is 75%
and for the magnesium-alloyed version 65% in both step thicknesses/cooling rates.
Selected thixocasting step samples of the two aluminium–lithium alloys underwent
T6 heat treatment (AlLi4Mg8, 2 h at 430 C solution annealed, quenched in
water, 15 h at 120 C precipitation heat treatment; AlLi4Cu4, 2 h at 510 C solution
annealed, quenched in water, 15 h at 120 C precipitation heat treatment). Even after
the heat treatment, the globulitic grain structure was retained. In the case of the alloy
AlLi4Cu4Ti, several large globulites collected near the surface, which, with
100–150 mm, had double the average grain size than that in the centre of the steps
(50–80 mm). In contrast, the alloy AlLi4Mg8Ti displayed a uniform grain size
distribution between the centre of the step and the step surface in all steps
(Figure 4.21).