Thixoforming : Semi-solid Metal Processing

128j 4 Design of Al and Al–Li Alloys for Thixoforming
Figure 4.14 Alloy compositions in the Al–Li–Mg system selected
for determination of pressure dependence.
The samples were investigated in a specialized DTA system at pressures from 1 to
2000 bar. The samples contained a hole of size 1.2 mm for a thermocouple. Three
other thermocouples assured the homogeneity of the temperature within the furnace
surrounding the sample. The chamber was evacuated and an argon pressure of
15–20 bar was applied in order to improve heat transfer. The samples were heated
to 780 C and for measurements under normal conditions they were cooled
(at 3–5 Cmin 1 ) for solidification.
For recording of the DTA curves under high pressure, a pressure of 2000 bar was
applied on the hot sample solidified again at the same rate of cooling. Two tests were
run for each billet. After DTA analysis, the samples and two untreated samples per
billet were used for tensile strength testing.
4.5.2
Results of High-pressure DTA
The results generally indicate that for almost all samples solidified under a pressure
of 2000 bar (except sample 6), a substantial increase in the liquidus temperature was
identified (Table 4.5). Figure 4.15 shows examples of DTA recordings for the Al3–Li
sample under normal pressure and under 2000 bar; all results are summarized in
Table 4.5.
The effect of the pressure on the increase in the liquidus temperature is most
significant in the binary systems Al–Li (DT ¼ 16.2–47.4 C). In the binary systems
Al–Mg the pressure effect is somewhat less (DT ¼ 16.2–24.2 C), and similarly for the