Thixoforming : Semi-solid Metal Processing

316j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
. components with tightness requirements (pressure connections, brake master
cylinders);
. thick-walled, highly loaded components;
. thin-walled components (structure components with high strength and
elongation);
. components of special materials (materials with particulate reinforcement).
Aluminium-based alloys have been almost exclusively used as the material. In
series applications, the preferred alloys are mainly A356 (AlSi7Mg0.3) and A357
(AlSi7Mg0.6) due to their advantageous mechanical characteristics and good processability
[5]. On the other hand, the wrought alloys 2024, 5083, 6012, 6082 and 7075
offered high potential due to their high strengths. The tendency for hot cracks and
microporosities induced by solidification can be significantly lowered in comparison
with conventional casting processes [18]. The thixocasting of aluminium-based
materials can show excellent mechanical qualities; Figure 9.5 presents the strengths
obtainable for a T6 heat treatment.
Since the end of the 1990s, the industrial use of the thixoforming has declined
steadily. The reasons for this change can be found in both the economic and
technological framework conditions. A main reason is that most applications need
specially made, expensive pre-material. The necessity for a larger number of singlecoil
heating devices leads to an increase in investment and operating costs. The
longer lifetime of the dies and the several seconds shorter cycle time do not
compensate for these costs completely. A solution would have been the extended
use of rheoforming plants, which were not widely available at that time. The smallest
deviations in the temperature of the billet were an important process-specific
disadvantage because they influence the flow behaviour considerably with a direct
change of the liquid phase content.
Figure 9.5 Mechanical properties for established aluminium alloys after T6 heat treatment [19].