It must also be noted that there exists a fundamental difference
between modified and unmodified alloys (where the
size and shape of the silicon particles are modified with additions
of strontium). Modified alloys undergo fast spheroidisation,
while complete spheroidisation is not achieved in
unmodified alloys, even after long solution treatment times.
Therefore, shorter solution heat treatments can be employed
with modified castings [5]. It is well known that aluminium
alloy A356 responds to (room temperature) natural aging -
the precipitation hardening that results from natural aging
alone produces the useful T4 temper [5]. Dewhirst [3] varied
the natural aging time of semi-solid processed A356 between
8 and 24 hours. It was found that increasing the natural aging
beyond 8 hours had a slight negative effect on the tensile
properties of the material. Rosso and Actis Grande [4] also
recently studied the optimisation of T6 heat treatment cycles
for rheocast A356. In their paper, the natural aging time employed
is not mentioned. Popular artificial aging treatments
for alloy A356 are either 6 hours at 160o
C [4] or 6 hours at
170o
C [4]. Both Dewhirst [3] and Rosso and Actis Grande
[4], however, have proposed that the optimum artificial aging
treatment for rheocast alloy A356 is 4 hours at 180o
C.
The objective of this study was to determine the influence of
solution treatment time (at 540o
C), artificial aging temperature
and time and prior natural aging time on the T6 heat
treatment response of rheocast A356.