The effect of solidification variables on the microstructure of hypereutectic Al-Si alloys

In this work, the effect of phosphorus, as a primary silicon refiner, and of
strontium, as a eutectic silicon modifier, individually and simultaneously, on the
microstructure ofhypereutectic AI-Si alloys was studied.
AI-18.6Si-0.35Fe-0.02Cu-0.0014P (wt%) alloy was used for casting and
phosphorus was added in a range of 0.015-0.08 wt% mainly by means of AI-6.75Fe4.91P
(wt %), and in a few cases by AI-17.lCu-0.89P (wt%), master alloys. Eutectic
silicon was modified using AI-5.93 wt% Sr over a range of 0.04-0.3 wt% Sr. Two
different casting methods were used: bottom casting into sand moulds and chamber
casting into steel moulds. The melt temperature was 800aC and holding time for each
addition was 15 min.
Adding 0.02 wt% P led to an increase in the number of primary silicon
particles per unit volume (NV) by 1.5 times. NV was trebled by adding 0.08 wt%
phosphorus during chamber casting into steel moulds. NV was decreased by about 20
times by adding 0.2 wt% Sr and 0.02 wt% Sr+0.02 wt% P to untreated alloy during
bottom casting into a sand mould. Adding strontium increased primary silicon
undercooling from 7.l±1.0 to 46.6±6.5 K, though phosphorus addition of 0.02, 0.04
and 0.08 wt% (for ingots chamber cast into steel moulds) gave a primary silicon
undercooling of 18.7±10A, 8.5±1.2 and 9.l±0.9 K respectively (compared to 9.0±5.0
K for untreated ingots).
The nucleation models (one surface-dependant and the other volumedependant)
of Perepezko were applied to the observed nucleation behaviour of
primary silicon. It was found that the nucleation temperature was the most crucial
variable in both models. Applying the models to our results showed that contact angle
from the surface-dependant model varied over the range of 24.3 to 30 degrees,
compared with 17.7 to 23.5 degrees from the volume-dependant model.
Additionally, Electron Back Scatter Diffraction (EBSD) was used to determine
possible crystallographic relationships between neighbouring primary silicons in
untreated and P-inoculated ingots. Between two connected primary silicon particles,
the outermost layers of each silicon particle were often found to be twin-related by a
60° rotation around axis <111>.