The aim of this work is to study the solidification conditions necessary to produce good quality/low
defect metal alloy strip when thixorolling directly from the semi-solid state. To facilitate the study
lead/tin alloys were chosen for their relatively low operating temperature. The objective is to
extrapolate these findings to the higher temperature aluminium alloys. Three alloys (70%Pb-
30%Sn, 60%Pb-40%Sn, 50%Pb-50%wtSn) were used particularly to study the influence of the
solidification interval. The equipment consists of a two roll mill arranged as an upper and lower
roller, where both rollers are driven at a controlled speed. The lower roller is fed with semi solid
alloy through a ceramic nozzle attached to the lower end of a cooling slope. Several types of nozzle
and their position at the roller were tested. This produced different solidifications and consequently
different finished strip. The alloys were first cast and then poured onto the cooling slope through a
tundish in order to create a continuous laminar flow of slurry and uniformity of metal strip quality.
The pouring was tested at different positions along the slope. The cooling slope was coated with
colloidal graphite to promote a smooth slurry flow and avoid the problem of adherence and
premature solidification. The metallic slurry not only cools along the slope but is also initially
super-cooled to a mush by the lower roller whilst at room temperatures, thus enabling thixorolling.
It was also found that the nozzle position could be adjusted to enable the upper roller to also
contribute to the solidification of the metallic slurry. However the rollers and the cooling slope
naturally heat up. Temperature distribution in these zones was analysed by means of three
thermocouples positioned along the cooling slope and a fourth in the base of the semi solid pool
within the nozzle. The objective being to design an optimum pouring and cooling system. The
formed strip was cooled down to room temperature with a shower of water. Microstructures of the
thixorolling process were analysed. The differences in solidification conditions resulted in differing
qualities of finished strip and corresponding defect types, all of which are a serious quality issue for
the rolled product.
A semi solid thin strip continuous casting process was used to obtain 50%wt
Pb/50%wtSn strip by single and twin roll processing at speed of 15 m/min. A 50%wt Pb/50%wtSn
plate ingot was also cast for rolling conventionally into strips of 1.4 mm thickness and 45 mm width
for comparison with those achieved non-conventionally. This hypoeutectic alloy has a solidification
interval and fusion temperature of approximately 31°C and 215°C respectively. The casting alloy
temperature was around 280°C as measured by a type K immersion thermocouple prior to pouring
into a tundish designed to maintain a constant melt flow on the cooling slope during semi solid
material production. A nozzle with a weir ensures that the semi solid material is dragged smoothly
by the lower roll, producing strip with minimum contamination of slag/oxide. The temperatures of
the cooling slope and the lower roll were also monitored using K type thermocouples. The coiled
semi solid strip, which has a thickness of 1.5 mm and 45 mm width, was rolled conventionally in
order to obtain 1.2 mm thick strip. The coiled thixorolled strip had a thickness of 1.2 mm and
achieved practically the same width as the conventional strips. Blanks of 40 mm diameter were cut
from the strips in a mechanical press, ready for deep drawing and ironing for mechanical
characterization. All the strips achieved from non-conventional processing had the same mechanical
performance as those achieved conventionally. The limiting drawing ratio (LDR) achieved was
approximately 2.0 for all strips. Microscopy examination was made in order to observe phase
segregation during processing.
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