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are only minor spaces between the fibres, but
there was a complete metallisation reached by
the chosen process conditions. There were no
voids at the carbon fibre-matrix-interface. To
sum up, the state of science and technique for
producing particle, wire or fibre reinforced
strip by twin-roll casting, several constructions
for feeding the reinforcement homogeneously
to the melt were suggested up to now.
In dependence from the process parameters,
very thin strip down to a thickness of 70 μm
and good wetting of the reinforcement were
achieved.
Future research directions
(1) Grain refinement: A significant advantage
of light metal strip produced by twin-roll
casting is the ability to obtain extremely fine
grained microstructure. Mechanical properties
gain from fine grained microstructure as well
as the formability. Superplasticity is discussed
in this context. A future successful development
of superplastic formable sheet, which
can be produced by twin-roll casting, would
ease the cost-efficient production of complex
shaped sheets.
One positive influence on grain refinement
is the very high cooling rate up to several
1000 °C/s. It can be promoted by the contact
conditions between the melt and the rolls surface
(see point (2). Several researchers prove
the application of ultrasonic treatment. It fosters
not only a fine grained, but a homogenous
microstructure. Therefore, it is recommended
to implement ultrasonic melt treatment in future
twin-roll cast equipment.
(2) Contact conditions between the melt
and the rolls surface: Solidification under pressure
seems to enhance the cooling rate and
the roll speed. Contact between the cooled roll
surface and the melt is improved by pressure,
and, therefore, the heat transfer is promoted.
Some researchers point to the advantage of
hydrostatic pressure of the melt pool in terms
of shortening the cooling time and increasing
the output of strip. Further application of additional
pressure on the melt pool or dynamic
mixing of the liquid metal in the pool between
the rolls seems to be a promising way to increase
cooling time and productivity of twinroll
casting light metal strip. For light metal strip,
copper rolls superior in comparison to steel [8].
(3) Fibre and particle reinforced light
metal strip: Strength and elastic modulus
of conventional light metals, i.e. for hybrid
materials structures of the future auto-body
could be enhanced by reinforcements. Up to
now, wire or fibres were inserted into twin-roll
cast strip successfully. It was demonstrated,
that very thin light metal strip of 70 μm thickness
can be produced by a laboratory twin-roll
caster. The up scaling of these results requires
new concept for fibre integration into the melt.
Here, downward melt drag twin-roll casting
offers the possibility to insert the fibre rovings
efficiently. The production of reinforced
light metal strip by downward twin-roll casting
should be investigated in future research.
Acknowledgement
The author thanks Dr. T. Haga of the Department
of Mechanical Engineering, Osaka Institute
of Technology, Japan, for the friendly
allowance to use and publish photographs and
drawings of his international appreciated research
work in twin-roll casting.
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