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a)
b)
c)
Fig. 3: Downward melt drag twin-roll caster according
to Haga et al. [3], (a) position of the nozzle
with the aluminium alloy melt, (b) photograph
around roll bie, (c) schematic illustration showing
solidification layer
unit cell. Frequent intervening reheating procedures
during hot rolling are required and
causes high manufacturing costs [17].
Kaya et al. [17] suggest AZ31 (magnesium
alloy with about 3wt% aluminium and about
1wt% zinc), which has a relatively narrow
equilibrium freezing range (ΔT = 66 °C), is a
good candidate for the twin roll casting process.
They describe a dendritic structure throughout
the thickness of AZ31 sheet after the twin
roll casting. The dendritic grain size is approx.
300 μm. The major problems with twin roll cast
magnesium alloy strips are coarse columnar
grains, coarse deleterious intermetallic phases
and surface/edge defects. [17, 19]. There are
several measures to reduce the grain size and
reach an uniform microstructure of twin-roll
cast magnesium alloys, like thermomechanical
treatment, addition of special alloying elements
and ultrasonic treatment.
Kaya et al. [17] successfully apply the thermomechanical
treatment. They report, that
after a heat treatment of AZ31 at 400 °C for
1 hour, homogenisation occurs. The dendritic
structure is changed to equiaxed grains. An
average grain size of around 25 μm was measured.
An additional warm / hot rolling process
reduces the grain size further to less than
10 μm. The mechanical properties benefit from
this thermomechanical treatment. The tensile
strength of the AZ31 is increased from 225
MPa to approx. 275 MPa by applying homogenisation
and rolling process. Moreover, the
elongation values are increased to more than
11%. Not only are the mechanical properties
improved due to a fine microstructure of twinroll
cast light metals, but also the formability
is positive influenced. As reported by Allen at
al. [20], the small grain size is expected to benefit
superplasticity at elevated temperatures.
They find a secondary dendritic arm spacing of
α-Mg in twin-roll cast AZ31, AZ61 and AZ91
of 7-9 μm, which was transformed into recrystallised
grains of 9-10 μm. Nakaura at al. [23]
describe dendritic arm spacing varies in the
range of 5.0-6.5 μm through the width and
thickness of as-cast AZ31 Mg alloy strip. The
relatively small dendritic arm spacing is attributed
to the high cooling rate, which was
estimated to be 239-556 K/s by the authors.
Additionally, they observe microsegregation
of Al and Zn atoms at the grain boundaries as
wall as centre-line segregation in the strip. Although,
the maximum solubility of aluminium
in Mg-Al alloy is 12,7% and AZ31 contains
about 3% aluminium only, heterogeneous distribution
of aluminium is known for several
die casting processes also. Microsegregation
is considered to be caused and promoted by
high cooling rates. Because twin-roll casting
provides higher cooling
rats in comparison
to die casting, segregations
can be expected
to be enhanced.
Therefore, a follow-up
homogenisation of the
structure by heat treatment
is recommendable.
In the example,
improvement of elongation
is correlated by
Nakaura et al. [23] to
changes of the microstructure
due to heat
treatment like homogenisation
and dynamic
recrystallisation.
Zhao et al. [24]
show, that due to
treating a AZ31 alloy
with ultrasonic
power of 800W during
the twin roll-casting,
the grain size of
α-Mg decreased from
136,3 μm to 44,7
μm. The morphology
changed from dendritic
to globular. The AZ31 was additionally alloyed
with 0,8wt% Ce and 0.3 wt% Mn. Grain multiplication
by fragmentation of dendrites and
caviation-induced heterogeneous nucleation
are the mechanisms of refinement due to the
impact of the ultrasonic waves. The needle-like
shaped intermetallic MgAlCeMn is modified
by the ultrasonic treatment as well. It became
a more globular shape with finer particles.
Ramirez et al. [25] as well as Qian et al. [26]
argued that the primary role of ultrasonic
treatment appeared to produce the initial crystallites
by enhancing the nucleation sites and
influencing the activating potential in the effectively
irradiated melt volume. Both groups
describe that ultrasonic treatment could lead
to a significant grain refinement only in the
presence of adequate solute elements. Increasing
the solute content at a low applied ultrasonic
power level above the cavitation level was
more effective than substantially increasing
the applied ultrasonic power. Although, grain
refinement is a routine practice and there are
many different methods, in casting industry
the most applied technique is inoculation [27].
However, ultrasonic treatment seems to be an
effective and promising way for grain refinement
of twin-roll cast strip because the melt
pool is small and the mechanical vibrations can
distribute very homogeneously.
Microstructure and mechanical proper-
Fig. 4: Cross-section of wire-inserted aluminium strip according to Haga et al.
[3]. The wire was inserted at different roll speeds.
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