special - Alu-web.de

RESEARCH
Fig. 22: Influence of casting speed on grain refinement with AlTiB master alloys
Fig. 23: Influence of casting speed on grain
refinement with AlTiC master alloys
rameter for DC casting is the casting
speed. Modern DC casting systems
for extrusion ingots make high casting
speeds possible. These are only
achievable because efficient grain
refiners comprising AlTiB master alloys
are available. Through grain refinement,
the occurrence of hot and
cold cracking, which becomes more
likely with increasing casting speeds,
can be effectively offset. Higher casting
speeds not only increase the occurrence
of cracks but also the formation
of columnar crystals in the
cast structure of the ingots. This is
shown in Figs. 22 and 23 for 157 mm
diameter billets. These figures also illustrates
the effect of different types,
and varying amounts, of grain refining
master alloys [29]. It can be clearly
seen that the formation of columnar
crystals are enhanced by increasing
casting temperatures. It is also evident
that AlTiC master alloys are, as in the
case of casting temperature, sensitive
to an increase in casting speed. It can
be assumed that the influence of increased
casting speed can be traced
back to an increase in melt turbulence
in the ingot pool and the temperature
gradient in the melt ahead of the solidification
front.
Thus, it is clear that an increase in
the casting speed necessitates an increase
in the amount of grain refiner
in order to achieve a fully equiaxed
grain structure. It also shows that, in
future mould development, engineering
a system that provides an environment
that encourages the formation
of an equiaxed grain structure will be
necessary in order to achieve higher
casting speeds.
In addition to the process parameters
discussed above, the as-cast
structure of a DC cast ingot is also influenced
by the flow of the melt into
the mould, and the resulting fluid flow
field in the liquid pool of the ingot.
Particularly in the higher temperature
gradient range, there is a risk of columnar
structure formation, notably in the
form of feathery crystals. An example
is shown in Fig. 24 for the level pour
casting of extrusion billets [30]. It can
be seen that columnar crystals form
opposite the mould gate in the area
where the highest temperature gradients
occur during casting as demonstrated
in the simulation in Fig. 25.
The same negative impact of high temperature
gradients also exists for the
DC casting of rolling ingots, as shown
in Fig. 26, were the molten metal is fed
by means of a nozzle and distributor.
In the susceptible area, the requirements
for improved grain refinement
increased, which in turn can lead to
excessive amounts of grain refiner being
added. In mould development, especially
where higher casting speeds
are desired, optimisation of the melt
feeding system must be effected in
order to achieve a flow distribution
in the ingot pool which favours grain
refinement. This, however, must also
take place in conjunction with the
optimisation of the melt temperature
at the mould gate and smoothing the
temperature distribution in multiple
ingot casting stations.
As reported in numerous other
places, grain refinement with TiB 2
is mainly influenced by the concentration
and the type of alloying elements.
Correspondingly, the “growth
restriction factor” values show that
the different alloying elements have
Fig. 24: Feathery crystal formation opposite
to the pouring gate for unsufficient grain
refinement
Fig. 25: Velocities and temperatures at a
horizontal cross section, referring to Fig. 24
76 ALUMINIUM · 6/2007