special - Alu-web.de

Fig. 19: Grain size of AA 6063 extrusion ingots of 8‘‘ diameter
in dependency of Ti concentration after addition of
different AlTiB master alloys
Fig. 20: Grain size of AA 6063 extrusion ingots of 8‘‘ diameter
in dependency of B concentration after addition
of different AlTiB master alloys
Ti-B master alloy needed to achieve
grain refinement can be reduced substantially
compared with a melt containing
no residual Ti as shown in Fig.
18 [2]. However, excess Ti content introduced
by the grain refining master
alloy is not enough to exert an influence
on the final grain refinement result
as shown in Fig. 19. It can be seen
here that, for the two wrought alloys
investigated, the master alloy with the
lowest Ti content (AlTi1.2B0.5) still
displays the best grain refinement efficiency.
Furthermore, the B content
in the form of alloyed Ti borides does
not appear in itself to be decisive to the
efficiency of the master alloys, as seen
in Fig. 20. Also here, the master alloys
with the lowest amounts of B display
the best grain refinement. According
to the latest investigations, the size
of the TiB 2 particles contained in an
Al-Ti-B master alloy are decisive to its
ALUMINIUM · 6/2007
efficiency [25, 27]. Coarser
TiB 2 particles require less
supercooling for the formation
of grain nuclei so that
a larger volume fraction of
Ti borides in the 2-3 μm in
size range substantially increases
the efficiency of a
master alloy. Consequently,
the notion that it is necessary
to have the smallest
TiB 2 particles possible, to
achieve the most efficient
master alloy, is being questioned.
The casting temperature
of the melt is an important
process parameter for the
quality of a DC cast ingot
particularly with respect
to the formation of the cast
structure. The effect that
casting temperature has on
grain refinement is shown
in Fig. 21 [28, 24]. This
displays the formation of a
fully equiaxed grain structure
for various AlTiB and
AlTiC grain refiner metal
alloys as a function of the
casting temperature and
the amount of grain refiner.
It can be seen that with
increasing temperature of
the melt at the mould gate
larger amounts of grain refiner are
required to achieve a fully equiaxed
grain structure. With increasing temperature,
the formation of columnar
crystals is promoted and the area
Billet Diameter 203 mm
RESEARCH
with equiaxed crystals reduced. The
difference between the efficiency of
the various AlTiB master alloys is also
clearly evident. This can be traced
back to not only the different quantity
of TiB 2 particles introduced but also
depends on the size distribution of the
Ti borides. Using alloy AlTi1.2B0.5 as
an example, this master alloy displays
the lowest temperature sensitivity despite
the small amount of TiB 2 introduced
compared with, for example,
AlTi3B1. This can be traced back to
a favourable distribution of TiB 2 particle
sizes. It can also be seen from
Fig. 21 that AlTiC master alloys appear
to react more sensitively to an
increase in casting temperature. This
is possibly connected with the fact
that active Ti carbides as nucleating
agents require more constitutional
supercooling to activate them as nucleating
agents [24]. In practice, this
means that in multiple-strand casting
stations, such as those used for the
casting of extrusion billets, particular
attention must be paid to small
temperature losses across the whole
mould table. If the temperature difference
is too large, the risk of different
cast structure formations in the
DC cast ingots arises. For instance to
avoid differences in the cast structure,
larger amounts of grain refiner need
to be added which in turn leads to
higher grain refinement costs and can
also result in quality problems due to
excessive amounts of TiB 2 or TiC in
the ingots.
The most important process pa-
Fig. 21: Influence of melt temperature on grain refinement with different AlTiB and AlTiC
master alloys
75
�