special - Alu-web.de

Fig. 26: Feathery crystal formation in sheet ingot casting with Combo
Bag and unsufficient grain refinement.
varying levels of efficiency. This is of
particular importance to industrialscale
practice since the required additions
can be optimised in accordance
with the particular alloy being cast
and, consequently, it may be possible
to achieve substantial cost savings. In
working out the additions required,
not only the concentration of alloying
elements and alloy type must be
taken into account but also, as already
reported, other process parameters,
such as melt temperature and casting
speed have to be determined.
Furthermore, it should be noted that
the influence of the melt temperature
and casting speed decreases as the
concentration of alloying elements
increases. This also means that alloys
with lower solute contents, which are
cast using the DC process, are more
difficult to grain refine.
Grain refinement and molten
metal treatment
When using grain refiner rod, the place
where the rod is added is of particular
importance in practice. At present,
ALUMINIUM · 6/2007
for quality reasons,
it is usual to
add grain refining
master alloy
before the melt
cleaning units to
ensure that oxide
impurities, coarse
Ti borides and
dense TiB 2 –agglomerates
in the
melt are removed.
Only in exceptional
cases an
addition is made
after the molten metal treatment unit.
When adding grain refiner before
the degassing unit, there is a risk of
agglomeration of the Ti borides or Ti
carbides due to the turbulence which
exists in the chamber and the subsequent
release of the agglomerates out
of the chamber. In investigations, it
was determined that the agglomeration
of Ti borides is enhanced when
degassing is effected by means of an
Ar/Cl 2 gas mixture [31]. In this case, a
downstream filter is necessary to remove
the harmful agglomerates. Another
problem is the removal of the agglomerates
by flotation using purging
gas bubbles in a degassing chamber.
In all cases, the melt loses useful Ti
borides or Ti carbides that are subsequently
no longer available for grain
refinement. The addition of master
alloys must be adjusted accordingly,
i.e. increased in order to achieve sufficient
grain refinement of the as-cast
structure during casting. This is also
valid for additions made before the filter.
In addition, the filter is filled with
deposited Ti borides or Ti carbides,
which reduce the capacity of the filter
for trapping
other impurities
and,
as a result,
its filtration
performance
is quickly
exhausted.
Besides this,
there is also a
risk of the alreadydeposited
Ti borides
or Ti carbides
being flushed
RESEARCH
out of the filter and entering the DC
cast ingots in the form of large “inclusion
clouds”. This, for example, can be
caused by vibration of the filter or by
a change in the melt flow rate through
the filter. Recent investigations using
ceramic foam filters and tube filters
have also shown that the addition of
Ti borides or Ti carbides before the
filter markedly reduces filtration efficiency
as shown in Fig. 27 [32]. The
reasons for this are due to a change
in the filtration mechanism. With no
addition of Ti borides or Ti carbides
before the filter, so-called “inclusion
bridges“ are formed in the filter pores;
the efficiency of these is very similar
to that of cake filtration and displays a
comparable filtration capability. When
Ti borides or Ti carbides are added to
the melt before the filter, the formation
of these bridges is prevented and
only depth filtration comes into play
for the removal of undissolved impurities.
As a result of the change in mechanism,
the performance of the filter is
reduced since “bridge filtration” appears
to be more efficient than depth
filtration. Specific investigations into
the influence of master alloy composition
on “bridge filtration” have also
shown that the negative influence of
the master alloy decreases in line with
the volume fraction of TiB 2 in the master
alloy [33].
As reported above, the exact location
of where to add grain refining
master alloys must be considered
very carefully. In the future, the addition
of grain refiner after the melt
cleaning units should be considered
in order to avoid the negative effects
of Ti borides and Ti carbides on the
cleaning efficiency of filters. The risk
today is not as grave as in the past
since the quality of master alloys,
especially with regard to the amount
of oxide impurities and the presence
of agglomerated nucleating particles
(TiB 2 , TiC), has improved considerably
in the last 20 years.
References
[1] W. Schneider “DC Casting of Aluminium
Alloys – Past, Presence and Future”, in
Light Metals 2002 (edited by W. Schneider),
pp. 953-960, TMS, Warrendale PA (2002)
[2] Men.G.Chu “Grain refining of commercial
aluminium wrought alloys”, in Light
Fig. 27: Removal efficiency of ceramic foam filters with grain refiner addition
before and after the filter �
77