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RESEARCH
Grain refinement in DC casting of aluminium, Part II
W. Schneider, Bonn; D. Granger, Murrysville
Process and casting parameters
affecting grain refinement
The grain refinement of aluminium
alloys with AlTiB and AlTiC master
alloys in DC casting is influenced by
a wide variety of process parameters.
The most important of these comprise:
• Amount of master alloy added,
• Size and distribution of nucleant
particles in the master alloy,
• Efficiencies with which particles
are dispersed in the melt,
• Ti and B (or Ti and C) content of
the master alloy,
• Composition of the alloy to be
refined, including Ti content,
• Casting speed,
• Casting temperature (or tempera-
Fig. 16: Influence of master alloy type and
master alloy addition rate on grain site of
AA 1050 extrusion ingots with 8‘‘ diameter
Fig. 17: Influence of master alloy type and
master alloy addition rate on grain size of
AA 6063 extrusion ingots with 8‘‘ diameter
ture of the melt at the mould inlet),
• Dimension of the DC cast ingot
(or local cooling rate).
To achieve optimum grain refinement,
it is necessary to understand
and adjust parameters with respect
to one another. The amount of master
alloy added in relation to its Ti and B
content is of particular importance to
achieving a fine equiaxed grain structure.
The influence of the relative
amounts of different master alloys on
the grain size of DC cast extrusion billets
in alloy AA 1050 is shown in Fig.
16 [25]. It can be seen that, depending
on the composition of the master alloy,
different minimum amounts need
to be added to attain a fully equiaxed
grain structure through the ingot
cross-section. If the amount added is
below this value columnar or feathery
crystals appear in the outer region of
the ingot. It can also be seen that the
master alloys when added in equal
amounts lead to different grain sizes
depending on their respective TiB 2
content and, from a certain amount
onwards, no significant difference in
the grain sizes can be observed. The
same relationships for alloy AA 6063
are shown in Fig. 17 [25]. It can be
seen here that, due to the higher solute
content, a fully equiaxed grain structure
is attained with lower amounts
of grain refiner. In addition, the difference
in the amount added for the
various master alloy compositions
disappears more quickly than with alloy
AA 1050. These
results highlight the
long-known fact
that, with an increasing
concentration of
alloying elements,
a lower amount of
grain refining master
alloy is needed
to achieve complete
grain refinement
of a DC cast ingot.
Furthermore, AlTiB
master alloys with
lower B contents,
can be used effectively when there are
higher concentrations of elements in
the alloy to be refined; this also gives
related quality advantages.
In practice the amount and type of
master alloy to be added should always
be in accordance with the alloy
being grain refined. In this way, the
cost of grain refinement in a casthouse
can be reduced substantially. It does,
however, necessitate the selection of
different addition amounts and different
master alloys to make this possible
and realise the unfulfilled cost-saving
potential. In addition, the Ti borides
or Ti carbides contained in the scrap
component of a furnace charge should
also be taken into account when working
out the amount of grain refiner
required. Newly developed measurement
and control methods suitable for
large-scale operations now facilitate
the optimisation of grain refiner additions
[26].
The alloy content greatly influences
the final grain size. More details about
this fundamental relationship appears
elsewhere. The Ti content plays a particulary
important role. Excess Ti occurs
either through the addition of the
master alloy or the Ti content of the
melt. It is well known that Ti has a considerable
influence on grain refinement
in conjunction with TiB 2 . Even
at a concentration as low as 0.013%
Ti in the melt, grain refinement with
TiB 2 can be improved considerably.
As a consequence, the amount of Al-
Fig. 18: Influence of residual Ti concentration on grain refinement
with AlTiB master alloys
74 ALUMINIUM · 6/2007