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ALUMINIUM SMELTING INDUSTRY
Optimisation of grain refinement in a
range of casthouse aluminium alloys
M. Bryant, MQP
Fig. 1: Instrumentation used in the Opticast system
MQP Ltd recently presented a paper at
the 2013 TMS Conference in San Antonio,
Texas, about optimising the grain
refining process for aluminium alloys [1].
The article is based on that presentation,
including some new information, and
looks at
• the optimisation technique used
• a new powerful and consistent grain
refiner, Optifine
• and their combined effect on a range
of aluminium alloys currently being
produced at a casthouse involved in
the study.
Optimisation
The Opticast system is proving to be an invaluable
tool in carrying out assessment and control
of grain refinement practice in industrial
Fig. 2: Grain refiner curves for two master alloys with different efficiency
casthouses by using data generated from sampling
the melt in real time. Originally conceived
by Lennart Backerud [2] and co-workers at
Stockholm University, and since that time extensively
developed, tested and refined, the
Opticast technology is today being used for
optimisation and control of grain refinement
practice in casthouses worldwide.
It produces rapid and reliable results so
that accurate conclusions can be quickly made
regarding implementation of optimised grain
refining practice.
Implementation of the Opticast system in
the casthouse entails the following steps: calibration
and sampling in casting furnace.
Calibration involves establishing how a
specific alloy responds to addition of fresh
nuclei via the grain refining rod, in other
words establishing the equations for the grain
refinement curves as shown in Fig. 2. It shows
a test carried out with
two different batches of
the same grain refiner
in the same alloy melt.
Obviously one batch
is more efficient than
the other and it follows
that if both grain refiners
are being used in a
casthouse, the calibration
must be done to handle
any variations in the grain
refiner efficiencies, that is
© MQP
a best and worst case scenario. This means that
the calibration equation must be set up for the
upper of the two curves and is for the lower
one as well. The practical implication of this is
that there is much to gain if the grain refiners
used have a consistently high efficiency from
batch to batch.
Optifine
Establishing that grain refiner variability is an
important consideration factor in achieving a
fully optimised practice has led to the development
of a consistently powerful titanium boron
master alloy grain refiner, Optifine.
Optifine is produced via a special production
route, which substantially optimises its
nucleation potential. In standard TiBAl grain
refiners only 1% of particles are active, and
large boride particles and agglomerates lead
to preferential formation of large grains
whereas studies have shown that particles in
the size range 1-3µ are optimum for nucleation.
Optifine has been found to have over six
times more active nuclei per ppm of boron
Fig. 3: Microstructure of a standard TiBAl
Fig. 4: Microstructure of Optifine
40 ALUMINIUM · 7-8/2013