Foundry Guide to using TiBor Aluminum Grain Refiner and Modification with Strontium

1. Foundryman’s Info
Structure control of aluminium castings
using AlTiB grain refiners and AlSr modifiers
FI.2.E

2. Grain refinement
A casting is a compact network of
individual grains. The size, shape and
bonding of these grains will determine
the mechanical properties and structure
of the casting so there is a real need to
control their formation.
The need for fine grains
A fine grain structure promotes:
casting homogeneity by uniform
dispersion of the alloying elements
better feeding during the final
solidification stages
resistance to shrinkage cracking
through stress distribution
reduced internal porosity
enhanced response to heat treatment
A fine grain size supports the
production of castings with superior
mechanical properties. A lower reject
rate results in improved yields and
overall cost saving.
Aluminium Titanium Boron
Grain refiners
Grain formation in aluminium castings
can be controlled by the addition of
Titanium and Boron before casting. In
combination these two elements are
known to be the most potent nucleating
agents available.
In the early days Ti and B were added
to molten aluminium in the form of
fluoride salts pressed into tablets.
This route has now been almost fully
replaced by the use of AlTiB master
alloys, combining the two elements in a
matrix of high purity aluminium
usually in the form of (cut) rod.
Compared to the salt tablets an AlTiB
grain refiner offers the following
benefits:
no evolution of toxic fumes
no uncertainty of reacting chemicals
no health hazards
no attack on refractories
no risk of salt inclusions
no reaction with Mg
no reaction with Sr and Na
no dross from reacting salts
reliable Ti and B yield
highly consistent results
optimum nuclei size
predictable grain refinement
unlimited shelf life
survival during recycling
simple foundry practice
less dependence on operator skill
Microstructure AlTiB 5/1 rod
Grain refinement with KBM AFFILIPS
AlTiB is a well established foundry
practice for general engineering,
automotive and aerospace applications.
Grain refinement
Structure control of
aluminium castings
The quality of a casting can be
judged by its dimensional accuracy
and its internal structure. Both
influence strength and performance
and will determine its value.
Foundrymen recognise that structure
control during solidification plays a
vital role in the achievement of high
integrity castings.
Two important metallurgical operations
influence the final structure of the
casting. These are:
Grain refinement
- to influence the macrostructure
Silicon modification
- to influence the microstructure
KBM AFFILIPS, with over 40 years
of experience in these products, have
developed a sophisticated series of
master alloys for effective grain
refinement and modification of
aluminium foundry alloys.
These master alloys have advanced
the concept of premium quality
castings to all aluminium foundries,
irrespective of their size and casting
techniques used.

3. Silicon modification
Most aluminium foundry alloys contain
silicon in the range of 6 - 12 %.
Uncontrolled solidification of such
alloys will favour the growth of a coarse
acicular silicon phase in the aluminium
matrix which will reduce the mechanical
strength and ductility of the casting.
Modification of the silicon phase from
coarse platelets into a fine fibrous
structure markedly improves the
soundness of the casting, increasing its
strength and in particular its ductility.
Microstructure AlSi7%
unmodified/modified
Although fast cooling will produce the
desired fine structure, it is not always
feasible. The addition of strontium
produces a similar effect that persists for
several hours under normal foundry
conditions.
Strontium can best be introduced as an
aluminium based master alloy during
melt preparation. KBM AFFILIPS’
experience has resulted in a comprehen-
sive range of AlSr master alloys, see
table 2.
For ease of handling most
foundries prefer to
use aluminium-
strontium
in the form
of cut rod,
9.5 mm dia-
meter rod cut
into lengths of
50 cm or 100 cm.
KBM AFFILIPS AlSr
cut rod is a true high
performance product.
It is made using a proprietary process
which ensures that the rod has an
extremely fine microstructure. This
results in very fast and consistent
modification. Full modifying effect is
achieved within two minutes after
addition at normal operating temperatures,
irrespective of operator skill.
Microstructure AlSr10 rod
Sr compared with Na
Although Na is still sometimes preferred
for certain applications, e.g for sand-
castings having considerable variations
in wall thickness, Sr has a number of
distinct advantages over Na.
These are:
predictable yield
higher ductility
retained effect over long holding times
effective over a wide cooling range
effective over a wide Sr range
insensitive to overmodification
no health hazards
no violent reaction during addition
no fume evolution
no melt losses
no refractory attack
unlimited shelf life
simple foundry practice
less dependence on operator skill
higher melt fluidity due to low
oxide skin formation
over 50% survival in recycled metal
option to use pre-modified foundry
ingot
reduced casting flow lines
minimum losses on N2 or Ar
degassing
Sr in high pressure diecasting
Because of the high cooling rates, high
pressure diecasting of aluminium
foundry alloys normally does not require
modification or grain refinement.
Any improvement of the metallurgical
structure would also be masked by the
porosity naturally present in
conventional HP-diecast parts.
There is a tendency towards the
production of high quality HP-diecast
components that have the mechanical
properties (density, strength, ductility)
of a gravity or low pressure diecast
part. Examples are parts that undergo
subsequent welding, safety structural
parts or parts that need to have a high
quality surface. Usually vacuum assisted
HP-diecasting is used to achieve the
required low porosity level. Such
castings greatly benefit from a small
strontium addition to modify the silicon
phase. Typically 100 - 150 ppm Sr
addition doubles the ductility.
Additions
The required quantity of grain refiner
and modifier is influenced by the
casting method, in particular the cooling
rate, and by the alloy composition.
Heavy-walled sandcastings demand
higher additions than thin-walled low
pressure diecastings.
Foundry alloys containing over
7% silicon need higher amounts
of strontium for proper modification
while lower silicon contents require
more grain refiner.
The following table provides a basis.
Modification
Typical additions for conventional gravity die casting
alloy master alloy addition in kg/mt
ISO AA AlTiB AlSr3.5 AlSr5 AlSr10
AlSi5Cu3 319 3 - 5 3 - 6 2 - 4 1 - 2
AlSi7Mg 356 2 - 4 3 - 9 2 - 6 1 - 3
AlSi12 413 1 6 - 12 4 - 8 2 - 4
AlSi12Mg 360 1 - 2 8 - 10 5 - 7 2 - 3
AlMg5 514 3 - 5 - - -
AlCu4 295 3 - 5 - - -
Table 1

4. Other forms available
For further information please contact:
KBM AFFILIPS B.V.
P.O. Box 799, 5340 AT Oss, The Netherlands
Tel: (31) 412 681311 Fax: (31) 412 635594
E-mail: info@kbmaffilips.com Website: www.kbmaffilips.com
KBM AFFILIPS is a wholly owned subsidiary of ROBA Holding B.V.
- subject to change without notification -
Copyright®KBMAFFILIPSB.V.TheNetherlands2000.03.2002
Foundry practice
AlTiB and AlSr are available in cut
lengths of 9.5 mm diameter rod which
offer the most convenient addition
method for foundries.
Actual working procedures are best
established on site but will follow
straightforward basic guide lines for melt
preparation.
The AlTiB and AlSr master alloys can be
added to the molten metal in the crucible
furnace or into the transfer ladle. Rotor
degassing will promote the distribution of
the grain refining and modifying
elements.
Procedure:
metal temperature 700º -750º C
rotor degassing is recommended
skim melt surface
add AlTiB cut lengths, stir
degas with nitrogen/argon
skim melt surface
add AlSr cut rod lengths, stir
final skim before casting
Using cut rod, both AlTiB grain
refinement and AlSr modification are
effective within 2 minutes after addition
and will survive up to 6 hours. Strontium
modified metal will survive remelting,
approximately 50% retention should be
included in batch calculations.
7 kg Waffle ingot 0.5 kg Piglets 0.5 and 2.5 kg Conticast™ Contiform™
100 gr and 200 gr cut rod
For more information on grain refining and modification refer to KBM AFFILIPS product brochures "Aluminium Titanium Boron grain
refiners" and "Aluminium Strontium for AlSi modification". Also available are specific Technical Information Sheets and Material Safety
Data Sheets.
Derived from specification
Application Product CEN AA Colour code
Grain refinement AlTiB 5/1 EN AM-92256 H2252 green
Modification AlSr3.5 EN AM-93800 H2012 light blue
AlSr5 EN AM-93802 - light blue
AlSr10 EN AM-93804 H2007 light blue 2x
AlSr15 - - light blue 3x
AlSr3.5Ti1B0.2 - - light blue/black
AlSr10 Ti1B0.2 EN AM-93850 H2017 light blue/red
Product range Table 2