Technology0614

Technology
June 201436
- AMIT KAMBLE, Manager R&D, Minex Metallurgical Co. Ltd.
Grain Refiners
& Modifiers for the
Aluminum Foundry

June 201437
Behaviour of Grain Refiner
ContactTimeandFade
Fig.1 shows the grain size vs. time curve
after grain refiner addition. The curve is
characterized by initial rapid decline in grain
size to ultimate grain size (UGS) at contact
time tc, followed by a rise in gain size at longer
times. This loss of refinement is termed as
'fade'. Fade can be ameliorated in part by
stirring the melt, through the degree of recover
depends on the melt compositions [6]. It is
known that grain refinement can be lost if a
melt is held above 750°C, but that this can be
regained if more titanium is added and melt is
stirred.
Fig.1 : Schematic form of grain size vs. log time
behaviour following addition of grain refiner to
Al alloy melt [6]
Fig. 2 : Schematic representation of dilute Al-Ti
phase diagram showing liquidus for TiAl3 on TiB2.
Sequence A to B to c shows how potency may be
lost and regained [6]
EffectofRefiningParticles
Following the rod addition, TiAl3
particles dissolves rapidly (40µm/min) and
with good stirring 90% Ti will be in solution
within 10s for a typical distribution of
aluminum sizes. Depending on the melt
temperature and composition, the TiAl3
overlayers on TiB2 may grow slightly, remain
stable, desorbs or undergo site competitive
substitution with other transition metal
elements present in the alloy melt. Fig.2 (Fig.3)
demonstrates how the absorbed TiAl3 layer
may be lost and reformed. Substitution and site
competition effects may be estimated from
dilute Al-X binary phase diagrams which are
shown super imposed in Fig.4 from this fig. it is
possible to see Zr and V compete strongly with
Ti at particularly lower temperatures and that
Ta will form a stable layer. Whilst aluminide
dissolution and TiB2 interfacial chemistry
changes are in progress, some degree of
particle collision, sticking and settling will
occur. If holding times are long then there will
be fewer particles remaining in the melt to
nucleate grains. It will also be likely that those
remaining in the suspension will be smaller on
average than immediately after addition as
largerparticlessettlefirst.
Fig. 3 : Schematic representation of processes
occurring on addition of TiBAl grain refiner to
aluminum melt showing how the characteristic
form of behavior emerges [6]
Solidification
Fig. 4 : Dilute portions of Al-Ti, Al-Zr, Al-V and Al-Ta
phase diagrams showing relative stabilities of
aluminides which affects preferred adsorption
behavior on TiB2 [6]
Grain refinement can be understood to be
directly related to the nucleation and growth
process of aluminum grains. This is based on
Volmer and Weber ideas for homogeneous and
heterogeneous nucleation [10]. Fig. 5 shows
the solid nucleating on the substrate on liquid.
The critical free energy for heterogeneous
nucleation is always less than or equal to that
for homogenous nucleation. The values of
undercooling, ? T is of order of 1 – 2 K for
observable nucleation rates in commercial
aluminum alloy with grain refiners. Therefore
clearly heterogeneous nucleation is taking
place. The expression for heterogeneous
nucleation rate per unit volume in m-3s-1 is by
equation(1).
Fig. 5 : Schematic representation showing the
formation of spherical cap of solid (s) on a
Fig. 6 : Schematic distribution of undercoolings
required substrate, contact angle and surface
tension by different nuclei within a grain refined
melt. [6] forces [10]
Under the heat flow conditions prevailing
during solidification, the melt becomes
undercooled until sufficient undercooling is
achieved for particles to nucleate á-Al by the
peritectic reaction (Fig. 6). In order to achieve
finest grain size, it will be necessary to activate
the median portion of the undercooling

June 201438
distribution. In reality undercooling will vary
from positions to positions because of local
heatflow and constitutionaleffects.
Behaviour of Grain Modifier & its
Selection for Al Casting
Comparison of Sodium & Strontium
Modifier
Modifiers are added to eutectic and hyper
eutectic Al-Si alloys to refine the eutectic Si
phase from one of angular platelets to fine
fibers. This change in microstructure results in
improved mechanical properties, particularly
strength and ductility [01]. Modification also
leads to refined porosity which leads to an
additional improvement in mechanical
properties.
The first hypoeutectic modifiers were
based on Na, and are added as Na metal or
encapsulated Na in Al foil. Due to high
reactivity and low solubility of Na in
aluminum (recoveries low up to ~ 20%) with
gradual loss of Na with time (fade) results in a
process thatisdifficulttocontrol.
In sodium modified melt the eutectic
growth front is typically planer, thus allowing
regular solidification through the dendritic
network and minimal entrapment of liquid
pockets [14]. With Sr modification however,
the growth front is more irregular, allowing the
possibility of liquid areas becoming trapped
leading to shrinkage microporosity. The
modifying action of Na practically disappears
after only few remelts. On the other hand, the
modifying effect of strontium does not fade on
holdingatelevatedtemperature[11].
However, longer holding time is required
at 750 C due to the difficulty in dissolution of
strontium, resulting in increased gas pickup.
Finally, although antimony has no fading effect
and the improvement in elongation and impact
strength is greater than that achieved by
sodium, its use has been precluded in most
countries because of environmental and safety
reasons. Thermal modification involves heat-
treatment of cast alloys at high temperature;
usually at the solid solution temperature
around 540 C for long times [12]. Solution heat
treatment results in a substantial degree of
spheroidizationof secondphaseparticles.
Variation of Silicon particles
morphology with modification [15]
In unmodified alloy, the silicon grows as
plates that can be lamellar or acicular. Lamellar
structures are nearly parallel and thin plates,
while acicular structures are randomly
distributed plates with much massive structure.
At an unmodified condition, thin silicon plates
Recovery
Contact Time
Fade
Environmental
Overmodification
Potency
Addition Level (Typical)
Sodium Metal
Low, Variable & Operator Dependent
Immediate
Fast (20-40 min)
Fumes and Refractory Attack
Possible Due to Difficulty of Control
Very High
50 ppm
Al-Sr Alloy
High and Consistent
Up to 10-20 min.
Slow (> 2 hours)
Clean and fume free
Unlikely
High
250 ppm
Unreactive with Moisture if
<20% Strontium Master Alloy
Ease of Handling Must be Protected from Moisture
TABLE 1 : COMPARISON OF SODIUM & STRONTIUM MODIFIER [11]
can be seen cutting through the aluminum
matrix(Fig.7).
Partially modified structures are the
coexistence of flake and fibrous silicon mixed,
cellular or banded structure. They can be
obtained by a combination of slow
solidification rates (bet. 1 & 25µm/s) and low
strontium concentration. At low strontium
concentration and during slow growth, the
eutectic is able to reject enough strontium
ahead its interface to avoid transforming to
fibrous morphology(Fig.8).
In well modified structure silicon grows in
a coral-like form (fibrous) that looks like small
individual round-shaped particles in polished
surface. The modification treatment affects not
only size and shape of the particle, but also its
distribution. Ideal concentration of strontium
results in a fibrous eutectic structure, well
refinedandhomogenous(Fig.9).
When the addition of strontium exceeds
the amount necessary to produce a fully
modified structure, it exerts a deleterious effect
on the mechanical properties of the alloy.
Strontium overmiodification is much more
subtle and difficult to identify by
microstructuralanalysis.
In overmodified alloys, elongated silicon
particles that no longer grow as well as refined
fibers can be observed. However the formation
of Sr-rich phase is reported in strontium
concentration above 0.03% and the eutectic
silicon grows in coarser fibrous form.
Although the morphologies of a well modified
alloy and an overmodified alloy are mostly
coral-like ones, their thickness are
considerablydifferent(Fig.10).
Fig. 7 : Unmodified Alloy (0.0003% Sr) Lamellar
Structure (a) 200X (b) 1500X
Fig. 8 : Partially Modified Structure (Mixed)
(0.0026% Sr) (a) 200X (b) SEM Deep Etched 1500X
Fig. 9 : Fully Modified Structure (0.0075% Sr)
(a) 200x (b) SEM Deep Etched 1000x
Fig.10 : Over Modified Structure (0.0130% Sr)
(a) 200x (b) SEM Deep Etched 1000x
Effect Of Strontium on Reliability Of
Castings
The average mechanical property results
after the various treatments are shown in Fig.
12 these show that the melt treatments lead to
improved mechanical properties in GDCA319
alloy. The addition of modifier gives an
increase in UTS and elongation values,
treatments have resulted in reduction in scatter
as evidenced by an increases in Weibull
modulus from 5.9 (untreated) to 17.6
(modified) [05]. Similarly the Fig.11 shows an
increased fracture toughness value for Sr-
modified alloy A357 compared to unmodified
alloy.
Selection of Grain Refiners
Grain refiners are added to Al-Alloy to
improve the melt processability and to reduce
the size of primary Al grains. These grain
Technology

June 201439
Fig.11 : Fracture Toughness of Alloy A357(al-
7si-0.5mg) with and without Sr Modification [14]
Fig.12 : Effect of Melt Treatment on Weibull Plots
of UTS of Gravity Die Castings A319 [05]
refiners can be in various forms. The most
suitable form is master alloys as AlTiB and
AlTiC, it contains TiAl3 particles and TiB2
particle which are responsible for grain
refinement as discussed earlier. Originally
these particles were created into the melt by
addition of compacted powder tablets i.e.
compacted mixture of K2TiF6 and KBF4 salts.
The tablets works as simple dissolution and
reaction of titanium boron tablets or by
dissolution and reaction of titanium and boron
salts in titanium boron tablets as indicated
below.
Because these reactions have to occur in
the melt to form TiB2 nuclei, this addition
method is not as quick or as controllable as
TiBAl master alloy approach, as comparison
shown below:
TABLE 2 : COMPARISON OF TIBAL & SALT
TABLET AS GRAIN REFINER IN FOUNDRY [5]
TiBAl Rod Salt Tablets
Recovery High and
Consistent
Variable and
Operator
Dependent
Contact Time Short (30 sec.) Long (5-10 min)
Efficiency High Low
To be Cont..
Fade 2-4 hours 1-1.5 hours
Dissolution Fast (<30 sec.) Slow (2-5 min.)
Addition
Clean and Fume
Free
Fumes and Flux
Residues
Cost
Comparison
High Low
Industrial Application of TiCAl &
TiBAl
TiCAl refiners (Al-3Ti-0.15C) have been
used industrially in aerospace plate (AA7xxx),
bright trim (AA5xxx), lithographic (AA1xxx),
canstock (AA5xxx) and foil (AA8xxx) alloy
applications. Operationally, the higher
sensitivity of TiCAl to temperature makes it
more suitable for slowly cooled products e.g.
large rolling slabs where the undercooling
driving grain growth is primary [08]. TiCAl
offers advantages in alloy containing Zr, Cr
which can otherwise poison TiBAl refiners. At
a given addition rate and low temperature
TiCAl master alloy is as efficient as TiBAl
alloys in grain refining AA6063. In terms of
quantity of particles added however, both
TiCAl andAl5Ti-0.2B are more potent thanAl-
3Ti-1B. TiBAl is used for strip cast application
with addition rate of 0.15 to 0.4 wt% of grain
refiner. [03]. Recycling test demonstrates that
TiBAl is more robust than TiCAl during the
thermal cycling of AA3004 at 800°C [02].
TiBAl andTiCAl bearing 3xxx and 5xxx alloys
can be recycled effectively with modest
additionofTiBAlafterexposureto800°C.
Comparison of Grain Refiners TiCAl
and TiBAl
It has been estimated typically only 1 in
500 TiB2 particles nucleates grain. Fig. 13
shows relation between no. of particles
required per grain and grain refiner addition
rate for AA1050 at different temp. In general,
with fewer particles present, the nucleation
efficiency of each particle becomes greater.
The loss of potency displayed by TiCAl at
higher temp. may be caused by poisoning.
Conversion of the surface of TiC particles to
Ti2AlC or Al4C3 would destroy the excellent
lattice matching between TiC and á-Al which
is basis of TiC's effectiveness as a nucleant for
aluminum[09].
Thermal cooling dominates at the edges of
castings and constitutional undercooling is
greatest at the centre of cast sections. AlTiB
produces finer grain sizes at billet edges and
therefore responds better to thermal
undercooling (Fig.14). The nuclei generated
near the mould walls will be relatively mobile
and rejected solutes will tend to be dispersed
leading to relatively low constitutional
undercoling. Under such condition most potent
Fig.13 : No of Particles (Calculated) Needed to
Nucleate A Grain in Aa1050 as a Function of
Refiner Type, Addition Rate and Temperature
(a) 670°c (b) 700°c [09]
nucleant will dominate solidification behavior
and so TiB2/TiAl3 produces finer grains at the
mouldwalls.
Fig. 14 Schematic of the constitutionally-
Conversely at billet centre, thermal gradient
supercooled region ahead of the solidification
will be low and the constitutionally-front
showing different behaviour of AlTiC and
AlTiB [10] supercooled zone will be large. The
zone of activation of TiB2 nucleants will be
larger than that for TiC and the opportunity for
nucleation well in advance of growth front will
be greater for TiBAl [09]. The relatively fine
grain size at billet centre is obtained with
TiCAl is therefore suggested to be due to
combination of its greater constitutional
supercooling to become active and its tighter
particle size distribution. ForTiCAl there is no
change observed in grain size with increasing

June 201440
mould temperature in the range 25500°C.
Whereas, TiBAl showed coarsening of grain
with rising mould temperature. Thus TiCAl is
more effective in situations where thermal
gradientarelow.
PoisoningEffect
Influence of Zr/Cr on Grain Refiners
Zirconium has a poisoning effect
(especially inTiBAl, not inTiCAl) on the grain
refiner efficiency more obvious at higher
melting temperature. G.P. Jones and J. Pearson
suggest that the fading mechanism is mainly
due to the formation of a thin layer of ZrB2 on
TiB2 particles in the melt They claim that the
nucleation is on TiB2 particles adsorbing of Ti,
when Zr element is present, the titanium
borides will be covered with a monolayer of
zirconium borides preventing the dissolved Ti
frombeingadsorbed[13].
On the contrary, when 0.12%Zr is added
into the melt, the grain refinement performance
of AlTiC refiner with 0.2% addition level was
affected at higher temp. (i.e. 800°C)[13]. Zr
element interact with both TiAl3 and TiC
phases, if both of them are present, Zr
preferentially reacts with TiAl3 phase. Thus
TiCAl is a effective grain refiner at lower
refiningtemperatures.
A combination of small quantities of Fe,
Cr, Si (0.2 wt% Cr) has shown early and
significant fading. At higher Cr concentration
(Cr 2wt %) has adverse effect of coarse ring
grainsize[07].
Agglomeration
The boride agglomeration takes place only
in case ofAlTiB grain refiner. The user can not
be certain that the agglomerate will disperse on
addition to aluminum. It is also possible that
the agglomerate may form within the
customer’s own process. The causes of boride
agglomeration are: TiB2 particles collide with
each other, TiB2 particles are attracted to oxide
films, to form complex agglomerate, halogen
elements “wet” TiB2 particles causing them to
easily agglomerate and TiAl3 layers on TiB2
cansticktheparticlestogether.
Inclusions, in grain refining master alloys
and aluminum alloys as TiB2 agglomerates,
manifest themselves in various forms such as
rings, clusters and strings. Boride
agglomerates are generally classified as
defects when they are above 5 micron in size.
They are often observed to be tiny clusters of
much smaller particles (½ to 1 micron).
Fig.13 : Optical Microstructure of TiBAl 5/1
at 1000x having TiB2 Agglomeration and String
Type Defects
Examination of fractured samples to
investigate the boride agglomeration (as
strings) contains fluorine at relatively higher
levels.TiB2 particles are susceptible to wetting
by fluorides [04]. This can also be identified in
grain refiner microstructure as shown (grain
refinerwithdefects)
The chances of agglomeration of TiB2
particles in lower boron containing grain
refiner is less (e.g.TiBAl 5/0.2). ReducedTiB2
agglomeration to allow thinner foils and
brighter trim alloys to be produced. In case of
TiCAl,TiC a particle doesn't agglomerate, thus
TiCAlissuitablefor allapplications.
Conclusion
lGrain refiners play a crucial role in
aluminum alloys; it improves mechanical
properties that are uniform throughout the
casting, better feeding to eliminate shrinkage
porosity, better strength and fatigue life and is
achieved by heterogeneous nucleation and
growthofgrains.
TiCAl offers advantages in alloys
containing Zr/Cr which can otherwise poison
TiBAl refiner. Al-3% Ti-0.15% C improves
melt fluidity and finished cast appearance in
comparison toAl-3% Ti-1% B in 5XXX series
alloysinparticular,butalsoin6XXX alloys
For lithographic/Strip casting/foil grade
application Al-5%Ti-0.2%B is more effective
grain refiner than compared to Al-3%Ti-
0.2%C and 3%TiAl. As lower boron
concentration leads to less agglomeration of
TiB2particles
And in modifiers Sr is preferred over Na
metal modifier in Al-foundry due to
environmental restrictions. But excess amount
of Sr cause overmodification by coarsening of
Silicon structure which is not easily assessejd
byopticalmicrostructurestudy.
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