Note: Descriptions are shown in the official language in which they were submitted.
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GRAIN REFINING AGENT FOR CAST ALUMINUM PRODUCTS
The present invention pertains to improvements in the field of cast
metals and metal alloys. More particularly, the invention relates to a grain
s refining agent for cast aluminum products.
Grain refiners are widely used to reduce the grain size and to
control the microstructure of cast metals and alloys. Adding grain refiners to
molten metal or alloy during casting enhances the heterogeneous solidification
and results in a fine-structured material with equiaxed grains. The resulting
io material shows improved mechanical properties such as high yield strength
and
toughness.
In the aluminum industry, different grain refiners are generally
incorporated in the aluminum as master alloys which are added to the aluminum
melt in solid form, for example, in the form of small ingots or a rod which is
15 continuously fed into the melt. The master allay can also be added in a
molten
state.
Typical master alloys for use in aluminum casting comprise from
1 to 10% titanium and from 0.1 to 5% boron or carbon, the balance consisting
essentially of aluminum or magnesium, with particles of TiB2 or TiC being
a o dispersed throughout the matrix of aluminum. Master alloys containing
titanium and boron are normally produced by dissolving the required quantities
of titanium and boron in an aluminum melt. This is achieved by reacting
molten aluminum with KBF4 and K2TiF6 at temperatures in excess of
800°C.
These complex halide salts react quickly with molten aluminum and provide
25 titanium and boron to the melt. This technique is currently used to produce
commercial master alloys by almost all grain refiner manufacturing companies.
However, there is a number of disadvantages encountered with this technology.
Tetrafluoroborate (KBF4) and hexafluorotitanate (K2TiF6) complex salts are
costly and have relatively low boron and titanium contents. KBF4 decomposes
3 o at relatively low temperatures to give gaseous BF3 which is toxic and thus
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requires special handling and filtering facilities during manufacturing of
master
alloys. The final master alloy contains potassium aluminum fluoride (KA1F)
salt and aluminum oxide impurities giving rise to local defects in grain
refined
aluminum. Traces of halide salts in master alloys produced by conventional
s processes enhance the agglomeration of TiB2 particles resulting in a
decrease
of effectiveness of the grain refiner. It is also very difficult to control
the TiB2
particle size and distribution in master alloy; these parameters are important
in
determining the effectiveness of grain refiner and affect the quality of the
grain
refined aluminum.
i o Many efforts have been made to overcome the above problems.
Hardman et al. proposed in Materials Science Forum, Vols. 217-222 (1966),
pp. 247-252, to manufacture Al-Ti-B master alloys from low cost B203 and
Ti02 starting materials where there is no need for KBF4 and K2TiF6 salts in
the
manufacturing process. However, the use of cryolite (a mixture of NaF and
15 A1F3) is unavoidable in this process. Megy et al. proposed in Light Metals
2001, pp. 943-949, an in-situ grain refining process called the fy-Gem
process.
This process is carried out by introducing an Ar/BC13 gas mixture into molten
aluminum in the form of fine bubbles using a rotating head. Borontricoloride
is
decomposed in molten aluminum and boron is dissolved into the aluminum and
a o combined with titanium and other solute elements to form heterogeneous
nuclei having an average particle size ranging from 0.5 and 5 Vim. Although
this technique does not use the complex halide salts and the problems
associated with these salts are resolved, it uses borontrichloride which
decomposes during the process, causing corrosive and toxic chlorine gas
a s emission. In addition, a rotating head with special design is required to
produce
bubbles with optimum size within the molten aluminum in order to achieve
good results. All this equipment and additional parameters to be controlled
make the fy-Gem process more complicated and less interesting in practice.
It is therefore an object of the present invention is to overcome
3 o the above drawbacks and to provide an effective, salt free and low cost
grain
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refining agent for cast aluminum products, especially aluminum alloys
containing >0.003% Ti.
According to one aspect of the invention, thre is provided a grain
refining agent for cast aluminum products containing titanium, comprising
s particles formed of a matrix of a ductile material, in which are uniformly
dispersed boron particles having an average particle size of 0.1 to 10 Vim.
The expression "cast aluminum product" as used herein refers to
a cast product comprising aluminum or an alloy thereof.
The average particle size of the boron particles must be within a
io range of from 0.1 to 10 ~,m. When the average particle size is greater than
10
~,m, the number of boron particles introduced into the melt for a given
addition
level is too small. On the other hand, when the average particle size is
smaller
than 0.1 Vim, the heterogeneous nucleation of molten aluminum is not
effective.
Typical examples of ductile material include aluminum, titanium,
15 chromium, copper and silicon. Aluminum is preferred.
Preferably, the particles of ductile material have an average
particle size of 0.5 to 5 mm. The boron particles, on the other hand,
preferably
have an average particle size of 0.5 to 2 ~,m.
When the grain refining agent of the invention is added to molten
z o aluminum containing titanium, the ductile material melts or dissolves into
the
melt and boron particles are released within the melt and are combined with
titanium to form heterogeneous nuclei which grain refine the aluminum during
solidification.
Where the cast aluminum products contain no titanium, use is
z s made of a ductile material comprising titanium.
Accordingly, the present invention provides in another aspect
thereof a grain refining agent for cast aluminum products containing no
titanium, comprising particles formed of a matrix of a ductile material
comprising titanium, in which are uniformly dispersed boron particles having
3 o an average particle size of 0.1 to 10 Vim.
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The grain refining agent according to the invention is quite
different from Al-B master alloys which are used occasionally in aluminum
alloys. Al-B master alloys are produced by reacting KBF4 salt with molten
aluminum and its microstructure consists of A1B2 or A1B12 particles with
extremely small amounts of boron in solid solution with an aluminum matrix. It
is believed that A1B2 particles are the effective nuclei of ca Al, however the
exact grain refinement mechanism with boron addition has not as yet been
clarified thoroughly. In addition, it was reported that Al-B master alloys has
no
effect on the grain refinement of Si-free A1 alloys. In contrast, in the grain
1 o refining agent of the invention, there is no chemical reaction between the
ductile
material and the boron. The grain refining agent of the invention can
effectively
grain refine all Al alloys including Si-free A1 alloys.
According to a further aspect of the present invention, there is
provided a method of preparing a grain refining agent for cast aluminum
products containing titanium. The method of the invention comprises the steps
of
a) mixing boron particles having an average particle size greater
than 0.1 ~m with particles of a ductile material to form a powder mixture; and
b) subjecting the powder mixture obtained in step (a) to high energy
a o ball milling to reduce the size of the boron particles to a size ranging
from 0.1
to 10 g.m and to uniformly disperse the boron particles of reduced size within
the ductile material, thereby obtaining particles formed of a matrix of the
ductile material, in which are uniformly dispersed the boron particles having
an
average particle size of 0.1 to 10 Vim.
2 5 As noted above, where the cast aluminum products contain no
titanium, use is made of a ductile material comprising titanium.
According to yet another aspect of the invention, there is thus
provided a method of preparing a grain refining agent for cast aluminum
products containing no titanium, comprising the steps of:
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a) mixing boron particles having an average particle size greater
than 0.1 ~,m with particles of a ductile material comprising titanium to form
a
powder mixture; and
b) subjecting the powder mixture obtained in step (a) to high energy
s ball milling to reduce the size of the boron particles to a size ranging
from 0.1
to 10 ~,m and to uniformly disperse the boron particles of reduced size within
the ductile material, thereby obtaining particles formed of a matrix of the
ductile material, in which are uniformly dispersed the boron particles having
an
average particle size of 0.1 to 10 Vim.
io Due to impact forces during ball milling, the boron particles are
broken to small particles with the desired average particle size. Depending on
the initial particle size of boron and the desired particles size thereof in
the grain
refining agent, the milling time can be adjusted. The milling time generally
ranges from 10 minutes to 20 hours. The impact ,forces, in addition, cause
15 plastic deformations of the ductile material and during these plastic
deformations hard boron particles are trapped in the ductile material to form
a
composite comprising a matrix of ductile material in which boron particles are
uniformly dispersed.
According to a preferred embodiment, step (b) is carried out in a
z o vibratory ball mill operated at a frequency of 8 to 25 Hz, preferably
about 17
Hz. It is also possible to conduct step (b) in a rotary ball mill operated at
a speed
of 150 to 1500 r.p.m., preferably about 1000 r.p.mo
According to another preferred embodiment, step (b) is carried
out under an inert gas atmosphere such as a gas atmosphere comprising argon or
a s nitrogen, in order to prevent oxidation of the grain refining agent. An
atmosphere of argon is preferred.
Since the grain refining agent according to the invention is in
powder form, it may be difficult to handle. Consolidation is thus preferred to
facilitate manipulations and also to ensure that the grain refining agent is
3 o homogeneously dispersed in the aluminum melt to be cast. For example, the
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powder can be compacted to form pellets, discs or bricks by uniaxial pressing,
hot or cold isostatic pressing, with or without a suitable binder. The powder
can
also be formed into a cored wire by wrapping the powder with a suitable foil
which is preferably made of the same metal or alloy to be cast or of an
element
s having a melting point lower than that of the metal or alloy to be cast.
The following non-limiting examples illustrate the invention.
EXAMPLE 1.
A grain refining agent was prepared by ball milling a 90%Al-
10%B powder mixture in a hardened steel crucible using SPEX 8000
to (trademark) vibratory ball mill operated at a frequency of 17 Hz. The
initial
particle size of aluminum powder was -100 mesh and that of the boron powder
was 1-Sp.m. The operation was performed under a controlled argon atmosphere
to prevent oxidization. The crucible was sealed with a rubber O-ring. The ball
milling was carried for O.Sh. The resulting grain refining agent in powder
form
i5 was uniaxially pressed and added into molten aluminum containing
0.15 wt% Ti.
EXAMPLE 2.
A grain refining agent was prepared by ball milling a 50%Ti-50%Al
powder mixture for 1 hour in a hardened steel crucible using SPEX 8000
(trademark)
z o vibratory ball mill operated at a frequency of 17 Hz. Al and Ti powders
with a
particle size of -100 mesh were chosen as the starting materials and the
operation was
performed under a controlled argon atmosphere to prevent oxidization. The
crucible
was sealed with a rubber O-ring. Two parts of the resulting powder were mixed
with
one part of the powder obtained in Example 1 and the powder mixture thus
obtained
a s was uniaxially pressed and added into a pure aluminum melt.
EXAMPLE 3.
A grain refining agent was prepared starting with the same raw
materials and with the same proportion as in Example 1. The ball milling was
performed in a ZOZ (trademark) rotary high energy ball mill operated at 1000
3 o r.p.m. The resulting grain refining agent in powder form was uniaxially
pressed
and added into molten aluminum containing 0.15 wt% Ti.
