Note: Descriptions are shown in the official language in which they were submitted.
PATENT APPLICATION
MANUFACTURE OF ALLOYS CONTAINING DISPERSED FINE
PARTICULATE MATERIAL
The present application is directed to a novel method of preparing metal
alloys
containing finely divided particulate metal or intermetallic compounds as a
substantially
insoluble second phase. A particular embodiment is directed to manufacture of
aluminum
base master alloys containing titanium together with boron or boron alone.
BACKGROUND TO THE INVENTION
Products containing dispersions of fine particulate non-metallic material in
metals
and alloys, commonly known as metal matrix composites, are well known in
metallurgical engineering. Examples are dispersions of metal oxides, nitrides,
carbides
and the like in a matrix of aluminum or alloys thereof. Commercial use of such
composites was delayed for many years by the need to develop consistent and
reliable
means of manufacture of sound defect-free components. Mass production of such
components required melting and casting methods for conventional metal alloys
to be
adapted to metal matrix composites. This required complete wetting of the
particle
surfaces by the molten matrix metal which, in the early stages of this
technology, was
difficult to achieve.
A number of different methods of achieving such wetting have been proposed. An
early proposal by the International Nickel Company was to coat the particulate
with
metallic nickel by exposure to gaseous nickel carbonyl at an appropriate
temperature.
Metallic zinc and lithium and magnesium oxide have also been proposed as
coating
materials.
A more successful approach has been the use of high shear stirnng of molten
metal containing solid non-metallic particulate using specially designed power-
driven
rotor systems to develop high relative velocity between particulate dispersate
and molten
metal. Such methods have been disclosed by Klier et al in U.S patent
4,961,3461 and
Skibo et al in U.S patent 4,786,467. Klier et al disclose an upright tapered
cylindrical
vessel containing a centrally located high speed rotor into which the molten
matrix metal
containing the particulate dispersate is introduced. The rotor shaft extends
below said
rotor to the bottom of the vessel in which a second rotor is positioned. The
second rotor is
conical in shape and positioned inside a fixed tapered wall. The narrow gap
between the
wall and the rotor serves as a zone of high shear and can be varied at will by
raising and
lowering the rotor. Skibo et al disclose a circular section vessel into which
a body of the
molten matrix metal is introduced and into which the required particulate
dispersate is
subsequently fed. The vessel contains a specially designed centrally located
dispersing
impeller to effect high shear stirring. This is optionally augmented by a
sweeping
impeller located at the periphery of the vessel to sweep the particulate to
within the orbit
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of the dispersing impeller. This operation is carried out under selected
temperature and
time conditions to promote wetting by the molten metal of the exposed
particulate
surfaces.
A conventional method for manufacturing metal alloys with which the present
invention concerned is to establish a stirred body of the molten metal into
which is
introduced a chemical compound of halides or double halides of the desired
metallic
additives) in powder form. Stirring is typically effected by electric
induction. The
compound reacts with the molten metal to form the desired metallic additive
which on
solidification and casting is present in particulate form within the cast
product matrix.
Examples of metal alloys of the present invention are aluminum base master
alloys
containing titanium and boron. These are widely used in aluminum and aluminum
alloy
manufacture by addition to molten metal prior to casting to effect refinement
of the as-
cast grain size. The titanium and boron in such master alloys are typically
present as
insoluble particles of titanium diboride, TiBsub2, suspended in the melt
whilst titanium in
excess of the stoichiometric proportions for formation of TiBsub2 (2.2:1 ) is
in solution in
the molten metal. Such titanium precipitates as the aluminide, TiAlsub3, on
cooling and
solidification. A typical and commonly used formulation for such a master
alloy is
aluminum 94%-titanium 5%- boron 1%.
A conventional method for manufacture of such master alloys is by addition to
a
stirred body of unalloyed aluminum of a mixture of potassium titanium
fluoride,
Ksub2AlFsub6 and potassium borofluoride, KBFsub4 as powders premixed in the
appropriate proportions to obtain the desired master alloy formulation.
Optionally, a
portion of the titanium in excess of the aforesaid stoichiometric requirements
can be
added as solid unalloyed titanium metal or as an aluminum titanium master
alloy (e.g.
containing 20 % titanium). Such additions are made after completion of the
mixed
powder addition. The entire process is typically earned out in a low or medium
frequency
electric induction furnace, the electric power providing both heat and
continuous stirring.
Such stirring is essential not only to effect uniform dispersion of the
alloying ingredients
but also to minimise gravity segregation of the titanium diboride particles
within the melt.
The final stage of the process is casting the master alloy in an appropriate
form for the
desired final product, e.g. an ingot for remelting or further working directly
to rod on a
continuous casting machine.
It is desirable that casting take place as soon as possible after the alloying
process
to minimise both gravity segregation and agglomeration of the titanium
diboride
particles. The particles serve to effect grain refinement when the master
alloy is
introduced into molten aluminum or alloys thereof. For this to be efficiently
carried out, it
is essential that the size of the titanium diboride particles in the master
alloy be strictly
controlled within given limits. For instance, a master alloy user may require
the particle
size to be within the range of 1 /4 to 3 microns. Moreover, any coarse
particles or
agglomerates may function as hard inclusions and impair the mechanical
properties of the
final product.
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Aluminum-base master alloys containing boron alone are widely used in the
aluminum industry for aluminum conductor alloys requiring high electrical
conductivity.
Such conductivity may be impaired by small quantities of the transition
metals, titanium,
chromium and vanadium commonly present in solution in commercial aluminum,
rendering the product unsuitable for its intended use. Boron will combine with
the
aforesaid impurities to precipitate them as the borides thereof, in which form
they have
little or no effect on conductivity.
In the conventional manufacturing process of the master alloy, boron is
introduced to molten aluminum in the form of potassium borofluoride powder in
a
process analagous to that described above for aluminum titanium boron master
alloy
manufacture. Boron combines with the aluminum to form the diboride, AlBsub2,
or the
dodecaboride, AlBsub 12, depending on boron content and production conditions.
Commercial master alloys typically contain either 3 or 4% boron. Particle size
requirements are less stringent than for Al-Ti-B alloys. However, the borides
must be fine
enough to react completely with the transition metals during the production
time period
available between addition of the master alloy and casting of treated product.
This can be
particularly critical when the master alloy is fed in rod form to the casting
trough of the
treated product.
The above-described manufacturing methods for both Al-Ti-B and Al-B master
alloys surer many serious disadvantages. Firstly, use of the double fluorides
of both
titanium and boron entail severe environmental problems. The reaction with
molten
aluminum generates volatile fluorine-containing gases which must be contained
and
disposed of. This requires elaborate emission control installations which are
expensive to
install and maintain and liable to malfunction. The reaction products captured
by the
installation must also be disposed of in an environmentally acceptable manner.
Volumes
of molten fluoride-containing stags are also generated. Whilst these may have
a
commercial value, elaborate equipment is required for their separation from
the molten
alloy product and in either disposal or processing and packaging. 'These
processes
increase labour requirements.
Particle size control, particularly in Al-Ti-B master alloy production
requires
close control of temperature and time conditions, not always successfully
achieved in an
industrial environment. Stringent microscopic examination may be required
before
packaging and shipment to ensure compliance with the user's specification.
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SUMMARY OF THE INVENTION
The present invention is based on the realisation that the various methods and
apparatus for high shear stirring originally developed and used for
manufacture of metal
matrix composites can also advantageously be applied to manufacture of certain
types of
metal alloys. The invention provides a method of preparing a dispersion of
particles of a
first metallic material comprising one or more refractory hard metals in a
matrix of a
second metallic material which comprises the metal or alloy matrix. The term
"refractory
hard metals" as used herein is based on the characterisation provided in
Schwarzkopf,
Paul et al, Refractor3r Hard Metals, New York, Macmillan, 1953, Chapter I and
includes
carbides, nitrides, borides and silicides of the transition elements of the
fourth to sixth
groups of the periodic table. The present invention also applies to bonides of
aluminum.
According to one aspect of the invention, a molten body of the second metallic
material is
established into which solid particles of the first metallic material are
introduced. During
this step, the molten body is stirred at a rate sufficient to establish
relative shearing of the
particle surfaces such that they become wetted by the molten metal. The
resultant alloy is
then cast by any desired method. The particulate first metallic material must
have a
higher melting point than the second metallic material and be substantially
insoluble
therein.
According to another aspect of the invention, a molten body of the second
metallic material is established into which are introduced solid particles of
a metalloid
which chemically reacts with the second metallic material to form particles of
said first
metallic material. The term "metalloid" is used here as defined in the McGraw-
Hill
Encyclopedia of Science and Technology, 8'~ Edition, Volume 11, 1997, page 67,
namely
an element which exhibits the external characteristics of a metal but behaves
chemically
both as a metal and a non-metal. During this process, the molten body is
stirred at a rate
sufficient to establish shearing of the metalloid particle surfaces such that
they become
wetted by the molten metal thereby facilitating the reaction. The resultant
alloy is then
cast by any desired method The particulate first metallic material must have a
higher
melting point than the second metallic material and be substantially insoluble
therein..
In both the above aspects of the invention, it is preferred that the molten
body be
established in an upright cylindrical vessel heated by any desired means.
Where fuel-fired
heating is used, however, the combustion products are preferably excluded from
contact
with the molten metal to avoid contamination or gas pick-up. Stirring of the
molten metal
body may be effected by a one or more power driven rotating impellers which
serve to
distribute the added particulate material and to effect shearing of the
particulate surfaces
and wetting thereof by the molten metal.
One embodiment of the present invention is directed to the manufacture of
aluminum-base master alloys containing titanium and boron in which the boron
is
combined as the intermetallic compound TiBsub2. The method comprises
establishing a
molten body of unalloyed aluminum, into which particulate titanium diboride is
introduced whilst effecting high speed stirring of said body. Shearing action
of the molten
aluminum on the titanium diboride particle surfaces is thereby effected which,
as a result,
become wetted by the aluminum. Titanium in excess of the stoichiometric
requirements
CA 02362023 2001-11-08
for TiBsub2 formation is also added in metallic form, but there is no
requirement for this
to be as powder. Such titanium, for example, may be added as solid ingot or
the like or as
an aluminum-titanium master alloy.
Another embodiment of the invention is directed to the manufacture of aluminum-
base master alloys containing boron present as one or both borides of
aluminum,
AlBsub2 and AlBsubl2. In one aspect of this embodiment, a molten body of
unalloyed
aluminum is established into which particulate aluminum boride is introduced
in the same
way as titanium diboride as heretofore. In yet a further aspect, elemental
boron is
introduced. Boron is classified as a metalloid in the McGraw-Hill Encyclopedia
of
Science and Technology, 8~'. Edition, Volume 3, 1997, page 11. The molten body
is
stirred a using a high speed rotor to effect shearing and wetting of the boron
particle
surfaces. A chemical reaction of the boron with the aluminum is thereby
promoted to
form one or both borides of aluminum.
Both titanium diboride and elemental boron are available on the market in
particulate form. Whilst most commercial material is micron sized, sub-micron
materaial
is also available. The previously mentioned disadvantages of the conventional
methods of
manufacture are substantially overcome by the present invention. Pollution
problems
associated with use of fluoride materials are eliminated resulting in major
cost savings in
emission control equipment. Particle size of the metallic alloying materials
can be more
closely controlled and finer average second phase particle sizes are feasible.
No major
quantities of slag are generated other than the dross normally generated by
melting
aluminum. In preferred embodiments of the invention, the foregoing processes
can be
operated either under vacuum or controlled inert gas atmosphere such as argon.
These
measures significantly improve alloy cleanliness.
The advantages of the invention and further embodiments thereof will now be
readily apparent to a person skilled in the art, the scope of the invention
being defined in
the appended claims..
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