Filtration of molten material

An apparatus for filtering molten material, such as a molten metal-ceramic particle mixture, includes a porous cloth filter located so that the mixture must pass through the cloth filter, and a mechanical filter shaker that prevents the accumulation of filtered solids on the porous cloth filter. Where a further degree of filtration is required, there is a second filter located so that material leaving the porous cloth filter passes through the second filter after it passes through the porous cloth filter, and a mechanism that prevents an accumulation of filtered solids on the second filter. The second filter is desirably a porous media filter.

BACKGROUND OF THE INVENTION 
This invention relates to metallurgical processing, and, more particularly, 
to the filtration of molten metal and composite materials to remove solid 
material therefrom. 
According to one approach, cast composite materials may be formed by 
melting a metallic matrix alloy in a furnace and adding particulate matter 
to the molten metal. The mixture is vigorously mixed to encourage wetting 
of the matrix alloy to the particles, and after a suitable mixing time the 
mixture is cast into molds or forms. The mixing is conducted while 
minimizing the introduction of gas into the mixture. The resulting 
composite materials have the particulate reinforcement distributed 
throughout a matrix of an alloy composition. 
Such cast composite materials are much less expensive to prepare than many 
other types of metal-matrix composite materials such as those produced by 
powder metallurgical technology and infiltration techniques. Composite 
materials produced by this approach, as described in U.S. Pat. Nos. 
4,759,995, 4,786,467, and 5,028,392, have enjoyed commercial success in 
only a few years after their first introduction. 
There are two types of solid matter that may be present in the composite 
material. A desirable particulate is the ceramic material intentionally 
added to the melt. This material is usually a carefully selected and sized 
ceramic. Typical types of ceramics are aluminum oxide and silicon carbide, 
and typical particle sizes are in the range of from about 5 up to about 35 
micrometers. An undesirable solid matter is an uncontrolled material that 
finds its way into the melt during the production operation. The 
undesirable solid matter may include, for example, pieces of the ceramic 
furnace lining that have broken off during mixing, pieces of impellers 
that have broken off during mixing, pieces of molten-metal furnace troughs 
that have broken off into the flow metal, pieces of oxide films that have 
formed on the melt surface and been enfolded into the melt during mixing, 
and pieces of reaction products between the desirable particulate and the 
melt that have become free floating in the melt, such as aluminum 
carbides. 
The undesirable solid matter is generally larger in size than the desirable 
particulate reinforcement, and may typically be on the order of 200 
micrometers or more in maximum dimension (i.e., about 10 times the size of 
the desirable particulates). If left in the melt, the undesirable solid 
matter is frozen into the composite material when it solidifies. The 
undesirable solid matter becomes inclusions that can adversely affect the 
mechanical properties of the final composite material. 
A similar problem is encountered in the more-conventional metallurgical 
industry that does not deal with composite materials. It has long been the 
practice to filter undesirable solid matter from melts of non-composite 
alloys that are to be used in sensitive applications. Different types of 
filters are used, depending upon the metal to be filtered and the 
cleanliness requirements of the product. 
In aluminum alloy melting practice the molten alloy may be passed through a 
glass-fiber sock filter having an open weave so that there are openings of 
a predefined size in the filter. The solid matter is trapped at the 
surface of the filter. The filter openings are typically on the order of 
400 micrometers or more in size, and are selected according to the 
cleanliness requirement of the final product and production 
considerations. Smaller openings remove smaller particles, resulting in a 
cleaner final product. On the other hand, the smaller the openings, the 
greater the flow resistance offered by the filter and the slower the 
filtration process. The filter may actually remove particles smaller than 
the filter mesh size due to the buildup of a filter cake. The filter size 
opening is usually selected to be a compromise between the requirements of 
metallurgical cleanliness and production efficiency. 
Another type of filter used in the aluminum industry for filtering 
conventional (non-composite) alloys is the porous media filter. The porous 
media filter is a block of a material such as a ceramic that has a 
controlled open-cell porosity therethrough. Pieces of undesirable solid 
material are trapped within the volume of the filter as the molten alloy 
is passed through the filter. 
In the course of the work leading to the present invention, conventional 
glass-fiber sock filters and porous media filters were used to filter 
molten mixtures of an aluminum alloy and 10-20 volume percent of desirable 
particulate such as alumina or silicon carbide, of a size distribution of 
about 5-20 micrometers. Coarse undesirable solid matter was mixed in to 
the melt. The conventional filtering practice could be used on a 
laboratory scale. However, it did not produce successful commercial-scale 
heats of the composite material. Variations of filter opening size were 
also tried, unsuccessfully. In short, conventional aluminum-alloy 
filtering practice was not operable with aluminum-based cast composite 
materials on a commercial scale. 
There is therefore a need for an improved filtering technology for removing 
undesirable large solid pieces from composite material melts, while not 
affecting the distribution of smaller particles in the melt and the final 
product. The present invention fulfills this need, and further provides 
related advantages. 
SUMMARY OF THE INVENTION 
The present invention provides an apparatus and method of particular value 
in filtering melts of composite materials on a commercial scale, but which 
can also be used for filtering non-composite materials. The filtering 
approach removes large-size, undesirable solid pieces, but does not change 
the amount or distribution of the smaller, desirable particulates in the 
composite material. Metal flows through the filter at the full rate, 
throughout the entire course of the filtering of a commercial-scale heat. 
There is no plugging of the filter. The apparatus and method are readily 
implemented in commercial operations, without changing the basic metal 
melting, distribution, and casting equipment. 
In accordance with the invention, an apparatus for filtering molten 
material comprises a molten material trough, a porous cloth filter located 
so that material flowing in the trough must pass through the filter, and 
means for preventing an accumulation of solids on the filter as material 
flows through the trough and the filter. In another embodiment, an 
apparatus for filtering molten material comprises a molten material 
trough, a porous media filter located so that material flowing in the 
trough must pass through the filter, and means for preventing an 
accumulation of solids on the porous media filter as material flows 
through the trough and the filter. 
Common to these two embodiments is some means for preventing an 
accumulation of filtered solids on the surface of the filter. (As used 
herein, "filtered" solids are those solids that have not passed through a 
filter, but remain upstream of the filter or on the surface of the 
filter.) Where solids, here the undesired solid matter that is removed by 
the filter, are permitted to accumulate on the filter surface as a filter 
cake, that accumulation can quickly block the filter and prevent further 
flow of the metal through the filter. Thus, the filter plugs and 
production stops. 
Conventional filters seem to work for small, laboratory scale filtering 
requirements, but are not acceptable for commercial scale work because the 
buildup of filter cake gradually reduces the metal flow rate and leads to 
plugging. Under the present approach, a buildup of solids is prevented, so 
that the filter is operable to remove large, but not small, solid pieces 
throughout the filtering operation, and plugging is avoided. 
The prevention of an accumulation of solids is to be distinguished from the 
common approach of permitting filtered solids to accumulate and to remove 
them periodically. This removal is not easily done for molten metal 
filtration, but in any event the present approach does not permit an 
accumulation of solids. 
Two techniques have been developed to prevent the accumulation of filtered 
solids on the surfaces of the filters. In one, of most interest for 
flexible porous cloth filters such as glass fiber sock filters, the filter 
is continuously shaken during the filtration operation. The shaking is 
preferably at a rate of about 0.1 to about 10 cycles per second, and with 
an amplitude of about 1/2 to 4 inches. In the other approach, of most 
interest for rigid filters such as porous media filters, an impeller is 
operated on the upstream side of the filter to stir and agitate filtered 
solids as they are removed from the metal. The filtered solids are 
retained in suspension upstream of the filter, so that they cannot settle 
on the filter and plug it. The filter is desirably oriented at an angle to 
the horizontal so that the solids cannot settle back onto the surface of 
the filter and instead gradually fall to a trap below the filter. 
The single filters of the invention are operable to remove a fraction of 
the undesirable solid matter. To achieve a higher degree of cleanliness, 
two filters may be placed in a serial relation so that the molten material 
passes through each in turn. The first filter is sized to remove 
larger-size undesirable solid pieces, and the second filter is sized to 
remove smaller-sized undesirable solid pieces. Selection of the filter 
types depends upon factors such as the composition of the molten material. 
The present invention has been demonstrated to provide good filtration for 
a variety of alloy types and cleanliness requirements. The filtration is 
achieved over long production filtering runs, which was not possible with 
the conventional filters. The final composite material product has a 
reinforcing particulate size, size distribution, and volume fraction 
substantially identical to the melted material in the furnace, but is 
freed of larger-sized, undesirable solid pieces such as broken furnace 
linings, surface oxides, and slag, for example. Filtration is achieved at 
acceptable commercial production rates. 
The present invention therefore provides an important advance in the art of 
cast composite materials. High-quality, clean composite material is 
prepared by filtration in acceptable production quantities and rates. 
Other features and advantages of the present invention will be apparent 
from the following more detailed description of the preferred embodiment, 
taken in conjunction with the accompanying drawings, which illustrate, by 
way of example, the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 schematically depicts a melting and casting operation 20. A mixture 
22 of desirable particulate and molten metallic alloy is prepared in a 
crucible 24. Any operable preparation and mixing procedures may be used. 
The preferred approach is as described in U.S. Pat. Nos. 4,759,995, 
4,786,467, and 5,028,392, whose disclosures are incorporated by reference. 
When the mixture 22 is prepared, the crucible 24 is tilted and the 
flowable mixture is poured into a trough 26. The mixture flows along the 
trough, through one or more filters in a filtering zone 28, to be 
discussed in more detail subsequently, and into a mold 30. The molten 
metallic alloy solidifies in the mold 30, producing a cast composite 
material. The trough 26 is depicted as relatively short, but in commercial 
practice may be quite long and split into multiple troughs in order to 
convey the mixture to multiple molds 30. The metal may also be conveyed to 
other casting devices, such as a continuous caster. The present invention 
is concerned with the filtration of the composite material, and not with 
the details of mixing or solidification. 
FIG. 2 is a drawing of the microstructure of a composite material that has 
not been filtered. The microstructure includes a matrix 40 and desirable 
small reinforcing particulate 42 distributed throughout the matrix 40. In 
this example, the matrix 40 is an aluminum-based alloy and the desirable 
particulate 42 is nearly spherical particles of aluminum oxide, silicon 
carbide, or other ceramic material of a size of 5-35 micrometers. 
Also found in the matrix 40 are large, irregular undesirable pieces of 
solid matter 44 and 46. These pieces are typically much larger than the 
desirable particulate 42, and often 10-100 times as large or more. The 
undesirable solid pieces 44 can be of many types. The solid pieces can 
include, for example, oxide stringers 44 that formed on the surface of the 
melt in the crucible 24 and were enfolded into the melt during mixing or 
pouring. The solid matter may also include pieces of the refractory lining 
46 of the crucible 24 or the trough 26 that break off during, mixing in 
the crucible or flow of the composite through the trough. Other types of 
undesirable solid pieces can also be present, and these two types are 
illustrated as exemplary. 
It is to be understood that the amount of undesirable solid matter is not 
as great as suggested by FIG. 2, and that this drawing shows the solid 
matter in greater fraction than is conventional for the sake of 
illustration. However, even small amounts of the undesirable solid 
material can have highly adverse effects on final product properties far 
out of proportion to the amount present in the structure. The undesirable 
solids can cause premature cracking of the composite material during 
solidification or in service, and only a single premature crack can lead 
to failure of the composite material. 
The undesirable solid matter is selectively removed from the matrix, 
leaving the desirable particulate 42 distributed throughout the matrix, by 
filtration in the filtration zone 28. FIG. 3 illustrates two preferred 
types of filters, here operated serially so that the mixture 22 first 
passes through one filter and then the other. The filters may also be 
operated singly, if preferred. The serial filtration produces a cleaner 
final composite product, with the production flow-through rate determined 
by the slower-flowing of the filters. In many applications, the use of a 
single filter is sufficient to provide the required degree of cleanliness. 
(As used herein, "cleanliness" of the composite is synonymous with the 
degree of absence of undesirable solids such as the particles 44 and 46.) 
Referring to FIG. 3, the molten flowable mixture 22 is supplied from the 
melting-and-mixing crucible 24, which is out of view to the left of the 
drawing. The unfiltered mixture flows through the trough 26 and thence 
into and out of the filtering zone 28. After lzone 28. After leaving the 
filtering zone 28, the filtered mixture flows to the mold 30, which is out 
of view to the right of the drawing, for solidification. 
A first filter 50 is formed of a porous cloth such as porous glass cloth, 
preferably shaped as a sock filter as shown. Porous glass cloth is widely 
used as a filter material in the aluminum industry, and is available 
commercially in a wide range of types and pore opening sizes. That is, the 
porous cloth can be ordered and purchased with a specified pore size, such 
as 400 micrometer, 500 micrometer, etc. size pores. Alternatively, the 
porous cloth can be purchased by specifying the number of openings per 
inch. In the present discussion, the glass cloth will be discussed in 
terms of pore size, and that is most easily compared with particle sizes. 
A useful porous glass filter for filtering molten aluminum-alloy composite 
material having about 5-35 volume percent reinforcement particles of size 
5-35 micrometers has a pore size of about 0.3-1.0 millimeters. 
In accordance with the present invention, there is provided means for 
preventing an accumulation of solids on an upstream side 52 of the porous 
cloth filter 50. In the preferred approach, the means for preventing is a 
mechanical vibrator or shaker 54 attached to the portion of the filter 50 
that extends above the surface of the flowing mixture 22. The shaker 54 
includes a motor and a mechanical linkage that causes the filter 50 to 
move back and forth relatively rapidly. The movement prevents undesirable 
solid matter from affixing itself to the upstream side 52 of the filter 
50. Instead, large particles such as the refractory lining particles 46 
that cannot pass through the porous cloth filter 50 remain suspended in 
the metal on the upstream side of the filter 50. 
As a result of the vibration, a filtering region 55 of the filter 50 
remains unclogged with filter cake or any other accumulation of separated 
solids. Thus, even after extending filtering as required in a commercial 
operation, the filtering region 55 responds as though the filtering 
operation has just commenced. The effective pore size of the filter 50 
does not decrease and the filter does not become blocked, inasmuch as 
filtered solids remain in suspension on the upstream side 52 of the 
filtering region 55. The solids do not plug the filter 50, which would 
otherwise be the case in the conventional approach wherein the filtered 
solids are allowed to accumulate on the filter. 
The important result of this use of a means for preventing an accumulation 
is that the flowthrough rate of the filter 50 does not decrease with 
increasing filtration time, and the filter does not become blocked with a 
filter cake. When solids are allowed to accumulate on the upstream side 52 
of the filter 50 as in the conventional practice, this filter cake can 
slow the flowthrough rate and soon block the filter entirely. 
Extensive experimentation has determined preferred amplitudes and 
frequencies for the shaking of the filter 50. The amplitude of vibration 
is preferably from about 1/2 to about 4 inches. Too small a vibration is 
unsuccessful in preventing accumulation of filtered solids, while too 
large a vibration can disrupt the flow of mixture 22 in the trough 26 and 
introduce gas into the mixture 22. The frequency of vibration is 
preferably from about 0.1 to about 10 cycles per second. Slower 
frequencies are unsuccessful in preventing the accumulation of solids, 
while higher frequencies can damage the filter, disrupt the mixture flow, 
and require overly large equipment. Lower frequencies are preferred for 
large opening sizes of the porous cloth, while higher frequencies are 
preferred for small opening sizes. 
FIG. 3 also illustrates a second filter 60, which in this case is a rigid 
porous media filter. Such filters are used commercially in the aluminum 
industry to filter molten materials. They are available in a range of 
porosity sizes and materials of construction. In most instances, the 
porous media filters are made of ceramics such as phosphate-bonded 
alumina. 
The porous media filter, sometimes known as a ceramic foam filter when made 
of ceramic, achieves filtration by a different filtration mechanism than 
the porous glass filter. The porous glass filter is essentially a sieve, 
while the porous media filter is a depth filter. The porous media filter 
permits material to enter the interior of the filter and pass through a 
tortuous porosity path. Undesirable solid matter is trapped within the 
interior of the filter, and the filter is thrown away after use. The 
porous media filter is particularly effective in capturing and removing 
elongated undesirable solid matter that otherwise typically slips through 
a porous cloth filter, such as the oxide stringers 44 of FIG. 2. The 
porous media filter usually has a maximum preferred metal flow rate, 
typically about 1 pound of aluminum alloy per minute per square inch of 
filter area. If there is an attempt to impose higher flow rates through 
the filter, entrapped solid matter may be forced through the filter and 
into the casting. 
Although the porous media filter achieves filtration by a different 
mechanism than the porous cloth filter, in conventional practice large 
solid pieces in the mixture that has passed through the first filter 50 
may accumulate on an upstream surface 62 of the filter 60. With an 
increasing amount of total metal flow through the filter as required in 
filtering commercial-size heats, the filter flowthrough rate falls and the 
filter becomes partially or totally blocked, much in the same manner as 
discussed for accumulations of solids on the porous cloth filter. 
To avoid this effect, means for preventing an accumulation of solids on the 
upstream side 62 of the filter 60 is provided. To prevent the accumulation 
of solids on the upstream side 62 of the filter 60, an impeller 64 turning 
on a shaft 66 is positioned Just above the upstream side 62. The impeller 
64 turns at a rate sufficiently high to prevent solids which have not 
passed into the filter 60 from settling onto the surface of the filter 60. 
The rate should not be so high as to create a vortex or enfold gas into 
the mixture 22, however. In practice, a rate of about 150 revolutions per 
minute has been found satisfactory. The impeller should not be close to 
contact with the filter surface, but is preferably about 1-2 inches from 
the surface of the filter. If the impeller is too close, it may tend to 
force filtered solids into the filter rather than maintain them in 
suspension. If the impeller is too far from the surface of the filter, it 
will be ineffective in maintaining the filtered solids in suspension 
upstream of the filter. 
The filter 60 is preferably oriented at an angle to the horizontal, as 
shown in FIG. 3. In the illustration, the filter 60 is angled upwardly by 
about 15 degrees from the horizontal, but it could be more if desired. The 
upward angle of the filter 60 has two beneficial effects. Bubbles on the 
downstream side of the filter 60 are able to float upwardly and escape to 
the surface of the molten mixture. Also, solids on the upstream side 62 
gradually settle toward the lower end of the filter to a collection region 
68. In this location, the solids upstream of the filter are not repeatedly 
forced into the filter 60, and can be cleaned out when the casting run is 
complete and the used filter 60 is replaced with a new filter in 
preparation for the next run. 
After passing through the filter 60, the flowable mixture flows along the 
remainder of the trough 26 to the casting station and into the mold. 
The resulting structure of the cast composite material is similar to that 
depicted in FIG. 4. The microstructure has only matrix 40 and the 
desirable particulate 42. The undesirable solid pieces in the form of 
stringers, refractory lining, and other types of solids are removed in the 
filter or filters. 
Although a particular embodiment of the invention has been described in 
detail for purposes of illustration, various modifications may be made 
without departing from the spirit and scope of the invention. Accordingly, 
the invention is not to be limited except as by the appended claims.