Note: Descriptions are shown in the official language in which they were submitted.
CON-123-M
,, .
98C~
~ ACXG~OUND''OF TH~ INVENTION
' The present invention relates to the ~iltration of
molten metal.
Molten metal, par~icularly alumlnum, ln practice,
generally contains entralned and dissol~ed impurities,
both gaseous and solid, which are deleterious to the final
cast product. The impurities may originate ~rom several
sources. For example, the impurities may include metallic
impurities such as alkaline and alkaline earth metals,
and occluded hydrogen gas and dissolved surface oxide
films which have become broken up and are entrained in
the molten metal. In addition, the inclusions may originate
as insoluble impurities such as carbides, borid~s and
others or eroded furnace trough refractories. In typical
sand, permanent mold, investment and die casting fou~ries, it is
common practice to use a pot or crucible type melting
and holding furnace ranging in capacity from 300 to 1500 lbs.
of molten aluminum alloy. Depending on the type of
' operation, this furnace may be filled with the molten
alloy from a larger ~urnace or the cold alloy may be
added to the crucible furnace and melted therein. After
the furnace ls full of molten metal, it is common practice
to treat the melt by fluxing with C12, N2, mixtures
thereof, or C2C16 and an added additional grain refiner
in the' form of salts or a 5% Ti-1% B aIuminum alloy
hardener. Th~' melt is then ad~usted to the desired
temperature and pourlng of castings is allowed to begin
Pouring is usually carried out with hand carried or
manipulated ladles. The ladle is sized to hold slightly
more metal than t~at required to pour one'or more of the
3~
,~
_ 1 --
CON-123-M
31 ~3~
molds. The operator dips the ladle into the melt and
fills it, wipes any skim from the melt surface and ladle
llp and pours the molten metal into the molds. The residue
of metal left in the ladle is dumped back into the parent
melt in the furnace. Numerous pours are made in this
manner until 1/2 to 2/3 of the molten rnetal has been
cast. The repeated operation of bailing, pouring and
dumping back results in the generation and entrainment
of large quantities o~ oxide films and particulate in
the parent melt due to metal turbulence. In addition,
during the inltial stages of ~illing the molds by
pouring molten metal from the ladle into the sprue there is
considerable rnetal turbulence generated until khe gates,
runners and sprue fill with the molten metal. Here
again~ the turbulence can result in the generation and
entrainment of oxide films and particuia~e.
The foundry alloy ingot used in preparing the melts
as described above are normally cast by a primary or
secondary producer in an inline piggin~ machine. This
operation involves the free fall o~ metal into the pig
mold cavity. In addition, the melt treatment practices
generally used in the production o~ the foundry alloy
ingot are less than adeqùate~ As a result, the ingot
usually contains entrained oxide ~ilms and non-metallic
particulate.
The o~ides generated and entrained in the melt as
described above can and do often produce defects in the
resulting castings that are a cause for re~ection~ This
is particularly true in speci~ication type work ~or critical
application where the foundryman must meet speci~ic
~3~
radiographic standards.
It is naturally highly desirable to filter the
molten metal in the crucible type furnace in order to remove
or minimize impurities in the -final cast product, especially,
for example, when the resultant metal is to be used in making
highly stressed castings bearing critical specifications such
as aircraft or aerospace structural components. Impurities
as aforesaid cause loss of properties such as tensile and
fatigue strength in the final cast products and lead to a
degradation of the processing efficiency.
An improved method and apparatus for the filtering
of molten metal in a crucible type furnace is disclosed in U.S.
Patent No. 4,124,506, issued November 7, 1978, by the inventor
herein and assigned to the assignee of the present invention.
As set forth in the aforenoted United States Patent, a ladling
crucible which is provided with a ceramic foam filter is inserted
into a furnace crucible which is filled with molten metal~ The
molten metal passes through the ceramic foam filter from the
furnace crucible and into the ladling crucible cavity. me
ceramic foam filter effectively removes the oxide films and other
impurities from the molten metal as the metal passes through the
filter. Thus, the molten metal ladled from the ladling crucible
is substantially free of oxide films and particulate.
While the method and apparatus of the aforenoted
U.S. Patent eliminates oxide films and particulate from the
, .~
1~3~
molten metal which is ladled from the ladling crucible,
there is no method or apparatus presently available for
eliminating the oxide films and particulates which are
generated by the metal turbulence which occurs during the
initial stages of filling the molds by pouring the molten
metal into the sprue.
Conventional steps taken by the foundryman to
minimize the aforenoted condition has been only partially
successful. These conventional steps include designing the
sprue so that it fills rapidly and remains full during most
of the pouring operation. In addition, pouring boxes~are
often added at the top of the sprue to further minimize
turbulent flow. Furthermore, the runner and yating system is
sized so as to insure so called "choked" feeding of the casting.
As a final measure, it is common practice to use screens located
in the sprue and/or runner system to catch oxide films and
moderately constrict flow so that the onset of "choke" feeding
occurs early. These screens usually consist of thin perforated
metal plates containing a multiplicity of holes ranging from
1/32" to 3/64" in diameter or other screen type fabric made
from heat resistant FIBERGLAS (products made of or with glass
fibers or glass flakes) with similar aperture sizes. These
screens are largely ineffective and only catch the very large
oxide films and inclusions. Furthermore, the other measures
are only partially successful in minimizing the turbulence which
causes the generation of the oxide films and particulate. As
a conse~uence, rejection of castings because of entrained oxides
and inclusions is still a commonplace occurrence in these cast-
ing processes.
a, _
1.. ~ _.~
CON-123-M
~3~
Porous ceramic foam materials are known to be
particularly useful in filtering molten metal, as described
in U~S. Patent 3,893,917 for "Molten Metal Filter" by
Michael J. Pryor and Thomas J. Gray, patented July 8, 1975
and also as described in U.S~ Patent 3,962,081 for "Ceramic
Foam Filter" by ~ohn C~ Yarwood~ James E Dore and Robert
K. Preuss, both of which patents are assigned to the
assignee of the present invention.
Porous ceramic foam materials are particularly useful
for flltering molten metal for a variety of reasons
included among which are their excellent filtration
efficiency, low cost, ease of use and the ability to use
same on a dispo~able, throwaway ba3is. The fact that
these ceramic foam filters are convenient and inexpensive
to prepare and may be used on a throwaway basis allows for
the development of means for easily assembling and removing
porous molten metal filters from a crucible type melting
furnace while providing a highly efficient filtration
assembly.
Accordingly~ it is the principal object of the present
invention to provide an improved method and apparatus for
positively filtering the molten metal after pouring and
pr~or to entering the mold cavity.
It is a particular ob~ect o~ the present invention to
provide means for rapidl~ filllng the sprue ~it~ molten
metal so as to obtain the desired choke feeding throughout
the pour,
It is a further ob~ect of the present invention to
provide an i~pro~ed filter means for use in the filtration o~ molten
metal prior to the metal entering the mold cavity.
_ 5 _
CON-123-M
~ ~ 3~ 9~
It is stlll a further o~ect of the present inventlon
to pro~ide improvements as aforesaid which are
convenient and inexpensive to ut~lize and which result
in high filtration efficiency.
Further ob~ects and advantages of the present
invention will appear hereinbelow.
SU~MARY OF THE INVENTION
,
In accordance wlth the present invention, it has been
found that the foregoing obJects and advantages may be
readily obtained.
The present invention provides a highly efficient
filtration assembly which utilizes a ceramic foam ~ilter
elcment or elements located in the sprue and/or ru~ner
.
system of the ca~t mold~ The filter means consist~ of
a ceramic foam plate with interconnected voids being
surrounded by a web of ceramic. The sprue design and the
permeability 9 thickness, pore size and area of the filter
element are designed in such a manner as to provide
non-turbulent fiow downstream of the filters between the
filters and the mold. Thus, all the metal is positively
~iltered prlor to entering the mold cavity and by
maintaining the proper flow rate downstream of the filters
.. .. . ., . .... , ~ . .............. .. . . . . .
the metal flowing through the runners and gates into the
mold cavity proper proceeds in a non-tur~ulent and quiescent
manner~ In accordance ~ith the preferred em~odiment of
the present inYention, when a ceramic foam filter ~s
utilized, e~tremel~ h~gh filtration efficiencies are
oht-alned.
-
~L3~98~
According to the invention, there is provided
an impro~ement in the filtration of molten metal with a
rernovable filter plate. The method comprises providing a
casting mold comprising sprue means~ runner means and mold
casting proper means' locating at least one ceramic foam filter
element having an open cell structure characterized by a
plurality of interconnected voids surrounded by a web of ceramic
downstream of said sprue means and upstream of said mold casting
proper means; and charging said sprue means with molten metal
at a first flow rate so as to prime and force said molten metal
through said at least one filter element at a second flow rate
thereby removing entrained oxide films and particulate while
maintaining a substantially constant metallostatic head over
said at leas~ one fil-ter element whereby said molten metal passes
- 'down said runner means and into said mold casting proper means
in a ~uiescent, non-turbulent fashion thereby preventing the
generation of oxide films and particulate downstream of said at
least one filter element.
- 6a -
CON-123~M
~3~ 0
BRIEF DESCR~P~ION OF THE DRAWTNGS
Figure 1 ls a schematic illustration of a casting
system~ '
Figure 2 i5 a first embodiment of the pre~ent
invention wherein the filter.is provided beneath the
sprue.
Figure 3 i5 a second embodiment of the present
invention illustrating a modified arrangement of a filter
beneath a sprue.
.. ... . ....... , ,,, . .
Figure 4 is a third em~odiment of the present .
invention wherein a plurality of ~ilters are provided in :
the r~nners.
DETAILED DESCRIPTION
Figure 1 is a detailed description of a castlng
system emplo~ing the details of the p~esent in~ention.
Figure 1 illustrates a casting mold system includlng a
sprue 1, runner system 3, gates 4, casting proper 5 and
the riser 6. In this figure a ceramic foQm filter ,
element 2 is positioned below the sprue 1 at the entrance
of the runners 3 so as to effectively filter the molten
metal which is ladled into the sprue 1.
Referring to Figure 2 which illustrates a first
embodiment o~ the present inYentiOn, the ceramic foam
fllter element 2 is located ~eneath the sprue I on a
di~ding ~ridge 7. loca~ed in the r~nners 3.
In the pre~erred embodiment~,the filter él'ement 2 is
a cer,amic foam filter as d~scribed in aforesaid U,S, 1,
Patent~ 3,8937917 and 3,962,081, In accordance w~th the
teachlngs of the aforesald patents., the ceramic foam
-- .
CON-123-M
~L~3~
filter has an open ceIl structure characterized ~y a
plurality o~ interconnected voids surrounded by a web of
ceramic materlal. The ceramlc filter has an air
permeability in the range of from 400 to 8,ooo x lo 7 cm2,
preferably from 400 to 2~;500 x lQ 7 cm2, a porosity or
void fraction of o.8 to O.g5 and ~rom 5 to 45 pores per
linear inch, preferably from 20 to 45 pores per llnear
inch. The molten metal flow rate through the filter
should be from 5 to 50 cubic inches per square inch of
. .
filter area per minute. The ceramic foam filter described
in U.S. Patent 3,962,081 is particularly suitable in the
present invention since it is of low C08t and may be
readily employed on a throwaway basis. Furthermore, this
filter is surprisingly effective in the filtration of
molten metal, especially aluminum, at a low cost
achieving surprising filtration efficiency with considerable
flexibility.
The ceramic foam filter may be prepared in accordance
with the general procedures outlined in U.S. Patent
3,893,917 wherein an aqueous ceramic slurry is prepared
and the foam material impregnated therewith so that the web
thereof is coated therewith and the voids substantially
filled. The impregnated material is compressed so that a
portion of the slurry ls e~pelled therefrom and the
balance uniformly distrlhuted throughout the foam material~
The coated foam material is then dried and heated to first
burn out the flexible organlc foam and then sinter the
ceramic coating there~y providing a fused ceramic foam
having a plurality o~ interconnected voids surrounded ~y a
web of ~onded or fused ceramic in the configuration of the
_ 8 -
CON 123-M
~ ~ 3~
flexible foam. Naturally, a wide variety of cer~mic
materials may be chosen depending upon the particular
metal to ~e filtered. Preferably, a mixture of alumina
and chromia is employed; however~ these materials may
naturally be utilized separately or ln combination with
other ce~amic materials. Other typical ceramic materials
which may be employed include zirconia~ magnesia, titanium
dioxide, silica and ~ixtures thereof. Normally, the
. . , .
slurry contains from about 10 to 40% of water and one or
more rheological agents, binders, or air setting agents,
Since the filter element 2 of the present invention
is designed to be a throwaway item, and in permanent
molds and the like it is desirous to insert a new filter
element before each casting, it is essential to provide
.an effective means for sealing the filter element 2 ln
plac~ in the bottom of the sprue 1 and runners 3. lt is
preferred to seal the filter plate in place using a
resilient seal means or gasket type seal as illustrated
in Figure 1 which peripherally circumscribes the filter
element at the bevelled portion thereof, ~he gasket
type seal must be of a material that is resistant to
molten aluminum alloys and resillent at elevated temperatures.
Resiliency is required to make a metal tight seal between
the filter element 2 and the ~rue l and hold the filter 4
element aecurely in place, Typical seal materials include
fibrous refractory type seals of a variety of compositions,
Some typical seal compositions are, but not limited to:
(1~ a seal containing ahout 45% alumina, 52% silica, 1,3%
ferric oxide and 1.7% titania; ~2) a seal containing a~out
55% silica, 40.5% alumina3 4% chromla and O,5% ferric
_ g _ .
g ~3L3~38V
oxide; and (3) a seal containing a~out 53% silica, 46%
alumina and 1% ferric oxide.
Referring to Figure 1, an operation at the start of
casting, molten metal ls poured into t:he sprue 1. The
molten metal bullds up in the sprue until a metallostatic
head is developed over the filter element 2 sufficient
enough to prime and force the liquid metal through the
filter. This usually requires a metaliostatic head from about
4 to 6" depending on the permeability, thickness, pore
.. .. . . ..
size and area of the filter elèment. Once the filter
element 2 is pr~med, llquid metal flows down the runners -
3 and through the gates 4 into the mold cavity proper 5
and riser 6 in a quiescent, non-turbulent fashion. The
llquid metal is continuously poured into the sprue at
that rate required to .keep the sprue f~ll until.the mold
cavity is filled. Upon filling the mold cavity the pouring
of the molten metal is terminated.
The inherent advantages of the present invention
offer significant improvements over other methods hereto-
rore known. The use of a ceramic foam filter in the method
described aforesaid allows for the molten metal to be
filtered in a positive fashion prior to the entry Or the
molten metal into the mold caYity proper, The liltration
is accomplished wlth a ceramic foam filter mater-lal which
is e~tremely efficient in remoYing the non-metallic
particulate present in aluminum alloy melts Thls
effective filtration is in contrast to the presently ~:;
used metal plates or FIsERGLAS type screens which are
planar in nature, contain large apertures and there~ore
3o remove only very large oxide films. In additionl since the
-- 10 -- .
CON-123-M
~ ~ 3~
ceramic foam fllter employed in the present in~entlon
requires considerable head to prime the filter, the sprue
fills rapidly at the start of pouring. By maintalnlng
the proper pouring rate, the sprue stays full of molten
metal throughout pouring. The ceramic ~oam ~ilter element
dissipates the veloclty head of the molten metal flo~ing
down the sprue and thus provides for a non-turbulent and
~uiescent flow of molten metal through the runners, gates
and into the mold cavity proper and risers. Thus, by
maintaining the proper molten metal pouring into the spruea
the non-turbulent flow downstream of the filter element
prevents the further generation and entrainment of oxide
films and particulate both before and after the filtration
step. Thus, from the above, it is clear that the present
in~ention will eliminate substantially all those conditions
in the mold that lead to the rejection of cast products
because of non-metallic inclusions.
In the preferred embodiment of the present lnvention,
the thickness of the ceramic foam filter element should
be in the range of about 1/4" to 1-1/2" and preferably
from 3/8" to 3/4". It should be noted that one or more
filter elements may be used in a single mold. In large mclds with one
or more sp~ues in a c ~ lex runner and gating system, a multiplicity
of filter elements may be employed. To insure sufficie~ priming
of the'filter el'ement at t~e start of pouring,the height
of the'sprue a~o~e the filter element should ~e ~rom'6" to
12" for most applications~ The flow area of the f'ilter
element must ~e sufficient to allo~ for filling of the
mold ca~ity and risers at the proper rate yet ~all ~ithin
the preYiously descri~ed flow rate'of 5 to 5Q cu~c inches
-- 11 --
CON-123-M
~3~
of llquid per square inch of filter area per minute,
Figures 3 and 4 illustrate alternate em~odiments
~f the present invention. The embodiment of Figure 3 is
substantially the same as that illustrated in Figure 2
except that the dividing bridge 7 shown in Figure 2 has
been eliminated. The elimination of the bridge allows for
unrestricted flow through the total surface area o~ the
filter element thereby inc~easing the effective surface ''
area of the filt0r while eliminatlng an~ turbulence which
may occur by the flow of the molten metal around the
bridge 7. Figure 4 illustrates a third embodiment of
the present invention in which a plurality of filte~
elements are positioned in the runners 3 as opposed to
being 'located directly under ~he sprue which is shown ln
the embodiments of Figures 2 and 3.
It is obvio~s that the present invention can be
employed in a number of con~igurations other than those
shown here. It is evident that the invention can be used
in a variety of casting processes including sand, permanent
mold, investment, low pressure permanent mold, die~ and
the like. It should further be noted that the invention
need not be limited to the casting of aluminum and its
alloys but may be used in the castlng of copper and other -
non-ferrous alloys as well as ferrous alloys~ It should
further be noted ~hat in sand type molds the filter element
would be directly inserted in elther the cope or drag
when the mold is assembled ? in the fashion illustrated in
Figures' 2-4~ In such`sand type molds a direct seal ~s
obt'ained between the sand and thb filter eIement and thus
no gasket type seal is required,
CON-123-M
~3~
.
It is to be understood that the invention is not
limited to the illustrations descr~bed and shown hereln,
which are deemed to be merely illustrative of the ~est
mode of carrying out the invention, and whlch are .
susceptl~le of modiflcation of form, size, arrangement of
parts and details of operation. The invention rather is
intended to encompass all such modifications wh~ch are
within its spirit and scope as defined by the claims~
.. . .
.
- 13 -
