CA1130980A

CA1130980A - Method for the filtration of molten metal

Info

Publication number: CA1130980A
Application number: CA314,841A
Authority: CA
Inventors: James E. Dore
Original assignee: Schweizerische Aluminium AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1977-11-25
Filing date: 1978-10-30
Publication date: 1982-09-07
Also published as: JPS5484803A; DE2848005A1; SE7812079L; NL7811581A; BE872280A; FR2409785A1; IT7830184A0; GB2008427B; IT1100591B; DK508478A; GB2008427A; ES475339A1; NO783965L

Abstract

ABSTRACT OF THE DISCLOSURE

An improved method for the filtration of molten metal is used in the productions of castings is disclosed in which a filter element is disposed upstream of a casting mold cavity so as to effectively filter molten metal which passes through the filter element while providing non-turbulent flow of the molten metal downstream of the filter element.

Description

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~ 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~
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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 , .~

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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.

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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.

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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.

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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.

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~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.
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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 -- .

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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 -~ ~ 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 _ .

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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 -- .

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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 ~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, ~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~

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the filtration of molten metal with a removable filter plate, the method which comprises:
providing a casting mold comprising spruce 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 least one filter element whereby said molten metal passes down said runner means and into said mold casting proper means in a quiescent, non-turbulent fashion thereby preventing the generation of oxide films and particulate downstream of said at least one filter element.

2. The method of claim 1 wherein said second flow rate is from about 5 to 50 cubic inches per square inch of filter area per minute.

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3. The method of claim 1 wherein said at least one filter element has an air permeability in the range of 400 to 8,000 x 10-7 cm2, a porosity of 0.80 to 0.95, a pore size of 5 to 45 pores per linear inch and a thickness from 1/4"
to 1-1/2".

4. The method of claim 1 wherein said metallostatic head is from about 4" to 6".

5. The method of claim 2 wherein said at least one filter element has an air permeability in the range of 400 to 8,000 x 10-7 cm2, a porosity of 0.80 to 0.95, a pore size of 5 to 45 pores per linear inch and a thickness from 1/4"
to 1-1/2".

6. The method of claim 3 wherein said metallostatic head is from about 4" to 6".

7. The method of claim 1 wherein said at least one filter element is located beneath said sprue means.

8. The method of claim 1 wherein said at least one filter element is located in said runner means.

9. In the filtration of molten metal the method which comprises:
providing at least one ceramic foam filter element in a molten metal passage; and passing molten metal through said at least one filter element at a first flow rate whereby molten metal passes through said passage downstream of said filter element in a quiescent non-turbulent fashion.

10. The method of claim 9 wherein said first flow rate is from about 5 to 50 cubic inches per square inch of filter area per minute.

11. The method of claim 10 wherein said molten metal passage comprises sprue means, runner means, and mold casting proper means and said at least one filter element is located downstream of said sprue means and upstream of said mold casting proper means.

12. The method of claim 11 wherein said sprue means is charged with molten metal at a second flow rate so as to prime and force said molten metal through said at least one filter element at said first flow rate while maintaining a substantially constant metallostatic head over said at least one filter element.

13. The method of claim 12 wherein said at least one filter element has an air permeability in the range of 400 to 8,000 x 10-7 cm2, a porosity of 0.80 to 0.95, a pore size of 5 to 45 pores per linear inch and a thickness from 1/4" to 1-1/2".

14. The method of claim 13 wherein said metallostatic head is from about 4" to 6".

15. The method of claim 14 wherein said at least one filter element is located beneath said sprue means.

16. The method of claim 14 wherein said at least one filter element is located in said runner means.