US3508601A - Molybdenum screen for molten non-ferrous metal - Google Patents

Description

A ril 28, 1970" G. D. M ARDLE 3,503,601

MOLYBDENUM SCREEN FOR MOLTEN NON-FERROUS METAL Filed April 17. 1967 INVENTOR.

United States Patent Otiice 3,508,601 Patented Apr. 28, 1970 U.S. Cl. 164-358 3 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to the art of casting molten metal and more particularly to the casting of non-ferrous metals using sand molds.

In all casting techniques, it is essential that the amount of slag and other drosses present in the cast article be minimized, and various techniques have been employed to accomplish this result. Since these impurities are, for the most part, lighter than the metal being cast they will rise to the surface of the molten metal if it is poured slowly enough. Slow pouring rates, however, not only decrease production line eificiency but can cause poor castings. With slow pouring rates, the metal being cast might solidify before the mold is completely filled causing chills or voids in the finished casting. Another technique that has been used for permitting these lighter impurities to rise to the surface is the use of blind risers or multiple risers. These excess risers, however, result in a high loss of material or the necessity of subsequent reheats to reclaim this material.

Another technique that has been employed for removing slag and dross from a cast metal employs a perforate member through which the molten metal is poured. Such perforate members, however, must be formed from a material that is able to withstand the extremely high temperatures at which the molten metals are poured. Normally, ceramic screen have been employed for this purpose but ceramic screens have several disadvantages. The ceramics become brittle at the high pouring temperatures and can fracture and enter into the cast article along with the other impurities.

It is, therefore, a principal object of this invention to provide an improved screen through which a molten metal may be poured to remove impurities during a casting process.

It is another object of the invention to provide a screen for removing impurities from a cast object, which screen will not reduce the pouring rates.

It is another object of the invention to provide a screen for metal casting that will remove impurities without being attacked by the molten metal.

SUMMARY OF THE INVENTION This invention is embodied in an expanded molybdenum screen particularly adapted for use in the casting of nonferrous metals in a mold that defines a cavity that is adapted to receive a molten metal and having the form of the object to be cast. The molten metal enters the mold cavity through a passage that extends at least in part through the mold and is adapted to receive poured molten metal. The expanded molybdenum screen is positioned in the mold and extends across the passage.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a cross-sectional view taken through a sand mold embodying this invention.

FIGURE 2 is an enlarged, perspective view of the area encompassed by the circle in FIGURE 1.

FIGURE 3 is an enlarged top plan view of the screen embodying this invention and supported in the mold shown in FIGURES 1 and 2.

FIGURE 4 is a cross-sectional view taken along the line 4-4 of FIGURE 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring in detail to FIGURES 1 and 2, the reference numeral 11 indicates a sand mold for use in the casting of articles formed from non-ferrous metals. The mold 11 is comprised of a drag 12 formed fro-m casting sand ram packed into a drag flask 13. Supported above the drag 12 and drag flask 13 is a cope 14, which is also formed from casting sand ram packed into a cope flask 15. The cope 14 and drag 12 engage each other along a parting line indicated generally by the reference numeral 16.

A cavity 17 is formed in the drag 12 and cope 14 in a known manner which cavity has the shape of the article to be cast. As is well known in casting techniques, one or more cores may be inserted into the cavity 17 if openings are desired in the finished casting. A runner 18 is formed in the drag 12 adjacent the parting line 16 and opening at one end into the cavity 17. The other end of the runner 18 terminates in an enlarged cavity 19 'which is adjacent to the lower end of a sprue 21 formed in the cope 14 and may be considered to be an extension thereof. The sprue 21 extends vertically through the cope 14 and terminates at its upper end in an enlarged pouring basin 22. At the side of the cavity 17 opposite to the runner 18, a riser 23 is formed which extends through a portion of the drag 12 and through the cope 14. The riser 23 terminates in the upper surface of the cope 14.

During casting the molten non-ferrous metal is poured into the pouring basin 22 from a ladle or the like at a temperature that will be considerably lower than the melting point of molybdenum. This molten metal passes down the sprue 21, across the runner 18 and into the cavity 17. Sufficient metal is poured to fill the riser 23 and insure that the complete volume of the casting 17 is filled. The metal thus poured contains slag or other drosses which might enter the cavity 17 and form impurities within the finished cast article. In order to exclude these impurities from the finished casting, an expanded molybdenum screen, indicated generally by the reference numeral 25, is positioned within the mold 11 at the parting line 16 and extending across the sprue 21.

Referring now in more detail to FIGURES 3 and 4, the molybdenum screen 25 is formed with a plurality of generally like size uniformly distributed polygonal openings 26 that are defined by unitary strands 27 of molybdenum. The strands 27 defining adjacent openings 26 are integrally connected to each other at the apexes of the polygonal openings, as clearly shown in FIGURE 3. This form of screen may be made by any conventional metal expanding technique.

In a preferred embodiment of the invention, a screen 25 was formed from a low carbon molybdenum sheet rolled from an arc cast ingot and expanded in a conventional manner. It is to be understood that certain alloying elements such as titanium, zirconium, columbium and tungsten may be added to the molybdenum as long as the metal retains sufiicient ductility to be expanded.

Molybdenum sheets made by rolling sintered powders also may be expanded as long as they have adequate ductility.

The size of the openings in the screen 25 and the ratio of the area of the openings with respect to the remaining strand area may be varied to suit particular requirements in a given application. This is also true with respect to the particular shape of the openings. The illustrated embodiment has a generally diamond shaped opening 26 and good results have been obtained with the use of such openings wherein the long dimension of the diamond was about 0.187 inch and the short dimension of the openings was in the range of about 0.048 inch to about 0.071 inch. -In this embodiment there were approximately 170 such openings per square inch of screen material and the thickness of the strands 2-7 in the plane of FIGURE 3 was in the range of about 0.007 inch to about 0.035 inch. The thickness of the original material before expansion was in the range of about 0.002 inch to about 0.015 inch.

It is to be understood that these dimensions represent only one range of examples that has proven satisfactory and that variations from this range may be made to suit a particular application. Due to the thinness of the screen 24, the screen 24 may be positioned in the mold without providing a special cavity for the screen 24. The thinness of the screen 24 will cause only very minor and insignificant displacement of the sand of the mold when the cope 14 is positioned upon the drag 12.

The use of the molybdenum screen as described herein has particular advantage in the casting of non-ferrous metals. It is believed that the expanded molybdenum screen can be used in the casting of any non-ferrous metal in which the molybdenum of the screen has a relatively low solubility at the casting temperature. Good results have been obtained to date in the casting of commercial grades of aluminum, which contain small amounts of silicon and other alloying materials, aluminum alloys, the Babbitt metals, lead, zinc, cobalt based hard facing alloys, copper and copper base alloys. The pouring temperatures of the aforenoted metals and their alloys are considerably lower than the melting point of the molybdenum and molybdenum is not Wetted by these metals and their alloys, this characteristic being particularly true of copper and the copper base alloys.

Molybdenum, however, had not previously been used in such applications, since another of its known characteristics is the catastrophic oxidation it undergoes in air at temperatures at or above 1400 F. The resulting molybdenum oxide would vaporize at normal casting temperatures of most non-ferrous metals. It has been found, however, that the molten metal apparently shields the screen from oxygen and hence these deleterious eifects do not occur. The use of expanded molybdenum also provides considerable advantage in this application. The high ratio of open mesh area to strand area provides for high flow rates with low flow resistance which means that hotter metal is delivered to the mold cavity. In addition, the hotter metal has greater fluidity and smaller runners can be used with a reduction in the amount of scrappage or amount of metal to be remelted. The expanded molybdenum screen further streamlines the flow of metal and reduces the tendency for swirling and turbulence. Hence,

the more laminar metal flow stream is less likely to dislodge sand particles into the metal being cast further reducing the amount of impurities present in the finished, cast article. Conventional screens formed from interwoven wires have a number of voids in addition to the flow openings. Woven screens cause capillary action to occur when the molten metal is poured, thus reducing the pouring rates.

In the described embodiment the expanded molybdenum screen has been described and illustrated as being positioned within a sand mold extending across the sprue passage at the parting line between the cope and the drag. It should be readily apparent that the use of an expanded molybdenum screen to remove slag and other drosses from molten non-ferrous metals may be used in connection with other types of casting techniques, such as those using a permanent mold. In these embodiments the expanded molybdenum screen also may be positioned within the mold. As an alternative arrangement, the expanded molybdenum screen may be positioned across the upper face of the sprue passage or interposed in another conduit that is employed to deliver the molten metal to the mold. This latter embodiment has been used successfully in the casting of the Babbitt metals.

While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change Without departing from the proper scope or fair meaning of the subjoined claims.

What is claimed is:

1. *In a mold for casting non-ferrous metals, the mold defining a cavity adapted to receive a non-ferrous molten metal and having the form of the object to be cast, the

non-ferrous metal being poured at a temperature substantially less than the melting point of molybdenum, a passage extending at least in part through the mold, said passage having an opening for receiving poured molten metal, said passage terminating at said cavity for conducting the poured molten metal to said cavity for the introduction of molten metal to said cavity from said passage, the improvement comprising an expanded metal molybdenum screen extending across said passage for separating slag and other impurities from the molten metal as it is poured through said passage.

2. The improvement as set forth in claim 1 wherein the expanded metal molybdenum screen is an integral member having a plurality of substantially like sized,

- uniformly distributed openings defined by molybdenum strands integrally connected to each other, said openings forming substantially the sole voids in said screen.

3. The improvement as set forth in claim 2 wherein the openings are generally polygonal in shape.

References Cited I. SPENCER OVERHOLS-ER, Primary Examiner

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