US2812562A - Method of casting metallic articles - Google Patents

Description

Nov. 12, 1957 R. F. DALTON METHOD OF CASTING METALLIC ARTICLES Filed June 5, 1956 United States Patent 2,812,562 METHOD OF CASTING METALLIC ARTICLES Robert F. Dalton, Chicago, 111., assignor to Hills-McCanna Company, Chicago, Ill., a corporation of Illinois Application June 5, 1956, Serial No. 589,464 22 Claims. 01. 22-400 The invention relates to the metal casting art and in particular to an improved method of making precise and complex shaped cored passageways in metal castings, with special reference to uniformly smooth and accurately cored passageways in magnesium and aluminum alloy castings.

There are three well known methods which have heretofore been used in forming passageways in the internal portions of castings. Probably the best known method resides in coring the passageway by using conventional oil bonded or resin bonded sand cores. Secondly, the passageway may be formed by the use of a cast-in-plaee metal tube, and thirdly, the passageway may be formed by subsequent machining operations on the casting. There are, of course, limitations and disadvantages to all three methods, as, for example, in cored cavities produced by the use of conventional sand cores, it is difficult to obtain smooth surface finishes, the requireddimensional accuracy, and reproducibility. Also as sand cores become smaller in diameter it becomes more diflicult to produce them with sufiicient strength and permeability. The strength may be increased with more binder but this results in more gas :being formed on pouring .and venting becomes more of a problem.

Metallic tubular inserts customarily used in magnesium :and aluminum castings are difiicult and expensive to fabricate because the assemblies must be made pressure-tight in order to prevent metal entry into the tube assembly, with resultant rendering of the passageway ineffective. Also metallic tubular inserts often produce discontinuities in the casting, particularly in the areas immediately adjacent the tube. Furthermore, on pouring magnesium or aluminum into the mold cavity containing metallic inserts, the tube may expand unduly and consequently be displaced from its intended position. It can also be noted that cast-in metallic inserts increase the weight ofthe cast ings, which is undesirable, particularly for aircraft application.

The formation of passageways by machining operations :has many disadvantages. Such operations not only introduce an extra element of cost, but the machining of such passageways is inherently limited in extent and thus design is limited with respect to the metal components of .an assembly. Furthermore, in some machining operations, such as drilling complex fluid passageways in metal castings, difficulties are encountered in drill run-out, spiraling and off-center junctions which cause restriction and :result in turbulence of fluid flow.

In view of the foregoing, it is an object of the invention to provide a new and novel commercial casting process whereby the desired result of producing cored passageways "in metallic castings, either straight or bent, can be accomplished in a better and more facile manner and with the elimination of the many disadvantages, complexities and expensive procedures of the methods as heretofore employed.

.A more specific object of the invention is to provide a method of producing intricate passageways in magnesium and aluminum alloys castings wherein metallic tubular inserts are not employed and wherein machining operations are not required to produce the aforementioned passageways.

In recently developed casting methods, inserts of preformed glass tubing have been employed as the preferred means of obtaining cored passageways characterized by smooth-surfaced interiors with the required dimensional accuracy throughout their length. Various compounds of silicon such as quartz, borosilicate glass and other glasses have been used in forming the core inserts. Upon solidification of the metal, the glass core constitutes the liner of the passageway, or, more accurately, the glass core constitutes the hole former in the solidified metal and said core remains intact without fracture or appreciable distortion or melting of the glass. Although in some cases it may be desirable to allow the glass tubing to remain in the cored passageway, particularly when the castings are to be used in conjunction with fluids whichare more corrosive to. the alloy than to the glass, the said casting methods specifically provide for removal of the glass tubing by subjecting the same to the action of hydrofluoric acid or aqueous solutions of the fluophosphoric acids, with or without inhibitors, such as arsenic oxide. The reagent is permitted to selectively or preferentially corrode the glass so that after a suitable passage of time the glass is removed from the casting, leaving a clean, precise and accurately formed passageway in the casting metal. Alloys of aluminum, magnesium and other metals having pouring temperatures as high as 1700 F. may be cast, using the herein described glass core technique and procedures for producing the intricatepassageways, it being necessary to select glass compositions which have the high resistance to thermal shock requiredto withstand the pouring temperatures, which will retain their predetermined designed shape, and which, in addition, can be subsequently removed by a selective reagent without appreciable corrosion of the metal of the casting. I

However, the glass core technique and procedures as described are not free from inherent disadvantages and problems. In the first place, certain molten metals at casting temperatures will react chemically with the borosilicate glass. In particular, certain magnesium alloys react with borosilicate glass to form magnesium silicide. Proof of the composition of this reaction product has been established by X-ray diffraction analysis. Experiments have indicated that the formation of magnesium silicide takes place at a greater depth in the glass than would normally be expected and since this formation resists the action of hydrofluoric acid during the core removal, it thus prevents complete removal of the glass core from within the passageway. It therefore becomes essential to prevent any reaction between the borosilicate glass and the molten metal.

Accordingly, the present invention has for its primary object the provision of new and novel coating procedures for the core material whereby the same is coated in order to prevent any chemical reaction between the core material and the molten metal at the mold casting temperatures. Thus, when the core material is subjected to the action of the selective chemical reagent, all of the core material will be removed, resulting in a smooth and clean passageway.

A more specific object of the invention-has reference to castings of magnesium alloys or aluminum alloys and resides in the disclosure of certain materials having the desired properties and which fulfill the requirements for protective coatings such as can be applied to the borosilicate glass core to prevent the undesirable reactions between the core material and the casting metal and which thus facilitates the complete removal of the glass core when subjected to the action of a selective chemical reagent.

In connection with the foregoing, another object of the invention is to provide a practical and reliable detection method to which the castingscan be subjected following the removal of the core material, whereby positive proof of freedom from glass or other foreign materials in the cored passageways can be established. 1

A further object is'to provide a detection method for the purposes above described, which consists in selecting surface coatings for the core material which are radiographically dense so that the coatings'can be subsequently detected radiographically.

Another object of the invention is to provide a detection method for the purposes herein mentioned, which consists in selecting surface coatings for the core material containing small amounts of radioactive ingredients whereby the coating can be subsequently detected by suitable instruments sensitive to the radiations common to such materials.

Other'objects of the invention will become apparent from the'following description and accompanying drawings, of which Figure 1 is an isometric view of a mold equipped with a precoated preformed glass core prior to casting;

Figure 2 is an isometric cross sectional view of the casting and precoated glass core with the mold removed, the section being taken through the center of the cored passageway;

Figure 3 is the same isometric cross sectional view of the casting as shown in Figure 2 with the pour gate and riser removed, said view also illustrating the smooth surfaced passageway which results following removal of the glass core and coating;,and

Figure 4 is an isometric view of a precoated glass core preformed by assembly of glass tubing components.

This application is a continuation-in-part of my copending application Serial No. 480,892, filed January 10, 1955, now abandoned, and entitled Method for Casting Metallic articles.

Principal applications for intricate cored passageways in magnesium and aluminum alloy castings are for the passage of lubricating oils, hydraulic fluids and coolants through mechanical parts such as gear covers, pump housings, pistons and brackets. Since the transmission of fluids through such passageways commonly requires contact between the fluid and moving mechanisms, it is essential that the cored passageways be entirely free of foreign materials so that the mechanisms can not be rendered inoperative by contamination with such materials. Any foreign material left in the, passageways might become dislodged and possibly enter intopnmps and cylinders, scoring or marring the machined surfaces thereof and causing failure of said device at a time when its successful operation is imperative. Accordingly, the invention involves the use of techniques which will prevent chemical reaction of the coring mateirals such as borosilicate glass, structural fused salts or similar materials with the molten metal of the casting at casting temperatures, whereby with respect to castings of magnesium alloys with core material of borosilicate glass, the borosilicate glass is prevented from reacting with the metal to form undesirable reaction products such as magnesium silicide. When the silicide is allowed to form, it penetrates the glass core for a thickness of several 'thousandths of an inch and since magnesium silicide is not readily soluble in hydrofluoric acid, the formation prevents the action of said acid from removing all of the glass core remove all of the glass core. for use in carrying out the invention should have the following properties:

(I) Freedom from formation of volatile material on contact with molten metal;

(2) Freedom from reaction with or solution in molten magnesium or aluminum alloys at or below the casting temperature;

(3) Solubility in aqueous hydrofluoric acid solution or other reagent which will not affect the magnesium or aluminum alloys during the preferential removal of the coring materials or susceptible to removal by mechanical means;

(4) Having suflicient coherence to resist impingement erosion from the action of flowing molten metal; and

(5) Freedom from fusion at metal pouring temperatures of the magnesium or aluminum alloy.

Coating materials which meet the above requirements and which have been used satisfactorily to prevent the magnesium silica reaction are as follows:

(1) Oxides of metals 'in the fully oxidized, maximum valence (ic) state such as titanium dioxide, barium oxide, tungstic oxide, selenium oxide, molybdenum oxide, hafnium oxide, osmium oxide, uranium oxide and germanium oxide.

(2) Salts of the above metals such as fluorides and in particular barium fluoride, lead fluoride and tungsten fluoride.

(3) Salts of the above metals existing in their amphoteric (acid) state as an (ate) such as barium selenate, barium molybdate, and lead tungstate, etc.

Coatings in finely divided form may be applied to the outer surface of the glass cores by dipping, spraying, dusting, ainting and sooting (carbonaceous). The finely ground material may be mixed with a liquid for application. Any readily volatile liquid carrier to which the glass is inert may be used, for example, a low boiling alcohol such as methyl, ethyl, or propyl alcohol, since after applying the liquid can be readily eliminated by burning or by heating the glass coring. It is not necessary for the coating material to go into solution in the liquid carrier. A suspension rather than a solution may be used to obtain a coating of sufiicient thickness and density for both protection and detection purposes. The coating process for the glass core may be likened to a painting operation, except that there should be no volatile material remaining at the time the casting is poured. It is also desirable to wet r r 1i1l the liquid carrier-solid coating mixturebefore application in order to assist in obtaining a uniform, smooth coating. Finely ground or precipitated tungstic oxide, barium oxide, andbarium fluoride in alco hol suspensions have been found to give superior results as protective coatings.

Metallic coatings which are the same as or inert to reaction with thecasting metal atcasting temperatures, may also be used as protective coatings. For example, in pouring magnesium or aluminum castings, either magnesium or aluminum coatings applied by metal spraying, painting,.dustin g or plating may be used.

Because of the nature of the complex, intricate cored passageways described herein, it is often impossible to inspect them with optical instruments to determine the degree of freedom from glass or other foreign materials. Accordingly, ;a positive method .of insuring absolute freedom from foreign materials, such as the glass core materials or :the core coating materials is essential for the practicalapplication .of the techniquesjherein disclosed. One practical inspection method for detection of foreign material following removal of the core from .the passageway consists in applying asurface coating to the core which may be subsequently detected radiographically. A second method contemplated by theinvention consists in apply- Coating materials suitable ing radioactive material as a coating to the core or incorporating in a suitable core coating a small quantity of radioactive material whereby the same can be subsequently detected by a suitable instrument such as a scintillation meter.

In selecting a core coating material for'radiographic detection, high radiographic density with respect to the casting alloy and the glass core material is necessary. Radiographic densities appear to be approximately proportional to the specific gravities of the materials. The following inorganic metallic compounds are listed with their specific gravities in order to compare them with borosilicate glass, aluminum, and magnesium metal. The

compounds listed are to be considered as typical and accordingly the invention is not limited thereto but may include other compounds within the scope of the claims.

Material Specific Gravity tungstic OxidebWOa The clarity with which the dense coating on the outside of the glass core may be seen in a radiograph is determined by the thickness of the coating, the comparative radiographic density of the coating material used, the density of the casting metal around the coating and the density of the borosilicate glass inside of the coating. The coating on the outside of the glass tube can not be removed until all of the glass coring immediately in contact with the coating material has been dissolved. Even though a very small thickness'of glass may remain, say .0001 of an inch, it would form a protective barrier and prevent the removal of the coating. Thus a radiograph will reveal the presence of a very thin coating of glass by showing the presence of residual radiographically dense coating material. It will. also reveal the presence of any residual coating material.

The materials of high radiographic densities are not necessarily satisfactory when used alone as coatings on glass cores, as they may react with the molten magnesium or aluminum alloys, leaving reaction products permanently in .the casting. This was found to be true with lead oxide, lead fluoride, chromium oxide, tin oxide, zirconium oxide, iron oxide and other metallic oxides. This disadvantage may be overcome by first coating the glass core with the lead oxide or other reactive compound and then applying a second coating such as titanium dioxide, which is inert to the molten casting alloy although both coatings may be chemically removed from the solid casting. An- .other method consists in coating the glass cores with a material having a selected physical or chemical aflinity for the material of the glass cores and which is soluble in the acid solution employed for chemically removing the said glass core. A second coating is then applied to the first coating so as to completely cover the same and said second coating should comprise a material of greater radiographic density than that of either the casting metal, theiglassu core material, or the said first coating. It is e v i further necessary that said second coating be inert to the metal of the casting to prevent any chemical reaction between the molten casting metal and the material of the first coating during the casting operation, and, further, said second coating should also be soluble in the acid solution employed for chemically removing the glass core. For example, lead oxide may be employed for the first coating and tungstic oxide has been found satisfactory for the second coating. The most satisfactory materials used as single coatings were found to be barium oxide, barium fluoride and tungstic oxide, since they are both sufliciently radiographically dense and inert to molten magnesium and aluminum alloys.

A second method of insuring freedom from foreign materials in cored passageways is by radioactive detection. A radioactive material may be applied to the outer surface of glass cores by mixture with core coating materials. A suitable radioactive material is one which has a sufliciently long half life, sufficiently high intensity of gamma radiation, and which is inert to the casting alloy or can be mixed with or protected by an inert coating. A typical suitable material is a compound containing radioactive zirconium. This typical material can be formed by chemically reacting a radioactive isotope of zirconium (atomic Number The radioactive material is applied to the core and then recoated with titanium dioxide in alcohol suspension; or a suspension of titanium dioxide in ethyl alcohol containing a very small amount of radioactive zirconium compound may be used. A suitable coating which gives adequate protection and can be detected by radioactive inspection is applied by spraying onto the glass core a suspension composed of 200 grams of titanium dioxide in 600 ccs. of ethyl alcohol to which has been added approximately 24 mg. of radioactive zirconium as an oxalate complex, wherein one gram of zirconium contains the activity of five curies (1470 micro curies per ml. solution). It is evident that for radioactive detection an extremely small amount of radioactive substance is required in a mixture with sufficient protective coating substance to prevent reaction between the casting metal and the glass coring and also between the casting metal and the radioactive substance, if the latter is not insoluble in or inert to reaction with the casting metal.

Removal of all core and coating materials, including the coatings containing the radioactive material, is insured when the radiation count reaches the prescribed minimum, as measured from the outer surfaces of the casting. The prescribed minimum is determined by destructive tests, that is, by sawing the casting through the cored passageways, and chemically and visually examining the cored passageways for presence of residual foreign materials. The reading on a suitable detection device, such as a scintillation meter, when the required removal of glass cores and coatings has been effected is then set up as a standard.

For a better understanding of the invention and the manner in which the same may be commercially practiced, reference is made to the drawings, wherein Figure 1 discloses a standard flask identified by numeral I mounted on a bottom board 2 with the flask filled with sand 3. Also shown is a precoated core preformed by assembly of borosilicate glass tubular components and identified in its entirety by numeral 4, the same having been set in the sand in the proper manner with the glass ends 5, 6 and 7 being firmly held in the proper position by core prints 8, 9 and 10. Also shown is the casting void or mold cavity 11 to be filled with molten metal delivered through the pouring gate 12. The riser 13 vents the mold cavity and feeds the casting. Alloys of aluminum, magnesium or other metals having pouring temperatures as high as 1700 F. may be cast in accordance with the present invention. Examples of said metals with their alloying ingredients in approximate ranges are as follows:

Aluminum Base Alloys Magnesium Base Alloys Alloy 34A Alloy GIOA Alloy AZ 92A Alloy AZ 63A Percent Percent Percent Percent Gu-4.0 5.Q. Oil-0.2 max. A18.5-9.5 Al5.56.5.

Bl1.5 1381..-. Si-0.2 max- Mn0.l3 min... Mil-0.18 min.

Ti0.2 max.-. Ti0.2 Other impurities Other impurities 0.30 max. 0.30 max. Other J5-.- Other 0.15. Mg-balance-- Mgbalance.

AI-balaneal Ail-balance.

Inpractice the molten alloy is poured into the prepared mold in an amount to completely fill the mold cavity 11. The metal thus completely surrounds and envelops the glass core assembly with the exception of the glass ends 5, 6, 7, and after a certain period of time the metal will solidify. The casting 14 is then removed from the mold and the same, as shown in Figure 2, will have the gate 12 and riser 13 attached thereto. The glass core embedded in the casting is then subjected to the action of chemical reagents for removal of the glass core and coating. Following this core removal operation, the final casting, as shown in Figure 3, is characterized by a smooth-surfaced cored passageway 15 of the complex shape desired and which is precise as regards dimensional accuracy throughout its extent.

Referring more particularly to the glass tubing core assembly 4, it will be understood that the same may be prefabricated by any conventional glass forming techniques. However, before the assembly is set in the mold the same is coated in accordance with the invention in the following manner:

To 600 ccs. of denatured alcohol is added 200 grams of powdered barium fluoride and the suspension is wet milled. This mixture is painted or sprayed onto the surface of the 6 mm. diameter borosilicate glass core 4 so that no visible spot of glass may be seen. The coating is then dried by baking, burning or other applications of heat, causing the alcohol to burn off or vaporize. The glass core is then set into the sand mold (either green sandor dry sand) in much the same manner as conventional foundry cores, and the mold is closed. The magnesiumalloy for the casting is melted, superheated and poured into the mold, at 1400 F. The cored casting is removed from .the sand when the metal has solidified (about /2 hour after pouring) and the passageway is tested .bY applying a stream of compressed air through it. The gates and risers are cut from the casting in the con voli nal manner The operations for the removal of the 6 mm. diameter borosilicate glass core consists in drilling into each of the outer ends of the passageway a .339 hole (Rdn'll) and tapping the holes with a A" pipe tap. Into these tapped holes are inserted A" nipples and necessary brass fittings to allow the attachment of /2" diameter copper tubing. The tubing is attached to a high alloy centrifugal pump having connection with a source of hydrofluoric acid and including a return line thereto. The said hydrofluoric acid (40-70% solution) is circulated through the interior of the glass tube and by its chemical action, the glass tube is gradually removed. The hydrofluoric acid is circulated by the pump through the glass tubing until all of the glass is removed and the acid is drained. Any remaining portions of the acid are removed by washing the passageway with a suflicient quantity of water. The casting is then X-rayed and the radiograph visually inspected to determine if the barium fluoride coating has been, completely removed. Any residual barium fluoride will show up in the radiograph as a relatively light area on the inner cored wall. Shouldsuch a light area show up, the flushing with hydrofluoric acid is continued or a supplemental procedure employing the flow of high pressure water is used until inspection reveals the casting to be free of the barium fluoride coating and therefore also free of the glass .coring.

It may be noted that although magnesium and aluminum alloys are the preferred metals'for the techniques of this invention, the same is not necessarily limited to such alloys but can also be used in connection with other metals having melting points below the softening point of the glass core so that the core will not become distorted during the casting operation. The use of cylindrical hollow borosilicate glass tubing of appropriate size is most satisfactory in producing fine cored passageways, otherwise notproduceable heretofore, or produceable to a limited extent. Here again the present invention is not specifically limited to the use of such tubing but the same can be practiced with various sections as to size and shape, such as round, oval, square, curved or bent, and wherein the cores have been preformed by heat or conventional glass-forming operations whereby the resulting glass cores may consist of various sections. Likewise, an assembly of formed glass sections can be set in the mold for subsequent pouring operations and for subsequent glass core extraction operations. The accurate retention of the glass core dimensions is the result of the high resistance to thermal shock of borosilicate glass in the conventional pouring temperature range used for alloys of magnesium and aluminum.

It will be seen, therefore, that the invention consists of a simple, practical and economical process for making aluminum, magnesium and other alloy castings having straight or complex cored passageways of predetermined directions and shapes and of large or small sizes, and which cored passageways will be smooth-surfaced and precise as to dimensions. Also in connection with the foregoing, the invention provides certain practical methods for insuring complete removal of all foreign matter from the passageways.

What is claimed is:

1. In a method of making metal castings with cored passageways in which the passageways are formed by placing glass cores in a mold, the said glass cores having a sufiiciently high melting point and resistance to thermal shock to remain intact under casting conditions, and wherein, said glass cores are chemically removed from the solid casting, the improvement which comprises ap plying to the outer surface of the glass core a coating of a substance which is non-volatile and which is inert to the casting metal at temperatures to which the coating is subjected during the casting operation.

2. A method of making metal castings as defined by claim 1, wherein the coating substance is soluble in the chemical used to remove the glass.

3. A method of making metal castings as defined by claim 1, wherein the coating substance is soluble in the chemical used to remove the glass, and wherein the casting is made from metal selected from the group of magnesium and aluminum alloys.

4. In a method of making metal castings with cored passageways in which the passageways are formed by placing glass cores in a mold, the said glass cores having a sufficiently high melting point and resistance to thermal shock to remain intact under casting conditions, and wherein said glass cores are chemically removed from the solid casting, the improvement which comprises applying to the outer surface of the glass core a coating of a material which is non-volatile and which is inert to the casting metal at temperatures to which the coating is subjected during the casting operation, and the said coating material also having a substantially greater radiographic density than that of the metal from which the casting is made and also higherthan that of the glass core.

5. A method of making metal castings as definodby claim 4, wherein the casting is made from metal selected from the group of magnesium and aluminum alloys.

6. In a method of making metal castings with cored placing glass cores in a mold and then pouring molten metal in the mold so that the glass cores become embedded in the casting, the said glass cores having a sufiiciently high melting point and resistance to thermal shock to remain intact under casting conditions, and wherein said glass cores are chemically removed from the solid casting, the improvement which comprises applying to the outer surface of the glass cores a coating of a material which is non-volatile, which also can be chemically removed from the solid casting and which is inert to the casting metal at temperatures to which the coating is subjected during the casting operation, the said material being selected from the group consisting of magnesium, aluminum, titanium dioxide, barium oxide, barium fluoride, barium titinate and tungstic oxide.

7. A method of making metal castings as defined by claim 6, wherein the glass cores are made from borosilicate glass.

8. A method of making metal castings as defined by claim 6, wherein the coating is applied to the glass coring in the form of a suspension of finely divided particles in a volatile carrier.

9. A method of making metal castings as defined by claim 6, wherein the coating is applied to the glass coring in the form of a suspension of finely divided particles in a low boiling alcohol.

10. In a method of making metal castings with cored passageways in which the passageways are formed by placing glass cores in a mold and then pouring molten metal in the mold so that the glass cores become embedded in the casting, the said glass cores having a sufficiently high melting point and resistance to thermal shock. to remain intact under casting conditions and wherein said glass cores are chemically removed from the solid casting, the improvement which comprises applying to the outer surface of the glass core a coating of a material which is nonvolatile, which is inert to the casting metal at temperatures to which the coating is subjected during the casting operation, which is soluble in the chemical used to remove the glass, and which has a substantially greater radiographic density than either the casting material or the glass coring.

11. A method of making metal castings as defined in claim 10, wherein the coating is applied to the glass coring in the form of a suspension of finely divided particles in a volatile liquid carrier.

12. A method of making metal castings as defined in claim 10, wherein the coating is applied to the glass coring in the form of a suspension of finely divided particles in a low boiling alcohol.

13. In a method of making metal castings with cored passageways and wherein said passageways are formed by placing glass cores in a mold and then pouring molten metal in the mold so that the glass cores become embedded in the casting, the said glass cores having a sufficiently high melting point and resistance to thermal shock to remain intact under casting conditions, and wherein said glass cores are chemically removed from the solid casting, the improvement comprising applying to the outer surface of the glass cores a first coating of a material which has a selected physical or chemical afiinity for the material of the glass cores, and then applying a second coating to completely cover said first coating, said second coating comprising a material of greater radiographic density than that of either the casting metal, the glass core material, or the first coating, and said second coating also being inert to the metal of the casting whereby to prevent any chemical reaction between the molten casting metal and the material of said first coating during the casting operation.

passageways in which the passageways are formed by 14. In a method of making metal castings with cored passageways in which the passageways are formed by placing glass cores in a mold and then pouring molten metal in the mold so that the glass cores become embedded in the casting, the said glass cores having a sufliciently high melting point and resistance to thermal shock to remain intact under casting conditions and wherein said glass cores are chemically removed from the solid casting, the improvement which comprises applying to the outer surface of the glass core a coating of a material containing a radioactive substance, said coating material being nonvolatile, capable of being chemically removed from the solid casting, and inert to the casting metal at temperatures to which the coating is subjected during the casting operations.

15. A method of making metal castings as defined in claim 14, wherein the radioactive substance in the coating is a zirconium compound prepared from a radioactive zirconium isotope.

16. A mold for use in casting metallic articles of the type having passageways formed therein, in combination, a mold body forming a mold cavity having a pouring opening, a glass tubing core assembly disposed within the mold cavity and having its respective ends properly supported in core prints, and said glass tubing core assembly having a coating which completely covers all the glass located within the mold cavity, said coating comprising a material which is non-volatile, which can be chemically removed from the solid casting and which is inert to the casting metal at temperatures to which the coating is subjected during the casting operation, the said material being selected from the group consisting of magnesium, aluminum, titanium dioxide, barium oxide, barium fluoride, barium titinate, and tungstic oxide.

17. A mold as defined by claim 16, wherein the glass tubing core assembly is made from borosilicate glass.

18. A mold as defined by claim 16, wherein the coating material contains a substance that is radioactive.

19. In a method of making metal castings with cored passageways and wherein the said passageways are formed by placing glass cores in a mold and then pouring molten metal in the mold so that the glass cores become embedded in the casting, the said glass cores having a sufficiently high melting point and resistance to thermal shock to remain intact under casting conditions and wherein said glass cores are chemically removed from the solid casting, the improvement which comprises applying to the outer surface of the glass cores a first coating of a material which has a substantially greater radiographic density than either the casting metal or the glass of the cores, and then applying a second coating to completely cover said first coating, said second coating comprising a material which is inert to the metal of the casting whereby to prevent any chemical reaction between the molten casting metal and the material of said first coating during the casting operation.

20. In a method of making metal castings with cored passageways in which the passageways are formed by placing glass cores in a mold and then pouring molten metal in themold so that the glass cores become em bedded in the casting, the said glass cores having a sufficiently high melting point and resistance to thermal shock to remain intact under casting conditions and wherein said glass cores are chemically removed from the solid casting, the improvement which comprises applying to the outer surface of the glass core a coating of a material which is non-volatile, which is inert to the casting metal at temperatures to which the coating is subjected during the casting operation, which is soluble in the chemical used to remove the glass cores, and which has a substantially greater radiographic density than either the casting material or the glass coring, and then, following the treatment of the casting for chemically removing the glass coring, subjecting the casting to radiographic inspection to determine the extent to which the coring has been removed by said treatment.

21. In a method of making metal castings with-cored passageways in which the passageways are formed by placing glass cores in a mold and then pouring metal in the mold so that the glass cores become embedded in the casting, the said glass cores having a sufiiciently high melting point and resistance to thermal shock to remain intact under casting conditions and wherein said glass cores are chemically removed from the solid casting, the improvement which comprises applying to the outer surface of the glass core a coating of a material containing a radioactive substance, said coating material being non-volatile, capable of being chemically removed from the solid casting, and inert to the casting metal at temperatures to which the coating is subjected during the casting operations, and then after treatment of the casting for chemically removing the glass coring, subjecting the casting to inspection by means of a device sensitive to emanations from a radioactive substance References Cited in the file of this patent UNITED STATES PATENTS 2,251,130 Haux July 29, 1941 2,304,879 Brazil Dec. 15, 1942 2,362,875 Zahn Nov. 14, 1944 2,475,194 Nyquist July 5, 1949 FOREIGN PATENTS 626,906 Great Britain July 22, 1949 OTHER REFERENCES Foundry Practice, Palmer (pp. 182-220), 1926.

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