US2903759A - Casting of refractory metals - Google Patents

Description

p 1959 J. B. BRENNAN Q 2,903,759

CASTING 01 REFRACTORY METALS Filed July 6, 1954 2 Sheets-Sheet 1 IN VE/V TOR JOSEPH E. BRENNAN ATTORNE' Y5 A p 1959 .1. B. BRENNAN CASTING OF REFRACTQRY METALS 2 Sheets-Sheet 2 Filed July 6, 1954 JOSEPH B. BRENNAN BYyMMM/ 3 2M United States Patent 7 2,903,759 CASTING. on REFRACTORY METALs Joseph B..Brennan, Cl'eveland,..0hjio;.Helen E Brennan, executrix of the estate of' said Joseph B. Brennan, deceased Applicationluly 6, 1954, Serial No. 441,415-

15 Claims. (Cl. 22.-73)j The present invention relates generally tozcasting metals,

2303359 Pate'nteol Sept. 1 5, 1 959 metal from the pools may be fed into the desired comstantially along the line 3-3 ofFigure l.

and more specifically to'a casting method and apparatus for shaping refractory; metals including group IV of the periodic table'ofelements, such'as titanium and its alloys.

In casting titanium, titanium alloys, and other highly refractory metals having ameltingpoint'inexcessof 1500 or l800 C. a primary problem is to prevent reaction for preheatingthesurface of the cavity to a temperature sufliciently highto de-gas the mold to prevent blowing, of the casting, said meansxalsobeingeffective to reduce sharply the temperature. of the high melting refractory metal poured into the cavity.

Another objectof .the'invention'is: the provision of a method for casting in. which a predetermined quantity Figure 4 is a cross-sectional view similar to Figure 2 showing? a modified form of mold.

Figure 5 is aview similar. to Figure 2 in which the mold cavity: has a special coating.

Figure 6 is a vertical cross-sectional diagrammatic view illustrating asmeltingiand casting apparatus through which the casting? molds shown in Figures .1 through 4 may be advaneedfor'preheating, casting and cooling.

Eigure-Tis a verticalcross-sectionaldiagrammatic view illustrating ahsmelting and casting apparatus wherein metal slugs or sinterings or powder may be placed above or: intacavitiedlmold member and for shaping by subsequentzmeltingi' Figure. Talso illustratesthe use of alternatenmolds and sealing: plugs for sealing the vacuum chamber; of theimold guide or sleeve.

I Figure; 8 is a vertical cross-sectional diagrammatic view ofithechigh frequency coil 148 shownin Figure 7 wherein the sealing. plug has been modified to include a cavity for containingcthelmassofmetalto'be melted, evacuated and cast:

of solid refractory metal to be shaped isplacediin a-mold I cavity, heated above its melting point tofill thercavity, and then rapidly cooledunder controlled conditions to solidify the metal.

Another object of the invention is to-provide a mold having relatively thinrefractory walls and contained within a thin-walled flask adapted. to. befilled with molten material at a temperature. substantially below the temperature of the metal cast in the mold, to cool the casting.

A further object is to provide a vacuum casting apparatus comprising an elongated vacuumchamber through whichthe molds are adapted to .pass, similar to pistons in a cylinder, means being provided for sealing the chamber between successive molds and between the side walls of the molds and the side walls of the. chamber to prevent air from entering the vacuum chamberas the molds- Another feature of this invention is the provision of a.

casting apparatus in which a succession of pools of. molten metal are placed in partially or fully encompassing relation to a mold guide, said guide having openings therethrough which align with corresponding, openings in a compartmented mold passing through said'guide, whereby Referring now to Figures 1 to 3, the mold shown, identifledgenerallyby the'numeral 10, comprises for example; two thin-wall complementary mating sections or halves: 11 and 12:which' divide a cavity 13 of any desired shape -:or size: A channel 14 may be provided in the top surface of the mold parallel with the parting line to facilitate the degassing of the melt andithe mold cavity through the. open upper end; thereof. The complementary sections ofvthe mold may be mechanically secured or may be cemented together as showninthe vdrawing'by means of a refractory cement. Each mold section or half maybe formed with a peripheral fiange,. section 11 having a peripheral flange designated as 15, and section 12 having a similar flange designated" as 16. The space behind the mold cavity andbetween the. flanges of each section is filled with a highly conductive material 17 such as aluminum, copper.or' silver, or other good thermal conductor.

Since the melting and casting is preferably carried out within a: vacuum chamber, it is desirable that the heat conductive material 17 beone which does not vaporize under the high vacuum as used. The mold sections 11 and 12 are formed from a refractory ceramic shell having a thin cross section, say, for example, less'than one-eighth ineh.. Suitable materials for forming the shell are graphite,

. zirconia, alumina and silica. All of these materials will withstand forvthe time required, the very high temperature of molten-titanium, which is in the vicinity of 1800" C., and will also withstand extreme'temperature changes without cracking.

The mold of Figure 1 is preferably enclosed in a flask 18 shown in-Figure 2, having a relatively thick bottom 18a andtthin sidewalls 18b. Thus, when the thermallyconductive material 17 is raised to a temperature above its melting point it remainsin contact with the thin walled mold halves 11 and 12, rather than flowing away. The flask preferably contains an opening through which liquid coolant such as molten aluminum metal may be introduced to fill the space surrounding the mold cavity.

As will appear hereinafter the molds are adapted to fit into aguide .tube or sleeve so that a vacuum may be applied to the mold cavity during casting. The opening a 180 in the flask may be located so as to align with a corresponding opening in the guide tube through which coolant may be introduced from a molten pool of thermally-conductive material. The flask has a smooth exterior surface so that it will slide within the guide tube, and may be made from graphite, alumina, zirconia and similar materials which are capable of resisting very high temperatures.

Figure 4 shows the mold of Figure 1 enclosed in a tight-fitting longitudinal sleeve 19, which cooperates with flanges 15 and 16 to enclose the conductive coolant material 17. The sleeve 19 serves the same purpose as the side walls of the flask 18 and prevents the molten coolant material from escaping from the space provided therefor behind the mold cavity. Sleeve 19 has a smooth external surface so that it will slide downward within the guide tube of the casting furnace. This sleeve may be made, for example, from stainless steel or refractory ceramic materials mentioned as suitable for the mold and flask.

In any case, there must also be a relatively thick, impervious, easy to seal plug of material such as graphite between or as a part of the molds so that these impervious plugs at the entrance and exit to the guide tube serve as sealing means to maintain the vacuum within the guide tube.

Other suitable means may be used for completely enclosing the conductive material 17. For example, the mold halves 11, 12 may have a refractory ceramic backing panel adhered to the upper edge of the flanges 15, 16 with the silica cement. This panel may be of any thickness desired and may also act as an impervious barrier to vacuum leakage or impairment. It is preferable that a resilient sealing member surrounding the mold at all times be in engagement with the perimeter of this panel so as to prevent leakage.

It should be borne in mind that the conductive material 17 will expand considerably when heated and a space, as indicated at 21, should be provided between the material and the confining sleeve 19 to accommodate the expan- S1011.

To render the surface of the mold cavity less reactive to molten titanium and similar refractory metals, it is desirable to coat the cavity with an aluminum oxide film, or other protective coating. For example, aluminum metal may be applied to the surface of the cavity 13 in the form of foil, or it may be cast or sprayed on the surface of the cavity. The aluminum metal film may then, or prior thereto, be converted into aluminum oxide as, for example, in accordance with the process of my Patent No. 2,346,658. In Figure the mold of Figure 3 is shown having such a coating, designated by the numeral 20. Nickel and/ or molybdenum may also be used as a coating for the mold cavity. Such coating is generally applied in a thickness not greater than 0.005 inch. The coating material may form an alloy on the surface of the casting or may comprise a separate film which later may be removed if it is not desired.

Another suitable material for coating the surface of the mold cavity to reduce and limit activity is carbon. Carbon may be applied in the form of a film of carbon black dusted onto the surface of the cavity, or by flame depositing or by colloidal coating.

According to one way of casting metals in accordance with my invention, the mold of Figure l is preheated higher temperatures are employed it is necessary to enclose the mold in flask 18 or sleeve 19 as shown in Figure 4. The molten refractory metal to be cast is poured into the cavity of the preheated mold while a vacuum is being applied to the cavity to prevent inclusion of any air. It is very important that the casting step be carried out in the absence of air and, if desired,

an inert atmosphere comprising a gas such as, for example, argon or other inert gas may be used. The temperature of the molten metal in contact with the surface of the cavity is immediately reduced as the heat is absorbed by the material 17. Hence, there is little opportunity for chemical reaction to take place. The mold is then subjected to external cooling which causes solidification of the casting and then of the molten material 17 Heat is continually conducted away from the mold through the mass conductive material 17 until the casting has been cooled to the point where it can be removed from the mold. The temperature at which the molten metal is poured, for example, may be upwards of 800 higher than the temperature of the mold surface, and consequently, the heat flow is from the molten metal to the mold, causing the surface of the casting to solidify almost immediately, and form at least a solid skin on the casting.

A suitable apparatus for carrying out the method of the invention is shown in Figure 6. This apparatus is particularly suitable for lower melting metals such as steel while the apparatus of Figure 7 is particularly suitable for refractory metals like titanium. Guide tube 30,

. having a cross section the same as that of the mold, is

1 ing excessively and the sealing rings mentioned hereinbelow from overheating. A series of circumferential slots 33, located within the cooled zone of the coils 29, are fitted with resilient gaskets or packings 34 to form a seal with the external surface of the mold, thus preventing atmospheric air from entering the guide tube 30.

If desired, rings like piston rings may be used in place of gaskets 34, provided they have a slight taper to give a lead to the mold being inserted. The central portion of the guide tube 30 is enclosed in a housing 35 wherein the preheating, casting and cooling steps are carried out. The upper portion of the tube 30 is enclosed in a separate housing 36, secured to the top of housing 35. The chambers defined by the housings 36 and 35 are connected to a vacuum line 40 through pipes 41 and 42, respectively. The housing 36 is provided primarily to insure a high vacuum within the housing 35 and evacuate the mold completely before casting takes place. Air carried into tube 30 between the molds and within the mold cavities is practically all removed through the vacuum line 41.

The guide tube 30 contains a series of apertures 38 through the wall thereof which permits gas escaping from the molds to flow into the chamber surrounding the tube and be removed through the vacuum lines. If desired, inert gas may be introduced into the housing 35 continuously and recirculated to insure an inert atmosphere during casting. O-ring 46 serves to seal tube 30 where it joins the housing 36. The gaskets, or O-rings, or packings 34 and 46 may be made from a resilient material, preferably one which is heat resistant, such as silicon r ubber. Heat resistant, impregnated and resilient asbestos packings which are commercially available are also satisfactory.

The preheating zone within housing 35 includes a high frequency coil 48 which surrounds the guide tube 30 and is adapted to operate at a frequency that will rapidly heat the conducting metal 17 within the molds. The central portion of the tube within the housing 35 serves as a casting zone in which a pool of molten metal 50, to be shaped in the mold, is provided in an at least partially surrounding crucible 52 surrounding the tube 30. The metal is introduced through tube 69. The crucible 52 is'ofthe usual'type which ismade from ceramic material surrounded by heating means, for example, a high frequency coil 55, so that the metal 50 is retained molten by the inductive effect of the highv frequency coil. Water circulated around the high frequency coil 55 keeps the coil cool. An opening 57 in the tube 30' communicates with the channel 14 in the top surface of the evacuated mold so that the molten metal to be shaped may flow therethrough into the mold cavity 13. It will be seen from the drawing that the molds fit snugly within the tube 30 so that the metal cannot flow out of the crucible 52 until such time as the channel 14 becomes aligned with the aperture 57. The loweri portion of the 'tube within the housing 35 comprises a cooling zone, said zone being equipped with coil 59 surrounding and contacting the tube 30. Water or other suitable cooling fluid is circulatedthrough the coil 59 to conduct away heat and reduce the temperature of'the casting to the point Where it can be removed from the mold. Chilling the molds immediately results in improved physical properties of the casting, particularly where the rate of heat transfer is controlled.

For purposes of keeping the high temperature metal 50 in.molten condition, radiant heating elements 60, as of graphite or silicon carbide, with a high frequency coil 62 surrounding them, are provided, for example, just above the surface of the metal 50 in the crucible. The end of the element 60 is parabolic and focuses the heat centrally on the pool of molten metal. Other suitable heating means may be employed, as for example, an electric arc or a parabolic silicon carbide resistor. A detailed description of the casting apparatus may be found in my copending application Serial No. 406,809, filed January 28, 1954.

In a casting apparatus of the type described, molds eight inches in length and having cavities of about three cubic inches in volume may be moved downwardly. through the guide tube 30 at a uniform speed of sixteen inches per minute. If desired, the molds may be moved intermittently through a distance of two inches, for example, and maintained in each of the preheating, casting and cooling zones for about four to five seconds. As soon as the channel 14 of'the mold becomes aligned with the aperture 57, the molten metal 50 flows. rapidly into the evacuated cavity 13, to fill the cavity.

Where the mold is made from two halves, there is a possibility that air may seep through. the joint where the halves are joined. Therefore, it is desirable to insert solid ceramic plugs 43 between successive molds to prevent such entry of air, unless, of course, the mold is enclosed in a flask, in which case the bottom of the flask serves as a plug. It is preferable that the length and number of theseplugs be such that one plug is always in sealing contact with the O-rings 34 at the entrance and exit of the guide tube to prevent atmospheric leakage.

To further improve the seal against atmospheric leakage a pool of molten sealant material 68 may be pro-' vided within the annular enclosure 37 near the lower end of the guide tube 30. The molten material may be glass, lead or another relativelylow temperature melting lubricant, and will flow through the openings 76'into the space between the molds and the guide tube. Molten material 68 is supplied through tube. 65, surrounded by. high frequency heating coils 67. Coolingcoils 58 maintain the lower end of tube 30 in cool condition and prevent thevsealantmaterial 68 from escaping. by solidifying it.

As indicated, the externalsurface of the mold or flask is smoothso that .a tight sliding contact is made with the inside surface of the guide tube 30. The finish of. such surface is preferably about 70 microns, or better.

In'operation, the air within the cavity 13, and air trappedzbetween molds and in the pores of the mold is evacuated within the housing 36. The molds are separated by plugs 43 which prevent air seepage between the mold halves. As the mold 10 passes intothe space surrounded by the high frequency coil 48-, the conductive metal 17 is heated to its melting point which, in the case of aluminum, is 659 C. The heat is immediately transferred to the lining of the mold cavity, and is effective to de-gas the mold, thus removing materials that would volatilize upon filling of the cavity with the high temperature metal and blow the casting. As the mold advances past the casting station, molten metal flows into the cavity through the aperture 57. The weight of the metal causes or assists the molds to move downwardly through the guide tube. Therefore, the walls of the mold cavity are relatively cool, and as soon as the high temperature casting metal contacts the wall it is immediately cooled. It will be noted that before the casting operation is complete the lower end of the mold 10 is already entering the cooling zone surrounded by the coil 59. Thus, cooling of the casting is effected from the bottom up to prevent trapping of any gases that might'evolve upon contact with the molten metal. effectively seals off the tube 30 by cooperation with the gaskets 34. The penultimate mold in the tube makes sealing contact with the first gasket before the lowermost mold is removed, and is further sealed by molten material 68. A clamping ring (not shown) surrounds the lowest mold to frictionally brake the advance of the stack of molds in the guide tube.

Obviously, other means may be employed in casting high temperature metals in molds of the kind disclosed, although the apparatus described has proved to operate very satisfactorily. If desired, means may be provided to permit draining of molten conductive metal 17 from the mold immediately after casting and the space vacated filled with a cooler fluid to effect complete solidification of the casting. Suitable apparatus for carrying out the casting process, including the introduction of a second coolant into the mold, is the rotary indexing mold transfer table disclosed inmy copending application Serial No. 406,809, filed January 28, 1954.

It will be apparent to those skilled in the art that the mold 10 may be of one-piece construction, formed over a destructible pattern or model. Such a pattern may be made from a low melting alloy, a thermoplastic resin, or a combustible material. The pattern is melted and poured out after the mold has set, and in the case of the as a slug or powder briquette.

combustible pattern, the same is burned out leaving a mold cavity 13 of the desired shape and size.

Figures 7 and 8 illustrate additional and somewhat modified apparatus for smelting and casting which is particularly suitable for shaping metals of group IV of the periodic table.

In accordance with the method employed in this modification, a predetermined quantity of the metal to be shaped is placed in the mold cavity in solid form, such The mold must have a sleeve or flask, enclosing the hollow space surrounding the thin-walled cavity. The mold is then heated to de-gas the cavity and melt the charge of metal therein. As it advances within the guide tube, molten coolant metal flows into the space surrounding the cavity when the opening provided for this purpose becomes aligned with an opening in the guide tube connecting to a source of molten metal. In this way the casting is cooled initially with final coolingtaking place as the mold passes a cooling coil surrounding the lower'portion of the guide tube.

Referring now to Figure 7, a hollow walled mold enclosed in a flask or sleeve, having a smooth surface exterior 119, is fed down into an embracing tube as of graphite. This mold assembly is referred to as mold 110 in this description of Figure 7. The mold 110 may be cast of material such as finely ground 96% silicon oxide, and has a cavity therein with treated surface, if desired, to prevent or reduce reaction with highly reactive refractory metal such as group IV metals, and is in con- The last mold in the tube 30 Mold 110 has one or more openings 115 leading to the enclosed chamber 112 surrounding the cavity 113 into which the final product, such as a group IV metal, may be melted. When the mold 110 is introduced into the tube 130, or prior to such introduction, a slug of metal 116, which it is desired to shape, is placed within the cavity in the mold 110 and as the mold 110 is moved down through the tube 130 it is sealingly embraced by surrounding resilient rings 134 in the recesses 133 of the tube 130. The metal may be in the form of powder or a sintered shape as well as a slug, and the weight thereof is predetermined to fill the mold cavity when melted. The tube 130 at the entrance section is water cooled by the cooling coil section 136 closely embracing the tube 130.

Plugs 111, which may be a part of the mold 110 or separate members shaped to fit the guide tube 130, are inserted alternately with the molds 110 to prevent leakage between the molds as they are progressively fed down into the vacuum chamber 135.

The chamber 135 is sealed with O-rings 146 about the tube 130 as described above and serves to evacuate tube 130 through openings 138 by means of the pipe 141 which is connected to a source of vacuum. A high frequency coil 148 is provided surrounding the porous section of the tube 130 so that as the molds 110 and the plugs 111 progress downward through the high frequency coil 148 they become highly heated, and the slug or quantity of powdered metal, or sintered metal 116 within the cavity 113 of the mold 110 immediately melts.

The mold cavity 113 is vented through opening 114 leading to the exterior of the guide tube 130 through openings 138 to assist in evacuation of the mold cavity as the mold is lowered through the high frequency coil 148. The mold 110 has also a wall opening 115 which aligns peripherally with an opening 157 in tube 130 so that the coolant metal 150 will flow by gravity through the opening 157, into the hollow space 112, surrounding the mold cavity. The at least partially surrounding pool of molten metal 150 is fully or partially enclosed within a ceramic crucible 152, and is retained molten by the high frequency coils 148 thereabout in the vacuum chamher 135. The cooling metal may be aluminum or other good thermal conductor which does not vaporize readily under vacuum. The coolant metal 150 is introduced into the crucible 152 through a wall of the vacuum chamber 135, preferably in the form of a solid ingot 150a pushed through a tube-like opening by means of a suitable pusher member 161. Water-cooled coils 162 surround the tube for the purposes of maintaining the ingot 150a solid until it reaches the zone inside the vacuum chamber where it is heated to above its melting point by the high frequency coil 148a. The solid, water-cooled section of the ingot 150a may also be sealed by sealing gaskets or O-rings 133a so as to prevent leakage of vacuum thereabout. The crucible 152 containing the molten metal may be open, or it may be sealed on top as shown in the drawing. The crucible 152 may be located outside of the vacuum chamber if desired.

After coolant metal 150 has entered the hollow space 112 of the mold 110, solidification of the molten metal 116 in the cavity 113, and also of the coolant metal, is effected rapidly and progressively as the mold passes through the embracing cooling coil 159 surrounding the tube 130 below the crucible 152.

A sealant 170, such as lead or glass, may be introduced around the mold as it is moved downward through the tube 130 and below the cooling coil. The at least partially surrounding crucible 171 which contains the molten sealant metal may be heated by the high frequency coil 172, and lead 170 may be fed in by a pusher 173 in the tube 174.- The tube 174 has a cooling and solification zone 175 through which water flows to retain the metal 170 in the solid state until it comes within the influence of the high frequency coil 172, whereupon it is melted and is fed through the openings 176 in the tube 130 to seal and lubricate the plugs and the molds as they move downward. Other sealing and lubricating materials can be substituted in place of lead, such as thermoplastic resins which are normally solid and which would solidify if by chance they were sucked upward inside the tube 130 within the embracing cooling coil 159. The sealing means may be fully or partially Within the vacuum chamber 135, rather than below it as shown in Figure 7.

An additional cooling coil 177 may be placed around the tube 130 in the cooling chamber 137. O-rings 147 within recesses in the wall of the tube elfect additional sealing and garter spring brake member 178 may be used to retard the molds and the impervious sections therebetween against the pull of gravity as they exit from the tube at the lower end thereof. When filled with metal, the molds are relatively heavy. Brake 178 may be of any suitable mechanical construction, generally a simple friction device will meet the purpose.

A number of molten material supply chambers or crucibles may be placed about the guide tube 130, with complementary openings in the tube which align with the mold openings. Thus, more than one type of material may be introduced successively either into the interior of the cavity of the mold itself or in the hollow chamber about this cavity, or to act as a sealant and lubricant for the moving molds and impervious plugs.

During all times of operation a top as well as a bottom surrounding sealing member is in contact with an axially impervious mold section or plug between the molds. Thus, it is important to provide enough O-rings at the top and bottom ingress and egress portions of tube 130, to prevent flow of atmospheric air around the molds as they enter and leave the tube. The O-rings and the entrance and exit vacuum chambers are fluid cooled, and in this way are protected from extremely high temperatures.

In practicing the invention it is important that the molten cooling fluid melt at a temperature substantially below the melting point of the metal being cast, and be a good heat conductor. Preferably, its mass should be two to three times that of the metal being cast. Under these conditions the molten cast metal is immediately cooled through the thin walled mold cavity to reduce to a minimum any reaction that might take place between the cast metal and the mold cavity or its lining.

Referring to Figure 8, which is the upper portion of the apparatus of Figure 7 at the heating coil 148, the normally solid plug 111 has been modified to include a cavity and is designated by the numeral 180. A slug or briquette of metal 116 to be shaped, larger than the mouth of the mold cavity 113, may be placed within the cavity of the plug to supply the cavity 113 of mold 110 with molten metal. As the slug passes within the electrical field of coil 148 it is melted in a very few seconds. For example, a slug weighing four ounces will melt in four to ten seconds, when the coil 148 has a power of kilovolt amperes and a frequency of 9600 cycles. The molten metal falls into the cavity 113 without splattering into the vacuum chamber since the cavity in the plug and in the mold are sealed off, though gas pervious.

The coils 148 may be enclosed in an active nonconductive ceramic material which insulates the coil and prevents scintillation. Under such conditions higher frequencies may be employed.

It will be understood that the method of feeding solid metals to the crucibles surrounding the guide tube and the method of feeding the molten metal to the mold, as shown in the vacuum chamber of Figure 7, can also be used in the apparatus of Figure 6.

Other modifications of my invention will occur to those skilled in the art and it is my intention not to limit the invention other than is necessitated by the scope of the appended claims.

This applicationisa'continuation in part of my applications Serial No; 225',949,-filed May'12, 1951, now Patent No. 2,716,790, issued- Sept. 6, 1955, and Serial No. 202,707, filed December 26, 1950, nowabandoned, insofar as they contain common subject matter.

What I claim is:

1-. The method of-casting refractory metal which comprises placing a' predetermined amount of said metal in a mold having a-thirr-walled cavity backed by a hollow enclosed-space, heating said metal to melt it and fill the mold cavity, and introducing molten thermally-conductive material at a temperature substantially below the melting point of said refractory metal into said hollow space to cool said casting through said thin wall.

2. The method of casting refractory metal which comprises placing a predetermined amount of said metal in a mold having a thin-Walled cavity backed by a hollow enclosed space, evacuating said cavity and maintaining said vacuum while heating said metal to melt it and fill the mold cavity, and introducing molten thermally-conductive material at a temperature substantially below the melting point of said refractory metal into said hollow space to cool said casting through said thin wall.

3. The method of casting refractory metal which comprises placing a predetermined amount of said metal in a mold having a thin-walled cavity backed by a hollow enclosed space, evacuating said cavity and maintaining said vacuum while passing said mold through a high frequency electrical field to melt the metal and fill the mold cavity, and introducing molten thermally-conductive material at a temperature substantially below the melting point of said refractory metal into said hollow space to cool said casting through said thin wall.

4. An apparatus for casting metals including in combination an elongated refractory guide tube, the interior of which is connected to a source of vacuum, a series of molds adapted to slide through said tube, said molds being spaced from each other by close fitting disc-like plugs which seal the tube against air seepage through the molds, heating means associated with said tube for melting metal charged into said molds, and cooling means spaced from said heating means for cooling said molten metal prior to discharge of the mold from said tube.

5. An apparatus for casting metals including in combination an elongated refractory guide tube, the interior of which is connected to a source of vacuum, a series of molds adapted to slide through said tube, said molds being spaced from each other by close fitting disc-like plugs which seal the tube against air seepage through the molds, heating means associated with said tube for melting metal charged into said molds, means for introducing molten materials between the walls of said mold and the walls of said tube to seal the space around the molds, and cooling means spaced from said heating means for cooling said molten metal prior to discharge of the mold from said sleeve.

6. An apparatus for casting metals including in combination an elongated refractory perforated guide tube having its central portion enclosed in a vacuum chamber, a series of molds adapted to slide through said tube, each having an enclosed hollow space surrounding the cavity thereof, heating means associated with said tube for melting a solid charge of metal within the cavity of the mold, a receptacle containing molten cooling material joining said tube, filling means for introducing said molten cooling material into said hollow space of each mold as it advances past said receptacle and cooling means for conducting heat from said molten charge through said cooling material.

7. An apparatus for casting metals including in combination an elongated refractory perforated guide tube having its central portion enclosed in a vacuum chamber, a series of molds adapted to slide through said tube, each having an enclosed hollow space surrounding the cavity thereof, sealing rings at either end of said tube for making close contactwith the side walls of said molds to seal the tube against air seepage, heatingmeans associated with said tube for melting a solid charge of metal within the cavity of the mold, a receptacle containing molten cooling material joining said tube, filling means for introducing said molten cooling material into said hollow space of each mold as it advances past said receptacle and cooling means for conducting heat from said molten charge through said cooling material.

8. In a casting apparatus, guide means for directing=a mold past-a plurality of stations, a mold disposedin said guide means having a relatively thin ceramic refractory shell backed by a conductive metal, preheating means associated with said guide means adapted to melt the conductive metal and thus preheat the mold shell, casting means adjacent said preheating means for pouring molten casting metal into said preheated mold and cooling means adjacent said casting means effective to solidify the conductive metal within the mold and thus cool the shell to solidify the casting.

9. A method for casting refractory metal which comprises providing a mold having a relatively thin refractory shell backed by a conductive metal, said conductive metal having a melting point substantially lower than the melting point of the refractory casting metal, melting said conductive metal to preheat said mold, pouring molten refractory metal into said mold, and resolidifying said conductive metal to cool the refractory metal cast within the mold.

10. The mold of claim 8 in which the refractory shell is made from a ceramic taken from the group consisting of alumina, silica and zirconia and the conductive metal is aluminum.

11. The method of claim 9 in which the resolidification of the conductive metal is effected from the bottom of the mold upwardly.

12. The method of continuously casting refractory metal which comprises providing a succession of molds having an unshaped mass of said refractory metal in the cavities thereof, advancing said molds through a high frequency electrical field to degas and melt said metal so that the molten metal will flow by gravity to assume the shape of the mold cavity, and continuing to advance said molds through an embracing cooling zone whereby the shaped metal is solidified.

13. The method of continuously casting refractory metal which comprises providing a succession of molds each having a cavity of relatively thin refractory material surrounded by an enclosed hollow space, placing a predetermined unshaped mass of said metal in each said cavity, advancing said molds through a guide tube in which the molds are subjected successively to a vacuum to evacuate the cavity, a high frequency field to degas and melt the metal in the cavity, and a two-stage cooling zone in which first said enclosed hollow space is filled with a molten coolant material followed by cooling of the entire mold, whereby the casting is solidified, and removing said molds one at a time from said guide tube.

14. In a method of casting a heat softenable material which comprises providing a guide tube maintained at subatmospheric pressure and a succession of molds, each slightly spaced from the interior of said tube and having a cavity for shaping said material, and advancing said molds through said tube past a filling station communieating with said tube for filling said cavities with said material, the step of sealing the interior of said evacuated guide tube from ingress of gas from the atmosphere which comprises casting a layer of fluid heat softenable material on the outside of the mold to seal the space between the mold and the guide tube.

15. In a method of casting molten refractory metal which comprises providing a guide tube maintained at subatmospheric pressure and a succession of molds, each slightly spaced from the interior of said tube and having a cavity for shaping said metal, and advancing said 7 References Cited in the file of this patent UNITED STATES PATENTS Rohn June 30, 1931 Sherwood et al. Dec. 18, 1934 12 Sherwood et al. Apr. 23, Payne Aug. 10, Jacklin June 26, Kennedy Mar. 17, Davis Dec. 1, Lutz Mar. 23, Kohl July 6, Findlay June 7, Brennan Sept. 6, Brennan Aug. 20,

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