US2880483A

US2880483A - Vacuum casting

Info

Publication number: US2880483A
Application number: US664982A
Authority: US
Inventors: Charles W Hanks; Charles D A Hunt; Jr Hugh R Smith
Original assignee: Stauffer Chemical Co
Current assignee: Stauffer Chemical Co
Priority date: 1957-06-11
Filing date: 1957-06-11
Publication date: 1959-04-07
Anticipated expiration: 1976-04-07

Description

April 7, 1959 c. w. HANKS ETAL VACUUM CASTING 2 Sheets-Sheet l MFLT 5706,@

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INVENTOM United States Patent() VACUUM CASTING Charles W. Hanks and Charles dA. Hunt, Orinda, and

Hugh R. Smith, Jr., Berkeley, Calif., assignors to Stauffer Chemical Company, New York, N .Y., a corporation of Delaware Application June 11, 1957, Serial No. 664,982 7 Claims. (Cl. 22-57.2)

This invention relates to the art of casting ingots, rods and the like by drip-melting compacted or solid melt stock into an annular, cooled mold from the bottom of which resolidied material is withdrawn continuously or semi-continuously to maintain a relatively constant level of molten material within the mold.

A crucial need has arisen for improved cold-mold casting means suitable for the large-scale commercial processing of metals, such as titanium and tantalum, that have intense chemical activities when in the molten state. In the production of such metals, the initial ingots may contain impurities, voids, porosities and surface irregularities. They may be porous compacted masses. Melting and casting is required for various purposes, including purication and the production of sound ingots, rods and the like, Ifree of voids, porosities, and surface irregularities, that are suitable for subsequent metal-Working and fabrication processes. Conventional melting and casting techniques used with less active metals are not feasible with the intensely active metals, which in the molten state attack ordinary crucibles and molds.

Heretofore, the most common technique for large-scale casting of the intensely active metals utilizes a consumable electrode and a water-cooled annular copper mold. An electric arc is established between a lower end of the consumable electrode and material within the mold. The arc heats and gradually melts the consumable electrode, which drips into a molten pool of metal within an upper portion of the annular mold. The metal held within the mold solidifies from the circumference inward, forming a bowl or skull of solidified metal that supports the pool of molten metal. Thus, contact between the intensely active molten metal and the copper mold is kept to a minimum. As the metal solidifies, it is withdrawn continuously or semi-continuously from the bottom of the annular mold to maintain a substantially constant level of molten metal within the mold.

The consumable-electrode arc-melting technique has several signiticant advantages. The rate of heat supply to the consumable electrode should be just suiiicient to maintain the desired rate of drip melting. The rate of heat supply to the pool of molten metal within the mold should be just suflicient to maintain a molten pool of adequate size for the formation of sound castings. An incorrect division of heating energy between the two electrodes, or an incorrect ratio between the heating rates and the rate of cooling within the mold, may result in either a deficiency or an excess in the pool of molten metal. A deficiency causes unsound castings, containing voids, porosities and surface irregularities. An excess causes accelerated chemical activity between the molten pool and the mold, rapidly destroys the mold, and contaminates the cast metal.

Where arc-melting is employed, the melting operation and the casting operation are interdependent, since the same electric arc supplies heat to both electrodes in a ratio that is not controllable. Hence, separation of the Significant variables affecting the separate operations is 2,880,483 4Patented Apr. 7, 1959 impossible, both experimentally and operationally. Further, the operation must be performed batch-wise because of the relatively large amount of spattering of metal which occurs as the consumable electrode melts under the influence of enormous localized current concentrations in the arc between the consumable electrode and the molten metal surface atop the cast ingot. Also, the consumable electrode must extend into the mold well past guiding supports of the electrode. The danger of shortcircuits to the copper walls of the mold is considerable, and explosions occur frequently in large-scale operations because of electric current concentrations at the side walls of the mold that melt the mold in spots and allow cooling liuid to contact molten metal.

Briefly stated, in accordance with certain aspects of this invention, an improved casting process is provided for drip-melting melt sto-ck into a molten pool maintained on the top of a resolidifying ingot or rod within a cooled mold. This process is carried out in a high vacuum, which for purposes of this patent application is defined as an absolute pressure not exceeding approximately one micron of mercury. Both the consumable melt stock and the molten pool are anodes, and are heated by highvoltage electron bombardment from one or more cathodes. Either anode (or both anodes) can be bombarded and heated at will, and at regulated rates, by regulating the electron ilow from the cathode or cathodes to the anodes. The cathodes can be made of metals or materials different than the consumable melt stock and the cast ingot Without danger of contamination of the cast ingot due to this dissimilarity. Unlike arc melting processes, the cathodes and cathode structures do not melt, nor do they touch the anodes at any time in the melting process. Their sole purpose is to supply electrons for bombarding the anodes. v

Because of the high vacuum that is maintained, the current between the electrodes is an essentially electronic current, which heats the anodes by conversion of the kinetic energy of the electrons into thermal energy as the electrons strike the anodes. Thus, the nature of the electrical discharge is very different from that of low-voltage discharges, such as electric arcs and the glow discharges that prevail at higher pressures. Metal vapors and other gaseous matter emanating from the anodes may partially or Wholly neutralize the electronic space charge.

Preferably, the consumable electrode of melt stock is a vertical rod positioned above and vertically almed with an annular mold containing the cast ingot with a molten pool of metal at its top end. In one embodiment of the invention a single annular cathode is employed for bombarding both the melt stock and the molten pool. In another embodiment, two annular cathodes are employed, one above the other, to provide better control of the heating ratio for producing ingots and rods of a higher quality and with better surfaces. In both cases focusing electrodes hereinafter described are employed for confining the electron beams to desired paths.

In the improved process, the heating rates can be controlled accurately by regulating the electron current and, particularly with the two cathode apparatus, the rate of heat supply to each anode can be independently regulated to maintain any desired heating ratio. The process can be operated continuously for long periods of time, without excessive splattering of the molten metal. The danger of short-circuits and explosions is substantially eliminated and the process operates with a high degree of safety.

According to another of its aspects, this invention provides an improved zone-refining process that is particularly useful for the purification of the intensely-active metals. As practiced heretofore, zone refining of mal terials that attack hot crucibles has been limited to thel refining of rods having suciently small diameters that Asurface tension can keep a floating zone of molten material attached to the rod. In the present invention, zone refining is accomplished by drip-melting the melt stock into a molten ypool of'jthe material being refined. Since there Vis a break or space between the melt stock and the molten pool, the present invention provides an improved interrupted zone refining process.

As material resolidiiies at the bottom and sides of the molten pool, impurities tend to remain in the molten pool and the resolidiied material is purer than the melt stock. The process can be repeated as many times as may be desired to effect successive purilications of a rod or ingot. Because of the relatively slow rates of drip melting and resolidilication used in the zone refining process, separate 'heating of vthe molten pool to maintain an adequate amount of material in the molten state is essential. Furthermore, the drip melting rate and the rate of heat supply to the molten pool are individually capable of precise regulation. The present invention makes this possible and also provides additional purilication through the evolution of volatile impurities into the high vacuum employed. With this invention, ingots and rods of large diameter can be handled, and large-scale commercial zone-refining is made feasible, even for materials having intense chemical activity when in the molten state.

vThe foregoing and other aspects of this invention may be better understood from the following description of illustrative examples taken in connection with the accompanying drawings. The Vscope of the invention is pointed out in the appended claims.

In the drawings:

Fig. 1 is a somewhat schematic vertical section of an improved casting apparatus;

Fig. 2 is a horizontal section taken along the line 2-2 of Fig. 1; and

Fig. 3 is a somewhat schematic vertical section of another improved casting apparatus.

Referring now to Fig. l of the drawings, the casting process is carried out within a closed chamber ll that is evacuated to a high vacuum through a passage 2 connected to a conventional vacuum pump (not shown). Melt stock in the form of a rod or ingot 3 is fed into the vacuum chamber through a conventional vacuum seal d. A suitable electrical connection 5 is provided for maintaining rod 3 at a reference electric potential herein called circuit ground. Circuit ground usually is established by a low-resistance electrical connection to the earth, and furnace. or building framework for safety reasons.

Rod 3 of the melt stock forms a consumable electrode disposed vertically with its lower end above and in vertical alinement with the center of an annular copper mold 6. Mold 6 is surrounded by a water jacket 7 that is connected to an inlet pipe 8 and an outlet pipe 9 arranged for the circulation of water or other liquid through the water jacket for cooling mold 6. Both the top and the bottom of annular mold 6 are open. Material melted from rod 3, as hereinafter explained, drips into the top of mold 6 and forms a pool of molten material 10. As heat flows from the molten material yto water-cooled mold 6, the molten material solidiiies inward from the circumference and upward from the bottom of the molten pool, and thus forms an ingot of resolidiied material 11 having a bowl or skull at its top which contains the pool of molten material. and minimizes Contact between the molten material and mold 6.

As melting and resolidiication proceed, rod 3 of melt stock is continuously or semi-continuously moved downward Vto keep the lower end ofV rod 3 Vat a substantially constant level within the vacuum chamber, and the ingot 11 of resolidiiied material is continuously or semi-continuously moved downward to maintain a substantially constant level of molten material within the upper portion of moldv 6. The ingot 11 of resolidiiied material is withdrawn through the open bottom of mold 6, and may be withdrawn from the vacuum chamber through a conventional'vacuum seal 12.

Thus the melt stock is continuously melted and cast into a new ingot of resolidiiied material. The melt stock may contain impurities, voids, porosities, and surface irregularities. It may be a porous compacted mass of material formed, for example, by pressing together powdered or pelletized material. If the process is properly regulated, the ingot il. of resolidilied material will be a sound casting, relatively free of voids, fissures, porosities and surface irregularities, Rod 3 may comprise a mixture of powders or pellets of different materials and ingot 11 an alloy of these materials. The process may also be used for purification or refining, to remove volatile impurities that are `evolved from the molten material into the high-vacuum, and to remove impurities that tend to remain in the molten pool as the cast material resolidifies.

For accomplishing the purposes of this invention., heat must be supplied to the consumable electrode 3 andthe pool of molten material lil. Heat is supplied to the ylower end of electrode 3 for the purpose of melting it. The rate of heat supply to the consumable electrode controls the rate of melting. Heat must be supplied to lthe molten material it) to maintain a molten pool of adequate size for the production of sound castings. The rate of heat supply to pool l0, in relation to other factors including the rate of heat transfer to mold 6, controls the depth yof the molten pool.

if the rate at which heat is supplied to pool 1d is too low, a molten pool of adequate size will not be maintained at the top of ingot 1l and each new drop of molten material that drips from electrode 3 will splash onto the top of ingot lll and there solidify quickly and irregularly. The end result will be an unsound casting, which may contain voids, fissures, surface irregularities and other undesirable characteristics.

ln accordance with the present invention, the consumable electrode 3 and the pool of molten metal lll are both heated by high-voltage electron bombardment. Electrode 3 is maintained at circuit ground through electrical connection 5, and pool lil is maintained at circuit ground through the grounded copper mold. rl`he lower end of rod 3 and the upper surface of pool 10 constitute anodes for the electron bombardment system.

An annular cathode is formed from a horizontal circular loop of tungsten wire 13. The cathode is somewhat larger in diameter than either of rod 3 and ingot l1, as is shown in the drawings. The cathode is in vertical axial alinement with rod 3 and ingot 11 and is disposed below the bottom of rod 3 and above the top of mold 6, so that electrons emitted by the cathode can bombard both of the two anodes. The two ends 13 and 13" of tungsten wire 13 pass through insulators 14 and 15 through a side wall of vacuum enclosure 1. The insulators are protected from the condensation of metallic vapors by suitable means such as shields 16 and 17.

A. transformer 13, having its primary connected to any suitable A.C. supply and having its secondary connected across the two ends 13 and 13 of wire 13, supplies to the cathode sutcient current to heat the same and pro'- duce thermionic emission of electrons from the cathode. A DC. supply 19 connected between circuit ground and the secondary of transformer 18 maintains the cathode at a negative potential relative to the two anodes, so that electrons emitted from the cathode bombard the lower end of rod 3 and the upper surface of molten pool 1l). The electrons that bombard the lower end of rod 3 gradually melt the consumable electrode, and melted metal drips into pool ll as indicated by drop 20. The electrons that bombard the upper surface of pool 10 keep an adequate amount of the material in a molten state to assure the production of sound ingots.

An important function is performed by the annular focusing electrode 2.1 which substantially surrounds cathode 13, as shown.v A metal bracket 22 helps to support cathode 13 and also provides an electrical connection between cathode 13 and focusing electrode 21, in consequence of which the focusing1 electrode is maintained at cathode potential. The annular focusing electrode 21 has a channel-shaped cross-section, as shown, which opens inwardly and shields the top, outer circumference and bottom of the annular cathode. Electrode 21 is supported by any suitable means, such as

straps

23 and 24 suspended from the top of vacuum enclosure 1 and insulated therefrom by insulators 25 and 26. The insulators are protected from the condensation of metallic vapor by suitable means such as

shields

27 and 28.

The focusing electrode 21 is maintained at cathode potential and because of the strong electric field existing between cathode and anode there is little tendency for the electrons emitted by the cathode to travel to the focusing electrode. Instead, most of the electrons emitted by the cathode 13 are directed inwardly from the cathode, and then upwardly toward the bottom of consumable electrode 3 and downwardly toward the top surface of molten pool 10. By this means the electrons are concentrated where they are needed and there is no substantial bombardment of mold 6, the sides of rod 3, and other parts that should not be bombarded.

Because of the high vacuum maintained within vacuum chamber 1, the current between cathode 13 and the two anodes is essentially electronic in character. The impedance of the space-discharge path is high and a relatively high-voltage, low-current discharge is maintained. This type of electron discharge permits regulation and control of the thermal power developed to a degree of precision that would be utterly impossible with an electric arc or similar low-impedance discharge. In the immediate vicinity of the two anodes the melting and the molten metal has sufficient vapor pressure to provide an ionizable vapor, which to some extent neutralizes the electronic space charge and provides a low-resistance discharge region immediately adjacent to each anode. This has the beneficial effect of spreading the discharge rather uniformly over both of the two anodes, and substantially eliminates localized current concentrations or hot spots that are a cause of unequal heating, metal spatterng, and other diiiiculties in the prior-art dripmelting techniques utilizing electric arcs.

The total power input to the heating system can be controlled by regulating the current supplied by D C. supply 19, and thus regulating the total electron bombardment current. The thermal power developed is equal to the square of the electron current times the resistance of the space-discharge path. The high vacuum discharge path has a high resistance. The total electron current is equal to the current supplied by supply 19. The ratio between the rate of heat supply to electrode 3 and the rate of heat supply to molten pool can be regulated by adjusting the relative distances of the two anodes from cathode 13. If the lower end of rod 3 is raised slightly from the position shown in Fig. l, a smaller percentage of the electrons emitted by cathode 13 will bombard the lower end of rod 3, and a larger percentage of the electrons will bombard pool 10. Consequently, the rate of heat supply to electrode 3 will be reduced relative to the rate of heat supply to molten pool 10. Conversely, if the lower end of rod 3 is lowered slightly from the position shown in Fig. l, a larger proportion of the electrons will bombard the lower end of rod 3, and the rate of heat supply to rod 3 will be increased relative to the rate of heat supply to molten pool 10. By controlling the total heating energy supplied to both anodes, through the adjustment of D.C. supply 19, and controlling the ratio of the heat energies supplied to the two anodes by adjusting the position of rod 3, it is evident that the heat energy supplied to each anode can be individually adjusted to approximately any desired value.

The single-cathode arrangement shown in Fig. l works quite satisfactorily if a moderate amount of variation as an irregular function of time can be tolerated in the rela'- tive distribution of heating energy between the two anodes. If a more precise regulation of the energy distribution is required (for the casting of ingots having fewer surface irregularities, or for zone refining, for eX- ample), the double-cathode configuration shown in Fig. 3 should be employed.

Referring to Fig. 3, the apparatus illustrated is essentially similar to that shown in Fig. 1 except for the greater separation between the consumable electrode 3 and molten pool 10, and the provision of two separate cathodes for bombarding respective Ones of the two anodes. To simplify and to clarify the description, parts of the Fig. 3 apparatus that are substantially the same as corresponding parts of the Fig. l apparatus are identified by the same reference numbers.

In the Fig. 3 apparatus the upper cathode 13 and fo cusing electrode 21 are substantially identical to the cathode and focusing electrode of the Fig. 1 apparatus, except that in the Fig. 3 apparatus the upper cathode is at a relatively great distance from the molten pool 10, in consequence of which most of the electrons emitted by cathode 13 bombard and heat the lower end of consumable electrode 3. Consequently, the rate of melting of electrode 3 is substantially a direct function of the input power provided by D.C. supply 19 and can be precisely regulated and controlled by regulating electrons supplied by the cathode.

A second annular cathode 29 and focusing electrode 30 are substantially identical to cathode 13 and focusing electrode 21 except that the second cathode is positioned in axial alinement with and just above molten pool 10 and is spaced a relatively great distance from the lower end of consumable electrode 3. Consequently, most of the electrons emitted by lower cathode 29 bombard and heat the upper surface of molten pool 10. For heating cathode 29 and producing a thermionic emission of electrons, a transformer 31 has its secondary connected across the two ends 29 and 29" of cathode 29 and has its primary connected to any suitable A.C. supply. A second D.C. supply 32 is connected between the Secondary of transformer 31 and circuit ground, as shown. Supply 32 provides the lower cathode with a negative electric potential relative to the pool of molten metal, so that electrons emitted by the lower cathode bombard the molten pool. With this arrangement the amount of heating energy supplied to the upper surface of molten pool 10 is a direct function of the input power provided by D.C. supply 32 (the square of the electron current times the resistance of the space-discharge path), and consequently the size of the molten pool can be precisely controlled and regulated by regulation of D.C. supply 32.

The two-cathode configuration shown in Fig. 3 makes it possible to control with a high degree of precision both the rate of melting of rod 3 and the rate of heat supplied to molten pool 10, each independently of the other. It is only necessary that the two

cathodes

13 and 29 be sufficiently separated to prevent any undesirable amount of cross-bombardment of each anode by the more distant cathode. It has been found that this can be accomplished quite satisfactorily by providing between the two cathodes a vertical separation about equal to or greater than the larger of the diameters of electrode 3 and ingot 11.

With the arrangement shown in Fig. 3, cast ingots of superior quality with a minimum of surface irregularities can be produced easily. The arrangement shown in Fig. 3 is also advantageous for zone refining processes in which the melting rate and the size of pool 10 should be quite uniform and precisely regulated. Where the requirements are less exacting the configuration of Fig. 1 may be employed with good results.

In a typical casting operation according to this invention, the configuration of Fig. 1 was operated successfully :usingB-inch diameter titanium electrodes and ingots, a f4-.inch diameter ring of 1A() inch diameter tungsten wire as a cathode, and a D.C. supply of 7000 volts providing -a total input power between about and 20 kilowatts.

The configuration illustrated in Fig. 3 was successfully operated using 3-inch diameter titanium electrodes and ingots, 4l-inch diameter rings of 1/10 .inch diameter tungsten Wire as cathodes, and a D.C. supply of about 7000 volts for each cathode. The threshold of melting of the consumable electrode was reached with an input power of about 5 kilowatts provided by supply 19. At 7 to 8 kilowatts of input power from supply 19, electrode 3 melted at a rate of approximately inches per hour. Sound and smooth-surfaced cast ingots were produced under these conditions with an input power of 12 to 15 kilowatts provided by D C. supply 32 for bombarding the moltenY pool. In addition, about l kilowatt of 'alternating current power was supplied to each cathode for heating the cathode to produce thermionic emission.

The process for zone refining is substantially the same as the casting process, except that for zone refining relatively slow melting and resolidification rates are employed to increase the tendency for impurities to remain in the molten pool as the cast material resolidifies. In Zone relining, the melting and casting process may be repeated several times, the cast ingot forming the consumable electrode for the following melting operation, to produce successive ingots of increasing purity.

The drawings are somewhat schematic, and illustrate only the essential parts of the apparatus. In practice, heat shields are employed to reduce heat flow between the hot parts and the Vacuum chamber walls.

It should be understood that this invention in its broader aspects is not limited to specific examples herein illustrated and described7 and that the following claims 'are intended to cover all changes and modifications within the true spirit and scope of the invention.

What is claimed is:

l. Casting apparatus comprising an annular mold open at its top and bottom, said mold being adapted to form a rod-like ingot of resolidilied cast material, means for supporting a rod-like consumable electrode o-f melt stock with a lower end above and in vertical alinement with the open top of said mold, substantially annular cathode means disposed between and in vertical alinement with thek bottom of said electrode and the top of said mold, means for enclosing and evacuating a space containing the bottom of said consumable electrode, the top of said mold and said cathode means, means for heating said cathode means to produce thermionic emission of electrous therefrom, means for establishing on said consumable electrode a positive electric potential relative to said cathode means so that electro-ns emitted by said cathode means bombard, heat and melt the lower end of said consumable electrode, material melted from said consumable electrode dripping into said mold, means for establishing on the material within said mold a positive electric potential relative to said cathode means so that electrons emitted by said cathode means bombard, heat and maintain a pool of molten material resting on top of the resolidified material within said mold, means foilowering said co-nsumable electrode as it melts, and means for withdrawing solidified material from the bottom of said mold to maintain a substantially constant level of molten material within said mold.

2. Casting apparatus comprising an annular mold open at its top and bottom, said mold being adapted to form a vertical rod-like ingot of resolidilied cast material, means for supporting a vertical rod-like consumable electrode of melt stock above and in vertical axial alinement with said mold, an annular filamentary cathode of larger diameter than the larger in diameter of said consumable electrode and said ingot, an annular focusing electrode having an inwardly opening channel-shaped cross-section, said focusing electrode shielding the top, outer circumference and bottom of said cathode, means for keeping said focusing electrode substantially at cathode potential for directing the electrons emitted by said cathode inwardly, means for enclosing and evacuating a space containing the bottom of said consumable electrode, the top of said mold, and said cathode, means for heating said cathode to produce thermionic emission of electrons therefrom, means for establishing on said consumable electrode a positive electric potential relative to said cathode so that electrons emitted by said cathode bombard, heat, and melt the lower end of said consumable electrode, material melted from said consumable electrode dripping into said mold, means for establishing on the material within said mold a positive electric potential relative to said cathode so that electrons emitted from said cathode bombard, heat and maintain a pool of molten material resting on top of the resolidified material within said mold, means for cooling said mold so that the molten material resolidilies inwardly from the circumference and upwardly from the bottom of said pool, means for lowering said consumable electrode as it melts, and means for withdrawing resolidified material from the bottom of said mold to maintain a substantially constant level of molten material within said mold.

3. Casting apparatus comprising an annular mold open at its top and bottom, said mold being adapted to form a rod-like ingot of resolidified cast material, means for supporting a rod-like consumable electrode of melt stock with a lower end above and in vertical alinernent with the open top of said mold, first and second substantially annular cathodes disposed between and in vertical alinement with the bottom of said electrode and the top of said mold, said first cathode being vertically spaced above said second cathode, means for enclosing and evacuating a space containing the lower end of said consumable electrode, the top ofsaid mold, and said two cathodes, means for heating both of said two cathodes to produce thermionic emissio-n of electrons therefrom, means for establishing on said consumable electrode a positive electric potential relative to said first cathode so that electrons emitted by said first cathode bombard, heat and melt the lower end of said consumable electrode, material melted from said consumable electrode dripping into said mold, means for establishing on the material within said mold a positive electric potential relative to said second cathode so that electrons emitted by said second cathode bombard, heat and maintain a pool of molten material resting on top of the resolidified material within said mold, means for lowering said consumable electrode as it melts, and means for withdrawing resolidified material from the bottom of said mold to maintain a substantially constant level of molten material within said mold.

4. Casting apparatus comprising an annular mold open at its top and bottom, said mold being adapted to form a vertical rod-like ingot of resolidilied cast material, means for supporting a vertical rod-like consumable electrode of melt stock above and in Vertical axial alinement with said mold, a first annular cathode of larger diameter Ythan said consumable electrode, a second annular cathode of larger diameter than said ingot of resolidified cast material, said cathodes being disposed between and in vertical axial alinement with the bottom of said consumable electrode and the top of said mold, said second cathode being disposed below said first cathode and vertically separated therefrom by a distance not substantially smaller than the larger of the diameter of said consumable electrode and the diameter of said ingot, means for heating both of said two cathodes to produce thermionic emission of electrons therefrom, first and second annular focusing electrodes each having an inwardly opening channel-shaped cross-section, said rst focusing electrode shielding the top, outer circumference and bottom of said first cathode, said second focusing electrode shielding the top, outer circumference and bottom of said second cathode, means for keeping said rst and second focusing electrodes substantially at the same potentials as said first and second cathodes respectively, whereby electrons emitted by said cathodes are directed inwardly, means for establishing on said consumable electrode a positive electric potential relative to said irst cathode so that electrons emitted by said irst cathode bombard, heat and melt the lower end of said consumable electrode, material melted from said consumable electrode dripping into said mold, means for establishing on the material within said mold a positive electric potential relative to said second cathode so that electrons emitted by said second cathode bombard, heat and maintain a pool of molten material resting on top of the resolidiied material within said mold, means for cooling said mold so that the molten material resolidiles inwardly from the circumference and upwardly from the bottom of said pool, means for lowering said consumable electrode as it melts, and means for withdrawing resolidiiied material from the bottom of said mold to maintain a substantially constant level of molten material within said mold.

5. The method of casting that comprises introducing a consumable electrode of melt stock into a high-vacuum chamber containing thermionically electron-emissive cathode means and a mold, evacuating said chamber to a high vacuum for maintaining an essentially electronic current to said cathode, causing electrons emitted by said cathode means to bombard, heat and melt said consumable electrode, causing material melted from said electrode to drip into said mold and there resolidfy, causing electrons emitted by said cathode means to bombard material within said mold for maintaining a pool of molten material within an upper portion of said mold, regulating the electron current between said cathode means and said consumable electrode to control the melting rate, and regulating the electron current between said cathode means and the material within said mold for controlling the amount of molten material in said pool.

6. The method of casting that comprises introducing a consumable electrode of melt stock into a high-vacuum chamber containing an electron-emissive cathode and a maintaining a pool of molten material within an upper portion of said mold, and adjusting the spacing between said cathode and said consumable electrode relative to the spacing between said cathode and said mold for adjusting the ratio of the electron currents lowing between said cathode and said consumable electrode and between said cathode and the material within said mold.

7. The method of casting that comprises introducing a consumable electrode of melt stock into a high-vacuum chamber containing lirst and second electron-emissive cathodes and a` mold, evacuating said chamber to a pressure not exceeding approximately 1 causing electrons emitted by said iirst cathode to bombard, heat and melt said consumable electrode, said consumable electrode being positioned above said mold so References Cited in the file of this patent UNITED STATES PATENTS Berghaus et al Dec. 3l, 1940 Herres June 2, 1953 Newcomb et al. Sept. 11, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NoD 2,88Gylr83 April 7'p 1959 Charles W Hanks et alo It is herehr certified 'that error appears in Jche printed specification of 'the above numbered patent requiring correction and thal Jthe said Letters .Patent should read as corrected below.

Column l,n line 53g for adventagew reed .um disadvantages uw Signed this llth of August l959 (SEAL) Attest:

KARL AXLINE ROBERT C. WATSON Commissioner of Patents Attesting Oicer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NQ, esoy/83 April '7, 195e Charles En Hanks im inthe printed specification It is hereby certified that error appears ion and that the said Letters of tlfeA above numbered patent requiring correct .Patent should read as corrected below.

Column l, line 53, for adventagess'f read 4diaetdvantage.e

Signed and this lltld day of August 19591,-

(SEAL) Attest:

KARL H, M1-LNE Attesting Ocer ROBERT C. WATSON Commissioner of Patents