US3234606A

US3234606A - Apparatus for melting and casting

Info

Publication number: US3234606A
Application number: US221807A
Authority: US
Inventors: Jr Hugh R Smith
Original assignee: Temescal Metallurgical Corp
Current assignee: Temescal Metallurgical Corp
Priority date: 1962-09-06
Filing date: 1962-09-06
Publication date: 1966-02-15
Anticipated expiration: 1983-02-15
Also published as: FR1368497A; GB980242A; SE313894B

Description

Feb. 15, 1966 Filed Sept. 6, 1962 H, R. SMITH, JR 3,234,606 APPARATUS FOR MEL'IING AND CASTING 3 heetsSheet l INVENTOR.

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zwzmw United States Patent 3,234,606 APPARATUS FOR MELTING AND CASTING Hugh R. Smith, Jr., Piedmont, Califi, assignor to Ternescal Metallurgical Corporation, Berkeley, Calif., a corporation of California Filed Sept. 6, 1962, Ser. No. 221,807 7 Claims. (Cl. 22-73) The present invention relates to an improved method and apparatus for pouring molten metals and particularly metals or materials having very high melting points.

The melting and casting of materials having high melting points, particularly the refractory metals, such as tungsten, titanium, tantalum, niobium, and molybdenum is difficult in that they are extremely reactive at high temperatures and are very susceptible to contamination during processing. Vacuum processing of these high-meltingpoint-materials avoids contamination and is advantageous over sintering processes, because, for example, certain residual impurities, such as hydrogen, oxygen, carbon, nitrogen or other volatile impurities, with relatively low boiling points, are easily removed. By electron bombardment heating these metals to a high temperature in a vacuum, impurities are easily evaporated therefrom and readily removed from the furnace. In many castings, the temperature gradient in the casting must be religiously controlled to obtain proper directional solidification. In order to accomplish this, additional metal is conventionally poured into the mold to form a large, heated hottop or riser, which transfers heat to the cooler metal in the bottom portion of the casting to prevent undesirable stresses that might otherwise develop during the cooling process. This problem is particularly acute in casting the refractory metals. Another serious difi'iculty is casting refractory metals is that upon termination of the heating process, the metal may solidify in the crucible before the crucible is emptied. This creates a peripheral mass of solid met-a1 about the walls of the crucible. Such solidification then necessitates additional melting, which is timeconsuming, expensive, and ineflicient. The foregoing is particularly undesirable in the formation of large castings and additionally demands a high degree of care in the casting process to remain within critical directional solidification limitations.

The above problems of peripheral solidification and directional solidification become accentuated when the ad vantages of vacuum processing are desired, since the repeated heating and pouring processes must be remotely controlled.

The present invention provides an improved method of melting high-melting-point metals, especially in a vacuum, and overcomes the problems present in this type of casting by continuing the heating of the metal in the crucible while the crucible is tilted for pouring the liquid metal into a mold. For this purpose, an electron gun is mounted upon one side and near the top of the crucible or upon some part of the structure that moves in unison with the crucible, while the crucible is tilted for pouring molten metal. When ultra purity is desired, the metal may be maintained in molten state within a vacuum for any desired period of time, either in the crucible or in the mold, causing impurities to evaporate into the vacuum system for removal by the vacuum pumps. Thus, the invention provides for melting, purification and casting in one continuous and uninterrupted process.

The entire process is completed within a vacuum chamber, so that the known advantages of vacuum operation are obtained. The electron beam is focused, by a magnetic field, onto the surface of the metal within the crucible to heat and melt the metal within the crucible and to maintain at least part of the metal in a molten state. In pouring liquid metal, the crucible is tilted and the electron 3,234,606 Patented Feb. 15, 1966 beam continues to bombard the metal, so that some of the metal is always maintained in a liquid state. In the case of the refractory metals, a solid residue is left about the inner surface of the crucible and protects the walls from the very high temperatures employed in melting. Since the special relation of the electron beam and the crucible remains unchanged during pouring, heat will be continuously applied to the solid residue for melting more metal as needed and providing a continuous source of liquid metal. This then eliminates the problem of premature solidification of substantial amounts of the material, and a large casting may be poured without interruption to obtain a freedom from cracking otherwise resulting from pouring liquid metal on top of solidified metal. Addi tionally, in the present invention, the need for a large hot top or riser is eliminated because the electron beam may be switched, periodically, from the metal in the crucible to the top of the casting in the mold, for supplying heat thereto, by adjusting the field strength or the focusing electromagnetic field. This provides improved directional solidification.

Various objects and advantages of the method and apparatus of the present invention will become apparent from the following description and from the accompanying drawings illustrating certain preferred embodiments of the apparatus hereof. In the drawings:

FIGURE 1 is a transverse sectional view of a vacuum furnace including a pouring crucible in accordance with the present invention;

FIGURE 2 is a transverse sectional view of the furnace showing a crucible in pouring position above a casting mold;

FIGURE 3 is a transverse sectional view showing the crucible in tipped or pouring position with the electron beam focused into the open top of a casting mold; and

FIGURE 4 is a transverse sectional view of an alternative embodiment of the present invention with the crucible in the pouring position and electron beams focused into both the crucible and casting mold.

The present invention overcomes the problems referred to above by providing an improved method of casting metals, and, in particular, there is provided an improved method of high temperature casting. In this method, heat is applied to the top of the solid stock material to form a molten pool of hot, liquid metal. Heat is then continuously applied to the top of the metal as the liquid is poured from the top for casting. Alternatively, or simultaneously, heat is applied to the top of the poured metal, within a mold below the solid material. As the liquid metal is poured from the top of the solid metal stock, more heat is applied, to melt more stock as more liquid is required or to maintain the metal molten until later needed. A variety of suitable heating means may be employed, but the preferred method employs bombardment heating means. For example, an electron beam is directed onto the top of metal stock contained Within a cooled crucible placed above the metal to be cast, which, for economic considerations, may be provided as pieces of scrap metal. The volume about the crucible and mold is evacuated and the electron beam is focused onto the metal, to heat and melt the metal. As the scrap metal melts, it drips through the spaces between the various chunks of scrap metal, until it reaches the cold crucible walls. There, the liquid metal will solidify and form a solid peripheral coating against the relatively cold walls of the cooled crucible. This process will hereinafter be referred to as peripheral solidification, and it should be noted that, without peripheral solidification, molten tungsten, for example, would melt the walls of most crucibles. As more electron energy is applied, a molten pool of metal is formed on top of the solid metal, and, when desired, the liquid is poured into the mold by tipping the crucible. At this point, peripheral solidification which was useful as an insulator and which aided in protecting the crucible from melting, becomes a hindrance, because the solid metal thereof is not available for casting, and conventionally repeated meltings and pouring are required. However, in this invention, the electron beam is continuously focused into the crucible and energy is applied to the solid peripheral layer of metal to melt the same, as desired. Thus, as more liquid metal is required, it is melted, or, when the mold is full, the beam is turned off and the melting and casting process terminated.

The same method may be applied to a solid block of material contained within a crucible, or, alternatively, a solid block may be heated until a pool is formed in the top thereof, and the remainder of the unmelted block serves as a consumable crucible and as a source of more liquid metal.

As an additional and optional step in the method, the electron beam is also focused onto the top of the casting, or a second electron beam is simultaneously employed for the same purpose. In either event, heat is directly supplied to the casting, through the use of electron energy. Thus, the temperature of the top of the casting is controlled, and, in castings requiring close control of the temperature gradient therein, directional solidification of the casting is facilitated by directly heating the top of the casting with the electron beam. In this manner, the present invention eliminates the need for a large mass of molten metal otherwise required in such castings and usually supplied by pouring an excessive amount of liquid metal to form a hot top or riser, which is later wastefully removed from the finished casting. Thus, less metal is required to form the same casting and greater control of the temperature of this hot top is available by employing an electron beam to heat the same.

A preferred embodiment of the apparatus of the present invention is illustrated in FIGURE 1 and includes a cup-shaped crucible 11 mounted upon a pivot 12, which is in turn carried by a supporting arm 13. An electron gun 14 is rigidly mounted on the crucible 11 and all of these elements are contained within a vacuum chamber 15 defined by structure 16 and evacuated by a conventional vacuum pump 17.

The crucible 11 is formed as a container with an open, or partially open, top capable of holding chunks of ore, pieces of scrap metal, or shavings of metal, and also capable of holding molten metal after melting. The crucible may have any desired configuration, and, in the preferred embodiment of FIGURE 1, it is shown as a hollow, cylindrical container closed at the bottom and open at the top. The crucible walls define cooling passages 18 connected to a plurality of outlets 19, through which cooling fluid 20 may be circulated. Flexible tubes (not shown) may be provided for circulating cooling fluid. Cooling means are provided because most available materials suitable for crucible construction are either reactive with the refractory metals at elevated temperatures, as of the order of 3500 F., or will melt for below this temperature.

The pivot 12 is mounted on the supporting arm 13 (which may be mounted against the housing 16, as shown) so that its pivotal axis is horizontally disposed, allowing crucible 11 to be rotated thereabout in the vertical plane. The pivot is attached to crucible 11 at any convenient point which allows the crucible to be tilted in the vertical plane for pouring out molten material. Crucible 11 is also provided with a tilting mechanism 23, which may, for example, include a hydraulic cylinder 24 with an internal piston 25 connected to a tilting arm 26 on the crucible 11 through a push rod 27. A pump 28 actuates the piston 25 through fluid feeding lines 30 and 31 with suitable valving or control means (not shown) for moving the push rod 27 to the right or left for pivoting the crucible.

Electron guns are well known n the and y C011- ventional type of electron gun may be employed herein for bombarding material in the crucible. For example, the electron gun 14 may be of the type containing an electron emissive cathode 35, an accelerating anode 36, and an electromagnet 37 establishing a magnet field for bending and focusing the electron beam 38. This magnetic field has lines of force generally in a direction perpendicular to the plane of the drawings and thus perpendicular to the direction of beam travel. The field causes the beam to bend, as shown, in accordance with well known electromagnetic theory. The electron source 14 is rigidly mounted on the crucible 11, opposite the intended pouring side, and the field strength of the electromagnet 37 is adjusted to bend the electron beam 38 around and onto the surface of a metal 39 contained in the crucible 11. The beam 38 provides sufficient energy to melt the stock metal 39, forming a pool 40 of hot, liquid metal atop of a solid residue 41 formed by peripheral solidification, and which is maintained solid by the relatively cold walls of the crucible 11. When the crucible 11 is tipped about the pivot 12, the electron source 14, being secured to crucible 11, will turn with the crucible so that the electron beam 38 is maintained at the same relative angle with respect to the crucible walls and remains permanently focused on top of the metal 39. The beam then continues to heat the metal as it is poured. It should be noted that the present state of the art in the field of electron sources is such that satisfactory use of electron beams in the higher pressure ranges has been successfully accomplished. Thus, if the advantages of vacuum purification are not required, the method and apparatus disclosed herein may be employed with pressures of the order of several millimeters of mercury when an electron beam is used as the heating source.

Operation of the apparatus hereof is initiated by charging the crucible 11 with material to be melted and evacuating the chamber 15. Additional material may be placed in the crucible through a vacuum lock 43, preferably disposed at the top of the housing above the crucible. With the crucible filled and in a horizontal position, as shown in FIGURE 1, the electron source is energized to generate an electron beam and to focus the beam into the crucible and onto material therein.

In the melting and casting of low temperature metal, such as copper, substantially all of the material may be melted, as the temperatures employed need not be sufficiently high to damage the crucible. With high temperature metals, the above-noted peripheral solidification provides an insulating layer of metal about the interior of the crucible. Electron beam energy is controlled to provide a desired amount of heat to the material for maintaining a molten pool thereof within the crucible, and the cooling fluid, which is passed through the crucible walls, maintains the crucible temperatures within a safe range. For example, the crucible may be formed of copper (melting point 1981 F.) and refractory metals (having melting points above 3000 F.) operated upon therein without crucible damage.

Following initial melting of the material within the crucible to form a molten pool 40, the crucible is pivoted into the position shown in FIGURE 2, so that molten metal flows from the crucible downward into the mold 44. During and following this pouring of material from the crucible, the electron beam 38 remains focused within the crucible so that material therein is maintained liquid to facilitate pouring. The mold 44 is also provided with passageways 45 and a plurality of outlets 46, through which cooling fluids may be passed to cool mold 44 and to provide the desired temperature gradient necessary to establish any required directional solidification.

Referring to FIGURE 2, it can be seen that, after the molten metal 40 has been poured into the mold 44 by tipping the crucible 11 in the vertical plane about the horizontally-disposed axis of the pivot 12, the frozen residue 41 will remain in the crucible. Since the electron source 38 is securely attached to the crucible 11, it will also be tipped and will remain focused within crucible 11 upon the frozen residue 41 for melting the same and providing a continuous supply of liquid metal 40, even in the tipped position. In order to compensate for any change in level of the molten pool 40 within the crucible because of tipping, the electron beam is adjusted so that it remains on any part of the surface of the pool 41? contained in crucible 11. Should no more liquid be desired, the electron beam 38 is extinguished by cutting off the supply of current to filament 35, thereby stopping the supply of heat to pool 40, causing pool 40 to solidify. On the other hand, if a greater amount of liquid be desired, the beam may be intensified, accelerating the melting of the solid residue 41 and providing more liquid without the necessity of restoring the crucible to its upright position for reloading or reheating. In this manner, a continuous source of liquid metal is provided and liquid may be poured off when desired, thereby obviating the problem of peripheral solidification.

In FIGURE 3 there is shown an alternative electron beam focusing wherein the beam 38 is directed onto the cast metal 47 which has been poured into the mold 44. As the liquid metal is poured, the beam may be switched back and forth from the frozen residue 41 in the crucible 11 to the cast metal 47 in the mold 44 as needed simply by switching the magnet energization or the accelerating voltage to vary the beam trajectory. In this manner, a desired amount of heat is supplied to the material in the mold so as to achieve the requisite directional solidification.

An alternative embodiment of the apparatus, as shown in FIGURE 4, provides a plurality of electron guns mounted upon the crucible. In this case, the electron gun 51 emits an electron beam 52, which is focused on the metal 53 in the crucible 54, so as to provide a continuous heat supply thereto. Another electron source 55 emits an electron beam 56, which is focused on the top of the cast metal 57, or the riser thereof in the mold 58, for providing thereat a continuous source of heat.

In either manner, a hot top may be maintained on top of the casting which will facilitate directional solidification of the metal in the mold. Since many molds must be designed to allow for directional solidification of the casting therein, the design problem is herein simplified because the added heat applied at the top of the casting alleviates the need for relying solely on heat absorbed from a large riser provided at the top of the casting to supply heat to the casting as it cools and to control its cooling rate. Because electron energy is used to supply heat, the size of the riser in the mold may be reduced, providing a substantial decrease in the amount of costly material required in the riser, especially when the expensive refractory metals are cast. This greatly reduces the expense and time in forming castings, and provides greater temperature control.

It should be noted that after melting the metal, the energy of the electron beam may be lowered so as to just maintain the molten metal in liquid state, allowing evaporation of impurities into the vacuum system. Thus, full advantage of vacuum casting and purification may be attained, and, if ultra purity is desired, as, for example, in the use of nickel-based, iron-based or cobalt-based super alloys, suitable for use in turbines, the present invention provides a faster and improved method of continuously casting high-melting-point metals in a vacuum where the impurities are driven off. Thus, this invention provides a method and apparatus whereby metals are maintained in a molten state throughout the process, without the disadvantages presented by undesirable peripheral solidification.

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

What is claimed is:

1. An apparatus for melting and casting materials in a vacuum, said apparatus comprising, an open-topped crucible defining a cavity adapted to contain materials for melting and movable between a material retaining and a material pouring position, means defining an enclosed chamber surrounding said crucible, means adapted to evacuate said enclosed chamber, electron gun means rigidly secured to said crucible and movable therewith between the material retaining and the material pouring positions adapted to emit an electron beam capable of melting said material, and means for focusing the electron beam upon the surface of the material contained in said cavity while said crucible is in said material retaining position and upon the surface of non-molten material remaining in said cavity While said crucible is in said material pouring position.

2. An apparatus for melting and casting high temperature materials in a vacuum, said apparatus comprising, a crucible including means defining an open-topped cavity therein adapted to contain materials for melting, said crucible being movable between a material retaining and a material pouring position and being provided with cooling jackets adapted to dissipate heat therefrom to prevent melting of the material in said crucible adjacent said cavity defining means, means defining an enclosed chamber surrounding said crucible, means adapted to evacuate said enclosed chamber, electron gun means rigidly secured to said crucible and movable therewith between the material retaining and the material pouring positions adapted to emit an electron beam capable of melting said material, and means for focusing the electron beam upon the surface of the material contained in said cavity while said crucible is in said material retaining position and upon the surface of non-molten material remaining in said cavity while said crucible is in said material pouring position.

3. An apparatus for melting and casting materials in a vacuum, said apparatus comprising, an open-topped crucible adapted to contain materials for melting and movable between a material retaining and a material pouring position, means defining an enclosed chamber surrounding said crucible, means adapted to evacuate said enclosed chamber, an open-topped mold located within said enclosed chamber and positioned below the crucible so as to accept the material poured therefrom when said crucible is in said material pouring position, electron gun means rigidly secured to said crucible and movable therewith between the material retaining and the material pouring positions adapted to emit an electron beam capable -of melting said material, focusing means for focusing the electron beam upon the surface of the material exposed at the open top of said crucible while said crucible is in said material retaining position, said focusing means further being adapted to selectively focus said electron beam while said crucible is in the material pouring position alternately upon the surface of the material exposed at the open top of said crucible and upon the surface of the material accepted by said mold and exposed at the open top thereof to maintain at least a portion of the material in said mold in a molten state.

4. An apparatus for melting and casting high temperature materials in a vacuum, said apparatus comprising, a crucible including means defining an open-topped cavity therein adapted to contain materials for melting, said crucible being movable between a material retaining and a material pouring position and being provided with cooling jackets adapted to dissipate heat therefrom to prevent melting of the material in said crucible adjacent said cavity defining means, means defining an enclosed chamber surrounding said crucible, means adapted to evacuate said enclosed chamber, an open-topped mold located within said enclosed chamber and positioned below said 7 crucible so as to accept the material poured therefrom when said crucible is in the material pouring position, electron gun means rigidly secured to said crucible and movable therewith between the material retaining and the material pouring positions adapted to emit an electron beam capable of melting said material, focusing means for focusing the electron beam upon the surface of the material exposed at the open top of said crucible while said crucible is in said material retaining position, said focusing means further being adapted to selectively focus said electron beam while said crucible is in the material pouring position alternately upon the surface of the material exposed at the open top of said crucible and upon the surface of the material accepted by said mold and exposed at the open top thereof.

5. An apparatus for melting and casting materials in a vacuum, said apparatus comprising, means defining an enclosed chamber, means adapted to evacuate said enclosed chamber, an open-topped crucible defining a cavity adapted to contain materials for melting and movable between a material retaining and a material pouring position pivotally supported within said chamber in a manner so as to cause said crucible to normally assume said material pouring position, means operatively connected to said crucible adapted to releasably support said crucible in said material retaining position, said last-mentioned means being operable to cause said crucible to assume said normal position to accomplish pouring of said material, electron gun means rigidly secured to said crucible and movable therewith between said material retaining and material pouring positions adapted to emit an electron beam capable of melting said materials, and means for focusing the electron beam upon the surface of the material contained in said cavity while said crucible is in said material retaining position and upon the surface of non-molten material remaining in said cavity while said crucible is in said material pouring position.

6. An apparatus for melting and casting materials in a vacuum, said apparatus comprising, means defining an enclosed chamber, means adapted to evacuate said enclosed chamber, an open-topped crucible defining a cavity adapted to contain materials for melting and movable between a material retaining and a material pouring position pivotally supported within said chamber in a manner so as to cause said crucible to normally assume said material pouring position, a hydraulic cylinder positioned externally of said chamber, said cylinder including a hydraulic piston selectively movable therein in response to hydraulic pressure applied thereto, and a piston rod connected to said piston and movable therewith extending into said enclosed chamber, said rod being operatively connected to said crucible whereby movement of said piston in said cylinder causes movement of said crucible between said material retaining and material pouring positions, electron gun means rigidly secured to said crucible and movable therewith between the material retaining and material pouring positions adapted to emit an electron beam capable of melting said material, and means for focusing the electron beam upon the surface of the material contained in said cavity while said crucible is in said material retaining position and upon the surface of nonmolten material remaining in said cavity while said crucible is in said material pouring position.

7. An apparatus for melting and casting materials in a. vacuum, said apparatus comprising, an open-topped crucible adapted to contain materials for melting and movable between a material retaining and a material pouring position, means defining an enclosed chamber surrounding said crucible, means adapted to evacuate said enclosed chamber, an open-topped mold located within said enclosed chamber and positioned below said crucible so as to accept the material poured therefrom when said crucible is in the material pouring position, plural electron gun means rigidly secured to said crucible and movable therewith between the material retaining and the material pouring positions adapted to emit electron beams capable of melting said material, focusing means for focusing one of said electron beams upon the surface of the material exposed at the open top of said crucible while said crucible is in both said material retaining and material pouring positions, and focusing means for focusing another of said electron beams while said crucible is in said material pouring position upon the surface of the material accepted by said mold and exposed at the open top thereof.

References Cited by the Examiner UNITED STATES PATENTS 2,937,217 5/1960 Baker et al Iii-31 3,068,309 12/1962 Hanks 2273 XR 3,157,922 11/1964 Gruber 2273 J. SPENCER OVERHOLSER, Primary Examiner.

MICHAEL V. BRINDISI, WILLIAM J. STEPHENSON,

Examiners.

Claims

1. AN APPARATUS FOR MELTING AND CASTING MATERIALS IN A VACUUM, SAID APPARATUS COMPRISING, AN OPEN-TOPPED CRUCIBLE DEFINING A CAVITY ADAPTED TO CONTAIN MATERIALS FOR MELTING AND MOVABLE BETWEEN A MATERIAL RETAINING AND A MATERIAL POURING POSITION, MEANS DEFINING AN ENCLOSED CHAMBER SURROUNDING SAID CRUCIBLE, MEANS ADAPTED TO EVACUATE SAID ENCLOSED CHAMBER, ELECTRON GUN MEANS RIGIDLY SECURED TO SAID CRUCIBLE AND MOVABLE THEREWITH BETWEEN THE MATERIAL RETAINING AND THE MATERIAL POURING POSITIONS ADAPTED TO EMIT AND ELECTRON BEAM CAPABLE OF MELTING SAID MATERIAL, AND MEANS FOR FOCUSING THE ELECTRON BEAM UPON THE SURFACE OF THE MATERIAL CONTAINED IN SAID CAVITY WHILE SAID CRUCIBLE IS IN SAID MATERIAL RETAINING POSITION AND UPON THE SURFACE OF NON-MOLTEN MATERIAL REMAINING IN SAID CAVITY WHILE SAID CRUCIBLE IS IN SAID MATERIAL POURING POSITION.