US3247554A - High vacuum casting with electron bombardment heating - Google Patents

Description

P 1966 c. w. HANKS ETAL 324-7 554 HIGH VACUUM CASTING WITH ELECTRON BOMBARDMENT HEATING Filed Aug. 6, 1962 Paws? JuPPLY SUPPLY INVENTORfi (MW; 55 w, firm/M Alva/1 miM/mg 1e;

3,247,554 HIGH VACUUM CASTING WITH ELECTRON BOMBARDMENT HEATING Charles W. Hanks, Orinda, and Hugh R. Smith, In, Oakland, Ca'lif., assignors to Staulier Chemical Company, New York, N.Y., a corporation of Delaware Filed Aug. 6, 1962, Ser. No. 219,353 1 Claim. (Cl. 22-572) The present application is a continuation-in-part of our copending application Serial No. 764,357, filed September 30, 1958, now abandoned, which is a continuation of our then copending application Serial No. 600,561, which issued December 6, 1960 as US. Patent No. 2,963,530.

This invention relates to the casting of metals and the like in a high vacuum, and in particular to improved apparatus for the continuous casting of sound, void-free rod-like ingots of highly devolatilized material.

Metals and the like that have been cast in a high vacuum, generally at an absolute pressure of one micron of mercury or less, have exceptional characteristics because of the high degree of purity and devolatilization which can thus be obtained. The object of this invention is to provide improved apparatus for casting metals and the like in a high vacuum, and in particular to apparatus capable of continuously casting sound, void-free ingots of highly devolatilized material.

Briefly stated, this object is achieved by continuously supplying molten material into the open top of a liquidcooled annular mold, situated in a high vacuum tank, from the bottom of which the cast material can be continually withdrawn as a rod-like ingot. In order to produce a sound, void-free ingot, it is necessary to maintain a pool of molten material in the top of the mold, so that the molten material supplied into the mold will not solidify too quickly, but rather solidification and crystallization of the material takes place continuously at the bottom of the pool. Maintaining such a pool presents a difiicult heating problem in the high vacua which must be provided for the desired degree of devolatilization. Electric arc heating requires too high a pressure, and induction heating and the like is prevented by the short circuiting action of the casting mold. According to the present invention the molten pool is maintained by electron bombardment heating from an annular cathode disposed in vertical alinement above the open top of the casting mold, and the molten material is continually supplied into the casting mold by dropping it through the annular cathode structure.

' The invention will be better understood from the following detailed description of an illustrative embodiment taken in connection with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an installation for the continuous production of homogeneous ingots from material initially supplied in sponge form,such as titanium sponge; and

FIG. 2 is a transverse section taken along the line 2--2 through the cathode structure disposed above the casting mold.

In the diagram of FIG. I, illustrative of a complete installation including associated equipment not directly a part of the present invention, the various elements are shown in purely schematic form, without indication of their actual structure. The elements illustrated are either conventional, so that their structure is well known, or will be described in'detail hereinafter.

In FIG. 1 the vacuum system comprises a main treatment chamber or tank 1, and a vacuum lock 3, these being provided with exhaust ports communicating with ducts 5 and 7 respectively, leading to exhaust pumps red Sates Patent lock 3 through a wide feed passage 13 that may be closed and sealed by a gate valve 15. The vacuum lock chamher 3 preferably has a relatively small volume as compared with the main tank 1. It is provided with a loading gate 17, also preferably a vacuum-tight gate valve, through which the material to be treated can be introduced.

Immediately below the gate 17 is positioned a feed hopper 19 that supplies a vibratory feed mechanism 21. These parts are so arranged that the apparatus 21 operates to feed divided material 22 from the hopper olf of its end and drop it through the channel 13 when the gate valve 15 is open. Material falling through the gate valve is received by a second hopper 23 within the main tank. Hopper 23 also is shown supplying a vibratory feed mechanism 25, adapted to supply a flow of material to the apparatus with which the present invention is directly concerned at the desired rate.

As stated, this apparatus is conventional and its oper ationis well understood. To place a charge of material for treatment in it, gate valve 15 is closed, valve 17 is opened and hopper 19 is filled with a charge of the material to be treated. Valve 17 is then closed and the lock is exhausted through duct 7 by pump 11 to substantially the same degree of vacuum as that at which the main tank 1 is maintained. This evacuation removes the major portion of the gas held in the interstices of the sponge and the interspaces between particles. Pump ill has sufiicient capacity to accomplish this evacuation be fore hopper 23 can be emptied at the prescribed rate of feed. When the required vacuum has been established within the vacuum lock 3, valve 15 is open and the feed '21 is activated. The operation of the feed is adjusted so that it operates much more rapidly than the feed 25 within the main tank, filling the hopper 23 before the charge in hopper 19 is exhausted. The operation is then repeated, feed 25 working continuously while feed 19 works intermittently and the vacuum in the main tank 1 is not broken at all during the operation of the apparatus.

The pump 9 should have sufficient capacity to reduce the base pressure in the tank 1 to a fraction of 21 micron, and to keep the operating pressure within the tank, when gas is being evolved, to less than one micron Hg. It follows that pump 11 should be able to reduce the pressure within the vacuum lock to the operating pressure within the time allowance given by the capacity of the hopper 23.

The continuous feed from the vibrator mechanism 2.5

supplies a third small hopper 27, the bottom of which opens directly into a duct 29 containing the feed screw 31 of a screw conveyor. The shaft 33 of the conveyor extends outwardly through the rear end of duct 29 and through a vacuum-tight seal or gland 35, where it connects to a drive motor 37. The latter is preferably of the adjustable-speed type, although this may not be necessary in an installation adapted to handle material of a constant grade and average gas content.

The feed screw 31 forces the material into the forward end 39 of the duct. This end at least of the duct must be of conducting material; the remainder of the duct may be, and usually will be, of metal as well. It must also be able to withstand the temperature of the molten metal at its outlet or discharge end and to withstand possible chemical attack or dissolution in the molten metal. some punposes graphite will meet these requirements. Preferably, however, this discharge end of the duct is made of metal of high heat conductivity, usually copper, and is fluid-cooled as, for example, by a water-jacket 4'1 connecting through leads 43 to a water circulating system. The cooling system must be fluid-tight; in one embodiment of the invention the jack and ducts are Heliarc" Welded of copper to insure continuity.

satia i- Patented-Apr. 26, 1966 I For i The duct is shown in the present case as being horizontal. It is generally preferable that it extend generally .transversely across the tank 1, but it may slant upward or downward. It is also generally preferable that the opening in the discharge end should face generally upward and in nearly all cases it should be inclined with respect to the horizontal as shown. The preferred angle of the opening for most purposes lies between 30 and 80 degrees with the horizontal. With a tubular duct the slanted orifice provides a pouring lip which guides the molten metal from the fusion zone to drop in a definite course instead of falling in a more or less random fashion from any point of the melted surface. It should be mentioned, however, that because of the high surface tension of the melted material it has proved possible to maintain a complete liquid seal covering an overhanging surface, and that it has also proved possible to maintain a seal and practice the invention using a vertical duct fed from below to form a horizontal liquid layer. The use of a transverse feed and upward-facing discharge are therefore precautionary measures that lead to simpler mechanical structures, rather than necessary features of the invention.

A thermionically-emissive cathode structure, indicated generally in FIG. 1 by the reference character 44, is mounted with its emissive surface spaced from and generally parallel to the discharge orifice of the duct. Various forms of structure have been used for the cathode, and have given successful operation.

An ingot mold 54 is positioned within the tank, immediately below the lip formed at the discharge end of the duct 39. The mold can be formed in the same general manner as the end 39 of the duct, i.e., of copper with a surrounding water-jacket supplied by tubing 54' connecting with a water supply. The mold is open at the bottom and immediately below it is a seal 55 through which an ingot 57, formed within the mold, can be continuously withdrawn without breaking the vacuum in the tank 1. A thermo-emissive cathode 59 is mounted immediately above the opening in the top of the ingot mold. The cathode can be formed of a single turn of tungsten wire or tape, substantially coaxial with the mold 54, and surrounded by an annular focusing shield 62, having an opening through which the material can fall to the top of the ingot.

In operation the conveyor screw 31 feeds the divided material continuously to the forward or right end 39 of the transverse feed duct. Here the material is melted by an electric discharge from the cathode structure 44, and as the material melts it drops, as shown, through the annular cathode 59 and into the open top of casting mold 54. Thus, as substantially continuous supply of molten material is fed into the top of the casting mold.

' The bottom of the ingot mold 54 is plugged by the ingot 57, so that the mold forms a cup into which the metal drops. In starting the apparatus for the first time it can be plugged by any other material of proper dimension, e.g., a steel shaft which extends through seal 55 and into the mold, later to be discarded. Where the melted material touches the water-chilled wall of the mold it immediately solidifies, as was the case with the water-cooled duct, and therefore does not attack the mold. Enough discharge is maintained from the cathode 59 to the forming ingot to maintain a small pool of melted material at the upper end of the mold, into which the discharge from the duct falls and is promptly diffused therein, even though it may have cooled slightly, by radiation, in its fall. -It is generally desirable that the molten pool just cover the top of the ingot 57, so that there is minimum contact between the molten material and the walls of mold 54, and yet the entire upper surface of the ingot is covered with molten material, which is in the continuous iii process of solidifying at its boundary with the ingot, whereby sound, void-free castings are formed. Only enough power is supplied to the discharge at the mouth of the mold to maintain a small liquid pool of this character at that point. This pool gradually freezes as the ingot is withdrawn, leaving no definite lines of cleavage or lamine but instead of a completely homogeneous ingot which can be rolled, drawn or subjected to any other treatment required for further fabrication.

A preferred form of cathode is a horizontal loop 59 of tungsten wire, which is connected through leads 60 and 61 to the electrical power supply and is heated to incandescence by current through the loop provided by the power supply. In consequence of its high temperature the tungsten loop 59 thermionically emits a copious supply of electrons. Furthermore, cathode 59 is maintained at a substantial negative potential, typically 15,000 or 20,000 volts, by the power supply 53, whereas the molten material within mold 54 is maintained at ground potential by electrical contact with the mold and the cast ingot 57, both of which are electrically connected to ground. Hence, the electrons are accelerated to high velocities by the applied voltage, and bombard and thereby heat the upper surface of the molten pool as an anode. For focusing the electrons on to the molten pool and preventing undesirable bombardment of the vacuum tank walls and other parts, a generally annular metal shield 62 surrounds the cathode in the manner shown. A connection 63 between this shield and the center of loop 59 maintains the metal shield substantially at cathode potential, and thereby the electrons thermionically emitted by loop 59 are focused on to the upper surface of the molten pool atop ingot 57 within casting mold 54.

The invention is not limited to the specific example herein illustrated and described, since it is contemplated that various changes and modifications will be apparent to those skilled in the art. The scope of the invention is defined by the following claim.

We claim:

Vacuum melting and casting apparatus comprising a vacuum tank, means for evacuating the tank, a casting mold with an open topdisposed in the tank, a horizontal annular cathode disposed within the tank in closely spaced relationship above and in vertical alignment with the open top of the mold, an annular metal shield open at the top and bottom thereof and surrounding said cathode, electrical connections for maintaining said shield substantially at cathode potential, means for supporting material to be melted over the mold and above the annular cathode, means for heating the material to melt it and cause it to drop in molten form through the annular cathode and into the open top of the mold, means for heating the cathode to a temperature to cause it to emit electrons, and means for accelerating electrons from the cathode and directing them into the top of the mold so they strike material in the upper portion of the mold to maintain a pool of molten material which receives molten material dropping into the mold.

References Cited bythc Examiner UNITED STATES PATENTS 2,554,902 5/1951 Godley 3 l5106 2,858,586 11/1958 Brennan 22-57.Z 2,880,483 4/1959 Hanks et al. 22-572 2,935,395 5/1960 Smith -10 2,963,530 12/ 1960 Hanks et al 13-31 J. SPENCER OVERHOLSER, Primary Examiner.

MICHAEL V. BRINDISI, MARCUS U. LYONS,

Examiners.

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