Melting furnace including wire-discharge ion plasma electron emitter

An apparatus for melting an electrically conductive metallic material includes a vacuum chamber and a hearth disposed in the vacuum chamber. At least one wire-discharge ion plasma electron emitter is disposed in or adjacent the vacuum chamber and is positioned to direct a wide-area field of electrons into the vacuum chamber, wherein the wide-area electron field has sufficient energy to heat the electrically conductive metallic material to its melting temperature. The apparatus may further include at least one of a mold and an atomizing apparatus which is in communication with the vacuum chamber and is positioned to receive molten material from the hearth.

BACKGROUND OF THE TECHNOLOGY

1. Field of Technology

The present disclosure relates to equipment and techniques for melting metals and metallic alloys (hereinafter “alloys”). The present disclosure more specifically relates to equipment and techniques utilizing electrons to melt metals and alloys and/or the materials while in a molten state.

2. Description of the Background of the Technology

The alloy melting process involves preparing a charge of suitable materials and then melting the charge. The molten charge or “melt” may then be refined and/or treated to modify melt chemistry, remove undesirable components from the melt, and/or affect the microstructure of articles cast from the melt. Melting furnaces are powered by either electricity or the combustion of fossil fuels, and selection of a suitable apparatus is largely influenced by the relative costs and applicable environmental regulations, as well as by the identity of the material being prepared. A variety of melting techniques and apparatus are available today. General classes of melting techniques include, for example, induction melting (including vacuum induction melting), arc melting (including vacuum arc skull melting), crucible melting, and electron beam melting.

Electron beam melting typically involves utilizing thermo-ionic electron beam guns to generate high energy substantially linear streams of electrons which are used to heat the target materials. Thermo-ionic electron beam guns operate by passing current to a filament, thereby heating the filament to high temperature and “boiling” electrons away from the filament. The electrons generated from the filament are then focused and accelerated toward the target in the form of a very narrow, substantially linear electron beam. A type of ion plasma electron beam gun also has been used for preparing alloy melts. Specifically, a “glow discharge” electron beam gun described in V. A. Chernov, “Powerful High-Voltage Glow Discharge Electron Gun and Power Unit on Its Base”, 1994 Intern. Conf. on Electron Beam Melting (Reno, Nev.), pp. 259-267, has been incorporated in certain melting furnaces available from Antares, Kiev, Ukraine. Such devices operate by producing a cold plasma including cations which bombard a cathode and produce electrons that are focused to form a substantially linear electron beam.

The substantially linear electron beams produced by the foregoing types of electron beam guns are directed into the evacuated melting chamber of an electron beam melting furnace and impinged on the materials to be melted and/or maintained in a molten state. The conduction of electrons through the electrically conductive materials quickly heats them to a temperature in excess of the particular melting temperature. Given the high energy of the substantially linear electron beams, which can be, for example, about 100 kW/cm2, linear electron beam guns are very high temperature heat sources and are readily able to exceed the melting and, in some cases, the vaporization temperatures of the materials on which the substantially linear beams impinge. Using magnetic deflection or similar directional means, the substantially linear electron beams are rastered at high frequency across the target materials within the melting chamber, allowing the beam to be directed across a wide area and across targets having multiple and complex shapes.

Because electron beam melting is a surface heating method, it typically produces only a shallow molten pool, which can be advantageous in terms of limiting porosity and segregation in the cast ingot. Because the superheated metal pool produced by the electron beam is disposed within the high vacuum environment of the furnace melting chamber, the technique also beneficially tends to degas the molten material. Also, undesirable metallic and non-metallic constituents within the alloy having relatively high vapor pressures can be selectively evaporated in the melting chamber, thereby improving alloy purity. On the other hand, one must account for the evaporation of desirable constituents produced by the highly-focused substantially linear electron beam. Undesirable evaporation must be factored into production and can significantly complicate alloy production when using electron beam melting furnaces.

Various melting and refining methods involve the electron beam melting of feed stocks using thermo-ionic electron guns. Drip melting is a classic method used in thermo-ionic electron beam gun melting furnace for processing refractory metals such as, for example, tantalum and niobium. Raw material in the form of a bar is typically fed into the furnace chamber and a linear electron beam focused on the bar drip-melts the material directly into a static or withdrawal mold. When casting in a withdrawal mold, the liquid pool level is maintained on the top of the growing ingot by withdrawing the ingot bottom. The feed material is refined as a result of the degassing and selective evaporation phenomena described above.

The electron beam cold hearth melting technique is commonly used in the processing and recycling of reactive metals and alloys. The feedstock is drip melted by impinging a substantially linear electron beam on an end of a feedstock bar. The melted feedstock drips into an end region of a water-cooled copper hearth, forming a protective skull. As the molten material collects in the hearth, it overflows and falls by gravity into a withdrawal mold or other casting device. During the molten material's dwell time within the hearth, substantially linear electron beams are quickly rastered across the surface of the material, retaining it in a molten form. This also has the effects of degassing and refining the molten material through evaporation of high vapor pressure components. The hearth also may be sized to promote gravity separation between high-density and low-density solid inclusions, in which case oxide and other relatively low-density inclusions remain in the molten metal for a time sufficient to allow dissolution while high density particles sink to the bottom and become trapped in the skull.

Given the various benefits of conventional electron beam melting techniques, it would be advantageous to further improve this technology.

SUMMARY

According to one non-limiting aspect of the present disclosure, an embodiment of an apparatus for melting an electrically conductive metallic material is described. The apparatus includes a vacuum chamber, a hearth disposed in the vacuum chamber, and at least one wire-discharge ion plasma electron emitter within or adjacent to the vacuum chamber. The wire-discharge ion plasma electron emitter is positioned to direct a wide-area electron field into the chamber, wherein the electron field has energy sufficient to heat the electrically conductive metallic material to its melting temperature. A mold or other casting or atomizing device is in communication with the chamber and is positioned to receive molten material from the hearth. Certain non-limiting embodiments of the melting apparatus may be operated at chamber pressures greater than chamber pressures of conventional electron beam melting furnaces so as to reduce or eliminate the vaporization in the chamber of volatile elements from the molten material.

According to another non-limiting aspect of the present disclosure, an electron beam cold hearth melting furnace is described including a vacuum chamber and a hearth disposed in the vacuum chamber, the hearth including a molten material holding region. The furnace further includes at least one wire-discharge ion plasma electron emitter that is disposed in or is adjacent to the vacuum chamber. The hearth and the at least one wire-discharge ion plasma electron emitter are disposed so that a wide-area electron field emitted by the wire-discharge ion plasma electron emitter at least partially impinges on the molten material holding region and any material disposed within the region. A withdrawal mold is in communication with the chamber and is positioned to receive molten material from the hearth. At least one material feeder is in communication with the chamber and is adapted to introduce electrically conductive feed material into the vacuum chamber in a position over a region of the hearth. Certain non-limiting embodiments of the electron beam cold hearth melting furnace may be operated at vacuum chamber pressures greater than chamber pressures of conventional electron beam melting furnaces so as to reduce or eliminate the vaporization in the vacuum chamber of volatile elements from the molten material.

According to yet another non-limiting aspect of the present disclosure, a method of processing a material is provided. The method includes introducing a material including at least one of an electrically conductive metal and metallic alloy into a melting chamber that is maintained at a low pressure relative to atmospheric pressure. The material is subjected to a wide-area electron field within the chamber so as to heat the material to a temperature above its melting temperature. The wide-area electron field is at least partially generated by at least one wire-discharge ion plasma electron emitter. Optionally, a casting or a powder is formed from the material subsequent to or simultaneous with subjecting the material to the electron field. In certain non-limiting embodiments of the method the pressure within the melting chamber is greater than the melting chamber pressures of conventional electron beam melting furnaces so as to reduce or eliminate the vaporization in the melting chamber of volatile elements from the molten material within the melting chamber.

According to a further aspect of the present disclosure a method of processing a material is provided including introducing at least one electrically conductive material selected from titanium, titanium alloys, tungsten, niobium, tantalum, platinum, palladium, zirconium, iridium, nickel, nickel base alloys iron, iron base alloys, cobalt, and cobalt base alloys into a chamber maintained at a low pressure relative to atmospheric pressure. The material is subjected to a wide-area electron field within the chamber to heat the material to a temperature above a melting temperature of the material, wherein the wide-area electron field is generated by a wire-discharge ion plasma electron emitter. Optionally, at least one alloying additive is added to the material within the chamber. A casting or a powder is formed from the material either subsequent to or simultaneous with subjecting the material to the electron field. In certain non-limiting embodiments of the processing method the pressure within the chamber is greater than the melting chamber pressures of conventional electron beam melting furnaces so as to reduce or eliminate the vaporization in the chamber of volatile elements from the molten material within the melting chamber.

According to yet a further aspect of the present disclosure a method is provided for decreasing or eliminating undesirable evaporation of volatile elements from a molten material during heating of the material in a vacuum chamber of an electron beam melting furnace. The method includes maintaining the pressure within the vacuum chamber at at least 40μ (5.3 Pa) during at least a portion of the time for which the material is being heated in the vacuum chamber.

The reader will appreciate the foregoing details, as well as others, upon considering the following detailed description of certain non-limiting embodiments of apparatus and methods according to the present disclosure. The reader also may comprehend certain of such additional details upon carrying out or using the apparatus and methods described herein.

DETAILED DESCRIPTION OF CERTAIN NON-LIMITING EMBODIMENTS

In the present description of non-limiting embodiments and in the claims, other than in the operating examples or where otherwise indicated, all numbers expressing quantities or characteristics of ingredients and products, processing conditions, and the like are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, any numerical parameters set forth in the following description and the attached claims are approximations that may vary depending upon the desired properties one seeks to obtain in the apparatus and methods according to the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

The present disclosure, in part, is directed to an improved design for an electron beam furnace for melting metals and metallic alloys and/or for maintaining the materials in a molten state for use in preparing metallic castings or powders. A conventional thermo-ionic electron beam gun melting furnace is schematically illustrated inFIG. 1. Furnace110includes vacuum chamber114surrounded by chamber wall115. Multiple thermo-ionic electron beam guns116are positioned outside and adjacent chamber114and direct discrete linear electron beams118into chamber114. Feed material in the forms of metallic bar120and alloying powder122are introduced into chamber114by a conventional bar feeder115and a conventional particle or granule feeder117, respectively. The linear electron beam118of one of the electron beam guns116impinges on and melts an end of bar120, and the resulting molten alloy124falls into water-cooled copper refining hearth126(a “cold hearth”) within chamber114. The thermo-ionic electron beam guns116are of a conventional design and generate electrons by heating a suitable filament material. The guns116focus the generated electrons to a point, and the electrons are projected from the guns116in the form of a tightly focused, substantially linear beam. Thus, the electrons projected from the guns116essentially impinge on the target as a point source. The heating of the target by the point source of electrons is facilitated by rastering the linear electron beams118across at least a portion of the targets, similar to the manner of rastering electrons across the phosphor screen of a cathode ray television tube. Rastering the substantially linear electron beam118of a thermo-ionic electron beam gun116across the end region of bar120, for example, melts the bar120.

With further reference toFIG. 1, the molten alloy124deposited in hearth126is maintained in a molten state by rastering certain of the substantially linear electron beams118across the surface of the molten alloy124in a predetermined and programmed pattern. Powdered or granulated alloying materials122introduced into the molten alloy124by feeder117are incorporated into the molten material. Molten alloy124advances across the hearth126and drops from the hearth by gravity into a copper withdrawal mold130. Withdrawal mold130includes a translatable base134so as to accommodate the length of the growing ingot132. Molten alloy124initially collects in withdrawal mold130as molten pool131, and progressively solidifies into ingot132. Impingement of electrons onto molten pool131by means of rastering one or more of the substantially linear electron beams118across the pool's surface advantageously maintains regions of the pool131, particularly at the pool edges, in a molten state.

In furnaces utilizing one or more substantially linear electron beams to heat material in the furnace chamber, such as a conventional thermo-ionic electron beam gun melting furnace, alloys including volatile elements, i.e., elements with relatively high vapor pressure at the furnace melting temperatures, tend to boil off from the molten pool and condense on the relatively cold walls of the furnace chamber. (Common alloying elements having relatively high vapor pressures at temperatures commonly achieved by electron beam melting include, for example, aluminum and chromium.) The substantially linear electron beam melting technique is particularly conducive to volatilization, which is a significant disadvantage of conventional electron beam furnaces when alloying, as opposed to refining or purifying, for at least two reasons. First, the overall and localized chemical composition of the alloy becomes difficult to control during melting due to unavoidable losses of highly volatile elements from the molten pool. Second, the condensate of the vaporized elements tends to build up on the furnace walls over time and can drop back into the melt, thereby contaminating the melt with inclusions and producing localized variations in melt chemistry.

Without intending to be bound by any particular theory, the inventors believe that the foregoing disadvantages of conventional electron beam melting furnaces result from the action of conventional substantially linear electron beams on the materials processed within electron beam furnaces. As suggested above in connection with the description ofFIG. 1, conventional electron beam cold hearth melting technology utilizes substantially linear electron beams to both melt the raw materials introduced into the furnace and to maintain the temperature of the molten material as it flows along and over the cold hearth, and into the casting mold. Such furnaces typically include multiple electron beam sources, wherein each source produces a substantially linear electron beam that is essentially a point source. These “points” of intense electron concentration must be rastered rapidly over the areas to be heated so that the average temperature needed to melt the material and allow the molten material to adequately flow is reached throughout the target area. Because of the point source nature of the linear electron beams, however, the spot at which the electron beam impinges on the alloy is heated to an extremely high temperature. This phenomenon of localized intense heating can be observed as visible white radiation emitted from the particular spot at which the electron beam impinges on the solid or molten alloy within the furnace. It is believed that the intense superheating effect that occurs at these spots, along with the high vacuum maintained in the furnace chamber, readily evaporates the relatively volatile elements within the alloy, resulting in the excessive evaporation of the volatile elements and concomitant condensation on the chamber walls. As noted above, such condensation risks contamination of the bath as the condensed material drops back into the molten alloy, and this can result in, for example, marked compositional heterogeneities in the cast ingots.

An improved design for an electron beam melting furnace described herein utilizes one or more wire-discharge ion plasma electron emitters as at least a part of the electron source in such a furnace. As used herein, the term “wire-discharge ion plasma electron emitter” refers to an apparatus that produces a relatively wide, non-linear field of electrons by impinging positively charged ions onto a cathode and thereby releasing electrons from the cathode. The electron beam produced by a wire-discharge ion plasma electron emitter is not a linear beam, but instead is a three-dimensional field or “flood” of electrons that, when impinged on the target, covers a two-dimensional region that is very large relative to the small point covered by impinging a substantially linear electron beam onto the target. As such, the electron field produced by wire-discharge ion plasma electron emitters is referred to herein as a “wide-area” electron field, with reference to the relatively much smaller point of contact produced by conventional electron guns used in electron beam melting furnaces. Wire-discharge ion plasma electron emitters are known in the art (for use in unrelated applications) and are variously referred to as, for example, “wire ion plasma (WIP) electron” guns or emitters, “WIP electron” guns or emitters and, somewhat confusingly, as “linear electron beam emitters” (referring to the linear nature of the plasma-producing wire electrode(s) within certain embodiments of the devices).

Wire-discharge ion plasma electron emitters are available in a variety of designs, but all such emitters share certain fundamental design attributes. Each such emitter includes a plasma or ionization region including a positive ion source in the form of an elongate wire anode to produce plasma including cations, and a cathode that is spaced from and positioned to intercept positive ions generated by the wire. A large negative voltage is applied to the cathode, causing a fraction of the positive ions in the plasma generated by the wire positive ion source to be accelerated toward and to collide with the cathode surface such that secondary electrons are emitted from the cathode (the “primary” electrons being present within the plasma along with the positive ions). The secondary electrons produced from the cathode surface form a non-linear electron field that typically has the three-dimensional shape of the positive ion plasma impacting the cathode. The secondary electrons are then accelerated from the vicinity of the cathode back toward the anode, experiencing few collisions in the process of passing through the low-pressure gas within the emitter. By properly designing and arranging the various components of the wire-discharge ion plasma electron emitter, a wide field of energetic secondary electrons can be formed at the cathode and accelerated from the emitter and toward the target.FIG. 2is a simplified depiction of components of a wire-discharge plasma ion electron emitter, wherein a current is applied to a thin wire anode12to generate plasma14. Positive ions16within plasma14accelerate toward and collide with negatively-charged cathode18, liberating wide-area secondary electron cloud20, which is accelerated in the direction of anode12by action of the electric field between the electrodes and toward the target.

According to one non-limiting embodiment according to the present disclosure, an apparatus for melting an electrically conductive metallic material in the form of an electron beam melting furnace includes a vacuum chamber (melting chamber) and a hearth disposed in the vacuum chamber and adapted to hold a molten material. At least one wire-discharge ion plasma electron emitter is disposed in or adjacent to the vacuum chamber and is positioned to direct a non-linear, wide-area field of electrons generated by the emitter into the chamber. The wire-discharge ion plasma electron emitter produces a non-linear field of electrons having sufficient energy to heat the electrically conductive metallic material to its melting temperature. A mold or other casting or atomizing device is disposed in communication with the chamber and is positioned and adapted to receive material from the hearth. The furnace may be used to melt any material that may be melted using a conventional electron beam melting furnace, such as, for example, titanium, titanium alloys, tungsten, niobium, tantalum, platinum, palladium, zirconium, iridium, nickel, nickel base alloys, iron, iron base alloys, cobalt, and cobalt base alloys.

Embodiments of an electron beam melting furnace according to the present disclosure may include one or more material feeders adapted to introduce electrically conductive materials or other alloying additives into the vacuum chamber. Preferably, the feeders introduce the materials into the vacuum chamber in a position over or above at least a region of the hearth so that gravity will allow the materials, in solid or molten form, to fall downward and into the hearth. Feeder types may include, for example, bar feeders and wire feeders, and the feeder type selected will depend upon the particular design requirements for the furnace. In certain embodiments of the furnace according to the present disclosure, the material feeder and at least one of the one or more wire-discharge ion plasma electron emitters of the furnace are disposed so that the electron field emitted by the wire-discharge ion plasma electron emitter at least partially impinges on the material introduced into the chamber by the feeder. If the material that is introduced into the vacuum chamber by the feeder is electrically conductive, then the electron field, if of sufficient strength, will heat and melt the material.

The hearth incorporated in embodiments of a melting furnace according to the present disclosure may be selected from the various hearth types known in the art. For example, the furnace may be in the nature of an electron beam cold hearth melting furnace by incorporating a cold hearth or, more specifically, for example, a water-cooled copper cold hearth in the vacuum chamber. As is known to those of ordinary skill, a cold hearth includes cooling means causing molten material within the hearth to freeze to the hearth surface and form a protective layer thereon. As another example, the hearth may be an “autogenous” hearth, which is a hearth that is plated with or fabricated from the alloy that is being melted in the furnace, in which case the bottom surface of the hearth also may be water-cooled to prevent burn-through.

The particular hearth included in the vacuum chamber may include a molten material holding region, in which the molten material resides for a certain dwell time before passing to the casting or atomizing device fluidly connected to the vacuum chamber. In certain embodiments of a furnace according to the present disclosure, the hearth and at least one of the furnace's one or more wire-discharge ion plasma electron emitters are disposed so that the electron field emitted by the wire-discharge ion plasma electron emitter at least partially impinges on the molten material holding region. In this way, the electron field may be applied to maintain the material within the molten material holding region in a molten state, and the heating action of the electron field may also serve to degas and refine the molten material.

Certain non-limiting embodiments of a furnace according to the present disclosure include a mold for casting the molten material. The mold may be any suitable mold known in the art such as, for example, a static mold, a withdrawal mold, or a continuous casting mold. Alternatively, the furnace may include or be associated with an atomization apparatus for producing a powdered material from the molten material.

One particular non-limiting embodiment of an electron beam melting furnace according to the present disclosure includes a vacuum chamber and a hearth disposed in the vacuum chamber, wherein the hearth includes a molten material holding region. The furnace further includes one or more wire-discharge ion plasma electron emitters disposed in or adjacent the vacuum chamber. The hearth and the at least one wire-discharge ion plasma electron emitter are disposed so that an electron field produced by the emitter at least partially impinges on the molten material holding region. A withdrawal mold communicates with the vacuum chamber and is positioned to receive molten material from the hearth. A least one feeder is included in the furnace and is adapted to introduce material into the vacuum chamber in a position over at least a region of the hearth.

Any suitable wire-discharge ion plasma electron emitter may be used in connection with apparatus according to the present disclosure. Suitable embodiments of wire-discharge ion plasma electron emitters are disclosed in, for example, U.S. Pat. Nos. 4,025,818; 4,642,522; 4,694,222; 4,755,722; and 4,786,844, the entire disclosures of which are hereby incorporated herein by reference. Suitable emitters include those capable of producing a non-linear, wide-area electron field that may be directed into the vacuum chamber of the furnace and that will heat electrically conductive feed materials placed into the furnace chamber to the desired temperature.

In one embodiment of a wire-discharge ion plasma electron emitter, the emitter includes a plasma region and a cathode region. The plasma region includes at least one elongate wire anode adapted to produce a plasma including positive ions. The cathode region includes a cathode which is electrically connected to a high voltage power supply adapted to negatively charge the cathode. In the wire-discharge ion plasma electron emitter, the electrode used to produce the plasma may be one wire or multiple wires positioned along a length of the plasma region. At least a portion of the cathode impacted by the positive ions is composed of a material suitable for generating electrons. Certain non-limiting embodiments of the cathode disposed in the cathode region of the emitter also may include an insert, such as, for example, a molybdenum insert, having a high melting temperature and a low work function so as to facilitate generation of electrons. The cathode and the anode are positioned relative to one another so that the positive ions in the plasma generated by the wire anode accelerate toward and impinge on the cathode under influence of the electric field between the electrodes, liberating the wide-area field of secondary electrons from the cathode.

Certain non-limiting embodiments of the wire-discharge ion plasma electron emitter include at least one suitably electron transmissive window, such as a thin electron transmissive titanium or aluminum foil, that opens through a wall of the furnace vacuum chamber. Alternative materials from which the electron transmissive window may be constructed include, for example, BN, diamond, and certain other materials composed of low atomic number elements. As discussed herein, other embodiments of the wire-discharge ion plasma electron emitter do not include an electron transmissive window, in which case the plasma region of the emitter fluidly communicates with the vacuum chamber holding the molten material. In either case, the wide-area electron field derived enters the furnace chamber and may be impinged on the material within the chamber. If an electron transmissive window does separate the interior of the electron emitter from the vacuum chamber (as discussed further herein), then the electron field passes through the window as it is projected from the electron emitter into the vacuum chamber. In certain non-liming embodiments of a wire-discharge ion plasma electron emitter, the high voltage power supply electrically coupled to the cathode powers the cathode to a negative voltage greater than 20,000 volts. The negative voltage serves the functions of accelerating the positive ions in the plasma toward the cathode and then repelling the secondary electron field from the cathode and toward the anode.

An electron transmissive window is necessary if the pressure within the wire-discharge ion plasma electron emitter differs significantly from the pressure within the furnace chamber, in which case the foil window serves to isolate the two adjacent regions of differing pressure. An advantage of wire-discharge ion plasma electron emitters relative to non-gas containing electron emitters, such as thermo-ionic electron beam guns, is that wire-discharge ion plasma electron emitters must include gas within the plasma region to serve as the plasma source. Although wire-discharge ion plasma electron emitters can operate at very low gas pressures, such devices also can operate effectively at relatively high gas pressures. In contrast, conventional electron beam melting furnaces commonly operate at ultra low vacuum conditions, and in that case an electron transmissive window would be necessary to separate the gas atmosphere within the wire-discharge ion plasma electron emitter and the near vacuum environment within the furnace chamber. It appears, however, that volatile element evaporation within the furnace chamber may be reduced by increasing the gas pressure within the furnace chamber beyond the ultra-low levels of conventional linear (thermo-ionic emitter) electron beam melting furnaces. Those conventional pressures levels are typically within the range of 10−3to 7.5μ (10−3to 1 Pa) and do not exceed 15μ (2 Pa). Increasing pressure within the furnace chamber beyond conventional levels, i.e., to pressures exceeding 40μ (5.3 Pa), or more preferably exceeding 300μ (40 Pa), increases the pressure at the surface of the molten material within the furnace and thereby reduces the driving force for undesirable vaporization. For example, data presented in H. Duval et al., “Theoretical and Experimental Approach of the Volatilization in Vacuum Metallurgy”, suggests that there is a significant reduction in chromium vapor transport at 66.7 Pa (500 mTorr) argon relative to 4.27 Pa (35 mTorr) argon. Because wire-discharge plasma ion electron emitters already require a partial gas pressure environment (typically of helium) to be operational, the present inventors consider it possible that both the wire-discharge plasma ion electron emitter and the furnace chamber could be operated at substantially the same pressure, wherein the pressure is sufficiently high to allow the electron emitter to operate and also is higher than in conventional electron beam furnaces, thereby reducing undesirable volatilization within the furnace chamber. In such case, the electron transmissive window may be omitted such that the gas environment within the emitter and the furnace chamber is substantially the same. Alternatively, in another embodiment of a wire-discharge ion plasma electron emitter the electrons generated by the emitter pass through a gas-impermeable window that is transparent to electrons, wherein the pressure of ionizable gas within the emitter is suitable for electron emitter operation and the furnace chamber is operated at a pressure greater than conventional pressures in electron beam furnaces and is suitable for minimizing or reducing undesirable volatilization. It will be understood that the reduction in undesirable elemental vaporization would be optimized by both utilizing one or more wire-discharge ion plasma electron emitters, which do not create points of intense heating, along with furnace chamber pressures greater than is conventional in electron beam furnaces.

Further discussion of possible embodiments of an electron beam melting furnace and possible embodiments of a wire-discharge ion plasma electron emitter useful in connection with a furnace according to the present disclosure are provided below.

FIG. 3schematically illustrates one possible non-limiting embodiment of an improved electron beam melting furnace according to the present disclosure. Furnace210includes vacuum chamber214at least partially defined by chamber wall215. Wire-discharge ion plasma electron emitters216are positioned outside and adjacent chamber214. The wire-discharge ion plasma electron emitters216project wide-area electron fields218into the interior of the chamber214. Similar to the conventional furnace110shown inFIG. 1, alloy bar220is introduced into chamber214by a bar feeder215. Molten alloy226is produced by impinging the wide-area electron field218of at least one wire-discharge ion plasma electron emitter216onto bar220. The molten alloy226melted from bar220drops into water-cooled copper hearth224, is resident in the hearth224for a certain dwell time, where it is heated, degassed, and refined by one or more of the wide-area electron fields218produced by emitters216. The molten alloy226ultimately drops from hearth224into copper mold230and forms a molten pool231. Molten pool231ultimately and progressively solidifies in mold230to form ingot232. At least one of the wide-area electron fields218preferably heats the molten alloy within pool231in a manner advantageous to controlling the solidification rate of the forming ingot232.

As discussed above, the wire-discharge ion plasma electron emitters216of furnace210are designed to generate a field or “flood” of energetic electrons covering a wide area relative to the spot coverage of the substantially linear beam produced by electron beam guns used in conventional electron beam furnaces. The electron field emitters216spread electrons over a wide area and impinge on the materials to be melted and/or maintained in the molten state within the furnace210. Because the electron field it produces will cover a wide area within the furnace chamber, a wire-discharge ion plasma electron emitter will maintain a more even temperature within the electron beam melting furnace relative to a conventional electron beam furnace, and also dispenses with the need to raster a highly focused spot of electrons. Nevertheless, certain embodiments of the electron beam furnace according to the present disclosure may include components generating electric fields or other suitable components to steer the field of electrons generated by the one or more wire-discharge ion plasma electron emitters as desired. For example, in furnace210it may be desirable to raster the broad field produced by a wire-discharge ion plasma electron emitter216from side-to-side to provide additional heat to edges of the hearth224. By flooding a relatively wide area with a field of energetic electrons, rather than rastering a point source of electrons across the area, the localized intense heating effect (e.g., power per unit area) associated with substantially linear electron beams that occurs when using conventional electron beam melting furnaces is significantly reduced. This eliminates or at least significantly reduces the extent to which relatively volatile alloying elements undesirably evaporate for the reason that points of relatively extremely high temperature are not produced. This, in turn, partially or wholly obviates the compositional control and contamination problems inherent in the conventional electron beam furnace design.

As noted above, various embodiments of wire-discharge ion plasma electron emitters generally include one or more elongate wire anodes producing positive ion plasma, wherein the plasma is impinged upon a cathode to generate a field of secondary electrons that may be accelerated to impinge on a target that is to be heated. A schematic representation of one known design of a wire-discharge ion plasma electron emitter, previously used in other, unrelated, applications, is shown inFIG. 4. This emitter310includes an ionization or plasma region314in which a positive ion plasma is produced, and a cathode region316that includes cathode318. The plasma region314is filled with an ionizable gas at low pressure, and the gas is ionized in the plasma region to produce the cation-containing plasma. For example, the ionization region314may be filled with helium gas at, for example, approximately 20 mTorr. A small diameter elongate wire anode319passes through a length of the plasma region314. A positive voltage is applied to wire anode319by power supply322, and this initiates ionization of the helium gas into a plasma comprising helium cations and free electrons (the “primary” electrons). Once ionization of the helium gas is initiated, the plasma is sustained by applying a voltage to the thin wire anode319. Positively charged helium ions within the plasma are extracted from the ionization chamber314through an extraction grid326maintained at a high negative electrical potential and accelerated through a high voltage gap into the cathode region316, where the cations in the plasma impact high negative voltage cathode318. Cathode318may be, for example, a coated or uncoated metal or alloy. Impingement of helium ions on cathode318releases secondary electrons from cathode318. The high voltage gap328accelerates the secondary electrons in a direction opposite the direction of movement of the helium cations, through the extraction grid326and into the plasma region314, and then through a thin metallic foil window329made of material relatively transparent to electrons. As noted above, depending on the relative gas pressures within the electron emitter and the furnace chamber, it may be possible to omit the foil window329, in which case the electrons produced by the emitter would enter the furnace chamber directly.

The wire electrode319and the cathode318may be designed and arranged to better promote movement of the positively charged helium ions to the cathode318. Also, the cathode318and the extraction grid326may be designed and arranged to maximize secondary electron transmission through the grid326and with a beam profile suitable for penetration through foil window329, if present. The wide-area field of energetic electrons exiting the emitter310may be directed to impinge on a target positioned opposite foil window329and within the vacuum chamber of a melting furnace. Also, the window329may be sized to be as thin as possible in order to maximize electron transmission from emitter310. An aluminum-type or titanium-type foil having a thickness allowing sufficient electron transmission, while also maintaining a soft vacuum environment within the emitter310, may be used as foil window329, if necessary. Other suitably strong and acceptably electron transparent materials that may be used as a window in the apparatus, if present, will be known to those having ordinary skill. As discussed generally herein, window329may be omitted if the pressure differences between the interior of the emitter310and the vacuum chamber containing the target are not significant.

According to the present disclosure, one or more wire-discharge ion plasma electron emitters, such, for example, emitter310, may be provided to supply the energetic electrons into the vacuum chamber of an electron beam melting furnace, as a substitute for electron beam guns producing a substantially linear electron beam. As shown inFIG. 5, one non-limiting embodiment of an electron beam melting furnace according to the present disclosure includes one or more wire-discharge ion plasma electron emitters310positioned adjacent vacuum chamber330. Wide-area electron field332exits emitter310through film window329and floods at least a region of the surface of the molten alloy334in hearth336, thereby heating the alloy to maintain it in a molten state. Because the electrons impinging on the alloy in hearth336are spread across a relatively wide area, the energy focused on the molten material in any particular localized region is not great enough to result in a problematic level of volatilization of elements from the alloy, thereby reducing or obviating the alloy contamination and heterogeneity problems inherent in the use of conventional electron beam melting furnaces. As noted above, film window329may be omitted if the operating pressure differential between the emitter310and the vacuum chamber330is not significant. Also, as noted above, the vacuum chamber330preferably is operated at a pressure higher than is conventional in order to further reduce or eliminate undesirable elemental vaporization, and in such case the need for a film window partitioning the electron emitter from the furnace chamber will, again, depend on the particular pressure differential inherent in the design. Optionally, components340for magnetically steering the wide-area electron field are provided so as to allow further improved control of the melting process within the vacuum chamber330.

AlthoughFIG. 5provides a simplified view of one embodiment of a wire-discharge ion plasma electron melting furnace according to the present disclosure including a single electron emitter, it will be apparent to those of ordinary skill that actual or alternate embodiments of such an apparatus may have multiple wire-discharge ion plasma electron emitters. It also will be apparent that one or more wire-discharge ion plasma electron emitters may be incorporated in such an apparatus to: (1) melt raw materials introduced into the furnace, in the form of, for example, an alloy bar or wire; (2) maintain molten alloy resident in the furnace hearth at a temperature above the alloy melting temperature (and possibly degas and/or refine the molten alloy); and (3) maintain desired regions of the molten pool on the surface of the incrementally advancing cast ingot in a molten state, thereby influencing ingot solidification rate in a desired manner. Also, in certain embodiments, one or more wire-discharge ion plasma electron emitters may be used along with one or more electron beam guns producing conventional substantially linear electron beams.

FIGS. 6 and 7provide additional details related to one possible non-limiting embodiment of a wire-discharge ion plasma electron emitter that may be adapted for use as the source of energetic electrons in an embodiment of an electron beam melting furnace according to the present disclosure.FIG. 6is a perspective view, partly in section, of the wire-discharge ion plasma electron emitter embodiment.FIG. 7is a schematic diagram illustrating, in a simplified way, the operation of emitter510. Emitter510includes electrically grounded enclosure513, which includes cathode region511, ionization or plasma region514, and electron transmissive foil window515. Elongate wire electrode516extends through a length of ionization region514. Foil window515is electrically coupled to chamber513and thereby forms an anode that operates to accelerate electrons within chamber513therethrough to exit chamber513in the general direction of arrows “A”. Chamber513is filled with helium gas at low pressure, such as 1-10 mtorr, and is supplied with the gas by gas supply517. Gas supply517is connected to enclosure513by conduit519, which passes through valve521. A soft vacuum environment is maintained in chamber513by pump523, which is connected to chamber513by conduit526.

Cathode region511includes cathode518, which in turn includes insert520mounted on a lower surface thereof. The insert520may be composed of, for example, molybdenum, but may be composed of any material with a suitably high secondary electron emission coefficient. Cathode518is suitably uniformly spaced from the walls of enclosure513to prevent Paschen breakdown. Cathode518is coupled to high voltage power supply522by cable525which passes through insulator526and into resistor528. Power supply522supplies high negative potential, for example, 200-300 KV, to cathode518. Cathode518and insert520may be suitably cooled, such as by, for example, circulating oil or another suitable cooling fluid through conduits524.

Ionization region514includes a plurality of thin metallic ribs which are coupled both electrically and mechanically. Each rib530includes a central cut-out region to allow wire electrode516to pass through the ionization chamber514. The sides of ribs530facing cathode518form an extraction grid534. The opposed side of all or a portion of the ribs530provide a support grid536for electron transmissive foil window515. Cooling channels540may be provided to circulate a cooling fluid through and in the vicinity of ribs530to allow for heat removal from ionization region514. Electron transmissive foil window515, which may be composed of, for example, aluminum or titanium foil, is supported on grid534and is sealed to enclosure513by an O-ring or other structures sufficient to maintain the high vacuum helium gas environment within enclosure513. In certain embodiments of emitter510, a gas manifold542is provided to cool foil window515, such as with pressurized nitrogen. As discussed generally herein, window515may be omitted if the pressure differences between the interior of the chamber513of emitter510and the chamber containing the target of the electron field are not significant.

An electrical control device548is connected to wire electrode516through connector549. On activation of control device548, wire electrode516is energized to a high positive potential, and helium within ionization region514is ionized to produce a plasma including helium cations. Once the plasma is initiated in ionization region514, cathode518is energized by power supply522. Helium cations in the ionization region514are electrically attracted to cathode518by the electric field that extends from the cathode518into the plasma region514. The helium cations travel along the field lines, through the extraction grid534, and into the cathode region511. In the cathode region511, the helium cations accelerate across the full potential of the electric field generated by the energized cathode518and forcefully impinge on the cathode518as a collimated beam of cations. The impacting cations free secondary electrons from the insert520. The secondary electron field produced by the insert520is accelerated in a direction opposite the direction of travel of the helium cations, toward the wire electrode516, and through foil window515, if present.

Means may be provided to monitor the actual gas pressure within the chamber513as changes in pressure may affect the density of the helium ion plasma and, in turn, the density of the secondary electron field generated at the cathode518. An initial pressure may be set within enclosure513by appropriately adjusting valve521. Once the cation-containing plasma is initiated in plasma region514, a voltage monitor550may be provided to indirectly monitor the instantaneous quiescent pressure within the chamber513. A rise in voltage is indicative of a lower chamber pressure. The output signal of the voltage monitor550is used to control valve521, through valve controller552. The current supplied to wire electrode516by control device548also is controlled by the signal of voltage monitor550. Utilizing the signal generated by voltage monitor550to control gas supply valve521and control device548allows for a stable electron field output from emitter510.

The current generated by emitter510may be determined by the density of the cations impacting the cathode518. The density of the cations impacting the cathode518may be controlled by adjusting the voltage on wire electrode516through control device548. The energy of the electrons emitted from the cathode518may be controlled by adjusting the voltage on the cathode518through power supply522. Both current and energy of the emitted electrons can be independently controlled, and the relationships between these parameters and the applied voltages are linear, rendering control of the emitter510both efficient and effective. In contrast, conventional thermo-ionic electron beam guns cannot be controlled in a corresponding linear manner when adjusting beam parameters.

FIG. 8is a schematic illustration of one embodiment of an electron beam melting furnace according to the present disclosure, wherein the furnace610incorporates two wire-discharge ion plasma electron emitters614,616having a design as generally shown inFIGS. 6 and 7and as discussed above in connection with those figures. Furnace610includes vacuum chamber620, material feeder622, and casting or atomizing device624. Current required for operation of emitters614and616, as discussed above, is fed to the emitters by power lines626, and the interface between emitters614,616and the vacuum chamber620includes electron transmissive foil windows634,636, which allow the electron fields638generated by the emitters614,616to enter the vacuum chamber620. The foil windows634,636may be omitted if the operating pressures within the emitters614,616and the vacuum chamber are identical or do not significantly differ. Means639for magnetically steering electron fields638may be included within vacuum chamber620to provide additional process control. A hearth640, which may be, for example, a cold hearth, is disposed in vacuum chamber620. In operation, wire-discharge ion plasma electron emitters614,616are energized and generate electron fields618. An electrically conductive feed material644is introduced into the vacuum chamber620by feeder622, is melted by electron field638emitted from emitter614, and drops to hearth640. Wide-area electron field638emitted by emitter616heats, degasses, and refines the molten material642while resident in hearth640. Molten material642advances along the hearth640and drops into casting or atomizing device624and is processed to a desired form.

Although the foregoing description has necessarily presented only a limited number of embodiments, those of ordinary skill in the relevant art will appreciate that various changes in the apparatus and methods and other details of the examples that have been described and illustrated herein may be made by those skilled in the art, and all such modifications will remain within the principle and scope of the present disclosure as expressed herein and in the appended claims. For example, although the present disclosure has necessarily only presented a limited number of embodiments of electron beam melting furnaces according to the present disclosure, and also has necessarily only discussed a limited number of wire-discharge ion plasma electron emitter designs, it will be understood that the present disclosure and associated claims are not so limited. Those having ordinary skill will readily identify additional wire-discharge ion plasma electron emitter designs and may design and build additional furnace designs along the lines and within the spirit of the necessarily limited number of embodiments discussed herein. It is understood, therefore, that the present invention is not limited to the particular embodiments disclosed or incorporated herein, but is intended to cover modifications that are within the principle and scope of the invention, as defined by the claims. It will also be appreciated by those skilled in the art that changes could be made to the embodiments above without departing from the broad inventive concept thereof.