Abstract:
A cold hearth melting and refining arrangement for reactive or refractory metals utilizes an electron beam to automatically clean the perimeter of the pool of molten material in the hearth. A programmable logic controller is used to sweep the electron beam along at least a portion of the perimeter. The electron beam delivers energy to remelt or reevaporate volatile impurities that evaporate from the pool of molten material and redeposit on the perimeter.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to cold hearth melting and refining of reactive and refractory materials. In particular, this invention relates to improvements to electron beam cold hearth melting and refining apparatus and processes.  
         [0002]     Cold heart refining processes are now commonly employed for the production of reactive and refractory metals such as tantalum, niobium, molybdenum, tungsten, vanadium, hafnium, zirconium, titanium and their alloys. In electron beam cold hearth refining (EBCHR) processes, electron beams are used to melt target raw metals that are placed under high vacuum in water-cooled hearths. Metallic and non-metallic impurities having vapor pressures higher than those of the base constituents selectively evaporate from the molten material. Thus, the base constituent of the target material is purified.  
         [0003]     In conventional EBCHR apparatus, the hearths are usually elongated copper troughs or crucibles, which are water-cooled. The hearths are configured to receive a charge of target material (e.g., titanium scrap or lumps) from one end or side (e.g., by gravity feed). One or more electron guns are configured to direct electron beams to the surface of the target material to melt and form shallow surface pools of liquid metal in the hearth. The molten metal in the surface pools flows along the hearth and then overflows from the hearth into a water-cooled mold. The hearths usually have distinct melting and refining regions. The melting region is at the end where the target material is introduced and the refining region extends from the melting region toward the overflow mold. The shallow pools of liquid metal (i.e., molten material) in the two regions are connected by a narrow channel through a skull of solid material that separates the two regions. Advantageously in the refining process, high- and low-density inclusions are also removed from the molten material in addition to the selective evaporation of volatile impurities. High-density inclusions settle and collect in the skull, while lighter inclusions either are dissolved in the liquid metal or are held back by a dam or other physical barriers prior to flowing into the mold.  
         [0004]     In the operation of EBCHR apparatus, the electron beams are scanned over the surface of the target material to sustain the liquid state of the molten material in the melting and refining regions. Conventional electromagnetic deflectors, which may be computer controlled, are used to move the electron beams. By scanning the electron beams in suitable geometrical patterns, heat is applied to selected portions of the surface of the target material so that a liquid stream of metals flows from one end of the hearth to the other. Exemplary EBCHR apparatus are described in co-assigned Harker U.S. Pat. Nos. 4,932,635 and 4,961,776, and Harker et al. United States patent No. RE32,932, all of which are incorporated by reference in their entireties herein.  
         [0005]     In the continuous operation of the EBCHR apparatus, the volatile impurities that evaporate from the liquid stream often recondense and accumulate along the cooler banks or perimeter of the liquid stream. Additionally, inclusions in the molten liquid may also collect and accumulate along the perimeter of the liquid stream. In time these accumulations can become large and interfere or even choke the flow of the liquid stream. To prevent this, it is necessary to clear the buildup of accumulated material on the perimeter of the liquid stream. In practice, a human operator observes the buildup of the accumulated material on the perimeter of the liquid stream, and periodically directs an electron beam to clear blocking or interfering accumulations from the perimeter of the liquid stream. The operator manually sets the position and the dwell times of the electron beam at specific spots, for example, using a joystick, to melt or vaporize on the undesirable accumulations. The continuity of the refining process is thus dependent on the availability and skill of a human operator.  
         [0006]     Consideration is now being given to ways of enhancing both the processes and apparatus to improve the overall efficiency of electron beam cold hearth refining systems. Particular attention is directed to cleaning or removal of undesirable accumulations on the perimeters of the liquid streams.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly it is an object of the present invention to provide a cold hearth melting and refining arrangement which overcomes the disadvantages of the prior art.  
         [0008]     This and other objects of the invention are attained by incorporating an movable electron beam for perimeter cleaning in a cold hearth melting and refining arrangement. The electron beam automatically sweeps at least a portion of the perimeter of the pool of molten material that is formed in the melting and refining operations. The electron beam supplies heat to reevaporate, remelt or otherwise disperse volatile impurities that evaporate from the pool of molten material and recondense on the perimeter. The movement and the timing of the electron beam is controlled by a program loaded in a programmable logic controller or similar programmable device 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     Further objects and advantages of the invention will be apparent from a reading of the following description in conjunction with the accompanying drawings in which:  
         [0010]      FIG. 1  is a schematic vertical sectional view illustrating a cold hearth melting and refining arrangement in accordance with the prior art;  
         [0011]      FIG. 2  is a schematic horizontal sectional view illustrating the cold hearth melting and refining arrangement of  FIG. 1 ;  
         [0012]      FIG. 3  is a schematic illustration of aluminum wings formed on the perimeter of the liquid stream in the cold hearth melting and refining arrangement of  FIG. 1 ;  
         [0013]      FIG. 4  is a schematic representation of a cold hearth refining arrangement in accordance with the invention; and  
         [0014]      FIG. 5  is a schematic representation of the path of an electron beam for cleaning the perimeter of a liquid stream in a cold hearth melting and refining arrangement in accordance with the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENT  
       [0015]     The present disclosure provides solutions for improved electron beam cold hearth melting and refining operations. A disclosed solution concerns cleaning the perimeter of the liquid stream of molten material that flows through the cold hearth.  
         [0016]     The invention is suitable for improving the operation of EBCHR hearths, whose configurations may vary, for example, based on considerations of the type of material to the refined, throughput and other manufacturing parameters. Exemplary hearths, which can be used for refining titanium alloys, are described in Harker U.S. Pat. Nos. 4,932,635 and 4,961,776 (hereinafter “Harker”). In order that the invention herein described may be easily understood, the subsequent description is set forth with reference to the prior art cold hearths described by Harker. It will, however, be understood that the invention is equally applicable to other types or configurations of cold hearths. As an aid to the understanding of the present invention, a limited description of the cold hearths described by Harker is included herein.  FIGS. 1 and 2 , which for convenience are reproduced herein from Harker, respectively show vertical and horizontal sectional views of a prior art hearth,.  
         [0017]     With reference to  FIGS. 1 and 2 , hearth  10  includes a melting region  17  and a refining region  18 . Melting region  17  may have, for example, square dimensions of about 36″ by 36″. Refining region may have rectangular dimensions of about 36″ wide by about 6′ long. Each region may be about 3″ to 4″ deep. In the operation of the hearth, both regions hold shallow pools of liquid material  19 . The liquid material is contained by a skull  21  of resolidified material formed on a hearth bed  11 , which is cooled by a coolant passing through cooling pipes  12 . At an inlet end of hearth  10 , a chute  13  directs pieces  14  of the raw material to be refined (e.g., titanium sponge or titanium alloy turnings) into melting region  17  of hearth  10 . Electron guns  15 , which produce controllable patterns of electron beams  16 , are directed selectively on to regions  17  and  18 . In the arrangement shown in  FIG. 1 , one of the electron beams  16  is directed on raw material  14  in region  17  of the hearth so as to melt that material. Liquid material  19  from region  17  can flow into region  18  through a narrow channel  28  extending through raised portions  27  of skull  21  between the two regions  17  and  18 . Liquid material  19  flows through refining region  18  and exits hearth  10  through pouring lip  20  into mold  21 . The refined liquid material is collected in mold  21  as liquid material  26  and then further cooled to solidify as refined ingot  23 .  
         [0018]     Two of the other electron beams  16  shown in  FIG. 1  (to the right of the electron beam directed on raw material  14  in region  17 ) are computer controlled to automatically scan over central portions of the surface of refining region  18 . These electron beams supply heat to sustain the liquid state of molten material  19  as it flows toward pouring lip  20 . The scan paths of the electron beam are pre-programmed so electron beams  16  may scroll, for example, up and down the length of refining region  18 . Some of the heat supplied by electron beams  16  is drawn out by the cooled sides of hearth  10  causing material adjacent to the cooled sides to solidify as skull  21 . Coolant pipes  12  that pass through hearth bed  11  are configured so that skull  21  forms or grows inward from the sides of hearth  10  with particular geometric shapes (see e.g. skull peninsulas  30  and  29 ,  FIG. 5 ) to confine the liquid stream of molten material  19  in a suitable flow path of channel. Optionally or additionally, the scan paths of electron beams  16  may be designed to avoid heating selected portions of the material in hearth  10  to encourage growth of skull  21  in particular geometric shapes. The portions of skull  21  extending inward from the sides of hearth  10  (see e.g.,  FIGS. 2 and 5 ) form the cooler banks or perimeter of the liquid stream of molten material  19  flowing through hearth  10  toward pouring lip  20 .  
         [0019]     During the refining operations, volatile constituents evaporate from molten material  19  as it flows as a liquid stream through regions  17  and  18 . The evaporated constituents can recondense on the cooler perimeter of the liquid stream. In the illustrative case of titanium or titanium alloy refining, aluminum is a known volatile constituent. The buildup or accumulation of recondensed aluminum on the perimeter of the liquid stream can be substantial. The recondensed or redeposited aluminum may initially form a slight crust and progressively accumulate as tall “skull wings” on the perimeter of the liquid stream.  FIG. 3  schematically shows aluminum wings  300  deposited on the perimeter of molten titanium material  19 . Over time, wings  300  may, for example, grow to be over 6″ inches high. Wings  300  can interfere with the electron beam heating of molten material  19  (e.g., by electrostatic charging). Wings  300  also may physically interfere with the flow of molten material  19 .  
         [0020]     In accordance with the present invention, the growth of skull wings and other redeposit of material on the perimeter of liquid stream is inhibited by an automatic perimeter cleaning process, which is integrated with the usual EBCHR operations. In the inventive cleaning process, an electron beam (hereinafter “the cleaning beam”) is programmed to automatically sweep over selected outward portions of the hearth (e.g., over the surface skull portions adjoining the liquid stream) during the EBCHR process. The cleaning beam may be generated by a pre-existing electron gun in a given EBCHR apparatus (e.g.,  FIG. 1  gun  15 ) or by an additional electron gun  410 , which is provided, for example, in EBHCR apparatus  400  ( FIG. 4 ). A programmable logic controller (PLC)  420  or similar programmable device may be used to control electron gun  410  and associated deflection optics (not shown) for moving cleaning beam  430  along a predetermined path. PLC  420  may be a stand alone controller or a controller which is in common with other electron guns (e.g.,  FIG. 1  electron gun  15 ) in the cold hearth arrangement.  
         [0021]     The cleaning beam may be programmed to follow a path along the perimeter of the liquid stream in either the refining region and/or the melting region of the hearth. The co-ordinates of the path along the perimeter of the liquid stream are pre-recorded and utilized by the PLC  420  for this purpose. The path of the cleaning beam may be optimized for a given hearth and refining operation by empirical process learning.  FIG. 5  schematically shows an exemplary path ( 50 ) of a cleaning beam along the perimeter of the liquid stream in refining region  18  and portions of melting region  17  of a prior art hearth  20  (which is also described in Harker, see e.g.,  FIG. 3  therein). The cleaning beam may traverse path  50  continuously or in step-and-scan mode in which the cleaning beam steps along a series of spots on each of which the cleaning beam stays for a “dwell time.” 
         [0022]     The cleaning beam directs energy to the skull surface to remelt, sublime, or reevaporate material that may condense as wings along the perimeter of the liquid stream. The amount of energy delivered by the cleaning beam is a process parameter that may be suitably selected on consideration of the physiochemical properties (e.g., sublimation temperatures) of the material of concern (e.g., aluminum), and/or determined by empirical process learning. The amount of energy applied to a spot (e.g., spot  50   a ) along the scan path of the cleaning beam is a function of the intensity and the dwell time of the cleaning beam at the spot. In principle either parameter can be adjusted. In practice, however, it is more convenient to supply constant power to an electron gun keeping the intensity of the electron beam substantially fixed. The energy applied to a spot along the path of the cleaning beam then depends primarily on the dwell time of the electron beam. The co-ordinates of a spot and the dwell time of the electron beam at the spot are parameters that may be coded in a program (e.g., PLC program), which causes the electron beam to automatically sweep in a defined path along the perimeter of the liquid stream in the cold hearth. It will be understood that the sweep rate is also a programmable parameter that is suitably selected for keeping the perimeter of the liquid stream clean during the melting and refining operations. The sweep rate (like the dwell time and the spot co-ordinates) may be optimized, for example, by empirical process learning. For hearth dimensions like those mentioned, for example, with respect to  FIG. 1 , the sweep rate may be such that time it takes for the cleaning beam to circumscribe a path around the refining region may be in range from about a few milliseconds to several seconds. Exemplary dwell times of the cleaning beam at spots along the path may be in the range of about a millisecond to about several hundred milliseconds.  
         [0023]     The automated cleaning of the perimeter of the liquid stream is likely to improve refined material throughput by eliminating flow interruptions caused by overgrown wings. Further, the automated perimeter cleaning is also likely to provide a consistent compositional environment along the liquid stream during the refining operations. This consistent compositional environment can advantageously result in compositional uniformity of the refined material produced at different times during the refining operations.  
         [0024]     Although the invention has been described herein with reference to specific embodiments, many modifications and variations therein will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention.